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

Abstract

Abstract Barrier-type anodic oxide films, with relatively uniform distributions of Al3+ and Zr4+ ions, have been formed on sputter-deposited, non-equilibrium Zr—Al alloys, containing 5, 15 and 43at.%Al, in 0.01 M ammonium pentaborate electrolyte at 293 K. The films formed on the last two alloys are primarily amorphous, with transport numbers of cations of about 0.18 and 0.25 respectively, 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 oxide 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 95at.% leads to the formation of a relatively uniform, crystalline anodic film with a monoclinic ZrO2 structure; film growth now proceeds predominantly due to migration of anions, similar to films formed on high purity zirconium