Hot Zone Design And Thermal Stress Analysis During Growth Of Bulk Crystals

During the past fifty years, bulk crystals have been widely used in many areas, involving in industry, environmental sciences, information processing, medical science, military, etc. Recently, with the rapid development of science and technology, demands of the high-quality and large-scale bulk crystal are ever growing. The bulk crystals, such as semiconductor crystals (e.g., Si, GaAs, InP), laser crystals (e.g., Nd:YAG, GGG), and optical crystals (e.g.,?-Al2O3, KNbO3), are usually grown using the melt growth methods, such as the Czochralski (Cz) method, the Bridgman method and the Floating Zone (FZ) method. Large thermal stresses are induced in the bulk crystals after the growth due to severe and complex thermal environments. The thermal stress may cause generation and multiplication of dislocations and cracking, which degrades the crystal quality. The non-linear temperature variation between the surface and the center of the crystal during the cooling process induces the thermal stress. Hence, generation of thermal stress is related to the temperature distribution and the solid/liquid (S/L) interface determined by the thermal environments of the furnace. Appropriate temperature distributions with the stable S/L interfaces are the key factor to grow high-quality bulk crystals. Therefore, optimizing the hot design will be of great significance in improving crystal quality.In the thesis, multi-physical transfer models have been developed on the basis of fluid mechanics, thermal science and materials science in combination of crystal growth theory. Then, the models are applied on optimizing the thermal design during the growth and cooling processes of sapphire and multicrystalline silicon (mc-Si) by improving the structure design and process parameters. In regard to the mc-Si, during the growing process, we investigated influences of the insulation design and heater arrangement on the S/L interface and on the thermal stresses inner the ingot, and then proposed the optimization strategies. During the cooling process, we detailedly analyzed effects of the inlet velocity and the moving rate of the bottom insulation on the thermal, the flow and the stress fields. Based on the study, the inlet gas velocity was further optimized. Regarding the sapphire single crystal, we firstly analyzed the characteristics of the temperature and velocity distributions in the melt at different growing stages, and the effects of marangoni convection on the melt flow. Then, causes of cracking in industrial production were discussed from the three aspects:the thermal stress, the geometric surface of the crystal and the three-dimensional effects. Effective methods avoiding the cracking were proposed. During the cooling process, influences of heater structure improvement and power arrangement on the temperature, thermal flux and thermal stress distributions in the crystal were analyzed carefully. Finally, the power ratio in the growth system was optimized.The result shows that, at the growing stage of the mc-si, the highest stress can be reduced by reasonably adjusting the thickness of the insulations or the heater position to obtain a slightly convex or horizontal solid-liquid interface. During the cooling process, thermal stress was influenced obviously by the inlet velocity and the moving rate of the bottom insulation. The peak value of the highest stress could be significantly decreased by reducing the moving rate of the insulation. The ventilation regime with a high inlet velocity for the initial cooling and a slow one for the later cooling was found useful to reduce the highest stress. The crystal quality could be improved by optimizing the inlet velocity without increasing the cooling time. During the Kyropoulos growing process, the melt flow is influenced by natural convection and marangoni convection. At the beginning of the growing, natural convection in the melt is dominant, and marangoni convection is negligible. At the end of the growth, natural convection is suppressed due to drop of the melt volume, while marangoni convection becomes dominant. Cracking during the growing stage is determined by thermal stress and crystal defects, such as polycrystalline growth independent on the crystal shape. Additionally, the three-dimensional effect caused by changes of thermal environments surrounding the crystal is one of the main causes of cracking appeared at one side of the crystal shoulder. Therefore, the heater and the thermal shield should be modified regularly. During the cooling process, the largest stress always appears in the regions near the 'throat' and the 'bottleneck' of the crystal. In addition, the highest stress usually turns up at the 'bottleneck'. An appropriate high power ratio at the beginning of the cooling is beneficial to reduce thermal stress during the cooling process.