SYNTHESIS OF CERAMIC MEMBRANES .1. SYNTHESIS OF NON-SUPPORTED AND SUPPORTED GAMMA-ALUMINA MEMBRANES WITHOUT DEFECTS

Non-supported gamma-alumina films are prepared from a boehmite colloidal suspension. After calcination at 600-degrees-C, the microstructure is characterized by a mean pore diameter of 3 nm, a porosity of 50% and a tortuosity of 5.5. The structure is formed by card packed, plate-shaped particles, giving rise to slit-shaped pores. Supported gamma-alumina films, made by a slipcasting process using the same boehmite precursor, have a similar structure as the non-supported films. The slipcasting process is very sensitive to support characteristics and frequently yields defect films. Polyvinylalcohol (PVA) (molecular weight 72000 g mol-1) is added to the boehmite precursor (0.25 g g-1 film) to improve this process. This addition results in a less critical and better controllable drying and calcining procedure. The addition of PVA is necessary to slipcast defect free gamma-alumina films on supports with pore diameters of 0.4-mu-m or smaller and on multi-layer supports. It has no significant effect on the resultant microstructure of the gamma-alumina film, provided all PVA is removed by appropriate thermal treatment. The slipcasting rate is slower, resulting in thinner films of 3-5-mu-m at identical slipcasting times.