Modern problems of processes burning, detonation, explosion: educational manual 9786010427952

The educational manual is devoted to the problems of soot formation and fullerenes in the flame of hydrocarbons. The res

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Modern problems of processes burning, detonation, explosion: educational manual

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М. Nazhipkyzy    



Almaty «Qazaq university» 2017


UDC 662 (075.8) LBC 35.51 я 73 N 32 Recommended for publication by the decision of the Academic Council of the Faculty of Chemistry and Chemical Technology, Editorial and Publishing Council of Al-Farabi Kazakh National University (Protocol №5 dated 11.07.2017); Educational and methodical association on groups of specialties «Natural sciences», «Engineering and technology» of Republican educational-methodical council on basis Al-Farabi Kazakh National University (Protocol №2 dated 29.06.2017) Reviewers: Doctor of chemical sciences, Professor R.A. Kazova Doctor of chemical sciences, Professor I.S. Irgibaeva Doctor of chemical sciences, Professor M.K. Aldabergenov

N 32 

Nazhipkyzy М. Modern problems of processes burning, detonation, explosion: educational manual / М. Nazhipkyzy. – Almaty: Qazaq university, 2017. – 134 p. ISBN 978-601-04-2795-2 The educational manual is devoted to the problems of soot formation and fullerenes in the flame of hydrocarbons. The results on the synthesis of superhydrophobic soot in the combustion of hydrocarbons and the production of waterproofing materials based on it are presented, and materials on the use of soot as a waterproof combustible additive in ammonia-nitrate explosives are also presented. The educational manual can be recommended not only to PhD doctoral students of the speciality 6D073400 – Chemical Technology of Explosives and Pyrotechnics, but also to undergraduates, PhD doctoral students of other training profiles, in addition, specialists mastering this field. Published in authorial release.

UDC 662 (075.8) LBC 35.51 я 73 ISBN 978-601-04-2795-2

© Nazhipkyzy M., 2017 © Al-Farabi KazNU, 2017


CONTENT  FOREWORD ............................................................................................. 4 INTRODUCTION ..................................................................................... 5 1. FORMATION OF FULLERENES C60 IN HYDROCARBON FLAMES ................................................................................................... 7 1.1. The main allotropic modifications of carbon ...................................... 7 1.2. The mechanism of formation of soot particles and fullerenes particles in a flame ..................................................................................... 10 1.3. The methods of synthesis of fullerenes in flames ............................... 19 1.4. The influence of external local impact on the processes of formation of combustion products ......................................................... 25 1.5. The influence rendered by a local effect of external acetylene – oxygen flame on temperature profile benzene – oxygen flame.................. 31 1.6. The effect of local action of external acetylene – oxygen flame on the mass yield of fullerene C60 during combustion of benzene – oxygen mixture. ......................................................................................... 34 1.7. The influence of an external impact on the processes of formation of fullerenes nuclei .................................................................... 44 2. FORMATION OF THE SUPERHYDROPHOBIC SOOTY SURFACE IN THE FLAME ..................................................................... 56 2.1. Hydrophobic and hydrophilic properties of materials ......................... 56 2.2. Modeling of the wetting angle for liquid which is in contact with a rough surface................................................................................... 74 2.3. Synthesis of hydrophobic soot in a flame ........................................... 80 2.4. Synthesis of hydrophobic soot in a flame under the effect of an electric field ...................................................................................... 91 2.5. Electron microscopic studies of hydrophobic soot samples obtained in the flame ................................................................................. 98 2.6. Investigations on the interaction of surface-active substances with the obtained hydrophobic soot surface............................................... 102 2.7. Application of hydrophobic soot in textiles ........................................ 105 2.8. Application of hydrophobic soot in the construction industry ............ 109 2.9 The use of nanoparticles in power sysmems of exlusive...................... 115 2.10. The use of hydrophobic soot in the ammonia-saltpeter explosive .................................................................................................... 118 BIBLIOGRAPHIC LIST ........................................................................... 124 References ................................................................................................. 124


FOREWORD  Recently, at the intersection of chemistry and physics of combustion processes, all over again much attention have been paid to the intensive development of investigations related to the study of soot formation processes in hydrocarbon flames. The use of hydrocarbon flames for synthesis of various nanomaterials is promising. In a flame, changing the modes of combustion and the design of torches, that is, varying conditions, it is possible to achieve a directed synthesis of the required materials. Soot formation in combustion processes is one of the multifaceted, inter-disciplinary phenomena the study of which is important both from the point of view of ecology and for formation of various nanomaterials which is rapidly developing in recent years. Despite numerous researches, there is no clear understanding of the mechanism of conversion of initial fuel to final products – soot, fullerenes. Changing experimental conditions (pressure, a ratio fuel/oxidizer, application of electric field, introduction of different additives), it is possible to obtain carbon materials of different properties. This educational manual deals with the mechanisms of formation of soot particles and fullerenes in a flame, influence of external local impact on the processes of formation of combustion products, influence of external impact on the process of formation of fullerene germs. The results on synthesis of hydrophobic soot with and without the influence of electric field are presented. Also, materials on the use of superhydrophobic soot as waterproof combustible additive in an ammoniac-saltpetre explosives are presented. The educational manual presents the data on the use of the obtained superhydrophobic soot as a modifying additive for creation of waterproof nanostructured materials. The data on investigation of interaction of surface active substances with the obtained hydrophobic soot surface are presented.


INTRODUCTION  The combustion process as the technological reactor has long been used for production of soot with specified properties, which is widely used in different field of industry. Being an important technological raw material, soot is produced on a commercial scale by different methods. To produce soot, the method of thermal decomposition is mainly used, when burning up liquid and gaseous hydrocarbons with the lack of oxygen at the temperatures of ~1500 ° C followed by rapid cooling of the decomposition products. Such soot consists of separate closed particles, wherein the primary globules are spherical with a diameter of 9 to 600 nm, which are able to chemically bind with each other to form a secondary structure aggregating in the form of linear branching chains, spirals, bunches, so-called fractal clusters. The size of soot particles, the specific surface and the degree of structurization (i.e., branching of soot chains) depend on the conditions of its formation [1-2]. The shape of most types of soot particles is close to spherical. However, microcrystallites in the soot particle are not ordered and come onto the surface at different angles, so that the soot surface is quite non-uniform. There may be free valence bonds of carbon atoms, side chains of both saturated and unsaturated hydrocarbons and compounds containing oxygen. Investigations on the methods of controlling the combustion process to produce products with the specified initial properties (soot particles, fullerenes) is an important aspect in the study of soot formation process. The methods of exerting an influence on the flame electric, magnetic and high-frequency electromagnetic fields are being intensively investigated. The flame has electrical properties, and this fact has been known for a long time. However, only in the twentieth century when the molecular and kinetic theory of substance was formulated, it became clear that electric properties of a flame are conditioned by the existence in them of charged particles – ions and electrons. The presence of charged particles in the flame allows influencing the processes taking place in the flame, when applying an 5

electric field. Specificity of electrical phenomena and the nature of the structure of the flame front during combustion is such that even a weak external electric field exerts an effect on all the processes occurring in the flame [3]. Currently, there already exist facilities for production of fullerenes in hydrocarbon flames, but problems remain. In particular, it is an expensive cost for their preparation and the lack of large-scale production. Therefore, the research of the conditions allowing intensifying the process of obtaining fullerenes with smaller costs and with the possibility of organizing their commercial production is an actual task. One of the methods allowing to create such conditions is a method of external energy impact on a flame. In some cases, for reliable functioning of products it is necessary to provide high water and oil repelling properties of their surfaces (for example, glasses of cars, planes, protective suits, walls of reservoirs for storage of liquids, building constructions, etc.). Therefore, production of hydrophobic substances for water resistant and waterproof properties to various materials is an actual task. Soot can have hydrophobic properties and be used as additives of materials of such profile. The process of producing hydrophobic soot in a flame has its own specifics being continuous technological and controllable. Development and research of the controllable synthesis of hydrophobic soot in flames is an important and actual task. It is of interest to use additives of nanocarbon materials in explosive materials for improvement of operational characteristics [4, 5]. Widely spread and at the same time the cheapest and safe explosives are compounds based on ammonium nitrate (ammonium saltpeter explosives - ASE) in which nitrate is an oxidizing agent and impurity is a fuel. The impurities can be both explosive and nonexplosive (wood flour, cake, pitch). The data on the use of superhydrophobic soot as additive to ammonium saltpeter explosives are presented. A modified explosive in the composition of ammonium nitrate soot without liquid combustible additives is obtained. Explosive Granulite N (Nano) is highly water resistant in comparison with other granular ammonium saltpeter explosives. Field testing of the explosive with addition of superhydrophobic soot for the completeness of detonation showed improved performance. 6


FORMATION OF FULLERENES C60   IN HYDROCARBON FLAMES  1.1. The main allotropic modifications of carbon Carbon is quite a widespread element. Until recently, it was known that carbon forms three allotropic forms – diamond, graphite, carbyne. Allotropy, from the Greek «allos» – different, «tropos» – turn, the property, the existence of one and the same element in the form of various structures differing in properties and structure. Now the fourth allotropic form of carbon, the so-called fullerene (polyatomic molecules of carbon Cn) is known. Graphite has a layered structure and is found in the form of two crystalline modifications – hexagonal and rhombohedral. Each layer consists of carbon atoms covalently linked to each other in regular hexagons with strong chemical bonds (Figure 1). The distance between the layers (0.334 nm) is much larger than the interatomic distances within the layer (0.142 nm), and the link between the layers is provided by van der Waals interactions, so they can easily slide over one another. A simple pencil can serve as an example: if you draw a graphite rod on paper, the layers gradually get «peeled off» from one another, leaving a trail. Diamond has a three-dimensional tetrahedral structure. The structure of diamond is presented in Figure 2. Each carbon atom is linked with the four others in a covalent union. All the atoms in the crystal lattice are arranged at the same distance (0.1544 nm) from one another. Each of them is linked with the others with a direct covalent bond to form in the crystal, no matter what size it is, one giant macromolecule. Due to the high energy of covalent C-C bonds, diamond has the highest strength and is used not only as a precious stone, but also as a feedstock for the production of cutting and grinding tools. 7

Figurre 1. The structure of graphite

Figure 2. The struccture of diamond

Successful syntheesis of the third aallotropic form of carbon was execcuted by Yu. P. Kudryavtsev, K A. M M. Sladkov, V. I. Kasatochkin and V. V. Korshak in 1960. The neew form was nam med carbine. A po ossible synthesis route was found when investigatiing the following g oxidation polym merization reactionn:

n be considered ass a molecule conssisting of two As acetylene can C-H fragments, it has been assumed that such reaction is possible for it, to oo. Really, the reseearch of this oppoortunity as a result has come to the end e with opening of the third allotroopic form of carbo on. The discovery off fullerenes – a new w form of existen nce of carbon, is reecognized as one of the most amaazing and the mo ost important disco overies in the scieence of the XX cenntury. Fullerenes are called closed molecules off the type C60 , С70 , С76 , С84 , in which all the atom ms are arranged at a the vertices of reegular hexagons or o pentagons coveering the surface of o a sphere or spheeroid. The origin of the term «fulllerene» is associaated with the namee of the Americcan architect andd mathematician Buckminster 8

Fuller, who designed hemispherical architectural constructions consisting of hexagons and pentagons. As an architect, he proposed constructions in the form of multi-faceted spheroids intended for covering a large area of the premises, and as a mathematician he used a systematic approach to the analysis of structures of different origin and showed that the structure is a self-stabilizing system. The most studied structure and properties of fullerene C60 a stable isomer of which is composed of 20 hexagonal and 12 pentagonal cycles. Compound C60 has a spherical shape similar to a soccer ball (Figure 3). The molecule contains 90 ribs. The radius of C60 molecule is 0.3512 nm, the length of a short C-C bond (common in pentagons and hexagons) is 0.1391 nm, the length of the other C-C bond (common in hexagons) is 0.1455 nm. Pentagons do not articulate with each other. Fullerene C70 («rugby ball«, «melon«) is the next after C60 fullerene (Figure 3), one of 8149 of isomers which corresponds to the rule of isolated pentagons. It contains 25 hexagonal and the same 12 pentagonal cycles.




Figure 3. Fullerene molecules: a) C60, b) C70, c) a forecast of a fullerene molecule containing more than 100 carbon atoms [6-7]

There are varieties of fullerene compounds: endofullerenes, heterofullerene and exofullerenes, shown in Figure 4. Endofullerenes Мm@Cn are fullerene molecules in which m atoms of other chemical elements (M) are located within the fullerene molecule (Cn), where m1 (Figure 4a.). Heterofullerenes МmCn-m are fullerene molecules in which m carbon atoms are substituted for the atoms of other elements (Figure 4b). 9




Figure 4. The structure of the moleccules of endofullerenee (a), heteerofullerene (b) and exxofullerene (c)

Exofullerenes МmCn are fullerene molecules, in wh hich m atoms of other o elements arre attached to thhe fullerene moleecule outside (Figu ure 4 c). When obtaining exofulllerenes, soluble in water the moleecules have a morre complex structuure. 1.2. The mechan nism of formation n of soot particlees and fulleren nes particles in a flame Up to the presentt time, the mechannism of soot form mation has not been n studied well. Thiis is explained byy the fact that form mation of soot partiicles takes place in n a very short tim me, fractions of a second s [8, 9]. The study of the meechanism of form mation of soot paarticles in the flam me and the structture of soot flam mes provides imp portant information on the physicaal and chemical processes that occu ur before and durin ng soot formation n. Investigation of the t processes takiing place in the course of soot form mation is still imp portant and actuaal. This is determ mined by the follo owing reasons: fiirst, soot is produuced on a large scale and is wideely used as an acttive filler of rubbber, ink componen nts; secondly, soot is a carcinogenicc pollutant of thee environment duee to combustion of hydrocarbon fu uel in power plantts [11, 12]. Thus far, polycycclic aromatic hydrrocarbons are con nsidered to be the main m intermediatee compounds in the processes of formation of comb bustion products and a act as nuclei oof soot particles and a fullerenes being g formed [12, 13]]. 10

During soot formation it is possible to distinguish three distinct stages: 1. Nucleation – transition from the molecular system to the fragment system, or to the formation of embryonic particles that grow faster than decay. 2. The growth of spherical particles with a diameter of 10 to 50 nm (100 – 500 A). 3. Aggregation of spherical units with formation of chains. A scheme of the growth from primary molecular particles to soot aggregates on the basis of the fact that the nuclei are ions is presented in work [14]. The scheme shows how the transition from molecular sizes to particles having a diameter by 3 orders of magnitude greater, which is equivalent to the molecular weights of more than 10 orders of magnitude, than the starting material over a period of milliseconds takes place. Soot is a product resulting from incomplete combustion of hydrocarbon fuels. The yellow-red glow in the flame volume is due to incandescent soot particles that eventually either burn or leaving the high-temperature oxidation region get rapidly cooled and deposit in the form of soot. [15]. It is known that soot with specified properties is an important technological raw material and is produced in an industrial scale by various methods [16-18]. To produce soot, the method of combustion of liquid and gaseous hydrocarbons with the lack of oxygen at temperatures of about 1500 0С, followed by rapid cooling of decomposition products is usually used. The resulting soot consists of separate spherical particles in the form of spherical globules with the diameter d  9...600  nm, which are chemically bound and form a secondary structure, coagulating into aggregates with linear branched chains, spirals, clusters, called fractal clusters. The size of soot particles, specific surface and structural ordering depend on the conditions of its formation [19, 1-3]. Soot is a polydisperse substance. In [20], the authors present the data on distribution of dispersed carbon particles obtained by a furnace method, and make a conclusion on a great degree of monodispersity of the obtained soot. There is evidence [11], characterizing the distribution of soot particles by molar masses and 11

it is stated that generally the law of distribution of soot particles by size is logarithmically normal. At the same time, the smaller the fragments of decomposition and the faster and deeper they are cooled, the smaller is the average size of soot particles (for example, an arc or laser spraying of graphite) [13]. On the other hand, in the process of incomplete combustion of hydrocarbons, which are characterized by relatively low temperatures and low cooling rate of thermal decomposition products, mainly larger soot particles grow [17, 18, 1-3]. Soot formation and combustion of soot particles are mainly determined by the local C / O ratios, local concentrations of hydrocarbons, temperature and residence time of soot particles both in the field of soot formation and in the field of burnout. It is known that soot particles (globules) are a disordered set of separate micro-crystallites having characteristic sizes by the thickness Lc = 12÷17 Å and the length Lа = 1530 Å [17]. However, depending on the method of producing carbon black, the characteristic sizes can vary in a wide range of values: Lc = 1080 Å and Lа = 15150 Å [3].The distance between the planes of parallel graphene layers of carbon atoms forming a microcrystallite is determined by a range of values d002 = 3,453,7 Å, i.e., a microcrystallite is formed by 3-5 layers [3, 16, 17]. There are soots, for example acetylene soot, which have seven or more planar layers of carbon atoms [15]. Flat graphene layers in the soot particles are shifted relative to one another, this increasing still more the disordering of the soot particle structure. By the degree of ordering of the structure a soot particle occupies an intermediate position between amorphous carbon and crystalline graphite. Radiographic results of investigations show that different soots give typical for amorphous and polycrystalline materials continuous by the angle scattering of X-rays with few diffuse maxima, corresponding to the position of diffraction lines of the graphite lattice with the reflexes of the type (hkl) being absent [18]. The decrease in the value of particle density may occur in accordance with the Smoluchowski equation [21]: 2 d [ n]   ktheory d [n] , dt



where, the rate constant ktheory depends on the particle diameter d  or the average particle volume V . It was found that for spherical particles G ~ 2 (a factor that takes into account the dispersion forces inside the particle), and for a self-similar distribution of particles by sizes   6.55 (the factor that takes into account the polydisperse nature of the system in question). For a constant value of the volume fraction fV of soot (when only coagulation provides the growth of particle), the average volume of a particle V  fV /[n] is determined by the ratio: dV 6 ktheory f  V V  dt 5

1/ 6



In [21], the authors report the investigations results on pyrolysis of aromatic hydrocarbons in the shock tube are presented by a 5 6

normal straight line, of V versus/t. Their experimental values of k correspond to the theoretical values of ktheory that indirectly confirm theoretical research. It was also shown that the particle size distribution in the pyrolyzed gases moves to form a self-maintaining form. Self – maintaining size distribution (SMSD) is preservation of the particle size over time in the coagulation system. In most cases this provides a significant simplification of the description dynamics of pure coagulating aerosols; however, the use of simple forms during condensation of synchronous surfaces sharply increasing f V is hardly probable. For prolonged reactions the quantity of particles does not depend on the initial amount n0  and decreases according to the equation: 1 n   k theory fV 6 t   





which shows weak dependence on fV , i.e. the number of particles depends on the loaded soot. Since ktheory is a weak function of temperature, n at a given time does not depend on the gas temperature. The density curve of the amount of particles in the ethylene 13

flame contains varying amounts of soot, and in the benzene flame it is barely distinguishable, this corresponding to the Smoluchowski equation. In early studies of acetylene flame at low pressures [22] it was found that a significant proportion of the observed growth of elementary soot particles occur due to coagulation, where the particles join with each other, thereby reducing their overall number concentration and the average size increases. These calculations and calculations of Graham [23] allowed to state that coagulation is the characteristic of soot itself and does not depend on the nature of origin of the molecules and on the way of their generation. The qualitative and quantitative results presented in [24] confirm the author’s arguments [25] that the chemistries of fuel and pyrolysis and the structure of fuel play an important role in soot forming diffusion flames. The identical chemical nature of soot formed from diffusion or mixed controllable systems shows that it is common in the chemical mechanisms of soot formation. Fuel independent general mechanism is modified only due to the next route to intermediates. It is simply influenced by the temperature of the combustion system and the general nature of the initial fuel. This idea implies that the relative arrangement of one fuel to soot is compared with the other, and possibly arises primarily from the difference in the initial rate of formation of the first and second ring structures, and that the mechanisms that control the growth of larger condensed aromatic rings soot nucleus, soot growth, etc., remain unchanged, and the growth of steps of large aromatic structures leading to soot nucleation is stronger than the formation of initial rings. Thus, formation of initial rings controls the rate of nascent soot formation. The concentration of soot forming particles determines the amount of soot fraction, or the full amount of the formed soot. In substrate of the overall dispute that the initial formation of the ring is a control step of new results in line with the trend of soot formation some fuels is measured by time of flight mass – spectrometer, coupled with the shock tube [26] and chemically formed in co-ring normal state in diffusion flames [27 ].In these cases, the presence of allin was stated by the kinetic way and formation of benzene directly by C [28] allows you to bring tendency of fuel to soot formation to correspondence. 14

In works [29, 30],the authors propose the basic principles of molecular growth of soot and mechanisms of soot formation concerning ions, growth of an aromatic ring, polyacetylene chains, reactions of Diels-Alder and neutral radicals. Analysis of the experimental results showed that the mechanisms of soot formation using different fuels and varying configurations of flames can be explained by a generalized mechanism based on the growth of soot precursors through the system depending on neutral radicals. The effect of electric field on the processes that occur in the flame volume is an important aspect in the study of soot formation processes. It is known that the process of combustion is accompanied by the appearance of positively and negatively charged particles, ions, radicals, and free electrons in the flame. In earlier studies, it was already stated that, in the flame, positively charged particles prevail and the soot formed becomes predominantly positively charged. So far, it is experimentally stated that negatively charged particles are also formed in sufficient amount in the flame [31-33]. Furthermore, there take place charge infer change of soot particles in a certain time interval. As a result of a greater mobility and a high sticking coefficient of electrons at the initial stage of the combustion process in the high temperature region of the flame the formed soot particles are charged negatively. Over a time, distribution of positively and negatively charged particles becomes more symmetrical. Due to the interaction of soot particles with ions (electrons), the total concentration of positive ions increases as compared to negative ones. It is known that even a very weak external electric field influences the processes that occur in the flame. When applying an electric field to the flame, general processes of soot formation shift considerably and the structure and shape of the flame change, the flame temperature grows, the density of charged particles increases and, finally, all this has an impact on the nucleation process of soot particles, on their growth, on the yield of definite products of combustion, their structural characteristics and composition [34]. It was found that with the application of electric field the size of the soot particles and the mass yield decrease, and distribution of soot particle sizes become more uniform. If the positive electrode is positioned at the beginning of the flame front and the negative 15

electrode at the top of the flame, the soot particles being formed become positively charged under the effect of thermionic emission and get accelerated by an external electric field. Soot particles, due to rapid passage of the combustion zone cannot grow to the sizes characteristic of the process without application of the field, this explaining the decrease in the mass and size of soot particle [35]. Thus, from a detailed analysis of particle sizes conducted on the basis of electron micrographs, it was stated that the sizes of soot particles with a diameter of 50 nm, which were obtained in the experiments without application of electric field, decreased to 10 nm in diameter upon application of electric field of several kilovolts [36]. Many authors investigated the effect of electric field on the combustion process at atmospheric pressure, form the position of influence on the yield and structure of soot particles [37-40]. It was found that, when applying a constant electric field to the diffusion acetylene flames, the yield of soot particles decreases: at the tension 200 kV / mA is reduced by 90% at the negative polarity and is reduced by 70% at positive polarity [41]. It follows that reduction in the mass yield of soot under the influence of electric field is caused by oxidation of soot particles on account of forming free atoms of O and O3. The increase of the applied voltage and the change in the polarity resulted in a significant change of the flame shape. At negative polarity the temperature at the end of the flame was higher by 773 K, than without application of electric field and made up 2073 K which was not observed at positive polarity. As all particles acquire the charge in the very early stages of the process, the weight and size of a soot particle will depend on the duration of stay in the combustion zone, which changes with application of electric field [36]. In works [42, 43], the authors studied the applied electric field to the reaction zone of the flame on formation of nuclei of soot particles, their growth in the zone of oxidation and sedimentation. Furthermore, the rate of formation of soot particles depends on polarity and magnitude of the ion flux through the oxidation zone. It has been found that with a large flow of positive ions crossing the oxidation zone, the masses of deposited particles increase while decreasing the particle sizes decrease, this indicating the increase in the amount of particles formed. In 16

comparison with the case of absence of the field, at higher intensities the rate of particle formation increased eight times. The research of the influence of an alternating current on diffusion propane – air flame in a coaxial flow was conducted [39]. When applying an electric field of an alternating current, oxidation of soot particles increased, this indicating the influence of electric field on formation and growth of soot particles. Fullerenes were discovered in 1985 and obtained in macroscopic quantities in 1990, in both cases by a graphite evaporation method arc discharge. Ions of C60 and C70 fullerenes were detected in soot carbon flames in 1987 and 1991 and taken in macroscopic quantities from a flame. The highest observed content of fullerenes in soot makes up 20%. The highest speed of 38 cm / sec formation of fullerenes was observed with a pressure of 69 Torr, at a ratio of C/O=0.989 and dilution of 25% of helium. Depending on the way of evaporation of graphite in flames the ratio of the formed fullerenes C70/C60 changes within 0.26...8.8 (at evaporation of graphite within 0.02...0.18 ). The atoms in fullerene molecules are located on the surface of a sphere or spheroid at the vertices of hexagons and pentagons. The interest in the study of fullerenes is due to the variety of new physical and chemical phenomena that take place with the participation of fullerenes, and the prospects for the use of a new class of materials that are created on their basis. At present, researches and developments in the field of fullerenes are one of the priority directions of world science and technology. In this regard, the creation of effective controlled synthesis of fullerenes is an actual task. Thus, this problem is closely connected with the study of the mechanism of fullerene formation. It is stated that formation of fullerenes competes with the process of soot formation and is closely related to the mechanism of formation of polycyclic aromatic hydrocarbons PAH [10, 44-51]. Numerous studies in premixed benzene / oxygen flames have shown that the presence of concentration profiles of polycyclic aromatic hydrocarbons with a molecular weight of more than 300 a.m.u, promotes the intensive formation of fullerenes [45]. In works [50, 52-59], there are different approaches to explaining the 17

mechanism of formation of PAHs and fullerenes with the presence of large amounts of experimental data allowing to test the kinetic models of formation of higher PAHs and fullerenes during combustion of aliphatic and aromatic hydrocarbons. The process of formation of PAHs, fullerenes, soot, and others in the combustion mode is a fast reaction, in which a solid phase is formed during a very short period of time. Currently, there is a significant progress in the understanding of the mechanisms of fullerene formation based on different kinetic models [60]. During combustion of hydrocarbon fuels, formation and further transformation of PAHs up to fullerenes and soot particles is a complex multi-step process, which is closely related to the overall kinetic mechanism of combustion. From the point of formation of PAHs, acetylene formed during high-temperature oxidation of any hydrocarbon fuels, is the most important intermediate combustion product. The process of formation of acetylene and its participation in the combustion reactions depends on the fuel used and on detailzation of kinetic models, as in the proposed kinetic models of combustion processes, differing in the number elementary reactions that are responsible for the combustion processes are taken into account. When using hydrocarbon fuels containing aromatic rings (e.g., benzene, toluene, and others) it is not necessary to take into account participation of the molecules or intermediate acetylene combustion products in the reactions of formation of the first aromatic ring and small aromatic molecules in benzene / oxygen flame [61]. After formation of the first aromatic ring the further growth of PAHs is related to interaction of aromatic radicals with the molecules of acetylene. The process is realized by NASA- mechanism (Habstraction-C2H2-addition) [62-65], which is a simplification of the kinetic mechanism of the growth of the aromatic structure of PAH molecules, and it more accurately presents the way of formation of aromatic rings: H atom separation due to interaction of an aromatic molecule An with hydrogen atoms and further attachment of C2H2 to the forming aromatic radical Rn. In the course of interaction of aromatic molecules and radicals with O atoms and molecules of O2 there takes place their destruction [68]. 18

Five-member rings are the main constituent elements of many molecules of PAH (e.g. C20H10 korannulene) [53]. It is now known that fullerenes along with hexagonal cells contain 12 pentagonal cells. Hence, the conclusion is made that the content of PAH with five-member rings is very important for formation of the fullerene structure, as the presence of a five-member ring creates a curvature required for closing of fullerenes [51, 52]. According to the kinetic model of Richter, flat PAHs will continue to grow to soot particles and curved PAH form a closed fullerene. 1.3. The methods of synthesis of fullerenes in flames In the flames with the excess of the stoichiometric content of carbon, the combustion process is accompanied by formation of soot. Under definite conditions such flames there can form different flames, fullerenes, and other nanostructures. Production of fullerenes in large quantities at low cost is of both industrial and commercial importance. And today, development of a more economical method for producing fullerenes of high yield is an actual task. One of the most promising methods in this direction is the process of synthesis of fullerenes in combustion process of hydrocarbons. The advantage of this method is the ability to control different technological parameters (pressure, temperature, the ratio of fuel – oxidant, the fuel feeding rate, etc.). In [66], the authors carried out extensive investigations on the properties of various flames to determine the optimal conditions of formation of those types of nanomaterials. The studies have shown that the most suitable for synthesis of fullerenes is a premixed laminar homogeneous flame. In many works, the results of investigations on different flames are presented [67-72]. It was determined that for formations of fullerenes in flames the most effective components of the combustible mixture are aromatic hydrocarbons and oxygen. It was found that addition of an inert gas exerts a significant influence on the process of formation of fullerene structures. In 1991, the patent [70] of the workers of the Massachusetts Institute of Technology (MIT) (Howard J.B. and McKinnon J.T.) 19

presents the conditions of experiments and their results on formation of fullerenes in a homogeneous flame. The highest yield of fullerenes 0.24% of the fuel carbon mass was obtained by the authors of this work in a benzene-oxygen flame at a pressure of 20 Torr, the ratio C / O = 0.995, with 10% of argon, a gas mixture flow rate of 49.1 cm / c, the flame temperature of 1800 K, the ratio of C 70 / C60 being equal to 0.86. This method for producing fullerenes is likely to provide a small percentage of conversion of the initial raw material into fullerenes, however, the range of the ratio change of C70 / C60 is very important (0.02 ÷ 0.18) compared to the arc methods. It is noted that the increase in temperature and the ratio C / O, as well as the decrease in pressure contribute to the increase in the yield of C60 + C70, therefore, it must be assumed that these results are not optimal for the yield of fullerenes, the dependence being not monotonous. In [46, 48], the authors reported on the full and detailed research on formation of fullerenes in the benzene-oxygen-argon (helium or nitrogen) flame. Also acetylene was used as a fuel. The flame parameters changed in the following ranges: chamber pressure 12-100 Torr; atomic ratio C / O = 0.717-1.082; the molar percentage of Ar (He, N2) 0-50%; gas velocity above the burner 14.6-75.4 cm / s. The combustion products contained carbon, polycyclic aromatic hydrocarbons (PAH) and fullerenes within. The ranges of these parameters the dependences of the content of fullerenes in soot on C / O, the gas velocity, the pressure in the chamber, the type and concentration of the diluent gas were measured. The main differences of fullerene formation in the flame from the method of arc evaporation of graphite were determined [46, 48]: in the flame, the molar ratio of C70 / C60 in soot is greater and it can be varied by changing the parameters of conditions; metastable isomers of C60 and C70, oxidized fullerenes C60O and C70O, hydrogenated fullerenes C60H2, C60H4, C70H2 [C60 (CH2) (H2)] or [C60 (H) (CH3)] and others are formed in the flame; hydrogen and oxygen are present in the field of generation of fullerenes. It was found that depending on combustion conditions, the percentage yield of fullerenes from soot varies in the range of values 0.0026-20%. The highest rate of production of fullerenes (C60 + C70) was 0.45 g / h under the conditions of pressure of 69 Torr , C / O = 0.989, rate 38 cm / s, 25% 20

helium. The mass of fullerenes made up 12.2% of the soot mass. The highest content of fullerenes in soot (20%) was observed at: 37.5 Torr pressure, C / A = 0.959, the rate of 40 cm / s, 25% helium [46, 48]. Fullerenes C60 and C70 were first discovered in 1985, in the mass spectra of the products obtained by laser vaporization of graphite [73]. In 1990, for production of fullerenes a simple method of obtaining large (bulk) quantities of soot containing fullerenes by sputtering graphite in an inert atmosphere of helium at low pressure in the plasma of electric arc was used [74]. For separation of fullerenes from soot, the methods of extraction and sublimation were used. Then the arc method was improved to increase the yield of fullerenes [75]. Currently the predominant way of industrial production of fullerenes is the method of arc evaporation of graphite, which gives 10-15% yield of fullerenes from 30-40% of the soot formed [76-78]. In fact, about 3-4% of the evaporating carbon material is consumed for formation of fullerenes that, taking into account the cost of purification, leads to a strong rise in the cost of fullerenes. Optimization of the arc method of production is in achieving of a definite ratio between carbon concentration, temperature of gas and flow rate form the intra-electrode space. However, it is practically impossible to obtain such optimum ratio in the arc as, when changing the arc current, all the three parameters change simultaneously. Thus, the standard electric arc method for producing fullerenes according to the opinion of some researchers, exhausted, apparently, all the possibilities of increasing efficiency. The advantages of the method for producing fullerenes by pyrolysis of acetylene are the continuity of production (as opposed to the arc method), the absence of UV radiation and oxygen that adversely affect the process of formation of fullerenes. Acetylene pyrolysis method is based on decomposition reaction of acetylene during which is soot containing fullerenes and hydrogen is formed: 30С2Н2 = С60 + Н2 [79, 80]. Acetylene-argon mixture is fed vertically downward through a graphite heat exchanger, placed in a quartz tube with inductive heating. The heating temperature of pyrolysis zone ranges from 1500 to 2500 K. The gas mixture is rapidly heated, the decomposition reaction occurs and the reaction products are 21

transferred by argon downward into the cooled toluene and dissolve in it. Further, the solution of fullerenes in toluene is filtered out. The highest yield of fullerenes  2% was obtained at 1500 K. The low yield of fullerenes does not allow using this method in an industrial scale. Nowadays, the method of obtaining fullerenes in a flame in the course of combustion of hydrocarbons is used for production of fullerenes, alongside with the arc method. Production of fullerenes in flames is the most promising method to solve the problem of a largescale production. Its distinctive feature is continuity, efficiency and adaptability to streamlined manufacture compared to the electric arc method of evaporation of graphite. In addition, controlling the processes occurring in the flame volume, it is possible to achieve the highest yield of various fullerenes and the possibility to change their ratio in soot containing fullerenes [81]. Since this method is similar to the method used for production of technical carbon, it is expected to be more productive for obtaining fullerenes than the arc process. In [48, 82], it is shown that as compared with toluene and other hydrocarbon fuels, the use of benzene as fuel gives the highest yield of fullerenes. It is stated that addition of the diluting nonflammable gas, for example, helium or argon, exerts a favorable effect on formation of fullerenes. It was determined that realization of the process for synthesis of fullerenes within the ratios C / O = 0.95-1.1, at a pressure of 40 Torr with addition of argon in the amount of 10% of the volume of the feed mixture of benzene and oxygen gives the highest yield of fullerenes [81]. In work [52], the authors conducted researches using probe sampling of products of burning from different areas of a flame.It was revealed that, generally, fullerenes are formed in the central region of the flame where the temperature is much higher than at the edge. In the central region of the flame soot particles are not formed, but their significant growth was observed at the edge of the flame. In [52], it is shown that if the peripheral region of the flame is heated by some external source such as a laser beam, which does not just burn soot, but creates the same conditions as in the central region of the 22

flame, the concentration of fullerenes increases. It is stated that electron density and ionization instability in the reaction zone renders a significant impact on the yield of fullerenes. It was found that using a conventional method for organization of burning, it is difficult to expect a significant increase in the efficiency of formation of fullerenes in flames [76, 78]. Taking this into account, to effectively increase the yield of fullerenes in flames, it is necessary to use methods that combine combustion with additional exposure of energy sources (the high-frequency electric field, gas discharge, electromagnetic field, etc.). As is known, fullerenes are formed only in certain flames and under strictly defined conditions of combustion: at low pressure in С2Н2/О2 [52, 83] or С6Н6/О2 flame [47, 69, 84] and at atmospheric pressure in a countercurrent diffusion flame. Therefore, an essential task now is to create an efficient controllable synthesis of fullerenes during combustion of hydrocarbons. Investigations on production of fullerenes at atmospheric pressure in a countercurrent flame were carried out during burning of the enriched methane-oxygen mixture with and without addition of acetylene using nitrogen at ratios [85, 86], 100 % СН4, 50 % О2 + + 50 % N2; 96 % СН4 + 4 % С2Н2, 50 % О2 + 50 % N2. The flow rate was varied in the range 20-60 m / s, and the maximum adiabatic temperature in the flame reached 2700 K. Sampling of combustion products was performed directly from the flame. It is stated that the addition of acetylene and reduced flow rate increase the yield of fullerenes, and the peak of fullerene formation is observed above the stagnation zone at adiabatic temperature of 1800 K. The maximum yield of fullerenes made up 1-1.5% of the soot produced. The study of countercurrent diffusion flame showed that fullerenes can be formed at atmospheric pressure, too. However, the use of this method due to the low yield of fullerenes does not allow to use it for industrial production. The combustion process of enriched hydrocarbon fuels in oxygen with the addition of inert gas (argon or helium) at low pressure is a more promising method for producing fullerenes in flames. This method was developed when for the first time in premixed soot forming flames of low pressure, positive and negative ions C60 and 23

C70 fullerenes were detected [51, 87-90]. In order to optimize the process and increase the yield of fullerenes, this served as an impetus for an intensive study of flames. With regard to the plasma method for producing fullerenes in an electric arc, a positive effect of electron density on the efficiency of fullerene formation in plasma was stated experimentally [91] and theoretically [92]. The dependence of C60 formation rate on the temperature and electron density is related to the dependence of carbon cluster collisions on these parameters of the section. Experimentally, it was also found that fullerenes are produced efficiently in the presence in plasma of ionization spontaneous or forced instability accompanied by a change in the concentration of electrons, for example in the range of 1010-1011 cm-3. Though the conditions for formation of fullerenes in flames differ from the conditions of their formation in the arc, they have a common origin – the presence of ionized initial products. The main parameters that one can use to change the combustion conditions in general, are the temperature and the concentration of electrons. These parameters influence the charge of intermediate combustion products, and therefore the cross section of their collision with each other. By varying the concentration of electrons and the temperature of the flame it is possible to create optimal conditions for synthesis of fullerenes. This phenomenon can be achieved by acting on the electric flame by a gas discharge, which can both increase the flame temperature and change the degree of its ionization. Now it can be considered beyond question that the impact of electric field on combustion causes numerous effects that can be used in technological processes. Here are several factors that arise under the influence of electric discharge and can have a positive influence on formation of fullerenes in flames. It is stated [52, 93] that the most important small radicals in rich hydrocarbonic flame are H, OH, O and CH3. At the same time under the influence of electric discharge there takes place an intensive formation of O, H, OH, radicals which take an active part in the process of branching of chain reactions [94], OH radical being the most important oxidant in flames. Considering that the concentration of OH sharply decreases beyond rich flames oxidation zone, its formation by electric discharge behind the oxidation zone will 24

increase the rate of PAHs oxidation at the beginning of the yellow glow zone and the flame edge that will slow down the process of soot formation and create the conditions for the preferential formation of fullerenes instead of soot. It is well known [95, 96] that the plasma formed by the flame, and has a good conductivity and tends to shield external electric influences, therefore to a more extent the electric discharge will act on the peripheral part of the flame, which is more preferable from the point of increasing the yield of fullerenes at the flame edge [ 51]. The time factor exerts an impact on the process of formation of fullerenes, too. Since synthesis of fullerenes occurs in the environment of negatively charged ions – aromers, the negative potential applied to the top of the flame, will prevent the rapid movement of aromers from the fullerene formation zone and increase the time of their effective interaction. There will simultaneously take place acceleration of the movement of positively charged ions of PAHs which are predecessors of aromers and that will contribute to a more intensive formation of aromers due to acceleration of coagulation processes and ion-molecular interaction. 1.4. The influence of external local impact on the processes of formation of combustion products The flame is a universal reactor of synthesis of nanostructured materials. During combustion of different hydrocarbons it is possible to synthesize a wide class of nanosize particles (nanotubes, carbon «bulbs» , fullerenes and their derivatives). One of the methods of controlling the combustion process is the method of introduction of additional energy to the volume of flame. Investigation on the influence of the external energy action on the flame is interesting both in terms of practical application of the obtained effects, and to explain the mechanisms of combustion products formation. As it known, the flame is a certain system with distributed space charge, and under the effect of electric field, it is possible to observe a change of the flame form, in a transverse electric field flame deviates from the anode to cathode, and at a longitudinal impact, this leads to the decrease or the increase in the flame height. 25

The presence of charged particles in the flame allows to influence the processes occurring in the flame when applying an electric field. Specificity of electric phenomena and the nature of the structure of the flame front during combustion are such that even a weak external electric field is already exerting an effect on all the processes occurring in the flame [93, 96]. The flame behavior depends on many factors: the direction of the applied electric field – along or perpendicular to the flame, the type of electric field – direct or alternating, position of the negative electrode – on top of the flame, or from the side of cold mixture, and the magnitude of the applied voltage, strength of current and pressure of the environment. Depending on the values of these parameters one can observe a transition from the action of electric field on the flame to the electrostatic discharge. The mechanism of influence of electric discharge on the flame will depend on the type and power of discharge and qualitatively differ from the effects of electrostatic field. Depending on the voltage, the strength of current and pressure of the environment, electrical discharges is divided into dark (Townsend), corona, spark, glow and arc discharges. When applying an external electric field to a flame, the following effects effecting the processes occurring in the flame [36, 97] can be observed: – «Gas-dynamic effect (ion wind)» – the movement of positive ions and neutral particles entrained by them to the negative electrode – the cathode and negative ions and electrons to the anode. As a result of the motion of charged particles there arise mass forces changing the gas dynamics of the combustion process. Since the concentration of charged particles in the flame can be millions of times exceed than the values corresponding to the thermodynamic equilibrium at the combustion temperature, mass forces prove to be significant in magnitude, this resulting in the change of the shape and size of the flame, the surface and burning rate; – «Thermal» mechanism when the electric field energy is converted into heat energy in the flame volume and in the border regions near the combustion surface, and, according to the Arrhenius law, the rate of chemical reactions increases; – «Kinetic» mechanism, when there takes place direct effect of the electric field on the kinetics of chemical reactions due to polarization and activation of reacting particles owing to collisions 26

with electrons which in an electric field acquire some extra energy compared to the state without the presence of the field. The distribution of the electric field intensity in the inter electrode space is not uniform. Furthermore, the greatest potential drop occurs in the region with predominance of a positive charge because of their immobility compared to free electrons. The flame front is characterized by a small electrical resistance and voltage drop on it is not large [98, 99]. Inequality of electric forces that act from the anode and cathode is caused by asymmetry of electric field: in the field of positive charges there prevail electric forces acting in the direction of the cathode. In [100-102], the authors analyzed the effects acting on the processes that occur in the flame with application of a gas discharge and assessed the degree of influence of the type and parameters of the discharge. The experimental facts of the increased yield of fullerenes depending on the type and place cathode application to the flame at different gas discharges are explained. The experiments showed the existence of ionization instability that arises in the flame plasma under the action of glow discharge, this confirming the hypothesis of electron density of the flame plasma – as one of the main parameters effecting the formation of a fullerene molecule. Under the influence of electric field on the ionized gas volume, the charged particles, which were in chaotic motion, being exposed to the action of the field, acquire an ordered accelerated motion. A charged particle is accelerated by the field only on the length of free path, which is defined as the distance between collisions. Additional average speed of directed motion of a charged particle, which it acquires in time  in the direction of the acting strength of the electric field, is determined by the expression [97, 103]:


qE ,  m


where, E is electric field intensity V / m;  is mean free time, s; m is mass of the particle, kg; q is the charge of the particle, C. Thus, under the influence of the electric field the charged particles have additional kinetic energy, which has a strong influence on the chemical processes occurring in the flame volume. 27

The influence of the applied electric field as an organizing factor on the process of formation of PAHs, aromers with transition into fullerenes may be related to the following reasons. Firstly, the applied electric field induces the dipole moment in the molecules formed owing to this, nearby molecules or associates with equally orientated dipole moments are attracted to each other due to dipoledipole interaction and thus can be arranged along the lines of force of the electric field into structures contributing to formation of PAHs, aromers and fullerenes. And, secondly, in a nonuniform electric field, which took place in experiments a dipole is effected by the force F = peΔE / Δl (here pe is a dipole moment of the molecule or associate, and ΔE / Δl is the electric field gradient). Under the influence of this force a dipole is drawn into the region of a strong field being attracted to the charged body, i.e., there takes place electrophoresis of particles with induced dipole moment, this contributing to their more effective interaction. Earlier experiments have shown a positive effect of electric discharge on synthesis of fullerenes, increasing the percentage yield of C60 up to 10-15% of the mass of soot formed [102]. From a physical point of view, without going in detail to the mechanisms of chemical interaction of the intermediate components of the combustion process, it is possible to enumerate changes in the flame under the action of electric discharge positively influencing on synthesis of fullerenes [51, 91]: the degree of ionization increases significantly; the flame temperature rises in the periphery and in the volume; the relative speed of the movement of the charged particles increasing the number of active collisions grows; ionization instability arises and electron density increases. These factors effect the chemical process of combustion and mechanisms of products formation in the flame. One of the main difficulties when constructing mechanisms of chemical reactions taking place in flames is that in the combustion process there form short – lived, but able to further transformations highly active intermediate particles – atoms, ions, radicals. The difficulty lies in the fact that it is impossible to describe all chemical processes taking part with participation of intermediate particles. For the final products to be formed, a sequence of reactions must take place. Therefore, by providing an external impact on certain flame 28

regio ons (electric fielld, thermal effecct, addition of organic o compoun nds) it is possible to initiate chemiccal processes in th he right direction, and relating parrameters of exterrnal source with the obtained resullts, we can assum me the probable m mechanisms of form mation of the finall products of comb bustion. Studies of the efffect of external iinfluence on the formation of comb bustion productss in the fuel-ricch hydrocarbon flames were carriied out on the experimental setup a schematic reprresentation of whicch is shown in Fig gure 5 [102].

1 – receiver, 2 – inj njection valve needle, 3 – reduction gear, 4 – valve, 5 – capillary flow meter, 6 – kilovoltmeter C-550, 10 – electric igniteer high voltage AC C, 11 – high-voltage DC D power supply, 7 – evaporator-mixer, 8 – buffer tank, 9 – fuel combustion cham mber, 12 – vacuum gaauge of EDC-1, 13 – soot collector, 14 – cryogenic trap, 15 – three-way valve, 16 – vacuum pump VN-2MG, V 17 – the type of liquid dispenser d electromechhanical syringe, 18 – ammeter a M95 F Figure 5. The experim mental setup

On installation, you can study thhe processes of formation of comb bustion products of both gaseous and liquid hydro ocarbon fuels. The main fullerene fo orming benzene-oxxygen flame can be b influenced by ex xternal acetylene--oxygen flame. 29

Consumption of the supplied fuel mixture components was controlled by rheometers. The combustion chamber is made of quartz glass. Acetylene-oxygen flame burner is an annular tube with a diameter of 0.4 cm, with the openings with a total area of output of acetylene-oxygen mixture S = 0.6 cm2. The output rate of the acetylene-oxygen mixture V = 18 – 20 cm / c. The main part of the apparatus for synthesizing fullerenecontaining soot during combined combustion is the reactor (Figure 5 (9)) made of quartz glass. Oxygen and argon are fed from cylinders. Benzene in an amount of 0.5 ml / min was fed from a special system to a heated glass vessel which was connected to the supply line of oxygen and argon. Benzene having evaporated mixes with oxygen and argon and the formed combustible mixture enters the reactor. The combustible mixture is additionally stirred in the chamber with balls of an inert substance that is in the lower part of the burner. The use of such a system allows to create a uniform flow of the fuel mixture for a stable flame burning. The matrix is made of a steel wheel, in which holes are drilled with a diameter of 0.8 mm. On the upper cover of the reactor, a special system for measuring temperature of a flame is provided as well as the possibility of vertical movement of the thermocouple along the length of the reactor for removal of the temperature profile of a flame. For collection of the formed soot, a cylindrical soot collector is used. Soot collector sleeve for filtration of soot is made of carbon cloth. After the experiment, the soot was collected, weighed and prepared for further studies. The combustible mixture is ignited by the spark arc, which was created by transformer of the brand HT-1020, the electrode was located at a distance of 7-10 mm over the burner matrix. The vacuum in the system was created by a backing pump. Observation on the change of pressure in the system was carried out by means of strain gauge EDC-1, which was connected directly to the burner. The required system pressure was created by pumping and control over the cock with the help of a weigher. To prevent water from entering the vacuum pump, in the system before the pumps traps with liquid nitrogen are installed. In general, the whole system is airtight. 30

In different areas, along the height of the main benzene-oxygen flame an effect was exerted by premixed acetylene oxygen-flame. The parameters of acetylene-oxygen flame are: acetylene flow Q4 = 200 cm3 / min, oxygen consumption Q5 = 500 cm3 / min. 1.5. The influence rendered by a local effect of external acetylene – oxygen flame on a temperature profile benzene – oxygen flame Researches on the temperature profile of benzene – oxygen flame were carried out under the effect of diffusion acetylene – oxygen flame on it. Different zones along the height of benzene – oxygen flame were effected. For this purpose, a ring torch with the possibility of moving it along the height of benzene-oxygen-argon flame was prepared. The combustion modes, when supplying a ring torch with acetylene – oxygen mixture were tested. The torch provided steady combustion in the estimated ranges. To measure the temperature profile along the height of the central part of the flame, a chromel – alumel thermocouple was used. To eliminate the deposition of soot on the bead of thermocouple, it was caused by quartz glass, this leading to a significant reduction of the measured temperature. In connection with the absorption of heat by a quartz case, thermocouple readings were corrected in the amount of 150 K. When you move the thermocouple along the flame height there was a danger of the vacuumed combustion chamber air – tightness failure. To eliminate this phenomenon, we developed a system of electromechanical reverse feeding of the thermocouple placed in the syringe and connected to the syringe piston. Its movement was realized jointly with the syringe piston, which ensured tightness of the system and allowed to monitor the temperature measuring point along the height of the flame. The axis of the piston is a rod with threads. By rotating the nut with the help of reversible motor wound on the rod, we moved the thermocouple placed into the flame, vertically to the desired height. Photo of the device for measuring the temperature profile along flame height in a vacuum environment is shown in Figure 6. 31

Figure 6. Photo of the device for measuring the temperature profile along the height of the flame

The experiments were conducted under the following conditions: ratio of benzene to oxygen C / O = 1, consumption of benzene Q1 = 138 cm3 / min, oxygen consumption Q2 = 415 cm3 / min, argon consumption Q3 = 55 cm3 / min (10% by volume). The rate of cold benzene – oxygen mixture blow from the stabilizer was equal to V = 16.9 cm / s. Parameters acetylene-oxygen diffusion flame: acetylene consumption Q4 = 200 cm3 / min, oxygen consumption Q5 = 500 cm3 / min. Acetylene consumption was varied from 30 to 50% of the volume of benzene-oxygen mixture. System pressure P = 40-60 Torr. To eliminate the backward puff of acetylene – oxygen mixture, we estimated the rate of acetylene-oxygen mixture flow from the ring torch moving along the height of benzene-oxygen flame. The ring torch has openings with the total area of acetylene-oxygen mixture flow S = 0.6 cm2. The flow rate of acetylene-oxygen mixture is equal to V = 19.4 cm / s. The effect of external acetylene – oxygen flame on benzene – oxygen flame results in the increase of its temperature. It is stated that the increase in temperature, generally occurs at the top of the flame above the moving ring torch. It is shown that the temperature of fullerene forming flame increases maximum by 50 °C. Tempe32

raturre profiles over th he height of the toorch were taken when w applying exterrnal oxygen-acety ylene flame flowiing around the in nner benzeneoxyg gen flame at distan nces 2, 3 and 5 cm m (Figure 7 and 8)). The Studies havee shown that the iimpact of externaal acetylene – oxyg gen flame on the periphery p of the bbenzene-oxygen- flame results in th he increase of its temperature onlyy at the top part by b 50 °C and spreaads to a distancee of 1-1.5 cm (Fiigure 8), then thee temperature decreeases.

1 – 2 cm; 2 – 3 cm; 3 – 5 cm Figure 7. Diagram of the com mbustion chamber

Temperaature profiles along thhe axis of the torch: 1 – 2 cm; 2 – 3 cm; 3 – 5 cm Figure F 8. The temperrature profile of benzeene-oxygen flame when exposed to oxygen-acetylenne flame


1.6. The effect of local action of external acetylene – oxygen flame on the mass yield of fullerene C60 during combustion of benzene – oxygen mixture If we divide the flame front into separate zones, on account of chemical reactions in each area there takes place splitting of fuel into separate components, neutral or ionized atoms, particles, radicals, fragments of molecules, etc. are formed. The nature, concentration and their ratio relatively to each other are strongly dependent on the type of fuel used, on the ratio C / O in a gas mixture, pressure, etc. Mainly due to the difference of the temperature gradient, the distribution and concentration of the forming intermediate particles produced in flame volume will be different. The general sequence of chemical transformations that lead to the formation of combustion products occurs predominantly upward along the height of the flame. The formation of a particular combustion product will depend upon the separate chemical reactions between intermediate particles and the process will depend on their presence and concentration in the volume. If the necessary intermediate particles are directed into a definite volume of the flame, it is possible to achieve the formation of the required final products of combustion. So, in order to increase the yield of fullerenes a new approach to organization of the combustion process was used. The essence of the new method was in the use of ring acetylene-oxygen torch, which was moved along the height of benzene-oxygen flame with the purpose of exerting a local effect on different zones. In the region of contact of flames, separately formed intermediate products of combustion of acetylene and benzene react with each other. IR – spectroscopic study of extracts of soot samples show different effects of oxygen-acetylene flame on the yield of fullerenes. The difference is observed depending on at what stage of the oxidation process of benzene – oxygen flame, the intermediate products of acetylene combustion exert an effect. The effect on percentage yield of fullerenes, depending on the experimental conditions was evaluated using infrared spectroscopic samples. In [104], the authors determined the influence of acetyleneoxygen flame on the processes of formation of fullerenes. The aim of 34

this work was to study the influence of the oxygen-acetylene flame at its local action on different areas of the benzene-oxygen flame. For this purpose, acetylene-oxygen ring torch moving along the height of benzene – oxygen flame was used. The point is that in the course of combustion there form short – living but able to further transformations intermediate particles – atoms, ions, radicals, which play an important role in the chemistry of the formation of the final products. It is now assumed that formation of fullerenes occurs via PAH. In the benzene flame, a benzene molecule itself is an important intermediate for formation of PAH, which are precursors of soot or fullerenes but benzene is completely consumed before the end of the oxidation zone. Therefore, in the area of temperature maximum benzene cannot independently act as a building block to form large molecules or particles which include fullerenes. Due to oxidation of any hydrocarbons, as a result of successive transformations of small hydrocarbon, fragments acetylene forms. Inside the benzene flame with the increase in temperature there also forms acetylene and at the end of the oxidation zone acetylene is present in the same concentrations as in acetylene flames [52]. Numerous studies carried out by different authors using probe sampling along the height of the flame show that fullerenes begin to form beyond the formation zone of soot particles, where acetylene is present in sufficient quantities. This indicates the fact that it is acetylene that acts as a building material during formation of the fullerene struc-ture. Based on these conclusions, artificially introducing acetylene or intermediate products of combustion in different zones of hydrocarbon flames, you can control the process of formation of fullerenes. In rich hydrocarbon flames the most important radicals involved in the oxidation processes are H, CH 3 and OH [32], and OH is the most important oxidant. The maximum concentration of these radicals is achieved before the end of the oxidation zone. In rich flames, the rate of formation of large particles in the oxidation zone is determined by the degree of their oxidative degradation. If they are formed rapidly, the majority of them burn up. If formation proceeds slowly and is still in progress when concentration of OH reduces, the large particles formed will represent the final products of combustion. Thus, the growth of large molecules begins in any case 35

within the oxidation zone, all possible precursors of soot and fullerenes are more or less subjected to oxidative degradation. The authors [53] propose a method of formation of fullerenes in flames, and emphasize the importance of PAH that form corannulene (С20N10). According to the authors, by means of acetylene bonds, there proceeds parallel formation of large planar PAH that contain several five-member rings not only on the periphery, but also inside the six-member rings. Continuous addition of five-member rings at simultaneous formation of more amounts of hydrocarbons containing a small amount of N leads to bending of the PAH formed, which increases before closing of a cell with formation of fullerenes. In this mechanism, on the way to C60, intermediate products С20Н10, С30Н10, С40Н10, и С50Н10 form. This mechanism predicted copious formation of C60 and C70 in flames saturated with acetylene that has not been verified experimentally. Our experimental data obtained at the local impact of the acetylene-oxygen flame on benzene-oxygen flame speak in favor of the proposed mechanism, but in the reactions of formation of fullerene structures, a greater role is played by intermediate products of acetylene flame combustion. This follows from the fact that at the moment of formation of other hydrocarbon flames, acetylene may have a different reactivity than acetylene artificially introduced into the reaction zone. Figure 9 shows a typical IR spectrum of soot extract obtained during combustion of benzene-oxygen flame at the ratio of C / O = 0.95-1.1, system pressure of 40 Torr and addition of 10% argon in relation to the fuel mixture [102]. Flames speak well for the offered mechanism, but in reactions of forming of fullerene structures the big role is rendered by the formed intermediate products of burning of an acetylene flame. It follows from the fact that acetylene at the time of education of others hydrocarbonic flame can have other reactionary capability, than the acetylene which is artificially entered into a reactionary zone In the figure 9 the typical IR spectrum of extract of the soot obtained when burning benzene – oxygen flame is given, at C / O=0.95-1.1 ratio, pressure in system 40 Torr and additive of 10% of argon in relation to gas mixture [102]. 36



Figure 9. IR spectrum of soot extract obtained during combustion of benzene – oxygen flame with C / O=0.95-1.1 ratio, and addition of 10% of argon in relation to gas mixture and pressure in the system 40 Torr



Figure 10. IR spectrum of soot extract obtained under the action of external acetylene – oxygen flame which is imposed on the pre – flame zone

The carried out researches show that under such conditions about 1% of fullerenes of the total mass of the formed soot (Figure 9) is formed. Figure 10 presents the IR spectrum of soot obtained under the same conditions of combustion of benzene – oxygen flame, but under influence of external acetylene – oxygen flame which was imposed on a flame zone, i.e. at the distance of 1 cm from the beginning of the flame front. In IR spectrum 4 peaks: 576 cm-1, 526 cm-1, 1182 cm-1, 1429 cm-1 are accurately observed which correspond to C60 fullerenes. The carriedout analysis on percentage content of fullerenes in the initial soot has shown that the content of fullerenes increases to 15%. For identification of the local influence of acetylene – oxygen flame on the process of formation of fullerenes in benzene – oxygen flame, the subsequent experiments were made by moving a torch along the height of acetylene-oxygen flame. Figure 11 presents IR spectra of soot extract obtained under the influence of acetylene – oxygen flame at the height of 2 and 4 centimeters from the beginning of the benzene – oxygen flame front. From the analysis of IR-spectra it is seen that the composition of functional groups dose not practically change, there are all the four peaks corresponding to C60 fullerenes and at the same time their ratio on intensity remains, but the percentage content of C60 fullerenes has decreased in comparison with the previous condition (Figure 11a). IR – spectroscopic researches show that the composition of functional groups in comparison with the previous condition does not undergo changes, but the percentage ratio of C60 fullerenes decreases and corresponds to less than 10% of initial soot (Figure 11b). Figure 12 presents IR – spectrum of soot extract obtained under the condition when acetylene – oxygen flame was imposed at the distance of 6 cm from the beginning of the front benzene – oxygen flame front. It IR spectrum there are all the four peaks corresponding to C60 fullerenes, but their percentage content strongly decreases and reaches minimum as at usual benzene – oxygen flame combustion. 39





Figure 11. IR spectrum of soot extract obtained when imposing acetylene – oxygen flame at the height of 2 and 4 cm from the beginning of the benzene – oxygen flame front

a – at the height of 2 cm; b – at the height of 4 cm



Figure 12. IR spectrum of soot extract obtained, when imposing acetylene – oxygen flame at the distance of 6 cm from the beginning of benzene – oxygen flame front

The investigations on the processes of fullerene formation in the flame show that fullerenes are generally formed in the center of flame where temperature is always higher, than on borders of the flame [100]. In the proposed by us method of organization of the combustion process, the first positive consequence for creation of optimum conditions for formation of fullerenes is additional heating of fullerene forming benzene – oxygen flame. When imposing acetylene – oxygen flame on benzene – oxygen flame, the temperature of the latter in the zone of warming up increases by 50 ˚C and this leads to the increase in the probable procedure of reactions in a definite area. The flame is poorly ionized plasma and the degree of flame ionization plays an important role in the process of formation of combustion products if we consider the fact that reactions between ions take place without activation energy. In hydrocarbon flames the concentration of ions considerably exceeds the concentration of ions possible in case of thermal ionization. It is especially characteristic of an acetylene flame. In rich oxygen – acetylene flames the concentration of ions reaches ~ 1010 an ion/cm3. In works Wan – Tiggelen with his co – workers [105] used a mass – spectrometric method, when studying acetylene – oxygen flame at the pressure of 10-40 Torr. According to their date, 70 – 90% of the total concentrations of ions are the share of H3O+. The mass spectra show HCO+ ions and small amounts of ions having the most probable formulas C2H+, NOH+, C2H3O+, and also ions weighing 41 and 44. Formation of a fullerene structure requires the presence of PAH containing five-member rings. In aromatic combustible flames, generally H-rich PAH with 30 or more C atoms are formed. They have structures which consist only of six-member rings. Poor in hydrogen PAH which contain at least three five-member rings in their structure are generally formed in acetylene flames up to C100 H x. This can also play the role of reactions initiating formation of fullerenes as when imposing an acetylene – oxygen flame on benzene – oxygen flame, the concentration PAH containing five – member rings which are nuclei of fullerenes sharply increases. Thus, impact of acetylene – oxygen flame on the periphery of benzene oxygen flame leads to intensification of the process of fullerene formation. The degree of influence is determined by at 43

what stage of the process of benzene – oxygen flame combustion this impact is made. The researches carried out with the flames of previously mixed benzene-acetylene-oxygen mixture do not result in a similar result. It is confirmed by the fact that the process of formation of fullerene structures takes place at the boundary of the flames contact. Positive influence of an acetylene flame results in the temperature increase of the basic benzene – oxygen flame, reaction zone, the increase in the ionization degree and in additional introduction in to the benzene flame reaction zone of HCO and HCCO radicals which are intensively formed during acetylene combustion. In more detail the mechanisms and the reasons of increase in the yield of fullerenes when acetylene – oxygen flame renders an external impact on benzene – oxygen flame are given in the final chapter of this work. 1.7. The influence of an external impact on the processes of formation of fullerenes nuclei According to the obtained date on an external power effect on the flame, this section presents the results of a theoretical investigation on the possible mechanisms of formation of PAH containing five-member rings which act as fullerenes nuclei. Researches on the influence of the chemical composition and various additives on the combustion rate, limits and temperature in the flame front facilitate to a certain extent understanding of the general processes taking place in a flame, but do not give the complete information on the mechanisms of chemical reactions leading to formation of combustion. It is impossible to describe accurately all chemical processes proceeding with participation of intermediate particles, nevertheless the final products do not appear instantly, but via a sequence of reactions. Therefore, rendering an external impact on definite areas of a flame (electric field, thermal impact, introduction of organic compounds), to some extent it is possible to initiate chemical processes in the necessary direction. If we know parameters of an external source and to relate them with the obtained results, it is possible to assume the probable chemical processes concerning formation of final combustion products. 44

It is possible to initiate chemical processes in the necessary direction in separately taken areas of a flame not only by the impact of electric field, but also introduction to this area of a flame of additional heat energy, chemical compounds, radicals and atoms necessary for the procedure of required chemical reactions. It is possible to activate the processes the reactions proceeding in a flame using the impact of strong electric fields when rapid electrons [106] become carriers of energy. During collision of an electron with a molecule resulting from an electron energy transfer an electron there appears an excited molecule, molecular ion or there takes place dissociation of molecules to neutral or ionized fragments (atoms, radicals). The excited state of such particles is unstable, and they are relatively long-living. The probability for such a molecule to react will be higher than for a usual excited molecule which quickly loses its energy in the form of fluorescence [107]. It follows from these conclusions that, applying an electric field or other powerful power source (heat, laser, etc.) to definite areas of a flame; it is possible to control the kinetics of chemical reactions and to achieve directed synthesis of final products of combustion. In the represented work, for synthesis of fullerene containing soot a common and combined methods of organization of the combustion process were used (benzene – oxygen fullerene forming flame burns in a ring of acetylene – oxygen flame). It was experimentally determined that optimum conditions for formation of fullerenes in benzene – oxygen flame are: C / O=0.95-1.1, pressure in the system 40-60 Torr and an additive of 10% of argon in relation to the supplied gas mixture [108]. About 1% of fullerenes of the total mass of the formed soot are formed under such conditions. In [106], the researches carried out investigations directed to the study of the influence of the type of the gas discharge, type of an electrode system and interelectrode distance on the efficiency of formation of C60 fullerenes. When imposing tension, there forms a volume electric discharge which covers the whole volume of a flame below the upper electrode leading to warming up of the flame from 950 °C to 1250 °C. The results of researches on the mass yield of C60 fullerenes revealed the advantage of a ring electrode in comparison with a needle and showed that the maximum yield of C60 fullerene (B≈15) 45

was observed in case of its location in the middle part of the flame at distance of 3.5 – 4.5 cm from a torch matrix. In this work, the influence of a combined method of organization of the combustion process on the mass yield of fullerenes was experimentally studied. Researches on local heating of the peripheral zone of benzene – oxygen flame by acetylene – oxygen flame showed that the yield of C60 fullerenes increases to 15% of the mass of the formed soot. The use of the combined method of organization of process the combustion provides a stable yield of fullerenes in the range of pressures 40-60 Torr. Experiments with expansion of the range of pressures from 60 to 100 Torr were made. With the increase in pressure more than 60 Torr the increase in a mass yield of a total quantity of soot up to 25%, is observed and a percentage yield of fullerenes decreases [109]. There is no single point of view about mechanisms of formation of fullerenes in a flame now, however most of researchers inclines to the opinion that in the course of formation of fullerenes a fundamental role is played by the polycyclic structures containing five-member rings [10, 46]. It should be noted that five-member rings are the constituent elements of some molecules of PAH (for example, for a korannulene C20H10, and others) and act as the centers of growth of fullerenes. In its turn, the process of formation of PAH with five-member structures in a flame is not fully cleared up yet. In [52], the authors describe a mechanism according to which formation of aromatic molecules containing five-member rings (aryl5) is the result of oxidation of six-member aromatic rings (aryl-6) in an oxi-PAH (aryl – 6-O), at their interaction with O2. Subsequently, aryl 6-0 decomposes to aryl-5 and CO. Further interaction of aril-5 with acetylene forms PAH with an odd number of C atoms in a molecule. Reaction of a formation of a five-member structure by splitting of CO particle with from a molecule of aril-O has a high barrier of activation energy. In a flame, also other mechanisms with low energy of activation which lead to formation of structures with five-member rings can take place. In this chapter, analyzing the obtained by us experimental data on synthesis of fullerenes using a usual and combined method of organization of the combustion process with application of electric field, other possible mechanisms of formation of these structures are proposed. 46

In [45], it is sho own that in a flam me there are two areas where theree is a formation off fullerenes take pplace (Figure 13). Formation of a fullerene nucleus requires the presence of a struccture containing a five-member rinng, and in [45] the question of its formation f is nott considered. Thhe process of formation f of fulleerenes in the firsst area can be rrelated to formattion of PAH containing five-memb ber rings some paart of which can be b completed to a fullerene molecu ule at the very bbeginning of the flame front. Specctroscopic researcches of hydrocarboon flame show the presence of the most m various radiccals and free atom ms: CH, C2, C3, HCO, H HCCO, CH2O, O CH2, H, and O in the flame [1033].

Figure 13. Profiles of o concentration of fulllerenes in the previouusly mixed flame [45]

The observed rad dicals are non-unniformly distributted along the heigh ht unevenly and d their concentrattion depends on the type of hydrrocarbon, temperaature and pressurre, on a ratio of gas mixture, and each of these values v plays an important role in chemical processes of formation n of combustion pproducts. Let us consider some possible mechanisms of formation of tthe structures con ntaining fivemem mber rings at inteeraction of radicaals and free atom ms which are form med in the coursse of the combuustion process. Arenes A of a 47

hydrroxyl (phenol, nap phthols, hydroxyppyrenes) are, if not n the initial, then the very first and d dominating oxiddation products in hydrocarbon flam mes [52]. One thee possible mechaanisms of formatiion of PAHs with h five – member rings r at the beginnning of the flamee front can be the reaction of inteeraction of phennol molecules with w oxygencontaining radicals, fo or example, with H – С˙ = C = O raadicals which are formed f due to intteraction of acetyylene (an intermed diate product of co ombustion of all hydrocarbon h fuels) with atomic oxy ygen: С2Н2 + О˙ → Н-С˙=С=О + Н Figure 14 presentts a scheme of reaaction of interactiion of phenol moleecules of phenol with w radicals.

Figure F 14. A scheme of a possible reactionn of interaction of pheenol and the H – С˙ = C = O radical leeading to formation oof PAH with five-mem mber rings (the reactiion proceeds with releease of water and eneergy ΔHH2O = -241.989 kJ/mol [105] – addiitional warming up off a reaction zone)

The reaction pro oceeds in several stages as with the t decreased presssure simultaneous collision of moore than two mollecules is not prob bable, and reactio ons the order off which is more than 2, are practtically impossiblee. Similar reactions are quite adm missible in the flam mes, they are exo othermic and havve a low barrier of activation 48

energ gy. The prime cau use of origin of cchains, i.e. formattion of highly activ ve radicals in the pre-flame zone iss diffusion of hyd drogen atoms, and, to a lesser exten nt, hydroxyl radiicals from a high h-temperature area of flame [111]. It follows from this that in a zo one of power preparation aryl radicaals are formed by the following reaactions [107]: ОН+С6Н6 = С6Н5 + Н2О Н+ С6Н6 = С6Н5 + Н2 Thus, in a zone of power preparaation of fuel, aryll radicals are preseent at sufficient concentrations. c Itt is known that th he maximum conccentration of C2 raadicals is observedd also in the loweer zone of the flam me, i.e. in a zone of o power preparattion of fuel [103]]. These radicals cause a greenish luminescence of the flame basis. Further F interactio on of aryl radicalss with the radical C2 in a zone of preparation p of fuel by the mechanism m presented in Fiigure 15 leads to formation of PAH H containing five-m member rings.

Figure 15. A schemee of formation of PAH H with five-member riings during innteraction of aryl radiicals and C2


The first maximu um of fullerenes yyield (Figure 16) along height of th he flame can be explained by thee increase in con ncentration of PAH H with five-memb ber rings due to innteraction of aryll radicals and C2 which w act further as a nuclei of fullereenes, and, it is quiite admissible that their small part manages m to be com mpleted to fulleren nes already at f beginning of o the flame front. the flame In [103], the autthors carried out the spectroscopic analysis of comb bustion of variou us flames. It is stated that in th he flames of mixttures of hydrocarb bons with oxygenn C2, CH, OH, HC CO bands are obseerved, however th hese bands have different relativee intensity in diffeerent zones of th he flame. Bands of CH and HCO O have high inten nsity in the middlle part of the flam me. Such partial division into zonees having different coloring and spectrum is chaaracteristic of many y flames. It is statted that «bands off hydrocarbon flam mes«, related to th he HCO radical arre best of all observed in spectra of o the flames burn ning at decreased pressures p and diluuted with inert gases. Bands of HCO O are observed in n the spectra of aany hydrocarbon flames f including g methane, acetyleene and benzene.. The binding energy calculation based on experim ments with a pulsse photolysis of formaldehyde f ws that HCO radiccal must be ratherr stable as dissociation energy show of H – CO bond exceeeds 27 kcal/mol. In the course of formation f of PAH H with five-memb ber rings, aryl radiccals and H – С˙= = O which are fformed during co ombustion of hydrrocarbon flames the t quantity of w which in additionaally increases with h addition of bufffer gas and the decrease of presssure of the systeem (that is an ind dispensable condittion for synthesis of fullerenes in th he flame) can take part. The reaction of formation of poolycyclic structurees with fivemem mber rings with participation p of arryl radicals and HCO H (Figure 16) proceeds p in severaal stages.

Figure F 16. The schem me of formation of PAH H with five-member rings r during innteraction of aryl radiicals and C2


The further growth of molecule of PAH with five-member rings (up to formation of a molecule of fullerene) goes with participation of molecules of acetylene and aryl radical. The analysis of the structure of HCO bands obtained using the method of pulse photolysis has shown that HCO radical in the main state has a nonlinear structure with HCO angle equal to about 1200 [103]. Nonlinearity of HCO radical creates a prerequisite for formation of polycyclic structures having not a linear, but a curved structure, thereby increasing the probability of participation of these structures in the process of formation of nuclei of fullerenes, not soot particles. Further, the scheme of reaction is presented. The above reactions are thermodynamically favorable due to energy evolution ∆ HH2O =-241.989 kJ/mol [111] during formation of molecules of water. It should be noted that reactions of free radicals with molecules differ in a low activation barrier. The experiments [112] show that the value of activation energy of these reactions varies in the range from 0 to 60 J (14.35 kcal). At the same time the activation barrier height lo decreases with the increase in the thermal effect Q of this reaction; this regularity for different classes of reactions can be presented in the form: е0 = 48.185-1.05 Q kJ or е0 = 11.5-0.25 Q Kcal. Apart format the fact that particles must have sufficient energy for overcoming a power barrier, they must correspondingly be oriented in relation to each other. Therefore, an important role for formation of combustion products is played by a steric factor. The influence of a steric factor is considered in the expression for determination of bimolecular reaction rate [113]:

Z  pZ 0 e



C1C2 ,

where: p – steric factor; Z0 – a factor of collisions; Е – energy of activation; R – universal gas constant; C1 – concentration of particles of A1; C2 – concentration of particles of A2. A positive influence of a steric factor on formation of fullerenes in our experiments is considered below. During combustion of hydrocarbon flames at a low pressure, oxides of fullerenes are formed significant amounts. Mass spectroscopic researches of the obtained by us fullerene containing soot have shown the presence of oxides of C70O, C94O C60O, C60O2 C60O3 fullerenes and the presence of higher fullerenes (C74, C78, C90, C94) [114, 115]. The results of identification by fullerenes on mass spectra are given in Table 1. When investigating the first batch of samples, 50 ml of benzene extract of a soot sample were added to 1 ml of acidified acetonitrile. For the second batch samples three series of experiments were conducted: – the sample was diluted with benzene with addition of acetic acid; – 50 ml of the a sample were added to 1000 ml of acidified methanol; – 50 ml of the sample were added to 1000 ml of acidified benzene then 600 ml of methanol were added. In our opinion, the main role in the process of formation of PAHs with five-member rings is played by oxygen-containing radicals (H- С˙=O, H- С˙=C=O) as they are very active and reactions with their participation have a low activation barrier. At interaction of these radicals energy is emitted that increases the probability of procedure of these reactions. Formation of oxides of fullerenes can be explained by participation of oxygen-containing radicals in the mechanisms of formation of nuclei of fullerenes (Figure 17). This scheme of education to an oxy-PAH with five-member rings as the possible centers nucleation of fullerenes oxides visually illustrates an important role of spatial orientation of the interacting particles. 52

Figure 17. The schem me of formation of oxxi-PAH with five-mem mber rings Table 1 Results of thee identification of fulllerenes by mass specctra Form mula of substances

Mass according too calculating, a.e.m m

Mass accordding to spectra а.e.m.

First batch of exaamples С24


2 288.3

С16Н10 (pyren, fluoranten)


2 202.4

С18Н10 (cyclopentapyren,, benzofluoranten)


2 226.4

С20Н10 (corannulen)


2 250.3



8 856.7



8 842.6



9 936.8







Second batch of exxamples С60О


7 736.5



7 768.5



7 722.5



8 888.6








Let us relate the proposed mechanisms with results of our researches of the process of fullerene formation, when applying electric discharge [106]. When applying electric discharge to the flame, depending on the distance between electrodes, different yields of fullerenes [100] were observed. At interelectrode distance equal to 1 cm the smallest yield of fullerenes is observed as in this case the electric field shields and complicates diffusion of H and OH atoms from the reaction zone to the zone of power preparation of raw materials, thereby, reducing the number of aryl radicals necessary for formation of PAH with five-member rings. Besides, the main action of discharge is directed to the blame area where there are no HCO, HCCO radicals as their most part is concentrated in the middle zone of the flame and, owing to it, there are no prerequisites for formation of PAH with five-member rings according to the proposed schemes (Figure 16, 17). At the interelectrode distance of 18 cm the influence of electric discharge on the processes taking place directly in a reaction zone of the flame where chemical reactions with participation of radicals proceed is weakened as a result of shielding of influence of the discharge by the charged particles localized on the flame surface. At the electrode location in the middle part of the flame (interalectrode distance of 4 cm) it is in the flame and influence of the discharge intensifies reactions according to the proposed schemes of formation of PAH with five-member rings and the further growth of the latter to fullerenes. A positive influence of electric discharge is in the temperature increase and increase in concentration of the required charged radicals and free atoms in the flame volume due to collisions with fast – moving electrons. In the electric field, the charged particles having the dipolar moment are oriented according to the direction of force lines of the electromagnetic field. It increases the importance of the steric factor which is of great importance formation of fullerenes. The experimental results obtained at different pressures do not contradict the proposed mechanisms [106]. Reduction of the total yield of fullerenes due to the increase in pressure of the system can take yield because of insufficient concentration of HCO radical necessary for formation of PAH with five-member rings the quantity of which strongly depends on pressure. In favor of the mechanism with participation of HCO and aryl speaks the fact the fast that at 54

addition of inert gas to the flame, the number of HCO radicals increases, and dilution of the system with inert gas (argon, helium) is an indispensable condition of formation of fullerenes. In this work, it has experimentally been stated that under the conditions of on fullerene forming benzene – oxygen flame action of external acetylene – oxygen flame the total yield of fullerenes increases. A positive influence of an acetylene flame is not only in temperature increase of the reaction zone of the basic benzene – oxygen flame and in increase the degree of ionization, but also in the additional introduction of HCO and HCCO radicals which are formed during combustion of acetylene. At the same time the total concentration of these radicals in the reaction zone of a benzene flame increases owing to that, the probability of formation of PAH with five-member rings increases. The understanding of ways of formation of nuclei of fullerenes in the flame will allow not only to increase the yield of fullerenes, but also to control the process of synthesis of different nanostructures in the flame. Here we have only made the assumption on possible ways of formation of PAH molecules with five-member rings in the flame, and also about the role of conditions of combustion, initial and intermediate products of oxidation in the process of formation of fullerenes nuclei. Control questions for self-examination 1. Explain the mechanism of formation of soot particles and fullerenes. 2. What kind of allotropic modifications of carbon do you know? 3. What methods of synthesis of fullerenes do you know? 4. What are the distinctive stages of soot formation? 5. Explain the mechanism of formation of fullerenes and soot particles in the flame. 6. Explain the impact of external effects on the local processes of combustion products formation. 7. Explain the scheme of formation of oxy-PAH with five-member rings. 8. Explain the scheme of formation of polycyclic aromatic five-member ring structures during interaction of aryl radicals and HCO. 9. Explain the scheme of formation of PAH with five-member rings during interaction of aryl radicals and C2. 10. Consider the effect of local influence of external acetylene-oxygen flame on the mass yield of fullerene C60 during combustion of benzene – oxygen mixture.



FORMATION O OF THE SUPERH HYDROPHOBIC C   SOOT SURFACE IN THE FLAM ME  2.1. Hydrophobiic and hydrophiliic properties of materials m In the most vario ous fields of enginneering there is an a imperative need d in waterproof and water-repellennt materials (a wiind-screen of planes and cars, binocculars, field-glassees, cases of torped does, hulls of subm marines, etc.). There are no abssolutely hydrophoobic substances ev ven the most hydrrophobic, hydrocarrbon and fluorocaarbon surfaces adssorb water. In this regard, water reepellency is conssidered as a smaall degree of hydrrophilicity as betweeen molecules of w water and any bod dy to a greater or lesser extent there taake place intermoleecular attractive fo orces.

Fig gure 18. Hydrophilic (a) ( and hydrophobic ((b) surfaces in a three-phase system waterr – a solid body – air; 1 – water; 2 – solid bbody; 3 – air; Ɵ – wettting angle [116]


The value of the binding energy of water molecules with the surface of a body serves as the common measure of hydrophilicity. Hydrophobicity and hydrophilicity can be estimated by spreading of a water drop on the smooth surface of a body (Figure 18) [116]. A drop on a hydrophilic surface spreads completely, on hydrophobic – partially, and the angular value between surfaces of a drop and the moistened body depends on how much this body is hydrophobic. All bodies in which intensity of molecular (atomic, ionic) interactions is great enough are hydrophilic. The hydrophilic nature of minerals with ionic crystal lattices (for example, carbonates, silicates, sulfates, clays, etc.) as well as silicate glasses [117] is especially sharply expressed. Water repellency and hydrophilicity can be estimated, as well as wettability of a surface with water (in the air medium) by the wetting angle value θ: for hydrophylic surfaces θ < 900 (for absolutely hydrophylic surfaces θ = 0); for hydrophobic surfaces 900  l g , there will take place the complete wetting. If

( s  g   s l ) is commensurable with  l g , the liquid will spread restrictedly on the surface. In case, when  s g <  sl , non-wetting of a solid body by a liquid is observed as emergence of a surface (a solid body – liquid) in the system instead of (a solid body – gas) would lead to the increase in the free energy of the system. In this case, a liquid drop ideally must take a spherical shape and contact with only a solid surface, but actually a drop of liquid is affected by gravity, flattening it a little therefore, the drop form is only close to spherical. 69

The degree of wetting or the wetting angle depends on a ratio of the surface tension values on borders: air – a solid body  s g , liquid – a solid body

 sl and air – liquid  l g .

Under equilibrium conditions this dependence is expressed by the equation of Young [120]:

 sg   sl   l g  cos .


The degree of humidification of a solid body is described by means of a contact angle θ, entering a formula with the surface energy σ on different interfaces according to the law of Cassie:

cos 

 s g   sl .  lg


From equation (5) it follows that if  s g >  sl ; cos >0; θ< 90 0 – wetting is complete or partial; if  s g <  sl ; cos 90 0 – non-wetting. Superhydrophobic materials have surfaces which are extremely not wettable (with the angle of contact with water exceeding 150 °). Many of similar materials found in nature obey [121] law of Cassie and are two-phase at the submicronic level, one of components being air. The effect of a lotus is based on this principle. A drop of water which fell on a leaf surface remove a particle of pollution form it particles do not penetrate inside the drop but are uniformly distributed on its surface. It is revealed that a hydrophobic substance is removed by a water drop from a hydrophobic surface. When considering the conditions of the lotus – effect at the nanoscale level, the mechanism of this phenomenon becomes clearer. With the help of Cassie's law it is possible to explain why the value of a contact angle for a surface, and, consiquently the condition of non-wetting can be changed changing the surfaces so that it has a size relief [121]. Let us imagine a massage brush on the teeth of which there lies piece of paper which represents a particle of 70

pollu ution. A spot of «dirt» is located only at the tops of teeth, not contacting with a bru ush surface, Figurre 26 b. The adheesive force of «dirtt» is conditioned by a surface areea of the mutual contact. If a surfaace is smooth or macrorelief, m in Fiigure 26 a one can n see that the contact area is significcant, and «dirt» iss held rather stron ngly. Because of th he sharp ends of teeeth the contact aarea is minimum, and «dirt» as if «h hangs on a leg». The T same happens also to a water drrop. It cannot «spreead» on edges and a therefore seeeks to be curtaileed in a ball, Figu ure 26 b.



Figure 26. The T position of a dropp of water on a – macrro and b – nanosurfacce [121]

According to Caassie [121] the prrotective water repelling propertiies of waterfowll feathers are geenerally condition ned by their special rib structure, not by existence of protective fat-like f substancees on feathers wh hereas in case wiith the surface off a lotus leaf thesee properties only y supplement eacch other. Water bugs b – water strid ders known by theeir ability to movve easily on the water w surface, also use this natural phenomenon. p The body and tips off legs of these inseccts are covered with w the hairs whhich are not wettaable in water prov viding them with these t surprising abbilities. As the lotus – efffect is based onnly on the physico o – chemical phen nomena and properties of plants aand is not coeditied only by a live system; it is tech hnically to producce not – wettablee surfaces for vario ous materials. In this regard, intennsive researches on the developm ment and producttion of resistant to pollution non n – wettable surfaaces and covering gs are carried out nnow. 71

Nanotechnologies based on the «effect of a lotus» are most widely used in motor – car industry, construction, production of protective cloth and in some other branches. A covering on the basis of titanium dioxide nanoparticles with the sizes of 20-50 nanometers and polymer binding is developed for this purpose [122]. This covering sharply reduces wettability of a surface with water, vegetable oil and alcohol solution. Hydrophobic properties of a surface, its wettability with this or that liquid depend on a wetting angle  . At the complete wetting, the wetting angle is equal to zero, at absolute unwettability it is equal to 180°. Water repellency of a surface is defined by angle  between a solid surface and a tangent line at the point of contact of phases. The energy of a drop of water retention on a surface is equal the product of work of adhesion by the contact area Wa S . Energy of separation is

proportional to gravity P   / 6Dê3 g , where Dd is diameter of a drop,  – liquid density. In [122, 123], it is shown that wetting angles larger than 1500 confirm a superhydrophobic behavior. Until recently it was considered that soot particles which are formed in the combustion chamber of aviation engines are hydrophobic, but in the course of their interaction with gases in an exhaust stream there take place modification of the surface properties of particles, and they begin to absorb vapors of water [124]. Recently the statement about initial water repellency of soot particles is questioned because usually in experimental investigations on hygroscopic properties of soot particles the researchers used either soot obtained during combustion of hydrocarbons and natural fuels in a diffusion torch or commercial soot, or soot created on graphite rods [125, 126]. At the same time, conditions of soot formation in the combustion chamber of a jet engine significantly differ. As hygroscopicity of soot particles is considerably defined by the nature of the surface, the physical and chemical characteristics of which depend on soot formation conditions, it is clear that different samples of soot can significantly differ in structure and reactivity. On the basis of silicon dioxide nanoparticles (quartz, silicon dioxide) about 40 nanometers in size, colloidal solutions for treatment of cloth are developed (Figure 27). 72

Figure 27. A drop of water on cloth treated with a hydrophobic composition [118]

This substance in the form of nanoparticles assumes new properties, in particular, a high surface energy, that allows SiO2 particles, colloidal solution dries out, to stick strongly to various surfaces, first of all, to glass which is similar in composition to them forming a continuous layer of nanosize protrusions. If we immerse a suit from this material into water, having taken it out, we can see that it is absolutely dry. And if we water this cloth from a hose, it is possible to notice the absence of any trace of liquid on the cloth, (Figure 28).

Figure 28. A water jet does not wet the surface of cloth


Besides, it is noted that this effect, though not eternal, is well conserved after mechanical manipulations with the material. Owing to the nanosize thickness, such coverings are absolutely invisible, and thanks to bioinertness of silicon dioxide are harmless for people and the environment. They are resistant to ultraviolet and withstand temperatures up to 400° C, and the action of water-repellent effect lasts for 4 months [118]. The increased humidity is the main reason for destruction of buildings and constructions. Hydrophobic waterproof materials proved to be able to cope with this problem best of all. These products provide the bases from porous materials: concrete, brick, plaster, gypsum, asbestos-cement, tile with the effect of non – wetting with water. Besides, thanks to water repellents, frost resistance and corrosion resistance of concrete and reinforced concrete constructions, especially in ocean water and other corrosive media increase. Hydrophobic compositions are used for coating of external surfaces with the aim to decrease water absorption of the latter and increase their frost resistance and heat stability. They are coated onto the finished products for protecting them from humidification, when storing them in factory warehouses, when transporting products to the building sites and at their operation in construction. 2.2. Modeling of the wetting angle for liquid which is in contact with a rough surface Superhydrophobic and self-cleaning surfaces must possess both high wetting angle, and low wetting angle of hysteresis. Liquid can also form a homogeneous interface with a solid body or a composition interface with air pockets between a solid body and liquid. In this section, mathematical models in which the relations between roughness and the wetting angle are will be considered. (a) Homogeneous interface (Wenzel) Let us consider a rough solid surface with a usual size roughness which is less than the size of a water drop as it is shown in Fig. 29. For a drop of water which is in contact with a rough surface without 74

an aiir pocket referring g to a homogeneoous interface, the wetting w angle is presented as follow ws (7) cos  R f co os 0 , wherre  is wetting ang gle for a rough suurface, 0 is wettin ng angle for a smoo oth surface, and Rf is a roughnesss factor which is defined as a ratio o of the area of a solid body – liquuid АSL to its pro ojection to an flat surface, s АF. (8) R A /A . f



Dependence of the t wetting anglee on a roughnesss factor of is preseented in Figure 29 b for the differrent sizes of 0 on n the basis of equaation (7). The model pred dicts the followinng: the hydroph hobic surface (090) becomes mo ore hydrophobic w with the increase in Rf and the hydrrophilic surface ( ( 090) becomees more hydrophilic with the increease in Rf [119]. (a)



Figu ure 29. (а) schematic representation of a drrop of liquid which is in contact with (i) a smooth solid surface (wetting angle, 0) annd (ii) a rough solid surface (wetting an ngle, ), (b) wetting anngle for a rough surface () as a function off a roughness facttor (Rf) for various wetting w angles of a sm mooth surface (0) andd (с) schematic repreesentation of pyramiddal roughnesses with a hemispherical top, with w a complete packing (with the finished packing) (Noosonovski and Bushann) [119]


As an example, in Figure 29 c the geometry with pyramidal roughnesses which have a hemispherical top and which are completely packed is shown. The size and form of these roughnesses can be optimized for the described roughness factor. (b) Composition (Cassie – Baxter) interface. For a rough surface, the wetting liquid can be completely absorbed by rough hollows on the surface while non – wetting liquid cannot get into surface hollows leading to formation of air pockets, forming a composition interface a solid body – liquid – air as it is shown in Figure 30 a Cassie and Baxter (1944) expanded (developed further) the Wenzel equation which was first created for a homogeneous interface a solid body – liquid for a case of a composition interface. For this case, there are two types of interface: a surface a solid body – liquid with the external environment which surrounds a drop and the composition interface including liquid-air and a solid body – interface air. To calculate the wetting angle for composition interfaces, the equation of Wenzel can be modified, having united the fractional area of wet surfaces and fractional area by air pockets ( = 1800): cos  = Rf cos 0 – LA (Rf cos 0 + 1),


where LA is the fractional flat geometrical area liquid-air of interfaces under a drop. According to equation (9), even for a hydrophilic surface, the wetting angle increases with the increase of LA. At high value of LA the surface becomes hydrophobic; however, the necessary value can be unattainable or formation of air pockets can be unstable. The Use of the equation of Cassie – Baxter, value of LA at which the hydrophilic surface could transform into hydrophobic, is presented as follows (Young and Bushan, 2006): LA  Rf cos 0 / Rf cos 0 + 1 for 0  90о.


In figure 30 b LA value of the condition as Rf function for four surfaces with different wetting angles 0 is shown. Hydrophobic surfaces can be obtained at a higher value of LA as it was predicted 76

in eq quation (10). The upper part over eaach line of the weetting angle is a hy ydrophobic area. For a hydrophobbic surface, the wetting w angle increeases with the inccrease in LA bothh for a smooth, an nd for a rough surfaace. (a)


Figure 30. (a) schematicc representation of forrmation of a composittion interface a d body – liquid – air foor a rough surface andd (b) LA condition foor a hydrophilic solid surrface to become hydrophobic as function oof a factor of roughnesss (Rf) and 0 ( (Young and Bushan, 22006) [119]

(c) Hysteresis of the wetting angle The hysteresis of the wetting anggle is another im mportant characteeristic of an interfface a solid bodyy – liquid. If a drrop settles on the tilted t surface (Figu ure 31), the wettinng angles ahead of o and behind the drop d correspond to t the increasing and decreasing wetting w angle, respeectively. The incrreasing angle is laarger than the decreasing angle that leads to a wettin ng angle hysteressis. The hysteresiis of wetting angle takes place because of the roughhness and heterog geneity of the surfaace.

Figure 31. The tilted surface profile (sloppe angle, ) with a liqquid drop; the increasing and decreasing wetting anngle inc and dic, resppectively


Though, for rough surfaces which are carefully controlled on a molecular level it is possible to obtain hysteresis of the wetting angle, equal to 10 or less (Gupta, et al; 2005), it is impossible to remove hysteresis completely as, automatically, smooth surfaces possess a particular roughness and heterogeneity. The hysteresis of the wetting angle is a measure of energy spattering during running off of a drop along a solid surface. Surfaces with hysteresis of a low wetting angle possess a very low angle of running off of water, this is an angle at which a surface must be tilted for water to run off (Nosonovski and Bushan, 2007, 2008, Bushan and Young, 2008). The low angle of running off of water is important when using a fluid flow, in micro/nanochannels and surfaces with a self-cleaning surface. Nosonovski and Bushan (2007) obtained the ratio for a wetting angle hysteresis in the form of function of roughness which has the following form: cos  a 0  cos  r 0   sin  cos  r 0  cos  a 0 . Rf 2R f cos  0  1

 adv   rec  1  f LA R f 

 1 f  LA


For a homogeneous interface LA =0, while for a composition interface of LA is not a zero number. It is observed in equation (11) for a homogeneous interface, the increasing roughness (high Rf) leads to the increase in hysteresis of the wetting angle (high adv – rec), while for a composition interface, approach to unity of LA gives as a hysteresis of both a high wetting angle, and a low wetting angle. Therefore, the composition interface is desirable for superwater repellency and self-cleaning. (d) Stability of a composition interface and the role of a hierarchical structure Formation of a composition interface is the phenomenon of multiscaliness which depends on the relative sizes of a liquid drop and peculiarities of roughness. Composition interface is metastable, its stability is important. Though, it can be quite geometrically possible for a system to become a composition one, for liquid to get 78

inside hollows between roughnesses and form a homogeneous interface it can be energetically favorable. The composition interface is friable and can be transformed irrevocably to a homogeneous interface, thus, preventing formation of super water – repellency. To form a steady surface with air pockets between a solid body and liquid, it is necessary to avoid destabilizing factors, such as capillary waves, condensation of nanodrops, inhomogeneity of a surface, and fluid pressure. For high LA, a nanopattern is desired as every time when the interface liquid-air is formed it depends on a distance ratio between two adjoining roughnesses and radius of a nanodrop. Moreover, nanoscale roughnesses can chop off liquid drops and, thus, without allowing liquid to fill hollows between roughnesses. High Rf can be obtained by both micro- and nanopatterns. Nosonovski and Bushan (2007) studied destabilizing factors for a composition interface and found that the convex surface leads to formation of a steady interface and high wetting angle. Also, they asserted that the influence of a drop weight and curvature are also factors which influence transition. Nosonovski and Bushan (2007) showed that combination of micro- and nanoroughnesses with a convex surface can help resist destabilization by fastening an interface. This also allows to prevent filling of cracks between roughnesses, with liquid even in case of hydrophilic material. The influence of roughness on wetting depends on scales and mechanisms which lead to destabilization of a composition interface, as well as on scales. To effectively withstand this scale – dependent mechanisms, it is expected that the multiscale roughness will be optimum for hydrophobicity. (е) Ideal surfaces with a hierarchical structure For stability of a composition interface, Bushan and Young (2008) proposed a structure of an ideal hierarchical surface which is shown in Figure 32. Roughnesses must be rather high so that drop did not concern hollows. As an example, for a structure with the repeated columns, it is necessare to use the following relation for a composition surface. For a drop with a radius about 1 mm or more, values H, D, P about 30, 15 and 130 m, must be optimum, respectively. 79

Nanoroughnessess can fasten an intterface liquid-air and a not allow the liquid to fill ho ollows between roughnesses. Th hey are also uired for substratting nanodrops w which can be condensed c in requ hollo ows between hig gh roughnesses. Therefore, nano oroughnesses mustt have a low throw w (platform) to coope with nanodrop ps, less than 1 mm is lower than seveeral nanometers oof a radius.

Figu ure 32. Schematic reppresentation of a struccture of an ideal hierarrchical surface. Miccroroughnesses consisst of round (repeated) columns with a diam meter D, height, H and a platform P. Nanoroughnesses N consist of pyramidal nanooroughnesses with h a height, hand diam meter d with the roundeed- tops (Young and Bushan, B 2008)

Values h and d about 10 and 100 nanometers can c be easily madee, respectively. 2.3. Synthesis of hydrophobic sooot in a flame Production of coaatings with hydroophobic propertiess is an actual direcction of research. Currently, many open publication ns of different scien ntific centers [127 7-134] are devoteed to creation of such s coatings and study s of their prop perties. Hydrophoobicity of the surfaace is directly related to nanoscale su urface roughness. New methods of synthesis and meth hods for determining the characteriistics of the materrials obtained are being b developed. To change wettiing of the surfacee, amorphous carbo on is often used [127]. Methods oof rapid deposition n of a carbon layerr are proposed by I.K. Puri [128]. T This method of syn nthesis allows to ob btain large aggreg gates consisting off separate particless (nanobeads) with sizes ranging fro om 20 to 50 nm [129], but syntheesis of larger partiicles is possible. This T method used combustion of acetylene flame (700 00 C) and a catalysst – nickel [130]. For creation of a carbon, layer silico on plates were useed as a substrate. A silicon substratee was exposed to flame f at a distan nce of 10 mm at different exp posure times. 80

Irrespective of the method of processing the surface facing the flame, nano-structures synthesized on carbon are stated to be identical [128]. The size and composition of the formation regions of carbon nanoparticles called by the authors nanobeads depend on exposure time (2.5, 5, 7.5 min), (Figure 33). It is shown that the radius of the region of nanoparticles with special hydrophobic properties decrease with the increase of exposure time. Thus, for exposure times of 5 minutes the first flame zone has a border of 23 mm, and for 7.5 minutes – 20 mm.

Figure 33. Carbon nanostructures obtained on silicon substrate after 5 minute exposure to the flame [10]

The closer to the center of the disc, the easier larger carbon chain structures – nanobeads are formed. Surface architecture renders a profound effect on the wettability [130]. It is known that hydro-phobicity of the system primarily depends on the kind of the surface coating – its roughness. Thus, a thin layer of carbonaceous particles (one dimensional beads of soot) provides a very low degree of surface roughness. Pozzato et al. [131] have found that when water droplets form very large wetting angles (1500), hydrophobicity of particles allows a drop to run off form the surface easily. TEM images for Si substrate subjected to treatment for 5 minutes show that carbon nanoparticles (nanobeads) are bound to each other. As the deposition process for nanobeads not catalytic, it was supposed that growth is the result of fuel pyrolysis in a gaseous phase. The pyrolysis products are transported in the layer adjacent to the relatively cool surface of Si, where they 81

cond dense and form nanobeads. Pyrolyssis is decisive in formation of carbo on, while oxidatio on takes place beyoond the beyond th he flame. Sharon et al. [132], who are am mong the first reesearchers of synth hesis of carbon nanoparticles, n havee shown that it iss desirable to havee a source of carbo on, which containns a combination of o sp2 and sp3 2 binding structure. Theey showed that soources with sp orrbital, such as graph hite, hinder form mation of sphericcal structures. Sp pongy carbon nano ostructures have been synthesizedd by pyrolysis of o vapors of camp phor at 10000 С in n the atmosphere oof argon. The cataalyst used was ferro ocene. Hard and hollow h carbon nannoparticles of two o sizes ≈ 250 nano ometers and 500-8 800 nanometers weere obtained. They y consisted of amorrphous layers thatt have been coatedd with graphite sh hells. Spectral analy ysis of electron en nergy loss (EEL) hhas shown that thee internal part of th he particles (beads) was amorphous.. X-ray analysis off the different specttra shows that thee peripheral part of nanobeads contaains 98.1% of carbo on by volume [132 2]. One of the mateerials with low suurface energy, which w is often used d to modify and co ontrol the surface for wettability arre amorphous carbo on films (AC) [12 27]. In [133], for obtaining a superrhydrophobic layerr consisting of homogeneous h carrbon nanobeads, a method of rapid d deposition on Sii substrate placed in ethylene-air diffusion flame was used. The obtain ned carbon particlles of nanobeads morphologically y are similar to carbon c nanopearl synthesized by Levesque L and his coworkers c [129] with w the help of diissociation of acettylene at 700 0 C on n nickel catalytic nanoclusters.

Figure 34. A schem me of the burner for ssynthesis of hydrophoobic soot on a substrate [1223, 128]


In work [134], th he authors determiined the optimal conditions c for form mation of a hydrop phobic surface onn silicon substratee coated with ferro omagnetic fluid ass well as on the m mesh or foil of sttainless steel, depeending on their loccation in the flamee. For hydrophobicc soot synthesis [123, 128] a burner device show wn in Figure 34 was w used. The process of hydrophobic sooot formation duriing diffuzion comb bustion of propaane-oxygen mixtuure was studied. Substrates – diskss diameter with th he of 7 cm and a thickness of 1-0,5 mm, made of different d materialls (silicon, nickeel and stainless steel) were instaalled over the flam me at a height of 3.2 cm from the burner b matrix and soot deposited up pon them. The exxposure time was varied in the range of 2-10 minutees. Investigationss on the influencce of electric field d on formation an nd properties of tthe soot surface being b formed weree carried out.


b Figure F 35. a picture off the process soot depoosition on the substratee (a) and the temperature profiile along the height of tthe flame C3H8 / O2 (b)) [123]


The figure shows the process of soot deposition on the substrate (Figure 35 a), and shows the temperature profile (Figure 35 b) in the middle part along the height of propane-oxygen diffusion flame at the consumption rate of propane – 150 cm3 / min, and that of oxygen – 260 cm3 / min. Flames can be visually divided into two zones (Figure 35 b). The first one is a blue zone which gradually turns to a red zone. The height of the blue zone is about 1 cm. In this zone, the temperature is equal to 770 K. At a distance of 1 to 2 cm from the burner matrix there takes place a sharp rise in temperature and in the range of 2 to 2.5 cm its stabilization from 2.5 cm the temperature rises to the maximum value of 1470 K at a distance of 4 cm from the burner matrix. Then a gradual fall of temperature is observed. It was found that when the exposure time is less than 4 minutes, ordinary soot is deposited on the substrate. Exposure time more than 4 minutes results in formation of soot with hydrophobic properties, and the increase in exposure time more than 10 minutes has no effect on the improvement of these properties.

Figure 36. A soot sample produced in the oxygen-propane diffusion flame on the silicon substrate [17]: 1 – the central gray zone; 2 – brown zone; 3 – black sooty outer zone

On the surface of the substrate, there takes place deposition of soot with the average thickness of 1-1.5 mm. Figure 36 shows a sample obtained using a silicon substrate. On the substrate surface, separation of the deposited soot into three zones can be visually observed. It is seen that the central gray area (1) is surrounded by a brown zone (2) which, in turn, is covered by black sooty outer zone (3). Zonal depositions of soot particles on the surface of silicon substrate suggest that in different 84

regions of the flame, soot particles with different properties are formed. The main cause of this phenomenon is the non – uniformity of the temperature gradient in the volume of the flame. Also, this dependence can be explained by the limitation of the oxidant diffusion into the flame: the middle part of the flame is richer in fuel than the edges. To study the properties of the soot samples for hydrophobicity a sessile drop method was used. The picture of a water drop placed onto the soot surface was taken and the wetting angle was measured geometrically. Figure 37 shows pictures of to the water droplet deposited on a clean silicon substrate (a) and with the soot into three different zones (b, c, d).




d Figure 37. A drop of water on the hydrophilic and superhydrophobic surface: a – hydrophilic; b, c – hydrophobic; d – superhydrophobic surface


It is stated that the placed on a clean silicon substrate water drop (Figure 37 a) has an outer contact angle of 500, this indicating the hydrophilicity of its surface. Figures 37 (b, c, d) show pictures of drops on gray, brown and black sooty areas. Consequently, the angle of the gray zone is 1350 С, brown zone – 1550 С and external sooty areas – 1450 С. The data obtained show that the soot deposited in the central brown zone has better hydrophobic properties compared with the soot deposited in the central and the outer zones. The carried out investigations [130] show that hydrophobic properties of the surface of soot are developed due to the presence of carbon nanobeads in the composition of soot. Electron microscopic images of soot, taken from the substrate surface showed the presence of nanobeads the concentration of which prevails in a brown zone. The average size of soot particles increases from the center to the edge. To gain full information on the structure and properties of the resulting soot on a silicon substrate, Raman spectroscopy was used [123]. For the research, samples of carbonaceous depositions were taken from the plates that have most clearly expressed zonal structure. The obtained spectra are shown in Figure 38. From the Raman spectra (Figure 38, a, b, c) it is evident that there are several carbon modifications in the obtained samples. In the first and second zones, two carbon modifications were found – 1350 cm-1 (D – amorphous) and 1590 cm-1 (G – graphite), which were also observed in [129]. Raman peaks near 1470 cm-1 in Figure 38 (a, b) correspond to typical peaks of nanospherical structures to which nanobeads can be referred. In the third zone one could observe the presence of only two peaks – 1350 cm-1 (D – amorphous) and 1590 cm-1 (G – graphite) that corresponds to an amorphous carbon phase (Figure 38, c). The results of radiographic studies of soot samples on DRON-4 (CuKα radiation, U = 40 kV, I = 40 mA, the angle range 5 – 60о 2θ, step 0.05о 2θ, time measurement – 2 ° C / min) are shown in Figure 39 and in Table 2 [135]. The substrate on which the monocrystal of silicon (400) that at small and medium angles of diffraction is free of interfering lines. For small diffraction angles (zone 1), there is a diffuse line with d = 10.56 Å, which can be referred to Graphite nitrate (ICDD № 74-2328 card). 86



c Figure 38. Raman spectra of a soot sample obtained on a silicon substrate: a – the central gray area (1); b – brown zone (2); c – black sooty outer zone (3)



The main phase of all samples is graphite, represented by two broad, diffuse diffraction lines. The first line is asymmetric; its small-angle side is flatter than the other. This may be due to the fact that it can be imposed by likes of other disperse carbon modifications. Therefore, when calculating the crystallite sizes, a symmetrical form of the diffraction line (002) was displayed.

Figure 39. Diffractograms of soot samples from zones 1, 2, 3, and Σ of the substance from three zones [135] Table 2 Phases and sizes of carbonaceous particles dẴ 10.5642

Zone 1 Phase


Poss. Graphite nitrate –

Zone 2 Phase –

dẴ –

Zone 3 Phase –

dẴ –

1-3, sample 2 Phase –

3.4432 Graphite 3.4801 Graphite 3.4791 Graphite 3.6675 Graphite – – 2.1348 Graphite – – 2.1264 Graphite 2,0360 Graphite 2.0405 Graphite 2.0450 Graphite 2.0349 Graphite Crystallite sizes found using WinFit program in Fourier analysis mode on the line (002) L=2.4 nm. L=2.3 nm. L=2.3 nm. L=1.5 nm*. * Because of the low intensity of the line, graphite crystallite sizes may be underestimated

The values found for the parameters of hexagonal graphite crystalline lattice are as follows: Zone 1: c = 2.461 Å and a = 6.886 Å; Zone 2: a = 2.465 Å and c = 6.96 Å; Zone 3: a = 2.470 Å and c = 6.958 Å. Carbonaceous deposit of the whole plate: a = 2.451 Å and 88

c = 6.934 Å. These parameters differ from the reference values, especially in the direction c (reference values of the parameters are: a = 2.470 Å and c = 6.790 Å). Thus, according to X-ray analysis, the bulk of the substance obtained by increasing the exposure corresponds to soot formations having well-defined diffraction reflexes. The size of crystallites of the individual particles is 2.3-2.4 nm, that is significantly less than the transmission electron microscopy shows. Probably, carbonaceous individuals consist of amorphized mass containing more structured carbon phases. The change in the wetting angle depending on the contact time with the surface of a water drop in the different zones of the surface obtained was studied (Figure 40). The experimental data on measuring the wetting angle are shown in Table 3. Table 3 Time t , min 0 2 6 10 14

Contact angle, Time t ,  ,  degrees min 132 0 132 2 131 6 130 10 127 14

Contact angle, Time t ,  ,  degrees min 155 0 155 2 155 6 154 10 153 14

Contact angle,  ,  degrees 160 160 153 152 139

Zone 1






Zone 2






Zone 3






Figure 40. The fluid droplets placed on the soot surface in different areas at different times of contact of the surface with a water drop


Dependences of the wetting anggle on time are presented as graphs in Figure 41. These dependences of the wettin ng angle of a wateer drop applied to o the surface of soot on a silicon n substrate at diffeerent time exposu ures from 2 minnutes to 14 minu utes with an interrval of 4 min show s that the hyydrophobic prop perty of soot decreeases with time.

a – the central gray area a (1); b – brown zoone (2); c – sooty blacck area (3) Figure 41. The deppendence of the wettinng angle on in differeent zones

This occurs beccause wetting deepends on the efficiency e of redu uction of the surfface tension undder dynamic cond ditions. After apply ying a drop of waater it starts graduually spreading on o the surface of so oot and during ab bout 14 min the pprocess of mutuall diffusion of surfaactant molecules at the interface between the liq quid and the surfaace of soot ends.. During 14 minuutes of exposure, the wetting process gets stabilized and we observee a true hydropho obic property of th he surface. 90

2.4. Synthesis off hydrophobic sooot in a flame und der the effect of an electric field In [17], the auth hors carried out investigations un nder the conditio ons of the effect of electric fieldd on flame which h allowed to deterrmine the effect of o external influeence on the hydro ophobicity or hydrro-philicity of thee resulting soot suurface. Below is a picture of a burn ner device (Figurre 42) and a schheme of applicatiion of direct electtric field on the fllame (Figure 43). The experimentss were carried out on o silicon, nickel and stainless steeel substrates. The experimental data on application off an electric field aare presented in Table T 4. The following are a installation ((Figure 42) and a schematic diagrram of application n of an electric fieeld (Figure 43).

Fig gure 42. Picture of thee experimental setup w with application of ann electric field


b a – with a positive polarity, b – w with a negative polaritty Figure 43. Schem matic representation off the flow of the electrric field


Table 4 The parameters of the direct electric field parameters and experimental conditions

2 3 4



+ + -






Stainless steel

6 +




Current, μА


Voltage, V


Time exposure τ, min

Carrier gas flow rate, Q, cm3 / min



Consumption of gas, С3Н8. Q, cm3 / min



Plate type












































At the voltage of 1000 V and higher, the flame undergoes visual changes: it becomes brighter, periodic overshoots of spark discharge 92

are observed and on the substrate there takes place abundant formation of soot which is deposited in large quantities (Figure 44) in the form of dendrites. Without application of electric field the flame spreads on the surface of the substrate. With application of electric field the flame propagates in the form of a cylinder with a diameter of 1.5 cm till it touches the substrate. This phenomenon is expressed brighter when applying a negative polarity to the substrate. The flame as if adheres to it. At a positive polarity there appears a small gap between the upper front of the flame and the substrate. At a long period of treatment of the flame with electric discharge the entire space between the matrix and the substrate. It is filled with the soot formed. The surface becomes rough, i.e. non – uniform (Figure 45). The current increases to 700 μA. Application of an electric field contributes to the increase in the thickness of the carbon hydrophobic layer and a narrowing of the field of its distribution.

Figure 44. Abundant formation of soot at application of electric field

Figure 45. A rough soot surface at application of electric field


Irrespective of the material of the substrate and the value of the applied voltage, the same dependence of the properties of the resulting surface is observed. In the middle part of the substrate, in the diameter of 2.5 – 3 cm, a superhydrophobic surface with the wetting angle higher than 1700 is format (Figure 46). A drop of water hardly wets the surface, and without any restraint, runs off form it like mercury (Figure 46). 93

Figure 46. A piccture of superhydrophhobic soot surface obttained w application of an eelectric field with

Nearer to the edg ge of the substratee the hydrophobic properties of soot deteriorate and d there takes pllace an abrupt transition of hydrrophobic soot to hydrophilic soott with complete wetting with wateer. The transition n of hydrophobicc soot to hydrop philic soot is show wn in Figure 47.

Fig gure 47. A picture of superhydrophobic s sooot surface obtained wiith application of an electric field, annd the transition from m hydrophobic soot inn the center too the hydrophilic soott at the edge

Thus, it is stated [123], that byy choosing the parameters p of electtric field, it is posssible to control thhe properties of hydrophilicity h and hydrophobicity h off the soot surface being formed. 94

X – ray diffractometric investigations on soot samples obtained with the application of an electric field of different polarity were carried out on diffractometer DRON-4 (SuKα – radiation, U = 40 kV, I = 40 mA, range of angles – 560o 2.θ, step – 0.05o 2.θ, time of measurement -2 grad / min). The results are shown in Table 5 and Figures 48 and 49 [18]. As a substrate onto which the sample was coated, a mono crystal of silicon (400), which is free of interfering lines at small and medium diffraction angles was used. The sizes of crystallites were determined by program WinFit.1.2. It is stated (Figure 48 a) that the main phase of the sample with negative polarity on a nickel substrate is graphite presented by two broad, diffuse diffraction lines. The first line is asymmetric. This is due to the fact that the lines of other disperse modifications of carbon are imposed on its small angle side. Therefore, when calculating the crystallite sizes, a symmetrical form of diffraction line (002) was marked out. At positive polarity, the main phase of the sample obtained on the nickel substrate is also graphite (Figure 48 b) presented by two broad, diffuse diffraction lines. The sample contains a dispersed modification of carbon which can be referred to soot particles in the form of nanobeads.




Table 5 Phases and sizes of carbonaceous particles under the conditions of application of electric field


Zone 1 graphite lattice parameters


Zone 2

Zone 3

graphite lattice dẴ parameters

graphite lattice parameters

3.3921 а=2.474 Å 3.4582 а=2.483Å 3.4656 а=2,461Å 2.0501 с=7,048 Å. 2.0038 с=6,784Å 2.0375 с=6,931 Å 2.0501 _ Size of crystallites found using program WinFit in Fourier analysis mode Оaccording to the line (002) L=2.3 nm L=2.4 nm. L=2.3 nm. 3.5390 а=2,469 Å 3.4612 а=2,468Å 3.5718 а=2.440Å 2.0472 с=7,077 Å 2.0424 с=6,922 Å 2.7373 с=7.147 Å + Size of crystallite found using program WinFit in Fourier analysis sizes of П+ mode on the line (002)



L=2.3 nm L=2.4 nm L=2.2 nm 3.4656 3.4656 3.4656 а=2.460Å а=2,460Å а=2.460 Å 2.1222 2.1222 2.1222 с=6.923 Å с=6,923 Å с=6.923 Å 2.0277 2.0277 2.0277 _ Size crystallite found using WinFit program in Fourier analysis sizes of Оmode on the line (002) L=2,4 nm 3.4539 а=2.449 Å 3.4539 а=2.449Å 3.4539 а=2.449Å 2.0279 с=6,908 Å 2.0279 с=6.908 Å 2.0279 с=6.908 Å + Size of crystallite found using WinFit program in Fourier analysis sizes of mode on the line (002) О+ L=2,4 nm















b Figure 48. A diffractogram of the soot sample obtained on a nickel substrate in the middle zone, at negative (a) and positive (b) polarity


Figure 49 (a) shows the diffraction pattern of the soot sample collected from the middle region of the silicon substrate, obtained at negative polarity. The main phase of this sample is graphite presented by two broad, diffuse diffraction lines. The First line is asymmetrical, as the liner of other disperse modifications of carbon the content of which is somewhat less than in the sample obtained on a nickel substrate at negative polarity are imposed onto its small – angle side (Figure 49, a).



b Figure 49. a diffractogram of the soot sample obtained on a silicon substrate in the middle zone, at negative (a) and positive (b) polarity


At positive polarity on the silicon substrate in the middle zone, soot characterized by the presence of graphite presented by two broad, diffuse diffraction lines is synthesized (Figure 49 b). The sample contains a dispersed modification of carbon the content of which is also slightly smaller than in the sample obtained on a nickel substrate at negative polarity (Figure 49 a), but higher than in the sample of the soot obtained on a silicon substrate at a negative polarity (Figure 49 a). 2.5. Electron microscopic studies of hydrophobic soot samples obtained in the flame Electron microscopic studies were carried out for the soot samples with a maximum display of hydrophobic properties obtained on the silicon and nickel substrates with a clear division into zones [123, 135]. Synthesis of hydrophobic soot was carried out in propane-oxygen flame under the following conditions: propane consumption – 50 cm3 / min, oxygen consumption – 260 cm3 / min, the plate height over the burner – 2 cm, exposure time – 10 minutes. Electron microscopic studies were carried out on microscope JOL – 100CX with an operating voltage of U = 100 kV. The preparations were prepared by the method of dry preparation on copper-palladium grids. A silicon substrate. It is stated that in zone 1 the main mass of particles form aggregates. They are composed of rounded, flattened carbon particles with a visible size of 15-30 nm (Figure 50, a). By the pictures of micro-diffraction (Figure 51 b) one can judge about the prevalence of volume three-dimensional ordered carbonaceous (nongraphite) structures. The bulk of the material in zones 2 and 3 are aggregates of rounded, «melted» particles without division into individuals (Figure 50, b, c). The morphology of the aggregates is similar to that of high-molecular compounds. The apparent size «of the individual particles is 50 nm and more. Pictures of microdiffraction indicate greater concentration of carbon by the height of formations. The diffraction patterns characterize two – three dimensional structural ordering of the carbonaceous material, i.e. indicates the presence of a less expressed (than in the first zone) crystal structure. 98

It is stated [135] that carbon deposition on a silicon substrate have differences in the morphostructure of the deposited particles. In the first zone, the particles are closer to fused scales, and in the second and third zones, individuality of particles is displayed very weakly. Aggregates formed by globular particles, are more like high-molecular compounds.



c Figure 50. Micro pictures of hydrophobic soot samples on a silicon substrate a- the first zone; b – the second zone; c – the third zone 18

Microdiffraction patterns obtained from accumulations of particles of all three zones indicate the presence of a bulk structure (developed not only in a plane). Chemical reactions accompanying the combustion processdid not provide the deposition of a pure carbon layer. Comparing the results with the data of [130], we can 99

conclude that the «sooty zone» corresponds to soot formations of zone 1 and «nanobeads» correspond to carbon formations of zones 2 and 3.



Figure 51. diffractogram of a soot sample on a silicon substrate: a – the first zone; b – the second zone [135]

A nickel substrate. When using a nickel substrate, and a catalytic effect develops and the share of the non – amorphous carbon increases, which is manifested in the structural ordering of soot samples. The carbonaceous deposition obtained from all three areas is rather homogeneous. The main mass is presented by aggregates consisting of rounded flattened particles (Figure 52) [123, 135]. At higher magnification it is seen that carbonaceous particles have asymmetrical shapes. Apparent diameter of particle varies slightly in the zones: in zone 1 it is in average of 25-40 nm (Figure 52 a), in zone 2, long chains of individuals of 40-50 nm are formed (Figure 52, b); in zone 3, they are sintered into aggregates of particles of 30-50 nm (Figure 52, c). Microdiffraction pictures (Figure 53) show higher carbon concentration by the height of formations, but less structured than in the first zone. It is stated that the morphological structure of soot samples from all zones is sintered similar aggregates of flattened particles size of which slightly increases from the first to the third zone (Figure 52). In the central and middle zones the soot is present in the form of nanobeads. Aggregates formed by these particles have a three100

dimensional structural ordering as indicated by the picture of microdiffraction (Figure 53 b).



c Figure 52. SEM images of hydrophobic soot samples on a nickel substrate

а b Figure 53. Diffraction pattern of the soot sample black on a nickel substrate: a – the first zone; b – the second zone [135]


Electron microscopic photographs of the hydrophobic soot samples produced with application of direct electric field (negative polarity on the substrate) with voltage U = 1000 are shown in Figure 54.




Figure 54. Micrographs of the morphostructure of hydrophobic soot samples obtained with application of the electric field [135]

It is stated that the soot particles produced with application of an electric field are sufficiently homogeneous, their mass being mainly composed of flattened, rounded particles with sufficiently clear boundaries (Figure 54 a). The diameter of the soot particles is in the range of 30-40 nm (Figure 54 b, c). Cloud and film particles are relatively rare. In the central part of the substrate, when applying an electric field the diameter of the produced soot particles exceeds the diameter of the particles obtained without the influence of the field, and the concentration of nanospherical structures in the form of nanobeads increases [135, 136]. 2.6. Investigations on the interaction of surface-active substances with the obtained hydrophobic soot surface The modifying effect of anionic – sodium dodecyl sulfate (C12H25OSO3 Na), cationic – cetylpyridinium bromide (C16H33C5H5N Br) and nonionic surfactant – oxyethylated alkylphenol (C16H33C6H4O (CH2CH2O) 10H) on the surface of a silicon substrate coated with a layer of soot was studred. The surface was superhydrophobic with its wetting angle of 1500. For comparison, 102

hydrophobic surfaces such as Teflon, have wetting angles of 105 – 1100. On wetting of the carbon surface in the presence of surfactants of different nature it was found that when changing the surfactant concentration from 10-5 to 10-1, a one can observe the change of the surface properties from hydrophobic to hydrophilic. Ionic surfactants have a great modifying effect than nonionic ones. The difference in the modifying effect of ionic and nonionic surfactants, is probably logically explained by the difference in the degree of hydration of the polar groups of the surfactant. Figure 55 shows schematic images of ionic (DDS Na, CP Br) and nonionic (OP-10) surfactant:

Figure 55. Micrographs morphostructure hydrophobic carbon samples obtained from the superposition of the electric field [135]

Figure 55 Schematic images of ionic (DDS Na, C P Br) and non – ionic (OP-10) SAS on the soot surface. Naturally, it can be supposed that the used groups of sodium dodecyl sulfate ( SDS) and cetylpyridin bromide (C P Br) will have a greater affinity to water that the polar groups of oxyethylated alkylphonoe (OP – 10) presented by an oxyethylated chain (Figure 56). Using the experimental date, the dependence of the wetting angle on the logarithm of concentrations was plotted (Figure 56). 103

Figure 56. Deppendence of the wettiing angle on the logarrithm of the surfactant conncentration Table 6 Th he experimental dataa on the conditions oof surfactant interacttion with the hydrophobic soot surface Namee of conceentration DDS


OP 10 0%

С 0.0001 0.001 0.01 01 1 0.0001 0.001 0.01 0.1 1 0.0001 0.001 0.01 0.1 1

lgC -4 -3 -2 -1 0 -4 -3 -2 -1 0 -4 -3 -2 -1 0

Ө, degrees 131 116 96 20 114 106 104 45 112 109 103 58 -


CosӨ -0.65 -0.43 -0.10 0.94 -0.40 -0.27 -0.23 0.70 -0.36 -0.36 -0.23 0.52 -

Σ erg/сm m2 72.113 70.556 66.224 39.334 30.333 69.006 61.882 42.999 36.226 33.229 70.556 69.006 54.009 33.446 30.005

Wa, erg/сm2 25.24 40.22 59.62 76.32 41.436 45.13 33.102 61.642 45.158 47.651 41.659 50.859 -

t values of the surface tension of o surfactants On the basis of the and wetting angles th he values of adheesion energy werre determined depeendences of wetting on the logaritthm of the concen ntration were plottted (Figure 57).

Figure 57. A plot p of the work of weetting versus the logarrithm of the concentraation

As seen in Figuree 57, the greatest work of wetting was w observed for sodium s dodecyl su ulfate (SDS Na), tthat is in good ag greement with the data d on wetting off surfaces. Thus, m modifying carbon nized surfaces with h surfactants of different d nature, it is possible to achieve the requ uired degree of hydrophilization, h tthat can be used d for targeted regu ulation of surfacce hydrophobiciity. This can be used in nano otechnology, in prreparation of hemoosorbents, biosensors, etc. 2.7. Application of hydrophobic soot in textiles The obtained sup perhydrophobic ssoot was coated on to cotton cloth h mechanically an nd using the methhod of Obruchev. Figure 58 a show ws a drop of wateer placed onto a cclean, not well washed w cotton cloth h and figure 58 b shows a drop of w water on cloth thaat was treated in alcohol solution con ntaining the obtaiined superhydroph hobic soot. 105



Figure 58. A drop of water on a clean cloth (a); a drop of water on the cloth treated in 70% alcohol solution containing superhydrophobic soot (b)

It is seen that both surfaces initially possess a hydrophobic property but with time a drop of water get soaked into cloth: it takes a drop 15 min to get soaked into clean cloth and 30 min – into cloth treated with soot. The dynamics of the process of a water droplet soaking into washed cloth with time is shown in Figure 59. It takes a drop of water 1 minute and 15 seconds to soak into a clean, washed cloth. Beginning

45 sec

15 sec

1 min

30 sec

1 min 15 sec

Figure 59. The dynamics of a water drop soaking into washed cloth [137]


Figure 60 shows pictures of the behavior of a water drop on the surface of washed cloth treated in 70% alcohol solution containing superhydrophobic soot. Beginning

3 min

1 min

4 min

2 min

5 min

Figure 60 – A drop of water on the washed cloth treated with 70% alcohol solution containing superhydrophobic soot [137]

The surface of the cloth has a hydrophobic property with the wetting angle of less than 1500. After five minutes a water droplet starts getting soaked into cloth. Subsequent experiments on treatment of cloth were carried out using the known method of Obruchev, which is used to create a waterproof cloth. Beginning

5 min

10 min

Figure 61. A drop of water on the washed cloth treated by the method of Obruchev


A washed cotton cloth was treated with a soap solution and then placed in a saturated solution of alum (potassium alum – KAl (SO4) 2 • • 12H2O) for 5 minutes. The cloth had been dried at room temperature for 3 hours. After complete drying, drops of water were placed on its surface and using the method of photographing, observations on the absorption of water drops were made. The results are shown in Figure 61. As can be seen in the photographs, a drop of water is initially retained on the surface of cloth but during 10 minutes there takes place complete soaking of water into the cloth. Then, a washed cloth was treated in a soap solution into which 100 mg of the obtained hydrophobic soot were added. Then, the cloth was immersed in to a saturated alum solution for 5 minutes. After drying at room temperature for 3 hours, the cloth was tested for hydrophobicity by placing a drop of water onto its surface (Figure 62). Beginning

15 min

30 min

Figure 62. A drop of water on the washed cloth treated by the method of Obruchev with addition of soot having a superhydrophobic property [137]

The investigation results showed that the cloth treated by the method of Obruchev with addition of into synthesized superhydrophobic soot a soap solution, has an exclusive waterproof property which is preserved for a long time. The wetting angle of more than 1.700 indicates a superhydrophobic property of the obtained surface. After that, we had the task to determine how much hydrophobic soot is required for 3 cm of washed cotton cloth. For this experiment, we took 25, 50, 75, 100 mg of superhydrophobic soot, and then treated it by the method of Obruchev (Figure 63). 108

25 mg

50 mg

75 mg



100 mg  

Figure 63. A drop of water on the washed cloth treated by the method of Obruchev with addition of soot of 25, 50, 75, 100 mg having a superhydrophobic property [137]

The results showed that hydrophobicity of the cloth, does not depend on the amount of superhydrophobic soot. According these data, we can take the least amount of superhydrophobic soot for treating cotton cloth [137]. 2.8. Application of hydrophobic soot in the construction industry Also, the obtained superhydrophobic soot was added as a filter to the putty of the type T-37 in concentrations of 1, 2, 3, 4, 6, 8, 10, 15% by weight of the base material. A thoroughly mixed mixture was coated onto a gypsum board and dried for 5 days at room temperature. After five days, the samples were examined for water repellency by the method of a «sitting drop». The Studies have shown that the addition of soot improves hydrophobic properties of the filler to a certain limit, the maximum wetting angle (higher than 150 °) was observed when the concentration of soot was equal to 8%. 109

Figure 64 shows the behavior of water drops placed onto the surface of the obtained hydrophobic fillers. It is see that the increase in the content of carbon of from 1 to 8% results in the growth of hydrophobic properties of putty coating, the further increase in the percentage content of soot in the total volume of the mixture to 1015% is not expedient because a sharp decrease of both hydrophobic and basic properties of putty is observed [136]. 1%








Figure 64. A drop of water on the layer of putty coated onto the gypsum board [136]

However, at 10% concentration, the surface of putty loses its hydrophobic properties and begins to absorb water easily. Further increase of the content of soot up to 15%, and deteriorates the plastic and strength properties of coating, it contributes to cracking and loss of the main purpose, as a material for primary treatment and protection of plasters (Figure 64). 110

Studies have sho own that the use oof superhydropho obic soot as a fillerr allows to impaart water repellinng properties to not only the surfaace layer of putty y, but also its whhole volume that significantly imprroves protection against a the ingresss of moisture. In ntroduction of smalll amounts of soot s also increaases the hardnesss and wear resisstance of the coatiing. Besides, the oobtained hydropho obic filler has a hig gh adhesion to varrious surfaces andd high penetrating ability. Figure 65 showss a plot of the coontact angle of th he percentage content of the obtained superhydrophobbic soot in the puttty.

Figure 65. Depenndence of the wetting angle on the percentaage ratio of soot, %

Investigations on n the effect of aaddition of superrhydrophobic ymer used as a primer were carried d out. Acrylic soot in to acrylic poly ymer of the brand al B 30H was useed we studied thee dynamics of poly wateer absorption on th he surface of 1) cllean gypsum boarrd, 2) gypsum boarrd theated with acrylic a polymer, aand 3) gypsum board b treated with h acrylic polymer, into which 10% of superhydropho obic soot in a masss ratio was added.. The prepared sam mples were dried d for 3 days at room m temperature. The behavior of water w on the surfaace of prepared gy ypsum boards is sh hown in Figure 66. 6 The untreatedd surface absorbs water easily 111

(Figure 66 a), the surface treated with pure polymer absorbe water during 1 hour, water on the surface coated with acrylic polymer with addition of soot did not absorb water during several days.




Figure 66. Behavior of water on the surface of prepared gypsum boards [138]

Then, we used the obtained by us superhydrophobic soot production of hydrophobic sand [139]. The technology of production of hydrophobic sand includes several stages. In the first place, the surface of the sand is coated with an adhesive agent, the next step is treatment with a hydrophobic filler. This is followed by the curing process. We used ordinary washed river sand, polyurethane glue SD600 dissolved in ethyl acetate was used as an adhesive agent. The content of the adhesive mass is not more than 5% by weight of the hydrophobic sand. The adhesive layer is applied to the surface of sand by sedimentation of polyurethane film from the solvent. For this, the sand with polyurethane adhesive dissolved in ethyl acetate is subjected to intensive mixing, the volatile solvent evaporates and a nanosize polyurethane film is formed on the surface of sand. Then, 1% of superhydrophobic soot is added into the obtained in this way sand and the obtained mass is mixed with the rate of 60 r/s at 40900C for 30 minutes. While stirring, the surface of sand grains is enveloped by a nanosize film from the mixture of hydrophobic soot. The role of superhydrophobic soot is to increase the degree of adhesion of the hydrophobic film to the surface of grains, their hydrophobic properties and decrease the curing time. The obtained sand has an exclusive hydrophobic property. Figure 67 shows a picture of the behavior of water droplets on the surface of the obtained hydrophobic sand. The wetting angle of the water droplets is more than 150 degrees. 112

Figure 67. Drops of water on the surface of hydrophobic sand [139]




Figure 68. Dynamics of water absorption: (a) – ordinary sand, (b) – the sand with polyurethane, (c) – hydrophobic sand [139]

The characteristics of the dynamics of water soaking (Figure 68) into the original sand, in to the sand with coated on its surface polyurethane film and the obtained hydrophobic sand. The initially taken original sand and the sand with coated on its surface polyurethane absorb water instantly to complete wetting. Water applied to the surface of the obtained hydrophobic sand is distributed on its surface in the form of drops and is not absorbed by hydrophobic sand until its complete evaporation. We measured the strength of the hydrophobic sand for the external pressure of water. The ability of hydrophobic fillers to withstand the water pressure is proportional to the cosine of the contact angle and inversely proportional to the radius of capillaries, or the radius of the gap between the grains. With the aim of measuring the ability of the obtained hydrophobic sand to withstand 113

the pressure of water, the following test was carried out. The tip of a long syringe with the diameter of 1 cm2 was cut so that to get a uniform cylinder. The syringe was inserted into the hydrophobic sand, located in the vessel so that the distance from any wall to the syringe was 2 cm. Then, the syringe was filled with water and a piston was inserted into it. Placing 200 gram load on the piston, left it to stand for a day, the pressure of water remained unchanged during this time period. Figure 69 vividly, shows the behavior of the original sand, the sand coated with polyurethane and the obtained hydrophobic sand with her falling asleep in the water. The obtained hydrophobic sand in an amount of 10 grams freely floats on the water surface. The proposed method for preparation of hydrophobic sand allows to hydrophobize not only the surface layer of grains of sand, but also its bulk mass, this greatly increasing the quality of protection against the ingress of moisture. The obtained hydrophobic sand is proposed to be used as a filler in building materials for exterior finishing and in agriculture for preventing seepage of irrigation water in to the lower layers of soil or its evaporation.

Figure 69. The behavior of sand on the surface of water [139]

Also, hydrophobic sand can be used to isolate the soil around plants from the salty soil and salty groundwater leading to destruction of the root system of plants. 114

2.9. The use of nanoparticles in power systems of explosives Nanotechnology burst into our lives about 50 years ago, but in practice only now we begin to realize what it means and what are its prospects. The Nanoworld corresponds to that part of the space in which a chemical substance of organic or inorganic nature is formed by self-organization. Today it is possible to formulate the main task of the 21st century for chemistry. While physicists are exploring how the universe originated, and biologists – how life appeared, chemists need to determine how the matter was formed. The place where it is generated is the nanoworld. Let the term «high-energy materials» denote a wide range of organic and inorganic substances, the chemical transformation of which allow to produce a large amount of energy used to perform mechanical work, heating to a high temperature, formation of highenthalpy gases, etc. Components of rocket propellants and pyrotechnical mixtures: oxidizing agents, polymeric binders, explosives and metals can serve as examples of such materials. A definition of nanotechnologies and nanoparticles should be given. It is easy to understand that nanotechnologies deal with nanoparticles – from the methods for their production and analysis of the properties to operations for the assembly of the objects containing these particles and analysis of the results of their application. The term nano means tiny (dwarf in Greek), and applied to the dimensions of length we have 1 nm = 10-9 m. For comparison, the average size of an atom is 0.2 nm, the thickness of the strands of DNA -. 2 nm, and the size of erythrocytes is 7000 nm. Figuratively speaking, 1 nm is as less than 1 m as the thickness of a finger is thinner than the diameter of the globe. The development of nanotechnologies as applied to high-energy materials is primarily concerned with questions of their production in the nanoscale form. You can then set up goals for the use of such materials in their original form and in the form of high-nano-composites. The use of nanoscale materials сan result in the change in the dependence of combustion rate of a mixture on pressure that in a number of cases is of particular interest for creators of rocket fuels. Let us consider a mixture of aluminium with water as an example of the change in reactivity. 115

It is known that micron aluminum powder can be kept for a long time in an aqueous medium without changes as the oxide layer on the metal surface protects it from reacting with the oxidant. If we take the nanosize aluminium powder, it may react with water, when heated, slightly, or even at room temperature, depending on the state of the surface layer of metal particles [5]. This effect is in the basis of practical application of a mixture of aluminum with water in a «fuel cell» for mobile phones and laptops, as the reaction product is hydrogen used for production of electric power: 2Al + 3H2O → Al2O3 + 3H2

Burning Rate U, mm/s

30 25 20 15 10 5 0 mAl/mAP







mAP-mAl nAP-mAl mAP-nAl nAP-nAl

30 20 10 0 10





Figure 70. The Data on combustion rates of nano- and macrodisperse double mixtures [5]


Furthermore, as hydrogen is an ideal gas to be used in jet engines (lowest molecular weight), there are proposals to use the mixture (aluminum + water) as the propellant. In this case, to prevent premature reaction of the metal with water, the mixture is frozen, and stored in a frozen form before using it. Another example of nanosize aluminium in power systems is a pyrotechnic mixture with an oxidant, for example,ammonium perchlorate. There is a significant dependence of the effect on the value of the total surface area of contact of the reactants. Figure 70 presents the data on the value of the combustion rate of the compacted samples of a mixture of aluminum with ammonium perchlorate, when varying the particle sizes of both components. It is seen that the maximum combustion rate is realized when the two components are particles of nanometer sizes. In case of metallothermic reactions (thermite compositions of the type metal-metal oxide, for example, Al + CuO) the use of nanoscale powders allows to realize a very fast and powerful heat, release. To investigate the mechanism of such reactions, in [140], the authors proposed an original method of electrical heating of metal wire coated with a oxide layer. Observations with the X-ray highspeed shooting and simultaneous recording of spectra on the time-offlight mass spectrometer showed that ignition of aluminum wire takes place at the moment of intensive decomposition of oxides (CuO or Fe2O3). The intensity of oxygen evolution from copper oxide was significantly higher than in case of iron oxide. And the ignition temperature in the first case was found to be approximately 1000˚ C, while in the second case is 1250˚ C the heating rate of wire was of the order of 105 K / s. Another example of unique results obtained in experiments with carbon nanotubes is a recently discovered powerful thermoelectric effect during combustion multiwall nanotubes coated with a nanometer (7 nm thick) layer of RDX [5]. Based on the total weight of the composition, electric power released during its combustion is equal to 7kW / kg, that is almost an order of magnitude greater than a similar parameter characteristic of the modern lithium-ion batteries. A side remarkable result of these experiments is the burning rate of such composition was 10,000 times higher than the rate of 117

combustion of pure RDX (!). The attempts to explain the potential difference (about 200mV) during combustion on the basis of Seebeck theory, thermoelectric coefficient, known for a carbon nanotube have been unsuccessful. And it opens up the field for future research. Furthermore, it was found that the characteristics of specific electric power are strongly inversely dependent on the total mass of the composition. Therefore, it is required to perform many more serious researches to use the observed effect for the good of mankind. It should be noted that the use of a number of energy materials in the nanoscale form has already shown the prospects of their use in terms of improved performance and impact on the parameters of the process of combustion and explosion. 2.10. The use of hydrophobic soot in the ammonia-saltpeter explosives In the balance of industrial explosives (ES) in the Republic of Kazakhstan the large place is occupied by the simplest ammonium saltpeter explosives. The simplest explosives of this type in the English transcription are denoted by the index AN – FO (ammonium nitrate – fuel oil), in the Russian transcription AC-DT (ammonium saltpeter diesel oil). These explosives are called the simplest because they do not contain nitro- compounds. Explosives are relatively safe to handle, easily yield of different types of technological treatment, can be prepared by hand and have a low cost. [4] One of the main disadvantages of this type of explosives is hygroscopicity, caking, low water resistance, insufficient sensitivity to the initial momentum especially in waterlogging and compaction. Mixtures are not waterproof under the influence of water, at a moisture content of more than 5%, they lose the ability to knock. Numerous attempts to impart the necessary degree of water resistance to a granulated mixture of explosives based on ammonium nitrate have been unsuccessful [140]. To import relative water resistance to granulated explosives of AS-DF type, low melting solid oil products having hydrophobic 118

properties, as well as various water-oil emulsions are introduced instead of liquid petroleum fuel into explosives. The reason of holding back a widespread introduction of AS-DF mixtures at mining plants was the instability of its composition with time. The stability of igdanite is understood as the pre – determined homogeneity of the mixture (94.5% and 5.5% DF). Igdanite stability depends on the specific surface and moisture of AS, its natural moisture makes up 2%. The specific surface depends on the diameter and structure of AS granules. Figure 71 shows a micrograph in which it is seen that on the AS granules of brand B there are practically no visible cracks and cavities in which liquid fuel additive could retain. The Research in the column of charge quality of AS-DF mixture, prepared on granulated ammonium saltpeter, shows that after 1.5 – 3 hours diesel fuel starts quickly running off from the surface of granules, 1.5 hours later after preparation of igdanite, the content of liquid fuel in it reduces by 30-70%, that leads to delamination of explosive composition, conditioning the decrease in the efficiency of ES, as a result, in upper part of the charge column the content of diesel fuel in the mixture makes up 2.5- 3%. At the bottom part of the charge column, the content of DF reaches 11-12%. The heat of the explosion of this mixture does not exceed 2928 kJ / kg, instead of the estimated heat for optimal mixture 3782 kJ / kg (94.5% of AS and 5.5% of DF). Therefore, the basic requirement is to maintain stability of the composition with time – from the moment of production to the moment of explosion.

Figure 71. Optical – microscopic picture of AS


Further, igdanites were developed in the form of emulsion granulites. At the enterprises of the Republic of Kazakhstan, emulsion granulite E and granulites of the type ETV are widely used. Granulite E is an industrial explosive prepared directly in the places of usage is a mechanical mixture of AS with water – oil emulsion (WOE). Granulite E is designed for mechanized and manual loading of blast holes, wells and chambers in the dry and drained faces of quarries and mines not dangerous on gas and dust, in all climatic regions of Kazakhstan. For blasting of rocks with the factor of hardness 6-18 according to prof. M.M. Protodjakonov [141]. WOE steadily keeps on AS granule (Figure 72.). In the photomicrograph with laser illumination it can be seen that WOE is steadily held on AS granules.

Figure 72. A micrograph with a laser illumination

Granulites E and granulites ETV refer to stabilized igdanites with all their drawbacks (except stabilization on granule of AS of fuel additive) [142]. Granulite E and granulites ETV have a complex technological scheme of preparation and a higher cost than igdanite. In the laboratory of energy-intensive materials, granulite N (nano) of absolute stability was developed. Soot steadily keeps on AS granule. When you introduce of 5% of water into the charge, it detonates reliably. The diameter of the crater in moist sand after the explosion is 130 cm. Standard – that of igdanite is 115 cm. Soot was prepared on a silicon (nickel) plate from the torch burning with acetylene and concentric supply of oxygen. Figure 73 shows a diagram of the torch. The Sample in a silicon substrate, the oxygen supply rate: 260 cm3 / min, acetylene: 150 cm3 / min, the 120

heigh ht of the plate over the torch is 2 cm, sputteering time is 10 minutes m at, the sub bstance is taken zoones (1, 2, 3).

F Figure 73. Diagram oof the torch

In Raman spectrroscopy: Zone 1 – carbonaceous substance is preseent with the amorrphous, graphite annd fullerene strucctures; Zone 2 – am morphous, fulleren ne structure and ann unidentified phaase; Zone 3 – amorrphous carbon maaterial. To prepare ES so oots from all three zones were used.. The carried out investigations i on combustion of accetylene – air mixtture showed that carbon c deposits onn the plates have differences d in the morphostructure of deposited pparticles. Regard dless of the cond ditions of burning g, there are longg chains of indiv viduals in the form m of nanobeads 15 5 – 30 nm [123]. A model the charrge column with a separating funneel (Figure 74) was created to study the t migration of ccarbon from ES with w A vertical posittion of the chargee was simulated. The charge colum mn is a glass cylin nder filled with a weighed amountt of explosive wh hich is closed by th he funnel to preveent the loss of eassily volatile fractiion. Soot was mixeed with AS. To determine d the migration of soot from the charge in th he construction, a separating funnel via with soot can n by collected in th he beaker is provid ded. When filling the explosive charge column, a smaall amount of soot was collected in the lower part of the funnel. Then movement of soot ceased. Part of soot deposited bettween the granulees, but part of it covered saltpeter grranules uniformly.. The charge was examined for sooot migration from m the charge colum mn for 30 days. Migration M of soott from the chargee column was not observed. o 121

Figure 74. Chargee column 1 – reverse funnel, 2 – glass cylinder, 3 – a weighted amount ES under study 4 – sepparating funnel, 5 – a glass – collector

Field studies Explosives were tested for frost – resistance in a cryochamber undeer field conditionss. Liquid nitrogenn was used as a refrigerant. Six explosive charges werre placed in a papper envelope with a diameter of 120 mm. The temperaature was -130 0C. Then blasting was w performed with h full detonation without w failure. We studied ES th he for complete detonation in a paper p shell of 120 mm in diameter, the initiation waas conducted by a cartridge of amm monite 6ZhV weig ghing 200 g. The diameter of th he crater was meassured in moist san nd. The charge was in the shell 120 mm m in diameter, weiight of the charge of granulite N (Nan no) – 6 kg. The cartridge was insttalled vertically. Initiation I was cond ducted by a cartridg ge of ammonite 6Z ZhV weighing 200 g. The explosive chaarge was supplied w with – 5% by weig ght of the charge. Six S charges were bllasted. Field testingg data are shown in Table 7. As a result of fieeld tests it is founnd that the develop ped modified ES granulite N has stability and reelative water ressistance. The diam meter of the crater in moist sand aafter the explosio on is 130 cm Stan ndard – Igdanite 115 cm. It does not have a complicated prepparation scheme. It I can be used in all climatic zones and a for special expplosions requiring low temperaturess up to 170 0C. It haas low cost compaared to emulsion grranulites. 122

Table 7 The data of field testing

1 2


4 5

6 7

The name of indexes

Moisture content, % The critical diameter (in paper envelope), mm Sensitivity: - To strike - To the excitation of the shock wave from the detonation of bombs ammonite 6ZhV weighing 200 g - To the detonator cap - To affectation fire fuse Bulk density, g. cm3 Completeness of detonation in a paper shell with the diameter of 120 mw of the cartridge of ammonite 6 ZhV weighing 200g. The sand is moistened. The diameter of the crater, cm. The sand is moistened. The depth of the crater, cm

Igdanite Mass fraction of components, % Ammonium salt peter of the brand grade B 94.5 DT 5.5 2

Granulite H Mass fraction of components, % Ammonium salt peter of the brand B soot 2 2



0% 0-2 сm

0% 0-2 сm

inadequate not sensitive

inadequate not sensitive


Not researched







Control questions for self-examination 1. Explain the lotus effect. 2. Explain the principle characteristics of hydrophobicity and hydrophilicity. 3. What is wetting? 4. What can be explained by Cassie law? 5. What is the hysteresis of the contact angle? 6. The methods of synthesis of hydrophobic soot in flame. 7. The effect of electric field on the process of formation of hydrophobic soot in flame. 8. The fields of application of hydrophobic soot. 9. Why are ammonium – saltpeter explosive substances called the simplest ES? 10. What are the disadvantages of ammonium – saltpeter explosive substances?


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