Crystallization of anodic alumina membranes studied by simultaneous TG-DTA/FTIR

The thermal change of anodic alumina (AA), particularly the exothermic peak followed by the endothermic peak at ca 950 degreesC was studied in detail by mainly using simultaneous TG-DTA/FTIR. The gradual loss of mass up to ca 910 degreesC is attributed to dehydration. When heated at a constant rate by using TG-DTA, an exothermic peak with subsequent endothermic peak is obs erved at ca 950 degreesC, but the exothermic peak becomes less distinct wit h decreasing heating rate. It has been found that gaseous SO2 accompanying a small amount of CO2 is mainly discharged at this stage. The reaction in t his stage can be considered roughly in two schemes. The first scheme can be said collectively as crystallization, in which the migration of S or C tra pped inside the crystal lattice of the polycrystalline phase (gamma-, delta -, and theta -Al2O3, which presumably accompanies a large amount of amorpho us or disordered phase) occurs. In the second scheme, the initial polycryst alline (+amorphous) phase crystallizes into a quasi-crystalline gamma -Al2O 3-like metastable phase after amorphization. Conclusively, after the distin ct exo- and endothermic reactions, the amorphous phase crystallizes into ga mma -Al2O3, presumably accompanying small amount of delta -Al2O3. It is als o found that, when maintained isothermally, the metastable phases undergo t ransformation into the stable alpha -Al2O3 at 912 degreesC.