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LEPL - FERDINAND TAVADZE METALLURGY AND MATERIALS SCIENCE INSTITUTE
SHS 2011 XI International Symposium on Self-Propagating High Temperature Synthesis

SHS 2011

XI International Symposium on Self-Propagating High Temperature Synthesis

 

5 - 9 September 2011

EDEN Beach Resort Hotel, Anavyssos, Attica, GREECE

web.ims.demokritos.gr/SHS2011

 

Method for Application of SHS-Compacting in Vacuum

 

D.V. Sakhvadze *1, G.F. Tavadze 1, A.S. Shteinberg 2, J. V. Khantadze 1, G.V. Jandieri 1

 

1 Ferdinand Tavadze Institute of Metallurgy and Materials Science, AS of Georgia;

ALOFT, Chem. Eng. and Mater. Sci. Consulting Inc., Berkeley, California, 94708, US

*david.sakhvadze@yahoo.com

 

SHS-compaction is a promising method for obtaining hard-alloy materials. SHS-compaction based technology including SHS-pressing and SHS-extrusion allows one to synthesize required material and to obtain end product by using an installation described in [1].

Due to high temperatures the process is characterized by high rates and conversion degrees, on the one hand, and high ductility of the end synthesis product on the other hand. That allows one to apply pressure to still hot combustion product to shape in desired manner thus in most cases directly obtaining end product. One of the main features of the process is that both reagents and final products are in condensed state. Under real conditions, even at SH-synthesis in mixtures of quite pure powders, combustion is always accompanied by emission of relatively small amounts of gases. This phenomenon may be related to such processes as desorption of gases from surface of the powders during their heating, evaporation of volatile impurities, de-oxidation of the oxide films, etc. [2]. 

The emission of gases breaks contacts between the reagent particles, causes shattering of the reaction mass and may finally cause decreasing of the target material conversion degree. It is also noteworthy that due to high rates of the combustion product cooling, some amount of gas fails to escape from the target material prior to the pressing stage beginning. This gas forms the closed pores in the compacted material.  

According to the literature data [3], at SHS-pressing of the TiC-TiB2 system the increase in impurity-related gas emission in the range of 30-60 cm3/g causes significant growth of the target

material porosity. M. Ponomaryov [ 4 , 5] showed experimentally that gas evolution takes place in the preheat zone that is before the chemical reaction responsible for SHS. The evolving gas separates surfaces of the reagents causing worsening diffusion conditions and slowing down macrokinetic processes. This mechanism is indicated by an unusual dependence of the combustion rate on the sample thickness (for a wide range of this parameter): the SHS front propagation velocity decreases as the sample thickness grows.
Emission of gas due to impurities during SHS-compaction is also responsible for distortion of desired shapes of even porous final products (items). Thus development of method ensuring gas removal from SHS hot product prior to the stage of its pressing would be of significant importance. This matter of interest is related to the fundamental study of the SHS-compaction theory. The study of capabilities and aspects of SHS-compaction involving forced (vacuum-assisted) extraction of impurity-born gas from still non-compressed mixture burning in a press-form is of great interest.

A new method of SHS-compaction by using a specifically developed vacuum press-form (Fig. 1) [6] is considered. Some experimental results are presented. It is demonstrated that application of the press-form facilitates gas escape from the SHS product which allows one to make a serious step towards solution of many practical SHS-compaction problems.

  

Fig. 1. Press-form for SHS-compaction in vacuum

1 – matrix with vacuum chamber, 2 –breeching holes, 3 – connecting pipe for gas pump-down, 

4 – heat-insulating mass, 5 – reaction system, 6 – the press-form base,

7 – initiating spiral, 8 – porcelain two-way system for spiral, 9 – die,10 – polyethylene film.

 

This study leads to increasing the process reliability and the quality of the end products with respect to their desired shape and size as well as the synthesized material characteristics.

 

References:

1.  Polyakov B.B. Computer modeling and selection of optimal regimes of SHS-compacting // works TSTU. Volume №22 / Tambov. Editorial TSTU/ 2009, http://www.tstu.ru/education/elib/pdf/st/2009/polyakov.pdf

2. Scherbakov V.A., Sychev A.E., Shteinberg A.S. Degassing macro-kinetics in the SHS-process, Fizika Goreniya i Vzryva, №4, v. 22, 1986. p. 55-61 (in Russian).

3. Scherbakov V.A., Vishniakova G.A., Kustova L.O., Borovinskaya I.P. Physico-mechanical properties of hard-alloy material СТИМ 1 B/3, obtained by the SHS-pressing method. Preprint. – Chernogolovka, 1986.

4. Effect of impurity degassing on solid flame structure: critical characteristics of solid flame propagation. Ponomarev, M. A. Shteinberg, A. S.,  Shcherbakov, V. A., Journal of Materials Synthesis and Processing (1995), 3(2), 83-91.

5. Ponomarev M. A., Shcherbakov, V. A.  Shteinberg, A. S. Combustion patterns of thin layers of Ti-B powder mixture. Dokl. Ross. Akad. Nauk (1995), 340(5), 642-645.

6. Sakhvadze D.V., Tavadze G.F., Okrostsvaridze O.Sh., Khvadagiani A.A., Lekishvili K.M. The press-form for self-propagating high-temperature synthesis in vacuum. Georgian Engineering News, №4, 2002. p. 250-252.

 

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