Basic Research On Removal Of Key Non-metallic Impurity Elements From Silicon By Al-Si Microalloy Solvent Refining

Purifying silicon by metallurgy method has attracted widespread attention due to its low cost,low energy consumption,simple process,environmental friendliness,and is expected to become a dedicated production technology for solar-grade Si.The Al-Si alloy solvent refining has become the most widely studied and applied solvent refining method because of its advantages of not forming intermediate compounds during the solidification process,low melting point temperature after alloying,and low raw material prices.However,the traditional Al-Si alloy method is difficult to efficiently remove non-metallic impurity elements B and P in silicon with a large segregation coefficient.In this work,the removal efficiency of the key non-metallic elements B and P in silicon is greatly increased by the Al-Si microalloying solvent refining.For the removal of B,we used the combination of kinetics and microalloying for the first time to efficiently remove B by the Al-Si solvent refining.After the Al-Si alloy is refined by combining two factors,the B content in the primary Si can be reduced from the initial 100 ppmw to below 1 ppmw.With the Zr-3000 ppmw addition and 0.4 mK·s-1 cooling rate,the removal efficiency of B in the primary Si can reach 99.36%.The analysis of the Si phase impurity content revealed how Zr remained in Si.Finally,the reaction process and interaction mechanism of Zr with B and Si in different reaction stages are analyzed.Furthermore,we investigated the effect of electromagnetic stirring on of Al-Si microalloy solvent refining.The flow field and the temperature field of the Al-Si melt under different lowering process were analyzed by a numerical simulation.Combined with the microstructure,it is revealed that the flow field and the temperature field in the Al-Si melt can cause the segregation of impurity phases in the sample.The distribution position of the impurity phases and the temperature gradient in the melt makes the B content a gradient distribution in the Si-rich area of the sample.For the removal of P,Sr was employed as the P-getter to further improve the removal effect of P during the solidification of the Al-Si solvent.By microstructure and EDS analysis,it is determined that the Al2Si2Sr phase precipitated in the melt is enriched with 0.8-1.2 at%P.The formation of the P-containing Al2Si2Sr phase changes the occurrence state and the distribution of the impurities in the Al-Si alloy,so that a high P removal fraction in the primary Si is obtained.The calculation of the formation enthalpy and the cohesive energy of the three different P substitutional solid solutions reveals the detailed crystal structure of the P-containing Al2Si2Sr phase.Finally,a quaternary alloy system was used in the directional solidification process to achieve the purpose of efficiently removing B and P in Si for the first time.The quaternary alloy system means that two metal elements(P and B-affinity elements)need to be added to the alloy,namely Zr and Sr as the B/P-getter elements in the Al-Si solvent refining process.In the Al-Si-Sr-Zr system,it was confirmed that the Sr and Zr co-addition in the Al-Si alloy can precipitate a P-containing Al2Si2Sr phase and ZrB2 phase in the melt.The precipitation of these two B and P rich phases enables the primary silicon to obtain a higher B and P removal efficiency.Through the comparison study of the Al-Si-Sr-Zr systems and the Al-Si-(Sr or Zr)systems,the effect of Sr and Zr co-addition on the removal fractions of P and B is revealed.An evolution model is established to describe the nucleation and growth stage of the Sr/Zr-containing compound phases.