SYNTHESIS OF CERAMIC MEMBRANES .2. MODIFICATION OF ALUMINA THIN-FILMS - RESERVOIR METHOD

Modification of the internal surface of gamma-alumina films was realised by an impregnation method employing an aqueous metal salt-urea solution, followed by drying and reaction at 90-degrees-C. Loads of non-supported films were found to be much lower than those of supported films. The support acts as a reservoir, supplying its metal salt under appropriate conditions to the thin film. This results in high loads. A model analysis identified the ratio of the drying time to the reaction time as the critical parameter. With silver, as well as with magnesia, a long reaction time compared to the drying time proved to be favourable. This is best realised by performing the drying (at 40-degrees-C) and the reaction (at 90-degrees-C) sequentially. Silver loads of 70% of the pore volume were realised. A high degree of dispersion upon heating is maintained only under a reducing atmosphere. Up to 85% of the pore volume could be plugged with magnesia. This does not lead to a considerable pore size decrease however, due to the slit shape of the pores.