Synthesis and preparation of silicon carbide ceramics

　　Due to the characteristics of covalent bonding, the diffusion rate of SiC is rather low. Even in the high temperature of 2100oC, the self diffusion coefficient of C and Si is only 1.5 * 10-10 and 2.5 * 10-13 cm2/s. So It is difficult to take the normal pressure sintering process of the simple compound used in the ionic bond material to make the high density material, which must be made by some special technological means or by the second phase material. SiC is hard to sinter. The ratio of grain boundary energy and surface energy is very high, and it is not easy to get enough energy to form the grain boundary. The diffusion rate of SiC is very low, and the oxide film on the surface of the sintering process also has a diffusion barrier. Therefore, silicon carbide need to use additives or pressure to get a dense material. This part uses Al-B-C as sintering aids.

　　Selective segregation of boron(B) at the SiC grain boundary decreases the grain boundary energy and improves the sintering driving force, but excess B can cause the abnormal growth of SiC grains. The addition of C (carbon) can reduce the SiO2 film which is hindered by sintering, and improve the surface free energy. But too much carbon will cause the weight lose and density decreased. Aluminum (Al) has the effect of inhibiting the growth of grain and enhancing the sintering aids of boron. But excess of Al will make the parts of the high temperature strength decreased. Therefore, it is necessary to determine the amount of Al, B, and C by experiment. At present, the main methods of producing SiC ceramics are non pressure sintering, hot pressing sintering, HIP sintering, reaction sintering and so on.