1.1 Hot pressing sintering
Hot pressing sintering is the sintering of silicon carbide ceramic by applying a certain amount of axial pressure while the SiC is heated and sintered. Hot pressing sintering increases the contact area between SiC particles, reduces the sintering temperature, shortens the sintering time and increases the densification of the sintered body to facilitate the sintering of SiC. In order to make SiC particles easier to sinter, hot pressing sintering usually requires the addition of B, C, Al, B4C, Y2O3, A12O3. and other sintering aids to the SiC powder to promote sintering . B, Al or BC are solidly soluble in SiC to reduce the interfacial energy of SiC, and C reacts mainly with SiO on the surface of SiC particles to form a low temperature liquid phase to promote the diffusion of B, A1. Liu et al. Liu et al. sintered SiC ceramics with Y2O3 and A12O3 as sintering aids at 2000 °C and 30 MPa, and the denseness of the sintered silicon carbide ceramic was 97-99.3%, while Y2O3 and A12O3 formed the second phase YAG with low thermal conductivity during sintering, resulting in a thermal conductivity of only 92 w/(m -K) at room temperature. On the one hand, the added YO. has the ability to repel oxygen, thus purifying the lattice, reducing lattice defects, increasing grain purity and improving thermal conductivity; on the other hand, the use of LaO. instead of A1O. On the other hand, replacing A1O with LaO can ensure that YAG with low thermal conductivity is not formed, thus improving its thermal conductivity. Hot-press sintering is an effective way to produce high-performance SiC ceramic materials, but it is a complex, expensive and costly process for manufacturing complex SiC parts, which is not conducive to industrial production.
1.2 Reactive sintering
Reactive sintering of silicon carbide ceramic is a process that uses C-containing powder and SiC powder molds to react with vapour or liquid phase Si at high temperatures to produce a sintered SiC. The sintering process does not require the addition of any sintering aids, has few defects in the grain, has high grain boundary purity and has little effect on the thermal conductivity of the material. The reaction between the C in the raw material and the external surface produces SiC on the one hand and causes a densification effect on the other.] Since the 1950s, when silicon carbide ceramic were prepared by reactive fusion sintering, researchers have been improving the performance of reactive sintering by refining the forming method and improving the process in order to reduce structural defects and improve the performance of the material. Liu Hong et al._j infiltrated molten Si into the blanks by capillary action and reacted it with carbon powder. The newly generated SiC combined the original SiC whiskers with SiC to obtain SiC/SiC composites with high density, low defects, bending strength of 243 MPa, fracture toughness value of 6.43 MPa and thermal conductivity of 125.3 w/(m -K).
