Hysteresis scans for Pd–H and Pd–alloy–H systems

Abstract

Hysteresis scans have been measured for Pd–H starting from both plateaux. Return point memory has been verified, i.e., a scan can always return to its starting point upon reversal. The symmetry of hysteresis scans with respect to a 180° rotation about the midpoint is pointed out for scans originating along a horizontal plateau. Scan behavior, which is not observed for Pd–H, has been found for some Pd alloy–H systems, i.e., starting from the desorption plateau, absorption scans level-off at plateau pressures, p_f, lower than for the preceding absorption plateau. This must be related to the cycling effect found for Pd–rich alloys where plateaux are observed to shift with cycling thereby reducing hysteresis. Cycling effects are shown here for the first time for the Pd_0.90Ag_0.10 and Pd–Rh alloys. A new way to measure hysteresis scans for metal–H systems has been employed for Pd–H by cooling or quenching at essentially constant r to within the hysteresis gap from above the two-phase critical temperature. Thus such a scan can commence from the midpoint of the hysteresis gap rather than from a plateau. Such scans appear to have a larger fraction corresponding to the Wagner mechanism than those starting from the plateaux where the Wagner mechanism refers to H₂ dissolving in or evolving from both co-existing phases. It is concluded from thermodynamic principles that states on either plateau cannot be at equilibrium because an equilibrium state must be “memoryless” and those on the plateaux are demonstrated to have “memories”, indeed, memory is the prime characteristic of hysteresis.