EFFECT OF CRYSTAL SIZE

In the previous sections, it is seen that the crystal growth rate is positively correlated with the local concentration gradients. The gradients in turn depend on the size of the crystal, ramp rate and the rate of rotation. It is of interest to know if the gradients are explicit functions of time. An absence of time-dependence would show the process to be quasi-steady, and the results obtained in the present work would be applicable with greater generality.
To test the experimental data for the presence of growth history and inertia effects, the following experiments were performed: Previously grown crystals of small, medium and large sizes were suddenly inserted into the supersaturated solution. The solution was subjected to cooling at the prescribed ramp rate (of ). The short time convection currents around a passive crystal have been compared with those that evolve gradually around a growing one when the growth process takes place from a seed, for a longer duration of time (~ 60-70 hours). A favorable comparison in terms of convection patterns is an indicator that the concentration gradients respond directly to the process parameters, and are not explicit functions of evolutionary time.

Figure 5.25: Schlieren images showing the instantaneous evolution of buoyancy-driven convection currents in the growth chamber when a crystal of (a) small, (b) medium and (c) large size is inserted into its supersaturated solution.