Measurements of hydrogen solubility and electrical resistance of some palladium–rhodium alloys by a gas phase technique

Abstract

The hydrogen solubility and the corresponding change in electrical resistance of Pd–5 and 10 at.% Rh alloys have been investigated by a gas phase technique. It was observed that low pressure hydrogen solubilities decreased with increasing Rh content, whereas, the increment of main plateau pressures showed opposite behavior. The α max and β min contents were increased with increasing Rh content but the widths of the miscibility gaps showed almost similar values. Higher hydrogen solubility was attained for higher Rh content alloy. It seemed that pressure hysteresis was increased with increasing Rh content but practically it does not, because the loss in useful available energy for the 10 at.% Rh–H system showed lower values than that for the 5 at.% Rh–H system. The ( R⧸ R 0) α max values are gradually increased with increasing temperature but decreased with increasing Rh content. During the initiation of β hydride, pronounced increments of R⧸ R 0 values are observed probably due to lattice deformation. However, extent of resistance increment due to β hydride initiation increased with increasing Rh content but decreased with increasing temperature. At higher temperature, a lowering of resistance occurred when the system entered from coherent state to incoherent state. In the main plateau region, change of R⧸ R 0 value is small but from H⧸ M contents about 0.55, a pronounced lowering of resistance was observed due to approaching of β phase structure to NaCl type. A remarkable resistance hysteresis appeared between 323–373 K, whereas almost equal R⧸ R 0 values were attained for the Pd–5 at.% Rh–H system at 398 K. It occurred probably due to the annihilation of plastic deformations during hydride decomposition which were created during β hydride formation. For the 5 at.% Rh–H system a larger resistance hysteresis appeared than that of the 10 at.% Rh–H system. A larger lattice expansion and concurrent creation of a higher lattice deformation in the β phase region of 5 at.% Rh–H system seemed responsible. At 348 K, an analogous R⧸ R 0 – c behavior was observed between the Pd–10 at.% Rh–H(D) systems.