Morphological variation of cadmium crystals grown from the vapor

In a cell filled with pre-purified hydrogen, cadmium crystals were grown from the vapor. The pyrex fiber substrate was placed perpendicular to the temperature gradient. As a result, crystals growing on the substrate manifested the morphology under a single condition of supersaturation and temperature. Growth temperature was directly measured by a thermocouple located next to the substrate. Supersaturation was calculated from the two interior temperatures of the two ends of the cell. These interior temperatures were estimated from the corresponding exterior temperatures which could be directly measured. This way, supersaturation could be estimated to +-0.05. Growth temperature ranged between 260 and 3200C, and at a given temperature, supersaturation varied from 0.05 to 0.5. Only above 2950C was whisker growth ative. At a supersaturation of 0.3, this whisker growth was most active. When the supersaturation was below 0.1, there was no visible crystal growth. Whiskers maintained characteristic shapes only when supersaturation was below 0.5. Occasional irregular growth was attributed to traces of oxygen and water vapor in the hydrogen atmosphere. The effects of these contaminants on crystal growh were summarized as follows (a) increased crystal populations, (b) a tendency for hexagonal blocks to become irregular nodules, and (c) enhanced whisker growth. (U.S.)