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

In the present study, robust -Al2O3 specimens (144 mu m thick) are obtained by the heat treatment at 1400 degrees 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 10.2nm. After heat treatment at 1400 degrees C the samples lose their porous structure and certain crystallites with average size of 5-6 mu m 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 degrees C, producing -Al2O3. The second one takes place at around 1194 degrees C, which corresponds to the formation of -Al2O3. For the as-anodized and samples treated at temperatures below 1200 degrees C the band at 420nm can be attributed to the emission of OH-related and other impurities centers. The fluorescence at 460nm relates to emission of oxygen defect centers, such as F and F-2 centers, and sharp bands at 678 and 693nm indicate the formation of highly crystalline alumina. For -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.