Hysteresis Behavior in Electric Resistance-hydrogen Concentration of Pd Thin Films

摘要

We report on hysteresis behavior in the electrical resistance-hydrogen concentration of Pd thin films. The variation of the electrical resistance has been investigated during the process of absorption and desorption of hydrogen gas (H_2) as a function of thickness of Pd thin films. The hysteresis behavior in the electrical resistance with H_2 concentration was found for Pd thin films and consists of a phase, alpha + beta phase, and beta phase regions. The sensitivity of Pd thin films with H_2 concentration was found to follow Sieverts' law in the a phase region. However, the sensitivity was observed to increase abruptly with H_2 concentration in the alpha + beta phase co-exist region. This is because Pd-H interaction is stronger in the beta phase than in the alpha phase and needs a higher concentration gradient as a driving force to desorb. The formation of the beta phase also was observed to cause the structural change because of the lattice expansion during absorption. The hysteresis height and the trace of structural change were affected by the thickness of the Pd film. As the film becomes thinner, the hysteresis height becomes lower and the amount of delamination on the surface becomes smaller. For films thinner than 20 nm in thickness, the delamination was not found but electrical resistance hysteresis was still observed.