Study on volatilization rate of silicon in multicrystalline silicon preparation from metallurgical grade silicon

摘要

The quantity of silicon lost during evaporation is greater than theoretical expectation during the purification of metallurgical grade silicon by vacuum evaporation. In this paper, silicon volatilization rates were measured for evaporation times of 30, 45 and 60 min at 1723, 1773 and 1823 K, respectively. Results indicate that volatilization rates determined in our experiments are one or two orders of magnitude greater than those from theoretical calculation. The equation for theoretical calculation was revised (ω = (2.23-6.30)× 10~(-1) p_(pa)(M/T)~(1/2)) using silicon evaporation coefficient of 8.5-24. The details of the experimental set-up were found to be important and the mass of silicon evaporated in particular was found to be related to the water-cooling system. The carbon/graphite inserts also and the presence of trace amounts of oxygen in the vacuum furnace could reduce the content of silicon in gaseous phase and support the evaporation of silicon. It was found under certain conditions that there are two principal stages involved: 1) Formation of vapor-liquid equilibrium, 2) Maintenance of the established vapor-liquid equilibrium during the silicon evaporation process. It was found that silicon process losses can be reduced by shortening the time of the first stage.