I. Jacob

Neutron diffraction study of ZrM2Dx deuterides (M=Fe, Co)

Abstract The structure of ZrFe2 and ZrCo2 deuterides prepared under very high deuterium pressure has been studied by neutron diffraction (ND) at 10 K. The patterns of both compounds are refined with a mixture of intermetallic compound and deuteride. For ZrFe2Dx, the cell volume increases with 23% and the 2.7 D/f.u. are located randomly in A2B2 sites. The Fe moments order in a ferromagnetic structure and increase from 1.8 to 2.2 μB/Fe upon D absorption. The ZrCo2 deuteride shows a doubling of the cell parameter due to deuterium order of 2 D/f.u. in 7 over 12 A2B2 sites with a cell volume increase of 12%.

The effect of vacuum annealing on the hydriding kinetics of zirconium

Abstract The effects of vacuum preannealing on the hydriding kinetics of zirconium metal at 400 °C and 1 atm hydrogen pressure were studied using combined metallographic and kinetic measurements. Two stages were identified in the reaction kinetics: a fast initial stage in which the apparent front velocity was 70 μm min−1 and a second stage in which the front velocity gradually decreased until it stabilized around 8 μm min−1. During the fast stage a special topochemical development of the hydride was observed, with the hydride progressing along grain boundaries which were previously oxidized during the vacuum annealing. This relatively fast process continued until a hydride overlayer was formed on the sample surface. The spatial extent of this stage was thus of the order of the grain size (around 40 μm). During the second stage of the hydriding the rate-controlling step was the diffusion of hydrogen through the product layer leading to a decrease in the apparent reaction rate. Hence, the main effect of the vacuum preannealing on the hydriding rate was introduced through grain boundary oxidation, affecting the initial rapid stage.

Neutron diffraction study of atomic bonding properties in the hydrogen-absorbing Zr(AlxFe1−x)2 system

Abstract Powder neutron diffraction of Zr(Al x Fe 1− x ) 2 intermetallics ( x =0–1) has been performed at room temperature in an attempt to study the variation of their bonding properties as a function of x . The Debye–Waller factors indicate a minimum of the bonding strength for both the Zr and the Fe/Al sites in the Zr(Al x Fe 1− x ) 2 compounds at x =0.2. A similar minimum has also been observed by other experimental techniques, and it corresponds to the maximum hydrogen absorption in the above intermetallic system.

The kinetics and mechanism of cerium hydride formation

Abstract The kinetics of the reaction between cerium metal and gaseous hydrogen were studied within the temperature range 0–100 °C and the pressure range 0.1–32 atm. Combined kinetic rate measurements and metallographic observations enabled the evaluation of the intrinsic kinetic parameters of the reaction, i.e. the velocities of the hydride front progression and their temperature-pressure dependence. Arrhenius temperature plots of the velocities yielded a pressure-independent apparent activation barrier of about 0.2 ± 0.05 eV per H atom. For each temperature an apparent power pressure dependence of about 1 3 was displayed. Different possible mechanisms are discussed. It has been concluded that the rate-limiting step for this reaction is the penetration of hydrogen from the surface (of the hydride layer) into the subsurface region. The activation energies for this penetration process and for the desorption of chemisorbed hydrogen (from the hydride surface) were estimated to be 0.24 and 0.09 eV respectively.

Standard molar enthalpies of formation of alloys in the pseudobinary system Zr(AlxCo1 − x)2 at the temperature 298.15 K

Abstract The standard molar enthalpies of formation ΔfHm° of alloys with compositions in the pseudobinary system Zr(AlxCo1 − x)2Zr have been determined by solution calorimetry in a mixture of hydrofluoric and nitric acids. The values of ΔfHm°(298.15 K) obtained are as follows: −(113±11) kJ mol−1 for x=0; −(123±8) kJ mol−1 for x= 1 12 ; −(120±8) kJ mol−1 for x= 1 6 ; −(130±7) for x= 1 4 ; and −(147±10) kJ mol−1 for x= 1 2 . Uncertainties correspond to twice the standard deviation of the mean. The following empirical relation was found to be applicable over the range of compositions studied: ΔfHm°{Zr(AlxCo1 − x)2} ≈ x·ΔfHm°(ZrAl2)+(1 − x)·ΔfHm°(ZrCo2).

Kinetics of hydride formation in massive LaNi5 samples

Abstract The hydriding kinetics of parallelepiped, 50–200 mg LaNi 4 samples were investigated as a function of temperature and pressure in the ranges 245–293 K and 5–70 atm H 2 respectively. Mathematical analyses of the time-dependent agreement with both shrinking core and low-dimensional nucleation and growth-type models. Visual and metallographic examinations of partially hybrided samples do not provide any additional evidence in support of either of the two hybriding models. The temperature dependence of the hydrogenation rate constants follows an Arrhenius-type behavior for a given driving force, determined by a constant ratio between the applied and equilibrium plateau hydrogen pressures. The pressure dependence of the rate constants indicates an interface-controlled phase transition as the rate-determining step of hybride formation. The activation energy for the hydriding process is estimated as 0.37 eV/H atom. This value is independent of the applied model.

Hydrogen-oxygen accumulation on polycrystalline uranium surfaces

Abstract The kinetics of accumulation of hydrogen on clean and on oxidized uranium surfaces was studied by combined direct recoil spectrometry (DRS), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) techniques. For the sputter-cleaned surface an island growth progression characterizes the accumulation kinetics of hydrogen, whereas on the oxidized surface a random two-site Langmuir model seems to fit the initial chemisorption process. Similar combined DRS and AES measurements were applied to study the kinetics of accumulation of oxygen on H 2 predosed uranium as compared to the clean (H-free) case. The pre-chemisorption of hydrogen modifies the mechanism of oxygen incorporation and the structure of the product overlayer.

Kinetics of hydrogen absorption in the intermetallic Zr(Al0.2Fe0.8)2

Abstract The hydriding kinetics of Zr(Al 0.2 Fe 0.8 ) 2 was studied at four different temperatures between 238 K and room temperature (298 K) and at an approximately constant pressure of 10 atm H 2 . This compound is characterized by the largest hydrogen capacity in the Zr(Al ξ Fe 1−ξ ) 2 intermetallic system, which exhibits an interesting and anomalous hydriding behaviour. The experiments were carried out with thin intermetallic pieces of definite thickness in order to facilitate the data interpretation. Special precautions were taken during the slicing of the brittle compounds. Visual and metallographic examinations of partly hydrogenated compounds imply a contracting envelope type of hydrogenation. A simple mathematical analysis of the time-dependent hydrogen absorption curves yields the interface velocity μ of the advancing hydride. The derived values are approximately in the range 10 −3 −5 × 10 −3 mm s −1 for the investigated temperatures and pressure. An activation energy of about 0.14 eV (H atom) −1 (27 kJ (mol H 2 ) −1 ) was estimated for the hydrogenation process.

Experimental elucidation of the anomalous hydrogen absorption in Al-containing pseudobinary intermetallic compounds

Abstract Experimental evidence supports an explanation of the peculiar x -dependence of the hydrogen absorption in Zr(Al x Co 1− x ) 2 , Zr(Al x Fe 1− x ) 2 and LaAl x Ni 5− x in terms of two opposing trends: increasing metal–hydrogen attraction but decreasing hydrogen–hydrogen attraction. The latter factor causes a vanishing of the hydrogen absorption at high Al concentrations. Such an explanation is proposed, to the best of our knowledge, for the first time. It may give a new perspective in understanding and tailoring hydrogen absorption in intermetallic compounds.

The hydriding kinetics of massive ZrCo

The kinetics of hydride phase development in massive samples of ZrCo was investigated at temperatures between 25 and 300°C and for hydrogen pressure ranged from 1 bar down to the absorption equilibrium pressures. Metallographic examinations of partially hydrided samples have shown that a hydride layer is formed on the sample exterior, advancing into the bulk of the sample during the hydriding process. The front velocity was found to be constant for given pressure and temperature, provided a proper vacuum annealing is performed prior to exposure. Under relatively high pressure (i.e., where P/Peq(T)≫1), the velocity approaches a saturation (pressure independent) asymptotic value (Usaturation(T)), whereas, at the relatively low pressure regime (i.e., where P/Peq(T) ≈1), the pressure dependence of the front velocity is linear obeying a pressure function (P/Peq(T)−1), with a slope U0(T). The activation energies obtained for the pressure independent constants U0(T), and Usaturation(T) are about the same, 45.4±0.1 kJ/mol. The bulk velocities measured in this work were compared to the surface growth velocities measured previously. It has been found that far from equilibrium, the bulk and surface velocities are much the same. Similar results were found for the bulk front velocities in well-annealed massive uranium samples and ZrCo, under similar experimental conditions. Some conclusions were drawn concerning the hydriding mechanisms in both systems.