Effect of buoyancy-driven convection upon crystal growth of KNbO 3 in the melt

Abstract

A high-temperature in situ observation method has been used to observe two different states of convection: diffusive–advective flow and diffusive–convective flow. Characteristic diffusion distance L can be easily measured from a Schlieren photograph and it ranges from 0.01 to 0.1 cm for two different states of convection. By observation of the rotating crystal growth processes in the KNbO 3 melt, the widths of interfacial concentration, heat and momentum transition zone are obtained, which are 7.5×10 −3, 8.6×10 −2 and 4.4×10 −1 cm, respectively. An investigation of growth kinetics of the KNbO 3 crystal related to the different states of convection in high-temperature melts has been performed by this melt. Irrespective of the state of convection in the melt, at the lower supercoolings two-dimensional nucleation growth has been obtained. At the same supercooling, the discrepancy between the growth rates for two different states of convection is assigned to the buoyancy convective enhancement of the interfacial mass flow. The free energy per unit length of a step ε, and the thermodynamic potential barrier for two-dimensional nucleation Δ G* are calculated from the experimental data for two different states of convection. These data are significant for consideration of the influence of convective flow on the interface growth kinetics. The effect of convection is to enhance the sharpness of the interface.