Hysteresis of pressure-composition and electrical resistance-composition isotherms of palladium–silver alloys–hydrogen system

In a gas phase study on the pressure (p)-composition (c) and the relative electrical resistance (R/Ro)-c relationships of Pd-5 and 9.9 at.% Ag–H systems, remarkable pressure and resistance hysteresis are observed between 298 and 323 K. On absorbtion, R/Ro in the (α+β) two-phase region showed a very small increase compared to the large increase reported earlier by the electrolysis method. A large resistance hysteresis, dissimilar to that of the electrolysis method is observed for the creation of lattice strain deformations accompanied of dislocations due to the β hydride formation. The resistance hysteresis is decreased with increasing temperature. For the Pd-5 at.% Ag–H system, no resistance hysteresis is appeared between 348 and 398 K. In this case, R/Ro value in the α phase region is increased abruptly up to αmax composition due to higher lattice strain deformation than that occurred at lower temperatures, which is created from the increment of lattice size. (H/M)αmax is appeared after a long holding but the absorbtion kinetics increases with increasing temperature. In the (α+β) two-phase region, R/Ro remains almost constant, and (R/Ro)(α+β) is decreased with increasing temperature. In this region, the dislocation densities for the α↔β transformations behave as equal in quantity and R/Ro is insensitive to small change in dislocations. The absence of resistance hysteresis is attributed to the fact that in the β phase region the plastic deformation created during the hydride formation is fully annihilated by the hydride decomposition. Presently observed (R/Ro)-c behaviors are analogous to that reported for the Pd–H system by the gas phase method.