Gamma-alumina membrane was prepared from anodic (amorphous) alumina (AA) obtained in a sulphuric acid electrolyte. The transformation
scheme, i.e., the crystallization to form metastable alumina polymorphs and the final transition to α-Al2O3 with heating was studied by TG-DTA and X-ray diffraction (XRD) using fixed time (FT) method. When heating at a constant rate,
the crystallization occurred at 900C or higher and the final formation of α-Al2O3 occurred at 1250C or higher, which temperatures were higher than the case of using anodic (amorphous) alumina prepared from
oxalic acid electrolyte. Relative content of S of the products was obtained by transmission electron microscope (TEM)-energy
dispersive spectroscopy (EDS). The proposed thermal change of anodic alumina membrane prepared from sulphuric acid is as follows:
1. At temperatures lower than ca 910C: Formation of a quasi-crystalline phase or a polycrystalline phase (γ-, δ- and θ-Al2O3);
2. 910–960C: Progressive crystallization by the migration of S toward the surface within the amorphous or the quasi-crystalline
phase, forming S-rich region near the surface;
3. 960C: Change of membrane morphology and the quasi-crystalline phase due to the rapid discharge of gaseous SO2;
4. 960–1240C: Crystallization of γ-Al2O3 accompanying δ-Al2O3; and
5. 1240C: Transition from γ-Al2O3 (+tr. δ-Al2O3) into the stable α-Al2O3.
The amorphization which occurs by the exothermic and the subsequent endothermic reaction suggests the incorporation of SO3 groups in the quasi-crystalline structure.