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Structural and Fluorescence Studies of Polycrystalline α‐Al2O3 Obtained From Sulfuric Acid Anodic Alumina

In the present study, robust α‐Al2O3 specimens (144 µm thick) are obtained by the heat treatment at 1400 °C of free‐standing sulfuric acid anodic alumina. Scanning electron microscopy analysis show that the as‐anodized anodic alumina films possess well‐ordered porous structure with the pore diameter of about 10.2 nm. After heat treatment at 1400 °C the samples lose their porous structure and certain crystallites with average size of 5–6 µm can be observed. These crystallites can be visualized by fluorescence imaging. According to differential scanning calorimetry data accomplished by X‐ray analysis, the first step of crystallization occurs at around 967 °C, producing γ‐Al2O3. The second one takes place at around 1194 °C, which corresponds to the formation of α‐Al2O3. For the as‐anodized and samples treated at temperatures below 1200 °C the band at 420 nm can be attributed to the emission of OH‐related and other impurities centers. The fluorescence at 460 nm relates to emission of oxygen defect centers, such as F and F2 centers, and sharp bands at 678 and 693 nm indicate the formation of highly crystalline alumina. For α‐Al2O3, its fluorescence is caused by both surface defects and oxygen defect centers. The fluorescence spectroscopy can be applied as a cheap, fast, nondestructive method for the identification of amorphous alumina crystallization.