S. Luo

Degradation behavior of LaNi5−xSnxHz (x=0.20–0.25) at elevated temperatures

Abstract Systematic studies of the hydriding behavior of LaNi 5− x Sn x alloys with tin contents in the range 0.20 x P – C – T ) isotherms measured after heating the hydrides above 450 K. Some loss in reversible capacity was observed along with reductions in the plateau pressures and hysteresis ratios while the slopes of the plateaus became greater. These changes are indications of degradation processes and increased disorder within the alloy structure. Additional experiments were performed for long periods (i.e. >1000 h) at elevated temperatures and hydrogen pressure to produce further degradation in the P – C – T isotherms. The impact of alloy composition on the isotherms has been determined. The crystal lattice properties of the alloys before and after hydrogen reactions have been studied using high-resolution X-ray powder diffraction with synchrotron radiation. Changes in these X-ray diffraction patterns are correlated to various structural modifications resulting from hydride formation and degradation.

The thermodynamics of hydrogen absorption/desorption by Pd–Co alloys

Abstract The thermodynamics of Pd–Co–H(D) systems have been investigated using H 2 (D 2 ) pressure–composition isotherms and reaction calorimetry. The enthalpies for hydrogen solution and hydride formation decrease in magnitude with increase of X Co and, because of this, the hydrogen pressures at a given value of H / M increase with X Co . This behavior is consistent with the decrease of the unit cell size with increase of X Co for these random solid solution fcc Pd–Co alloys. Repeated hydride formation and decomposition at ambient temperature without intermediate annealing treatments, causes the plateau pressures to converge thereby decreasing the extent of hysteresis.

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.

Calorimetric enthalpies for the reaction of H2 with Pd-Cu alloys at 303 K

Abstract Thermodynamic parameters for the reaction of H 2 with Pd 1− x Cu x ( x =0, 0.05, 0.096, 0.15, 0.2 and 0.3) alloys have been determined via reaction calorimetric and isotherms measurements at 303 K. Both the hydrogen solubilities in the dilute phase and the lengths of the plateaux decrease with increase of x , while the plateau pressures increase with increase of x . The magnitude of the enthalpies for the plateau reaction decreases with increase of x . In the single hydride phase region at the end of the plateau the |Δ H H | values decrease with H content. Relative entropies have been determined from the enthalpies and the equilibrium pressures. The entropies for the plateau reaction do not change very much with x in Pd 1− x Cu x

Hydrogen solution in homogeneous Pd–Fe alloys

Abstract Thermodynamic properties for H2 solution and hydride formation in Pd–Fe alloys have been determined from pressure–composition isotherms and from reaction calorimetry. The dilute phase solubility in the Pd–Fe alloys appears to be anomalous because the Sieverts’ constants for XFe≥0.0374 do not extrapolate linearly to the value for pure Pd–H. Substitution of Fe in the Pd lattice causes the unit cell size to decrease and thus this system is contracted with respect to Pd. Contracted Pd-rich alloys generally have increased plateau pressures as compared to Pd and the Pd–Fe alloys are no exception. Enthalpies for the plateau reaction have been determined by both p–c–T and reaction calorimetric methods. The enthalpy magnitudes decrease with atom fraction Fe from 19.1 kJ/mol 1 2 H 2 (XFe=0) to 13.5 (XFe=0.10) as determined from reaction calorimetry.

Thermodynamic properties and the degradation of ZrNiHx at elevated temperatures

Abstract The hydrogen absorption and desorption behavior of ZrNi was measured over the temperature range from 525 K to 700 K. The isotherms demonstrate that the (β + γ) two-phase region extends to these temperatures, in contrast to a recently published phase diagram. The hysteresis ratio of the absorption to desorption plateau pressures decreases with temperature and approaches unity just above 700 K. The pressure-composition data were reproducible, even at temperatures greater than 680 K, provided the isotherm measurements were completed within a few hours. If the hydrided ZrNiHx sample was held above 680 K for more extended periods, significant changes in the pressure and isotherms were observed. The extent of these changes is dependent on both the temperature and hydrogen content. Only minor changes were seen after 24 h at 700 K for x 1.5. X-ray diffraction studies of several ZrNiHx samples revealed that the degradation in the isotherms can be attributed to the formation of ZrHx and Ni via disproportionation.

Hydrogen isotherms for LaNi4.6M0.4 alloys where M=group 4A elements

Abstract Hydrogen isotherms have been measured for activated LaNi 4.6 M 0.4 alloys from 300 to 473 or 493 K where M=Si, Ge, Sn. Thermodynamic parameters have been obtained from the isotherms using van’t Hoff plots for hydride formation and decomposition. The Δ H plat values are more exothermic than for the parent compound, LaNi 5 , but the Δ S plat values are similar. As for the LaNi 5− x Sn x alloys, LaNi 4.6 Ge 0.4 and LaNi 4.6 Si 0.4 have a greater resistance towards degradation than the parent compound.

On hydrogen solubilities in Pd/alumina composites prepared by internal oxidization of Pd/Al alloys

Internal oxidation of a Pd-Al alloy results in a composite consisting of Al O precipitates within a Pd matrix. In accord with earlier 23

Thermodynamics of hydrogen absorption / desorption for Pd-group 6B alloys and some anomalous isotherms

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 XM where M=Cr, Mo and W, and, because of this, the hydrogen solubilities at a given pH2 in the dilute phase decrease with XM. This behavior is consistent with the decrease in the unit cell size with increase in XM for these random solid solution fcc Pd–M alloys. 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 H2 absorption because they are the most effective in increasing the plateau pressures for a given atom fraction M.

Hydrogen segregation to internal interfaces of Pd–yttria composite prepared by internal oxidation of Pd–Y alloys

Abstract Pd –Y alloys have been internally oxidized in the atmosphere at T ≥973 K to form Pd–yttria composites. Dilute phase H 2 isotherms and reaction calorimetry have been used to characterize the trapping of dissolved H at the Pd–yttria interface. Although H 2 isotherm 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 0.98 Y 0.02 alloy decrease in magnitude with r =H/Pd, i.e., Δ H H =−85 kJ/mol H, r →0, −60 kJ/mol H, r =0.004, −40 kJ/mol H, r =0.0055 and −20 kJ/mol H, r =0.0065. After evacuation at 303 K, the enthalpies were found to be Δ H H =−35 kJ/mol H at r →0 and −15 kJ/mol H at r =0.0025, i.e., some of the trapped H is not removed by evacuation (303 K). For H 2 solution in well-annealed Pd, Δ H H =−10.3 kJ/mol H at r =0. The amounts of strongly and weakly trapped H at the interface are 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 H 2 for hydride formation; thus there is significant expansion on the inside of the sample before the outer zone becomes hydrided. The present results show that this does not affect the plateau pressure for hydride formation for the alloy.