Field crystallization of anodic films on tantalum

Abstract

The radial growth rate of crystalline areas during the anodization of tantalum is found to be related to the nature and concentration of both the solute and the solvent and also to the pH of the electrolyte. In general, the use of high solute concentrations or the addition of certain organic solvents produces reduced growth rates in aqueous electrolytes.

Structural examinations indicate that for bright rolled tantalum foil of nominal purity 99.99%, the density of nucleation sites is approximately 10⁷ cm⁻², of which about 1% undergo active radial growth. The nucleation sites, which can be related to impurity inclusions, are believed to maintain contact with the electrolyte through narrow pores. As the amorphous oxide is rolled back by the growing crystals, a layer of relatively uniform thickness is stripped from the top surface of the crystalline area and remains in intimate contact with the detached amorphous oxide.

The use of field crystallization to study the location and distribution of impurities at both exterior and interior surfaces of sintered powder anodes is described.