Thermodynamic properties of titanium and iron in molten silicon
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TRODUCTION
SOLAR energy shortly will be in great demand, because it is inexhaustible and cleaner than any conventional energy resources. At the moment, very expensive off-grade silicon for semiconductor is used for a solar cell to convert solar energy to electric power. Consequently, the solar cell system has not been developed widely until now. Using relatively inexpensive metallurgical grade silicon as a starting material for solar grade silicon (SOG-Si) is believed to be one of the ways to produce inexpensive solar cell. Titanium and iron in silicon are known as harmful ‘‘lifetime killer’’ impurities, which shorten the lifetime of exited carriers in silicon solar cell and disturb electric generation, and the titanium and iron contents for SOG-Si are required to be less than 1029 (mass pct Ti) and 1027 (mass pct Fe), respectively.[1] Hence, the removal of titanium and iron is one of the most important issues on the purification of molten silicon for solar cell. However, little is known about their thermodynamic properties owing to the experimental difficulties arising from the fact that the activity coefficients of both titanium and iron in molten silicon are considerably small. Especially for iron, which is nobler than silicon, a metal-slag equilibration technique cannot be applied. To overcome these difficulties, in the present study, thermodynamic properties of titanium and iron at 1723 K were determined by measuring equilibrium distributions of titanium and iron between molten silicon and lead, which have little mutual solubility, and by eliminating the effects of mutual solubility by making smart use of interaction coefficients in both the silicon and lead phases. II.
EXPERIMENTAL
A. Thermodynamics of Titanium in Molten Silicon Various alloys composed of 3 g of silicon and 0.5 to 15 mass pct titanium were equilibrated with 10 g of lead, which has little solubility in silicon, in a graphite crucible
TAKAHIRO MIKI, Graduate Student, KAZUKI MORITA, Associate Professor, and NOBUO SANO, Professor, are with the Department of Metallurgy, The University of Tokyo, Tokyo 113, Japan. Manuscript submitted July 8, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
for 14.4 ks at 1723 K in an argon atmosphere. According to Kirkwood and Chipman[2] lead solubility in Si-rich liquid is 2.1 at. pct Pb and silicon solubility in Pb-rich liquid is 3.2 at. pct Si at 1693 K for Si-Pb binary alloys. A graphite lid was placed on top of the crucible to avoid evaporation of lead. After equilibration, the sample was quenched and titanium and lead contents of silicon, and titanium and silicon contents of lead, were determined by chemical analysis. Titanium content was analyzed by spectrophotometry, and silicon and lead content was analyzed by using radio frequency inductively couple plasma (ICP) emission. At equilibrium, titanium activities in silicon and lead phase are equal and can be expressed by Eqs. [1] and [2]. aTi in Si 5 aTi in Pb
[1]
ln gTi in Si 1 ln XTi in Si 5 ln gTi in Pb 1 ln XTi in Pb
[2]
where g i in M is the activi
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