Self-propagating high-temperature synthesis: textbook 9786010434691

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AL-FARABI KAZAKH NATIONAL UNIVERSITY and INSTITUTE OF COMBUSTION PROBLEMS

Z. A. Mansurov A. S. Mukasyan A. S. Rogachev

SELF-PROPAGATING HIGH-TEMPERATURE SYNTHESIS Textbook

Almaty «Qazaq University» 2018

UDC 544.1 (075.8) LBC 21.1 я 73 M 24

The Academic Council of Department Chemistry and Chemical Technology and RISO of al-Farabi Kazakh National University was recommended for publication (Protocol №7 dated 05.07.2018)

Reviewer Doctor of Chemical Sciences, Associate Professor N.N. Mofa We express great gratitude to Dr. S.M. Fomenko for discussion and thanks to Ph.D-student Makpal Seitzhanova for her assistance in preparation of this textbook for publication.

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Mansurov Z.A., Mukasyan A.S., Rogachev A.S. Self-propagating high-temperature synthesis: textbook / Z.A. Mansurov, A.S. Mukasyan, A.S. Rogachev. – Almaty: Qazaq University, 2018. – 164 p. ISBN 978-601-04-3469-1 The textbook is devoted to the problems of self-propagating high-temperature synthesis. This textbook can be useful to a wide range of professionals involved in nanotechnology as well as bachelors, masters and Ph.D students and doctors. Published in authorial release.

UDC 544.1 (075.8) LBC 21.1 я 73

ISBN 978-601-04-3469-1

© Mansurov Z.A., Mukasyan A.S., Rogachev A.S., 2018 © Al-Farabi KazNU, 2018

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ONTENTS

PREFACE ............................................................................................................................... 5 Chapter 1. SELF-PROPAGATING HIGH-TEMPERATURE SYNTHESIS ................... 7 1.1. General definitions ...................................................................................... 7 1.2. Fundamentals of SHS .................................................................................. 8 1.3. Structural macrokinetics of SHS processes ................................................. 10 Control questions ................................................................................................... 11 References ............................................................................................................. 11 Chapter 2.DIFFERENT PROCESSES OF SELF-PROPAGATING HIGH-TEMPERATURE SYNTHESIS ............................................................. 12 2.1. Gasless combustion synthesis ...................................................................... 12 2.2. Combustion synthesis in gas–solid systems (infiltration combustion)......... 15 2.3. Combustion synthesis with a reduction stage .............................................. 20 2.4. Solution combustion synthesis .................................................................... 22 2.5. Mechanical activation of initial powder mixtures for SHS .......................... 23 2.6. Example of SHS with preliminary mechanical treatment of the powders ... 25 Control questions ................................................................................................... 26 References ............................................................................................................. 26 Chapter 3. THERMODYNAMICS AND DRIVING FORCE OF SHS PROCESSES ..... 30 3.1. General principles ........................................................................................ 30 3.2. Equilibrium, reversibility, stationary and stability of the SHS processes and products ........................................................................ 36 3.3. The thermodynamics of the reaction cell ..................................................... 38 Control questions ................................................................................................... 41 References ............................................................................................................. 41 Chapter 4. KINETICS OF HETEROGENEOUS REACTIONS........................................ 42 4.1. Solid-state reactions..................................................................................... 44 4.2. Solid–gas reactions ...................................................................................... 49 4.3. Reactions with the liquid phase ................................................................... 57 4.4. Reactions with gasification of the initial solid phase reagent ...................... 58 4.5. Methods of high-temperature kinetics of heterogeneous reactions .............. 60 Control questions ................................................................................................... 64 References ............................................................................................................. 65 Chapter 5. STRUCTURE FORMATION IN HYBRID SOLID–GAS SYSTEMS ............ 68 5.1. Model of structure formation during combustion of metals with nitrogen........................................................................................................ 70 5.2. Models of structure formation during combustion of non-metals in nitrogen ................................................................................................... 73 Control questions ................................................................................................... 77 References ............................................................................................................. 77

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Chapter 6. SHS OF TiB2-Al2O3 and CrB2-Al2O3 CERAMICS ........................................... 78 6.1. Self-propagating high-temperature synthesis of boron containing ceramic materials: the state of art ................................................................ 78 6.2. Experimental ............................................................................................... 81 6.3. Results and discussions ............................................................................... 84 6.3.1. Layer-by-layer X-ray structural analysis of hardened samples. SHS-process was stopped by “Hardening” method followed by analysis of partially and completely burnt part of the charge in 0.75TiO2-0.25Ti-2B-Al system ............................................................... 84 6.3.2. SHS parameters ........................................................................................... 85 6.3.3. Analysis of the composition and morphology of the combustion products ....................................................................................................... 87 6.4. Conclusions ................................................................................................. 91 Control questions ................................................................................................... 91 References ............................................................................................................. 92 Chapter 7. SOLUTION COMBUSTION SYNTHESIS....................................................... 94 7.1. Synthesis gas production on glass cloth catalysts modified by Ni and Co oxides .............................................................................................. 94 7.2. Synthesis of catalysts ................................................................................... 95 7.2.1. Study of the catalytic activity ...................................................................... 96 7.2.2. Physicochemical examination of samples ................................................... 96 7.3. Characteristics and properties of the catalysts ............................................. 97 7.4. The catalytic activity of samples in the reaction of dry reforming of methane ................................................................................................... 101 Control questions ................................................................................................... 104 References ............................................................................................................. 104 Chapter 8. COMBUSTION SYNTHESIS OF SILICON AND SILICON CARBIDE NANOPOWDERS ............................................................................ 106 8.1. Combustion synthesis of silicon nanopowders ............................................ 106 8.2. Investigation of the silicon nanopowders .................................................... 108 8.3. Combustion synthesis of silicon carbide...................................................... 113 Control questions ................................................................................................... 117 References ............................................................................................................. 118 Chapter 9. SHS REFRACTORY MATERIALS “FURNON” AND THEIR PRACTICAL IMPLEMENTATIONS IN KAZAKHSTAN AND RUSSIA ... 119 9.1. SHS-refractories: “Furnon” ......................................................................... 119 9.2. New carbon-containing refractories............................................................. 123 Control questions ................................................................................................... 132 References ............................................................................................................. 132 Laboratory work 1. Measuring of the combustion wave velocity ............................................ 134 Laboratory work 2. Measuring of the maximum combustion temperature .............................. 140 Laboratory work 3. Combustion wave profile ......................................................................... 151 Laboratory work 4. SHS of nitrogen containing materials by using chromium oxide based concentrate ............................................................................................................ 157 Laboratory work 5. Synthesis of the boron-based composites by using SHS method .............. 160 Laboratory work 6. SHS synthesis of superconductors: magnesium diboride ........................ 162

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REFACE

Self-Propagating High-Temperature Synthesis,сombustion synthesis is an attracttive technique to synthesize a wide variety of advanced materials including powders and near-net shape products of ceramics, intermetallics, composites, and functionally graded materials. SHS-produced materials have found their application in different branches of modern science and technology, including mechanical engineering, aerospace engineering, electrical engineering, chemical industry, ferrous and nonferrous metallurgy and electronics. Self-propagating high-temperature synthesis is a subject of discussion on different international symposiums and meetings on combustion, as well as materials science forums, where different aspects in the field of SHS are regularly presented and discussed. It is worth noting that a major contribution to the investigation of SHS processes was made by A.G. Merzhanov and members of his scientific school, such as V.M. Shkiro, I.P. Borovinskaya, and Yu.M. Maksimov, as well as by the world recognized leaders in the SHS field, including Z. Munir, K.C. Patil, A. Varma, T.P. Weihs and other researchers. Theory of this process is based on the classical fundamental works of N.N. Semenov, Y.B. Zeldovich and B.I. Khaikin. This textbook is written on the base of special course delivered to PhD-students of Department of Chemical Physics and Materials. In the first chapter, we consider the fundamentals of self-propagating high-temperature synthesis and structural macrokinetics of SHS processes. Chapter 2 is devoted to the different routes of selfpropagating high-temperature synthesis, including gasless combustion synthesis, combustion synthesis in gas-solid systems and with reduction stage, solution combustion synthesis, mechanical activation of initial powder mixtures for SHS and the effects of mechanical treatment of the reaction mixtures on SHS. In the third chapter, we describe the thermodynamics and driving force of SHS processes, their general principles, equilibrium, reversibility, stationary and stability of the SHS processes and products, the thermodynamics of the reaction cell. Chapter 4 is dedicate to the kinetics of heterogeneous reactions, including solid-state and solid–gas reactions, reactions with the liquid phase, reactions with gasification of the initial solid phase reagent. The fifth chapter describes the structure formation in hybrid solid–gas systems, focusing on general models of structure formation during combustion of metals and non-metals in nitrogen. Chapter 6, describes the specifics of self-propagating high temperature synthesis and structure formation of boron containing, ceramics. The seventh chapter considers the solution combustion synthesis of catalysts. It is demonstrated that fine catalysts fabricated by SHS method can be effectively used for the solution of ecological 5  

problems in processes of CO and hydrocarbon combustion, as well as for the utilization of the components, which lead to the, so-called, “greenhouse effect”. Chapter eighth is dedicated to combustion synthesis of silicon nanopowders. In the ninth chapter we overview the SHS refractory materials from “Furnon” family and their practical implementations in Kazakhstan and Russia. At the end of this textbook, we also provide the laboratory works that are related to the different aspects of SHS process. Finally, for those who seek for the deeper understanding of the theory and mechanisms of SHS-processes, as well as other combustion-based technologies, we highly recommend reading additional literature cited in this book (cf. Chapter 1, Ref. 1-10).

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HAPTER

1

SELF-PROPAGATING HIGH-TEMPERATURE SYNTHESIS

1.1. General Definitions Self-propagating high-temperature synthesis (SHS) is a science-intensive process for fabrication of materials. It was established in the second half of XX century by group of Russian scientists leaded by A.G. Merzhanov [1]. The history of this discovery and development of its technological applications have been described in many books and reviews [2-6]. Comprehension of SHS requires fundamental knowledge in thermodynamics, chemical kinetics, general and structural macrokinetics, materials science, and other related fields. SHS involves processes in which the reactive mixture of the initial reagents (usually powders), when ignited, spontaneously transformed into the valuable solid products due to the exothermic self-sustained reactions. Several other terms – such as combustion synthesis, gasless combustion, self-propagating exothermic reaction – are used to describe the process. A well-known example of SHS reaction is the thermite reaction given below: Fe2O3 + Al → 2Fe + Al2O3 This reaction generates temperatures above the melting point of alumina and is used in the welding process for joining railway lines. Considering the SHS as a variety of heterogeneous combustion, three stages of the process can be outlined: 1) mixing the components typically at room temperature, when the chemical reaction does not yet take place; 2) the initiation of an exothermic chemical reaction (ignition or self-ignition); 3) a self-sustaining chemical reaction which occurs without external heat sources and leads to the formation of combustion products (chemical compounds, powders, materials or net shape articles important from the practical applications) [4, 7-9]. The initiation of a chemical reaction is usually carried out by heating the reaction mixture and there are two entirely opposed methods of heating. The first way is to heat the whole sample slowly so that temperature has time to equalize over the entirety of its volume. In this case the reaction should develop simultaneously and uniformly at all points of the sample and at a specific temperature one should observe a sharp self7

acceleration of the process – the whole sample evenly ‘self-ignites’. This SHS mode is called volume combustion or thermal explosion, it is illustrated in Fig. 1.1. SHS in the thermal explosion mode has much in common with reaction sintering in powder technology, but there is a fundamental difference. It consists in the fact that in reaction sintering it is necessary to avoid spontaneous heating of the sample by chemical reaction, whereas in SHS this warm-up is utilized. The theory of thermal explosion was developed in the works of the outstanding Russian scientist and Nobel Prize winner N.N. Semenov and his followers.

Fig. 1.1. Self-propagating high-temperature synthesis modes: volume combustion.

The second way of ignition is optimal rapid heating of only a small volume of the sample (e.g. ~1 mm3) that results in local reaction initiation of an exothermic reaction, which then self-propagates to the rest of the sample in the form of a combustion wave. This SHS mode is called the wave or auto-wave combustion mode. Fig. 1.2 shows schematically such a process. Fast heating is needed to ensure that the heat from the local area of ignition has no time to spread to nearby areas and create an uneven temperature distribution in the sample. The point is that the route of the reaction and thus the properties of the combustion products depend on the initial temperature of the medium. Consequently, if the initial temperature is not uniform, the products of combustion synthesis are varied in the volume of the sample. Typically, the nonuniformity of the product is undesirable, except in special cases where the purpose of synthesis is to provide materials with a gradient structure and properties.

Fig. 1.2. Self-propagating high-temperature synthesis modes: auto-wave combustion.

1.2. Fundamentals of SHS Theory has played an important role in combustion science. Early examples, include Chapman-Jouguet detonation theory; the Burke-Schumann fast-chemistry approxi-mation for diffusion flames (derivable in a limit process that called 8

Damkohler-number asymptotic); Frank-Kamenetskii's steady-state theory of spontaneous combustion (the origin of activation-energy asymptotic); Zeldovich's early contributions to deflagration theory (equivalent to use of activation-energy asymptotic for achieving spatial scale separation); and the Darrieus-Landau hydrodynamic limit for deflagrations (which could be termed Peclet-number asymptotic). It is no an accident that most of these theoriespresent anasymptotic approximations. Indeed, the combustion problems, like those of fluid mechanics, can seldom be linearized, and so analytical strategies require mathematical tools capable of dealing" with nonlinearities. Asymptotic is the only universal tool, requiring only a large parameter Asymptotic, whether matched asymptotic expansions (such as boundary-layer theory), Poincare-Lighthill strategies (as in perturbed orbital mechanics and sonicboom theory) or multiple-scale techniques (justifying Krylov-Bogoliubov averaging, WKB approximations, and adiabatic invariances), essentially emerged strongly after World War II (although with roots extending back to Laplace and Newton) and was vigorously developed by the fluid-mechanics community. It is difficult to pick up copies of the Journal of Fluid Mechanics from the 1950s, 1960s, and 1970s, and understand the theoretical work discussed there without at least a rough grasp of asymptotic. In combustion, the development of asymptotic was slower and for many years was restricted to the Russian school associated with names of Semenov, Zeldovich, and Frank-Kamenetskii. The achievements of this school are summarized by Zeldovich, Barenblatt, Librovich, and Makhviladze in a book [10], which in many ways is a compendium of that work. These contributions are characterized by rich physical discussion, and they challenge anyone who might feel that physical understanding and intuition are necessarily in conflict with formal mathematical strategies. It is a fact that simple mathematical models that incorporate a minimum of physics, when solved in a manner that makes transparent the physical interactions in various parts of the combustion field, and when the results are presented in a physical context, can be a source of physical insight superior to any other. It is difficult, for example, to see the specific role of radiation in the stabilization of flame balls [11] without an examination of the mathematical stability theory. In fact, a little thought along the lines of radiant-loss influences on flame speeds, without carefully considering Lewis-number effects, can easily lead to an apparently plausible, but incomplete and possibly misleading picture. However, the Russian school may have had one flaw, i.e.an apparent unwillingness, once the mathematical model was posed, to push analysis to the limits. Some hint of why this had happened can be found on page 369 of [10], which suggests that additional details may have little physical validity. But, in fact, there is no reason to believe that the omitted physics necessarily would undo the subtle details predicted by the physics that is retained. Thus, a legitimate strategy is to push the mathematics to the limit, but be prepared to adjust the model should the solutions be at variance with the experimental record or fully detailed numerical solutions. Flame balls provide one example of rich behavior generated by a simple model consistent with the experimental record: there is a lean flammability limit [11]; onedimensional stability only if heat losses by radiation, convection or conduction are present [12,13]; the disappearance of an interval of stable solutions as the Lewis number of the deficient reactant is increased from small values to unity [14]; threedimensional instabilities at mixture strengths well removed from the lean limit [15]; 9

repulsion of one flame ball by another to generate drift [16]; and stabilization by fluctuating velocity gradients of appropriate amplitudes and frequencies [17]. The theoretical papers in the Journal of Fluid Mechanics today look quite different from those of fifty years ago. These days, the applied mathematician wrestling with mechanics problems is far more likely to use scientific computation strategies than asymptotic. The same trend is now apparent in combustion (albeit this review contains counterexamples), naturally so since asymptotic has its limitations. In combustion, most asymptotic treatments are either one-dimensional or small perturbations thereof; exceptions include descriptions of the dynamics of combustion fronts for flames (such as the Michelson-Sivashinsky equation or the KuramotoSivashinsky equation) or more recently for detonations, in which multidimensional combustion problems are reduced to a partial differential equation or an integral differential equation for a single scalar, an equation that must be solved numerically, for the most part, but a numerical task that is much simpler than the unreduced problem. It must be emphasized that the trend towards computation is not simply an abandonment of analytical strategies for computational approaches of a kind long pursued in the past. Typically, the models are still incomplete, the algorithmic investment is comparatively small, and there is an applied-mathematician's sensibility (for good and bad) that permeates the endeavor. Recent monographs and review articles [18,19] summarize many of the main achievements in combustion theory over the past fifty years. This literature documents attainment of rather a high level of conceptual coherence. Combustion theory is, in fact, perhaps one of the most elegant areas of classical phenomenology, presenting a graphic example of the wide range of natural phenomena that can be deduced from a few fundamental principles [20]. 1.3. Structural macrokinetics of SHS processes In recent years strict requirements demanded by modern materials led to intensive studies of structures and mechanism of structure formation of the SHS products. This is not accidental since the structure of a materialto a great extent determines its properties, especially. It would not be overestimation to state that problem of controlling product structure is one of the most important tasks in further development of SHS. Here the structure means a wide range of characteristics including macrostructure (composition distribution, macroscopic defects), microstructure (arrangement of phases and crystals with respect to each other, grain size, porosity and pore structure, localization of impurities), and crystal structure (crystal lattice type and lattice parameters, presence of defects, ordering with formation of super lattices, amount and distribution of dislocations). The structure of combustion wave itself needs to be added to the above classification since distribution of temperature and concentration in the combustion and after-burning zones markedly affects the composition and structure of the synthesized materials. It is well recognized that structural macrokinetics (SMK) studies evolution of structure in the course of chemical transformation taking into account heat and mass transfer processes [7,8]. A place of SMK in the number of other fields of science can be presented by the following equations: 10

Classical macrokinetics = chemical kinetics + heat and mass transfer theory Structural macrokinetics = classical macrokinetics + kinetics of structural transformations Since the concept of ‘SHS’ covers many different processes, each of which has its own history, in the next Chapter we examine some of them using the classification well recognized by researchers, which is based on the physical state of the initial precursors. CONTROL QUESTIONS 1. 2. 3. 4. 5. 6.

What does the abbreviation “SHS” mean? How wave propagation mode of SHS differs from volume combustion mode? Who discovered phenomenon of SHS (“solid flame”)? Could you describe the structure formation of the SHS products? What study structural macrokinetics? Could you explain differences between volume combustion and auto-wave combustion? REFERENCES

1. Merzhanov, A.G. Borovinskaya, I.P. Self-spreading high-temperature synthesis of refractory compounds, Dokl. Chem., 1972, vol. 204, No. 2, pp. 429–431. 2. Merzhanov A.G. Self-propagating high-temperature synthesis: Twenty years of research and findings, in: Combustion and Plasma Synthesis of High-Temperature Materials, Munir Z., Holt, J.B., Eds., New York: VCH, 1990, pp. 1–53. 3. Varma A., Rogachev A.S., Mukasyan A.S., Hwang S., Combustion synthesis of advanced materials: Principles and applications, in: Advances in Chemical Engineering, Wei, J., Ed., New York: Academic Press, 1998, Vol. 24, pp. 79–226. 4. Merzhanov A.G., Mukasyan A.S., Solid-Flame Combustion, Moscow: Torus Press, 2007. 5. Merzhanov A.G., 40 years of SHS: Good Fortune of Scientific Discovery (story-presentation with elements of a scientific paper), Chernogolovka: Izd. ISMAN, 2007 (Russ). 6. Hlavacek V., Combustion synthesis: A historical perspective, Amer. Ceram. Soc. Bull., 1991, Vol. 70, No. 2, pp. 240–243. 7. A.S. Rogachev, A.S. Mukasyan. Combustion for Material Synthesis // 2015, CISP, 394 p. 8. A.S. Rogachev, A.S Mukasyan. Burning for the synthesis of materials. Introduction to Structural Macrokinetics // Moscow: Fizmatlit, 2013. – 400 p. (in Russian) 9. Concise Encyclopedia of Self-Propagating High-Temperature Synthesis, Elsevier, Amsterdam, Holland, 2017. 10. Y.B Zeldovich, G.I Barenblatt, V.B. Librovic, G.M. Makhviladze, The Mathematical Theory of Combustion and Explosion. Consultants Bureau, New York, 1985. 11. J. Buckmaster, G. Joulin, P. Ronney, Combust. Flame 79 (1990) 381. 12. J. Buckmaster, G. Joulin, /. Fluid Mech. 227 (1991) 407. 13. J. Buckmaster, G. Joulin, Combust. Sci. Technol. 89 (1993) 57. 14. C.J. Lee, J. Buckmaster, SIAM J. Appl. Math. 51 (1991) 1315. 15. J. Buckmaster, P. Ronney, Proc. Combust. Inst. 27 (1998) 2603. 16. G. Joulin, P. Cambray, N. Jaouen, Combust. Theory Modelling 6 (2002) 53. 17. J.D. Buckmaster, G.S.S. Ludford, Theory of Laminar Flames. Cambridge University Press, Cambridge, UK, 1982 18. G.I. Sivashinsky, Annu. Rev. Fluid Mech. 15 (1983) 179. 19. F.A. Williams, Combustion Theory, second ed. Addison-Wesley, Redwood City, CA, 1985. 20. Rao CNR: Combustion synthesis. In Chemical Approaches to the Synthesis of inorganic Materials. New Delhi: Wiley Eastern Limited; 1994:28-30.

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DIFFERENT PROCESSES OF SELF-PROPAGATING HIGH-TEMPERATURE SYNTHESIS

Accounting the state of the precursor one may suggest different types of SHS systems. Let us briefly overview the main ones, which include gasless combustion synthesis (CS), CS in gas-solid media, solution CS, as well as SHS with mechanical activation. 2.1. Gasless combustion synthesis Gasless combustion was the first type of the SHS which was used for fabrication of different compounds [1]. This is understandable, since, as noted above, the discovery of the SHS was made when searching for combustion systems that can burn without a gas flame. At that time, it was shown that a mixture of two or more powders of refractory elements could be an ‘active’ medium in which the front of the reaction may self-propagate with formation, for example, of titanium carbide: Ti + C = TiC + 310 kJ

(2.1)

The heat released in the process is so high that the temperature of solid and molten products may reach up to 2500–3500 K, so that the reaction does not depend on any external sources of heat and may spread like a combustion wave, which selfproduces energy for its propagation. It is interesting that in many systems, despite the high temperature, the transition of any of the components of the mixture to the gas phase is extremely small and thus can be neglected. In the example mentioned above, i.e. during combustion of the Ti + C mixture, the total vapor pressure at a temperature of 3300 K is only 0.0058 atm with the main contribution through evaporation of titanium. Also note that gases may be emitted not only by evaporation of the main reagents, but also due to gasification of impurities. For example, powders of transition metals (Ti, Zr, Hf) usually contain an appreciable amount of adsorbed hydrogen (2–3 wt. %) and also the fractions of a percent of CO, N2, which transform to the gas phase during heating. Although these impurity gases may also affect the mode of the reaction wave propagation, they typically do not participate in the main combustion reaction and have no significant effect on the heat of reaction. The main precursors and combustion products (including intermediates) are in the solid or liquid (melt) 12  

state in such systems. Based on the above, it was decided to neglect the impurity gas evolution and assume that this type of combustion is a gasless process. The first product of gasless combustion mentioned in the literature was a molybdenum silicide (MoSi2). A note was placed in 1959 in a scientific journal, which is now difficult to find, and consisted of 78 words in the German language and it is therefore appropriate to present it in its complete translation [2]: ‘‘molybdenum disilicide was obtained in a water-cooled closed container from elements. Argon was passed through the reaction space to create an inert (shielding)atmosphere, and the ignition of the reaction was carried out electrically by means of a molybdenum spiral. This method was selectively complemented by hot pressing at 1550–1750°C. The sample compacted at high temperature was immediately placed in an oven preheated to 1200°C and slowly cooled. The achieved density was equal to 6.15 against the theoretical value of 6.24. MoSi2 blends especially well with Al2O3. Such material has metallic conductivity to the Al2O3 content of 70%, after that the conductivity drops sharply.” A year later, the same author published a communication in the book [3] where he expanded on the description of the process, noting the uniformity of reaction propagation and that the product is strongly ‘red-hot’. As we can see, these brief notes contain many features of gasless SHS: synthesis of a mixture of elementary powders, a water-cooled reactor and the atmosphere of an inert gas to initiate locally the reaction with a heated electric spiral. Hot pressing and furnace annealing of the synthesized material were also tried. However, the author, a British materials scientist J.B. Huffadine was interested mainly in the final product and its properties; therefore, he did not examine the process of synthesis and did not propose to use gasless combustion for the production of other materials. Perhaps the fact that he did not work in an academic institution, but in the laboratory of a private company Plessey in Caswell (Scotland), and thus was focused on getting new materials, primarily for the electronics industry, may explain why J.B. Huffadine missed the discovery of a new class of combustion processes. The next time gasless combustion was mentioned in the works of A.G. Merzhanov and his co-workers, who well understood the scientific importance and application capabilities of the new process. They synthesized dozens of compounds by SHS and secured the priority of this discovery by patents in several countries [4]. Among the first scientific studies, one should mention the proceedings by V.M. Shkiro on the Ti–C system at a conference of young scientists [5] in 1970, which have become now a bibliographic rarity, virtually inaccessible to the reader. The first publication in the generally available journal was the article by A.G. Merzhanov and I.P. Borovinskaya [1], which is cited today by most authors as a pioneering work in the field of SHS. This paper reported on two types of gasless SHS systems: metal– carbon and metal–boron. Around the same time, reports appeared about the possibility of gasless combustion in the metal–metal systems, first in the thermal explosion mode [6], and then in the mode of the self-propagating reaction wave [7]. The synthesis of silicides was also studied on a new scientific level [8]. From a chemical point of view, the process of gasless SHS includes four main types of chemical reactions: –metals and non-metals with carbon producing carbides; –metals with boron to make borides; –metals with silicon, giving silicides; –metals with other metals to synthesize intermetallic compounds. 13  

In general, these reactions can be described by the chemical formula: A+ xB = ABx,

(2.2)

A – the first reagent, usually a metal (Ti, Zr, Hf, Nb, Ta, Mo, Ni, Co, Fe, Cu, Si); B – the second reagent, typically a non-metal (C, B, Si, Al, Ni); x – stoichiometric ratio. Some element may play the role of both the first and second reagent, e.g. Ni + Al = NiAl and Ti + Ni = TiNi. The diagram in Fig. 2.1 shows the heat of formation from the elements and the adiabatic combustion temperature (i.e. the maximum temperature of the products of combustion in the absence of heat loss) of some of the compounds for the above four types of reactions. The diagram indicates the general pattern: all compounds with a large heat of formation can be synthesized in the gasless combustion mode. The compounds with moderate heat of formation can also be obtained by SHS but require additional external preheating. For the compounds with a relatively low heat of formation there are usually no reports on the synthesis in the combustion mode.

Fig. 2.1. The adiabatic combustion temperature and the heat of formation of elements for some two-component systems.

For the overwhelming majority of gasless SHS compositions the combustion temperature exceeds the melting point of at least one of the reactants, products or exceeds the lowest eutectic temperature of the system. However, there are exothermic compositions for which even the adiabatic combustion temperature is below the minimum melting temperature in the system. The most striking example is the Ta + C composition which burns at the maximum temperature of 2734 K, while the eutectic temperature (Ta–Ta2C eutectic) is equal to 3120 K. In other words, the gasless combustion in the Ta–C system should take place solely by means of solid state reactions. This fact played an important role in the development of SHS. The point is 14

that the combustion processes with melting of the reactants have long been known, although it was not synthesis from the elements (e.g. burning of thermites). But the auto-wave combustion processes only by solid state reactions were not known at that time! This was the basis for registration in 1984 (with a priority of 05.06.1967) of the invention titled ‘The phenomenon of wave localization for auto-retarded solid-state reactions’. The essence of the invention is formulated as follows [9]: “Experiments established the previously unknown phenomenon of wave localization for self-retarded solid-state reactions, consisting in the fact that the chemical interaction between the solid dispersed components occurs without melting and gasification of the reactants and products, after thermal initiation is localized in a zone moving spontaneously in the space of the reagent in the form of a combustion wave’’. As will be shown below, the mechanism of solid-phase combustion is much more complicated than it was initially considered, but patenting the invention played a positive role in the development of SHS as a scientific and technological direction and attracted the attention of researchers in many countries. To date the mechanisms of gasless combustion and properties of the synthesized products were thoroughly investigated for dozens of simple compounds, indicated in Fig. 2.1. A large number of ternary and multicomponent systems, products and materials obtained thereof, are also being studied. As examples of practical applications it is worth noting the double carbide powder (Ti, Cr) C, is used for applying heat-resistant coating on metals, or ceramic material TiC–TiB2, has high hardness and wear resistance. Gasless systems remain also a popular target for experimental and theoretical research, which expands our understanding of the processes of heterogeneous combustion. These issues will be discussed in more detail in the subsequent chapters. 2.2. Combustion synthesis in gas–solid systems (infiltration combustion) Combustion synthesis in the systems involving both solid and a gas-phase reagents is one of the most important types of SHS. Attempts to find the origins of this class of reactions take us back to 16th century. The famous alchemist, physician and expert in the occult sciences Phillippus Aureolus Theophrastus Bombastus von Hohenheim, better known as Paracelsus, showed that combustion requires air, and metals transforming to oxides increase in weight. One of the fathers of modern chemistry Antoine Lavoisier in 1775 explained the phenomenon of combustion and firing as a process of interaction of substances with oxygen. Great chemist Jens Jakob Berzelius first produced zirconium powder in 1824 and a year later reported that zirconium burns in air, turning to the oxide [10]. The second most important gas phase precursor for the SHS processes is hydrogen. Combustion of metals in hydrogen in the SHS mode to form hydrides was first reported in [11]. Recently, interest in this direction has increased due to the development of prospects of hydrogen energetic: hydrides of some metals and alloys can be effective for hydrogen storage. Metals, as is well known in chemistry, also readily burn in gases such as chlorine and fluorine, but these reactions cannot be used for SHS because of the extreme aggressiveness and toxicity of these gases. Besides the elementary gaseous reactants, many transition metals can burn in an atmosphere of gaseous hydrocarbons, e.g. C2H2 or CH4 (thus obtaining metal carbides and releasing 15  

hydrogen), as well as in CO and CO2 (to form carbides and oxycarbides), but these systems have not been studied in details. The gas-solid SHS process is described by the chemical equation: A + xB = AB,

(2.3)

A – the first reagent, usually a metal (Ti, Zr, Hf, Nb, Ta, Mo, Ni, Co, Fe, Cu, Si); reagent B is a gas; x – stoichiometric ratio. Some element may play the role of both the first and second reagent. However, all these works of the great chemists can hardly be viewed as prototypes of SHS in the gas-solid systems. It is clear that in these classical papers, there was no mention of the organization of the burning regime, since the theory of combustion and thermal explosion appeared much later. It should also be noted that the combustion of elementary reactants (metals and nonmetals) with oxygen is not used widely in SHS for the synthesis of materials. The reasons are the same as for the above-mentioned sulphides. Indeed, why, for example, burn zirconium in oxygen to obtain an oxide, if the mineral based on this oxide is present in nature, and it was the merit of Berzelius that he was able to separate metallic zirconium from the oxide? Note that an oxygen-containing atmosphere is more frequently used in the synthesis of complex oxide compounds, such as, for example, high-temperature superconducting ceramics YBa2Cu3O7–x, as well as in the processes of synthesis in combustion of solutions. Analysis of the literature suggests that the main gas reagent used in the SHS is nitrogen (N2)! Despite the fact that classical chemistry considered this major component of air incapable of supporting combustion (in contrast to oxygen), the reaction of metals with nitrogen taking place with intensive self-heating were known to chemists for a long time. For example, the reaction of titanium with nitrogen taking place from an intense glow and the formation of a nitride was reported by Henri Moissan [12]. However, this outstanding chemist who got the Nobel Prize in 1906 for his invention of the electric arc furnace and for his work on the chemistry of fluorine did not attempt to arrange the combustion process either in the auto-wave mode or in the bulk combustion mode: he simply heated the titanium powder in a flow of nitrogen. The same conclusion can be related to the early work on the combustion of cerium in nitrogen [3]. Thus, the first experiments in which the combustion of metallic powders in a nitrogen atmosphere was organized as the auto-wave process represent early stages of SHS development. A pioneering article on the synthesis of nitrides of transition metals (Zr, Ti, Nb, Ta) was published in 1972 [14]. The next fundamentally important steps were the combustion synthesis of silicon nitride Si3N4, [15,16], boron nitride [17] and aluminum nitride [18], which are the basis of many advanced ceramic materials. At present, all binary refractory nitrides, as well as many ternary and multicomponent materials based on nitrides, which have practical value, have been produced by SHS. The second most important gas phase precursor for the SHS processes is hydrogen. Combustion of metals in hydrogen in the SHS mode to form hydrides was first reported in [11]. Recently, interest in this direction has increased due to the development of prospects of hydrogen energetics: hydrides of some metals and alloys can be effective for hydrogen storage. Metals, as is well known in chemistry, also 16  

readily burn in gases such as chlorine and fluorine, but these reactions cannot be used for SHS because of the extreme aggressiveness and toxicity of these gases. Besides the elementary gaseous reactants, many transition metals can burn in an atmosphere of gaseous hydrocarbons, e.g. C2H2 or CH4 (thus obtaining metal carbides and releasing hydrogen), as well as in CO and CO2 (to form carbides and oxycarbides), but these systems have not been studied in details. All SHS processes that involve a gas reagent are considered to be an infiltration type of combustion. Infiltration combustion (IC) is the phenomenon of propagation of the combustion wave (front of a chemical reaction) in porous media with ‘feeding’ (infiltrating) of the gas to the reaction front. Generally speaking, IC may occur both in the self-propagating and the thermal explosion regimes. The auto-wave regime is schematically presented in Fig. 2.1, which shows a sample of compacted metal (or non-metal) powder placed in a chemical reactor of constant pressure (its volume is much higher than the sample volume).

Fig. 2.2. Auto-wave mode of infiltration combustion.

The sample has a relative density (thus relative porosity of the medium is П = 1 –Δ). For example, if the titanium powder (Ti theoretical density ρt = 4.5 g/cm3) is compacted to the actual density ρ = 2.25 g/cm3 then = ρ/ρt ≈ 0.5, which means that half the volume of the sample is occupied by the pores. The porosity can be open, providing gas access from the external environment, and closed. The pressure of the reaction gas (e.g. nitrogen) in the reactor power is equal to P and coincides with the initial gas pressure in the pores of the sample. Initiation of the reaction leads to the propagation of the combustion wave front with velocity U owing to interaction between the initially solid phase of porous skeleton (A) and the gas (B) according to the reaction (2.2). The combustion wave is followed by the formation of the solidphase product (ABx). It is necessary to distinguish between the gas initially contained in the pores of the sample (internal reagent) and in the environment (external agent). At relatively low pressures in the reactor, the amount of internal nitrogen may be insufficient to ensure adequate heat required for the propagation of the wave. In this case, combustion wave propagation maybe due to infiltration of nitrogen from the environment to the combustion front through pores of the solid reaction media. Infiltration takes place due to the pressure difference arising between the combustion zone, in which nitrogen is 17  

absorbed (P = 0), and the surrounding atmosphere with constant pressure (P0). Indeed, the gas contained in the pores is quickly absorbed in the front of the combustion wave due to the reaction with the solid particles. As a result, the gas pressure in the pores sharply decreases (in the limit to zero), while the external pressure in the reactor remains constant and equal to P0. There is a pressure gradient P/x (where x is the characteristic scale, such as the radius of the sample), which is a driving force in the supply of the gas-phase reactant from the outside to the reaction front. This process can be compared to the effect of a chemical pump, which supplies the gas from the environment to the reaction surface in the sample volume. The question naturally arises: is the infiltration of gas from the outside necessary for the existence of the self-sustained combustion wave? Is it possible to ensure that the gas, initially located inside the pores of the sample, is sufficient for the synthesis process? Obviously, this gas must be at high pressure, but what is the magnitude of this pressure? One can make some simple estimates. Consider the same reaction (2.3) with respect to systems with a single gas reagent: A(sol) + 0.5xB2(gas) = ABx(sol)

(2.4)

Here it is taken into account that the most commonly used gas reagents, i.e. N2 and H2, are diatomic molecules. The relative porosity of the medium (Π) is the total pore volume divided by the total sample volume. If the mass of the gas in this volume (pore) is such that per mole of solid reactant A we have 0.5x mole of the reactant gas B2, the reaction will be complete without the participation of the gas from the outside, but solely by the gas stored in the pores. It is easy to show that to fulfill this condition the density of the gas in the pores must be equal to the critical value ρB:

B 

xM B 1  P A MA P

(2.5)

Fig. 2.3. The critical density of nitrogen in the pores of the sample, sufficient for 100% conversion of the solid reactant to the corresponding nitride.

18  

where ρA and ρB are the densities of the solid reactant A and gas reactant B2, respectively; MA, MB are the molecular (mole) weights of the reagents. The results of calculation by formula (2.5) for the ‘solid reagent – N’ systems are shown in Fig.2.3. Typically, the relative porosity of the sample does not exceed 0.5–0.6 (samples with greater porosity crumble when handled), but the samples with a porosity of less than 0.35–0.4 are also rarely used in the combustion mode. As can be seen from Fig. 2.2, in the range of actually used porosities (0.35 to 0.6) the value of the critical density of nitrogen is very high. For most systems, it is higher than the density of liquid nitrogen and for some is higher than the density of solid nitrogen! It is difficult to estimate how much pressure is required to compress the nitrogen to such a density. The van der Waals equation for real gases, of course, does not apply to this density range. A highly rough estimate can be made, perhaps, with the help of the so-called virial equation of state, but even this equation does not work when approaching the density of the liquid and the gas becomes difficult to compress. The results of estimates of the critical pressure of nitrogen by using the virial equation:

PВ 

RT VB

 C D 1   2   VB VB 

(2.6)

whereVB = MB/ρB – molar volume of the reactant gas; T = 298 K – initial room temperature; virial coefficients for nitrogen C = – 5.47 cm3/mol, D = 1437 cm6/mol2. It can be seen that the critical pressures are in the range of tens and hundreds of thousands of atmospheres for the Zr–N2 and Hf–N2 systems, while for other systems the critical gas density is higher than the density of liquid nitrogen. Thus, calculations show that it is impossible to store the reactant gas inside the sample in an amount necessary for 100% completion of the reaction. However, is it always necessary to complete the reaction for the propagation of combustion wave? For example, in the Ti–N2 system reaction heat is so high that the adiabatic combustion temperature reaches 3446 K, when the reaction product TiN melts completely and partially dissociates. Thermodynamic calculations show that if only one tenth of titanium reacts with nitrogen, the temperature in the reaction zone may already exceed 1300 K, and if one-fifth of titanium reacts, the temperature is over 2000 K. And it is shown that the combustion wave can already self-propagate at these temperatures. Although these critical densities and gas pressures remain pretty high (hundreds or thousands of atmospheres), they can be achieved in high-pressure reactors. Thus, the SHS wave with a single gas phase reactant may in principle propagate due to the gas stored in the pores. However, to complete the reaction during post-combustion stage it is required to ensure infiltration supply of the gas reagent from the external atmosphere. This is the typical route for the final SHS-product formation in the gas-solid (hybrid) reaction systems. There is another method to increase the degree of conversion in the reaction front of the combustion wave. Assume that in addition to solid reactant (A) and pores, filled with reagent gas (B), the original sample already contains some amount of final solid ABx, i.e. the reaction equation is as follows: А(sol)+0.5xB2(gas) + yABx(sol) = (1 + y)ABx(sol) 19  

(2.7)

For fixed pressure and porosity, the greater the amount of ABx in the initial mixture, the more gas per solid reactant in the pores of the skeleton of the sample. In this case, next equation for the critical density of the gas takes the form

B 

xM B 1 P  A  AB M A  AB  yM AB  A P

(2.8)

where ρAB is the density of the product ABx. Dilution of the initial mixture by the reaction product is a very effective way to increase the ratio of gaseous and solid reactants inside the sample and is often applied in practice. Its advantage is that incomplete combustion is reduced to a minimum, i.e. the combustion product does not retain any initial component A, and the entire phase consists of the desired phase ABx. On the other hand, in this case some amount of released energy is used for preheating of the inert additive, which does not participate in the reaction; therefore, this approach is applicable only to systems with large energy generation. For example, for the Ti–N2 system for y = 8 (Eq. 2.8) the adiabatic combustion temperature is ~1610 K, and for y = 9 it decreases to 990 K, and the combustion mode becomes unobtainable. Thus, in most cases, infiltration of the reactant gas from the environment into the porous sample is essential for the synthesis of the single-phase product by SHS in the solid-gas systems. It turns out that in such systems there are typically two stages of the chemical interaction. During the first stage, the reaction proceeds with a degree of conversion that is significantly less than 100%, however this stage controls the velocity and temperature of the combustion wave. The second stage, the so-called volume post combustion stage, under optimal conditions may lead to complete conversion of the solid phase reactant to the final product with the desired composition. In concluding this section, it is worth noting that the concept of ‘infiltration combustion’ refers not only to the SHS processes. Some examples are: infiltration combustion of gases in chemically inert porous media [19]; propagation of the reaction wave during infiltration of the reaction gas mixture through the bed of a solid catalyst [20]; infiltration combustion of oil- and gas-bearing fields [21]; burning of debris in infiltration combustion reactors with super-adiabatic heating [22], and others. Infiltration combustion is also a favorite object of study in the field of combustion theory. 2.3. Combustion synthesis with a reduction stage A simple chemical scheme for the synthesis of inorganic compounds and materials involving the reduction stage is described by the equation ABx + yC+ zD= ADz +BxCy.

(2.9)

where A – the first element, usually a metal: Fe, Ni, Co, Cr, Cu, W, Mo, Bi, Ti, Ta, etc., but may be a non-metal, such as boron or silicon; B – a second element, usually oxygen (O); C – a third element is a metal – reducing agent, typically Al or Mg, less often Ti or Zr. D – a fourth element which can react with component A, after the latter 20  

is reduced from compound, is often represented by carbon; x, y, z – stoichiometric coefficients. An example of the reaction (2.9) is the process: 3Cr2O3 +6A1+4C = 2Cr3C2 +3A12O3

(2.10)

that may occur in the combustion mode, with the temperature reaching 2320 K, which is above the melting point of the carbide Cr3C2 (2100 K) and is equal to the melting point of the oxide Al2O3 (2320 K). The combustion products are in the molten state, so the process is often referred to as SHS metallurgy [23]. Another popular name is the thermite type system that indicates the relationship of the reaction with the metallother-mic processes. The boiling points of aluminum and magnesium are 2773 K and 1380 K respectively, and many aluminum and magnesium thermite reactions occur at a temperature above 3500 K, therefore, the thermite type processes are obviously not gas-free. The thermite mixtures are most often referred to in the literature as prototypes of SHS systems, and an opinion exists that the synthesis of materials by SHS is not a new scientific and technological direction, but only the further development of the thermite area. In Russian literature, it is believed that metallothermy was developed by N.N. Beketov in 1859–1865 [24,25], and in the foreign literature it is stated that metallothermy was invented by G. Goldschmidt in 1895 or in 1898. In 1895, H. Goldschmidt took out a patent for his process [26], which he called metallothermy and later actively promoted its use [27,28]. The process was rapidly introduced into practice and began to be used primarily for welding of rails. The first commercial use of aluminothermy for welding rails was done in Essen (Germany) in 1899. Figure 2.4 shows the metallothermic welding of rails which was performed at the beginning of the 20th century (a) and at present (b). As one can see, the Goldschmidt technology has remained nearly the same for a hundred years! The company ‘Goldschmidt still exists today, but produces other products. a

b

Fig. 2.4. Metallothermic welding of rails: a – in Graz, Austria, 1914; b – allTEC Engineering Service, Austria 2011

The development of SHS has brought many new advances in the field of metallothermy. The search for gasless flammable compounds, referred to in section 1.1, was first based on the use of iron–aluminium thermite heavily diluted by alumina in order to lower combustion temperature and suppress evaporation of the components [29]. In 21  

this work, as well as in a preceding work [30], the metallothermic process was studied using the methodology, which had been developed to study the combustion of gunpowder. This approach involves the organization of the plane front of the combustion wave propagating in one direction; recording the linear velocity of reaction front propagation; the study of the thermal structure of the wave; the identification of controlling experimental parameters and patterns of burning. Thus, the novelty was already in that metallothermy was not studied as a process of metallurgy, but as a process of combustion. With expansion of research in the field of SHS, the ‘SHS metallurgy’ systems form a separate research area [31]. In recent years, the thermite subject received a new impetus, due to the developments of nanothermites. The development of various nanotechnologies, including metals and their oxides, allowed the preparation of the reaction heterogeneous structures with nanostructured reagents. The range of available nanopowders is now quite large (aluminium, boron, carbon, silicon, nickel, many oxides). Thermite compositions prepared using nanopowders are called nanothermites or superthermites [32]. Therefore, using the same raw nanopowders but contained and mixed under different conditions usually leads to different results for the characteristics of the ignition and combustion of such systems. The data on the chemical composition and particle size of some reagents of the studied superthermites systems are summarized in Table 2.1. Superthermite mixtures are highly sensitive to ignition and have an unusually high rate of combustion (according to some data more than a kilometer per second!). Table 2.1 Some characteristics of superthermite systems System

Al–MoO3

Al–WO3 Al–Bi2O3 Al–CuO2 Al–Fe2O3

Metal particle size, nm 17, 25, 30, 40, 53, 76, 100, 108, 160, 200 50, 80, 120 44, 80, 120 44 30, 45, 140, 170 80 80 80 40, 100 80 52

Oxide particle size Sheets 10 × 10 μm × 10 nm 10 × 10 μm 1 × 1 μm × 20 nm 15.5 nm Ssp. = 66 m2/g 200 × 200 × 30 nm 100 × 100 × 20 nm filaments – 25 × 2 μm 40, 108, 321, 416 nm 100 × 100 × 20 nm Ssp. = 50 – 300 m2/g

Reference [33, 34] [35] [36] [37] [38] [39] [40] [40] [41] [40] [42]

2.4. Solution combustion synthesis We can define solution combustion synthesis (SCS) as a complex self-sustained chemical process, which takes place in a homogeneous solution of precursors. SCS starts with dehydration and thermal decomposition of the homogeneous solution and involves several thermally coupled exothermic reactions, which result in the formation of at least one solid product and a large amount of gas. It is important to note that this process allows for the synthesis of a variety of nanoscale materials, including oxides, metals, alloys, and sulfides, which are currently used in many important applications [43,44]. SCS systems can be classified according to the chemical composition of fuel, oxidizer, and solvent (Table 2.2). 22  

It can be seen that SCS makes use of salts, such as nitrates, metal sulfates and carbonates, as oxidants and, reducing reagents, fuels such as glycine, sucrose, urea, or other water soluble carbohydrates. Nitrate acts as an oxidizer for the fuel during the combustion reaction. As shown by thermodynamic calculations, such system is highly exothermic [45]. The overall reaction scheme for combustion in air can be written as follows: 5 15 Fe( NO3 )3   CH 2 NH 2CO2 H  (  1)O2  3 4 1 10 25 15   Fe2O3( sol )   CO2( gas )   H 2O ( gas )     3  N 2( gas ) 2 3 6 23 

(2.11)

where φ = 1 corresponds to the stoichiometric composition when the reaction proceeds completely without oxygen, φ > 1 (< 1) – fuel rich (lean) systems, respectively. Table 2.2 Most Frequently Used Components for the Solution Preparation oxidizer metal nitrates nitrate hydrates Meν(NO3)ν·nH2O ν – metal valence ammonium nitrate (NH4NO3)

nitric acid (HNO3)

fuel urea (CH4N2O) glycine (C2H5NO2) sucrose (C12H22O11) glucose (C6H12O6) citric acid (C6H8O7) hydrazine-based fuels

solvent water (H2O) Hydrocarbons Kerosene benzene(C6H6) alcohols: ethanol (C2H6O)

carbohydrazide (CH6N4O) oxalyldihydrazide (C2H6N4O2) hexamethylenetetramine (C6H12N4) Acetylacetone (C5H8O2)

methanol (CH4O) furfuryl alcohol (C5H6O2) 2-methoxyethanol (C3H8O2) formaldehyde(CH2O)

In the absence of water the adiabatic combustion temperature for the stoichiometric mixture exceeds 2200 K and during combustion a large amount of gaseous products form, which increases as the parameter φ increases. Local initiation of the reaction in these solutions, similarly to gasless systems, leads to the propagation of the self-sustained combustion wave. All above, i.e. molecular mixing of the reagents and intensive release of gaseous species results in formation of solid products with very fine nanostructure. See more details in Chapter 6. 2.5. Mechanical activation of initial powder mixtures for SHS Another rapidly developing direction in the field of combustion synthesis of materials is the mechanical activation of SHS green mixtures. Mechanical activation (or simply activation) of combustible mixtures is a treatment in high-speed planetary ball mills, vibratory mills, and other devices in which the particles in the mixture are subjected to mechanical effects with the force sufficient for cracking of brittle and plastic deformation of ductile components [46]. Brittle reagents are ground to finer particles and ductile reagents (usually metals) are subjected to multiple flattening deformations forming laminated composites, in which the layer thickness decreases with increasing 23  

duration of activation. Often fine pieces of brittle components of the mixture are inside the particles of the ductile reactants. Activation not only reduces the size of the reactants but also increases the contact area between them, cleans the contact surface from oxide layers and other impurities, accumulates the crystal defects and all these increase the chemical activity of the reactive mixture [47,48]. Already at the stage of activation there may be partial or complete dissolving of one reagent in the other (mechanical alloying), and even a chemical reaction between the components of the mixture to form a new compound (mechanosynthesis) may take place. In some cases, mixture auto-ignition occurs directly during activation. Over the last decade, a vast number of publications have been devoted to the mechanical activation of flammable mixtures. Comparison of the results of different authors are difficult because of the fact that the mechanical activation process depends on many parameters, including speed, acceleration, mass, size and shape of the grinding media, geometrical dimensions of the unit, the ratio of the mass of milling bodies (balls) to the mass of the activated mixture, the environment in which the activation takes place (air, inert gas, vacuum, liquid) and many others. However, it can distinguish three parameters, which most obvious express the physical (not technical) aspect of the process: the energy of an impact, (collision between the balls or of balls with the wall), the frequency of collisions and the total activation time. If it multiplies all three parameters, it can obtain the amount of total energy that is spent on activation. Of course, this does not mean that all this energy is ‘stored’ in the activated mixture, since most of it is converted into heat, however, the three parameters and their product can serve as a physical basis for comparing the results. The published data can be divided into two distinct groups on the basis of the impact energy. The first group can be called low-energy activation, the energy of collision is 0.1-0.2 J and the activation time – from minutes to tens of hours [49,50], the second group of results is related to high-energy activation when the impact energy is 1-2 J and the activation lasts from a few seconds to minutes [51,52]. Many studies report that mechanical activation leads to a large decrease in the ignition temperature, which is usually determined by differential thermal analysis. The ignition temperature was found to fall from 1600 K for non-activated mixtures to 770 K after 5–10 h of low-energy activation of the Ti–C system [49]; from 1670 K to 870 K (a few hours of activation) for the Ti–Si system [53]; from 1190 K to 430 K after 90 min of activation of the Ti–Si–C system (3Ti + Si + 2C composition); and after 106 min, the last named mixture self-ignites directly in the milling jar at a temperature of 340 K [49]. The nature of the effect of mechanical activation on combustion parameters is not yet fully understood, but several possible explanations can be found in the literature. One is that MA allows one to decrease the reagents size to a nano scale, which leads to increase of their reactivity. Normally the size of crystallites in the activated mixture is inferred from broadening of XRD peaks using the Scherrer formula [54] or the Williamson–Hall method [55]. These methods are indirect since broadening of the peaks can be caused not only by a small crystallite size and crystal strains but also by non-uniformity of the size or chemical composition. A direct method is transmission electron microscopy (TEM) but this method is expensive and time-consuming, and only few authors report TEM results for mechanically activated reaction mixtures [55,56] and these data confirm the presence of particles or crystallites with a size of about 100 nm or less. Thus, we have to recognize that the presence of nanostructured 24  

components in activated reaction mixtures is supported by indirect evidences, viz. the broadening of diffraction peaks and sharp change in properties (ignition temperature) of these compounds. Note that for abrupt change in the properties it is not necessary to convert the entire volume of the mixture into a nanostructured composite. It can be assumed that the nanostructured regions with changed composition form locally during mechanical activation and act as active sites for the initiation of the reaction at reduced temperature. Further studies are required for more definite conclusions to be drawn. Another line of reasoning admits the formation of unstable solid solutions. A 3 : 1 Al–Ti mixture was subjected [52] to mechanical activation under Ar in the presence of stearic acid, CH3(CH2)16COOH, as an anti-cold-welding agent. After 100 hours of activation, diffraction patterns showed the emergence of a new phase identified (by lattice parameters) as a supersaturated solution of Ti in the fcclattice of Al. The size of its crystallites, as determined by using the Scherrer formula [57], was very small (3.8 nm). Such a phase is absent in the equilibrium phase diagram. In [58] the above method was used to prepare a non-equilibrium solution in the Mg–Al system (10–50% Mg). The ignition temperatures of the solid solutions (about 1000 K) were significantly smaller than that for aluminum (~2300 K). Similar results were also obtained for the Al–Ti, Al–Li, and Al–Zr systems [59-60]. Many researchers also point to the fact that mechanical deformation is accompanied by the formation of a new clean contact surface between the reagents, which are free of oxide layers and impurities, thereby increasing the reactivity of the compounds. This hypothesis also needs experimental confirmation. To sum up this section, it may be concluded that almost all researchers agree that high energy ball milling reduces the ignition temperature of various combustible systems, expands their combustion limits, promotes more complete reactions, and typically leads to an increase in the velocity of combustion wave propagation. 2.6. Example of SHS with preliminary mechanical treatment of the powders The dependence of the reactivity of redox process in the SiO2+Al system as a function of the parameters for preliminary mechano-chemical treatment (MCT) was investigated. Quartz of the purity up to 99.7% was used in the work. Aluminum of the brand PA4 was used as a reducer. Mechanochemical treatment of powders was carried out in a centrifugal-planetary mill “Pulverizette 5” (FRITSCH, Germany) in the 500 mm3 jar, with the rotation rate of the platform 400 r/m, acceleration of the movement of milling balls 40g and energy power 1.5 kW/h. In the course of MCT, the time of milling was varied. MCT was carried out with a carbon modifier (activated coal), which was introduced in the amount of 5% to the powder being treated. As is shown earlier [61,62], during MCT of quartz with carbon modifier, the surface of milled particles is saturated with carbon imparting them high activity in the process of technological combustion. After MCT, mixture of powders with the reducer was compacted into cylindrical samples with the diameter of 20 mm and height of 20-25 mm, when introducing a binder in the amount of 5%. The samples were molded on the laboratory press “Carver” with the force of 10 ton. SHS was carried out in a muffle furnace with the 25  

pre-determined temperature from 570 to 900°C. The temperature of the sample was measured by a pyrometric thermometer “RaytekRaynger 3i”. The phase composition and strength of synthesis products were also studied. According to different literature data the self-ignition of the mixture (SiO2+Al) of a stoichiometric composition changes from 670 to 720°C. Synthesis of aluminum with non-activated quartz at the temperature of self-ignition 670°C provided only partial reduction (up to 13%) of silicon. After activation of SiO2 without a modifier, the furnace temperature providing self-ignition of the sample decreases to 570°C and the amount of reduced silicon is ~17%. The increase in the temperature of furnace heating to 800-900°C contributes to the increase in the amount of reduced silicon up to 2435%, respectively [64,65]. However, the reaction occurs in the thermal explosion mode, which is in consistency with the work [66] where the SiO2+Al aws investigated with pre-activation of quartz. With the change of the reaction state after mechanical treatment of quartz, not only the self-ignition temperature but also the induction period of ignition, rate and temperature of the system combustion changed. The temperature of the furnace made up 900°C. With the time of preliminary mechanical treatment of quartz being equal to 5-10 min, the combustion temperature increases up to 1620°C, with the increase in the time of MCT to 20 min and more, one can observe the decrease in the maximum combustion temperature to 1250°C. The use of carbon modifier in mechanical treatment of quartz contributes to the increase of combustion temperature and the longer the treatment, the more intensive is the process and the higher is the combustion temperature. The power intensity of the system increases after mechanochemical treatment of aluminum with carbon both independently and together with quartz. In this case, the combustion temperature of the system grows to 1700°C after MCT during 20 min and more. CONTROL QUESTIONS 1. 2. 3. 4. 5. 6.

List main classes of SHS products. What types of chemical SHS-reactions do you know? Give examples. Why filtration of gas reactant is a necessary condition for the solid-gas SHS? Could you describe the chemical scheme for the synthesis of inorganic compounds and materials involving the reduction stage by the equation? How define the solution combustion synthesis? Explain the example of SHS with preliminary mechanical treatment of the powders. REFERENCES

1. 2. 3. 4.

Merzhanov, A.G. Borovinskaya, I.P. Self-spreading high-temperature synthesis of refractory compounds, Dokl. Chem., 1972, vol. 204, No. 2, pp. 429–431. Huffadine J.B., Caswell B., The Fabrication and properties of molybdenum disilicide, Angew. Chem., 1959, Vol. 71, No. 20, pp. 653. Huffadine, J.B., The Fabrication and properties of molybdenum disilicide and molybdenumdisilicide–alumina, in: Special Ceramics, Popper P., Ed., New York: Academic Press, 1960, pp. 220. Merzhanov A.G., Shkiro V.M., Borovinskaya I.P., A method for synthesis of refractory inorganic compounds, USSR Inventor's Certificate 255 221 (1967), Byull. Izobr. No. 10, 1971; French Patent 2 088 668, 1972; US Patent 3 726 643, 1973; UK Patent 1 321,084, 1974; Jap. Patent 1 098 839, 1982.

26  

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

28. 29.

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30. Belyaev A.F., Komkova L.D., Presure dependence for the burning velocity of thermites, Zh. Fiz. Khim., 1950, No. 11, pp. 1302–1311 (Russ). 31. Merzhanov A.G., Yukhvid V.I., Borovinskaya I.P., Self-propagating high-temperature synthesis of cast refractory inorganic compounds, Dokl. Akad. Nauk SSSR, 1980, Vol. 255, No. 1, pp. 120–124 (Russ). 32. Puszynski J.A., Processing and characterization of aluminum-based nanothermites, J. Therm. Anal. Calorim., 2009, Vol. 96, No. 3, pp. 677–685. 33. Granier J.J., Pantoya M.L., Laser ignition of nanocomposite thermites, Combust. Flame, 2004, Vol. 138, pp. 373–383. 34. Pantoya M.L., Granier J.J., Combustion behavior of highly energetic thermites: Nano versus micron composites, Propell. Explos. Pyrotech., 2005, Vol. 30, No. 1, pp. 53–62. 35. Pantoya M.L., Granier J.J., The effect of slow heating rates on the reaction mechanism of nano and micro composite thermite reactions, J. Therm. Anal. Calorim., 2006, Vol. 85, No. 1. pp. 37–43. 36. Bockmon B.S., Pantoya M.L., Son S.F., Asay B.W., Mang J.T., Combustion velocities and propagation mechanisms of metastable interstitial composites, J. Appl. Phys., 2005, Vol. 98, No. 6. pp. 064903–1–7. 37. Asay B.W., Son S.F., Busse J.R., Oschwald D.M., Ignition characteristics of metastable intermolecular composites, Propell. Explos. Pyrotech., 2004, Vol. 29, No. 4, pp. 216–219. 38. Walter K.C., Pesiri D.R., Wilson D.E., Manufacturing and performance of nanometric Al/MoO3energetic materials, J. Propuls. Power, 2007, Vol. 23, No. 4, pp. 645–650. 39. Son S.F., Asay B.W., Foley T.J., Yetter R.A., Wu M.H., Risha G.A., Combustion of nanoscale Al/MoO3 thermite in microchannels, J. Propuls. Power, 2007, Vol. 23, No. 4. pp. 715–721. 40. Sanders V.E., Asay B.W., Foley T.J., Tappan B.C., Pachero A.N., Son S.F., Reaction propagation in four nanoscale energetic composites (Al/MoO3, Al/WO3, Al/CuO, and Bi2O3), J. Propuls. Power, 2007, Vol. 23, No. 4, pp. 707–714. 41. Puszinski J.A., Bulian C.J., Swiatkiewicz J.J., Processing and ignition characteristics of aluminum–bismuth trioxide nanothermic system, J. Propuls. Power, 2007, Vol. 23, No. 4, pp. 698–706. 42. Plantier K.B., Pantoya M.L., Gash A.E., Combustion wave speeds of nanocomposite Al/Fe2O3: The effect of Fe2O3 particle synthesis technique, Combust. Flame, 2005, Vol. 140, pp. 299–309. 43. Patil K.C., Hedge M.S., Rattan Tanu, Aruna S.T., Chemistry of Nano-crystalline Oxide Materials: Combustion Synthesis, Properties and Applications, New Jersey: World Scientific, 2008. 44. Vrama A., Mukasyan A.S, Rogachev A.S. Manukyan K., “Solution Combustion Synthesis of Nanoscale Materials”, Chemical Review, 116, 14493-14586 (2016). 45. Mukasyan A.S., Epstein P., Dinka P., Solution combustion synthesis of nanomaterials, Proc. Combust. Inst., 2007, Vol. 31, No. 2, pp. 1789–1795. 46. Grigorieva T.F., Barinova A.P., Lyakhov N.Z., Mechanochemical synthesis of intermetallic compounds, Usp. Khim., 2001, Vol. 70, No. 1, pp. 52–71. 47. Grigorieva T., Korchagin M., Lyakhov N., Combination of SHS and mechanochemical synthesis for nanopowder technologies, KONA, 2002, No. 20, pp. 144–158. 48. 107. Bernard F., Gaffet E., Mechanical alloying in SHS research, Int. J. SHS, 2001, Vol. 10, No. 2, pp. 109–132. 49. Maglia F., Anselmi-Tamburini U., Deidda C., Delogu F., Cocco G., Munir Z.A. Role of mechanical activation in SHS synthesis of TiC, J. Mater. Sci., 2004, Vol. 39. pp. 5227–5230. 50. 111. Maglia F., Milanese C., Anselmi-Tamburini U., Combustion synthesis of mechanically activated powders in the Nb–Si system, J. Mater. Res., 2002, Vol. 17, No. 8, pp. 1992–1999. 51. Korchagin M.A., Lyakhov N.Z., Self-propagating high-temperature synthesis in mechanically activated compositions, Russ. J. Phys. Chem., 2008, Vol. 27, No. 1, pp. 73–82. 52. 116. Korchagin M.A., Dudina D.V., Application of self-propagating high-temperature synthesis and mechanical activation for obtaining nanocomposites, Combust. Explos. Shock Waves, 2007, Vol. 43, No. 2, pp. 176–187. 53. Anselmi-Tamburini U., Maglia F., Doppiu S., Monagheddu M., Cocco G., Munir Z.A., Ignition mechanism of mechanically activated Me–Si (Me = Ti, Nb, Mo) mixtures, J. Mater. Res., 2005, Vol. 19, No. 5, pp. 1558–1566. 54. Birks S.L., Friedman H., Particle size determination from X-ray line broadening, J. Appl. Phys., 1946, Vol. 17, pp. 687–692.

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55. 244. Williamson G.K., Hall W.H., X-ray line broadening from filed aluminum and wolfram, Acta Metall., 1953, Vol. 1, pp. 22–31. 56. Wen C.E., Kobayashi K., Sugiyama A., Nishio T., Matsumoto A., Synthesis of nanocrystallite by mechanical alloying and in situ observation of their combustion phase transformation in Al3Ti, J. Mater. Sci., 2000, Vol. 35, pp. 2099–2105 57. Clevenger, L.A. Tompson C.T., Tu K.N., Explosive silicidation in nickel/amorphous-silicon multilayer thin films, J. Appl. Phys., 1990, Vol. 67, No. 6, pp. 2894–2898. 58. Shoshin Y., Mudryi R., Dreizin E., Preparation and characterization of energetic Al–Mg mechanical alloy powders, Combust. Flame, 2002, Vol. 128, pp. 259–269. 59. Shoshin Y., Dreizin E., Laminar lifted flame speed measurements for aerosols of metals and mechanical alloys, J. AIAA, 2004, Vol. 42, No. 7, pp. 1416–1426. 60. 249. Schoenitz M., Zhu X., Dreizin E.L., Mechanical alloys in the Al-rich part of the Al–Ti binary system, J. Metast. Nanocryst. Mater., 2004, Vol. 20–21, pp. 455–461. 61. Ksandopulo G.I., Mofa N.N., Ketegenov T.A., Chervyakova O.V., Tyumentseva O.A. Combustion of oxide systems based onquartz modified with organic compounds during mechanical treatment // Physics of combustion and explosion. – 2002.V.38, №1, pp 51-59. 62. Mofa N.N. Mechanochemical treatment is a progressive technological process for creation of new composition materials // Chemistry and Chemical Technology. Modern problems: Annual of review articles of scientists-chemists / under the Ed. prof. Z.A.Mansurov. – Almaty: Kazak universiteti, 2004, pp 189-232. 63. Mofa N.N., Shabanova T.A., Antonyuk V.I., Mansurov Z.A. Mechanochemical control of the process oftechnological combustion is creation of composition systems of different structural levels // A space challenge of the XXI century. 2011. - Volume 4. - pp. 144-149. 64. Mansurov Z.A. Mofa N.N. Сarbon is an effective modifier of silicon dioxide and a reagent when obtaining nanostructurized SHS-composites //Eurasion Chemico-Technological Journal 2012. -V 14, №1 – Р31-36. 65. Mansurov, Zulkhair A., Mofa, Nina N., Sadykov, Bakhtiyar S., Shabanova, Tatyana А. Activation of the Technological Combustion Process of Oxide Systems by Different Modifying Additives //Advances in Ceramic Science and Engineering (ACSE) Volume 2 Issue 3, August 2013. – Р.106-112. 66. Mansurov Z.A., Mofa N.N., Sadykov B.S., Antonyuk V.I. Mechanochemical treatment, peculiarities of the structure, properties and reactivity of SHS-systems on the basis of natural materials. Part 3: The effect of mechanochemical treatment and modification of oxide materials on technological combustion // Engineering-physical Journal. - 2014. - T. 87, №5. - pp. 10511059.

29  

C

HAPTER

3

THERMODYNAMICS AND DRIVING FORCE OF SHS PROCESSES

3.1. General principles After initiation, the SHS process proceeds self-sufficiently without any external energy sources. Consequently, the driving force of the process should be sought in the SHS system itself. What is this driving force? The simplest and most obvious answer is that the driving force of the process is to reduce the internal energy of the system, more precisely, by conversion of its chemical potential into heat. In general, this is true, but it does not reflect all the specifics of the thermodynamics of the SHS. To understand these particular features, one needs to analyze the different structural levels of reactive systems. Let us first consider the SHS system as a whole, on a macroscopic scale, under the assumption that the interaction with the surrounding environment can be neglected. Conditions in which there is no exchange of heat between the considered system and the environment are called adiabatic conditions. If in addition the amount of matter in the system remains constant, the system is called the isolated system. So, assume that a reaction mixture consisting of the powder particles of the solid reagents is placed in a sealed adiabatic vessel (optionally the vessel may also contain a gaseous or liquid reagent). It is experimentally confirmed that at room temperature such an initial mixture can typically exist indefinitely long, without noticeable changes in composition, microstructure, temperature or pressure. The condition in which the system does not change its characteristics over time is called the stationary state. However, such an initial stationary state of the SHS system is ‘deceptive’. If the temperature of the system is raised to the point at which the mutual diffusion of the reagents could occur at an appreciable rate, a chemical reaction with heat generation starts in the vessel. As a result of the exothermic reaction, the temperature in the vessel increases even more, which in turn leads to an increase in the reaction rate, i.e. the process becomes self-accelerating. This process stops when the reagents are completely transformed into the reaction products and the system reaches a new steady state. It is natural to ask how the new state is different from the initial stationary state. Since any exchange with the environment is excluded (isolated adiabatic system), then from the energy conservation law it follows that the total energy of the system remains unchanged. At the same time, the reaction has been completed and common sense sug30  

gests that the system does not spontaneously return to its original state. Which characteristic of the system has undergone irreversible changes? In thermodynamics, this characteristic is called the Gibbs free energy and can be presented as follows [1]: G U = + PV −TS,

(3.1)

whereU and S are the internal energy and the entropy of the system respectively, P is pressure, V is volume, and T is temperature. Josiah Willard Gibbs (1839–1903) was a great American scientist with a broad range of scientific interest, one of the founders of chemical thermodynamics and statistical mechanics in the second half of the 19th century, J.W. Gibbs proved that any spontaneous irreversible process occurs with a decrease in free energy G. And this decrease in free energy is the driving force of the process. Since this statement is fundamental for the SHS, including both auto-wave and thermal explosion modes, we discuss it in more details. First of all, it should be noted that the free energy G is not energy in a physical sense; otherwise its decrease in an isolated system would contradict the law of conservation of energy. In our view, it is a matter of terminology, and if one is confused by the term ‘energy’, other terms, such as ‘isobaric–isothermal potential’ or ‘Gibbs potential’ can be used. For a system with a single type of particles, if G is divided by the number of particles in the system, N (or moles), a quantity called the chemical potential (μ) can be obtained: μ = G/N

(3.2)

Let us consider the physical meaning of each term in equation (3.1). The internal energy, U, is the total energy contained by a thermodynamic system. This concept is fundamental, like the concept of energy over-all, so it is not possible to derive its definition from some general principles. Classical thermodynamics does not raise the question of the nature of internal energy [2]. In real physical and chemical processes one can measure only the change in internal energy (ΔU), which occurs when the system transfers from one state to another. To measure ΔU the concepts of heat (Q) and the work (A) should be used. The second term on the right-hand side of equation (3.1) is just a measure of mechanical work: for example, at a constant pressure the work done by the system (or with the system) is equal to A=PΔV, (3.3) where ΔV is the change of the volume of the system. In the analysis of SHS processes, the mechanical work is almost always negligible It is undoubtedly true for gasless chemical systems, since the volume change of the liquid or solid phases during the reaction is insignificantly small. But even in solid–gas systems, in which the volume of the gas reagent due to the reaction varies considerably, mechanical work can be neglected compared to the heat of the chemical reaction. Consider, for example, the reaction А(sol)+xB2(gas) = AB2x(sol), where the solid reactant A bonds with the diatomic gas B2 at a constant pressure P (constant pressure condition means that the initial volume of gas in the vessel is much larger than that required for the completion of the reaction). 31  

Due to the formation of solid product AB2xthe amount of the reactant gas is reduced by x moles, i.e. the corresponding change of the gas volume can be estimated in accordance with the Mendeleev–Clapeyron equation:

V r 

xRT P

and the work is done

Ar  PVr  xRT

(3.4)

(R = 8.314 J/mol·K is the universal gas constant). Here we have neglected a change in the solid product volume compared to that for the gas phase and also used the ideal gas approximation, due to which the Ar in the expression (2.4) does not depend on pressure. For approximate evaluation these assumptions are well justified. According to the formula (3.4) we can calculate, for example, that the work done during the reaction Ti + 0.5N2 = TiN is 1.25 kJ/mol. Comparing this value with the energy of formation of TiN from the elements, which is equal to 383 kJ/mol, one may conclude that for engineering calculations the mechanical work of the reaction can be neglected. Another important thermodynamic function, which for a system with a constant number of particles can be written as F =U −TS,

(3.5)

is called the Helmholtz free energy (Hermann Ludwig Ferdinand von Helmholtz, 1821–1894, was a German physicist and physiologist) or the isochoric–isothermal potential. Change of this function, ΔF, in any process, shows the maximum work which the system may carry out over the other bodies. Classical thermodynamics has historically developed, responding to the needs of creating heat engines (steam and internal combustion engines), which convert the internal energy to work. Therefore, the fundamental differences of the G and F functions in conventional thermodynamic processes are essential. However, as it is mentioned above, the mechanical work in the SHS processes is negligible, therefore the change of the free energy at constant volume, ΔF, and the change in free energy at constant pressure, ΔG, tend to have similar values. The other thermodynamic function H =U + PV,

(3.6)

is called enthalpy (from the Greek ενθαλπα – heating). The enthalpy is of great importance for the analysis of combustion and thermal explosion processes including SHS. The change of this function, ΔH, when the system moves from one state to another, shows the amount of internal energy and work that can be transformed into heat (note again that since the mechanical work in SHS processes is small, it is mainly the internal energy of the system that is converted to heat). The absolute value of the enthalpy is unknown, and, in any process only the enthalpy change is important (sees above about the internal energy). To facilitate practical calculations, an arbitrary selected ‘zero’ reference point, which is called the standardmolar enthalpy of 32  

formation from elements ΔH, or simply the standard enthalpy is usually introduced. It is considered that the standard enthalpy of elemental substances, consisting of identical atoms, is zero under normal conditions, i.e. at temperature of 298 K and a pressure of1 atmosphere. If a substance exists under normal conditions in several crystalline modifications (polymorphism), the zero standard enthalpy is attributed to the most stable modification (typically, this modification is most common in nature). For example, for carbon the graphite has a zero standard enthalpy ΔH (graphite) = 0 kJ/mol, and for diamond ΔH (diamond) = 1.828 kJ/mol. A similar approach is used for molecules and gases; for example, for molecular oxygen ΔH (O2) = 0, and for ozone ΔH (O3) = 142.3 kJ/mol. In contrast to the G and F functions, which remain unchanged only in equilibrium states, the enthalpy H remains unchanged in the isolated system, no matter what type of processes take place. This follows from the first law (the first origin) of thermodynamics. The wording of the first thermodynamic law differs, depending on to which system it is applied, but in any formulation this law is a consequence of the fundamental law of conservation of energy. For an open system, which exchanges heat and work with the environment, the formulation is as follows: the quantity of heat Q transmitted to the system (or taken away from it), is used for changing the internal energy of the system ΔU and for carrying out work A against external forces, i.e. Q = ΔU + A

(3.7)

For an isolated system Q = 0, and therefore, ΔU + A = 0. If the process takes place in an isolated system at constant pressure, it is obvious that ∆U + A = ∆U + P∆V = ∆H = 0, i.e. the first law of thermodynamics in this case takes the form of the law of conservation of enthalpy. The standard enthalpies of formation from elements were measured for essentially all known chemical compounds using different calorimetric approaches and can be found in handbooks. Therefore, based on the first law of thermodynamics and knowing the composition of the starting components and reaction products, one can calculate the amount of heat generated during the reaction and thus the temperature of the products. The practical use of such calculations will be discussed below, but now let us return to the formula (3.1) and consider the third and final term of the right-hand side of this equation. The product TS in expression (3.1) characterizes the part of the free energy which the system lost when transferring from one state to another and cannot be used for any useful work or to increase the temperature of the system. The concept of entropy (from the Greek εντρoπα – turning, transformation) was introduced in 1865 by the German physicist and mathematician Rudolf Julius Emmanuel Clausius (1822–1888) to determine the measures for irreversible dissipation of energy, the measure of the deviation of the real process from the ideal one. From a macroscopic standpoint for reversible processes, entropy (DS) is introduced as a state function, where changes (DS) are determined by the ratio of the amount of heat (Q) transferred to or out of the system at constant temperature T, to this temperature

S  33  

Q T

(3.8)

Clear understanding of the physical meaning for this thermodynamic function came later, with the development of statistical physics and the theory of the structure of matter. It was found that the entropy is a measure of disorder of the system, or a measure of the number of specific ways in which a thermodynamic system may be arranged. The concept of entropy is essential for the analysis of the reversibility or irreversibility of the processes that occur in nature and technology. It is the basis of the second law of thermodynamics, which states that in an isolated system only such processes can spontaneously proceed that lead to an increase of disorder of the system, i.e. increase of its entropy. This principle is fundamental, like the law of conservation of energy, as it is a generalization of scientific experience and cannot be derived from any other laws. At the same time, statistical physics makes it possible to consider some of the physical aspects of this principle. Let the number of different ways by which the given state of the system can form be W; obviously, for macroscopic systems, this number is very large. The value of W is called the thermodynamic probability. In statistical physics it is shown that the entropy S is proportional to the natural logarithm of the thermodynamic probability of the system: S = k ln W

(3.9)

Note that in principle the constant k can be any value, because the absolute value of the entropy is not defined and in any process only the change of this function is important (similar to the functions U and H discussed previously). However, to ensure that the statistical definition (3.9) is consistent with the thermodynamic one (3.8), it is that k is equal to the Boltzmann constant (kB = 1.38·10–23 J/K). For practical calculations of S and G values of fundamental importance is the third law of thermodynamics. Based on established benchmarks for the enthalpy and entropy, and using the results of measurements of some thermophysical properties, it is possible to calculate the change of free energy (ΔG) Based on established benchmarks for the enthalpy and entropy, and using the results of measurements of some thermophysical properties, it is possible to calculate the change of free energy (ΔG) in any chemical process at any temperature T. There are several ways for such calculations: 1. Calculation according to the Gibbs–Helmholtz equation: ∆GT = ∆HT − T∆ST

(3.10)

with the value ΔHTobtained by solving the Kirchhoff equation dH  c p dT

(3.11)

and ΔSTis calculated from the formula:

S  c p

T , T

(3.12)

where cP is the heat capacity of the system at constant pressure, and ΔcP is the heat capacity difference between the reaction productsand the precursors determined from 34  

the calorimetric measurements. Heat capacity, by definition, is the amount of heat that must be transferred to system to raise its temperature by one degree. Usually, the specific heat per gram or one mole of constant pressure (cp) or constant volume (cv) is determined. For solids and melts the difference between cp and cv is insignificant, so that the heat capacityof solid bodies is often denoted just by letter c. 1. Calculations by the Temkin–Schwartzman method are carriedout by integration of the expression in the range from 298 K to T:

H  G  d    2T d T  T  this gives:

0 GT  H 298  T  i 0 ai M i N

(3.13)

(3.14)

To calculate ΔGT by this formula, it is sufficient to know the heat of the reaction, the standard enthalpies of formation at 298.15 K for all substances, and the coefficients A, B, C in terms of the temperature dependence of heat capacity c(T) = A + BT + CT2.

(3.15)

These values, as well as the integrals designated Mi can be found in thermodynamic reference books. 2. Calculations using the reduced isobaric potentials:

Ф 

0 GT0  H 298 T T

(3.16)

which are also given in the thermodynamic handbooks. Combining the tabulated values of Ф’ for the initial reagents and products, ΔGTcan be determined as follows 0 GT0  TФ  H 298

(3.17)

All spontaneous reactions occur with a decrease in free energy, which reaches the minimum at equilibrium, and under constant conditions remains further unchanged. Depending on the nature of the processes, the criterion for equilibrium is defined by the extreme value of one of the thermodynamic functions (see Table 3.1). As can be seen from the table, the first two cases are related directly to the combustion processes. Note that some above defined values, such as G, U, F, H and S are functions of the state, i.e. for the given parameters of the system– the component concentrations, temperature, pressure, volume – they are uniquely determined. During transition from one state to another the difference of their values in the initial and final states is independent of the pathway between the states (this rule, known as Hess’s law, which is another specific case of the law of conservation of energy): ΔG = G2 – G1; ΔU = U2 – U1; ΔH = H2 – H1; ΔF = F2 – F1; ΔS = S2 – S1. At the same time the concepts of heat (Q) and work (A) do not relate to the system but relate to the 35  

processes. They are not energies but the forms of energy transfer [2]. Since the work in the considered combustion processes is negligibly small, the energy is transferred in the system only in the form of heat, which leads to high temperature of the products. Table 3.1 Criteria for spontaneous occurrence of a chemical process and the establishment of equilibrium Constant parameters

Equilibrium criterion

Process examples

U and V

Criterion for occurrence of direct process ΔS> 0; S → max

ΔS = 0

H and P

ΔS> 0; S → max

ΔS = 0

T and P

ΔG < 0; G → min

ΔG = 0

T and V

ΔF< 0; F → min

ΔF = 0

S and P S and V

ΔH< 0; H → min ΔU< 0; U → min

ΔH = 0 ΔU = 0

Adiabatic combustion in a constant volume reactor (the volume of the reaction mixture is comparable with the volume of the reactor), combustion of hybrid SHS systems solid–gas Adiabatic combustion in the constant pressure volume or with expansion to constant pressure (in a rocket engine, in a furnace of power equipment), combustion of gasless SHS systems Isothermal reaction at constant pressure, for example, reactive sintering in the absence of thermal explosion Isothermal reaction at constant volume, for example, slow reaction inside a thermostatically controlledhermetic vessel, filled with the reaction mixture Isoentropic compression or expansion of gases Isoentropic heating or cooling of the gas in a constant volume vessel

3.2. Equilibrium, reversibility, stationary and stability ofthe SHS processes and products With the required thermodynamic values defined, we are ready to discuss the concepts of equilibrium, reversibility, stationary and the stability of the real SHS processes and products on a macroscopic scale. Can we consider the combusting sample to be an isolated system (Fig. 3.1)? The answer depends on the specific times of reactions and product formation, as well as the rate of heat exchange with the environment. The time of chemical transformation in the combustion wave is in the range of treact = 0.001–0.1 s. The total time required for the process to be completed along the whole sample depends on the sample size and the velocity of combustion wave propagation; for the most common laboratory conditions this time is of the order tcomb = 1–10 s. The heat generated by the exothermic reaction in the sample is transferred to the environment by radiation, heat conduction along the contact surface with the solid parts of the reaction chamber, and by convective heat transfer in the surrounding gas atmosphere (Fig. 3.1, b, c). Due to these heat losses the sample cools down to ambient temperature with typical cooling time tcool = 100–1000 s (the larger the sample and smaller the heat loss, the longer the cooling time). Thus, treact100 μm) layers, one can follow the sequence of structural transformations in the diffusion regime of interaction. In the diffusion regime the kinetics of product layer growth in the gas–solid system obeys a parabolic law, which can be written as

v

D (C a  C min ) , C min 72

 

(5.7)

where D is the diffusion coefficient of nitrogen in the nitride phase. When the pressure of nitrogen over the surface is significantly greater than Pe, the problem (5.2) and (5.3) with a kinetic law (5.7) has similar solution * Ydiff   ( P0  Pe )  t 1 / 4 ,

* where Ydiff is the distance at which the pressure changes from P0 to Pe and





  2BCmin / 3(Cmax  Cmin ) 2D(Cmax  Cmin ) . Complete penetration Y* = Y*K+Y*diff of the infiltration front into the sample at various nitrogen pressures is shown in Fig. 5.3. Thus, at T = 2200 K and P = 105 Pa this value does not exceed Y* = 400 μm. Clearly if y T01. The non-reacting parts of the silicon and silicon nitride surfaces are in thermal contact with nitrogen through the Newtonian type heat exchange with characteristic heat transfer coefficients α1(Si) and α2(Si3N4). The following processes take place on the reacting surfaces of Si, y1(t), and Si3N4, y2(t), respectively, – phase transition: Si condense → Si gas with the heat of evaporation L; – heterogeneous chemical reaction: Si gas + N2 → Si3N4 with the heat of reaction Q.

Fig. 5.4. The VLS mechanism of crystal growth of silicon nitride in an SHS wave

The gas-dynamic relaxation time of the cell is of the order of 10–6 s, which is significantly less than the evaporation time of the silicon particles, so the pressure gradients along the length of the capillary are insignificant. Typically, the local partial vapour pressure of silicon is much smaller than the nitrogen pressure ( PN 2  PSi ) , and if there are no infiltration difficulties, then PN 2  Pexternal . The behavior of such a reaction cell in the first approximation can be described by a system of one-dimensional equations derived from a two-dimensional problem by averaging over the x coordinate (for details see [3]): 74  

с1 1

T 2  2T T 1  2T   22 ;   21 ; с2  2 t y t y

y   y1 , y 2  :

 g



t

 (U g ) y

 f (t ),

 2Tg  T  2T  с g  g  1  U 21    g  c g f (t )Tg  T0  y 2 y   t a (Ua)   a     D . t y t  y 

with the boundary conditions y = y1(t):

 y1 1  U  y1  g  y1 1  U  y1 1a  1 D

a y

 y1 1 L  (1   )(c1  c2 )T10   1 a = asat (T10);

T T1  g g y y

T10 = Tg (y1) p;

y = y1(t):

 y 2  2  U  y 2  g  y 2  2 v  U  y 2 1a  1 D

a y

 y 2  2 vL  Q  (1   )(c1  c2 )T20   2 a = 0, y = 0: y = y3:

T20 = Tg (y2) T 1 1   T1  T0 ; y

2

T2   T2  T0 ; y

P = P1 + P2;

a  a sat T  

v

g 

2P RT

 1   2 a

P0 exp  L / RT  a ;  1 RT 

g 31 ; D  Le 31   2 c 75

 

T T2  g g y y

wherein T1(y) – the temperature of the silicon particle; T2(y) – the temperature of the nitride crystal; Cg, ρg, λg – heat capacity, density and thermal conductivity of the gas; ci , ρi , λi (i = 1.2) – the thermal physical constants of silicon and silicon nitride, respectively; P – total gas pressure; a – molar concentration of the silicon vapour; cρ1, cρ1 – heat capacities of Sig and N2 ; a sat – saturated vapour pressure of silicon; μ1, μ2 – molecular masses of Sig and N2 ; f(t) – the average mass flow rate of nitrogen into the reaction capillary; P0 – pre-exponential factor. As noted above, this problem is formulated for the conditions in which the thermal relaxation time of the cell is much less than the evaporation time of silicon particles. This means that the steady-state distribution of temperatures and pressures is established in the cell. This solution was studied analytically [3] and the following generalized macrokinetic estimates for the average mass evaporation rate of the particles m ( m ) and mean local heating of condensed phases relative to the temperature of nitrogen (ΔTi = Ti0 – T0) have been obtained m

g   c

T10 



P0  exp L / RT0 , P

d Q P0    exp L / RT0 ,  c P

where d and Δ are the typical diameters of the reactive and transport pores. From the analysis of these expressions we can make the following conclusions: – In the diffusion regime, the reaction rate of silicon with nitrogen in the SHS wave has the Arrhenius dependence on temperature with activation energy equal to the heat of silicon evaporation (L). For the typical conditions of the combustion process (P = 6–10 MPa, T = 2000–2300 K) the evaporation time of the Si particle with a radius of 10 μm is about 40–50 s, which is in good agreement with experimental observations. By reducing the heat of evaporation by doping the silicon (e.g. by aluminium), one can significantly accelerate the process; this conclusion was experimentally confirmed in [2]. – An even more surprising feature of the proposed mechanism is that in order for the process to become self-sustaining in the quasi-stationary mode it is absolutely necessary to have the thermal inhomogeneity of the reaction cell, i.e. ΔTi = Tio – T0 ≠ 0. The estimations made for the characteristic combustion conditions show that the degree of overheating of the reaction surfaces should be small, ~20 K, which is ~1% of the combustion temperature. But such small overheating may ensure the evaporation and the reaction of silicon with nitrogen through the VLS mechanism. – The excess pressure of nitrogen in the pores reduces the growth rate of crystals, slowing the process of transfer of silicon into the gas phase. In the absence of infiltration difficulties, the decrease of nitrogen pressure should increase the rate of nitride formation. This conclusion was experimentally confirmed in the studies of the mechanism of reaction sintering of silicon in nitrogen [4] and combustion of Si particles at high gas pressures [5,6], where the effect of reducing the rate of the process with increasing pressure of the gaseous reagent was observed. – Finally, the reaction in gas–solid systems can occur in the gas phase, as shown in Fig. 5.1d. The vapour of the condensed reagent (e.g. silicon) reacts with the gaseous 76  

reagent (nitrogen), and the crystals of the product nucleate and grow in the gas phase and are deposited on the surface of the particles, the walls of the reaction vessel, etc. Probably, the SHS α-Si3N4 phase forms by this mechanism [7]. To date, no theoretical models describing the formation this type of structure in the combustion wave have been developed. CONTROL QUESTIONS 1. 2. 3. 4. 5. 6.

List and briefly describe main mechanisms of microstructure formation in SHS. What methods of experimental diagnostics know? What does it mean “quenching of the combustion wave” What are differences between primary and secondary structure formation? Is it possible to monitor process of crystal structure formation during SHS (in situ)? Describe general features and schemes of time-resolved methods of XRD and SRD analysis. REFERENCES

1. Borovinskaya I.P., Merzhanov A.G., Mukasyan A.S., et al., Macrokinetics of structural formation during filtration combustion in titanium-nitrogen system, Dokl. Akad. Nauk SSSR, 1992, Vol. 322, No. 5, pp. 912–917 (Russ). 2. Mukasyan A.S., Combustion synthesis of nitrides: Mechanistic studies, Proc. Combust. Inst., 2005, Vol. 30, pp. 2529–2535. 3. Stepanov B.V., Mukasyan A.S., Shkadinskii K.G., Macrokinetics of Si3N4 formation during the gas-phase silicon transfer, Dokl. Akad. Nauk SSSR, 1988, Vol. 302, No. 1. pp. 145–149 (Russ). 4. Andrievskii R.A., Spivak I.I., Silicon Nitride and Related Materials, Moscow: Metallurgiya, 1984 (Russ). 5. Skibska M., Szulc A., Mukasyan A.S., Rogachev A.S., Microstructural peculiarities of silicon nitride formation under high nitrogen pressures. I: The influence of initial Si particle size distribution on Si3N4 morphology, Int. J. SHS, 1993, Vol. 2, No. 1, pp. 39–48. 6. Skibska M., Szulc A., Mukasyan A.S., Shugaev V.A., Shiryaev A.A., Microstructural peculiarities of silicon nitride formation under high nitrogen pressures. II: The effect of nitrogen pressure on morphology and phase composition of SHS-produced Si3N4, Int. J. SHS, 1993, Vol. 2, No. 3, pp. 247–251. 7. Mukasyan A.S., Borovinskaya I.P., Structure formation in SHS nitrides, Int. J. SHS, 1992, Vol. 1, No. 1, pp. 55–63.

77  

C

hapter

6

SHS OF TiB2-Al2O3 and CrB2-Al2O3 CERAMICS

Among the variety of refractory and heat resistant ceramics, of special interest are materials from metal borides [1-3]. Transition metal borides possess a unique complex of physical and chemical properties including high hardness, heat resistance, high temperature strength, high electric and heat conductivity, resistance to the action of melts, radiation and wear resistance. Such materials are widely used as the promising candidates in many fields of engineering, electronics and power industry [2,3]. However, strong covalent bonds inherent to transition metal borides lead to a low plasticity and low bending strength, which limiting their application. In this regard, currently much attention is paid to the technology of production of composites based on transition metal borides in combination with more plastic phase. For example, aluminum oxide can play the role of a high temperature binder and also decrease the content of expensive borides in composite materials. 6.1. Self propagating high temperature synthesis of boron containing ceramic materials: the state of art

At present, the main methods for production of borides are methods of physical and chemical vapor deposition, as well as self-propagating high temperature synthesis (SHS). In work [4], using two methods of ionic deposition, nanostructural boride and nitride nanofilms based on transition metals were obtained on different supports. Under optimum conditions the boride and nitride films were fabricated, with the thickness of 3200 and 8600 nm, respectively, and the transition layer of ~100 nm consisting of boron and nitrogen compounds with atoms of the corresponding supports. The authors of [5] obtained filament-like crystals of titanium, zirconium and hafnium borides with the length of fibers in the range 0.4-0.8 µm. However, above methods for production of refractory ceramics are labor-consuming and expensive. Bulk materials from the powders are manufactured by conventional methods of powder metallurgy, i.e. sintering or hot pressing, which are also energy consumption methods. Frequently, production of these materials cannot be realized within conventional notions on equilibrium states and hence requires new approaches and methods for synthesis of a special class of ceramic composites [1,2]. Specifically, the synthesis of borides directly from elements is one of the simplest method providing the most accurate composition and maximum degree of purity but at the same time the most expensive one. Typically, a reactive sintering with hot 78  

compac-tion takes place at temperatures lower than the melting points of the precursors and thus occurs by solid phase interaction. The controlling stage of the process is believed to be diffusion of boron into metal through the layer of the formed product. Main disadvantages of such sintered boride-based alloys are their friability and insufficient thermal stability, which are related to specific properties of transition metal borides [1,3]. Production of transition metal borides by the SHS method is widely used in many branches of national economy. However, their usage in pure form is impeded due to their cold brittleness. Therefore, attention is paid to the development of SHS technology for production of materials based on borides and one of the promising directions is related to liquid phase recative sintering with metal binding [4-6]. At the Merzhanov’s Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences(Chernogolovka, Russia), the SHS-technology was developed to produce boron containing powders such as BN, TiB2, B4C, B13C2, TiB2Al2O3, B4C-Al2O3, using oxides of the corresponding elements and magnesium or aluminum as a reducer. Technical characteristics of boron containing powders are not inferior to the best furnace analogs and by most properties even exceed them. The cost of SHS powders is 1.2-3.0 times lower than that for their analoges [7]. The use of powder aluminum as an active reducer allows to reach high reaction temperatures (1500-2000K), which are necessary for self-sustained in SHS-processes [8,9]. Formation of TiB2-Al2O3 and NbB2-Al2O3 compounds under natural conditions with a wide range of phase composition was carried out by SHS method including thermite reactions [10]. Thermite mixture Al-TiO2 and Al-TiO2-B2O3 were introduced into Ti-B system for formation of TiB2-Al2O3 cermets. It was shown that the increase of the Al2O3 content in the thermite mixture leads to decrease of the reaction temperature and the velocity of combustion front propagation. For synthesis of NbB2-Al2O3 cermet, two thermite mixtures Al-Nb2O5 and AlNb2O5-B2O3 were used. It was demonstrated that the usage of B2O3 effectively improves formation of the product. The Al-TiO2-B2O3 thermite mixture possesses a slightly higher combustion velocity than that for Al-TiO2 mixture (Fig. 6.1) and allows synthesis of a wider range of the compositions.

Fig. 6.1. The dependence of combustion front propagation velocity in SHS process for two different thermite mixtures for formation of TiB2-Al2O3 [11].

79  

It was suggested that this may be caused by formation of a liquid phase of B2O3, which improved the contact between the reacting particles [11]. The reaction mechanism for synthesis of TiB2-Al2O3 was proposed in [12-14]. The subsequent reactions necessary for formation of TiB2-Al2O3 compounds in Ti-B-Al-TiO2-B2O3 system are presented below: Ti + B = TiB

(6.1)

4Al + 3TiO2 = 3Ti + 2Al2O3

(6.2)

Al + B2O3 = 2B+ Al2O3

(6.3)

TiB+ B= TiB2

(6.4)

Interaction of Ti with B is the first step resulting in formation of TiB, followed by reduction reactions between termite reagents according to reactions 6.2 and 6.3. Then the intermediate TiB phase transforms into TiB2 via the reaction (6.4) [11, 14]. In work [15], ceramic powder of TiB2 was synthesized using Mg-TiO2-B2O3 mixture. The effect of TiB2 additive as a dilutor on the SHS process was studied. The result of thermodynamic calculations and experiments showed that the increase in the content of the TiB2 from 0 to 20 mass % decreases adiabatic temperature from 3100 K to 2896 K and the combustion temperature from 2139 K to 1621 K, respectively. The average size of synthesized particles and width of the particle size distribution increase with the increase of the TiB2 content. In work [16], the processes of high temperature binding of titanium diboride-aluminium oxide powder composite during wetting of cathode for aluminium electrolyzer was investigated. XRD analysis and electron microscopy were used to study the formation of contact surfaces and boundary phases. It was shown that a determining role in the processes of binding the grains with a monolitic body is played by boron anhydride, which is a product of TiB2 oxidation during thermal treatment of aluminium borates Al4B2O9 and Al18B4O33. Electrical resistance of the obtained material with the relative density of 0.60-0.62 sharply decreases with the increase in burning temperature, but above 1200 K it changes only slightly, having the values within (1-3)*10-3 Ohm*m. The authors of work [17] carried out investigations on the structure of TiB2-CrB2, TiB2-W2B5 ceramics and analyzed relations between their structures and mechanical properties. They concluded that: 1) in these systems during formation of a solid solution the dominant structure is TiB2 phase; 2) anisotropic changes in the lattice structure during formation of solid solution (Ti, W) B2 can be caused by the competing effect of dissolved atoms of tungsten in the excess of boron; 3) the increase in the temperature of hot compaction resulting in formation of one phase solid solutions and leads to production of dense ceramic materials with enhanced strength characteristics. Boron containing SHS powders (BN, TiB2, B+Mg3B2) can be used as biological shielding in nuclear engineering, abrasive powders and pastes, ceramic products with high temperature strength, solid lubricants (BN hexagonal) and oil additives [8]. In work [18], the authors proposed a method for synthesizing refractory compounds of rare earth metals MeB2 in two stages. The first stage is high temperature synthesis from elements at high pressures and the second stage is additional annealing in argon 80  

atmosphere. As a result, samples of terbium, erbium, thulium and lutetium diborides were produced with the content of impurity phases less than 3 wt. %. Composite materials based on chromium and aluminum oxides were obtained by SHS method in [19,20]. The authors studied the reactions of aluminothermy oxidation of chromium oxide under the SHS conditions at different ratio of reagents. It was shown that chromium-aluminum termite burns by a complex mechanism and the final product forms via several subsequent transformations. Levashov. et al. [8] investigated fabrication of ceramic materials based on chromium and titanium borides by SHS-compaction from preliminary mechanical activated reactive mixtures. The use of mechano-chemical activation (MA) allows SHS-process in low exothermic systems such as Mo-B, Cr-B. Bulk chromium boridesbased articels with large dimensions (125 mm in the diameter) were produced. Addition of titanium into the reaction mixture allowed decreasing the residual porosity to 6% in Cr-B mixture to 2% in Ti-Cr-B mixture, which results in the increase of the mechanical properties [8,21]. The size of the reagents is played an important role to fabricate SHS materials with desired morphology [22,32]. In this regard, SHS is similar to nanotechnology. The SHS materials with nanocrystalline grains possess unique physical, chemical and functional properties [22,33]. The structure and, correspondingly, properties of nanomaterials are formed at the stage of their production [22, 32-37]. The use of mechanical activation (MA) before the SHS results in formation of nanostructural materials. Mechanical activation of reagents is a very important stage. It results in enhance of the reactivity of the reagents due to the increase of the defects density, as well as the reaction surface. All above allows one to decrease apparent activation energy of the chemical transformations. The authors of work [22] developed a method and technology for production of (Ti, Cr)B2-based materials by high temperature mechano-chemical synthesis (HMS). This technology allows obtaining TiB2 and (Ti, Cr)B2 cermets [23,24]. This technology allows obtaining TiB2 and (Ti, Cr)B2 with the size in the range 40-100 μm. Also, borides were obtained by the method of mechanical activation in works [23,24]. Formation of the microstructure of the materials is of no less interest than formation of their crystalline structure. The microstructure is formed due to destruction of structural components of the initial reaction mixture, nucleation and growth of grains (crystals) of products, recrystallization, sintering and other process. To study micro structural transformations, it is necessary to “quench” the combustion wave so that to investigate the intermediate micro structures on the different stages of synthesis. The existing methods for quenching of SHS-process allow us to obtain a sufficiently reliable pattern of micro structural transformations in SHS wave. Its advantages are relative simplicity and low cost. For example, one may quenched the combustion wave controlling the heat losses by changing angle of the wedge in the massive copper block [21, 25]. 6.2. Experimental

The powders of the following reagents were used: ‒ amorphous boron, B, (brand B-99A, the content of boron is 99,1 %, the specific surface area is 12 m2/g, an average size of particles is 4μm); 81  

‒ boric acid (H3BO3 Inder deposit, West Kazakhstan, the content of H3BO3 not less than 98%); boron oxide (B2O3; of 99,9% purity); – titanium (Ti; purity of 99%, average size of the particles 80 μm); – titanium oxide (TiO2; rutile; 99,9% purity); – chromium oxide (Cr2O3; 99,8% purity); – aluminium oxide (Al2O3; f 99% purity); – aluminium, (Al, PA-4; 99% purity, particels size 65 μm); – magnesium (Mg; ACD-4; purity 99%, particle size less than 40μm). To reveal the mechanisms of formation of TiB2-Al2O3 composites, thermite mixtures based on titanium, boron and their oxides were studied according to the following recations (6.5-6.13): (1-x)(Ti+2B)+x(TiO2+2B+4/3Al)= TiB22/3x Al2O3

(6.5)

X=0.75 0.25Ti+0.75 TiO2+2B+Al= TiB2+0.5 Al2O3

(6.6)

X=0.85 0.15 Ti+0.85 TiO2+2B+1,13Al= TiB2+0.56Al2O3

(6.7)

X=1 TiO2+2B+4/3Al= TiB2+2/3Al2O3

(6.8)

(Ti+2B)+y(TiO2+2 B2O3+10/3Al)=TiB2+5/3yAl2O3

(6.9)

Y=0.4 0.6 Ti+1.2 B+0.4 TiO2+0.4B2O3+4/3Al=TiB2+2/3Al2O3

(6.10)

Y=0.8 0.2 Ti+0.4 B+0.8 TiO2+0.8 B2O3+8/3Al= TiB2+4/3Al2O3

(6.11)

Y=1 Ti+B2O3+10/3 Al= TiB2+20/3Al2O3

(6.12)

(Ti+2B)+n Al2O3 = TiB2+nAl2O3,

(6.13)

where x, y, n are stoichiometric coeficients characterizing mole fractions of Al2O3 being formed in TiB2-Al2O3 composite. Parameters x and y were changed within the range of 0.4-1. In reactions (6.13), the content of aluminium oxide in the (Ti+2B) +x Al2O3 mixture was changed from 0 to 50%. SHS was carried out in a chamber of conctant pressure (Fig. 6.2) in the atmosphere of argon at pressure 1 atm. Air was evacuated from chamber with the help of a vacuum pump (1), then it was filled with argon. Pressure in the chamber was controlled with the help of a vacuum guagu (2). In case when it was necessary to realize combustion at elevetaed initial temperature, sample (13) placed on a ceramic support (14) was heated by molybdenium furnace (3) to which electric current was applied from power unit (4). Heating was controlled by a tungsten-rhenium thermonouple (5) with the junction thickness of 200 μm connected to controller (6). Combustion was initiated from the upper surface of the sample by an incandescent tungsten coil (7) heated by electric current supplied from power unit (8). The process was registered by a video camera (10) connected to video tape recorder Panasonic NV-SD450 (11) and TV Supra (12) through peephole (9) and light filter (16). Combustion rate is determined by video recording. Video camera shots with the velocity of 30 frames per second. 82  

Thus, the experimental unit allows to keep a video record of combustion processes at different initial temperatures of the sample and register the combustion temperature. The dependence of combustion velocity (u) as a function of combustion temperature (Tc) was determined.

1–roughing-down pump, 2–vacuum meter, 3–molybdenium furnace, 4–electric power unit of molibdenium furnace, 5– thermocouple, 6–thermocouple coutroller, 7–tungsten coil, 8–power unit of tungsten coil, 9–peephole, 10–video camera, 11–video tape recorder, 12–television set, 13–sample, 14–ceramic supporp, 15–bottle with argon, 16–light filter Fig. 6.2. A scheme of the experimental unit.

Then, effective kinetic parameters, i.e. effective activation energy (Eeff) was calculated based on the knowledge of the temperature coefficient of combustion velocity (αT): αT =d(ln(u))/dTc

(6.14)

Eeff =2RTc2αT,

(6.15)

where: R-universal gas constant 8.314 J/molK.

Fig. 6.3. Quenching of SHS wave in a wedge-shaped sample obtained using a conventional video camera (left) and thermal imaging system (right).

83  

“Quenching” of the combustion wave in SHS process was carried out by using the massive cooper (Cu) blocks. The reaction mixture is ignited under a wide edge of Cu blocks and in the course of combustion, specific heat losses increase resulting finally in reaction extinction (Fig. 6.3). By this method, it is possible to reach cooling rates of ~103 K/s. Adiabatic combustion temperatres were calculated by using software package “Thermo” (ISMAN, Chernogolovka, Moscow region). Electron microscopic investigations (SEM) and energy dispersion spectral analysis (EDX) were carried out on scanning electron microscope Hitachi S-4800 FE-SEM (Japan) and Quanta 200i3D. XRD analysis was performed with the help of diffract meters DRON-3 and Rigaku RINI-200 in CuKa1-radiation. 6.3. Results and discussions 6.3.1. Layer-by-layer structural analysis of quenched samples

SHS process was arrested by “quenching” method described above and followed by layer-by-layer analysis of the sample in 0.75TiO2-0.25Ti-2B-Al system. The magnified fragment of reaction zone of quenched SHS wave presented in Fig. 6.4a. It can be seen that the initial reagents of the mixture melt and particles of titanium with boron form titanium diorite crystals. Aluminum oxide grains wet the plates and grains of titanium diorite. Inside the pore of the system under study there were detected crystalline filamentary structures of aluminum dioxide (Fig. 6.4b) formed during melting of aluminum with titanium dioxide. Thus, the primary structure formation syage involves the dissolution of refractory reagent in the melt of the other reagent and separation of the solid granules of the product from the supersaturated melt. a

Zone No.

Atomic ratio, % O Al

B

1

b

Ti

57.32

39.69

2.99

2

7.76

52.99

37.93

1.32

3

44.13

24.23

20.69

10.95

4

49.98

13.43

7.44

29.14

Z on e  No. 1 2 3

A tomic  ratio, % B

O

Al

Ti

19.01 13.42

42.87 29.45 40.40

56.45 50.00 42.60

0.67 1.54 3.58

Fig. 6.4. (a) Reaction zone with formation of TiB2 grains, (b) formation of Al2O3 fibres in pores of the reaction zone.

84  

SEM analysis confirmed the formation of α-Al2O3 fibers with the length of 5 μm and diameter d=700nm. It is known that formation of filamentary crystals may occur by two mechanisms, either “vapour-crystal” or “vapour-liquid-crystal”. The process of formation and peculiarities of the mechanisms of aluminum oxide fibers were studied in work [25-29]. For example, it was suggested that Si3N4 crystals in the reaction zone grow by the mechanisms “vapour-liquid-crystal” (VLC). The presence of small amount of Fe, Ca, Zn, Al impurities contribute to realization of VLC mechanisms. There impurities form stable droplets of eutectic melt, which work as the nucleation centers for silicon nitride crystals [24].

6.3.2. SHS parameters

Figure 6.5 presents dependences of adiabatic combustion temperature of combustion as a function of Al2O3 content in TiB2-Al2O3 composites. For all systems, the increase in Al2O3 content (X) leads to decrease of adiabatic combustion temperature. Maximum Tad~3200K is observed for elemental system Ti-2B. Also, due to a higher heat of B2O3 formation in comparison with TiO2, the adiabatic temperature in Al-Ti-TiO2-B-B2O3 system is higher than that in Al-Ti-TiO2-B and decreases from 2662 to 2500°C with the increase in the content of Al2O3 from 40 to 50 mass %. Ti-B-Al2O3

3200

Ti-TiO2-B-Al Ti-TiO2-B-Al-B2O3

Y Axis Title

3000

2800

2600

2400

2200

2000 0

10

20

30

40

50

60

70

X Axis Title

Fig. 6.5. The dependence of adiabatic temperature on the content of aluminium oxide

SHS was carried out for all mixture in the chamber at constant argon pressure of 1 atm. Measuring the combustion velocity was made by treatment of the frames obtained by video recordings of the SHS process. The dependences of combustion velocity as a function of the content of aluminum oxide for all systems, both preactivated and non-activated, are shown in Fig. 6.6. It can be see that the highest combustion velocity of 3.8 cm/s is observed for elemental reaction of TiB2 formation in Ti-2B-0% Al2O3 system. For the system with the alumina content of 20%, the Uc is higher for pre-activated mixture (4 min), than that that for a non-activated mixture. This effect can be explained by the effect of MA-acceleration of volume and surface 85  

diffusion process, i.e formed active surfaces contribute to acceleration of the heterogeneous reactions [23]. The combustion velocity decreases with the increase in the content of aluminum oxide. This effect can be explained by decrease of the combustion temperature (see Fig. 6.5). Also, Fig. 6.6 shows that system Ti-TiO2-B-B2O3-Al system possesses a slightly higher rate of combustion and a wider range of composition that shows a better level of the reaction stability. In reaction (6.9) with y=1, i.e. when the mixture involves only Al, TiO2 and B, it was possible to achieve stable composition with the rate of 0.48 cm/s. 4,0

Ti-2B-Al2O3 Ti-2B-Al2O3 activated.

3,5

Flame-front velocity,sm/s

Ti-TiO2-B-Al 3,0

Ti-TiO2-B-Al-B2O3

2,5 2,0 1,5 1,0 0,5 0,0 0

10

20

30

40

50

60

70

Al2O3 Content in TiB2-Al2O3 composites (mol%)

Fig. 6.6. The dependence of combustion rate on the content of aluminium oxide.

Changing of the combustion temperature by pre-heating of the initial mixture or by its dilution with the inert product and measuring the combustion wave velocity, it is possible to determine the the process-activation energy (see Eq. 6.14 and 6.15). As a rule, the measured activation energy is compared with that for some elementary process, which may limit the reaction rate. Figure 6.7 shows the calculated values of effective activation energy for SHS-systems under study.

Fig. 6.7. Determination of the effective activation energy.

86  

6.3.3. Analysis of the composition and morphology of the combustion products To get a deeper notion on the reaction products, they were studied by XRD analysis. According to the XRD data, the products primarily consist of titanium diboride and aluminum oxide. However, it is found that they also involve small amounts of aluminum nitride (Fig. 6.8a, 6.8b). This is likely due to interaction of a small part of aluminum with nitrogen of air. Finally, it can be seen that peaks for Ti-TiO2-B-B2O3-Al system are more intensive compared to those of TiO2-2B-1.33 Al system. The XRD data of SHS products in Cr-B-Al2O3 system is presented in Fig. 6.9. It can be seen that main phases are chromium boride and alumina, as well as traces of aluminum nitride was detected. TiB2

400

300

Intensiry, counts

Intensity, counts

600

TiB2

350 TiB2

250 200 Al2O3

150

Al2O3 Al2O3 TiB2 Al2O3

100

Al2O3 Al2O3

TiB2

TiB2

Al2O3 Al2O3

50

500

400 TiB2

300

200 TiB2

TiB2 TiB2

Al2O

Al2O3

100

TiB2

Al2O

Al2O

Al2O

Al2O Al2O

TiB2

Al2O3

TiB2 TiB2

0

0 20

30

40

50

60

70

80

2, deg

20

30

40

50

60

2 deg

Fig. 6.8. X-ray pictures of SHS products in Ti-TiO2-B-B2O3-Al (a) and TiO2-2 B-1.33 Al (b) systems.

Fig. 6.9. X-ray phase analysis os SHS products for Cr2O3-B2O3-Al system.

87  

AlTiB O2 2 Al2O

70

Al2O

80

Figure 6.10 shows the typical microstructure (a) and results of elemental EDS analysis (b) of the SHS product. In addition EDS mapping of all elements are presented in Fig. 6.11. It can be seen that product involves alumina fibers and small (~ 5 µm) particles of chromium boride.

Fig. 6.10. Microstructure (a) and EDS spectra of SHS product in Cr2O3- B2O3-Al system

Fig. 6.11. Distribution of elements on the surface of SHS-product. Energy dispersion X-ray spectral analysis (EDS).

Close look on the morphology of the alumina fibers shows that they have peculiar structure (Fig.6.12). These α- Al2O3 fibers have length in the range of 10-25 μm and submicron (200-500 nm) diameter. Such dispersion in size can be explained by selfturbulence of the system, caused by non-uniformity of temperature [30]. Formation of fibers is typically related to gas phase reactions proceeding with partial gasification of reagents [31]. Aluminium can serve as a concentrator of the growth for filamentary crystals by VLC mechanism. I.P.Borovinskaya with coworkers, observed formation of 88  

filamentary fibres in SHS reactors in the pilot units for production of refractory powders. Formation of fibres was related to gas phase reactions proceeding with partial gasification of reagents [31]. To reveal the structure in the cross-section, micro section metallographic specimen was prepared. Figure 6.13 presents the corresponding pictures of the obtained structures. The regions containing aluminum oxide and titanium diboride in the system were determined. For a cross section metallographic specimen, using the method of energy dispersion X-ray spectral analysis, maps of distribution of elements in the microstructure of the synthesis product were also obtained for the cross-section of the sample (Fig. 6.14). It can be seen that the structural components contain aluminium, oxygen, boron and chromium. a 

b

c 

d

e 

f

Fig. 6.12. Microstructure of Cr2O3-B2O3-Al mixture products. (a) general picture of final products, (b) filamentary crystals (nibs) from aluminium oxide, (c,d,e)-a straight, wave and twested form of filamentary crystals, (f)-branching of several crystals.

89  

Fig. 6. 13. The results of SEM and EDS in different points of the sample.

Fig. 6.14. Distribution of elements in the cross-section of SHS product and energy dispersion X-ray spectral analysis.

90  

As a result we may suggest the following mechanism of chemical reactions and micro structural transformations in the combustion wave for Cr2O3-B2O3-Al system. In SHS process, chromium and boron formed as intermediate products by the reactions (21-22), followed by formation of chromium diboride (23): 2Al+ B2O3=2B+ Al2O3

(21)

2Al+ Cr2O3=2Cr+ Al2O3

(22)

Cr+2B=CrB2

(23)

The initial mixture is heated in the combustion wave front to the melting point of boron oxide (753 K). At this temperature boron oxide melts and quickly wets solid particles of aluminum and chromium oxide, thereby significantly increasing the specific surface of the contact between the reagents contributing to reduction of boron (21). The process of the system combustion starts with melting of aluminum (933 K), then a solid phase of aluminum oxide starts crystallizing out of the melt. The melting point of aluminum oxide is 2317 K, i.e. higher than the maximum temperature of combustion of the system under study. The further increase in temperature leads to final formation of titanium diboride and aluminum oxide phases. 6.4. Conclusions

1. The regularities and peculiarities of aluminotermic combustion of boron containing compounds with titanium and chromium oxides has been investigated. The dependences of the macrokinetic parameters of SHS vs the composition of initial reagents were determined 2. The effect of preliminary mechanical activation on maximum temperature, combustion rate and phase composition of SHS final products is revealed. 3. It is found that in the course of aluminotermic SHS in air of boron containing systems, beside diborides, there submicron filament-like crystals of aluminium oxide are fromed. It is suggested that formation of such submicronic particles occurs through the “vapour-liquid-crystal” mechanism. 4. A possible mechanism of interaction of components during SHS of boride containing composition materials is proposed. 5. Refractory SHS boride-based matarials on the basis of TiB2-Al2O3, CrB2-Al2O3 were developed. Such materials can withstand temperatures up to 2000°C, having compressive strength ~30-40 MPa and specific electric conductivity is 36*10-6Ohm*m [32-34]. CONTROL QUESTIONS 1. What do you know about the self-propagating high-temperature synthesis of boron containing compounds? 2. What is the “Quenching” method? 3. How would you describe thelayer-by-layer X-ray structural analysis of quenched samples? 4. Could you list the SHS parameters?

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5. What kind of analysis of the composition and morphology of the combustion products can be performed? 6. What do you know about the “vapour-liquid-crystal” mechanism? REFERENCE 1. Tavadze G.F., Steinberg A.S. production of special materials by the methods of SHS. – Tbilisi: Meridiani. – 2011. – P. 111-112. 2. Gu M., C.Huang C., Xiao S., H.Liu Improvements in mechanical properties of TiB2 ceramics tool materials by dispersion of Al2O3 particles // Mater.Sci.Eng. – 2008. – A 486. – P. 167-170. 3. Merzhanov A.G., Mukasyan A.S., Rogachev A.S. Some aspects of commercialization and industrial use of SHS products // Combustion and plasmochemistry. – 2010. – V. 8. – №4. – P. 265. 4. Ignatenko P.I. The factors determining formation of nanostructural boride and nitride films on the basis of transition metals // Solid state physics. – 2009. – V. 51. – №8. – P. 1632-1638. 5. Nicholas J., Weiham N. Formation of nanometric TiB2 // J.Am.Ceram.Soc. – 2000. – №83. – P. 1290-1292. 6. Nechepurenko A.S., Shamrikov V.M., Lasychenkov Yu.Ya., Samun S.V., Kislitsyn V.I. Boron, its oxygen free compounds and their use in modern engineering // Cool. Of works “Unichim with 03”. – 2005. – №72. – P. 478. 7. Amosov A.P., Borovinskaya I.P., Merzhanov A.G. / Powder ethnology of SHS-materials. – M.: Machine engineering-1, 2007. – 567 p. 8. Levashov E.A., Kurbatkina V.V., Patsera E.I., Pogozhev Yu.S., Rupasov S.I., Rogachev A.S. Selfpripagating high temperature synthesis of promising ceramic materials for deposition technologies of functional nanostructural coatings // Non-ferrous metallurgy. – 2010. – №5. – P. 27-53. 9. Abdulkarimova R.G., Suleimenova A.S., M.T. Doszhanova., Kapizov U.S. The peculiarities of alumotermic combustion of mechanoactivated silicon dioxide of different modifications // Vestnik KazNU. Chem.series. – 2012. – №1. – P. 21-24. 10. Yeh C.L., Li R.F. Formation of TiB2-Al2O3 and NbB2-Al2O3 composites by combustion synthesis involving thermite reactions // Chemical Engineering Journal. – 2009. – №147. – P. 405-411. 11. Wang L.L., et al. Thermite reactions: their utilization in synthesis and processing of materials // J. Mater.Sci. – 1993. – №28. – P. 3693-3708. 12. Vallauri D., Adrian I.C., Chrysanthou A. TiC-TiB2 composite // J. Eur.Ceram.Soc. – 2008. – №8. – P. 1697-1713. 13. Locci A.M., Orrfj R., Cao G., Munir Z.A. Simultaneous spark plasma synthesis and densification of TiC-TiB2 composite // J. Am.Ceram.Soc. – 2006. – №89. – P. 848-855. 14. Raimkhanova D.S., Fomenko S.M., S. Mihaylovich, Abdulkarimova R.G., Mansurov Z.A. Effect of Argon Pressure and Aluminium Content (in TiO2-H3BO3-Al mix) on combustion and formation of chemical composition in combustion products advanced materials research. – 2013. – Vol. 746. – P. 62-67. 15. He J., Wang W., Fu Zh. And Sun H. Combustion synthesis of TiB2 ceramics powder from TiO2-B2O3-Mg system in air atmosphere // J. of Wuhan university of tehnology, Materials sci. edition. – 2005. – Vol. 20. – №2. – P. 90-93. 16. Ivanov V.V., Chernousov A.A., Kirik S.D., Nagibin G.E. Some aspects of physic-chemistry and binding in TiB2 / Al2O3 composite // Refractories and tehnical ceramics. – 2011. – №10. – P. 19-25. 17. Gladkih L.I., Grigoriev O.N., Sobol O.V., Pugavhev A.T., Sobol E.A., Martynyuk S.V. The structure and strength of composition ceramics TiB2-CrB2 and TiB2-W2B5 obtained by the method of hot compaction // The problems of science and engineering. Series: Physics of radiation damage and radiation materials science. – 2002. – №6(82). – P. 139-142. 18. Matovnikov A.V., Urbanovitch V.S., Chukina T.A., Sidorov A.A., Novikov V.V. A combined method of synthesis of rare-earth metal diborides // Inorganic materials. – 2009. – V. 45. – №4. – P. 414-416. 19. Sviderskii A.K. reactions of alumitermic reduction of chromium (III) under the conditions of auto wave synthesis // Izvestia of Tomsk polytechnical university. – 2009. – V. 315. – №3. – P. 28-31. 20. Raimkhanova D.S., Abdulkarimova R.G., Mansurov Z.A. Research of Nanostructures Formation during Self-Propagating High-Temperature Synthesis of Boride-Containing Composite Materials // Journal of Materials Sci.and Chem.Eng. – 2014. – Vol. 2. – P. 66-69.

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21. Levashov E.A., Rogachev A.S. The promising materials and technologies of Self-Propagating High-Temperature Synthesis. – M.: MUCCUC, 2011. – 377 p. 22. Arestov O.V., Ruzhutskaya E.V. High temperature mechanochemical synthesis of double (Ti, Cr) B2 // Vestnis of engineering school. – 2012. – №4(13). – P. 20-26. 23. Korchagin M.A., Grigorieva T.F., Bokhonov B.B., Sharafutdinov A.P., Barinova B.B., Lyakhov N.Z. Solid phase regime of combustion in mechanically activated SHS-systems. The effect of duration of mechanical activation on the characteristics of the process and composition of combustion products // Physics of combustion and explosion. – 2003. – V. 39. – №1. – P. 51-68. 24. Sychev A.E., Merzhanov A.G. Self-Propagating High-Temperature Synthesis of nanomaterials // Suc. of chemistry. – 2004. – V. 73. – №2. – P. 157-170. 25. Chen H.Q., Fan Q.C. Microstructural evolution during the ignition / quenching of preheated Ti/3Al powders // Journal of Alloys and Compounds. – 2009. – №475. – P. 184-190. 26. Longland P.L., Moulson A.I. The growth of α and β-Si3N4 accompanying the nitring of silicon compacts // J. Mater.Sci. – 1978. – Vol. 13. – №10. – P. 2279. 27. Amonosov A.P., Bichurov G.V. Azide technology of Self-Propagating High-Temperature Synthesis micro- and nano- powders of nitrides. – M., 2007. – P. 337-338. 28. Valcalcer V., Development of single-crystal α- Al2O3 fibers by Vapor-Liquid-Solid deposition (VLS) // Adv.Mater. – 1998. – №2. – P. 10. 29. Mansurov Z.A. Producing Nanomaterials in Combustion // Combustion, Explosion, and Shock Waves. – 2012. – Vol. 48. – №5. – P. 561-569. 30. Andrievski R.A., Ragulya A.V. nanostructural materials. – M.: Academy, 2005. – 187 p. 31. Merzhanov A.G. Self-Propagating High-Temperature Synthesis // Physical chemistry. Modern problems / under the edition of Kolotyrkin. – M.: Chemistry, 1983. – P. 5-44. 32. Peculiarities of self-propagating high-temperature synthesis and structure formation of TiB2-Al2O3 and CrB2-Al2O3 composites // Eurasian Chemico-Technological Journal. – Vol. 13 (3-4). – 2011. – P. 161-168 / Co-author: D. Abdulkarimova, I.M. Vongai, A. Gubarevich, Wu WenWen, O. Odawara. 33. Abdulkarimova D.S. Synthesis of composition materials based on borides under the conditions of solid flame combustion: dissertation. – Almaty, 2012. – 116 p. 34. Abdulkarimova D.S. Synthesis of composition materials based on borides under the conditions of solid flame combustion:dissertation. – Almaty, 2012. – 116 p.

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C

HAPTER

7

SOLUTION COMBUSTION SYNTHESIS

7.1. Synthesis gas production on glass cloth catalysts modified by Ni and Co oxides

Nanosized catalytic systems can be effectively used for the solution of ecological problems in processes of CO and hydrocarbon combustion as well as for the utilization of the components which cause the so-called “greenhouse effect” [1, 2]. The main components in a greenhouse gas are water vapor, CO2, CH4 and O3 [3]. The utilization of CO2 and CH4 is of great interest because these substances can be used as reactants for the dry reforming of methane to yield hydrogen or synthesis gas [4-6], which are valuable feedstocks for the production of ultraclean fuels[7]. In general, the dry reforming of methane may be the most effective process wherever carbon dioxide is a byproduct and available for utilization, for instance, in power plants which emit a large amount of CO2 at relatively high temperature, and in petrochemical industries where effluents of light gases can be processed with waste streams of CO2 [8,9]. In the metallurgical industry, the excess coke oven gases, consisting mainly H2, CH4, CO and N2, may be turned into synthesis gas by means of the dry re-forming. RostrupNielsen [10] has reported that most of the catalysts for the DRM process consist Ni, Co and noble metals supported on oxide supports. Nickel catalysts exhibit high activity in the carbon dioxide reforming of methane. However, when using such catalysts, a serious problem is the evolution of a great amount of carbon, which blocks active sites, result-ing in a rapid deactivation of the catalysts [11]. Noble metal catalysts are much more stable [12, 13], but their high costs limit a wide commercial use of these systems. As with nickel, noble metals show smaller equilibrium constants for methane decomposition than those calculated on the basis of graphite, but the effect is even more pronounced and could probably be ascribed to the even smaller metal particles in these catalysts and the smaller dissolution of carbon [10]. Different methods have been used to suppress carbon de-position on nickel catalysts. First of all, attention should be paid to the works on the minimization of acid sites of the carrier. It is stated that coke formation decreases on carriers with a high Lewis basicity [14]. Addition of alkaline and alkaline-earth elements [15] and uranium oxides [16], special methods of preparation and bimetallic systems [17] are used. The main idea in this kind of work [18] is based on the supposition that carbon is deposited on the large particles of nickel. Small particles interact with the carrier and provide high stability to aggregation, thus, creating preconditions for high stability of the 94  

catalyst. The authors in Ref. [18] developed a stable low percentage nickel catalyst with Ni particles of the size less than 4 nm in the system of Ni0.03Mg0.97O. Results obtained by Guo et al. [19] were in good agreement with Ref. [18]. Com-pared to Ni/γ-Al2O3, the Ni/MgO-γ-Al2O3 and Ni/MgAl2O4 catalysts exhibit higher activities and better stabilities in the DRM reaction. The high sintering-resistance ability and the low acidity of MgAl2O4 compared to γ-Al2O3 and the inter-action between Ni and MgAl2O4 might be responsible for its high activity and its resistance to coking and sintering, because it can produce a highly dispersed active Ni species. The importance of the high dispersion of Ni is emphasized in Ref. [20]. They showed that the dispersion of the Ni/MgAl2O4 catalysts increased when the Ni content decreased from 15 to 5 wt%, and the small Ni particle size was responsible for catalytic activity and stability. A 7% Ni/MgAl2O4 catalyst was very effective in the DRM reaction and exhibited a relatively stable performance for 50 h of reaction. In recent years many papers directed to the study of the efficiency of DRM reaction on Ni-Co-catalysts have appeared. Co-Ni bimetallic catalysts gave the best performance in terms of conversion and carbon resistance in a range of the Ni-X bimetallic catalysts, when X = Co, Ca, K, Ba, La, Ce [21] or X = Co, Fe, Cu, Mn [22]. Surplus Co in the Ni-Co-catalysts reflected lower levels of deactivation, sup-pressed carbon formation and improved H2 yield [23, 24]. The performance of bimetallic catalysts was determined also by the surface areas of the supports, such as Al2O3 [23], CeO2 [25], CeO2-ZrO2 [26,27], MgO-ZrO2 [21], MgAl2O4 [28], and Al2O3-ZrO2 [29]. The main idea of these works was on the material which exhibited both small crystallite sizes of the employed support and small active bimetallic particles, which demonstrated high catalytic activity and good resistance to carbon accumulation on the catalyst surface. The oxide sup-ports with good redox properties and high oxygen mobility within the crystalline lattice [21,25−29] enabled good dispersion of the active metal, formation of a strong metal/support interface and minimizing carbon accumulation on the catalyst surface. Quite many data [30−34] on the use of catalysts on the basis of glass fiber in many other chemical processes have appeared lately. It is shown that catalysts on the basis of glass fiber are highly active even at a low content of the active component on the surface of the support. This work is devoted to the development of nanosized catalysts via the forming of low-percentage CoO-NiO active components on a glass fiber support by a “solution combustion” (SC) method and their study in the reaction of dry re-forming of methane with CO2. The chosen method of preparation will allow uniform heating of the catalyst at a low temperature (400°C) that initiates the combustion front on the surface of catalyst, and the low content of the active components will limit the development of a high temperature wave by preventing the overheat of the catalyst. 7.2. Synthesis of catalysts

A silica glass fiber (SGF) used in the experiments consisted of filaments with a diameter of 6−7 µm. After removing of the lubricant, the glass fiber was leached in a 5.5% solution of nitric acid for 1 h at 90°C, washed with water to obtain pH = 5.5−7, dried and calcined at 300°C in air. The properties of the SGF after the pretreatment were as follows: SBETArwas 1 m2/g, Vporewas 0.0006 cm3/g, amorphous phasein XRD. 95  

Ni and Co oxides were deposited onto the surface of a glass fiber matrix by the method of “solution combustion”, which is one of the variants of a self-propagating high temperature synthesis [35]. Low percentage (no more than 1%) species were synthesized by the SC method (Table 1, IK1-IK6). Glass fibers of a definite size were impregnated with a solution of cobalt and nickel nitrates, then dried for 30 min in air at 100°C and then calcined in an air atmosphere at 400°C. At this temperature the self-propagating high temperature synthesis took place, resulting in the formation of nanoparticles with the size ranging from 30 to 100 nm [36−37]. The SC method starting from initial components (mixed Co and Ni nitrates+glycine) corresponded to the following reaction. 3Co(NO3)2· 6H2O + 2NH2CH2 COOH + 5.5O2=Co3O4+ 4CO2+ 8NO2+ 23H2O Ni(NO3)2· 6H2O + 2NH2CH2COOH + 6O2=NiO + 4CO2+ 4NO2+ 11H2O Table 7.1 List of catalysts on glass cloth Glass fiber catalysts IK1 IK2 IK3 IK4 IK5 IK6

CoO content (wt%) 0.97 0.67 0.62 0.50 0.32 −

NiO content (wt%) 0.28 0.43 0.54 0.72 1.05

CoO/NiO (%/%) 100/0 70/30 60/40 50/50 30/70 0/100

7.2.1. Study of the catalytic activity

The catalytic activity in the DRM process was examined using a fixed bed quartz reactor with an inner diameter of mm under the following conditions: CH4, 50%; CO2, 50%; temperature, 600-800°C; flow rate, 60 cm3/min; gas hourly space velocity, 1440 h−1; and time of activity measurement, 2-4 h.

7.2.2. Physicochemical examination of samples

The specific surface area (SBET, m2/g) of the samples was determined by the method of thermal desorption of argon (the BET method) on the device of SORBI N.4.1 and by comparing between volumes of the gas-adsorbate (argon) sorbed by the sample under study and the standard sample of the mate-rial with a known specific surface area. Elemental composition of deposited active components on the glass fiber samples was determined by the method of atomic-absorption spectroscopy with inductivelycouple d plasma (AAS ICP) (OPTIMA-4300 DV). X-ray diffraction (XRD) patterns were recorded using a HZG-4C diffractometer with Co Kα (λ = 1.79021 A) radiation. Sample patterns were taken by a point scanning with 0.05 degrees and with accumulation of 18 s at each point in the range of angles of 96  

20o–80o. To evaluate a certain phase composition and to determine parameters of the lattice, we carried out additionally the point scanning with the step of 0.05o and with accumulation of 36−60 s at each point in the range of angles of 70o–80o. The average size of crystallites was estimated in accordance with the Sherrer equation. An analysis of the crystalline structure of the samples was carried out using the JCPDS Database. The SEM study was performed using a JSM 6460LV (JEOL, Japan) microscope with an accelerating voltage of 25 kV. For local analysis of chemical composition of a sample, the microscope is equipped with an EDAX energy dispersive X-ray spectrometer (INCA OXFORD Instrument). The particle size and the state of the active component were estimated by HRTEM using a JEM 2010 (JEOL, Japan) microscope with lattice resolution of 0.14 nm and connected to a Phoenix EDAX spectrometer for microanalysis. An additional assessment of the catalyst dispersion was made by AFM using a SolverP47Bio instrument. DR UV-Vis spectra of glass-fiber samples were registered by a UV-2501 PC (Shimadzu) spectrophotometer equipped with a ISR-240A DR unit in the range of 11000−54000 cm−1. The spectra are presented in coordinates: Kubelka-Munk function vs. wavenumber. Thermogravimetric analysis (TGA) was made with a ≪NETZSCH STA 449C≫. TGA was reformed in air at a heating rate of 10 o/min and a temperature range from 22 to 1000°C with 20 mg samples. 7.3. Characteristics and properties of the catalysts

The specific surface of the studied samples was up to 1.0 m2/g, which was close to that of the catalyst carrier-glass fibers due to the fact that the amount of the deposited components did not exceed 1 wt%. To study the samples by X-ray analysis, the samples with the active component deposited onto the surface of the sup-port were ground to powder. XRD patterns (Fig.7.1) of the samples IK1 (CoO/NiO = 100/0), IK2 (CoO/NiO = 70/30) and IK5 (CoO/NiO = 30/70) show the presence of an X-ray amorphous phase of glass fibers and the phases of the CoO-NiO catalytic systems. In the case of the IK1 catalyst, a spinel phase Co3O4 with the size of crystallites of 20 nm has been observed (Fig. 7.1, Curve 1). The study of the oxide film without the support (without a glass fiber matrix) by the X-ray phase analysis also showed the presence of the spinel phase Co3O4, and this can serve as a verification of the obtained data. Addition of NiO to CoO leads to a dilution of NiO in Co3O4 and the formation of a substituted shpinel phase (Ni, Co)Co2O4. The XRD pattern of the catalyst IK2 consisting CoO/NiO = 70/30 represents only the phase (Ni, Co)Co2O4 (Fig. 7.1, Curve 2). An increase in NiO amount in the catalyst IK5 (CoO/NiO = 30/70) results in the formation of two phases: spinel (Ni, Co)Co2O4 and NiO with the same particle size of 20 nm. Arrows on Fig. 7.1 point out inserts of XRD patterns obtained in the range of spinel 4.4.0 at signal accumulation of 36 s for the IK1 sample and during 60 s for the IK5 sample. The fiber surface of the initial support (Fig. 7.2a) and the glass cloth with the deposited active component (Fig. 7.2b) were studied by the SEM method. As seen in Fig. 7.2(a), the initial sample is fibers with the diameter of 7−8 µm. There are inclusions on the surface, showing probably the presence of impurities. Comparison with Fig. 7.2(b) (IK3 catalyst) shows that deposition of cobalt and nickel oxides onto 97  

the surface by the SC method results in the formation of a coating on the surface of the glass fibers.

Fig. 7.1. XRD pattern of catalysts: (1) IK1 (CoO/NiO = 100/0), (2) IK2(CoO/NiO = 70/30), (3) IK5 (CoO/NiO = 30/70)

The samples IK1 and IK3 were studied by SEM equipped with an EDX device. The area of X-ray generation was 1 µm. In accordance with the EDX spectra, the amount of Co in the IK1 sample was 2.2−2.5 wt% (Fig. 3a) and the content of Co-Ni in the IK3 sample was 1.5−2.4 wt% (Fig. 7.3b). The content of active components in these samples is three times higher than that obtained by AAS-ICP. It can be explained by the localization of the active component in a surface layer.

Fig. 7.2. SEM images of pure glass fibers (a) and sample of IK3 (b)

Fig. 7.3. EDX spectra of catalysts IK1 (CoO/NiO = 100/0) (a) and IK3 (CoO/NiO = 60/40) (b).

98  

One can see in the pictures given by TEM (Fig. 7.4a) that the active component is dispersed on the surface of the fiber mainly in the form of separated particles with the size of about 10−20 nm. Figure 7.4(b) presents a HR-TEM picture of separated particles and their digital diffractions from selected areas (black square). The morphology of the particles is characteristic for spinel structures. Observed interplanar distances are close to that of NiCo2O4 (111) 4.6846°A, (222) 2.3423°A,2.4465°A, (331) 1.8615°A (JCPDS diffraction data). It is seen in the picture that some particles are on the surface of the SGF, the others are localized inside the glass cloth matrix.

Fig. 7.4. TEM pictures of 0.6Co-0.4Ni catalyst (a) and HR-TEM picture of separated particles consisting of NiCo2O4 (catalyst IK3) and digital diffractionfrom selected area (black square) (b); consisting of metal Ni and Co (c).

Fig. 7.5. EDX spectrum of the spherical particle shown in Fig. 4(c).

According to the data of EDX analysis of a spherical particle shown in Fig. 7.4(c), the particle contains amounts of Ni and Co corresponding to the chemical composition of this sample (Fig. 7.5). Interplanar distances characterize the metallic state of the elements. It should be noted that, ac-cording to data of radiographic studies, metallic phases of the active component were not detected. It cannot be excluded that metallic particles can be obtained from oxides under the action of a beam of electrons. One can note that particles are strongly retained on the support surface. In a special experiment with a pronounced mechanical effect followed by an ultrasonic treatment, we managed to detach some particles from the support. An AFM study of these particles shows (Fig. 7.6) that their size does not exceed 8 nm. 99  

Fig. 7.6. AFM picture of the IK3 catalyst.

Figure 7.7 presents a DR UV-Vis spectrum of the CoNi-catalyst containing 1.0 wt% metals (Curve 1, IK4). This spectrum contains absorbed species as shoulders at 15100 and 23000 cm−1. For the understanding of the Co- and Ni-state in supported CoNi-catalysts we have presented on Fig.7.7 the spectra of the SGF sample and the monocomponent Ni- and Co-catalysts. One can see that the intensities of the bands observed for bicomponent CoNi-catalyst are significantly lower than that of the Cocatalyst with a close cobalt content, but it is higher in comparison with the Ni-catalyst (Curves 3 and 4, respectively). An intense absorption at 40000 cm−1 attributed to a fundamental absorption edge (FAE) is observed on the unmodified SGF sample (Curve 2). In the DR UV-Vis spectrum of the Co-catalyst (Curve 3, IK1) two intense bands of 14000 and 23200 cm−1 are observed. The energies of these bands agreed with d-d transitions of 4A2(F)-4T1(P) of Co2+ ions in tetrahedral coordination (Co2Td+) and 1 A1g-1T2g of Co3+ ions in octahedral coordination (Co3Oh+), respectively. The presence of such cobalt states indicates the formation of spinel structures, for example [Co2+](Co3+)2O4 or [Co2+](Al3+, Co3+)2O4. In the spectrum of the Ni-catalysts (Curve 4, IK6) a doublet of the band 12900 and 13700 cm−1 as well as a shoulder of 23900 cm−1 are observed. Energies of these bands correspond to d-d transitions 3 T1g-3A2g, 1Eg-3A2g and 3T1g(P)-3A2g of Ni2+ ions in octahedral coordination (Ni2Oh+), respectively.

Fig. 7.7. DR UV-Vis spectra of Co-Ni-catalyst.

100  

So, the spectrum of the CoNi-catalyst is a superposition of bands typical of Ni2Oh+ (doublet 13000−15000 cm−1) and Co2Td+ (14000 cm−1), as well as Ni2Oh+ (25000 cm−1) and Co3Oh+ (23000 cm−1). In this case, the CoNi-catalyst can contain Ni2Oh+ cations in NiO particles, and Co2Td+ and Co3Oh+ cations in the spinel structure Co3O4. The features of our supported Ni-catalyst and Co-catalyst in comparison with the wellknown NiO/MgO system and bulk Co3O4 are a shift of the absorption bands to the low frequency region (about 600−1400 cm−1) and a decrease or an absence of the splitting of the multiplet bands at 14000 cm−1. They point out to a strong interaction of the Ni2Oh+ and the Co2Td+ ions with the support. Besides, the higher intensity of the band 23200 cm−1 in comparison with the band 14000 cm−1 indicates a higher amount of Co3Oh+ and a high defectiveness of the spinel structure. 7.4. The catalytic activity of samples in the reaction of dry reforming of methane

The catalytic activity in the reaction of DRM was determined for samples of catalysts presented in Table 7.1. Figures 7.8 and 7.9 present the results of the study of the dependence of the conversion of initial substrates, methane and carbon dioxide, as well as values of yields of target reaction products, hydrogen and carbon monoxide, on the temperature and time on stream.

Fig. 7.8. Changes in the yield of reaction products (a), initial componentsof the reaction mixture (b) depending on the time on stream at different temperatures on the catalyst IK1.

101  

The IK1 sample (Fig. 7.8) shows quite high an activity. The yield of synthesis gas reaches 46% by H2 and 52% by CO at 750°C (Fig. 7.8a). The conversion of CH4 reaches 42% and the CO2 conversion is 67% at the same temperature (Fig. 8b). Considerable changes in the yield of synthesis gas for 3 h are not observed, and this indicates the fact that the operation of this catalyst is stable in the studied temperature range. The activity of IK1 is a little lower than that of a similar 1% Co/MgO-Al2O3 catalyst with specific surface area of 160 m2/g. The latter catalyst at similar conditions exhibits H2 and CO yields of 59% and 65%, respectively. The decrease in cobalt oxide concentration results in the decrease of the catalyst activity in the reaction of DRM.

Fig. 7.9. Changes in the yield of reaction products depending on the timeon stream at different temperatures on the catalysts.

102  

An industrial Ni catalyst (NiO = 6%-8%, Al2O3 = 90%) was studied for comparison with the glass fiber catalysts in the reaction of DRM. The ratio of initial gases was CH4/CO2 = 1.25/0.75. The yield of synthesis gas was observed to reach 53% for H2 and 24% for CO at 630°C, the conversion of CH4-58%, CO2 - 75%. However, the life time of the industrial Ni catalyst appeared to be short, and after approximately 3 h of operation the yield of carbon monoxide decreased down to 8% and the conversion of carbon dioxide was about 82%. This indicated the process of carbonization occurred. Figure 7.10 presents the influence of the temperature of the reactor on the activity of the catalysts. Sample IK1 shows quite high an activity among the series of the studied low percentage catalysts. It is seen that synthesis gas including 32% of H2 and 46% of CO is produced at 745°C, the conversion of CH4 is about 30%, and the conversion of CO2 is 80% at the same temperature. Inconsiderable decrease in the yield of CO with the increase of the temperature up to 780°C is connected with the formation of carbon on the surface of the sample. The catalyst IK2 is less active in the dry reforming of methane. The yield of H2 and CO is observed to be 25% and 45% at 660°C, respectively, and the yields of the gases increase up to 35% and 50% with the increase of the temperature up to 800°C. In this case the conversion of methane decreases from 35% to 10%, which is connected with a change in reaction mechanism at this temperature. It is seen from the figure (IK3) that maximum yields of H2 and CO can be reached only at 800°C and a decrease in methane conversion from 33% to 22% is observed. Thus, the given catalyst is less active that the IK1, which includes 100% of CoO. The sample IK4 shows average values of conversion and yield for target products among the studied systems.

Fig. 7.10. Temperature dependences between the yield of synthesis gas and the conversion of CH4and CO2on different catalysts.

103  

For demonstrative comparison of catalytic activities from the active component composition point of view, values of H2 and CO yields are summarized in Table 7.2. These data demonstrate higher activity of the catalysts enriched with Co. The data are in good agreement with[23] that has studied alumina supported Ni, Co and Ni-Co catalysts with 9 wt% nominal metal content in the DRM reaction, and it was shown that higher activity and stability is exhibited by cobalt-rich catalysts. Table 7.2 Yields of H2 and CO in DRM reaction on the CoO-NiO catalysts on glass cloth DRM reaction products H2

CO

Temperature (◦C) 690 725 760 790 690 725 760 790

Yields of reaction products (%) on catalysts IK1 IK2 IK3 IK4 IK5 IK6 26 25 20 24 13 9 30 25 23 19 16 16 32 27 26 17 15 10 30 33 33 17 16 13 37 44 29 24 20 14 43 44 33 23 22 18 44 41 35 23 22 13 43 41 38 22 22 16

After the DRM reaction, the surfaces of the glass fiber catalysts were studied by the TGA method. According to the obtained data, the amount of carbon on the surface is 0% for the sample IK 1 (Fig. 7.11a), 0.51% for IK2 and 3.1% for IK6 (Fig. 7.11b). A tendency of decrease in stability of the catalysts relating to carbonization with increase of the concentration of Ni in the catalysts takes place. As was shown by the XRD method, an increase in the concentration of Ni leads to the formation of the NiO phase, and the amount of which in-creases up to 100% in the sample of IK6 (CoO/NiO = 0/100). We suppose that the presence of the NiO phase in the catalysts contributes to the formation of carbon. According to [24] surplus Co in the Ni-Co-catalysts can display lower levels of deactivation due to suppressed carbon formation. But even for the catalyst IK6, in which the amount of formed carbon is more relative to the mass of the active component, a considerable decrease of its activity is not observed (Fig. 9c). Probably, carbon is distributed in the bulk of the carrier and does not block the active sites of the catalyst, and that is important for its stable operation. CONTROL QUESTIONS 1. What is the catalytic activity? 2. What do you know about the characteristics and properties of the catalysts? 3. What type of synthesis of catalysts do you know? 4. How would you describe physicochemical characterization of sample’s properties? 5. Describe the synthesis gas production on glass cloth catalysts modified by Ni and Co oxides. 6. Describe the catalytic activity of catalystduring dry reforming ofmethane. REFERENCES 1. Ross J.R.H. Catal Today, 2005, 100(1-2): 151. 2. Ismagilov Z.R., Kuznetsov V.V., Ismagilov I.Z. E-MRS Fall Meeting A “Carbon Dioxide as a Raw Material for Sustainable Development”, 2010. – Р. II.

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3. In: Houghton J.T., Ding Y., Griggs D.J., Noguer M., Dai X., Maskell K., Johnson C.A. ed. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. United Kingdom and New York: Cambridge University Press, 2001. 4. Liu S., Xiong G., Dong H., Yang W. Appl Catal A, 2000, 202(1): 141. 5. Ma L., Trimm D.L. Appl Catal A, 1996, 138(2): 265. 6. Ismagilov Z.R., Kuznetsov V.V., Shikina N.V., Gavrilova A.A., Kuntsevich S.V., Parmon V.N., Kerzhentsev M.A., Balakhonov V.G., Lazarchuk V.V. RU Patent 2350386. – 2007. 7. Dry M.E., Hoogendoorn J.C. Catal Rev-Sci Eng, 1981, 23(1-2): 265. 8. Wang S.B., Lu G.Q.M., Millar G.J. Energy Fuels, 1996, 10(4): 896. 9. Gadalla A.M., Bower B. Chem Eng Sci, 1988, 43(1): 3049. 10. Rostrup-Nielsen J.R. Catal Today, 1993, 18(4): 305. 11. Fan M.S., Abdullah A.Z., Bhatia S. ChemCatChem, 2009, 1(2): 192. 12. Rezaei M., Alavi S.M., Sahebdelfar S., Yan Z.F. J Nat Gas Chem, 2006, 15(4): 327. 13. Ocsachoque M., Pompeo F., Gonzalez G. Catal Today, 2011, 172(1): 226. 14. Hu Y.H. Catal Today, 2009, 14(3-4): 206. 15. Nagaraja B.M., Bulushev D.A., Beloshapkin S., Ross J.R.H. CatalToday, 2011, 178(1): 132. 16. Ismagilov Z.R., Kuntsevich S.V., Kuznetsov V.V., Shikina N.V., Kerzhentsev M.A., Rogov V.A., Ushakov V.A. Kinet Catal, 2007, 48(4): 511. 17. Michalkiewicz B., Strenscek´-Nazzal J, Ziebro J. Catal Lett, 2009, 129(1-2): 142. 18. Tomishige K., Fujimoto K. Catal Surv from Jap, 1998, 2(1): 3. 19. Guo J.J., Lou H., Zhao H., Chai D.F., Zheng X.M. Appl Catal A, 2004, 273(1-2): 75. 20. Hadian N., Rezaei M., Mosayebi Z., Meshkani F. J Nat Gas Chem, 2012, 21(2): 200. 21. Fan M.S., Abdullah A.Z., Bhatia S. ChemSusChem, 2011, 4(11): 1643. 22. Zhang J.G., Wang H., Datai A.K. J Catal, 2007, 249(2): 300. 23. San-Jos´e-Alonso D., Juan-Juan J., Ill´an-Gomez´ M.J., Ro m´an-Mart´ınez M.C. Appl Catal A, 2009, 371(1-2): 54. 24. Gonzales O., Lujano J., Pietri E., Goldwasser M.R. Catal Today, 2005, 107-108: 436. 25. Luisetto I., Tuti S., Di Bartolomeo E. Int J Hydrogen Energy, 2012, 37(21): 15992. 26. Osojnik Crnivec I.G., Djinovi´c P., Erjavec B., Pintar A. Chem EngJ, 2012, 207-208: 299. 27. Djinovi´c P., Osojnik Crnivec I.G., Erjavec B., Pintar A. Appl CatalB, 2012, 125: 259. 28. Chen L., Zhu Q.S., Wu R.F. Int J Hydrogen Energy, 2011, 36(3): 2128. 29. Li X.H., Ai J., Li W.Y., Li D.X. Front Chem Eng Chin, 2011, 4(4): 2128. 30. H ¨oller V., Yuranov I., Kiwi-Minsker L. Catal Today, 2001, 69(1-4): 175. 31. Kiwi-Minsker L., Yuranov I., H ¨oller V., Renken A.Chem Eng Sci, 1999, 54(21): 4785. 32. Yuranov I., Kiwi-Minsker L., Slin'ko M., Kurkina E., Tolstunova E.D., Renken A. Chem Eng Sci, 2000, 55(15): 2827. 33. Zinfer I., Nadezhda S., Vladimir K., Nina R., Vladimir U., Nikolai V., Hubert V. Catal Today, 2005, 102: 85. 34. Patil K.C., Aruna S.T., Mimani T. Curr Opin Solid state Mat Sci, 2002, 6(6): 507. 35. Aldashukurova G.B., Mironenko A.V. Chem J Kazakh, 2007, 16: 288. 36. Aldashukurova G.B., Mironenko A.V., Mansurov Z.A., Rudina N.A., Itshenko A.V., Ushakov V.A., Ismagilov Z.R. Eur Chem-Technol J, 2010, 12(2): 97. 37. Mansurov Z.A., Aldashukurova G.B., Mironenko A.V., Shikina N.V., Yashnik S.A., Kuznetsov V.V., Ismagilov Z.R. Synthesis gas production on glass cloth catalysts modified by Ni Co oxides // Journal of Energy Chemistry. – 2013. – №22. – P. 811-818.

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C

HAPTER

8

COMBUSTION SYNTHESIS OF SILICON AND SILICON CARBIDE NANOPOWDERS

8.1. Combustion synthesis of silicon nanopowders [1,2]

With intensive development of advanced technologies, such as electronics and solar energy, pure silicon becomes an important material for the industry, and the demand on this product is constantly rising. On the industrial scale, silicon is typically produced by the reduction of silicon dioxide with carbon in high-temperature furnaces [3]. However, this method is energy consuming and requires relatively long processing time. The investigations are still going on to improve this process [4,5]. Also, the final product usually should be subjected to additional purification. Moreover, the main byproduct of this technology is carbon dioxide responsible for the undesired greenhouse effect. Attractive “green” approach to silicon and silicon nitride production based on using the solar energy was suggested in [6]. But this process requires a lot of time and thus is ineffective. Two other similar methods for silicon production, aluminothermy and magnesium reduction are also well known. The former one leads to the formation of alumina which is a chemically stable phase and thus the process of silicon purification is difficult and expensive. The latter one, i.e. conventional magnesium-reduction method, is more efficient [7,8]. However, this energy consuming and long-term heat treatment approach does not afford production of submicron powders. For a stoichiometric composition (Mg : SiO2 = 2:1), the overall SHS reaction can be written as follows: SiO2 + 2Mg = Si + 2MgO

(8.1)

The results of thermodynamic calculations for the adiabatic combustion temperature (Tad) and amount of equilibrium products as a function of inert gas (Ar) pressure (Pin) in the reactor are shown in Fig. 8.1. It can be seen (Fig. 8.1a) that when Pin increases, Tad also increases while the amount of gas-phase products, such as Mg(g) and SiO(g), decreases. This effect can be explained by the fact that high Pin “suppresses” gas phase formation, thus leading to an increase in Tad. Note, that in the range of Pin = 1-20 atm, the values of Tad (21002200 K) are well above the melting points (m.p.) of magnesium (922 K), silicon (1683 K), and silicon oxide (1923 K) as well as the boiling point (b.p.) of Mg (1363 K), but 106  

below the m.p. of magnesium oxide (3073 K). Finally, while Pin increases, the amount of undesirable Mg2Si4O decreases (Fig. 8.1b) giving almost 99.999% pure silicon. It can be seen (Fig. 8.1a) that when Pin increases, Tad also increases while the amount of gas-phase products, such as Mg(g) and SiO(g), decreases. This effect can be explained by the fact that high Pin “suppresses” gas phase formation, thus leading to an increase in Tad. Note, that in the range of Pin = 1-20 atm, the values of Tad (21002200 K) are well above the melting points (m.p.) of magnesium (922 K), silicon (1683 K), and silicon oxide (1923 K) as well as the boiling point (b.p.) of Mg (1363 K), but below the m.p. of magnesium oxide (3073 K). Finally, while Pin increases, the amount of undesirable Mg2Si4O decreases (Fig. 8.1b) giving almost 99.999% pure silicon.

а) adiabatic combustion temperature Tad and total amount of gaseous products; b) equilibrium amounts of condensed products Fig. 8.1. Thermodynamic characteristics as a function of inert gas pressurePin.

Magnesium (Mg) powder from Alfa Aesar, USA (99.8% purity, mean particle size d ≤ 44 μm) was used in all experiments. Three different types of silicon oxide (SiO2) powder were used: (i) from the Yerken deposit, Kazakhstan, marked hereinafter as KZ (98.8% purity, d ≤ 100 μm), (ii) from Cerac, WI, USA, marked as Cerac (99.5%, d ≤ 44 μm); and (iii) from Cabot Corporation, USA, nano Untreated Fumed Silica marked as UFC (99.9%, BET ≈ 200 m2/g). Typical microstructures of these SiO2 powders are presented in Fig. 8.2.

a) KZ; b) Cerac; c) UFS Fig. 8.2. Microstructure of starting SiO2powders.

107  

Starting reactants were thoroughly mixed in desired ratios and compacted in an automatic press plant (Carver, Inc., USA) into cylindrical samples (10, 30, 40 mm in diameter and 25-50 mm long) with a relative density of up to 70%. Thus prepared samples were horizontally placed onto a graphite tray and inserted into a cylindrical steel reactor with transparent windows for monitoring the process. The reactor was evacuated to 10-3 atm and then filled with argon up to 1-20 atm. Ignition was carried out by passing a short (1770

2.0

8.0

10.0

4.4 «Furnon-3MM»

>1770

2.0

10.0

14.0

4.5 «Furnon-7»

1500

2.2

10.0

12.0

4.6 «Furnon-7H»

1300

2.5

12.0

-

4.7 «Furnon-8M»

>2000

4.5

20-30

-

4.8 «Furnon-8K»

>2000

4.5

20-30

-

*-new materials

Indeed, the SHS refractory mortars are a new generation of refractory materials, which differs from the traditional ones. They provide essentially complete synthesis of working ceramic body at heating stages of linings through internal energy of components. Further developments of SHS refractory technology can be continued in a number of directions: ‒ Development of refractory materials having non-traditional phase composition: carbides, nitrides, borides combined with oxides and carbon; ‒ Extended use of wastes of mining and processing industries for purposes of their utilization; ‒ Development of methods and technique of manufacturing molded refractory materials and shaped products of diverse purposes applications. 121  

The absence of significant refractory production in Kazakhstan and the presence the most important consumers in the republic promote further intensive investigations and developments in the field of refractory technology. Based on the above we can conclude that researches and developments in the field of technology for SHS of refractories are extremely important. The global economic crisis exerts a negative impact on mining-metallurgic complexes of the CIS countries. Falling of demand for metals on world commodity exchanges has led to curtailment of production and export of basic production of the metallurgical enterprises. In these conditions, each separately taken enterprise tries to find all possible ways to reduce of the costs of the closed production cycles and as a whole to achieve economic benefit. One way to achieve such benefits is to increase the period of in between repairs and refusal of the "hot" repairs. It is caused by the fact that constructional durability and thermal stability of seams lining completely excludes loss of separate shaped refractories from a laying at any operating modes of heat generating unit, including, for example, the faults in raw materials or fuel delivery [9]. High mechanical durability of lining allows to maintain rotating furnaces with the bent bandages or flared drum. With occurrence of a through circular crack along the whole panel of lining of the rotating furnace, which typically occurs at a sharp curvature of its case and caused by hit of foreign subjects in space between rollers and bandages, this panel remains in operation, i.e. the laying collapse does not occur. The lining is not influence either by sticking of a burnt material to walls, that periodically occurs in rotating installations for roasting of chamot, rotating cement or stationary mine furnaces of roasting of limestone – at mechanical removal of these rat holing, nothing happens to a bricklaying from shaped refractories. Masonry seams subjected to the reaction synthesis, have much higher durability, than shaped refractories, therefore they also prevent cleavage crack surfaces of bricks. And necessity to repair the furnace arises either at uniform abrasive deterioration of lining till the thickness of 50-60 mm, or at occurrence of mechanical malfunctions in a reducer, rollers, the electric motor, gas flue, etc. [10]. Some HS refractories which found industrial applications are listed in Table 9.2. Table 9.2 Industries using “Furnon” № 1 1 2 3 4 5 6 7 8 9 10

Units 2

Enterprises Years 3 4 The enterprises of nonferrous metallurgy Furnaces for sintering anode mass JSC “Krasnoyarsk Since 1991 to the present Aluminium” Furnaces for sintering anode anode mass Furnaces for sintering anode mass Mine furnaces for roasting limestone Mine furnaces for roasting limestone Copper fusing converters Furnaces for sintering alumina Furnaces for sintering anode mass Magnesium electrolysis Rotating furnaces for roasting synthetic chromium spinel

Novokuznetsk Aluminum JSC “UkrGrafit” JSC “UKTMK” JSC “AVISMA” Balkhash GMK JSC “AGK” JSC “Volgograd Aluminium” JSC “UKTMK” AZHS

122  

Since 1999 to the present Since 2001 Since 1991 to the present Since 2000 to the present 1990-1992

1 11 12 13 14 15 16 17 18 19 20 21 22 23 24

2 3 4 THE ENTERPRISES OF FERROUR METALLURGY Vaults, arches and lateral walls of “Serp I Motor” 1999 electro-arc steel-smelting furnaces (Moscow) Vaults, arches and lateral walls of JSC “Elektrostal ” 1999-2000 electro-arc steel-smelting furnaces Covers of heating wall JSC “NTMK” Since 2000 to the present THE ENTERPRISES OF THE CEMENT INDUSTRY Cement furnaces, shafts of JSC “Iskitimcement” Since 1990 to the present refrigerators Cement furnaces Ustkamenogorsk 1993-1995 cement factory Cement furnaces JSC Semipatainsk 1993 cement” Cement furnaces JSC “AGK” 1990-1995 Cement furnaces AO “Oskol cement” Since 2000 to the present THE ENTERPRISES ON MANUFACTURE OF CERAMIC AND BUILDING MATERIALS In refractory blocks-light-weight in JSC “DZKSM” 1992 the tunnel furnace Protective layer of the tunnel furnace Kislovodsk farfor 1991 zavod Rotating ceramsite furnaces Lisakovsk 1993 THE ENTERPRISES ON MANUFACTURE REFRACTORIES Rotating installations on roasting of OGP NTMK Since 1999 to the present chamot and dunit THE ENTERPRISES OF THE CHEMICAL INDUSTRY Installations of manufacture of the JSC “TNHK” 1994 methanol ENTERPRISES OF COKE AND CHEMICAL OF MANUFACTURE Coke furnaces KHP NTMK Since 2000 to the present

Also, on the basis of SHS-technology, mixtures for hot torkret process were developed and applied to hot repair of a fire-resistant laying coke furnaces, in particular, for elimination or considerable reduction of defects such as "bowl", "crack", "burnout", "cleavage crack", "groove" and others on vertical and inclined surfaces of a laying. In view of its high abrasive firmness, use of the given mixtures allow the hot repairs of other types of industrial heat generating unit. Solution of a refractory is poured into the torkret device and is periodically barbotaged by means of a compressed air flow. An under repair surface of coke of furnaces is carefully cleaned from a film of graphite and the remains of former torkret -mass. A refractory solution is applied to a defective site of the lining, with the thickness of the layers between 3-6 mm. In case of liquidation of a defect of big sizes, torkret-mass is applied several times. Pressure of air in the torkret-device must not exceed 3 atmospheres. Reaction of synthesis in the applied coating proceeds during 1-3 min from the moment of the achievement of the ignition temperature [10]. 9.2. New carbon-containing refractories

A general approach to the synthesis of carbonaceous refractory materials is based on aluminothermic solid-phase combustion of metal oxides in the presence of carbon [11]. Typically, amount if used carbon exceeds the amount of the stoichiometric mixture of Al and metal oxide. At high synthesis temperatures (~1500°C), the reduced metal reacts with carbon and forms refractory carbides, for example: 123  

4Al + 3TiO2 + 3C = Al2O3 + 3TiC

(9.1)

4Al + 3SiO2 + 3C = Al2O3 + 3SiC

(9.2)

Transition metal oxides, such as chromium, titanium, niobium, zirconium, and others, are used as oxidation agents, and it is possible to obtain chemically stable, high-refractory carbonaceous materials. SHS composite materials consist of sveral refractory compounds, e.g. aluminum oxide, metal carbide, and carbon. Some investigations on SHS in systems containing carbon, aluminum powder, refractory clay or mortar, and magnesium sulfate as binder and oxidant were carried out [12,13]. The obtained material had characteristics that allowed its use for bonding of graphite and silicon carbide products. However, despite the obvious advantage over conventional carbon-containing refractories, the SHS material has negative features, e.g., the presence of aluminum sulfides formed in the course of secondary reactions during SHS. This compound reacting with water vapor leads to distortion of the structure throughout a monolithic refractory mass. There is a possibility to avoid the formation of carbides and aluminum sulfides during SHS by optimizing the oxygen content in the initial mixture of components and increasing the oxygen supply for the aluminothermic reduction recation of of silicon. Simultaneously, it is desirable eliminate the sulfatecompounds in the composition of the initial mixture. To avoid undesirable effects, the use of colloidal silica was suggested, which was obtained by the hydrolysis of ethyl silicate (brand ES-40) in the solutions of sulfuric acid with different concentrations (Fig. 9.1). As a basic chemical process, during the development of carbonaceous SHS refractories, the process of SHS with an intermediate stage has been accomplished. The silicon carbides, titanium, zirconium, etc., were fabricated in the second step of the synthesis. An important element of the developed technology is a combination of solgel and SHS technology in the manufacturing process, resulting in pre-encapsulation of components of the SHS system by aggregated nanodispersed oxides, such as xerogel. The use of silica sol as a binder introduced a new effect: hetero-coagulation of the sol. The phenomenon of sol hetero-coagulation leads to the formation of ultradispersed silica with particle sizes of 20-30 nm or less in the system. An important consequence of sol hetero-coagulation is the fact that the surface of aluminum particles is blocked by nano-dispersed silica and its activity is suppressed both in an alkaline medium (pH 10-11) and in an acidic environment (pH 1-2). In the absence of a sol the immediate release of hydrogen is observed. Note that the silica sol was obtained by weakly acidic hydrolysis of ethyl silicate. The optimum composition of hydrolyzate, possessing adequate stability to coagulation of sol, is determined: ethyl silicate (ES), 40%–55%; 0.5% solution of sulfuric acid, 45%. For SHS of the masonry mortar compositions for fastening of graphite and silicon carbide products the aluminothermal system with aluminosilicate oxidants was chosen as a filler. Filler consists of the waste of electrode graphite in the form of grains about 1-3 mm in size and the powder with size less than 90 μm. The exothermic mixture was diluted by a hydrolyzed ethyl silicate solution with a 20%-25% content of SiO2. The particle sizes of silica sol were 20-30 nm. The obtained initial mass demonstrated much better rheological characteristics than described in research by Ksandopulo et al. (1996). It moistened well the surface of fastened graphite samples when exposed for 124  

4 h at 40°C–50°C, and it is completely converted to solid state due to heterocoagulation and the formation of a spatially structured matrix of silica gel, which includes the remaining components of the mixture.

Fig. 9.1. Optimal conditions for obtaining silica sol by the hydrolysis of ethyl silicate. (а) Dependence of hydrolysis temperature on the concentration of H2SO4: 1, 0.5%; 2, 1.0%; 3, 2.0%; 4, 3.0%; and 5, 4.0%, (b) Stable time of silica sol.

Thus, the material after drying is an exothermic composition containing a significant amount of ultra-dispersed silica oxide. After heating in a muffle furnace at 1100°C–1150°C the SHS process is initiated. The velocity of the combustion wave was 2–5 mm/s at a combustion temperature of 1400°C–1700°C. After completion of the synthesis, formed material has negligible porosity and fastens well the graphite products, as shown in Fig. 9.2.

(a

(b

Fig. 9.2. Modifications of a carbonaceous crucible: (а) carbonaceous SHS composite and (b) graphite samples “welded” by SHS synthesis.

125  

The fasten seam remains without alterations up to the working temperature of 1800°C. The strength properties of synthesized SHS refractory material were similar to those for graphite blocks and reached 5–6 MPa. These values are comparable and even better than for conventional high-temperature methods for bonding of graphite materials. It is noteworthy that the strength and adhesion properties of the dried mixture are also high before the realization of SHS. According to XRF results the phase composition of carbonaceous SHS refractories involve silicon carbide, aluminum oxide, graphite, and elemental silicon. It is important that aluminum carbide and aluminum were not detected. SHS refractory composites are a promising materials for use not only as masonry mortar for fastening of graphite and silicon carbide plates and blocks [13], but also as a ramming mixture for the production of high-resistance carbonaceous products used in the metallurgy industry [14]. The use of SHS carbonaceous masses for the production of crucibles for melting metals is also of great interest. It is known that the main wear of graphite crucible is due to its burning out at periodic thermal cycling during the operation process. In this regard, characteristics such as burning in air are very important for determination of the carbonaceous material’s applications. Investigation was conducted on cylindrical samples with a diameter of 2 cm and a height of 4 cm pressed from exothermic mixtures diluted by humidified silica sol. In all samples titanium oxide and aluminum silica in different ratios were used as oxidizers. The carbon content in the original system was 20, 30, and 40 wt.%. Then SHS samples were exposed to thermal cycling at 950°C in air atmospheric, with 30 min exposure in each cycle. The nature of the relative change of the sample weight at thermal cycling is shown in Fig. 9.3. For comparison, the graph represents the relative variation of crucible graphite mass, which indicates its much higher burning out rate in air. The burning out of carbonaceous SHS refractory materials is 3–5 times lower than that for crucible graphite.

Fig. 9.3. The burning degree of carbonaceous SHS refractory as the result of thermal cycling in the air. Carbon content: 1, 20%; 2, 30%; 3, 40%; and 4, graphite.

126  

Table 9.3 Compression strength of carbonaceous SHS refractories before and after cyclic thermal treatment Sample No.

1

2

3

4

5

6

as1, МPа

4.0

6.4

12.0

7.2

3.2

6.4

Graphite chamotte 11.2

bs2, МPа

4.4

6.8

12.4

6.4

6.4

7.2

6.0

Graphite refractory 3–5 —

as1: Compressive strength immediately after SHS bs2: Compressive strength after 5 thermal cycles at 900 °С Table 9.4 Influence of graphite content on compressive strength Carbon content in mixture

Compression strength after SHS, MPa

Compression strength after thermal treatment at temperature 1400°С, МPа

20

3.1

18.2

30

4.8

13.9

40

4.2

8.9

50

3.4

8.6

The physical and mechanical properties of synthesized refractories are very important characteristics. Table 9.3 shows the compression strength for carbonaceous SHS refractories after thermal cycling. Table 9.4 shows that as the result of heat treatments the strength of SHS refractories usually increases, while the crucible graphite shows an almost two times reduction of compressive strength. As it turns out, the multiple thermal impact hardens the SHS materials in contrast to graphite and graphite-containing traditional refractories. Fire resistance measurements were also carried out. Measurements have shown that even at maximum heating mode of Tamman furnace at temperatures in the range 1850°C – 1900°C softening and falling cones are not observed for SHS materials. The results of XRD analysis are presented in Table 9.5. It can be seen that the high-refractory material after SHS contain essentially only refractory: aluminum oxide, titanium carbide, silicon, spinel, and carbon. These phases provide refractoriness of samples above 1850 °C. Table 9.5 Phase composition of SHS products Silicon-containing systems Corundum 24–49 Quartz 7–22 Christobalite 0–3 Silicon 2.5–28 Carbon 6–44 Silicon carbide 4–10

Titanium-containing systems Corundum 20–45 Quartz 0–2 Christobalite 0–1 Titanium-disilicide 0–4 Carbon 9–48 Titanium carbide 6–8

127  

Table 9.6 Corrosion resistance of SHS refractories to molten bronze Element O C Al Si Ti Carbon content in the initial No. mixture, % 1

20

wt.% 30.29 4.68 55.08 5.77 4.18 The samples synthesized in an argon atmosphere, mass variation, % 4.3

at.% 43.23 4.09 46.60 4.69 1.99 The samples synthesized in an air atmosphere, mass variation, % 4.5

2

30

3.4

3.0

3

40

0

2.3

4

50

0

4.0

The experimental samples were held in molten bronze for 15 min at a temperature of 1450°C. The resistance to melting is determined by the change of sample mass by dissolving or chemical interaction with the liquid phase. Table 9.6 shows the mass variation in the tested specimens at high-temperature with presence of liquid metal (bronze), depending on the carbon content in the starting mixture and the synthesis conditions. All the samples of carbonaceous SHS refractory materials have shown high corrosion resistance to melting with the weight loss in the range 0% – 5%.

Fig. 9.4. Electron micrographs of SHS products synthesized in an air atmosphere.

For tests on corrosion resistance to melted materials, the refractory products were fabricated in the form of cylindrical cups with the height of 80 mm, outer diameter of 50 mm, wall thickness of 7 mm, and bottom thickness of 15 mm. SHS was carried out in an argon atmosphere (inert) and in an air atmosphere. The sample that was synthesized in an inert atmosphere was denser and more carbonized. Electron micrograph of a sample that was synthesized in the air atmosphere show that the basis of the metal-ceramic matrix is penetrated in random ways by fibers of different thicknesses from 30 to 1000 nm (Fig. 9.4). 128  

Element C O Al Si Ca Ti Total

wt.% 23.80 26.14 13.10 3.06 0.86 33.05 100.00

error% 0.96 3.12 0.77 0.92 1.65 2.37 —

atom% 40.26 33.21 9.87 2.81 0.43 14.02 100.00

Fig. 9.5. SEM micrographs and EDS analysis of SHS products synthesized in an argon atmosphere.

Such a picture is typically observed during the formation of composite material carcasses from aluminum oxide, during a partial oxidation of metallic aluminum in air with participation of its sub-oxides. The samples that were synthesized under an argon atmosphere at pressure of 10 atmospheres have entirely different microstructures (Fig. 9.5). Cubic and orthorhombic particles with a size of 1 micron and less are uniformly distributed throughout the volume of the composite. Furthermore, the degree of carbonation in this case is much higher because the inert atmosphere prevents the graphite oxidation and promotes the formation of carbide compounds around the carbon particles. In these cylindrical crucibles, experimental melting was performed (Tamman furnace): duralumin at 950°C, copper and bronze at 1400°C–1450°C, and iron at 1600°C.Figure 9.6 shows the photographs of the obtained ingot. Only on the iron ingot 129  

are metal droplets visible: the reaction product of the melt with the inner surface of the crucible. The other melts do not react with the materials.

(a)

(b)

Fig. 9.6. Ingot metals melted in cylindrical crucibles: (а) crucible is synthesized in an air atmosphere; (b) crucible is synthesized in an argon atmosphere; 1, duralumin; 2, bronze; 3, copper; and 4, iron.

Thus the laboratory experimental crucibles showed unique properties and by their technical properties are considerably superior to traditional graphite. However, for the application of SHS technology at the lining of induction melting furnaces, similarity of electro-physical properties between SHS material and traditional crucible graphite is also a necessary condition. The measurements showed that the lower electric conductivity of the used material compared to graphite crucible may lead to deviations of electric modes if the standard furnaces. For the removal of such nonconformities the composition of the exothermic mixtures was optimized, with the purpose of obtaining carbonaceous composites, satisfying the operation conditions of induction furnaces. Also their electrical characteristics were determined. To accomplish this task SHS exothermic mixtures containing: reduction precursors (aluminum, silicon, and titanium); oxidation agents (silicon and titanium oxides); fillers (the bout of electrode graphite and aluminosilicate mortar) and binding material (silica hydrolysis of Degussa AG production or a sol is obtained from ethyl silicate). Dry mixtures were etched by silica sol. The samples in cubic form were prepared by the compaction. After drying, the samples were heated in an electric furnace at temperatures of 850°C– 950°C, and at the same time during which the SHS process occurred. For obtained samples the specific electric resistance was measured in the apparatus as shown in Fig.9.7.

Fig. 9.7.Installation diagram on measurement of specific electric resistance.

130  

Table 9.7 The content of initial components in exothermic systems, % mass Silicon-containing systems Titanium-containing systems Mixed titanium silicon systems Aluminum PА-4 12–24 Aluminum PА-4 10–16 Aluminum PА-4 12–18 Graphite powder 0–425 μm 7–15 Graphite powder 0–425 μm 5–20 Graphite powder 0–425μm 5–10 Graphite grain0.425–4.5 μm Quartz < 90 μm

0–50 0–17

Graphite grain0.425–4.5 μm Titanium oxide

0–50 5–15

Graphite grain0.425–4.5 μm Titanium oxide

30–40 5–10

Siliconpowder Fire-resistant clay

0–12 0–13

Titaniumpowder Fire-resistant clay

0–12 0–3

Silicon powder Fireclaymortar

0–8 30–40

Measurements showed that all synthesized materials had significant electrical conductivity. For two series of SHS products from exothermic mixtures using silicon oxides and titanium as main oxidizing agents (Table 9.7) the values of specific electric resistance at a temperature of 298 K were 5·10-1–2 10-4ohm·m and 4·10-3–3·105 ohm·m, respectively. For the samples of crucible graphite the specific electric resistance is determined within 8·10-6–1·10-5ohm·m. Measurements of specific electric resistance for silicon- containing composites and graphite (Fig. 9.8) reveal the increase of their conductivity with temperature increase. For titanium-containing systems, on the contrary, the temperature dependence of electrical resistance shows a decrease of conductivity as temperature increase. Such behavior is characteristic for metallic type conductivity (Fig. 9.9).

(a)

(b)

Fig. 9.8 Temperature dependence of specific electric resistance (а) for silicon-containing composites and (b) for crucible graphite.

Fig. 9.9. Temperature dependence of specific electric resistance in titanium-containing composites.

131  

On the basis of XRF results it can be concluded that in silicon- containing systems materials, electrical conductivity is provided by silicon, carbon, and silicon carbide-substances that have a semiconducting conduction mechanism. In the case of titanium-containing systems the basic electrical conducting phases are metallic components such as Ti5Si3 and TiSi2, having a significantly lower specific electric resistance than carbon and titanium carbide. Accordingly, their contribution to the total conductivity is predominant, which is reflected in thermal dependence of the resistivity.

Figure 9.10. Specific electric resistance of carbonaceous titanium-silicon composite (the shaded area corresponds to the crucible graphite resistivity).

The difference of electrical resistance between investigated systems and crucible graphite is quite significant, and reduction of these differences is achieved by introducing silicon powderinto the titanium system. At a silicon content of 6-8wt.% the electrical resistances of SHS composite and crucible graphite are equalized (Fig. 9.10). Thus, above results on SHS of carbon-containing refractories have shown the possibility of fabrication of fire-resistant carbon-containing composites that can replace the expensive graphite elements of lining in induction melting furnaces of ferrous and nonferrous metals without significant change of the electric mode of their operation. CONTROL QUESTIONS 1. 2. 3. 4. 5. 6.

What do you know about the oxide SHS refractories “Furnon”? What are specifics for SHS carbon-containing refractories? Comporise the characteristic of SHS refractory with their analogues. What do you know about new carbon-containing refractories? List the content of initial components in exothermic system. What kind of apparatus used to measure specific electric resistance? REFERENCES

1. Merzhanov A.G., Borovinskaya I.P.: Self-Propagating high-temperature synthesis of refractory inorganic connections. Reports (AS the USSR 204, p. 366-369, 1972). 2. Mansurov Z.A., Dilmukhambetov E.E., Ismailov M.B., Fomenko S.M., Vongai I.M. (2001). New refractory materials on the SHS technology. La Chimica eI’ Industria, 83: 1–6. 3. Mansurov Z.A. (2010). SHS refractory materials “furnon” and their practical implementations in Kazakhstan and Russia. Adv. 4. Mansurov Z.A., Fomenko S.M. (2014). Carbonaceous refractory materials on SHS-technology. Adv. Sci. Technol. Mater. Res., 88: 112–128.

132  

5. Ksandopulo G., Ismailov M.: Combustion processes involving mineral raw materials and some problems of refractory synthesis Jnt. Journal of SHS. – 1992. – V. 1. – №3. – Р. 496-507. 6. Ismailov M.B.: Processes of metallotermic burning and their application in the technology of refractories. Thesis for a scientific degree of Dr.Sci. – Alma-Ata, 1992. 7. Dilmukhanbetov E., Vongay I., Fomenko S. Abstract. International Symposium “Chemistry of flame front”. – Almaty, p. 62 (1997). 8. Ksandopulo G.I., Ismailov M.B.: SHS-refractories "Furnon". – Almaty, p. 16 (1993). 9. Dilmukhanbetov E., Ismailov М., Fomenko S.: Solid phase combustion of oxide systems Bulletin KazGU. – №3 (20). – Р. 28–42 (2000). 10. Information on http://www.kianit.narod.ru, Ekaterinburg, Gamma Hi-Tech SHS Refractories. 11. Ksandopulo G.I., Dilmukhambetov Е.Е., Ismailov М.B., Fomenko S.М., others. (1996). Fireresistance ramming mixture for limning. Institute of Combustion Problems, Republic of Kazakhstan, СОUВ 35/00, 35/04, Patent 4013, Publ. 16.12.96, Bul. – №4. – Р. 2. 12. Vongay I.M., Dilmukhambetov Е.Е., Fomenko S.М., Mansurov Z.А. (2004). SH-synthesis of carbonaceous refractory in the presence of silica sol. Combust. Plasmachem.,2(2): 109–113. 13. Dilmukhambetov Е.Е., Fomenko S.М., Mansurov Z.А., Vongay I.М. (2006). Method of material connection, Patent 18058, Publ. 15.12.06, Bul. – №12, 4 p. with illustration. 14. Fomenko S.М., Dilmukhambetov E.E., Mansurov Z.А. (2010). Manufacturing method of refractory crucible of induction furnace. Applicant and patent holder is the Institute of Combustion Problems, Republic of Kazakhstan, №2010/0775.1, Innovative patent 24126, application form 10.06.2010, publ. 15.06.11, p. 3.

133  

Laboratory work 1 MEASURING OF THE COMBUSTION WAVE VELOCITY

Work objectives: Experimental measuringof the average combustion velocity in the considered SHS system. Used materials and equipments: 1. Powders to make initial reaction mixture of the desired composition. 2. Press die 3. Press for sample preparation 4. Calipers to measure sample dimensions. 4. Balance to measure sample weight; 5. A tool for fixing the samples. 6. An apparatus for ignition the sample (stationary or portable electric igniting device); 7. Manual stopwatch to measure time of combustion front propagation from the top to the bottom of the sample, Preparation for laboratory work: to get acquainted with the safety rules when working with chemical reagents; to master the rules and techniques of safe operation with the used devices. Theoretical part

The basic formula for determining of combustion velocity in approximation of narrow reaction zone is called Zeldovich’s formula and can be presented as follows: T

c 1 U 2Q   (T )dT , cp Tc  T0  T0

(1)

where:Т –current temperature; T0 – temperature of initial mixture; Tс – adiabatic combustion temperature; Q –heat of chemical reaction; Ф(Т) – kinetic function, which describes the rate of chemical conversation as a function of temperature; λ – thermal conductivity; с– heat capacity; р– density. Physical parameters λ,с,р, Q и Ф(Т) are the properties of reaction mixture. However, formula (1) does not account possible phase transformations during reactions, as well as a degree of conversion (η). Self-propagating high temperature synthesis (SHS) is characterized by: а) extremely high heating rate (105-106 с/с); b) high combustion temperature (2000-4000 K), which is typically above phase transformations (melting, dissociation and etc) temperatures in the system ; c) heat release takes place uniformly along the combustion front, but not necessary full conversion is achieved [1]. These specifics require the innovative ways for synthesis control. B.I. Khaikin and A.G. Merzhanov [2] proposed the model involving the kinetic function F(T,η), 134  

which accounts above features of heterogeneous combustion. In general this function can be written as follows:   E   F (T , )  A ( ) exp     L (T ) t  RT 

(2)

Here the reaction rate depends not only on the combustion temperature (Arrhenius equation), but also on degree of conversion level (φ(η)), which is definded. The conversion level is defined as η = (c0 - c)/c0, where c0– initial reagent concentration; с – its instantaneous concentration. Furthermore, in Eq.(2) the possibility of heat generation and heat absorption as the results of phase transition is considered; here L – is the heat of phase transition, function Ψ(Т1η) is describing the transition kinetics. A combustion analysis of heterogeneous system of type (2), including computer modeling, and a correlation with experimental results of combustion wave structure is allowed to A.G. Merzhanov together with co-workers to suggest the generalized formula for front propagation velocity of combustion:  E , U 2  A(T* ,* ) exp     RT 

(3)

where Т* and η* are so-called leading combustion temperature and degree of conversion in the combustion front, which are determined based on the additional assumptions for considered system, e.g. T* can be equal to the dissociation temperature of the product, which may be much less than adiabatic combustion temperature; A(Т*, η*,) weak power function. The combustion wave propagation velocity is influenced by various other factors: density of sample, reactants particle size, reactive mixture dilution by inert additives etc. Fig. 1 represents the typical dependences of combustion velocity in gasless systems versus different experimental parameters.

a

d

b

g

c

e

а) the size of metal particles; b) ratio of reagents; c) sample diameter; d) initial temperature; g) inert dilution; h) gas pressure Fig. 1. The dependence of burning rateof experimental parameters.

135  

Experimental approaches To study the structure of the SHS wave, laboratory set-upis used, which general scheme is shows in Fig. 2. In general, thisset-ups, allows to study both the combustion wave propagationregime and the thermal explosion regime. The velocity of reaction front propagation, the dynamics of changing the dimensions of the sample (expansion, shrinkage) are usually determined by video recording. The use of macro-lenses makes it possible to obtain an image of a reacting medium with a resolution of the order of 10 μm, if higher resolution is required (1 μm), the video is taken through a microscope (see, for example, [5-7]).

1 – manometer, 2 – transformer, 3 – high-speed video camera, 4 – lock nut, 5 – computer, 6 – data acquisition systems LTR-U-1, 7 – bottle with nitrogen, 8 – sample, 9 – thermocouple, 10, 11 – inlet and outlet valves, 12 – window for observation of the synthesis process Fig. 2. Experimental installation for studying SHS process.

The most accurate method for measuring the temperature time profile of the combustion process is the usage of the thermocouples. Due to the high temperature developed during combustion, it is necessary to use the most refractory thermocouples from tungsten-rhenium alloys. For example, W/Re thermocouples BP5/20 with a diameter d = 100 μm (thermal relaxation time t ≈ d2/4a ≈ 4 ∙ 10-5 s;a ≈ 0.65 cm2/s – thermal diffusivity of tungsten) are often used to measure the temperature in the combustion wave. If one needs a more detailed measurement of the T-profile of the combustion front a micro thermocouple with a thickness of the order of 10 μm or less is required [3,4]. In addition, they must be coated with a protective coating layer, typically boron nitride. To incorporate such thin thermocouples into the samples is rather complicated and laborious task. The error in high-temperature thermocouple measurements in the SHS wave is approximately 10-50 K. Non-contact measurements with a pyrometer are more efficient [8,9], but the measurement accuracy is worse than the thermocouple, mainly due to the uncertainty of the radiation coefficients of the reacting sample at different wavelengths. Combining the photosensor with a microscope allows micro-pyrometric measurements 136  

when the brightness temperature measured in a region of about 10 μm [10]. Thermal vision is the most modern and promising method of measuring temperature in SHS. The thermal imaging camera (thermal imager) detects the thermal radiation of the sample in the far infrared range (wavelength 2-5 μm) and shows the dynamic temperature distribution in the field of view. The shooting frequency of the thermal imager is usually 30 frames per second. Recently, high-speed thermal imagers have appeared, with a shooting frequency of up to 15,000 frames per second, as well as special infrared lenses, which increase the spatial resolution up to tens of micrometers. However, such thermal imagers are still quite expensive and require the use of special expensive windows (fluorite, NaCl, etc.) in the reaction chamber. All the methods considered work well in the study of synthesis in the thermal explosion regime [11].

Fig. 3. High-speed camera (EVERCAM 1000-4с).

High-speed cameras EVERCAM is a high-performance device in a compact and lightweight case that allows you to shoot 4000 frames per second (for EVERCAM cameras) at a resolution of 1280x800 (up to 85000 fps at a reduced resolution) and 2800 frames per second (for EVERCAM F-series cameras) at a resolution of 1920x1080 (82000 fps at a reduced resolution) (Fig. 3). Steps for the laboratory work

The precursors are weighed on electronic scales and thoroughly mixed in a porcelain mortar. Then, a certain amount of water is added, sufficient to prepare a semimoist mixture for the purpose of making the samples by pressing. Humidity of mixtures should be 5-10%. The samples are prepared in the form of cylinders with a diameter of 20 mm and a height of 40 mm. Such cylinders are made by pressing powder mixture in a mold at a pressure of about 70 MPa. After compaction, the samples are left for natural drying at room temperature for about 24 hours, then kept in a drying oven at a temperature of 70-80°C for 5 hours. Before starting the tests, samples should be weighed on a scale and determine their length and diameter using a caliper. The dried samples are placed in a reactionchamber (Fig. 2) where self-propagating high-temperature synthesis is carried out. Secure the specimen in the combustion chamber. Bring the spiral of the electric igniter to the surface of the sample. Set up and test instruments for measuring time. The hand-held stopwatch must be checked for proper start-up and stopping, to avoid delay when rotating on the dial. 137  

The experimenter's duty is to make sure before burning on the absence of people near the combustion chamber. The sample is ignited by applying a voltage to the spiral of the igniter. The control button of the igniter, the student should be located behind the protective glass. When reaction is initiated start on the stopwatch and record the burning time of the sample on its whole height. Record your results in the table 1. Make at least three experiments for each investigated compositions. Table 1 Observation recording form No. sample

Sample height, l (mm)

Sample diameterd (mm)

Sample weight, m (g)

Burning time, τ (s)

Linear combustion velocity, u (mm/s)

Mass burning rate, um (g /s cm2)

DataAnalysis

Calculate the linear combustion rate for each experiment by formula:

l u ,



where l is the length of the sample, mm τ – burning time, s Calculate the mass burning rate by formula:

um 

m ,  d 2 / 4

where m is the mass of the composition, g; τ – burning time, sec; d – is the diameter of the sample, Calculate the average linear velocity and average mass velocity from a series of parallel tests. Calculate the absolute and relative errors in determining the linear and mass velocities. All the results obtained are also included in the table. Make conclusions. REFERENCES 1. Levashov Ye.A., Rogachev A.S., Yukhvid V.I. Borovinekaya I.P. Fiziko-khimicheskiye i tekhnologicheskiye osnovy samorasprostranyayushchegosya vysokotemperaturnogo sinteza. ‒ M.: Binom, 1999. ‒ S. 16-19. [in Russian] 2. Merzhanov A.G., Borovinekaya I.P., Yukhvid V.I. Samorasprostranyayushchiysya vysokotemperaturnyy sintez tugoplavkikh neorganicheskikh soyedineniy / DAN SSSR, 1980, 255. ‒ S. 120-124. [in Russian]

138  

3. Zenin A.A., Merzhanov A.G., Nersisyan G.A. Structure of the thermal wave in some SHS processes // Reports of the Academy of Sciences of the USSR. – 1980. – T. 250. – №4. – Pp. 880-884. 4. Zenin A.A., Merzhanov A.G., Nersisyan G.A. Investigation of the structure of the thermal wave in SHS processes (by the example of boride synthesis) // Physics of Combustion and Explosion. – 1981. – Vol. 16. – No. 1. – Pp. 79-90. 5. Rogachev A.S. Macrokinetics of gasless combustion: the old problems and new approaches // Intern. J. of SHS. – 1997. – V. 6. – №2. – P. 215-242. 6. Hwang S., Mukasyan A.S., Rogachev A.S., Varma A. Combustion wave microstructure in gassolid reaction systems: experiment and theory // Combustion Science and Technology. – 1997. – V. 123. – P. 165-184. 7. Varma A., Rogachev A.S., Mukasyan A.S., Hwang S. Complex behavior of self-propagating reaction waves in heterogeneous media // Proc. Natl. Acad. Sci. USA. – 1998. – V. 95. – P. 11053-11058. 8. Andreev V.A., Maltsev V.M., Seleznev V.A. Study of combustion of mixtures of hafnium and boron by the method of optical pyrometry // Physics of Combustion and Explosion. – 1980. – V. 16. – №4. – Pp. 18-23. 9. Anselmi-Tamburini U., Magila F., Spinolo G., Munir Z.A. Use of two-color array pyrometry for characterization of combustion synthesis waves // J. of Materials Research. – 2000. – V. 15. – №2. – P. 572-580. 10. Garkol D.A., Gulyaev P.Yu., Evstigne ev V.V., Mukhachev A.B. A new technique for highspeed brightness pyrometry for investigating SHS processes // Physics of Combustion and Explosion. – 1994. – V. 30. – №1. – Pp. 72-77. 11. Rogachev A.S., Mukasyan A.S., Varma A. Volume Combustion Modes in Heterogeneous Reaction Systems // J. Materials Synthesis and Proc. – 2002. – V. 10. – №1. – P. 31-36.

139  

Laboratory work 2 MEASURING OFTHE MAXIMUM COMBUSTION TEMPERATURE

Aim of work: To measure the combustion temperatures of SHS process. Used materials and equipment: Experimental set up, powder of aluminum grade PA-4, silicon powder, silicon oxide grade CP, titanium oxide or chromium oxide grade CP, tanks of nitrogen and argon gases; thermocouples, igniter (laboratory transformer), pyrometer. Preparation for laboratory work: to get acquainted with the safety rules when working with chemical reagents; to master the rules and techniques of safe operation of the set-up. Theoretical part

For realization of SHS is necessary to have reactive system with enough exothermicity and rapid kinetics, in order, the heat released in the combustion front may maintain its self-propagation along the media. Not all exothermic chemical reaction fit such requirements, therefore, not every exothermic system can be used for SHS. Unfortunately there are no easy criteria which allow to predict the possibility of SHS type of processes. The first step in prediction of the possibility of SHS is the calculation of adiabatic combustion temperature of considered system. This temperature should be high enough (>1800 K: criteria suggested by Z. Munir) to provide the intensive heterogeneous reaction. It is desirable that this temperature would be above the melting point of at least one of the components of the reaction mixture. Thermodynamics are typically used for chemical processes to calculate the energy balance of the reaction, to define possible irreversible reactions and the direction of the reversible reactions under considered conditions, to determine the conditions of chemical equilibrium and their change under the influence of external factors. In the case of the combustion process, thermodynamic approaches are used, primarily for calculating the adiabatic combustion temperature and equilibrium reaction products. The calculation of the adiabatic combustion temperature assumes the absence of heat loss in the reaction zone and the complete conversion of the reactants. The basic condition for the calculation of adiabatic combustion temperature that heat released during reaction (Q) Tad is equal to differences of enthalpies of starting materials at the initial temperature T0 and the combustion products Tad. a

 H T   H T   Q , i 1

0

ad

140  

(1)

whereTad– adiabatic combustion temperature; T0 – initial temperature; Q – reaction heat, summarization is carried out in all reaction products. Consider, for example, the reaction: Ti + 2B = TiB2 The starting materials are simple elements and therefore their standard enthalpies equal to zero. The standard enthalpy of formation for titanium diboride phase from the elements is known from handbooks and is ΔH0(TiB2) = –293 kJ/mol. This value is negative because during exothermic reaction the system moves into a lower energy state. The difference between the standard enthalpies of the reactants and reaction products is the amount of heat released during the reaction, which is called the heat of reaction (recall that the work in this case is negligible.) In this example, the heat of reaction Qr = –ΔH0(TiB2) = 293 kJ/mol = 4.21 kJ/g. Under adiabatic conditions, i.e. in the absence of heat loss, all this heat is used to preheat the reaction product to a temperature from T0 to Tad, which is called the adiabatic temperature, and can be determined by solving the following equation

Q

Tad

c

p

(T ) dT ,

(2)

t0

where cp(T) is the specific heat of products at constant pressure as a function of temperature. The temperature dependence of the specific heat of substances and compounds can be found in handbooks, and usually are presented in the form of polynomials: (3) c(T) = A + BT + CT2 Substituting (3) for (2), we obtain an algebraic equation for determining Tad:

C 3 B 2 C B   Tad  Tad  ATad  Q  T03  T02  T0   0 3 2 3 2  

(4)

For example, for TiB2: c(T) = 30.2 + 0.048T J/mol·K, and assuming T0 = 300 K, we obtain a quadratic equation with respect to Tad:

0.024Tad2  30.2Tad  304220  0

(5)

The solution of this equation is Tad = 2986 K. As it is shown below, the obtained temperature value is approximate, but it immediately suggests the possibility of titanium diboride synthesis in the combustion mode. In general, the more accurately defined that the c(T) function is, the more precise the value of the adiabatic temperature can be calculated. However, for rough estimates the specific heat can be considered even constant, then from (2) we obtain the expression for adiabatic temperature, which is often used in studies of combustion processes:

Tad  T0  Q / c 141  

(6)

It should be noted, however, that eq. (6) is only acceptable for rarified gas mixtures whose heat capacity depends weakly on the temperature. The use of formula (6) for systems with solid and liquid phases can lead to a large error. For instance, the heat capacity of TiB2 at room temperature is 44.18 J/mol·K. If we use this value in the formula (6), we obtain the calculated adiabatic temperature Tad = 6900 K, which more than twice (!) exceeds the actual value of the adiabatic temperature of the system. The phase transitions of the first-order (melting, boiling, etc.) are common for most SHS systems. In this case, the latent heats of these phase transformation should be taken into account in the balance of enthalpy, and the formula for determining the adiabatic temperature (2) becomes more complex: n

Q   H   ph H ph  i ph

i 1

Tad

 c(T )dT

(7)

t0

where Q is the heat of formation of the product at T0 , i.e. the difference between the standard enthalpy of the reactants and products, ΔHiph is the enthalpy of phase transitions at Tad the expression (7) can be approximately presented in a form widely used in combustion theory

Tad  T0 

Qeff

(8)

c

where

c  Tad  T0 

1

Tad



n

c (T ) dT ; Qeff  Q   H iph

(9)

i 1

t0

Two-component compositions are only a small part of SHS systems. In practice, compositions consisting of 3 to 7 precursors are often used and these mixtures include not only the elements but also compounds. The calculations Tad in multicomponent systems are quite complex but this is not the only problem. In order to perform the above calculations, one has to assume in advance what products are formed by the reaction. Sometimes this assumption is obvious, as in the chemical equation between Ti and B, but in general, setting the initial composition of the reaction system, we do not know what equilibrium products are obtained by the reaction. Thus, the product composition should also be defined by thermodynamic calculations. Such calculations are possible only with the help of a computer. At present many software packages exist for calculation of the thermodynamic equilibrium in chemically reacting systems and the principle of all algorithms is the search for the minimum of free energy. Some of them are discussed below. Popular software packages CHEMKIN were created by Sandia National Laboratory, USA (Sandia National Laboratories). Since 1997, the software for these 142  

programs is being developed by Reaction Design (http://www.ReactionDesign.com). The main goal of these packages is to help engineers to develop more efficient and environmentally friendly chemical processes. CHEMKIN includes not only the thermodynamic but also kinetic type of calculations. To investigate extreme phenomenon such as combustion and explosion, the National Agency for Aeronautics and Space Administration USA (NASA) developed a program called CEA (Chemical Equilibrium with Applications), which is used to calculate the equilibrium composition and thermodynamic properties of complex systems (http://www.lerc.nasa.gov/www/caeweb/). The applications of the CEA program include rocket engines and shock waves. For ease package, the independent database for the transport and the thermodynamic properties of substances was also created. This accompanying database contains information on the thermodynamic properties of more than 1900 substances. The program was written by Bonnie J. McBride and Sanford Gordon in standard ANSI FORTRAN. It is widely used to solve different thermodynamic and aerodynamic problems. Table 1 The adiabatic temperatures for some SHS systems Systems

Experimental Combustion Temperature

Eutectics or melting Point of the Less Refractory Compound K

2735 3290 1873

2550 3070 2000*

3295 (Ta) 1921 (eut) 1690 (Si)

3193 3349

3190 2500

1810 (eut) 1810 (eut)

1925 1690 2403

1920 1770 2350

1673 (eut) 1600 (eut) 1600 (eut)

1912 1586 1517 1418 1042

1900 1600 n/a n/a n/a

921 (eut) 921 (eut) 933 (Al) 1215 (eut) 1358 (eut)

3165 3322 3446 3639 2430 3437

2500 2800 2700 2300 2250 2600

3000 (Ta) 2673 (NbN) 1943 (Ti) 933 (Al) 1690 (Si) 2350 (B)

Adiabatic Combustion Temperature

Carbides Ta + C Ti + C Si + C Borides Ti + 2B Ti + B Silicides Mo + 2Si Ti + 2Si 5Ti + 3Si Intermetallics Ni + Al 3Ni +Al Ti + Al Ti + Ni Ti + Fe Nitrides↑ 2Ta + N2 2Nb + N2 2Ti + N2 2Al + N2 3Si + 2N2 2B + N2

ad c

T

,K

In turn a variety of problems in metallurgy, chemistry, materials science, and geochemistry can be solved by the MTDATA software package, developed by the National Physical Laboratory in Great Britain (Materials Thermochemistry Section at National Physical Laboratory, Teddington, UK). This package is designed for the calculation of phase equilibrium in multi-phase multi-component systems; it includes a database containing critically selected thermodynamic properties (http://www.npl.co.uk/npl/cmmt/mtdata/). 143  

The software package Thermo-Calc was developed by Thermo-Calc Software, which was founded in 1997 as a branch of the Department of Materials Science and Technology at the Royal Institute of Technology, Stockholm, Sweden (Materials Science and Engineering at KTH, Stockholm, Sweden). This software is designed to calculate the phase equilibriums and phase diagrams (http://www.thermocalc.com/; http://www.thermocalc.se). In developing the program special attention was paid to the possibility of investigating thermodynamic systems with strongly non-ideal behaviors. This software can be used for analysis of thermodynamic systems in areas such as chemistry, metallurgy, materials science, geochemistry, etc. depending on the database, which is attached to the package. Thermo-Calc contains a number of modules, which enable the researcher to solve problems of interest. For example, there is a module for selecting a database, viewing and editing the information in it. The most important module for calculating the equilibrium composition provides an opportunity to build various charts. Very useful is a module designed to estimate the parameters of thermodynamic models based on the experimental data. There is also a module, which presents the thermodynamic properties and chemical reactions in tabular form. The system is open, so that the users have the opportunity to develop their own modules using the documented interface of the system. Finally, for studies of SHS processes the Institute of Structural Macrokinetics and Materials Science Russian Academy of Sciences (ISMAN), designed software package THERMO (http://www.ism.ac.ru). It comprises a data bank which collects the thermodynamic functions (free energy, enthalpy, entropy) and thermal properties (temperature dependence of heat capacity, temperature and thermal effects of phase transitions) for the majority of individual substances and compounds that may be involved in the SHS processes. The bank may be supplemented by the user. To calculate the adiabatic temperature of combustion and the equilibrium composition of the products, it is necessary to set the concentration of the starting materials and their temperature, as well as choose from the data bank proposed values for various compounds in the system that may actually form (one can select all proposed compounds, this only slightly increases the calculation time). To determine the equilibrium state, the program searches for the minimum of the thermodynamic potential (free energy) of the system taking into account the contributions of the thermodynamic potentials of all compounds contained in the system proportional to their concentrations. The calculation of the equilibrium combustion products can be carried out in two regimes: constant pressure (pressure in the system is set P = const) and the constant volume regime (V = const). In the first case the program searches the minimum Gibbs free energy (isobaric–isothermal potential) G, in the second case the minimum of the Helmholtz free energy (isochoric–isothermal potential) F. The table shows the adiabatic temperatures of some SHS systems. The most common method for measurement of actual combustion temperatures is the use of the thermocouples. Athermocoupleis an electrical device consisting of two dissimilarelectrical conductorsformingelectrical junctions at differing temperatures. A thermocouple produces a temperature-dependentvoltageas a result of thethermoelectric effect, and this voltage can be interpreted to measure temperature. Thermocouples are a widely used type oftemperature sensor. For the systems with reaction temperature below the temperatures of 1400 K the chromel-alumel K-type of thermocouple can be used (AH195/NMtsAK2-21). During conduction of SHS process with combustion temperature not higher than 1700 K the 144  

platinum-rhodium thermocouples are applicable (Pt⋅6/ph, Pt⋅30). Typically, for measu-rement of maximum combustion temperatures the tungsten-rhenium thermocouples (5⋅VR / 20⋅VR) are used with diameter of 50-200 μm. Nonisolated tungsten-rhenium thermocouples exhibit a high stability up to 2600-2800 K during a short period of time (15 sec). We use a chromel-alumel thermocouple.

Fig. 1. K-type thermocouple (chromel–alumel) in the standard measurement configuration

The standard configuration for thermocouple usage is shown in the Fig. 1. Briefly, the desired temperature Tad is obtained using three inputs – the characteristic function E(T) of the thermocouple (calibration curve), the measured voltage V, and the reference junctions' temperature Tref. The solution to the equation E(Tad) = V + E(Tref) yields Tad. These details are often hidden from the user since the reference junction block (with Tref thermometer), voltmeter, and equation solver are combined into a single product. The thermocouple contains two electrodes of dissimilar materials. In total, there are about a dozen thermocouples of various types, according to the international standard, denoted by the letters of the Latin alphabet. Each type has its own characteristics, which is mainly due to the materials of the electrodes. For the manufacture of thermocouples, metals and their alloys are mainly used [1]. Thermocouples of semiconductors are characterized by high sensitivity, but they have a large internal resistance and low mechanical strength and find limited application [2]. The features and applications of some thermocouples are given in Table. 2. Table 2 Applications of thermocouples Thermoelectrode material Chromel-alumel Chromel-kopel Platinumrhodium (10%) platinum

Tungsten rhenium (5%) tungsten rhenium (20%)

Application features Possess: the closest to the direct characteristic. Designed to work in oxidizing and inert media Possess: - the greatest sensitivity; - high thermoelectric stability at temperatures up to 600°C. Designed to work in oxidizing and inert media. Disadvantage: high sensitivity to deformations Possess: - good resistance to gas corrosion, especially in air at high temperatures; - high reliability when operating in a vacuum (but less stable in neutral environments). Designed for long-term use in oxidizing environments. Disadvantage: high sensitivity of thermoelectrodes to any contamination that appeared in the manufacture, installation or operation of thermocouples Possess: - the possibility of long-term use at temperatures up to 2200°C in non-oxidizing environments; - resistance in argon, helium, dry hydrogen and nitrogen. Thermocouples with thermoelectrodes from a platinum alloy with 10% rhodium relative to an electrode made of pure platinum can be used as standard for establishing the nominal static characteristics of thermocouples by comparison. The disadvantage is the poor reproducibility of thermal EMF, which forces to group thermoelectrode pairs into groups with nominal static characteristics A-1, A-2, A-3

145  

1 – chamber for burning; 2 – cover; 3 – sample; 4 – thermocouple; 5 - oscilloscope; 6 – nozzles for supply and gas outlet; 7 – manometer; 8 – fire spiral Fig. 2. Diagram of reaction chamber for laboratory investigation of SHS.

Fig. 3. Digital Multimeter UT50B.

The Meter uses large scale of integrated circuit with double integrated A/D converter as its core and has full range overload protection. Can measure AC/DC Voltage, AC/DC Current, Resistance, Capacitance, Temperature, Frequency, Diodes and Continuity, has Data Hold, Full Icon Display and Sleep Mode features.Adopted advanced "co-injection" technique in order to provide sufficient insulation and anti-shaking. The automatic display backlight feature enables users to work in a dim condition (Fig. 3.). 146  

Table 3 Calibration tables for chromel-alumel Thermocouple (Type K) Temperature range 0 – 1370°C. The output voltage is given in mV °с 1 0

0 1 2 3 0.000 0.039

2 4 0.079

3 5 0.119

4 6 0.158

5 7 0.198

6 8 0.238

7 9 0.277

8 10 0.317

9 11 0.357

10 12 0.397

10

0.397 0.437

0.477

0.517

0.557

0.597

0.637

0.677

0.718

0.758

0.798

20 30 40 50

0.798 1.203 1.612 2.023

0.838 1.244 1.653 2.064

0.879 1.285 1.694 2.106

0.919 1.326 1.735 2.147

0.960 1.366 1.776 2.188

1.000 1.407 1.817 2.230

1.041 1.448 1.858 2.271

1.081 1.489 1.899 2.312

1.122 1.530 1.941 2.354

1.163 1.571 1.982 2.395

1.203 1.612 2.023 2.436

60

2.436 2.478

2.519

2.561

2.602

2.644

2.685

2.727

2.768

2.810

2.851

70 80 90

2.851 2.893 3.267 3.308 3.682 3.723

2.934 3.350 3.765

2.976 3.391 3.806

3.017 3.433 3.848

3.059 3.474 3.889

3.100 3.516 3.931

3.142 3.557 3.972

3.184 3.599 4.013

3.225 3.640 4.055

3.267 3.682 4.096

100 4.096 4.138

4.179

4.220

4.262

4.303

4.344

4.385

4.427

4.468

4.509

110 4.509 4.550

4.591

4.633

4.674

4.715

4.756

4.797

4.838

4.879

4.920

120 4.920 4.961 130 5.328 5.369 140 5.735 5.775

5.002 5.410 5.815

5.043 5.450 5.856

5.084 5.491 5.896

5.124 5.532 5.937

5.165 5.572 5.977

5.206 5.613 6.017

5.247 5.653 6.058

5.288 5.694 6.098

5.328 5.735 6.138

150 6.138 6.179

6.219

6.259

6.299

6.339

6.380

6.420

6.460

6.500

6.540

160 6.540 6.580

6.620

6.660

6.701

6.741

6.781

6.821

6.861

6.901

6.941

170 180 190 200

6.981 7.380 7.779 8.178

7.021 7.420 7.819 8.218

7.060 7.460 7.859 8.258

7.100 7.500 7.899 8.298

7.140 7.540 7.939 8.338

7.180 7.579 7.979 8.378

7.220 7.619 8.019 8.418

7.260 7.659 8.059 8.458

7.300 7.699 8.099 8.499

7.340 7.739 8.138 8.539

210 8.539 8.579

8.619

8.659

8.699

8.739

8.779

8.819

8.860

8.900

8.940

220 8.940 8.980 230 9.343 9.383 240 9.747 9.788

9.020 9.423 9.828

9.061 9.464 9.869

9.101 9.504 9.909

9.141 9.545 9.950

9.181 9.585 9.991

9.222 9.626 10.031

9.262 9.666 10.072

9.302 9.707 10.113

9.343 9.747 10.153

250 10.15 10.19

10.235

10.276

10.316

10.357

10.398

10.439

10.480

10.520

10.561

260 10.56 10.60

10.643

10.684

10.725

10.766

10.807

10.848

10.889

10.930

10.971

270 10.97 11.01 280 11.38 11.42 290 11.79 11.84

11.053 11.465 11.877

11.094 11.506 11.919

11.135 11.547 11.960

11.176 11.588 12.001

11.217 11.630 12.043

11.259 11.671 12.084

11.300 11.712 12.126

11.341 11.753 12.167

11.382 11.795 12.209

300 12.21 12.25

12.291

12.333

12.374

12.416

12.457

12.499

12.540

12.582

12.624

310 12.62 12.66

12.707

12.748

12.790

12.831

12.873

12.915

12.956

12.998

13.040

320 13.04 13.08 330 13.46 13.50 340 13.87 13.91

13.123 13.540 13.958

13.165 13.582 14.000

13.206 13.624 14.042

13.248 13.665 14.084

13.290 13.707 14.126

13.331 13.749 14.167

13.373 13.791 14.209

13.415 13.833 14.251

13.457 13.874 14.293

350 14.29 14.33

14.377

14.419

14.461

14.503

14.545

14.587

14.629

14.671

14.713

360 14.71 14.75

14.797

14.839

14.881

14.923

14.965

15.007

15.049

15.091

15.133

370 15.13 15.17 380 15.55 15.59 390 15.97 16.01

15.217 15.638 16.059

15.259 15.680 16.102

15.301 15.722 16.144

15.343 15.764 16.186

15.385 15.806 16.228

15.427 15.849 16.270

15.469 15.891 16.313

15.511 15.933 16.355

15.554 15.975 16.397

6.941 7.340 7.739 8.138

147  

1 2 3 400 16.40 16.44 410 16.82 16.86

4 16.482 16.904

5 16.524 16.947

6 16.566 16.989

7 16.608 17.031

8 16.651 17.074

9 16.693 17.116

10 16.735 17.158

11 16.778 17.201

12 16.820 17.243

420 17.24 17.28 430 17.67 17.71 440 18.09 18.13

17.328 17.752 18.176

17.370 17.794 18.218

17.413 17.837 18.261

17.455 17.879 18.303

17.497 17.921 18.346

17.540 17.964 18.388

17.582 18.006 18.431

17.624 18.049 18.473

17.667 18.091 18.516

450 18.51 18.56

18.601

18.643

18.686

18.728

18.771

18.813

18.856

18.898

18.941

460 18.94 18.98

19.026

19.068

19.111

19.154

19.196

19.239

19.281

19.324

19.366

470 19.36 19.41 480 19.79 19.83

19.451 19.877

19.494 19.920

19.537 19.962

19.579 20.005

19.622 20.048

19.664 20.090

19.707 20.133

19.750 20.175

19.792 20.218

Steps for the laboratory work

The precursors are weighed on electronic scales and thoroughly mixed in a porcelain mortar. Then, a certain amount of water is added, sufficient to prepare a semi-moist mixture for the purpose of making the samples by pressing. Humidity of mixtures should be 5-10%. The samples are prepared in the form of cylinders with a diameter of 20 mm and a height of 40 mm. Such cylinders are made by pressing powder mixture in a mold at a pressure of about 70 MPa. After compaction, the samples are left for natural drying at room temperature for about 24 hours, then kept in a drying oven at a temperature of 70-80°C for 5 hours. Before starting the tests, samples should be weighed on a scale and determine their length and diameter using a caliper. Temperature measurement on the samples is carried out using an optical infrared pyrometer Raytek 3i (measurement interval 600-3000°С) (Fig. 3) and a thermocouple method. Fabrication of the thermocouples

Making a thermocouple is to create a strong connection (welding, Fig. 3) of two materials (wires). To do this, you can use a voltage source of sufficient power (for example, LATR - laboratory autotransformer, car battery). A thermocouple (both free ends) with pre-mechanically connected wires is connected to one pole of the voltage source, and the output connected to a piece of graphite is connected to the other. When the connected ends of the graphite thermocouple come into contact, an electric arc arises and welding of the thermocouple wires takes place. The voltage necessary for welding is selected experimentally, starting with small voltages of 3-5 V. The optimum value of the voltage for welding depends on the thermocouple material, diameter, length and, as a rule, does not exceed 40-50 V. When working, it is necessary to observe safety precautions: do not use too much stress, do not touch bare sections of the electrical circuit. For convenience, a small section of thermocouple wires can be covered with electrical tape, ceramic tube, etc. A sufficiently good connection can also be obtained by heating the wires of the thermocouple with an arc discharge ignited between them and a strong aqueous solution of common salt. 148  

a

B

c

Fig. 3. The process of making of chromel-alumel thermocouple: (a) mechanical connection of two wires before welding, (b) wiring diagram for welding thermocouples, (c) made thermocouple.

Experiment 1 Measurements of the combustion temperature-time profile by thermocouple method

To perform the experiment on the measurements of the combustion temperaturetime profile one has to accomplish the following steps (see also Fig. 2): (a) To prepare the reactive mixture of the desired composition; (b) To press a cylindrical samples with desired density; (c) In the middle of the high of the cylindrical sample drill a hole 3 mm in depth; (d) Install thus prepared sample (3) on the stage and constrain its position by the ignition coils; (e) Insert the tip of the thermocouple (4) into the drilled hole and attached the other ends to the electrical output on the reactor cover (2) (f) After thermocouple installation its operating status should be checked by measuring device (multimeter, oscilloscope); (g) Thus prepared set-up insert into the reaction chamber and seal the reactor; (h) Pump the system and then filled the reactor with the desired gas; (i) The SHS process is initiated by an ignition coils. (j) Measure and store the voltage-time (V-t) characteristics provided by the thermocouples during combustion; (k) By using table of EMF find the voltage values to temperature values; (l) Made temperature-time profile plot of SHS process; (m) Define the maximum combustion temperature and compare it with adiabatic value. (n) Make conclusions on the observed results Thermocouple method is typically used at temperatures below 2600 K. At higher temperature ranges, the optical method, i.e. optical pyrometer is used. Optical method is easy to use than the thermocouple one, but it provides information only on the surface temperature. Also, the special calibration methodology should be established to define the right values for the sample surface emissivity. Experiment 2 Pyro metric measurements of the combustion temperature-time profile

The required steps are similar to those for experiment 1 but instead of thermocouples the Rytek 3i pyrometer is used to measure maximum combustion 149  

temperature. The pyrometer is mount on a tripod at a distance of 50-100 cm from the test sample, the measurement button is turned on, after which the SHS in the sample is initiated. The maximum value of the combustion temperature is shows on the pyrometer display.

Fig. 3. Pyrometer Raytek 3i

REFERENCES 1. Rogelberg I.L. Alloys for thermocouples: a reference book / I.L. Rogelberg, V.M. Beilin. ‒ M.: Metallurgy, 1983. ‒ P. 328. 2. GOST 6616-94. Thermoelectric converters. General specifications. ‒ M.: publishing standards, 1994. ‒ P. 8. 3. GOST 6616-94. Thermoelectric converters. General specifications. – M.: publishing standards, 1994. ‒ P. 8.

150  

Laboratory work 3 COMBUSTION WAVE PROFILE

Aim of work: To measurement the temperature-time profile of the combustion wave during SHS process. Used materials and equipment: computer-assisted temperature recording system. Preparation for laboratory work: to get acquainted with the safety rules when working with chemical reagents; to master the rules and techniques of safe operation with the installation. Theoretical part

The conventional application of combustion theory to gasless synthesis processes is based on an assumption that heat conduction in such media occurs much faster than mass diffusion. As a result, mass transfer by diffusion at the macroscopicscale may be neglected, and an average concentration of the reactants in any local region of the heterogeneous mixture may be used. Thus, the physical and thermal properties (e.g., density, thermal conductivity, heat capacity) are the average of reactant and product values. In this case, the combustion synthesis process is controlled only by heat evolution from the exothermic reaction and heat transfer from the reaction zone to the unreacted mixture. Also assuming a flat combustion wave, its propagation is described by the one-dimensional energy continuity equation with a heat source [1]: сp 

T   T   t X  X

     k ( , T ) ,  k

(1)

where Фк represents a heat source or sink, and η denotes reactant conversion. For example, heat evolution from a single-stage chemical reaction, along with the chemical kinetic equation, would be presented as

(2) where Q represents the heat of reaction. Phase transformations with a significant thermal effect (e.g., melting, evaporation) can be are also accounted. For example, the effect of melting can be accounted for by introducing a negative heat source [2]: 151  

where ΔHfus is the latent heat of fusion and ζ is the fraction of reactant melted.

Fig. 1.Temperature and heat generation profiles in the combustion zone for a single reaction

For a combustion wave that propagates through the reactant mixture with a constant velocity U, we can change to a coordinate system attached to the wave (Fig.1): (3) x=Xo+Ut Substituting Eq. (3) into Eqs. (1) and (2) yields:

(4) Assuming that the temperature and concentration profiles do not change with time and further that there are no heat losses (i.e., constant-pattern propagation), we can set dT/dt and dη/dt equal to zero. Then Eq.(4) takes the following form: (5)

along with the boundary conditions (BCs):

152  

The temperature profile for the preheating zone of the combustion wave can be derived readily by introducing the variable q=λdT/dx for the heat flux, so that Eq. (5) rearranges as

(6) with BCs:

(7) Because there is no heat evolution in the preheating zone, we can neglect the third term of Eq. (6) thus obtaining:

(8) Direct integration of Eq. (8) with BCs [Eq. (7)] yields the Michelson solution for the temperature profile in the preheating zone: (9) where α=λ(ρχπ), and xT=α/U denotes the characteristic length scale of the combustion wave. Classic combustion theory does not describe the temperature profile in the reaction zone, since it is assumed to be close to Tad. The theory discussed here applies mainly to kinetic-controlled reactions. However, combustion synthesis reactions involve many processes, including diffusion control, phase transitions, and multistage reactions, which result in a complex heat release function, Analysis of the system of combustion equations for various types of heat sources was compared with experimental data, which led to the conclusion that in general, the combustion wave velocity can be represented as follows [3]: (10) where T* and η* are the values of temperature and corresponding conversion, respectively, which control the combustion front propagation, and A(T*, η*) is a function weaker than the exponential. Based on Eq. (10), four types of combustion reactions can be identified [4]: 1. Combustion with a thin reaction zone (discussed earlier), where η*=1, T*=Tad=To+Q/cp, and for which the combustion velocity is determined by the adiabatic combustion temperature [5]. 2. Combustion with wide zone, which occurs when a product layer strongly inhibits the reaction, and T*=To+(Q/cp)η*, where η* is determined from 153

(11) 3. Combustion with phase transformations (e.g., melting reactants), where T*=Tm (melting point) and η*=(Tm-To)/(Q/cp) 4. Combustion with multistage spatially separated reactions, where η1* = 1 and T*=To+(Ql/cp), where the subscript 1 refers to the first low temperature stage of the complex reaction. For the four types of combustion synthesis reactions, just discussed above, the corresponding temperature profiles are shown schematically in Fig. 2a-d, the last shown profile (2e) corresponds for so-called micro heterogeneous combustion model [6].

Fig. 2. Time temperature profiles: (a) thin reaction zone; (b) wide reaction zone; (c) with phase transformation; (d) multistage reaction; (e) heterogeneous combustion

All the above types of combustion wave structures can be observed in SHS experiments. Example of T-t profiles with melting is shown in Fig. 3 for Тi + 3Si system. It can be seen that at eutectic temperature of the system (~1300 C) melting starts which results in appearance of the plateau on the profile. Reaction simultaneously is taking place at this temperature and dose not complete after all media melted. The latter leads to the increase of temperature after melting process is accomplished. For measurement of temperature profiles of combustion wave in various zones of SHS, the micro thermocouple methodology is used, which was developed [7]. According to this methodology, the tungsten-rhenium thermocouples with thickness of 5-7 microns are prepared, so to avoid the interaction of thermocouples with surrounding reagents, it is covered with a thin layer of boron nitride and then is pressed into the sample. Such thin thermocouples are required in order the characteristics relaxation time of thermocouple (trelax=d2/α, where d-diameter of thermocouple, α – thermal diffusivity of the thermocouple material) be smaller that the reordered process characteristic time. For example melting process, which can be as small as 1 ms. 154  

Fig. 3. Temperature profile in systemTi + 3Si

Fig. 4. The temperature combustion profiles ofМп02- Alsystem with different amount of Al2O3: 1 - 18 mass. %; 2 - 20 mass. %; 3 - 30 mass. %.

One can influence the T-t profiles by changing different experimental parameters, e.g. Initial temperature, porosity of the medium, by diluting the reaction mixture by inert phases, changing reactants particle’s size. The example of such profiles for Мn02 - Al – Al2O3 system with different amount of alumina content is shown in Fig. 4. It can be seen, that the temperature profile can be significantly change by using dilution. Steps for the laboratory work Technique for measurement of combustion wave profile and operational procedure

All steps are exactly the same as for laboratory work 2. However, to monitor the measurement of temperature data of SHS processes a computer temperature recording system is used.In this case, the signal from the thermocouple installed inside the sample is transmitted through the conductive connections of the bottom cover via 155  

shielded wires on the LTR-U-1 crate system with the LTR27 module and the H-27T submodule (L-CARD firm). On the functional diagram of Fig. 5 shows how the counters of pulses count the number of pulses arriving during a time of 1 ms. Pulses come from voltage-tofrequency converters (SPDs) of submodules H-27x. The counters interrogate the AVR, counting the number of pulses for specified time periods and send the calculated codes to the interface. Therefore, the maximum sampling frequency from each channel at the worst measurement accuracy is 1 kHz.The registration interval is -25 + 75 mV, which allows using all known types of thermocouples. The measurement results were processed on a computer.

Fig. 5. Functional diagram of the LTR27 module.

REFERENCES 1. Varma A., Rogachev A.S., Mukasyan A.S., Hwang S., Combustion Synthesis of Advanced Materials: Principles and Applications", Advances in Chemical Engineering. – 1998, 24, 79-226 (1998). 2. Aldushin, A.P., and Merzhanov. A.G., Gasless combustion with phase transformation. Dokl. Phys.Chem., 236, 973 (1978). 3. Merzhanov, A.G., Self-propagating high-temperature synthesis: ’henty years of search and findings. In “Combustion and Plasma Synthesis of High-Temperature Materials” (Z.A. Munir and J.B. Holt, eds.). VCH Publishers. – New York, 1990. – Р. 1. 4. Merzhanov A.G., New elementary models of the second kind. Dokl. Phys. Chem., 223, 430 (1977). 5. Zeldovich Y.B., Barenblatt G.I., Librovich V.B., and Makhviladze G.M., “Mathematic Theory of Combustion and Explosions” (D.H. McNeil, trans.). Consultant Bureau, New York, 1985. 6. A.S. Mukasyam A.S. Rogachev, Discrete Reaction Waves: Gasless Combustion of Solid Powder Mixtures”, J. Progress in Energy and Combustion Science 34(3), 377-416 (2008). 7. Zenin A.A., Merzhanov A.G., and Nersisyan G.A., Structure of the heat wave in some selfpropagating high-temperature processes. Dokl. Phys. Chem., 250, 83 (1980).

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Laboratory work 4 SHS OF NITROGEN CONTAINING MATERIALS BY USING CHROMIUM OXIDE BASED CONCENTRATE

Objective of work: To synthesize SHS material and measure the combustion wave parameters. Used materials and equipments: aluminum powder (PA-4), chromium oxide, zircon concentrate Obukhov GOK (SKA, Kazakhstan) and nitrogen gas. Particle size of all used powders less than 90 µm. Preparation for laboratory work: to get acquainted with the safety rules when working with chemical reagents; to master the rules and techniques of safe operation of the set-up. Procedure of the experiment

First, the components are weighed on the electronic balance VLE-134 in desired amounts, followed by their thorough mixing in the porcelain ware. Before compacting the mixture, a certain amount of silica sol is added. The humidity of the obtained mixture components is in the range 5-10 %. Table 1 The composition of the initial experimental samples in the systems Al - ZrSiO4– Cr2O3 – N2 Composition Content,% wt.

Al 17 34

ZrSiO4 33 24

Cr2O3 50 50

Second, thus obtained mixture is pressed to a cylindrical samples with a diameter of 20 mm (D) and a height of 30 mm (h) at a pressure of about 70 MPa. After compacting, the samples should be dried at room temperature for 24 h and then kept in a drying box at a temperature of 70-80°C for 5 hours. The mass (mo) and geometry (D, h) should be carefully measured. Based on these measurements, the initial relative porosity (Пo) of the sample should be calculated. For example if you have Al-Cr2O3 mixture with mass ratio of mAl: mCr2O3 = ε, theoretical density of Al: ρAl=2.7 g/cm3 and atomic weight AAl=27g/mol, theoretical density of Cr2O3: ρAlCr2O3 = 5.22 g/cm3 and ACr2O3= 252+316 =152g/mol Π

1

1 157

 

Also the stoichiometric ratio for the reduction reaction Cr2O3 + 2Al = Al2O3 + 2Cr

(1)

can be calculated as follows: εst = 54/(152) = 0.355, which corresponds to 26.2 wt.% of Al and 73.8 wt.% Cr2O3 Third, the dried samples are placed in a gas high-pressure reactor (HPR), which allows SHS reaction under the high nitrogen pressure (Figure 1). The HPR is a metal spherical vessel, with a wall thickness of 60 mm and a capacity of 45 liters, which can be filled with nitrogen up to the pressure 100 atm. To increase the concentration limits for investigated system the sample can be placed into a tubular heating furnace, which allows pre-heat the sample up to ignition temperature. A set of electrodes for placing thermocouple and for supplying of electrical power for the furnace is installed in the bottom cover of the reactor. Temperature time profile of the SHS process can be measured both by optical infrared pyrometer Raitek 3i (600-3000°C), and a thermocouples. The mechanical properties, i.e. compression strength, of the obtained SHS products is determined on an apparatus consisting of a press and the dynamometer according to the standard procedure.

1 – compressor; 2 – transformer; 3 – ampermeter; 4 – top cover; 5 – bottom cover; 6 – tubular furnace; 7 – thermocouples; 8 – sample; 9 – main body; 10 – manometer; 11 – valves; 12 – gas; 13 – data registering machine LTR-U-1; 14 – computer. Fig. 1. High pressure reactor.

Experiments are carried out in in the nitrogen pressure range 5-20 atm. In order to initiate reaction one has to preheat the sample to self-ignition temperature and SHS occurs in so-called thermal explosion mode. It can be see from able 1 that the first composition correspond to the stoichiometric ratio between Al and Cr2O3 for reduction reaction (1). For the second mixture, we do have excess of Al, which may react with nitrogen. One of the main parameter which allows to evaluate contribution of the gas solid reaction is the relative change of the mass of the sample: 158  

m = (mf-mo)/mo

One may hypotheses that for the first mixture just reduction reaction (1) between Al and Cr2O3 controls the combustion process and thus m should be small and weakly depends on the nitrogen pressure. While for the second composition reaction between Al and nitrogen: (2) 2Al+N2=2AlN May also contribute to combustion process thus m value should be larger and increase with gas pressure increase. In order to check above hypothesis three different nitrogen pressures, i.e. 5 atm, 15 atm and 20 atm should be used for each investigated mixture. After each experiments, mass (mf) and geometry (Df, hf) of the sample after SHS should be measured. Please plot these parameters for both mixtures as a function of nitrogen pressure. Make your conclusions. If samples after SHS process keep their regular cylindrical from, please measure the compression strength of the sample (σcomp) by measuring applied load (F, N) by dynamometer and dividing this load to the cross section of the sample S= πDf2/4: σcomp = F/πDf2/4 (MPa) Please make a plot of the obtained values of πcomp as a function of nitrogen pressure for both compositions. Make the conclusion based on the observed trends.

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Laboratory work 5 SYNTHESIS OF THE BORON-BASED COMPOSITES BY USING SHS METHOD Objective of the work: to synthesized boron-based materials by using SHS method. Used materials and equipment: the following precursors are used: boron (B) containing up to 40wt. % of boron oxide (B2O3), metal oxides (MeO2: Me – Ti, Zr, Cr, V.), as well as magnesium (Mg) or aluminum (Al). There are equipment to be used: electronic balance – to prepare mixture of the desired composition; press die and “Carver” laboratory press – to compact sample of desired dimensions and relative density; chemical reactor – to make SHS; muffle furnace to heat treat the samples; pyrometer – to monitor temperature time history of the process; dynamometer to measure the mechanical property of the synthesized materials; X-ray Diffraction (XRD) – to characterize the phase composition of obtained products; scanning electron microscopy (SEM) – to investigate the microstructure of the SHS materials. General scheme of the experiments is shown in Fig. 1. Preparation for laboratory work: to get acquainted with the safety rules when working with chemical reagents; to master the rules and techniques of safe operation with the used set-up. Theoretical part The SHS process involves self-sustained heterogeneous non-catalytic reactions, which lead to fabrication of variety of valuable solid products. Three main stages of the SHS process can be outlined, (i) reaction initiation; (ii) reaction front propagation; (iii) cooling.

1 – window of the camera; 2 – cover of the camera; 3 – spiral; 4 – sample; 5 – substrate; 6 – pressure gauge; 7 – inlet and outlet of argon gas; 8 – optical pyrometer; 9 – transformer; 10 – argon gas; 11 – the latch of gas; 12 – outlet of gas. Fig.1. Scheme of camera of SHS process in constant pressure.

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The “simplicity” of the SHS process hides the highly complex chemical and physicochemical transformations taking place in combustion wave, which influence both the combustion characteristics (velocity of combustion wave, T-t profile, reaction mode, etc.) and structure of the obtained products. To obtain the SHS product of desired phase composition and microstructure, shape and dimensions, mechanical and physical properties, an enormously work should be accomplished to understand the fundamentals of SHS phenomenon. Steps for the laboratory work

(a) Prepare the reaction mixture: the desired amounts of powders are weighted on the electron balance and thoroughly mixed. For example, let us considered B+(B2O3)+Al+Ti system to produce TiB2-Al2O3 composite. In this case the following reaction should be used: yB + 0.4yB2O3 + 0.8y Al + 0.9yTi = 0.9yTiB2 + 0.4yAl2O3, where y – mole amount of used boron; (b) Press reaction mixture to the sample with desired initial density. Put desired amount of mixture (mo) into the press die and compact plying load ~ 100 KN to make cylindrical sample. Measure weight, height (h) and diameter (D) of the sample to calculate its density: =mo/2πh(D/2)2. (c) Insert the sample into reactor, attached the ignition coils on the top of sample and close the reactor. Vacuumed the reactor volume and then fill it with argon gas. (d) Properly install the pyrometer to be able to record temperature change on the surface of the sample at the middle of it height; (e) Initiate the chemical reaction by passing the current through the ignition coils. (f) After the sample cooled down open the reactor and take the sample. Now you can characterized synthesized material for it phase composition (XRD), microstructure (SEM) and its compression strength. Determination of compressive strength

The strength of synthesized articles measuring 20×20×20 mm was determined on an installation consisting of a press and a dynamometer (Fig. 2). The pressing pressure was registered by a dynamometer.At the moment of sample destruction, the dynamometer readings decreased sharply from the maximum value to zero. The experiments were carried out on 2 to 3 identical samples, the mean of the maximum Fig. 2. Dynamometer DOSM-3-1. values (N) of the dynamometer readings are used to calculate the compressive strength of the sample by the formula: σсж = 0,16 N, 161  

Laboratory work 6 SHS SYNTHESIS OF SUPERCONDUCTORS: MAGNESIUM DIBORIDE

Objectiveof the work: to fabricate the bulk MgB2 material by using SHS method and measure it properties. Used materials and equipments: amorphous boron powder, magnesium metal powder. All equipment listed in laboratory 4. Preparation for laboratory work: to get acquainted with the safety rules when working with chemical reagents; to master the rules and techniques of safe operation with the experimental set-up. Theoretical part Recent studies on electronic structure of several transition metal diborides indicated that the electron phonon coupling constant for these materials is much smaller than that in superconducting intermetallics. In addition, experimental studies show an exceptionally large superconducting transition temperature of ~40 K for MgB2. In order to understand such unexpected behavior of this compound we have made electronic structure calculations for MgB2 and closely related systems. Calculated Debye temperature suggest that the average phonon frequency is very large in MgB2 as compared to other intermetallic superconductors and the exceptionally high Tc in this material can be explained through BCS mechanism only if phonon softening occurs or the phonon modes are highly anisotropic. Based on this result, we have searched for similar kinds of electronic feature in a series of isoelectronic compounds such as BeB2, CaB2, SrB2, LiBC and MgB2C2 and found that MgB2C2 is one potential material from the superconductivity point of view [1].

1 – compressor; 2 – transformer; 3 – ampermeter; 4 – top cover; 5 – bottom cover; 6 – tubular furnace; 7 – thermocouples; 8 – sample; 9 – main body; 10 – manometer; 11 – valves; 12 – gas; 13 – data registering machine LTR-U-1; 14 – computer. Fig. 1. High pressure reactor

162  

The available high-pressure apparatuses (Fig.1) with 45 cm3 working volume allow to obtain the bulk MgB2. The main element of the setup is a thick walled metallic spherical reactor vessel without welded seams, with a wall thickness of 60 mm and a capacity of 45 l. Corps is mounted on a metal frame, made of corner 60×60 mm, and provided with an upper and a lower cap which fasten with nuts on eight studs 35 mm in diameter. For the thermocouple wires and the power supply are installed in the bottom cover current fittings. Filing and release of argon is carried out through a flexible high-pressure hoses fitted with quick connections (manufacturer "HANSA FLEX HIDRAVLIK ALMATY"), which are mounted on the top cover. A tubular heating furnace is placed inside high pressure reactor that allows preheating the sample to 1000°C. The heater furnace is made of an alundum tube with diameter of 70 mm and a height of 250 mm and wound on it’s of a nichrome wire with a diameter of 2 mm. The furnace power was 1.2 kW. For the control of measurements of temperature change of SHS processes are used computer desk setting of temperature. The method of direct measurements of signals from thermocouples installed inside the reactor, passed through an electrically conductive stoker of a bottom cover on the shielded wire to the system LTR-U-1 module and sub-module LTR27 H-27T (L-CARD). Steps for the laboratory work

Magnesium diboride is synthesized from the following powder reactants: metallic magnesium (>98.0% purity), amorphous boron (94% purity). Powder mixture in stoichiometric ratio (55.3% Mg and 44.7% B) should be prepared. The homogeneous powder mixture was compacted under a pressure of 40 ton to obtain disk 30 mm in diameter and 15 mm in thickness. Thus prepared sample is inserted to the highpressure reactor. The used argon pressure in the reactor is 25 atm. The sample is ignited by heating tubular furnace (Fig. 1) which is located in the reactor. Note that self-sustaining synthesis for this system is initiated at about 650°C. The composition and crystal structure of the product at room temperature should be evaluated by X-ray diffraction (XRD) with CuKα radiation (λ = 1.54056 Å). The particles morphological features and microprobe analysis are determined by thermal field emission Scanning Electron Microscopy (SEM, Jeol-7100F). The sample magnetization over a broad range of temperatures (1.9‒300 K) and magnetic field up to 9 Tesla are carried out with a Quantum Design PPMS EverCool-II system having Vibrating Sample Magnetometer (VSM) attachment. REFERENCES 1. Mansurov Z.A., Fomenko S.M., Alipbaev A.N., Abdulkarimov R.G., Zarko V.E. Features of alumothermal combustion of systems based on chrome oxide in the conditions of high pressure of nitrogen // Physics combustion and explosion. – 2016. – V. 52. – №2. – Р. 1-9.

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Еducational issue

Mansurov Zulkhair Aimukhametovich Mukasyan Aleksandr Sergeevich Rogachev Aleksandr Sergeevich SELF-PROPAGATING HIGH-TEMPERATURE SYNTHESIS Textbook Computer page makeup and

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Signed for publishing 31.05.2018. Format 70x1001/12. Offset paper. Digital printing. Volume 13,6 printer’s sheet. 100 copies. Order №3041. Publishing house «Qazaq University» Al-Farabi Kazakh National University KazNU, 71 Al-Farabi, 050040, Almaty Printed in the printing office of the «Qazaq University» publishing house.

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