Fabrication Of Porous Anodic Alumina Films By Large Current Density And High Voltage Anodization Process

Porous anodic alumina（PAA: Porous Anodic Alumina） film is one of the most wildly used templates for the fabrication of various nanostructured materials due to its advantages such as high technical maturity, adjustable parameters and controllable process. In this paper, large current density and high voltage anodizing process will be investigated by using oxalic acid as electrolyte system. The main contents and results are as follows:1. In this paper, anodization process under large constant current density was studied by using 50 m A/cm2 as example, and thus we established a large constant current density anodizing process system. Based on the voltage-time curve, the rising process of regime voltage has been divided into four stages:（1） linear rising stage,（2） a slow transition phase,（3） relatively stale stage,（4） index rising stage. The growth status of PAA films under different stages was also analyzed.2. PAA films with complex microstructure such as taper-shaped, funnel-shaped, branch pores were prepared by using large transient current density of 100 m A/cm2, 300 m A/cm2 in the process of two-step anodization. The experiment results show that the ordering degree of concaves on the Al substrates which was prepared by removing the PAA films of one-step anodization have an important influence on the two-step anodization process under large constant current density. The higher degree of ordering concaves, the more homogeneous structural units of the prepared PAA film. The concaves on the Al substrates with different ordering degrees and sizes can be obtained by adjusting experimental conditions of one-step anodization. And further adjusting the transient current density of the anodization, anodization time and pore opening time of two-step anodization can result in different pore shapes of PAA films. Changes of morphology after pore widening have confirmed the theory that the pore walls of PAA films are made up of two layers: higher purity inner layer and contaminated outer layer respectively. Through the corrosion during pore widening process, rough outer layer can be removed, thus remaining the high purity inner layer.3. Applying large constant current density of 30 m A/cm2⁶0m A/cm2 in the anodization process can obtain stable voltage stages（CCSV process）. Under this stage we can fabricate long-range ordered PAA with large structural units films. In this paper, the self-organizing optimal voltage is between 135V¹83V and the corresponding pore distance is between 307⁴15nm, widening the self-organizing optimal voltage window. The ratio of anodization voltage interpore distance under this method was found between that of mild and hard anodization(ζ_HA<ζ_ccsv<ζ_MA), which means CCSV under large constant current density form a new self-organizing regime. By adjusting the current density, the anodization temperature, the ratio of oxalic acid and ethanol and the initial reaction setting, the influence of the anodization conditions on the anodization process was investigated.4. By applying large constant current density of 30 m A/cm2⁶0m A/cm2 in the anodization process, the anodization voltage can be increased to a value of more than 300 V. Anodizing process with high constant voltage will lead to the competitive growth phenomenon among structural units, thus resulting in the micron scale clusters on the barrier layer, which are made up of irregular structural units. In this paper, we proposed an concept of the initial competitive growth starting site（CGSS）, the CGST pores have an advantage of the growth and will have branch pores, which will occupy the growth space of other pores. We also explored the influence of anodization condition such as anodization voltage, ethanol adding, ending current density on the competitive growth process under high voltage anodization. Through the high voltage anodization process, PAA films with micron / submicron hierarchical structure can be obtained. The as-prepared PAA films can be divided into three types:（1） close-paced,（2） inter-linked,（3） separated. Combining mild anodization, PAA films with micron/submicron/nano multi-scale structures can be prepared. Regulating voltage of the two-step mild anodization（30⁵0V）, we found that nanoscale pore distances nearly follow the mild anodization law of 2.5nm/V, and the larger of the applied voltage, the greater of the interpore distances. Using high voltage anodization process and mild anodization process is a facile method for preparing PAA films with multi-scale structures, which has the advantages such as simple operation, controllable process, without using of complicated equipment.