Hysteresis of pressure-composition and electrical resistance-composition relationships of palladium–hydrogen system

The equilibrium hydrogen(deuterium) pressure and the relative electrical resistance, R/R0, as a function of H(D)/Pd content of Pd–H(D) systems have been investigated by a gas phase method. Measurements were carried out at 323, 333 and 343 K for Pd–H system, and 333 and 343 K for Pd–D system. R/R0, in the α phase region for the absorbtion process showed a large increase with increasing H(D)/Pd contents up to αmax composition compared with the small increase observed previously by the electrolytic charging method and gas phase method. Increase of R/R0 in this phase showed a strong relationship with the equilibrium pressures. About 200 h were required to achieve the equilibrium (H/Pd)αmax and (R/R0)αmax values at 323 K. The absorbtion kinetics gradually increased with increasing temperature. For Pd–D system slow kinetics was observed and the required time for equilibrium is about two times longer than those of Pd–H system. Nearly constant R/R0 was observed in the (α+β) two-phase region, instead of the steep increase reported for the electrolytic charging method. Pressure and R/R0 versus time study showed that previous electrochemical and gas phase studies were in quasi-equilibrium states. At a temperature 323 K, hysteresis was observed in the R/R0 versus composition plots for Pd–H system, whereas for 343 K such a phenomenon did not take place. For Pd–H(D) systems, R/R0–c relationships, in the α and (α+β) two-phase regions exhibited analogous behavior. A higher R/R0 value was observed for the Pd–D system than those of Pd–H system due to the lower zero point motion of deuterium compared with hydrogen.