Thermodynamic properties of aluminum, magnesium, and calcium in molten silicon
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I.
log g oAl(l ) in Si 5 2
1570 1 0.236 (1723 to 1848 K) T
log g oMg(l ) in Si 5 2
4900 1 1.96 (1698 to 1798 K) T
log g oCa(l ) in Si 5 2
7670 1 1.53 (1723 to 1823 K) T
INTRODUCTION
Solar energy will shortly be in great demand, since it is inexhaustible and cleaner than any conventional energy resource. However, very expensive off-grade silicon for semiconductors (SEG-Si) has been used for solar cells, which prevents the solar cell system from being in wider use. Hence, inexpensive metallurgical-grade silicon (MG-Si) is considered to be more suitable as a starting material for low-cost solar cell production. Since impurities in silicon will lower conversion efficiency, substantial purification of MG-Si is necessary to produce solar-grade silicon (SOG-Si). Suzuki et al.[1] have reported that vacuum treatment is effective for removing some impurities from molten silicon such as phosphorus, calcium, and aluminum. Little is known about the thermodynamic properties of those impurities in molten silicon, the knowledge of which is indispensable when considering the possibility and efficiency of an elimination process for such elements. Previously, the present authors have investigated the thermodynamic properties of phosphorus,[2] titanium,[3] and iron[3] in molten silicon. In the present work, the activity coefficients of aluminum, magnesium, and calcium in molten silicon were determined by using a chemical equilibration technique at 1723 to 1848 K, 1698 to 1798 K, and 1723 to 1823 K, respectively.
TAKAHIRO MIKI, Graduate Student and Research Fellow, Japan Society for the Promotion of Science, and KAZUKI MORITA, Associate Professor, are with the Department of Metallurgy, The University of Tokyo, Tokyo 113-8656, Japan. NOBUO SANO, Emeritus Professor, is with Department of Metallurgy, The University of Tokyo, and is Executive Adviser, Nippon Steel Corporation, Chiba 293-8511, Japan. Manuscript submitted October 20, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
II.
EXPERIMENTAL
A. Thermodynamics of Aluminum in Molten Silicon Two grams of a 1 to 2 mass pct aluminum-silicon alloy was equilibrated with a crucible made of an Al2O3Al6Si2O13 mixture for 18.0 to 130 ks at 1723 to 1848 K in an argon atmosphere. The crucible was prepared by compressing 15 mass pct SiO2-Al2O3 powder into a crucibleshaped pellet, followed by sintering at 1773 K in an argon atmosphere. By X-ray diffraction, it was confirmed that the crucible consisted of only Al2O3 and Al6Si2O13 phases. After equilibration, the sample was quenched, and the aluminum content of the silicon was analyzed using radio frequency inductively coupled plasma (ICP) spectroscopy. The chemical reaction of this technique can be expressed as follows: 13 3 Al2O3 (s) 1 Si (l) 5 Al (l) 8 4 1
3 Al6Si2O13 (s,Al2O3 saturated) 8
[1]
DG o1 5 138,600 2 23.89 T (J/mol) Most of the thermodynamic properties of the substances used in the present work were chosen by using those of, Eriksson and Pelton,[4] who estimated the thermodynamic properties of the Al2O3-SiO2 system. For Al
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