Fluorescence spectroscopic study of the phase transformation of γ-Al2O3 at high temperatures

The phase transformation of gamma-Al2O3 at high temperatures was characterized using X-ray diffraction (XRD) and fluorescence techniques. The results of a comparative fluorescence spectroscopic study of commercial and pure Al2O3 show that with laser excitation at 632.8 and 514.5 nm the absorption bands at 683.8 and 686.7 nm (R-line luminescence) are attributed to the presence of Cr3+ ions in octahedral coordination in a theta-Al2O3 environment. The other two bands at 692.8 and 694.4 nm (R-2 and R, lines) are attributed to Cr3+ fluorescence in an alpha-Al2O3 environment. Compared with XRD measurements, fluorescence spectroscopy has a higher sensitivity for identification of phase transformations in theta-Al2O3, and alpha-Al2O3 According to the different absorptions of Al2O3 with two laser excitations, the laser excitation at 514.5 nm is found to be more sensitive at the surface region while the laser excitation at 632.8 nm supplies information about the bulk. The fluorescence spectra together with UV-visible diffuse reflectance spectra and XRD patterns indicate that the gamma-Al2O3 starts transforming to theta-Al2O3 and alpha-Al2O3 simultaneously at 800 degrees C. However, the theta-Al2O3 and alpha-Al2O3 Compositions of the surface region and the bulk are different. The alpha-Al2O3 content at the surface region is higher than that in the bulk. The gamma-Al2O3 sample transforms to alpha-Al2O3 completely at 1250 degrees C. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.