Material strength study on cracking of oxide bulk single crystals for optelectronic use

用LiNbO_3(LN)和LiTaO_3(LT)等氧化物单晶作为光电材料。这种大块单晶是用提拉法生长的。在生长过程中，有时会出现宏观裂纹。这种开裂是由单晶中的热应力引起的。利用张量变换技术得到了任意拉伸方向下的弹性常数矩阵和热膨胀系数向量，并将其并入有限元程序，编制了用于分析LN和LT等三角单晶热应力的有限元程序。利用该程序分析了东北大学材料研究所在拉力装置中生长的LN大块单晶的热应力。通过对整个系统的热传导分析，得到了晶体的温度边界条件。对a轴和c轴的拉伸方向进行了热应力分析。结果表明，c轴拉伸时的最大热应力低于a轴拉伸时的最大热应力，这说明a轴拉伸比c轴拉伸更容易开裂。研究还发现，在沿a轴方向拉伸的大块单晶中，较低的应力区分布较广。这预示着，如果能够防止宏观开裂，a轴拉伸可以提供比c轴拉伸更高质量的大块单晶。通过观察a轴拉伸获得的单晶中的微观缺陷，验证了这一预测。为了定量研究LN单晶的开裂，利用圆盘试件进行了热震实验。试验结果表明，裂纹主要发生在解理面上，垂直于解理面上的应力会导致裂纹的产生。

Oxide single crystals such as LiNbO_3(LN) and LiTaO_3(LT) are used as optelectronic materials. Such bulk single crystals are manufactured by the Czochralski growth method. Macroscopic cracking sometimes occurs during growth process. Such cracking would be caused by thermal stress in a single crystal. In the present study, a finite element computer program was developed for the thermal stress analysis of trigonal single crystals such as LN and LT.A tensor transformation technique was used to obtain the elastic constant matrix and the thermal expansion coefficient vector corresponding to an arbitrary pulling direction, and they are incorporated into the finite element program. Using this program, we analyzed the thermal stress in LN bulk single crystals, which were grown in a pulling apparatus in Institute for Materials Research, Tohoku University. The temperature boundary conditions of the crystals were obtained from the heat conduction analysis of the global system. Thermal stress analyzes were performed for the pulling directions of both the a-axis and the c-axis. The results show that the maximum thermal stress is lower in the c-axis pulling than in the a-axis pulling, which indicates that the a-axis pulling is easier to crack than the c-axis pulling. It is also found that a lower stress region spreads widely in the bulk single crystal pulled in the a-axis direction. This predicts that the a-axis pulling could provide a bulk single crystal of higher quality than the c-axis pulling, if macroscopic cracking can be prevented. This prediction was verified by observing microscopic defects in a single crystal obtained by the a-axis pulling.In order to study the cracking of LN single crystals quantitatively, thermal shock experiments were carried out using circular disk speciments. The experimental results show that the cracking mainly occurs at the cleavage planes, which indicates that the stresses normal to the cleavage planes would induce the cracking.