INFLUENCE OF ELECTROLYTE ON COMPOSITION OF ANODIC OXIDE FILMS ON TANTALUM

We have studied the influence of the electrolyte on the composition of dielectric films formed by anodic polarisation of tantalum. The electrolytes used were mainly aqueous solutions of salts and acids (H2SO4, H3PO4, HNO3 etc.) at various concentrations. A new method of microanalysis of the light elements (O16, O18, N14and D) contained in the films has been coupled with the more classical methods of study (growth rate, capacity measurements, and the use of Redox systems in solution). The absence of large incorporation of elements other than oxygen and tantalum in films formed in aqueous salt solution and very dilute acids has been shown. In films formed in concentrated acids the incorporation of anions of the type Ax Oyn-is so important - 30 per cent of the total number of anions in the case of films formed in 95% H2SO4-that it is no longer possible to consider these films as oxides. The results obtained by deduction from the analysis of oxygen are in good agreement with those obtained from direct analysis for nitrogen. The film properties (rate of chemical attack, dielectric constant, formation field, molar volume etc.) depend on the nature of the electrolyte used for formation. © 1969.