Z. Hadari

The reaction of hydrogen with magnesium alloys and magnesium intermetallic compounds

The hydrogenation characteristics of various dilute magnesium alloys and magnesium intermetallic compounds were studied. The rate of the reaction between hydrogen and activated magnesium containing small amounts of group IIIa metal additives (aluminium, gallium, indium) was investigated and was compared with the rate for pure (unalloyed) magnesium. The results are interpreted in terms of a three-dimensional diffusion mechanism in which magnesium cation vacancies are the more mobile species controlling the rate of the process. Thermodynamic and crystallographic studies of the hydride Mg2NiH4 revealed the presence of two allotropie forms of this compound. An orthorhombic structure which is stable in the ambient temperature range transforms at 210–245 °C to a cubic pseudo-CaF2-type structure. The transition is not accompanied by a change in the hydrogen composition which remains the same for both structures. The enthalpy change associated with the transition is 0.80 ± 0.05 kcal (mol H2)−1 (1.60 kcal (mol Mg2NiH4)−1). The solubility of hydrogen in Mg2Ni (i.e. the α-phase region) was investigated a and the relation between the lattice parameters and the hydrogen concentration was determined. We also studied partial substitution of the nickel atoms in Mg2Ni by cobalt and iron. Reactions of hydrogen with Mg-Al intermetallic compounds (Mg2Al3 and Mg3Al2) were studied. The results indicate the occurrence of reversible disproportionation reactions similar to those observed for Mg2Cu. An irreversible disproportionation was observed for the reaction of hydrogen with Mg2Ca yielding MgH2 and CaH2.

Experimental measurements and general conclusions on the effective thermal conductivity of powdered metal hydrides

Abstract The effective thermal conductivity K e of Mg 2 NiH 4 and MmNi 4 FeH 5.2 (Mm ≡; misch metal) was measured under steady state conditions as a function of the hydrogen pressure and the temperature. The effective thermal conductivity of the two hydrides increases significantly as a function of the hydrogen pressure up to approximately 40 atm and then remains almost constant. The K e values for Mg 2 NiH 4 and MmNi 4 FeH 5.2 are 0.83 W m −1 K −1 and 1.05 W m −1 K −1 at 373 K and 273 K respectively and a hydrogen pressure of 40 atm. The results were analysed using the Yagi-Kunii model for the effective thermal conductivity in powder-fluid beds. The solid thermal conductivity K s of the above two hydrides was then estimated. The most important and significant result of the present work is the conclusion that K e “saturates” at relatively low values of K s . For example, the maximum K e values (above the hydrogen breakaway pressure) at 300 K and a typical hydride void fraction (ϵ = 0.5) change from about 0.4 to about 1.5 W m −1 K −1 for a corresponding K s change between 1 and 500 W m −1 K −1 . A survey of the existing data in the literature for the effective thermal conductivities of powdered metal hydrides supports the above conclusion. The practical meaning of this result is that, in engineering applications of hydrides in which a knowledge of K e is necessary, it can safely be assumed that K e ≈ 1–2 Wm −1 K −1 .

The initial kinetics of uranium hydride formation studied by a hot-stage microscope technique☆

Abstract The initial stages of the formation of uranium trihydride on the surface of uranium samples reacted with gaseous hydrogen (about 1.5 atm) were studied utilizing a hot-stage microscope. The nucleation and growth processes of the product hydride were continuously monitored with a television camera and were recorded on a videotape. The formation kinetics and the morphological characteristics of the developing hydride phase are discussed. A comparison with the kinetic results obtained in the more advanced bulk hydriding stage is made.

Hydrogenation of oxygen-stabilized Ti2MOx (M Fe, Co, Ni; 0 ⩽ x < 0.5) compounds

The hydrogenation characteristics of Ti2mo inx (M  Fe, Co, Ni; ifo ⩽ x < 0.5) compounds were investigated using a combined thermogravimetric analysis and differential thermal analysis technique, pressure-volume-temperature measurements and X-ray diffraction analysis. Pure Ti2M (M Co, Ni) intermetallics react with hydrogen in an irreproducible manner, reaching various H/Ti2M composition ratios. Surface decomposition reactions catalysed by surface impurities may account for this irregular behaviour. Heating hydrogenated Ti2M samples in a hydrogen atmosphere results in initial decomposition followed by disproportionation reactions which convert part of the Ti2MHx into TiH2 and TiM. The oxygen-containing Ti2MOx compounds, however, do not display such disproportionation reactions. The addition of oxygen to the Ti2M structures decreases the stability of the corresponding hydrides; the substitution of nickel by cobalt and iron also results in decreased hydride stability. These effects point to the participation of two factors which determine the stability of the Ti2M hydrides: 1. (1) the electron concentration in the conduction band and 2. (2) local interactions between hydrogen and nearest-neighbour metal atoms.

Hydrogenation characteristics of Ti2NiOx compounds (0 ⩽ x < 0.5)

The hydrogenation characteristics of Ti2NiOx compounds (0 ⩽ x < 0.5) were investigated using combined TG-DTA techniques. The presence of oxygen in Ti2Ni affects the hydrogen uptake capacity, the stability of the hydrides and the phase transition behavior of the corresponding hydrogen-containing compounds. Oxygen atoms occupy some of the available hydrogen interstitial sites and also influence the electron concentration, and possibly they reduce the enthalpy of formation of the hydrides. The cooling rate affects the maximum hydrogen uptake at room temperature, with rapid cooling giving the highest hydrogen content. These rate effects are tentatively explained on the basis of a model involving two types of hydrogen-occupied interstitial sites.

Hydrogen absorption in ANiAl (A ≡ Zr, Y, U)☆

The hydrogen or deuterium absorption capacities of the Fe2P-type hexagonal compounds ZrNiAl, YNiAl and UNiAl were measured at pressures of approximately 70 atm and room temperature. The maximum capacities of these compounds were found to be 0.7 H atoms (formula unit)−1, 1.5 H atoms (formula unit)−1 and 2.5 H atoms (formula unit)−1 respectively. Desorption isotherms obtained at several temperatures for the UNiAl-H2 system revealed the existence of at least two hydride phases. Their heats of formation were estimated to be −63.6 kJ (mol H2)−1 and −46.5 kJ (mol H2)−1. The phase limits of the hydride phases in UNiAlHx seem to coincide approximately with the hydrogen capacities of ZrNiAl and YNiAl. The hydrogen absorption of the intermetallic compounds investigated is examined with respect to their crystal structures and the available interstitial sites.

NMR study of hydrogen diffusion and phase determination of the Mg2NiHx system

We have measured the temperature dependence of the hydrogen spin lattice relaxation time in the hydride (β, β′, γ, δ) and solid solution phases of the Mg2NiHx system. These gave the temperature ranges of the various phases. The hydrogen diffusion activation energy for the β, β′, γ and δ hydride phases were found to be 10.6±0.4, ?17, 12.0±0.3 and ?20 kcal/mole, respectively, and 8.7±0.7 kcal/mole for the α phase solution.

Effects of nickel and indium ternary additions on the hydrogenation of MgAl intermetallic compounds

Abstract Alloying small amounts (2–3 wt%) of Ni and In with MgAl intermetallics causes a significant increase in the hydrogenation rates of these compounds. The catalitic effects of these ternary additions are accounted for by two mechanisms: (a) The formation of surface locations enriched with the ternary constituent which provide preferred dissociation sites for hydrogen and (b) More extensive cracking of the reacting particles during repeated hygrogenation-dehydrogenation cycles. The hydrogenation reaction of both, binary and ternary MgAl β-phase alloys, has been found to follow a law of a three dimensional diffusion through a growing product layer. The thermodynamic parameters associated with the hydrogenation reaction of the MgAl compounds, are evaluated.

The topochemistry of hydride formation in rare earth metals

Abstract The topochemistry of hydride formation in the rare earth metals was investigated by metallographic examination of partially hydrogenated samples. The effects of surface overlayers (“oxides”) on the development of the hydride phases are discussed and are related to measured overall reaction rates. Four different types of hydride topochemical forms are identified. The development of each of these types is accounted for by the interplay of the hydrogen diffusion rates in the metallic matrix and the nucleation and growth rates of the hydride phases. Temperature-induced changes in the type of progression of the hydride phases are discussed. The effects of higher (trihydride) phases are illustrated. A method for the estimation of the diffusion constants of hydrogen in the α phase metallic solid solutions is presented.

NMR study of hydrogen diffusion in uranium hydride

Abstract The diffusion of hydrogen in uranium hydride is studied employing the NMR technique. From measurements of spin-spin relaxation time T2, the activation energy for hydrogen diffusion in β-UH3 is determined to be E a = (19.25 ± 0.4) kcal mole and the preexponential factor to be A0 ≈ 5 × 1014 Hz. It is shown that these results are in fair agreement with spin-lattice relaxation time T1 data. Assuming that hydrogen diffusion proceeds via vacancies whose concentration is temperature dependent, it is concluded that Ea is the sum of the energies of vacancy formation and barrier height, and that A0 contains an entropy change factor. Using vacancy concentration data calculated by Libowitz, we estimate the barrier height energy to be Eb ≈7 kcal/mole. Using a value for the frequency of hydrogen vibration v0 determined from inelastic neutron scattering by Rush et al., we estimate the entropy change due to vacancy formation and the hydrogen atom jump to be about S k B ≈3 . Similar measurements on samples containing less hydrogen than is needed to compose stoichiometric UH3, show that the rate of diffusion is enhanced by the presence of excess metal in the sample. The jump frequency at 500°K in UH3 is found to be approximately 106 Hz while for the two-phase samples of H/U = 2.8 and 2.5, it is larger by a factor of about 3 and 3.5, respectively.