PHYSICAL CHEMISTRY ON THE REFINING OF SOLAR GRADE SILICON USING THE AL-SI SOLVENT

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In the project, physical chemistry of SOG-Si refining using the Al-Si solvent aiming at the establishment of its low cost production process. In prior to the optimization of the solvent composition, solid solubility of Al in the solid Si was precisely determined by TGZM method at 1016 - 1622 K, showing quantitatively that the addition of Cu was effective for the reduction in the Al contamination during the solidification refining due to the strong interaction between Al and Cu.Then the possibility of P and B removal with such solvents was assessed. Considerable removal of P and B was clarified through the solid solubility of P and B determined by TGZM together with the partition ratios of them between solid Si and Si-Al solvent at 1273 - 1673 K. In the case of B bearing system, equilibria between borides and Si-Al melts as well as thermodynamic properties of AlB_<12> solid solution were clarified. Furthermore, Ti addition was found to be significantly effective for the removal of boron due to the ultra low solubility product of TiB_2 in molten Al-Si.Then, the removal of several impurities by the solidification method was assessed. Equilibrium partition ratios of them between solid Si and Al-Si melts were smaller than those for pure Si, which indicated that the recommended process was favorable also from the standpoint of the refining efficiency. Finally, small scale solidification tests were conducted and the utilization of electromagnetic force was found to be effective to recover the primary crystal of Si precipitation in the Al-Si melts. Based on the present results, a low cost refining process for SOG-Si was proposed.