Study On The Effect Of Cu On The Purification Of Solar Grade Polysilicon By Al-Si Alloy Method

The Al-Si alloy purification method has the advantages of low energy consumption,environmental friendliness,simple operation,and low equipment and plant construction costs.It is an up-and-coming method for preparing solar-grade polysilicon raw materials,and small-scale industrial production has already been achieved.However,the conventional Al-Si alloy method is difficult to efficiently remove the non-metallic elements B and P with large partition coefficients in the primary silicon,and Al as a solvent in the Al-Si alloy method can pollute Si.Therefore,the development of the Al-Si alloy method in solar polysilicon preparation is limited.Improving the removal rate of B,P,and Al from primary silicon prepared by the Al-Si alloy method is of great importance for industrial production.Therefore,in this paper,Al-Si-Cu ternary alloy purification systems with different alloy compositions were prepared by adding Cu to Al-50wt.%Si alloy based on the study of Al-50wt.%Si.Analyzed the effect of Cu on the nucleation growth of primary silicon and revealed the effect of Cu on the removal efficiency of impurity atoms B,P,and Al in primary silicon.The following results were obtained:Firstly,Al-Si-Cu ternary alloys’ microscopic morphology and elements distribution were systematically investigated.SEM and EDS analyzed the elemental composition,and the software simulated the solidification phase diagram of the ternary alloy.The results indicate that a large amount of the Al2Cu phase precipitated in the alloy melt,and the precipitation mode and growth characteristics of the Al2Cu phase are clarified.The variation of alloy melting point,eutectic temperature,alloy viscosity,and melt diffusion coefficient with Cu content were calculated based on JmatPro software.It pointed out that adding Cu to the ternary alloy has the "eutectic effect" and"confinement effect" on primary silicon’s nucleation and growth process.The average size of the primary silicon increases with increasing Cu content,and the yield decreases with increasing Cu content.The average length and thickness of the primary silicon increased by 21.6%and 17.24%,respectively,with the addition of 1%Cu compared to that without the addition of Cu.This finding indicates that the "eutectic effect" caused by Cu addition is stronger than the "confinement effect"inhibition.When the Cu addition amount is 1%～2%,the experimental yield can reach more than 80%,close to the theoretical yield of 85.58%of primary crystalline silicon.These results show that the Al-Si-Cu alloy purification method is feasible.Secondly,the variation of the activity coefficient of Al in the alloy with Cu content was analyzed by thermodynamic calculations.With the increase of Cu content,the activity coefficient of Al in the alloy melt decreases.The activity coefficient of Al in the alloy is 0.7445 when Cu is added at 1%;when Cu is added at 10%,the activity coefficient decreases to 0.7148.The decreased activity coefficient results in a lower solid solution of Al in Si,reducing the Al content in the primary silicon.In addition,the precipitation temperature of Al2Cu is lower than the eutectic temperature,indicating that Al2Cu does not "contaminate" the primary silicon in the form of inclusions.The impurity analysis shows that the Al content in the primary silicon decreases with increased Cu content.When Cu was added at 10%,the Al content in the primary silicon was 181.637 ppmw,much lower than that of 250.96 ppmw without Cu addition.These results will enrich the theory of A1 impurity removal in primary silicon and show that using Al-Si-Cu ternary alloy purification system and the optimized pickling process is a proven idea to suppress Al contamination in primary silicon.In order to further analyze the removal behavior of Cu on B and P impurity atoms in the primary silicon,it is shown in this paper that the content of B and P in the primary silicon decreases with the increase of Cu content by analyzing the impurity content of B and P.Among them,the B content in the primary silicon can be reduced to 1.92 ppmw,with a removal rate of 94.09%;the P content can also be reduced to 1.236 ppmw,with a removal rate of 96.23%.Furthermore,the distribution of B and P in the sample was analyzed by energy spectroscopy,revealing that the Al2Cu phase precipitated in the Al-Si-Cu alloy can be used as B and P absorption centers.Furthermore,the results show that Al2Cu has a more substantial enrichment effect on P.Adding Cu can form Cu-based B and P compounds at high temperatures,effectively reducing the chance of the AIP phase as a nucleation site for primary silicon.Meanwhile,adding Cu enables the removal of other major metal impurities in the primary silicon to reach more than 90%.Therefore,the Al-Si-Cu alloy system can effectively remove the high partition coefficient impurities B,P,and other metallic impurities from the primary silicon,proving the practicality of the Al-Si-Cu alloy method for purification.In summary,the best Al-Si-Cu alloy composition ratio was determined as Al-Si-2wt.%Cu.This solvent alloy can achieve a primary Si recovery rate of more than 80%.Additionally,when the Cu addition was below 2%,the residual Al content in the primary silicon dramatically decreased,and the removal fractions of B,P,and other major metallic impurities were higher than 90%.These results showed that employing the Al-Si-Cu solvent alloy is a potential refining solution for producing solar-grade Si feedstock.