FLOATING-ZONE CRYSTAL-GROWTH WITH A HEATED RING COVERING THE MELT SURFACE

Although free from gravity-induced natural convection, the melt zone in microgravity floating-zone crystal growth can still be subjected to strong, oscillatory thermocapillary convection. In order to help reduce thermocapillary convection, and meanwhile improve the cross-sectional control and surface quality of resultant crystals, the conventional floating-zone process has been modified. In this modified process a heated ring covers most of the surface of the melt zone. This process was studied using NaNO3 as a test material. Single crystals were grown from 6 mm diameter cast rods of NaNO3 using two different types of heated rings. It was observed that with one type of ring, wetting of the ring outer surface by the melt caused significant variations in the crystal cross-section. With the other type, however, this wetting problem did not occur and the resultant crystals exhibited a very uniform cross-section and smooth surface. Thermocapillary convection, instead of prevailing in the bulk melt zone as in conventional floating-zone crystal growth, was limited to near the free surface.