Mechanical instability and pore generation in anodic alumina

Abstract

This paper examines the mechanism of pore formation in anodic films on aluminium. For this purpose, the dimensional changes of specimens during growth of porous films on aluminium in phosphoric and sulphuric acid electrolytes are examined using transmission and scanning electron microscopies. Further, the compositions of films and the efficiencies of anodizing are determined by Rutherford backscattering spectroscopy and nuclear reaction analysis. Significantly, the efficiency of anodizing is about 60%, while the surface of the anodic film is located above the original aluminium surface, i.e. before anodizing. The ratio of the thickness of the anodic film relative to the thickness of the consumed aluminium is about 1.35 for the selected conditions of anodizing. The behaviour runs counter to the widely accepted mechanism of pore formation by field-assisted dissolution of alumina. It is explained by the high plasticity of the anodic alumina in the barrier region in the presence of ionic transport, with film growth stresses displacing material from the barrier layer towards the cell wall region during anodizing. The response of the film to volume constraints on growth indicates a major role of stress and stress-relief processes in determining the morphology and self-regulating organization of pores.

Anodic TiO2 nanotube layers: Why does self-organized growth occur—A mini review

“…The diameter of the tubes then correlates well with the approach taken in Fig. 2f.Another interesting phenomenon is that under self-organizing growth conditions, for porous alumina and TiO 2 nanotubes an unexpected lengthening during growth is observed [34,73,74]. This has been ascribed (based on convincing experimental observations) to stress-induced viscoelastic flow of anodic oxides [73,75] that may become possible under certain electrochemical conditions.…”

“…2e). More recently, a number of fundamental quantitative approaches -namely using perturbation analysis [32][33][34] or numerical treatments [72] -deal with initiation and growth of self-ordered porous alumina and TiO 2 nanotubes. In the initial phase the main question is how a morphological instability such as a corrugated surface can be stabilized.…”

“…directional charge transport, orthogonal carrier separation, optimized and directional diffusion profiles, defined membranes, biological cell interactions, possible quantum size and tubular core shell devices). A particular advantage of anodic nanotube arrays is that tubes are grown from their metallic substrate and thus can be used directly as back-contacted oxide electrode.While a number of excellent literature reviews exist on the immense efforts that are directed towards an ever increasing control over growth conditions as well as exploiting the tube layers in applications [1,[5][6][7], only relatively little work deals with the question after the origin of self-organization in anodic oxide formation processes [29][30][31][32][33][34][35]. It is thus the goal of the present review to particularly address this point and give an overview of factors dictating self-organization and tube formation.…”

A tracer investigation of chromic acid anodizing of aluminium

“…The formation of a compact alumina film of the same thickness as that of the consumed aluminium can be achieved by anodizing at an efficiency of close to 60%. 11 The efficiency of anodizing in the chromic acid electrolyte falls below this value due to the presence of the porosity. The difference between the residual aluminium that was not oxidized, 1 Following almost complete oxidation of the deposited aluminium during anodizing for 700 s, the tungsten and aluminium contents of the anodic film correspond to 1.50 ð 10 16 and 2.07 ð 10 18 atoms cm 2 respectively, indicating a cation charge of 1.01 C cm 2 ( Fig.…”

Theoretical Pore Growth Models for Nanoporous Alumina

“…However, it should be noted that direct loss of Al 3+ ions (without oxide formation) can take place during anodization [139], which provides some spacing for the expanded volume and relief of the compression stress. For instance the measured ratio of the formed oxide thickness to the consumed aluminum thickness was 1.35, which still indicates a large extent of volume expansion [140]. Thus, the mechanical compression stress due to volume expansion was recently regarded as the driving force for pore growth in AAO, which was proposed to result in the plastic flow of oxide from the bottom to the walls of the pores [120,124,126].…”