Study On The Purification Of Metallic Silicon By Unidirectional Solidification

With the global economy development,demand for energy is growing for human.Oil,coal and other non-renewable resources are increasingly exhausted.As a kind of clean renewable energy,solar energy has a rapid progress in research and development.The explosive growth of Photovoltaic industry makes solar grade polysilicon materials a huge gap in supply.Traditional method of Siemens can not satisfy the market demand for PV.Physical metallurgy method with low investment and low cost has been widespreadly concerned in all countries.Directional solidification process is an important part in the physical preparation of metallurgical-grade polycrystalline silicon.By controlling the temperature field changes,ingot growth in a one-way was made,the impurity elements enriched at the top of the ingot by segregation effect,the purpose of purification was achieved.Using self-designed vacuum induction melting furnace,directional solidification purification of industrial silicon has been conducted and process has been optimized through directly drawing the ingot after silicon melt.The effect of purification through directional solidification has been evaluated through ingot organization,composition and resistivity analysis system,the results show that:In the One-time directionally solidified silicon ingots,about 50%of the region achieved 4N,Fe,Cu,Ni,Ti and other metal impurity removal were more than 96%;the two-times directional solidification was made in the same region,the majority of Metallic impurities reduced to less than 10^-6,the total content of metal impurities were less than 5×10^-6;reducing the rate of pulling ingot or increasing size of the crucible,solid-liquid interface can become straight,the effect of purification can be increased。A one-time directional solidification,the continuous enrichment of impurities in the solid-liquid interface made the coarse columnar crystals stop growth at a certain height,faults appearing.Lower part was P-type silicon material,the resistivity increased in the vertical direction with the P element increasing,achieving maximum at the fault;the upper part was N-type silicon material,the resistivity decreased rapidly to 0.In the second directional solidification of ingot,the columnar crystal growth extends to the top of ingot,no fault appeared.The overall ingot resistivity was lower than P-type region in a one time ingot.