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 0.90 Ag 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 twophase 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 H 2 dissolving in or evolving from both coexisting 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.

Hydrogen in thin Pd-based layers deposited on reticulated vitreous carbon—A new system for electrochemical capacitors

“…3, the plots obtained in procedures (a) and (b) are similar in the regions of ␣ and ␤ phases but differ in the phase transition region, which for absorption series (solid symbols) is placed at a lower potential than for the desorption series (open symbols). This behavior corresponds to the effect of hysteresis well known for gas-phase measurements, where hydrogen pressure connected with the ␣-␤ equilibrium during absorption course is higher than during desorption course [1,7,51,52]. It is visible that the potential ranges of ␣-␤ and ␤-␣ transitions are almost identical for deposits on RVC (solid line in Fig.…”

Thermodynamic aspects of hydrogen electrosorption into Pd–Rh alloys

“…One of them assumes that the thermodynamic functions can be calculated separately for absorption and desorption processes [19,20,30,37,38,44]. The other approach assumes that only one of the curves, i.e., the absorption or (usually) desorption curve represents the true equilibrium [60,61], or the true equilibrium is represented by pressures/potentials located between absorption and desorption curves in the hysteresis region [38,50,59,[62][63][64][65].…”

Hydriding and structural characteristics of thermally cycled and cold-worked V–0.5at.%C alloy

“…Therefore, a similarity in the microstrain distribution between the cold-worked and thermally cycled hydrides was observed that applies to both the ␤-and ␥-phases. Birnbaum et al [27], Flanagan et al [28,29] and other researchers [61][62][63][64][65][66][67] proposed that metals and alloys undergo plastic deformations and dislocation creation both during hydride formation and decomposition. During the hydrogen absorption cycle, the ␤-phase is in a state of increasing compressive strain due to incipient formation of the ␥-phase [65].…”