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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.

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The thermodynamic parameters for solution of hydrogen at infinite dilution and the (a+B) plateau thermodynamic properties for hydrogen absorption at moderate temperatures for a series of Pd‐based f.c.c binary alloys with relatively low solute metal content have been correlated from the viewpoint of whether the alloy lattice is expanded or contracted to parent Pd lattice. Newly determined data for Pd‐Zn, Pd‐Sb, Pd‐Bi, Pd‐Cr, Pd‐Mo and Pd‐Mn alloys are included. When alloys with solute metals in the same groups are compared, the lattice parameters of Pd binary alloys with the 4 d and 5 d transition elements are, in general, larger than those with the 3 d elements. The relative partial molar enthalpy, ΔH0H, at infinite dilution becomes more exothermic with an increase of lattice parameter for the lattice‐expanded alloys with the exception of Pd‐Pt alloys. Conversely, for contracted alloys, with the exceptions of Pd‐Li, Pd‐Zn, Pd‐Al and low Ti content Pd‐Ti alloys, the enthalpy becomes less exothermic with increasing lattice contraction. The dilute phase solubility at the same temperature as reflected by the relative chemical potential of dissolved hydrogen at infinite dilution, Δμ0H, increases with increasing lattice expansion for the expanded alloys, with the exceptions of Pd‐Pt and Pd‐Nb (Ta), whereas for the contracted alloys, the solubility decreases with increasing lattice contraction, with the exception of the Pd‐Li alloys. The standard free energy change, ΔG0plat' for B‐hydride formation in the expanded alloys decreases with increase of lattice expansion with the exceptions of the Pd‐Pt, Pd‐Zr, Pd‐Sb, Pd‐Nb(Ta) and Pd‐Mn alloys. For the contracted alloys, the B‐hydride becomes less stable with increasing lattice contraction except for the Pd‐Li alloys. The isobaric hydrogen solubilities in Pd alloys at relatively high pressures may be attributed to the influence of the solute metal atoms on the Pd band structure and to the exclusion of H from site occupation due to solute atoms in the nearest neighbor shells; both of these effects are independent of the lattice expansion and contraction.

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H diffusion constants, D H , have been obtained from steady-state fluxes through Pd membranes with the downstream side maintained at p H 2 ≈ 0. Good linearity of plots of H flux versus (1/d), where d is the thickness, attests to H permeation being bulk diffusion controlled in this temperature (423-523 K) and p H 2 range (≤0.2 MPa). D H values have been determined at constant p up and also at constant H content. H fluxes through Pd membranes with three different surface treatments have been investigated (polished (unoxidized), oxidized and palladized) in order to determine the effects of these pre-treatments. The palladized and oxidized membranes give similar D H values but the polished membranes give values about 12% lower.

H diffusion constants have been determined from steady-state fluxes through Pd-Ag alloy membranes. The upstream side is maintained at a nearly constant pup (and consequently at a nearly constant rup=H/(Pd(1-x)Agx)) atom ratio, whereas the downstream side is at pH2 approximately 0 (rdown=0) (423-523 K). It is shown that the permeability is a maximum for atom fraction Ag, XAg=0.23 (423-523 K) at both pup=20.3 and 50.6 kPa. DH has been determined for some Pd-Ag alloys as a function of r in the dilute region, and it decreases with r even at small H contents for alloys with XAg<0.35. The concentration dependence of DH(cH) has been determined for the Pd0.77Ag0.23 alloy over a large concentration range. The effect of nonideality on DH(r) and ED(r) has been systematically determined as a function of XAg, where XAg is the atom fraction of Ag in the H-free alloy. (dDH/dr) increases with XAg up to XAg=0.35 and then changes from negative to positive at approximately 0.35. The activation energies for diffusion, ED(r), have been determined as a function of H content in the dilute range for several Pd-Ag alloy membranes, and the conversion to concentration-independent E*D values has been carried out in several different ways.

Hydrogen isotherms have been measured for a series of solid solution Pd-Au alloys in the temperature range from 393 to 523 K. Standard partial thermodynamic parameters at infinite dilution of H, DeltaH(H) degrees, and DeltaS(H) degrees, have been determined from these equilibrium data; both standard values for H(2) absorption become more negative with increase of atom fraction Au, X(Au). An interesting result is that the dilute phase isotherms at 423 and 523 K are all very similar for alloys with X(Au) = 0.15 to about 0.30 although their DeltaH(H) degrees and DeltaS(H) degrees differ. This is due to a compensating effect of the two thermodynamic parameters leading to (partial partial differentialDeltaG(H)/partial partial differentialr) = RT(partial partial differential ln p(1/2)/partial partial differentialr) approximately constant for the alloys from X(Au) approximately 0.15 to 0.30 at low r where r = H-to-metal atom ratio. Calorimetric enthalpies and isotherms at 303 K have been determined for a series of Pd-Au alloys over a range of H contents including, for some of the low Au content alloys, the plateau regions. These calorimetric data are the most complete reported for the Pd-Au-H system.