J.D. Clewley

Hysteresis in metal hydrides

Abstract A detailed model for hysteresis is presented for metal hydride systems based on the requirement for dislocation production during both hydride formation and hydride decomposition. A quantitative illustration of the model is presented using the regular interstitial solution approximation. The relationship between the solvus hysteresis and pressure hysteresis is given. The model predicts that calorimetrically measured enthalpies for hydride formation and decomposition will not exhibit any differences arising from hysteresis but that the enthalpies determined from the temperature variations of the plateau pressures for hydride formation and decomposition will show differences due to hysteresis.

The effect of lattice defects on hydrogen solubility in palladium: I. Experimentally observed solubility enhancements and thermodynamics of absorption

Abstract Measurable, and reproducible solubility enhancements have been observed for hydrogen in cold-worked palladium in the low content α-phase (273–363 K). It is shown that the solubility of hydrogen is unaffected in palladium samples which had been quenched from near their melting points. It is concluded that the stress field about the dislocation array is the principal cause of the solubility enhancement. Absorption of hydrogen at the cores of dislocations is believed to be insignificant. From absorption isotherms measured at various temperatures, the relative partial molar enthalpies and entropies of solution of hydrogen into deformed palladium have been determined. The heat of solution of hydrogen by 78% deformed palladium is approximately 880 J/H more exothermic than for well-annealed palladium in the low hydrogen content α-phase.

The effect of the α-β phase change on the α phase solubility of hydrogen in palladium

Abstract Following the α-β phase change in palladium, there is an enhanced solubility of hydrogen at a given equilibrium pressure within the low hydrogen content α phase. There are marked similarities between the absorption isotherms for hydrogen following the phase change and those exhibited by heavily cold-worked palladium. Partial thermodynamic parameters for hydrogen absorption are reported for palladium plastically deformed by the phase change.

Hydrogen-induced phase separation in PdRh alloys

The principal result of this research is the demonstration that phase separation takes place in RdRh alloys at moderate to elevated temperatures in the presence of H2: it occurs to a much lesser extent under similar conditions but in the absence of hydrogen. The main technique employed for determining whether or not lattice changes have occurred are ‘diagnostic’ isotherms measured before and after exposure to hydrogen. These isotherms are measured at a much lower temperature than those employed for the hydrogen heat treatment (HHT). This diagnostic technique has the advantage that it is more sensitive than other methods to lattice changes in alloys: however, it has the disadvantage that the nature of the changes must be inferred. Direct evidence for segregation following HHT in these alloys has been obtained from electron microprobe analysis via a wavelength-dispersive spectrometer. X-ray diffraction revealed that the original f.c.c. reflections of the XRh = 0.20 alloy were broadened after HHT so as to suggest the presence of two sets of f.c.c. reflections. It is suggested that the segregation corresponds to phase separation according to the PdRh binary phase diagram, at least in the elevated temperature range employed for HHT. 673 to 873 K, where the hydrogen solubility is small and where consequently the binary phase equilibrium should not be perturbed by the small amounts of dissolved hydrogen. The lattice changes were more marked in the presence of hydrogen than in its absence in all of the experiments which were carried out. This research demonstrates the potential utility of employing H-induced changes for phase diagram determination of Pd alloys and possibly for other alloy systems, e.g. Ni-based alloys.

Hydrogen-induced lattice migration in Pd-Pt alloys

It was recently found that hydrogen-induced segregation takes place in initially homogeneous, fcc Pd-Rh and Pd-Pt alloys. From the results for the Pd-Rh alloys it was established that dissolved hydrogen induces metal atom diffusion because phase separation is very slow in its absence at the same temperatures. For the Pd-Rh alloys the driving force for the segregation is the miscibility gap which exists in the binary Pd-Rh system. In the case of the Pd-Pt alloy system it is not clear whether the driving force for the segregation is a miscibility gap which has been postulated, but not experimentally verified, or else a result of a ternary (Pd + Pt + H) equilibrium. This research attempts to resolve this question.

Thermodynamics and isotope effects of the vanadium–hydrogen system using differential heat conduction calorimetry

The V–H (D) systems have been investigated using reaction calorimetry at 298 and 323 K over hydrogen contents from H (D)/V=0→≊1.8. Relative partial molar enthalpies have been determined by absorption of small increments of hydrogen in the single phase regions; for regions where two solid phases coexist, integral or plateau enthalpies are obtained by absorption (desorption) of either large or small amounts of hydrogen. The following calorimetric results were obtained for the two solid phase plateau regions: ‖ΔH‖=40.6 kJ/mol H, 40.8 kJ/mol D for (α + β) and ‖ΔH‖=19.1 kJ/mol H and 23 kJ/mol D for (β’+γ) and (α’+e), respectively. Calorimetric data, relative partial molar enthalpies are also given for the single phase regions. Several phase boundaries are intersected as the hydrogen contents increase in this temperature range and the behavior of the enthalpies at the phase boundaries has been determined and is discussed in relation to the nature of the coexisting phases.

Enhanced rates of hydrogen absorption resulting from oxidation of Pd or internal oxidation of Pd-Al alloys

The goal of this research was the determination of the relative rates before and after internal oxidation of Pd--Al alloys and oxidation (Pd) and this is independent of whether heat transfer is the rate-limiting step for the internally oxidized Pd--Al alloys rather than a more fundamental step.

The thermodynamics of the ErFe2H system using differential heat conduction calorimetry

Abstract A differential twin-cell heat flow calorimeter has been constructed for the measurement of the enthalpy changes of the reaction of hydrogen gas with intermetallic compounds as a function of their hydrogen contents. It was designed especially for use with those intermetallic compounds which form very stable hydride phases; therefore prior to the calorimetric studies it must be possible to evacuate the sample in situ at elevated temperatures to remove all of the hydrogen resulting from the activation procedure. The calorimeter was employed in this research for the determination of the thermodynamics of the reaction of H 2 with ErFe 2 H x over the range of hydrogen contents x = 0.4. ErFe 2 H is a multiplateau system; the enthalpy change for hydride formation corresponding to the plateau near x = 3 is more exothermic than that corresponding to the preceding plateau near x = 2. In addition to the calorimetric determination of the enthalpies of reaction, the free energies were determined simultaneously from the equilibrium hydrogen pressures. Using these free energy and enthalpy data the entropies were also obtained over a range of hydrogen contents.

Thermodynamic properties of hydrogen and deuterium dissolved in palladium at low concentrations over a wide temperature range

The equilibrium pressures of the Pd/H2 and Pd/D2 systems have been measured in the temperature range from –100°C to 350°C for values of H(D)-to-Pd, atomic ratio, in the approximate range from 0.0005 to 0.005. These data were obtained in an ultra-high vacuum apparatus with large samples of low surface area. Partial thermodynamic data have been obtained from these equilibrium data; a significant temperature dependence is reported for the relative partial molar enthalpies and entropies.

Thermodynamic characterization of the Zr-Mn-H system part 1. Reaction of H2 with single-phase ZrMn2+χ C-14 Laves phase alloys

Abstract The reaction of hydrogen with ZrMn 2+χ has been investigated using reaction calorimetry for χ = −0.2, 0, 0.5 and 1.0. The enthalpies for hydride formation and decomposition are equal in magnitude over the two-solid-phase region; their magnitudes decrease with increasing χ. Hysteresis is large for all these intermetallics, approximately 3.0 kJ (mol H) −1 , and independent of χ.