THERMO-INDUCED PHASE-MORPHOLOGICAL TRANSFORMATIONS IN THE FILMS OF POROUS ANODIC ALUMINA FROM SODIUM HYDROXIDE SOLUTIONS

Abstract

Porous anodic aluminium oxide must be widely used in new types of micro- and nanoelectronics devices, including those operating at elevated temperatures. Therefore, it is of great importance to study the behaviour of anodic alumina fi lms upon heating. This paper studies phase and morphological changes in the fi lms of anodic aluminium oxide from sodium hydroxide solutions which occur under thermal treatment. The study uses a set of physical and mathematical methods, including colorimetric, x-ray phase and fractal analyses. Colorimetric analysis showed that after annealing, the values of refl ection coeffi cients reduced from 65 to 57%, which may be due to structural changes. The samples of initial and annealed anodic aluminium oxide fi lms were studied by X-ray phase analysis. It has been established that annealing anodized samples at a temperature of 500 ° C for 40 minutes leads to a number of changes in the hydrated alumina forms. Low-temperature modifi cation of aluminium oxide γ - Al2О3 is formed by thermal decomposition of “hydroxide precursors”. The study proposes an explanation of the thermally induced phase transformations of porous alumina which is based on the statement about different crystallization rate of Al2O3 enriched with electrolyte anions and anion-free oxide. The interrelation between changes in micro- and macrostructures was determined by comparative fractal analysis of images of anodized samples before and after annealing. The calculation showed that the fractal dimension of the surface increased from 2.7621–2.8724 (initial samples) to 2.8193–2.8824 (after annealing). The process of annealing involves the dehydration of aluminium hydroxides accompanied by the destruction of primary crystallites, the formation of platelet particles, and an increase in the specifi c surface area. The interrelation between micro- and macrostructures is of great importance for understanding the mechanisms of the formation and interdependent transitions of aluminium hydroxides and aluminium oxides. The detailed study of these mechanisms will make it possible to obtain aluminium oxides with set properties.

Keywords

• anodic alumina
• alkaline electrolyte
• X-ray phase analysis
• colorimetry
• fractal analysis.