Hydrogen Induced Abrupt Structural Expansion at High Temperatures of a Ni32Nb28Zr30Cu10 Membrane for H2 Purification.

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.

Temperature dependence of H permeation through Pd and Pd alloy membranes.

H permeabilities (normalized fluxes), have been measured through Pd and some Pd alloy membranes at a series of constant upstream H(2) pressures with the downstream pressure being ~0 in the temperature range from 393 to 573 K. From these data, activation energies for H permeation, E(P), have been determined. Conditions of constant upstream p(H(2)) are of most interest since most determinations of E(P) in the literature have employed this boundary condition. Permeabilities have also been measured at a series of constant upstream H concentrations with the downstream concentration being ~0 and, under these conditions, the slopes of the Arrhenius plots give activation energies equivalent to those for H diffusion. It is shown here that under constant upstream p(H(2)) conditions, nonideality of the H leads to nonlinear Arrhenius plots of P for Pd and especially for some Pd alloy membranes where the H(2) solubilities are significant even at moderate p(H(2)). For example, the permeabilities of a Pd(0.77)Ag(0.23) alloy membrane and a Pd(0.94)Y(0.06) alloy membrane are found to be nearly independent of temperature (423 to 523 K) in the range of constant upstream pressures from 16.1 to 81 kPa.

Thermodynamics of hydrogen in fcc Pd-Au alloys.

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.

Diffusion of H through Pd-Ag alloys (423-523 K).

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.

Thermodynamics of H in disordered Pd-Ag alloys from calorimetric and equilibrium pressure-composition-temperature measurements.

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.

Thermodynamics of hydrogen solution and hydride formation in Pd-Mn alloys. 1. Disordered alloys and a correlation effect.

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).

Thermodynamics of hydrogen solution and hydride formation in Pd-Mn alloys. 2. Ordered alloys.

There are marked differences in H(2) solubilities between ordered and disordered Pd-Mn alloys with the largest difference found between the L1(2) and the disordered form of the Pd(3)Mn alloy. The thermodynamics of H(2) solution have been determined for the L1(2) form, the long-period superstructure (lps), and the disordered forms of the Pd(0.80)Mn(0.20) and Pd(0.75)Mn(0.25)(Pd(3)Mn) alloys. Relative partial molar enthalpies and entropies were determined mainly by reaction calorimetry over the range of H contents accessible from p(H)()2 approximately 10 Pa to approximately 0.3 MPa (303 K). The enthalpies for absorption of H(2) are more exothermic over most of the range of H contents for the L1(2) forms of the Pd(3)Mn and Pd(0.80)Mn(0.20) alloys than for their other forms. The reaction enthalpies are constant across a relatively wide range of H contents for the L1(2) form of the Pd(0.80)Mn(0.20) and Pd(3)Mn alloys indicating that there are two-phase coexistence regions (303 K). The H-H attractive interaction, which leads to hydride formation, is much greater for the L1(2) than for the other forms of the Pd(3)Mn alloy and for Pd itself. It has been found that the H-H interaction always decreases in magnitude and, accompanying this, the THS (terminal hydrogen solubility) always increases by alloying Pd.(1) The L1(2) ordered Pd(3)Mn alloy is an exception to this, and therefore, the generalization about THS must be restricted to disordered face centered cubic (fcc) Pd alloys.