Importance of amorphous-to-crystalline transitions for ionic transport andoxygen generation in anodic films

Barrier-type anodic oxide films, with relatively uniform distributions of A l3+ and Zr4+ ions, have been formed on sputter-deposited, non-equilibrium Z r-Al alloys, containing 5, 15 and 43 at.% Al, in 0.01 M ammonium pentaborat e electrolyte at 393 K. The films formed on the last two alloys are primari ly amorphous, with transport numbers of cations of about 0.18 and 0.25 resp ectively, i.e., increasing with increase in aluminium content. Thus, as for other amorphous anodic oxides, both cation and anion transport contribute significantly to film growth. However, nanocrystals, suggested to be based on cubic ZrO2, are formed near the alloy-film interface in the amorphous ox ide matrix, in association with bubbles due to generation of oxygen gas in the vicinity of the alloy-film interface. Increase in the zirconium content to 95 at.% leads to the formation of a relatively uniform, crystalline ano dic film with a monoclinic ZrO2 structure; film growth now proceeds predomi nantly due to migration of anions, similar to films formed on high purity z irconium.