TEXTURAL EVOLUTION AND PHASE-TRANSFORMATION IN TITANIA MEMBRANES .2. SUPPORTED MEMBRANES

Nanostructural evolution and phase transformation in supported and uns upported titania membranes have been studied using Raman spectroscopy, X-ray diffraction (XRD) and field-emission scanning electron microsco py (FE-SEM). Densification of unsupported membranes started at ca. 450 -degrees-C and reached more than 97% density at 600-degrees-C, whereas the supported membranes had a density of only ca. 70-75% even at 700- degrees-C when calcined for 8 h. At 700-degrees-C the average crystall ite size of supported and unsupported membranes was ca. 20 and 70 nm, respectively. This behaviour is primarily attributed to the decrease i n the driving force for sintering due to the stress developed during t he constrained sintering of a film attached to a rigid support and to the inhibition of the reorganization process within the film, resultin g in lower coordination numbers in supported membranes. Supported memb ranes showed a higher transformation temperature (slower rate of trans formation) than did the unsupported. Supported and unsupported membran es, calcined for 8 h, transformed to ca. 90% rutile (calculated from R aman spectrum) after calcination at 850 and 650-degrees-C, respectivel y. This difference in phase transformation behaviour is attributed pri marily to the large stress which is developed in a constrained environ ment owing to the negative volume change during the anatase-rutile tra nsformation.