液桥内热质耦合对流不稳定性及旋转磁场法控制

INSTABILITY OF COUPLED THERMO-SOLUTE CAPILLARY CONVECTION IN LIQUID BRIDGE AND CONTROL BY ROTATING MAGNETIC FIELD

浮区法因具有无坩埚接触污染的生长优点而成为生长高完整性和高均匀性单晶材料的重要技术.但熔体中存在的毛细对流会给浮区法晶体生长带来极大挑战,这是由于对流的不稳定会导致晶体微观瑕疵的产生和宏观条纹等缺陷的形成.为了提高浮区法生长单晶材料的品质,研究浮区法晶体生长中毛细对流特性及如何控制其不稳定性显得尤为重要.本文采用数值模拟的方法对半浮区液桥内SixGe1-x体系中存在的热质毛细对流展开研究并施加旋转磁场对其进行控制.结果表明:纯溶质毛细对流表现为二维轴对称模式,温度场主要由热扩散作用决定,而浓度场则由对流和溶质扩散共同支配;纯热毛细对流呈现三维稳态非轴对称流动,浓度分布与熔体内热毛细对流的流向密切相关,等温线在对流较大的区域发生弯曲;耦合溶质与热毛细对流则为三维周期性旋转振荡流.施加旋转磁场后,熔体周向速度沿径向向外增大,熔体内浓度场和流场均呈现二维轴对称分布.

Floating zone method is an important technology for growth of high-integrity and high-uniformity single crystal materials due to its free of crucible contamination.However,the capillary convection in the melt brings a great challenge to the floating zone crystal growth.This is because the instability of convection will cause the formation of some crystal defects such as microscopic imperfections and macroscopic stripes.Therefore,it is very important to investigate the behaviors of the capillary flow and control its instability in order to improve the quality of the produced single crystal materials.In this paper,numerical simulations are performed to investigate all kind of the capillary convection in the half floating liquid bridge on the SixGe1-x system.And the impact of the external rotating magnetic field is also investigated on the stability of capillary convection.The results show that the purely solute capillary convection is a two-dimensional axisymmetric model,and the temperature field is mainly determined by thermal diffusion while the concentration field is dominated by convection and solute diffusion together.On the other hand,the purely thermo-capillary convection presents three-dimensional unsteady axisymmetric flow.The concentration distribution is closely related to the flow direction of thermo-capillary convection.The isotherms bend in the region with strong convection.The coupled solute and thermocapillary convection is a three-dimensional periodic rotating oscillatory flow.When the rotating magnetic field is applied,the circumferential velocity of the melt increases with increasing radius.Both the concentration field and the flow field in the melt show a two-dimensional axisymmetric distribution.