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Geoffrey A. Landis
There are many different process sequences currently used for manufacture of silane [1]. We need to evaluate these methods considering availability of materials on the moon. All of the reactions involve materials which are not available on the moon. It is therefore important that the recycling efficiency of the process be high. The largest scale process, developed by Union Carbide [2,3], converts powdered, 98% metallurgical grade silicons to trichlorosilane at 500 C in a fluid-bed reactor and then successively disproportionates the trichlorosilane to an end product of silane according to the following sequence of reactions:
Si + 2 H2 + 3 SiCl4 --> 4 HSiCl3
6 HSiCl3 --> 3 SiCl4 + 3 H2SiCl2
4 H2SiCl2 --> 2 HSiCl3 + 2 H3SiCl
2 H3 SiCl --> H2SiCl2 + SiH4
overall reaction: Si + 2 H2 --> SiH4
The intermediate reaction products are recycled. Metallic impurities in the initial metallurgical grade silicon are converted to chlorides and settle out in the first distillation step. For maximum recycling efficiency, these should be electrolyzed to recover the chlorine for reuse.
(2) Union Carbide, "Feasability of the Silane Process for Producing Semiconductor Grade Silicon," Final Report, June 1979, DOE/JPL Contract 954334.
(3) G.A. Landis, "Thin, Light-Trapping Silicon Solar Cells for Space," 20th IEEE Photovoltaic Specialists Conference, Las Vegas, NV, 708-712 (1988).
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