(Keynote) Actual Pore Morphology of Anodic Alumina Films: Straight or Branching?

Abstract

Anodic oxide film on aluminum has attracted attention as a starting structure for nanomaterials because of its nano-sized straight tube shape. In this case, a straight pore of the films formed in oxalic acid electrolyte or sulfuric acid electrolyte is generally used. On the other hand, a film with a branched pore structure, like a chromic acid film, is used for practical aluminum materials such as aircraft alloy containing silicon and copper. This is because the branching action of the pores has the effect of suppressing the inhibition of uniform film growth due to the non-uniform base composition of the alloy material. Although the branched structure of the chromic acid film is considered to be unique as anodic film, it has been revealed that when the solubility of the electrolyte is low, the pore branching often appears accompanied by minute dielectric breakdown [1, 2].Recently, however, the authors found that by performing detailed TEM observation of the pore wall of anodic films, considerable branching of the pores occurred even when electrolysis was performed at 40 V in oxalic acid electrolyte, which is the self-ordering condition. The branching of the pores stops at the early stage of the process and remains as traces in the pore walls as radial fine pores. When the subsequent anodic oxidation continues, these radial micropores disappear due to chemical dissolution reaction of the oxide to the electrolyte. In this study, the actual morphology and controlling factors of pore branching in not only chromic acid electrolyte, but also of pore branching found in films formed by other typical anodizing electrolytes such as sulfuric acid, oxalic acid and phosphoric acid will be reported.[1] S. Ono, S. Chiaki, T. Sato, J. Surf. Finish. Soc. Jpn, 26, 456 (1975) DOI:10.4139/sfj1950.26.456[2] S. Ono, T. Kawaguchi, H. Ichinose, Y. Ishida, N. Masuko, J. Surf. Finish. Soc. Jpn, 40, 1361 (1989) DOI:10.4139/sfj.40.1361