Production and Application of AlCl as a Reductant for Solar Grade Silicon Manufacture

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Skrecky, Kristin
Metallurgy , Solar Grade Silicon
Solar grade silicon is currently produced mainly through blending semiconductor grade silicon waste with metallurgical grade silicon. As the demand for solar cells continues to increase rapidly, soon demand will outstrip supply of semiconductor grade silicon waste. A process for producing solar grade silicon efficiently and without relying on other industries is needed. It is proposed to produce solar grade silicon of 6N purity (99.9999%) by reacting silicon tetrachloride with aluminum monochloride via the following reaction: 2 AlCl(g) + SiCl4(g)= Si(s) + 2 AlCl3(g) Aluminum monochloride is proposed as the reductant for silicon tetrachloride because it is an extremely strong reducing agent and the reaction will produce all gaseous by-products. Additionally, the aluminum trichloride produced can be recycled to form more aluminum monochloride, which is produced by reacting aluminum metal with aluminum trichloride in the following reaction: AlCl3(g) + 2 Al(l)= 3 AlCl(g) High yields of AlCl have only been found above 1200°C, with very little AlCl present in equilibrium with Al and AlCl3 at lower temperatures. The high temperatures under which AlCl can be found in larger quantities makes it difficult to determine if the AlCl3 reacting with Al is actually producing AlCl as opposed to another subhalide such as AlCl2. Numerous IR spectroscopy studies have been undertaken to confirm that the reaction of aluminum trichloride gas with molten aluminum does produce aluminum monochloride, with all such studies confirming that this theoretical path is correct. Unlike previous studies, which pass the AlCl3 gas over molten aluminum, it is proposed to bubble the AlCl3 gas into the molten aluminum. This should increase yield of aluminum monochloride, which was not a priority in previous studies. In order to achieve the project objectives a literature review of silicon manufacturing techniques as well as aluminum monochloride production was completed. Experiments to determine the rate of sublimation of aluminum trichloride were to be done in order to determine what temperature at which to sublime the aluminum trichloride. Aluminum trichloride was bubbled into aluminum metal to form aluminum monochloride with experimental conditions being varied to increase yield. Yield was determined through analysis of the reaction products, which was difficult due to the instability of aluminum monochloride, which dissociates at room temperature back into aluminum trichloride and aluminum metal. After the yield of aluminum monochloride was maximized, silicon tetrachloride was introduced into the reactor to react with the aluminum monochloride to form silicon metal.
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