| With the steady growth of the capacity of global solar PV generating, photovoltaic cells industry are facing both big challenges and great opportunities. Under the large market demand, solar grade multicrystalline silicon (SG-Si) which is the dominant material of photovoltaic cells encounters a serious situation of shortage, and becomes as the development bottleneck of PV industry. So it is needed to develop some new technologies in order to break the limitations of the feedstock of SG-Si. At present, the modified Siemens is the main production technology of SG-Si, but the core part of this technology has been monopolized almost by the United States, Japan and Germany. And as a chemical purification method, it has some disadvantages, such as large investment in equipment, higher production costs, severe environmental pollution and so on. Therefore, more and more attentions have been paid to physical methods, especially physical metallurgy process. Metallurgy process consists of vacuum melting, slagging, hydrometallurgical process, plasma beam melting, directional solidification and other methods are used to purify low-cost metallurgical grade silicon (MG-Si) up to SG-Si directly. It is considered as one of cost-effective methods used commonly to cast SG-Si materials because of its short production cycle, little pollution, low cost and simple process.The multicrystalline silicon (mc-Si) ingot was prepared by vacuum-induction melting and directional solidification in this paper using self-designed vacuum-induction melting furnace. It is well known, vacuum melting can not only prevent oxidation but also remove volatile impurities from molten silicon. Directional solidification is widely used to remove the impurities with small segregation coefficient according to the segregation phenomenon. Meanwhile it can control the heat flow through one-way to make the grains grow as columnar structure nearly arranged along the vertical direction, thus reducing the harmful effects of the grain boundary.Silicon feedstock with purity of 2N was used to make a series of mc-Si ingots under different temperature fields in order to investigate the effect of directional solidification process on the morphology of silicon ingots and the behaviors and removal mechanisms of volatile impurities in molten silicon through the analysis of microstructure, composition and temperature field. The results show that:the heat spread through the crucible wall is inevitable, but it can be supplemented by adjusting the relative position of the heater and liquid-solid interface. To get the straight (or convex slightly) solid-liquid interface, the interface should be located near to the bottom of induction coil (or slightly higher than the induction coil) in theory. The satisfactory growth morphology of silicon ingot was received when the liquid-solid interface is 1～1.5cm higher than the bottom of the induction coil in our experiments.The concentration of phosphorus, aluminum and calcium decreased significantly due to the obvious evaporation in the stage of thermal insulation (≥1723 K,0.1 Pa,1.5 hours), and the evaporation rates in theory increased up to 21.1%,22.4% and 33.3% respectively. Further studies showed that, segregation mechanism of the three impurities played a dominant role in the directional solidification stage, and aluminum has the best purification result of these impurities because of its smallest segregation coefficient. Compared to calcium, the evaporation effect of phosphorus and aluminum on their concentration and distribution exist, to an extent, but not obvious. And for calcium, the evaporation becomes stronger owing to the increase of calcium concentration in silicon melt based on the Scheil equation with the solidification of silicon. Therefore, the concentration and distribution of calcium are mainly determined by both segregation and evaporation of calcium in the stages of solidification. |
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