DIRECT OBSERVATION OF ANODIC FILMS FORMED ON TANTALUM IN CONCENTRATEDPHOSPHORIC AND SULFURIC-ACID-SOLUTIONS

The anodic films formed at 1 mA cm(-2) on tantalum in concentrated H3P O4 (85%) and H2SO4 (95%) solutions at 25 degrees C have been examined directly in the transmission electron microscope employing ultramicrot omed sections. In the former electrolyte, duplex films comprising an o uter PO43--doped layer, constituting about 70% of the film thickness, and an inner relatively pure Ta2O5 layer are developed, contrasting wi th the uniformly SO42--doped films developed during anodizing in conce ntrated H2SO4. The doped layers contain high concentrations of PO43- a nd SO42- ions, with compositions represented by Ta2O5(1-x)(PO4)(10x/3) and Ta2O5(1-x)(SO4)(5x), where x = 0.27 and 0.30 respectively. The in creased incorporation of electrolyte species relative to films formed in dilute electrolytes significantly enhances the resistivity of the f ilm material with an associated reduction in the dielectric constant. (C) 1998 Elsevier Science Ltd. All rights reserved.