Membrane-based separations are becoming increasingly more important in a variety of applications such as those in the biotechnology and energy production areas. As a result, the demand for high quality, defect-free membrane materials has grown, fostering a synergistic relationship between membrane synthesis and characterization. Within this relationship, membrane characterization has become a “feedback control,” thereby providing vital information about membrane quality that can be incorporated back into membrane synthesis. In this work, a series of metal oxide membranes were synthesized and characterized in an effort to optimize the synthesis process and gauge the performance of these membranes for potential ultrafiltration applications.

The metal oxides were synthesized using sol-gel chemistry and the selective metal oxide layers were deposited on commercially available symmetric and asymmetric gamma-alumina supports via spin-coating. During the first-pass synthesis process, gas permeation testing results showed both Knudsen and Poiseuille flow contributions to overall membrane permeance. This indicated the presence of defects within the metal oxide active layer. After several successive depositions of additional metal oxide layers on the gamma-alumina support, the Poiseuille flow contribution to total membrane permeance decreased. In general, the membrane effective pore size seemed to approach its nominal pore size as the number of metal oxide layer depositions increased. These results indicate defect healing with successive layer addition. Transient porosimetry analysis appeared to agree with these findings, indicating that the average pore size decreased with the addition of successive metal oxide layers. The porosimetry results also showed some narrowing of the pore size distribution with increasing number of metal oxide layers.

Overall, the deposition of additional active layers has proven to be effective for healing defects and reducing defect flow contributions to total membrane permeance. Further, membrane characterization proved its worth as a feedback control: the information garnered from gas permeation testing and transient porosimetry analysis proved to be instrumental for the future synthesis of quality membrane materials.