M.H. Mintz

Kinetics and mechanisms of metal hydrides formation—a review

Abstract The microscopic mechanisms which may control the rate of the reaction of gaseous hydrogen and hydride-forming metals are reviewed. A distinction is made between the early stages of the reaction associated with the nucleation and growth of the hydrides on the surface of the reacting metal and the subsequent massive stage. For the very early stage, factors affecting the ability of hydrogen gas to penetrate surface passivation layers are considered. Different types of nucleation groups are demonstrated. For the latter, massive stage, possible morphological forms of the hydride phase development are summarized. A special case, frequently encountered in binary metal–hydrogen systems, is the contracting envelope (or shrinking core) morphology. For this case, a simple evaluation of the reaction front velocity can be deduced from the overall rate measurements and from the known geometry and dimensions of the metal sample. A detailed analysis of this hydride-front velocity dependence on sample temperature and gas pressure can then point to the controlling mechanism. Some characteristics of Arrhenius-type curves and possible deviations from linear relation are discussed. Examples for possible surface-controlled (Ce), diffusion-controlled (Th, Ti, Zr, Hf) and interface-controlled (U) reactions are presented, as well as limited bulk intermetallic hydriding reactions. Certain symptomatic aspects of the kinetic behaviour related to some of the above mechanisms are discussed.

Kinetic study of the reaction between hydrogen and magnesium, catalyzed by addition of indium

The kinetics of formation of magnesium hydride from pure activated magnesium and from some indium containing magnesium alloys has been studied. The experimental data fit the rate equation [(1−2α/3)−(1−α)2/3] = Kt where α is the fraction of metal reacted at time t. The form of this rate expression indicates a three-dimensional diffusion-controlled reaction. The addition of small amounts of indium to magnesium lowers appreciably the activation energy of the process.

Hydriding kinetics of powders

Abstract Kinetic measurements of gas-solid reactions performed on powders, are usually interpreted according to single-particle analysis (SPA) models. The use of SPA functions is not a priori justified, since deviations from that functional dependence may occur in the powder, owing to three factors: (i) particle size distributions, (ii) particle shape variations and (iii) time distributions for the beginning of the reaction on each one of the particles composing the powder. The effects of these factors are quantitatively analysed and their interference in the SPA procedure is estimated. It is concluded that under certain circumstances the SPA can yield the correct reaction mechanism and even enables a reasonable estimate of the corresponding intrinsic kinetic parameters. This analysis is relevant generally to gas-solid kinetics, with particular emphasis on hydriding reactions.

Kinetics and mechanism of the U-H reaction

Abstract The kinetics of the hydrogenation reaction of uranium were studied over a wide range of pressures (10 2 –10 7 Pa) and temperatures (0–500 °C). The results are discussed in terms of two possible models: model 1, hydrogen diffusion through a protective product hydride layer; model 2, hydride growth at the hydride-metal interface. A two-dimensional ( i.e. pressure-temperature) grid of the experimental data was successfully fitted to the pressure-temperature rate relations derived from model 2. This model relates the microscopic distribution of hydrogen atoms in the metal to the phase transformation probability, from which the pressure-temperature dependence of the hydride growth rate is obtained. The model also qualitatively accounts for the negative apparent activation energy reported in the literature for the hydrogenation of the powdered metal.

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.

Site related nucleation and growth of hydrides on uranium surfaces

Abstract The characteristics of hydride nucleation and growth on certain surfaces of pure uranium and of U-0.1 wt.% Cr samples were studied (under I atm H, at temperatures of 50–75 C) using the hot-stage microscope, microprobe analyzer and atomic force microscope techniques. Different preparation procedures of the samples were applied, in order to check the effects of surface oxidation layer variations on the nucleation and growth characteristics. Four families of hydride nuclei, differing in density, size and growth rates, were observed and classified. The smalles and most dense is in the form of submicron blisters formed instantaneously along mechanical polishing scratches. A larger (1 – 10 μm) blister-like family is formed beneath the oxide at point defect sites (but not discontinuities of the oxide), growing very slowly probably due to the compression of the coating oxide layer. The third family is characterized by preferential nucleation and rapid growth around carbide inclusions due to the discontinuity in the oxide at the carbide/oxide interface. The fourth family is found only on the samples having a thick oxide layer, and is characterized by a rapid growth rate, but is not located around inclusions. In this case, the nuclei probably originate at some other oxide discontinuities, such as twins or grain boundaries.

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.

The initial interactions of oxygen with polycrystalline titanium surfaces

Abstract The interactions of gaseous oxygen and different types of polycrystalline titanium surfaces were studied at room temperature within the exposure range of 0–1000 L. Combined measurements utilizing direct recoils spectrometry (DRS), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and work function variations enabled the distinction between processes occurring on the topmost atomic layer and those associated with subsurface incorporation of oxygen. Also, the different chemical forms (oxidation states) developing during the exposure course were identified. The results were compared for three types of surfaces, each prepared by a different cleaning procedure. It has been concluded that: 1. (i) Oxygen initially accumulates on the topmost atomic layer, regardless of the type of the studied surface. No preferred subsurface occupation has been observed. 2. (ii) The kinetics of initial accumulation (up to a complete surface coverage) are similar for all the different types of surfaces. 3. (iii) Mixtures of different oxidation states of titanium (0, +2, +3, +4) are present during the whole course of exposure. Qualitatively, increasing proportions of the higher valence states are displayed for higher oxygen exposures. However, the quantitative estimates of their relative amounts indicate a strong dependence on the type of surface, with preferred high oxidation (+4) states obtained for high temperature annealed samples (as compared with room temperature sputtered surfaces). 4. (iv) Topmost oxygen atoms which terminate the oxides surfaces are less negatively charged than the underlying (i.e., subsurface) “oxidic” atoms. These results may account for some of the controversies presented in the literature.

THE EFFECT OF THERMAL ANNEALING ON THE HYDRIDING KINETICS OF URANIUM

Abstract The effects of vacuum heat pretreatments on the hydriding kinetics of uranium (pure and chromium alloyed) were studied. Two pretreatment temperature regimes were correlated with changes displayed in the two stages of the hydriding reaction. The lower pretreatment regime (below about 200 °C) has a drastic effect on the initial stage of the reaction (the “induction period”. However, it does not alter the rate of the subsequent massive reaction stage. This low temperature region is associated with surface modification processes, the most prominent process being gas desorption. However, the higher pretreatment regime (above about 450 °C) affects the rate of the massive reaction stage (i.e. the velocity of the hydride reaction front) and is attributed to heat-induced changes in the microstructure of the metal (grain growth). The increased rate induced by chromium addition is attributed to the precipitation of chromium at grain boundaries.

High-pressure studies of the TiCr1.8–H2 system Statistical thermodynamics above the critical temperature

Abstract Pressure–composition isotherms of the TiCr 1.8 –H 2 system were measured within the temperature range 298–433°K and over a wide pressure range up to 1000 atm H 2 . The above temperature range is well above the critical temperature, T C , of the system. Hence, partial molal enthalpies and entropies of formation were evaluated as a function of hydrogen composition. Both thermodynamic quantities obeyed a linear decrease (i.e. becoming more negative) with increasing H/M atomic composition ratio (with M=Ti+Cr atomic content). The experimental isotherms were compared to calculated expressions derived by a rigid-metal sublattice statistical thermodynamics model. Two approximations applied in solving the model, the Bragg–Williams (B.W.) and the Quasi-Chemical (Q.C.) were compared, respectively. The pairwise nearest neighbors H–H interaction parameter, η , was evaluated for each isotherm. For both approximations a similar temperature dependence of η ( T ) was obtained, with η changing from attractive (i.e. negative) to repulsive (i.e. positive) with increasing isotherms temperatures. A good agreement was obtained between the calculated T C values (derived from the η ( T ) parameters) and the experimental observations.