Ionic gradients at an electrode above the equilibrium limit current. 3. Stabilization of ion depleted conduction by a nanoporous alumina layer during electrophoretic deposition

The constant current electrophoretic deposition (EPD) of alumina powder from an acidic suspension is accompanied by an anomalous voltage rise across the deposited particulate layer. This voltage rise is much greater than can be accounted for by the simple blocking of the electrode by nonconducting particles. It is shown here that this voltage rise can be accounted for by the formation of a high resistance ion depleted conduction layer in the solvent at the cathode. This layer, which is highly unstable and therefore not seen in the pure solvent, is stabilized by the adsorption/desorption equilibria of ions on the alumina powder surface. This also explains instabilities seen in the EPD coating process. The stabilization of this type of layer is of interest beyond EPD for its ability to produce large pressure gradients in a fluid medium on the micrometer scale and a stable region of unbalanced charge and extreme voltage gradients in a nanoporous alumina layer.