Abstract
The growing field of silicon solar cells requires a substantial reduction in the cost of semiconductor grade silicon, which has been mainly produced by the rod-based Siemens method. Because silicon can react with almost all of the elements and form a number of alloys at high temperatures, it is highly desired to obtain high purity crystalline silicon at relatively low temperatures through low cost process. Here we report a fast, complete and inexpensive reduction method for converting sodium hexafluorosilicate into silicon at a relatively low reaction temperature (∼200°C). This temperature could be further decreased to less than 180°C in combination with an electrochemical approach. The residue sodium fluoride is dissolved away by pure water and hydrochloric acid solution in later purifying processes below 15°C. High purity silicon in particle form can be obtained. The relative simplicity of this method might lead to a low cost process in producing high purity silicon.
Highlights
In 1823, Berzelius obtained iron-free silicon by reducing SiF4 gas, which came from the heat decomposition of K2SiF6, with red-hot potassium metal above 520uC
Conversion method of sodium hexafluorosilicate into silicon particles by metal sodium was investigated as follow: Under nitrogen atmosphere, the certain amount of Sodium and sodium hexafluorosilicate (Analytical reagent, Alfa Aesar) which had been dried at 120uC for 2 hours to remove the moisture, were put into the round bottom flask with three-necks
The situation that nobody had carried out the preparation of silicon under these conditions was probably due to two preconceived notions: (1) Na2SiF6 begins to decompose at 647uC [6] and has no melting point; (2) silicon tetrafluoride, which is a byproduct of producing superphosphate fertilizer from phosphate rock [6], is not pure
Summary
In 1823, Berzelius obtained iron-free silicon by reducing SiF4 gas, which came from the heat decomposition of K2SiF6, with red-hot potassium metal above 520uC. The processes of producing silicon with precursor silicon tetrafluoride or trichlorosilane have been extensively studied. Current industrial manufacturing of high purity silicon adopts the Siemens method using purified trichlorosilane, SiHCl3. SiHCl3, obtained by converting crude silicon with hydrogen chloride, decomposes and deposits silicon onto highpurity silicon rods and enlarges the rods at 1150uC. The well-known Stanford Research Institute International (SRI) reduction process involves that purified silicon tetrafluoride (SiF4) gas through fractional distillation is reduced to silicon by metal sodium above 500uC. SiF4 is from the heat decomposition of sodium hexafluorosilicate (Na2SiF6) at 647uC: Na2SiF6(s)~SiF4(g)z2NaF (s) ð1Þ
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