Thermodynamics of Impurity Removal in Solvent Refining of Silicon

In a new approach to metallurgical refining of silicon by solvent refining, iron has been successfully utilized as the getter element. Unlike most metallic impurities, phosphorus and boron have relatively large distribution coefficients that render them unresponsive to solidification refining processes. Consequently it is crucial to study the efficiency of phosphorus and boron removal through solvent refining with iron. This research focuses on thermodynamics of phosphorus and boron distribution between silicon crystals and the iron-silicon melt. Aiming to establish the thermodynamic fundamentals of the process, the present study was performed to investigate the distribution coefficient of phosphorus and boron between silicon and the alloy melt. Based on the values obtained for distribution coefficient of phosphorus and boron at different temperatures, impurity removal is 2-3 times more effective compared with conventional directional solidification process. The values of interaction parameter of phosphorus on iron were obtained as -5297±359 (1310 °C), -4324±292 (1260 °C) and -2893±268 (1210 °C). The values of interaction parameter of boron on iron were: -813±53 (1310 °C), -830±92 (1260 °C), -863±91 (1210 °C). Large negative values obtained for both interaction parameters, indicate strong affinity between iron and either phosphorus or boron. Self interaction parameter of phosphorus were as follows: 243±38 (1310 °C), 137±22 (1260 °C) and 18±2 (1210 °C), indicating the repulsion between phosphorus atoms. Self interaction parameter of boron were as follow: -96±12 (1310 °C), -111±28 (1260 °C) and -159±45 (1210 °C) implying an attraction between boron atoms in silicon. The distribution coefficient of phosphorus at infinite dilution were obtained as: 0.34±0.01 (1310 °C), 0.28±0.01 (1260 °C) and 0.24±0.00 (1210 °C). The same parameter for boron were: 0.49±0.01 (1310 °C), 0.41±0.03 (1260 °C) and 0.33±0.04 (1210 °C). Henrian activity coefficient of phosphorus and boron in solid silicon were also estimated.Ph.D