FORMATION OF REACTION-BONDED SILICON-NITRIDE FROM SILANE-DERIVED SILICON POWDERS - MACROSCOPIC KINETICS AND RELATED TRANSPORT PHENOMENA

Reaction-bonded silicon nitride (RBSN) with little or no residual silicon was formed from laser-synthesized silicon powders and N2, at low temperatures and in short times (< 10 min at 1250-degrees-C, < 1 h at 1150-degrees-C). The nitriding kinetics were studied with thermogravimetric analysis (TGA). Samples formed in the TGA furnace between 1150-degrees and 1350-degrees-C were completely converted to silicon nitride near their centers, with residual silicon near the external surfaces. These unusual composition profiles were not observed in samples that were formed in a higher-purity atmosphere. Various experimental observations were compared to analyses of heat and mass transport, and it was determined that these composition gradients were probably caused by oxygen contamination in the nitriding gas. These analyses also indicate that heat transfer, N2 diffusion, and Si evaporation and transport all affect the nitriding kinetics under certain conditions.