Numerical Simulation Of Sapphire Growth By Kyropoulos Method For Process Optimization

Sapphire is a kind of intraocular lens with good properties, it has been widely used in military optical materials, semiconductor substrate and smart phones. Kyropoulos method is one of the main methods for sapphire crystal growth process. In this article, the numerical simulations using Fluent software are performed to investigate temperature distribution, flow field and shape of the melt/crystal interface during the Kyropoulos sapphire crystal growth process, and the effect of different production processes and system architecture on the crystal growth process is studied.In this article, the sapphire growth theory is summarized, the physical and numerical models are established of sapphire crystal growth by Kyropoulos method, numerical simulations are used to study shoulder, earlier equal-diameter and later equal-diameter growth stage which is the three main crystal growth process. The result indicated that: In the three stages, the isotherm value of the system increases with the depth increases, since the seed is cooled by water and the crucible is heated by heaters. The isotherms in the melt convex to the crucible bottom, the melt flows upward along the crucible wall to the center of melt/crystal interface then flows downward along the central axis of symmetry, forming a counter-clockwise vortex. With the crystal growth process continuing, the isotherms within the system and the center of the vortex in the melt move downward, the melt convection and maximum velocity subsides. The position of maximum velocity in the melt moves from center axis of symmetry to crucible side wall.The selection of process parameters and thermal field structure has great influence on the quality of crystal during the Kyropoulos sapphire crystal growth process. In this article, the influences of the shoulder angle, the radius of crucible bottom and the power ratio of side heater to bottom heater on the temperature distribution, flow field and shape of the melt/crystal interface are investigated. The result shows that: the average temperature in the system is reduced and the axial temperature gradient increases, the maximum velocity is reduced within the melt and the melt convection becomes weakened, the convexity of the melt/crystal interface reduced with the decrease of shoulder angle. With the increase of the crucible bottom radius, isotherm in the system moves upward and the average temperature of the melt increases,the maximum velocity near the crystallization front decreases, the flow pattern in the molten sapphire is changed to a round shape when the curved bottom is introduced within the crucible, the convexity of the melt/crystal interface reduces, since it is conducive to maintain a stable free interface. With the power ratio of side heater and bottom heater decreasing, isotherm moves downward in the system, the average temperature and temperature gradient in radial direction of the melt the decreases near the melt/crystal interface, the maximum velocity in the melt is decreased and the convexity of the melt/crystal interface reduces.