INTERRELATIONSHIPS BETWEEN IONIC TRANSPORT AND COMPOSITION IN AMORPHOUS ANODIC OXIDES

The inter-relationships of alloy composition, film composition and ion ic transport for formation of amorphous anodic oxide films are address ed quantitatively through systematic study of sputter-deposited Al-Ta alloys containing up to 39 at.% Ta. The work reveals the dependence of electric field, ionic transport number, incorporation of species into the anodic film at the alloy-film interface and mobility and distribu tion of species within the anodic film on alloy composition. Anodic ox idation, at high current efficiency, of alloys containing 2.8, 15, 32 and 39 at.% tantalum results in formation of two-layered anodic films by migration of cations outwards and by migration of anions inwards: a n outer layer, 20% or less of the total film thickness, composed of re latively pure alumina and an inner layer containing units of Al2O3 and Ta2O5 distributed relatively homogeneously. Two-layered films develop due to the slower migration rate of Ta5+ ions relative to Al3+ ions i n the inner layer of the growing anodic films, which changes progressi vely from about 0.6 for dilute alloys to about 0.9 for Al-39 at.% Ta. The average nm V-1 ratios, total transport numbers of cations and aver age Pilling-Bedworth ratios for the films change almost linearly with alloy composition between the values for anodic alumina and anodic tan tala. A tantalum-enriched layer, about 1 nm thick, is formed in the Al -2.8 at.% Ta and Al-15 at.% Ta alloys just beneath the anodic film, in dicating prior oxidation of aluminium in the initial stages of anodizi ng. In contrast, aluminium and tantalum in the alloys containing more than 30 at.% tantalum are immediately incorporated into anodic films i n their alloy proportions, without development of a tantalum-enriched layer, at the available resolution. Boron species, incorporated from t he electrolyte into the outer parts of the films, are immobile in film s on alloys up to 15 at.% Ta but migrate outwards in other films, poss ibly due to the increased Lorentz field. Though the inter-relationship s between film parameters and alloy composition are established for Al -Ta alloys specifically, the findings are considered to be equally rel evant to amorphous anodic oxides formed on alloys and semiconductors g enerally.