The thermal gradient method
The procedure is performed in a small, vertically elongated autoclave that can operate under pressures of 100-2,000 bar and temperatures of 300- 1,000°C according to requirements.
The highest temperature is in the lower part of the autoclave, which is where the raw material is to be dissolved. Crystallization occurs at the other cooler, end on a seed placed beforehand. Transport is performed by convection movements of the solvent.
If the solvent is an aqueous solution, we refer to this as the hydrothermal method. The use of water allows us to operate at below the melting temperature in the space that we want to crystallize.
However, the solvent can be a metallic oxide and subsequently water soluble, which allows for recovery of the crystal, which is not water soluble.
The silicon carbide does not melt, but sublimes. A mixture of silica and coke fuse at 2,500°C under an argon atmosphere. The result is a SiC vapor, which is condensed into plates of 1 cm2 in surface on a cooler wall. This is the Lely process.