Effect of aspect ratio on coupled solute-thermocapillary convection instability in half-zone liquid bridge

Abstract

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, capillary convection in the melt is a great challenge to floating zone crystal growth. In this paper, numerical simulations are performed to investigate the coupled solute-thermocapillary convection in SixGe1-x system of the half-zone liquid bridge. The impact of aspect ratio, As, is also investigated on stability of capillary convection. For As = 0.5, the results show that pure solute capillary convection is very weak, which presents 2-D axisymmetric structure. The temperature field is mainly determined by thermal diffusion, while the concentration field is dominated by convection and solute diffusion together. Coupled solute-thermocapillary convection exhibits 3-D periodic and rotating oscillatory flow with the azimuthal wavenumber m = 4, while the pure thermocapillary convection presents a 3-D steady non-axisymmetric flow while solute capillary convection is absent. This means that instability of convection will increase when two kinds of capillary convection are coupled. When the height of the liquid bridge is changed from 5 mm to 10 mm with a constant radius of 10 mm, azimuthal wavenumber, m, of coupled capillary convection shows a strong dependence on aspect ratio. The relationship between the azimuthal wavenumber and aspect ratio can be written as m ? As = 2 or m ? As = 2.2. Further results indicated that when velocity of the monitoring point is large, corresponding concentration is also high at that moment, but the phases of concentration and velocity are not completely synchronized.