Study On The Processing Technology To Improve The Recovery Rate Of Poly-crystalline By Al-Si Alloy Refining Method

It is a huge demand for poly-crystalline Si feedstock with high purity due to the development of photovoltaic industry.Based on the advantage of high removal rate of impurity and environment friendly,purification by alloy refining method is a promising technology.The Al-Si alloy is a good candidate.In this route,the silicon raw material was wasted because of the high eutectic composition,i.e.Al-12.6 wt.%Si.Thus,improving the recovery rate and decreasing the impurity content of primary Si is the key factor for its scale-up application.In this work,the semi-solid treatment and Al-Si-Sn ternary composition have been adopted to improve the recovery rate of primary Si.Following results have been summarized.In the study of semi-solid treatment,the semi-solid treatment was introduced into the purification of poly-crystalline Si,in turn,the recovery rate was increased.It was found that lower annealing temperature(<600℃)is beneficial to the grain growth of primary Si.The improved recovery rate is resulted from ripe effect.During the heat treatment process,the dissolved eutectic Si atoms agglomerated together to form Si clusters,which deposited on the surface of neighbored primary Si.This could result in grain coarsening.The impurity content decreases with increasing the annealing time.The impurity is easier to remove due to the separation effect,which suppresses the contamination of Al to purified Si.For the Al-50Si alloy,the optimized semi-solid treatment parameter is annealed at 590 ℃ for 35min,leading to a recovery rate of 88.8%(higher than the theoretical value).The removal rate of B is 94.6%,and that values are higher 99%for P and majority metal impurities.In the study of Al-Si-Sn ternary alloy,both eutectic and diffusion effects affect the recovery rate.One hand,the oriented grain growth along<111>and<112>direction was promoted by Sn addition.The nucleation rate was decreased since the decreased melting point and eutectic composition in Al-Si-Sn alloy.On the other hand,the diffusion ability of Si atoms was weakened in the thmary alloy,which results in hindering the grain growth of Si.Moreover,the precipitating process of Si was prolonged when the cooling rate is low.For the Al-50Si alloy,90%recovery rate,which is 5%higher than that in the binary alloy,could be obtained using 10-15%Sn addition and cooling rate of 4 ℃/min.The B content in the purified Si is below 1 ppmw,and that is in the range of 1.8～2.8 ppmw for the P impurity.The absorbing effect of Sn on the impurities of Ca,Cu,Ti and P results in the increase of removal rates.The B atoms distributed along the boundary between Si and matrix,resulting in the decrease of removal rate.No absorbing effect was observed on Fe.In conclusion,the recovery rate of primary Si was focused in this work.New purification technology has been proposed.Firstly the enhanced recovery of Si was accomplished by introducing the semi-solid treatment into purification.The ripe process of silicon grains was quasi-situ characterized.Secondly,the purification process was optimized for Al-Si-Sn ternary alloy,leading to 5%increase in recovery rates.The absorbing effect of Sn on impurities was illustrated.Based on the research background of Al-Si solvent refining,the obtained results in this work are innovative and practical,which would facilitate the industrial-scale production of poly-crystalline Si by solvent refining.