Coarse-grain alpha-alumina films with highly ordered porous structure

Abstract Application of porous anodic alumina as membranes and templates is restricted by its rapid degradation in acidic and alkaline media. Chemical stability of the anodic alumina could be strongly enhanced by annealing leading to crystallization of as-prepared amorphous material into a mixture of low-temperature Al2O3 polymorphic modifications and then to α-Al2O3 phase. Here an approach to choosing an annealing protocol for crystallization of porous anodic alumina into the corundum phase preserving its starting morphology is suggested. Formation of corundum films with hexagonally-packed 27-nm-diameter channels by multi-step annealing of anodic alumina prepared in 0.3 М sulfuric acid at 25 V is demonstrated. According to electron backscatter diffraction, α-Al2O3 films consist of porous single-crystalline grains with an average size of ∼5 ÷ 10 μm. Crystallization of α-Al2O3 phase from amorphous anodic alumina results in the enhancement of its chemical stability by two orders of magnitude in comparison with porous films consisting of low-temperature polymorphic modifications of Al2O3.