Characterization of Technological Materials [1 ed.] 9783038134572, 9783037850121

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Characterization of Technological Materials

Edited by R. Saravanan

To my mother and (late) father who taught me humanity and life And to my family who provided the environment

Characterization of Technological Materials

Special topic volume with invited peer reviewed papers only.

Edited by:

R. Saravanan M.Sc., M.Phil., Ph.D. Asst. Prof. (S.G.) Research Centre and PG Department of Physics The Madura College Madurai – 625 011 Tamil Nadu, India Email: [email protected] [email protected] Website: http://www.saraxraygroup.net/

TRANS TECH PUBLICATIONS LTD Switzerland UK USA

Copyright  2011 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of the contents of this publication may be reproduced or transmitted in any form or by any means without the written permission of the publisher. Trans Tech Publications Ltd Laubisrutistr. 24 CH-8712 Stafa-Zurich Switzerland http://www.ttp.net

Volume 671 of Materials Science Forum ISSN 0255-5476 Full text available online at http://www.scientific.net

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About the Editor Dr Ramachandran Saravanan, has been associated with the Department of Physics, The Madura College, affiliated to the Madurai Kamaraj University, Madurai, Tamil Nadu, India from the year 2000. He worked as a research associate during 1998 at the Institute of Materials Research (http://www.imr.tohoku.ac.jp/eng/), Tohoku University, Sendai, Japan and then as a Visiting researcher at Centre for Interdisciplinary Research (http://www.cir.tohoku.ac.jp/e/index_e.html), Tohoku University, Sendai, Japan up to 2000. Tohoku University was adjudged as Asia’s second best University in standards–Ref. Multi- disciplinary University Ranks - (http: //cgi.cnn.com/ ASIANOW/ asiaweek/ features/ universities2000/ schools/ multi.overall.html). Earlier, he was awarded Senior Research Fellowship by CSIR, New Delhi (http://www.csir.res.in/), during Mar.1991- Feb.1993; awarded Research Associate ship by CSIR, New Delhi, during 1994 – 1997 (In CSIR research Project). Then he was awarded a Research Associateship by CSIR, New Delhi, during 1997- 1998. Later he was awarded the Matsumae International Foundation (http://www.mars.dti.ne.jp/~mif/) Fellowship - 1998 (Japan) for doing research at a Japanese Research Institute (Not availed by him due to the simultaneous occurrence of other Japanese employment). As on 2010, about 10 researchers are working under his guidance on various research topics in materials science, crystallography and condensed matter physics and he has published around 60 research articles in reputed Journals apart from around 45 presentations in conferences, seminars, symposia. He has attracted government funding in the form of Research Projects. He has completed one CSIR (Council of Scientific and Industrial Research, Govt. of India) project successfully and proposing various projects to Government funding agencies like CSIR, UGC and DST. Recently (2009), he has been awarded a CSIR research project for his research proposal on Dilute Magnetic Materials (DMS). He has written two books in the form of research monographs with details as follows; (i) “Experimental Charge Density - Semiconductors, oxides and fluorides”, Lambert Academic Publishing (LAP) AG & Co. KG, Saarbrücken, Germany, 2010 (204 pages), ISBN-13: 978-3-8383-8816-8 (ISBN-10:3-8383-8816-X) and (ii) “Experimental Charge Density - Dilute Magnetic Semiconducting (DMS) materials” Lambert Academic Publishing (LAP) AG & Co. KG, Saarbrücken,

Germany, 2010 (147 pages), ISBN-13: 978-3-8383-9666-8 (ISBN-10: 3-83839666-9). He has committed to write several books in the near future. His expertise includes various experimental activities in crystal growth, materials science, crystallographic, condensed matter physics techniques and tools as in slow evaporation, gel, high temperature melt growth, Bridgman methods, CZ Growth, high vacuum sealing etc. He and his group can handle various equipments, different types of cameras; Laue, Oscillation, Powder, Precession cameras; Manual 4-circle X-ray diffractometer, Rigaku 4-circle automatic single crystal diffractometer, AFC-5R and AFC7R automatic single crystal diffractometer, CAD-4 automatic single crystal diffractometer, Crystal pulling instruments, and other crystallographic, material science related instruments. He and his group have a sound computational capabilities too working on different types of computers. IBM – PC, Cyber180/830A – Mainframe, SX-4 Supercomputing system – Mainframe. He can handle various softwares related to crystallography and materials science. He has written many computer software programs too. Around twenty of his programs (both DOS and GUI versions) have been included in the SINCRIS software database of the International Union of Crystallography, (http://www.iucr.org/)

which

(http://www.saraxraygroup.net/).

are

also

available

at

his

website

Contributors to the topical volume, Characterization of Technological Materials Dr. T. Kajitani, Dr. Y. Miyazaki, Dr. K. Hayashi Department of Applied Physics, Graduate School of Engineering, Tohoku University Sendai 980-8759, Japan Email: [email protected] Dr. K. Yubuta Institute for Materials Research, Tohoku University, Sendai 980-8759, Japan Dr. X.Y. Huang Shanghai Institute of Ceramics, Chinese Academy of Science, 1295 DingXi Road, Shanghai 200050, China Dr. W. Koshibae Cross-Correlated Materials Research Group (CMRG), RIKEN, Wako 351-0198, Japan Dr. C. Sanjeeviraja, Dr. S. Nagarani, Dr. L. C. Nehru School of Physics, Alagappa University, Karaikudi-630 003 Tamil Nadu, India Email: [email protected] [email protected] [email protected] Dr. G. J. Shyju, Dr. S. Dawn Dharma Roy Department of Physics, N.M. Christian College, Marthandam - 629 165 Tamil Nadu, India Email: [email protected] [email protected] Dr. M. Jayachandran ECMS Division, Central Electrochemical Research Institute, Karaikudi-630 006 Tamil Nadu, India Email: mjayam54@gmail Dr. V. Swaminathan School of Materials Science and Engineering, Nanyang Technological University Singapore –639798 Email: [email protected] Dr. R. Saravanan, Mrs. M. Prema Rani, S. Saravanakumar, M. Jeya Priya, K. J. Lakshmi Sri PG Department and Research Centre of Physics, The Madura College Madurai - 625 011, Tamil nadu, India Email: [email protected] [email protected] [email protected] Website: http://www.saraxraygroup.net/ Dr. A.Xavier, Dr. R. Sathya, Dr. R. Nagarathnam Post Graduate and Research Department of Chemistry, The Madura College (Autonomous), Tamil Nadu, India, India Email: [email protected] [email protected] [email protected] Dr. T. K. Thirumalaisamy Department of Physics, H.K.R.H. College, Uthamapalayam - 625 533 Tamil Nadu, India Email: [email protected] Dr. J. GandhiRajan Post Graduate Department of Chemistry Vivekananda College (Autonomous) Chennai, Tamil Nadu, India.

 

Note from the Editor The study of materials in terms of their effectiveness/usefulness in the device application is important for the technological development of a country. It is obvious that countries which concentrated on the growth and characterization of technologically important materials have become developed countries.

Hence the growth and

characterization of new and novel materials is highly important for many countries. This special topical volume (STV) on Characterization of Technological Materials reports the methods, technology, growth and characterization of important technological materials prepared by reputed researchers around the region. The systems characterized are thermo electric materials, thin films, nano sensor materials, semiconductors etc. Many review articles have been incorporated in this volume written by eminent scientists in the field of condensed matter physics. To mention a few, Prof. T. Kajitani has written a high profile review article on Thermoelectric Energy Conversion and Ceramic Thermoelectrics. He is an expert in the growth and characterization of many important materials being used for device applications including thermo electric materials. I believe that the readers can gain sufficient knowledge and ideas for further research works through his report. Prof. C. Sanjeeviraja, is an expert in the field of thin film preparation and characterization. He has submitted two review articles on the preparation of thin film coatings and one on the preparation of nano materials. Various technological materials have been discussed in this volume. For example, the

thermo

electric

materials,

γ-Na0.7CoO2,

Na0.35CoO2·1.3H2O,

NaCo2O4,

Ca2CoO3]0.62CoO2, Ca2 (Co0.65Cu0.35)2O4]0.624 CoO2, CaCoO2]0.62CoO2, CaMnO3, etc., thin films like ZnO, IZO, Zn3In2O6, In2O3(ZnO)3, Ga-ZnO (GZO), grown using electron beam evaporation. The characterization techniques involved are electrical, structural, optical studies. Preparation of SnO2 nano material by microwave assisted technique is reported in this volume. Synthesis and electron density distribution of nano SnO2 is also reported here. The local structure of Al2O3, Cr:Al2O3, and V:Al2O3 are studied using powder XRD and reported. The thermal conductivity of different grades of semiconducting GaAs has also been reported. The studies reported here will enhance our understanding and on the respective materials and induce new ways for further development and utilization of these technological materials. In that sense, I believe, this topical volume will be highly useful to

researchers and scientists working in the fields of materials science, condensed matter physics, crystal growth, thin films and related fields. Any constructive research needs tireless efforts, motivation, hard work and implementation. The articles presented in this volume are the results of those constant efforts by the respective authors who have dedicated themselves for constructive research useful for the human society. I also dedicate this volume to hard-working researchers. R. Saravanan (www.saraxraygroup.net) Madurai- 21

Date:25/10/2010

Table of Contents About the Editor, Contributors, Note from the Editor Thermoelectric Energy Conversion and Ceramic Thermoelectrics T. Kajitani, Y. Miyazaki, K. Hayashi, K. Yubuta, X.Y. Huang and W. Koshibae Review on Indium Zinc Oxide Films: Material Properties and Preparation Techniques G.J. Shyju, S. Dawn Dharma Roy and C. Sanjeeviraja Review on Gallium Zinc Oxide Films: Material Properties and Preparation Techniques S. Nagarani, M. Jayachandran and C. Sanjeeviraja Nanomaterial Preparations by Microwave-Assisted Solution Combustion Method and Material Properties of SnO2 Powder - A Status Review L.C. Nehru, V. Swaminathan, M. Jayachandran and C. Sanjeeviraja Synthesis and Electron Density Analysis of SnO2 Nano Particles S. Saravanakumar, M. Jeya Priya and R. Saravanan Local Structural Analysis of Al2O3, Cr:Al2O3 and V:Al2O3 Using Powder XRD T.K. Thirumalaisamy, K.J. Lakshmi Sri and R. Saravanan Influence of Silicon and Boron Doping on the Thermal Conductivity of N-GaAs Single Crystals M. Prema Rani and R. Saravanan Studies on the Removal of Malachite Green Dyes by Adsorption onto Activated Carbons – A Comparative Study A. Xavier, D. Usha, J. Gandhi Rajan and M. Malarvizhi Dynamic Study of Adsorption for the Removal of Bismark Brown – Using Activated Carbons A. Xavier, R. Sathya, J. Gandhi Rajan and R. Nagarathnam

1 21 47 69 121 131 153 165 187

Materials Science Forum Vol. 671 (2011) pp 1-20 © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.671.1

Thermoelectric Energy Conversion and Ceramic Thermoelectrics T. Kajitani1,a, Y. Miyazaki1, K. Hayashi1, K. Yubuta2, X.Y. Huang3, W. Koshibae4 1

Department of Applied Physics, Graduate School of Engineering 2Institute for Materials Research Tohoku University, Sendai 980-8759, Japan 3 Shanghai Institute of Ceramics, Chinese Academy of Science, 1295 DingXi Road, Shanghai 200050, China 4 Cross-Correlated Materials Research Group (CMRG), RIKEN, Wako 351-0198, Japan a [email protected] (Submitted on Sept. 20th, 2010)

Keywords: Thermoelectricity, Seebeck effect, Spin-Seebeck effect, 2D-conduction, Thermionic effect, Oxide thermoelectrics, Cobalt-oxide superconductor, Misfit-layered oxides

Abstract. Oxide thermoelectrics are relatively new materials that are workable at temperatures in the range of 400K≤T≤1200K. There are several types of thermoelectric oxide, namely, cobalt oxides (p-type semi-conductors), manganese oxides (n-type) and zinc oxides (n-type semi-conductors) for high temperature energy harvesting. The Seebeck coefficient of 3d metal oxide thermoelectrics is relatively high due to either high density of states at Fermi surfaces or spin entropy flow associated with the carrier flow. The spin entropy part dominates the Seebeck coefficient of 3d-metal oxides at temperatures above 300K. Introduction of impurity particles or quantum-well structures to enhance thermionic emission and energy filtering effects for the oxide semiconductors may lead to a significant improvement of thermoelectric performance. Introduction Due to global warming, environment-friendly green technology and reproducible energy production technology have become subjects for urgent discussion. Electrical energy generation from the heat current can be realized by means of thermoelectric devices. The efficiency of thermoelectric energy conversion has an upper limit at the Carnot efficiency, ηCarnot = (Th-Tl)/Th, in the same way as mechanical generators, where Th and Tl are the absolute temperature of the hot and cold heat reservoirs, respectively. To get high efficiency electrical energy generation, we must develop devices with high heat resistance, high Seebeck coefficient and low thermal conductivity. Other characteristics for practical usage of devices include being mechanically reliable, non-toxic and cheap. The Japanese Ministry of Economy, Trade and Industry unveiled a “Cool earth, Energy Innovation Technology, Technology Development Roadmap” [1] in March 2008. According to the section on “Innovative Solar Energy Panels” of that roadmap, the target cost of solar energy conversion is at present 30 US-cents to around 10 cents per kilowatt-hour for the end of 2010. Thus, there is strong motivation for development of new semiconductors to meet the above target. Thermoelectric devices provide mutual exchange between the heat flux and the charge current; they function as a kind of heat pump. High performance chemical heat pumps are also considered. The performance of the heat pump is targeted to decrease by about 25% in 20 years while a 50% increase

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Characterization of Technological Materials

of its efficiency is expected. The International Energy Agency (IAEA) affiliated to the Organization for Economic Cooperation and Development (OECD) announced a road-map for the development of new technologies settled in the group of eight (G8) meeting of 2008 [2] including development of the focused solar heat electrical generators and wider utilization of the solar panel technology. Thermoelectric devices are useable either for solar heat power generators or for heat pumps. Since thermoelectric devices have no need for mechanical components such as electrical motors, or boiler/turbine systems, they are reliable and cheap. Thermoelectric phenomena Typical thermoelectric effects are Seebeck and Peltier effects, being cross effects between the heat current and electrical current formulated in the Onsager equation. There are three other thermoelectric phenomena, namely the Alkaline-Metal-Thermoelectric Converter (AMTEC) [3, 4], the spin-Seebeck effect [5-9], the energy filtering effect and the thermionic effect. Alkaline-Metal-Thermoelectric Converter (AMTEC) Figure 1 shows a schematic presentation of the AMTEC device reported by Onda et al. [4]. Metallic sodium is used for the heat exchange media. From the hot sodium reservoir, heated sodium vapor travels and condenses on the hot side of the ionic conductor (separator), consisting of the beta’’-alumina, then transfers to the cold side driven by the vapor pressure difference between the cold side where the sodium vapor catch is situated. The cooled sodium liquid trapped in the vapor catch is circulated to the hot sodium reservoir by a capillary pump. Electro-motive force (EMF) is generated due to the difference in the chemical potential of sodium ions at the hot and cold sides of the beta’’-alumina separator. Metallic contacts are plated on both sides of the separator. The essential idea of this system is the sodium ion concentration cell. The liquid sodium can be replaced by any other working fluid which is easily evaporated from the hot reservoir, ionized in the beta’’-alumina separator, running through the separator, then sublimated again from the back surface of the beta’’-alumina and finally condensed in the cold vapor catch.

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Fig.1 Schematic diagram of alkali-metal thermoelectric conversion (AMTEC) system [4]. (Liquid Na is evaporated at Th and Ph at the high temperature part. Na vapor penetrate into the beta” –alumina ionic conductor separator. From cold side of the separator, Na vapor is extracted into the cold part at Tl and Pl then recovered to liquid state. Liquid Na is pumped up to the high temperature part again) The best efficiency for the electrical power generation by the AMTEC was estimated as 22% at Th=1200K, Tl=500K with a current density of 18A/cm2 by Onda et al. [4]. Figure 2 shows the voltage and efficiency vs. current density curves of the AMTEC generator. This technology is interesting but presently difficult to realize. In future, a similar mechanism, to be operated in a lower temperature range using organic liquids and ionic conductors, could be considered.

Fig.2 Power generation efficiency and voltage vs. current density (A/cm2) relationship of the AMTEC generator [4]. Spin-Seebeck effect The spin-Seebeck effect was discovered recently by Uchida et al. [5, 6]. The temperature dependent difference in the chemical potential of the carriers having parallel and anti-parallel spins, i.e., µ↑ − µ↓ = δµF0 , generates the EMF, which is generated in the platinum electrodes due to their strong spin-orbit coupling effect. Figure 3 shows a schematic set-up of the spin-Seebeck constant measurement system.

4

Characterization of Technological Materials

Fig.3 Schematic representation of Spin-Seebeck effect thermoelectric power generation set-up. Js, σ, H, T and ESHE show the spin-flux, Pauli’s spin-matrix element, magnetic field, temperature and electric field due to the spin-Hall-effect, respectively [5]. A soft ferromagnetic alloy Ni-Fe slab is set parallel to the temperature gradient, δT, and magnetic field H. Two platinum electrodes are plated at the hot and cold ends of the slab. The EMF is generated perpendicular to the temperature gradient. The spin-Seebeck voltage, VISHE, being generated by the inverse spin-Hall effect between the non magnetic electrode, Pt, and the magnetic slab is given as,

 LN  0 (1) δµF e  dN  where, θ N , LN and dN are the spin-Hall angle in the non-magnetic electrode, Pt, the length and depth of V ISHE= −

θN

ηF / N 

the Pt layers, respectively. ηF/N is a coefficient, being linearly related with the spin current penetrating from the magnetic slab to the Pt layers. The longer the spin-diffusion lengths in the Pt and in the magnetic slab, i.e. λN and λF, the higher the spin-Seebeck voltage generated. When the magnetic field is applied normally to the slab surface, the spin-Seebeck effect and Nernst effect override. The direction of parallel and anti-parallel spins are polarized by the external magnetic field H, then picked up by the platinum electrodes due to the inverse spin-Hall effect [7] at the hot and cold ends of the Ni-Fe slab. An important point about the spin-Seebeck effect is that no electrical charge flows parallel to the temperature gradient, but parallel and anti-parallel spins flow in the opposite directions, so charge neutrality is maintained. Recently, the spin-Seebeck coefficient has been theoretically calculated [8] and compared with the Nernst coefficient. The spin-Seebeck voltage is generated perpendicular to the external magnetic field as well as the temperature gradient. The efficiency of the spin-Seebeck effect contributing to the thermoelectric power generation is almost of the same order of Nernst voltage in the ideal quasi-one-dimensional ballistic electron systems [9].

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Energy Filtering Technology and Thermionic Emission Devices The energy filtering technology and the thermionic emission devices are presently hot subjects among researchers working in the field of multi-layer materials and nano-electronics. High efficiency thermoelectric devices, especially coolers usable at around room temperature, have been proposed. Energy filtering technology in the form of a two-dimensional quantum well superstructure was proposed by Hicks et al. [10, 11]. An anisotropic two-band metal was assumed for the calculation. The thermoelectric performance of a two-band metal in the form of a two-dimensional quantum well was considered. In the metallic bismuth two-dimensional well with the width, a, in the z-direction, the dispersion relation was simply given as, 2 2 ℏ2 k x2 ℏ k y ℏ2 k z2 ε (k x , k y ) = + + (2) 2mx 2my 2mz where kx and ky are wave numbers in the x-y plane. The effective masses of electrons in the x, y and z

directions are mx, my and mz, respectively. The electrical carriers are assumed to conduct only in the lowest energy level in the quantum well. Due to low phonon thermal conductivity in the z-direction and the energy filtering effect, the Z2DT values was expected to be high. High Z2DT values were calculated for the thin Bi2Te3 quantum well at the width of a < 20Å as shown in Figure 4 [10]. Two Z2DT lines for the conduction parallel (1) to the a-b plain and (2) to the a-c plain were calculated for the Bi2Te3 wells, respectively.

Fig.4 Calculated dimension-less thermoelectric figure of merit of 2-dimensional Bi2Te3 lattices as functions of well width a, being less than 100.0Å. Two Z2DT lines (1) and (2) correspond to the electrical conduction parallel to the a-b and a-c planes, respectively [10]. High performing thermoelectric devices were proposed by Mahan et al. [12-14] and Shakouri et al. [15] based on the classic thermionic emission formulation, i.e. Richadson-Dushman’s equation for the emitting current density js from the metallic surface at T(K) and the work function –eφ(eV).

6

Characterization of Technological Materials

 eϕ  js = AT 2 exp− with A ≅ -120A/cm2 (3)   kB T  The devices, i.e. the hetero-structure integrated thermionic (HIT) devices, have a multi-layer semiconductor structure. Each layer has a relatively low Schottky barrier height, ⎜eφ⎟ ≤ 0.4 eV, between the adjacent layers. The thermoelectric efficiency, η, of a metal/semiconductor, e.g. Au/In0.7Ga0.3As, multi-layer thermionic devices was calculated by Mahan et al. [13, 14] as a function of the Schottky barrier height. The thickness of the barrier, L, is less than the mean-free path of the carrier, λ. Figures 5(a) and 5(b) show the calculated thermoelectric efficiency of the multi-layer thermoelectric cooler (a) and power generator (b), respectively. A more promising prediction is given for the cooler rather than the power generator. The temperature difference between the hot and cold sides of the device was Th=300K and Tl=260K, respectively. The ballistic electron conduction was assumed in the barrier layers. A typical thickness of the barriers is Lt=10nm, which is shorter than the mean free path length of the carrier, λ≈100nm at about 300K. High thermal resistance of the multilayer device at a level 10 times higher than the bulk was assumed for the calculation. (a)

(b)

Fig.5 Calculated efficiency of a multi-layer (well width a 80%. The optical band gap increased linearly from 3.22 to 3.43 eV with increasing RF portion of the total power. They reported that, this widening of the band gap energy can be attributed to the well- known Burstein–Moss shift (BM shift) caused by the filling of excessively charged carriers in the lowest levels of the conduction band and the transitions to energies above the Fermi level. Higher deposition temperature, sputtering gas of light mass and heat

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treatment resulted in decreased extinction coefficients and increased transmittance. However, excessive amount of oxygen flow during deposition brought about the decrease of transmittance as reported by Y.S. Jung et al. [95]. Lee and Park [96] fabricated transparent conducting indium zinc oxide (In2O3–ZnO) thin films by a sol–gel method using zinc acetate dehydrate [Zn(CH3COO)2 .2H2O]

and indium nitrate trihydrate [In(NO3)33H2O] as starting precursors, and 2-methoxyethanol

as a solvent and monoethanolamine as a stabilizer. The optical transmittance was only slightly dependent upon the variation of atomic ratio in the visible region and the value of transmittance was 80-85%. However, the optical window was directly affected by the film composition and the narrowest was obtained in a film of the atomic ratio of 0.5. Effects of introduction of argon on transparent conducting properties of ZnO–In2O3 thin films prepared by pulsed laser deposition is studied by Moriga et al. [97]. They reported that, for the film deposited at x=0.71, the optical transmission seemed to be improved only in the violet–blue wavelength region though the film became amorphous in the presence of argon. The band gap did not increase in spite of the increase in carrier concentration in the film. The same phenomenon was observed in the homologous films deposited at x=0.78. On the other hand, at x=0.43 where the films were both amorphous, the band gap of the films was increased by the introduction of argon gas, based on the onset of transmittance. This suggests argon gas may improve the optical band gap only for amorphous films of lower composition x. The optical properties of the a-IZO anode, which was prepared by BCS at room temperature, were compared to those of commercial ITO anode films by Kim et al. [99]. The transmittance of the a-IZO films in the green region is much higher than that of the c-ITO films. Although the transmittance of the a-IZO anode film in the blue region is lower than that of the cITO anode films, the average optical transmittance (~88.9 %) of the a-IZO anode in the green region between 500 and 550 nm is higher than that of a commercial c-ITO (84.4%). G. Machado et al [100] obtained indium doped ZnO thin films by co-electro deposition (precursor and dopant) from aqueous solution. The band gap energy of the films varied from 3.27 eV to 3.42 eV, increasing with the In concentration in the solution. Ramamoorthy et al. [101] optimized and developed high quality transparent IZO (In2Zn2O5) thin films considerably greater than that of state-of-the-art of ITO and other commercially valid TCOs by using PLD. It is observed that the transmission of the IZO thin films improves from 87% to 95% with increasing in the substrate temperature from RT to 450 0C.

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Characterization of Technological Materials

Conclusion The various preparation technique and the material properties of the films are reported in this review article. The structural, electrical and optical properties of the films prepared by various techniques reveals that those properties are strongly dependent on the method of deposition, the substrate

temperature,

the

rate

of

deposition,

the

background

pressure

etc.

References [1]. [2]. [3]. [4]. [5]. [6]. [7]. [8]. [9]. [10]. [11]. [12]. [13]. [14]. [15]. [16]. [17]. [18]. [19]. [20]. [21]. [22]. [23]. [24]. [25]. [26].

Hoffman RL, Norris BJ, Wager JF. Appl Phys Lett 2003;82:733. Nomura K, Ohta H, Ueda K, Kamiya T, Hirano M, Hosono H. Science2003; 300:1269. Nomura K, Ohta H, Takagi A, Kamiya T, Hirano H, Hosono H. Nature 004;432:488. Munsee CL, Presley RE, Park CH, Hong D, Wager JF, Keszler DA. J Phys D Appl Phys 2004;37:2810. Fortunato E, Pereira LMN, Barquinha PMC, Rego AMB, Goncalves G, Vila A, et al. Appl Phys Lett 2008;92:22103. Kenichi Inoue, Kikuo Tominaga, Takashi Tsuduki, Michio Mikawa, Toshihiro Moriga Vacuum 83 (2009) 552. Yen-Chin Huang , Zhen-Yu Li , Hung-hsin Chen , Wu-Yih Uen , Shan-Ming Lan, Sen-Mao Liao , Wu-Yih Uen , Shan-Ming Lan , Sen-Mao Liao, Tsun-Neng Yang, Chin-Chen Chiang Thin Solid Films 517 (2009) 5537. A.F. Kohan, G. Ceder, D. Morgan, C G. Van de Walle, Phys. Rev. B 61 (2000) 15019. K.-K. Kim, S. Niki, J.-Y. Oh, J.-O. Song, T.-Y. Seong, S.-T. Park, S. Fujita, S.-W. Kim, J. Appl. Phys. 97 (2005) 066103. T. Makino, Y. Segawa, S. Yoshida, A. Tsukazaki, A. Ohtomo, M. Kawasaki, Appl. Phys. Lett. 85 (2004) 759. T. Minami, H. Nanto, S. Takata, Jpn. J. Appl. Phys. 23 (1984) L280. K.L. Chopra, S. Major, D.K. Pandya, Thin Solid Films 102 (1983) 1. Y. Li, G.S. Tompa, S. Liang, C. Gorla, Y. Lu, John Doyle, J. Vac Sci. Technol. A 15 (1997) 1063. X.L. Chen, B.H. Xu, J.M. Xue, Y. Zhao, C.C.Wei, J. Sun, Y.Wang, X.D. Zhang, X.H. Geng, Thin Solid Films 515 (2007) 3753. K. Ramamoorthy, K. Kumar, R. Chandramohan, K. Sankaranarayanan, Mater. Sci. Eng. B 126 (2006) 1. N. Ito, Y. Sato, P.K. Song, A. Kaijio, K. Inoue, Y. Shigesato, Thin Solid Films 496 (2006) 99. Y.S. Jung, J.Y. Seo, D.W. Lee, D.Y. Jeon, Thin Solid Films 445 (2003) 63. N. Naghavi, A. Rougier, C. Marcel, C. Guery, J.B. Leriche, J.M. Tarascon, Thin Solid Films 360 (2000) 233. W.J. Lee, Y.K. Fang, J.J. Ho, C.Y. Chen, L.H. Chiou, S.J.Wang, F. Dai, T. Hsieh, R.Y. Tsai, D. Huang, F.C. Ho, Solid State Electron. 46 (2002) 477. G. Hu, B. Kumar, H. Gong, E.F. Chor, P. Wu, Appl. Phys. Lett. 88 (2006) 101901. S.A. Aly, N.Z. El Sayed and M.A. Kaid, Vacuum 61 (2001) 1. R. Asmar, G. Ferblantier, F. Mailly, P. Gall-Borrut and A. Foucaran, Thin Solid Films 473 (2003) 49. Rakhi Khandelwal, Amit Pratap Singh, Avinashi Kapoor, Sorin Grigorescu, Paola Miglietta, nadya Evgenieva Stankova and Alessio Perrone, Optics & Laser Technology 40 (2008) 247. Yan Zhao, Yijian Jiang and Yan Fang, Journal of Crystal Growth 307 (2007) 278. Jie Zhao, Lizhong Hu, Weifeng Liu and Zhaoyang Wang, Applied Surface Science 253 (2007) 6255. Mattox, D. M., Deposition Technologies for Films and Coatings: Developments and Applications, Noyes Publications, (1982) 244.

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[27]. [28]. [29]. [30]. [31]. [32]. [33]. [34]. [35]. [36]. [37]. [38]. [39]. [40]. [41]. [42]. [43]. [44]. [45]. [46]. [47]. [48]. [49]. [50]. [51]. [52]. [53]. [54]. [55]. [56]. [57]. [58]. [59]. [60].

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Knodle, W., Research and Development, 28(8):(1986) 73. Neugebauer, C. A., Handbook of Thin Film Technology , McGraw-Hill, Ch. 8,. 8-3. Peters, J. W., Gebhart, F. L., and Hall, T. C., Solid State Technol., 23(9): (1980)121. Osgood, R. M., and Gilgen, H. H., Ann. Rev. Mater. Sci., (R. Huggins, ed. ), 15:549–576, Annual Reviews, Inc., New York (1985) Ghandhi, S. K., VLSI Fabrication Principles, John Wiley & Sons, Ch.6 (1983). 299. Preetam Singh, Ashwani Kumar, Deepak and Davinder karur, Journal of Crystal Growth 15(2007) 303. Campbell, D. S., Handbook of Thin Film Technology, McGraw-Hill, Ch. 5, (1970) 5-1. P.A.Lee, G. Said, R. Davis and T.H. Lim, J Phys Chem Solids, 30 (1969) 2719. D.I. Bletskan, I.F. Kopinets, P.P. Pogorsh, E.N. Salkova and D.V. Chepor, Kristallografiya, 20 (1978) 1008. M.F. Ladd and R.A. Palmer (Eds), Structure determination by X-ray Crystallography, Plenum Press, New York (1964) 71. H.E. Bennet and J.M. Bennet in G. Hass and R. E. Thun (Eds), Physics of Thin Films Vol 4, Academic Press, New York (1967). V.P. Bhatt, K. Gireesan and C.F. Desai, Cryst Res Technol, 24 (1989) 187. D. Bhattacharya, S. Chaudhuri, A.K.Pal and S.K.Bhattacharya, Vacuum, 42 (1991) 1113. D.Y. Ku, I.H. Kim, I. Lee, K.S. Lee, T.S. Lee, J.-h. Jeong, B. Cheong, Y.-J. Baik, W.M. Kim, Thin Solid Films 515 (2006) 1364. Do-Geun Kim, Sunghun Lee, Dong-Ho Kim, Gun-Hwan Lee, Minoru Isshiki, Thin Solid Films 516 (2008) 2045. N. Ito, Y. Sato, P.K. Song, A. Kaijio, K. Inoue, Y. Shigesato, Thin Solid Films 496 (2006) 99. Y.S. Jung, J.Y. Seo, D.W. Lee, D.Y. Jeon, Thin Solid Films 445 (2003) 63. Chongmu Lee, Wangwoo Lee, Hojin Kim, Hyoun Woo Kim, Ceramics International 34 (2008) 1089. Shang-Chou Changa, Ming-Hua Shiao, Microelectronics Journal 38 (2007) 1202. Chongmu Lee, Anna Park, Youngjoon Cho, Minwoo Park, Wan In Lee, Hyoun Woo Kim, Ceramics International 34 (2008) 1093. Jiansheng Jie, Guanzhong Wang, Xinhai Han, Qingxuan Yu, Yuan Liao, Gongpu Li, J.G. Hou, Chemical Physics Letters 387 (2004) 466. L.L.Chen, Z.Z.Ye, J.G.Lu, H.P. He, B.H. Zhao, L.P. Zhu, Paul K. Chu, L. Shao, Chemical Physics Letters 432 (2006) 352. Edit Pal, Imre Dekany, Colloids and Surfaces A: Physicochem. Eng. Aspects 318 (2008) 141. Edit Pal, Viktoria Hornok, Albert Oszko, Imre Dekany, Colloids and Surfaces A: Physicochem. Eng. Aspects 340 (2009) 1. Chang Eun Kim, Hyun Soo Shin, Pyung Moon, Hyun Jae Kim, Ilgu Yun, Current Applied Physics 9 (2009) 1407. V. Senthilkumar, P. Vickraman, Current Applied Physics 10 (2010) 880–885. Se Hun Park, Ji Bong Park, Pung Keun Song, Current Applied Physics 10 (2010)S488. J.F. Chang, H.L. Wang, M.H. Hon, J. Cryst. Growth 211 (2000) 93. S.Y. Kim, J.M. Seo, H.W. Jang, J.S. Bang, W. Lee, T.Y. Lee, J.M. Myong, Appl. Surf. Sci. 255 (2009) 4616. C.E. Benouis, M. Benhaliliba, A. Sanchez Juarez, M.S. Aida, F. Chami, F. Yakuphanoglu, Journal of Alloys and Compounds 490 (2010) 62. Susan Huang, Tatiana Kaydanova, Alex Miedaner, David S. Ginley, U.S. Dept. Energy J. Undergraduate Res. 4 (2004) 70. F. Paraguay, D.J. Morales, W. Estrada, L.E. Andrade, M. Miki-Yoshida, Thin Solid Films 366 (2000) 16. Malle Krunks, Enn Mellikov, Thin Solid Films 270 (1995) 33. P.M. Ratheesh Kumar, C. Sudha Kartha, K.P. Vijayakumar, T. Albe, Y. kashiwaba, F. Singh, D.K. Avasthi, Semicond. Sci. Technol. 20 (2005) 120.

44

[61]. [62]. [63]. [64]. [65]. [66]. [67]. [68]. [69]. [70]. [71]. [72]. [73]. [74]. [75]. [76]. [77]. [78]. [79]. [80]. [81]. [82]. [83]. [84]. [85]. [86]. [87]. [88].

Characterization of Technological Materials

M. Caglar, Y. Caglar, S. Ilican, J. Optoelectron. Adv. Mater. 8 (2006) 1410. Naoki Ohashi, Tsuyoshi Ogino, Isao Sakaguchi, Shunichi Hishita, Mamabu Komatsu, Tadashi Takenaka, Hajime Haneda, Journal of Crystal Growth 237–239 (2002) 558. Xingping Peng, Hang Zang, Zhiguang Wang, Jinzhang Xu, Yinyue Wang, Journal of Luminescence 128 (2008) 328. Le Hong Quang , Lim Swee Kuan,Gregory Goh Kia Liang, Journal of Crystal Growth 312 (2010) 437. Juan Liu, Yue Zhang, Junjie Qi, Yunhua Huang, Xiaomei Zhang, Qingliang Liao, Materials Letters 60 (2006) 2623. T. Ushiro, D. Tsuji, A. Fukushima, T. Moriga, I. Nakabayashi, K. Murayama, K. Tominaga, Materials Research Bulletin 36 (2001) 1075. K. Ramamoorthy, C. Sanjeeviraja, M. Jeyachandran, K. Sankaranarayanan, P. Misrac, L.M. Kukreja, Mater. Chem. Phys. 84 (2004) 14. N.Naghavi , C.Mar cel, L.Dupont , C.Guery , C.Maugy , J.M.Tarascon, Thin Solid Films 419 (2002) 160. Shang-Chou Chang, Ming-Hua Shiao, Microelectronics Journal 38 (2007) 1202. Saliha Ilican, Yasemin Caglar, Mujdat Caglar, Fahrettin Yakuphanoglu, Physica E 35 (2006) 131. L.P.Peng , L.Fang , X.F.Yang , H.B.Ruan , Y.J.Li , Q.L.Huang , C.Y.Kong, Physica E 41 (2009) 1819. H. Gomez, A. Maldonado, R. Asomoza, E.P. Zironi, J. Canetas-Ortega, J. Palacios-Gomez , Thin solid Films 293 (1997) 117. M.A. Lucio-Lo´ pez, M.A. Luna-Arias, A. Maldonado, M. de la L. Olvera, D.R. Acosta, Solar Energy Materials & Solar Cells 90 (2006) 733. M.A. Lucio-Lo´ pez, A. Maldonado, R. Castanedo-Pe´rez, G. Torres-Delgado, M. de la L. Olvera, Solar Energy Materials & Solar Cells 90 (2006) 2362. M. de la L. Olvera, H. Go´mez, A. Maldonado, Solar Energy Materials & Solar Cells 91 (2007) 1449. Keun-Young Son, Dong-Hyuk Park, Joon-Hyung Lee, Jeong-Joo Kim, Jai-Sung Lee, Solid State Ionics 172 (2004) 425. Yasemin Caglar, Saliha Ilican, Mujdat Caglar, Fahrettin Yakuphanoglu, Spectrochimica Acta Part A 67 (2007) 1113. A. Bougrine, A. El Hichou, M. Addou, J. Eboth´e, A. Kachouane, M. Troyon, Mater. Chem. Phys. 80 (2003). J.-H. Lee, B.-O. Park, Thin Solid Films 426 (2003) 94. E.J. Luna-Arredondo, A. Maldonado, R. Asomoza, D.R. Acosta, M.A. Mele’ndez-Lira, M.de la L. Olvera, Thin Solid Films 490 (2005) 132. X. Zi-qiang, D. Hong, L. Yan, C. Hang, Mater. Sci. Semiconduct. Process. 9 (2006) 132. N. Naghavi, A. Rougier, C. Marcel, C. GueÂry, J.B. Leriche, J.M. Tarascon, Thin Solid Films 360 (2000) 233. F. Paraguay D, J. Morales, W. Estrada L, E. Andrade, M. Miki-Yoshida, Thin Solid Films 366 (2000) 16. Tae Young Ma, Dae Keun Shim, Thin Solid Films 410 (2002) 8. Elin Hammarberg , Anna Prodi-Schwab , Claus Feldmann, Journal of Colloid and Interface Science 334 (2009) 29. Naoki Ohashi, Tsuyoshi Ogino, Isao Sakaguchi , Shunichi Hishita, Mamabu Komatsu, Tadashi Takenaka, Hajime Haneda Journal of Crystal Growth 237–239 (2002) 558. Buguo Wang, M.J. Callahan, Chunchuan Xu, L.O. Bouthillette, N.C. Giles, D.F. Bliss, Journal of Crystal Growth 304 (2007) 73. Ji Bong Park, Se Hun Park, Pung Keun Song, Journal of Physics and Chemistry of Solids 71 (2010) 669.

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[89].

45

S. Kikkawa, H. Sasaki, H. Tamura, S. Hosokawa, H. Ogawa, Materials Research Bulletin 39 (2004) 1821. [90]. K. Ramamoorthy , M. Jayachandran , K. Sankaranarayanan, Pankaj Misra, L.M. Kukreja, C. Sanjeeviraja, Solar Energy 77 (2004) 193. [91]. W.J. Lee , Y.-K. Fang , J.-J. Ho , C.-Y. Chen , L.-H. Chiou , S.-J. Wang , F. Dai , T. Hsieh , R.Y. Tsai , D. Huang , F.C. Ho, Solid-State Electronics 46 (2002) 477. [92]. Kim H, Gilmore CM, Pique A, Horwitz JS, Mattoussi H, Murata H, Katati ZH, Chrisey DB. Electrical, optical, and structural properties of indium-tin-oxide thin films for organic lightemitting devices. J Appl Phys 1999;86(11): 6451. [93]. Tae Young Ma, Dae Keun Shim, Thin Solid Films 410 (2002) 8. [94]. M.S. Tokumoto , A. Smith, C.V. Santilli, S.H. Pulcinelli, A.F. Craievich, E. Elkaim, A. Traverse, V. Briois, Thin Solid Films 416 (2002) 284. [95]. Yeon Sik Junga, Ji Yoon Seo, Dong Wook Lee, Duk Young Jeon, Thin Solid Films 445 (2003) 63. [96]. Seung-Yup Lee, Byung-Ok Park, Thin Solid Films 484 (2005) 184. [97]. Toshihiro Moriga, Michio Mikawa, Yuji Sakakibara, Yukinori Misaki, Kei-ichiro Murai, Ichiro Nakabayashi, Kikuo Tominaga, James B. Metson, Thin Solid Films 486 (2005) 53. [98]. E.J. Luna-Arredondo, A. Maldonado, R. Asomoza, D.R. Acosta, M.A. Mele´ndez-Lira, M. de la L. Olvera, Thin Solid Films 490 (2005) 132. [99]. Han-Ki Kim Surface & Coatings Technology 203 (2008) 652. [100]. G. Machado, D.N. Guerra, D. Leinen, J.R. Ramos-Barrado, R.E. Marotti, E.A. Dalchiele, Thin Solid Films 490 (2005) 124. [101]. K. Ramamoorthy , K. Kumar , R. Chandramohan , K. Sankaranarayanan, R. Saravanan , I.V. Kityk , P. Ramasamy, Optics Communications 262 (2006) 91.

Materials Science Forum Vol. 671 (2011) pp 47-68 © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.671.47

Review on Gallium Zinc Oxide Films: Material Properties and Preparation Techniques S.Nagarani1,a, M. Jayachandran2,b and C. Sanjeeviraja1,c 1

School of Physics, Alagappa University, Karaikudi-630 003, India. Electrochemical Materials Science Division, Central Electrochemical Research Institute, Karaikudi -630 006, India. a [email protected]; [email protected]; [email protected].

2

Keywords: Thin film, Gallium zinc oxide, Electron beam evaporation, Structural properties, Electrical properties, Optical properties.

Abstract. Thin films continue to become more and more integral to numerous applications in today's advancing technologies. In recent years, thin film science has grown world-wide into a major research area. The importance of coatings and the synthesis of new materials for industry have resulted in a tremendous increase of innovative thin film processing technologies. Thin film properties are strongly dependent on the method of deposition, the substrate temperature, the rate of deposition, the background pressure etc. Hardness, adhesion, non porosity, high mobility of charge carriers / insulating properties and chemical inertness, which are possible with a selection of suitable functional materials and deposition techniques. There are number of different techniques that facilitate the deposition of stable thin films of oxide materials on suitable substrates. Material properties of gallium zinc oxide thin films and all the techniques used to deposit thin films are summarized with an elaborative account along with our results. Introduction Transparent conductive oxide films have been developed for use in optoelectronic devices such as solar cells, flat panels, and window coatings [1]. ZnO has gained great attention from researchers [2]. Zinc oxide (ZnO) is a wide band gap semiconductor with a hexagonal wurtzite structure. It demonstrates high thermal and chemical stability, good electrical conductivity and high optical transparency. In addition, it also exhibits the features of non-toxicity, low cost and high abundance, and therefore can be considered for widespread applications, such as a transparent conductive layer [3]. Undoped ZnO films usually show n-type conductivity but with a high resistivity due to the intrinsic defects of oxygen vacancies and zinc interstitials [4]. Therefore, high conductive films can be obtained only by doping metal elements that substitute zinc sites [5-6]. Compared to undoped ZnO, the doped one has a lower resistivity and better stability of electrical properties. It is well known that group III elements such as Al [7], In [8], Ga [9] and B [10] act as donors in ZnO. Among these metal dopants, Ga seems to be a promising one because the covalent bond length of Ga–O (0.192 nm) is slightly smaller than that of Zn–O (0.197 nm) and only small ZnO lattice deformations are caused even high concentrations of Ga are introduced. ZnO has large exciton binding energy of 60 meV and high refractory and chemical stability [11]. Doping it with Ga led to films with the highest quality [12]. Most of the works related to zinc oxide use aluminum as dopant. Nevertheless, aluminum presents a very high reactivity leading to oxidation during film growth, which may become a problem. Gallium is less reactive and more resistant to oxidation

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compared to aluminum [13, 14]. Recently, it was demonstrated by the present authors that doping with gallium led to films both with low resistivity and a high transmittance in the visible region [15]. Methods of thin film deposition Thin film of amorphous metal alloys, semiconductors, oxide and chalcogenide glasses have been rapidly prepared by common physical vapor deposition as well as chemical vapor deposition methods. A simple and brief discussion about the various methods of film preparation is given here. Thermal Evaporation Thermal evaporation is one of the most well known physical vapor deposition techniques. It is a simple technique and one can evaporate large variety of materials on different substrates. In thermal evaporation, the material is created in vapor form by means of resistive or RF heating. The vapor atoms thus created are transported through vacuum to get deposited on the substrates. The ambient is vacuum because otherwise the vapor species will get scattered by collision with gas atom. The evaporant material is supported on a source which is then heated to a sufficiently high temperature, to produce desired vapor pressure. In most of the cases, this temperature is in the range of 10000 C to 20000 C. The requirements for source material are that it should have negligible vapor pressure at the deposition temperature and should not react with the evaporant. The shape of source should be such that it should be possible to hold the evaporant materials in any available form (powder, wires etc.) The film deposition is not uniform, because the amount of material reaching the substrate depends on the angle (θ) between the source and the substrate area. If a uniform thickness is desired, then the substrate has to be rotated in a manner that each point on the substrate receives almost same amount of material during the deposition. The advantage of technique is that the amount of impurities included in the growing layer will be minimized and straight line propagation will occur from the source to the substrate. The pressure that is required in a vacuum system to obtain satisfactory results in terms of included impurities and the fabrication of sharply defined patterns is about 1.33x10-3 Torr. Electron Beam Evaporation Disadvantages of resistively heated evaporation sources include contamination by crucibles, heaters and support materials and limitation of relatively low input power levels. This makes it difficult to deposit pure films or evaporate high-melting-point materials at appreciable rates. Electron beam heating eliminates these disadvantages and has, therefore, become the preferred vacuum evaporation technique for depositing films [16]. In electron beam evaporation, an electron beam is accelerated through a potential of 5 to 10 kV and focused on the material. The electron loses their kinetic energy mostly as heat and the temperature at the focused spot can become as high

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as 30000C. At such a high temperature, most of the refractory metals and compounds can be evaporated. The electron gun used for evaporation consists of a heated tungsten filament for electron emission. Depending on how these emitted electrons are accelerated, the guns are called work accelerated or self accelerated. In work accelerated guns, the electric field is applied between the cathode and the evaporant. If the separated anode is used having an aperture to allow passage of electron beam, the gun is called self accelerated. The schematic diagram of electron beam gun is shown in Fig.1. These guns can be used at higher voltages and are most commonly used. In both the electron guns, electrostatic and electromagnetic focusing is used to focus the electron beam.

Fig 1. Schematic diagram of electron beam gun

Sputtering The process of ejection of atoms by energetic, bombarding ions as a result of momentum transfer from them to the atoms on the surface of a target material is termed as sputtering [17-18]. In brief, sputtering is obtained by generating a beam of inert-gas ions, accelerating the beam to energies of a few KeV, and directing the beam at a target composed of a material to be deposited. Atoms or molecules from a target are ejected on to the substrate [19].

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Characterization of Technological Materials

Different types of sputtering techniques are as follows:a) RF Sputtering The RF sputtering system has been used as to prepare thin films of fused quartz, aluminum oxide, boron nitrate and various glasses. The technique of RF sputtering has been shown to yield insulator films of high quality and excellent stability. This method has been successfully applied for the preparation of Si films. Sputtering at low pressures (~ 10-3) is also possible by enhancing gas ionization with the help of an inductively coupled external rf field. Sputtering cannot sustain the glow discharge because of the immediate build up of a surface charge of positive ions on the front side of the insulator. The dc voltage power supply of the dc diode sputtering is replaced by rf power supply. This is called RF diode sputtering. b) DC Sputtering DC sputtering is also known as diode or cathodic sputtering. The DC sputtering system is composed of a pair of planar electrodes. One of them is cold cathode and the other is the anode. The front surface of the cathode is covered with target materials to be deposited. The substrates are placed on the anode. The glow discharge is maintained under the application of dc voltage between the electrodes. The sputtered particles collide with gas molecules and then arrive at the substrates. During DC-diode sputtering the atoms that leave the target with typical energies of 5 eV undergo gas scattering events in passing through the plasma gas; this is so even at low operating pressures. As a result of repeated energy-reducing collisions they eventually thermalize or reach the kinetic energy of the surrounding gas. This happens at the distance, where their initial excess kinetic energy, so necessary to provide bombardment of the depositing film, has dissipated. No longer directed, such particles now diffuse randomly. Not only there is a decrease in the number of atoms that deposit, but there is little compaction or modification of the resulting film structure [20]. c) Triode Sputtering Over the years several variants of DC sputtering have been proposed to enhance the efficiency of the process. In one, known as triode sputtering system, the filament cathode and anode assembly is installed closed to the target, parallel to the target-substrate electrodes. When the filament is heated to high temperatures, thermionic emission and injection of electrons in to the plasma increase the gas-ionization probability. The resulting ions are then extracted to by the negative target potential. A disadvantage of triode sputtering is the non uniform plasma density over target surfaces, which leads to uneven erosion of metal. However, the discharge can be maintained at low pressures, and this is an advantage [21].

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d) Magnetron sputtering Magnetron sputtering is the most widely used variant of DC sputtering. One to two orders of magnitude more current is typically drawn in magnetron than simple DC discharges for the same applied voltage. Important implications of this are higher deposition rates (e.g., ~ 1µm per minute for Al metallization alloys) or alternatively, lower voltage operation than for simple DC sputtering. Another important advantage is reduced operating pressures. At typical magnetron-sputtering pressures of a few milli torr, sputtered atoms fly off in ballistic fashion to impinge on substrates. Avoided are the gas phase collisions and scattering at high pressures which randomize the directional character of the sputtered-atom flux and lower the deposition rate [22]. e) Reactive Sputtering In reactive sputtering, thin films of compounds are deposited on substrates by sputtering metallic targets in the presence of a reactive gas usually mixed with an inert working gas. The most common compounds reactively sputtered and the reactive gases employed are briefly listed: 1.Oxides (oxygen)-Al2O3, In2O3, SnO2, SiO2, Ta2O5. 2.Nitrides (nitrogen, ammonia)- TaN, TiN, AlN, Si3N4,CNx. 3.Carbides (methane, acetylene, propane)- TiC, WC, SiC. Sulfides (H2S)-CdS, CuS, ZnS. 4.Oxy carbides and oxy nitrides of Ti, Ta, Al and Si. Irrespective of which of the above materials is being considered, during reactive sputtering the resulting film will usually be a solid solution alloy of the target metal doped with the reactive element, a compound or some mixture of the two [23]. f) Ion beam Sputtering Sputtering deposition under controlled high-vacuum conditions can be achieved by using an ion beam source. In this method, the ions of the required material are produced and condensed on a surface to form a thin film.

Hot wall epitaxy Hot wall epitaxy [24-25] is a vacuum deposition technique whose main feature is the growth of epitaxial layers or thin films under conditions as near as possible to thermodynamic equilibrium with the least loss of material. The main feature of the hot wall epitaxy is the heating of the walls of the chamber which serves the purpose of enclosing and directing the vapor from the source onto the substrate by reducing markedly the deposition of the vapor atoms onto the walls of the chamber. This leads to the following advantages: i) Loss of material is avoided. ii) High vapor pressures of the compound or its different components can be maintained.

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iii) The difference between the substrate temperature and the source temperature can be reduced to a minimum value. Activated Reactive Evaporation If the evaporated material (by resistive heating or electron beam) is transported through reactive gas (oxygen, acetylene etc.) plasma, the deposition technique is called activated reactive evaporation. The technique has been mainly used to deposit highly adherent films of oxides and carbides. Activated reactive evaporation technique is divided in to two main types. a. Biased ARE In this technique, the substrate is biased; normally negative to attract the positive ions and a positively biased electrode between the source and the substrate is used to create a plasma. b. Enhanced ARE The plasma in activated reactive evaporation (ARE) is enhanced by accelerating electron emitted from a tungsten filament under electric field perpendicular to the vapor beam. The ionization can be further enhanced using a magnetic field. This has the advantage of that the deposition can be done at low pressure. Ion Plating Ion plating refers to process in which the substrate and the film are exposed to a flux of high energy ions during the deposition. The energy of the ions is high enough to cause the changes in the interfacial region and film properties such as adhesion of the film, its morphology and density, stress in the film and coverage of the substrate surface by the film. Ion plating has been used to get a film adhesion especially for an incompatible substrate film combination. This technique also gives a better electrical contact for films of Pt, Au and Si. The technique has also been used to deposit films for lubrication, wear resistance and corrosion resistance. Chemical Vapour Deposition Chemical deposition techniques are the most important methods for the growth of films owing to their versatility for depositing a very large number of elements and compounds at relatively low temperature. It has been quite successfully used to prepare epitaxial layers of elemental semiconductors and also the compound semiconductors such as GaAs, GaP, various chalcogenides etc. [26-27]. The process is very economical and has been industrially exploited to a large scale. Spray Pyrolysis Spray pyrolysis is a process in which thin film is deposited by spraying a solution on the heated surface, where the constituents react to form chemical compound. The chemical reactants are

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selected such that the products other than the desired compound are volatile at the temperature of deposition. The process is particularly useful for the deposition of oxides. Spray deposition process can be classified according to the type of reaction. In process A, the droplet resides on the surface as a solvent evaporates, leaving behind a solid that may further react in the dry state. In process B, the solvent evaporates before the droplet reaches the surface and the dry solid impinges on the surface, where decomposition occurs. In process C, the solvent vaporizes as the droplet approaches the substrate; the solid then melts and vaporizes, and the vapor diffuses to the substrate there to undergo a heterogeneous reaction. In process D, the entire reaction takes place in the vapor state. In all the process, the significant variables are the ambient temperature. An ultrasonic atomizing system has been used to narrow down the distribution of droplet size for better surface uniformity [28]. Electro deposition This process is also known as "electroplating" and is typically restricted to electrically conductive materials. There are basically two technologies for plating: Electroplating and Electrodeless plating. In the electroplating process the substrate is placed in a liquid solution (electrolyte). When an electrical potential is applied between a conducting area on the substrate and a counter electrode (usually platinum) in the liquid, a chemical redox process takes place resulting in the formation of a layer of material on the substrate and usually some gas generation at the counter electrode. In the electrodeless plating process a more complex chemical solution is used, in which deposition happens spontaneously on any surface which forms a sufficiently high electrochemical potential with the solution. This process is desirable since it does not require any external electrical potential and contact to the substrate during processing. Unfortunately, it is also more difficult to control with regards to film thickness and uniformity. Anodization Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. The process is called "anodizing" because the part to be treated forms the anode electrode of an electrical circuit. Aluminum alloys are anodized to increase corrosion resistance, to increase surface hardness, and to allow dyeing (coloring), improved lubrication, or improved adhesion. The anodic layer is non-conductive. The metal to be anodized is made an anode and immersed in an oxygen-containing electrolyte, which may be aqueous, nonaqueous, or fused salt. Growth may take place at constant voltage or at constant current. Solution Growth process The solution growth process was pioneered by Bode [29], Kitaev et al. [30] and Chopra [31]. The process itself was first used in 1946 to prepare PbS films for infrared applications. Films are grown on either metallic or nonmetallic substrates by dipping them in appropriate solution

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containing metal salts without the application of any electric field. Deposition may occur by homogeneous chemical reaction, usually reduction of a metal ion in solution by a reducing agent. If this occurs on a catalytic surface, it is called an autocatalytic or electrodeless deposition. Silvering is perhaps the most widely used of this technique. The rate of growth and degree of crystallinity depends on the temperature of the solution which is kept between 40 to 1000C. Typical rates of growth of 500Å/min are obtained. One of the chief advantages of such a method is that it is possible to deposit films on non-accessible surfaces, i.e., inside a glass tube, where they will be protected from physical damage. Brush plating Brush plating [31-32] is an electroplating process that is performed with a hand held plating tool rather than using a tank of solution. Brush plating systems vary in degree of sophistication and capabilities. The sophisticated system use power packs up to 500 A output and have the capacity for producing excellent quality deposits up to 0.25 inch thick on large areas. Brush plated coatings are widely used on air-craft for the corrosion protection of steel parts [33]. Brush plated rhodium and gold deposits on copper slip rings are used in submarines and gold plated antenna [32] in the “Voyager” spacecraft. Crystal structure of zinc oxide and gallium oxide Zinc oxide crystallizes in three forms: hexagonal wurtzite, cubic zincblende, and the rarely observed cubic rocksalt. The wurtzite structure is most stable at ambient conditions and thus most common. The zinc blende form can be stabilized by growing ZnO on substrates with cubic lattice structure. In both cases, the zinc and oxide centers are tetrahedral. Figure 2 represents the crystal structure of zinc oxide. The β-Ga2O3, with a melting point of 1740˚C, is the most stable crystalline modification. The oxide ions are in a distorted cubic closest packing arrangement, and the gallium (III) ions are in distorted tetrahedral and octahedral sites, with Ga-O bond distances of 1.83 and 2.00 Å respectively. These distortions are in fact the reasons for the great level of stability of β-Ga2O3 [34].

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Fig 2: Crystal structure of ZnO Table 1 Physical properties of zinc oxide and gallium oxide Property Zinc Oxide Gallium Oxide Molecular formula Molar mass

ZnO

Ga2O3

81.408 g/mol

187.444 g/mol

Density

5.606 g/cm3

6.44 g/cm3, alpha 5.88 g/cm3, beta

Melting point

1975 °C 1900 °C, alpha (decomposes) 1725 °C, beta

Solubility

In water

In most of the acids

Structural Properties of GZO films Structural properties of the material play a dominant role in the performance of the devices and a knowledge of the influence of various deposition parameters on the structural properties of thin films is essential before the application of these materials in devices [35-40]. Sophisticated characterization techniques have emerged to understand the multifaceted properties of thin films. Xray diffraction method is one of the best methods for the estimation of the crystallographic parameters. Figure 3 shows the electron beam evaporated 1% gallium doped zinc oxide film deposited at room temperature.

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Characterization of Technological Materials

X. Yu et al. [41] deposited gallium doped zinc oxide films (ZnO:Ga) on glass substrates by rf magnetron sputtering at room temperature. According to them, the crystal structure of the ZnO:Ga films is hexagonal wurtzite and the films are highly oriented along the c-axis perpendicular to the substrates. As film thickness increases, the crystallinity is improved and the crystallite sizes become larger. Gallium doped zinc oxide films have been deposited on glass substrates using RF magnetron cosputtering, followed by H2 ambient annealing at 623K by S. Kim et al. [42]. They have reported that all the ZnO:Ga thin films have the preferred c-axis orientation due to selftexturing regardless of the deposition temperature and that the crystallinity would be improved when the growth temperature is increased from RT. Many authors [43, 44, 45] revealed that the formed GZO films are polycrystalline and preferentially oriented in the [0 0 2] orientation.The Xray diffraction diagram of the GZO films prepared by vacuum arc plasma evaporation [46] shows that the (0 0 0 2) diffraction intensity observed from the GZO film showed a tendency to increase with doping up to an appropriate Ga content. Yamada et al. [47] reported the structural properties of gallium doped zinc oxide thin films prepared by ion plating. From XRD analysis, with increasing Ga2O3 content in the source, FWHM of (002) diffraction peak began to increase from 6 wt. % at an O2 gas flow rate of 10 sccm, leading to the deterioration in crystallinity of GZO film. K.Y. Cheong et al. [48] reported that the introduction of the dopant seems to reduce the relative intensity of the (101) plane and increase the intensity of the (002) plane. As the post-heat treatment temperature increased, the structure orientation remained the same at 450 and 500 0C, but started to alter at 550 0

C. Zinc oxide layers grown by spray pyrolysis at a substrate temperature of 350 0C with a gallium

concentration of 5.0 at.% studied by K.T. Ramakrishna Reddy et al. [49] had a strong {002} preferred orientation with a grain size of 98 nm and exhibited the wurtzite structure. 700

(1 0 1) 600

Intensity (a.u)

500

400

300

200

100

0 0

10

20

30

40

50

60

70

80

90

2θ ( degrees)

Fig 3: XRD pattern for 1% Ga doped ZnO deposited at room temperature.

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Song et al. [50] observed that all films exhibited ZnO-polycrystalline structure with (0 0 1) preferred orientation. The intensity of the (0 0 2) peak increased remarkably for the films deposited at 300 and 4000C, but the peak intensity decreased with further increase in Ts (500–700 0C). The concentration of Ga showed no clear changes in the Ts range from RT to 400 0C, and then increased remarkably for the films deposited at Ts higher than 400 0C. Khranovskyy et al. [51] investigated the position of the (002) peak. A small shift towards larger angles was found, which suggests a decrease of the c-lattice parameter of ZnO. Ma et al. [52] observed that all the films exhibit the (002) and (004) peaks, indicating that all of the obtained films (the film thickness is around 600 nm) with hexagonal wurtzite structure have a preferred orientation with the c-axis perpendicular to the substrates. No Ga2O3 phase was found from the XRD patterns. Buyanova et al. [53] reported that the structural quality improves significantly with increasing Ga content, which is evident from a substantially reduced density of extended defects and micro cracks. X. Yu et al. [54] studied that all the sputtered ZnO:Ga films are highly textured, with the c-axis perpendicular to the substrates. Only (002) diffraction peak located at 2θ=34.300 is observed. These values are very close to that of the standard ZnO crystal (34.450). With increasing annealing temperature, the positions of measured diffraction peaks were not changed significantly, but the (002) diffraction peaks become more intense and sharper. This is due to the crystallite size becoming larger and the crystallinity of the films improved with an increase in the annealing temperature. At lower contents, Ga stimulates growth of (002) oriented textured films and the smallest FWHM was obtained as low as 0.170 for ZnO:Ga with 1 wt. %. [55]. Nano crystalline gallium doped zinc oxide (ZnO:Ga) thin films were synthesized by plasma-enhanced chemical vapor deposition (PECVD) by J.J. Robbins et al [56]. They observed that gallium doping had a profound impact on film orientation. Using pulsed laser deposition (PLD), two kinds of targets, InGaZnO4 and InGaZn3O6, were deposited by K. Inoue et al. [57]. They observed that below 250 0C, the films showed no clear peaks and the diffraction intensities were very weak. At the higher substrate temperature of 380 0C, clear peaks were assigned to bixbyite In2O3. XRD patterns studied by Huang et al. [58] showed that the undoped film and the films deposited with the TEG flow rate lower than 7.5 sccm by metal-organic chemical vapor deposition method exhibit grain structures with a dominant orientation of (101). The intensity of (101) peak retains while that of other peaks decrease with increasing TEG flow rate. According to X. Yu et al. [59] and Ma et al. [60], the obtained films were polycrystalline with the hexagonal structure and had a preferred orientation with the c-axis perpendicular to the substrates. No diffraction of Ga2O3 phase was found from XRD patterns. Gomez et al. [61] reported that annealed films are polycrystalline and fit well with the wurtzite crystal structure. All films presented a preferred orientation along the (0 0 2) direction. Some other additional peaks associated with the (1 0 3), (1 0 0), (1 0 1), (1 0 2), (1 1 0), and (1 1 2) directions also appear, but their intensities are

58

Characterization of Technological Materials

weaker than the (0 0 2) signal. No extra peaks corresponding to some Ga compounds, in asdeposited or annealed films, were detected. The influence of sputtering power on the structural properties of ZnO:Ga films were investigated by X-ray diffraction by Q.-B. Ma et al. [62]. They observed that as the sputtering power increases, the FWHM first decreases and then increases and reaches its minimum value of 0.36 at 140 W. The grain size along the c-axis is found between 15.4 nm and 27.3 nm. The film deposited at 140 W has the narrowest FWHM and the largest crystal grain. Ahn et al. [63] studied the annealing effect on ZnO and GZO films. After annealing in O2, (002) peak of ZnO and GZO films shifts towards the higher diffraction angle because of chemisorbed oxygen. According to Hirasawa et al. [64], above 19 at% Ga content in GZO film, the (0 0 2) peak broadened and shifted to a lower angle. At low Ts of 150 0C, the (0 0 2) peak was dominant, but at 350 0C, the (1 0 3) peak was dominant. D.-H. Kim et al. [65] compared the structural properties of undoped ZnO and Ga doped ZnO. Both undoped ZnO and GZO films showed a preferential orientation along the (002) plane at 2θ = 34.340 (d = 2.608) and 2θ = 34.320 (d = 2.61), respectively. These values were very close to those of standard ZnO crystal (34.450).A weak XRD peak in both of the films was observed at 2θ = 36.40, d = 2.466, corresponding to the (101) plane. Ga-doped ZnO (GZO) films with a thickness of 100 nm were prepared on cyclo-olefin polymer (COP) and glass substrates at various temperatures below 100 °C by ion plating by A. Miyake et al.[66]. Analysis of the XRD patterns of the films showed that all polycrystalline GZO films on both cyclo olefin polymer and glass substrates have a preferred orientation along the c-axis regardless of Ts, although the quality of GZO films on COP substrates is lower than that of GZO films on glass substrates at any given Ts. With increasing Ts, thermal stress originating from the difference in the thermal expansion coefficients between the GZO film and the COP substrate increases, causing the reduction of the a-axis lattice constant of GZO films. This may result in GZO films with a small grain size on COP substrates compared with GZO films on glass substrates. de la Olvera et al. [67] deposited gallium, aluminum, and indiumdoped ZnO (ZnO:Ga, ZnO:Al, and ZnO:In) by the chemical spray method on sodocalcic substrates. The effect of different dopant elements, a post-annealing treatment in vacuum, and the film thickness on the structural properties of the films has been investigated. The structural and morphological properties of ZnO:Ga and ZnO:Al films are similar, as in both cases the (0 0 2) orientation is dominant on the rest of the peaks, and both surfaces have a rough appearance. In the case of ZnO:In films, the (1 0 1) was the preferential growth orientation, and the surfaces seem to be smoother than the corresponding ZnO:Ga and ZnO:Al films. J.-Y. Tseng et al. [68], suggests that the better crystalline quality was obtained at lower sputtering pressure. As reported by Q.-B. Ma et al. [69], the peak position of the (0 0 2) plane is linearly shifted to the lower 2θ value with the

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59

increase of Ga content. The c-axis orientation in ZnO films was explained by the ‘‘survival of the fastest’’ model [70]. The temperature dependence of crystalline structure of ZnO:Ga was also observed by Suzuki et al.[71]

who explained this dependence as the suppression of c-axis

orientation of the crystalline structure that resulted in milky films with a tetra pod-like surface. Electrical properties of GZO films X. Yu et al. [41] prepared gallium doped zinc oxide films (ZnO:Ga) on glass substrates by rf magnetron sputtering at room temperature. They reported that the increased mobility and carrier concentration makes the resistivity of the film to decrease from 4.9 × 10 -3 to 3.1 × 10 -4

cm when

the film thickness increased from 50 to 1000 nm. The resistivity of ZnO:Ga thin films annealed in H2 ambient at 623 K decreased to the extent of 2–3 orders of magnitude compared to as-deposited thin films. The lowest resistivity of 5.2 × 10 -4

cm was obtained with the carrier concentration of

2.5 × 10 21 cm-3 [42]. Fortunato et al. [44] present results concerning the thickness dependence (from 70 to 890 nm) of electrical properties presented by gallium-doped zinc oxide (GZO) deposited on polyethylene naphthalate (PEN) substrates by rf magnetron sputtering at room temperature. With increasing thickness up to a value approximately 300 nm, the resistivity decreases to a minimum value of 6×10-4

cm. After that point ρ is kept almost constant, independent of the thickness of the

film used. The electrical resistivity of GZO films is reduced from 9:23 ×10-3to 5.77 ×10-3

cm.

The ZnO buffer layer can reduce the electrical resistivity of GZO films from 5.77 ×10-3to 2.38 ×103

cm. It can be anticipated that room temperature deposition enables film deposition onto

polymeric substrates for flexible optoelectronic devices. According to Assuncao et al. [45], with increasing thickness up to a value near 400 nm, the resistivity decreases to a minimum value of 5.7× 10 -4

cm for a substrate to target distance of 15 cm and to 2.7× 10 -4

cm for the distances of

10 and 5 cm. T. Yamada et al. [47] observed the lowest resistivity of 2.23×10−4

cm was obtained

for the GZO film with a thickness of 179 nm prepared with a Ga2O3 content of 4 wt.% at a O2 gas flow rate of 2.5 sccm. Additional heat treatment under reduced atmosphere (4%H2–96%N2) did not change the structure orientation, microstructure shape and size but it can beneficially improve the electrical performance by reducing the sheet resistance further by one order of magnitude. The electrical conductivity of ZnO layers grown with a gallium doping concentration of 5.0 at.% varied in the range, 1.0×102– 4.0×103 0

-1

cm-1 with a variation of substrate temperature from 200 to 450

C [49]. The resistivity of GZO films deposited on RT substrate in 100% Ar gas increased

drastically with increasing water partial pressure (PH2O) of residual gas, due to a decrease in both carrier density and Hall mobility. On the other hand, the electrical properties of the GZO films deposited in Ar+H2 mixture were improved, even though the grain size of film was degraded in proportion to the H2 flow ratio. A lower resistivity was obtained for the films deposited at an H2 flow ratio of 8.5%. P.K. Song et al. [50] deposited Ga doped ZnO films under various conditions of

60

Characterization of Technological Materials

substrate temperature (RT–700 0C), residual water pressure (1.61×10-4 –2.2×10-3 Pa), and H2 / (Ar+ H2) flow ratio (0–15%). The resistivity of GZO films deposited at RT in 100% Ar gas increased drastically with increasing water partial pressure (PH2O) of residual gas, due to a decrease in both carrier density and Hall mobility. On the other hand, the electrical properties of the GZO films deposited in Ar+H2 mixture were improved, even though the grain size of film was degraded in proportion to the H2 flow ratio. A lower resistivity was obtained for the films deposited at an H2 flow ratio of 8.5%. Khranovskyy et al. [51] observed the resistivity reduction of (ρbefore/ρafter ~ 80) after annealing at optimal regime and the final film resistivity was approximately ~ 4 × 10−4

cm, due

to effective Ga dopant activation. Q.-B. Ma et al. [52] studied the influence of Ar/O2 ratio on the properties of transparent conductive ZnO:Ga films prepared by DC reactive magnetron sputtering. The lowest resistivity of 3.58×10−4 cm obtained at the Ar/O2 gas ratio of 15:1. As the Ar/O2 gas ratio further increases, the resistivity increases and reaches 5.45×10−4

cm at the Ar/O2 gas ratio of

20:1. As annealing temperature increases, the resistivity gradually decreases from 1.13×10−3 to as low as 5.4×10−4

cm

cm [54]. The electrical properties of ZnO:Ga films obtained at various flow

rates of TEG ranging from 1.5 to 10 sccm were investigated by Y.-C. Huang et al. [58]. It can be seen that the resistivity of ZnO:Ga thin films are much lower than that can be achieved in normally undoped ZnO. The resistivity of doped films decreases initially with increasing TEG flow rate and achieves a minimum value of 3.6×10−4

cm at 7.5 sccm. Then it increases conversely with a

further increase of TEG flow rate. Effects of sputtering power on the properties of ZnO:Ga films deposited by rf magnetron-sputtering at low temperature is studied by X. Yu et al. [59]. It is seen that as the sputtering power increases from 50 to 200 W, both carrier concentrations and Hall mobility values increased from 6.5×10−19 to 1.12×1021cm-3 and 3.7 to 12.8 cm2 v -1 s-1, respectively, and the resulting resistivity values of the films decreased from 2.58×10−2 The lowest resistivity achieved was 3.9×10−4

cm to 3.9×10−4

cm.

cm for a thickness of 800nm (sheet resistance

~4.6 /cm). The effect of deposition pressure on the electrical properties of GZO films is studied by Q.-B. Ma et al. [60]. As the deposition pressure increases from 0.5 to 1.0 Pa, Hall mobility and carrier concentration increase from 3.40 to 7.52 cm2/V s and from 1.05×1021 to 1.78×1021 cm-3, respectively, resulting in the decrease of resistivity from 1.75×10

-3

to 4.48×10−4

cm. The effect

of the deposition temperature, Ts, dopant concentration [Ga/Zn], and a vacuum annealing treatment on the electrical properties of the ZnO:Ga thin films were analyzed by H. Gomez et al. [61]. The minimum resistivity (7.4×10−3

cm), and the maximum figure of merit was obtained for vacuum-

annealed ZnO:Ga films deposited at a substrate temperature of 425 0C from a solution containing a [Ga/Zn] ratio of 2 at. %. The influence of sputtering power on the electrical properties of ZnO:Ga films were investigated by Hall measurement by Q.-B. Ma et al. [62]. They observed that as the

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61

sputtering power increases from 120W to 140 W, both Hall mobility and carrier concentration increase respectively, and resulting in decrease of resistivity. The resistivity reaches the minimum value of 4.48×10−4

·cm at 140 W. The resistivity of ZnO film annealed at 400 °C increased by

two orders of magnitude, in case of GZO film was relatively stable up to at 400 °C. The passivation of surface, more defects annealing out, and the decrease of oxygen vacancies made the resistivity of ZnO films increase substantially as observed by Ahn et al. [63]. A. Miyake et al. [66] deposited GZO thin films on cyclo-olefin polymer (COP) and glass substrates by ion plating. The values of ρ for the GZO films on the glass substrates are smaller than those for the films on the COP substrates at any given de la Olvera et al. [67] compared the resistivity of Al, Ga and In doped ZnO films. According to them, the as-deposited ZnO:Al films presented the highest resistivity, however after the vacuum-annealing process, the resistivity is competitive with the obtained in ZnO:Ga films. The minimum electrical resistivity, on the order of 10-3 cm, was obtained in annealed ZnO:In films. QB Ma et al. revealed that the lowest resistivity of 3.51×10−4

cm is obtained under the Ga content

of 3.0 at%. As the Ga content further increases, the resistivity increases and reaches 1.54×10−3

cm

at the Ga content of 5.0 at%. Figure 4 shows the variation of resistivity for the different substrate temperature for 3 wt. % Ga doped ZnO films.

55

Resistivity (Χ 10−4 Ω cm)

50

45

40

35

30

25 20

40

60

80

100

120

140

160

Substrate Temperature

Fig 4: Dependence of the electrical resistivity as a function of substrate temperature. Matsubara et al. [72] compare the resistivity of aluminum doped zinc oxide and gallium doped zinc oxide. The minimum resistivities for ZnO:Al and Ga-doped ZnO (ZnO:Ga) films were obtained at the impurity contents of 1.0 and 2.7 wt. %, respectively, and these values were similar; approximately 2.5× 10

-4

cm. At the minimum resistivity compositions, ZnO:Al had higher

mobility and a lower carrier concentration than ZnO:Ga. Cheong et al. reported the result of sheet resistance, Rs, for samples (0–5.0 at.% Ga) post-heat-treated in air at

450 0C. There was a

62

Characterization of Technological Materials

significant reduction in Rs for samples doped with 1.0 at. % Ga (16.61 k /cm) compared to the undoped sample (1114 k /sq.)). Beyond this dopant concentration (1.0 at. %), Rs increased gradually to 267.1 k /sq. (samples doped with 5.0 at. %). A similar trend was reported by Miyazaki et al. [73] using magnetron sputtering to deposit ZnO:Ga (6 wt. % of Ga) films and Ohyama et al. [74] who deposited ZnO:Al (0.8 at. % of Al) films via the sol–gel technique. Undoped ZnO, Aldoped ZnO (AZO) and Ga-doped ZnO (GZO) thin films were deposited on silica glass substrate at room temperature in a vacuum by KrF excimer laser (λ=248 nm) pulsed laser deposition method by N. Sakai et al. [75]. Both AZO and GZO thin films showed low resistivity of the order of ~103 cm. It was emerged that resistivity decreases by doping Al or Ga into ZnO. The improvement of conductivity with gallium doping concentration can be attributed to the increasing free carrier density. Chen et al. [76] indicate this result with the grey relational Taguchi method. Assuncao et al. [77] achieved the lowest resistivity of 2.6 × 10-4

cm (sheet resistance ~ 6

/ sq. for a thickness ~

600 nm) and was obtained at an argon sputtering pressure of 0.15 Pa and a rf power of 175 W. For higher sputtering pressures an increase on the resistivity was observed due to both, a decrease on the mobility and carrier concentration, associated to a change on the surface morphology. Iwata et al. [78] reported that the carrier activation of Ga-doped ZnO thin films changed around a Ga atomic concentration of 1×10-21 cm-3 and the optimum Ga2O3 composition in target was approximately 3%. According to Shirakata et al. [79], electrical properties in 3 wt.% Ga-doped ZnO films are very similar to those in 4% Ga-doped samples. For Ga-doping concentrations of 3 and 4 wt.%, the resistivity is low (2.6× 10-4 to 3.0× 10-4

cm). The lowest resistivity is 2.6× 10-4

cm for 4 wt.%

Ga-doped films. Shirakata et al. [80] revealed that Ga acts as a donor effectively. It can be seen that the increase of the Ga-doping level from 2 to 3 wt. % causes the decrease in resistivity and increase in carrier concentration. However, no remarkable change in resistivity and carrier concentration was found for the increase of the Ga-doping level from 3 to 4 wt. %. Yamamoto et al. [81] obtained ntype films with a low-resistivity of almost 2–3 ×10−4

cm. Excess O2 gas flow rate substantially

decreased the carrier concentration of the GZO films and consequently increased resistivity up to 6×10−4

cm with a carrier concentration of 4.1×10 20 cm-3 . The maximum carrier concentration of

GZO films was 8.6×10 20 cm-3. Nunes et al. [82] investigated the effect of different dopants on the properties of ZnO thin films. It can be observed that the resistivity decreases rapidly with the increase of dopant concentration (up to 1 at. %), being this decrease more accentuated for ZnO films doped with In. The resistivity almost stabilizes for the ZnO film doped with In, while for those doped with Al and Ga an increase in the resistivity is observed. This behavior can be explained by the increase of defects due to the fact that some of the dopant atoms occupy interstitial sites [83], as well as by the appearance of some non-conducting Al2O3 and Ga2O3 oxides. T. Ogi et al. [84] deposited aluminum, gallium and indium doped zinc oxide films by spray low-pressure

Materials Science Forum Vol. 671

pyrolysis method. The resistivity of AZO and GZO as zinc oxide-based TCOs were 2.7 ×104 and 2.2 ×104

63

m

m, respectively, which is higher than that of the indium oxide-based ones. AZO

resistivity was found to be higher than that of GZO. A calculated crystal size of 8 nm was found in AZO, which was smaller than that of GZO. This is considered to be the cause of the higher resistivity of AZO compared with that of GZO. The rf sputtering power varied from 50 to 200 W, and the argon gas pressure controlled from 0.5 to 5 Pa was used to deposit the high quality GZO films by X. Yu et al. [85]. They reported that with increasing sputtering power, the locations of the measured

(002) diffraction peaks do not change significantly and the intensities of the peaks

become more intense and sharper.

Optical properties of GZO films: The average transmittance for all prepared GZO samples prepared by rf magnetron sputtering is over 83% in the wavelength range of the visible spectrum [41]. S. Kim et al.[42] reported that the values of optical band gap obtained from absorption coefficient vs. hν for the annealed thin films were red-shifted and spread out corresponding to activated dopant concentration (carrier concentration), while the values of it for as-deposited thin films were almost identical. Assuncao et al. [45] observed that the average transmittance in the visible range of all the films produced is in the range of 85%, reaching even the value of 90%, for the films with very low resistivities. The effects of dopant (Ga) concentration, post-heat treatment temperatures, and different heat treatment environments on the optical properties of ZnO films were studied by K.Y. Cheong et al.[48] Films doped with 5.0 at.%, 3.0 at.% and 0 at.% Ga recorded average optical transmissions of 78%, 82% and 77%, respectively. Ma et al. [52] studied the influence of Ar/O2 ratio on the properties of transparent conductive ZnO:Ga films prepared by DC reactive magnetron sputtering. With increasing Ar/O2 gas ratio, the band gap of the films increases approximately linearly up to ~3.78 eV. The obtained optical gaps of these films are much larger than those of pure ZnO (~3.3 eV). X. Yu et al [54] discussed the influence of annealing on the properties of ZnO:Ga films prepared by radio frequency magnetron sputtering. When the films are annealed at 573 K for 20 min, the average transmission of the ZnO:Ga samples (in the visible) increases from 85% to more than 90%, and the optical absorption edge for the films shifts to longer wavelength and the absorption edge becomes steeper. Inoue et al. [57] studied the properties of transparent conductive In–Ga–Zn oxide films produced by pulsed laser deposition. The optical band gap energy shifted to higher energies and the transmittance at the blue range was improved dramatically as compared with similar amorphous IZO films. The optical properties of ZnO:Ga films obtained at various flow rates of TEG ranging from 1.5 to 10 sccm were investigated by Y.-C. Huang et al. [58]. As the TEG flow rate increases from 0 to 10 sccm, the average transmittance increases from 75% to more than

64

Characterization of Technological Materials

85% and the absorption edge shifts to a shorter wavelength. Q.-B. Ma et al. [60] fabricated ZnO:Ga films by DC reactive magnetron sputtering and studied the effects of deposition pressure on the properties of those transparent conductive films. He observed that the average optical transmittance of all the films is over 90% and the optical band gap energy of the ZnO:Ga films was greatly influenced by the deposition pressure. Figure 5 shows a plot of (αhν)2 vs hν for ZnO:Ga films. It indicates a band gap of 3.47 eV. 40

30 25 20

2

(α hν) ( cm

-1 eV)2

35

15 10 5 0 1.5

2.0

2.5

3.0

3.5

Photon energy, hν (eV)

Fig 5: Plot of (αhν)2 vs hν for ZnO:Ga films Gomez et al. [61] prepared Ga-doped Zn oxide (ZnO:Ga), thin films by the chemical spray technique using Zn acetate and Ga pentanedionate as precursors of Zn and Ga, respectively. Films deposited at 425 0C from a starting solution containing a [Ga/Zn] = 2 at. % ratio showed higher transmittance values respect to rest of the films. The average optical transmittance in the visible range exceeds 90% regardless of the sputtering power for all the films deposited on glass substrate by DC reactive magnetron sputtering [62]. The estimated optical band gap of the films is larger than that of undoped ZnO (~ 3.30 eV). The optical properties of undoped zinc oxide (ZnO) thin films of various thicknesses were compared with those of Ga-doped (GZO) thin films by D.-H. Kim et al. [65]. Undoped ZnO and GZO samples showed interference fringe patterns in their transmission spectra. The average transmittance in the visible spectrum (400–700 nm) for the glass substrate was 85% after deposition of a ZnO thin film. For GZO deposition, an average transmittance of 80% was obtained. The transmittance of the glass and COP substrates themselves at a wavelength of 550 nm were 92% and 91% respectively as observed by Miyake et al. [66]. Q-B Ma et al. [69] reported that all films exhibit an average optical transmittance of higher than 90% and a sharp fundamental absorption edge. As the carrier concentration in the films increases, the absorption edge shifts to shorter wavelength. The obtained optical gap of these films is much larger than that of pure ZnO. Highly transparent ZnO conducting films for thin film solar cell applications were fabricated at low temperature by pulsed laser deposition. Al-, B- and Ga-doped ZnO films were deposited on Corning

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7059 glass substrate at a substrate temperature of 200 0C and their optical properties were studied by K. Matsubara et al. [72]. The transmittance of 5.0 at.% Ga doped ZnO films deposited by spray pyrolysis method was higher than 85% with an optical energy band gap of 3.34 eV in the visible region with a high reflectance in the infra-red region [75]. Gallium-doped zinc oxide films have been grown on glass substrates with and without ZnO buffer layers by rf magnetron sputtering at room temperature by Chen et al. [76]. According to them, the suitable sputtering pressure and O2 gas flow rate can increase the optical transmittance of GZO films. Assuncao et al. [77] and Shirakata et al. [79, 80] observed that the films present an overall transmittance in the visible spectra of approximately 90%. Fortunato et al. [86] observed that the average transmittance at the visible range is located between 80 and 90%, for the thickest and thinnest film, respectively. The near-infrared transmittance decreases as the film thickness increases. According to Fortunato et al. [87], the films are highly transparent (between 80% and 90% including the glass substrate) in the visible spectra with a refractive index of about 2, very similar to the value reported for the bulk material. Conclusion The material properties of the films prepared by techniques like sputtering, spray pyrolysis, electro deposition and electron beam evaporation are reported. The films prepared by most of the methods are polycrystalline and show n-type semiconducting nature. The structural, electrical and optical studies reveal that device quality semiconducting thin films can be prepared. References [1]. [2]. [3]. [4]. [5]. [6]. [7]. [8]. [9]. [10]. [11]. [12].

Kenichi Inoue, Kikuo Tominaga, Takashi Tsuduki, Michio Mikawa, Toshihiro Moriga Vacuum 83 (2009) 552. H.L. Hartnagel, A.L. Dawar , A.K. Jain, C. Jagadish, Semiconducting Transparent Thin Films, p16, Institute of Physics Publ, Bristol & Philadelphia, 1995, pp. 202. Yen-Chin Huang , Zhen-Yu Li , Hung-hsin Chen , Wu-Yih Uen , Shan-Ming Lan, Sen-Mao Liao , Wu-Yih Uen , Shan-Ming Lan , Sen-Mao Liao, Tsun-Neng Yang, Chin-Chen Chiang Thin Solid Films 517 (2009) 5537. A.F. Kohan, G. Ceder, D. Morgan, C G. Van de Walle, Phys. Rev. B 61 (2000) 15019. K.-K. Kim, S. Niki, J.-Y. Oh, J.-O. Song, T.-Y. Seong, S.-T. Park, S. Fujita, S.-W. Kim, J. Appl. Phys. 97 (2005) 066103. T. Makino, Y. Segawa, S. Yoshida, A. Tsukazaki, A. Ohtomo, M. Kawasaki, Appl. Phys. Lett. 85 (2004) 759. T. Minami, H. Nanto, S. Takata, Jpn. J. Appl. Phys. 23 (1984) L280. K.L. Chopra, S. Major, D.K. Pandya, Thin Solid Films 102 (1983) 1. Y. Li, G.S. Tompa, S. Liang, C. Gorla, Y. Lu, John Doyle, J. Vac Sci. Technol. A 15 (1997) N. Ito, Y. Sato, P.K. Song, A. Kaijio, K. Inoue, Y. Shigesato, Thin Solid Films 496 (2006) 99. K.Ch.Park, D.Y.Ma, K.H.Kim, The physical properties of Al-doped zinc oxide films prepared by RF magnetron sputtering. Thin Solid Films 305 (1997) 201. R.G. Gordon, MRS Bull. 25 (2000) 57

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[13]. [14]. [15]. [16]. [17]. [18]. [19]. [20]. [21]. [22]. [23]. [24]. [25]. [26]. [27]. [28]. [29]. [30]. [31]. [32]. [33]. [34]. [35]. [36]. [37]. [38]. [39]. [40]. [41]. [42]. [43]. [44]. [45]. [46]. [47]. [48].

Characterization of Technological Materials

H.J. Ko, Y.F. Chen, S.K. Hong, H. Wenisch, T. Yao, Appl. Phys. Lett. 77 (2000) 3761. R.G. Gordon, MRS Bulletin 25 (2000) 52. V. Assuncao, E. Fortunato, A. Marques, H. Aguas, I. Ferreira, M.E.V. Costa, R. Martins, Thin Solid Films 427 (2003) 401. Milton Ohring, Material Science of Thin Films (Academic press, NewYork 2002) 121. L. Maissel, Handbook on Thin Film Technology, P4-1, (Eds) L I Maissel and R. Glang McGraw Hill Book Co, New York, USA, (1970). J.C.C. Fan, F.J. Bachner and G.H. Poley, Appl Phy Lett, 31 (1997) 73. Lie mieo, Sakaljanemore, Shoichitoh, journal of crystal growth, 264(2004) 246. Milton Ohring: Material Science of Thin Films (Academic press, NewYork 2002) 207. Milton Ohring: Material Science of Thin Films (Academic press, NewYork 2002) 209. Milton Ohring: Material Science of Thin Films (Academic press, NewYork 2002) 222. Milton Ohring: Material Science of Thin Films (Academic press, NewYork 2002) 216. R.W. Cahn (Ed), Materials science and Technology-A comprehensive treatment, Vol 15, VCH, Weinheim, FRG (1991). R.K.Bedi, Proc Conference on the Physics and Technology of Semiconductor Devices and Integrated Circuits, (Eds) B.S.V. Gopalam and J.Majhi, SPIE, Vol 1523, Tata Mc Graw Hill, New Delhi (1992) 104. S.K.Gandhi, R. Sivily and J.M. Borrego, Appl Phys Lett, 34 (1979) 833. T.M. Ray Koov, Thin Solid Films, 164 (1987) 301. E.Shanthi, A.Banerjee and K.L.Chopra, Thin Solid Films, 88 (1982) 93. D.E. Bode, Proc Natl Electron Conf, 19 (1963) 630. G.A. Kitaev, A.A. Uritskaya and S.G. Mokrushin, Russ J Phy Chem, 39 (1965) 1101. J.C.Norris, Met Finish, 86(7) (1988) 45. J.C.Norris, Met Finish, 86(8) (1988) 45. K.R. Baldwin and C.J.E Smith, Plat and Surf Finish, July (1997). King, R; Encyclopedia of Inorganic Chemistry. 1994, 3, 1256. P.A.Lee, G. Said, R. Davis and T.H. Lim, J Phys Chem Solids, 30 (1969) 2719. D.I. Bletskan, I.F. Kopinets, P.P. Pogorsh, E.N. Salkova and D.V. Chepor, Kristallografiya, 20 (1978) 1008. M.F. Ladd and R.A. Palmer (Eds), Structure determination by X-ray Crystallography, Plenum Press, New York (1964) 71. H.E. Bennet and J.M. Bennet in G. Hass and R. E. Thun (Eds), Physics of Thin Films Vol 4, Academic Press, New York (1967). V.P. Bhatt, K. Gireesan and C.F. Desai, Cryst Res Technol, 24 (1989) 187. D. Bhattacharya, S. Chaudhuri, A.K.Pal and S.K.Bhattacharya, Vacuum, 42 (1991) 1113. Xuhu Yu,, Jin Ma, Feng Ji, Yuheng Wang, Chuanfu Cheng, Honglei Ma, Applied Surface Science 245 (2005) 310. Sungyeon Kim , Jungmok Seo , Hyeon Woo Jang , Jungsik Bang , Woong Lee , Taeyoon Lee , Jae-Min Myoung, Applied Surface Science 255 (2009) 4616. F. Wu, L. Fang , Y.J. Pan , K. Zhou , L.P. Peng , Q.L. Huang , C.Y. Kong, Applied Surface Science 255 (2009) 8855. Elvira Fortunato, Alexandra Goncalves, Vitor Assuncao, Antonio Marques, Hugo A guas, Luıs Pereira, Isabel Ferreira, Rodrigo Martins, Thin Solid Films 442 (2003) 121. Vitor Assuncao, Elvira Fortunato, Anto´nio Marques, Alexandra Gonc¸alves, Isabel Ferreira, Hugo A´ guas, Rodrigo Martins Thin Solid Films 442 (2003) 102. Tadatsugu Minami, Satoshi Ida, Toshihiro Miyata, Youhei Minamino, Thin Solid Films 445 (2003) 268. T. Yamada, K. Ikeda, S. Kishimoto , H. Makino, T. Yamamoto, Surface & Coatings Technology 201 (2006) 4004. K.Y. Cheong, Norani Muti, S. Roy Ramanan, Thin Solid Films 410 (2002) 142.

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[49]. [50]. [51]. [52]. [53]. [54]. [55]. [56]. [57]. [58]. [59]. [60]. [61]. [62]. [63]. [64]. [65]. [66]. [67]. [68]. [69]. [70]. [71]. [72]. [73]. [74]. [75]. [76]. [77].

67

K.T. Ramakrishna Reddy, T.B.S. Reddy, I. Forbes, R.W. Miles, Surface and Coatings Technology 151 –152 (2002) 110. P.K.Song , M.Watanabe, M.Kon , A.Mitsui , Y.Shigesato, Thin Solid Films 411 (2002) 82. V. Khranovskyy, U. Grossner, V. Lazorenko, G. Lashkarev, B.G. Svensson, R. Yakimova, Superlattices and Microstructures 42 (2007) 379. Quan-Bao Ma, Zhi-Zhen Ye , Hai-Ping He, Li-Ping Zhu, Jing-Rui Wang, Bing-Hui Zhao, Materials Letters 61 (2007) 2460. I.A. Buyanova, X.J. Wang, W.M. Wang, C.W. Tu, W.M. Chen, Superlattices and Microstructures 45 (2009) 413. Xuhu Yu, Jin MaT, Feng Ji, Yuheng Wang, Xijian Zhang, Honglei Ma, Thin Solid Films 483 (2005) 296. V. Khranovskyy , U. Grossner , O. Nilsen , V. Lazorenko , G.V. Lashkarev , B.G. Svensson , R. Yakimova , Thin Solid Films 515 (2006) 472 . J.J. Robbins, J. Harvey, J. Leaf, C. Fry, C.A. Wolden, Thin Solid Films 473 (2005) 35. Kenichi Inoue , Kikuo Tominaga , Takashi Tsuduki, Michio Mikawa , Toshihiro Moriga, Vacuum 83 (2009) 552. Yen-Chin Huang , Zhen-Yu Li , Hung-hsin Chen , Wu-Yih Uen , Shan-Ming Lan , Sen-Mao Liao , Yu-Hsiang Huang , Chien-Te Ku , Meng-Chu Chen , Tsun-Neng Yang , Chin-Chen Chiang , Thin Solid Films 517 (2009) 5537. Xuhu Yu, Jin Ma, Feng Ji, Yuheng Wang, Xijian Zhang, Chuanfu Cheng, Honglei Ma, Journal of Crystal Growth 274 (2005) 474. Quan-Bao Ma, Zhi-Zhen Ye, Hai-Ping He, Li-Ping Zhu, Bing-Hui Zhao, Materials Science in Semiconductor Processing 10 (2007) 167. H. Gomez, M. de la L. Olvera, Materials Science and Engineering B 134 (2006) 20. Quan-Bao Ma, Zhi-Zhen Ye, Hai-Ping He, Jing-Rui Wang, Li-Ping Zhu, Bing-Hui Zhao, Materials Characterization 5 9 ( 2 0 0 8 ) 1 2 4. Byung Du Ahn , Jong Hoon Kim , Hong Seong Kang , Choong Hee Lee , Sang Hoon Oh, Kyoung Won Kim , Gun-eik Jang , Sang Yeol Lee, Thin Solid Films 516 (2008) 1382. Hiroshi Hirasawa, Makoto Yoshida, Susumu Nakamura,Yoshio Suzuki, Satoshi Okada, Ken-ichi Kondo, Solar Energy Materials & Solar Cells 67 (2001) 231. Do-Hyun Kim , Hoonha Jeon, Geumchae Kim, Suejeong Hwangboe ,Ved Prakash Verma , Wonbong Choi, Minhyon Jeon, Optics Communications 281 (2008) 2120. A. Miyake , T. Yamada, H. Makino, N. Yamamoto, T. Yamamoto, Thin Solid Films 517 (2009) 3130. M. de la L. Olvera, H. Gomez, A. Maldonado, Solar Energy Materials & Solar Cells 91 (2007) 1449. Jiun-Yi Tseng , Ming-Yi Yang , Cheng-Yi Wang , Pin-Chou Li , Wang-Chieh Yu,Yung-Fu Hsu , Sea-Fue Wang , Yuan-Tsung Chen, Thin Solid Films 517 (2009) 6310. Quan-Bao Ma, Zhi-Zhen Ye, Hai-Ping He, Shao-Hua Hu, Jing-Rui Wang, Li-Ping Zhu, Yin-Zhu Zhang, Bing-Hui Zhao, Journal of Crystal Growth 304 (2007) 64. X. Yu, J. Ma, F. Ji, Y.Wang, X. Zhang, C. Cheng, H. Ma, J. Cryst. Growth 274 (2005) 474. Suzuki, T. Matsushita, T. Aoki, Y. Yoneyama, M. Okuda: Jpn. J. Appl. Phys. 38 (1999) 71. K. Matsubara, P. Fons, K. Iwata, A. Yamada, K. Sakurai, H. Tampo, S. Niki, Thin Solid Films 431 –432 (2003) 369. M. Miyazaki, K. Sato, A. Mitsui, H. Nishimura, J. Non-Cryst. Solids 218 (1997) 323. M. Ohyama, H. Kozuka, T. Yoko, J. Am. Ceram. Soc. 81(1998) 1622. Norihiro Sakai , Yoshihiro Umeda, Fumiaki Mitsugi, Tomoaki Ikegami, Surface & Coatings Technology 202 (2008) 5467. Ding-Yeng Chen, Chun-Yao Hsu, Superlattices and Microstructures 44 (2008) 742. V. Assuncao , E. Fortunato, A. Marques, H. Aguas, I. Ferreira, M.E.V. Costa, R. Martinsa, Thin Solid Films 427 (2003) 401.

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[78]. [79]. [80]. [81]. [82]. [83]. [84]. [85]. [86]. [87].

Characterization of Technological Materials

K.Iwata , T.Sakemi , A.Yamada, P.Fons , K.Awai, T.Yamamoto, M.Matsubara , H.Tampo, K.Sakurai , S.Ishizuka , S.Niki , Thin Solid Films 451 –452 (2004) 219. S. Shirakata, T. Sakemi, K. Awai, T. Yamamoto, Thin Solid Films 451 –452 (2004) 212. S. Shirakata, T. Sakemi, K. Awai, T. Yamamoto, Thin Solid Films 445 (2003) 278. T.Yamamoto, T.Sakemi , K.A wai, S.Shirakata, Thin Solid Films 451 –452 (2004) 439. P. Nunes, E. Fortunato , P. Vilarinho, R. Martins, International Journal of Inorganic Materials 3 (2001) 1211. Nunes P, Femandes B, Fortunato E, Martins R. Thin Solid Films, 1998;333:1. Takashi Ogi, Darmawan Hidayat, Ferry Iskandar, Agus Purwanto, Kikuo Okuyama, Advanced Powder Technology 20 (2009) 203. Xuhu Yua,, Jin Maa, Feng Jia, Yuheng Wanga, Xijian Zhanga, Chuanfu Chengb, Honglei Ma, Applied Surface Science 239 (2005) 222. E. Fortunato, V. Assuncao , A. Goncalves, A. Marques, H. Aguas, L. Pereira, I. Ferreira, P. Vilarinho, R. Martins, Thin Solid Films 451 –452 (2004) 443. E. Fortunato , L.Raniero, L.Silva, A.Gonc-alves, A.Pimentel, P.Barquinha,H.Aguas, L.Pereira, G. Gonc-alves, I.Ferreira, E.Elangovan, R.Martins, Solar Energy Materials & Solar Cells 92 (2008) 1605.

Materials Science Forum Vol. 671 (2011) pp 69-120 © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.671.69

Nanomaterial preparations by microwave-assisted solution combustion method and material properties of SnO2 powder —A status review L.C.Nehru1,a, V. Swaminathan2,b, M.Jayachandran3,c and C.Sanjeeviraja1,d* 1

School of Physics, Alagappa University, Karaikudi-630 003, India School of Materials Science and Engineering, Nanyang Technological University, Singapore-639798 3 ECMS, Central Electrochemical Research Institute, Karaikudi-630 006, India a [email protected]; [email protected]; cmjayam54@gmail; [email protected] *Corresponding author: [email protected]

2

Keywords: Nanocrystalline; SnO2; Microwave-assisted solution combustion

Abstract. A nanocrystalline tin oxide (SnO2) powders have been prepared by a simple, lowtemperature initiated, self-propagating and gas producing by microwave-assisted solution combustion process. The effects of temperature on crystalline phase formation and particle size of nanocrystalline SnO2 and its structure have been investigated. It is observed that heat-treated upto 800ºC shows tetragonal phase SnO2. It was observed that the average crystallite size of the annealed SnO2 samples is in the range 9 - 43 nm through controlled heat treatment process. The crystal density of the as-prepared powder is 5.850g cm-3 where as the bulk density is 6.998 g cm−3. The microstructure and morphology were studied by scanning electron microscope (SEM) and HRTEM it is interesting to note that asprepared SnO2 sample are almost spherical in shape and average agglomerate crystal size of 0.2 – 0.4 m with increase in calcination temperature, the samples become better morphology than the asprepared sample. The crystallographic parameters were refined by XRD pattern and Rietveld refinement using TOPAS-3 and Diamond software was used to construct the structural parameters. History of nanosized materials In 1962, Ryogo Kubo reported that the energy levels of the ultra-tiny metal particles vary from the bulk materials and change with the particle diameters [1]. This interesting phenomenon leads the scientist to pay much attention in the researches of the nanoscale materials. Then in 1972, Norio Taniguchi used the brand-new word “Nanotechnolohy” to describe the manufacture in the precision machinery for the first time in the international conference. Ever since, the word “Nanotechnology” becomes a proper noun of the new-generation technology in the 21th century. Then, K.E. Drexler described the future atom and molecule machine detailed in his publication “Engine of Creation” at the last 70s. But there was no instruments could distinguish the single atom and molecule directly and this publication is not bring great influence upon the scientific field. Till 1982, G. Binning and H. Rohrer fabricated the first one surface analysis instrument to observe the single atom and the atomic arrangement successfully. Since, the human being can really observe the materials in ultra-tiny scale and a series of researches in the nanoscale materials were reported one by one.

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In 1991, Sumio Iijima first discovered the carbon nanotubes [2] and the one-dimension materials interested the scientists. Then, in 1998 the scientists used the similar synthesis method to synthesize the silicon nanowires (SNWs). Since many research groups used the similar synthesis theories to produce nanowire composed of other materials both in the gas phase and the liquid phase. Due to the different properties in the nanoscale materials such as optical property [3], magnetic property, electric property, thermal conductive property [4, 5], diffusive property [6, 7] and mechanical property [8], several nanoscale metals, ceramics and semiconductors are synthesized successively. Nanotechnology and nanomaterials Nanoscience and nanotechnology is a highly multidisciplinary field of applied science and technology covering a broad range of topics [9]. The main unifying theme is the control of matter on a scale smaller than one micrometer, normally between 1-100 nanometers, as well as the fabrication of devices on this same length scale. These smaller materials are referred to as nanomaterials and defined as a particle less than 100 nm in at least one direction. At this length scale, nanomaterials have very large surface to volume ratio, and quantum confinement starts to play an important role. As a result, nanomaterials are found to possess unique or enhanced properties compared with their bulk counterparts and more and more devices continue to be fabricated to utilize these properties. Top-down and bottom-up are two distinct strategies for fabricating nanostructures and devices. The top-down method starts from bulk materials, which are sculpted into nanosized features by carving, milling, etching and patterning. Lithography methods such as e-beam lithography, photolithography, focused ion beam lithography, dip pen lithography, etc, are getting more important to realize nanostructures in this top-down approach. Benefited by increasingly powerful computers, software, and well-developed techniques, the top-down method can usually achieve good control over the device dimension, location, and organization with high precision. However, high precision induces greater cost when the size of device is reduced, especially when the size is below 100 nanometers. In comparison, bottom-up methods construct structures or devices from the basic building blocks, atom by atom or molecule by molecule. This method utilizes the self-assembly concept, in which the building blocks, atoms or molecules, automatically arrange themselves into desired conformation. Bottom-up approaches usually are able to produce devices in parallel and much cheaper than top-down methods. However, this method could potentially be outgrown as the size and complexity of the desired assembly increases. Sometimes scaling-up can be problematic, for bottom up methods. Because both top-down and bottom-up have their own advantages and drawbacks, hybrid methods combining both

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approaches should be desired techniques in the future to achieve molecular resolution and functionality on large areas. Physical properties of nanomaterials Nanomaterials are materials with particle sizes less than one micrometer, usually less than 100 nm. These small particle sizes impart different physical and chemical properties compared to the bulk forms. Different phases are also found in some nanocrystalline materials. For example, bulk Er2O3 exists in two hexagonal phases, but its nanocrystalline Er2O3 exhibits two phases (fcc cubic and monoclinic) that are not found in the bulk [10]. A well-known property of nanomaterials is that their surface areas are tremendously increased. Their surface-to-volume ratios are very high, so that most of the molecules/atoms are on the surface or at the grain boundaries. Since surface molecules/atoms don't have any force above the particle surface to balance the attractive force from inside the particle, they are in high energy states [11]. In addition, molecules/atoms at the grain boundaries are in highly distorted lattice structures, and forces exerted on a molecule/atom from surrounding species are not balanced, so molecules/atoms at the grain boundaries are also in high energy states. Therefore, the surface energy of a nanomaterial is very high [11]. The large surface area and number of grain boundaries of nanomaterials provide a high concentration of low-energy diffusion paths. Therefore, nanomaterials have higher self-diffusivity and solute diffusivity than the bulk forms [8]. Nanoparticles have electrical and optical properties that are not observed in the bulk. These "quantum-size" effects appear when particle sizes are comparable with or smaller than some characteristic lengths, such as a phonon wavelength, an electron de Broglie wavelength, or an effective Bohr radius around impurity centers. The energy states of doped impurity atoms are strongly modulated in nanocrystallites, whose sizes are smaller than the Bohr radius of the impurity atoms. This phenomenon is called quantum confinement. Quantum confinement effect changes overlaps of the wave functions of the impurity atoms with those of host atoms, leading to more efficient interactions between impurity atoms and the host atoms. Materials which are less than 100 nm in size begin to exhibit changes in their fundamental properties. For example, the melting point [12] of nanostructured materials can be much less than those of the bulk phase. Their magnetic properties may also vary from those of the bulk metal, as in the case of iron [13]. The electronic properties of nanostructured semiconductors can be changed with grain size. The optical properties of nanometer-sized particles also exhibit changes from those in the bulk. Wilcoxon et al. have developed techniques for the synthesis of metal colloids with fine grain

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dimensions [13, 14], it was noticed that the wavelength of light absorbed by different grain sizes shifted more than was predicted by various theories. It is obvious that as the grain size of nanostructured materials decreases, the absorbed wavelength blueshifts toward shorter wavelengths (i.e. the materials appear more blue). Metal oxide semiconductor nanomaterials The researches of the semiconductor materials have been started since early 19th century, in addition to the single-element semiconductors such as Si and Ge, there are some semiconductors composed of two or more different elements, like III-V GaAs and the II-VI ZnS. The major difference between the semiconductors is the energy band gap. The energy band gap is considered as the interval energy between the conduction band and the valence band of the semiconductors, and also be the minimum energy for an electron to escape from its chemical bond. This energy band gap dominates the conductivity of the semiconductor. If the band-gap energy is smaller, the number of free electron in the conduction bend is larger and the conductivity is better relatively. Currently, metal oxide nanomaterial’s are among the most highly produced nanomaterials [9]. Metal oxide nanomaterial’s applications include catalysis, sensors, environmental remediation, and personal care products [15]. Metal oxide nanomaterials have proven to be effective in treating hazardous substances such as chlorinated solvents, microbes, pesticides, and mustards [11]. Metal oxide nanomaterials have demonstrated beneficial properties, but metal oxide nanomaterial exposure has also caused toxic effects in cells and organisms. Metal oxide nanomaterials have increased cell death with increasing concentrations, affected mitochondrial function, induced lactate dehydrogenase (LDH) leakage, and generated abnormal cell morphology at concentrations as low as 50-100 µg/L [16, 17]. There is a potential for metal oxide nanomaterials to enter aquatic ecosystems through use of industrial products and waste. Another source of metal oxide nanomaterials is personal care products as they are washed off during recreational aquatic activities and during bathing. The aforementioned studies indicate that aquatic organisms could be at risk if exposed to metal oxide nanomaterial. This could be detrimental to aquatic ecosystems as metal oxide nanomaterials have caused toxicity to organisms from all levels in an aquatic food chain. There is little information regarding toxicity of metal oxide nanomaterials on aquatic vertebrates, however studies with aquatic invertebrates and microbes show that it is possible that amphibians and fish may be at risk as the metal oxide nanomaterials could induce stress on the organism as well as deplete their food source. Such a stress on amphibians could create a larger problem as amphibians have been suffering from declining

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populations for many decades, since the 1950s [18]. In this study, attention is mainly focused to the nano-scaled metal oxide particles. Types of nanoparticles Nanoparticles are available with a wide range of morphologies and states of agglomeration. They include:
Nanotubes mostly of carbon fullerenes with diameters from 1 to 20 nm and lengths greater than 1mm,
Nanowires of metals, semiconductors, etc., comprising a single crystal structure with diameters of 10s of nanometres and large aspect ratio
Nanocrystals and quantum dots of semiconductors, metals and metal oxides comprising 1000 to 100,000 atoms,
Spherical and dendritic aggregated nanoparticles made from a range of materials including carbon black, fumed silica, metals, metal oxides, ceramics, semiconductors and organic materials. They can range in size from 10-50 nm to 100s of nanometres. Nanoparticles are very rarely found as single particles and readily form aggregates in which the particles can tightly be bound by covalent bonds. These aggregates can then clump together to form agglomerates held together by relatively weak forces including van der Waal’s forces. Agglomerates can range in size from 5 to 25 nm. Applications of nanomaterials Nanomaterials and most of the applications derived from them are still in an early stage of technical development. Much work still needs to be done in this newly born field of science. Nanocrystalline materials are characterized by a microstructural length or grain size of up to 100 nm [19, 20], and have distinctly different properties than bulk materials. The number of atoms or molecules on the surface of nanoparticle is comparable to that inside the particles, therefore nanoparticles can be used to develop materials with unique properties [21]. It is reported that to meet the technological demands in the areas such as electronics, catalysis, ceramics magnetic data storage, structural components etc, the size of the materials should be reduced to the nanometer scale. Recently, the synthesis of nanoparticles has become very important. Nanoparticles synthesized using different methods may have different internal structures that affect the properties of materials consolidated from them. One of the most critical characteristics of nanoparticles is their very high

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surface-to-volume ratio, i.e. large fractions of surface atoms. The large fractions of surface atoms together with ultra-fine size and shape effects make nanoparticles exhibit distinctly different properties from the bulk [19]. The percentage of surface atoms increases as the size of the nanoparticles is decreased. Controlling the size, shape and structure of nanoparticles is technologically important because of strong correlation between these parameters and optical, electrical, magnetic and catalytic properties [22, 23]. Because of the novel properties of nanomaterials compared to their bulk forms, they are promising candidates for many advanced technical applications. Nanomaterials inherently have a very high surface-to-volume ratio. Therefore, nanometer-sized catalyst supports, or nanometersized catalysts have greatly improved efficiencies [24]. Nanoparticles of magnetic materials exhibit greatly improved magnetic properties and much smaller particle sizes, which find many potential applications in magnetic recording, magnetic refrigeration, and ferrofluids [25]. Nanometersized semiconductor clusters are promising materials to prepare devices for efficient conversion of light into electricity [26] or electricity into light (for example, nanocrystalline Si [27]. Nanocrystallites of semiconductor materials are considered as quantum dots due to quantum confinement effects, and doped quantum dots are candidates for advanced displays (High Definition TV, Field Emission Display, Plasma Display, Electroluminescent Display), ultra-fast sensors, and lasers [28]. Superplasticity of nanometer-sized ceramic materials creates a new processing technology for ceramics, the super-plastic forming technology [29]. Superior hardness and fracture toughness of some nanomaterials make them ideal materials for cutting tools [30]. The mechanical properties of nanocrystalline ceramics lead them to be called "ceramic steel" [31]. Commercial realization of ceramic engines also depends on the development of such nanocrystalline ceramics [32]. Methods of synthesis of nanopowders There are several ways to synthesize semiconductor nanopowders have been employed. In particular, a large number of nanomaterials were prepared by these following methods: sol-gel technique [33], hydrothermal synthesis [34], precipitation [35], and combustion synthesis [33].

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Sol-gel Process The sol-gel process is also a controlled precipitation process [36]. The particles formed have such small sizes that they don't precipitate but rather disperse in the solution as stable colloids (sol). The colloid particles can agglomerate with each other to form precipitate of particle networks (gel) [37]. Solubility variations with temperature can be used to prepare sol. First, to prepare substance is to add or dissolved or mixing into a solution slowly in a solvent. which is either a poor solvent or a precipitant of the solute, so that a sol forms once the solubility limit is exceeded. Then, the mixed solution is placed within capped polystyrene tubes to gel. Solvent is allowed to evaporated from the gel during stage. During densification the sol-gels is to shrink. After the shrink sample is annealing completes, the nanopowder is formed (Fig. 1). But, the sol forms were aged for several days. The gel powder samples were further dried and annealed at elevated temperatures to calcine hydroxides into oxides.

Homogeneous Precipitation In normal precipitation processes, aqueous solutions of reactants are mixed to produce precipitates of insoluble substance by exceeding the solubility limits. Since the mixing processes are not controlled, large concentration gradients during mixing produce a broad distribution of particle sizes. The mixture, with magnetic stirring, was then heated to boiling. Due to this process NH3 molecules during thermal hydrolysis of urea, there was essentially no concentration gradient in the solution. The resulting precipitate was washed with water and acetone. The average particle sizes ranged from less than 100 nm to several hundreds of nanometers. They found that urea played a critical role in the precipitation process. In sol-gel processes, precipitation conditions, such as solution pH,

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temperature, reagent concentration, stirring rate, addition rate, and aging time, must be controlled precisely to achieve uniform nucleation throughout the solution. Most of these methods involve complicated steps, expensive chemicals and need longer processing time. Combustion Synthesis Combustion method as one of the gas phase (aerosol) methods generally produce powders “ready to use”, is largely investigated and widely employed in the large-scale production of several metal oxide semiconductors. Combustion synthesis has been used to prepare many simple and complex oxide ceramics, such as aluminates [38], ferrites [39] and chromites [40]. In combustion synthesis, an aqueous solution of an oxidizer, typically a mixture of metal nitrate of the metal elements in the target product, and organic fuels is dehydrated and ignited in a muffle furnace or on a hot plate at temperatures less than 3500C. The dehydrated mixture undergoes a vigorous, exothermic oxidationreduction reaction. The heat created causes a flame for few minutes, resulting in voluminous and foamy powder product occupying the entire reaction container. The exothermic combustion reaction releases a large amount of heat, which can quickly heat up the system to reach a temperature higher than 16000 C [41]. The combustion method results in uniform and pure powders of high surface-to-volume ratio. The mechanism of nanoparticle formation in the combustion synthesis is not complicated. Many parameters need to be considered, including the type of fuel, the fuel-to-oxidizer ratio, the use of an assistant oxidizer, the ignition temperature, and the water content. Among this combustion method processing offers many advantages which include low processing temperature, high purity, molecular level homogeneity and more flexibility in the components. Due to its advantages, the combustion approach was used in this study, and it is discussed in detail in the above section. Combustion Basics Combustion is the conversion of a substance called a fuel into chemical compounds known as products of combustion by combination with an oxidizer. The combustion process is an exothermic chemical reaction, i.e., a reaction that releases energy as it occurs. Thus combustion may be represented symbolically by: Fuel + Oxidizer → Products of combustion + Energy Here the fuel and the oxidizer are reactants, i.e., the substances present before the reaction takes place. This relation indicates that the reactants produce combustion products and energy. Either the chemical energy released is transferred to the surroundings as it is produced, or it remains in the combustion products in the form of elevated internal energy (temperature), or some combination thereof.

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Principles of Combustion The basis of the combustion synthesis technique comes from the thermo-chemical concepts used in the field of propellants and explosives. The reaction releases the maximum energy when the reductive mixture follows its chemical formula. That is, when the element valences are balanced, irrespective of whether they are present in the oxidizer or the fuel components of the mixture. Thus, the method consists on establishing a simple valence balance and the assumed valences are those presented by the elements in the usual products of the combustion reaction, which are CO2, H2O and N2. The extrapolation of this concept to combustion synthesis of ceramic oxides means that metals should also be considered as reducing elements with the valences they have in the corresponding oxides. Experimentally, the chemical balance is used to calculate the appropriate amounts of the selected starting materials (i.e. the cationic precursors and the fuel) following its chemical formula designed for expected product composition. This concept is particularly useful when thermodynamic calculations are difficult to carry out for lack of the relevant parameters and it has been shown that there is a direct correlation between the results derived from the valence balance and those based on heat of formation or bond energies. When both the precursor salts and the fuel are water soluble, a good homogenization can be achieved in the solution.

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There are number of reaction parameters which affect combustion synthesis reactions, e.g. reactant particle size, stoichiometry (including the use of diluents or inert reactants), green density, thermal conductivity, ignition temperature, heat loss and, therefore, combustion temperature, heating and cooling rates and physical conditions of reactants (solid, liquid, gas). Many of these parameters are interdependent and have a significant effect on the final product morphology and properties. Establishing the optimum reaction parameters for synthesizing a material is based on obtaining a fundamental understanding of the controlling reaction mechanisms in each combustion reaction system. This has been one of the most active research areas for combustion synthesis. Combustion synthesis has emerged as an important technique for the synthesis and processing of advanced ceramics, catalysts, composites alloys inter-metallic, and nanometer-sized materials. Depending on selections of characteristic reactants, the combustion synthesis technology can be classified as shown in Fig. 2, where the reactant elements or compounds are varied in solid, liquid or gas phase and each nature of exothermic reaction characterizes combustion propagation and system temperature controlled processes.

Three T’s of combustion The objective of good combustion is to release all of the heat in the fuel. This is accomplished by controlling the "three T's" of combustion which are (1) Temperature high enough to ignite and maintain ignition of the fuel, (2) Turbulence or intimate mixing of the fuel and oxygen, and (3) Time, sufficient for complete combustion. Commonly used fuels like natural gas and propane generally consist of carbon and hydrogen. Water vapor is a by-product of burning hydrogen. This removes heat from the flue gases, which would otherwise be available for more heat transfer. Natural gas contains more hydrogen and less carbon per kg than fuel oils and as such produces more water vapor. Consequently, more heat will be carried away by exhaust while firing natural gas. Too much, or too little fuel with the available combustion air may potentially result in unburned fuel and carbon monoxide generation. A very specific amount of O2 is needed for perfect combustion and some additional (excess) air is required for ensuring complete combustion. However, too much excess air will result in heat and efficiency losses. Not all of the fuel is converted to heat and absorbed by the steam generation equipment. Usually all of the hydrogen in the fuel is burned and most boiler fuels, allowable with today's air pollution standards, contain little or no sulfur. So the main challenge in combustion efficiency is directed toward unburned carbon (in the ash or incompletely burned gas), which forms CO instead of CO2.

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Overview of Combustion Synthesis Combustion synthesis (CS) or self-propagating high-temperature synthesis (SHS) is an effective, low-cost method for production of various industrially useful materials. Today CS has become a very popular approach for preparation of nanomaterials and is practiced in 65 countries. Recently, a number of important breakthroughs in this field have been made, notably for development of new catalysts and nanocarriers with properties better than those for similar traditional materials. The extensive research carried out in last five years emphasized the SHS capabilities for materials improvement, energy saving and environmental protection. The importance of industrialization of the SHS process is also realized. Several books [42, 43] and reviews [44 - 47] have been published on this subject in recent years. Combustion synthesis (CS) has attracted considerable interest in the last two decades due to its unique combination of technologically relevant characteristics. The method, in fact, makes possible the rapid synthesis of several highly refractory inorganic materials and advanced ceramics, thus avoiding the prolonged high temperature treatment, known as sintering, usually required in their conventional preparation. Refractory materials are resistant to thermal shock and are used to make crucibles, incinerators, insulation, and furnaces, particularly metallurgical furnaces. Advanced ceramics are inorganic, nonmetallic, crystalline materials of rigorously controlled composition and manufactured with detailed regulation from highly refined raw materials giving the products precisely specified attributes desired by the producer. Reaction sintering is the welding together of small particles of a ceramic material applying prolonged heat below the melting point resulting in improved mechanical and physical properties of the material. In this novel approach, the synthesis is obtained through an extremely rapid self sustaining process driven by the large heat release by the internal energy of the reactants. The macroscopic characteristics of CS procedure resemble those observed in conventional combustion processes. The reactants, in the form of fine powders, are usually dry-mixed. These mixtures are then placed in a controlled atmosphere and ignited through a resistively heated wire, a laser beam, or an electric discharge. Due to the enthalpy change between the reactants and products SHS reactions generally result in high combustion temperatures. The combustion synthesis reaction can be conducted in two modes [48]. Combustion synthesis methods can be separated into three categories there are: - Conventional self-propagating mode (SHS) of nanoscale materials, i.e. initial reactants are in solid state (condensed phase combustion).

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- Solution-combustion synthesis (SCS) of nanosized powders, i.e. initial reaction medium is aqueous solution. - Volume combustion synthesis (VCS). Conventional SHS: condensed phase combustion It is not an easy task to produce nanomaterials by conventional SHS, where the typical scale of heterogeneity for the initial solid reactants is on the order of 10–100 nm. This feature, coupled with high reaction temperatures (>2000 K), makes it difficult to synthesize nanosize structures with high surface area. However, several methods were suggested for synthesis of nanomaterials by using this approach: (i) SHS synthesis, followed by intensive milling; (ii) SHS + mechanical activation (MA); (iii) SHS synthesis followed by chemical treatment, so-called chemical dispersion; (iv) SHS with additives; (v) carbon combustion synthesis (CCS). Since the first method is common and well known [49], and different combinations of SHS and MA have already been well documented [50], the abilities of three other methods are briefly discussed below. The process of etching SHS-powders in an appropriate dilute acid (e.g. HNO3 or H2SO4) solution, thus dissolving the defect-rich layers between the crystallites and removing impurities, followed by ball milling, is termed as chemical dispersion. This approach was suggested by the group from Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences [51]. A variety of fine powders including boron, aluminium and silicon nitrides were produced by this technique. The SHS method with additives for nanomaterials synthesis is known as alkali metal molten salt assisted combustion [52, 53]. In this process, the reducing metal, (e.g. Sn) reacts with transition metal oxide (SnOx) in the melt of alkali metal salt (e.g. NaCl) to form fine reduced metal particles (Sn). Solution combustion synthesis (SCS) Solution combustion synthesis (SCS) is a versatile, simple and rapid process, which allows effective synthesis of a variety of nanosize materials. This process involves a self-sustained reaction in homogeneous solution and stoichiometric amounts of different oxidizers (e.g., metal nitrates) and fuels. Depending on the type of the precursors, as well as on conditions used for the process organization, the SCS may occur as either volume or layer-by-layer propagating combustion modes. This process not only yields nanosize oxide materials but also allows uniform (homogeneous) doping of trace amounts of rare-earth impurity ions in a single step. Among the gamut of papers published in recent years on SCS, synthesis of luminescent materials and catalysts occupy the lion share. The latest developments in SCS technique are discussed based on the materials applications. The synthesis of nanophosphors is

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currently a hot topic in the field of CS and the range of nanophosphorbased materials prepared by SCS were discussed by Singanahally et al. [47]. Single step SCS has been used for the preparation of nanosized ZnO/carbon composite for supercapacitor application, which showed higher specific capacitance, compared to micron sized ZnO powder [54]. It is well recognized that the fuel is an important component for the preparation of oxides by SCS. A organic fuels are the most popular and attractive fuels for producing highly uniform, complex oxide ceramic powders with precisely controlled stoichiometry. The glycine nitrate process (GNP) has been billed as ‘environmentally compatible’. This solution combustion can be initiated by microwave, which yields uniform, narrow size distribution products. Volume combustion synthesis (VCS) The reactants are heated by an external source (e.g., tungsten or molybdenum coil, laser) either locally in SHS or uniformly in a furnace or microwave in VCS to initiate an exothermic reaction. It is interesting to note that many combustion synthesis reactions lie between these two types described. If the combination of thermo chemical and thermo physical properties of the system are appropriate, a high temperature reaction front (usually 1500-4000°C) is initiated which then propagates through the mixture with a rate ranging from some millimeters to several centimeters per second for micron sized powders.

The combustion wave propagates as shown in Fig. 3 for mixtures consisting of two or more precursor powders. The initial mixture heats up and the contact between particles of starting mixtures is

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too limited for a chemical reaction to occur. Once the mixture’s temperature rises to a certain point, impurities escape. Next, the reactant with the lower melting point coats other particles increasing the contact among the powders. At this stage, the chemical reaction is ignited. At the combustion zone, the leading edge of the heat wave promotes a full-blown reaction. Here, initial products are formed which may or may not be the same composition as the final products. Next, the final products begin to form from the heat released from the combustion wave followed by crystal growth and organization. Finally, the mixture cools into the final product [55]. A variant of this scheme involves one gaseous reactant. In this case, the reaction mixture is made of the powder(s) of the other non-gaseous reactant(s). This approach allows the synthesis of nitrides, hydrides, and oxides. Beside low energy requirements and the high reaction rate, the method has other advantages over the traditional methods, such as the simplicity of the experimental apparatus. Another demonstrated advantage is represented by the high purity of the products, which is largely due to the expulsion of volatile impurities under the extremely high temperatures in the wave. The reaction products are generally porous, but densification can easily be obtained through the application of a mechanical load just after the end of the reaction or simultaneous to it. Also, the high thermal gradients and rapid cooling rate can give rise to new nonequilibrium or metastable phases. Characteristic temperatures During the combustion synthesis reaction there are four important temperature factors that may affect the process of the reaction and final product properties •

Initial temperature (To) which is the average temperature of the reactant sample before the reaction is ignited in the propagating mode

•

Ignition temperature (Tig) which represents the point at which the SHS reaction is dynamically activated without further external heat supply

•

Adiabatic combustion temperature (Tad), which is the maximum combustion temperature achieved under adiabatic conditions

•

The actual combustion temperature (Tc), which is the maximum temperature achieved under normal configuration and non-adiabatic conditions.

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The reactant mixture is generally ignited locally and the combustion front leaves behind the reaction products. In an idealized representation, the trend of macroscopic parameters representing the combustion process is reported in Fig. 4 (a). The reaction is limited to a very narrow region, 10-50 µm wide, in which the degree of conversion (η) goes rapidly from 0 to 1 while the temperature increases to its maximum theoretical value i.e, the adiabatic combustion temperature (Tad). Heat is conducted ahead of the combustion front and the temperature corresponding to the onset of the chemical reaction is generally referred to as the ignition temperature (Tig). The symbol φ in Fig. 4 (b) represents the rate of heat release, corresponding to the rate of the chemical process. In real processes, however, the reaction zone can be wider. This happens in the case of processes with a kinetic limitation. In this case the chemical reaction continues after the passage of the combustion front producing the so-called “afterburn phenomenon” (Fig. 4 (b)) [56]. Other effects of combustion conditions In addition to the reaction parameters already considered, there are other conditions that affect the CS process. These parameters range from the initiation of the SHS process (i.e. using different temperatures to ignite the combustion wave), initial temperature of the reactants (i.e. generally, higher initial temperature, yields a faster combustion wave velocity), and the effects of gravity on the system. The stoichiometry of the reactant powder mixture is another important process parameter, which significantly affects the SHS reaction and product properties. Any deviation from the stoichiometric

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ratio required to produce a certain product results in a decrease of the adiabatic temperature. The addition of excess product as a diluent is mostly used to control the reaction process. In the present research work, we have proposed a new method called “microwave-assisted combustion” method to prepare the nanocrystalline powders. In this microwave-assisted combustion is a novel method to produce nanopowders of inorganic compounds, since microwave heating process is fundamentally different from the convention heating process. In the microwave-assisted combustion method, heat will be generated internally within the material, instead of originating from external sources. The applications of microwave heating irradiation in the preparation of nanocrystalline-advanced materials have been attractive in recent years. The nanocrystalline powders synthesized by this method are ultrafine, pure and more homogeneous and low-agglomerated. Microwave-assisted combustion synthesis Microwave-assisted combustion synthesis of preparing metal oxide materials is a fairly recent development compared to solid-state combustion synthesis or self-propagating high-temperature synthesis technologies. Today, microwave-assisted combustion synthesis is being used all over the world to prepare metal oxide materials for variety of applications. In the case of dry processing, product powders form agglomerates of non-uniform size and distribution. This could lead to poor mixing of the multi-component reactants and to packing density variations, which may result in inhomogeneous structures in the synthesized products. However, in wet processing, the particles are dispersed in a liquid and are free to move in relation to each other so better mixing is achieved. It is possible to prepare oxide materials with desired composition, structure, etc. by microwave-assisted combustion synthesis. The oxide materials prepared and investigated during the review period have been along with the fuels used and their applications. Two important aspects of microwave-assisted combustion synthesis are its instantaneous and volumetric heating characteristics. Therefore, the entire mixture reacts uniformly throughout, at the moment of heating. Microwave heating has other advantages as well. The heating is outward from the center, giving rise to inverse temperature gradients. This can lead to different microstructures. Also, since the heating is instant, microwave power can be used to control the extent of reaction. The potential of microwave heating to control the extent of the reaction and possibly yield nano-overlayers of ceramic on metal substrates can be explained as follows. Microwave energy can interact with metal powders as well as non-metals. Microwave-assisted combustion synthesis does recently gather reputations as its effective synthesis technique, which can neglect such steps as washing, filtration, drying, etc. This microwave-assisted combustion synthesis categorized with solid or a gas reaction has

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almost been made clear without any consideration of in-situ pressure changes. Since this synthesis process proceeds with large temperature gradients in a short time, the difference of the decomposition temperature may not affect so much to the feature of products. The characteristics of microwaveassisted combustion reactions, such as reducing power and generated gas amount, can be controlled by the selection of organic fuels. Background details of microwave oven Microwave technique has the potential to be a useful method for the preparation of nanoparticles. The potential advantages include more uniform temperatures in the sample, improved product uniformity, higher energy efficiency, less total time and space. Also, smaller grain sizes are achieved at any given density, leading to better and more uniform properties. A microwave oven works by passing non-ionizing microwave radiation, usually microwaves refer to the electromagnetic waves, in the frequency range of 300 MHz and 300 GHz (million cycles per second). Electromagnetic waves are waves of electrical and magnetic energy moving together through space as shown in Fig 5. They include gamma rays, x-rays, ultraviolet radiation, visible light, infrared radiation, microwaves and the less energetic radio waves. Ceramics are insulators, thus electrons do not flow in response to an electric field. However, an electric field can cause a reorientation of dipoles, which can lead to heating. The rapidly changing electric fields associated with microwaves lead to rapidly changing orientations of the dipoles in the material. There is a natural frequency that causes maximum reorientation, thus maximum heating of the material. This is often referred to as "coupling". This is usually a very broad maximum, so that the frequency dependence is not strong. Each material has intrinsic properties relative to the absorption of microwave energy. Microwaves in this frequency range have another interesting property: they are not absorbed by most plastics, glass or ceramics and metal reflects microwaves [57]. Because that metal reflects microwaves is that no electronic waves resident in inside of conductor because conductor’s conductivity is infinity. In a microwave oven, molecules of the sample are excited directly by the electromagnetic field. As energy is absorbed by the sample, its temperature increases. The surface temperature becomes higher than the surroundings, and loses heat by convection. Therefore the surface temperature is lower than the core temperature. Because heat is being generated in the sample and conducted out of the sample, a stable temperature gradient can be achieved. If the goal is uniform heating, then a combined approach may be a solution. Since conventional ovens lead to higher surface temperatures and microwaves lead to higher core temperatures, the right combination of the two could theoretically lead to equal core and surface temperatures. This would likely be a very difficult combination to arrive at

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experimentally, because the combinations would change over a period of time. Also, the temperature would still probably not be constant throughout the entire sample, so the best case scenario would be to minimize the temperature gradient to achieve the most even heating. Unlike conventional ovens, microwaves have "penetrating radiation, controllable electric field distributions, rapid heating, selective heating of materials through differential absorption, and self-limiting reactions" [58].

There are some potential problems with the use of microwaves. Materials with high electrical conductivity generally are difficult to process because of poor penetration of the microwave energy. Insulators with low dielectric loss are hard to heat because of poor absorption of microwave energy. Some materials have highly temperature dependent dielectric properties, often causing uneven heating and thermal runaway. In recent years, extensive research has been done in the area of microwave processing of nanoparticles. Part of the reason for the increase in study in this area is the expectation of different, and perhaps better, properties than those achieved in traditional convection ovens. The fact that microwaves heat materials by internal absorption that resembles heat generation makes uniform heating, and thus uniform properties. Experimental evidence shows that uniform heating is not achieved, and very large temperature gradients can result. This is caused by the loss of heat due to

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convection at the surface. The non-uniformity is then compounded by the temperature dependence of the absorption of microwave energy, which increases with temperature in some materials. Microwave-assisted combustion synthesis Two important aspects of microwave assisted combustion synthesis are its instantaneous and volumetric heating characteristics. Therefore, the entire mixture reacts uniformly throughout, at the moment of heating. Microwave heating has other advantages as well. The heating is outward from the center, giving rise to inverse temperature gradients. This can lead to different microstructures. Also, since the heating is instant, microwave power can be used to control the extent of reaction. The potential of microwave heating to control the extent of the reaction and possibly yield nano-overlayers of ceramic on metal substrates can be explained as follows. Microwave energy can interact with metal powders as well as non-metals. The power absorbed per unit volume (W/m3) provides the following basis for heating [59, 60].


=σ 




Where σ = total effective conductivity = 2πfεoεr tanδ E = local electric field f = frequency of microwave εo = permittivity of free space εr = relative dielectric constant tan δ = loss tangent = ε″/ε′ where materials complex permittivity = ε* = ε′ - iε″

   =   −    where d Eo

= skin depth or depth to which microwaves penetrate = maximum electric field generated

The skin depth is a strong function of frequency of the field

=



 ( ) ε  +  δ − 

Since the effective conductivity σ for metallic samples is 106 to 1016 higher than for non-metal dielectric materials, at 2.45 GHz the skin depth for metallic samples is sometimes a few micrometers or less. Hence the absorption of microwave energy is a relatively thin outer layer, which then transmits

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energy by conduction to inner layers [59]. According to Cherraddi et al. [61], “the heating of metals is attributed to eddy current losses from the magnetic fields. The demonstrated heating of metal particles by microwaves leads to the following hypothesis: since the heating of the metals is limited to a thin surface layer the probability of reaction with a nonmetal at this surface layer is a maximum and would lead to a ceramic over layer formation. Furthermore, since microwave heating is instantaneous, switching the microwave on or off can control the reaction. Thus switching the microwave on would lead to reaction and if the reaction has not reached the self-sustaining stage, switching the microwave off would stop the reaction. For reactions that would not be self-sustaining because of a very low adiabatic flame temperature, microwave energy may also be used to drive these reactions at stoichiometric conditions. For reactions having a low flame temperature because of extremely low concentration of one reactant, microwave heating may also be used to promote reaction. Principles of microwave oven Microwaves are a form of energy which is absorbed, polar molecules and ions inside the sample will rotate or collide according to the alternating electromagnetic field and heat is subsequently generated for preparation for nanoparticles. Microwaves make the water molecules inside the sample vibrate so that they rub against each other causing friction. These friction produce the heat that actually makes the sample. Energy → Vibration → Friction → Heat → Sample In a microwave, the heat is produced inside the sample. Microwave energy penetrates the sample to a depth of about 25 mm. Microwaves are similar to light and they are emitted downwards from the top of the microwave oven cavity. The waves bounce off the metal sides of the cavity, hitting the sample from different angles. These microwaves do not hit the sample evenly. To minimise this effect, the oven either has a rotating turntable or a paddle wheel in the roof to stir up the microwaves. Working principles of microwave ovens In a microwave oven, sample is heated by exposing it to microwave radiation. The source of the radiation in a microwave oven is the magnetron tube which is the heart of a microwave oven. A magnetron converts electrical energy to microwave radiation. To do this, it uses low-voltage alternating current and high-voltage direct current. A transformer changes the incoming voltage to the required levels and a capacitor, in combination with a diode, filters out the high voltage and converts it to direct current. Inside the magnetron, electrons are emitted from a central terminal called a cathode. A

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positively charged anode surrounding the cathode attracts the electrons. Instead of traveling in a straight line, permanent magnet forces the electrons to take a circular path. As they pass by resonating cavities, they generate a continuous pulsating magnetic field, or electromagnetic radiation (Fig. 6). The microwave energy from the magnetron is transferred to the oven cavity through a waveguide section. A mode stirrer spreads the microwave energy more or less evenly throughout the oven as shown in Fig. 6 and 7. The microwave radiation produces heat inside the sample kept in the oven. Heat is produced when the water molecules in the sample vibrate (at a rate of 2,45 GHz times per second) when the sample absorbs the microwave radiation. The movement of the molecules produces friction which causes heat. This heat produces the nanoscale materials.

Microwave radiation is

measured as power density in units of milliwatts per square centimetre (mW/cm2) which is essentially the rate of energy flow per unit area.

Details about the Microwave Oven A microwave oven can be divided into two fundamental sections, the control section and the high-voltage section. The control section consists of a timer (electronic or electromechanical), a system to control or govern the power output, and various interlock and protection devices. The components in the high-voltage section serve to step up the house voltage to high voltage. The high voltage is then converted microwave energy. The working of the oven is shown in Fig. 7. i.e electricity from the wall outlet travels through the power cord and enters the microwave oven through a series of fuse and safety protection circuits. These circuits include various fuses and thermal protectors that are designed to deactivate the oven in the event of an electrical short or if an overheating condition occurs. If all systems are normal, the electricity passes through the interlock and timer circuits. When oven door is

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closed, an electrical path is also established through a series of safety interlock switches. Setting the oven timer and starting the operation extends this voltage path to the control circuits Generally, the control system includes either an electromechanical relay or an electronic switch called a triac. Sensing that all systems are "go," the control circuit generates a signal that causes the relay or triac to activate, thereby producing a voltage path to the high-voltage transformer. By adjusting the on-off ratio of this activation signal, the control system can govern the application of voltage to the high-voltage transformer, thereby controlling the on-off ratio of the magnetron tube and therefore the output power of the microwave oven. Some models use a fast-acting power-control relay in the highvoltage circuit to control the output power. In the high-voltage transformer along with a special diode and capacitor arrangement serve to increase the typical household voltage, of about 115 volts, to the shockingly high amount of approximately 3000 volts! While this powerful voltage would be quite unhealthy -- even deadly -- for humans, it is just what the magnetron tube needs to do its job, i.e dynamically convert the high voltage in to undulating waves of electromagnetic energy. The microwave energy is transmitted into a metal channel called a waveguide, which feeds the energy into the cooking area where it encounters the slowly revolving metal blades of the stirrer blade. Some models use a type of rotating antenna while others rotate the sample through the waves of energy on a revolving carousel. In any case, the effect is to evenly disperse the microwave energy throughout all areas of the sample compartment. Some waves go directly toward the sample, others bounce off the metal walls and flooring; and microwaves also reflect off the door. So, the microwave energy reaches all surfaces of the material from every direction. All microwave energy remains inside the working cavity. When the door is opened, or the timer reaches zero, the microwave energy stops-just as turning off a light switch stops the glow of the lamp. Microwave heating The absorption of microwaves by a material results in the microwaves giving up their energy to the material. This transfer of energy causes the temperature of the material to rise. The microwaves themselves do not heat up materials. The heat generation in a microwave field is caused by ionic polarization and dipole rotation of the water molecule. Molecules of all samples consist of a dipole and have positive charge in one side and have negative charge in another side. If we put electromagnetic fields in this, all molecules are rearranged: positive charge is to negative pole and negative charge is to positive pole [57, 62]. In this process molecules heat is produced by friction. The frequency of microwave oven is 2,500 MHz as we saw before. Then microwave of this frequency change the

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direction of electromagnetic fields 2,5 giga times in 1 second. Consequently the heat efficiency of a microwave oven is greatly high. Figure 8 shows, a polar molecules in the presence of microwave field attempt to orient themselves according to the rapidly changing field. The rotation of the molecule leads to friction with surrounding medium and heat is generated. The rotation of the molecule also produces kinetic energy, which produces additional heat. Ionic conduction is another important microwave heating mechanism. When an electrical field (i.e. microwave field) is applied to solutions containing ions, the ions move at an accelerated pace due to their inherent charge. The ions collide and the collisions cause the conversion of kinetic energy of moving ions into thermal energy. A solution with higher concentration of ions will have more frequent collisions and therefore heat faster than a solution with lower concentration.

Structure of a microwave oven Nowadays, microwave oven generally consists of the following basic components [1] (i)

power supply and control: It controls the power to be fed to the magnetron as well as the sample time,

(ii)

magnetron: It is a vacuum tube in which electrical energy is converted to an oscillating electromagnetic field. Frequency of 2.45 MHz has been set aside for microwave oven,

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(iii)

waveguide: It is a rectangular metal tube which directs the microwaves generated from the magnetron to the sample cavity. It helps prevent direct exposure of the magnetron to any type of materials would interfere with function of the magnetron, stirrer: It is commonly used to distribute microwaves from the waveguide and allow more

(iv)

uniform heating of material, turntable: It rotates the materials through the fixed hot and cold spots inside the working

(v)

cavity and allows the materials to be evenly exposed to microwaves, working cavity: It is a space inside which the materials is heated when exposed to

(vi)

microwaves, and (vii)

door and choke: It allows the access of material to the working cavity. The door and choke are specially engineered that they prevent microwaves from leaking through the gap between the door and the working cavity.

The microwaves readily pass through some materials, such as glass, most plastics, paper and china, with little or no effect [57]. Generally, these materials make excellent waves for preparation of nanomaterials in a microwave oven. Microwave-assisted combustion method is quick process to prepare the materials so time-saving, simple and more convenient as well as energy-saving, low cost method and prepared a large amount of nanopowders. A few publications described the preparation of SnO2 by microwave-assisted combustion method using different fuels. In the present context, it is challenging to overcome the above drawbacks through a controlled particle size by microwave-assisted combustion. Nevertheless some experimental and theoretical studies have been performed on SnO2 and are reviewed. Finally, the recently synthesized nanocrystalline tin oxide by microwave-assisted combustion method of preparation powder materials is summarized and its particular interest in these studies was the influence of the crystallographic parameters. The SnO2 nanoparticles synthesized through a chloride solution combustion synthesis (CSCS) [63] whereas, the molar ratio of sorbitol-to-ammonium nitrate divided by that of stoichiometric value was varied and to find the optimum values of specific surface area for the CSCS technique. SnO2 nanoparticles were prepared by citric acid assisted hydrothermal route which produces materials of narrow size distribution and large specific surface area as 206m2/g. The blue emission increased remarkably after calcined at 800ºC [64]. The microwave-assisted method [65, 66] did not use precise

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control of temperature and these results were not compared to conventional-hydrothermal method but Varela’s group [67] reports on the microwave-assisted hydrothermal synthesis of nanocrystalline SnO powders using SnCl2·2H2O, under precise control of temperature , processing time, and mineralizing agents, nanocrystals having different sizes and morphologies have been reported. Chen-Tao Lee et al. [68] reports on rapid microwave-assisted solvothermal route to synthesize SnO2:Eu3+ which excites at 314 nm due to the host absorption, and generated emission peaks in the 570–670nm range due to the 5D0→7FJ transition of Eu3+. The luminescent intensity was found to increase with urea concentration. A microwave-synthesized SnO2 colloids to thin film deposition has been reported [69]. Yong Wang and Jim Yang Lee [70] have been reported microwave-assisted synthesis of SnO2–graphite nanocomposites for Li-ion battery applications have been used in the preparation of urea-mediated hydrolysis of SnCl4. Crystal phase analysis of SnO2-based varistor ceramic using the Rietveld method was elabored [71]. Rapid synthesis of nanocrystalline SnO2 powders using microwave heating method was elaborated by Jun-Jie Zhu et al, [72]. Microwave-assisted synthesis of tin oxide nanoparticles took only 10 min by microwave radiation. They are easily converted to SnO2 nanoparticles crystallized in the Cassiterite structure by simple annealing process at relatively low temperature (300°C) [73]. Microwave-assisted synthesis of tin dioxide nanoparticles with particles size ranging from 10 to 11 nm was done within 10 min [74]. Srivastava et al. [75] has obtained tin dioxide after thermally treating at 600°C for 3 h by using microwave technique. Cirera et al. [76] have prepared tin dioxide nanoparticles by microwave technique after conventionally treating at higher temperatures of order of 450°C to 1000°C for 8 h.Wu et al. [77] have prepared tin mono oxide (SnO) nanoparticles by using microwave technique. Properties of nanocrystalline SnO2 was obtained by means of a microwave process. Further stabilisation treatments conventional heating, OH-stimulated microwaves and combined treatments were also considered. Material structural characterisation of Pt and Pd in situ catalysed, and pure SnO2 is presented, showing the suitability of this procedure for gas sensor applications [78]. Borom and Lee [79], using a low thermal mass resistance heater rather than microwave, showed that for alumina titanium carbide composites, the higher the heating rate, the higher the final density for the same temperature. A microwave absorption for sintering of Sb-doped SnO2 led to the development of fine uniform microstructure and complete densification with improved electrical properties [80]. H. Hallil et al. [81] reports on the microwave gas sensor using dielectric resonator with SnO2 as high sensitivity in the frequency response of the proposed gas detector device based on high-Q whispering-gallery-modes

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which is a very interesting for developing unique gas detection with ease of integration of the RF function. Yamazoe et al. [82] proved that the sensors response increases significantly if the crystallite size (D) of the nanocrystalline tin dioxide is about twice the adsorption depth. Shi [83] analyzed the importance of surface diffusion on densification during the sintering process and proposed that surface diffusion is the most probable mass transport mechanism to promote particle coarsening and center approaching between particles or grains. Morimitsu and co-workers found to enhance the long-term stability of the sensor in the manner that the sensor’s surface was modified by sulfuric acid [84], thiourea solution [85] and a salt solution containing platinum and ruthenium [86]. The various metal catalyst including Pt, Pd, Ni etc. have been introduced in the SnO2 for realization of gas sensors with enhanced response for LPG [87]. The response magnitude of LPG sensors are reported over the range 2–200 and operating temperature is very high (350–800ºC) [88]. Enhancement in sensitivity of surface ruthenated tin oxide towards hydrocarbons by microwave irradiation [89]. Microdeposition of microwave obtained nanoscaled SnO2 powders for gas sensing with thermal distribution in the drop does not seem to be affected, the transient response of them substrate after heating are reported [90]. Improvement of dynamic gas sensing behavior of SnO2 acicular particles by microwave calcinations by heat treating acicular SnC2O4 precursors at 500ºC were 90% response time to 30 ppm CO was 5–27 s was reported [91]. Huang et al. [92] studied responses of a SnO2 nanotube sheet sensor to 100 ppm H2, 100 ppm CO, and 20 ppm ethylene oxide. The sheet (0.2 mm in thick) was of about 2mm×4mm in size and was fixed with gold paste onto an alumina substrate attached with gold electrodes having a gap of 1 mm. Comparison between stannic and stannous oxide There are two main oxides of tin: stannic oxide (SnO2) and stannous oxide (SnO). The existence of these two oxides reflects the dual valency of tin, with oxidation states of 2+ and 4+. Stannous oxide is less well characterized than SnO2. For example, its electronic band gap is not accurately known but lies somewhere in the range of 2.5–3 eV. Thus SnO exhibits a smaller band gap than SnO2, which is commonly quoted to be 3.6 eV. Also, there are no single crystals available that would facilitate more detailed studies of stannous oxide. Stannic oxide possesses the rutile structure and stannous oxide has the less common litharge structure. Stannic oxide is the more abundant form of tin oxide and is the one of technological significance in gas sensing applications and oxidation catalysts. In addition to the common rutile (tetragonal) structured SnO2 phase there also exists a slightly more dense orthorhombic high pressure phase. Suito et al. showed that in a pressure– temperature diagram the regions of

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tetragonal and orthorhombic phases can be separated by a straight line of the equation p (kbar) = 140.0 + 0.022T (ºC). [93]. Figure 9 shows the Sn–O phase diagram for atmospheric pressure [94]. This diagram indicates the presence of an intermediate tin-oxide phase between SnO and SnO2 at elevated temperature. Sn3O4 is often given for its composition [95] but Sn2O3 [96, 97] has also been considered. In these intermediate oxides Sn is present as a mixture of Sn(II) and Sn(IV) [95, 97]. Also, the SnO2 phase can accommodate a significant amount of oxygen vacancies. Li-Zi et al. measured the variation of the bulk oxygen vacancy concentration as a function of the oxygen partial pressure by coulometric titration [98]. They found a relationship of the oxygen vacancy concentration X with the oxygen partial pressure PO2 via the proportionality X α P-1/n O2 with n varying between 5.7 and 8.3 for temperatures between 990 K and 720 K respectively. In these studies a maximum oxygen deficiency of x = 0.034 in SnO2-x at 990 K was observed before metallic Sn is formed. At lower temperatures less oxygen vacancies could be accommodated. The heats of formation for stannous and stannic oxides at 298 K were determined to H = -68 cal/mol and

H = -138 cal/mol, respectively [99]. This results in

H = -70 cal/mol for the reaction

SnO(c) + 1/2 O2(g) ) SnO2(c). Also the disproportionation reaction of SnO(c)) SnxOy(c) + Sn → SnO2(c) + Sn has been reported to occur at elevated temperatures. This disproportionation of SnO into Sn and SnO2 proceeds via the aforementioned intermediate oxides [95, 96]. This indicates that stannic oxide is the thermodynamically most stable form of tin oxide. Geurts et al. [100] explain this by the structural similarities between the tin matrix of the SnO (001) plane and that of the SnO2(101) plane and the oxidation of SnO films to SnO2 has been studied by Raman scattering, IR reflectivity and X-ray diffraction. It was found that the oxidation starts with an internal disproportionation before external oxygen completes the oxidation to SnO2. More importantly, (001)-textured SnO layers convert into (101)-textured SnO2 films. The same behavior was observed by Yamazaki et al. [101]. Because of this structural similarity essentially only the incorporation of an additional oxygen layer is required to obtain the final SnO2 structure. For comparison Fig. 10 shows top views of SnO (001) and SnO2(101).

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Crystal structure of stannic and stannous oxide The crystal structure of stannous oxide (SnO) is shown in Fig. 11. It has a tetragonal unit cell with the litharge structure, isostructural to PbO. The symmetry space group is P4/nmm and the lattice constants are a = b = 3.8029 A˚ and c = 4.8382 A˚ [102]. Each Sn and O atom is fourfold coordinated with a bond length of 2.23 A˚ . The structure is layered in the [001] crystallographic direction with a Sn1/2–O–Sn1/2 sequence and a van-der-Waals gap between two adjacent Sn planes of 2.52 A˚. The

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positive charge of the Sn2+ ions is screened by electron charge clouds between the Sn planes, thus reducing the Coulombic repulsion between adjacent Sn layers [103-105].

These charge clouds, or charge hats, arise from Sn 5s electrons that do not participate in the bonding for Sn(II) and thus can be described as a lone pair. Galy et al. [106] define this lone pair as an intermediate state between an inert spherical s2-type orbital that is centered on the nucleus and a nonbonded hybridized-orbital lobe that is not spherical but localized far from the atomic nucleus. Thus, the valence shell electronic pair repulsion theory [107] takes this spatial effect into account to predict local environment symmetries. The local environment can be very different for different Sn(II) compound materials and therefore the electronic charge can be either spherically centered around the Sn atom like for example in the case of SnTe (cubic NaCl structure) or be strongly directional, forming charge hats, like in the case of SnO [108]. Stannic oxide (SnO2) is much better characterized than stannous oxide. As a mineral, stannic oxide is also called Cassiterite. It possesses the same rutile structure as many other metal oxides, e.g. TiO2, RuO2, GeO2, MnO2, VO2, IrO2, and CrO2. The rutile structure has a tetragonal unit cell with a space-group symmetry of P42/mnm. The lattice constants are a = b = 4.7374 A˚ and c = 3.1864 A˚ [109]. In the bulk all Sn atoms are six fold coordinated to threefold coordinated oxygen atoms. The structure and composition of SnO2 surfaces is discussed in below. Tin oxide (SnO2) is an n-type wideband-gap semiconductor (Eg =3.6 eV). When tin oxide is oxygen deficient, oxygen vacancies act as donors, and create free charge carriers. Fig. 12 shows a schematic of tin oxide electronic band structure,

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which crystallizes in the rutile structure (space group D144h or – in another notation – P42/mnm with ο

a=b≠c, α=β=γ=90 ) [110].

The unit cell of SnO2 contains six atoms, two tin and four oxygen atoms (Fig. 13) [111].

Each tin atom is bound to six oxygen atoms placed approximately at the corners of a regular slightly deformed octahedron, and every oxygen atom is surrounded by three tin atoms at the corners of 4+

an equilateral triangle see figure. The metal atoms (Sn

cations) are located at (0,0,0) and (½,½,½)

2-

positions in the unit cell, and the oxygen atoms (O anions) at ±(u,u,0) and ±(½+ u,½- u,½), where the internal parameter u, takes the value 0.307. Lattice parameters are: a = b = 4.737 Å and c = 3.186 Å

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[112]. The value of the direct band gap has been estimated to be 3.6 eV [97]. SnO is a n-type, wide 2

band-gap semiconductor. The origin of the n-type behavior is the native non-stoichiometry caused by 6

oxygen vacancies. The electrical resistivity, varies from 10 to 10 Ω.cm, depending on the temperature and the stoichiometry of the oxide [112-114]. A property of SnO2 is given in Table. 1.

Electronic Band structure of SnO2 In SnO2, chemical bonding is mainly governed by the linear combination of oxygen 2s and 2p orbitals with tin 5s and 5p orbitals. Whole calculations are given for every point of Brillouin zone in Fig. 14. As seen, the upper valence band consists of a set of three bands (Г+ 2, Г+ 3, and Г+ 5), and the top of the valence band is a state of Г+ 3 symmetry. The bottom of the conduction band is Г+ 1, which is a 90% tin s-like state and is very similar to that for a free electron in spite of the overall ionic character of SnO2. Therefore the gap is E (Г+ 3 - Г+ 1) = 3.6eV and its variation with respect to temperature is 1.2 x10-3 eV K-1 between 300 and 1300 K. With respect to its chemical properties, tin oxide has a high chemical stability, as it is only attacked by hot concentrated alkalis, being this one of the reasons of the wide research done with this material.

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Comparison between the bulk electronic structure of SnO and SnO2 Comparison between SnO and SnO2 is complicated by the reliable preparation of SnO and difficulties in discriminating it from SnO2. Lau and Wertheim [115] proposed that such a negligible chemical shift is observed because the change in the free ion potential between Sn2+ and Sn4+ is cancelled by the change in Madelung potential at tin sites in SnO and SnO2. However, there also exist reports of a sizeable shift of 0.5–0.7 eV of the Sn 3d core level between the two tin oxides. Nevertheless, the probably most reliable way to distinguish SnO from SnO2 is by comparing their valence band spectra or by measuring the energy separation between the Sn 4d5/2 peak and the leading edge of valence band [116]. For SnO2 a separation of 21.1–21.4 eV and for SnO of 22.4–23.7 eV is reported [117]. In contrast to SnO, SnO2 does not exhibit any Sn 5s character at the VBM. Fig. 15(a) shows DOS calculations for SnO2 [118, 119]. Three energy regions can be differentiated. The VBM is mainly of O 2p character, while the center region results form hybridization of Sn 5p with O 2p, and only the bottom of the valence band has some Sn 5s character. Experimental results for a SnO2(110) surface are also shown in Fig. 15(a) for comparison. Fig. 15(b) shows the orbital character of the valence band and conduction band more schematically. It can be seen from this representation that for SnO2 the empty Sn 5s orbital make up the bottom of the conduction band. For a more detailed band structure calculation for SnO2, see also Fig. 14(a). With these general properties for the bulk electronic structure in mind we now turn to the electronic structure of stoichiometric and reduced low index SnO2 surfaces.

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Experimental Chemicals Use of suitable raw material in solution combustion synthesis ensures stability of the chemical composition and high quality of products. More importantly, it must be readily available and .

convenient to be used. In addition, the reactants proportions can be tuned in order to have a non-violent reaction and should only produce non-toxic gases (CO2, H2O and N2). In the present, starting materials used for preparation of SnO2 were of Analar grade pure tin metals (Sn, 99%, MERCK), and nitric acid (HNO3, 99%, MERCK), were the source of Sn(NO3)2. The organic fuel such as urea (CO(NH2)2, 99%, MERCK), used for solution combustion synthesis, was prepared in our laboratory. It is also worthwhile to mention that this process uses an exothermic and usually very rapid chemical reaction to form the final reaction products. Since its price is inexpensive and its combustion heat of urea is −2.98 kcal/g. On the other hand, tin nitrate [Sn(NO3)2] is utilized in the present study because of its dual role of being the tin source and the oxidant. Calculation of stoichiometry The stoichiometry of the redox mixture used for combustion process was calculated using the total oxidizing and reducing valencies of the ingredients which serve as numerical coefficients for the stoichiometric balance so that the equivalence ratio (Φe) is unity and the energy released by the combustion is maximum [120]. According to the concept used in propellant chemistry [121], reducing elements with the valencies of Sn =+2 and H=+1 respectively. The element oxygen (O) is considered as an oxidizing element with valency −2. The valency of nitrogen is considered to be zero. Based on these considerations, Sn(NO3)2 has an oxidizing valence of −10. These valences should be balanced by the total “reducing valencies” of the urea (fuel) is +6. The water molecules do not affect the total valencies of the nitrate and are, therefore, irrelevant for the chemistry of the combustion. Thus, the stoichiometric composition of the redox mixture, in order to release the maximum energy for the reaction, leads to: (10) + (+6) ψ = 0 and the stoichiometric composition of the redox mixture, to release the maximum energy for the reaction, would demand that ψ =1.667 mole of urea were used. Synthesis of SnO2 nanopowders Nitric acid (HNO3, 99%, MERCK), were dissolved in distilled water and mixed in an appropriate ratio to form a pure tin (Sn) metals and the solution in the beaker was heated (∼100oC) with magnetic stirrer with stirring for 10 min. As the distilled water evaporated, the solution became viscous and generated small bubbles to remove excess nitrate. The viscous solution was then transferred to silica container. Then urea was used as a fuel, in order to synthesize SnO2 nanopowders. The solution

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was homogenized by magnetic stirring for ½ hour. This clear solution was taken in a silica container and was irradiated with microwaves in a modified domestic microwave oven (BPL India Limited, Bangalore, India, Model No. IFB, 17PG1S, microwave 700W, input range 210-230 V-ac 50 Hz, microwave frequency 2.45 GHz) to produce ZnO nanomaterial. Within a few seconds of irradiation, reaction mixture was converted into a clear solution and started boiling, and after about 2 min of irradiation white fumes started coming out from the exhaust opening provided on the top. After about 3 min of irradiation, the mixture solution boils, ignites, burst into flames and resulted into a foamy white powder. During this process, the solution became more and more viscous and changed from a clear solution. Then, the clear solution was swelling into foam and undergoing a strong self-propagating combustion reaction with the evolution of large volume of gases.

These powders were referred to as ”as-prepared”. The entire combustion process ended in a few seconds. The as-prepared SnO2 nanopowder was washed with distilled water and the resulting powder were calcined in a muffle furnace at temperatures from 200-800o C for 1 h in air atmosphere. The color of the as-prepared is changed to yellow with the increases of the calcined temperatures. The method of preparation is represented by a flowchart as presented in Fig. 16. A photograph of microwave-assisted combustion reaction of SnO2 is shown in Fig. 17. Theoretical equation assuming complete combustion of the redox mixture used for the synthesis of SnO2 may be written as (but over-simplified) Sn(NO3)2(c) + ψ CO(NH2)2(c) + (1.5 ψ −2) O2(g) → SnO2(c) + (1 + ψ) N2(g) + ψ CO2 (g) + (2 ψ) H2O(g) ~ (1 + 4ψ) mol gases/ SnO2

(1)

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Instruments used The as-prepared and annealed nanocrystalline SnO2 powder was characterized by powder X-ray diffraction (XRD) method using a X’Pert PRO PANalytical diffractometer with CuKα radiation of wavelength 1.5405 Å. Diffraction patterns were recorded with a step size of 0.02°, scan speed of 2 s/step and scan angle 2θ from 10° to 80°. Crystallographic parameters were refined by XRD pattern and Rietveld refinement using TOPAS-3 and Diamond software was used to construct the structural parameters. The dislocation density (δ), defined as the length of dislocation-lines per unit volume of the crystal, was evaluated using the formula [122].

δ=






(2)

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where D is the crystallite size and the number of unit cells is calculated using the formula [123], (3)

where D is the crystallite size and V is the cell volume of the SnO2 sample. Morphological of asprepared and annealed samples was examined by SEM (JEOL-6360) and high-resolution transmission electron microscopy (HRTEM) were prepared by grinding the powder under ethanol; drops of the suspension were deposited on grids. Images and diffracted rings pattern were collected using a JEOL2100F microscope operated at 200 kV and fitted with a low-background Gatan double tilt holder. The photoluminescence measurements were carried out from 280 nm to 480 nm at room temperature using Varian Cary Eclipse spectrophotometer with 265 nm as the excitation wavelength of a 15 W Xenon pulse lamp. All emission spectra were recorded for the detector response and excitation spectra for the lamp profile. Structural characterization History of Powder X-ray diffraction The powder diffraction method has been used among scientists to determine structures of a range of materials including ionic solids, organic framework structures, fullerenes, zeolites, and coordination compounds. The method was understood shortly after Laue and Von Knipping developed X-ray diffraction in 1910. The powder diffraction method was developed by Debye and Scherrer in Germany in 1916 and Hull in the United States in 1917 independently. Hull in 1917 suggested a simple model of a powder diffractometer and later it was used to determine simple structures of graphite, diamond, iron, etc. Even then the use of metal foils to filter Kβ radiation was understood. With later developments, crystallographers used the powder diffraction method to determine the structures of compounds such as minerals, metals, and simple organic molecules. During that time, a crystal structure of simple molecules such as NaCl could be determined from powder diffraction alone [124]. The first attempts to determine the crystal structure of a non-cubic compound from powder diffraction were reported by Zachariasen in 1940s. Zachariasen 1948, [125] reported the structure of UCl3 by locating the uranium atoms first by trial and error method and then determining chlorine atomic positions through careful observation of classes of reflections. This approach by Zachariasen laid the foundation for determining crystal structure from powder patterns based on systematic absences. Using this approach, Zachariasen reported structures of several uranium halides while Mooney [126] followed the same process and reported the structure of tetragonal UCl4. The

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fundamental limitation of powder diffraction involves loss of information due to the peak overlap. Peak overlap due to high symmetry can be treated by partitioning peak intensities according to multiplicity. When peaks with different intensities overlap, de convolution is not straight forward. It was a large problem in powder diffraction for many years although later developed approaches and software packages have greatly improved the de convolution of non-equivalent overlapping reflections [124]. With a steady progress in powder diffraction, in 1960s, the abinitio structure determination was overlooked by crystallographers as a new approach for structure determination from powder data. During that time, modern methods such as direct methods and Patterson methods were understood and used for structure determination. Zachariasen and Ellinger 1963 solved the structure of monoclinic I2/m β-plutonium by direct method through manual phasing [127]. As an interesting approach, they used anisotropic thermal expansion to de convolute overlapping peaks into individual reflections by the diffraction patterns collected at different temperatures. Debets (1968) determined the structure of orthorhombic UO2Cl2 in space group Pnma by the Patterson method [128]. Both approaches are similar to the modern approaches of abinitio structure determination. Modern Advances in Powder Diffraction The advances in powder diffraction that have taken place over the last several decades are associated with the developments in several areas of the powder diffraction. That includes the development of the Rietveld method, development of structure solution methods, development of instrumentation and new software packages. The new software packages have advanced due to the availability of high power computers. The development of non traditional diffraction methods such as variable temperature in situ powder diffraction and high pressure diffraction has been a breakthrough in the field. Since the early development of powder diffraction, structures of unknown compounds have been determined by reciprocal space methods based on extracted intensities from powder patterns. The major problem associated with reciprocal space methods is the peak overlap. Within the last two decades, with the development advanced algorithms, the quality of the intensity extraction process and therefore the structure determination process has improved significantly. Modern reciprocal space structure solution software packages are capable of deconvoluting non-equivalent overlapping reflections accurately. With the development of real space methods within the last two decades, scientists have embraced structure determination from powder data with new enthusiasm. This is due to the fact that real space methods do not rely on the intensity extraction process. Instead they create a

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structural model based on the composition, space group, and unit cell parameters. Next calculated powder patterns based on trial structures are compared with observed powder pattern to test the accuracy of the structural model. With the advancement of computer power, thousands or millions of trial structures can be calculated and compared in real time [124]. Therefore, nowadays the real space method is widely used to solve structures, especially for compounds with highly overlapping reflections. K. Anandan et. al [129] have been reported as-prepared samples of SnO2 at different temperatures are discussed. And Eu doped SnO2 samples at different temperatures are discussed in Yujie Fenga et. al [130]. Ü. Kersen has been reported as a crystallite size of the SnO2 powder at different temperatures [131]. The effect of calcinations temperature on powder size of the SnO2 samples are reported in Novinrooz et. al, [132, 133]. The lattice parameters of SnO2 at different temperatures are discussed by K. Nombra et. al, [134-136]. Lattice parameters of differente temperatures SnO2 samples are calculated with XRD peaks are reported in H. S. ZHUANG et. al [137]. X. Q. Pana and L. Fu has reported unit cells of the effect of calcinations temperature on SnO and SnO2 powders [138]. As the temperature increases, a smaller number of unit cells start to grow, which results in large grain size. Fluorine doped SnO2 samples at various substrate temperatures are reported in Chanipat Euvananont et al. [139] and Mn doped SnO2 samples at various temperatures are reported in Feng Gu et. al, [140]. The grain growth results of annealing of SnO2 samples was discussed in J.K.L. Lai et. al, [141]. The increasing grain growth with increasing temperature was observed in L.C. Tien et. al [142]. Figure. 18 shows the XRD results of our as–prepared SnO2 nanocrystalline powders obtained from stoichiometry ratio (Φe) of urea as a fuel at different temperatures from 200 to 800°C. For asprepared and annealed powders, the XRD data showed that the tetragonal SnO2 phase was successfully produced, its lattice parameters were ‘a’ = 4.7493 nm and ‘c’ = 3.1898 nm (as-prepared). The obtained pattern showed majority (hkl) peaks of (110), (101), (200), (111), (211), (220), (002), (310), (112), (301), (202) and (321) were corresponding to SnO2 and no traces of unidentified peaks were present in this pattern.

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From the XRD pattern, as-prepared and annealed nanocrystalline SnO2 powders were compared with bulk commercial (MERCK) powder. The commercial powder pattern showed that the high intense with narrow peaks were observed. The width of the as-prepared Bragg peaks are considerably broadened which indicates a small crystallite size. As the annealing temperature increases from 200 to 800°C the intensity of the peaks were increased, where as the FWHM were reduced. The peak broadenings decrease and the intensities of the peaks are gradually sharper with increasing temperatures led to nanocrystalline powders with average crystallite sizes (in nm) varying from 8 to 43 nm. The changing the annealing temperatures from 200 to 800°C which decreased the density of nucleation centers as shown in Fig. 19. A smaller crystallite size exhibits larger cell volume and minimum of number of unit cells were present. Thereafter, crystallite size is increased, the cell volume is also decreased due to the heating temperature is increased and the number of unit cells is also increased. Major advances in powder diffraction were realized following the development of the Rietveld method in late 1960s by Rietveld and after that refinement of complex structures from powder data increased significantly. Cheetham and Taylor 1977 reviewed the refined structures of over 150 compounds showing the advantages in powder diffraction that occurred following the development of the Rietveld method [143]. Although many reported structures during that time were refined with

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neutron data, the extension of the Rietveld method to lab X-ray data in late 1970s greatly expanded the use of the powder diffraction method to determine structures. In the early 1980s the Rietveld method was developed to analyze time-of-flight neutron data and synchrotron data, which allowed scientists to refine structures more precisely and accurately from powder data. These advances were accompanied by development of software packages for Rietveld refinement i.e. DBWS, GSAS and TOPAS. Pre-Rietveld Refinement Methods The use of least squares methods to refine structures in the 1960s paralleled the development of digital computers. These methods were first applied to refine single crystal structures and later used to analyze powder diffraction data. Some crystallographers used single crystal structure refinement codes to refine structures using intensities estimated from powder data, while some codes had capabilities even to decompose non-equivalent overlapping reflections. Although those methods had major disadvantages due to collapse of three dimensional crystallographic information to one dimensional powder pattern, they were used to characterize high symmetry inorganic solids until the Rietveld method was widely accepted. Rietveld Method The Rietveld method was developed by Hugo Rietveld in late 1960s and first reported at the seventh congress of the International Union of crystallography (IUCr) in Moscow in 1966 [144].

Since then it has become the most widely used method for structure refinement from powder diffraction data. While least square methods, developed prior to the Rietveld method, minimize the difference between the calculated and observed intensities of individual peaks, the Rietveld method optimizes the fit of the entire diffraction pattern at once. In the early days, the Rietveld method was developed for constant wavelength neutron data where the Gaussian peak shapes are relatively easy to

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model. In the 1970s the method was extended towards lab X-ray data [145]. Since lab X-ray peaks generally do not possess Gaussian peak shapes, more complex functions such as Lorentzian and pseudo-Voigt were used to model the peak shapes. Subsequently, the Rietveld method was successfully applied to synchrotron X-ray data. Having synchrotron X-ray data instead of lab X-ray data enhances the quality of refinements drastically due to the high resolution of the synchrotron data [146]. The Rietveld method has also been extended to refine time-of-flight neutron data collected with pulsed sources. In this method, different peak shape functions are employed to model the peak profiles. In the present investigations, for the SnO2 powder samples a structure and crystallographic parameters has been performed to obtain and refined from Rietveld analysis of the nanocrystalline SnO2 powders and were shown in Table 2. Rietveld refinement was made on the SnO2 at different heattreatment temperature values are reported by [147]. The convergence parameters of the Rietveld refinement and the results for microstructural characterization of different treatment temperatures of the SnO2 nanopowders are reported in [148]. Crystallite size, cell mass and density, number of unit cells and dislocation density were dependent on thermal conditions. Humberto V. Fajardo et al. [149] has reported the X-ray diffraction patterns, associated with the Rietveld refinement method, were used to determine the crystallite size of the tin oxide samples was 12 nm at 550 ºC and 65 nm at 1000 ºC. M.L. Moreira et al [150] the obtained Rietveld refinement value of doped SnO2 sample are Rp = 9.82%; Rwp = 14.42%; S=1.29 and RB=2.33 and pure SnO2 RWp = 1.77, RB = 6.32 was observed by Evandro A. de Morais et al. [151]. Using the refined data, the structural model of SnO2 was constructed and its bond length is obtained and comparable with the earlier reports [148, 151]. The Rietveld indexes and parameters of the pure and doped SnO2 are reported in [152]. The Rietveld refinement values of different phases for SnO2 are reported in [153]. The dopent phase was discuss in C. B. Fitzgerald et al [154] by using Rietveld. The structural and profile parameters of pure and doped SnO2 were refined by the Rietveld method was Rwp = 11.93% and RBragg = 4.19% as well as interatomic distances and angles are discussed in [155] and the unit cell changes isotropically since no significant alteration was observed in the tetragonality factor (c/a). X.M. Liu et al. [156] experimental data of SnO2 powder is not complete at a temperature of 350ºC, but perfect crystals can be obtained at higher treating temperatures at 500 and 700ºC. A SnO2 performed experiments results of lattice parameters and crystallite size were obtained for different heat treatments by Mahesh Bhagwat et. al [157]. In our experimental results using Rietveld method of the SnO2 nanocrystalline powders heat treated at 8000 C was refined (Fig. 20). The weight percentage, Rbragg and GOF were ~11%, ~1% and ~1% were calculated for as-prepared and heat treated powders are summarized in Table. 2.

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Scanning electron microscopy The spherical morphologies were carried out by SEM analysis as another tool which is used to resolve the nanosized particulate microstructure of the powders as shown in Fig. 21. SEM micrograph of SnO2 powders by different temperatures conditions, the grains have a diameter below 100 nm and with aggregated status. It is interesting to note that on calcination the hallow sphere SnO2 powder undergoes swelling and becoming more agglomerate. Particle size of the nanoparticles was drastically increased and strong agglomerated by increasing the calcination temperature. L.B. Fraigi et. al., [158] have been reported the synthesis of nanocrystalline SnO2 morphology of powders by combustion routes, the aggregates were observed are mainly constituted by clusters of several tiny spheres weakly agglomerated. The effects of urea of microwave-derived SnO2 microstructures in various kinds of solvents after 1200ºC calcinations were obtained in Chen-Tao Lee et. al. [159]. L. Fraigi et. al [160] have been reported the SEM images are mainly constituted by clusters of several tiny spheres weakly agglomerated.

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TEM studies The microstructure of the sample was examined by the high-resolution transmission electron microscopy (HRTEM). Typical TEM and HRTEM images of the as-prepared sample are given in Fig. 22(a). It is obvious that as-prepared SnO2 powders are crystalline with 12 nm particle size in HRTEM image, and the particles are held together by an irregular network. This is in good agreement with the XRD result. After heating at different temperatures, the particle size of SnO2 increases gradually. A large increase in particle size was observed after sintering at 600ºC, with the particles having the appearance of rounded hexagons Fig. 22. (b), exhibiting a high degree of crystallinity. The corresponding selected area electron diffraction (SAED) images are shown as inset in the TEM images. The diffraction rings (showed ‘hkl’ planes of (110), (101), (200), (111), (211), (220), (002), (310), (112), (301), (202) and (321)) clearly confirm that tetragonal structure of SnO2 has been synthesized successfully. Average particle sizes of about 17 ,24, 28, 32, 37, 43 and 51 nm were obtained for the annealing material at 200, 300, 400, 500, 600, 700 and 800ºC, respectively. Such data are in agreement with those obtained by the computation of the Scherrer formula and Rietveld method applied to XRD data within the experimental error range. The similar result was observed in the Jun-Jie Zhu et. al. [161]. TEM images of the samples prepared with urea under microwave-assisted conditions at different temperature conditions were observed in Malika Krishna and Sridhar Komarneni [162]. A. Cirera et. al. [163] discussed the TEM micrographs allow us to compute the statistical grain sizes, the value obtained using other techniques [164]. T. Krishnakumar et. al. [165] have been observed the morphology and particle size of the tin dioxide nanoparticles by Microwave-assisted synthesis method. Microwave-assisted synthesis and characterization of tin oxide nanoparticles, SEM and TEM analyses showed that the nanoparticles present a platelet-like shaped particle or, a pseudo spherical morphology, after calcination at moderate temperature during which the phase transformation from SnO to SnO2 was observed by T. Krishnakumar et. al. [166]. The as-prepared TEM image of SnO2 nanoparticles from the CEM microwave reactor to be heavily agglomerated particles was observed by Yong Wang et. al. [167].

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Photoluminescence Figure 23 shows a typical excitation and emission spectra of SnO2 nanocrystalline powder prepared by direct chemical precipitation method. The excitation spectrum of the SnO2 shows a strong band at 265 nm under an emission of 417 nm. In general, except the sharp excitonic emission, semiconductors have another broad trapped emission, which often contains multiple luminescent centers. There are various types of surface states that give rise to different energy states inside the semiconductor band gap. As for SnO2, the trapped emission is complicated. For example, SnO2 nanocrystalline powder has two distinct PL emissions at 400 and 430 nm [168]. Three emission peaks at 439, 486 and 496 nm were observed from the as-synthesized SnO2 nanoribbons [169]. Two main emissions at 452 and 560 nm were found by cathodoluminescence (CL) spectroscopy for the SnO2 nanowires and nanobelts grown on Al2O3, SiO2 and Si substrates [170]. Up to now, the mechanisms of observed emissions are not yet clear. However, they should be associated with defect energy levels within the band gap of SnO2. Oxygen vacancies are well known to be the most common defects in oxides and usually act as radiative centre in luminescence processes. Thus, the nature of the transition is tentatively ascribed to oxygen vacancies, Sn vacancies or Sn interstitials, which form a considerable number of trapped states within the band gap [169, 171-173].

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In this work, all the SnO2 nanocrystalline powders heated at different temperatures and all samples were excited with a dominant emission peaks which was observed at 417 nm (blue) as ascribed as luminescence centers. However, in our studies, emission maximum of 417 nm is lower than the band gap of the SnO2 bulk of 3.6 eV. These peaks can be attributed to electron transition, mediated by defects levels in the band gap, such as oxygen vacancies and the luminescence centers formed by such tin interstitials or dangling in the presence of SnO2 nanocrystals. From the Fig 13, it represents rutile tetragonal SnO2 structure by a tin atom at the centre and surrounded by six oxygen atoms in the vertex. The tin atom is bonded to four oxygen Sn1O1 (4x) atoms with the same bond length in the basal plane and another two apical oxygen formed Sn1O1 (2x) atoms with different bond length as mentioned in Table 2. The Sn-O bond length shows a very high in as-prepared nanocrystalline SnO2, it was strong and maximum emission intensity is appeared at 417 nm. When the temperatures increased the bond length is decreased due to the reduction of oxygen vacancies. So the emission intensity is also decreased and the peak is shifted towards red shift. Conclusion In recent few years of studies on the preparation methods by microwave-assisted combustion method and their property analysis for reliability and reproducibility of nanocrystalline SnO2 powder have been reviewed. The XRD patterns confirmed that SnO2 nanocrystalline powders possess a tetragonal rutile structure. The average crystallite size of the nanopowders are in the nanometer range, upon increasing the temperatures the crystallite size of the SnO2 was found to increase from 9 to 43 nm. The heating effect has influence the size of the crystallite on the nanocrystal powders. When

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samples were heated the lattice parameters, dislocation density was decreased, so a smaller number of unit cells start to grow. Then, the obtained Rietveld refinement indexes values of weight percentage is ~11%, R

bragg

value is ~1% and good of fitness is ~1% presenting good agreement in earlier reports.

Photoluminescence emission exhibits a band at 417 nm. It is related to the recombination of electrons in singly occupied oxygen vacancies with photoexcited holes in the valence band. Then the heating temperature can also affect the luminescence process, the emission resulting in the decrease in the oxygen vacancies, as revealed by the decrease in luminescence at 417 nm. Further experiments are planned and the results will be reported in future. Acknowledgements One of the authors, L.C.Nehru would like to thank University Grants Commission (UGC), India for the financial support under Research Fellowship in Sciences to Meritorious Students (RFSMS) to carry out this research work at School of Physics, Alagappa University, Karaikudi-3, India. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

[16]

R. Kubo, J. Phys. Soc. Jpn., 17 (1962) 975. S. Iijima, Nature, 354 (1991) 56. L.E.J. Brus, Phem. Chem. 98 (1994) 3575. J. Rupp, R. Birringer, Phys. Rev. B 36 (1987) 7888. E. Hellstern, H. Fecht, Z. Fu, W.L. Johnon, Appl. Phys. 65 (1989) 305. J. Horvath, R. Birringer, H.Gleiter, Solid State Commun. 62 (1987) 319. R. Birringer, H. Hahn, H.J. Hofler, J. Karch, H. Gleiter, Diffus. Defect. Date: Defect. Diffus. Forum 59 (1998) 17. H. Gleiter, Prog. Mater. Sci. 32 (1989) 223. Kumar, Challa. (ed) 2006. Nanomaterials: toxicity, health, and environmental issues. 1st ed. Nanotechnologies for the Life Sciences Vol 5. Weinheim: Wiley-VHC. Z Li,. H Hahn,. R. W. Siegel,.Mater. Lett. 6 (1988) 342. A. W Adamsom, A. P Gast, In Physical Chemistry of Surfaces 6th ed., Wiley Interscience, New York, 257 (1997). R.W. Siegel. Encyclopedia of Applied Physics, VCH, 11 (1994) 173. E. L. Venturini, J. P.Wilcoxon, and P. P. Newcomer. Mater. Res. Soc., Symp. Proc. volume 351 (1994) 311. J. P. Wilcoxon, R. L. Williamson, and R. Baughman. J. Chem. Phys., 98 (1993) 9933. Franklin, N. M., N. J. Rogers, S.C. Apte, G.E. Batley, G.E. Gadd, and P.S. Casey. 2007. Comparative Toxicity of Nanoparticulate ZnO, Bulk ZnO, and ZnCl2 to a Freshwater Microalga (Pseudokirchneriella subcapitata): The Importance of Particle Solubility. Environmental Science and Technology 41: 8484-8490. J Chen,., J. Zhu, H.-H. Cho, K. Cui, F. Li., X. Zhou, J.T. Rogers, S.T.C. Wong, and X. Huang. 2007. Differential Cytotoxicity of Metal Oxide Nanoparticles. In NSTI Nanotech 2007; May 20-24, 2007; Santa Clara, California.

116

[17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49]

Characterization of Technological Materials

Jeng, H. A. and J. Swanson 2006. Toxicity of Metal Oxide Nanoparticles in Mammalian Cells. Journal of Environmental Science & Health, Part A: Toxic/Hazardous Substances & Environmental Engineering 41(12): 2699-2711. Houlahan, J. E., C. S. Findlay, B. R. Schmidt, A. H. Meyer, and S. L. Kuzmin. 2000. Nature 404(6779): 752. S.C. Tjong and H. Chen, Materials Science and Engineering: Reports, 45 (2004) 2. C.J. Murphy, Material Science, 298 (5601) (2002) 2139. S. Pratsims, Particle technology laboratory, www.ptl.ethz.ch, 2005. H. Zhang, D. Yang, X. Ma, Y. Ji, S.Z. Li, and D Que, Materials Chemistry and Physics, 93 (2005) 65. A. Martucci, J. Fick, Serge-Emile LeBlanc, M. LoCascio, A Hache, J. of Non-Crystalline Solids, 345&346 (2004) 639. Masui, T.; Fujiwara, K.; Machida, K.; Adachi, G.; Sakata, T.; Mori, H.: Chem. Mater. 9 (1997) 2197. Dagani, R.: Chem. & Eng. News 70 (1992) 18. Argazzi, R.; Bignozzi, C. A.: J. Am. Chem. Soc. 117 (1995) 11815. Chin, R. P.; Shen, Y. R.; Petrova-Koch, V.: Science 270 (1995) 776. Bhargava, R. N.: J. Lumin. 70 (1996) 85. Wittenauer, J.: In Plastic Deformation of Ceramics, R. C. Bradt, C. A. Brookes, and J. L. Routbort (eds.), Plenum, New York, 321 (1995). Skandan, G.; Kear, B. H.: Mater. Sci. Forum 243-245 (1997) 217. Jack, K. H.: High Technology Ceramics, Past, Present, and Future; Ceramics and Civilization 3, W. D. Kingery (ed.), 259 (1986). Cahn, R. W.: Nature 332 (1988) 761. L. Sun, C. Qian, C. Liao, X. Wang, C. Yan, Solid State Communications, 119 (2001) 393. Y.J. Yang, L.Y. He, Q.F. Zhang, Electrochemistry Communication, 7 (2005) 361. R.S. Patil, C.D. Lokhande, R.S. Mane, T.P. Gujar, S-H. Han, J. Non-CrystallineSolids, 353 (2007) 1645. P. Aragon-Santamaria, M.J. Santos-Delgado, A. Maceira-Vidan, L.M. Polo-Diez, J.Mater. Chem., 1 (3) (1991) 409. A.C. Pierre, Introduction to sol-gel processing, Kluwer Academic Publishers, Boston, 1998. Ravindranathan, P.; Komarneni, S.; Roy, R.: J. Mater. Sci. Lett. 12 (1993) 369. Zhang, Y.; Stangle, G. C.: J. Mater. Res. 9 (1994) 1997. Kingsley, J. J.; Pederson, L. R.: Mater. Lett. 18 (1993) 89. Kingsley, J. J.; Pederson, L. R.: Mater. Res. Soc. Symp. Proc. 296 (1993) 361. Patil KC, Hegde MS, Tanu Rattan, Aruna ST. Chemistry of nanocrystalline oxide materials: combustion synthesis, properties and applications. Singapore: World Scientific; 2008. Mukasyan AS, Martirosyan K, editors. Combustion of heterogeneous systems: fundamentals and applications for material synthesis. Kerala, India: Transworld Research Network; 2007, 234. Segadaes AM. Eur Ceram News Lett 9 (2006)1. Varma A, Diakov V, Shafirovich E. Heterogeneous combustion: recent developments and new opportunities for chemical engineers. AIChE J 51 (2005) 2876. Patil KC, Aruna ST, Mimani T. Current Opinion in Solid State Mater Sci 6 (2002) 507. Singanahally T. Aruna, Alexander S. Mukasyan, Current Opinion in Solid State and Materials Science 12 (2008) 44. A. Varma, A.S. Rogachev, A. S. Mukasyan, Adv. in Ch. Eng., 24, 79-226 (1998) Stobierski L, Wegrzyn Z, Lis J, Buck M. Int J Self-Prop High-Temp Synth 10 (2001) 217.

Materials Science Forum Vol. 671

[50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71] [72] [73] [74] [75] [76] [77] [78]

117

Bernard F, Gaffet E. Int J Self-Prop High-Temp Synth 10 (2001) 109. Borovinskaya IP, Ignat’eva TI, Vershinnikov VI, Khurtina GG, Sachkova NV. Inorg Mater 39 (2003) 588. Nersisyan HH, Lee JH, Won CW.. Int J Self-Prop High-Temp Synth 12 (2003) 149. Nersisyan HH, Lee JH, Won CW. Mater Chem Phys 89 (2005) 283. Jayalakshmi M, Palaniappan M, Balasubramanian K. Int J Electrochem Sci 3 (2008) 96. A. Varma, “Form From Fire,” Scientific American, 45 (2000) 58. Z.A. Munir, U. Anselmi-Tamburini, Mater.Sci.Reports, 3 (1998) 277. Hill, A and ILSI Europe Microwave Oven Task Force. Microwave Ovens. Brussels: ILSI Europe; 1998. National Research Council (U.S.). Committee on Microwave Processing of Materials: An Emerging Industrial Technology. 1994. Microwave Processing of Materials. National Academy Press. Washington, D.C. Sutton W. H., “Microwave Processing of Ceramic Materials,” Ceramic Bulletin, 68 (2), (1989) 376-386. Roy R., Agrawal D., Cheng J., Gedevanishvili S., Nature, 399, (1999) 668-670. Cheraddi A., Desgardin G., Provost J., Raveau B., Electroceramics IV, II, (1994) 1219-1224. Ohlsson, T. Domestic use of microwave ovens. In: Macrae R, Robinson, RK and Sadler, MJ, editors. Encyclopaedia of food science food technology and nutrition. Vol. 2. London: Academic Press; (1993) 1232-1237. Sajjad Habibzadeh, Amin Kazemi-Beydokhti, Abbas Ali Khodadadi, Yadollah Mortazavi, Sasha Omanovic, Mojtaba Shariat-Niassar, Chemical Engineering Journal 156 (2010) 471–478. Zhijie Li, Wenzhong Shen, Xue Zhang, Limei Fang, Xiaotao Zu, Colloids and Surfaces A: Physicochem. Eng. Aspects 327 (2008) 17–20 J.J. Zhu, J.M. Zhu, X.H. Liao, J.L. Fang, M.G. Zhou, H.Y. Chen, Mater. Lett. 53 (2002) 12–19 D.S. Wu, C.Y. Han, S.Y. Wang, N.L. Wu, Mater. Lett. 53 (2002) 155–159 F.I. Pires, E. Joanni, R. Savu, M.A. Zaghete, E. Longo, J.A. Varela, Mater. Lett. 62 (2008) 239–242 Chen-Tao Lee, Fu-Shan Chen, Chung-Hsin Lu, Journal of Alloys and Compounds 490 (2010) 407–411 E. Michel, D. Chaumont, and D. Stuerga, Journal of Colloid and Interface Science 257 (2003) 258–262 Yong Wang, Jim Yang Lee, Journal of Power Sources 144 (2005) 220–225 M.L. Moreira, S.A. Pianaro, A.V.C. Andrade, A.J. Zara, Materials Characterization 57 (2006) 193–198 Jun-Jie Zhu , Jian-Min Zhu, Xue-Hong Liao, Jiang-Lin Fang, Miao-Gao Zhou, Hong-Yuan Chen, Materials Letters 53 2002 12–19 T. Krishnakumar, Nicola Pinna, K. Prasanna Kumari, K. Perumal, R. Jayaprakash, Materials Letters 62 (2008) 3437–3440 T. Krishnakumar, R. Jayaprakash, M. Parthibavarman, A.R. Phani, V.N. Singh, B.R. Mehta, Materials Letters 63 (2009) 896–898 Srivastava Abhilasha, Lakshmikumar ST, Srivastava AK, Jain Rashmi Kiran. Sens Actuators B 2007;126:583–7 Cirera A, Vila A, Cornet A, Morante JR. Mater Sci Eng C 2001;15:203–5 Wu Dien-Shi, Han Chih-Yu, Wang Shi-Yu, Wu Nae-Lih, Rusakova IA. Mater Lett 2002;53:155–9 A. Cirera, A. Vila, A. Cornet, J.R. Morante, Materials Science and Engineering C 15 2001 203– 205

118

[79] [80] [81] [82] [83] [84] [85] [86] [87] [88] [89] [90] [91] [92] [93] [94] [95] [96] [97] [98] [99] [100] [101] [102] [103] [104] [105] [106] [107] [108] [109] [110] [111] [112] [113] [114] [115] [116]

Characterization of Technological Materials

M.P. Borom and M. Lee, Advan. Ceram. Mater. 1 ( 1986) 335 F. Selmi, S. Komarneni, V.K. Varadarn and V.V. Varadan, MATERIALS LETTERS Volume 10, number 6 December (I990) H. Hallil, P. Ménini and H. Aubert, Procedia Chemistry 1 (2009) 935–938 Xu, C.; Tamaki, J.; Miura, N.; Yamazoe, N., Sensors & Actuators B-Chemical, 1991,3, 147155 J. L. Shi, J. Mater. Res. 14, 4 (1999) 1378-88 Y. Ozaki, S. Suzuki, M. Morimitsu, M. Matsunaga, Sens. Actuators B 62 (2000) 220–225 Y. Ozaki, S. Suzuki, M. Morimitsu, M. Matsunaga, J. Electrochem. Soc. 147 (2000) 1589–1591 M. Morimitsu, Y. Ozaki, S. Suzuki, M. Matsunaga, Sens. Actuators B 67 (2000) 184–188 S. Gupta, R.K. Roy, M. Pal Chowdhury, A.K. Pal, Vacuum 75, 2004, pp 111 – 119 M.S. Wagh, G.H. Jain, D.R. Patil, L.A. Patil, Sens. Actuators B 122, 2007, pp 357 – 364 V. A. Chaudhary, S. R. Sainkar, I . S. Mulla, Journal of Materials Science Letters 19 (2000) 249– 252 J. Puigcorbe´, A. Cirera, J. Cerda`, J. Folch, A. Cornet, J.R. Morante, Sensors and Actuators B 84 (2002) 60–65 Pyeong-Seok Cho, Ki-Won Kim, Jong-Heun Lee, Sensors and Actuators B 123 (2007) 1034– 1039 J. Huang, N. Matsunaga, K. Shimanoe, N. Yamazoe, T. Kunitake, Chem. Mater. 17 (2005) 3513–3518. K. Suito, N. Kawai, Y. Masuda, Mater. Res. Bull. 10 (1975) 677. L. Luxmann, R. Dobner, Metall (Berlin) 34 (1980) 821. F. Lawson, Nature 215 (1967) 955. G. Murken, M. Tro¨mel, Ueber das beider, Z. Anorg. Allg. Chem. 897 (1973) 117. K. Hasselbach, G. Murken, M. Tro¨mel, Z. Anorg. Allg. Chem. 897 (1973) 127. Y. Li-Zi, S. Zhi-Tong, W. Chan-Zheng, J. Solid State Chem. 113 (1994) 221. Matthias Batzill, Ulrike Diebold, Zh. Prikl. Khim. (Leningrad) 32 (1959) 273. J. Geurts, S. Rau, W. Richter, F.J. Schmitte, Thin Solid Films 121 (1984) 217. T. Yamazaki, U. Mizutani, Y. Iwama, Jpn. J. Appl. Phys. 21 (1982) 440. J. Pannetier, G. Denes, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 36 (1980) 2763. G.W. Watson, J. Chem. Phys. 114 (2001) 758. J. Terra, D. Guenzburger, Phys. Rev. B 44 (1991) 8584. M. Meyer, G. Onida, M. Palummo, L. Reining, Phys. Rev. B 64 (2001) 045119. J. Galy, G. Meunier, S. Andersson, A. A ˚ stro¨m, Ste´re´ochimie des, J. Solid State Chem. 13 (1975) 142. R.J. Gillespie, Molecular Geometry, Van Nostrand Reinhold, London, 1972. I. Lefebvre, M. Szymanski, J. Olivier-Fourcade, J.C. Jumas, Phys. Rev. B 58 (1998) 1896. A.A. Bolzan, C. Fong, B.J. Kennedy, C.J. Howard, Acta Crystallogr. Sect. B—Struct. Sci. 53 (1997) 373. Z. M. Jarzebski, J. Electrochem. Soc. 123 (1976) 199. V. W. H. Baur. Acta Crystallographica, 9: (1956) 515. G. McCarthy, J. Welton, Powder Diffraction 4 (1989) 156. C. G. Fonstad, R. H. Rediker, J. Appl. Phys. 42 (1971) 2911. M. Nagasawa, S. Shionya, J. Phys. Soc. Jpn. 30 (1971) 1213. C.L. Lau, G.K. Wertheim, J. Vac. Sci. Technol. 15 (1978) 622. J. M. Themlin, M. Chtaib, L. Henrard, P. Lambin, J. Darville, J.-M. Gilles, Phys. Rev. B 46 (1992) 2460.

Materials Science Forum Vol. 671

119

[117] Matthias Batzill, Ulrike Diebold, Progress in Surface Science 79 (2005) 47. [118] S. Munnix, M. Schmeits, Electronic structure of tin dioxide surfaces, Phys. Rev. B 27 (1983) 7624. [119] J.M. Themlin, R. Sporken, J. Darville, R. Caudano, J.M. Gilles, Phys. Rev. B 42 (1990) 11914. [120] J.J. Kingsley, K.C. Patil, Mater. Lett. 6 (1988) 427. [121] S.R. Jain, K.C. Adiga, V.R. Verneker, Pai, Combustion Flame 40 (1981) 71. [122] S. Velumani, Xavier Mathew, P. J. Sebastian, Sa. K. Narayandass, D. Mangalaraj, Solar Energy Materials & Solar cells 76 (2003) 347. [123] Preetam Sing, Ashvani Kumar, Ajay Kaushal, Davinder Kaur, Ashish Pandey and RN Goyal, Bull. Mater. Sci., 31(3) (2008) 573. [124] Landford, I. J.; Louer, D., Reports on Progress in Physics 59 (1996) 131. [125] Zachariasen, W. H., Journal of Chemical Physics 16 (1948) 254. [126] Mooney, Rose C. L., Acta Crystallographica 2 (1949) 189. [127] Zachariasen, W. H.; Ellinger, F. H., Acta Crystallographica 16 (1963) 369. [128] Debets, P. C., Acta Crystallographica B: 24 (1968) 400. [129] K. Anandan, S. Gnanam, J. Gajendiran, V. Rajendran, Journal of Non-Oxide Glasses 2(2) (2010) 83. [130] Yujie Fenga, Yu-Hong Cuia, Junfeng Liua, Bruce E. Logana, Journal of Hazardous Materials 178 (2010) 29. [131] Ü. Kersen, Appl. Phys. A 75 (2002) 559. [132] Novinrooz, Abdoljavad, Sarabadani, Parvin Iran. J. Chem. Chem. Eng. Research Note. 28(2) (2009)113. [133] L´aszl´o K˝or¨osi, Szilvia Papp, Vera Meynen, Pegie Cool, Etienne F. Vansant, Imre D´ek´any, Colloids and Surfaces A: Physicochem. Eng. Aspects 268 (2005) 147. [134] K. Nomura, C. Barrero, J. Sakuma, M. Takeda, Czechoslovak Journal of Physics, Vol. 56 (2006), Suppl. E A E75. [135] Jochan JOSEPH, VargheseMATHEW, Jacob MATHEW, K. E. ABRAHAM, Turk J Phys, 33 (2009) 37. [136] Y. Wang1, M, Aponte, N. Le´on, I. Ramos, R. Furlan, N. Pinto, J.J. Santiago-Avil´es, REVISTA MEXICANA DE F´ISICA S 52 (2) (2006) 42. [137] H. S. ZHUANG, H. L. XIA, T. ZHANG, D. C. XIAO, Materials Science-Poland, 26(3) (2008) 517. [138] X. Q. Pana and L. Fu, JOURNAL OF APPLIED PHYSICS 89 (11) (2001) 6048. [139] Chanipat Euvananont, Thamrong Chansawang, Yot Boontongkong, Chanchana Thanachayanont Journal of Microscopy Society of Thailand, 23(1) (2009) 79. [140] Feng Gu, Shu Fen Wang, Meng Kai L€u, Yong Xin Qi, Guang Jun Zhou, Dong Xu, Duo Rong Yuan, Inorganic Chemistry Communications 6 (2003) 882. [141] J.K.L. Lai, C.H. Shek, G.M. Lin, Scripta Materialia 49 (2003) 441. [142] L.C. Tien, D.P. Norton, J.D. Budai, Materials Research Bulletin 44 (2009) 6. [143] Cheetham, A. K.; Taylor, J. C., Journal of Solid State Chemistry 21 (1977) 253. [144] Rietveld, H. M., Journal of Applied Crystallography 2 (1969) 65. [145] Cox, D. E.; Hastings, J. B.; Thomlinson, W.; Prewitt, C. T., Nuclear Instruments and Methods in Physics Research 208 (1983) 573. [146] Young, R. A.; Mackie, P. E.; Von Dreele, R. B., Journal of Applied Crystallography 10 (1977) 262. [147] N.L.V.Carrno, A.P.Maciel, E.R. Leite, P.N.Lisboa-Filho, E.Lango, A. Valentini, L.F.D.Probst, C.O.Paiva-Santos, W.H.Schreiner, Sensors and Actuators B 86 (2002) 185.

120

Characterization of Technological Materials

[148] A.P. Maciel, P.N. Lisboa-Filho, E.R. Leite, C.O. Paiva-Santos, W.H. Schreiner, Y. Maniette, E. Longo, Journal of the European Ceramic Society 23 (2003) 707. [149] Humberto V. Fajardo, Luiz F. D. Probst, Antoninho Valentini, Neftalí L. V. Carreño, Adeilton P. Maciel, Edson R. Leite and Elson Longo, J. Braz. Chem. Soc., 16(3B) (2005) 607. [150] M.L. Moreira, S.A. Pianaro, A.V.C. Andrade, A.J. Zara Materials Characterization 57 (2006) 193. [151] Evandro A. de Morais, Luis V.A. Scalvi, Alberto A. Cavalheiro, Américo Tabata, José Brás B. Oliveira, Journal of Non-Crystalline Solids 354 (2008) 4840. [152] C. O. Paiva-Santos, H. Gouveia, W. C. Las, J. A. Varela, Materials Structure, 6(2) (1999) 111. [153] Sean R. Shieh, Atsushi Kubo, Thomas S. Duffy, Vitali B. Prakapenka and Guoyin Shen PHYSICAL REVIEW B 73 (2006) 014105. [154] C. B. Fitzgerald, M. Venkatesan, A. P. Douvalis, S. Huber, and J. M. D. Coey, T. Bakas, JOURNAL OF APPLIED PHYSICS, 95(11) (2004). [155] J. A. G. Carrió, T. J. Masson, A. H. Munhoz, M. M. de Jesus, L. Perazolli, U. Coleto, S. Gutierrez-Antonio, R. F. C. Marques, C. O. Paiva-Santos, Z. Kristallogr. Suppl. 26 (2007) 467. [156] X.M. Liu, S.L. Wu, Paul K. Chu, J. Zheng, S.L. Li, Materials Science and Engineering A 426 (2006) 274. [157] Mahesh Bhagwat, Pallavi Shah, Veda Ramaswamy, Materials Letters 57 (2003) 1604. [158] L.B. Fraigi, D.G. Lamas, N.E. Walsoe de Reca, Materials Letters 47 2001 262–266. [159] Chen-Tao Lee, Fu-Shan Chen, Chung-Hsin Lu, Journal of Alloys and Compounds 490 (2010) 407. [160] L. Fraigi, D.G. Lamas, and N.E. Walso¨e de Reca, NanoStructured Materials, 11(3) (1999) 311. [161] Jun-Jie Zhu, Jian-Min Zhu, Xue-Hong Liao, Jiang-Lin Fang, Miao-Gao Zhou, Hong-Yuan Chen, Materials Letters 53 (2002) 12. [162] Malika Krishna and Sridhar Komarneni, Ceramics International 35 (2009) 3375. [163] A. Cirera, A. Vila, A. Cornet, J.R. Morante, Materials Science and Engineering C 15 (2001) 203. [164] A. Dieguez, A. Romano-Rodrıguez, J.R. Morante, U. Weimar, M. Ž . Schweizer-Berberich, W. Gopel, Sens. Actuators, B 31 (1996) 1. [165] T. Krishnakumar, R. Jayaprakash, M. Parthibavarman, A.R. Phani, V.N. Singh, B.R. Mehta, Materials Letters 63 (2009) 896–898. [166] T. Krishnakumar a, Nicola Pinna b, K. Prasanna Kumari a, K. Perumal a, R. Jayaprakash, Materials Letters 62 (2008) 3437. [167] Yong Wang, Jim Yang Lee, Journal of Power Sources 144 (2005) 220. [168] Feng Gu, Shu Fen Wang, Chun Feng Song, Meng Kai Lu, Yong Xin Qi, Guang Jun Zhou, Dong Xu, Duo Rong Yuan, Chemical Physics Letters 372 (2003) 451. [169] J Q Hu, X L Ma, N G Shang, Z Y Xie, N B Wong, C S Lee and S T Lee, J. Phys. Chem. B 106 (2002) 3823. [170] D Calestani, L Lazzarini, G Salviati and M Zha, Cryst.Res. Technol. 40 (2005) 937. [171] J H He, T H Wu, C L Hsin, K M Li, L J Chen, Y L Chueh, L J Chou and Z L Wang, Small 2 (2006)116. [172] H W Kim, N H Kim, J H Myung and S H Shim, Phys. Status Solidi A 202 (2005) 1758. [173] S Brovelli, N Chiodini, F Meinardi, A Lauria and A Paleari, Appl. Phys. Lett. 89 (2006) 153126.

Materials Science Forum Vol. 671 (2011) pp 121-129 © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.671.121

Synthesis and electron density analysis of SnO2 nano particles S. Saravanakumara, M. Jeya Priya, R. Saravananb PG Department and Research Centre of Physics, The Madura College, Madurai-625011, Tamil nadu, India a [email protected]; [email protected]; URL: http://www.saraxraygroup.net/ Corresponding author: [email protected] Keywords: MEM, SEM, XRD, Reitveld method, Electron density, Sensor.

Abstract Tin oxide material (SnO2) is synthesized in nano scale range and is characterized. The refined X-ray intensity data was obtained from the Reitveld method. The electron density of nano SnO2 is determined using MEM (Maximum Entropy Method). Using one, two and three dimensional MEM maps, the bonding within the atoms is clearly understood. The particle size of SnO2 is also analyzed using XRD and SEM. Introduction In recent years, intensive research works are being carried out for the development of semiconducting nanoparticles because of their potential applications. Tin oxide is a kind of wide energy gap semiconductor and has many technological applications in solid-state gas sensing, optoelectronic device, dye based solar cells and electrodes in thin films [1-4].


SnO2 is the most used sensing material in commercial sensor devices for toxic gases

detection. It is well known that the sensing properties of SnO2

based materials depend on their

chemical and physical characteristics, which are strongly dependent on the preparation conditions, dopant and grain size. In lithium-ion-battery science, recent research has focused on nanoscale electrode materials to improve electrochemical performance. Materials based on tin oxide have been proposed as alternative anode materials with high-energy densities and stable capacity retention in lithium-ion batteries [5-7]. Various SnO2 based materials have displayed extraordinary electrochemical behavior, that the initial irreversible capacity induced by Li2O formation and the abrupt capacity fading caused by volume variation could be effectively reduced when in nano scale form [8-10]. From this point of view, tin oxide nano wires can also be suggested as promising anode materials because the nano wire structure is of special interest with predictions of unique electronic and structural properties [11].

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SnO2 powders can be prepared by a variety of synthesis methods [12]. In the present research work, we have analyzed the bonding nature of SnO2. The electron density for the tin oxide is determined through a statistical approach by using MEM (Maximum Entropy Method) [13]. Synthesis of tin oxide (SnO2) nanoparticles Tin chloride (2.2563 gms) is dissolved in de-ionized water (100 ml) to attain 0.1 molarity condition. Ammonium solution is added drop by drop to the above solution till the pH value reaches 8. The resulting powder was washed 4 to 5 times with de-ionized water. The final light yellowish colour product was dried at 120°C. Powder XRD on nano SnO2 Synthesized SnO2 nano powder was characterized using powder XRD using Cu-Kα radiation. The powder X-ray data set was collected in the 2θ range from 10° to 120° with step size 0.05° using XPERT PRO (Philips, Netherlands)

X-ray diffractometer with a monochromatic

incident beam of wavelength 1.54056 Å offering pure Cu-Kα stripping procedures. The Reitveld refinement is the standard tool which is devised by Hugo Reitveld [14] for use in the characterization of crystalline materials. In the present work, the cell parameters and other structural parameters were refined using the software, JANA 2006[15]. The micro absorption effects, surface roughness, zero shift, scale factor, temperature factors, occupancy of atoms, composition of atom, are examples of some other parameters that can be refined using JANA 2006 [15]. The cell and other structural parameters are given in table 1. Table 1: Structural parameters of nano SnO2 Parameter a (Å) c (Å) Cell Volume(Å3) Density (gm/cc) Robs(%) wRobs(%) Rp(%) wRp(%) GOF

Value 4.7649 (0.0018) 3.1842 (0.0012) 72.269 (0.0548) 6.9239(0.4904) 0.98 1.20 4.01 5.12 0.34

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The fitted powder profile and the positions of Bragg peaks are shown in figure 1.

Fig. 1: Rietveld refined powder profile for nano SnO2 The refined structure factors are given in table 2 h 1 1 2 1 2 2 2 0 3 2 1 3 3 2 3 2 3 4

k 1 0 0 1 1 1 2 0 1 2 1 0 1 0 2 1 2 0

l 0 1 0 1 0 1 0 2 0 1 2 1 1 2 0 2 1 0

Fobs 89.68 73.79 60.82 18.62 12.08 75.82 78.38 80.45 60.60 5.66 63.87 73.15 6.40 51.12 4.20 6.93 49.80 58.15

Fcal σ(Fobs) 89.83 1.158 74.14 0.947 62.38 1.063 19.04 0.355 11.93 0.993 74.98 0.934 78.79 1.434 82.37 2.541 60.97 1.198 5.68 0.108 64.93 0.924 74.50 1.050 6.49 0.240 50.60 1.366 4.16 0.112 7.17 0.422 49.71 0.914 58.60 1.080

h 2 4 3 3 4 4 3 1 3 1 4 4 4 2 4 5 3 5 4 2

k 2 1 3 1 1 2 3 0 2 1 2 0 3 1 1 1 3 0 3 2

l 2 0 0 2 1 0 1 3 2 3 1 2 0 3 2 0 2 1 1 3

Fobs 60.94 9.84 61.11 49.71 51.53 50.05 2.70 46.16 2.92 7.73 5.24 49.18 4.40 49.46 7.70 49.23 51.89 37.97 47.34 3.16

Fcal σ(Fobs) 61.57 1.275 9.93 0.204 61.38 1.167 50.13 0.708 51.94 0.728 50.28 0.746 2.68 0.044 47.69 1.167 3.05 0.097 8.19 0.285 5.63 0.204 49.35 0.714 4.41 0.063 49.57 0.701 7.74 0.101 49.48 0.644 51.91 0.656 37.95 0.484 47.32 0.603 3.16 0.042

Table 2: Structure factors for nano SnO2 obtained from Reitveld refinement

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Electronic charge distribution of nano SnO2 The MEM is an exact tool to study the electron density distribution because of its resolution. The bonding nature and the distribution of electrons in the bonding region can be clearly visualized using this technique [13]. For the numerical MEM computations on SnO2, the software package PRIMA [16, 17] was used. For the 2D and 3D representation of the electron densities, the program VESTA package was used [18]. The MEM refinements were carried out by dividing the unit cell into 48 × 48 × 32 pixels. The initial electron density at each pixel is fixed uniformly as F000/a03=1.8265 e/Å3, where F000 is the total number of electrons in the unit cell and a0 is the cell parameter. The MEM parameters are given in Table 3. Parameters SnO2 Number of cycles 2079 Number of electrons in the unit 132 cell Number of pixels in the unit cell 73728 ( 48 × 48 × 32) Lagrange parameter (λ) 0.001757 RMEM (%) 1.51 1.70 wRMEM (%) Table 3: Parameters from MEM refinement Figure 2 shows the 3D electron density distribution of SnO2.

The interaction of Sn atom

and O atom is clearly visible in nano SnO2.

Fig. 2: Three dimensional electron density of nano SnO2 The 2D electron densities are shown in figure 3 for SnO2 on the (001) plane. The interaction of Sn and the two oxygen atoms are visible in SnO2. Slight residual electron clouds are also visible in SnO2.


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Fig. 3: Two dimensional electron density of SnO2 on (001) plane

Figure 4 shows the two dimensional density distribution of SnO2 on the (110) plane. The bonding in SnO2 is seen to be covalent by the elongation of electron clouds from Sn atom (on both sides) towards the oxygen atoms




Fig. 4: Two dimensional electron density of SnO2 on (110) plane

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Characterization of Technological Materials

O

Sn

. Fig. 5: One-dimensional profile on SnO2

Direction

SnO2 Position(Å)

Electron density(e/Å3) [001] 1.5841 0.6319 [110] 1.1174 1.1474 [111] 2.1722 0.2193 Table 4: Mid bond electron densities of SnO2 In order to have a more clear understanding of the electron densities, one dimensional electron density profiles were drawn along [001], [110] and [111] directions along the unit cell for SnO2 and are shown in figure 5. The bonding nature can be determined from the strength of the electron density at the mid-bond position. In SnO2, the mid-bond density is high, so that the bonding can be declared as highly covalent though there is no non-nuclear maximum. Pair Distribution Function Refinement The local structure of SnO2 has been analyzed in terms of the bond lengths, concentration of atoms at a particular distance etc, using the software package PDFFIT[19] using which the Pair Distribution Function can be refined leading to various structural parameters like thermal vibration of atoms, bond lengths, occupancy, atomic concentration etc. The determination of average structure based on powder diffraction data is routinely done using Rietveld method [14] method, which is very similar to the full profile refinement of the atomic Pair Distribution Function. The analysis of Bragg scattering assumes a perfect long-range order of the crystal. The Rietveld [14] analysis is based on this assumption. But, many materials are quite disordered, particularly nowadays variety of new materials is not highly ordered. The deviation from the average structure should be studied in order to understand the physical properties

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of the disordered materials which are not so uncommon during these days. The observed Pair Distribution Function can be obtained from a software program PDFgetX [20]. The observed raw powder intensities of SnO2 material is used for getting the observed PDF’S using the above software package Multiple scattering corrections. The local structures of SnO2 are studied using pair distribution function analysis. Figure 6 gives the fitted pair distribution function of SnO2.This figure shows almost clear matching of the observed and calculated pair distribution function. Table 5 shows the nearest neighbor distances in SnO2 obtained using PDF analysis. Distance Distance % Difference Distance calculated* calculated ( (r2 - r1) / r1) × observed r1(Å) r3(Å) from PDF 100 r2(Å) from PDF 1.83 Sn – Sn 3.64 3.77 3.82 1.96 (or) O - O 6.66 6.53 6.64 1.83 7.68 7.54 0.37 8.40 8.43 8.45 2.71 11.62 11.31 11.58 7.32 Sn – O 2.14 2.31 2.34 5.64 2.27 5.68 5.81 10.14 0.79 10.16 10.24 13.70 0.66 13.70 13.79 Table 5:Bond lengths from PDF analysis together with calculations [21].

Bond length

Fig. 6: Fitted pair distribution of SnO2 Particle size The size of the SnO2 nano particles is analyzed using XRD and SEM (Scanning Electron Microscopy). SEM analysis was carried out under different magnifications one of which is shown in figure 7. From this figure, the average size of the particles can be compared with the size calculated from XRD. The particle size can also be evaluated using the Full width at half maximum of the powder XRD peaks. The size of the nano SnO2 is analyzed using XRD and their values are

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Characterization of Technological Materials

given in the table 6. It is interpreted that there are approximately 7 domains in a single particle of SnO2.


System

Particle size Grain Size** (SEM) (XRD) SnO2 134 nm 20.4993 nm Table 6: The average particle size from SEM and XRD [22].

Fig. 7: SEM picture of Nano SnO2 with a Magnification of 33,000 Acknowledgements The authors wish to thank the authorities of Madura College for the basic infrastructure provided for active research in the Dept. of Physics. This work was not supported by any funding agencies. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

O. K. Varghese, L.K.Malhotra: Sens. Actuators B:Chemical: 53 (1998), 19. N. L. Wu, L. F. Wu, Y. C. Yang, S. J. Huang: Mater. Res.11 (1996) 813. S. Ferrere, A. Zaban, B. A.Gsegg: J. Phys. Chem, B 53 (1997) 19. J. Zhu, Z. H. Lu, S. T. Aruna, D. Aurbach, A .Gedanken: Chem.Mater.12 (2000) 2557. Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka: Science 276 (1997), 1395. J. O. Besenhard, J. Yang, M. Winter: J. Power Sources 90 (2000), 70. I. A. Courtney, J. R. Dahn: J. Electrchem. Soc. 144 (1997), 2045. N. Li. C. R. Martin: J. Electrchem. Soc. 14 (2001), A164. J. Fan, T. Wang, C. Yu, B. Tu, Z. Jiang, D. Zhao: Adv. Mater. 16 (2004), 1432. S. Han, B. Jang, T. Kim, S. M. Oh, T. Hyeon: Adv. Funct.Mater. 15 (2005), 145.

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[11] [12] [13] [14] [15] [16]

[17]

[19]

[20] [21] [22]

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Min-Sik Park, Guo-Xiu Wang, Yang-Mook Kang, David Wexler, Shi-Xue Dou, and Hua-Kun Liu: Angew. Chem. Int, Ed 46 (2007), p. 750-753. N. S. Baik, G. Sakai, N. Miura, and N. Yamozoe: Sensors and Actuators B, vol.63 (2000), pp.74-79. D.M. Collins: Nature 49 (1982) 298 H.M. Rietveld: J. Appl. Crystallogr. 2 (1969) 65 V. Petrˇı_cˇek, M. Dusˇek, L. Palatinus, in: JANA2000, The Crystallographic Computing System, Institute of Physics, Academy of Sciences of the Czech Republic, Praha, 2000. A. D. Ruben, I. Fujio: Super-fast Program PRIMA for the Maximum-Entropy Method, Advanced materials Laboratory, National Institute for Materials Science, Ibaraki, Japan (2004), p. 305 0044. F. Izumi, R.A. Dilanian: Recent Research Developments in Physics, Part II, 3, Transworld, Research Network, Trivandrum, 2002, pp. 699–726. [18] K. Momma, F. Izumi: J. Appl. Crystallogr. 41 (2008) 653. C.L. Farrow, P. Juha´ s, J. W. Liu, D. Bryndin, E. S. Bozin, J. Bloch, Th. Proffen, S. J. L. Billinge, PDFfit2 and PDFgui: computer programs for studying nano structure in crystals, J. Phys. : Condens. Matter19 (2007) 335219. I.K. Jeong, J. Thompson, Th. Proffen, A. Perez, S.J.L. Billinge: PDFGetX, A program for obtaining the atomic pair distribution function from X-ray powder diffraction data (2001). Jean Laugier et Bernard Bochu: GRETEP, Domaine universitaire BP 46, 38402 Saint Martin d'Hères http:/www.inpg.fr/LMGP R.Saravanan: GRAIN Software available at http://www.saraxraygroup.net/

Materials Science Forum Vol. 671 (2011) pp 131-152 © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.671.131

Local structural analysis of Al2O3, Cr:Al2O3 and V:Al2O3 using Powder XRD T.K.Thirumalaisamy1,a, K.J.Lakshmi Sri2 and R.Saravanan2,b* 1

Department of Physics, H.K.R.H.college, Uthamapalayam - 625 533 PG Department and Research Centre of Physics, The Madura College, Madurai - 625 011. Tamil Nadu, India. a [email protected], [email protected] URL: http://www.saraxraygroup.net *Corresponding author: [email protected]

2

Abstract The electron density distribution and the local structure of aluminum oxide (Al2O3), chromium doped aluminum oxide (Cr:Al2O3) and vanadium doped aluminum oxide (V:Al2O3) have been studied. Powder X-ray data set of Al2O3 , Cr:Al2O3 and V:Al2O3 is analyzed in terms of cell parameters, thermal vibration parameters, 1D, 2D and 3 Dimensional electron density distributions. The bonding between the atoms using the maximum entropy method (MEM) and bond length distribution using pair distribution function (PDF) has been analyzed. The particle size of Al2O3 , Cr:Al2O3 and V:Al2O3 is also analyzed using XRD and SEM. Keywords: Electron density, Local structure, MEM, PDF, SEM Introduction Aluminum oxide (Al2O3), commonly referred to as alumina, possesses strong ionic interatomic bonding giving rise to its desirable material characteristics. Native alumina is found as the mineral corundum. It is also commonly referred to as alumina, sapphire, ruby or aloxite in the mining and ceramic. Alumina can exist in several crystalline phases, namely α, η, χ, γ, δ and θ alumina. Each has its own crystal structure and unique properties. Alpha phase alumina is the strongest and stiffest of the oxide ceramics. Its hardness, excellent dielectric properties, refractoriness and good thermal properties make the material of choice, for a wide range of applications. Aluminum oxide is an electrical insulator but has a relatively high thermal conductivity (30 Wm−1K−1). Additionally, it is extremely resistant to wear and corrosion. Addition of chromium oxide or manganese oxide improves its hardness and results in the change of color. Metallic aluminum is very reactive with atmospheric oxygen, and a thin passivation layer of alumina (4 nm thickness) forms in about 100 picoseconds on any exposed aluminum surface [1]. This layer protects the metal from further oxidation. The thickness and properties of this oxide layer can be enhanced using a process called anodizing. A number of alloys, such as aluminum bronzes, exploit this property by including a proportion of aluminum in the alloy to enhance corrosion resistance. The alumina generated by anodizing is typically amorphous, but discharge assisted

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oxidation processes such as plasma electrolytic oxidation result in a significant proportion of crystalline alumina in the coating, enhancing its hardness. Aluminum oxide was taken off the United States Environmental Protection Agency's chemicals lists in 1988. Aluminum oxide is widely used in the fabrication of superconducting devices, particularly single electron transistors and superconducting quantum interference devices (SQUID), where it is used to form highly resistive quantum tunneling barriers. Aluminum oxide (Al2O3), is a medium refractive index [2], low absorption material usable for coatings in the near-UV ( 1

Unfavourable

RL = 1

Linear

0 > RL > 1

Favourable

RL > 0

Irreversible

In the present study, the computed values of RL (Table 3) are found to be fraction in the range of 0-1,MG = 0.056- 0.007) indicating that the adsorption process of MG dye by CAC is favourable. Hence, CAC could be used as an adsorbent for the removal of MG dye. Kinetics of Adsorption The kinetics of adsorption of dyes by CAC has been studied by testing the applicability of the first order kinetic equations proposed by Natarajan – Khalaf [35], Lagergren – as cited by Pandey et al[36]. and Bhattacharya and Venkobachar[37]. Natarajan and Khalaf equation :

k =(2.303/t) log (Ci / Ct)

(7)

Lagergren equation

log (qe - qt) = log (qe) - (k/2.303)t

(8)

:

Bhattacharya and Venkobachar equation: log [ 1- U(t)] Where, U(t)

=

= - (k/2.303)t

(9)

[(Ci - Ct) / (Ci – Ce)] ; Ci, Ct and Ce are the concentrations of dye (in

mgL-1) at time zero, t and at equilibrium time (MG=35min); qe and qt are the amount adsorbed per unit mass of adsorbent (in mgg-1) at equilibrium contact time and at time t, respectively, and k is the first order rate constant for adsorption of dyes on CAC. The values of first order rate constant (in min.-1) are given in Table.5. All the linear correlations are found to be statistically significant at 95% confidence level as evidenced by r-vales close to unity. The results (Tables 6, 7) indicate the first order nature of adsorption process and applicability of these first order kinetic equations. The k values calculated from Bhattacharya and Venkobachar equation are noted to be close to that of the k

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Characterization of Technological Materials

values computed from Lagergren equation, for any given dye – CAC system. This concludes that, in future any one of these two kinetic equations can be employed to calculate the k values in adsorption process of MG dye. The first order rate constant of adsorption is found to be high in MG (0.199 min-1) for CAC. The adsorbate / dye species are most probably transported from the bulk of the solution to the solid phase through intra-particle diffusion / transport process, which is often the rate limiting step in many adsorption processes, especially in a rapidly stirred batch reactor [38]. The possibility of the presence of intra-particle diffusion as the rate limiting step was explored by using the intraparticle diffusion model[39] as suggested by Weber and Morris by the following equation : qt = kpt1/ 2 + C

(10)

where qt = amount adsorbed in time t, C = intercept and kp = intra-particle diffusion rate constant (in mgg-1 min1/2). The values of qt are found to be linearly correlated with values of t1/2. The computed kp values are also given in Table 10. The dye could easily penetrate into the internal pores of CAC as evidenced by the high values of kp. The values of intercept (C) given an idea about the boundary layer thickness i.e., the larger the intercept, greater is boundary layer effect [39] (C is greater in MG). The correlation of the values of log (% removal) and log (time) also resulted in linear relationships, as evidenced by r-values close to unity (MG = 0.990).G = 0.047- 0.278) indicate the presence of intra-particle diffusion as one of the rate limiting steps [39] in the adsorption of MG dye besides many other processes controlling the rate of adsorption, all of which may be operating simultaneously [39]. The results of the present study conclude that, CAC could be used as a standard adsorbent material in effluent treatment, especially for the removal of MG dye. The results will be highly useful in designing efficient effluent treatment plant to treat effluent from textile industry. Acknowledgement The authors thank the Management and Principal of The Madura College, Madurai for providing facilities and encouragement. The authors also gratefully acknowledge to HOD, and members of the chemistry department.

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References [1]

E.R. Krishnan, P.W. Utrecht, A.N. Patkar, J.S. Davis, S.G. Pour and M.E. Foerst, Recovery of Metals from Sludges and Wastewaters, Noyes Data, Park Ridge, NJ (1993).

[2]

W. Brooke-Devlin, Mercury and arsenic wastes removal, Recovery, Treatment and Disposal, US Environmental Protection Agency, Noyes Data, Park, Ridge, NJ (1992).

[3]

J.M. Grau and J.M. Bisang, J. Chem. Technol. Biotechnol. 62 (1995), p153.

[4]

D.E. Beck, W.D. Bostick, K. Bowser, D.H. Bunch, R.L. Fellows, P.E. Osborne and G.F. Sellers, Presented at the Ninth Symposium on Separation Science and Technology for Energy Applications Gatlinburg, TN (1995).

[5]

S.Adina Raducan, A.AleXandra olteanu, Mi haelapuiu, Dumitru oancea “ Influence of surfactants on the fading of malachite green”.central European journal of chemistry, 2008, volume 6. p.895.

[6]

Thomas Gessner & Udomayer “Triarylmethane & Diaryl methane Dyes” in Ullmann’ encyclopedia of industrial chemistry 2002, Wiley-VCH, Weinheim doi:10,1002/14356007a27-179

[7]

Protocal 2.18 leuco malachite green presumptive test for blood National Forensic science technology center, July 8, 2010, Retrieved on July 8,2010.

[8]

K.K. Mitsui Cyanamid, Jpn. Patent Appl. 193,846 (1985).

[9]

Y. Baba, K. Inoue, K. Yoshizuka, Y. Ritzu, T. Masato, Jpn. Kokai, Koyyo, Koho, JP 04,164,817 (1992).

[10]

M. Grayson, Kirk–Othmer Encylopedia of Chemical Technology, vol. 15, third ed., New York, 1981, p. 143.

[11]

A. Junker-Buchheit and J. Witzenbacher, J. Chromatogr. A 737 (1996), p. 67.

[12]

K. Pyrzynska and M. Trozanowicz, Crit. Rev. Anal. Chem. 29 (1999), p. 313.

[13]

J. Mary Gladis, C.R. Preetha and T. Prasada Rao, Met. News 20 (2002), p. 14.

[14]

T. Prasada Rao and C.R. Preetha, Sep. Purif. Rev. 32 (2003), p. 1.

[15]

A. Alexandrova and S. Arpadjan, Analyst 118 (1993), p. 1309.

[16]

H.A.M.Elmahadi and G.M.Greenway, J.Anal. At. Spectrom. 8 (1993), p. 1011.

[17]

G.J. Ramelow, L. Liu, C. Himel, D. Fralick, Y. Zhao and C. Tong, Int. J. Environ. Anal. Chem. 53 (1993), p. 219.

[18]

H. Emteborg, D.C. Baxter, M. Sharp and W. Frech, Analyst 120 (1995),

[19]

A. Alexandrova and S. Arpodjan, Anal. Chim. Acta 307 (1995), p. 71.

[20]

S. Peraniemi and M. Ahlgren, Anal. Chim. Acta 302 (1995), p. 89.

[21]

R. Shah and S. Devi, React. Funct. Polym. 31 (1996), p. 1.

[22]

S. Arpodjan, L. Vuchkova and E. Kostadinova, Analyst 122 (1997), p. 243.

[23]

M.L. Wang, G.Q. Huange, S.H. Qian, J.S. Jiang, Y.T. Van and Y.K. Chan, Fresnius J. Anal. Chem. 358 (1997), p. 856.

[24]

P.C. Rudner, J.M.C. Pavon, F.S. Rojas and A.G. Deforres, J. Anal. At. Spectrom. 13 (1998), p. 1167.

p. 69.

186

Characterization of Technological Materials

[25]

Wendy C.Anderson,sherri B Tumpseed & Jose E.Roybal “Quantitative and confirmatory Analysis of MG & Leuco MG residues in fish & shrimp “ J. Agric. Food chem. 2006,volume 54,pp 4517-4523

[26]

Chinese fish crisis shows seafood safety challenges USA today 7/1/2007

[27]

Veterinary Residues committee Annual report on surveillance for veterinary residues in food in the UK for 2001,2002 &2003.

[28]

J.L. Manzoori, M.H. Sorouraddin and A.M.H. Shabani, J. Anal. At. Spectrom. 13 (1998), p. 305.

[29]

.E. Mahmoud, M.M. Osman and M.E. Amar, Anal. Chim. Acta 415 (2000), p. 33.

[30]

B.C. Monda, D. Das and A.K. Das, Anal. Chim. Acta 450 (2001), p. 223.

[31]

M. Shamsipur, A. Ghiagvand and A. Sharghi, Int. J. Environ. Anal. Chem. 82 (2002), p. 23.

[32]

M. Bouyanne, J. Svie and I.A. Voinovitch, Analysis 7 (1979), p. 62.

[33]

H. Koshima and H. Onishi, Talanta 27 (1980), p. 795.

[34]

S. Nagatsuka and Y. Taniazaki, Radioisotopes 27 (1978), p. 379.

[35]

M.A.H. Hafez, I.M.M. Kanawaz, M.A. Akl and R.R. Lashein, Talanta 53 (2001), p. 749.

[36]

N. Malcick, O. Oktar, M.E. Ozcer, P. Cagler, L. Bushloy, A. Vaughan, B. Kuswadi and R. Narayanaswamy, Sens. Actuators B 53 (1998), p. 211.

[37]

V. Ramakrishna, G. Aravamudan and M. Vijayakumar, Anal. Chim. Acta 84 (1976), p. 369.

[38]

A.I. Vogel, A Textbook of Quantitative Inorganic Analysis, The ELBS & Longmans Green & Co. Limited, 1962.

[39]

R. Mathew, A.M. Starvin and T. Prasada Rao, Ind. J. Chem. 43 A (2004), p. 569.

Materials Science Forum Vol. 671 (2011) pp 187-204 © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.671.187

Dynamic Study of Adsorption for the Removal of Bismark Brown – Using Activated Carbons A.Xavier1,a, R. Sathya1,b, J. GandhiRajan2, R.Nagarathnam1 1

Post Graduate and Research Department of Chemistry, The Madura College (Autonomous), Madurai-625 011, Tamil Nadu, India 2 Post Graduate Department of Chemistry, Vivekananda College (Autonomous), Chennai, Tamil Nadu, India. a

[email protected]; [email protected]

Keywords: Removal of BB; adsorption and isotherms; kinetic equations; thermodynamics parameters, SEM studies.

Abstract Many industries use dyes and pigments to colorize their products. Large amount different types of dyes enter in to the environment. These dyes are invariably left in the industrial wastes. As a part of removal of Bismark Brown dye from textile and leather industrial wastes, using activated carbon as adsorbents namely, commercial activated carbon (CAC), rose apple carbon (RAC), coconut shell carbon (CSC) and saw dust carbon (SDC). The percentage removal of Bismark-Brown adsorbed increases with decrease in initial concentration and particle size of adsorbent and increased with increase in contact time, temperature and dose of adsorbent. The pH is highly sensitive for dye adsorption process. The adsorption process followed first order kinetics and the adsorption data with Freundlich and Langmuir isotherm models. The first kinetic equations like Natarajan Khalaf, Lagergren, Bhattacharya and Venkobhachar and intra-particle diffusion were found to be applicable. A comparative account of the adsorption capacity of various carbons has been made. These activated carbons are alternative to commercial AC for the removal dyes in General and Bismark-brown (BB) is particular. These results are reported highly efficient and effective and low cost adsorbent for the BB. The thermodynamics parameters are also studied and it obeys spontaneous process. The results are confirmed by before and after adsorption process with the help of the following instrumental techniques viz., FT-IR, UV-Visible Spectrophotometer and SEM analyze. Introduction Dyeing and finishing are some of the important processes in textile, paper making and leather manufacturing process. The use of dyes and pigments to calories their products. The effluents from dye stuff manufacturing and certain other industries like textile, paper and pulp, dyeing, printing and leather manufacturing contain small amount of dyes [1] discharged in to the environment which are poisonous and carcinogenic and causes toxicity for aquatic micro-organisms

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Characterization of Technological Materials

[2]. Dyes also can prevent from photosynthesis in aqueous system by absorbing sunlight. These effluents exceed the tolerance limit prescribed by ISI or BIS [3] (Limit : 1ppm). They impart colour to the water and thereby lowering its aesthetic value. Dye stuff and pigments, except a few have been found to be very harmful to the aquatic life and to the human beings [4]. Therefore, it is highly essential to remove the dyes from water and wastewater. Among the various treatment methods [5], the conventional treatment methods are reported not to be suitable for the removal of dyes and colour but adsorption is found to be effective. Dyes are not always removed through conventional physicochemical and biological treatment methods. This led to the study of other effective methods [6-7]. The adsorption process is one of the efficient methods to remove dyes from effluent [8]. Activated carbon is the most widely used adsorbent for this purpose because of its extended surface area, micro-porous structure, high adsorption capacity and high degree of surface reactivity. However commercially available activated carbons are very expensive [9-10]. This has led to search for cheaper substituent. The literature contains several reports, on the removal of dyes / colour from textile effluent and the use of various adsorbents for the treatment of effluents containing dyes / colour. Mention may be made on the use of chitosan for the removal of methylene blue [11], Bismarkbrown direct dyes [12], chitin for the removal of acidic and direct dyes [13], various low-cost adsorbents like vermiculite, saw dust, barbaceue charcoal, maize stalks, sand, rice hulls and pea nut mass for the removal of colour from textile effluents [14], activated carbon (AC) for the removal of colour from textiles effluents [15], granular AC for the removal of ultra zone blue [16], diatomaceous earth, wood, peat, bagasse pith, Fuller’s earth, alumina and silica for the removal of dyes / colour [17], sulphonated coal and Genoderma lucidum for the removal of rhodamine – B [18], mixed adsorbent (fly ash and commercial AC) for the removal Bismarkbrown [19], fly ash for the removal of chrome dye [20] and colour [21]. Indigenously prepared activated carbons (ACs) like straw carbon, bamboo dust carbon, coconut shell carbon, ground nut shell carbon and rice husk carbon sugarcane bagasse [22] biomass [23],eucalyptus bark [24] for the removal of dyes like congo red [25] rhodamine-B [26] and Bismarkbrown [27]. Bismarkbrown is a diazo dye and it is widely used in dyeing of synthetic fibers, leather, wool and cotton. The azo dyes represent the largest and the most important group of dyes. They are characterized by the presence of one or more azo groups (-N=N-) which form bridges between two or more aromatic rings. So they can produce harmful health effects and it is essential to have a proper method to remove this dye from waste waters. Among

these

various adsorbents the commercial activated carbon (CAC) possesses the maximum adsorption capacity. CAC is thus used as a standard adsorbent in the treatment of water and wastewater.

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The present work is an attempt to study the suitability of CAC for the removal of dye (BB) by determining the effect of various process parameters on the extent of removal of dye and to model the adsorption data with various isotherms and first order kinetic equations. Materials and methods CAC, RAC, CSC and SDC were procured from BDH, India and activated by acid digestion with 2N H2SO4 solution for 2hrs. All the other chemicals used were of analytical grade reagents obtained from either SD fine chemicals or Fischer (India). Double distilled (D.D) water was used throughout the experiments. Dye viz., BB supplied by BDH, India, was used as such. Preparation of AC CAC, RAC, CSC and SDC were activated by digesting it with 2N H2SO4 solution for 2 hrs at 80º C and finally activated in an air-oven for 5 hrs at 120º C. CAC, RAC, CSC and SDC were sieved by using 90 micron sieve (Jayanth, India) and then stored in an air-tight wide mouth reagent bottle and used for adsorption studies. Experimental Procedure Adsorption experiments were carried out at room temperature (30 ± 1º C) under batch mode [28]. Stock solution of dye viz.,BB (1000 ppm) were prepared, suitably diluted and calorimetrically estimated by using Systronics Spectrophotometer (Model No. 105) at its λmax (BB = 468 nm) [29]. Exactly 50 mL of dye solution of known initial concentration (Ci range: BB = 45-60 ppm) was shaken for a required period of contact time (BB = 5-40 min.) with required dose of adsorbent (0.54 gL-1 of all ACs) of a fixed particle size 90 micron in a thermostatic orbit incubator shaker (Neolab, India) at 200 rpm after noting down the initial pH of the solution. The initial pH was varied to the required value (range: BB = 2.0-10) by adding 1M solution of HCl or NaOH. After equilibrium, the final concentration (Ce) of dye was also measured spectrophotometrically [30]. The value of percentage removal and amount adsorbed (q, in mgg-1) were calculated using the following relationships. Percentage removal

=

100(Ci – Ce) / Ci

(1)

Amount adsorbed (q) =

Vol (Ci – Ce) / m

(2)

where, Ci and Ce are initial and equilibrium (final) concentration of dye (in gL-1) respectively and ‘m’ is the mass of adsorbent used in ‘g’ in the volume taken.

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Characterization of Technological Materials

Results and discussion Effects of process parameters The results on the extent of removal (% removal and q) of dyes viz., BB under various experimental conditions are given in Table 1. The percentage removal of dye increases with the decrease in initial concentration and increase in contact time, and dose of adsorbent (ACs). The amount adsorbed (q) increases with increase in initial concentration and contact time, but decreases with increase in dose of adsorbents. Table: 1 Variation process parameters for the removal of Bismark brown S.No 1

2 3 4

Concentration Contact time Dose pH -1 (ppm.) (min.) (g L ) CAC= 60-87 35 2 7 RAC= 45-72 CSC= 42-69 SDC= 60-87 60 for CAC and SDC 45 for RAC 5 – 40 2 7 42 for CSC 60 for CAC and SDC 35 0.5-4.0 7 45 for RAC 42 for CSC 60 for CAC and SDC 35 2 2-10 45 for RAC 42 for CSC

This is an expected behaviour [31]. The percentage removal of dye at 35 min. of contact time is 94.16 for BB at optimum initial concentration (BB = 60 ppm) with a dose of 2gL-1 of CAC at solution pH. But, at 70 min. of contact time the maximum removal of dyes occurred (BB = 97.3%). Similar results have been reported in literature for the removal of dyes [32], organic acids [33] and metal ions. The optimum conditions are initial concentration of BB = 60 ppm and dose = 2gL-1 of CAC; contact time = 35 min. The values of percentage removal were found to be maximum at2gL-1 of CAC (BB = 97.3). The values of log (% removal) are found to be linearly correlated to log (dose) values with correlation coefficient which are almost unity (r values: BB = 0.999). This is in accordance with the fractional power term of the dose as given below: q = (dose)-n + C

(3)

Where, n = 0.725 and 0.210, respectively for the removal of BB by CAC and others ACs. This suggests that the adsorbed dye may either block the access to the internal pores of carbon or may cause particles to aggregate and thereby minimizing the availability of active sites for adsorption [34].

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Effects of concentration variation Concentration increases with decrease in rate of adsorption for the removal of BB, the relevant data are given in (Table 2.) The amount adsorbed exponentially increases while the percentage removal exponentially decreases (Fig. 1) with increase in initials concentration of BB. Table: 2 Effect of Initial Concentration on the percentage removal and the amount adsorbed of Bismark Brown (BB) on ACs (Contact Time: 35 min; Dose of adsorbent: 2gL-1; Temperature: 32±1ºC; pH =7) Con. ppm 60 63 66 69 72 75 78 81 84 87

CAC Con % Amt. ppm Rem. Ads. 94.16 28.25 45 92.06 29.00 48 90.15 29.75 51 89.13 30.75 54 87.50 31.50 57 85.33 32.00 60 84.61 33.00 63 84.56 34.25 66 82.74 34.75 69 81.03 35.25 72

RAC Con. % Amt. ppm Rem. Ads. 83.33 18.75 42 81.25 19.50 45 78.43 20.00 48 76.85 20.75 51 74.56 21.25 54 72.50 21.75 67 69.80 22.00 60 67.42 22.25 63 65.94 22.75 66 65.27 23.50 69

CSC Con. % Amt. ppm Rem. Ads. 91.66 19.25 60 88.90 20.00 63 84.40 20.25 66 82.40 21.00 69 79.60 21.50 72 78.10 22.25 75 75.80 22.75 78 73.80 23.25 81 72.70 24.00 84 72.50 25.00 87

SDC % Amt. Rem. Ads. 91.66 27.50 89.68 28.25 87.87 29.00 86.95 30.00 84.72 30.50 84.00 31.50 83.97 32.75 82.09 33.25 80.04 33.75 79.30 34.50

Fig.1 Concentration Variation for Bismark Brown Dye 100 95

% removal

90 85 80 75 CAC RAC CSC SDC

70 65 60 30

40

50

60

70

80

90

Concentration (ppm)

Which indicates that the reduction in solute adsorption due to lack of available active sites on the adsorbent surface. Adsorption isotherm models namely Freundlich (Table: 3, Fig. 2).

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Characterization of Technological Materials

Table: 3 Freundlich Adsorption isotherm for the removal of BB Con . ppm

CAC

Con. ppm

Log Ce

log x/m

60 63 66 69 72 75 78 81 84 87

.5441 .6989 .8129 .8751 .9542 .0414 .0792 1.0969 .1614 .2175

1.4510 1.4624 1.4735 1.4878 1.4983 1.5051 1.5185 1.5346 1.5409 1.5472

45 48 51 54 57 60 63 66 69 72

RAC

Con. ppm

log Ce

log x/m

0.8751 0.9542 1.0414 1.0969 1.1614 1.2175 1.2788 1.3324 1.3711 1.3979

1.2730 1.2900 1.3010 1.3170 1.3274 1.3375 1.3424 1.3473 1.3569 1.3711

42 45 48 51 54 57 60 63 66 69

CSC

Con. ppm

log Ce

log x/m

0.5440 0.6989 0.8751 0.9542 1.0414 1.0969 1.1614 1.2175 1.2553 1.2788

1.2844 1.3010 1.3064 1.3222 1.3324 1.3473 1.3569 1.3664 1.3802 1.3979

SDC

60 63 66 69 72 75 78 81 84 87

log Ce

log x/m

1.4393 1.4510 1.4624 1.4771 1.4843 1.4983 1.5152 1.5218 1.5283 1.5378

1.0863 1.1003 1.1122 1.1254 1.1383 1.1507 1.1613 1.1625 1.1761 1.1832

Fig.2 Freundlich Adsorption Isotherm for BB 1.60

1.55

1.50

1.45

log x/m CAC RAC CSC SDC

1.40

1.35

1.30

1.25 0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

log Ce

Table: 4 Langmuir Adsorption isotherm for the removal of BB Con. ppm 60 63 66 69 72 75 78 81 84 87

CAC Ce Ce/qe 3.5 1.00 5.0 1.42 6.5 1.85 7.0 2.14 9.0 2.57 11.0 3.14 12.0 3.42 12.5 3.57 14.5 4.14 16.5 4.71

Con. ppm 45 48 51 54 57 60 63 66 69 72

RAC Ce Ce/qe 7.5 1.00 9.0 1.20 11.0 1.46 12.5 1.66 14.5 1.93 16.5 2.20 19.0 2.53 21.5 2.86 23.5 3.13 25.0 3.33

Con. ppm 42 45 48 51 54 57 60 63 66 69

CSC Ce Ce/qe 3.5 1.00 5.0 1.43 7.5 2.14 9.0 2.57 11.0 3.14 12.5 3.57 14.5 4.14 16.5 4.71 18.0 5.14 19.0 5.43

Con. ppm 60 63 66 69 72 75 78 81 84 87

SDC Ce Ce/qe 5.0 1.00 6.5 1.30 8.0 1.60 9.0 1.80 11.0 2.20 12.0 2.40 12.5 2.50 14.5 2.90 16.5 3.30 18.0 3.60

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Fig. 3 Langmuir Adsorption Isotherm for BB 6

CAC RAC CSC SDC

5

Ce /Qe

4

3

2

1

0 0

5

10

15

20

25

30

Ce

and Langmuir (Table: 4, Fig. 3) were tested which is highly significant and also compare the adsorption capacity of various adsorbents. Adsorption Isotherms Adsorption data are modeled with the help of Freundlich [35] and Langmuir [36] isotherms. The adsorption data are fitted with these isotherms (a) by plotting the values of log q versus log Ce and (Ce / qe) versus Ce and (b) by carrying out the correction analysis between the values of (i) log qe and log Ce and (ii) qe and Ce in (Table 3). Freundlich Isotherm : log q

= log K + (1/n) log Ce

(4)

Langmuir Isotherm

= (1/ab) + (Ce/a)

(5)

: (Ce/qe)

where, K and (1/n) are the measures of adsorption capacity and intensity of adsorption, respectively; qe, is the amount adsorbed per unit mass of adsorbent (in mgg-1); ‘a’ and ‘b’ are Langmuir constants, which are the measures of mono layer adsorption capacity ( in mgg-1) and surface energy (in Lmg-1), respectively. The results of correlation analysis along with the isotherm parameters are given in Table 3. The observed linear relationships are found to be statistically significant at 95% confidence level as evidenced from correlation coefficient (r-values) close to unity, which indicate the applicability of these two adsorption isotherms and mono layer coverage of dye species on the carbon surface. The high values of r indicate that Langmuir isotherm is better than Freundlich.

194

Characterization of Technological Materials

The monolayer adsorption capacity (Qo) of dye by CAC are noted to be very high (BB= 2 gm/lit). This indicates that the removal of dye by CAC is very effective and efficient. Further, the essential characteristics of the Langmuir isotherm can be described by a separation factor, RL, which is defined by the following equation [37] (Eq. 6): RL

=

1

(6)

(1 + bC i )

The separation factor, RL, indicate the shape of the isotherm and the nature of the adsorption process as given below: RL Value

Nature of the adsorption process

RL > 1

Unfavourable

RL = 1

Linear

0 > RL > 1

Favourable

RL > 0

Irreversible

In the present study, the computed values of RL (Table 3) are found to be fraction in the range of 01 (BB = 0.007) indicating that the adsorption process of dye by CAC and others ACs are favourable. Hence, CAC could be used as an adsorbent for the removal of dye. Effect of Contact time variation Adsorption process contact time plays vital role irrespective of other parameters in order to study the kinetics and the dynamics of BB by various adsorbents. The data present in (Table 5). The amount of adsorbed BB by adsorbent is rapid at the initial period and become slow, stagnates and then decreases probably due to desorption process with the increase in contact time. This indicate that the extent of removal of BB with time is higher in initial stages due to the greater availability of adequate surface area of the adsorbent, and with the increase in contact time, it is lesser owing to lesser availability of surface area. The relative increase in the extent of adsorption of BB is very low after 35min. which is the optimum contact time (Fig.4). Table: 5 Effect of contact time on the % removal and the amount adsorbed BB on ACs (Dose of adsorbent: 2g/L; Concentration: 45ppm for RAC, 42ppm for CSC; Temperature: 32ºC; 60ppm for CAC, SDC) % Amt. % Amt. % Amt. % Amt. Min. Rem. Ads. Rem. Ads. Rem. Ads. Rem. Ads. 5 85.00 25.50 67.70 15.25 65.50 13.80 75.83 22.75 10 86.66 26.00 72.22 16.25 70.23 14.80 79.16 23.75 15 87.50 26.30 73.33 16.50 73.80 15.50 81.66 24.50 20 89.16 26.80 75.55 17.00 78.50 16.50 83.33 25.00 25 91.66 27.50 80.00 18.00 82.14 17.30 85.00 25.50 30 92.50 27.80 82.22 18.50 88.09 18.50 87.50 26.25 35 94.16 28.30 83.33 18.75 91.66 19.30 91.66 27.50 40 95.00 28.50 85.55 19.25 95.20 20.00 94.16 28.25

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Fig. 4 Effect of Contact Time for the Removal of BB 100 CAC RAC CSC SDC

95

% Removal

90 85 80 75 70 65 60 0

10

20

30

40

50

Time (min.)

The following first order kinetic equations are used in our system. The data observed in (Table: 6, and Fig. 5) for Natarajan Khalaf equation.

Time Min.

Ct

5 10 15 20 25 30 35 40

9.0 8.0 7.5 6.5 5.0 4.5 3.5 3.0

Table: 6 Natarajan - Khalaf Equation CAC RAC CSC log Ct log Ct log C0/Ct C0/Ct C0/Ct 0.8239 14.5 0.4918 14.5 0.4618 0.8751 12.5 0.5563 12.5 0.5263 0.9031 12.0 0.5740 11.0 0.5819 0.9652 11.0 0.6118 9.0 0.6690 1.0792 9.0 0.6989 7.5 0.7482 1.1249 8.0 0.7501 5.0 0.9243 1.2341 7.5 0.7782 3.5 1.0792 1.3010 6.5 0.8403 2.0 1.3222

SDC log C0/Ct 14.5 0.6167 12.5 0.6812 11.0 0.7368 10.0 0.7782 9.0 0.8239 7.5 0.9031 5.0 1.0792 3.5 1.2341 Ct

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Characterization of Technological Materials

Fig. 5 Natarajan - Khalaf Equation 1.4 CAC RAC CSC SDC

Log Co/Ct

1.2

1.0

0.8

0.6

0.4 0

10

20

30

40

50

Time (min.)

The data observed in Table.7 (Fig.6) for Lagergran equations. CAC

Table: 7 Lagergren equation RAC CSC

SDC

Time 3+log Qe-Qt 3+log Qe-Qt 3+log Qe-Qt 3+log Qe-Qt min 5 0.7597 0.8129 0.8751 0.8893 10 0.7202 0.7404 0.8293 0.8293 15 0.6989 0.7202 0.7959 0.7782 20 0.6532 0.6767 0.7597 0.7404 25 0.5740 0.5740 0.6989 0.6989 30 0.5441 0.5119 0.5740 0.6284 35 0.4771 0.4771 0.4771 0.4771 Fig. 6 Lagergren Equation 1.0

CAC RAC CSC SDC

3+Log Qe-Qt

0.9

0.8

0.7

0.6

0.5

0.4 0

5

10

15

20

25

30

35

40

Time (min.)

The data observed in Table.8 (Fig.7) for Bhattacharya and Venkobachar equation.

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Table: 8 Bhatacharya Venkobachar Equation Time Min

5 10 15 20 25 30 35 40

U(t)= CiCt/CiCe 0.9026 0.9204 0.9292 0.9469 0.9735 0.9823 1.0000 1.0088

CAC Log3+1U(t)

0.4909 0.4885 0.4873 0.4847 0.4809 0.4797 0.4771 0.4721

U(t)= CiCt/CiCe 0.8133 0.8667 0.8800 0.9067 0.9600 0.9867 1.0000 1.0267

RAC Log3+1U(t)

0.5033 0.4960 0.4942 0.4904 0.4839 0.4790 0.4771 0.4621

U(t)= CiCt/CiCe 0.7662 0.8052 0.8312 0.8571 0.8961 0.9610 1.0000 1.0389

CSC Log3+1U(t)

0.5097 0.5044 0.5009 0.4973 0.4919 0.4827 0.4771 0.4601

U(t)= CiCt/CiCe 0.8273 0.8636 0.8909 0.9091 0.9273 0.9545 1.0000 1.0273

SDC Log3+1U(t)

0.5041 0.4964 0.4926 0.4901 0.4875 0.4837 0.4771 0.4631

Fig. 7 Bhatacharya Venkobachar Equation 0.52

0.51

CAC RAC CAC SDC

Log3+1-U(t)

0.50

0.49

0.48

0.47

0.46

0.45 0

10

20

30

40

50

Time (min.)

All the data were fitted with these kinetic equations which obey first order dependence of the adsorption process of BB on various adsorbents. The data observed in Table.9 (Fig.8) for intra particle diffusion model. Table: 9 Intra Particle Diffusion Model Time Min

√t

5 10 15 20 25 30 35 40

2.2360 3.162 3.872 4.472 5.000 5.4770 5.9160 6.3240

CAC Amount Adsorbed 25.50 26.00 26.30 26.80 27.50 27.80 28.30 28.50

RAC Amount Adsorbed 15.25 16.25 16.50 17.00 18.00 18.50 18.75 19.25

CSC Amount Adsorbed 13.80 14.80 15.50 16.50 17.30 18.50 19.30 20.00

SDC Amount Adsorbed 22.75 23.75 24.50 25.00 25.50 26.25 27.50 28.25

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Characterization of Technological Materials

Fig. 8 Intra Particle Diffusion Model 30 28

Amount Adsorbed

26 24 22 CAC RAC CSC SDC

20 18 16 14 12 2

3

4

5

6

7

t1/2

Intra particle diffusion process is one of the controlling the rate of adsorption process. Kinetics of Adsorption The kinetics of adsorption of dye by CAC, RAC, CSC and SDC have been studied by testing the applicability of the first order kinetic equations proposed by Natarajan – Khalaf [38], Lagergren – as cited by Pandey et al [39] and Bhattacharya and Venkobachar[40]. Natarajan and Khalaf equation:

k =

(2.303/t) log (Ci / Ct)

Lagergren equation: log (qe - qt) = log (qe) - (k/2.303)t

(7) (8)

Bhattacharya and Venkobachar equation: log [ 1- U(t)] =

- (k/2.303)t

(9)

where, U (t) = [(Ci - Ct) / (Ci – Ce)]; Ci, Ct and Ce are the concentrations of dye (in ppm) at time zero, ‘t’ and at equilibrium time (BB = 35 min); qe and qt are the amount adsorbed per unit mass of adsorbent (in mgg-1) at equilibrium contact time and at time t, respectively, and k is the first order rate constant for adsorption of dye on CAC. The values of first order rate constant (in min.-1) are given in Table 4. All the linear correlations are found to be statistically significant at 95% confidence level as evidenced by r-values close to unity. The results (Table 4) indicate the first order nature of adsorption process and applicability of these first order kinetic equations. The ‘k’ values calculated from Bhattacharya and Venkobachar equation are noted to be close to that of the k values computed from Lagergren equation, for any given dye – CAC system. This concludes that, in future any one of these two kinetic equations can be employed to calculate the k values in

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adsorption process of dye. The first order rate constant of adsorption is found to be high in BB (35 min.) for all ACs. The adsorbate / dye species are most probably transported from the bulk of the solution to the solid phase through intra-particle diffusion / transport process, which is often the rate limiting step in many adsorption processes, especially in a rapidly stirred batch reactor [41]. The possibility of the presence of intra-particle diffusion as the rate limiting step was explored by using the intraparticle diffusion model [42] as suggested by Weber and Morris by the following equation: qt = kpt1/ 2

(10)

where, qt = amount adsorbed in time t, C = intercept and kp = intra-particle diffusion rate constant (in mgg-1 min1/2). The values of qt are found to be linearly correlated with values of t1/2. The computed kp values are also given in (Table 4). The dye could easily penetrate into the internal pores of ACs as evidenced by the high values of kp. The values of intercept (C) given an idea about the boundary layer thickness i.e., the larger the intercept, greater is boundary layer effect (C is greater in BB). The correlation of the values of log (% removal) and log (time) also resulted in linear relationships, as evidenced by r-values close to unity (BB = 0.981). The divergence of the value of slope from 0.5

(slope : BB= 0.278) indicate the presence of intra-particle diffusion as one of the

rate limiting steps[42], in the adsorption of dye (BB) besides many other processes controlling the rate of adsorption, all of which may be operating simultaneously. The results of the present study conclude that, CAC could be used as a standard adsorbent material in effluent treatment, especially for the removal of dye (BB). The results will be highly useful in designing efficient effluent treatment plant to treat effluent from textile industry using various ACs. Effect of Dose of Adsorbent The percentage of removal of BB increases with increase dose of adsorbent. This is due to increase in availability of active sites on the surface area of the adsorbent. The data observed in Table.10 (Fig.9) for dose variation.

200

Characterization of Technological Materials

Table:10 Effect of Dose of adsorbent on the removal and the amount adsorbed dye by adsorption on ACs (Contact Time: 35 min; Concentration: 45 ppm for RAC; Temperature: 32ºC; 42 ppm for CSC,60 ppm for SDC; pH = 7; 60 ppm for CAC) Amount Adsorbed 0.025 0.050 0.075 0.100 0.125 0.150 0.175 0.200

CAC % Amt. Rem. Ads.

RAC % Amt. Rem. Ads.

CSC % Amt. Rem. Ads.

SDC % Amt. Rem. Ads.

89.16 91.66 92.50 94.16 95.00 96.66 -

75.55 77.77 80.00 83.33 88.88 90.00 92.22 95.55

82.14 88.09 89.28 91.66 96.43 -

85.00 87.50 89.16 91.66 94.16 96.66 -

26.80 27.50 27.80 28.30 28.50 29.00 -

17.00 17.25 18.00 18.75 20.00 20.25 20.75 21.50

17.30 18.50 18.80 19.30 20.30 -

25.50 26.25 26.80 27.50 28.25 29.00 -

Fig. 9 Effect of Dose of Adsorbent for the Removal of BB 100 CAC RAC CSC SDC

95

% Removal

90

85

80

75

70 0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

Amount of Dose (g/l)

The effect of initial pH on the extent of removal of dye (BB) by adsorption on CAC is found to be highly pH dependent (Table 11). As pH increases, the percentage removal increases, and reaches a maximum value for BB dye. The optimum pH for removal of dyes fixed as 7.0 for BB. Strongly acidic pH is found to be favourable for BB. The change in pH ( pH = initial pH – final pH) values are noted to be in the order of 0.3 – 0.5 pH units. This suggest that during the adsorption of dye species, protons are released from the surface functional groups present on the carbons (Surface area: CAC = 604 m2g-1). The surface functional groups like -C = O, -OH (Phenolic), are found to be present on the surface of adsorbent (CAC). As pH of the solution varies the charge on the surface functional group also varies. Dye nature and the surface functional groups account for the change in the extent of removal of dye. The data observed in Table.11 (Fig.10) for pH variation.

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Table: 11 Effect of pH variation on the percentage removal and the amount adsorbed dye Bismark brown by adsorption on ACs (Dose of adsorbent: 2gL-1; Concentration: 45ppm for RAC, 42ppm for CSC; Temperature: 32◦ C; pH = 7) pH

RAC %R Amount adsorbed 16.25 2 12.5 72.20 17.00 3 11.0 75.50 18.00 4 9.0 80.00 18.75 5 7.5 83.30 19.25 6 6.5 85.50 20.00 7 5.0 88.88 20.75 8 3.5 92.22 21.00 9 3.0 93.33 21.50 10 2.0 95.55 Ce

Ce

CSC %R

14.5 12.5 12.0 11.0 9.0 7.5 5.0 3.5 2.0

65.48 70.23 71.43 73.80 78.57 82.14 88.09 91.66 95.23

Amount adsorbed 13.75 14.75 15.00 15.50 16.50 17.25 18.50 19.25 20.00

Fig. 10 pH Variation of BB 100 95 RAC vs BB CSC vs BB

% removal

90 85 80 75 70 65 60 0

2

4

6

8

10

12

pH

Above pH 8.5, a colour change was noted and hence the removal of dyes was not studied at pH 7.0. These results suggest that CAC could be used as an adsorbent for the removal of dye (BB).

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Characterization of Technological Materials

SEM for BB: RAC

RAC-BB

Powdery nature of surface of an adsorbent and its porosity that will determine the particle size in microns roughly 90-100 microns. UV for BB: Bismark brown absorbs UV light maximum in the region of 468nm. ( λmax= 468nm) which is confirmed by Schimadzu UV 1700 series model. FT-IR for BB: In the spectrum of the dye N=N stretching frequency occurs at 1576 cm-1. C-N stretching frequency is observed at 1250 -1020 cm-1. The aromatic C-C stretching frequency is observed at 770-735cm-1. It indicates that the primary amine present n the aromatic ring system. RAC

RAC-BB

Materials Science Forum Vol. 671

SDC

203

SDC-BB

Acknowledgement The authors thank the Management and Principal of The Madura College, Madurai for providing facilities and encouragement and also thank Prof. Dr. R. Nagarathnam., HOD of Chemistry, The Madura College, Madurai, India for his timely help and encouragement. References [1] [2] [3] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

E.R. Krishnan, P.W. Utrecht, A.N. Patkar, J.S. Davis, S.G. Pour and M.E. Foerst, Recovery of Metals from Sludges and Wastewaters, Noyes Data, Park Ridge, NJ (1993). Y.C. Wong, Y.S. Szeto, W.H. Cheung and G. Mc Kay, Adsorption of acid dyes on chitosan –equilibrium isotherm analyses. Process Biochem., 39 (2004), 693-702. W. Brooke-Devlin, Mercury and arsenic wastes removal, Recovery, Treatment and Disposal, US Environmental Protection Agency, Noyes Data, Park, Ridge, NJ (1992). [4] J.M Grau and J.M. Bisang, J. Chem. Technol. Biotechnol. 62 (1995), p. 153. D.E. Beck, W.D. Bostick, K. Bowser, D.H. Bunch, R.L. Fellows, P.E. Osborne and G.F.Sellers, Presented at the Ninth Symposium on Separation Science and Technology for Energy Applications, Gatlinburg, TN (1995). Mc Kay G. Adsorption of dyestuffs from aqueous solutions with activated carbon, part-I, equilibrium and batch contact time studies. J Chem Technol Biotechnol (1982) 32: 759-772. Mc Kay G. Waste colour removal from textile effluents. Am Dyestuff Rep (1979) 68: 29-34. Nigarn P, Banat LM, Singh D, Marchant R. Microbial process for decolourisation of textile effluents containing azo, diazo and reactive dyes. Process Biochem (1996), 31: 435-442. Bhattacharya AK, Venkobatcher C. Removal of cadmium (11) by low cost adsorbents. J Environ Eng ASCE (1984), 110: 110-122. H.L.R. Yeh, L.H.R. Liu, H.M. Chiu, Y.T. Hung, Int.J.Environ. stud. 44 (1993), p.259 K.K. Mitsui Cyanamid, Jpn. Patent Appl. 193,846 (1985). Y.Baba, K. Inoue, K. Yoshizuka, Y. Ritzu, T. Masato, Jpn. Kokai, Koyyo, Koho, JP 04,164,817(1992). M. Grayson, Kirk–Othmer Encylopedia of Chemical Technology, vol. 15, third ed., New York,1981, p. 143. A. Junker-Buchheit and J. Witzenbacher, J. Chromatogr. A 737 (1996), p. 67.

204

Characterization of Technological Materials

[15] [16] [17] [18] [19] [20]

K. Pyrzynska and M. Trozanowicz, Crit. Rev. Anal. Chem. 29 (1999), p. 313. J. Mary Gladis, C.R. Preetha and T. Prasada Rao, Met. News 20 (2002), p. 14. T. Prasada Rao and C.R. Preetha, Sep. Purif. Rev. 32 (2003), p. 1. A. Alexandrova and S. Arpadjan, Analyst 118 (1993), p. 1309. H.A.M. Elmahadi and G.M. Greenway, J. Anal. At. Spectrom. 8 (1993), p. 1011. G.J. Ramelow, L. Liu, C. Himel, D. Fralick, Y. Zhao and C. Tong, Int. J. Environ. Anal. Chem. 53 (1993), p. 219. H. Emteborg, D.C. Baxter, M. Sharp and W. Frech, Analyst 120 (1995), p. 69. I. Abd-El-Thalouth, M.M. Kamel, K. Haggag, M.El-Zawahry, Am. Dyestuff Reporter 36, (July 1993) J.A. Laszlo, Am. Dyestuff Reporter 17 (August 1994) L.C. Morais, E.P.Goncalves, L.T. Vasconcelos, C.G. Gonzalez Beca, Environ. Technol.21 (2000) 577. A. Alexandrova and S. Arpodjan, Anal. Chim. Acta 307 (1995), p. 71. S. Peraniemi and M. Ahlgren, Anal. Chim. Acta 302 (1995), p. 89. R. Shah and S. Devi, React. Funct. Polym. 31 (1996), p. 1. S. Arpodjan, L. Vuchkova and E. Kostadinova, Analyst 122 (1997), p. 243. M.L. Wang, G.Q. Huange, S.H. Qian, J.S. Jiang, Y.T. Van and Y.K. Chan, Fresnius J. Anal. Chem. 358 (1997), p. 856. P.C. Rudner, J.M.C. Pavon, F.S. Rojas and A.G. Deforres, J. Anal. At. Spectrom. 13 (1998), p. 1167. J.L. Manzoori, M.H. Sorouraddin and A.M.H. Shabani, J. Anal. At. Spectrom. 13 (1998), p. 305. M.E. Mahmoud, M.M. Osman and M.E. Amar, Anal. Chim. Acta 415 (2000), p. 33. B.C. Monda, D. Das and A.K. Das, Anal. Chim. Acta 450 (2001), p. 223. M. Shamsipur, A. Ghiagvand and A. Sharghi, Int. J. Environ. Anal. Chem. 82 (2002), p. 23. M. Bouyanne, J. Svie and I.A. Voinovitch, Analysis 7 (1979), p. 62. H. Koshima and H. Onishi, Talanta 27 (1980), p. 795. S. Nagatsuka and Y. Taniazaki, Radioisotopes 27 (1978), p. 379. M.A.H. Hafez, I.M.M. Kanawaz, M.A. Akl and R.R. Lashein, Talanta 53 (2001), p. 749. N. Malcick, O. Oktar, M.E. Ozcer, P. Cagler, L. Bushloy, A. Vaughan, B. Kuswadi and R. Narayanaswamy, Sens. Actuators B 53 (1998), p. 211. T.V. Ramakrishna, G. Aravamudan and M. Vijayakumar, Anal. Chim. Acta 84 (1976), p. 369. A.I. Vogel, A Textbook of Quantitative Inorganic Analysis, The ELBS & Longmans Green & Co. Limited, 1962. R. Mathew, A.M. Starvin and T. Prasada Rao, Ind. J. Chem. 43 A (2004), p. 569.

[21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42]

Keywords Index 2D-Conduction

1

A Adsorption

165, 187

1

153

E Electrical Properties Electron Beam Evaporation Electron Density

21, 47 21, 47 121, 131

G Gallium Zinc Oxide

47

I Indium Zinc Oxide Isotherm

21 165, 187

K Kinetic Equation

165, 187

L Local Structure

131

M MEM Microwave-Assisted Solution Combustion Misfit-Layered Oxides

121, 131 69 1

N Nanocrystalline

69

O Optical Properties Optoelectronic Devices

PDF

131

R

D Doping

1

P

C Cobalt-Oxide Superconductor

Oxide Thermoelectrics

21, 47 153

Removal of BB Removal of MG Rietveld Method

187 165 121

S Scanning Electron Microscope (SEM) Seebeck Effect SEM Study Semiconductor Sensor SnO2 Spin-Seebeck Effect Structural Property

121, 131 1 165, 187 153 121 69 1 21, 47

T Temperature Gradient Thermal Conductivity (TC) Thermionic Effect Thermodynamics Parameters Thermoelectricity

153 153 1 165, 187 1

X X-Ray Diffraction (XRD)

121

Authors Index D Dawn Dharma Roy, S.

21

T

165, 187

U

G Gandhi Rajan, J.

H Hayashi, K. Huang, X.Y.

1 1

47, 69 121

K Kajitani, T. Koshibae, W.

1 1

L Lakshmi Sri, K.J.

131

M Malarvizhi, M. Miyazaki, Y.

165 1

N Nagarani, S. Nagarathnam, R. Nehru, L.C.

47 187 69

P Prema Rani, M.

153

S Sanjeeviraja, C. Saravanakumar, S. Saravanan, R. Sathya, R. Shyju, G.J. Swaminathan, V.

Usha, D.

131

165

X Xavier, A.

165, 187

Y

J Jayachandran, M. Jeya Priya, M.

Thirumalaisamy, T.K.

21, 47, 69 121 121, 131, 153 187 21 69

Yubuta, K.

1