Laser driven reactions, Sinterable powders from.

Extremely fine, uniform ceramic powders have been synthesized from Sil4 NH3 and C2H4 gas phase reactants that are heated by absorbing optical energy emitted from a C02 laser. Resulting Si, Si3N4 and SiC powders have been characterized in terms of parameters which are important for densification processes. They are virtually ideal. The spherical particles typically have mean diameters from 120 to 1500 A. The standard deviation is typically 25-45% and the diameter of the largest observed particle is typically less than twice that of the smallest particle. Purities are extremely high. The laser heated process has been modeled in terms of fluid flow and heat transfer criteria and characterized with respect to particle growth rates and temperature distribution. Many fundamental property measurements were made to provide data for these calculations. The process is extremely efficient, ~ 95% of the SiH4 is reacted in a single pass through the laser beam and approximately 2 kwhr. of energy are required per kilo of Si3N4.The resulting powders have been processed into dense pieces using several shaping techniques. The Si powders were densified to precisely controlled levels designed to yield high density reaction bonded silicon nitride (RBSN). Nitriding kinetics were rapid at low temperatures because of the small particle sizes. Characteristic dimensions of RBSN microstructures approximated the initial particle dimensions. Sintering experiments with pure Si3N4 powder indicated that densification occurred without pressure or sintering aids. Characterization of mechanical properties showed that, with lower densities, strengths and KIC's equalled or exceeded those of high quality RBSN materials.U.S. Dept. of Defense