Effect of Si Content on the Morphology Evolution of the Si Primary Dendrites in Al-Si Alloy Solvent Refining Process

Mangmang Gao,Rui Li,Min Gao,Xu Zhao,Sen Liang,Haibo Li,Wenyu Chen,Ang Gao

doi:10.1007/s12633-021-01233-x

Abstract

Solvent refining with Al-Si alloy is a promising purification method for the production of solar-grade silicon (SoG-Si) feedstock owing to the advantages of low production cost and high impurity removal efficiency. In this process, larger refined Si primary dendrites should be easily collected after acid leaching, which is favorable to recovery, thereby reducing the production cost. Hence, the growth behavior of the precipitated Si crystal must be investigated systematically. In the present work, the morphology evolution of solidified Al-Si alloys with a wide range of Si content (30 ~ 70 wt%) was analyzed. The typical plate-like Si primary dendrites grown following the twin plane re-entrance edge (TPRE) mechanism formed in all alloy compositions. As increasing the Si content from 30 wt% to 50 wt%, the Si primary dendrites underwent a coarsening process attributed to the preferred growth along with < 211 > and < 111 > directions, leading to an increase in the experimental recovery rate. However, the preferred growth along < 211 > direction was inhibited when the Si content is higher than 55 wt%. Moreover, the broken effect originating from grain collision and thermal stress on the Si primary dendrites was enhanced by further increasing the Si content, resulting in a decrease in the experimental recovery rate. Therefore, the optimum composition is determined as Al-50 ~ 55 wt% Si for solvent refining solution, based on the cost reduction consideration.

Highlights

Solar cell production based on Si wafers has increased significantly over decades, as there is a growing demand for clean energy
Since the investigated alloy composition is addressed at hypereutectic alloy, the shape of primary Si appears plate-like structure, which is a characteristic morphology for the precipitated grains in hypereutectic Al-Si alloy with high Si content [22]
A series of Al-Si hypereutectic alloys with Si content of 30 ~ 70 wt.% were employed to investigate the effect of Si content on the evolution of grain morphology and recovery rate during Al-Si alloy solvent refining process

Summary

Introduction

Solar cell production based on Si wafers has increased significantly over decades, as there is a growing demand for clean energy. Material resources for this application are mainly high-purity solar-grade silicon (SoG-Si), either for single crystal wafers or for multi-crystalline wafers. The SoG-Si feedstock is mainly produced via the traditional Siemens process or its modified alternatives [1], which is fairly energy intensive and environment-unfriendly [2]. There include, but no mean complete, slag treatment [3], plasma treatment [4], acid leaching [5], directional solidification [6], solvent refining [7], and so on. Solvent refining is a promising technique for the SoG-Si feedstock production. It demonstrates the potential to removal almost all impurities in MG-Si, including B and P elements, relying on the decrease of segregation coefficients of impurities due to the lowered liquidus temperature [8]

Results

Discussion

Conclusion