基于Al-Si合金精炼硅技术的微量金属添加剂对硼的捕获行为及其热力学研究

How to deeply remove B (boron) from silicon is the difficult problem that the technology of electromagnetic solidification refining with Al-Si alloy has to break through. As V, Zr and Ti have strong affinity for B, we propose a method of strengthening B removal process by adding tiny amount of V, Zr and Ti to the Al-Si alloy: B is firstly captured by V, Zr and Ti (form borides) in the refining process which makes it easier to move to the silicon grain boundaries and liquid phase (because the segregation coefficient of B decreases); thereafter, the borides in grain boundaries and liquid phase are removed by hydrometallurgy. The added V, Zr and Ti can be easily removed in this process because of their small segregation coefficients. This study will theoretically predict the ability of V, Zr and Ti capture boron and the thermodynamic conditions to form their borides in Al-Si alloy by measuring the thermodynamic data (solubilities, activity coefficients, and interaction coefficients). Meanwhile, using the thermodynamic prediction results as theoretical basis, this work will study the effects of cooling rate, melt composition and induction heating field on the behavior of V, Zr and Ti capture B in the electromagnetic solidification refining with Al-Si alloy. Combined with microscopic observation and analysis, the key factor and the restrictive links which influence the process of V, Zr and Ti capture B will be revealed. The research results will provide new metallurgical methods for solar-grade polysilicon production.

如何深度去除硅中B（硼）是Al-Si合金电磁凝固精炼技术需突破的难题。由于V、Zr和Ti与B的亲和力强，本课题提出往Al-Si合金中添加微量的V、Zr和Ti强化除B的方法：B在精炼时先被V、Zr和Ti捕获（形成硼化物）而更容易往硅晶界和液相迁移（降低B的分凝系数），再用湿法除去硅晶界和液相的硼化物而达到强化除B的目的，添加的V、Zr和Ti因分凝系数小可被轻易地去除。本课题将通过热力学数据的测定（溶解度、活度系数、相互作用系数），从理论上预测V、Zr和Ti捕获B的能力及计算相应的硼化物在Al-Si合金中稳定存在时的热力学条件。同时，以热力学预测结果为理论根据，研究Al-Si合金电磁凝固精炼过程的冷却速度、熔体成分、感应热场等因素对Zr、V和Ti捕获B行为的影响规律，结合微观观察和分析手段，揭示影响捕获B过程的关键因素和限制性环节。本课题研究成果将为冶金法制备太阳能级多晶硅提供新方法。

如何深度去除硅中B杂质是Al-Si合金法制备太阳能级硅需突破的难题。由于过渡族金属元素与B的亲和力强，本课题创新性地提出往Al-Si合金中添加微量的Ti、V、Zr或Hf（简称M）强化除B的方法，实验结果证实了M在除B的同时也被去除，不会对硅造成二次污染。本课题测定了M与B在1173 K和1273 K时的平衡浓度（溶解度），确定了M与B在Al-Si熔体中形成稳定的TiB2、VB2、ZrB2或HfB2。由实验结果计算了M与B在973 K-1323K时的平衡浓度。根据绘制的平衡浓度图，添加M去除Al-Si熔体中B的能力由强到弱为Ti、Hf、Zr和V。另外，由平衡浓度图确定了在Al-Si-M-B熔体中形成硼化物的热力学条件。由B在Al-Si熔体中活度及活度系数计算了M与B的相互作用系数。当温度为1173 K时，Ti、V、Zr和Hf对B的相互作用系数分别为889、2208、-772和-159。当温度为1273 K时，对B的相互作用系数分别为238、1112、-461、-53。由相互作用系数计算了添加M时B在固体Si和Al-Si熔体之间的分凝系数，并确定M除B的能力由强到弱为Zr、Hf、Ti和V。因此，去除Al-Si熔体中B的最佳添加剂为Ti，但去除硅中B杂质的最佳添加剂为Zr。添加Zr和Hf强化除B的机理是Zr和Hf不仅能够降低Al-Si熔体中B的含量，也可以降低B的分凝系数。添加Ti和V强化除B的机理是Ti和V可以降低Al-Si熔体中B的含量。过渡族金属添加剂对B的捕获行为研究结果表明，当添加1025 ppma的Hf、1057 ppma的Zr和1018 ppma的Ti后，B的去除率由59.5%分别提高到97%、97.2%和75.8 %；当添加800 ppma的V后，B的去除率由51.9 %提高到76.8 %。M强化除B的能力由强到弱为Zr、Hf、Ti和V。对B的捕获行为研究结果与热力学研究结果一致。定向凝固速度（或下拉速度、冷却速度）以及Al-Si合金液相线温度对除B效果影响显著。凝固速度越低越有利于除B过程，Al-Si合金液相线温度越低，越有利于B的去除。Al-Si合金液相线温度是影响除B效果的关键因素。B由固相向液相的迁移是影响除B的限制性环节。本课题研究内容为冶金法制备太阳能级多晶硅提供了新的思路和理论依据，共计获授权发明专利2项，共发表高水平期刊论文9篇。

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