Synthesis and sintering of SiC under high pressure and high temperature
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Synthesis and sintering of SiC under high pressure and high temperature S. K. Bhaumik, C. Divakar, S. Usha Devi, and A. K. Singh High Pressure Laboratory, Materials Science Division, National Aerospace Laboratories, Bangalore 560 017, India (Received 30 March 1998; accepted 17 August 1998)
Starting from elemental powders, simultaneous synthesis and compaction of SiC were conducted at 3 GPa pressure and temperatures in the range 2100–2900 K. The sintered compacts were characterized by x-ray diffraction, microhardness measurements, and microscopic studies. The efficiency of formation of SiC was dependent on the particle size of the silicon powder, crystallinity of the reactant carbon, molar ratio of silicon and carbon, and synthesis temperature and time. Carbon in excess of the stoichiometric amount was required to obtain compacts free from residual silicon. The SiC samples, with a Si : C molar ratio 1 : 1.05, prepared at 2100 K for 300 s had a density and hardness of 3.21 gycm3 (98.8% of theoretical density) and 22 GPa, respectively. The crystal structure of the SiC depended on the synthesis temperature. Pure b –SiC in the temperature range 2100–2500 K, and a mixture of a – and b –SiC above 2500 K were obtained. The b –SiC was highly crystalline and nearly defect-free.
I. INTRODUCTION
A highly exothermic reaction, once started, can run to completion by the propagation of combustion zone through the reactant mixture. Such reactions have been long since used in welding,1 fabrication of cermets,2 and ceramic composites.3,4 More recently, Merzhanov and Borovinskaya5 systematically exploited this principle in synthesizing a large number of compounds. This process, commonly known as the self-propagating high temperature synthesis (SHS), has now been used to synthesize over 200 compounds. Extensive reviews on SHS can be found in the literature.6–9 The SHS process has two distinct advantages. As the heat of reaction is used to complete the reaction, considerable heat energy savings are expected. The reaction product is free from volatile impurities as these are driven out when the reaction zone reaches high temperature. The reaction product, however, is porous. The porosity is caused by a number of factors, such as, the volume change inherent in the reaction, short duration (insufficient for sintering) of the high temperature reaction zone, porosity present in the reaction mixture, and escape of the gases during the reaction. For any practical application, the product of SHS has to be compacted and sintered. Miyamoto et al.10 suggested that the synthesis and compaction can be carried out in a single step if SHS is conducted under high pressure. This process is termed as high-pressure self-combustion sintering (HPCS). The HPCS of SiC at 3 GPa and 2270 K for 1 s gave compacts with 90% theoretical density which contained b-phase (cubic) and had high hardness (21 GPa).11 In this paper, we present the results of simultaneous synthesis and compaction of SiC under high pressure. A 90
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