Features of hydrogen storage in a solid and phase transformations in metal hydrides with different crystal structures

Concepts of ordering, phase separation, and phase transformations in metallic solid solutions are discussed in the context of the latest experimental data on the microstructure of Fe-M alloys. It is shown that the type of chemical interaction between alloy component atoms depends on the degree of the localization of 3d-valence electrons on atoms rather than on the ratio of energies of ordering in different coordination shells. The morphology of the products of phase transformations in alloys with a tendency to ordering or phase separation is considered, and the known mechanisms of phase decomposition are analyzed. The decomposition of alloys having a tendency to ordering and to phase separation is described in thermodynamic terms. It is shown that solid-solution fields in phase diagrams are in fact regions where the alloys have a tendency to phase separation and where, at certain correlations between free-energy components, separation microstructures are formed. It is noted that phase transformations in alloys can occur at different levels of the structure of matter: the 'primary' occur at the level of changes (ordering–phase separation) in the electronic structure, and the 'secondary' are realized at the level of changes (order–disorder) in the microstructure.

Switchable mirrors1,2,3 made of thin films of the hydrides of yttrium (YHx), lanthanum (LaHx) or rare-earth metals exhibit spectacular changes in their optical properties as x is varied from 0 to 3. For example, α-YHx 2.85 is a yellowish transparent semiconductor. Here we show that this concentration dependence of the optical properties, coupled with the high mobility of hydrogen in metals, offers the possibility of real-time visual observation of hydrogen migration in solids. We explore changes in the optical properties of yttrium films in which hydrogen diffuses laterally owing to a large concentration gradient. The optical transmission profiles along the length of the film vary in such a way as to show that the formation of the various hydride phases is diffusion-controlled. We can also induce electromigration of hydrogen, which diffuses towards the anode when a current flows through the film. Consequently, hydrogen in insulating YH3−δ behaves as a negative ion, in agreement with recent strong-electron-correlation theories4,5. This ability to manipulate the hydrogen distribution (and thus the optical properties) electrically might be useful for practical applications of these switchable mirrors.

Analysis of electrochemical techniques for studying the diffusion of hydrogen in metals

Realistic quantitative calculations on atomistic models for hydrogen in metals are difficult because quantum effects must be included. We show that quantum simulation based on Feynman's path-integral formulation of quantum mechanics gives a powerful way of making such calculations. After summarizing some essential facts about metal-hydrogen systems, we explain the principles of pathintegral simulation. We present the results of classical molecular-dynamics and quantum path-integral simulations of hydrogen in Pd and Nb based on simple empirical interaction models. Results are given for the probability density ρ(r) of hydrogen as a function of position r in the unit cell, and for a quantity proportional to the diffusion coefficient. We point out that diffraction measurements of ρ(r) would allow an important test of the interaction models. The simulated diffusion coefficient for H in Nb shows the experimentally observed break in Arrhenius slope at ∼250 K, which we argue represents a cross-over from quantum to classical behaviour; the low- and high-temperature activation energies are in reasonable agreement with experiment. We conclude with a general discussion of the significance of quantum simulation for metal-hydrogen studies.

Hydrogen is a key element in the energy transition. Hydrogen–metal systems have been studied for various energy-related applications, e.g., for their use in reversible hydrogen storage, catalysis, hydrogen sensing, and rechargeable batteries. These applications depend strongly on the thermodynamics of the metal–hydrogen system. Therefore, tailoring the thermodynamics of metal–hydrogen interactions is crucial for tuning the properties of metal hydrides. Here we present a case of large metal hydride destabilization by elastic strain. The addition of small amounts of zirconium to yttrium leads to a compression of the yttrium lattice, which is maintained during (de)hydrogenation cycles. As a result, the equilibrium hydrogen pressure of YH2 ↔ YH3 can be rationally and precisely tuned up to five orders of magnitude at room temperature. This allows us to realize a hydrogen sensor which indicates the ambient hydrogen pressure over four orders of magnitude by an eye-visible color change.

The nature and properties of transition metal hydrides

This article reviews the experimental and theoretical studies of the properties of hydrogen included in metals. The problem is discussed of diffusion of hydrogen atoms in a metallic matrix, and the theory of coherent and incoherent diffusion. Phase transitions caused by elastic interactions are considered in a system of hydrogen atoms included in the interstices of a metallic matrix. The electronic properties of compounds of hydrogen with different metals, the experimental studies of the electronic band structure and of phase transitions in the electronic subsystem (metal-dielectric in cerium hydride, magnetic transitions) are reviewed. Special attention is paid to superconductivity in hydrogen-metal systems. Detailed results are presented of experimental and theoretical studies of the superconducting properties of compounds of palladium with hydrogen and deuterium, and also compounds of palladium with the noble metals and hydrogen. The effect of inclusion of hydrogen on the superconductivity of other transition metals is discussed.

Mobility and retention of implanted hydrogen in Ti225 titanium alloy

Metal hydrides are considered a promising means of hydrogen storage mainly because they store hydrogen at low pressure while having volumetric densities comparable to liquid hydrogen (Reilly, 1979). Furthermore, the release of hydrogen in metal hydrides being an endothermic process, this makes them inherently safe. In these lectures, we will discuss how the hydrogen reacts with metals. More specifically, we will see how hydrogen diffuse into the metal and forms a solid solution or a metal hydride. The first lecture treats the basic phenomena of hydrogen-metal systems such as solid solution of hydrogen in metals and formation of metal hydrides with particular emphasis on their structure and thermodynamics. The location of hydrogen in the crystal structure of the host metal will also be reviewed. In the second lecture, the effect of alloying, the electronic structure of metal hydride alloys and kinetics of hydrogen in metals will be studied. In the third lecture, we will see three specific metal hydrides; palladium, niobium and magnesium. Finally, the last lecture will have for object the nanocomposite system Mg-Nb and the possible hydrogen desorption mechanism of this system.

The effect of nanometer grain size and extensive grain boundary regions in nanocrystalline alloy systems was investigated for the chemical order-disorder, structural, precipitation, and spinodal phase transformations. The kinetic paths for approach to the chemically ordered phase from the disordered phase in FeCo-Mo alloys were observed to be the same at different temperatures due to grain boundaries acting as short-circuited diffusion paths for atom movements. The structure of Fe 3 Ge was bcc for small crystallite size and the equilibrium fcc phase developed only after a critical grain size was attained. This was understood as a manifestation of the Gibbs Thomson effect. The precipitation phase transformation in Fe-Mo alloys proceeded by a rapid movement and clustering of the Mo atoms to the grain boundaries that was correlated to the size of the nano grains, and subsequent formation of the Mo rich lambda phase directly in the grain boundary regions. The composition fluctuation domains for spinodal decomposition in nanophase Fe-Cr alloys were observed to be linearly correlated to the growth of grains.

In previous chapters, the laws of plastic and creep deformation of metals due to applied forces have been studied. At the same time, irreversible strains can be induced by other, non-force impacts. This chapter concerns with the mathematical modeling of irreversible deformation due to jump-wise temperature changes, irradiation, and phase transformations.

On the importance of equilibrium in studies on the transport of hydrogen in metals

An analytic relation for the terminal solid solubility of hydrogen in a stressed metal is derived, based on finite deformation theory. Phase transformation is assumed to be a reversible process, which occurs under local chemical equi- librium among hydride, metal and hydrogen in solid solution. Hydrogen terminal solid solubility depends on stress due to the interaction of the applied stress field with the field of the expanding hydride as well as due to the reduction of hydro- gen chemical potential in solid solution, caused by hydrostatic stress. The present analysis is complementary to hydride- induced embrittlement studies in metals, under conditions, which require the use of finite deformations.

KINETICS OF HYDROGEN ABSORPTION AND DESORPTION

Hydrogen related peaks in Cu–LaNi 4.7Al 0.3