INFLUENCE OF RAMP RATE AND CRYSTAL ROTATION ON CONVECTION PATTERNS

The present section discusses the application of the laser schlieren technique to monitoring convection in a crystal growth process from its aqueous solution, when the process parameters are varied. The cooling rate of the solution determines the amount of excess salt available in the solution for deposition on the crystal, and hence the potential difference for driving the convection currents. Ramp rates of 0.05 and have been studied in the present work. These values are smaller than the ramp rate of Section Comparision of Interferometry, Schlieren and Shadowgraph in a Crystal Growth Experiment; consequently the stratification of the solution seen in those experiments was considerably delayed. The rpm of rotational motion fixes the degree of homogenization of the solution and hence, indicates a reduction in the strength of buoyant convection. Crystal rpm of 0 and 15 are studied through experiments. These values have been selected on the basis of their ability to permit growth of crystals of meaningful quality. The effects of ramp rate of the solution, crystal rotation and the size of the growing crystal have been correlated with the growth rate of the crystal. Results of the transient evolution of the convective field in the growth chamber in the form of two-dimensional schlieren images are reported. The images are quantitatively interpreted in terms of concentration contour maps and concentration gradient profiles. In order to bring out the influence of the process parameters, results have been presented in the following sequence: 1. convection currents at a ramp rate of with rotations of 0 and 15 rpm; 2. convection currents at a ramp rate of with rotations of 0 and 15 rpm; and 3. effect of crystal size.

The nature of fluid motion around the crystal gives rise to in the flow and solutal concentration fields. These are relatively thin zones adjacent to the crystal faces where large changes in velocity and concentration take place. Large concentration gradients are revealed in a schlieren image as a brightened region against a darker background. The images discussed in the following sections show that the stable growth regime of crystal growth is accompanied by thin high intensity zones that originate at the crystal surface. Thus, it is clear that fluid motion and transport occur in the bulk of the solution, but are governed by the physical conditions imposed by the crystal. These are 1. a prescription of fluid velocity in terms of crystal rpm, and 2. concentration levels fixed by salt depletion from the aqueous solution.