H. Noh

The effect of annealing pretreatment of Pd-Rh alloys on their hydrogen solubilities and thermodynamic parameters for H2 solution

Abstract Pressure-composition hydrogen isotherms in the temperature range 273 ⩽ T ⩽ 343 K have been determined for some Pd-rich Rh alloys with various degrees of homogeneity as a result of different cooling rates from above the miscibility gap. The following techniques have been employed to obtain different cooling rates: spin casting, fast quenching, slow quenching, furnace cooling (250 °C h−1) and prolonged annealing at 873 K; this order of cooling leads to an increasing extent of inhomogeneity. The dilute phase hydrogen solubilities are found to increase with increasing alloy inhomogeneity. The isotherms in the dilute region for the spin-cast alloy are the same as for the fast-quenched one. The partial molar enthalpy with ΔHHo at infinite dilution is less exothermic for the fast-quenched alloys as compared with the slow-quenched and furnace-cooled ones, but the enthalpy change for the plateau reaction, ΔHplat, does not change. Generally, for all these Pd-Rh alloys the hydrogen solubilities decrease with increasing Rh content and the relative partial molar enthalpy at infinite dilution, ΔHHo, and the enthalpy for hydride formation, ΔHplat, become less exothermic with increasing Rh content.

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.

A POSSIBLE ROLE FOR HYDROGEN-INDUCED LATTICE MIGRATION IN ALLOY MATERIALS PROCESSING

Abstract Evidence for hydrogen-induced lattice migration leading to rearranged lattices is given based on recent studies of palladium-rich alloys. In f.c.c. initially homogeneous Pd Rh, Pd Ni and Pd Pt alloys, hydrogen heat treatment (HHT) causes segregation. In Pd Rh alloys the segregation is clearly due to the miscibility gap which exists in this system; it takes place at both low hydrogen contents and high temperatures, approximately 873 K, and at moderate temperatures, approximately 473 K, and relatively high hydrogen contents. In the Pd Ni and Pd Pt alloys, however, segregation from HHT takes place only at moderate temperatures and relatively high hydrogen contents. In Pd Ni alloys it is shown that HHT at elevated temperatures promotes metal atom diffusion leading to the elimination of compositional variations in an initially inhomogeneous alloy. Dissolved hydrogen plays a dual role in some of these alloys, i.e. it catalyzes metal atom diffusion and also affects the equilibrium state.

Solubility and thermodynamics of hydrogen in homogeneous f.c.c. PdPt alloys

Abstract Hydrogen solubility data have been determined for homogeneous f.c.c. PdPt alloys. In comparison with pure Pd, alloying with Pt leads to a decrease in the dilute phase hydrogen solubility at a given p H 2 and an increase of the plateau pressures. This makes this alloy-H system an exception to the classification of Pd alloys with regard to their hydrogen absorption behavior as contracted or expanded; the former type of alloys have lower dilute phase solubilities and greater plateau pressures than Pd, and the latter have larger dilute phase solubilities and lower plateau pressures. The enthalpies for hydrogen solution at infinite dilution of hydrogen do not change much with X P1 but the corresponding entropies do change in such a way which would indicate that only interstices with six nearest neighbor Pd atoms are occupied. The enthalpies for hydride formation decrease in magnitude with X P1 .

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.

Hydrogen-induced segregation in PdPt alloys

Abstract In this research an initially homogeneous, f.c.c Pd Pt alloy, Pd 0.9 Pt 0.1 , is shown to segregate partially into Pd-rich and Pd-poor regions as a result of being subjected to a hydrogen pressure of 5.5 MPa at 448 K leaving the metastable segregated alloy. It can be returned to its initial homogeneous state by annealing at T ⩾ 673K. The presence of segregation resulting from the hydrogen treatment is demonstrated from the changes in the dilute phase hydrogen solubility and from Auger depth-profile analysis.

The effect of cycling through the hydride phase on isotherms for fcc Pd-rich alloys

Abstract The Pd–Ni, Pd–Co, Pd–Cr, Pd–Au, Pd–Rh, Pd–Mo and Pd–Pt fcc Pd-rich alloys have been found to exhibit large effects of cycling through the hydride phase. The effect of cycling Pd–Mo alloys is noticeable even at a composition of X Mo =0.02 whereas for Pd–Rh alloys it is not significant until X Rh >0.10. The dependence of the effect of the cycling on alloy composition will be reported here for the alloy systems Pd–Cr and Pd–Mo. The effect of cycling for a Pd–Ag alloy has also been determined. The temperature dependencies of the plateaux pressures of the cycled and the annealed alloy forms are reported for the Pd 0.95 Cr 0.05 alloy.

Hydrogen solubility in PdAl2O3 composites prepared by internal oxidation of PdAl alloys

Abstract Internally oxidized PdAl alloys have been studied at 323 K using p H 2 -composition isotherms. Internal oxidation of Pd 0.97 Al 0.03 alloys results in alumina precipitates within an essentially Pd matrix, i.e. internal oxidation results in a Pd Al 2 O 3 composite. The dilute hydrogen solubilities (323 K) in the composite are enhanced compared to annealed, pure Pd. Solubility enhancements have been measured after oxidations at 983 K, 1073 K, 1173 K and 1273 K for different times. Thermal residual stresses develop when the internally oxidized alloys are cooled from the high oxidation temperatures. The solubility enhancements are attributed to the presence of these residual stresses about the ceramic particles. The solubility enhancements for completely internally oxidized alloys are greater after lower, rather than higher, oxidation temperatures. Transmission electron microscopy indicates that alumina precipitates are nanometer-sized after oxidation at the lower temperatures and coherent with the Pd matrix. After oxidation at 1273 K, they precipitate as triangular prisms with their lengths being much greater than their sides while after oxidation at 1073 K, their sizes are smaller and their lengths are similar to their sides. Theoretical analysis of solubility enhancements about spherical and cylindrical precipitates reveal that the thermal stresses about the former are greater. The results of the analysis are consistent with the thermal residual stresses as the principal source of the experimental solubility enhancements.

Characterization of internal oxidation and permeability of oxygen in Pd/Al alloys using hydrogen isotherms

Eastman and Ruehle have shown that substitutional fcc Pd-Al alloys can be internally oxidized to produce Al{sub 2}O{sub 3} precipitates within a metal matrix. Emphasis in this study will be characterizing of internally oxidized alloys using hydrogen at contents r > 0.01; these are easily accessible by gas phase measurements which will be carried out to p{sub H{sub 2}} {le} 5 bar. The hydrogen isotherms of palladium and a Pd{sub 0.97}Al{sub 0.03} alloy both exhibit plateau pressure regions where a dilute and hydride phase co-exist. If complete internal oxidation takes place, it is expected that the isotherm for the Pd{sub 0.97}Al{sub 0.03} alloy will change to that for pure Pd. Partial oxidation should lead to two plateau pressures, one characteristic of the alloy and one of pure Pd and their relative breadths will be proportional to the fractions of each.

The disappearance of hysteresis for the hydride phase transition in palladium-nickel alloys

Hysteresis generally accompanies first order phase transitions which occur in the solid state and for this reason it is important. Its study has, however, been somewhat neglected. It is almost always present in the formation and decomposition of metal hydrides. The vanishing of hysteresis for such a system is noteworthy and this research is concerned with the disappearance. This paper reports that Pd-Ni alloys are reported to form a continuous series of fcc solid solutions over their entire concentration range. The present research concerns hydrogen absorption by several of these, initially homogeneous, Pd-rich alloys before and after they are subjected to a series of hydride formation and decomposition (cycling) phase transitions without intermediate annealing treatments. Pd-Ni alloys are unusual with regard to their hydrogen solubility behavior because the plateau pressures for hybrid formation/decomposition are greater than for pure Pd and the extents of these plateau regions do not markedly decrease with increase of % substituted metal, i.e. Ni. (The latter aspect of their behavior is more unusual than for former.) The increase in plateau pressure seems to be a characteristic of Pd substitutional alloys which have contracted volumes as compared to Pd itself.