SUBJECT: Ph.D. Dissertation Defense
BY: Logan McLeod
TIME: Tuesday, October 28, 2008, 10:00 a.m.
PLACE: Love Building, 311
TITLE: Hydrogen Permeation Through Microfabricated Palladium-Silver Alloy Membranes
COMMITTEE: Dr. Andrei Fedorov, Co-Chair (ME)
Dr. Levent Degertekin, Co-Chair (ME)
Dr. J. Rhett Mayor (ME)
Dr. Meilin Liu (MSE)
Dr. William Koros (CHBE)


Energy efficient purification of hydrogen is an important technological challenge. Palladium-alloy membranes are particularly suited to this problem due to their high hydrogen permeability, thermal stability, and virtually infinite selectivity. In current systems hydrogen flux is observed to be inversely proportional to membrane thickness which is indicative of the interstitial diffusion mechanism of hydrogen permeation. This observation, along with the high cost of palladium, has motivated continuous efforts to decrease membrane thickness. Theoretical modeling of membrane performance predicts that as membrane thickness continues to decrease, eventually the permeation rate will no longer be limited by diffusion through the bulk Pd but will become limited by desorption from the permeate surface. If it exists, this is a vital transition to pinpoint due to the fact that below this thickness membrane operating conditions will have a drastically different effect on performance and no further performance enhancements will result from further decreasing thickness. A handful of experimental results in the open literature contradict these modeling predictions. A new model is developed considering the non-ideal behavior of hydrogen solution into metals to explain these contradictions. Additionally, it has been demonstrated that hydrogen permeation through bulk Pd depends on membrane microstructure, making deposition conditions and post-deposition thermal treatment important issues for repeatable performance. The interplay of these issues on the performance limitations of ultra-thin hydrogen separating membranes will be experimentally investigated. It is demonstrated that the hydrogen permeation behavior of sub-micrometer thick Pd-alloy membranes exhibits diffusion limited behavior in the context of the new model. The microstructure evolution during annealing is characterized and the impact of grain size on hydrogen permeation rate is determined.