多晶硅铸锭内嵌杂质引发热应力的数值分析

Numerical Analysis of Thermally-induced Stress Caused by Inclusions Embedded in Polycrystalline Silicon Ingot

对制造光伏电池用多晶硅锭中的主要硬质夹杂SiC、Si3N4引起的热致应力进行数值分析。首先用晶体生长软件CGsim模拟定向凝固,获得铸锭凝固完成时温度场及夹杂分布,再基于此用有限元分析软件ANSYS分别分析这两种嵌于硅基体内的夹杂在硅基体内引起的热应力。夹杂颗粒模型形状设计依据其实际形状特征。由于SiC与硅均为立方结构,SiC夹杂影响可处理为各向同性;对于六方结构的Si3N4夹杂,通过对弹性矩阵的坐标转换考虑了其力学性能的各项异性。结果表明,多晶硅锭由1685 K降至室温的过程中,夹杂引起的最大热致应力SiC颗粒约为16 MPa,Si3N4颗粒在13-21 MPa之间,SiC团簇约为21 MPa,多颗粒在18-21 MPa之间。基于此,计算出多晶硅锭内最小失稳临界裂纹尺寸在286-676μm之间,小于夹杂体的尺寸,因此在铸锭冷却过程中夹杂引起的裂纹发生可能性较大。

A numerical analysis was performed to investigate the thermally-induced stress caused by SiC and Si3N4,the main hard inclusions in polycrystalline silicon ingot uesd in fabricating Photovoltaic solar cells. Firstly The temperature field and impurities（ SiC,Si3N4） distribution after solidification were obtained through the simulation of directional solidification of polycrystalline silicon ingot by using the crystal growth software CGsim. Then based on it,the thermally-induced stress was studied caused by these two kinds of hard inclusions embedded in silicon matrix by using the finite element analysis software ANSYS. Model shape was designed in terms of true shape of the inclusion particles. Cause SiC and silicon were all the cubic structure,the action of SiC inclusion can be treated as isotropy; For Si3N4 inclusion with hexagonal structure,anisotropy of mechanical properties were considered through the coordinate transformation of the elastic stiffness matrix. It indicated that,in the process of cooling of polycrystalline silicon ingot from 1685 K to room temperature,the maximum thermally-induced stress caused by inclusions were： 16 MPa by SiC particle,13-21 MPa by Si3N4 particle,15 MPa by SiC clusterand 18-21 MPa by multi-particles,respectively. Bsed on it,the calculation result showed that the minimum size of instability critical crack in polycrystalline silicon ingot was in the range from 286 to676μm,less than the size of inclusions,so the crack caused by inclusions was easy to appear in the process of cooling of ingot.