The interaction of dissolved H with internally oxidized Pd–Rh alloys (original) (raw)
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The interaction of hydrogen with internally oxidized Pd alloys as illustrated by Pd–Fe alloys
Journal of Alloys and Compounds, 2003
Hydrogen interactions in internally oxidized Pd alloys have been illustrated using Pd-Fe alloys. Internal oxidation of Pd-Fe alloys results in composites consisting of fine dispersions of nano-sized Fe O precipitates in a matrix of pure Pd. Hydrogen solubility 2 3 measurements can be used as a probe to characterize the reversible and irreversible trapping at the metal-oxide interfacial regions. The effect of hydriding and dehydriding (cycling) on solubilities has been characterized. Hydrogen isotherms can be used to characterize the extent of internal oxidation, and understand the effect of the dispersed internal oxides on hysteresis and H capacity. .in (R. Balasubramaniam).
Insights into internal oxidation of binary and ternary Pd alloys using hydrogen solubilities
Journal of Alloys and Compounds, 2004
A Pd 0.97 Al 0.03 alloy was partially internally oxidized and the extent of its oxidation determined from the weight gain. Its hydrogen isotherm was measured which exhibited two plateau regions corresponding to the oxidized zone and the unoxidized alloy. The percent internal oxidation may be estimated from the relative lengths of the two plateaux. In order to determine the viability of this method to determine the % internal oxidation the isotherm of the partially internally oxidized alloy was compared to one for a sample consisting of a combination of a Pd 0.97 Al 0.03 alloy and a pure Pd specimen with similar percentages as those estimated from the plateau lengths. Since the plateau lengths are similar for the partially internally oxidized alloy and the mixture which shows that the fractions of internal oxidation based on the relative lengths of the plateaux are valid. In addition, the percent oxidation based on the plateau widths generally agree with those based on weight gains. The Pd 0.97 Al 0.03 alloy was partially oxidized to various extents and the % advancement of the interface agreed with a square-root time dependence. The internal oxidation of ternary alloys with two oxidizable solutes sometimes leads to two oxidation zones and sometimes to one. A ternary Pd 0.90 Rh 0.05 Al 0.05 alloy was partially internally oxidized after which three plateau regions were found corresponding to the unoxidized alloy, a zone where only the Al solute was oxidized and one where both Al and Rh were oxidized. The determination of two oxidation zones rather than one illustrates the usefulness of hydrogen solubilities for characterizing internal oxidation of ternary alloys because the weight gain after internal oxidation does not provide information about the internal oxidation of the individual solutes.
Journal of Alloys and Compounds, 2003
Isotherms have been measured for internally oxidized Pd-M alloys where M5Al or Y. After internal oxidation the alloys become composites of nanosized oxide precipitates within Pd matrices. Following the internal oxidation (1073 K) of the alloys H isotherms are 2 closely identical to those of Pd-H, however, differences develop after hydriding / dehydriding (cycling) which increase with %Al (Y). H 2 isotherms for the cycled forms of Pd and a Pd / alumina composite have been measured and compared at 323, 473, 513 and 553 K. At the latter two temperatures the characteristic plateaux appear to disappear for the cycled composite although hysteresis is still present indicating hydride formation / decomposition. Large differences are also observed between the isotherms for Pd and those for Pd composites at lower temperatures especially in the dilute phase and the two-phase region where conversion to the hydride phase is ,50%. Similar behavior is found for a Pd / yttria composite obtained from internal oxidation of a Pd Y alloy. The differences between H 0.98 0.02 2 isotherms for cycled Pd and internally oxidized (1073 K), cycled Pd-Al(Y) alloys are due to microstructural changes in the Pd matrix resulting from cycling in the presence of the small and closely spaced precipitates. Since internal oxidation at 1273 K results in larger and more widely-spaced precipitates than internal oxidation at 1073 K, it would be expected that after cycling the former alloys, their isotherms would be more similar to Pd-H than those internally oxidized at 1073 K. This has been confirmed experimentally. 2002 Published by Elsevier Science B.V.
Journal of Alloys and Compounds, 2000
It is shown that a convenient method for obtaining very fast rates of H absorption by Pd is by its oxidation or by the internal oxidation 2 of Pd-M alloys where M5a solute more readily oxidizable than Pd such as Al. While the oxidation of Pd enhances the rates it does not affect its H solubility because its oxidized layers are rapidly reduced upon exposure to H even at low temperatures. The internal 2 2 oxidation of Pd-M alloys, gives a matrix of pure Pd containing internal oxide precipitates, e.g., a Pd / alumina composite. The solubility of H in the Pd matrix should be equivalent to that in pure Pd.
On hydrogen solubilities in Pd/alumina composites prepared by internal oxidization of Pd/Al alloys
Journal of Alloys and Compounds, 2002
Internal oxidation of a Pd-Al alloy results in a composite consisting of Al O precipitates within a Pd matrix. In accord with earlier 2 3 studies (Huang et al., 1988. Scripta Met. 22, 1114, H dissolved in the Pd matrix becomes strongly and weakly trapped by the alumina precipitates. It is shown that the extent of strong H trapping is only weakly dependent on the Al content from X 50.005 to 0.06. H can be Al removed from the strong traps by evacuation at 573 K (2 h) but not at 473 K. After evacuation at 973 K, the strong traps themselves disappear but when the alloys are hydrided / dehydrided (cycled) at 323 K, traps reappear along with weak-dislocation traps due to dislocation formation resulting from the cycling. Although the former traps, which appear after cycling, are deep enough so that the corresponding P (equil) cannot be readily measured (273 K), they are not as deep as the initial ones produced after internal oxidation H 2 since H can be removed from them by evacuation at 323 K. The number of weak-dislocation traps due to cycling the composite formed from internal oxidation (1073 K) is greater than for cycled pure Pd. A higher temperature is needed to anneal out the weak-dislocation traps arising from cycling the composites as compared to Pd; thus the alumina precipitates hinder the movement and annihilation of the dislocations. The enthalpies for H solution in the dilute phase of the internal oxidation Pd-Al alloys have been determined directly by 2 reaction calorimetry and they have a spectrum of trapping enthalpies.
Effect of Hydrogen on the Internal Oxidation of a Pd–Cr Alloy in Dual-Atmosphere Conditions
Oxidation of Metals, 2022
The effect of hydrogen on oxygen permeability has been studied in a diluted Pd-Cr alloy in dual-and single-atmosphere conditions between 600 and 950 °C. The 0.3 mm thick Pd-1.5Cr foil was exposed in dry and humid air as well as in dualatmosphere conditions, with one sample surface being exposed to air and one to hydrogen, as encountered in solid oxide fuel cells. At all temperatures, Cr oxidized internally forming internal oxidation zones which were measured in metallographic cross sections. Below 800 °C, an external layer of PdO formed on the surface decreasing the internal oxidation kinetics. No measurable effect of hydrogen on the internal oxidation of Cr in Pd has been detected.
Journal of Alloys and Compounds, 2002
Pd-Y alloys have been internally oxidized in the atmosphere at T $973 K to form Pd-yttria composites. Dilute phase H isotherms and 2 reaction calorimetry have been used to characterize the trapping of dissolved H at the Pd-yttria interface. Although H isotherm 2 measurements have been used before for this purpose for other metal-oxide composites [1,2], this is the first report of reaction calorimetric measurements used to characterize H segregation to interfaces in Pd-based composites. The relative partial enthalpies for H 2 solution (303 K) in the internally oxidized Pd Y alloy decrease in magnitude with r5H / Pd, i.e., DH 5285 kJ / mol H, r→0, 260 0.98 0.02 H kJ / mol H, r50.004, 240 kJ / mol H, r50.0055 and 220 kJ / mol H, r50.0065. After evacuation at 303 K, the enthalpies were found to be DH 5235 kJ / mol H at r→0 and 215 kJ / mol H at r50.0025, i.e., some of the trapped H is not removed by evacuation (303 K). For H H 2 solution in well-annealed Pd, DH 5210.3 kJ / mol H at r50. The amounts of strongly and weakly trapped H at the interface are H determined for Pd-Y alloys internally oxidized at different temperatures. Internal oxidation takes place by the inward movement of the internal oxidation front [3]. In contrast to other Pd-M alloys which have been partially internally oxidized such as Pd-Al [2], the inner, unoxidized Pd-Y region becomes hydrided before the outer oxidized zone because of its lower p for hydride formation; thus there is H 2
Hydrogen-induced rearrangements in Pd-rich alloys
Journal of Alloys and Compounds, 1999
Homogeneous fcc Pd alloys have been found to undergo lattice rearrangements such as phase separation in the presence of dissolved H. For Pd-Rh alloys the phase-separated form is the thermodynamic stable state but for others which have been investigated, for example, Pd-Ni, Pd-Pt, the binary homogeneous substitutional solutions are the thermodynamically stable state. Phase separation for the latter alloys is a consequence of a ternary (Pd1M1H) equilibrium. The rearranged lattices are metastable after evacuation of the dissolved H at a temperature where the recovery back to its homogeneous state does not take place at a measurable rate. These H-induced changes differ from the lattice rearrangements which accompany hydride formation in metals and alloys where the hydride phases are very stable and which have different structures from the parent alloy, e.g. the formation of dihydrides in Zr or Zr-based alloys such as Zr-Nb (5 at.%). These very stable hydride phases will return to their initial states after removal of H at the required elevated temperatures. By comparison, H can be removed at moderate temperatures from the fcc Pd-based alloys because H is not very strongly held in the lattices. The metastable, H-free phase-separated Pd alloys return towards their homogeneous state after annealing in vacuo at, for example, 673 K, demonstrating that the phase-separated binary alloy is metastable. Results of H-induced changes in Pd-rich alloys are discussed for different ranges of temperature and hydrogen pressure for several Pd alloy systems. A ternary-phase diagram is calculated using known thermodynamic parameters for hydrogen solution in the homogeneous binary alloys and the thermodynamics of mixing of the metals. Using the (Pd1Pt) system as an example, a ternary (Pd1Pt1H) equilibrium is shown to cause phase separation.
Journal of Alloys and Compounds, 2005
Hydrogen has been successfully used as a probe to characterize internally oxidized Pd alloys. Hydrogen isotherms indicated enhanced hydrogen solubilities in the dilute phase region for internally oxidized Pd-Cr alloys. The microstructures of internally oxidized Pd-Cr alloys have been studied by transmission electron microscopy. In the case of IOed Pd 0.98 Cr 0.02 and Pd 0.96 Cr 0.04 alloys, TEM revealed large chromia particles, in the size range 100-200 nm, on high angle grain boundaries. The size of loops and precipitates was larger and their number density smaller compared to those present in the grain interior where there was a uniform distribution of precipitates and dislocation loops throughout the matrix. After hydrogen cycling the internally oxidized Pd 0.96 Cr 0.04 alloy, a high density of dislocations, in the form of dislocation cells, and sub-grain formation were observed. The microstructures were indicative of the high degree of cyclic stress induced during the hydrogen cycling process. The hydrogen solubility enhancement in internally oxidized Pd-Cr alloys have been co-related with the presence of dislocation loops.