Assessment of the thermal stability of anodic alumina membranes at high temperatures

The thermal stability of anodic alumina membranes (AAMs) annealed in air from 750 degrees C up to 1100 degrees C was investigated. AAMs were produced by single-step anodising of laminated AA1050 in 0.30M oxalic acid medium. The barrier layer provided thermal stability to the membranes, since it avoided or minimized bending and cracking phenomena. X-ray diffraction (XRD) analyses revealed that as-synthesized AAMs were amorphous and converted to polycrystalline after heat-treating above 750 degrees C. However, porous and barrier layers did not re-crystallize in the same way. The porous layer mainly crystallized in the gamma-Al2O3 phase within the range of 900-1100 degrees C, while the barrier layer was converted to the alpha-Al2O3 phase at 1100 degrees C. Different grain sizes were also estimated from Scherrer's formula. Scanning electron microscopy (SEM) images pointed out that cell wall dilation of the porous layer explained membrane cracking, which was avoided in presence of the barrier layer. (C) 2008 Elsevier B.V. All rights reserved.