T. Kuji

Isobaric and isothermal hysteresis in metal hydrides and oxides

An analysis is presented of the thermodynamics of both isothermal and isobaric hysteresis, leading to equations for the calculation of the irreversible entropy production and the loss of free energy in hysteresis cycles of both kinds. The status of ‘apparent’ enthalpies derived from the temperature dependence of isothermal data is examined and two extreme types of behaviour are identified. An analysis of experimental data shows that the palladium–hydrogen system exhibits characteristics lying between these extremes. However, this does not seriously affect the use of either isothermal or isobaric data to derive the entropy production and free-energy loss. Existing published work on rare-earth metal–oxygen systems is also analysed, but no firm conclusions can be drawn.

Electrical resistance anomalies and hydrogen solubilities in the disorder-order system Pd3Mn

The maximum in electrical resistance is caused by a combination of two factors which affect the resistance oppositely, i.e., the scattering of electrons from the boundaries between the partially ordered domains and the disordered matrix increases resistance, and the growth and ordering of the domains decreases the resistance. Electron diffraction and TEM studies of samples quenched from temperatures corresponding to the maximum in resistance confirm that ordering occurs and that domains grow as resistance decreases.

The effect of isotopic substitution on the thermodynamic properties of palladium-hydrogen alloys

Abstract An attempt has been made to predict the thermodynamic properties of the Pd-D and Pd-T systems from those of the Pd-H system. By considering only the isotopic dependence of the optic modes, a satisfactory interpretation of the infinitely dilute solutions is not possible. However, using only the isotopic dependence of the optic modes together with an empirical equation for the infinitely dilute solution yields calculated results for the concentrated solutions in both the single and two phase regions which are in excellent agreement with the experimental results for the deuteride and tritide systems.

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

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, pf, 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 Pd0.90Ag0.10 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.

The thermodynamic properties of the niobium–hydrogen system measured by reaction calorimetry

Calorimetric enthalpies for reaction of hydrogen with niobium in the α‐, and (α+α’)‐, α’‐, (α’+β)‐, (α+β)‐, β‐ and (β+δ)‐phase regions are reported. The free energies and entropies are reported for those regions where the equilibrium pressures can be measured. The enthalpies for reaction with (1/2)H2(g) in the plateau regions for (α+β) and (α+α’) are −47.4 and −42.8 kJ/mol H, respectively. The magnitude of the calorimetric (absorption or desorption) enthalpy for the (β+δ) plateau region is 17.3 kJ/mol H which is about 3 kJ/mol H smaller than the literature value based on a van’t Hoff plot of the decomposition pressures. This difference arises because of the large hysteresis in this system and because of the irreproducibility in the plateau pressures; neither factor affects the present calorimetrically determined enthalpies.

Hysteresis in metal hydrides evaluated from data at constant hydrogen content: Application to palladium-hydrogen

Abstract Equations for the entropy production and free-energy dissipation for iso-hydrogen-content hysteresis are derived for metal hydrides. These values should be equal to those previously derived for isobaric, thermal and isothermal, pressure hysteresis. Experimental data for iso-hydrogen-content hysteresis are obtained for the palladium-hydrogen system, and these results confirm that, to within experimental error, the values of entropy production and free-energy dissipation are equal to the values from the other two methods of measuring hysteresis.

Hydrogen potential-temperature diagrams for the V-H, Nb-H and Ta-H systems

Abstract Hydrogen potential-temperature diagrams are presented for the V-H, Nb-H and Ta-H systems. The diagrams have been constructed from published thermodynamic data for the single-phase and two-phase regions in conjunction with the temperature-composition phase diagrams.

Hydrogen Solubility in Ordered and Disordered Palladium Alloys

A comparison is made of the solubility of hydrogen in ordered and disordered forms of palladium alloys. In Pd7Ce the solubility of hydrogen is greatest in the disordered form and in Pd3Mn and Pd3Fe the solubility is greatest in the ordered forms. In both Pd3Mn and. Pd7Ce there are more interstices surrounded by only palladium atom nearest neighbors in the disordered form than in the ordered, form. In Pd3Fe there are more such interstices in the ordered, form. There seems to be no simple pattern of behavior for the explanation of the solubility differences between the ordered and disordered forms of these alloys.

Temperature dependence of the chemical potential of hydrogen in the two-phase coexistence region of the palladium–hydrogen system

The chemical potential of hydrogen in the coexisting dilute α and hydride α′ phases of palladium–hydrogen has been examined as a function of temperature under conditions where there is essentially no hydrogen transferred between the gaseous and solid phases. The behaviour of ΔµH is governed by the redistribution of hydrogen between the two coexisting phases following the temperature change. If the chemical potentials of the two phases cannot become equal by transfer of hydrogen within the hysteresis gap, then the hydride phase must either form or decompose. This determines whether the system ends up within the hysteresis gap or on one or other of the plateau branches. This model is developed mathematically and confirmed by experimental results for the palladium–hydrogen system.