Study On Pore-Crack Defects Of Quartz Crucibles For Continuous Czochralski

As the global drive towards carbon neutrality gains momentum,China’s photovoltaic industry is spearheading the country’s energy sector towards green and low-carbon development.Monocrystalline silicon cells have emerged as the preferred choice,holding close to 90%of the market share.With the market share of monocrystalline silicon on the rise and silicon crystals increasing in size,single crystal pulling technology has shifted from multi-loading pulling to continuous Czochralski.Quartz crucibles,which serve as vessels for melting polycrystalline silicon,are indispensable consumables in the production process.However,during continuous Czochralski,the inner wall of the quartz crucible develops more air bubbles,either by bubble disturbance of the silicon melt creating crystal dislocations or by creating pores due to the corrosive effect of the silicon melt.These pores generate significant transient thermal stresses under cyclic high-temperature loading,which in turn cause the crucible surface to crack,resulting in unforeseeable damage.As a result,this article delves into a comprehensive investigation of the transient temperature and stress field distribution of quartz crucibles during continuous Czochralski,including pore-crack and pore coalescence,with a focus on the following aspects:(1)This study focuses on analyzing the temperature and stress fields of a quartz during continuous Czochralski growth.A three-dimensional transient finite element model of the quartz crucible is established using heat transfer theory and the finite element method.The model is used to investigate the transient temperature and stress fields during the four stages of shoulder setting,constant diameter,tapering,and cooling,with emphasis on different characteristic positions of the crucible,including the side wall,bottom corner,and bottom.The results reveal that the highest temperature of the quartz crucible is concentrated at the bottom and corners,with a maximum temperature of 1590 ℃.The cyclic temperature changes during continuous Czochralski lead to cyclic thermal stress acting on the quartz crucible.The bottom corner experiences the maximum thermal stress,while the stress on the side wall is relatively small.The thermal stress at each position of the quartz crucible during the shoulder setting stage is relatively large compared to other stages,which may have a certain impact on the lifespan of the quartz crucible.(2)In this study,stress intensity factors were investigated for pore-cracks at three locations on the inner surface of a quartz crucible,namely the sidewall,bottom corner,and bottom.Nine pore-crack models were established,incorporating three pore depth-to-radius ratios(d/r<1,d/r=1,d/r>1)and three crack aspect ratios(a/c<1,a/c=1,a/c>1).The explored the variation law of the stress intensity factor of the nine models at different crack inclination angles θ(0°,30°,45°,60°,and 90°),and analyzed the stress intensity factors during the four stages of shoulder,isodiametric,tailing,and cooling.The results showed that the stress intensity factor KI of the sidewall decreased with increasing pore depth-to-radius ratio and crack aspect ratio,while it increased with increasing crack inclination angle θ.When d/r≤1,a/c<1,and θ=0°,the KI value was highest,which had the greatest impact on damage to the bottom corner of the quartz crucible.The KI value at the bottom of the quartz crucible gradually increased with decreasing depth-to-radius ratio(d/r),and was distributed approximately symmetrically about the center of the crack,with the maximum value appearing at the two endpoints of the crack.As the crack aspect ratio(a/c)increased,the stress intensity factor KI continuously decreased,and the difference between KI values at the crack site increased,leading to a more imbalanced distribution around the crack.(3)The aim of this study was to investigate the phenomenon of pore defect coalescence in quartz crucibles and an equivalent defect model was proposed to characterize pore coalescence,by establishing two finite element models with adjacent pore defects to evaluate the influence of complex porosity on the crucible.The study considered two types of pore configurations,involving different defect arrangements(axial and circumferential)and defect spacings(dedge=0.1 mm,0.5 mm,1 mm,3 mm,4 mm,5 mm,6 mm).The stress distribution around the two defects was analyzed under cyclic high-temperature loading.Results revealed that the spacing of pores had a significant impact on the fatigue strength of the crucible,with interactions between adjacent pores observed regardless of the arrangement(axial or circumferential).Furthermore,the fatigue resistance of the quartz crucible weakened as the edge distance between pores decreased.As the edge distance increased,equivalent stresses became independent of each other,and the mutual influence of the local stress fields decreased.Lastly,it was found that the maximum equivalent stress of the equivalent defect model was consistently lower than that of adjacent pore defect.