Measurements of the Pressure-Concentration and Pressure-Resistance Temperature Relations of Thin Films of the Palladium-Hydrogen System.

The pressure-concentration isotherms (PCT) of thin films (thicknesses less than 1000(ANGSTROM)) of the palladium-hydrogen system were measured with a novel adaptation of the volumetric technique. The results are comparable with bulk data being steep in the dilute ((alpha)) and saturated ((beta)) phases and flat in the mixed phase region. We find that the maximum solubility decreases with decreasing film thickness. The volumetric results are also compared with those obtained by the quartz crystal microbalance technique. Also studied were resistivity measurements, which show that the maximum fractional resistivity change from hydrogen addition decreases with decreasing film thickness and deposition substrate temperature. The results can be explained with a simple two-mechanism model by considering the resistivity increase due to the increased scattering centers from hydrogen addition, and the resistivity decrease due to the grain expansion from hydrogen addition. We suggest that electron tunnelling between grains could be responsible for the second mechanism. We also find a thickness dependence on the number of absorption-desorption cycles required to obtain stabilized PCT and resistivity relations. We attribute this to a constraint due to the presence of the substrates.