Abstract
A small amount of Hf was employed as a new additive to improve B removal in the electromagnetic solidification refinement of Si with an Al–Si melt, because Hf has a very strong affinity for B. The segregation ratio of Hf between the solid Si and Al–Si melt was estimated to range from 4.9 × 10−6 to 8.8 × 10−7 for Al concentrations of 0 to 64 at.%, respectively. The activity coefficient of Hf in solid Si at its infinite dilution was also estimated. A small addition of Hf (<1025 parts per million atoms, ppma) significantly improved the B removal. It was confirmed that the use of an increased Hf addition, slower cooling rate, and Al-rich Al–Si melt as the refining solvent removed B more efficiently. B in Si could be removed as much as 98.2% with 410 ppma Hf addition when the liquidus temperature of the Al–Si melt was 1173 K and the cooling rate was 4.5–7.6 K min–1. The B content in Si could be controlled from 153 ppma to 2.7 ppma, which meets the acceptable level for solar-grade Si.
Highlights
Reducing the cost of producing solar-grade Si (SoG-Si) is important for manufacturing low-cost Si solar cells
Some attempts have been made to improve B removal and Si separation based on this solvent-refining process, such as using Si–Al–Sn (10–30 mol%) [11] and Si–Al–Zn (
The chemical potential of Hf is identical between solid Si and the Al–Si melt during the solidification refining process, which can be expressed by Equation (1): RT ln
Summary
Reducing the cost of producing solar-grade Si (SoG-Si) is important for manufacturing low-cost Si solar cells. An impurity in MG-Si, cannot be removed through solidification refining and vacuum melting because of its large segregation coefficient (0.8 at 1687 K) [1] and low vapor pressure (lower than that of Si) [2]. Some processes such as oxidation through H2O-added plasma melting [3, 4] and slag treatment [5,6,7,8] are currently employed for B removal. B removal by Hf addition in electromagnetic solidification refining of Si with an Al–Si melt was investigated in this study
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