Formation Of Hydride Phase Research Articles

Neural network potential for Zr-H

The introduction of Hydrogen (H) into Zirconium (Zr) influences many mechanical properties, especially due to low H solubility and easy formation of Zirconium hydride phases. Understanding the various effects of H requires studies with atomistic resolution but at scales that incorporate defects such as cracks, interfaces, and dislocations. Such studies thus demand accurate interatomic potentials. Here, a neural network potential (NNP) for the Zr-H system is developed within the Behler-Parrinello framework. The Zr-H NNP retains the accuracy of a recent NNP for hcp Zr and exhibits excellent agreement with first-principles density functional theory (DFT) for (i) H interstitials and their diffusion in hcp Zr, (ii) formation energies, elastic constants, and surface energies of relevant Zr hydrides, and (iii) energetics of a common Zr/Zr-H interface. The Zr-H NNP shows physical behavior for many different crack orientations in the most-stable ε-hydride, and structures and reasonable relative energetics for the 〈a〉 screw dislocation in pure Zr. This Zr-H NNP should thus be very powerful for future study of many phenomena driving H degradation in Zr that require atomistic detail at scales far above those accessible by first-principles.

Deciphering Hydrogen Embrittlement Mechanisms in Ti6Al4V Alloy: Role of Solute Hydrogen and Hydride Phase.

Ti6Al4V (Ti64) is a versatile material, finding applications in a wide range of industries due to its unique properties. However, hydrogen embrittlement (HE) poses a challenge in hydrogen-rich environments, leading to a notable reduction in strength and ductility. This study investigates the complex interplay of solute hydrogen (SH) and hydride phase (HP) formation in Ti64 by employing two different current densities during the charging process. Nanoindentation measurements reveal distinct micro-mechanical behavior in base metal, SH, and HP, providing crucial insights into HE mechanisms affecting macro-mechanical behavior. The fractography and microstructural analysis elucidate the role of SH and HP in hydrogen-assisted cracking behaviors. The presence of SH heightens intergranular cracking tendencies. In contrast, the increased volume of HP provides sites for crack initiation and propagation, resulting in a two-layer brittle fracture pattern. The current study contributes to a comprehensive understanding of HE in Ti6Al4V, essential for developing hydrogen-resistant materials.

A sensitivity analysis of twinning crystal plasticity finite element model using single crystal and poly crystal Zircaloy

The popularity of crystal plasticity finite element method (CPFEM) models is increasing due to their ability to predict the mechanical response of crystalline materials such as metals and metal alloys more accurately than traditional continuum mechanics models. This is since the crystal plasticity models consider the effect of atomic structure, microstructural morphology, and properties of individual grains. These CPFEM models use a large number of material parameters in order to capture the mesoscale physics which comes with the downside of the tedious calibration process. In this paper, a CPFEM code was developed to include the twinning induced grain reorientation and subsequent crystallographic slip for HPC material. The developed code is incorporated in a large-scale, parallelized nonlinear solver WARP3D. A sensitivity analysis with respect to 22 material parameters was then conducted using single crystal and polycrystal representative volume element (RVE) of Zircaloy material. Loading was applied along five different crystallographic orientations for single crystal RVE and along three directions namely, rolling (RD), transverse (TD), and normal (ND) direction for polycrystal RVE. Results obtained from the sensitivity analysis were used for the calibration of material parameters for Zircaloy. Finally, developed code along with calibrated material parameters was used to investigate the effect of the hydride phase formation in Zircaloy which is a typical case observed for nuclear applications. It was found that the volume fraction of the hydride phase has a significant impact on the mechanical properties of Zircaloy.

The formation of a lithium-iridium complex hydride toward ammonia synthesis.

Noble metal elements are focal catalytic candidates for many chemical processes, but have received little attention in the field of nitrogen fixation except ruthenium and osmium. Iridium (Ir), as a representative, has been shown to be catalytically inactive for ammonia synthesis because of its weak nitrogen adsorption and severe competitive adsorption of H over N that strongly inhibits the activation of N2 molecules. Here we show that, upon compositing with lithium hydride (LiH), iridium can catalyze ammonia formation at much enhanced reaction rates. The catalytic performance of the LiH-Ir composite can be further improved by dispersion on a MgO support with a high specific surface area. At 400 °C and 10 bar, the MgO-supported LiH-Ir (LiH-Ir/MgO) catalyst shows a ca. 100-fold increase in activity compared to the bulk LiH-Ir composite and the MgO-supported Ir metal catalyst (Ir/MgO). The formation of a lithium-iridium complex hydride phase was identified and characterized, and this phase may be responsible for the activation and hydrogenation of N2 to NH3.

Ultra-thin nickel oxide overcoating of noble metal catalysts for directing selective hydrogenation of nitriles to secondary amines

Hydrogenation of nitriles represents an efficient and environmentally benign route to synthesis of high-value amines. However, the amine selectivity is commonly low, and the mixtures of amines, imines, and even low-value hydrogenolysis byproducts are generally obtained. Herein, we report that overcoating of Pd and Pt catalysts with an ultra-thin NiOx layer via atomic layer deposition enables to close up the undesired hydrogenolysis pathway completely and to prompt a yield of secondary amines drastically to 96% under mild conditions in the hydrogenation of nitriles. The NiOx-coated catalysts also exhibit a much higher recyclability than the uncoated ones. Through detailed structural characterization and control experiments of catalytic performance tests, we show that the NiOx overcoat plays at least three important roles in the promotion of catalytic performance: (i) Enhancement of the adsorption of both imine intermediates and primary amines, to stimulate the subsequent condensation of these two molecules to form secondary amines exclusively with a high activity; (ii) Isolation of large Pd and Pt ensembles to inhibit the hydrogenolysis byproduct; and (iii) Acting as a protective layer to stabilize metal nanoparticles against leaching, and to prevent the formation of undesired metal hydride phase, thus leading to a significant improvement in catalytic stability. These findings provide a robust methodology for directing selective hydrogenation of nitriles to secondary amines exclusively, and the insights here would facilitate the rational design of oxide overcoating layers for advanced catalysis.

In-situ scanning electron microscope thermal desorption spectroscopy (SEM-TDS) analysis of thermally-induced titanium hydride decomposition and reformation

Hydrogen embrittlement (HE) micro-mechanisms are intrinsically challenging to investigate even for metallic materials with simple microstructures, due to the spatial and temporal scales involved. This underlying complexity is increased, when hydrogen uptake induces microstructural changes, which precede the apparent embrittlement. Unraveling these micro-mechanisms calls for integrated characterization approaches that can provide spatially- and temporally-resolved insights regarding hydrogen effects. For this purpose, we present an integrated scanning electron microscopy - thermal desorption spectroscopy (SEM-TDS) technique here, which is employed to carry out an in situ study of hydrogen induced effects in Ti-6Al-4 V. Focusing on the interplay among the neighboring α and β phases in this material enables a deeper understanding of the effects of cyclic heat treatments: When this hydride forming alloy is heated, the dissociated hydrogen from the hydride diffuses into the neighboring α-Ti grains. Upon cooling, the hydrogen content in the α-Ti is supersaturated, which leads to the formation of a new hydride phase and an increase in the defect density in the grain. These insights provided by the SEM-TDS technique can be used to guide HE resistant microstructure design efforts, as well.

Dynamics of cavitation zone development during sonication of suspensions of magnesium particles

The cavitation activity during ultrasonic treatment of magnesium particles has been investigated. The cavitation activity recorded in a continuous mode of ultrasonic treatment altered in a wide range at constant output parameters of the generator. The rate and nature of cavitation activity variation depended on the mass fraction of particles in the suspension. It has been demonstrated that during the ultrasonic treatment of magnesium aqueous suspensions it is possible to determine the following stages: growth of cavitation activity, reaching a maximum followed by a decrease and reaching a plateau (or repeated cycles of increasing or decreasing cavitation activity). The complex nature of the cavitation activity dynamics is associated with the participation of hydrogen released as a result of the chemical interaction of magnesium particles with water in the formation of the cavitation zone. The magnesium particles modified with ultrasound were characterised with the use of scanning electron microscopy, X-ray phase analysis and thermal analysis. It has been found that ultrasonic treatment of magnesium particles resulted in the formation of magnesium hydroxide and magnesium hydride phases.

Hydrogenation of ethylene over palladium: evolution of the catalyst structure by operando synchrotron-based techniques.

Palladium-based catalysts are exploited on an industrial scale for the selective hydrogenation of hydrocarbons. The formation of palladium carbide and hydride phases under reaction conditions changes the catalytic properties of the material, which points to the importance of operando characterization for determining the relation between the relative fractions of the two phases and the catalyst performance. We present a combined time-resolved characterization by X-ray absorption spectroscopy (in both near-edge and extended regions) and X-ray diffraction of a working palladium-based catalyst during the hydrogenation of ethylene in a wide range of partial pressures of ethylene and hydrogen. Synergistic coupling of multiple techniques allowed us to follow the structural evolution of the palladium lattice as well as the transitions between the metallic, hydride and carbide phases of palladium. The nanometric dimensions of the particles resulted in the considerable contribution of both surface and bulk carbides to the X-ray absorption spectra. During the reaction, palladium carbide is formed, which does not lead to a loss of activity. Unusual contraction of the unit cell parameter of the palladium lattice in the spent catalyst was observed upon increasing hydrogen partial pressure.

Hydrogenation-induced lattice expansion and its effects on hydrogen diffusion and damage in Ti–6Al–4V

Hydrogen uptake in titanium alloys leads to the formation of hydride phases, which in-turn cause severe embrittlement upon further deformation. The embrittlement micro-mechanisms are complicated by the distinct hydrogen diffusion and solution characteristics of the α and β phases, which arise from their different crystal structures. To improve the fundamental understanding of the room temperature interplay among hydrogen, the present phases and the hydride that is formed at their interface, we investigated the hydride formation process in Ti–6Al–4V alloy by in situ scanning electron microscopy and electron backscatter diffraction analyses during hydrogen charging, and post-mortem synchrotron diffraction tests. These investigations reveal that the lattice expansion of the hydrogenated β phase introduces a localized stress field near the phase boundary, which, in turn, induces hydrogen confinement therein, eventually facilitating the hydride growth along the boundary. The volumetric expansion of the α-to-hydride transformation extends the local stress field, influencing the work-hardening and damage evolution in the alloy. We discuss these findings and the overall damage initiation and propagation taking place in the hydrogenated alloy, based on the interplay among the present phases and hydrogen.

Kinetics of Hydrogen Absorption from a Gas Phase by Diffusion Filtering Pd–Y Membranes

Diffusion filtering metallic palladium–yttrium membranes are subjected to hydrogenation from a gas phase and are studied by X-ray diffraction using synchrotron radiation. Boundaries of the formation of hydrogen-enriched phases are improved for a range of alloying component content, which is critical for their formation. The effect of the initial state of the alloy on conditions of hydride phase formation in the system is demonstrated. The content of hydrogen occluded in the membrane structure and hydrogen-induced lattice dilatations are determined. The parameters of the alloy substructure are calculated.

Model of hydrogen diffusion in titanium with the formation of hydride phases

The problem of predicting the behavior of structural materials in hydrogen-bearing media (including extreme operating environments) cannot be solved without a thorough understanding of the mechanisms of hydrogen accumulation and transfer inside these materials. This issue is especially pressing for hydride-forming materials, such as widely used titanium alloys. Titanium and its alloys are used, in particular, in designing power and chemical reactors, for the geological disposal of highly radioactive waste. The paper presents a nonlinear mathematical model of hydrogen permeability taking into account the possible formation of hydride phases (after certain concentrations of dissolved atomic hydrogen are attained), and the iterative method for solving the corresponding boundary value problem based on implicit difference schemes.

Features of Studying Atomic Hydrogen – Metal Systems

All the main areas of energy development suggest or are already implementing the use of metal-hydrogen systems. For nuclear energy, this is associated with the creation of thermostable moderators and special-purpose construction materials, for thermonuclear energy, with the behavior of the so-called first wall of fusion reactors, for hydrogen energy — storage, transportation and extraction of hydrogen. Hydrogen is the most effective moderator of fast and thermal neutrons, especially at high volumetric concentrations of hydrogen atoms in the material, i.e. at a high value of the ratio of the number of hydrogen atoms to the number of metal atoms, taking into account the heat resistance of the hydride. This paper discusses the modern methods of experimental studies of heterogeneous reactions, the topochemistry of metal – hydrogen reactions, the dependence of the interaction rate on pressure and temperature, models of surface processes occurring during the interaction of hydrogen with metal. Methods for determining the probability of adsorption of hydrogen on a metal surface, methods for measuring the activation energy of dissociation of a hydrogen molecule on a surface are also discussed. The paper describes the fea-tures of the preparation of the reactor, experimental samples and the method of their study in the study of atomic hydrogen-metal systems, the method of plasma-chemical thermogravimetry used to study heterogeneous reactions occurring in a hydrogen plasma electrodeless discharge. In order to study the mechanism of interaction of hydrogen with hydride-forming metals, a kinetic method of research is proposed. The essence of the kinetic method is that the elimination of the limiting influence of surface and diffusion processes on the rate of hydride formation using atomic hydrogen and metal foil makes it possible to directly record the formation of the corresponding phases using hydro-gen-metal kinetic curves, and also study the effect of various parameters on the rate of interaction and the formation of hydride phases.

Hydrogen storage properties of nanostructured 2MgH2[sbnd]Co powders: The effect of high-pressure compression

In this study, MgH2 and Co powders were mechanically milled in the molar ratio 2:1 and compressed to hard-packed cylindrical pellets. The microstructure, phase changes, and hydrogen storage properties of the mechanically milled 2MgH2Co powder and the 2MgH2Co compressed pellet were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and synchronous thermal (DSC/TG) analyses. Dehydrogenation of the 2MgH2Co compressed pellet is mainly due to the decomposition of Mg2CoH5 while it is the dehydriding of MgH2 for the milled 2MgH2Co powder. Pressure composition absorption isotherms of the 2MgH2Co powder and 2MgH2Co compressed pellet show two and three plateaus, respectively, corresponding to the formation of Mg6Co2H11 and Mg2CoH5 hydride phases. For the compressed 2MgH2Co pellet, enthalpies of formation/decomposition were measured to be −58.11±7.69 kJ/mol H2/55.70±3.34 kJ/mol H2 for Mg2CoH5 and -81.89±10.39 kJ/mol H2/74.47±5.27 kJ/mol H2 for Mg6Co2H11. In contrast, hydrogenation/dehydrogenation enthalpies of Mg2CoH5 and Mg6Co2H11 mechanically milled 2MgH2Co powder were −73.98±10.1 kJ/mol H2/71.67±1.38 kJ/mol H2 and -96.86±8.73 kJ/mol H2/89.95±10.81 kJ/mol H2, respectively. Fast hydrogenation was observed in the dehydrided 2MgH2Co compressed pellet with about 2.75 wt% absorbed in less than 1 min at 300 °C and a maximum hydrogen storage capacity of 4.43 wt% (2.32 wt% for the 2MgH2Co powder) was achieved. The hydrogen absorption activation energy of the 2MgH2Co compressed pellet (64.34 kJ-mol−1 H2) is lower than the mechanically milled 2MgH2Co powder (73.74 kJ-mol−1 H2). The results show that mechanical milling followed by high-pressure compression is an efficient method for the synthesis of Mg-based complex hydrides with superior hydrogen sorption properties.

Molecular dynamics study of hydrogen-vacancy interactions in α-zirconium

Hydrogen is known to have detrimental effects on the mechanical properties of zirconium and its alloys, mainly due the formation of brittle hydride phases. The behaviour of hydrogen dissolved in zirconium is not fully understood at the atomic scale. In particular, the influence of hydrogen on the behaviour of defects in zirconium still needs to be clarified. In this paper, we report a molecular dynamics study of hydrogen behaviour in zirconium using an improved Zr-H interatomic potential. We determined the diffusivity of hydrogen in zirconium, and the interaction of hydrogen atoms with a single or a pair of atomic vacancies. The computed hydrogen diffusion is found to be in good agreement with experimentally determined values. Our results also indicate that hydrogen interactions with vacancies in zirconium promote vacancy clustering. Moreover, hydrogen is found to substantially decrease the mobility of divacancies. From the present simulation findings, it is concluded that the presence and concentration of hydrogen impurities may have a significant influence on the evolution of irradiation-induced defects in zirconium alloys.

Interstitial-atom-induced phase transformation upon hydrogenation in vanadium

The effect of interstitial atoms (nitrogen, carbon) on hydrogen storage properties in vanadium was investigated. When the N concentration was below 0.4 wt%, the plateau pressures increased with increasing N concentration during absorption and desorption and vanadium samples (body-centered cubic (BCC)) transformed to VH0.5 (body-centered tetragonal (BCT), c/a = 1.1) and then VH2 (face-centered cubic (FCC)). When the N concentration exceeded 0.6 wt%, a new single-phase region appeared in the pressure-composition isotherm, suggesting the formation of a new hydride phase. The X-ray diffraction data indicated that this new hydride phase was VH1.0 with a BCT structure and c/a = 1.24, and the phase transformation took place as V (BCC) became VH0.5 (BCT, c/a = 1.1), followed by VH1.0 (BCT, c/a = 1.24) and then VH2 (FCC). Density functional theory calculations indicated that the BCT structure model with hydrogen atoms fully occupying the octahedral sites (denoted as the Oz site) can explain the experimentally obtained crystal structure for VH1.0; they also indicated that the VH1.0 phase was stabilized by the addition of nitrogen. In addition, the N occupation site changed from the Oz site in VH0.5 and VH1.0 to the tetrahedral site (denoted as the T site) in VH2 in coordination with hydrogen during hydrogen absorption. A similar phenomenon was observed in carbon-containing vanadium. It can thus be concluded that the phase transformation pathway and stability of the hydride phases in the V–H system are highly sensitive to the addition of interstitial C and N atoms.

HYDRIDE FORMATION KINETICS ON GASEOUS HYDRIDING OF ZIRCALOY-4 USING AVRAMI EQUATION

HYDRIDE FORMATION KINETICS ON GASEOUS HYDRIDING OF ZIRCALOY-4 USING AVRAMI EQUATION . Zirconium alloys are mainly material for fuel cladding. Mechanical properties degradation of zircaloy-4 as nuclear fuel cladding is inevitable due to its interaction with hydrogen during normal reactor operation or transient condition. Kinetics analyses of Zircaloy-4 cladding materials at operating temperatures of 500, 550 and 600 o C under hydrogen atmosphere by using Avrami’s method to simulate brittle hydride phase formation and physical observation of the cladding after hydriding have been performed in order to observe the cladding to hydrogen interaction. Kinetic analyses showed that hydriding reaction rate follows the second order with activation energy of 35 kJ/mol. The formation of e-ZrH phase, an indication of the physical degradation of the tubes, consumed time of 180 minutes at a temperature of 500 o C, 150 minutes at 550 o C, and 100 minutes at 600 o C, respectively. Degradation mechanical properties could be seen visually on the higher hydriding temperature. The cladding materials showed that the tube swelling occurs at a temperature of 550 o C and the tube degraded into pieces occurs at 600 o C. The value of mechanical degradation will be determined by mechanical testing in the next research.

Inhibitive effect of Pt on Pd-hydride formation of Pd@Pt core-shell electrocatalysts: An in situ EXAFS and XRD study

In situ EXAFS and XRD have been used to study the electrochemical formation of hydride phases, Habs, in 0.5 M H2SO4 for a Pd/C catalyst and a series of Pd@Pt core-shell catalysts with varying Pt shell thickness, from 0.5 to 4 monolayers. Based on the XRD data a 3% lattice expansion is observed for the Pd/C core catalyst upon hydride formation at 0.0 V. In contrast, the expansion was ≤0.6% for all of the core-shell catalysts. The limited extent of the lattice expansion observed suggests that hydride formation, which may occur during periodic active surface area measurements conducting during accelerated aging tests or driven by H2 crossover in PEM fuel cells, is unlikely to contribute significantly to the degradation of Pd@Pt core-shell electrocatalysts in contrast to the effects of oxide formation.

Microstructure, defect structure and hydrogen trapping in zirconium alloy Zr-1Nb treated by plasma immersion Ti ion implantation and deposition

The effect of low energy plasma immersion ion implantation and deposition of titanium on microstructure, defect structure and hydrogen trapping in zirconium alloy Zr-1Nb was studied. Defect structure and distribution were analyzed by Doppler broadening using slow positron beam. The surface microstructure after modification is represented by nanostructured Ti grains with random orientation. The gradient distribution of titanium as well as vacancy type defects were analyzed. The concentration of vacancy type defects is rising with increasing bias voltage. Gas-phase hydrogenation of the Ti-modified Zr-1Nb alloy was performed at 400 °C for 60 min. The strong interaction of hydrogen with vacancy type defects was demonstrated. Two different changes in the defect structure after hydrogenation were observed: when a titanium film is formed on the surface (after deposition at 500 V) hydrogen trapping occurs with the formation of titanium hydride phases, while in the implanted layer (deposition at 1000 and 1500 V) hydrogen is trapped due to interaction with vacancy type defects. The physical basis of Ti diffusion and its influence on the evolution of defect structure after surface modification and hydrogenation were discussed.

Core–Shell Structure of Palladium Hydride Nanoparticles Revealed by Combined X-ray Absorption Spectroscopy and X-ray Diffraction

We report an in situ, temperature and H2 pressure-dependent, characterization of (2.6 ± 0.4) nm palladium nanoparticles supported on active carbon during the process of hydride phase formation. For the first time the core–shell structure is highlighted in the single-component particles on the basis of a different atomic structure and electronic configurations in the inner “core” and surface “shell” regions. The atomic structure of these particles is examined by combined X-ray powder diffraction (XRPD), which is sensitive to the crystalline core region of the nanoparticles, and by first shell analysis of extended X-ray absorption fine structure (EXAFS) spectra, which reflects the averaged structure of both the core and the more disordered shell. In the whole temperature range (0–85 °C), XRPD analysis confirms the existence of two well-separated α- and β-hydride phases with the characteristic flat plateau in the phase transition region of the pressure-lattice parameter isotherms. In contrast, first shell in...

Mechanisms of hydrogen embrittlement and fracture of nanocrystalline materials

The physical mechanisms of hydrogen embrittlement and fracture of nanocrystalline materials due to the molecular hydrogen pressure in cavities and the formation of hydride phases are proposed and justified mathematically.