Crystals and Crystal Growing
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The conditions necessary to grow zinc single crystals of 2. When a constant rate of growth is used and the crystal is initially given the desired orientation by starting it on a suitable nucleus the determining factor for the successful growth of a single crystal rod 10 cm or more in length is found to be the temperature gradient existing in the column of liquid zinc just below the growing crystal. The appropriate temperature gradient is a function of the orientation of the crystal.
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The growth of hopper crystals is observed for many substances, but the mechanism of their formation remains ill understood. Here we investigate their growth by performing evaporation experiments on small volumes of salt solutions. We show that sodium chloride crystals that grow very fast from a highly supersaturated solution form a peculiar form of hopper crystal consisting of a series of connected miniature versions of the original cubic crystal. Above this threshold, the growth rate varies as the third power of supersaturation, showing that a new mechanism, controlled by the maximum speed of surface integration of new molecules, induces the hopper growth of cubic crystals in cascade. The beauty of snowflakes is partly due to the amazing variety of shapes they come in. Yet, the underlying crystalline structure of snow is always the same: the thermodynamic equilibrium shape is hexagonal. The variety of shapes is due to the kinetics of ice growth that is clearly more important in determining the snowflake shape than the equilibrium shape dictated by the crystal structure and surface energy.