Effect of Zr addition on B-removal behaviour during solidification purification of Si with Si–Sn solvent

Abstract

This paper details a novel attempt for B removal from Si by adding Zr as the trapping agent in solidification refining with Si–Sn solvent. The premise of the study is as follows: (i) B becomes more thermodynamically unstable in Si–Sn melt and (ii) Zr has a strong affinity for B, which is expected to enhance the formation of the B–Zr intermetallic compound. The B-removal mechanism is discussed from the aspects of theoretical analysis and experimental design. The effects of different variables, such as the moving direction, moving rate, Zr addition content, and initial Sn content in Si–Sn alloys, on the B-removal fraction are observed. Furthermore, the solubility product of the B–Zr compound in the Si–Sn melt is measured using equilibrium technology, to be ≥ 9.62 × 10−10 (1605 K, Si-50 at.% Sn) and ≥3.32 × 10−10 (1575 K, Si-65 at.% Sn). Enriched B and Zr regions are found in the mushy zone, but separated from each other. The maximum removal fraction of B is 73.6%; further, the added Zr is almost completely eliminated and does not contaminate the refined Si. This study provides an alternative perspective for B removal via Si–Sn solvent refining to produce solar-grade Si.