TOMOGRAPHIC RECONSTRCUTION OF SCHLIEREN DATA

The question of a correlation between the solutal distribution in the solution and the topography of the crystal can be partially addressed by reconstructing the solute concentration contours on select planes above the crystal. Reconstruction from schlieren images using principles of tomography is discussed in the present section. Unlike the hanging crystal configuration in Sections Comparision of Interferometry, Schlieren and Shadowgraph in a crystal Growth Experiment and Influence of Ramp Rate and Crystal Rotation on Convection Patterns, the crystal in the set of experiments was mounted over a platform. With this arrangement, the interference of the rising convection plume with the crystal holder is eliminated. Recording projections requires the apparatus to be turned; in this respect, what is collected is time-lapsed data. For meaningful analysis, it is thus necessary that the convection pattern be steady in time. Only those experiments where this requirement was fulfilled have been included in the discussion.

One parameter that defines the quality of the growing crystal is the symmetry of its faces. Since growth from an aqueous solution is governed by the strength and movement of buoyancy-driven convection currents, the convective field plays an important role in transporting solute from the bulk of the solution to the crystal faces. Hence, to ensure symmetric growth of the crystal, it is necessary to ensure that the convective field in the growth chamber retains a symmetry pattern. In the present discussion, a viewpoint that an axisymmetric convective field is favorable for crystal growth has been adopted. Thus, conditions under which the solutal distribution is axisymmetric have been delineated from the tomographic reconstructions.

Results are presented for experiments on crystal growth in the diffusion as well as steady convection regimes. In Section CONVECTION AROUND A GROWING KDP CRYSTAL , results have been presented in the form of a time sequence of schlieren images recorded at four view angles, corresponding to four pairs of optical windows of the growth chamber. The crystal is not disturbed during the growth process. In this respect, the measurement process is truly non-invasive, though with a drawback of allowing only a limited number of projections. The emphasis is towards understanding the diffusion-dominated and the stable growth regimes of the crystal. In both regimes, the concentration field is practically steady, particularly during the recording of the individual projections. The reconstruction of the concentration field over selected planes above the crystal and their relationship to the crystal geometry are discussed. The influence of crystal rotation on the convective field is not covered in the present discussion.