Improvement of Hydrogen Absorption Characteristics of Pd Using Irradiation of Heavy Ions

The introduction of vacancies in palladium (Pd) was found to be effective for an increase in the initial hydrogen absorption reaction rate in a previous study. And it also has been reported that the initial hydrogen absorption reaction rate depends strongly on the surface conditions of metals. For the surface modification of materials, ion irradiation is known to be a quite useful method. Especially, heavy ions with keV ranges can create severe damage and high densities of vacancy near the surface of materials. As is further known, the formation of hydride phases can be facilitated by the presence of vacancy since vacancy acts as hydrogen trapping site to form hydrides. Therefore, the hydrogen absorption characteristics of Pd could be heavy ions with an energy range of keV such as chromium (Cr) ions. The Pd sample was irradiated with these ions at room temperature at TIARA (Takasaki Ion Accelerators for Advanced Radiation Application) of JAEA (Japan Atomic Energy Agency). The initial hydrogen absorption reaction rate of Pd was investigated before/after ion irradiations using an electrochemical method at room temperature. As a result, the initial hydrogen absorption reaction rate increase due to ion irradiation and the value became 3 ～ 10 times higher than un-irradiated Pd sample by ion irradiations. In order to obtain the information on electric charge of the hydriding process, the work function (contact potential) was also measured at several spots on the surface of Pd samples before/after ion irradiations by use of Kelvin probe. In this paper, the correlation among ion irradiation conditions, work function and initial hydrogen absorption reaction rate of Pd are reported.