The thermal change of anodic alumina (AA), particularly the exothermic peak followed by the endothermic peak at ca 950C was
studied in detail by mainly using simultaneous TG-DTA/FTIR. The gradual loss of mass up to ca 910C is attributed to dehydration.
When heated at a constant rate by using TG-DTA, an exothermic peak with subsequent endothermic peak is observed at ca 950C,
but the exothermic peak becomes less distinct with 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 this 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 trapped inside the crystal lattice of the polycrystalline
phase (γ-, δ-, and θ-Al2O3, which presumably accompanies a large amount of amorphous or disordered phase) occurs. In the second scheme, the initial
polycrystalline (+amorphous) phase crystallizes into a quasi-crystallineγ-Al2O3-like metastable phase after amorphization. Conclusively,after the distinct exo- and endothermic reactions, the amorphous
phase crystallizes intoγ-Al2O3, presumably accompanying small amount of δ-Al2O3. It is also found that, when maintained isothermally, the metastable phases undergo transformation into the stable α-Al2O3 at 912C.