Effect of Buoyancy Driven Convection and Crystal Growth

A high temperature in-situ observation method was used to observe two different states ofconvection: diffusive-advective flow and diffusive-convective flow. Characteristic diffusion distanceL can be easily measured from Schlieren photograph and it ranges from 0.01-0.1cm for two differentstates of convection. By observation of rotating crystal growth processes in the KNbO3 melt, thewidths of interfacial concentration, heat and momentum transition zone were obtained, which are7.5 × 10--'cm, 8.6 × 10--2cm and 4.4 × 10--1 cm respectively.An investigation of growth kinetics of KNbO3 crystal related to the different states of theconvection in high temperature melts was performed by this method. Irrespective of the state ofconvection in the melt, at the lower supercooling two dimensional nucleation growth was obtained.At the same supercooling, the discrepancy between the growth rates for two different states ofconvection was assigned to the buoyancy convective enhancement of the interfacial mass flow. Thefree energy per unit length of a step e, and the thermodynamic potential barrier for two--dimensionalnucleation △G*, were 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 interfacegrowth kinetics. The effect of convection is to enhance the sharpness of the interface.