HYSTERESIS BEHAVIOR OF ELECTRICAL-RESISTANCE OF THE PD-H SYSTEM MEASURED BY A GAS-PHASE METHOD

The hysteresis behaviour of electrical resistance during the absorptio n-desorption processes in the alpha-, (alpha + beta)- and beta-phase r egions of the Pd-H system, respectively, have been investigated at 323 K by a gas-phase method. A small extent of hysteresis in relative ele ctrical resistance R/R(0) over the cyclic absorption-desorption proces ses in the alpha single-phase region has been observed, showing slight ly larger R/R(0)-values for desorption processes than for absorption p rocesses, where R(0) is the initial resistance of a hydrogen-free samp le. In cyclic desorption-absorption sews commencing from the absorptio n branch of p-c isotherms in the alpha + beta two-phase plateau region , similar hysteresis loops to those of p-c isotherms have been observe d for plots of R/R(0) versus [H]/[Pd]. On the other hand, for cyclic d esorption-absorption scans starting from absorption pressures, corresp onding to hydrogen concentrations near the beta(min)-phase boundary co mposition, R/R(0)-values do not return to those of desorption plateau in the alpha + beta two-phase region. Values of relative resistance ov er subsequent desorptions from absorption pressures with pH(2) > 4.7 x 10(-2) MPa decrease gradually with decreasing H content down to beta( min)-phase boundary compositions and exhibit almost the same values as those observed for the absorption processes and, on entering the alph a + beta two-phase region, the R/R(0)-values remain almost constant wi th decreasing [H]/[Pd], i.e. at (R/R(0))((alpha+beta)) similar or equa l to 1.73 at 323 K. The large hysteresis of resistance relationships c an be attributed to creations of lattice strains accompanied by disloc ation formation arising from alpha --> beta-phase hydride transitions and by further highly dissolved hydrogen in the beta-phase region.