The surface tension driven-flow in BaB 2 O 4 (BBO) melt-solution is visualized by differential interference microscope coupled with Schlieren technique, and the streamline of the steady thermocapillary convection is found to be in form of an axially symmetric pattern. Based on the observation of BBO crystal rotation caused by the convective vortex, the widths of interfacial concentration, heat and momentum boundary layer are calculated. The effect of thermocapillary convection on boundary layer thickness is also investigated. Results show that the width of boundary layer decreases linearly with the increasing of dimensionless Marangoni number.

The evolution of solid-liquid interface during BBO single crystal growth was studied by the differential interference microscopy. And the step morphology on (0001) surface of the as-grown crystal was observed by the atomic force microscopy as well. It was found that the transition from a flat solid-liquid interface to a skeletal shape will occur in case of rapid growth. However, AFM images of surface steps revealed morphology differences correlated with crystallographic directions. The steps advancing along < 1010 > direction form the step flow, whereas those steps propagating along < 0110 > direction shape into step segments. Measurements of step heights by AFM indicated that it is the high anisotropy of the dimension of growth unit and step bunching due to the enlargement of concentration difference along the surface that results in the anisotropy of step morphologies.crystal growth, morphological instability, growth step, AFM observation

This paper reports that the rapid solidification of mixed Li 2 B 4 O 7 and KNbO 3 melted in a Pt loop heater has been performed experimentally by the method of quenching, and various morphologies of KNbO 3 crystals have been observed in different regions of the quenched melt-solution. Dendrites were formed in the central region where mass transfer is performed by diffusion, whereas polygonal crystals with smooth surface grew in the marginal region where convection dominates mass transport. Based on measurement of KNbO 3 concentration along crystal interface by electronic probe analysis, it finds the variety of crystal morphologies, which is the result of different solute distributions: in the central region the inhomogeneity of solute concentration is much sharper and morphological instability is easier to take place; nevertheless in the marginal region the concentration homogeneity has been greatly enhanced by convection which prevents the occurrence of morphological instability. Additional solute distribution in the melt along the primary dendrite trunk axis as well as that in mushy zones has also been determined. Results show that the solute concentration in the liquid increases linearly with distance from the trunk tip and more solutes were found to be concentrated in mushy zones. The closer the mushy zone is to trunk tip, the lower the solute concentration will be there.