Showing papers on "Overpotential published in 1969"

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 observedThe basic model used depended on the increase in cd 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

Electrocrystallization of Metals under Ideal and Real Conditions

Abstract: Ideal conditions during electrocrystallization, which are only approximately attainable, would lead to a perfect single crystal, whereas real conditions yield a monocrystalline or polycrystalline product containing many structural defects. The electrolytic production of quasi-ideal single crystals requires an overpotential that supplies at least the activation energy necessary for the formation of two-dimensional nuclei, since growth otherwise proceeds via screw dislocations with formation of a “real” crystal. The potential deviation in electrocrystallization under real conditions is due to electrochemical and/or crystallization overpotential, which can be determined separately. The investigation of electrocrystallization also offers possibilities for a systematic study of the formation of imperfections in real crystals.

Mathematical Theory of Electrochemical Machining 1. Anodic Smoothing

Abstract: Further solutions to the potential equation with moving boundary conditions, applicable to the electrochemical machining process, are presented. A theory is proposed for the machining of an arbitrary distribution of irregularities on to an anode for the cases where the cathode profile is specified and the resultant anode shape must be found, and where a cathode profile must be designed to give a required anode shape. Limitations on the application of this theory are discussed. The effects of overpotential are also discussed; overpotential only at the cathode is shown to reduce the limiting amplitude which can be obtained on the anode, and to increase the machining time required to achieve it. Overpotential only at the anode has no effect on this limiting amplitude, but again increases the necessary machining time.

Method of controlling zinc dendrite growth

Abstract: The growth of zinc dendrites during charge of a silver-zinc cell is a particularly undesirable feature of the zinc electrode as it eventually leads to shorting of the cell through growth of metallic trails of zinc through the separator. A method is disclosed of preventing the growth of zinc dendrites and more particularly of preventing zinc dendrite penetration of the separator. The method involves controlling the overpotential in the cell, i.e., limiting the overpotential to a value below the critical magnitude at which the zinc deposits as dendrites instead of as a moss. When zinc deposits as a moss, penetration of the separator does not occur. The new method involves so limiting the charge overpotential and using a charge rate that the cell is fully charged by the time the critical overpotential is reached. The overpotential is measured between the working zinc electrode and a built-in zinc reference electrode. When the critical overpotential is reached, charging is terminated. The subject matter of the invention also includes an improved electrochemical apparatus whose efficiency and useful life derive from the prevention of zinc penetration of the separator.

Cathode Overpotential and Electrosorption Effects of Straight‐Chain Carboxylic Acids during Electrodeposition of Copper

Abstract: The effects of straight-chain carboxylic acids on cathode overpotential during electrodeposition of copper were studied by galvanostatic methods at current densities up to 0.020 A-cm -2. No modification of the chargetransfer mechanism of copper deposition occurred. Overpotential increments caused by lengthening of the carbon chain can be correlated with the reduced concentrations of the carboxylic acids in accordance with Traube's rule. This can be rationalized in terms of the general relation between adsorbability and solubility of the additive. The general behavior is consistent with the blocking theory of additive function according to which overpotential increments are directly dependent on additive adsorbability. The fractional surface coverage calculated from overpotential increments appears to fit a Langmuirtype isotherm from which the free energies of adsorption of the carboxylic acids can be determined. The magnitudes of these free energies are consistent with physical adsorption forces. A relation was derived between the free energy of adsorption and the free energy contributions from the carboxyl group and the carbon chain. This shows that the longer the chain, the higher the adsorbability.

A Theory for Porous Electrodes Undergoing Structural Change by Anodic Dissolution

Abstract: A theoretical model is proposed for the description of flooded porous metal electrodes which undergo anodic dissolution by electrochemical reaction. Equations are developed to represent the pseudosteady state which prevails during dissolution after the concentration gradients have become fully established within the pores. The analysis leads to an understanding of how mass transfer, kinetic and geometric parameters of the system determine the electrode overpotential and its change during dissolution. The most uniform current distributions are predicted to occur not at vanishingly small currents but at finite anodic currents because of mass transfer limitations to the cathodic back reaction. Thus, for a range of anodic currents, an increase of applied current will result in a more uniform distribution of the electrochemical reaction throughout the porous electrode. Calculations illustrating the behavior are presented for the acid‐copper system.

Overpotentials for Intercalation into Graphite

Abstract: BEFORE intercalation of other molecules between the carbon hexagon networks in graphite can be started, they must first be given a sufficient chemical or electrical overpotential. The excess potential needed depends in a sensitive way on the textural perfection of the graphite used, which makes measurements of overpotential a valuable diagnostic test with near ideal graphites1.