LSA silicon material task closed-cycle process development. Interim summary report, August-December 1978

Abstract

This program was Phase I of an effort to achieve production of low-cost solar-grade polysilicon from metallurgical-grade silicon in a closed-cycle process. Impure silicon would be converted to trichlorosilane by reaction with by-product silicon tetrachloride, and the trichlorosilane thermally decomposed in a rotary drum reactor. The technical restrictions to development of the program approach were based on minimizing new technology requirements by using existing technology to reduce the timing, risk, and cost of achieving production capability. The deficiencies in the established Siemens polysilicon process were identified and systematically eliminated to achieve a simple, continuous, easily scaleable, low-cost process. The process is closed cycle with high reactant concentration for material, equipment, and energy efficiency. The initial effort on feasibility of the closed-cycle process was begun with the design of the two major items of untested equipment, the silicon tetrachloride by-product converter and the Rotary Drum Reactor (RDR) for deposition of silicon from trichlorosilane. The design criteria of the initial laboratory equipment included consideration of the reaction chemistry, thermodynamics, and other technical factors. Design and construction of the laboratory equipment was completed. Preliminary silicon tetrachloride conversion experiments confirmed the expected high yield of trichlorosilane, up to 98% of theoretical conversion. A preliminarymore » solar-grade polysilicon cost estimate, including capital costs considered extremely conservative, of $6.91/kg supports the potential of this approach to achieve the cost goal. The closed-cycle process appears to have a very likely potential to achieve LSA goals.« less