Influence of process parameters on crystal growth

The dependence of the convection patterns, concentration gradients and the crystal growth rate on ramp rate, rotation and crystal size is examined. Laser schlieren technique is employed as the measurement tool for determining the concentration distribution in the solution. Major conclusions to emerge from the study are:

1. The stable growth regime of the crystal at 0 rpm comprises symmetric buoyancy-generated convection plumes. When rotation is imparted to the crystal, the stable growth regime reveals a helical flow path.
2. Buoyancy-driven convection intensifies with increasing size of the crystal as well as the ramp rate. It is ultimately limited by salt depletion and solute stratification in the growth chamber that bring the fluid particles to rest. Crystal rotation diminishes the concentration gradients, equalizes them over all faces, and improves the symmetry of the concentration distribution in the solution. This leads to a crystal of better quality. The rpm of the crystal required to improve crystal quality depends on the cooling rate of the solution.
3. Over the range of parameters studied, the large-scale convection pattern is buoyancy-driven, even when the crystal is rotated.
4. Growth rates of the crystal correlate with the magnitude of the concentration gradients over its individual faces. The gradients in turn respond to the convective field around the crystal.
5. Experiments with crystals of various sizes immersed in an aqueous solution show that the convection transients are short-lived. Thus, the growth process is quasi-steady and can be controlled by adjusting the ramp rate and crystal rotation.