Supersaturation and hydride formation in the dilute phase of Pd–H and Pd–Mn–H alloys (original) (raw)
Related papers
Supersaturation and hydride formation in the dilute phase of Pd–H
2005
Annealed Pd exhibits supersaturation of the dilute phase before the hydride phase forms and this is characterized here by measurements of both | H H | and equilibrium p H 2. Dilute phase | H H | and p H 2 values were determined for annealed Pd for both absorption and desorption of H 2 and the | H H | were the same for both and the dilute phase H 2 isotherms were also the same, i.e., absorption/desorption was reversible. Pd, which has a high dislocation density, however, does not exhibit supersaturation, and its | H H | values are a continuous function of H/Pd in contrast to annealed Pd where there is a discontinuity at the phase boundary. In addition, absorption/desorption of H 2 is irreversible for this "cycled" Pd. The same phenomena were examined for several substitutional fcc Pd-Mn alloys; supersaturation was also observed for the three alloys: X Mn = 0.01, 0.05 and 0.075.
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, 2003
The solution of hydrogen and hydride formation in FCC substitutional solid solution Pd Rh Al alloys have been examined. In 0.9 0.12x x Ni Ni esis, complete equilibrium is not attained and there is not a absorption of hydrogen by Pd-Al alloys has been studied by Sakamoto et al. [9] who found that the plateau pressures increase because these are contracted fcc lattices compared to Pd and the H capacities decline markedly with Al (T.B. Flanagan). content such that there is no plateau for the Pd Al 0.90 0.10 0925-8388 / 02 / $ -see front matter
Thermodynamics of hydrogen absorption/desorption for Pd-group 6B alloys and some anomalous isotherms
Journal of Alloys and Compounds, 2001
The thermodynamics of Pd-Group 6B / H (D) systems have been investigated employing pressure-composition isotherms and for the Pd-Cr / H system, reaction calorimetry. The enthalpies for hydrogen solution and hydride formation decrease in magnitude with increase in X where M5Cr, Mo and W, and, because of this, the hydrogen solubilities at a given p in the dilute phase decrease with X . This M H2 M behavior is consistent with the decrease in the unit cell size with increase in X for these random solid solution fcc Pd-M alloys. M Repeated hydride formation and decomposition at a moderate temperature without intermediate annealing treatments causes the plateau pressures to change so as to decrease the extent of hysteresis. The group 6B elements-Pd alloys are of interest for H absorption because 2 they are the most effective in increasing the plateau pressures for a given atom fraction M.
Hydrogen-induced lattice migration in Pd–Pt alloys
Acta Materialia, 1998
AbstractÐInitially homogeneous, f.c.c. Pd±Pt alloys are shown to partially separate into Pd-rich and Pdpoor regions as a result of exposure to hydrogen gas at, e.g. 5.5 MPa, and at moderately high temperatures, e.g. 448 K. After this hydrogen heat treatment, HHT, the hydrogen can be removed by evacuation at T 448 K, leaving a metastable alloy which can then be analyzed for the degree of its phase separation. The presence of the phase separation is demonstrated primarily from changes of diagnostic hydrogen isotherms measured at a lower temperature before and after HHT, but also from Auger depth-pro®ling analysis, high resolution electron microscopy and small angle neutron scattering (SANS). After heating at the same temperature in the absence of the dissolved hydrogen, no evidence for phase separation is found. The X Pt =0.15, 0.20 and 0.25 alloys, which do not form hydride phases in their homogeneous forms at Tr 273 K, develop plateau pressure regions indicative of hydride formation in their diagnostic isotherms (r273 K) after HHT at 100 MPa (673 K). This is possible because signi®cant phase separation has occurred due to HHT resulting in a phase capable of forming a hydride at 273 K, i.e. X Pt <0.15. These alloys, which have undergone phase separation at 673 K, return to their initial, homogeneous states after annealing in vacuo at the same temperature, i.e. 673 K. This proves that there is no binary Pd±Pt miscibility gap for the temperatures and compositions examined. It is suggested, instead, that phase separation results from a ternary (Pd + Pd + H) equilibrium which causes hydrogen-induced segregation of the metal atoms relative to the initial, homogeneous alloy. The rates of recovery of the phase separated, metastable alloys back towards their homogeneous form are signi®cantly enhanced by the presence of smaller concentrations of dissolved hydrogen than required for the signi®cant phase separation. Thus at a moderate temperature, e.g. 673 K, large concentrations of dissolved hydrogen cause phase separation, whereas smaller concentrations enhance the return towards an intermediate, more homogeneous, state. #
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).
The interaction of dissolved H with internally oxidized Pd–Rh alloys
Acta Materialia, 2002
Homogeneous fcc Pd-Rh alloys have been internally oxidized in the atmosphere at several temperatures from 1023 to 1123 K forming oxide precipitates within a Pd matrix. As shown from electron diffraction patterns of the internally oxidized Pd 0.97 Rh 0.03 alloy, the oxide that forms is a mixed oxide, PdRhO 2 . Internally oxidized Pd-Rh alloys can be reduced with H 2 (573 to 623 K) forming PdRh precipitates within a Pd matrix. This represents a way of segregating components of a substitutional solid solution binary alloy. The segregated alloy can be returned to the homogeneous Pd-Rh alloy by annealing at an elevated temperature and thus, in contrast to the internal oxidation of, e.g., Pd-Al alloys, the process can be readily reversed. The oxidation and (reduction + H 2 O loss) were monitored from weight changes.
Hydrogen in disordered and ordered palladium alloys☆
International Journal of Hydrogen Energy, 1994
The solution of hydrogen in palladium-rich substitutional alloys has been of continuing interest since the pioneering studies of Thomas Graham on palladium-silver alloys, in the nineteenth century. The various palladium alloys exhibit different characteristics of hydrogen absorption compared with palladium (1, 2). Graham demonstrated considerable prescience in selecting the palladium-silver alloy system for his investigation of hydrogen absorption by a palladium alloy, since this alloy has subsequently proven to be the alloy of most interest.
Some microstructural aspects of hydriding internally oxidized Pd–Al alloys
Journal of Alloys and Compounds, 1999
Microstructural studies of internally oxidized Pd-Al alloys have been carried out using scanning electron microscopy. Pd nodules form by extrusion of Pd from the bulk while vacancies are transported from the surface during the formation of the internal oxide precipitates. The result is a dimpled honeycomb appearance in the subsurface region as seen along grain walls. Extensive intergranular cracking occurs during internal oxidation. When the internally oxidized alloys are hydrided / dehydrided, cracking is accentuated with transgranular cracks developing. Significant enhancement in the rates of hydriding and dehydriding have been observed after internal oxidation.