The Anodization Behavior Of Al Substrate Coated By Pre-existed Anodic Alumina And Fabrication Of AAO Film With Hierarchiral Pore Structure

The structure and composition of AAO film strongly depend on theelectrolysis conditions. Different type of electrolyte have different dissolutionperformance to AAO film and thus result in different electrolysis parameters, such asanodization potential or current density, which determines the structure of AAO film.In this paper, we explored the effect of different structure of the pre-existed anodicalumina film on successive anodization behavior and fabricated three-dimensional （3D）AAO film with step-enlarge-pore structure or periodic variations in pore diameter bychanging the electrolyte and electrolysis parameters. The main achievements in thispaper are as follows:1. Effects of different structure of pre-existed anodic alumina film on the successiveanodization behavior were investigated in a mixed acid solution. The results showthat the successive anodization current density dramatically decreased with thethickness of pre-existed anodic alumina film increasing. But the variation of porediameter has little effect on the current density. This phenomenon reveals that thelimitation effect of length of nanochannel on diffusion of Al³⁺, O^2- and OH^-is morepronounced than that of pore diameter. A retarded time was needed when the Alsubstrate was sequential anodized in two different electrolytes. And this retardedtime was related to the pore bottom morphology. The smaller the radius, the moreretarded time was needed, even successive anodization could not be carried out.2. The growth mechanism of successive AAO film formed in oxalic acid solution（denoted as OAAO） systematically researched when the Al substrate was coated bya pre-existed anodic alumina film formed in sulfuric acid solution （denoted asSAAO）. We found that a pre-existed anodic alumina film not only affected thevoltage and current density of stable anodization in oxalic acid, but also made thedistribution of electric field more uniform and thus resulte the voltage range wasenlarged obviously. The growth of OAAO was affected by the pre-existed SAAO.The as-obtained AAO film has a continuous change in the morphology fromSAAO-like, then to OAAO-like, finally to OAAO structure through a long periodof adjusting, accompanying by a process from disorder to order.3. A3D AAO film with step-enlarge-pore structure or periodic variations in porediameter was fabricated successfully by changing the type of electrolyte andelectrolysis parameters. The Al substrate was sequential anodized in sulfuric acid, oxalic acid, malonic acid and tartaric acid to fabricate step-enlarge-pore structureAAO film. The through-hole density was determined by the pore density of thelast layer of as-obtained AAO film. In addition, the aluminum substrate anodizedalternately in phosphoric acid and tartaric acid to fabricate AAO with periodicvariations in pore diameters, which will become more pronounced after enlargingpore size treatment conducted in phosphoric acid solution.