Physico-chemical origin of superpermeability — Large-scale effects of surface chemistry on “hot” hydrogen permeation and absorption in metals

Abstract

Superpermeability, the absorption of amounts of hydrogen much exceeding the equilibrium concentration, dependence of permeability upon characteristics of metal surface (for membranes of any thickness) and other particular effects, are exhibited when a metal membrane is acted upon by non-equilibrium “hot” hydrogen. The reason for this is that the process of thermal evolution of the dissolved, or implanted, hydrogen involves the associative step to form the diatomic molecule on the metal surface. This step may become effectively slow because of the presence of the activation barrier resulting, for transition metals, from non-metal adlayers on the surface. The effects of metal surface chemical composition on hydrogen uptake and permeation are demonstrated experimentally on the membranes of palladium, niobium and austenitic stainless steel interacting with hydrogen molecules, thermal atoms and fast ions. In the case of fast ions, hydrogen permeation and absorption increase and re-emission decreases, by orders of magnitude, if chemically active gases (O 2, H 2O, H 2S or C 2H 2) are present in the vacuum System. The effect is accompanied by a deep minimum in the energy dependence of permeation and absorption corresponding to the maximum in the sputtering yield.