硅合金精炼过程杂质的化学重构及其动态演变行为研究

One of the biggest difficulties in the preparation of solar grade silicon (SOG-Si) is how to deeply remove the trace nonmetallic impurities (B、P) and metal impurity. The effectively applied chemical refactoring makes the alloy refining become a hot research topic because it can change the occurrence position and form of impurities to strengthen the separation of impurities from the silicon substrate. The choice of production process and refining solvent has certain blindness due to the lack of thermodynamic data related to multi-element alloy. The evolution of composition and structure of impurities is the basic of understanding the efficient control during the alloy refining process. In this study, the refactoring process is understood more clearly with online monitoring combined with offline detecting which can guide the establishment of new method of chemical refactoring for removing impurities with high efficiency. Firstly, the basic separation properties and evolvement of impurities before and after chemical refactoring are understood by building different silicon alloy system. Secondly, the impurities’ generation and migration behavior at the interface is measured with online detecting at under different refining systems, while the structures and compositions of the impurities separated from the alloys system under super gravity are analyzed. Thus the effect of chemical refactoring on the impurities is confirmed. Finally, by correlating all the factors in this process such as the change of composition impurity，silicon purity and kinetic parameters, the mechanism of the chemical refactoring of impurities in the refining is revealed and also the technical prototype of the process for metallurgical silicon is established.

硅中微量非金属杂质（B、P等）与金属杂质的深度去除是冶金法制备太阳能级多晶硅的技术难点。化学重构能改变杂质赋存位置与形态，加大杂质与硅基体的分离性质差异，因此硅合金精炼提纯成为研究热点。但限于多元合金相关热力学数据的不足，目前合金精炼介质与工艺选择还缺乏针对性。杂质相组成与结构演变是认识精炼过程及对其高效调控的基础。本研究将通过在线观测联合离线检测的方式更清晰直观地认识重构过程，进而指导化学重构高效除杂新方法的建立。首先，通过构建不同硅合金体系，掌握杂质相重构前后分离特性变化规律；其次，通过高温在线监测方法捕捉不同精炼制度下杂质相生成及界面迁移行为，同时通过精炼实验利用超重力将合金体系析出的特征杂质相进行离线分离，分析其结构和成分，完成化学重构结果的确认；最后，关联化学重构过程杂质相组成结构、硅纯度与动力学参数，揭示硅合金精炼过程杂质相态化学重构机理，建立工业硅化学重构提纯工艺技术原型。

随着光伏行业的快速发展，冶金法提纯多晶硅制造技术已在高纯硅材料制备领域占据了重要地位。冶金硅提纯过程伴随着元素的迁移和相转化，多元杂质组分（元素+相）的高效定向迁移是提高冶金硅净化效率的重要保证。为突破微量杂质的分离限制，本项目利用合金精炼与化学重构协同作用的方式强化了非金属杂质B、P与基体硅/合金的分离，同时对杂质相的生成和迁移机理进行解析，为冶金法提纯硅的工艺技术提供理论支撑。. 通过构建硅合金的熔析体系，可改善杂质的分离属性。利用Factsage和Thermo-cal热力学软件，引入Scheil非平衡结晶方程，获得了硅中非金属杂质B、P在Al-Si体系中不同温度下的有效分凝系数，验证了金属熔析剂的引入可极大提高非金属杂质的分凝能力。通过向铝硅熔析体系中添加杂质捕集剂Ti，与硅中难除非金属杂质B进行化学重构，在此过程中非金属杂质B元素（均匀分布）转化为明显区别于基体的准金属化合物TiB2相（非均匀分布），促进分凝的同时进一步强化了杂质硼的分离。基于其高熔点高密度的特性，通过设计高温过滤坩埚，在超重力作用下强化分离，成功实现了光洁硅片的获取和杂质相的富集，大幅降低了酸耗。. 通过定向凝固技术构建合金提纯体系的近平衡凝固环境，在液固界面富集含P的特殊杂质相CaAl2Si2。统计分析可知，该杂质相中含磷量可达0.7 at%。杂质P以置换固溶体的形式分布其中。作为硅熔体降温过程的主要析出相，CaAl2Si2在结晶前沿的存在加大了杂质元素P在界面处的浓度梯度，进而强化P的进一步分凝。通过优化实验条件，强化了富P相的生成，从而实现了难除杂质P的有效去除。. 对铝硅合金感应定向凝固后不同位置处硅形貌及纯度进行关联，解析了液相合金组成的演变对硅晶体生长和杂质分配的作用本质。不同阶段析出的硅晶体受液相成分过冷和溶质扩散的影响呈现出不同形貌（块状-多孔硅-针状硅-颗粒硅），低速凝固下硅中B/P去除率可分别达到76.9%和85.1%。重构杂质相的界面赋存成为硅提纯过程的重要影响因素。