Pore Embryos in Porous/Compact/Porous Anodic Alumina by Three-Step Anodization

In order to study the formation process of the pore embryos in porous anodic alumina (PAA) and compact aluminum oxide (CAO), a new three-tier structure (PAA-CAO-PAA) was designed and fabricated in 3 wt% H2C2O4 and 8 wt% ammonium sebacate aqueous solutions. By exactly controlling the time of the third anodization in 3 wt% H2C2O4 solution, the formation process of the pore embryos is explicitly seen for the first time. The hemispherical pore bottom and hemispherical barrier-oxide base generated in the first anodization. After the second anodization, two hemispherical structures disappear and two interfaces become flat. When the third anodization finished under given voltage and anodizing time, the hemispherical structures reappear. A bigger hemispherical barrier-oxide base and spiny oxide formed in the third anodization. Besides, the newly formed pore embryos are not connected with the above pores and the channels cannot be observed in the compact oxide layer. All these facts cannot be explained by the traditional field-assisted dissolution model. Based on the oxide flow model and the electronic current theory, the formation process of pore embryos under the compact layer can be elucidated by the oxygen bubble mould effect.