1. COMBUSTION SÑIENCE: DIAGNOSTICS AND MODELING OF SHS

 
 

O-1-01: Ignition of Heterogeneous Systems Overburdened with Infiltration

V.V. Barzykin

Institute of Structural Macrokinetics and Materials Science,Russia Academy of Science, Chernogolovka, 142 432 Russia, E-mail: barzykin@ism.ac.ru
 
  The results of the experimental and theoretical studies performed by the author and his colleagues and literature data on the process of infiltration of heterogeneous systems under the condition of a gaseous component infiltration are discussed. Various ignition modes affected by the gas infiltration, e.g., thermal explosion and porous fuel ignition with a gaseous oxidant, surface ignition and the flame propagation along the interface of a condensed body and a gaseous oxidant, friction-induced ignition, surface spot ignition of a metal powder in oxygen etc., are considered. The influence of the oxidant infiltration on the specific features of the ignition transition to steady-state combustion and the ignition stability are also discussed.

The validity of the mathematical models describing nonstationary transition processes and taking into account the cases of either the oxidant infiltration along a porous system due to the external pressure gradient or the gaseous reagent infiltration resulting from gasifying of one of the components of the green mixture in the heating zone of the combustion wave is evaluated.

Acknowledgement. The present study was supported by the International Science and Technology Center (Project 355-97).
 
 

O-1-02: Mathematical Modelling Of Titanium-Carbon Mixture Ignition

V. Rosenband1, A. Aoufi2, A. Gany1

1 Faculty of Aerospace Engineering, Technion-Israel Institute of Technology

Haifa 32000, Israel

2 CNRS-LIMHP, Avenue J-B Clement, Villetaneuse 93430, France
 
 

 A two-scale mathematical model of the ignition of equimolar mixture of titanium and carbon particles is presented. At a macroscopic level a cylindrical sample with a homogeneous composition of titanium and carbon particles is placed into a furnace heated up to a given temperature. At a microscopic level the titanium particle is considered surrounded by an interfacial layer of titanium carbide TiC which itself is surrounded by carbon. The mass transfer between carbon and titanium is described which comprises both formation of TiC and dissolution of carbon in titanium. The heat released by these processes is taken into account at the sample macroscopic level. Formation of TiC increases the size of the TiC layer and induces a Stefan-like problem.

Three stages of an ignition process are being considered, which include a solid state interaction between titanium and carbon particles, titanium melting and reaction of carbon with liquid titanium. The titanium melting results in change of the system heterogeneous scale and increasing of the reaction heat release.

Mathematically a parabolic non linear coupling is being considered between two mass diffusion balances on a moving frame at a particle level and the heat balance at the macroscopic level. An implicit finite volume scheme on a moving mesh is used to discretise the non-linear system. Numerical simulations on the microscopic/macroscopic levels correlation are presented and discussed. Results of calculations are compared with known experimental data on the titanium-carbon mixture ignition.



 
 

O-1-03: Mathematical Modeling of the Process of Dissipative

Structure Self-Assembly During Liguid Flame Combustion

A.I.Lesnikovich1, S.A.Kirillov2

1Belarus State University, Minsk , Belarus

2Institute of Technical Acoustics, Vitebsk , 210717, Belarus
 
 

 The wave combustion front is transformed into a complex geometrical configuration during the combustion of compounds of mixtures containing a lot of nitrogen. The structure of the reaction zone is a liguid sphere. The interior of the sphere has a complex foamshaped structure with average size of 2mm of a dispersed drop. The heating with spraying of condenced products procedes thå combustion. Thenthe necleus of the sphere is formed in a developed crater. And the necleus of the sphere enlarges its sizes up to stationary meaning of 10 - 17mm, which are constant up to the end of the reaction. During the reaction the sphere makes stochastic roaming at the bottom of the crater and rotates. The Pulsation of the sphere sizes accomponied by the break of gas sphere also occures. It is experimentally established that the whole sphere with its interior foam structure is the zone of the reaction. The sphere temperature is of time higher then that is the temperature of a melting on which the sphere with enlarging of the sphere temperature the sphere size diminishes. The mathematical model of the dissipative structure above described is based on averaged equations of the dynamics of heterogeneous medium with phase transformations. In this case kinetic equations of the reaction of the mixture combustion were used for averaged intensity of phase transormations on interfaces. For the combustion front of a liqiud sphere type the using of the kinetic equations of the reaction is locally correct. The determination of the sphere temperature in the equations of chemical kinetics is not correct. As a result of samplifying assumptions the equation for average temperature of the liguid sphere reduces to Sheringer's nonlinear equation. The density of gas bubbles in the sphere acts as potential. The numerical integration of the equation demonstrates a solitary heat wave concentrated in the liguid sphere as a solution. Thus the liguid sphere is heat soliton in the variables temperature-space co-ordinates. The physical reason of stability of hear soliton is the crises of heattransfer of the first sort which takes place during such regime of the reaction. Owing to this fact heat field forms a space topologicully nontrivial configuration.
 
 

O-1-04: On Attaining Maximum Output of Material Synthesis in Filtration

Combustion Waves

O.S. Rabinovich, I.G. Gurevich, P.S. Grinchuk, A.V.Luikov

Heat and Mass Transfer Institute (HMTI), National Acedemy of Sciences of Belarus, Minsk, Belarus, E-mail: orabi@ns1.hmti.ac.by
 
  Two possibilities for the optimization of efficiency of material synthesis in filtration combustion waves with forced gas flow are considered in the present study. These are 1) the choice of the most efficient single-wave regime (co-current or counter-current relative to the gas flow) and 2) the use of multi-wave regimes in which the required conversion is achieved by successive passage of several reaction wave. In the both case, the optimization becomes possible only when the reaction rate depends on such factors as gaseous oxidant or solid product concentrations, or when the thermal conductivity of the system varies with the conversion degree. The objects of the study are the synthesis processes with the prescribed temperature level of the reaction and the given final conversion degree.

A very important circumstance which should be taken into account choosing a most effective regime and which was not taken into consideration in the previous works [1,2] is that just under those conditions, when the optimization is possible, the steady-state problem for counter-current filtration combustion waves has, generally speaking, two solutions and the solution with the higher conversion degree is not stable and can not be practically realized [3].

Comparative analysis and optimization are performed within the three classes of synthesis regimes, i.e. a) co-current and counter-current single-wave regimes, b) multi-wave regimes with the same direction of the combustion wave propagation in each wave, and c) “mixed” multi-wave regimes, when the waves have different direction of propagation.

References:

1. O.S. Rabinovich, S.N. Krasilshchikov, N.I. Stetyukevich and I.G. Gurevich. Heat and Mass Transfer in Phase and Chemical Conversions, pp. 103-115. Izd. ITMO AN BSSR, Minsk (1983).

O.S. Rabinovich, S.N. Krasilshchikov and I.G. Gurevich. J. Engng. Phys., 1984,v.46, N 1, p. 71-76.

3. O.S. Rabinovich, S.N. Krasilshchikov and I.G. Gurevich. Heat and Mass Transfer - VII, Vol III, Heat and Mass Transfer in the Presence of Chemical Reactions, pp. 153-158. Izd. ITMO AN BSSR, Minsk (1984).
 
 

O-1-05: Heterogeneous Mathematical Simulation of

Self-Propagating High-Temperature Synthesis

in the Mixed of Metallic Powders

O.B. Kovalev, V.M. Fomin

Institute of Theoretical and Applied Mechanics, Russian Academy of Sciences

SD 4/1, Institutskaya Str., 630090, Novosibirsk, Russia.
 
 

 A new physico-mathematical model of self-propagating high-temperature synthesis (SHS) of intermetallides is build. The derivation of the governing equations includes for the first time the principles of mechanics of heterogeneous media that involve at an elementary level the metallo-physical notions about the phase interaction in metallic systems and the diffusion mechanisms of formation of transient phases and final products. The condensed phase is presented as consisting of components corresponding to all transient and final states including the solid and liquid states of initial reagents. The suggested approach has allowed the obtaining of a number of qualitatively new results in description of the SHS processes. In particular, the numerical study of the problem about the laser ignition and propagation of an SHS wave in the mixture of nickel and aluminum particles has revealed SHS wave structure containing (experimentally observed) inflections, isothermal portion, etc. The numerical results are in good qualitative agreement with experimental data available in the literature concerning the effects of the initial sample temperature, size of initial particles, sample diameter, its porosity and the ratio of reacting components on the SHS wave velocity. It is shown that within the framework of the model concepts one can calculate the internal structure of the final sample consisting of isolated (spherical) and bound pores with a certain graininess and chemical composition. Multidimentional formulation of the SHS problem for the finite-sized samples has been numerically realized. It allows the study of 2D and 3D pictures of formation of the structure, composition and porosity of the final products under the conditions of frontal combustion.
 
 

O-1-06: Dynamics in Layer Models of Solid Flame Propagation

A.P. Aldushin1, A. Bayliss2, B.J. Matkowsky2

1 Institute of Structural Macrokinetics and Materials Science,

Russia Academy of Science, Chernogolovka, 142 432 Russia

2 Northwestern University, Evanston, IL USA
 
 

 Like many other systems, SHS is a pattern forming system. The problem of describing experimentallyobserved patterns and of predicting new, as yet unobserved, patterns continues to attract the attention of theoreticians due to the fundamental significance of the phenomena in combustion in particular, and in nonlinear science in general. Here, we analyze the dynamics of solid flame propagation in a 2D region by considering the region to be composed of parallel, identical layers aligned along the direction of propagation and having thermal contact. Each layer is then described by wave propagation in 1D, with the transverse Laplacian replaced by a term describing heat exchange between neighboring layers. This configuration is the simplest model of a 2D system because it accounts, in a simple way, for the principle feature of the problem, i.e., heat exchange between neighbors in the transverse direction. For simplicity, we describe the situation for two layers. Because the layers are identical, uniformly propagating waves in each layer must be identical, independent of the heat exchange rate. When the Zeldovich number exceeds a critical value, which depends on the heat exchange rate, uniformly propagating waves become unstable. The stability diagram for the two coupled layers reproduces that for the full 2D problem after appropriate identification of parameters in the two problems. Depending on parameter values, we determine three different steady state dynamical behaviors (though additional behaviors are also expected to occur). The three behaviors are: (i) waves in each layer which pulsate in phase as they propagate, so that together they form a single pulsating propagating wave, (ii) the waves in each layer are no longer identical, and antiphase pulsations occur, with the two waves alternately advancing and receding as they propagate. This wave is the analog of the spinning wave in circular cylinders. Finally, there is (iii) a region of bistability between the in phase and antiphase waves, with each having its own domain of attraction, so that which of the two behaviors occurs depends on the conditions of initiation of the wave. Our analysis of the layer model shows that as the heat exchange rate decreases to zero, e.g., due to increasing the width of the layers, themean velocity of the antiphase modes approaches that of the in phase modes. The results of the layer analysis suggest results for solid flames in cylindrical samples. In particular, we focus on large diameter samples. The suggested results are then verified by computations of the cylindrical problem.

Supported in part by NASA Grant NAG3-2209 and NSF Grants DMS-9705670, DMS-9530937
 
 

O-1-07: Model of Surface Spin Combustion in a Hybrid System

I. Filimonov1, N. Kidin2

1Institute of Structural Macrokinetics and Materials Science, Russia Academy of Science,

Chernogolovka 142432 Russia

2Institute for Problems in Mechanics RAS, Moscow 117526 Russia
 
 

Proceeding from the common system of filtration combustion equations ( Aldushin & Merzhanov, 1988 ), in which the approach of two interpenetrating continuums for the description of processes in porous medium is used , and neglecting losses of heat outside the reacting sample the authors have developed mathematical model of surface spin combustion in a hybrid system .

Alongside with the effects of heat diffusivity the model takes into account the gas phase filtration , heterogeneous chemical interaction of the condensed fuel with an active component of the gas flow (oxidizer) , which can proceed by two alternative ways , the difference in the temperature of the gaseous and condensed phases.

The formulated problem has been solved at the limit of a negligibly weak dependence of the chemical reaction rate on the oxidizer concentration ( but not on the pressure in the system, the index of the condensed fuel conversion and temperature ) and low Semenov numbers (Se << 1) . Various ranges of the filtration Peclet numbers (Pefi) , determining the relation of filtration flows to a convection flow of the gas have been considered . In spite of the fact that in practice the Pefi numbers are rather high (Pefi >> 1) , the influence of the gas - oxidizer on the combustion wave appears to be essentially various depending on a ratio between Pefi and Se.

At infinitely large Pefi ( 1 / (Pefi) << Se << 1 ), the pressure can be considered as a constant equal to the initial gas pressure everywhere and the effects of filtration can be neglected. Therefore one can reduce the problem to investigation of the thermal factors only ( if the effects of the reacting medium structure and temperature on its effective heat capacity and diffusivity are negligible ) , i.e. one can obtain the classical problem of thermal spin combustion theory ( Novozhilov, 1992, 1995 ). At moderately large Pefi ( Se << 1 / (Pefi) << 1 ) , the filtration flows essentially affect both the pressure and temperature distribution in the system . The work is financially supported by RFBR within the scope of Grant N 99-03-32020a .
 
 
 
 

O-1-08: The Peculiarities of Product Structure Formation

in Combustion Wave of Low Gas Systems

V.K. Smolyakov

Tomsk Branch of the Institute of Structural Macrokinetics and Materials Science,

Russian Academy of Sciences, Tomsk, 634021, pr. Academicheskii, 10/3, Russia.

E-mail: maks@fisman.tomsk.su
 
 

 The adiabatic combustion of two-component mixture, forming the refractory and gaseous products, is considered. One of the components is presumed to be easily melting, the other is refractory one. The quasi-stationary combustion model, considering the influence of force action of gas, being filtered on the pores, liquid-phase sintering and volume change of condensed phase in chemical transformation upon the porosity and size of the sample was built for mixture parameters, excluding the limiting effect of capillary speading. The melting temperature of easily melting reagent is the natural “cutting” of chemical interaction and structural transformations. After appearance of liquid phase and enlargement of the interface of reaction, of condensed phase is defined as the assemblage of suspension drops from liquid, refractory reagent and product. The volumeric fraction of solid phase is changing in the interaction process, that changes the suspension viscosity. Newton's law for viscous liquid that links lineary stress and deformation rate of the condensed phase was chosen as the rheological law, determining the substance deformation. The loosening action of gas, being formed in reaction and filtering in pores retards the coalescence of drops. The combustion of armoured and non-armoured samples was analysed. The formular for calculation of combustion rate, porosity and sample size were obtained analytically with previous evaluations of characteristic action times of various factors. The values of structural and physical and chemical parameters which disturb the media continuity were determined.

Numerical modeling verified the results of analytical investigations on stepped action of the factors under consideration on product structure. The calculated dependencies of rate combustion, product macrostructure and cracking formation on the starting parameters are in good correspondence to the ones, determined analytically.

The results of theoretical analysis are in good correlation with the known experimental data.


O-1-09: Study of Kinetics of Nonisothermal Reactions

Using Cellular Automata

V.V.Klubovich, S.A. Kirillov, A.S. Kirillov, I.S. Chebotko

Institute of Technical Acoustics, Vitebsk , 210717, Belarus
 
 

 In this paper the kinetics of nonisothermal heterogeneous reactions is simulated with the help of cellular automata having changing architecture of connections. In order to use the existing formal theory of the kinetics of nonisothermal transformations for the analysis of experimental data it is necessary to satisfy a set of conditions (additivity conditions) the validity of which is not verified in most transformations. Particularly random coincidence of the temperature dependencies of the speeds of origin and grows is actually needed. The transformation degree allowing in the kinetic equations is the result of physical volume averaging; due to this averaging a heterophase system geometrical structure of is degenerated.On the other hand physical and mechanical properties are precisely determined by the geometrical structure of a heterophase system. In SHS processes the structure transformations are inaccessible for a direct observation. Thus the registered parameters of the process such as temperature, pressure and others may be caused by various combinations of the geometrical structure of heterophase system. For the analysis of experimental data it is essential to know the degeneration degree of a set of the heterophase system structure combinations relatively to the macroparameters which experimentally observed. In other words what's the reliability degree of determination for instance a type and other peculiarities of the structure transformations by the combustion wave front microstructure. And what can we say about the change of the processes of the structure macrokinetics in electric, ultrasound fields and under a force influence with different stress tensor structure. The cellular automata system developed by the authors is designed for numerical simulation of the processes of the transformation kinetics from the first principles and the study of the above mentioned tasks.The architecture of the automata is a discreet analog of bundle. The cellular automata simulating the temperature front of the combustion wave acts as a base manifold of the bundle. The cellular automata with changing architecture of connections is the layer of this discrete stratification. The changing architecture of connections permits to realize a heterophase structure of any geometrical configuration as at an initial moment as during the process of synthesis. The statistical data about the structure transformation kinetics are mapped onto graphical windows.The work of stratificated cellular automata demonstrates the wave combustion front propagation, the front microstructure evolution and the structural transformations in the microvolume of a sample.Within the framework of the model the ultrasound and electrical fields influence on the kinetics of structural transformations has been studied.


O-1-10: Synchrotron Radiation in Research of SHS:

Yesterday, Now, Tomorrow

V.V.Aleksandrov

Institute of Structural Macrokinetics and Materials Science,

Russian Academy of Sciences, Chernogolovka, 142432 Russia
 
 

 Methods using Synchrotron Radiation (SR) are powerful tools for real time (“in situ”) studying phase and chemical transformations in reaction zones of SHS - -processes(1). First such investigations were fulfilled in 1979 - 1983 (2-5). These and other works published during the last 20 years are reviewed. The obtained results are compared and some general regularities are revealed. Promising research directions connected with the newest development of instruments and methods are discussed.
 
 

Referenses:

1. A.G.Merzhanov. Int.J. of SHS, 1997, v.6, N 2, p.119.

2. M.A.Korchagin, S.N.Gusenko et al. In: Gorenie Kondensirovannyh i Geterogennyh System, Chernogolovka: Izd. Inst. Khim. Fiz., 1980, p.93 (Ser.: Khim. Fiz. Protsessov Goreniya i Vzryva).

3. V.V.Aleksandrov, M.A.Korchagin et. al In: Otchet o rabotah po ispol’zovaniyu sinchrotronnogo izlucheniya v IYaF SO AN SSSR, Novosibirsk: IYaF, 1981, p.35.

4. V.V.Boldyrev, V.V.Aleksandrov et al. Doklady AN SSSR, 1981, v.259, N5, p.1127.

5. V.V.Aleksandrov, M.A.Korchagin B.P.Tolochko, M.A.Sheromov. Fizika Goreniya

i Vzryva, 1983, v.19, N4, p.65.
 
 

O-1-11: Intermetallic-Ceramic Composites Synthesis by SHS. Time-Resolved

Studies Sing Synchrotron Radiation X-Rays
    1. Curfs1, I.G. Cano2, M.A. Rodriguez2, X. Turrillas3, Å. Kvick1, G.Vaughan2
1 European Synchrotron Radiation Facility (ESRF), B.P. 220, 38043 Grenoble cedex, France

2 Instituto de Cerámica y Vidrio (CSIC), Arganda del Rey, Madrid, Spain

3 Instituto Eduardo Torroja (CSIC), Madrid, Spain
 
 

Self-propagating high-temperature synthesis (SHS) has been performed in the quaternary Al-Ni-Ti-C system in order to obtain intermetallic-ceramic composites. Some of the obtained products have properties that make them very attractive for many industrial applications. They have low density, excellent oxidation resistance and high melting point (~ 1900 K). Unfortunately polycrystalline AlNi has poor ductility at room temperature and low wear resistance due to poor creep strength at high temperatures. These problems may be overcome by the use of reinforcement by aluminium oxide fibers or ceramic powders.

The Materials Science beamline (ID11) at the European Synchrotron Radiation Facility in Grenoble, France has been employed to follow the reactions in-situ on a time-scale of ~ 100 milliseconds. Powder diffraction patterns were recorded in this time-interval using a high-speed CCD camera coupled to an image intensifier X-ray sensitive detector with 1000 x 1000 pixel frames. The pixel resolution is ~ 100 microns. As the reactions proceed patterns from the pre-heated, reaction front, post-heated and cooling portions of the reaction were sampled. The phases occurring during the reactions were identified and information of the reaction mechanism and the nucleation kinetics were obtained. SEM studies were used to characterize the final microstructure and EDX was used as a complement to identify the metastable phases occurring.


O-1-12: The Reaction Dynamics and Structure Formation in Shs of Tic/Fe

Z.G. Zou, Z.Y. Fu,Q.J. Zhang, R.Z. Yuan State Key Lab of Advanced Technology for Materials Synthesis

and Processing Wuhan University of Technology, Wuhan 430070,China
 
 

In the paper, the reaction dynamics and structure formation in Self-propagating High-temperature Synthesis(SHS) of TiC/Fe system were studied by combustion front quenching(CFQ), thermal and ignition analysis. It was found that in SHS process, Fe2Ti co-melted liquid formed first, then carbon dissolved into it at high temperature and reacted with it to form non-stoichometric TiCx. After the main reaction carbon continued to react with TiCx during the high temperature period and formed TiCx with a near stoichometric ratio. The flow of melted metals resulted in agglomerates and then the TiC grains continued to grow to larger size after combustion wave.
 
 

O-1-13: Some Peculiarities of Combustion and Structure Formation in the

Ternary Systems Ti--Si--C (Ti--SiC), Ti--Si3N4 and Ti--BN

H.E. Grigoryan, A.S. Rogachev

Institute of Structural Macrokinetics and Materials Science,Russian Academy of Sciences

Chernogolovka, Moscow, 142432 Russia
 
  New precursors for producing nitrides, carbides, and borides in SHS regime are proposed. The dependence of combustion velocity on composition of the initial mixture have been studied. The critical significance of mass ratios of the green mixture has been found. The maximum of combustion velocity has been found in the study of the dependence of the combustion velocity on density. Upon study of density effect on the SHS mode and combustion velocity, some new types of front instability were observed. They arise due to increase of green mixtures porosity. Microgravity conditions allows us to attain lower porosities for both the green mixture and product. This task was resolved experimentally on board Mir space station in January 1998.

The optimum concentration ranges favorable for the production of the Ti3SiC2-based ceramics during combustion were determined.
 
 

O-1-14: Structure Formation Relationships for SHS- Iron Monoalumide

P.A. Vityaz, A.V. Belyaev, T.I. Talako, A.A. Dmitrovich

Powder Metallurgy Research Institute, 41, Platonov St., 220071, Minsk, Belarus,

Tel: (0172) 39-98-27,Fax: (0172) 32-63-40
 
 

 Iron monoaluminide is of principal interest for structural applications because of its excellent corrosion resistance, retention of good strength to intermediate temperatures, low density compared with most iron or nickel-based alloys and potentially lower cost. This paper presents the results of structure and property investigations for iron monoaluminide based powder materials produced using SHS-method. The influence of material composition and powder pretreatment on the conversion completeness and on the degree of ordering for iron aluminide with B2 type structure is analyzed. Possibilities of monophased intermetallide formation as well as perspectives for iron monoaluminide containing composite material produced using SHS are discussed.
 
 

O-1-15: Experimental Determination of Heat Release

Function for Gasless Combustion

S.L. Kharatyan1, H.A. Chatilyan1, A.G. Merzhanov2

1 Nalbandyans Institute of Chemical Physics NAS RA, Yerevan, 375044, Republic of Armenia, E-mail: Suren@ichph.sci.am

2 Institute of Structural Macrokinetics and Material Sciences RAS, Chernogolovka, 142432, Russia. E-mail:Merzh@isman0.unicon.msk.su
 
 

 The results of in situ measurements of heat release function in conditions, simulating combustion of powdered mixture of metals with silicon for a number of systems Me-Si (Mo, W, Ta, Nb) are presented in this work. Thin metallic wires covered by silicon layer as samples for studies, were heated up by direct passage of electrical current. The heating modes were corresponded to thermograms of Me-Si powdered systems combustion. The heat release data are correlated with product’s phase and microstructure formation laws. It was established that the basic distinctions between the heat release functions for various Me-Si systems are caused by special features of phase diagram near to MeSi2-Si eutectics - on the one hand, and with other - diffusive-kinetic aspects and mechanism of silicide phases formation. In all cases the basic heat release is caused by formation of disilicide phases and proceeds by the mechanism of solid metal + liquid silicon. However, in Ta-Si, Nb-Si systems, a thin layer of Me5Si3 silicide (thickness no more than 1 mm) is formed practically just after silicon melting, limiting and supporting heat release rate constant for a long time. In the noted stage of process the zones of disilicide phases’ (TaSi2 and NbSi2) formation are not yet compact. It is represented, what just because of presence of lowest silicides layers, the values of stationary rate of heat release much concede to similar values for system Mo-Si, at which the formation of Mo5Si3 phase is observed on later stages of process. In the certain moment the continuous fall of heat release rate upon time and thickness growth of lowest silicide layer take place indicating that the process passes in to diffusive mode. For Mo-Si system the transition of process in a diffusive mode takes place simultaneously with Mo5Si3 silicide layer formation. Besides, in case of Nb-Si system heat release is registered also at temperature, much lower of silicon melting point (T~1300oC), caused by contact melting of silicon in the area of NbSi2-Si eutectics. The existence of chemical interaction in area of Teut. ~1300oC is proved also by direct SEM observations of contact zone of metal surface with silicon. In the case of W-Si system - in a stage of heat release the W5Si3 layer is not formed at all: formation of this phase and the transition of process in a diffusive mode takes place only after the full consumption of silicon - termination of chemical heat release. On the contrary, owing to microstructural peculiarities of formed WSi2 layer, self-accelerated growth of rate of process on finished stages of heat release takes place, to be caused by crystallization of WSi2 from oversaturated eutectic melt.
 
 

O-1-16: Travelling Waves of Filtration Combustion with Combined

Homogeneous and Heterogeneous Reactions.

V.S.Babkin1, Yu.M.Laevsky2, Z.R.Ismagilov3

1 Institute of Chemical Kinetics & Combustion Novosibirsk, 630090, Russia.

2 Computing Center, Novosibirsk, 630090, Russia.

 3 Institute of Catalysis, Novosibirsk, 630090, Russia.
 
 

Among the combustion processes used in SHS purpose the important role play filtration processes with heterogeneous reaction. The allied type of gas phase combustion proceeding in an inert porous medium is filtration gas combustion. This combustion is also prospective in SHS technologies [V.S.Babkin, Pure & Appl. Chem. Vol. 65, No 2, pp. 335-344, 1993]. It is due to specific features of this type of combustion.

They are wide variety of realized combustion regimes, possibilities of controlled supplying of chemical energy to chemical reaction zone and adaptable controlling of chemical reaction rate, realization of filtration gas combustion in various in material and structure porous media. An experimental study was carried out of steady-state filtration combustion waves simultaneously occurring in catalytic heterogeneous and homogeneous gas-phase reaction (travelling hybrid waves). The hybrid wave velocities, and the maximum and equilibrium temperatures in the combustion zone were measured. It had been shown that it is possible for a hybrid wave to be transformed into a wave where either the heterogeneous or homogeneous reactions dominate, depending on whether the gas filtration velocity is decreasing or increasing. It has been further shown that, under conditions of vanishing catalytic activity of the porous medium surface, the hybrid wave is transformed into a wave with dominant homogeneous reaction. On the basis of a proposed theoretical model, a parametric analysis of the hybrid wave was carried out. An interpretation of the experimental results is presented.

The work was supported by the grant from the European Union (INTAS - 96-1173).
 
 

O-1-17: Generalized Mathematical Model of Kinetics of Chemical

Reactions with Heat and Mass Transfer

Yu.A. Kuznetsov

Scientific Research Institute of Mechanics of N.I.Lobatchevsky Nizhny Novgorod

State University, 23 Gagarin Avenue, Building 6, Nizhny Novgorod, 603600, Russia
 
 

 At the present time technological process of Self-Propagating High-Temperature Synthesis (SHS-technology) is one of the most often used processes in various areas of modern chemistry and physics. This technology makes it possible to obtain a wide class of the new materials and compounds. One of the most characteristic features of SHS processes is a realization of the wave regime of the propagation of exotermic reaction of combustion in the mixtures of the reacting substunses; in this case reaction is concentrated in an narrow moving zone. As pointed out in [1,2], the future progress in the SHS-technology is based on the investigation of the macrokinetic laws and nonlinear effects of SHS processes. This requires, among other things, mathematical studies of the methods of calculation of the mass and the chemical reaction rates and the velocity of the propagation of combustion wave, the distinguishing properties of evolution of the concentration and the temperature of the components of the reacting mixtures and so on. The present paper is concerned with one of the generalized mathematical models of the chemical and physical processes described above. This model involves in general the nonlinear system of differetial equations with partial derivatives including equations of heat and mass transfer in reacting mixtures and general model of kinetics of chemical reactions. Some questions concerning solvability and qualitative properties of the solutions of general nonlinear initial-boundary value problems arising in the combustion theory are considered.

References:

1. A.G.Merzhanov. Ten Reseach Directions in the Future of SHS.Int.J. of SHS, 1995, v.4, N 4, p.323-350.

2. S.G. Vadchenco, A.G. Merzhanov . A model of heterogeneous flame propagation. Book of abstracts. 4-th Int. Symp. SHS, Toledo Spain, October 1997, 1997, p.48.
 
 

O-1-18: Thermodynamics of Quasi-Adiabatic SHS

I.B.Kudinov 1, D.G. Lemesheva 1, V.V. Klyucharev 2, S.V. Klyuchareva2, R.A Shuba3, A.A.Vakulenko4 , A.N. Ryabov4 , O.V.Guz' 4, M.V. Shunina4 1Institute of General and Inorganic Chemistry, RAS, 117907, Moscow, GSP-1, Leninskii prospect 31.

2Institute of New Chemical Problems, RAS, 142432, Moscow Region, Chernogolovka, Russia.

3Higher School of Materials Sciences, Moscow State University, 119899, Moscow, Russia.

4Department of Chemistry, Moscow State University, 119899, Moscow, Russia.
 
 

 The paper presents one of results of the five-year program (1994-1998), which was carried out by schoolchildren, students and the researchers of RAS with the purpose of creation and development of the non-isothermal chemistry of substances and materials. It has been established, that the critical section of combustion stationary wave made a body of unit thickness. The internal structure of this body can be uniform or non-uniform. When the anisotropy inherent in any heterogeneous mixtures shows up only at a microscopic level, the equation (1) determines the opportunity or impossibility of the quasi-adiabatic SHS (Q-SHS) in a body of unit size. However, when a flow shapes percolation cluster, the expression (2) defines the critical conditions of the Q-SHS realisation. The thermodynamic laws (1) and (2) are shared by quasi-adiabatic processes in heterogeneous mixtures with random distribution of components.

Tc Tc

Q = Qi + ò (Ci + Ck) dT } (1) Q = QR + K * {QE +Qi + ò (7 Ci + Ck) dT } (2)

To To

Here: Q is the calorific value of mixture containing N initial substances at a rate of ni mole with summary heat capacity Ci which turn into M products at a rate of mk mole with summary heat capacity Ck; To and Tc are the initial temperature of mixture, and its ignition temperature after Mallard and LeChatelier; QR is the calorific value of the medium comprising the quasi-adiabatic cells (QC-s); K » 0.16 is Scher-Zallen’s coefficient, that is the volume fraction of QC-s in the mixture; Qi is the heat of phase transitions and chemical reactions between To and Tc for raw material, which is localized within quasi-adiabatic layer (1) or QC-s (2); QE is the appropriate heat reserves in QC-s, which must compensate the heat absorption at endothermic decomposition of oxidiser, which lies outside the QC-s (in the special case that there is a deficiency of oxidizer in QC-s and macroscopic excess of the oxidizer in all mixture).

The authors thank for support: ISF of the Soros and Government of Russia (Grants: N9K-000, N9K-300), ISSEP (Grants: S97-638, S97-730, S98-1752, S98-1767), FP "Integration" (Project: 2.1.-855), RFBR (Grant: 98-03-32593 a), Fraunhofer ICT (Berghausen), Russian academy of education.
 
 

O-1-19: Complex Behavior of Self-Propagating Reaction Waves

in Heterogeneous Media

A.Varma, A.S. Rogachev, A.S. Mukasyan

University of Notre Dame, Notre Dame, IN 46556, USA


The results of digital high-speed microscopic video recording experiments in a variety of reaction systems have shown unequivocally that microstructural mechanism of high-temperature reaction wave propagation in heterogeneous media has unique features which have not been considered to-date [1-4]. While on the macroscopic length (~1mm) and time (~10-1s) scales, the front appears to move in a steady mode, the observed thermal structure of reaction waves on microscopic level (length ~ microns and time ~10-4s) has a complex character. The local overheating (scintillation) on the frontier of the reaction wave indicates areas of extremely high rates of reaction, where time of reaction is much less than characteristic time required for heat dissipation into surrounding areas. Discovery of this new type of fast chemical reaction propagation (Scintillation Reaction Wave) with low activation energy allows us to classify all gasless reaction waves in heterogeneous mixtures into two types, according to their microstructures: quasihomogeneous reaction wave (QRW) and scintillation reaction wave (SRW). In different systems, the reaction wave may propagate by either a single mode or by a combination of the two modes, where the SRW mode precedes the QRW.
 
 

Since the SRW mode occurs in systems where at least one reactants melts, it is expected to be the prevalent mode for gasless reaction wave propagation in heterogeneous media.
 
 

References:

1.Mukasyan A.S., Hwang S., Rogachev A.S., Sytchev A.E., Merzhanov A.G., and Varma A., Combust. Sci. Tech., 1996, v. 115, p. 335-353.

2.Hwang S., Mukasyan A.S., Rogachev A.S., and Varma A., Combust. Sci. Tech., 1997, v. 123, p.165-183.

3.Hwang S., Mukasyan A.S., and Varma A., Combust. Flame, 1998, v. 115, p.354-363.

4.Varma A., Rogachev A.S, Mukasyan A.S., and Hwang S., Proc. Natl. Acad. Sci. USA, 1998, v. 95, p.11053-11058.


O-1-20: Microscopic Mechanisms of Pulsating Combustion in Gasless Systems

A S. Mukasyan, A.S. Rogachev, A.Varma

University of Notre Dame, Notre Dame, IN 46556, USA


In the present work, we use a digital high speed microscopic video recording method [1] for direct observation and quantitative characterization of pulsating combustion in gasless systems. Three systems were studied, which are known to react in pulsating combustion mode: Ta + C; Nb + B and Ti + xC (where x was varied in the range 0.4 to 1.0). Our main goal was to measure variations of instantaneous combustion velocity during the pulsating period and study the microstructural aspects of the pulsation process.

For Nb-B and Ta-C systems, experiments show unequivocally that the cause of oscillations has microstructural nature: cracks form periodically just ahead of the combustion front, resulting in hesitation of its movement. For example, in the Nb+2B system (dNb=5 mm, dB=0.1 mm; porosity = 50%) the space frequency of cracks appearance is about 2/mm. The reaction front propagates uniformly between cracks, with velocity 100 mm/s, and then it stops at the crack for ~0.3 s. After this hesitation, the reaction wave initiates again at the other side of the crack and the cycle repeats.

The crack-induced oscillations described above are qualitatively different from those observed in other systems, such as Ti+xC mixtures, with x<0.6. For example, in Ti+0.4C mixture, we have observed periodic extinction of the combustion wave followed by ignition of the unreacted layer ahead of the front. The delay between extinction of the previously burned layer and initiation of the next is about 0.1 s. In this case, no cracks appear in the sample and the reaction follows the scintillation reaction wave mode [2]. Therefore, we may conclude that in this case oscillations occur because the heat of reaction is not high enough to maintain a steady propagation of the reaction wave (so-called thermo-kinetic oscillations).
 
 

References:

1.Mukasyan A.S., Hwang S., Rogachev A.S., Sytchev A.E., Merzhanov A.G., and Varma A., Combust. Sci. Tech., 1996, v.115, p. 335-353.

2.Varma A., Rogachev A.S, Mukasyan A.S., and Hwang S., Proc. Natl. Acad. Sci. USA, 1998,v. 95, p.11053-11058 .
 
 

O-1-21: Structure of Hcs and its Influence on Combustion Wave

Yu.V . Frolov, A.N. Pivkina

117977 Moscow, Kosygin St.4, Institute of Chemical Physics RAS, Russia
 
 

 Heterogeneous condensed system (HCS) generally represent a wide range of energy release composition, which include SHS-composition, pyrotechnics, SRP and etc.

Combustion of HCS-SHS is a complex multistage process depending on many parameters and characterizing by some peculiarities :burning rate dependence on a particle size ,non-simultaneous burn-out of the components, combustion limit of combustion, agglomeration and dispergation of condense phase, destruction and phase transition on combustion wave ,etc.

All the phenomena are interconnected and result in the combustion wave formation and its propagation with certain burning rate. The order and completeness of deflagration and chemical reaction in the combustion wave depend on not only system energy, but on a composite structure.

Problem of the non-simultaneous burn-out of the HSC-SHS components was investigated in different aspects , but there is no study of the combustion front shape with point of view the sample initial structure. Intuitive impression of the chaotic structure and non uniformity of combustion front was used to plane combustion surface .

Present paper is focused on the structure of HCS sample and its influence on the peculiarities of formation combustion front. Some phenomenological aspects are analyzed on base of fractal and percolation theory. Fractal dimension could be a one of key point of the non-simultaneous combustion process investigation.

Novel computer program is used to analyze of combustion front optical imaginations of the combustion propagation . It permit to receive the connection between the green structure of the sample and color card of combustion front . The brightness of the image could be attributed to the certain temperature level. The information which is produced by fractal analysis give opportunity to reconstruct the temperature-time history of any point of combustion sample.
 
 

O-1-22: A New Combustion Mode in SHS: Reverse Burning Phenomenon

Chien-Chong Chen

Department of Chemical Engineering National Chung Cheng University

Chia-Yi 621, Taiwan
 
 

A new combustion mode: reverse burning phenomenon was observed during the synthesis of Zirconium-based materials. When an external heat was applied to one end of a green pellet, surprisely, the ignition was ignited at the other end. Moreover, the ignition position, measured from the heated end, was proportional to the apparent green density of the pellet. The underlying cause was found to be a combination of nonuniform density distribution, heat transfer and oxidation of Zirconium.
 
 

O-1-23: Combustion synthesis of Al0.25-xNixTi0.75 (0 £ x £ 0.1)

by Time Resolved X-Ray Diffraction

O.Held1, Ch.Gras2, F.Charlot2, D. Vrel3, J.C.Gachon1

1 Laboratoire de Chimie du Solide Minéral, UMR 7555, Groupe Thermodynamique Métallurgique Université Henri Poincaré, Nancy1, Faculté des sciences, BP 239, F54506 VANDOEUVRE CEDEX, France

2 Laboratoire de Recherche sur la Réactivité des Solides, UMR 5613, Groupe Matériaux à Grains Fins Université de Bourgogne, UFR Sciences et Techniques, BP 400, f21011 DIJON CEDEX, France

3 Laboratoire d'Ingéniérie des Matériaux et des Hautes Pressions, UPR 1311, CNRS - Université Paris Nord Institut Galilée, F93430 Villetaneuse, France

 We studied SHS reactions between elemental aluminum, nickel and titanium powders with the following proportions 20/5/75 and 15/10/75 (respectively) which correspond to three phase compositions at 800 °C. Using a synchrotron intense X-ray beam (LURE D43) in Orsay (France) and fast detectors we determined the reaction paths by time resolved X-ray diffraction, with a resolution of 50 ms. We used an infrared recording system to study the surface temperature evolution of the sample.

Our main results are:

Ä the observation of an intermediate phase (Ti b ) during few seconds.

Ä the influence of nickel on the formation of AlTi3.

Complementary observations by electron microprobe analysis showed the occurrence of a eutectoid reaction : the formation of AlTi3 + NiTi2.
 
 

Experimental setup in the LURE, Orsay, France.
 
 

O-1-24: Effect of Impurities on Wave Propagation and Reaction Mechanism

V.M. Shkiro, A.S Rogachev

Institute of Structural Macrokinetics and Materials Science Russian Academy of Sciences

Chernogolovka, 142432, Russia, E-mail: shkirov@ism.ac.ru
 
 

 Powders of metals and nonmetals used as initial reactants for SHS contain some amount of impurities, which depends on the method of production. Commonly, these powders are contaminated by other metals, absorbed or dissolved gases (H2, O2, CO, etc.). Notable volume of gases are generated due to evaporation of the impurities in a combustion wave. It is known that these gases can strongly influence the combustion wave propagation, as well as the product structure. In the present work, we compiled and generalized experimental data published in the literature as well as our own results in order to describe the behavior of various impurities in the SHS wave. It is shown that gasifying impurities may affect the combustion velocity by expanding of the reaction medium (which cause decreasing of the thermal conductivity), formation of co-flows or gas-phase transport of the main reactants. Some impurities may result in occurrence of low-melted eutectics, which accelerate the combustion. Small amounts of impurities can be used specially to vary regimes of synthesis and to modify product structure.
 
 

O-1-25: The Absorption of Active Gases by Nonevaporating Getters on the Base

of Intermetallides in Combustion

Yu.S. Naiborodenko, E.G. Sergeeva, N.G. Kasatskiy

Tomsk Branch of the Institute of Structural Macrokinetics and Materials Science,

Russian Academy of SciencesTomsk, 634021, pr. Academicheskii, 10/3, Russia.

E-mail: maks@fisman.tomsk.su
 
 

 The ability of many metals and alloys to interact activity with gases lays in the background in the development of high effective means of oilless vacuum evacuation, which are widely used to bind the aggressive gases in the pressure range 10-2- 10-5 Pa.

High efficiency and reliability of getters mean of evacuation create prerequisites for their more wide use in practical purpose in solving of a number of ecological problems, combined with trapping and absorption of harmful and toxic gases.

But high concentration of gaseous phase requires the conduction of special investigations, since at gas phase pressure ³ 106 Pa, the interaction has already proceeded in un-exthothermic conditions of SHS wave [1]. Sherefore in the present paper the investigation of gas absorption by non-evaporating getters under normal conditions (P » 760 torr, 105 Pa) in air is carried out. The gas absorption was investigated in closed valume in combustion. The degree of gas absorption (sorption capacity) was determined by the residue gas pressure and was controlled by gain in weight of samples.

The investigations conducted, showed that among non-evaporating getters on the titanium base, pure titanium has best sorption properties and provide residual pressure of 2,2x104 Pa (230 torr), gain in weight 13,5 - 14%, sorption capacity (the amount of gas absorbed) - 89 L× torr/g.

Among the non-evaporating zirconium-based getters Zr+16%Al alloy on the base of zirconium aluminides, i.e. obtained by SHS method showed the best absorption properties. Residual pressure on the alloy, being melted = 2x104 Pa (210 torr), gain in weight 18%, sorption capacity 129 L× torr/g, and on SHS alloy the pressure was 225 torr, and sorption capacity was 123 L× torr/g.

The residual pressure obtained were achieved as the result of gas absorption up to the complete getters cooling. But in use of more large samples ( ~ 50 g) it appeared, that gas absorption in layer-to-layer combustion goes on Zr-Al, as lighting zone, up to the pressure of 315 torr, and then the absorption as lighting zone put to an end and occurs already due to the volumeric interaction up to the pressure of 210 torr. The obtained values of sorbtion capacity in the front were equal to 61,4 L× torr/g (Zr-Al) and 47 L× torr/g (Ti), that corresponds to appr. 50% of sorptiom ability realization.

The effect of porosity and dispersivity on gas absorption was also investigated in the work and phase composition of gas products was determined.
 
 

Referense:

1. Merzhanov A.G., Borovinskaya I.P., Volodin Yu.Ye. About combustion

mechanism of porous metal samples in nitrogen. Dokl. AN SSS, 1972,v.206, N 4,

p. 905.
 
 

O-1-26: Combustion of Al under a Nitrogen Pressure up to

200 ÌPa (Mechanism, Synthesis of Items Properties)

V.E. Loryan, I.P. Borovinskaya

Institute of Structural Macrokinetics and Materials Science Russian Academy of Sciences,

Chernogolovka, 142432, Russia
 
 

 Combustion of aluminum and aluminum-containing mixtures in nitrogen under pressure up to 300 MPa has been investigated. It is shown that the combustion proceeds in accordance with the following mechanism – a low melted component spreads over the surface of a refractory component. Temperatures, combustion rate, chemical and phase compositions as well as microstructure of samples obtained under various combustion conditions have been studied.

It is shown that at SHS of aluminum nitride and composite materials based on it (AlN–MenOm, AlN–TiB2) the combustion proceeds with formation of aluminum nitride melt in combustion zone.

A method of direct synthesis of items (bricks and plates) has been developed. It is shown that the specific feature of the item structure is existence of a matrix of melted aluminum nitride. In particular, the structure of AlN–TiB2 composite material makes up a matrix of melted aluminum nitride with inclusions of titanium diboride grains. Porosity is 25¸ 20% for items of pure aluminum nitride and below 7% for items of AlN–TiB2 composite materials.

Mechanical, thermal and electrical characteristics of synthesized materials were studied. Refractoriness, heat resistance and resistance of AlN–TiB2 in aggressive metallurgical atmospheres also were studied. It is shown that characteristics of this material are comparable to those of currently applied for production of main refractory materials.
 
 

O-1-27: Study on the Reaction Models of SHS in Al-TiO2-C System

Zeng Songyan, Zhang Erlin, Yang Bo

National Key laboratory of Precise Heat processing of Metal

P.O.Box 428, Harbin Institute of Technology Harbin, 150001 P.R.China
 
 

The combustion characteristic in Al-TiO2-C system has been investigated by computer image collecting and date processing technique. It has been shown that there are three kinds of combustion models of SHS in Al-TiO2-C system: stable plane combustion, unstable multiple-point combustion model and unstable single-point combustion model. With the addition of dilute, such as TiC and Al2O3, the combustion model changes from unstable multiple-point combustion model to unstable single-point combustion model. With the increasing of preheat temperature, the combustion model changes from unstable multiple-point combustion model to stable plant combustion model. Then analyses have been done on the combustion behavior on the basis of thermodynamics and kinetics. The results have shown that the reason for the change of reaction model is due to the change of the reaction enthalpy and the inhomogeneity of the reaction in the reaction front. In addition, a mathematic model has been built and numerical simulation has been done. The calculation results were in good agreement with the experimental results. In the end, based on the experimental research and theory analyses, a SHS figure in this system which displaying the change law of the reaction model has been made.