W. Luo

The effect of annealing pretreatment of Pd-Rh alloys on their hydrogen solubilities and thermodynamic parameters for H2 solution

Abstract Pressure-composition hydrogen isotherms in the temperature range 273 ⩽ T ⩽ 343 K have been determined for some Pd-rich Rh alloys with various degrees of homogeneity as a result of different cooling rates from above the miscibility gap. The following techniques have been employed to obtain different cooling rates: spin casting, fast quenching, slow quenching, furnace cooling (250 °C h−1) and prolonged annealing at 873 K; this order of cooling leads to an increasing extent of inhomogeneity. The dilute phase hydrogen solubilities are found to increase with increasing alloy inhomogeneity. The isotherms in the dilute region for the spin-cast alloy are the same as for the fast-quenched one. The partial molar enthalpy with ΔHHo at infinite dilution is less exothermic for the fast-quenched alloys as compared with the slow-quenched and furnace-cooled ones, but the enthalpy change for the plateau reaction, ΔHplat, does not change. Generally, for all these Pd-Rh alloys the hydrogen solubilities decrease with increasing Rh content and the relative partial molar enthalpy at infinite dilution, ΔHHo, and the enthalpy for hydride formation, ΔHplat, become less exothermic with increasing Rh content.

Correlations between phase diagrams and thermodynamic data for metal hydride systems

A series of correlations between the phase diagrams and enthalpic data are developed for metal hydride systems. The value of these correlations lies in their providing criteria for checking the consistency of the thermodynamic behavior for these systems. The correlations are illustrated using some experimental thermodynamic data and the accompanying phase diagrams for metal hydrides from the literature.

Thermodynamics and isotope effects of the vanadium–hydrogen system using differential heat conduction calorimetry

The V–H (D) systems have been investigated using reaction calorimetry at 298 and 323 K over hydrogen contents from H (D)/V=0→≊1.8. Relative partial molar enthalpies have been determined by absorption of small increments of hydrogen in the single phase regions; for regions where two solid phases coexist, integral or plateau enthalpies are obtained by absorption (desorption) of either large or small amounts of hydrogen. The following calorimetric results were obtained for the two solid phase plateau regions: ‖ΔH‖=40.6 kJ/mol H, 40.8 kJ/mol D for (α + β) and ‖ΔH‖=19.1 kJ/mol H and 23 kJ/mol D for (β’+γ) and (α’+e), respectively. Calorimetric data, relative partial molar enthalpies are also given for the single phase regions. Several phase boundaries are intersected as the hydrogen contents increase in this temperature range and the behavior of the enthalpies at the phase boundaries has been determined and is discussed in relation to the nature of the coexisting phases.

The thermodynamics of the ErFe2H system using differential heat conduction calorimetry

Abstract A differential twin-cell heat flow calorimeter has been constructed for the measurement of the enthalpy changes of the reaction of hydrogen gas with intermetallic compounds as a function of their hydrogen contents. It was designed especially for use with those intermetallic compounds which form very stable hydride phases; therefore prior to the calorimetric studies it must be possible to evacuate the sample in situ at elevated temperatures to remove all of the hydrogen resulting from the activation procedure. The calorimeter was employed in this research for the determination of the thermodynamics of the reaction of H 2 with ErFe 2 H x over the range of hydrogen contents x = 0.4. ErFe 2 H is a multiplateau system; the enthalpy change for hydride formation corresponding to the plateau near x = 3 is more exothermic than that corresponding to the preceding plateau near x = 2. In addition to the calorimetric determination of the enthalpies of reaction, the free energies were determined simultaneously from the equilibrium hydrogen pressures. Using these free energy and enthalpy data the entropies were also obtained over a range of hydrogen contents.

Thermodynamic characterization of the Zr-Mn-H system part 1. Reaction of H2 with single-phase ZrMn2+χ C-14 Laves phase alloys

Abstract The reaction of hydrogen with ZrMn 2+χ has been investigated using reaction calorimetry for χ = −0.2, 0, 0.5 and 1.0. The enthalpies for hydride formation and decomposition are equal in magnitude over the two-solid-phase region; their magnitudes decrease with increasing χ. Hysteresis is large for all these intermetallics, approximately 3.0 kJ (mol H) −1 , and independent of χ.

Stored energy resulting from the formation and decomposition of the hydride phase in the intermetallic compound: LaNi5

Abstract Stored energy in activated LaNi 5 has been determined from the difference in the heats of solution of activated and high temperature annealed samples. The hydrogen pressure needed for hydride formation is greater for annealed LaNi 5 than for the activated material. The stored energy, which is large, is related to the pressure needed for hydride formation.

The Thermodynamics of the ZrMn2+x— Hydrogen System*

The hydriding characteristics of ZrMn2+x has been studied by a number of workers(l-9). This intermetallic compound, IMC, can exist over a wide range of nonstoichiometry, i.e., from = —0.2 to 1.8 (3) and it has been shown by a number of workers that the plateau pressures increase markedly with x. Another feature of behavior in these systems is that their plateaux slope and the degree of slope appears to increase with .

The thermodynamic properties of the niobium–hydrogen system measured by reaction calorimetry

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.

Thermodynamic characterization of the Zr-Mn-H system Part 2. Reaction of H2 with two-phase alloys

Abstract The reactions of H 2 (g) with the two-phase alloys (a) α -Zr+ZrMn 2+ χ and (b) α -Mn+ZrMn 2+ χ , have been investigated. The homogeneity range of the Laves phase ZrMn 2+χ intermetallic compound has been determined from the dependence of its hydrogen absorption behavior upon χ and found to lie within the limits χ = −0.3 ± 0.1 and 1.0 ± 0.1. Normally H 2 (g) does not react with α-Zr below about 773 K, but it has been found that when it exists within the two-phase mixture, it reacts readily with hydrogen at room temperature. For alloys in the zirconium-rich two-phase field the hydrogen first reacts to form ZrH −2 and then it reacts with the ZrMn 2+χ phase. The former reaction leads to equilibrium pressures which are too small to measure, but the latter leads to measurable pressures. Results for alloys within the manganese-rich two-phase field, χ ≥ 1.0, are also reported. Calorimetric determinations of the enthalpies of reaction with hydrogen for several zirconium-rich alloys within the two-phase fields are reported for both the zirconium-rich and ZrMn 2+χ phases.

Calorimetrically measured enthalpies for the reaction of Hf with H(D) 2 (g)

The enthalpy for the direct reaction of H2 (g) with Hf has been measured by calorimetry for the first time at both moderate, 334 K, and elevated, 919 K, temperatures. The enthalpy for the reaction: 1/2 H2 (g) + 1/(b - a)HfHa(α) → 1/(b- a)HfHb(δ) is -70 ± 2.0 kJ/mol H at 334 K over a range of H contents from (H/Hf) = 0.5 to 1.5 with similar values found for D. The quantities α and δ are the coexisting phases anda andb are the corresponding (H/ Hf ) ratios, respectively. The magnitude of the enthalpy decreases from (H/Hf) = 0 to 0.5 and is then stable from 0.5 to 1.7. The value of δHf‡ (HfH1.5) = -107.5 kJ/mol and δHf‡ (HfH2.0) = -142.0 kJ/mol. In the elevated temperature range, calorimetric and equilibrium hydrogen pressure were determined over the range of H contents from 0 to 1.6. The enthalpy for the plateau reaction is -74.5 kJ/mol H and after the two-phase region, |δHH| increases with the increase of (H/Hf) passing through a maximum at about (H/Hf) = 1.3.