UPHILL HYDROGEN DIFFUSION EFFECTS OF HYDROGEN INTERSTITIAL STRAIN GRADIENTS IN PALLADIUM AND PALLADIUM ALLOYS

Utilisations of palladium alloys as hydrogen purification membranes pr ovide technological background circumstances for a phenomenon known as ''Uphill Hydrogen Diffusion Effects''. The membranes utilised have be en in the form of. both tubes and sheets. In experimental procedures, permeating hydrogen is introduced from external surfaces and hydrogen permeation through membrane walls, is monitored by changes in either t he hydrogen pressure within the tubes or the electrode potentials of t heir opposite internal surfaces. For clear observations of Uphill Effe cts, both membrane surfaces have been coated with catalytically active palladium black. When membrane walls initially contain hydrogen, furt her increases of hydrogen contents at their outer external surfaces co uld produce initial reductions of hydrogen pressure or equivalent pres sures calculated from values of electrode potentials measured at oppos ite internal surfaces before the expected onset of gradually increasin g pressure; Such Uphill Effects could be satisfactorily explained in t erms of overall expansions of the interstitial sites by hydrogen havin g been responsible for corresponding strain gradients that extended fr om outer to inner surfaces and produced complementary Grosky Effect hy drogen transfers. Studies of Uphill Effects have now been extended to include measurements with various experimental arrangements and with d ifferently activated surfaces of membranes of palladium, nickel and ra nges of compositions of alloys of palladium with platinum, silver and cerium. Copyright (C) 1996 International Association for Hydrogen Ener gy.