O-2-01: Usage of Filtrational Combustion for Accelerated Synthesis

Based on results of studies performed by the authors, specific one-stage technique of obtaining carbon adsorbents from timber wastes is proposed. It is based on thermocemical pyrolysis of raw material in the steam-gas flow from gas radiation porous burners. Under such conditions

cellulose containing materials are exposed simultaneously to intensive heat radiation and interaction with gaseous products of combustion. During contact heating high active radicals H, OH, HO₂ which are formed during gas filtrational combustion initiate destruction of bonds in polymeric chains with forming liquid and gaseous products and char residue. Combining pyrolysis and steam-gas activation under speedy heating (40 to 50 degrees per min) with short-term endurance at final processing temperature (near 600 - 700⁰C) leads to carbonization of impregnated timber and formation of porous absorbent's structure. Impregnating additives contain catalysts of charring and pore-forming substances. Again char residue yield increases 2 to 2.5 times and iongenic functional groups are introduced at the same time. Chars obtained in such a manner possess high consumption characteristics. They can absorb both organic compounds and ions of heavy metals and are comparatively cheap in the production. The process is characterized by high degree of ecological safety and is the basis to develop compact mobile devices to produce carbon adsorbents from timber wastes and other vegetation raw materials.
 
 

O-2-02: Kinetic Study of Synthesis of SiC Powders and Whiskers

The effect of KClO₃ and Teflon on maximum combustion temperature, reaction heating rate, and the combustion front velocity have been studied in detail. With increasing concentrations of KClO₃ (5 ~ 20 wt%) and Teflon (8 ~ 20 wt%), the maximum combustion temperature increases from 1400^oC to approximately 2000^oC. However, the combustion front velocity in the case of KClO₃ varies from 0.5 cm/sec to 2 cm/sec, and it is ten times faster than that when Teflon is used as the additive.

It has been found that the gas pressure has a very significant effect on the combustion characteristics. When the concentration of additives is greater than the minimum concentration, three different combustion regimes might exist. If the gas pressure is lower than minimum pressure, no propagation can be achieved. However, when the gas pressure is higher, low-temperature and high-temperature combustion waves might be generated. The propagation regime depends on the absolute value of the gas pressure.

The activation energy of SiC formation in the presence of KClO₃ and Teflon were estimated using non-isothermal technique and they were found to be 380 kJ/mole and 520 kJ/mole respectively.

It has also been demonstrated that SiC whiskers can be formed in the presence of above discussed additives and other growth promoters, such as Fe, Co and Ni. Mechanism of whisker growth and their morphology will be presented in detail.
 
 

O-2-03: Magnesium Thermite Method for Preparing Hydrides of Alkali Metals

The most known method for preparing hydrides of alkali metals is hydrogenation of molten alkali metals with molecular hydrogen at temperatures up to 700^oC during long time and that is the principal disadvantage of such methods. Besides, in order to prepare some hydrides it is necessary to use stirring means. In some cases hydrogenation is performed in inert organic solvents and that makes the process significantly more complex.

The proposed magnesium thermite method for preparing hydrides of alkali metals consists in reduction of hydroxides of alkali metals with magnesium that is perfomed in the combustion regime after ignition of a mixture and further hydrogenation of a formed in an active form alkali metal with evolved hydrogen at cooling the combustion products. In fact, both reactions proceed as a unique process.
 
 

MeOH + Mg = Me + MgO + 1/2H₂ (1)

Me + 1/2 H₂ = MeH (2)

MeOH + Mg = MeH + MgO (3)
 
 

The effect of various technological parameters (ratio of components, their dispersity, the relation between free volume and a mass mixture, etc.) on the yield and chemical activity of a hydride was studied. It was established that hydrides of alkali metals may be prepared by the proposed method with high yields (up to 90-95% LiH, 70-80% NaH and KH) at rather low hydrogen pressures

(5-10 ata) growing as a result of the reaction (1).

It was experimentally shown that inseparable mixtures of hydrides of alkali metals with magnesium oxide possess higher chemical activity than crystalline hydrides of alkali metals, which were prepared by direct hydrogenation of alkali metals. in particular, in exchange reactions with aluminium chloride at reduction of organic compounds and particularly upon using solvents or gases as drying agents.
 
 

Under high pressure nitrogen, AlN containing higher nitrogen content (33.5wt%) has been prepared by SHS method. The combustion reaction characteristics in the nitridation of Al was investigated by measuring nitrogen content. Based on calculating the free energy ((rGm() of reactions, the reaction mechanism of combustion synthesis of AlN is possibly the reactions between N2 and Al vapor and/or liquid Al. Based on experiments, the reaction mechanism of combustion synthesis of AlN is proposed. The primary structure of the combustion wave in the nitridation of Al is proposed. the reaction is divided into four zones, i.e. preheat zone, reaction zone (which, in turn, is divided into kinetics control stage, thermal explosion stage, transition stage and diffusion stage), after-burn zone and cold zone. The Activation Energy of the Kinetic Reaction Process is 6 KJ/mol.

SEM morphology of combustion products and its production mechanism have also been studied simply.

To apply AlN whisker commercially, it is necessary to produce it by an economic means. The AlN whiskers used in this investigation have been produced by SHS technique. Here, the morphology, chemistry, and growth mechanism of AlN whiskers have been investigated using high-resolution electron microscopy, and X-ray diffraction techniques. Emphasis has been placed on the crystallographic aspect of the mechanism of growth of the whiskers. The growth mechanisms of AlN whiskers are VLS and VS mechanism.
 
 

O-2-05: About Possibility of Use of SHS Nitrided Alloys for Production of

Nitrides

To extract the nitride phases through the acid enrichment, the solutions by hydrochloric and sulphuric acids in a wide concentration range (5-25%) were used. The kinetic curves of accumulation of iron, chromium, niobium, vanadium, titanium in the solutions of various concentrations are presented. Owing to the results of the acid enrichment investigation, treatment conditions of SHS nitrided ferroalloys were chosen; under these conditions the content of iron in metal nitrides and the losses of the latter are minimal.

Physical and chemical properties of the nitrides, being extracted, were investigated. Phase and elementary composition of obtained niobium, chromium, vanadium, titanium nitrides was determined with X-ray-phase and chemical analysis. The oxidation nitrides in air up to 1000^oC was investigated with the help of complex thermal analysis. The specific surface of the nitrides obtained was determined.

Thus, the nitrides of chromium, niobium, vanadium and titanium were produced through acid enrichment of SHS nitrided ferroalloys. Some of their physical and chemical properties were investigated.
 
 
 
 

O-2-06: Combustion Synthesis of Ternary Carbides Ti_xSc-xC_y Scandium

Hydride, Ti and C

The investigation of the combustion systems xTi + (1-x)ScH_2-z(Sc) + yC in hydrogen and argon atmosphere were carried out in wide ranges of Ti/ScH_2-z and Ti/Sc ratios at various carbon content (z<1; 0.4 < y < 1) .

Preliminary, the combustion process in Sc-H system was studied. The hydride of Sc (fcc structure, a = 4.76A, H-content - 2.0 %) was synthesized.

The thermal dissociation of ScH_2-z was studied using Derivatograph Q-1500. It was established that the dissociation began at 600^oC. The complete dissociation occured at 1000^oC. The powder of Sc of hcp structure (a=3.308; c=5.268) was obtained.

In this work ScH_2-z and Sc were used as initial reagents.The obtained experimental data prove that the combustion process in the above mentioned system is a two-stage process. In the first, high temperature stage, Ti + xC reaction takes place (when x > 5 and Ti/Sc > 0.5). Because of high temperature arised in the reaction zone, the dissociation of scandium hydride and its interaction with carbon begin. As a result, the scandium carbide is formed. Then depending on combustion temperture, full or partial dissolvation of ScC in TiC takes place. It has been found that the full dissolvation of this carbides is possible at Ti/ScH_2-z < 4. On the solubility of these carbides, the content of carbon in the initial mixture also influences.

This work was supported by ISTC. Grant A-192.
 
 

O-2-07: Hydriding Shs of Mg₂Nih₄

two pairs of the modifications of Mg₂NiH₄.
 
 
 
 

O-2-08: Transition Metal Nitrides Prepared by HP-SHS

References:

1. L.E. Toth,Transition Metal Carbides and Nitrides, Academic Press, 1971.

2. K. Schwarz, N. Rysch, J. Phys. C: Solid State Phys., 9 (1976) L433.

3. G. Ries, H. Winter, J. Phys. F: Metal Phys., 10 (1980) 1-7.

4. V.N. Lipatnikov, W. Lengauer, P. Ettmayer, E. Keil, G. Groboth, E. Kny, J. Alloys Comp., 261 (1997) 192-197.

5. K. Frisk, CALPHAD, 15 (1991) 79-106.

6. W. Mayr, W. Lengauer, P. Ettmayer, D. Rafaja, J. Bauer, M. Bohn, Def. Diff. For., 143-147 (1997) 569-574.

7. W. Huang, Metall. Trans. A, 27A (1996) 3591-3600.

8. M. Joguet, W. Lengauer, M. Bohn, J. Bauer, J. Alloys Comp., 269 (1998) 233-237.

9. Y. Du, R. Schmid-Fetzer, H. Ohtani, Z. Metallkd., 88 (1997) 545-556.

10.V. Buscaglia, F. Caracciolo, M. Ferretti, M. Minguzzi, R. Musenich, J. Alloys Comp., 266 (1998) 201-206.

11. R. Musenich, P. Fabbricatore, G. Gemme, R. Parodi, M. Viviani, B. Zhang, V. Buscaglia, C. Bottino, J. Alloys Comp., 209 (1994) 319-328.

12. M.W. Williams, K.M. Ralls, M.r. Pickus, J. Phys. Chem. Solids, 28 (1967) 333.

13. K.S. Keskar, T. Yamashita, Y. Onodera, Jap. J. Appl. Phys., 10 (1971) 370.
 
 

O-2-09: Peculiarities of SHS Fine-Dispersive Complex Titanium-

Chromium Carbide in Complex System

S.S. Mamyan, D. Belov, I.P. Borovinskaya

Thermodynamic researches have shown the possibility of the formation of that complex compound at the combustion of Ti - Cr - C - TiO₂ - NiO system. The experimental researches have shown that we can regulate the particle size, replace elementary Ni by its oxide and replace approximately to 20-30% elementary Ti by TiO₂ at SHS of this system.

The effect of the initial synthesis parameters on the product quality (particles morphology, size and distribution components, chemical composition, etc.) has also been shown.
 
 

O-2-10: Formation of Metal Sulfides in Combustion of Metal Nitrates Complex

Compounds with Sulfur-Containing Organic Ligands

Complex compounds of metal nitrates with hydrazine and its derivatives are knows as energy-rich substances able to undergo self-sustaining combustion [1, 2]. The presence of metal atoms in the complexes leads to the formation of solid products during combustion, along with gas-phase ones. This allows to consider these complexes as initial substances to obtain definite target products during combustion.

It was shown earlier that the combustion of complex compounds of nickel and copper nitrates with some hydrazine derivatives, e.g. semicarbazide, aminoguanidine, and oxalyl dihydrazide, leads to the formation of metal nickel and copper with porous, carcass structure [3-5].

In the present work we demonstrate the possibility to obtain sulfides of different metals during the combustion of such complexes. The formation of sulfides occurs if a sulfur-containing hydrazine derivative, thiosemicarbazide, is used as a ligand. Nickel, cobalt, copper, zinc and cadmium sulfides were obtained by this method. We also found that complexes of some metal nitrates with thiocarbamide (which are not so rich in energy) are also able to burn. Their combustion is also accompanied by the formation of the sulfides of corresponding metals.

It is important that the morphology and dispersity of the combustion products can be governed. This can be done by changing the conditions of their formation in the combustion wave by varying some parameters, e.g. external pressure, temperature and combustion rate. This effect is especially vivid in obtaining sublimable sulfides with rather high vapour pressure at the temperature of complex combustion.
 
 

References:

1. Shidlovsky A.A., Gorbunov V.V., Shmagin L.F. Izv.VUZov. Ser. Khim. i Khim. Tekhnolog.,1978, v.21, N 9, p.1249 (in Russian).
2. Sinditsky V.P., Fogelzang A.E. Rossiyski Khimicheskii Zhurnal, 1997,N 4, p.74 (in Russian).
3. Gavrilov A.I., Tukhtaev R.K., Larionov S.V., Lavrenova L.G., Saveljeva Z.A., Boldyrev V.V. Dokl.RAN, 1996, v 348, N 2, p.201.
4. Tukhtaev R.K., Gavrilov A.I., Larionov S.V., Saveljeva Z.A., Lavrenova L.G., Boldyrev V.V. Journal of Materials Synthesis and Processing, 1997, v.5, N 5, p.371.
5. Boldyrev V.V., Tukhtaev R.K., Gavrilov A.I., Larionov S.V., Saveljeva Z.A.,Lavrenova L.G. Russian Journal of Inorganic Chemistry, 1998, v.43, N 3, p.302.

O-2-11: SHS of Metal Carbides; Reactions of Calcium Carbide with Anhydrous

(> 2,000°C) under controlled atmospheres for prolonged time periods. We have developed two new routes to a wide range of transition metal carbides (TiC, ZrC, HfC, V8C7, NbC, TaC, Cr3C2, Mo2C, WC, Fe3C) from the self propagating reactions of aluminium carbide or calcium carbide with anhydrous metal chlorides (Eqn.1, Eqn.2). In the calcium carbide reactions crystalline metal carbides such as TaC can be made in less than ten seconds and the reaction promoted in air. The solid state metathesis reactions have been extended to the synthesis of solid solutions of mixed metal carbides and metal carbonitrides. Microstructure of the metal carbides has been investigated in detail by TEM and ELNES studies.

In the present work, the effect of the amount and chemical composition of the ammonium salt used, on the physic-chemical characteristics of the final products are studied.

Compositions using different amounts of Ammonium Chloride, Ammonium Fluoride and mixes of them are added to the raw materials to improve the Synthesis. Final products are characterized by SEM and XRD.
 
 

O-2-13: Chemistry and Technology of Combustion Synthesis of Complex

Oxides

O-2-14: Synthesis of Perovskites (ABO₃) by Combustion Process

CEP 13565-905, Sгo Carlos, Spain – Brazil, E-mail:morelli@power.ufscar.br
 
 

The development of ceramic materials with characteristic of laser properties, electrical conductivity, electro-optical and piezoelectric behaviour is a quite attractive subject of fundamental and technological research of the last years, due the properties highly peculiar that has been observed in these classes of materials. The work searches through the synthesis by combustion reaction the preparation of ABO₃ perovskite-type compounds, namely, LaAlO₃, LaCoO₃, LaCrO₃, LaNiO₃, LiNbO₃ powders, with adapted characteristics of high crystallinity and phase stability. The main objective is to evaluate the combustion technique for powder preparation as an alternative method to the sophisticated and complex processes, which involve high costs, besides the acquaintance conventional mixing of oxides that produces powders of low reactivity and/or with the presence of undesired secondary phases. Powder’s characteristics prepared by combustion reaction are presented and discussed. The purpose of this work is to attempt to bring together the information obtained from this study and to evaluate the combustion process as an alternative rote for preparing new perovskite compounds with new and improved properties.

The authors are thankful to FAPESP - State of Sгo Paulo Research Support Foundation - Brazil.
 
 

O-2-15: Self-Propagating High-Temperature Synthesis of Complex Chromium-

Containing Alkaline and Alkaline-Earth Metals Oxides

Synthesizing SHS products can be used in pigment mixtures; in the tunnery industry; like dry electrolytes; corrosion inhibitors etc.
 
 
 
 

O-2-16: Synthesis of Lattice Matched Cd-In-Ga Oxides for Gan Thin Film

Growth

³ Chemical Engineering Department, University of Houston,Houston, TX 77204, USA
 
 

Gallium nitride and associated III-N materials have various potential electro-optical applications. The lack of cost-effective latticed-matched substrates for growing these nitride films is a recognized problem. CdIn2-dGadO4, with lattice constants from 8.39 Е (d =2) to 9.17 Е (d =0) may be suitable substrates for growing various nitride thin films. For examples, CdIn1.673Ga0.327O4, having a lattice constant of 9.04 Е (2x cubic GaN), and CdIn0.952Ga1.048O4, having a lattice constant of 8.76 Е (2x AlN), may be substrates for growth of GaN and AlN thin films, respectively. The sointering/calcination processes to produce Cd-In-Ga oxides are time consuming and expensive. We studied the use of Self-propagating High-temperature Synthesis (SHS) to fabricate Cd-In-Ga oxides to circumvent these synthesis problems. We studied the impact of the standard SHS variables on the product quality at normal pressures using various precursor powders, pellet diameters and densities, oxygen flow rates, and additives. A high-pressure reactor was used to investigate the impact of oxygen pressure on product quality. The crystal structure and local stoichiometry of the product were determined by XRD and EMPA. The thermal behavior of the reactants at high temperatures was determined by TG/DTA. The reaction network and kinetics for the reaction were deduced from the temperature history during the combustion.
 
 

O-2-17: Disilicides Elaboration by Combustion Synthesis: Interest

of Using a Mechanical Activation Step

The mechanically activated SHS process is able to produce via a very fast combustion front (> 20 mms^-1) a pure “ bulk ” a-MoSi₂ with a nanometric structure (66nm). In addition, the nanostructure of the starting powder mixture (Mo+2Si) influences the propagation of the combustion front and modifies the values of thermal parameters during the process. In the case of Fe-Si system, mechanical activation allows to initiate a self-sustaining reaction after heating up to 400°C a cylindrical sample.

Thermal and structural informations describing the combustion front initiated in Fe-Si or in Mo-Si green samples were clearly determined using a Time-Resolved X-ray Diffraction (TRXRD) experiment coupled to an infrared camera. Mo+2Si-> MoSi₂ appears to be the only reaction, which is monitored with a temporal resolution of 30 ms inside the combustion front during SHS process. The existence of a combustion wave in Fe-Si system was assumed to be linked to the formation of FeSi compounds during gasless reaction.
 
 
 
 

O-2-18: The Effect of Mechanical Treatment on the Rate and Combustion

Limits of Shs Processes

It is known that it is difficult (or even impossible) to obtain a single-phase product by means of SHS in many intermetallic systems. MT of SHS products allows to carry out phase homogenisation. For example, the mixture of Ni₂Al₃ with NiAl after MT for 3 min is transformed into a single-phase product composed only of NiAl with the size of coherent scattering domains 8-10 nm. Electron microscopic studies of the changes in the morphology of particles in the reaction mixture during MT have been conducted.
 
 

O-2-19: Effect of High-Energy Milling Reactant on the Combustion Synthesis of

Ti₃SiC₂

O-2-20: Shs Synthesis of the Oxide Systems Based on the "Mechanochemical"

Quartz Modified by Organometallic Compounds

The use of divers alcohols and other organic additives in the mechanical quartz processing as a surface active substances makes possible the formation of polymer compound particles on the surfaces. During further heating of the particles having their modificated surfaces there are occurred organic compound decomposition and high-active radicals formation which participate in the redox processes and effect on the starting and kinetics of such processes running out.

In this paper the evaluation analysis of energy saturation of the quartz particles resulted of the mechanochemical processing in the centrifuge-planetary mills is carried out. This investigation showed that the variations of mechanical action conditions and its time duration (5-100 minutes) result in the observed stage-by-stage modification of the processed material structure and state. The quartz activation during 15 minutes results in the most pronounced effects, the synthesis starting temperature being decreased and making possible the most complete reaction running out at minimum charge content of aluminum (up to 6%). The energy saturation of the quartz processed individually or in the mixture with other oxides substantially depends on the used organic additions. The buthanol as a modifying additive is the most efficient to speed up the reactions between quartz and aluminum.

It is shown that the kinetic combustion characteristics, phase composition, structure and properties of the end synthesis products of the analyzed materials based on the SiO₂ + Al system depend on the organic and organometallic compound types formed on the surfaces of the quartz particles during preliminary mechanochemical processing and that the most pronounced effects on the SH-synthesis are attributed to the action of aluminum and organic compounds.

O-2-21: Mechanically Stimulated Self-Propagating Syntheses of

Coordination Compounds

In the course of systematic investigations we have shown wide possibilities of solid-state mechanochemical methods in the synthetic chemistry of coordination compounds such as metallocenes, b -diketonates, carboxylates, metallocarboranes etc. Physico-chemical (IR, XRD, TGA, DTA etc.) and preparative investigations of the reaction mixtures consisting of solid metal chlorides and salts of organic ligands allowed us to draw a corollary that all systems studied can be divided into three main groups depending on their response to mechanical loading.

The first group includes ordinary mechanochemical reactions, formation of products in which takes place directly in the course of mechanical loading, as, for example, formation of ferrocene in the system “FeCl₃ — TlC₅H₅”. The second group consists of the systems, mechanical loading of which leads initially to the formation and accumulation of activated mixture without formation of the final products. Formation and relative quantity of activated mixture can be detected by the appearance of a new exothermic event on DTA curves of mechanically loaded reaction mixtures. Prolongation of mechanical treatment after accumulation of the activated mixture in the systems leads to “explosive mechanochemical synthesis” - a sudden initiation of a fast exchange reaction leading to a vigorous self-heating and formation of final products. Bulk heating of the activated reaction mixtures leads to the exothermic interaction of reactants with formation of final products which can occur in the form of heat explosion. Exothermic interaction of reactants in such systems can be organized in the self-propagating mode: the action of a local thermal impulse on an end face of the specimen formed from the activated mixture causes the appearance of reaction zone which propagates along the specimen up to a full exhaustion of a material with formation of final products. “CrCl₃ — NaC₅H₇O₂” is an example of such systems.

The majority of the systems investigated takes an intermediate position between these extreme cases. That is, formation of final products and intermediate activated mixture occurs in the course of mechanical loading. Further transformation of mechanically activated mixtures to the products can be organized in various modes, including self-propagating, depending on conditions of the mechanical loading, organization of the process and chemical identity of reactants.

Characteristic features of self-propagating syntheses of coordination compounds are discussed.

We acknowledge support from RFBR (grants No 99-03-32253; 99-03-32900).
 
 

O-2-22: Mechanism of Interaction and Combustion Limits for Niobium and

The purpose of presented work is to study the interaction mechanism and expansion of combustion limits of Nb-C system on various parameters in conditions of activation.

Combustion limits of Nb-C system on following parameters are experimentally investigated: the size of metal particles, ratio of initial reagents, tablet diameter, amount of inert additive. Various activators of combustion with the purpose of activation both metal, and carbon were tested.

It was shown, that at introduction in an initial mixture Nb with C a few (less than 1%) chemical activator successfully selected for the given system - polytetrafluoroethylene, steady-state combustion in a wide range of values of various parameters takes place: the size of niobium particles (m _NbЈ 80 mm), initial tablet diameter (dі 10 mm), ratio of initial reagents (0,3Ј a Ј 1,0), amount of the inert additive in a mixture (up to 50 % mass.) etc.

It was shown, that the observable expansion of combustion limits is result of intensification of gas-transport transfer of niobium to carbon particles, by means of niobium fluorides. The similar studies on activation of carbon using various gas-generating additives, including of oxalates of metals, appreciable results have not brought.

It was established by X-ray and SEM analyses, that it’s possible to synthesize single-phase carbide NbC in the form of half-perfect crystallites by the sizes 5 ё15 microns at rather wide range of process parameters.

Complete distributions of temperature on combustion zones for Nb - aC mixtures in conditions of activation are received. Leading stages of process are established, sizes of reaction zones are estimated.
 
 
 
 

O-2-23: Shs in Fabrication of Composites

O-2-24: SHS of Composite Materials in Thin Multilayer Systems

O-2-25: Reactive-Casting of NiAl-Base Intermetallic Compounds

O-2-26: Processing of in-situ Nitride Reinforced Aluminum Matrix

Composites

H.J. Brinkman, J.Duszczyk, L. Katgerman

O-2-27: Hybrid SHS Technologies Based for Production of

Composite Materials and Coatings

- the development of new materials and chemical products using known technological types of the SHS processes;

- the development of novel technological versions and hybrid SHS-based technologies which in combination with other methods allows achieving super effects.

Let's take as super effect the creation of unique materials with a new, earlier inaccessible, level of properties and also a considerable growth of the technological process efficiency with a heightened product quality. Taking this point of view the method ofthermoreactive electrospark surface strengthening (TRESS) developed at the SHS- Center of the Moscow Steel and Alloys Institute can be classified with the hybrid technologies. The production of gradient ceramic and metal ceramic substrates for the subsequent precipitation of highly adhesive thick film coatings on them is another way of the effective SHS application. The report presents the results of the research connected with the synthesis of diamond containing graded materials with a thick diamond film. According to the force SHS- pressing technology poreless plates are produced with a variable through the thickness concentration of diamond grains from 0 to 60 vol. ^o in the cramic binder based on TiB₂, TiC etc. Then polycrystalline diamond films more then 30 u,m in thickness were precipitated on the plate surface by the CVD technology (in an arc discharge). A high adhesion of a coating to the substrate is an advantage of such composites. In contrast to the other technologies (powder metallurgy, HIP, HTHP) SHS allows to "seal up" a diamond grain into various ceramic binders, in particular into a highly heat conductive binder of titanium diboride. The composite's structure and properties were studied in the paper.
 
 

O-2-28: Multilayer Component Prepared by Combining Tape

Casting and SHS Process

J.P. Bonnet, S. Desiles, A.L. Dumont, T. Chartier, D.S. Smith

MoSi2-Al2O3 based composites can be prepared by S.H.S. reactions between MoO3, Al and Si or SiO2. Addition of Al2O3 or MoSi2 to the reactants allows to the relative amount of each phase in the final product to be controlled. It is then possible to obtain either electrically conducting (addition of MoSi2) or insulating composites (addition of Al2O3).

The addition of additives, which contribute to a decrease in the maximum temperature reached during the process, allows the amount of transient liquid to be altered. Therefore this parameter can be used as a processing variable.

Tapes of about 250 m m thickness of both MoSi2-rich and Al2O3-rich compositions are prepared by tape casting. Ten layers of each composition are then stacked alternately. After debinding, the S.H.S. reaction is initiated in the self-propagating mode and densification is achieved by the application of a small mechanical load.

The quality of the multilayer structure, as well as the efficiency of the densification step, are strongly dependent on the amount of transient liquid formed during the S.H.S. process. After optimisation of this parameter, dense multilayer plates were obtained. The final layers thickness is about 120 m m. Local electrical measurements confirmed the presence of alternating conducting

(0.1 W ) and insulating (10 MW ) layers.
 
 

O-2-29: Preparation of Carbide Fiber-Reinforced Composites

O-2-30: Characterization of Li-Cu Ferrite Powders Prepared by Self-

propagating High Temperature Synthesis

O-2-31: Mathematical Model of SHS Welding

S.A. Bostandgiyan

The whole of SHS- welding process may be divided into two stages. The first stage is complited out with electrothermal explosion with forming a charge melt. The second one involved in charge melt squeezing with simultaneous cooling and solidifying. Itis supposed that there is no reaction before melting temperature of charge metallic component is reached and heating is realized by Joule heat. The electrothermal explosion takes place when this temperature is reached. A reaction is proceeded instantly and the melt or suspension of reaction products are formed. Two postprocesses are realized after the electrothermal explosion: a) squeezing of a part of SHS-products; b) cooling and solidifying.

Squeezing in one’s turn may be divided into two stages. Simultaneously with melt squeezing melt cooling is occured. The first stage involved a period up to reaching of solidifying temperature. In this stage the problem is two-layer (solid electrode – liquid charge melt). After reaching of solidification temperature the problem transforms inton three layer (solid electrode – solidified reaction product – charge melt) with mooving boundary at which there is phase change. The systems of differential equations discribing different stages of SHS-welding process are developed.
 
 

O-2-32: In-Situ Joining of Nickel monoaluminide to Ni-base Superalloys

High gas pressure combustion synthesis is a new way for welding. In this process, a Ni-base braze is placed between a substrate of Ni-base superalloy and a compact made of 50at% Ni-50at% Al mixture. The heat released during the combustion synthesis of the Ni+Al compact gives rise to interdiffusion of the elements and consequently to a joining interface. The main advantage of this new process is an important decrease of welding temperature which allows to increase the number of repairs by turbine blade.

In this presentation, we will develop the results obtained for a Ni-base superalloy. In a first part, we will describe the main experimental conditions to optimise the joint’s quality. Then, using scanning electron microscope (SEM) and electron probe microanalyser, we have studied microstructures and concentration distributions across the joining interface. Finally, we will present first results of mechanical characterisation of the joining interface by distribution of hardness across the interface, three point bending test and shear test.

This work was supported by DGA (n°97-2527A/DSP/STTC) and by Sochata-SNECMA service (Chatellerault-France)

O-2-33: Influences of Mechanical Vibration on SHS Ceramic -Lined Pipes

and ceramic solidification process.
 
 
 
 

O-2-34: SHS Process for Manufacturing Aluminum Master Alloys by Use

of Flux

The influence of flux agents on the SHS process, technological and structural parameters of the master alloy has been studied. The flux function is based on the SHS process activation through the dissolution of oxide films from a surface of aluminum powder particles. It is determined that the increase in flux quantity leads to the rise of the SHS reaction temperature and reduction of its delay time. It makes possible to reduce the temperature of master alloy production by 100-200⁰C without the degradation of its quality, to increase the completeness of the running reaction. The quantity variation of the flux and aluminum powder in a SHS mixture provides a wide range of different morphologies of TiAl₃ particles- from needle-like, mixed to blocky and globular ones with the size of 10...15 m m. The small difference in the use of the two kinds of fluxes has been revealed. The NOCOLOK flux gives higher temperature of the SHS reaction (by 15-20⁰C), rises the melt fluidity, results in a finer structure of master alloy.

The optimal modes of the master alloy production with the cheaper Russian flux Na₃AlF₆. have been found. The flux application makes possible to control the SHS process in an aluminum melt and thereby effect on structural and technological parameters of a grain refiner.

____________________

NOCOLOK^TM is a registered trademark of Alcan Aluminum Ltd, Canada
 
 

O-2-35: Reaction Characteristics of the Fillers for SHS Welding of SiC Ceramic

O-2-36: Influence of Plastic Deformation on Microstructure and Properties

of Shs-Materials

²Institute of Structural Macrokinetics of RAS, Chernogolovka, 142432, Russia
 
 

The formation of a microstructure and mechanical properties in the nonstoichiometric titanium carbide was investigated by using three various methods of processing (C/Ti ratio was equal to 0.47): 1) the samples were prepared by SHS-reaction with the subsequent force compacting at high strain rates; 2) the samples were received after hot deformation of billets prepared by the 1^st method; 3) the samples were prepared by combining SHS-reaction and hot deformation under temperature-rate conditions of superplasticity of nonstoichiometric titanium carbide. The porosity of the samples processed by the given methods did not exceed 3%.

The processing of titanium carbide by the first method led to formation of an extremely non-equilibrium two-phase microstructure. The average size of carbide grains was 10-12 m m and the volume fraction of excess titanium phase amounted to 12%. The ductile-brittle transition (DBT) of such material was observed at the temperature 750° C.

The subsequent deformation of such material showed its microstructure is non-equilibrium. Concurrent development in titanium carbide of phase transformation TiC_0,47 ® b -Ti and dynamic recrystallization resulted in formation of a "microduplex" type microstructure with the average grain sizes of carbide and titanium phase about 2 m m at deformation in conditions of the enhanced strain rate sensitivity of flow stress (T=950° C, e =10^-3 s^-1). At that the volume fraction of titanium phase increased up to 30 vol.%, and the DBT-temperature decreased up to 650° C. Such material showed a much lower flow stress level, than that flow stress of titanium carbide produced by the 1^st method.

The processing of titanium carbide by the third method promoted the formation of an equilibrium microstructure with an average size of carbide grains of 10-12 m m. The DBT-temperature of such material was observed at 750° C. This material showed an essentially higher level of flow stress at high temperatures (900° C and higher). The chemical composition of titanium carbide did not change during the further deformation, and the microstructure retained its stability.

Thus, the comparison of three methods of preparation of nonstoichiometric titanium carbide using SHS-reaction and plastic deformation showed that the temperature-rate conditions of plastic deformation essentially influence the formation of microstructure during self-propagating high-temperature synthesis. The control of conditions of plastic deformation might allow us to influence the completeness of chemical reaction during SHS.
 
 

O-2-37: Densification of Condensed Systems During Combustion

Is allowed, that the capacity of combustion products to compaction is controlled by process of viscous flow at the expense of formation of hyperthermal fluid phases. In a general problem on combustion and densification the problems on a place of compacting are esteemed: behind front, in front and before combustion front.

The dependencies of combustion rate and final porosity on pressing efforts, and also on different ratio of characteristic times of densification both chemical reaction and cooling of products are analyzed. Influencing the fusible component on compaction of a material is during combustion. The analysis of transformation in time of initial distribution of density allows to observe propagation of densification boundary at moving of combustion wave along pattern.

This work was supported by RFBR Grant №98-03-32110a.
 
 

O-2-38: Formability of SHS-Materials

In the present activity the outcomes on usage of a method of free SHS-compaction for analysis of capacity to forming some widespread classes of SHS- materials are set up.

Are investigated of regularity of forming of some classes of SHS-systems with a different ratio of combustion temperature and melting of initial reactants. As the characteristic of capacity to forming it is offered to use parameter - degree of deformation having sense of relation final to the initial area of a sample. The analysis of relations of this testimonial from of quantity and structure of flow bundle is conducted.

This work was supported by RFBR Grant №98-03-32110a.
 
 

O-2-39: Production of Dense Molybdenum Disiliside Materials

Nearly fully dense (95 - 99 %) MoSi₂ materials were produced by combustion synthesis (SHS mode) of the elemental powders. Formation reaction for MoSi₂ has rather low adiabatic temperature (1627^oC), which makes it more difficult to obtain full density. Both the manufacturing process and the material composition have been developed to get high density and good high temperature properties. Experiments to alloy MoSi₂ by zirconium oxide, titanium carbide and silicon carbide were also carried out. The phase composition, impurity content and density of the materials were evaluated. Depending on the composition and process parameters, small amounts of Mo₅Si₃ phase were found in some samples but most samples had a single phase MoSi₂ structure. The materials have homogeneous microstructure and fine grain size. The microstructure and mechanical properties are compared to those manufactured by conventional processes.
 
 
 
 

O-2-40: New Technical Decisions Improving SHS Pressing Method

New technical decisions are developed and patented to improve the dimension accuracy of the article. To reduce the nonuniformity of the thermal field the side edges of the green compact are chamfered with dimension of 2 to 3 mm. To increase the axis contact stiffness of the tool: 1) the sheath thickness is reducer up to 2-3 mm simultaneously with cladding the green compact by a layer of heat-generating substance or 2) the sheath of the compact is performed thin rigid steel sheets.

At SHS pressing two objects are deformed plastically: the article and the sheath. The pressing power is distributed proportionally with energy consumption for deformation of the SHS product and the ring sheath around the article. To reduce the stiffness of the ring sheath and increase the press efficiency: 1) the ring sheath is made of material with a low stiffness or 2) the height of the ring sheath is increased in comparison with the central zone to reduce the rate of the axis deformation and compaction of the zone.
 
 

O-2-41: Metal-Like SHS Catalysts as Analogues of Noble Metals

E.H. Grigoryan

References:

D. reidel Publ. Comp., The Netherlands, 1983.

of SHS Catalysts

The proportion of spinels to simple oxides in Cu-Cr-O catalysts did not appear to depend to a substantial extent on the quenching media used. But significant differences were observed for their catalytic activity in the processes of deep methane oxidation and carbon monoxide and hydrocarbon oxidation. Detailed investigations using Scanning Electron Miscroscopy, EDX analysis and XRD showed that the underlying reason for the activity variation is the significant changes that take place in the microstructure in the materials.
 
 

O-2-43: Chemistry Reaction Processing and Microstructure Formation

In this paper, the TiO₂₂ B₂O₃₃ Mg SHS reaction system was chose as research objective ,the chemistry reaction processing and microstructure formation mechanism of this SHS reaction system were studied by DTA-TG,X-ray diffraction ,SEM ,EDS and Combustion Front Quenching (CFQ) . It was found that the B₂O₃ was reduced to B at the first step, and TiO₂ was reduced to Ti through following step: TiO₂2wTi₃O₅5iTi₂OOiTi . In this combustion processing ,the Mg content have a key role .The microstructure formation processing was studied in detail by SEM and EDS.
 
 

O-2-44: Nanocrystalline Ti (Zr) Hydrides Produced Combustion Synthesis

For obtaining the nanocrystalline hydrides the SHS is very advantageous in compare with the known methods i.e. reactive mechanical grinding or alloy under H₂ gas pressure of 1 MPa. We were successfull in synthesizing SHS nanohydrides. Nanocrystallization of hydrides of metals and alloys directly at combustion of metals in H₂ is an unusual phenomenon. Why does it take place, what is the SHS hydrogenation kinetic? These questions are discussed in the present work.

The results of analysis of materials by TEM, SEM and XRD methods are presented. It was established that for SHS technique the crystal grain of sizes smaller than 0.1 mkm are available. The complete mechanisms of event are not known yet, but these preliminary results can become the starting point of future experiments directed to explain the conditions of SHS process resulting in nanocrystalline hydride formation (hydrogen pressure, grain size, temperature and rate of combustion etc.) Nevertheless, now we are ready to say that apparently excellent peculiarities of SHS-hydrides are the results of instataneous rapid formation of nanocrystalls in combustion regime. From this point of view it will be useful and important to turn to the study of their physico-chemical properties (thermal, mechanical, magnetic, electrochemical, electrical resistance).

This work was supported by ISTC. Grant A-192.
 
 

O-2-45: SHS prepared BaSnO3; Comparison of SHS and Conventionally Prepared Materials as Sensors for CO2