Development of processes for the production of solar grade silicon from halides and alkali metals. Phase II. Fifth quarterly report, 1 October-31 December 1980

The objective of the program is to characterize the kinetics and mechanism of the formation and growth of silicon particles from the decomposition of silane at high temperatures. The experiments are aimed at determining the rates at which gas-phase species form silicon particle precursors, the time required for silane decomposition to produce silicon particles, and the competing rate of growth of silicon seed particles injected into a decomposing silane environment. The AeroChem high-temperature fast-flow reactor (HTFFR), modified to study the decomposition of silane and the subsequent growth of particles, was tested and particle growth measurements were made as functions of temperature (600 to 1200/sup 0/C), pressure (50 to 550 Torr), and residence time (0.5 to 30 ms). Optical diagnostics consisting of attenuation and 90/sup 0/ Mie scattering of the light from He-Ne or Ar/sup +/ lasers were used to determine the appearance growth rates and absolute sizes of the particles. The extent of silane decomposition was measured by infrared absorption spectroscopy. Representative data obtained at 900/sup 0/C indicate that: (1) particles formed from silane decomposition are spherical in shape and have a narrow size distribution at a given time in the decomposition/growth process; (2) silane disappearance and particle growth are controlled by heterogeneous gas-particle interactions for particles greater than 0.05 ..mu..m radius; and (3) tentatively, particles larger than 0.05 ..mu..m radius have a cellular structure (from SEM of collected particles). These results led to the formation of a simplistic model for the particle growth process.