Numerical Study On Transport Mechanism And Optimization Control Of Impurities Driven By Argon During Casting Silicon Process

Energy shortage and environmental pollution are major problems faced by countries all over the world.The only way to solve this problem is to develop and utilize renewable new energy.Solar photovoltaic power generation is one of the most promising technologies of renewable energy.Casting silicon ingot has become the most important solar cell material due to its high efficiency and low cost.However,it is inevitable to introduce oxygen(O)and carbon(C)in casting process for silicon ingots,which will lead to a variety of crystal defects,and then seriously affect the efficiency of solar cells.Therefore,it is important to reduce the concentration of O and C for improving the quality of silicon ingots and the efficiency of solar cells.The flow intensity and pattern of argon significantly affect the concentration and distribution of O and C.It is very limited to study the complex heat and mass transfer in the ingot furnace with high temperature.Therefore,the mechanism of impurity transport driven by argon in the casting process for silicon ingots is studied by numerical simulation,and based on the discovered mechanism,the process parameters and gas guidance structure are optimized.This can provide basic understand and guide for the growth of low-cost and high-quality silicon ingots.The main research contents and conclusions are as follows:(1)A global two-dimensional transient model is established,which is coupled with heat conduction,convection,radiation and phase change.The grid and time step independent validation of the numerical model is carried out,and the accuracy of the simulation results are tested.The temperature monitored by the thermocouple is basically the same as the simulation results.Based on the established models,a global three-dimensional steady model was developed coupled with impurity transport and chemical reaction.(2)The phenomena of argon flow and impurity transport are compared and analyzed in threedimensional and two-dimensional models.The induction mechanism of backflow at crucible outlet was found by analyzing the driving force of argon flow.The results show that the natural convection outside the crucible results in the backflow near the symmetry plane at the crucible outlet.The impurities outside the crucible are introduced into the crucible by backflow,and the chemical reaction at the cover is effectively inhibited by the backflow.Forced convection mainly affects the flow intensity and pattern of argon above the melt free surface and the kinetic rate of reaction at the cover.(3)The influence of argon flow rate on impurities transport driven by argon is studied numerically.The results show that the increase of argon flow rate will strengthen the flow intensity of argon above the melt free surface,promote the chemical reaction at the cover and effectively restrain the argon backflow at crucible outlet.The amount of CO flowing into the crucible from the outside decreased significantly,and finally the CO concentration at the melt free surface decreased.When the flow rate of argon is more than 50 L/min,the CO concentration decreases slightly,while the increase of flow rate means the increase of cost.Therefore,50 L / min is selected as the best flow rate of argon.(4)The influence of furnace pressure on impurities transport driven by argon is studied numerically.The results show that the increase of furnace pressure has little effect on the backflow at crucible outlet,while it can increase the flow intensity of argon,thus enhancing the kinetic rate of reaction at the cover.Therefore,CO concentration increases with the increase of furnace pressure.When the furnace pressure is small,the diffusion coefficient of impurities in argon increases rapidly with the decrease of furnace pressure.The CO concentration increases from 36.2 to 37.0 ppm due to the enhancement of diffusion when the furnace pressure decreases from 200 mbar to 50 mbar.Therefore,200 mbar is selected as the best furnace pressure.(5)The structure of susceptor is designed and the influence of the structure parameters on the argon flow and impurity transport is analyzed.The results show that the optimized susceptor structure effectively blocks the backflow at crucible outlet.At the same time,the vortex at the junction of the cover and the susceptor disappears where the argon flows out smoothly,which can avoid the chemical reaction and the accumulation of impurities.Although the local kinetic rate of reaction of the cover was increased,the CO concentration at the melt free surface decreased obviously.When a/b is 0.5,the CO concentration at the melt free surface decreases by 15.2%.(6)The argon flow,chemical reaction and impurity transport under the improved structure were compared and analyzed.The structure of designed device is optimized by the orthogonal experiment.The results show that the region of backflow at crucible outlet does not change under the new designed structure.However,the side flow effectively limits the flow area of backflow above the melt free surface and reduces the kinetic rate of reaction at the cover.Finally,the CO concentration at the melt free surface decreased by 31%.The results of orthogonal experiments revealed that when H = 50 mm,L = 50 mm and Q = 50 L / min,the argon guidance system has the most obvious effect of removing impurities.