Activity of calcium in dilute liquid Si-Ca alloy
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I. INTRODUCTION
(2) oxidation of calcium by the silica film,
IN recent years, the high-purity silicon market has been increased by the production growth of organo-silicon compounds, such as silicones. The silicon needed for the production of these compounds is commercially named chemical grade silicon and its degree of chemical purity lies between that of electronic grade and the traditional metallurgical grade On the other hand, the prospective growth of silicon use in photovoltaic applications, as well as in the aforementioned applications, has stimulated the study of silicon solution thermodynamics with low concentration of elements such as aluminum, magnesium, and calcium;[1–3] titanium and iron;[2,4] boron;[2,5] phosphorus;[6,7] and sodium.[2] An erratic decrease of calcium content in silicon can occur due to the runoff from the furnace and during tapping. To better understand this phenomenon, which could be explained by volatilization or oxidation, and to assure controlled calcium removal, knowledge about the calcium activity coefficient in dilute silicon solutions is necessary. However, the calculations made using the refining model with available data[8] in comparison with that of experimental results showed some disparity. Then, it was decided to undertake an investigation for measuring as accurately as possible the activity coefficient of calcium 0 g Ca in silicon, which is the aim of this article. The removal of calcium from silicon is attained by selective oxidation followed by interoxide reaction between formed CaO and SiO2. These two oxides, the first a strong base and the second an acid component, react to form calcium silicate. The following two reactions depict this oxidation and slag formation: (1) oxidation of silicon (high concentration) and formation of a silica slag film covering continuously the silicongas interface, Si (silicon) 1 O2 (gas) 5 SiO2 (slag)
[1]
E.C. de OLIVEIRA PINTO, Professor, and C. TAKANO, Associate Professor, are with the Department of Metallurgical and Materials Engineering, University of Sa˜o Paulo, Sa˜o Paulo, S.P., Brazil, CEP 5508-900. Manuscript submitted February 8, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
SiO2(slag) 1 2 Ca (silicon) 5 Si 1 2 CaO (slag) [2] If these reactions describe the phenomena associated with the decrease of calcium content mentioned previously, then the following global reaction can be proposed: 2Ca (silicon) 1 SiO2 (slag) 5 2CaO (slag) 1 Si (silicon)
[3]
Reaction [3] shows that if the activity coefficient of calcium in dilute silicon solution is known, then it will be possible, knowing the activities of the CaO and SiO2 in the CaO-SiO2 slag that form during refining, to estimate the residual equilibrium content of calcium after oxidation treatment. For low concentrations of calcium (up to 0.03 wt pct) in silicon, the Henry’s law can be applied with good confidence and, therefore, the activity coefficient of calcium in dilute silicon solution can be calculated at a given temperature by 0 gCa 5 aCa/XCa 5 constant, where aCa and XC
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