Numerical Simulation Of The Temperature Field Of The Growth Of Sapphire Crystals

Sapphire crystal, which shows attracking physical and chemical properties, has been widely used in fields of aerospace and military. Kyropoulos method is a major growth method for sapphire crystal. In this paper, numerical simulations for sapphire crystal growth by Kyropoulos method were conducted, which provides theoretical guidance for optimizing the furnace structure and adjusting fabrication parameters.The temperature distributions of crystal were calculated. By varying the layers of wall insulation, opening radius of radiation shield, and heater position, the effects of furnace structure on temperature distributions were investigated. The results show that, to increase radial temperature gradient of crystal, layers of top wall insulation need to be decreased, opening radius of radiation shield be increased, heater position be lowered. On the other hand, to decrease axial temperature gradient of crystal, layers of top wall insulation need to be increased, opening radius of radiation shield be increased, heater position be raised. However, layers of side heat insulation had little effect on temperature.The temperature field and flow field of sapphire growth by Kyropoulos method at different growth stages were simulated, and the results were analyzed. Temperature increases from top to bottom and from inside to outside. Isotherm distribution density on the top of crystal is more intensive. Melt isotherm distribution is complicated under the influence of melt convection. As crystal grows, melt convection changes from two vortex cell at shoulder into a vortex cell at diameter growth stage. Combined with experiments of crystal growth, the actual temperature were measured by the radiation thermometry system. The validity of the simulation result was proved.The shape and temperature gradient distributions of melt-crystal interface for different stages were calculated. By varying the growth rate, the effect of velocity on melt-crystal interface was investigated. The results show that, in shoulder growth process, the convexity of melt-crystal interface is larger, while it is smaller in diameter growth process. With value in the range of 0.5 to 2 mm/h in shoulder stage, and 2 to 5 mm/h in diameter stage, the axial and radial temperature gradient at the melt-crystal interface increases, and convexity increases, as the growth rate is reduced. During the Kyropoulos sapphire crystal growth process, it has a growing influence of growth rate on melt crystal interface.Macroscopic defects in crystals were analyzed and found that higher temperature gradient may lead to crack, and asymmetric temperature field lead to adhesive crucible phenomenon. Referring to simulation results, high-quality sapphire was successfully obtained. The results of microcosmic experiment showed that the optimized sapphire crystal has a good infrared transmittance, high lattice perfection, and low dislocation density.