Optimization of the SiC Powder Source Material for Improved Process Conditions During PVT Growth of SiC Boules

We have studied the influence of different SiC powder size distributions and the sublimation behavior during physical vapor transport growth of SiC in a 75 mm and 100 mm crystal processing configuration. The evolution of the source material as well as of the crystal growth interface was carried out using in situ 3D X-ray computed tomography (75 mm crystals) and in situ 2D X-ray visualization (100 mm crystals). Beside the SiC powder size distribution, the source materials differed in the maximum packaging density and thermal properties. In this latter case of the highest packaging density, the in situ X-ray studies revealed an improved growth interface stability that enabled a much longer crystal growth process. During process time, the sublimation-recrystallization behavior showed a much smoother morphology change and slower materials consumption, as well as a much more stable shape of the growth interface than in the cases of the less dense SiC source. By adapting the size distribution of the SiC source material we achieved to significantly enhance stable growth conditions

Slip Casting of Solid State SiC

A study of the parameters influencing the rheology of silicon carbide slurries containing boron carbide and carbon sintering additives was done. Carbon black and Novolac were used as carbon additives and their effect on rheology and sintering was compared. The dispersant concentration, pH and solid loading were varied to minimize viscosity and maximise solid loading. Several samples were slip cast and the green body density, fracture surface morphology and thermal behaviour were determined. The samples were densified by pressureless sintering and the density, linear shrinkage, fracture mechanisms and phase composition were ascertained. The viscosity of SiC slurries with carbon black additives was minimised by the addition of 0.5 wt. % anionic polymeric dispersant and pH adjusted to 9. The solid loading could be increased to 76 wt. %. The highest green body and sintered densities were achieved for 75 wt. % solid loading. The highest sintered density reached was 98 % TD. The viscosity of SiC slurries with Novolac phenolic resin additives was minimised by the addition of 1.5 wt. % dispersant and the pH adjusted close to 8. The solid loading could be increased to 74 wt. %, though this did not lead to increased density. The highest sintered density attained was 93.5 % TD. Linear shrinkage was reduced with increased green body density in all the samples. Density gradients were uncovered within the green bodies due to packing differences. The effect of the dissolution of Novolac at high pH was determined to be an important contributing factor to the high viscosity of SiC-Novolac slurries