Formation Of Metal Hydrides Research Articles

Tribology in Gaseous Hydrogen

Hydrogen is expected as a clean and renewable energy carrier for future environment-friendly society. Many machine elements in hydrogen energy systems should be operating within hydrogen gas and tribological behavior, such as friction and wear, of bearings and seals are affected by the hydrogen environment through some interactions between material surfaces and gaseous hydrogen, i.e., physisorption of hydrogen molecules and following chemisorptions of dissociated atoms on metal surfaces, formation of metal hydride and reduction of metal oxide layer by hydrogen atoms diffused into bulk. Therefore, friction and wear characteristics of tribomaterials in the hydrogen environment should be appropriately understood to establish a design guideline for reliable hydrogen utilizing systems. This paper reviews the current knowledge about the effect of hydrogen on friction and wear of materials, and then describes our recent progress of hydrogen research in the tribology field.

Mechanistic aspects of the protonation of [FeFe]-hydrogenase subsite analogues

The formation of transient metal hydride(s) at the metallo-sulfur active sites of [FeFe]-hydrogenase is implicated in both hydrogen evolution and uptake reactions. Using a combination of time-resolved NMR, stopped-flow UV and stopped-flow IR, we have begun to unravel the mechanisms for protonation of synthetic electron-rich analogues of the di-iron subsite of the enzyme: Fe(2)(mu-pdt)(CO)(4)(PMe(3))(2), Fe(2)(mu-edt)(CO)(4)(PMe(3))(2), (NEt(4))(2)[Fe(2)(mu-pdt)(CO)(4)(CN)(2)], (NEt(4))(2)[Fe(2)(mu-edt)(CO)(4)(PMe(3))(2)] and (NEt(4))[Fe(2)(mu-pdt)(CO)(4)(CN)(PMe(3))] (pdt = propane-1,3-dithiolate, edt = ethane-1,2-dithiolate). The mechanistic role of isomer interconversion and how this critically relates to steric access to the di-iron bridge are revealed.

Effect of Hydrogen Pressure on the Size of Nickel Nanoparticles Formed during Dewetting and Reduction of Thin Nickel Films

We report a study of the formation of nickel nanoparticles by the dewetting of nickel metal films under different reduction conditions. The effects of both film thickness and temperature of dewetting on the size distributions of Ni nanoparticles are in agreement with previously reported studies. However, our work evidences that the hydrogen pressure applied during the reduction process has a drastic influence on the size of the formed Ni nanoparticles, the hydrogen pressure effect being all the more important that the film thickness is small. We interpret this effect by the formation of nickel metal hydride displacing the surface free energy balance of the system in favor of the dewetting. Moreover, the replacement of hydrogen by an inert gas allows us to rule out Ostwald’s ripening as the mechanism originating the sintering of the nanoparticles.

Formation of transition metal hydrides at high pressures

Silane (SiH 4) is found to (partially) decompose at pressures above 50 GPa at room temperature into pure Si and H 2. The released hydrogen reacts with surrounding metals in the diamond anvil cell to form metal hydrides. A formation of rhenium hydride is observed after the decomposition of silane and reaction of hydrogen with Re gasket. From the data of a previous experimental report [M.I. Eremets, I.A. Trojan, S.A. Medvedev, J.S. Tse, Y. Yao, Science 319 (2008) 1506], the claimed high-pressure metallic and superconducting phase of silane is identified as platinum hydride, that forms after the decomposition of silane. These observations show the importance of taking into account possible chemical reactions that are often neglected in high-pressure experiments.

Uranium–zirconium hydride fuel properties

Properties of the two-phase hydride U 0.3ZrH 1.6 pertinent to performance as a nuclear fuel for LWRs are reviewed. Much of the available data come from the Space Nuclear Auxiliary Power (SNAP) program of 4 decades ago and from the more restricted data base prepared for the TRIGA research reactors some 3 decades back. Transport, mechanical, thermal and chemical properties are summarized. A principal difference between oxide and hydride fuels is the high thermal conductivity of the latter. This feature greatly decreases the temperature drop over the fuel during operation, thereby reducing the release of fission gases to the fraction due only to recoil. However, very unusual early swelling due to void formation around the uranium particles has been observed in hydride fuels. Avoidance of this source of swelling limits the maximum fuel temperature to ∼650 °C (the design limit recommended by the fuel developer is 750 °C). To satisfy this temperature limitation, the fuel-cladding gap needs to be bonded with a liquid metal instead of helium. Because the former has a thermal conductivity ∼100 times larger than the latter, there is no restriction on gap thickness as there is in helium-bonded fuel rods. This opens the possibility of initial gap sizes large enough to significantly delay the onset of pellet-cladding mechanical interaction (PCMI). The large fission-product swelling rate of hydride fuel (3× that of oxide fuel) requires an initial radial fuel-cladding gap of ∼300 m if PCMI is to be avoided. The liquid-metal bond permits operation of the fuel at current LWR linear-heat-generation rates without exceeding any design constraint. The behavior of hydrogen in the fuel is the source of phenomena during operation that are absent in oxide fuels. Because of the large heat of transport (thermal diffusivity) of H in ZrH x , redistribution of hydrogen in the temperature gradient in the fuel pellet changes the initial H/Zr ratio of 1.6 to ∼1.45 at the center and ∼1.70 at the periphery. Because the density of the hydride decreases with increasing H/Zr ratio, the result of H redistribution is to subject the interior of the pellet to a tensile stress while the outside of the pellet is placed in compression. The resulting stress at the pellet periphery is sufficient to overcome the tensile stress due to thermal expansion in the temperature gradient and to prevent radial cracking that is a characteristic of oxide fuel. Several mechanisms for reduction of the H/Zr ratio during irradiation are identified. The first is transfer of impurity oxygen in the fuel from Zr to rare-earth oxide fission products. The second is the formation of metal hydrides by these same fission products. The third is by loss to the plenum as H 2. The review of the fabrication method for the hydride fuel suggests that its production, even on a large scale, may be significantly higher than the cost of oxide fuel fabrication.

Hydrogen sorption in transition metal modified NaETS-4

Transition metals nickel (Ni), rhodium (Rh) and palladium (Pd) both exchanged and in situ loaded in NaETS-4 samples were synthesized and characterized. Hydrogen adsorption studies in the synthesized NaETS-4 samples were performed at 77.4 K up to 1 bar and at 303 K up to 40 bar pressure. The reversible adsorption isotherms at 77.4 K showed physisorption of hydrogen but at 303 K chemisorption was the major mechanism for hydrogen uptake, which is clear from the irreversible nature of adsorption isotherms at 303 K. Rhodium loaded NaETS-4 showed the highest hydrogen adsorption capacity of 66 cc/g at 77.4 K and 35 cc/g at 303 K. The hydrogen adsorption in transition metal modified NaETS-4 at 303 K is driven by the chemical interactions leading to the formation of transition metal hydrides inside the micropores of NaETS-4. The absorbed hydrogen at 303 K can be desorbed by heating the modified titanosilicates up to 423 K.

Electrochemical Charge for the Formation of Metal Hydrides from LiH+M (M=Mg, Al)

AbstractFor a formation of metal hydride of MgH2 or AlH3 under room temperature and ambient pressure, the cathode electrodes of metal and lithium hydride are electrochemically charged with Li anode electrodes in the system of Li-ion extraction. For MgH2 formation, the VC (Voltage-Composition) curve of Mg + 2LiH during charge shows a plateau voltage at 0.6 V until the final composition of 1.05 Li extraction. After charge MgH2 phase is observed by the XRD measurement. Therefore MgH2 is produced by the electrochemical charge from Mg and LiH. For AlH3 formation, Al + 3LiH is charged until the final composition of 0.6 Li at a plateau voltage of 0.8 V which corresponds to the reaction between Al and LiH for the formation of AlH3. In the XRD profile after charge AlH3 phase is not detected although the intensities of Al and LiH decrease compared with these before charge, which suggests the reaction leading to the formation of AlH3.

The formation of a new phase in nanoparticles

First-order phase transitions in nanoparticles (e.g., metal hydride formation) occur via absorption of species from or release to the gas or liquid phase. If the lattice strain is negligible, the uptake corresponding to a new phase exhibits a stepwise rise or drop with increasing or decreasing pressure or chemical potential of the absorbed species. With decreasing nanoparticle size, L, and increasing role of the surface and subsurface layers, this feature becomes less manifested because the system is less ideal. We present 3D lattice Monte Carlo simulations, explicitly illustrating this effect for L in the range from 100 down to 10 ( L is measured in units of lattice spacing).

Temperature dependence of the enthalpy of formation of metal hydrides characterized by an excess volume

A universal framework to calculate the temperature dependence of the excess enthalpy present in regions characterized by an excess volume is calculated for metals and metal hydrides. At high temperatures, the different contributions from the pressure–volume, heat capacity, entropy and work associated with the thermal expansion are studied separately and their magnitudes and signs are compared. It is found that the pressure–volume contribution opposes and dominates the other three contributions at both high temperature and excess volume, and it is thus found that this contribution becomes the leading temperature dependent contribution to the enthalpy of a material. The conditions under which a temperature change will reduce the enthalpy of formation of metal hydrides are also given and the Mg/MgH 2 system is studied as an example. Excluding the heat capacity contribution, an increase in temperature tends to offset the effect of the excess volume on the enthalpy of formation. It is also demonstrated that the impact of temperature will be more favorable to a reduction of the enthalpy of formation if a large fraction of the metal hydride is in a state of small excess volume compared to a small fraction of the hydride in a state of high excess volume.

Impact of nanostructuring on the enthalpy of formation of metal hydrides

High-energy ball milling has been shown to decrease the release temperature, increase the reaction kinetics, and lower the enthalpy of formation of metal hydrides in certain cases. This paper discusses several potential mechanisms for the reduction of the enthalpy of formation. Although the increased surface and grain boundary energy could play a role in reducing the enthalpy of formation, the predicted magnitude is too small to account for experimental observations. Structural deformation and the associated volume change provide another mechanism for the change in this thermodynamic property. We employed three equations of state models to characterize the excess energy present in the deformed regions and found that the excess volume provides a plausible explanation for the experimentally observed change in thermodynamic properties.

Hydrogen Storage in Pd Nanodisks Characterized with a Novel Nanoplasmonic Sensing Scheme

A novel nanoplasmonic sensing scheme is introduced based on remote real-time detection of induced electronic and shape/structural changes in a metal nanoparticle during the metal-hydride formation process. The localized surface plasmon resonance (LSPR) of the nanoparticle is utilized as signal transducer for optical readout. As a model system, hydrogen storage through metal-hydride formation is studied in Pd nanodisks. The experimentally obtained plasmonic response to hydrogen uptake yields pressure-LSPR-response isotherms. These isotherms are found to obey Sievert's law in the low-pressure range and exhibit a characteristic "plateau" at 18 Torr upon hydrogen charging and 7.5 Torr upon hydrogen discharging. An additional experiment also clearly shows the typical temperature dependence of the plateau pressure. Conversion of the LSPR signal to absolute hydrogen concentration, based on a proposed linear dependence of the LSPR response to hydrogen uptake, results in p-C isotherms in excellent agreement with those in the literature. This puts forward that the LSPR response is an extremely sensitive, remote, and real-time probe for "bulk" changes in a metal nanoparticle and can readily be used to study processes such as metal-hydride formation for hydrogen storage applications, alloying on the nanoscale, thermal reshaping, and so forth.

Study about sorption in sponge and powder titanium of hydrogen isotopes obtained from a cryogenic distillation process

Currently, hydrogen may be stored as a compressed gas or as cryogenic liquid. Neither method appears to be practical for many applications in which hydrogen use would otherwise be attractive. For example, gaseous storage of stationary fuel is not feasible because of the large volume or weight of the storage vessels. Liquid hydrogen could be used extensively but the liquefaction process is relatively expensive. The hydrogen can be stored for a long term with a high separation factor, like solid metal hydride. Using hydride-forming metals and intermetallic compounds, for example, recovery, purification and storage of heavy isotopes in a tritium-containing system, can solve many problems arising in the nuclear fuel cycle. This paper presents a comparative study about hydrogen sorption obtained from a cryogenic distillation process, on two titanium structures: powder and sponge. Also presented is the characterization, by X-ray diffraction, of two structures, before and after the sorption process.

Literature review of thermal and radiation performance parameters for high-temperature, uranium dioxide fueled cermet materials

High-temperature fissile-fueled cermet literature was reviewed. Data are presented primarily for the W–UO2 as this was the system most frequently studied; other reviewed systems include cermets with Mo, Re, or alloys as a matrix. Failure mechanisms for the cermets are typically degradation of mechanical integrity and loss of fuel. Mechanical failure can occur through stresses produced from dissimilar expansion coefficients, voids created from diffusion of dissimilar materials or formation of metal hydride and subsequent volume expansion. Fuel loss failure can occur by high temperature surface vaporization or by vaporization after loss of mechanical integrity. Techniques found to aid in retaining fuel include the use of coatings around UO2 fuel particles, use of oxide stabilizers in the UO2, minimizing grain sizes in the metal matrix, minimizing impurities, controlling the cermet sintering atmosphere, and cladding around the cermet.

Coordination Chemistry of H2 and N2 in Aqueous Solution. Reactivity and Mechanistic Studies Using trans-FeII(P2)2X2-Type Complexes (P2 = a Chelating, Water-Solubilizing Phosphine)

The reactions of the trans-Fe(DMeOPrPE)2Cl2 complex (I; DMeOPrPE = 1,2-bis(bis(methoxypropyl)phosphino)ethane) and its derivatives were studied in aqueous and nonaqueous solvents with a particular emphasis on the binding and activation of H2 and N2. The results show there are distinct differences in the reaction pathways between aqueous and nonaqueous solvents. In water, I immediately reacts to form trans-Fe(DMeOPrPE)2(H2O)Cl+. Subsequent reaction with H2 or N2 yields trans-Fe(DMeOPrPE)2(X2)Cl+ (X2=H2 or N2). In the case of H2, further reactivity occurs to ultimately give the trans-Fe(DMeOPrPE)2(H2)H+ product (III). The pathway for the reaction I --> III was spectroscopically examined: following the initial loss of chloride and replacement with H2, heterolysis of the H2 ligand occurs to form Fe(DMeOPrPE)2(H)Cl; substitution of the remaining chloride ligand by another H2 molecule then occurs to produce trans-Fe(DMeOPrPE)2(H2)H+. In the absence of H2 or N2, trans-Fe(DMeOPrPE)2(H2O)Cl+ slowly reacts in water to form Fe(DMeOPrPE)32+, II. Experiments showed that this species forms by reaction of free DMeOPrPE ligand with trans-Fe(DMeOPrPE)2(H2O)Cl+, where the free DMeOPrPE ligand comes from dissociation from the trans-Fe(DMeOPrPE)2(H2O)Cl+ complex. In nonaqueous solvents, the chloride ligand in I is not labile, and a reaction with H2 only occurs if a chloride abstracting reagent is present. Complex III is a useful synthon for the formation of other water-soluble metal hydrides. For example, the trans-[Fe(DMeOPrPE)2H(N2)]+ complex was generated in H2O by substitution of N2 for the H2 ligand in III. The trans-Fe(DHBuPE)2HCl complex (DHBuPE = 1,2-bis(bis(hydroxybutyl)phosphino)ethane, another water-solubilizing phosphine) was shown to be a viable absorbent for the separation of N2 from CH4 in a pressure swing scheme. X-ray crystallographic analysis of II is the first crystal structure report of a homoleptic tris chelate of FeII containing bidentate phosphine ligands. The structure reveals severe steric crowding at the Fe center.

The effect of hydrogen on the shear modulus of polycrystalline aluminum

Electrolytic hydrogen saturation of polycrystalline aluminum leads to a significant (up to about 30%) decrease in the shear moduli G of samples. This change in G is accompanied neither by the loss of plasticity (no hydrogen-induced brittleness) nor by a change in the electric resistance (in contrast to that observed during the hydrogenation of typical hydride-forming metals and alloys, including those in the amorphous state).

Cocondensation reactions of heterocyclic aromatic compounds with lithium, calcium and magnesium atoms at 77 K

Calcium and magnesium atoms were cocondensed with aromatic heterocycles containing five- and six-membered rings in the presence of THF at 77 K. In the case of calcium the cocondensation with five-membered heterocyclic compounds resulted in C–H bond activations and led to the corresponding aryl calcium compounds, while magnesium did not show comparable reactions. When six-membered heterocyclic compounds, e.g., pyridine and 4-methylpyridine (4-picoline) were cocondensed with calcium, magnesium and lithium atoms, all reactions led to the formation of non-metallated aromatic products and the formation of metal hydride. DFT calculations at the B3LYP/6-31G** level of theory were carried out in order to interpret the pathways of the cocondensation reactions using calcium atoms and identify the possible intermediates involved. In all reactions π- and σ-complexes between calcium atoms and the heterocyclic aromatic reactant were found as stable intermediates on the energy hypersurface.

Electrochemical activation and electrocatalytic enhancement of a hydride-forming metal alloy modified with palladium, platinum and nickel

The influence of the addition of polycrystalline Pt, Pd and Ni powders into hydride-forming metal alloy electrodes on the activation process, rate-capability and cycling stability has been evaluated. The metal powder additives were found to improve the activation rate and to decreased the overpotentials of the charge and discharge processes. The electrochemical properties of these modified electrodes were analysed in terms of a simplified form of a previously developed model. Electrochemical impedance spectroscopy measurements were performed to allow the validation of the model and the identification of kinetic and physicochemical parameters.

Study of the kinetics of hydrogen sorption and desorption from titanium

A method of thermocycling in a hydrogen medium for studying the kinetics of the interaction between hydride-forming metals and hydrogen is proposed. The method is applied to the titanium powder with a grain size of 15–20 μ m. Thermocycling curves were obtained in a wide range of temperatures and concentrations for α - and β -titanium. Evaluations of the kinetic parameters of absoprtion and desoprtion as well as the heat of sorption in α - and β -titanium are presented.

An experimental system to investigate kinetics and isotopic properties of the electrolytic metal hydride formation

We present in this paper an experimental set-up based in a mass spectrometer connected to a closed electrolytic cell. Calibrations accomplished with a Pt cathode and H 2O/D 2O mixtures have shown new kinetics in galvanostatic electrolysis. These findings may be relevant in some important processes such as the hydrogen evolution reaction, isotopic separation factors or the electrolytic formation of metal hydrides.

Finite element analysis for steady-state hydride-induced fracture in metals by composite model

Delayed hydride cracking, which is observed in hydride-forming metals, due to the precipitation of hydrides near the crack tip, is investigated under conditions of constant temperature and crack velocity, plane strain and small-scale hydride-precipitation. The coupling of the operating physical processes of hydrogen-diffusion, hydride precipitation and material deformation is taken into account. The material is assumed to be an elastic composite made of hydrides and solid solution, with properties depending locally on the volume fraction of the hydrides. In the present analysis, the composite elastic properties have been derived by a generalized self consistent model for particulate composites. With respect to hydride-precipitation, two cases have been considered: (i) precipitation in a homogeneous medium with elastic properties, equal to the effective properties of the composite and (ii) precipitation in an inhomogeneous medium, where the expanding hydride has different elastic properties than those of the surrounding solid solution. The differences between the near-tip field distributions, produced by the two precipitation models, are relatively small. The effect of the hydrogen concentration far from the crack tip, on the near-tip field is also studied. It is shown that for small crack growth velocities, near the threshold stress intensity factor, the remote hydrogen concentration weakly affects the normalized stress distribution in the hydride-precipitation zone, which is controlled by the thermodynamically required hydrostatic stress, under hydrogen chemical equilibrium. However, for values of the applied stress intensity factor and the crack tip velocity, away from the threshold stress intensity factor and crack arrest, the effect of remote hydrogen concentration on the normalized near-tip stress field is strong. Reduction of the remote hydrogen concentration generally leads to reduction of the hydride-precipitation zone and increase of the near-tip stresses. Also reduction of the remote hydrogen concentration leads to distributions closer to those under hydrogen chemical equilibrium.