P-1-01: Stochastic Modeling of SHS in Porous Systems

Recently, novel regimes of the SHS in porous systems have beendiscovered, namely, the so-called relay-race and quasi-homogeneous patterns. Besides, recent experiments on SHS in the conditions of microgravitation, when the sample porosity exceeds the poured-in porosity, necessitate a deeper insight into the dynamic behavior of SHS waves in highly porous systems.

In this work, a stochastic model of SHS in highly porous systems is developed using the so-called "cellular automata" approach. The model considers solid-state reactions in the constituting particles, conductive heat transfer in the charge mixture via the solid-state contacts of the particles and radiative heat transfer across the pores. Besides, heat transfer by gas molecules for the gas-filled pores is taken into account. The particle size is implied to be small enough, thus mass transfer between the particles is not considered in this model. The numerical procedures used in modeling permit simulating both chaotic and ordered distribution of pores throughout a sample.

Both 2D and 3D models have been developed in this work. The 3D model permits comparing the temperature distribution over the outer surface of a sample, which is registered in experimental studies, with an actual 3D temperature field in the sample.

Computer simulation of different patterns of the SHS wave dynamic behavior has been carried out, and their effect of the SHS product structure is evaluated.
 
 

P-1-02: The gas phase ionization in the SHS wave

The present paper reports the experimental data on the gas ionization in the combustion wave. The experiment were carried out according to the Langmure probe method, which is widely used in the diagnostics of low-temperature plasma. In contrast to the earlier studies a saw-wise voltage was applied to the probe and the volt-ampere characteristics were obtained in the during experiments.

The results obtained allow determination of the dynamics of the volt-ampere characteristics evoked by the combustion front propagation and calculation of the parameters of the gas ionization.

The present work was supported by RFFI ( grant N 98-03-32147).
 
 

References:

1.V.A.Koudryashov, A.S.Mukasyan, I.A.Filimonov. Chemoionization Waves in Heterogeneous Combustion Processes., Journal of Materials Synthesis and Processing,1996, v.4, N 5

2.O.K.Kouzmina, V.A.Koudryashov, I.A.Filimonov. To the Nature of Charged Particles in the Heterogeneous Reactions of Combustion. 27th International Symposium on Combustion, University of Colorado at Boulder, August 2-7, 1998
 
 

P-1-03: The Mathematical Modeling of the High-Temperature

The mathematical model of the high-temperature synthesis of Ni₃Al in regime of the heat explosion of the initial powder mixture was developed in a approximation of the model representation about macrostructure of a powder mixture nickel with aluminum, on the basis on equations of the diffusion kinetic of formation of the intermetallic compound and on the basis on equilibrium diagram of state. The influence of the nucleation center of the intermetallic phase on the temperature and time of the ignition of a powder mixture was investigated. Numerical calculations of the main characteristic of the heat explosion in the system Ni - Al as function of dispersivity of the nickel component of the initial powder mixture and the volume fraction of the inert filler were carried out. The numerical estimation of the technological parameters of the ignitions of the powder mixture allowing to synthesize the final product of the certain phase compound.
 
 

P-1-04: Computer Simulation of Behavior of Ni-Al SHS-System under

Impact Loading

Zh.A. Eremina,V.A. Gorelski,V.B. Nikulichev,A.S.Shteinberg

Tomsk Branch of the Institute for Structural Macrokinetics and Materials Science

10/3 Akademicheskii prospect, Tomsk, 634021, Russia,E-mail: gorelski@ismtb.tomsk.su
 
 

Chemical reactions in mixtures of elemental powders are often accompanied by rapid release of large amount of energy. The energy release, unlike that in detonation of explosives, manifested by the generation of high temperature, often exceeding the melting temperature of reaction products. The behavior of porous material under shock compression is significantly different from that of solid - density materials in that a large amount of extra energy is required to plastically deform and crush the particles in the process of void annihilation. This work is concerned with the problem of using of phenomenological kinetic model of chemical reactions in the porous Ni-Al powder mixture under high velocity impact. Effects of high shock pressure and high temperature are presented by variation of constants involved in the model. A powder mixture is treated as a single distended solid having thermomechanical properties based upon the mass-averaged quantities of the powder mixture. The density changes resulting from chemical reactions have been considered. Numerical calculations were carried out using a finite element code.A plane steel projectile impact onto the steel capsule with porous stoichiometric Ni-Al mixture was modeled. Calculations were performed for impact velocities up to 1200 m/s. The effects of chemical reaction on the pressure and temperature were studied. Comparison with experimental tests was made.
 
 

P-1-05: Forming of Composition and Structure in SHS Processes

of Carbide-Aluminide Systems

The main attention was given to influence of ratio of carbides and metal binder in combustion products and so nickel and aluminum in the metal binder on particulars of chemical and phase composition of composite materials and its structures and microstructure.

The influence of the scale factor on sputtering process, completeness of phase separation and extraction of target elements from a mixture was studied. The uniformity degree of distribution of elements in an ingot, change macro and microstructure in horizontal and vertical section of an ingot was investigated.
 
 

P-1-06: Pattern Formation of Combustion Waves

Chien-Chong Chen, Chia-Ling Li, Li-Wen Chung

Dept. of Chemical Engineering National Chung Cheng UniversityChia-Yi 621, Taiwan
 
 

In this work, we studied the pattern formation of combustion waves via the combustion of large green samples consisted of Ti and graphite. Large and thin green samples were used to observe the processes of the two-dimensional spatio temporal combustion waves. As one corner of the sample ignited, the combustion wave propagated forward with a circular wavefront, as expected. Combustion wave slowed down as it reached the sample edges, due to the heat losses. When TiC was used as a diluent, the velocity of combustion wave decreased as the amount of diluent increased. Samples failed to ignite when the wt. % of diluent exceeded 35% and an interesting combustion phenomenon was observed at a 32 wt. % of diluent. A hot spot traveled back and forth along the progressing circular combustion wavefront in an irregular manner. This indicated the combustion wave was moving both longitudinally and transversely. When dilution ratio was more than around 225 wt. %, the deformation of wavefronts were also observed, in which transverse instability occurred. Next, if samples were ignited simultaneously at the opposite corners, two circular wavefornts progressed toward each other and accelerated later on, comparing to the combustion with only one ignited spot. Finally, two combustion waves merged near the central of samples and temperature was increased due to superposition of two waves. The merged wave split and traveled to the unreacted part of samples showing a square-wave characteristic. The surface of combusted samples also revealed the history of the progress of combustion waves. Different degrees of dilution revealed distinct combustion patterns. Also, if inert was intentionally planted in the samples to account for the effect of incomplete mixing, the number of inert, the size of inert and the locations of inert could cause the deformation of combustion waves, the splitting of waves and the combustion patterns, too.
 
 

P-1-07: About Mechanism of Chromium Nitriding in Combustion

B.Sh. Braverman, M.Kh. Ziatdinov, Yu.M. Maksimov

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 results of study of chromium combustion in nitrogen are presented. In Cr-N system two nitrides Cr₂N and CrN are known, which may be formed from the elements in exothermal reactions with heat effects of 25,2 kcal/mol and 28,2 kcal/mol, respectively. Relatively low heat effects define low adiabatic combustion temperatures: 2063^oC in CrN formation and 1290^oC in Cr₂N one. The highest chromium nitride CrN is characterized by low thermal stability. The dissociation CrN temperature at pressures up to 10 MPa is lower then 2060^oC. The nitrading of metal powders with dissociating product in combustion is weakly studied. Chromium powders with purity of 99,5 mass.% and nitrogen with purity of 99,9 mass.% were used in the work. In combustion rate, transformation depth, as well as temperature profiles of combustion wave were measured. The products were analysed with help of X-ray phase and chemical analyses. The stop of the combustion front by quenching of burning samples was conducted. It was found out that the chromium combustion in nitrogen occured stage by stage. The first stage is the reaction of Cr₂N formation

Cr + N₂ = 2Cr₂N (1)

During the second stage the reaction takes place

P-1-08: Electrothermographic Neutralizer of Combustion Gases from the

Internal Combustion Engine

L.B. Mashkinov

In this report we present a neutralizer of Combustion gases from the internal combustion engine with a constant temperature (resistance) of a catalytically active wire (net) heating element.

The disign of an experimental neutralizer is provided and the test results are reported.
 
 

Referenses:

1. A.G.Merzhanov, V.G.Abramov, V.V.Barelko. J.of Phys. Chem., 1969, v.43, p.2828.

3. A.G Merzhanov, Yu.M.Grigor’ev , S.L.Kharatyan, L.V. Mashkinov, Zh.S.Vartanyan. Study on heat-evolution kinetics at high-temperature nitration of zirconium wires. Fiz. Goreniya Vzryva, 1975, v.11, N 4, p.563-568.

4. V.V.Barelko,Yu.E.Volodin. Proceedings on Thermal Analysis,Moscow,1976, p.46.
1. L.B.Mashkinov, L.N.Galperin, Merzhanov, Abstracts of the VI=th All-Union Conference on Thermal Analises,, Moscow, 1976, p.46.

P-1-09: Combustion Wave Propagation During SHS of BI-Layered System

P-1-10: Influence of Heating Rate on Kinetics of Rapid High-Temperature

Reactions

The extreme conditions of combustion wave (i.e. high temperatures and heating rates) make it difficult to study the intrinsic kinetics of chemical reactions during CS. We have built a computer-assisted electrothermography setup to determine the intrinsic kinetics of reactions under conditions similar to those realized during combustion synthesis of materials. In particular, we have studied the influence of heating rate on the reaction behavior. Some qualitative results are available in the literature which indicate that temperature-time history of the system may influence the reaction kinetics. However, the influence of heating rate in the range pertinent to CS processes, on the kinetics of rapid high-temperature interactions has not been reported previously.

Two reaction systems were investigated, Nb-N₂ and Ti-N₂, in the temperature range characteristic for their SHS regime of combustion (1800-2800 K). It was found that in Nb-N₂ system, parabolic law holds and reaction rate increases up to 60 percent, while the apparent activation energy decreases by about 20 percent, when heating rate changes from 3^.10⁴ to 2^.10⁵ K/s.

The reaction mechanism during titanium nitridation is different at sufficiently high temperature, when the metal melts and reaction accelerates due to higher diffusion coefficient of nitrogen in liquid phase. In this case once again, faster preheating favors more rapid nitridation of titanium. Reaction accelerates by a factor of 2.5 when heating rates increases from 3.5^.10⁴ to 1.1^.10⁵ K/s.
 
 

P-1-11: The Mechanism of Combustion and Degassing in the System

Titanium-Carbon-Polystyrene

The work was supported by Russian Fond of Basic Research (grand No. 97-03-32240).
 
 

P-1-12: Measurement of the Impurity Gas Pressure in the Combustion Front

at SHS of Long-Sized Samples in Cylindrical Shells

The initial sample was composed of two detached compacts, which were prepressed into quartz shells (d=4.2 mm) at the relative density r₀=0.62 and highly uniform density along the compact length (±2 %). The model green mixtures Ti+C and Ti+2B were used. The total length of the sample was constant (180 mm) throughout all experiments. It was important that only one part (l₀) of the sample took part in the combustion process. The other part (l_с) was used for enhancement of the filtration resistance. The length of both compacts, l₀ and l_с, varied in the wide range. The gas pressure responsible for transverse rupture and making shift of the remainder of the first compact was calculated by using the length l_B of the displaced part of the compact. The nonmonotonous dependence P=f(l_A) (l_A is the length of the burnt part of the first compact) for mixture Ti+C was calculated. The maximum pressure was (53± 6) MPa at l_А=12cm and the specific gas release from the green mixture V=39cm³/g. The changes in the properties of the combustion product (strength, porosity, capillary effective size) were evaluated.

The study was supported by the Russian Academy of Sciences and the Russian Foundation for Basic Research.
 
 

Referense:

1. Ponomarev M. A. and Sapronov Yu. A. Direct measurements of the impurity gas pressure in the SHS front (a titanium carbide study). In: 4th Int. Symp. on SH Synthesis, Book of Abstracts, Toledo, Spain, October 6-9, 1997. p.153.
 
 

P-1-13: Temperature Limitation of the Phase States of Condensed Systems

Mendeleev University of Chemical Technology,125043,Moscow,Miusskaya sq.9,

Russia,Fax: (095)2004204, Tel: (095)3676382
 
 

The reagents temperature at fast heating during intensive technological processes may reach the vicinity of spinodal line, i.e. the boundary between the metastable and labile phase states. On spinodal line the initial condensed system cuts off its existance: either it evaporates or dissociates with high rate (explosionlike). Sharp increasing of process rate near the spinodal line appears as result of homogeneous nucleation, or changing of chemical reaction mechanism, or weakening of intermolecular interaction etc. Many mathematical models in physical chemistry doesn't contain any restrictions with respect to spinodal line, because they have been developed for the gas systems beeing in equilibrium condition.

In this communication both theoretical and experimental ways of determination of the spinodal position for given matter are descriebed. The teoretical mathod is based on using of a wide-range caloric state equation. The experimental mathod is based on using an early developed technique of the contact thermal analysis. The experimental method and some experimental results are presented.

The corrections of mathematical models of Van't Hoff, Arrhenius, Knudsen - Langmuir et.al. have been proposed with respect to the temperature limitations of the phase states for providing technological calculations thermal and mass transferring processes.

The examples of calculations of some technological processes in combustion science are given to show the influence of the suggested corrections and their correlation to checking experimental data.
 
 

Referenses:

1.E.F.Vainstein, G.E.Zaikov and O.F.Shlensky, Polym.-Plast.Technol.Eng., 1996, v. 35, N 5,

p. 669 - 696.

2.O.F.Shlensky. Combustion Science and Technology,1996, v. 120, p.383 - 391.
 
 

P-1-14: CO₂ Laser Systems to Investigate Combustion Synthesis Reactions

The possibilities to obtain a propagating combustion mode instead of a simultaneous one (explosion mode) depends on the temperature distribution in the pellet at the ignition moment.

For that reason, the capability to control the ignition transient features is the necessary condition that guarantees to obtain results characterized by the same ignition conditions and thus a comparisons among them is more consistent.

In this work the CO₂ laser system has been used, as external energy source, to ignite different kind of powder mixtures (NiAl, NiTi and ZrB₂) and the ignition transient has been described by the temperature history of the irradiated pellet surface.

The temperature increase, during the heating process, has been measured by recording the signal generated by microthermocouples (50 micron) and from these data it is possible to estimate the value of the ignition temperature and energy required to reach the ignition conditions. Results point out that the meltability of one of the compounds play an important role in the ignition process.