CONVECTION AROUND A GROWING CRYSTAL

The apparatus in which experiments have been conducted on convection around a crystal in a top-hanging arrangement is discussed. The apparatus in which the crystal is held over a platform is described in the section titled Crystal Growth Chamber For Tomographic Projections.

Experiments that compare images of convection around the crystal recorded by interferometry, schlieren, and shadowgraph have been conducted in a growth chamber made of glass (17 cm diameter and 22 cm height) as shown in Figure 5.13. Glass used for the experimental chamber has the advantage of being a smooth surface on which unwanted nucleation cannot take place. In addition, glass is chemically non-reactive towards the KDP solution. The crystal growth chamber is in turn, placed in a Plexiglas tank. Temperature of the solution inside the growth chamber is maintained at prescribed levels by thermostatically controlled water circulating in the outer tank. The temperature of water is stabilized in the outer tank to within with the help of a programmable temperature controller (Eurotherm). A K-type thermocouple wire fixed to the outer surface of the growth chamber provides the feedback to the controller. With this arrangement, ramp rates could be applied to cool the solution over long periods of time. For example, a linear change in temperature of the solution from over a period of 60 hours was closely realized in the present set of experiments. Sufficient insulation is provided on the outer surface of the entire assembly to reduce thermal interaction between the growth cell and the ambient.

For visualization of the concentration field by the schlieren technique, circular optical windows (Stranda, BK-7, 60 mm diameter, 5 mm thickness, flatness) have been fixed on the walls of the growth chamber at opposite ends. The growth process is initiated by introducing into the growth cell a spontaneously crystallized seed crystal fixed at one end of a thin glass rod. The glass rod (diameter 3 mm) is small enough not to disturb the convection plume rising above the crystal. Rotation to the growing crystal is imparted with the help of a stepper motor (12V, 0.6A, RS components).

Figure 5.13: (a) Schematic diagram of the crystal growth chamber; and (b) Complete assembly.