Distribution And Transfer Mechanism Of Iron Impurity During Silicon Purification By Directional Solidification

There is a big gap between the supply of and demand for solar grade silicon which used as the main raw material for photovoltaic industry in future.It is urgent to develop a way of green manufacture with high efficiency,low cost and stable process for solar grade silicon production.Metallurgical route is an exclusive purification process for the preparation of solar grade silicon.Based on the differences of physicochemical properties between silicon and impurities,this method uses metallurgical silicon as raw material and integrates many metallurgical technologies,such as acid pickling,slag refining,vacuum refining,electron beam melting,directional solidification,etc.,to reduce the impurity content in silicon and obtain solar grade silicon.The impurities can fall into three categories,the volatile impurity,the oxidizing impurity and the metal impurity according to the differences of physicochemical properties between silicon and impurities,which are mainly in such characteristics as evaporation,segregation and oxidation.Each purification process in metallurgical route is based on only one of these characteristics.Three kinds of impurity can be removed from silicon through vacuum refining technology,slag refining technology and directional solidification technology respectively.The Fe element is a kind of metallic impurity in silicon which has the highest content,the most difficult to remove from silicon and the most significant effect on the photoelectric conversion efficiency of solar cells.In addition,the removal process of iron impurity from silicon has a synergistic effect with other metal impurity,that is,the other metal impurity can be removed before the iron impurity is removed.Directional solidification is the key technology in the preparation of solar grade silicon by metallurgical route,which can be used to remove the metal impurity from silicon.The principle of directional solidification technology is segregation effect during solidification process.The degree of segregation effect is represented by effective segregation coefficient which is the function of crystal growth rate for specific experimental process and impurity.Thus,the crystal growth rate is the key parameter,which closely related to the production cost,the production efficiency and the product quality,during silicon purification by directional solidification.The crystal growth rate usually is fluctuant in industrial production and has a negative impact on production.Based on the surface characteristics of solid silicon,quartz crucible and graphite plate,the relationship between thermal contact resistance and temperature during silicon purification by directional solidification were revealed.A theoretical model,which considers thermal contact resistance,for investigating the crystal growth rate is proposed in this thesis,which can reveal the reasons and rules of the fluctuation on crystal growth rate.The experimental results show that the calculation accuracy has improved about 32.6%compared with other researchers due to the contact thermal resistance had consider in our model and the calculation deviation less than 10%.The results show that crystal growth rate is affected by the surface temperature of silicon melt which has an approximate linear relationship with the temperature of heater in the same horizontal plane.The crystal growth rate can be calculated with the surface temperature of silicon melt.Thus,the theoretical model provides a theoretical basis for precisecontrolling the crystal growth rate and impurity distribution during directional solidification process.Based on characteristics of the iron impurity content on the solid-liquid interface during the silicon purification by directional solidification,a theoretical model for investigating the back diffusion of iron impurity is proposed in this thesis.The results show that back diffusion,which can affect the distribution of iron impurity seriously and weaken the purifying effect,exists on the on the solid-liquid interface during the directional solidification process.The degree of back diffusion mainly depends on the solidified fraction,the height of silicon ingot and the crystal growth rate.Its value increases with the increase of the solidified fraction and the height of silicon ingot,the crystal growth rate affect the back diffusion of iron impurity through the segregation effect and the diffusion time,the effect of crystal growth rate on the degree of back diffusion depends on which factors dominate.The results show that the degree of back diffusion approximate linear increases with the increase of the crystal growth rate when its value less than 5×10-8 m·s-1;the degree of back diffusion unchanged when the crystal growth rate increase from 5×10-8 m·s-+1 to 5×10-6 m·s-1;the degree of back diffusion decreases sharply with increase of the crystal growth rate when its value larger than 5×10-6 m·s-1.In addition,the degree of back diffusion increases with the increase of the solidified fraction and relative to back diffusion ignored reaches nearly 200%at the end of solidification.Experimental verification reveals that the calculation that considers back diffusion is more consistent with experimental results than when back diffusion is ignored.However,the calculations by the theoretical model are still substantially lower than experimental results at the end of solidification.Based on the segregation effect of impurity and mass conservation,the idea of discretization is proposed in the process of directional solidification,which is that the solidification process is decomposed into several sub processes and solved separately,and then integrated.A theoretical model for investigating the distribution of iron impurity during silicon purification by directional solidification with fluctuating crystal growth rate is proposed in this thesis.The results show that the iron impurity be enriched to silicon melt during directional solidification.The enrichment degree in the silicon melts and iron impurity distribution in the silicon solid are closely related with the concrete process of solidification.The fluctuating crystal growth rate is the main reason for the fluctuation distribution of iron impurity in silicon ingot.There is a large optimization space for traditional purification process in industrial production,for instance,the solidified time of traditional purification process and optimized purification process are 58.25h and 33.99h,respectively,which base on assuming that the initial iron impurity content in raw silicon is 659.4ppmw,the height of silicon ingot is 0.248m,the qualifying criteria of iron impurity is 0.1ppmw and the production yield is 80%.The initial impurity content in raw silicon seriously affect the product quality and the production yield,for instance,the production yields are 93%,70%,64%and 61%for the initial iron impurity content in raw silicon are 500ppmw,1000ppmw,1500ppmw,2000ppmw,respectively,which base on assuming that the height of silicon ingot is 0.248m and the qualifying criteria of iron impurity is 0.1ppmw.Based on the mass conservation and coupled temperature field and concentration field,a theoretical model for investigating the diffusion of impurity during cooling stage is proposed in this thesis,the effect of iron impurity diffusion during cooling stage on the iron impurity distribution is analyzed,the relationship between impurity diffusion during cooling stage and production parameter is discussed and found that the cooling rate and the height of silicon ingot can affect the diffusion of iron impurity seriously.The quantitative distribution of iron impurity in the final solidification region after directional solidification is revealed.The research result also shows that the diffusion of impurity can be effectively inhibited by increasing the height of silicon ingot or increasing the cooling rate during cooling stage,thereby,the production yield of slicon purification by directional soldidification can be improved.For iron impurity,the production yield can be improve 11.4%by increasing the cooling rate during cooling stage,which base on assuming that the initial iron impurity content in raw silicon is 659.4ppmw,the height of silicon ingot is 0.183m,the crucible charge is 300kg,the crystal growth rate is 1×10-6m/s and the qualifying criteria of iron impurity is 1 ppmw;the production yield can be improve 6.47%by increasing the silicon ingot height,which base on assuming that the initial iron impurity content in raw silicon is 659.4ppmw,the cooling rate is 0.03 ?/s during cooling stage,the crystal growth rate is 1×10-6m/s and the qualifying criteria of iron impurity is 1 ppmw.In addition,the diffusion of impurity can be inhibited completely by separating silicon solid and melt before solidification finish,which can completely avoid the affect of impurity diffusion on the production yield during cooling stage.In the process of studying the distribution and transport mechanism of iron impurity,firstly,the back diffusion phenomenon at the solid-liquid interface was considered.Secondly,the influence of the fluctuation of solidification rate and the diffusion of iron impurity during cooling stage were also considered.The experimental results show that the calculation accuracy has been improved 53.3%relative to Scheil equation separately,which more agree with experimental data and the distribution and transfer mechanism of iron impurity during silicon purification by directional solidification were revealed.