Study On The Interaction Mechanism Between Hydrogen And Impurity Boron In The Process Of Hydrogen Blowing And Boron Removal Of Aluminum-silicon Alloy

The depletion of traditional fossil energy sources and the increasingly serious environmental pollution problems have forced mankind to continuously develop renewable and clean energy.Solar energy is gradually gaining attention due to its green and safe characteristics.Solar grade silicon is the main raw material for the preparation of solar cells,which is currently mainly produced by the modified Siemens method,but the method has high investment,high cost and easy to cause environmental pollution.Therefore,the development of a new green and efficient silicon purification technology is the main development direction in this field at present.The alloy melt refining method is an ideal technology for the large-scale production of solar-grade silicon because of its low cost,low pollution and simple equipment operation.However,the current technology is still difficult for the removal of non-metallic impurities boron in silicon.In this paper,we propose to explore the possibility of deep removal of impurity boron in silicon based on aluminum-silicon alloy melt refining by using the method of melt hydrogen blowing in combination with directional solidification technology.The purpose of this paper is to reveal the mechanism of hydrogen removal from impurity boron in silicon by exploring the interaction between impurity boron and hydrogen,so as to provide a practical reference for the development of hydrogen-blowing boron removal technology.Firstly,the structure and properties of hydrides（BH₂,BH₃,B₂H₆,Al H₃,Si H₄）and borides（Ti B₂,Al B₂,Si B₆）were analyzed by first principles calculation method based on density functional theory.The results showed that the binding energies of hydrides were from high to low:B₂H₆,Si H₄,BH₃,BH₂,Al H₃,indicating that B and H may react to form B₂H₆ and make B be removed.The stability of borides in order from high to low:Ti B₂,Si B₆,Al B₂,indicating that Ti B₂ is also a stable compound that may exist in the refining process.Next,the energy band structures of hydrides and borides were calculated.The results show that BH₂ and Si B₆ have semiconductor properties,Ti B₂and Al B₂ have the properties of conductors,and BH₃,B₂H₆,Al H₃ and Si H₄ belong to insulators.In addition,the density of states calculations for hydrides and borides show that the valence band width of borides is larger than that of hydrides,and borides have obvious pseudo-energy gaps near the Fermi energy level,indicating that borides are more stable than hydrides.The results of Mulliken Bourget calculations for hydride and boride show that H is an electron donor when H interacts with B in hydride,and H is an electron acceptor when H interacts with Si and Al.In borides,when B interacts with Si,Al and Ti to form borides,respectively,B is the electron acceptor.The covalency of hydride is B-H,Si-H and Al-H in the order of strongest to weakest,where the covalency of B-H bond is B₂H₆>BH₃>BH₂.The B-B bonds in Si B₆,Al B₂ and Ti B₂have the Buju values of 0.35,2.21 and 2.44,respectively,and their covalency is enhanced in order.Based on the above theoretical calculation results,the removal behavior of impurity boron during the directional solidification of Al-Si alloys was investigated by using the method of high temperature melt hydrogen blowing with electromagnetic directional solidification.The results show that the removal rate of impurity boron gradually increases as the rate of directional solidification decreases.In addition,the removal rate of impurity boron increased with the increase of hydrogen gas passage time.When the pull-down speed was 5μm/s and the hydrogen pass time was 150 min,the best boron removal effect was achieved,and the boron content in silicon was reduced from 103 ppmw to 3.8 ppmw,and the boron removal rate reached 96.3%.Meanwhile,the results of scanning electron microscopy with energy dispersive spectrometer（SEM-EDS）characterization showed that impurities such as C,O and B adhered to the bubble wall at the solidification end of the sample after solidification,indicating that the impurity boron in the melt moved to the upper part of the sample with the gas phase.In addition,the thermodynamic calculations show that the formation of B₂H₆ gas in the Al-Si melt is feasible,which is consistent with the previous first-nature principle calculations.Through the above theoretical analysis and experimental verification,it is shown that hydrogen blowing and boron removal in Al-Si melt is feasible,and this study can provide a corresponding technical reference for Al-Si alloy refining hydrogen blowing and boron removal.