Large-scale effects of H 2O and O 2 on the absorption and permeation in Nb of energetic hydrogen particles

Abstract

The influence of chemically active gases (O 2 and H 2O) on hydrogen re-emission was investigated by and permeation through a Nb membrane acted upon by 20 keV H 2 + ions. Both the initial retention of ions after switching on the ion beam and the build-up of re-emission with time can be directly monitored with the new UHV apparatus by the changes of hydrogen pressure in the vacuum chamber in front of the membrane, since this pressure is determined by the incoming beam. In the absence of O 2 or H 2O, only a small portion of the flux absorbed penetrates the membrane, and the re-emission flux virtually equal to the flux of incident ions is established following the absorption of a relatively small dose. The admittance of O 2 or H 2O on the inlet side of the membrane continuously acted upon by the H 2 +-ion beam results in a dramatic increase of both the retention and the permeation of implanted hydrogen. With the pressures of O 2 and H 2O corresponding to about 0.03 O-atoms supplied to the sample surface per impinging proton, the time to reach the steady-state re-emission sharply rises, indicating a drastic increase of the dose absorbed. The steady-state re-emission flux is not changed appreciably, while the permeation flux, though increased by orders of magnitude, remains small compared to the incident ion flux. With greater pressures of the admitted gases (more than 0.1 O-atom supplied to the surface per proton), not only the sorptional capacity is increased, but also the permeation flux reaches up to the magnitude comprising the major part of the absorbed flux (i.e. superpermeability develops), and the re-emission flux correspondingly falls.