Global Simulation Of Micro-pulling-down Crystal Growth

The global numerical simulation including induction heating of yttrium aluminum garnet(YAG) crystal growth via the micro-pulling-down method was carried out in this research, which was simulated by using the finite volume method, and the effects of various parameters on the temperature field in the furnace and inducted heat in the crucible were analyzed.The mathematical model and physical model for YAG crystal growth were established. Induction heating, convection of both gas and melt, heat transfer of solid/melt/gas were solved simultaneously. In the melt zone, buoyancy convection and thermocapillary flow were considered. In order to uniformly discrete the controlling equations by using finite volume method, the electromagnetic field was modelled by using complex function method, the computation on the electromagnetic field was verified with the result modelled by stream function method. Both the temperature and flow fields in the global furnace(including gas and melt) were investigated.The crystallization rate of the crystal is mainly influenced by the temperature gradient on the solid/liquid surface, especially the axial temperature gradient. For the optimization of furnace design, the influence of coil axial and radial position, coil turns, current and frequency in the coil and material of afterheater on the temperature distribution and temperature gradient on the solid/liquid zone were analyzed in detail. The results show that:(1)With the coil moving up, the entire temperature of solid/liquid zone decreases, while the axial temperature gradient increases. With the radial distance of the coil increasing, the entire temperature of solid/liquid zone decreases, while the axial temperature gradient changes irregularly. When the radial position is 80 or 85 mm, the axial temperature gradient increases significantly. With the coil turns decreasing, the entire temperature of solid/liquid zone decreases. When the coil turns is 4 or 6, the effective induction surface is too small to produce high induction heat efficiency.(2)When the material of the afterheater is Zirconia(ZrO2), same with the insulation, the axial temperature gradient is higher than the original model, while the temperature of solid/liquid zone is lower. When the afterheater is not used, the axial temperature gradient is increased obviously, while the temperature decreased sharply leading to a low producing induction heat efficiency, the current in the coil have to increase.(3)With the current increase, the maximum temperature in the furnace increases near-linearly, the temperature of solid/liquid zone increases uniformly. With the frequency increasing, the maximum temperature in the furnace increases. There is a sharp increase of maximum temperature when frequency is increased from 1 kHz to 10 kHz, and a gentle increase of maximum temperature when frequency is increased from 20 kHz-100 kHz. The frequency of highest induction heating efficiency is in the range of 10 kHz to 20 kHz.The present model can well predict the actual circumstance of crystal growth of micro-pulling-down method, and it's useful for the optimal furnace design.