The Mechanism of the Dendritic Electrocrystallization of Zinc

Abstract

Measurements have been made of the growth rate of zinc dendrites in alkaline zincate solutions as a function of overpotential (η), concentration , and temperature . The tip radii have been measured by electron microscopy. At constant potential, an initiation time of between 5 and 100 min is observed, depending on η, c, and T. The dendrite grows linearly with time, at a rate depending on η, c, and T. The total current to base and dendrite was independent of time until a time , where (the time for initiation obtained from the growth rate vs. time relation). Thereafter, . A critical overpotential was determined, . Below this , sponge was formed. Dendrites were observed up to ; above this the deposition was heavy sponge. At a given , the growth rate of a given dendrite increased with η according to an exponential law. The growing tip is parabolic, where . No twinning was observed.The basic model used depended on the increase in c.d. possible for an electrodic reaction when the diffusion current depends on a radius of curvature of the substrate, rather than the linear diffusion layer thickness, . When the tip of a dendrite‐precursor attains this condition, its growth is released from the diffusion control characteristic of it in the predendrite situation, and it grows further under predominantly activation control at a rate far greater than that possible in any other direction, where the radii of curvature are much greater. The Gibbs radius‐dependent overpotential term is also present, although it has a minimized influence. The initiation of the dendrite is treated in terms of growing pyramids on the substrate surface. At first the growth is linear‐diffusion controlled, but it is shown that the rotation of the spiral, within the linear diffusion boundary surrounding the sphere, gives rise to a decrease of the effective radius of curvature of the dendrite tip until the value is attained, which is effectively the condition for the dendrite initiation. The theory of the propagation in terms of the activation, diffusion and Gibbs overpotential is consistent, in terms of , with experiment. A derived growth‐time line is also numerically consistent with experiment. The dendrite growth rate as a function of and η are numerically calculated with reasonable consistency. The tip radius can also be approximately calculated in terms of the present model.