Crystal Growth of Silver Metal. Mechanism and Kinetics

The crystal growth of silver-metal whiskers and ``ferns'' occurring in the reduction of Ag(I) with Fe(II) from aqueous perchlorate solution was directly observed under the microscope. Whiskers frequently grew at well-defined angles to the previous direction and occasionally stopped growing abruptly although nearby whiskers continued to grow. In fern growth, short spikes grew in a regular manner from only one side of a parallel-sided main stem. Linear growth rates of whiskers were essentially independent of an estimated eightfold variation in supersaturation, but the growth rate of a whisker occasionally increased abruptly in approximately integral ratios. Whisker and fern growth are qualitatively attributed to perfect and twinned screw dislocations, respectively, but no explanation is offered for the formation of whisker and fern habits. It is shown that an interpretation of growth habit will require the Burton, Cabrera, and Frank treatment of infinite surfaces to be extended to the edges and corners of a crystal. Limited kinetic data suggest that the size of the critical surface nucleus was independent of supersaturation, which is contrary to the classical theory of crystal growth. A possible explanation for constant-sized critical surface nuclei is offered.