Design of Refining Slag Based on Structural Modifications Associated with the Boron Removal for SoG-Si

Solar grade silicon (SoG-Si) is the core material of solar cells. The removal of boron (B) has always been a challenge in the preparation of high purity Si. Slag refining has always been considered as one of the effective methods to remove B, but the design of refined slag has been limited by the cognition of the relationship between slag structure and impurity removal, and can only rely on the apparent basicity and oxygen potential adjustment of slag based on a large number of conditional experiments. In order to clarify the B removal mechanism of slag refining from Si, nuclear magnetic resonance (NMR) and Raman vibrational spectroscopy were used to investigate in detail the behavior and state of B and aluminum (Al) in the SiO₂-CaO-Al₂O₃-B₂O₃ slag. The role of the degree of B-Si cross linking on the B activity in slag was highlighted by comparing the partition ratio (L_B) between slag and Si. Q² structural unit of slag is an important site for capturing B. BO₄ (1B, 3Si) species is the main form of connection between B and silicate networks, which determines the activity of B in the slag. The addition of Al₂O₃ into SiO₂-CaO slag can change the relative fraction of Q² and BO₄ (1B, 3Si). Increasing Al₂O₃ content from 0 to about 20 wt% can lead to the overall increase of Q² population, and a tendency to decrease first and then increase of BO₄ (1B, 3Si) fraction under both basicity conditions (0.6 and 1.1). When Al₂O₃ content is less than 10 ± 1 wt%, the decrease of BO₄ (1B, 3Si) population plays a major role in deteriorating the connectivity between B and aluminosilicate network, which leads to a higher activity of B. When the Al₂O₃ content is greater than 10 ± 1 wt%, B is incorporated into the silicate network more easily due to the formation of more Q² and BO₄ (1B, 3Si), which contributes to a rapid decline in activity of B in slag.