Experimental results compiled from the literature were compared to thermodynamic calculations of the most stable proportion of condensed phases to deposit from gas mixtures of Si-C-CI-H. The calculations indicated that the predominant gas molecules participating in a deposition process are chlorides and chlorosilanes for silicon and methane and acetylene for carbon. The mismatch of the calculated and experimentally determined phase boundaries at 1473 and 1600 K led to the conclusion that silicon deposition occurs faster than carbon deposition in proportion to their partial pressures. The probable reason is that silicon-bearing gas molecules have a greater sticking probability on polar Si and Sic surfaces because of their asymmetric geometries.

A careful examination of the dissolution/diffusion through liquid/precipitation steps of the well-accepted vapor-liquid-solid whisker growth mechanism has been conducted. Various diffusional processes involved in the whisker growth, such as solid surface diffusion, liquid surface diffusion, and bulk liquid diffusion, have been evaluated. It is noted that the diffusion along liquid alloy droplet surfaces, driven by both a concentration gradient and surface energy decrease, should not be ignored. Mass transport rate calculations using a simplified one-dimensional model show that the surface diffusion becomes more important with decreasing whisker diameter, decreasing solubility of growth species in the liquid phase, and increasing ratio of surface diffusion coefficient to bulk liquid diffusion coefficient. The diffusion along the liquid alloy droplet surface may play a major role in the growth of compound whiskers. Inclusion of the liquid droplet surface diffusion in the vapor-liquid-solid mechanism is proposed.

The sintering sequence of Y2O3-AI2O3-doped Si3N4 was investigated with respect to the relationship between densification, (Y -+ p transformation, and microstructural development. Quenching studies were performed to reveal these interactions during a complete sintering cycle. Isothermal studies were conducted to examine the sintering kinetics and compared to Kingery's liquid-phase sintering model. The bulk density increased to >90% of theoretical density with only minor transformation taking place. Major transformation occurred in a late sintering stage and was accompanied by the development of elongated grains. The kinetic order of the densification process, taking into account an appropriate correction, was larger than any of the rate exponents predicted by the Kingery model, indicating that other single or mixed mechanisms were active. [