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Isotherms have been measured for internally oxidized Pd–M alloys where M=Al 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 H2 isotherms are closely identical to those of Pd–H, however, differences develop after hydriding/dehydriding (cycling) which increase with %Al (Y). H2 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 Pd0.98Y0.02 alloy. The differences between H2 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. [Copyright &y& Elsevier]

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 Pd0.97Rh0.03 alloy, the oxide that forms is a mixed oxide, PdRhO2. Internally oxidized Pd–Rh alloys can be reduced with H2 (573–623 K) forming PdRh precipitates within the Pd matrix. This is a technique for 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+H2O loss) were monitored from weight changes.After internal oxidation/reduction “diagnostic” hydrogen isotherms (323 K) were measured to follow the extent of oxidation and to determine the extent of H trapped at the internal interfaces from the positive intercepts along the H/Pd axis in the dilute phase. The H capacities in the steeply rising hydride region of the H2 isotherms for the internally oxidized alloys are smaller than for Pd unless the Pd in the PdRh precipitates is considered to be inactive for H2 absorption for the calculation of H/Pd values.In the two-phase region the absorption plateau pressures were significantly greater than for Pd–H, however, they were much closer to the Pd plateau pH2 than to that of the unoxidized Pd–Rh alloys. The desorption plateaux were very close to those of Pd–H for XRh=0.01 to 0.05 alloys and therefore it can be concluded that the matrix is pure Pd whose absorption plateaux are affected by the presence of the precipitates.The techniques of TEM, SEM and SANS (small angle neutron scattering) were employed to examine the alloys after internal oxidation both before and after reduction. SANS confirmed directly that internal precipitates form from the internal oxidation and that they are rather large with an appreciable fraction having diameters greater than 100 nm. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The influence of hydrogen as an agent to accelerate the thermal recovery of cold-worked palladium has been investigated. The techniques used to characterize the effects of hydrogen on the thermal recovery of palladium were hydrogen solubility and mechanical property measurements. Results show that the presence of modest amounts of hydrogen during annealing of cold-worked palladium does enhance the degree of thermal recovery, with a direct correlation between the amount of hydrogen during annealing and the degree of recovery. The results indicate that the damage resulting from cold-working palladium can be more effectively and efficiently reversed by suitable heat treatments in the presence of appropriate amounts of hydrogen, as compared to heat treatment in vacuum. The somewhat novel technique of using changes in the hydrogen solubility of palladium as an indicator of thermal recovery has been validated and complements the more traditional technique of mechanical property measurements. [ABSTRACT FROM AUTHOR]

Ni-Nb-Zr amorphous membranes, prepared by melt-spinning, show great potential for replacing crystalline Pd-based materials in the field of hydrogen purification to an ultrapure grade (>99.999%). In this study, we investigate the temperature evolution of the structure of an amorphous ribbon with the composition Ni32Nb28Zr30Cu10 (expressed in atom %) by means of XRD and DTA measurements. An abrupt structural expansion is induced between 240 and 300 °C by hydrogenation. This structural modification deeply modifies the hydrogen sorption properties of the membrane, which indeed shows a strong reduction of the hydrogen capacity above 270 °C. [ABSTRACT FROM AUTHOR]

Abstract: The glass transition and crystallization kinetics of melt-spun Ni60Nb20Zr20 amorphous alloy ribbons have been studied under non-isothermal and isothermal conditions using differential scanning calorimetry (DSC). The dependence of glass transition and crystallization temperatures on heating rates was analyzed by Lasocka''s relationship. The activation energies of crystallization, E x , were determined to be 499.5kJ/mol and 488.6kJ/mol using the Kissinger and Ozawa equations, respectively. The Johnson–Mehl–Avrami equation has also been applied to the isothermal kinetics and the Avrami exponents are in the range of 1.92–2.47 indicating a diffusion-controlled three-dimensional growth mechanism. The activation energy obtained from the Arrhenius equation in the isothermal process was calculated to be E x =419.5kJ/mol. The corresponding three dimensional (3D) time–temperature–transformation (TTT) diagram of crystallization for the alloy has been drawn which provides the information about transformation at a particular temperature. In addition, the intermetallic phases and morphology after thermal treatment have been identified by X-ray diffraction (XRD) and scanning electron microscope (SEM). [Copyright &y& Elsevier]

Abstract: Amorphous Ni-based alloys show great promise as inexpensive, hydrogen-selective membrane materials. One of the main barriers to the commercial uptake of these materials is their tendency to crystallize during a long-term operation at elevated temperatures, a process which decreases both strength and permeability. In this study, the crystallization kinetics of a series of Ni100−x Zr x (where x = 36.3 or 70 at.%) and (Ni0.6Nb0.4)100−x Zr x (where x = 0, 10, 20 or 30 at.%) amorphous alloy membranes were examined by thermal analyses under isothermal and non-isothermal conditions. The activation energy of crystallization and the crystallization mechanism for binary Ni–Zr alloys were determined using the Johnson-Mehl-Avrami (JMA) equation. The Kissinger and Ozawa methods were applied to the non-isothermal kinetics derived from the heating rate dependence of the crystallization temperature. The hydrogen permeation properties of the Ni–Nb–Zr membranes were investigated over the 573–673 K temperature range at different hydrogen partial pressures. The linear relationship between the hydrogen permeation flux and the hydrogen partial pressure difference across the membrane indicates that permeation was controlled by diffusion through the bulk membrane. The effect of Zr concentration on the hydrogen permeability and the thermal stability was also investigated. The hydrogen permeability improved while the thermal stability decreases with the addition of Zr. [Copyright &y& Elsevier]

Abstract: Thermodynamic modelling of hydrogen interactions with the Cu–Pd B2 alloys is used to set the stage for the development of membrane alloys with improved thermal and chemical stability for the separation of pure hydrogen from coal gasifier exhaust or syngas within advanced water gas shift membrane reactors. Thermodynamic descriptions of the binary Cu–Pd, Pd–H, Cu–H, and ternary Cu–Pd–H systems are obtained by combining experimental investigation, first-principles calculations, and thermodynamic modeling. The H solubilities in the Cu0.56Pd0.44 B2 alloy have been measured from 100 to 350  ∘C up to 7 bar H2 pressure. The stabilities of the ternary B2 phase at different compositions are predicted by first-principles calculations. The model parameters for the Gibbs energy of individual phases are optimized using experimental phase equilibrium and thermochemical data, and the first-principles predictions. The B2 phase is described with a three-sublattice model, (Cu,Pd)0.5(Cu,Pd)0.5(H,V a), which allows for ordering. The Pd–H fcc miscibility gap is well described and the partial H thermodynamic quantities of the Pd–H system are in agreement with experiments. The ternary para-equilibrium and full equilibrium fcc miscibility gap are calculated for the Cu–Pd–H system. Extensive comparisons are presented between the thermodynamic modelling and experimental data. [Copyright &y& Elsevier]

Internal oxidation of a Pd–Al alloy results in a composite consisting of Al2O3 precipitates within a Pd matrix. In accord with earlier 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 XAl=0.005 to 0.06. H can be 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 PH2 (equil) cannot be readily measured (273 K), they are not as deep as the initial ones produced after internal oxidation 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 H2 solution in the dilute phase of the internal oxidation Pd–Al alloys have been determined directly by reaction calorimetry and they have a spectrum of trapping enthalpies. [Copyright &y& Elsevier]

Hydrogen isotherms have been measured for a series of solid solution Pd-Au alloys in the temperature range from 393 to 523 K. Standard partial thermodynamic parameters at infinite dilution of H, DeltaH(H) degrees, and DeltaS(H) degrees, have been determined from these equilibrium data; both standard values for H(2) absorption become more negative with increase of atom fraction Au, X(Au). An interesting result is that the dilute phase isotherms at 423 and 523 K are all very similar for alloys with X(Au) = 0.15 to about 0.30 although their DeltaH(H) degrees and DeltaS(H) degrees differ. This is due to a compensating effect of the two thermodynamic parameters leading to (partial partial differentialDeltaG(H)/partial partial differentialr) = RT(partial partial differential ln p(1/2)/partial partial differentialr) approximately constant for the alloys from X(Au) approximately 0.15 to 0.30 at low r where r = H-to-metal atom ratio. Calorimetric enthalpies and isotherms at 303 K have been determined for a series of Pd-Au alloys over a range of H contents including, for some of the low Au content alloys, the plateau regions. These calorimetric data are the most complete reported for the Pd-Au-H system.

H diffusion constants have been determined from steady-state fluxes through Pd-Ag alloy membranes. The upstream side is maintained at a nearly constant pup (and consequently at a nearly constant rup=H/(Pd(1-x)Agx)) atom ratio, whereas the downstream side is at pH2 approximately 0 (rdown=0) (423-523 K). It is shown that the permeability is a maximum for atom fraction Ag, XAg=0.23 (423-523 K) at both pup=20.3 and 50.6 kPa. DH has been determined for some Pd-Ag alloys as a function of r in the dilute region, and it decreases with r even at small H contents for alloys with XAg<0.35. The concentration dependence of DH(cH) has been determined for the Pd0.77Ag0.23 alloy over a large concentration range. The effect of nonideality on DH(r) and ED(r) has been systematically determined as a function of XAg, where XAg is the atom fraction of Ag in the H-free alloy. (dDH/dr) increases with XAg up to XAg=0.35 and then changes from negative to positive at approximately 0.35. The activation energies for diffusion, ED(r), have been determined as a function of H content in the dilute range for several Pd-Ag alloy membranes, and the conversion to concentration-independent E*D values has been carried out in several different ways.

In this research, the thermodynamics of H2 solution and hydride formation in a series of disordered Pd-Ag alloys has been determined using both reaction calorimetry and equilibrium PH2-composition-T data. Trends of DeltaHH and DeltaSH with both H and Ag concentration have been determined. For the Pd0.76Ag0.24 alloy, which does not form a hydride phase, DeltaHH and DeltaSH both exhibit minima with H/(Pd0.76Ag0.24) followed by a linear increase of the former. A linear increase of DeltaHH is found for all of the alloys in the high H content region beyond the two-phase region or, if, there is no two-phase region, in the high H content region. DeltaHH degrees at infinite dilution of H decreases with atom fraction Ag, XAg, up to about 0.40 and then increases. Enthalpies for hydride formation/decomposition, 1/2H2(g) + dilute <--> hydride, have been determined calorimetrically for alloys which form two phases (303 K). The enthalpies for hydride formation become more exothermic with XAg while the corresponding entropy magnitudes are nearly constant, 46 +/- 2 J/K mol H.

The thermodynamics of H(2) solution and hydride formation/decomposition have been determined by reaction calorimetry (303 K) for disordered face centered cubic (fcc) Pd-Mn alloys. This alloy system belongs to the expanded lattice category which predicts that and DeltaH(plat) for H(2) absorption should be more exothermic than those for Pd; the experimental results are that the former is more exothermic, at least at the higher Mn contents, but the latter is not. There is a regular decrease in the H capacity (at p(H)2 = 0.2 MPa) with atom fraction Mn. A linear dependence of log p(H)2 upon H content is found in the single hydride phase for all of these alloys suggesting that DeltaH(H) and DeltaS(H) are also linear functions of r in this region. This is confirmed using the Pd(0.875)Mn(0.125) alloy which has no two-phase region (303 K). It is shown for the Pd(0.875)Mn(0.125) alloy and for Pd that the changes of partial enthalpies and entropies with H content are correlated so as to minimize changes of mu(H).