Preparation of γ-Alumina Membranes From Sulphuric Electrolyte Anodic Alumina and Its Transition to α-Alumina

Abstract

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 900°C or higher and the final formation of α-Al₂O₃ occurred at 1250°C 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 910°C: Formation of a quasi-crystalline phase or a polycrystalline phase (γ-, δ- and θ-Al₂O₃);

2. 910–960°C: 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. 960°C: Change of membrane morphology and the quasi-crystalline phase due to the rapid discharge of gaseous SO₂;

4. 960–1240°C: Crystallization of γ-Al₂O₃ accompanying δ-Al₂O₃; and

5. 1240°C: 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.