CONVECTION PHENOMENA DURING CRYSTAL GROWTH

For a purely buoyancy-driven growth process, the driving potential of flow is the maximum concentration difference occurring in the solution. In addition, the strength of convection is also governed by the length scale, typically the size of the growing crystal. With time, the solution is depleted of the salt, though there is an increase in the length scale related to the change in size of the crystal. Jointly, the strength of convection can increase with time, till the solution in the growth chamber is fully depleted of salt. This state is characterized by stable stratification of the solution, the convection currents diminishing in strength to negligible levels. The growth of the crystal practically stops at this stage. Growth can be resumed when the grown crystal is immersed in a fresh supersaturated solution and the ramp rates are re-introduced. The resulting convection patterns would be different from the first stage because of a change in the crystal size. When the crystal is imparted rotation, velocities are created in the angular direction in the horizontal plane, in addition to the buoyancy-driven motion in the vertical plane. However, the two motions are interlinked through the radial component of velocity. The linkage is such that rotational motion leads to Coriolis forces that re-direct fluid motion. However, homogenization of the solution is the dominant factor that reduces concentration gradients, diminishes the driving potential and hence suppresses fluid motion arising from buoyancy. The critical speed at which buoyancy is practically suppressed will depend on the crystal size and overall concentration difference available in the solution. The resulting solutal concentration distribution at the surface of the crystal influences the growth rate and quality.

The process of solute deposition leading to crystal growth occurs on a hierarchy of length and time scales. At the small scale, solute particles arrange themselves as a part of the crystal structure. The pyramidal structure seen at later stages of growth is initiated at this point. The experiments conducted in the present work do not yield information on this aspect of the growth process. At the larger scale (the length scale of the crystal itself), concentration gradients are set up that feed solute to the crystal. These gradients naturally control the rate of crystal growth. The uniformity in distribution of the concentration gradients determines the crystal quality. The present research aims at investigating physical mechanisms at the scale of the crystal in terms of the solutal concentration distribution.