Engineering topics
Publications and source records attributed to Kapur, V..
New acid leaching process purifies silicon produced in reaction between silicon fluoride and sodium. Concentration of sodium fluoride and other impurities and byproducts remaining in silicon are within acceptable ranges for semi-conductor devices. Leaching process makes sodium reduction process more attractive for making large quantities of silicon for solar cells.
High purity silicon can be prepared from the reaction of Na with SiF4 to form Si and NaF. The SiF4 is obtained from inexpensive H2SiF6 by precipitation and decomposition of Na2SiF6. Concentrations of B and P are each in the 0.1 ppm wt range in the product Si, separated from NaF by aqueous leaching.
Silicon is produced in a NaF, Na2SiF6, and Na matrix when SiF4 is reduced by metallic sodium. Hydrogen is evolved during acid leaching to separate the silicon from the accompanying reaction products, NaF and Na2SiF6. The hydrogen evolution reaction was studied under conditions simulating leaching conditions by making suspensions of the dry silicon powder in aqueous fluoride solutions. The mechanism for the hydrogen evolution is discussed in terms of spontaneous oxidation of silicon resulting from the cooperative effects of (1) elemental sodium in the silicon that reacts with water to remove a protective silica layer, leaving clean reactive silicon, and (2) fluoride in solution that complexes with the oxidized silicon in solution and retards formation of a protective hydrous oxide gel.
Progress in the development of low-cost solar arrays is reported. Topics covered include: (1) development of a simplified feed system for the Na used in the Na-SiF4 reactor; (2) production of high purity silicon through the reduction of sodium fluosilicate with sodium metal; (3) the leaching process for recovering silicon from the reaction products of the SiF4-Na reaction; and (4) silicon separation by the melting of the reaction product.