cell of equal efficiency could be produced, it would weigh more than its crystalline equivalent by the weight of the supporting substrate, assuming the silicon thickness was equal. Only gallium arsenide, whose minimum absorption thickness is less than silicon, has the potential for weight savings, and it is not as great as one might expect. Gallium arsenide has a specific gravity of 5.32 compared to silicon which is 2.34. Thus, a film of gallium arsenide cannot exceed 0.044 mm or it will weigh as much as silicon. Since only 0.005-0.010 mm are theoretically required, the difference in mass is all that is possible for the substrate. This significantly restricts the choice of substrate materia to lightweight materials. (5) MATERIAL AVAILABILITY AND PROCESSING The solar array of each 5GW SPS will have a total area of approximately 71.7 x 10bm2 of which half or 35.8 x 106m2 will be active. The large quantities of materials required raise questions concerning the availability of the materials required for an SPS, the required production manufacturing facilities, and cost. This section analyzes these questions for silicon, gallium arsenide, and cadmium sulfide. (a) SILICON Silicon is derived from silica ores (sand), and thus there is an abundant supply of the element in the earth's crust. The steel industry uses over 90 x 103 metric tons of metallurgical grade silicon (98% pure) per year. Table IV.B.l.a.l is a listing of the 1975 production of semiconductor grade silicon by the major U.S. and world manufacturers and shows that their output would have to be increased by more than an order of magnitude to be adequate just for finished cells. At present, their production output is used almost solely for electronic device fabrication. [1] PRODUCTION OF HIGH PURITY SILICON Silicon dioxide, in the form of silica ore, is mixed with carbon and heated in an arc furnace (Figure IV.B.l.a.11) to produce a metallurgical grade of silicon (99% Si). The reaction is described by the following reaction. The metallurgical grade of silicon must be purified in order to obtain semiconductor grade material. The preferred method of obtaining high purity silicon involves converting silicon into trichlorosilane according to the following reaction: In order to eliminate the by-products of the process and improve the purity, the trichlorosilane is distilled. Liquid
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