Microstructure of porous anodic oxide films on aluminium
Microstructure of porous anodized films of aluminium prepared in sulphuric acid solution are different from those prepared in an oxalic or phosphoric acid solution. Transmission electron microscopy reveals a multilayer or higher order structure in the former films. Infrared spectra and specific surface area were also studied for these films and new functional properties of the films suitable for new materials were found. In contrast to the fibrous colloidal structure in the cells and barrier layer in the conventional films anodized in a sulphuric acid solution at d.c. 15 V, a network structure is formed in the cells and barrier layer in the hard films prepared at higher voltage of d.c. 25 V. The microstructure changes according to the anodizing conditions. A new model for these sulphuric acid films is presented, i.e. the cell walls are constructed from five layers and the fracture of the films occurs at the centre of the cell walls. Centre barrier layer (4 to 6 nm in thickness) composed of aluminium oxide of high crystallinity was found in a barrier layer at the bottom of the pore, and the thickness is independent on the applied voltage of the anodizing. Increase in thickness of the barrier layer due to applied voltage is governed by that of the outer barrier layer.