Influence Of Process Parameters On Temperature,Stress And Solidification Structure Of Multi-Crystalline Silicon Continuous Casting Billet

The conversion efficiency of multi-crystalline silicon solar cells is closely related to the lifetime of minority carriers.Low dislocation density,small number of grain boundaries,and grain boundaries perpendicular to the growth interface in the multi-crystalline silicon continuous casting billet will increase the lifetime of minority carriers.However,the dislocation density and the number of grain boundaries of the traditional multi-crystalline silicon continuous casting billet are not ideal,and the traditional multi-crystalline silicon casting technology has the limitations of high production cost and low efficiency.The low stress of the multi-crystalline silicon continuous casting billet,the straight shape of the solidification interface,the columnar grains,and the large grain size are beneficial to reduce the dislocation density,make the grain boundary perpendicular to the growth interface,and reduce the number of grain boundaries.Moreover,continuous casting technology has the advantages of improving material utilization rate and realizing continuous operation.The stress,solidification interface shape,grain shape,and grain size of multi-crystalline silicon continuous casting billets are closely related to the temperature field,stress field,and solidification structure of the solidification process.Therefore,the systematic research on the evolution of the temperature field,stress field and solidification structure during the multi-crystalline silicon continuous casting is of great significance to improve the conversion efficiency of the solar cell.The core part of the multi-crystalline silicon continuous casting equipment is the mold.This thesis refers to the characteristics of the shape of the continuous casting mold of other materials,and based on the characteristics of the solidification and expansion of multi-crystalline silicon,and through the design principle of the mold taper,a set of multi-crystalline silicon continuous casting equipment was manufactured.Based on the measured temperature data of the continuous casting experiment,the boundary conditions of the multi-crystalline silicon continuous casting process were obtained by combining the calculation of the inverse heat conduction problem and the numerical simulation.Based on the ProCAST finite element software and the boundary conditions of the multi-crystalline silicon continuous casting process,a mathematical model of the temperature field of the round billet continuous casting process was established,and it was validated by the measured temperature data recorded during the continuous casting experiment.On this basis,,this thesis applies the thermophysical parameters,initial conditions,and boundary conditions used in the mathematical model of the round billet continuous casting process to the mathematical model of the temperature field of the square billet continuous casting process,and the influence of pouring temperature,cooling intensity in the secondary cooling zone,and continuous casting speed on the temperature field of the round billet and square billet continuous casting process was studied.The research results show that with the increase of the pouring temperature,the temperature of the continuous casting billet increases slightly.the area of the incompletely solidified melt increases slightly,and the tendency of the solidiification interface shape to be flat decreases.With the increase of the cooling intensity in the secondary cooling zone,the temperature of the continuous casting billet decreases significantly,the area of the incompletely solidified melt decreases significantly,and the tendency of the solidification interface shape to be flat increases.With the increase of the continuous casting speed,the temperature of the continuous casting billet increases significantly,the area of the incompletely solidified melt increases significantly,and the tendency of the solidification interface shape to be flat decreases.The numerical simulation results of the temperature field were used as the thermal load to establish a mathematical model of the stress field of the round billet and square billet continuous casting process,and the mathematical model of the stress field of the round billet and the square billet during continuous casting process were validated by experimental method and theoretical calculation method.On this basis,the effects of the pouring temperature,the cooling intensity in the secondary cooling zone,and the continuous casting speed on the stress field of multi-crystalline silicon round billet and square billet continuous casting process were studied.The research results show that the stress of the continuous casting billet decreases slightly with the increase of the pouring temperature.The stress of the continuous casting billet increases significantly with the increase of the cooling intensity in the secondary cooling zone.The stress of the continuous casting billet decreases significantly with the increase of the continuous casting speed.The reference point is located on the center line of the continuous casting billet and 70mm above the mold outlet.When the pouring temperature increases from 1500℃ to 1800℃,the maximum stress at this point of the round billet is reduced from 47.2MPa to 44.8MPa,and the maximum stress at this point of the square billet is reduced from 35.6MPa to 34.0MPa.When the cooling intensity in the secondary cooling zone increases from Q/2 to10Q,the maximum stress at this point of the round billet increases from 36.5MPa to 93.8MPa,and the maximum stress at this point of the square billet increases from 31.1 MPa to 62.0MPa.When the continuous casting speed increases from 0.5mm/s to 1.5mm/s,the maximum stress at this point of the round billet is reduced from 85.8MPa to 28.3MPa,and the maximum stress at this point of the square billet is reduced from 53.3MPa to 27.6MPa.Based on the continuous casting equipment of multi-crystalline silicon,the continuous casting billet obtained from the experiment,the measured temperature data,the calculated boundary conditions of the continuous casting process,the macro heat transfer model of the temperature field of the continuous casting process,through the calculation method combining cellular automata and heat transfer,a mathematical model of the solidification structure of the multi-crystalline silicon round billet continuous casting process was established,and the model was validated by the solidification structure on the longitudinal section of the continuous casting billet obtained from the experiment,and the grain size on the cross section.On this basis,the influence of the pouring temperature,the cooling intensity of the bottom block,the continuous casting speed,the maximum nucleation density on the mold wall surface,and maximum nucleation undercooling on the solidification structure of the multi-crystalline silicon round billet continuous casting processs was studied.The research results show that:the pouring temperature,the cooling intensity of the bottom block and the continuous casting speed mainly affect the columnar grain zone areas growing perpendicular to the bottom of the continuous casting billet.With the increase of the pouring temperature,the columnar grain zone areas growing perpendicular to the bottom of the continuous casting billet decreases slightly.When the continuous casting speed is 0.1mm/s-1.5mm/s,with the increase of the continuous casting speed,the columnar grain zone areas growing perpendicular to the bottom of the continuous casting billet decreases significantly.When the continuous casting speed is 1.5mm/s-2.5mm/s,with the increase of the continuous casting speed,the columnar grain zone areas growing perpendicular to the bottom of the continuous casting billet almost remains unchanged.With the increase of the cooling intensity of the bottom block,the columnar grain zone areas growing perpendicular to the bottom of the continuous casting billet increases significantly.The maximum nucleation density on the mold wall surface and the maximum nucleation undercooling on the mold wall surface have a significant impact on the grain size of the continuous casting billet.With the increase of the maximum nucleation density on the mold wall surface,the grain size of the continuous casting billet decreases.With the increase of the maximum nucleation undercooling on the mold wall surface,the size of grains on and near the bottom block cooling surface decreases and those far from the bottom block cooling surface increases.In the development and production of multi-crystalline silicon continuous casting billet,reasonable configuration of process parameters is the key to the production of high-quality continuous casting billet with high efficiency and low cost.Based on the self-designed multi-crystalline silicon continuous casting equipment,this thesis uses continuous casting experiment and multi-scale numerical simulation technology to explore the influence of process parameters on the temperature field,stress field and solidification structure of multi-crystalline silicon continuous casting process,which provides an important theoretical and technical basis for the preparation of high-performance and low-cost multi-crystalline silicon continuous casting billet.