Silicon Refining by Solidification from Liquid Si–Zn Alloy and Floating Zone Method

Abstract

This study evaluated the refining ability of a proposed production process for solar-grade silicon utilizing the electrolytic reduction of SiO 2 on a liquid zinc electrode in molten salt. The distribution behaviors of impurity elements during the precipitation of solid silicon from a liquid SiZn alloy were studied by thermodynamic calculations at 923 K. In the precipitation experiment, silicon granules were recovered from a liquid SiZn alloy, which was prepared from metallurgical-grade silicon. The impurity removal ratios exceeded 99% for C, Al, and Ca, and 90% for Fe. High removal ratios were attained for B and O as well. As the post-processing, a silicon ingot was produced from the precipitated silicon granules by the floating zone method. The Zn residue in the precipitated silicon was completely evaporated during the floating zone refining. The total content of metallic elements (Al, Ca, Fe, Ti, and Zn) was lower than 0.2 ppmw, even though metallurgical-grade silicon was used as the starting material.

Electrodeposition of Crystalline Si Using a Liquid Zn Electrode in Molten KF–KCl–K2SiF6

“…Here, the deposition mechanism of crystalline Si is discussed in terms of crystal size. In previous reports of Si precipitation by decreasing the temperature of liquid Si-Zn alloys, the precipitated Si grains became smaller at a higher cooling rate (18). Even at the temperature decreasing rate of 20 K h −1 , only Si grains smaller than 1 mm were obtained (18).…”

Electrodeposition of Crystalline Si Using a Liquid Zn Electrode in Molten KF–KCl–K2SiF6

“…Previous reports on Si precipitation showed that decreasing the temperature of liquid Si-Zn alloys decreases the size of the precipitated Si grains at a higher cooling rate. 38 Even at the temperature-decreasing rate of 20 K h −1 , only Si grains smaller than 1 mm were obtained. 38 In the present study, the Si-Zn liquid alloy was cooled through natural cooling, and the temperaturedecreasing rate was extremely high (approximately 230 K s −1 ).…”

“…38 Even at the temperature-decreasing rate of 20 K h −1 , only Si grains smaller than 1 mm were obtained. 38 In the present study, the Si-Zn liquid alloy was cooled through natural cooling, and the temperaturedecreasing rate was extremely high (approximately 230 K s −1 ). Therefore, Si powders with small grain sizes could have been deposited during natural cooling.…”

“…Third, many impurity elements have small distribution coefficients between solid Si and liquid Zn, making it difficult for impurity elements to contaminate the solid Si. 38 Electrodeposition of Si with larger grain size was performed using a liquid Zn electrode in a KF-KCl-K 2 SiF 6 melt at 923 K. First, cyclic voltammetry measurements were conducted to observe the electrochemical behavior of Si(IV) ions on a liquid Zn electrode. Second, potentiostatic electrolysis was performed to electrodeposit Si via Si-Zn alloying.…”