The Effect Of Solidification Rate On The Precipitation And Removal Of Typical Metallic Impurities In Industrial Silicon Grain Boundaries

Industrial silicon is the basic material of solar cells.Its purification is mainly through physical metallurgy.The acid pickling and alloy remelting methods can achieve good results,but there is still room for further improvement.Most researchers pay more attention to the experimental parameters,and lack of systematic research on the distribution and occurrence of impurities in industrial silicon.Based on solidification theory and out-of-furnace refining technology of industrial silicon,the segregation and enrichment of impurities at grain boundaries and the change of microstructures under different solidification conditions were studied.In the solidification process,the solidification rate has an important influence on the diffusion and segregation of materials,so the main experimental parameter changed in this study is the solidification rate.(1)The experimental equipment is a resistance furnace with argon gas.The setting solidification rates are as follows: 0.5?/min,1.25?/min,2?/min,2.5?/min,4?/min and 10?/min.In each group of experiments,the amount and distribution characteristics of impurities at grain boundaries were observed under scanning electron microscopy after the samples were obtained.According to the atomic percentages,the main phases were conjectured,and the object of analysis is metal impurities.(2)The results show that the segregation ratio and distribution of impurities at grain boundaries are different when the solidification rate increases from 0.5?/min to 4?/min.Fe and Al mainly exist in the form of Fe-Al-Si phase,while Ti and V mainly exist in the form of Si-Ti-V phase.When the cooling rate is further increased to 10?/min,the resulting phases are also different.The boundary of cooling rate exists between 2?/min and 4?/min.(3)The segregation diffusion of elements is not only affected by solidification conditions and their own factors,but also by other elements.The interaction between Fe,Al,Ti,V,Mn and the segregation ratio of Fe: Al: Si and Ti: V are analyzed in detail.(4)In order to ensure the reliability of the experimental data,in addition to theoretical analysis,this paper also carried out acid pickling on the experimental samples to prove the theoretical analysis.The content of impurities was measured by inductively coupled plasma emission spectrometer(ICP-AES).The results show that the removal rate of impurities is the highest when the solidification rate is 2?/min.Through theoretical analysis and experimental data,the segregation law of impurities in industrial silicon is obtained.When the proportion of element content is different,the kinds of phase at grain boundary and the relative number of various phases are quite different.