Ground-based experiments and theory in preparation for floating zone melting and directional solidification of cadmium telluride in space

The objective of this program is to apply theoretical and experimental methods to optimize the equipment and procedures for floating zone melting and Bridgman-Stockbarger growth of cadmium telluride crystals in space. Computer codes were developed for the computation of heat transfer in the furnace and thermal stress in the resulting crystal. The predictions for the temperature field are being compared with experimental measurements. It was found that if the crystal sticks to the ampoule wall, differential thermal expansion between crystal and ampoule contributes much more to the stress than does the temperature field in the crystal. Thus, one goal of solidification of cadmium telluride in space is to reduce or eliminate contact of the crystal with the ampoule wall. Another goal is to find coatings and linings which reduce sticking of the grown crystal onto the ampoule. We developed techniques for measuring the surface tension and contact angle of molten cadmium telluride vs temperature and stoichiometry. The surface tension decreased with increasing temperature and with decreasing cadmium concentration. Wetting increased in the following order: pyrolytic boron nitride, carbon-coated quartz, sandblasted quartz, HF-etched quartz, and plain quartz. Additional coatings and potential ampoule lining materials are being developed and will be tested both for wetting by the melt and for sticking by the solid. Techniques are being developed for measuring sticking. We are also developing techniques for measuring the mechanical properties of cadmium telluride and for the direct observation of defect formation and evolution vs temperature. X-ray topography will be done in real time using the National Synchrotron Light Source at Brookhaven National Laboratory, in collaboration with the National Institute of Standards and Technology. Techniques are being developed for floating zone melting of cadmium telluride in space. We have successfully float zoned 5 mm rods on Earth. Evaporation of cadmium and tellurium from the molten zone is prevented by adding excess cadmium combined with heating of the entire ampoule. Still needed is a suitable automatic method to assure that the proper zone length is achieved in larger diameter rods in space.