PREPARATION OF FUNCTIONALLY GRADIENT CERAMIC MEMBRANE SUBSTRATES

A method for fabricating functionally gradient porous ceramic microstr uctures was conceived and investigated. Colloidally unstable or metast able suspensions of broad particle size distribution were prepared and slip cast to form ceramic discs. The suspensions were designed to pro mote segregation during consolidation and thereby produce the function ally gradient cross-section. Preliminary results pertain to slip cast discs of alpha-alumina. A number of different size samples of alumina powder ranging in mean diameters from 0.2 mu m to 3.4 mu m were used t o prepare the slips. Dispersing agents were ammonium salts of polymeth ylacrylic acid (PMAA). A number of experiments were conducted varying the particle size distribution of the powders in the slips, and the am ount of polyelectrolyte along with pH control. Slurry solids content w as also investigated as a means to control the rate of segregation whi ch occurred in the consolidating slips. Finally the temperature profil e of the sintering operation which the green bodies were subjected to contributed to the ultimate microstructure of the ceramic disc. Ultima tely it is desired to develop a process for making a substrate which i s relatively tight-skinned on one side, thereby facilitating subsequen t thin coating operations required for ceramic membrane making. Altern atively, the method can be refined to make microfiltration or ultrafil tration membranes in a single processing step.