摘要
在能源危机和环保压力双重作用下,21世纪以来光伏行业以平均每年37%的速度高速发展。目前,全球98%以上的太阳能电池是利用硅材料制备的,光伏行业基本是建立在高纯硅材料基础之上。随着光伏行业的快速增长,作为基础材料的高纯多晶硅需求日益紧张,传统的高纯多晶硅来源已不能满足光伏行业快速发展的需求,原料供应不足已经成为太阳能电池产业发展的瓶颈之一,这无疑限制了太阳能光电产业的更为广泛应用。为此,世界各国都在积极探索制备高纯硅材料的全新工艺方法,其中冶金法制备多晶硅由于具有生产周期短、污染小、成本低、工艺简单、规模大小可控等特点,被认为是最能有效地降低多晶硅生产成本的技术之一,目前已成为世界各国竞相研发的热点。冶金法是利用硅和硅中杂质物理性质的差异来使之分离,把定向凝固除杂、真空熔炼除杂、氧化除杂、酸浸除杂、造渣除杂、合金化除杂和电解还原除杂等方法中的一种或几种有机地组合起来形成一种工艺路线,达到提纯制备太阳能级硅的目的。在众多工艺方法中,电子束熔炼由于具有高温、高真空、无污染等特点,被认为可以用于出去硅中挥发性杂质元素。但由于相关基本问题及理论解释仍然存在不足,使得该技术目前还仅仅停留在学术研究或试运行阶段。
本文使用60 kW的电子束熔炼炉进行系列实验,研究硅中各种杂质元素的挥发去除行为及去除机理,以期为该工艺实现大规模工业化生产提供基本理论支持。实验结果表明电子束熔炼可以有效去除硅中挥发性杂质元素(P、Ca、Al)；非挥发性杂质元素(Fe、B)不但不会被去除,由于硅自身的挥发损失反而会在熔炼过程中浓缩。硅中重要杂质元素P的去除反应速率随电子束功率、熔炼时间增加而增大；21 kW下熔炼30 min后,P含量可以降至0.1×10-4 wt%以下,达到太阳能级硅要求的水平。除磷反应遵循一阶反应速率方程,即P以单原子形式挥发出硅熔体,且反应速率由气／液界面层的挥发反应控制。Ca、Al的含量随熔炼时间延长呈指数衰减形式逐渐降低；熔炼30 min后,Ca含量由7.8×10-3wt%降至1.5×10-4 wt%、Al含量由3.1×10-3wt%降至8.5×10-5wt%；Ca、Al的挥发去除遵循一阶反应方程,气／液界面层的挥发反应是其反应的速率控制步骤。
断面组织观测结果表明电子束熔炼制备的多晶硅铸锭是由大量柱状晶组成,同时在柱状晶间伴随生长很多孪晶。金属杂质Ca、Al、Fe在整个铸锭中并不是均匀分布,而是呈现出由底部到顶部、由边缘向中心的富集趋势；非金属杂质P、B由于平衡分凝系数较大,其在铸锭轴向上会向顶部做一定富集,但并不明显,径向分布则十分均匀。
The photovoltaic (PV) industry passed the 100 MW per year production milestone during 1997 and has been growing rapidly toward GW per year production at an annual growth rate of approximately 37%. At present, more than 98%of commercial PV modules are made from crystalline silicon, and predicted growth of PV market is very dynamic and entirely depends on polycrystalline silicon feedstock. Currently, the off spec, pot-scrap, re-melt, tops and tails and "out of spec" crystals are the born feedstock materials for the PV industry. The rapid growth of the solar cell market, however, predicts the lack of availability of low-cost solar grade silicon feedstock in the near future. To tackle this shortage problem, novel purification techniques for low-quality Si feedstock material have recently been investigated all of the world, and are meant to open alternative routes towards a solar grade silicon feedstock, exclusively destined for the PV market.
Among the different routes that are being explored, upgrading metallurgical grade silicon (MG-Si) was considered as the most attractive approach for its short production cycle, little pollution, low cost, simple process. However, this approach involves reduction of several impurities from high levels so that each impurity is reduced to less than 0.1 ppm level.
It is well known that electron beam melting (EBM) process has an excellent metallurgical characteristic in producing high-purity materials. Impurities with high vapor pressure under the condition of high vacuum can be evaporated effectively, because of the high local temperature near the beam impingement zone. Casenave firstly used EBM process to purify polycrystalline silicon and confirmed the technical feasibility of removing some impurities, such as phosphorus and calcium. Several investigations in EBM process have been reported. However, the process still has not resulted in a commercially viable product by now for the lack of the fundamental studies on the basic problems. In this paper, the effects of various melting condition on the impurities removal behavior were investigated, the impurity removal process during EBM process was also studied theoretically in thermodynamic and kinetics considerations.
It is found that the contents of phosphorus, calcium and aluminum decreased significantly after EBM process. However, no significant change was found in content for boron and iron. The content of phosphorus decreased by evaporation with the increase of melting time and evaporation rate increased with raising electron beam (EB) power density. The content of phosphorus decreased below 0.1 ppm by EBM traetment at 21 kW for 30 minutes. The dephosphorization reaction was analyzed to follow the first order rate law, and the dephosphorization rate was was considered to be controlled by free evaporation. The content of aluminum and calcium gradually decreased with the increase of melting time, and the lowest content of both the above impurities were 1.5 ppm and 0.85 ppm respectively obtained during EBM process at 15 kW for 30 minutes. The removal rates of these elements were also expressed by the first order rate law.
Metallic impurities including iron, calcium and aluminum was dragged from the bottom to the top and from the edge to the center of the ingot, following the direction and characteric of solidification. The purest area existed on the edge followed by the central area, coming from the bottom to the top with maximum concentration at the top of the sample. As for phosphorus and boron, the segregation effect is not siginificant due to the large segregation coefficient of phosphorus and boron.
引文
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