4. Unconventional SHS: New Ideas, Mehtods, and Directions of Research
 
 

O-4-01: Electroexplosion of Metastable Metal Powders with Stored Energy

for Self-Propagating High-Temperature Synthesis

G.V. Ivanov, V.G. Surkov

Akademichesky av.3, 634021, Tomsk, Russia, E-mail: pio@ipc.tsc.ru

Energy that is 1.5-2.0 times higher than sublimation energy is input in metals in 1 m sec as wires are electroexploded. The metal overheated to 20,000 – 40,000 ° C broadens in shock waves in aerosol form and cools at a speed of 106 – 108 deg/sec. Aerosols deposited (the installation productivity is to tens kilograms per month) represent electroexplosion powders (EEP) of a size less than 100 nm. They are stable at storage on air in open jars. The powders possess some unusual physicochemical properties:

Their excessive stored energy is 2-5 times higher than metal melting heats. This energy is released in the heat waves mode at the temperatures much lower than melting temperatures (400 ° C for Al, Cu, 150 ° C for Ag, Sn, 120 ° C for In) [1]. Al EEP reacts with water similar to alkali metals at 50-70 ° C with the release of hot hydrogen [2]. In standard calorimetric bombs Al EEP reacts with nitrogen in SHS mode at the pressures of 20 atm. at a 100 % AlN yield. Nitrides not oxides are formed in mixtures with lead and barium nitrides at burning.

Aluminium boride is formed in mixtures with commercial boron in SHS mode. Al EEP - amorphous boron pellets were put on an electrically heated plate. The process begins at 730 ° C and occurs at a speed of 5 mm/sec, at a self-sintering to 1,000 ° C. Brass is formed in SHS mode in mixtures of a commercial Zn powder and Cu EEP, at the temperatures within 170-200 ° C and self-heating to 800 ° C.
 
 

References:

1. G.V. Ivanov, B. G Ivanov, V.P. Kuznetsov. Formation of heat waves in dispersed metal media at the metastable state relaxation. 1st All-Union Symposium on Macrokinetics and Gas Dynamics. Abstracts, Alma-Ata, 1984.

2. B. G Ivanov., S.N Leonov., G. L. Savinov et al. Burning of ultrafine aluminium with gel-like water. Physics of Burning and Explosion, 1994, N 4, p. 167.
 
 

O-4-02: Near Net-Shaped Alkaline-Earth-Bearing Ceramics

via te Oxidation of Solid, Metal/Oxide Powder Precursors

K. H. Sandhage*

Dept. Materials Science & Engineering, The Ohio State University, Columbus, OH, USA

*on sabbatical at the Technische Universtitat Hamburg-Harburg;

E:Sandhage@tu-harburg.de
 
 

Alkaline-earth(AE)-bearing ceramic compounds are in use, or are being considered for use, in a variety of chemical, electronic, magnetic, biomedical, and structural applications. A novel, recently-patented method for fabricating AE-bearing ceramic components is the oxidation of shaped, AE-metal-bearing precursors. Ba, Sr, Ca, and Mg are relatively ductile metals that can act as inorganic binders in ceramic-bearing green bodies. Unlike conventional organic binders/plasticizers, however, AE metals are retained in the green body upon oxidation; that is, AE binders are "burned in", which results in less porosity than organic binder burnout. Certain AE oxides (i.e., barium peroxide) can react with other metals and oxides in the precursor to form binary or ternary ceramic compounds via novel reaction paths at quite modest temperatures (e.g., ¾ 650°C). Perhaps the most unusual feature of AE elements is the reduction in molar volume that occurs upon oxidation (e.g., Vm[Ba] > Vm[BaO]). Since the molar volumes of most other oxides are larger than for the corresponding metals (e.g., 2Vm(Al) < Vm(Al2O3), the phase content of solid, AE-metal-bearing precursors can be tailored so as to yield ceramic parts that retain the shape and dimensions of the precursor upon complete transformation (e.g., the volume of a Ba-Al-Al2O3-SiO2 precursor can be matched to that of celsian, BaAl2Si2O8). Important features of precursor fabrication (by powder metallurgical processing) and oxidation processing will be discussed. Unique reaction sequences and near net-shape component fabrication will also be illustrated with examples drawn from work on refractory AE aluminates and aluminosilicates, electronic cerates and titanates, and biocompatible phosphates.
 
 

O-4-03: Influence of Anti-Combustibles Additive on the Inflammability

of Foam Polymer

G. Ksandopoulo, E. Mazhitov

Combustion Problems Institute, 172 Bagenbay Bat., Almaty,Kazakstan, 480012
 
 

Now foam-polymers find wider application in the industry and construction of the most advanced countries of the world. However, high inflammability and light ignition of the most foam-polymers are very dangerous to life and health of the people. The foam-polymers ignition begins with intensive thermal decomposition. The self-ignition is caused by the thermal destruction processes of polymers and thermo-oxidizing destruction of thermal decomposition products at the surface layer of polymer.

Two ways of foam-polymers inflammability reduction have been shown as the most perspective. The first way consists in drawing fire-protective layer on the material surface . However this layer can be destroyed at further use of the product. The second way consists in addition in the foam-polymers the various chemical compounds, which will create some kind of obstacle to access the material surface oxidation as a result of thermal decomposition. The present paper were guided by the second direction. The additive for foam-polyuretan and foam-polysterol on the basis of urea-formaldehyde pitch (UFP) and diammonium phosphate (DAP) has been developed as anti-combustibles additive. These substances were added in polymers in frothing process in various percentage ratio. The advantages of these additives are cheapness, insignificant influence on frothing process and complete ecological safety. Solid UFP which has been used as the additive is a waste production and was not suitable for all industrial applications.

The tests of the synthesized material have been carried out on installations " Fire pipe " and " Oxygen index ". The obtained results have shown increase of the foam-polyuretan and foam-polysterol oxygen index more than twice. Now we may consider the received materials as not light igneous but hard igneous under the given inflammability characteristics (GOST 12.1.044-84). The found results are compared to the world analogues.

These days work on foam-polyuretan and foam-polysterol production with proposed anti-combustibles additive is conducted in industrial scale.
 
 

O-4-04: Microstructure and Mechanical Properties

of the SiO2-- Al2O3--Li2O--MgO--ZrO2--TiO2 Glass Ceramic

H. Satha, A.R. Nemamcha, C. Mai

1 Departament of Materials Science, University of Guelma, BP 401, Algeria

2 GEMPPM,U.A. 341, Insa-69621 Villeurbanne, Lyon, France
 
 

Glass ceramics could be considered as composite materials having interesting properties of both ceramic and glass. For instance, the mechanical properties of glass ceramic depend on their microstructure.

The objective of this study is to investigate the two aspects:

  1. the microstructural evolution from glass to glass ceramic state during heat treatment.
  2. The mechanical behaviour corresponding to each state.
The microstructural studies are done using several techniques: small angle X-ray scattering (SAXS), X-ray difraction and transmission electron microskopy. These techniques enable as to follow the early stages of nucleation and growth processes. The main results are:
  1. the first stage corresponds to a precipitation of small particles of ZrTiO4.The maximum particle size of ZrTiO4 observed is about 55 A° .
  2. The second stage corresponds to the nucleation of crystalline phase from the ZrTiO4 nuclei. The crystalline phase is identified as the Virgilite (LiAlSi4O8).
  3. The last stage corresponds to the growth of the Vrigilite according to an Oswald ripening mechanism.
The mechanical behavior, corresponding to these stages of nucleation and growth, is studied by the microhardness, Young’s modulus and creep and recovery test.
 
 

O-4-05: Heat Explosion in a Heterogeneous Medium

C. Barillon1, G.M. Makhviladze2, V. Volpert1

1 Analyse Numé rique, Université Lyon, UMR 5585 CNRS 6922 Villeurbanne Crdex, France

2 Center for Research in Fire and Explosion Studies, University

of Central Lancashire, Preston, PR1 2HE, England
 
 

In this work we study heat explosion in a heterogeneous medium consisting of reacting particles surrounded by a chemically inert medium. Chemical reaction can occur inside the particles or between the particles and inert medium. The particles are sufficiently small and there is no heat conduction in this phase. The inert medium is heat conducting and there is heat exchange between the 2 phase. Under the gravity conditions the temperature distribution can be influenced by convection. The influence of convection on thermal explosion for a homogeneous medium has been studied in [4]. In our model of heat explosion in heterogeneous medium, we assume that the particles are sufficiently small and 2 phase have the same velocity. The problem is of interest for SHS, and it can be used for example to study combustion of metal particles in sulfur [3]. Hence we consider a two-temperature and one-velocity model with the Navier - Stokes equations written under the Boussinesq approximation:


 
 

We consider the square geometry and assume that the side walls are heat isolated while the temperature at the upper and lower walls is equal to the ambient temperature. We study first the thermal model [1,2] of heat explosion (stationary solutions of (1) - (4) with zero velocity), relevant to condensed medium. Dependence on the coefficient of heat exchange, critical conditions of heat explosion, continuous branches of solutions were obtained as well as complete results on stability for a general positive and convex rate. These results can be applied to the case of heat explosion in a homogeneous medium. We study numerically the influence of natural convection on behavior of the two-phase system. If the Rayleigh number is sufficiently small, then the pure thermal regime [1,2] is stable and the medium is unmovable. If the Rayleigh number exceeds a critical value, then this solution losers its stability and convective structure appear. We study critical conditions of

convective instability and the interaction with convection.
 
 

Referenses:

  1. C. Barillon, G.M. Makhviladze, V. Volpert, Existence of solutions for an elliptic-algebraic
system describing heat explosion in a two-phase medium. Submitted.

2. C. Barillon, G.M. Makhviladze, V. Volpert, Stability of solutions for a model of heat explosion in

a two-phase medium. Submitted.

3. S. Goroshin, J.H.S. Lee, D.L. Frost, Combustion synthesis of ZnS in microgravity, 26h

International Symposium on Combustion, The Combustion Instotute, pp. 1651-1657, 1994.

4. A.G. Merzhanov, E.A. Shtessel, Free convection and thermal explosion in reactive systems,

Astronautica Acta, 1973, v.18, N 3, p.191-199.
 
 

O-4-06: Dynamics of thermal explosion in the post-induction period.

A.G.Merzhanov, N.I.Ozerkovskaya, K.G.Shkadinsky.

Institute of Structural Macrokinetics and Materials Science,

Russia Academy of Science, Chernogolovka, 142 432 Russia
 
 

In classical theory of thermal explosion the investigations were concentrated on heating up and thermal self-acceleration of reaction in the induction period. The post-induction period remained low-investigated since it was not necessary in that. Determination of the safety conditions required the nonexplosion regimes. In addition the further processes needed in the more complicated mathematical models for their description.

Development of problems of self-propagating high-temperature synthesis (SHS) intensified interest to thermal explosion as a synthetic method in inorganic media. Due to conservation of hard mixture during all time of process one can investigate the dynamics of complete thermal explosion by means of the uniform macrokinetic mathematical model - model of gasless combustion.

In present report by computing simulation the dynamics of thermal explosion was considered in the post-induction period and the complete nonstationary analysis was realized. The depth of the volume of medium and the depth of conversion in frontal regime were investigated in the induction period.

Two mechanisms of the front propagation in the post-induction period were defined. We showed that SHS in conditions of thermal explosion ( samples in kiln or in the grafhite press-form ) has essential stage of self-propagating front. Therefore it is more correct to considere thermal explosion like way of the front synthesis initiation together with changing the initial temperature of mixture. The investigation were carried out for the wide diapason of changing parameters Fk, Bi, Td, Ze and for the different macrokonetic laws of chemical conversion.

Supported by Int’l. Sci. & Tech.Ctr. Grant 355-97.
 
 

O-4-07: Electrothermal Explosion and Gasless Combustion in

SHS Systems Containing a Melting Reagent

A.S. Shteinberg, K.V. Popov

Institute of Structural Macrokinetics and Materials Science

Chernogolovka, 142432 Russia
 
 

Macrokinetics of high-temperature interaction in SHS systems under conditions of electrothermal explosion (ETE) is studied. In the experiments, both powder and layered samples were used. The latter were made by rolling up of metal foils (Ni, Al, Ti). The intensity of heat release during the first stage (responsible for the SHS front propagation) of the reaction was shown to be limited by the rate of refractory reagent dissolution in the melt of low-melting reagent (Ni in Al, Ti in Al). Differences in macrokinetics of heat release were found to depend on differences in phase diagrams. In particular, for the Ti–Al system, nonlinear dependence of the liquidus temperature on the green mixture composition courses exponential (pseudo-Arrhenius) increase in the rate of dissolution with temperature increase. Macrokinetic data obtained by ETE were used for analysis of mechanism of gasless combustion in SHS systems containing melting reagent.

In this connection, some data on SHS in layered samples obtained by PVD (material widely applied in semiconductor industry) are considered. Good agreement of theoretical and experimental results testifies that joint application of kinetic method of ETE and SHS research is an exceptionally promising direction for study of layered samples with limiting layer thickness (down to 100 Å).
 
 

O-4-08: Modeling of Reactive Synthesis in Consolidated Blends of

B4C-Ti and BN-Ti

L.Klinger, I.Gotman

Department of Materials Engineering, Technion, Haifa 32000, Israel
 
 

The model was developed to describe kinetics of reactive synthesis of ceramic matrix in situ composites from consolidated blends of B4C-Ti and BN-Ti powders. Interaction of B4C and of BN with Ti results in formation of multiphase diffusion zone when part of the phase act as diffusion barriers hampering formation and growth of other phases. Kinetics of phase growth was calculated taking into account temperature change and local heat evolved. Experimental data for rate of growth of phases obtained on planar systems as well as obtained by measurement of temperature change with time during exothermic reactions in the systems above were used in determination of kinetic parameters necessary for development of the model. Effect of interface barriers as well as external parameters such as powder particle size, initial temperature, heat exchange conditions , on synthesis kinetics were investigated. The model predicts conditions for synthesis via thermal explosion mode of SHS for the systems considered.
 
 

O-4-09: Computer Simulation of Electrothermal Explosion in the Ni-Al System

V.A. Gorelski, A.Yu. Smolin, V.S. Vladimirov

Tomsk State University,Tomsk Branch of the Institute of Structural Macrokinetics

and Materials Science of Russian Academy of Science, Tomsk (Russia)
 
 

A problem of selfignition using Joule heating including starting of flame propagation in a cylindrical sample of Ni-Al mixture is considered in two-dimensional axisymmetrical statement. The mathematical model consists of the equation of energy conservation and the equation for chemical reaction rate. Following experimental ETE data the rate of the main first stage of the reaction for Ni-Al mixture is assumed to be limited by solution of nickel in melted aluminum. The temperature at the end faces of the cylinder is assumed to be constant and equal to the temperature of electrodes, heat loss at the side surface of the cylinder is described by the Newton law. The problem is solved numerically using the finite element method. The intensity of Joule heating and the parameter of heat exchange at the side surface are varied in wide range. The dependence of the critical conditions of ignition on the varied parameters are investigated. The distributions of temperature in the cylinder at different times are presented. It is shown that when the intensity of Joule heating is small the reaction is initiated at the central part of the axis of the cylinder and propagates to the boundaries. When the intensity of heating is high the electrothermal explosion of Semenov regime takes place.
 
 

O-4-10: Effect of Shock Waves on SHS

Yu.A. Gordopolov, R.M. Shikhverdiev

Institute of Structural Macrokinetics and Materials Science

Chernogolovka, 142432 Russia
 
 

The effect of shock waves on chemical reactions in the Ti–C and Ti–B systems was investigated. The particle size was 20 (m for Ti powder, 5 (m for graphite, and 7 (m for amorphous B. Shock-driven reactions were carried out in cilyndrical or planar recovery systems. In cylindrical geometry, shock waves in the axial direction were initiated by an explosive. In planar geometry, shock wave was applied through an Al impactor. At relatively low pressures, shock-driven reactions were found to yield stoichiometric end products (TiC or TiB2). At higher pressures, chemical reactioon did not proceed altogether. Upon further increase in applied pressure, reaction was found to proceed first partially and then completely. Such a behavior was attributed to changes in the reaction root caused by shock loading. For the Ti–C system, chemical reaction under high pressure above 107 kbar was found to proceed (at least, partially) within the shock wave front.
 
 

O-4-11: Modeling of Solid-Phase Detonation

H. J. Viljoen

Department of Chemical EngineeringUniversity of Nebraska – Lincoln

Lincoln, NE 68588-0126, USA
 
 

Mixtures of solid reactants are usually ignited thermally, setting of a slow deflagration through the medium, and progressive layers of material are heated by thermal conduction. When the system is shocked to pressures above its Hugoniot elastic limit, the solids behave like fluids, and strong heating by compression also results. In perfectly mixed systems, this leads to the propagation of the reaction front at velocities which are very high, even exceeding the sound velocity in the medium. The chemical reaction can track the shock wave, completely converting reactants and propagating at velocities of several km/s. Several underlying issues will be discussed in more detail: 1) modeling the pore collapse, 2) the equation of state (EOS) and its validity over a range of porosities, including anomalous Hugoniots, 3) steady and oscillatory propagation, 4) mixing on particulate and atomic levels, and 5) sustenance of the shock wave by the chemical reaction and the interchange between internal and translational energies.
 
 

O-4-12: Detonation Type Autowave in Phase and Chemical Transformation

Processes in Condensed Matter (Gasless Detonation)

A.Pumir 1, V.Barelko 2

1 Institut Nonlineaire de Nice, CNRS, Nice, France

2 Institute of Chemical Physics Researches RAS, Chernogolovka, Russia
 
 

Fast self-sustained waves (autowaves) associated with chemical or phase transformations are observed in many situations in condensed matter: wave modes in cryochemistry, wave decay of metastable amorphous phases in semyconductors and tempered glasses, explosions of metal azids, geotechtonics phenomena and also cavitation phenomena in boiling. We propose a common simple model to describe these phenomena. The model is based on the concept of a coupling (feedback) between phase(chemical) transformation and apearing of the threshold value of stresses in solid matrix and may be classified as gasless detonation autowaves in solids. The model rests on the classical equation of elastic deformation in a 1-dimentional solid bar, with the extra as sumption that the phase (chemical) transformation induces a change of the elasticy modul (sound velocity) in matter. The transformation are assumed to occur through a chain-branched mechanism, which starts when the mechanical stress exeeds a given threshold.

Our investigation shows that supersonic autowaves exit in this gasless model (so as classical detonation). In the absence of a dissipation factor (losses), continuum of travelling wave solutions is found. In the presence of one( for example, diffusion) a steady state supersonic wave solution is found.
 
 
 
 

O-4-13: Application of Explosive Shock Compacting to Functionally Graded

Materials Produced by SHS Reaction

R. Tomoshige1, H. Tanaka 1, A. Kato1, K. Imamura2, A. Chiba2

1 Department of Applied Chemistry, Kumamoto Institute of Technology, 4-

22-1 Ikeda, Kumamoto-shi, Kumamoto 860-0082, Japan.

2 Department of Mechanical Engineering and Materials Science, Faculty of Engineering, Kumamoto University, 2-39-1 Kurokami, Kumamoto-shi, Kumamoto 860-8555, Japan.

The authors have been performing explosive shock compaction, utilizing simultaneously and complementarily SHS reaction. This technique is called a hot compaction method. Actually, we have tried thus far to produce some kinds of ceramics, intermetallic compounds and their composites from elemental or compound powder mixtures by using this technique, for example, TiC, TiAl, Ti4Al2C2 monolithic compounds, and TiC-Al2O3 and TiB2-TiN ceramic composites, etc. On the other hand, functionally graded materials (FGMs) consist of various materials which have different properties, e.g. coefficients of thermal expansion, melting points, and chemical stability, and so forth. Therefore, it is hard to fabricate FGMs by conventional sintering methods.

In this study, the hot-shock compaction technique was also applied to the production of FGMs. The study shows it to be possible to readily fabricate the dense TiC/Ti4Al2C2/TiAl and TiB2/TiNi/Cu FGMs by the hot shock compaction method. Especially, TiNi shape memory alloy was utilized in the TiB2/TiNi/Cu system FGMs as an effective material, in order to reduce the thermal thermal stress produced between both end materials by a reversible shape change of the alloy, so-called pseudo-elastic effect. The FGMs were evaluated on their microstructural characteristics and thermal properties. The particles in the FGMs bonded strongly with each other. EDS analysis revealed that each element was graded smoothly. Thermal shock tests revealed that the as-compacted FGMs had high interlayer bonding strength, no exfoliation and no microcracks. Thus, the SHS reaction had the potential to easily overcome the difficulties in FGM fabrication
 
 

O-4-14: Optimum Choice of Three-Parameter Expression to Approximate

Dynamic Adiabatic Curves of Shs Systems and other Condensed Media

V.S.Trofimov

Institute of Structural Macrokinetics and Materials Science Russian Academy of Sciences Chernogolovka, 142432, Russia
 
 

Analysis of shock and detonation adiabatic curves thermodynamic equations was carried out at additional as sumption that was not used till now at the present context. Namely, it was assumed that Grtneisen coefficient monoto-nously approaches to its limit corresponding infinite increase in pressure at constant volume. It turned out, that this assumption essentially restricts type of expressions describing dynamic adiabatic curves of condensed media. As a result it is succeed to describe uniformly equations of state (dependence temperature of volume and pressure) and caloric equations of state (dependence specific energy of volume and pressure) of various condensed media. On this basis new simple methods of shock and detonation waves parameters calculation in SHS-systems are proposed.
 
 

O-4-15: About Possibility of Gasless Detonation in Some Pyrotechnic Mixtures

E.A. Dobler, A.N. Gryadunov, S.A. Bostandjiyan, A.V. Utkin, V.E. Fortov

Institute of Structural Macrokinetics and Materials Science,

Institute of Chemical Physics Russia Academy of Science, Chernogolovka, 142 432 Russia
 
 

Possibility of detonation in high energetic reactive systems determined by simple equation

QPV<0 (1),

where QPV is the heat of the reaction at constant volume and pressure. This demand can be easily satisfied for reacting systems produces gases. But condition (1) holds and for some systems which are not producing gases systems. Most energy of the reaction in such systems are revealed in the form of the heat rather then in the form of the mechanical work and all work produced are performed by heat expansion of condensed products. So, detonation in such systems can be designated as gasless or heat detonation.

Satisfaction of (1) means that detonation can occur at some (may be infinite) diameters of reactive substance greater then some critical value. To provide detonation in the reasonable amount of substance the second demand must be satisfied: the time of reaction must be extremely small-a some microseconds (condition 2).

Current report devoted to the evaluation of possibility of the gasless detonation in metal - metal oxide pyrotechnic mixtures. Theoretical modelling of shock adiobates of reacting powders and detonation adiobates of products was used to test the possibility of the detonation regimes in some systems at different initial porosity’s. Necessary conditions for satisfying of (2) are discussed on the base of the experimental dates. As a result, the list of some perspective for gasless detonation systems with parameters of the possible detonation regime is presented.

The work was supported by Russian Foundation for Basic Research (grants ¹ 96-03-32703a, ¹ 98-03-32201a, ¹ 99-03-32262a) and the Russian Academy of Sciences (within the competition of the young scientists projects in the field of chemistry, physical chemistry, chemical physics and chemical engineering initiated by the RAS Presidium, Project No.47, Decree 272, of July 13, 1998).
 
 

O-4-16: Liquid Flame in Gravitational Field

K.G. Shkadinsky1, G.V. Shkadinskaya1, B.J. Matkowsky2

1 Institute of Structural Macrokinetics and Materials Science, Institute of Chemical Physics Russian Academy of Sciences, Chernogolovka, 142432, Russia

2 Northwestern University, Evanston, IL USA
 
 

In this work we investigate some phenomena of the self-propagating high-temperature synthesis (SHS) in the heterogeneous mixture of solid reactants which initially forms a solid matrix. The combustion temperatures of the high-caloric inorganic mixture are extremely high. The initially solid matrix is destroyed by the phase transitions and the chemical conversions of part of components. In this case the exothermic conversion, what stimulates the front propagation, is realized in the melt, which is gas-liquid-solid suspension. This kind of combustion is named “liquid flame”. The initially fixed components of the solid hard matrix receive freedom of the relative motion.

Multiple experiments on centrifuges, in the microgravity conditions and on earth showed existence (or absence) of separation in liquid products due to difference of the component densities. However the gravity separation begins after the solid matrix destruction (where components receive freedom of relative motion), i.e. in the preheat zone of the combustion front. Distribution of concentration and temperature is changed in the reaction zone. That influences on the front propagation velocity. In the present report the influence of the gravity fields on the propagation of liquid flame is investigated. We formulated a nonstationary mathematical model of liquid flame in the gravitational force fields. We carried out numericalsimulation and the analitical approximations of the model and compare to the experimental results. We studied the factors increasing (or decreasing) the combustion velocity with increase of the gravity acceleration. We showed that increase of the gravity acceleration can change the adiabatic temperature of combustion and the product structure. As for solid flame there are instability and nonuniqueness of the front propagation of liquid flame.

Supported in part by NASA Grant NAG3-1608, Int’l. Sci. & Tech. Ctr. Grant 355-97.
 
 
 
 
 

O-4-17: SHS processing of the Metal-Sulphides Refractory Glass-Ceramics. Ground-Based and Microgravity Experiments

J.H.S. Lee1, S. Goroshin1, R. Herring2

1McGill University, Montreal, Quebec Canada,E-mail: goroshin@mecheng.lan.mcgill.ca

2Canadian Space Agency, St-Hubert, Quebec, Canada,E-mail: rodney.herring@space.gc.ca
 
 

The progress in telecommunication technologies, infra-red sensors, windows and medical devices requires development of new materials transparent in the 8-22 m m wavelength range. Chalcogenid glasses based on the rare-earth sulphides such as La-Zn-S and La-Ca-S with heavy ions and relatively weak covalent bonding are the primary candidates for long wavelength IR materials. Lanthanum-based complex refractory sulfide glass ceramics are traditionally prepared from binary sulphides by hot sintering under higher pressure. It is difficult to avoid crystal growth, loss of sulphur and to obtain substantial amount of the glass phase during long-duration sintering process. An SHS process for manufacturing lanthanum-zinc-sulfide glass ceramics was developed recently at McGill University and produced the desired compound in a molten state. Melt solidification rendered a more superior product, with smaller crystals and higher volume of the glass phase than the sintering process. The SHS of Lanthanum-Zinc-Sulfide glass ceramic was performed in a high pressure argon environment using pressed La2S3-Zn-S initial powder mixtures with an SHS heating blanket using a Mn-Cr-S mixture. Detailed characterization of the produced glass ceramics samples included petrographic microscopic examination and SEM-EDS-BSE (Scanning Electron Microscope - Energy Disperse Spectrometry and Back-Scattered-Electron Imaginary) analysis. Even though lanthanum concentration in the samples produced was limited to 18 at. % ( since higher concentrations of La2S3 will decrease flame temperature below the product melting point in the present scheme) analysis shows that glass phase composes 70% to 80% of the sample volume. Microgravity environment (parabolic flight aircraft) will eliminate particle sedimentation in molten sulphur and therefore permit the synthesis of La-Zn-S from elemental precursors (i.e. La-Zn-S powder mixture) using commercially available coarse lanthanum powders. It is expected that the higher flame temperatures will result in higher lanthanum concentrations in the SHS process in microgravity. Thus the quality of the synthesized chalcogenid glasses should be more superior. The microgravity experiments are planned for April 1999 and the results will be reported at the conference.
 
 

O-4-18: Combined Effect of Magnetic Field and Gravity on SHS

Yu.G.Morozov

Institute of Structural Macrokinetics and Materials Science,RAS,

Chernogolovka, 142432 Russia

The new phenomenon during SHS under magnetic compensation gravity of lithium based ferrite was discovered and magnetic properties of the synthesized ferrite were studied. Two sets of experiments were worked out. (i) - a gradient magnetic field (1 kOe near the attached bottom surface of the permanent magnet) was used to hang up the slight compacted mixture of raw materials during the combustion. After local initiation (from free surface of the samples) auto oscillating combustion wave front propagation took place in open air with relevant burning velocities and combustion temperatures. (ii) - the sample was disposed on the top surface of the magnet to force the gravity and magnetic attraction to work together. After SHS the all samples were crushed to study bulk magnetic properties of the material.

Final products exhibited magnetization and coercive force whose value were strong dependent on the orientation of attractive magnetic field with respect to the direction of gravity. (i) samples have greater magnetization and strong reduced values of coercive force compared to (ii) and convenient SHS condition samples. The most obvious effect of magnetic suspension took place when the combustion temperature was below the Curie point of both the raw material and final product (Li0.5Fe2.5O4). It is assumed that the observed increase in the magnetization and decrease in coercivity are caused by difference in magnetic particles separation phenomena in the gravity and magnetic field.
 
 

O-4-19: Gasless SHS in Microgravity

A.G. Merzhanov, A.S Rogachev, A.E. Sytschev

Institute o Structural Macrokinetics and Materials Science Russian Academy ofsciences

Chernogolovka, Moscow, 142432 Russia
 
 

It is known, that for the most SHS systems, steady combustion is observed even at the bulk charge density (minimum relative density of common mixtures attained under gravity, -30%). Therefore, we can also expect that combustion will continue at still lower charge density. In this work, we investigated SHS processes in the pressed samples as well as in the bulk (loose) powders. In microgravity, the bulk powder forms a cloud of free-moving suspended particles. The goals of our experiments are elucidation of the possibility of SHS in such clouds and comparative analysis of SHS products produced under normal and microgravity conditions. Our experiments were conducted both under terrestrial conditions and aboard the MIR Space Station in 1997-98.

In our experiments, we used spherical particles of AI cladded with Ni (the thickness of a Ni coating corresponded to a ratio of Ni/A1=1/1 for each cladded particle). Two types of samples were used: (a) cylindrical pellets of cladded powder and (b) loose powders with a different bulk density.

The pressed samples burnt in space and on the ground retained their cylindrical shape and size. According to the data of electron probe microanalysts and X-ray diffraction, the chemical and phase compositions are identical (NiAl). As follows from ESM analysis, the NiAl grains are larger in the space-produced material. For this material, the intercrystalline fracture is prevailing. Meanwhile, the ground-produced material exhibits marked transcrystalline fracture. Apparently, microgravity affects the crystallization process due to the absence of convection.

Burning of the loose powder at normal gravity proceeds without volume change. The space-produced product (NiAl) exhibits higher porosity and acquires the shape of the ampoule. Combustion velocity in the loose powder (under terrestrial condition) is about 1.5 cm/s. At microgravity, for the first time we observed the propagation of gasless SHS wave in the particle cloud in vacuum. The space-produced material exhibits a high-porosity skeleton (bound) structure.

The experimental results shown that space-produced pressed samples of NiAl possess more perfect microstructure with larger crystal grains. Gasless combustion of the vacuum particle clouds is the most interesting result in the present work. A mechanism of this process must be studied. High-porous continuous structure is formed due to change in the shape and size of particles during combustion.
 
 

O-4-20: Effect of Microgravitation on Structure Formation of Refractory

Inorganic Materials in Cumbustion Wave

V.N Sanin, V.I. Yukhvid, A.G. Merzhanov.

Institute of Structural Macrokinetics and Materials science of RAS

Chernogolovka, Moscow Region, 142432 RUSSIA., E-mail: svn@ism.ac.ru
 
 

Two approaches to study of influence of microgravitation on SHS processes was considered. First way consists in realization of experiments in conditions of real space (station "MIR", the team of T. Musabaeva). Another way consists in realization experiments on the Earth in a centrifugal machine at positive (compression of the combustion zone) and negative (stretching) action of gravity forces and to interpolate the obtained data to point with zero gravity (microgravity). For experiments was chosen Ti-C, Ti-C-Ni and NiO-Ni-Al as model systems.

In the work our attention was focused on the effect of amount of a liquid phase in final and intermediate combustion products on gravitational sensitivity SHS processes and structure of resultant products. The detailed investigation of differences in course of SHS process and formation of final products in space and terrestrial conditions was carried out. The accuracy of the forecasting interpolated diagnostics was analyzed.
 
 

O-4-21:Self-Propagating High-Temperature Synthesis of Foam Cermet in Space

A.G. Merzhanov, V.A. Shcherbakov

Institute of Structural Macrokinetics and Materials Science,RAS, Chernogolovka, Moscow, 142432 Russia,Tel.: (7-095) 962-8000; Fax: (7-095) 962-8025; E-mail: head@ism.ac.ru SHS of cermet foam was performed in the Optizon-1 apparatus aboard the Mir space station by using the Ni–Al–Ti–C green mixture. We investigated the effect of gravity on the macro- and microstructure of the NiAl + TiC foam. The density of a porous product synthesized on the ground was found to change alongside the sample length. The same product synthesized in space was found to be uniform but comprise of several large fragments and large number of smaller pieces. Under zero-g conditions, the foam bubbles were found to form large voids at some sites in the material. The samples synthesized in space and on the ground were found to contain the well-developed crystals of TiC and NiAl. The mean grain size of TiC synthesized on the ground was one order of magnitude lower than that synthesized in space. In the former case, the TiC grains are distributed uniformly while in the latter case, nonuniformly.
 
 

O-4-22: Investigation of Effects of Electric Field on the Combustion Behavior in

the SHS Process by Use of the Heterogeneous Theory

Atsushi Makino

Shizuoka University, Hamamatsu 432-8561, Japan

Relevant to the Self-propagating High-temperature Synthesis (SHS) process, an analytical study has been made to investigate effects of electric field on the combustion behavior because the electric field is indispensable for some systems to sustain flame propagation. In the present investigation, use has been made of the heterogeneous theory which can satisfactorily account for the premixed mode of the bulk flame propagation supported by the nonpremixed mode of particle consumption. Although it is well-known that the burning velocity is enhanced by the application of the electric field, due to an increase in external energy supply, it is found that there exists the limit of the electric field for the steady flame propagation. In addition, it is found that even under the application of the electric field, the heat loss exerts influences on both the burning velocity and the range of flammability. Since the heat loss is closely related to particle size and dimensions of compacted specimen, this identification provides useful insight into manipulating the SHS flame propagation and/or extinction when the electric field is applied for the SHS process, by choosing an appropriate combination of the particle size and the dimensions of compacted specimen. A comparison between the predicted and experimental results further demonstrates appropriateness of the present study through a fair degree of agreement, as far as the trend and the approximate magnitude are concerned.
 
 

O-4-23: A Splitting Model for Interaction between Intense HF

Radiation and SHS Systems

S. E. Zakiev

Institute of Structural Macrokinetics and Materials Science,
Russian Academy of Sciences, Chernogolovka, 142432 Russia
 
 

A model based on splitting mathematics (V.Weston, G.Kristensson, S.Ricte, I.Aberg, B.Shevtsov) was used to describe interaction between HF radiation (including radio frequencies) and SHS systems on the macrokinetic level.

The model involves (1) the constitutive relations of the Maxwell equation for a green mixture (nonquasi-stationary approach), (2) specially transformed Maxwell equations (suggested for the first time), and (3) formal scheme for nonstationary energy dissipation in SHS systems. The model was applied to a real SHS system taken as an example. In view of complexity of the processes taking place in real SHS systems, predictions of the model are only qualitative in their character. Instead of existing "refined" modifications of the splitting technique, the model utilizes a more rough approximation based on the Laplace–Karson operational calculus. By its simplicity and physical clearness, the suggested technique was found to be comparable with macrokinetic modeling. The model has two advantages important for practical applications of SHS: (1) it is based on simple mathematics (convenient for experimental checking of results) and applicable to both direct and reverse problems of energy dissipation and (2) it provides new convenient approaches to classification of the electrical properties of SHS systems.
 
 

O-4-24: On the Field-Activated Combustion Synthesis of Titanium Aluminides

R. Orru’1#, G. Cao2, Z.A.Munir1

1Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA.

2Dipartimento di Ingegneria Chimica e Materiali, University of Cagliari, Piazza D'Armi, 09123 Cagliari, Italy.

# On leave from (2)
 
 

The feasibility of synthesizing the titanium aluminides Ti3Al and TiAl through field-activated self-propagating combustion synthesis is demonstrated. A self-sustaining combustion wave can be initiated only when the imposed field is above a threshold value for each of these two aluminides. At the threshold values, wave propagation resulted in an incomplete reaction between the metals and the products contained several phases in addition to the desired one. As the field strength was increased, the reaction approached completion and the amounts of the secondary phases decreased. At a sufficiently high field, a single-phase product was obtained in the case of Ti3Al but not in the case of TiAl. For the latter, the product contained Ti3Al as a secondary phase even with the highest imposed field. The effect of reactant compact density was investigated for the case of Ti3Al synthesis. At a fixed value of imposed field, the degree of reaction completion and the conversion to the desired phase increased as the relative density decreased. These observations are discussed in light of the role of the electric field in activating the self-propagating combustion synthesis reactions and the effect of relative density on this activation. The results show that the synthesis by SHS can be optimized by the combination of field strength and relative density.

The reaction mechanism of the field-activated combustion synthesis (FACS) of the phase Ti3Al from elemental powders is also investigated. Intermediates of the reaction leading to final product are identified by performing X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM) of "quenched" samples obtained by turning off the applied field during wave propagation. The combustion reaction is preceded by the melting of aluminum, which then spreads over the titanium particles. A reaction between liquid aluminum and titanium results in the formation of a layer of TiAl3 which grows around the original Ti particles until the aluminum disappears completely. Subsequently, the remaining (unreacted) Ti interacts with TiAl3. At this stage, the particles consist of Ti cores surrounded by a layer of Ti3Al which in turn is surrounded by another layer of TiAl. This interaction further proceeds leading to a shrinkage of the Ti cores, the gradual disappearance of the TiAl layer, and the simultaneous increase of the Ti3Al content. A single Ti3Al phase product is finally obtained.
 
 
 
 

O-4-25: Induction Field-Assisted SHS of AlN-SiC Solid Solutions

M. Ohyanagi1, T. Mizoshita1, K. Yamaguchi1, M. Koizumi1, Z.A.Munir2

1 High-tech Research Center and Dept. of Materials Chemistry,

Faculty of Science and Technology, Ryukoku University, Ohtsu 520-21, Japan

2 Facility for Advanced Combustion Synthesis, Department of Chemical

Engineering and Materials Science, University of California, Davis, CA 95612, USA
 
 

To overcome the thermodynamic and kinetic limitations of SHS reactions, the method of field-activation was developed by Munir et al.[1] to prepare materials which heretofore could not be synthesized by SHS. Field-activation is realized by the simultaneous imposition of an electric field and an ignition source on compacted reactant powders. Enhancement of the combustion process by field-activation was shown to have a strong dependence on the electrical conductivity of the reactants and products. It has been utilized in the synthesis of ceramic, intermetallic, and composite phases. An example of the latter is the synthesis of AlN-SiC composites and solid solutions [2].

In this work we describe another modification of the field-activated process: the use of an induced current. Samples covered with sheets of carbon foil are placed inside an induction coil. Depending on the electrical conductivity of the reactants, the induced current will be localized in the foil or in both foil and the outer sample layer. Analogous to the case of regular field-activation, two sources of energy can be present inside the reaction zone: chemical and electrical. Moreover, the use of the induction field-activation facilitates post-combustion annealing of the product. In this paper we describe the results of an investigation on the induction field-activated synthesis of AlN-SiC solid solutions and compare these results with those obtained under normal field-activation.
 
 

Referenses:

1. Z. A. Munir, W. Lai, and K. Ewald, , U. S. Patent No. 5,380,409, January 10,

(1995); A. Feng, and Z. A. Munir, J. Appl. Phys.,1994, v. 76, p.1927..

2. H.Xue and Z.A.Munir, Ssripta Mater., 1996, v. 35, p.979; J. Euro. Ceram. Soc.,1997,v. 101, p.1787
 
 

O-4-26: SHS in an External Magnetic Field (1.1T, 2T, 4T, 6T, 10T, 12T, 16T,

20T); Preparation and Characterization of Ferrites.

I.P.Parkin*1, M.V.Kuznetsov2, L.Affleck1, Q.A.Pankhurst3, M.Baumfa4,J.Perenbloom4.

1 Department of Chemistry, University College London, 20 Gordon Street, London, UK WC1H 0AJ. (_ HYPERLINK mailto:i.p.parkin@ucl.ac.uk __i.p.parkin@ucl.ac.uk_)

2 Institute for Structural Macrokinetics and Materials Science RAS, Chernogolovka, 142432 Russia

3 Department of Physics, University College London, Gower Street, London, UK WC1H 6BT

4 Department of Physics, University of Nijmegen, Holland
 
 

An extensive selection of industrially important ferrites have been made by the SHS reactions of metal oxides with iron powder. Materials made include the hexagonal ferrites MFe12O19 (M = Ba, Sr, Pb); the spinel ferrites M’Fe2O4 (M’ = Mg, Ca, Ba, Zn, Cr, Ni, Li) and the ferrite garnets Y3Fe5O12. In particular the effects of chromium substitution has been investigated, especially with regard to the observation of dramatic changes in structural and magnetic properties. All of these reactions have also been studied in an external magnetic field. The use of an external magnetic field during SHS, when using iron powder as a fuel source, increases the reaction temperature and propagation velocity. Studies of this effect using a 1.1T Halbach cylinder magnetic and a variable field Bitter coil magnet (0, 2, 4, 6, 10, 12, 16, 20T) will be presented. Preliminary analysis of the data indicates that saturation magnetization, coercivity and remanent magnetization of the materials prepared in an external field are greatly different to those prepared in zero field. In particular a fourfold reduction in the coercivity of BaFe12O19 is observed. The microstructure of BaFe12O19 prepared powders in an external magnetic field show a “metallic” fused component. Electron composition maps of this material show the pathway for the SHS reactions. In the lithium ferrite series, samples prepared in an external magnetic field showed 85% superstructured Li0.5Fe2.5O4 as compared to the materials synthesized in zero field which showed 15% superstructured component.
 
 

O-4-27: Mathematical Modeling of Frontal Polymerization

V.A. Volpert

Department of Engineering Sciences and Applied Mathematics, Northwestern University

Evanston, Illinois 60208-3125, U.S.A.

Frontal polymerization (FP) is a process in which a spatially localized reaction zone propagates into a monomer, converting it into a polymer. The process is currently under investigation as a novel method to produce polymers.

We develop mathematical models of FP and determine the structure of the polymerization wave and its propagation velocity as well as their dependence on the parameters of the problem. Our analytic results are in good quantitative agreement with both numerical simulations of the model and experimental data.
 
 

O-4-28: The Autowave Modes of Solid Phase Polymerization in

Two- and Three-Dimensional Composition Matrixes on the

Base of Fiberglass Fillers Appreting by Metal-Containing Monomers

V.V. Barelko, A.D. Pomogailo, G.I. Dzhardimalieva,

S.I. Evstratova, A.S. Rozenberg, B.M. Zuev

Institute of Chemical Physics Russian Academy of Sciences

Chernogolovka, 142432, Russia
 
 

The phenomenon of autowave (frontal) solid phase polymerization of metal-containing monomers based on metal-acrylamide complexe is considered. The comparison of the features of autowave processes realized in both the single-component matrixes of the monomer and the matrixes filled by the fiberglass materials is performed. The unstable regimes of the front travelling of the polymerization wave as well as the conditions for the stabilization of the flat front in the filled matrices are described. The peculiarities of the frontal regimes in the three- and two-dimensional media are studied. Some possibilities for using of the autowave polymerization in the fabrication technology of the polymer-fiberglass composites and composition prepregs are discussed.
 
 

O-4-29: Organoelemental Synthesis in an SHS Mode

E.G. Klimchuk, A.A. Khodak, V.Kh. S’undukova, A.G. Merzhanov

Institute of Structural Macrokinetics Russian Academy of Sciences, Chernogolovka, 142432, Russia, tel/fax:(095)9628040, E-mail: klim@ism.ac.ru

In the previous publications we established the opportunity of synthesis of useful organic products at realization of the protonation, oxidation and halogenation reactions in SHS mode into mixes of organic powder compounds.

The important task of the subsequent researches is increasing of number of reactions classes, in which organic SHS is possible. In this connection, the attempts to organize autowave modes of reactions in organoelemental synthesis systems triphenylphospin/chloramine "B"and ferrozen/phtalic anhydrid/AlCl3 were undertaken.

The thermogravmetric research has shown existence of exothermic interaction near to melting point of low-melting reagent. In equimolar mixes of powders was a success to organize the SHS wave. The main schemes of chemical interaction are offered on the basis of the analysis data:

                                                                                                                                                         | ---------->  (C6H5)3 PNSO2C6H5+NaCl+3H2O
                                                                                                                   |
                                                                                (C6H5)3 P + C6H5SO2NNaCl· 3H2O---- |
                                                                                                                                                          |
                                                                                                                                                          | ----------> (C6H5)3PO+C6H5SO2NH2+NaCl+2H2O
 
 

(wave velocity u @ 4,0 mm/sec, max temperature Tm @ 1900C, conversion a = 100 %, output of a target product triphenyl-(phenylsulfonamide)phospinimide b@ 40 %);

AlCl3
Fe(C5H5)2 + C6H4C2O3    -------------------------->  C5H5Fe C5H4COC6H4COOH



(u @ 1 mm/sec, Tm @ 190 C, a = 100 %, output of a target product o-carboxybenzoilferrozene b @ 16 %).

The results on study of the various factors influence on the processes macrokinetics and structure of products, microstructure of synthesized substances, date on DTA, NMR, X-ray analysis are discussed.
 
 

O-4-30: Frontal polymerization under conditions of weak convection

K.Kostarev, T.Yudina

Institute of Continuous Media Mechanics, RAS, 1, Korolyov st., Perm,

614013, Russia;Tel. +7 (3422) 391 365; Fax: +7 (3422) 336 957
 
 

Realization of frontal polymerization in real conditions is essentially restricted by a number of factors introducing characteristic changes in the course of reaction, in contrast to its physical-chemical models. One of these factors is free convection of the reaction mixture initiated by the very polymerization process [1]. The paper presents the results of experimental study of photoinitiated polymerization under conditions of weak convection when the process still preserves its frontal character. The investigation has been carried out using interferometry technique, which makes it possible to visualize distribution of monomer conversion. The reactors used in the experiments were horizontal rectangular glass cuvettes of different thickness (to allow control over the intensity of convective motion). During these experiments we realized the convection free- and weak convection- reaction regimes. We determined that in reactive systems, essentially differing by the response of polymerization front, the convection operates in a similar manner. It generates a thin monomer layer with a high degree of conversion, which spreads with a greater rate than the polymerization front. With increase of convection intensity, the amount of involved monomer is also increased, which finally leads to blocking of the reactor channel and generation of the secondary polymerization front.
 
 

Referense:

1. Briskman V.A., Kostarev K.G., Lyubimova T.P. et al. Polymerization under different gravity conditions. Acta Astronautic, 1996, v.39, N 5, p.395-402.