Equilibrium Hydrogen Pressure Research Articles

A pressure-composition-temperature study of Zr-Nb-H system

The solubility of hydrogen was determined in the (Zr + 5 wt pct Nb)-H2, (Zr + 10 wt pct Nb)-H2, and (Zr + 20 wt pct Nb)-H2 systems as a function of composition, temperature (700° to 950°C) and hydrogen equilibrium pressure (0.5 to 760 mm Hg). The position of boundariesβ - (β + δ) and(β + δ)-δ were determined in each of the above three systems. Niobium significantly reduces the solubility of hydrogen in theβ andδ phases and increases the equilibrium hydrogen pressure for any fixed concentration. The equilibrium pressure-temperature relations in the two phase region (β + δ) were derived and the heat of formation ofδ-hydride from saturatedβ-Zr, ΔH β → δ, were determined. The value of ΔH β → δ increases up to about 5 wt pct Nb after which the effect of niobium seems to be insignificant. The maximum hydrogen pick-up of zirconium at room temperature decreases with increasing niobium content of the alloy.

Influence of a magnetic field on the hydrogen equilibrium pressure of RCo 5H x

The hydrogen equilibrium abessure above a two-phase mixture RCo 5H x + RCo 5H y can be changed slightly by applying a magnetic field. For H=10 kOe a relative change Δp/p ≈ 6×10 −3 has been measured for R=La, x=3.35, y=4.3, T=295 K.

The preparation and crystallographic characterization of two new types of ternary metal hydrides

Ternary metal hydrides of thorium with zirconium or titanium have recently been prepared and identified. One of these compounds, ThZr2H7±x, has the cubic C15 type of structure. This hydride shows a high hydrogen density, e.g., 6.7 to 7.7 x 1022 hydrogen atoms per cu cm i.e., NH 6.7 to 7.7) and relatively low equilibrium hydrogen pressures at elevated temperatures. The second new type of hydride is ThTi2H6±x which crystallizes with the hexagonal C14 structure. This hydride also has a high hydrogen density of 8.0 to 8.8 x 1022 hydrogen atoms per cu cm and is relatively stable at elevated temperatures. High hydrogen density (high NH) and low equilibrium hydrogen pressure are a generally sought after combination of characteristics for hydrides to be used in nuclear reactors and in other applications. Accordingly, these two new kinds of hydrides and their possible analogs may be of future use in a variety of applications.

A pressure-composition-temperature study of the zirconium/2.5 wt % niobium+ hydrogen system

The solubility of hydrogen has been determined in the ( Zr + 2.5 wt % Nb)- H 2 system as a function of composition, temperature (300–950 °C) and hydrogen equilibrium pressure (0.5–760 mm Hg). From the isotherms, the position of the boundaries β−( β + δ) and ( β + δ)− δ were determined. The presence of niobium seems to shift these boundaries to lower hydrogen values and raise the hydrogen equilibrium pressure in the two phase region (β + δ). The equilibrium pressure temperature relation in the two-phase region (β + δ) was derived to be, log 10 p = −12 020/ T + 13.39, where p is measured in mm Hg and T in degrees Kelvin. The heat of formation of δ-hydride from saturated β-Zr is −54.8 kcal/mole H 2, as compared to −49.1 kcal/mole H 2 in unalloyed zirconiumhydrogen.

Electrochemical exchange currents of palladium and palladium alloy electrodes as a function of their hydrogen contents

Electrochemical exchange currents i0 have been measured on palladium/hydrogen and on gold (18.8 %)/palladium/hydrogen electrodes as a function of their hydrogen contents. The values of i0 are markedly dependent upon the hydrogen content of the electrodes. Results are interpreted in terms of the expected dependence of i0 upon the equilibrium hydrogen pressure corresponding to the hydrogen content of the electrodes.

Metal hydride reactions: II. Reaction of hydrogen with CaMG 2 and CaCU 5 and thermodynamic properties of the compounds

The equilibrium hydrogen pressure for the reaction CaMg 2 + H 2 ai CaH 2 + 2 Mg was measured in the temperature range 450°–637°C. The data below 517°C can be represented by log PH 2 ( atm) = −6919 T + 5.95 and that above 517°C by log PH 2 ( atm) = −6666 T + 5.56 The change in slope at 517°C is to be expected on the basis of a eutectic horizontal at this temperature. The data in the temperature range 450°–517°C were combined with known thermodynamic data for calcium hydride to give the relations ΔF o CaMg2 = −10,624 + 4.28 T (440°–517° C) and ΔF o CaMg2 = −9,735 − 2.46 T + 1.93 T log T (25°–440° C) The equilibrium hydrogen pressure for the reaction CaCu 5 + H 2 ai CaH 2 + 5 Cu was measured for the temperature range 492°–648°C. The data can be represented by log PH 2 ( atm) = −6700 T + 7.11 These data were combined with thermodynamic data for calcium hydride to obtain the relations ΔF o CaCu5 = −11,600 − 1,0 T (440°–780° C) and ΔF o CaCu5 = −10,740 − 7.70 T + 1.93 T log T (25°–440° C)

Statistical Model for the Beta Zirconium Hydrides

It appears reasonable to consider the hydrogen as bonded covalently to zirconium in the zirconium hydrides. After consideration of the known structure of ZrH2, the hypothesis is made that the nonstoichiometric ``beta'' (bcc) Zr–H phases have four symmetrical (tetrahedral) hydrogen sites on each face of the unit cell, with simultaneous occupancy restricted to pairs not closer than the shortest H–H distance in ZrH2. The complex configurational problem is solved for all concentrations by developing a simple process of building up the lattice, after making tests of the amount of detail required for accurate solutions in one and two dimensions. The total entropy, when decreased by the corresponding configurational entropy, corresponds to values of the hydrogen vibration frequency averaging 1440 cm—1 and showing no consistent trend with temperature or with a fourfold change in hydrogen concentration. A semiempirical energy model is constructed by introducing the hypothesis that zirconium bonded to no hydrogen is ``abnormally stable,'' and the two energy parameters are derived from the experimental data. The agreement with the curves derived from thermodynamic data is fairly good for the partial molal entropy and energy of hydrogen, and excellent for the hydrogen equilibrium pressures. Alternative models, however, are not eliminated unambiguously.

Ternary compounds between thorium monocarbide and thorium dihydride

The thorium-hydrogen-carbon system was studied by measuring hydrogen equilibrium pressures at various compositions and temperatures. Two compounds, ThC·ThH 2 and ThC·2ThH 2, were found and characterized. ThC·ThH 2 has a hexagonal closest-packed lattice with a o = 3·816 A ̊ and c o = 6·302 A ̊ . ThC·2ThH 2 has a monoclinic lattice with a o = 6·50 A ̊ , b o = 3·80 A ̊ , c o = 10·91 A ̊ and β = 119°. The enthalpies of formation for each compound were determined.

Adsorption of Hydrogen on a (110) Nickel Surface

Hydrogen adsorption at room temperature on a clean (110) surface of a nickel single crystal is accompanied by reconstructive rearrangement of the surface metal atoms to form a (2×1) surface structure. It is possible to remove this adsorbed hydrogen and obtain a planar surface by heating the crystal slightly above room temperature. A determination of the temperatures and equilibrium hydrogen pressures necessary to remove this adsorbed hydrogen yields an isosteric heat of adsorption of 1.2 eV. Oxygen displaces the hydrogen in the (2×1) structure from the surface and results in the formation of the (1×2) structure characteristic of adsorbed oxygen. This oxygen can be removed from the surface, however, by heating the crystal to 200°C in hydrogen.

Nonstoichiometry and Lattice Defects in Transition Metal Hydrides

The relationships between equilibrium hydrogen pressure, hydrogen content, and temperature have been derived for nonstoichiometric transition metal hydrides from both statistical mechanical and thermodynamic considerations. By comparing the derived equations with experimental pressure-composition isotherms, the type of lattice defect causing deviations from stoichiometry as well as the energies of defect formation and interaction can be calculated. This was done for uranium hydride and palladium hydride. The energies obtained were 69 kcal/mole for the vacancy formation energy, and 4.4 kcal/mole for the attractive vacancy interaction energy in uranium hydride. The corresponding energies in palladium hydride were 58.0 kcal/mole and 0.35 kcal/mole. The derived relationships are of general applicability to any nonstoichiometric binary compound which is deficient in the volatile component.

Hysteresis in the palladium + hydrogen system

Hysteresis in the relationship between the uptake of hydrogen by palladium sponge and the equilibrium hydrogen pressure has been studied at 120°C. The boundary curves of the hysteresis loop have been obtained together with several absorption and desorption scanning curves. The main loop is analyzed in terms of Lacher’s (1937 a ) statistical mechanical theory of hydrogen in palladium : the α - and β -phases are characterized by different parameters in his equation. This implies that the energies of absorption of a hydrogen atom, and the interactions between absorbed protons are different in the two phases. No single equation of the form deduced by Lacher exists which represents the system throughout the whole range of hydrogen concentrations. The generally accepted interpretation of earlier absorption data seems to be incorrect, and doubt is thrown upon the possibility of the existence of a reversible transition path between the two phases. By combining our data with a knowledge of the critical point of the α-β -phase change, the points of onset of the phase transitions are calculated as a function of temperature and compared with the values obtained by other workers. The co-existence of both α - and β -phases over a range of pressures is probably associated with the large volume change accompanying the transition and a tentative theory is put forward which accounts for the ranges of co-existence of the two phases both in absorption and desorption. The shapes of the scanning curves are shown to be generally consistent with the predictions of the domain theory of hysteresis.

Determination of Hydrogen in Titanium

A simple, rapid, precise method for determination of hydrogen in titanium is described. It is based on measurement of the equilibrium pressure of hydrogen over the metal in a closed system under predetermined conditions. Construction, calibration, and operation of the analytical equipment are described. Hydrogen can be determined in the range 0.001–0.33 wt % with a probable error of about 5%. The physical form of specimens is unimportant. Twenty analyses can be made in an 8‐hr period.