Removal of B from Si by Hf addition during Al–Si solvent refining process

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.

Leaching behaviors of impurities in metallurgical-grade silicon with hafnium addition

“…One approach involves the addition of large amounts of one or more additives to form a solvent. As the liquidus temperature of the solvent is normally much lower than the melting point of Si, MG-Si can be purified at low temperature, Hf can be employed as an impurity getter because its segregation coefficient is extremely low (4.9×10 -6 at the melting point of Si ) (Lei et al, 2016c). This indicates that the added Hf can be eliminated along with other impurities, and will not contaminate the Si.…”

Enhancing Segregation Behavior of Impurity by Electromagnetic Stirring in the Solidification Process of Al-30Si Alloy

“…To investigate the influence of electromagnetic stirring on the segregation behavior of the impurities, the distribution of impurities at the interface of primary Si phase/eutectic Al-Si matrix was measured through EPMA and the results are given in Figure 6. It was difficult to remove the nonmetallic impurity B from Si; thus, it was the main object to be investigated in the field of alloy solidification purifying [14][15][16][17][18]. Therefore select B element as research object carried on the EPMA surface and line analysis.…”

“…One idea is to improve the diffusion kinetics of impurities in the growth interface front of primary Si by reducing cooling rate [9,11,12]. Another strategy is to decrease the solubility of impurities in the Al-Si melt by chemically reacting with the other added elements for the target impurity to form the intermetallic compounds [13][14][15][16][17][18]. Although the removal efficiency of impurities can be improved by reducing cooling rate and chemical reaction, it will spend several weeks to finally remove impurities effectively and increase the risk of secondary pollution largely due to the excessive addition of other elements.…”

Separation and Recovery of Refined Si from Al–Si Melt by Modified Czochralski Method

“…The principle is that impurities in MG-Si have the tendency to segregate into the alloy melt to obtain the high-purity Si because of small segregation coefficients. The Al-Si solvent-refining method [14][15][16][17] has the advantage of low melting point, without intermediate phase, low-energy consumption, which has been investigated by many researchers. However, the density difference between solid Si (2.3 g/cm 3 ) and Al-Si melt (~2.4 g/cm 3 ) is small [18], which inevitably generates much adhesive Al on the surface of the Si dendrites and consumes lots of acid solution to remove it, generating considerable waste of acid solution and Si powder.…”