High-temperature annealing of porous anodic aluminium oxide prepared in selenic acid electrolyte

Abstract

Selenic acid electrolyte provides porous anodic aluminium oxide (AAO) films with low porosity and high optical transparency. Here, the high-temperature behaviour of AAO obtained in a selenic acid electrolyte is reported for the first time. The crystallization of as-prepared amorphous AAO at about 800 °C is accompanied by the elimination of electrolyte impurities, an increase in pore diameter from 15 to 21 nm, and an increase in specific surface area to 55 m2·g−1 due to the mesopore formation inside the cell walls. The second crystallization stage at about 1150 °C results in the formation of coarse-grained corundum (α-Al2O3) films with a mean grain size of about 11 μm. Each of the grains consists of regularly arranged 27-nm-diameter pores, which are stable up to 1300 °C. AAO physicochemical properties depend on the annealing temperature. In particular, chemical stability is up to three orders of magnitude higher after two-step crystallization. Optical properties and thermal expansion of selenic-acid-made porous AAO films with different phase composition are quantified. The crystallization of as-prepared AAO to low-temperature Al2O3 polymorphs has a minor influence on optical transmittance and effective refractive index of AAO, whereas the coarse-grained structure of corundum causes an order of magnitude decrease in average transmittance in the visible region. The coefficient of thermal expansion increases by 25% to 8.5 × 10−6 K−1 for α-Al2O3 porous films compared to as-prepared AAO.