The morphology and mechanism of formation of porous anodic films on aluminium

Abstract

The morphology of porous anodic oxide films formed on aluminium in phosphoric acid electrolytes at constant current density or voltage, and under changing electrical or electrolytic conditions, has been studied quantitatively by electron microscopy. Replicas from film sections and from both film interfaces have been prepared, as well as transmission micrographs of thin films, produced under accurately defined conditions. During formation at constant current density, pore initiation occurs by the merging of locally thickening oxide regions, which seem related to the substructure of the substrate, and the consequent concentration of current into the residual thin areas. The pores grow in diameter and change in number until the steady-state morphology is established. The film barrier layer thickness has been measured directly for the first time. The steady-state barrier-layer thickness, cell diameter and pore diameter are all observed to be directly proportional to the formation voltage. It becomes evident that the barrier-layer thickness, decided largely by an equilibrium established between oxide formation in the barrier-layer and field-assisted dissolution (probably thermally enhanced) at the pore bases, determines the cell and pore sizes by a simple geometrical mechanism. Anion incorporation into the film and its hydrogen-bonded structure play secondary roles to these factors in determining the actual film morphology, although not its subsequent properties. A consequence of the mechanism is that, at constant current density, relatively non-aggressive electrolytes give thicker barrier layers, larger cells and larger pores next to the barrier layer than aggressive media, although subsequent pore widening at the outer surface of the film by simple chemical dissolution is more severe in aggressive electrolytes.

Carbon nanotubes prepared by anodic aluminum oxide template method

“…Now, AAO films with a uniform and adjustable channel sizes of 20-200 nm can be reproducibly synthesized [78]. As a result, AAOCNTs with outer diameters ranging from 20 to 200 nm can be easily prepared [46,47,[79][80][81]. AAO film with channel size smaller than 20 nm can also be synthesized by selecting low voltage or in dilute sulfuric acid at low temperature (near 0°C) [76,82].…”

“…Generally, the pore diameter is linearly proportional to the anodizing potential with a proportionality constant of approximately 1.29 nm V 1 [46]. It is also related to the current density, electrolyte temperature, and acid concentration [45][46][47][48]. The depth (length) of the parallel AAO channels can be controlled in the range of 0.1300 m, which is mainly determined by the anodizing period.…”

“…The pore diameter, which can be controlled by altering the anodizing conditions, are in the range of a few nanometers to hundreds of nanometers. Generally, the pore diameter is linearly proportional to the anodizing potential with a proportionality constant of approximately 1.29 nm V 1 [46]. It is also related to the current density, electrolyte temperature, and acid concentration [45][46][47][48].…”

Growth behavior of anodic oxide formed by aluminum anodizing in glutaric and its derivative acid electrolytes

“…6c). Typically, there is a linear relationship between cell diameter and anodizing voltage for anodic porous alumina, and this relation is described as follows [4,39]:…”

“…Anodic porous alumina with numerous vertical, nano-scale pores can easily be fabricated via aluminum anodizing in several types of acidic aqueous solutions and has been widely applied in the fields of corrosion protection and electronic applications [1][2][3][4][5][6]. In particular, the development of highly ordered anodic porous alumina, which is fabricated via self-ordering and two-step anodizing under appropriate electrochemical conditions, has expanded the applicability of porous alumina [7].…”