Preparation of functionally gradient ceramic membrane substrates

A method for fabricating functionally gradient porous ceramic microstructures was conceived and investigated. Colloidally unstable or metastable suspensions of broad particle size distribution were prepared and slip cast to form ceramic discs. The suspensions were designed to promote segregation during consolidation and thereby produce the functionally 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 to prepare the slips. Dispersing agents were ammonium salts of polymethylacrylic acid (PMAA). A number of experiments were conducted varying the particle size distribution of the powders in the slips, and the amount of polyelectrolyte along with pH control. Slurry solids content was also investigated as a means to control the rate of segregation which occurred in the consolidating slips. Finally the temperature profile of the sintering operation which the green bodies were subjected to contributed to the ultimate microstructure of the ceramic disc. Ultimately it is desired to develop a process for making a substrate which is relatively tight-skinned on one side, thereby facilitating subsequent thin coating operations required for ceramic membrane making. Alternatively, the method can be refined to make microfiltration or ultrafiltration membranes in a single processing step.