DSC study of alumina materials — applicability of transient DSC (Tr-DSC) to anodic alumina (AA) and thermoanalytical study of AA

Thermo-analytical studies were performed on two types of porous anodic alumina (AA) membranes prepared from sulfuric acid. Both samples were 150 μm in total thickness; the first sample AA-1 consists of a 50 μm thick layer having 10 nm diameter pores and a 100 μm thick layer of 25 nm diameter pores, and the second sample AA-2 consists only of 25 nm pores. From the high temperature DSC run, the AA membranes as received were found to undergo dehydration up to 350°C where they exhibit a plateau, and at ca. 970°C, they yield a sharp exotherm immediately followed by a distinct endotherm. The apparent Cp values obtained by Tr-DSC at 350°C differed depending on the contact area of the samples with the sample pan or on the impurity content. The contact area depends on the pore diameter that is a function of applied voltage, and the impurity content similarly depends on the applied voltage. It is therefore presumed that Tr-DSC is advantageous in that it is a quick and handy method in obtaining apparent Cp as a parameter to identify samples differing in the properties which depend on the voltage applied at the preparation. In addition, the thermal changes obtained on the present AA membranes were found to be different from those obtained by oxalic acid known in the literature. That is, the present AA membranes exhibit a sharp exothermic reaction followed by a broad endothermic reaction apparently attributed to an amorphous to polycrystalline (AA → γ-+δ-Al2O3) transition, and, a exothermic reaction at ca. 1400°C, presumably due to the final transformation from the metastable ccp polycrystalline alumina to the most stable phase, i.e., the hcp α-Al2O3. The transformation temperatures are higher than those of AA prepared from oxalic acid by 150–300°C.