1.1.1 Solid-phase sintering
The sintering of single ceramic powders is often typical of solid-phase sintering, i.e. no liquid phase is formed during the sintering process. The densification of ceramic blanks is mainly achieved by evaporation and coalescence, diffusion and mass transfer. The sintering process consists mainly of the stages of particle rearrangement, pore filling and grain growth. At the same time, solid-phase sintering can be achieved by suitable particle gradation, appropriate sintering temperatures and short holding times. Solid-phase sintering has been of interest since the 1970s, when Prochazkal6 successfully solid-phase sintered silicon carbide ceramic with a density higher than 98 at 2050°C by adding small amounts of B and C to high-purity SiC as a sintering aid. Although the solid phase sintering of SiC in the SiC-B-C system requires high sintering temperatures, coarse sintered grains and poor homogeneity, and silicon carbide ceramic have low fracture toughness, high susceptibility to crack strength and a typical fracture mode through the crystal, the solid phase sintering has low sintering aid content, few impurities and almost no residual low melting point material at the grain boundaries, and the sintered SiC ceramics have good high temperature stability and high thermal conductivity. The sintered SiC ceramics have good high temperature stability and strong thermal conductivity. Therefore, solid-phase sintering has potential applications in the sintering of SiC ceramics. At present, the main manufacturers of SiC ceramics using the SiC-B-C sintering system for solid phase sintering are GE in the USA.
1.1.2 Liquid phase sintering
As there are always small amounts of impurities in ceramic powders, most materials will have a more or less liquid phase during the sintering process. In addition, even in a pure solid-phase system without impurities, “contact” melting can occur at high temperatures, so pure solid-phase sintering is actually not easy to achieve and most sintering is actually liquid-phase sintering. Liquid-phase sintering is a sintering process in which a certain number of multiple low eutectic oxides are used as sintering aids and the sintering aids form a co-solution phase at high temperatures, with the sintered grains being fine and uniformly equiaxed. The sintering system has a flowing mass transfer mode, which reduces the energy required for densification and makes it easier to achieve sintering densification at low temperatures and shorten the sintering time. The high thermal conductivity of BeO and the formation of the liquid phase between SiC and BeO during sintering were exploited by Nakano et al. to produce silicon carbide ceramic with a thermal conductivity of up to 270 W/(m -K). In the process of sintering SiC ceramics with an average powder size of 0.5 Fro at 2200°C, Takada et al. added sintering aids 2 BeO, 0.2 to O.4 BC and 0.2 to O.3 C (mass fraction) and sintered the material without pressure for 0.5 h. The resistivity and thermal conductivity of the material were 5 x l0^12Q -cm and 140 w/(m -K), respectively. During the sintering process, the B and C atoms, which are uniformly distributed on the SiC surface, react with Si atoms to form GB-C, Si-B-C, Si-Si and Si-DSi bonds, which promote the diffusion of Si atoms and increase the densities of silicon carbide ceramic.