Numerical Simulation Of Sapphire Crystal Growth By The Kyropoulos Method Of Three Temperature Zones

In this paper, CGSim is used to analyze the effect of internal radiation, melt convection, heater power ratio on three temperature-zone KY sapphire crystal growth and the problems in actual KY sapphire crystal growth process are discussed.(1) The effect of internal radiation on crystal growth. The existence of crystal internal radiation makes temperature field and flow field of system more uniform and lowers the crystal stress; With the increase of internal radiation absorption the heat transfer capacity of crystal lowers, the melt heat cannot conduct through crystal and the average temperature of melt lowers, which lowers the solid-liquid convexity and increases the temperature gradient inside the crystal; the existence of melt internal radiation increases the heat transfer capacity of system and intensifies the melt convection which makes the temperature distribution more uniform and lowers the temperature gradient of system and crystal stress.(2) The effect of melt convection on crystal growth. In the shoulder turning stage of crystal growth process, the existence of Marangoni convection makes the interface convexity increase by 39.8% and influences the melt convection state, which causes the temporal hollow. When the crystal grows to middle part, the clockwise vortex exposes near the triple point, which gives rise to the beginning of remelting. At the last stage of crystal growth, Marangoni convection increases the strength of vortex near the free surface and the the vortex differentiates to two vortexes, which causes the interface inversion. The interface inversion increases the dislocation density and thermal stress and causes the crystal cracking at the bottom. The experimental photo coincides to the simulation result.(3) The effect of heater power ratio on interface stability. At the last stage of crystal growth, the contribution of Marangoni convection on heat transfer of melt keeps pace with natural convection, so the flow mode variation of melt is the direct reason of interface instability. The lower interface forefront temperature gradient lowers the interference ability of interface and the complicated melt flow mode can influence the interface stability, so the lower interface forefront temperature gradient is the indirect reason of interface instability.In addition, the increase of bottom heater power can the occurrence of interface inversion,but because of the weaker crystal conductivity the interface is still unstable; the interface stability can be further improved by the lower of the upper heater power.(4) Several key problems of Kyropoulos method sapphire crystal growth. By the thermal expansion coefficient and thermal conductivity of sapphire and W crucible, it can be founded that the usage of Tungsten and molybdenum alloy crucible is beneficial to the sapphire crystal growth;The sticky-crucible at the side portion is more dangerous than the sticky-crucible at the bottom;A modified KY in this paper is used to produce larger size and square sapphire crystal in order to improve the crystal utilization and the related patents are implemented by a famous manufacturer of sapphire crystal.