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Effect of Crucible Geometry on Melt Convection And Interface Shape during Kyropoulos Growth of Sapphire Single Crystal

The Kyropoulos (KY) method is commonly used to grow large sized sapphire single crystals. The shape of the sapphire crystal thus grown is determined by the heater arrangement and the power reduction history in the Kyropoulos furnace. In order to grow high-quality sapphire single crystal, the heater arrangement should allow different power inputs in different sections in order to control the thermal field in the melt during the growth process. In this study, a numerical computation is performed to investigate the effects of the heater arrangement on the thermal and flow transport, the shape of the crystal–melt interface, and the power requirements during the Kyropoulos sapphire crystal growth process in a resistance heated furnace. Four different power ratio arrangements in a three-zone heater are considered. The results show that for the power arrangements considered herein, the temperature gradients along the crystallization front do not exceed 0.05 K/mm, and that, after the growth of the crown, the crystal maintains an almost constant diameter. The remelting phenomenon may occur during growth when the input power of the upper side of the heater is higher than that of the lower side of the heater.

In this work, the numerical simulations were performed to investigate the rolls of crucible shape on temperature distribution, melt convection behavior and shape of the melt/crystal interface during the Kyropoulos sapphire crystal growth process. Cylindrical crucibles with different bottom corner radii were considered, and the Kyropoulos growth process was simulated using the finite volume method. Through numerical analyses, it was found that an increase in bottom corner radius of the crucible decreased the convexity of melt/crystal interface and the velocity of convectional flows of the molten sapphire during crystallization. As a consequence, it was concluded that the crucible with a curved bottom had advantages for the crystallization of the sapphire using the Kyropoulos method.


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