Stable Hydride Phase Research Articles

Effect of addition of rare earth element La on the hydrogen storage properties of TiFe alloy synthesized by mechanical alloying

In this paper, we report the effect of adding the rare earth element lanthanum on the hydrogen storage properties of TiFe synthesized by mechanical alloying (MA). A remarkable improvement was observed in the first hydrogenation (activation) of La-added TiFe. Milling led to the refinement of the crystallite size of the TiFe alloy leading to nanocrystallinity. The formation of a stable hydride phase LaH3 facilitated heterogeneous nucleation and acted as a gateway for the hydrogenation of the TiFe phase. No change in thermodynamics was observed for the rare earth-added TiFe compared to pure TiFe reported in the literature. The microstructure revealed a uniform distribution of the elements. However, La was completely immiscible with the TiFe alloy-a conclusion drawn from the thermodynamics study of the sample. Also investigated was pure TiFe synthesized by MA and arc melting to compare these two synthesis methods.

Zirconium hydride phase mapping in Zircaloy-2 cladding after delayed hydride cracking

Hydrogen uptake in zirconium alloys is a major limiting factor of nuclear fuel cladding as subsequent hydride precipitation can lead to potential cladding failure via delayed hydride cracking (DHC). The specific mechanisms of DHC have been a focus in earlier research, however there is still no clear definition of the resulting crystallography and limiting hydride sizes at various temperatures of crack propagation. In this work, a novel thermo-mechanical testing procedure was used to induce radially propagating DHC cracks in thin-walled cladding in a temperature range from 100 to 320 °C. Focused ion beam (FIB)- prepared lamellae were extracted from samples containing the arrested DHC crack for synchrotron micro-beam X-ray diffraction (µXRD) mapping of crystallographic phases. The µXRD phase maps provided information about the required hydride cluster size that would lead to fracture (less than 5 µm), i.e. the critical hydride size. Phase mapping shows that DHC-responsible hydrides primarily consist of the δ phase, with slightly increased ɣ/δ ratios at lower cracking temperatures. In multilayer cladding, the liner material shows significantly increased ratios of ɣ-hydride, while on average, the δ-phase remains predominant. This work also highlights that the ɣ-hydride is indeed a stable zirconium hydride phase, which can precipitate during DHC propagation and within the inner liner of multilayer cladding.

Hydrogen storage performance of LaNi3.95Al0.75Co0.3 alloy with different preparation methods

Rare-earth AB5-type La–Ni–Al hydrogen storage alloys are widely studied due to their extensive application potentials in hydrogen isotope storage, hydrogen isotope isolation and hydrogen compressors, etc. Good hydriding/dehydriding kinetics, easily activation, high reversibility are important factors for their practical application. However, their overall hydrogen storage performance, especially plateau pressure and hydrogen absorption/desorption durability need to be further optimized. In this study, the microstructures and the hydrogen storage properties of as-cast, annealed, and melt-spun LaNi3.95Al0.75Co0.3 alloys were investigated. The experimental results of XRD and SEM showed that all alloys contained a pure CaCu5 type hexagonal structure LaNi4Al phase. The cell volume increased in an order of annealed ​> ​melt-spun ​> ​as-cast, resulting in a lower hydrogen absorption/desorption plateau pressure and a more stable hydride phase. The hydrogen storage capacity of three alloys was almost the same. The slope factor of the annealed and melt-spun alloys is smaller than the as-cast alloy, indicating that heat-treatment process can make the alloys more uniform. For the cycle stability of the alloys, the hydrogen absorption rate of the annealed alloy and melt-spun alloy was much faster than that of the as-cast alloy after 500 cycles. The melt-spun alloy showed high pulverization resistance during hydrogen absorption/desorption, and exhibited an excellent cycling retention of 99% after 500 cycles, suggesting that melt-spinning process can enhance the cycle stability and improve the cycle life of the alloy.

Towards resolving a long existing phase stability controversy in the Zr-H, Ti-H systems

In the binary Zr-H, Ti-H systems, there are three common hydride phases, namely γ-, δ-, and ε-hydride, with different ratios of H to Zr/Ti atoms. Among them, the relative stability of γ-hydride remains controversial, although it has been observed in many studies. In this study, we demonstrate that γ-hydride is the only stable hydride phase in high purity Zr and commercial purity Ti, using electrolytically loaded hydrogen and ex-situ synchrotron x-ray diffraction. Further, in-situ synchrotron heating/cooling experiments were conducted with hydrided pure Zr sample demonstrating the co-existence of γ- and δ-hydride phase, from a furnace cooling heat treatment followed by three days room temperature aging. It is shown that during heating γ-hydride dissolved first, while δ-hydride remained stable at temperatures up to 280°C, followed by dissolution of δ-hydride until all hydrogen was in solution. Under subsequent slow cooling, δ-hydride precipitated first, followed by a slow formation of γ-hydride when temperatures were below 210°C, which continued during a subsequent 60 hours aging at room temperature. The phase stability when hydride forms as precipitates in Zr is therefore determined to be δ-hydride as the high temperature phase, and γ-hydride as the low temperature phase. The microstructural characteristics of γ-hydride in Zr and Ti were studied by electron microscopy. The observed co-existence of γ- and δ-hydride as well as change from δ to γ during long term room temperature aging in the TEM confirmed the synchrotron data.

Stability of LaNi5-xCox alloys cycled in hydrogen — Part 1 evolution in gaseous hydrogen storage performance

In this study, the evolution trends in various hydrogen storage properties as well as kinetics performance of LaNi5−xCox (x = 0, 0.25, 0.50 and 0.75) alloys up to 1000 cycles are studied, and the effects of Co on the long-term hydrogen absorption/desorption properties are revealed. The alloys have single LaNi5 phase structure. The cell volume increases with increasing Co content, resulting in a lower hydrogen absorption/desorption plateau pressure and a more stable hydride phase. The alloys have good kinetics performance and can fully absorb hydrogen within 300 s. The hydrogen absorption rate increases with either cycling or Co addition, but neither of them changes the rate limiting step which remains to be diffusion controlled model in the temperature range of 343–383 K. With cycling, an intermediate γ phase emerges with the coexistence of α and β phases indicated by an extra higher plateau in P-C-T curves. This γ phase acts as a buffer releasing the microstrains which is reflected from the slower capacity degradation, and the significant decrease in hysteresis and slope factor upon its appearance. Moreover, the c/a value increases after 1000 cycles which also relieves the microstrains in the alloys. Co addition not only enhances the buffer effect of γ phase by partially combining it with β phase, but also promotes the increasing degree of c/a value, contributing to a better crystal structure, bigger particle size and higher cycle stability during long-term cycling.

Structure and hydrogen storage properties of mechanically alloyed Ti-V alloys

Ti-V alloys are potential candidates for hydrogen storage materials. In this study, mechanical alloying under an argon atmosphere was used to produce Ti2−xVx nanocrystalline alloys (x = 0.5, 0.75, 1, 1.25, 1.5). Shaker type ball mill was used. An objective of the present study was to investigate an influence of chemical composition and method of production on hydrogenation and dehydrogenation properties of Ti-V alloys. X-ray diffraction analyses revealed formation of BCC solid solution after 14 h of milling. It is the first time of obtaining this phase directly from mechanical alloying method. HRTEM images confirmed formation of nanocrystalline materials. Synthesized materials were studied by a conventional Sievert's type apparatus at 303 K. It was observed that the maximum hydrogen storage capacity is increased with increased Ti content in the alloy. Ti1.5V0.5 alloy showed high hydrogen storage capacity at room temperature, which reached about 3.67 wt.%. Simultaneously, it was noticed that Ti-rich alloys form more stable hydride phases than V-rich alloys. Observed properties resulted mainly from structure of studied materials.

Effect of hydrogen absorption/desorption cycling on hydrogen storage properties of a LaNi3.8Al1.0Mn0.2 alloy

The effect of long-term hydrogen absorption/desorption cycling up to 3500 cycles on the hydrogen storage properties of LaNi3.8Al1.0Mn0.2 alloy was investigated. The pressure-composition (PC) isotherms for absorption/desorption and the absorption kinetics were measured at 433 K, 453 K and 473 K. X-ray diffraction analysis revealed that the alloy had a homogeneous hexagonal CaCu5 type structure and kept this structure even after 3500 cycles, but the diffraction peaks were broadened. The degree of peak broadening was increased with increase of the cycle number, but exhibiting a maximum after initial activation. The shapes of PCT curves after 300, 2000 and 3500 cycles were similar to that after initial activation. It was found that the alloy subjected to 300 cycles did not exhibit significant changes in hydrogen storage capacity, but the long-term cycling up to 2000 and 3500 cycles resulted in obvious decrease in hydrogen storage capacity. The degradation of the hydrogen capacity might be resulted from the formation of the irreversible sites and more stable hydride phase, though no new phase was found after absorption/desorption cycling from XRD pattern as shown in Fig. 6 because of the limitation of XRD analysis sensibility. The hydrogen absorption kinetics after 300 cycles was deteriorated but improved again after 2000 and 3500 cycles compared with that of after initial activation. The changes in hydrogenation properties of the alloy induced by cycling were discussed by considering the crystal structure, lattice strain and pulverization of the sample.

Synthesis, characterization and hydrogen storage behaviour of AB2 (ZrFe2, Zr(Fe0.75V0.25)2, Zr(Fe0.5V0.5)2 type materials

In this paper, we describe and discuss the synthesis, structural-microstructural and hydrogen storage behaviour of three AB2 type storage materials namely (a) ZrFe2, (b) Zr(Fe0.75V0.25)2 and (c) Zr(Fe0.5V0.5)2. These alloys were synthesied by radio frequency induction melting in argon atmosphere. X-ray diffraction and transmission electron microscope have been employed for structural and microstructural characterizations. The XRD study reveals that the lattice constants and the unit cell volume of ZrFe2, Zr (Fe0.75V0.25)2, Zr(Fe0.5V0.5)2 alloys, which has C14 type hexagonal Laves phase. The Surface morphology and elemental composition of these alloys were investigated by scanning electron microscope and energy dispersive X-ray analysis. The pressure composition isotherms of these alloys were investigated at room temperature and pressure ranges of 0–100 atm respectively, measured through a fully computerized PCI apparatus. As we increase the concentration of V (substituted for Fe), the total hydrogen storage capacities increased up to 1.45 wt%. This capacity is achieved in Zr(Fe0.5V0.5)2 alloy, while the reversible hydrogen storage capacity decreases due to the formation of a stable hydride phase. It has been found that the lattice constants increase with higher vanadium concentration. This is indicating that the majority of vanadium atoms reside in the B-site. The broader X-ray diffraction peaks observed in Zr(Fe0.5V0.5)2 alloy indicates a higher degree of disorder for alloys with the higher V-content. The yet another interesting feature observed in our present study is that the plateau pressure remains well below 1 atm for all the compositions.

Structural and desorption characterisations of milled [formula omitted] powder mixtures for hydrogen storage

( MgH 2 + Y , Ce ) mixtures were processed by mechanical milling in an attempt to improve the hydrogen sorption properties of MgH 2 . The structural evolution and hydrogen desorption properties were investigated by a range of experimental techniques. XRD analysis shows the formation of a stable hydride phase, CeH 2.35 or YH 3 , in the relevant mixture during milling. Hydrogen desorption of the milled mixtures was characterised by coupled thermogravimetry (TG) and differential scanning calorimetry (DSC). Milled ( MgH 2 + Ce ) samples show higher desorption kinetics than the ( MgH 2 + Y ) . The formation of CeO 2 in the ( MgH 2 + Ce ) mixture benefits the desorption kinetics, likely due to the formation of surface defects on MgH 2 and surface catalysis by the oxide.

Hydrogen-induced rearrangements in Pd-rich alloys

Abstract Homogeneous fcc Pd alloys have been found to undergo lattice rearrangements such as phase separation in the presence of dissolved H. For Pd–Rh alloys the phase-separated form is the thermodynamic stable state but for others which have been investigated, for example, Pd–Ni, Pd–Pt, the binary homogeneous substitutional solutions are the thermodynamically stable state. Phase separation for the latter alloys is a consequence of a ternary (Pd+M+H) equilibrium. The rearranged lattices are metastable after evacuation of the dissolved H at a temperature where the recovery back to its homogeneous state does not take place at a measurable rate. These H-induced changes differ from the lattice rearrangements which accompany hydride formation in metals and alloys where the hydride phases are very stable and which have different structures from the parent alloy, e.g. the formation of dihydrides in Zr or Zr-based alloys such as Zr–Nb (5 at.%). These very stable hydride phases will return to their initial states after removal of H at the required elevated temperatures. By comparison, H can be removed at moderate temperatures from the fcc Pd-based alloys because H is not very strongly held in the lattices. The metastable, H-free phase-separated Pd alloys return towards their homogeneous state after annealing in vacuo at, for example, 673 K, demonstrating that the phase-separated binary alloy is metastable. Results of H-induced changes in Pd-rich alloys are discussed for different ranges of temperature and hydrogen pressure for several Pd alloy systems. A ternary-phase diagram is calculated using known thermodynamic parameters for hydrogen solution in the homogeneous binary alloys and the thermodynamics of mixing of the metals. Using the (Pd+Pt) system as an example, a ternary (Pd+Pt+H) equilibrium is shown to cause phase separation.

CeMnA1H x, a new metal hydride

In contrast to CeAl 2, which does not form hydrides, and CeMn which does not form binary intermetallic compounds at all, CeMnAl absorbs considerable amounts of hydrogen. A stable hydride phase (β-phase) is formed at low pressures (<10 mbar) with a capacity of about 1.2 H/formula unit (f.u.). Hydrogen is absorbed up to 2.4 H/f.u. at pressures > 10 mbar (at 300 K). In this work we show the absorption characteristics and structural properties of this new metal hydride. The intermetallic compound CeMnAl crystallizes in the cubic C15 Laves phase as observed by X-ray diffraction. Neutron scattering experiments on this intermetallic compound show that the Mn and Al atoms are distributed randomly on the B sites in this AB 2-type C15 Laves phase compound (A = Ce; B = Mn, Al). Upon hydride (deuteride) formation the C15 structure is preserved. The deuterium atoms occupy only the 2A-2B-tetrahedral interstitial sites (96g-positions) over the investigated concentration range 0.0<×≤2.5 in CeMnAID x .

Some peculiarities of interactions in the Hf2Fe−H2 system at low temperatures and high pressures

Hydride formation was studied in the Hf2Fe−H2 system at hydrogen pressure of up to 2000 atm in a temperature range from 195 to 295 K. Hydride phases of different compositions were studied by the X-ray diffraction method. The hydrogenation reaction in the system can take two pathways to form two stable hydride phases depending on the conditions of initial hydrogenation. Absorption of hydrogens at a pressure of about 2000 atm yields a hydride which contains two H atoms per metal atom. Models of the arrangement of hydrogen atoms in the crystal lattice of hydride phases were suggested.

Hydrogenation of titanium-based quasicrystals.

Measurements of hydrogen loading and unloading in a quasicrystal are reported. The icosahedral phase (i phase) in a Ti-Zr-Ni alloy demonstrates hydrogen absorption from the gas phase at a temperature of 260 \ifmmode^\circ\else\textdegree\fi{}C and a pressure of 40 atmospheres. The incorporation of hydrogen causes the quasilattice to expand, with an increase in quasilattice constant of almost 7%, from ${\mathit{a}}_{\mathit{i}}$=5.18 to 5.52 \AA{}. Similar abilities for storing hydrogen are found in high-order crystalline approximant and amorphous phases of like composition. Of the phases examined, the i phase absorbs the most hydrogen, giving a hydrogen atom to metal atom ratio ([H]/[M]) of 1.6. Initial studies indicate that hydrogen desorption is hindered by phase transitions to stable hydride phases. Hydrogen loading at lower temperatures with higher ${\mathrm{H}}_{2}$ pressures favors higher [H]/[M] ratios for the i phase and the crystal approximant, and also minimizes the growth of more stable crystalline hydride phases.

The intrinsic degradation behaviour of the Laves phase alloy Zr 0.9Ti 0.1Cr 0.9Fe 1.1 upon temperature-induced hydrogen absorption-desorption cycling

The intrinsic degradation behaviour of the Zr-based Laves phase alloy Zr 0.9Ti 0.1Cr 0.9Fe 1.1 upon temperature-induced hydrogen absorption-desorption cycling is investigated by pressure-composition isotherm measurements, X-ray diffractometry and thermal analysis on the degraded samples. After 7000 thermal cycles between room temperature and 270 °C the reversible hydrogen absorption capacity decreases by about 15%. As the cycling temperature and/or applied hydrogen pressure increase, the degradation phenomenon becomes more severe. The cause of the intrinsic degradation is found to be a stable crystalline hydride phase with a higher Cr content than the original matrix. Under severe cycling conditions it is observed that transformation to an amorphous phase is also a possible reason for the degradation. These structural modifications are strongly activated by the internal accumulation of elastic strain due to the repetitive hydrogenation treatments and the applied thermal energy. It is suggested that the corresponding mechanism is related to the motion of Cr and Fe atoms.

Hydrogen absorption characteristics of Pd 1− xLi x ( x = 0.055 and 0.072) alloys and ordered Pd 7Li

Thermodynamic quantities were determined for the absorption of hydrogen in Pd-5.5at.%Li and Pd-7.2at.%Li alloys, the latter alloy being in the two-phase field of saturated α-Pd(Li,s) solid solution and the ordered Pd 7Li phase. Data were also determined for the Pd-10.6at.%Li alloy which consists only of the ordered Pd 7Li phase. The data were determined from measurements of pressure-composition isotherms at temperatures between 273 K and 463 K and hydrogen pressures up to 1000 Torr. The PdLi alloys dissolve considerable amounts of hydrogen and form a more stable hydride phase than Pd despite the lattice contraction which occurs on alloying Pd with Li. The hydrogen solubility in the ordered Pd 7Li phase alone is a little smaller than that in the two-phase mixture. The large hydrogen absorption of PdLi alloys, which does not depend on the presence of ordered Pd 7Li, may be attributed to (i) an attractive HLi pair interaction, (ii) a decrease in the strain energies necessary for hydrogen occupation of the interstices, because of the similar lattice parameters of the α-Pd(Li,s) and ordered Pd 7Li phases, and the large compressibility of PdLi alloys, and (iii) the valence of 1 of Li in Pd compared with 3 for, for example, Y in Pd.

Thermal hydriding of some Ni-based intermetallic compounds

The behavior of various types of Ni-based intermetallic alloys, namely NiZr, NiZr 2 and LaNi 5 , in thermal hydriding-dehydriding cycles has been investigated from room temperature to 1173K at various initial pressures ranging from 0.2 to 3.3 MPa absolute. It has been found that, upon hydriding, the rate of hydrogen absorption becomes enormous at specific temperatures, regardless of the initial hydrogen pressure over the samples. The same phenomenon has been observed upon dehydriding (i.e. the rate of hydrogen release becomes high at certain temperatures). The incident has been found to be reproducible. The specific temperature changes from one alloy to another during hydriding cycles. A method by which to record and present the phenomenon has been suggested, plotting the rate of change of hydrogen in the hydride with the temperature, where these sudden changes appear as sharp and distinctive peaks. This phenomenon has been attributed to the formation of a stable hydride phase in the case of LaNi 5 , and a phase transition in both NiZr and NiZr 2 .

Studies of sorption-desorption processes in the Ce xLa 1− xNi 5 −H 2 system by DSC

The dependence of the integral enthalpy of hydrogenation (or dehydrogenation) of the intermetallic compounds (IMCs) Ce x La 1− x Ni 5 ( x = 0, 0.05, 0.1 or 0.3) on pressure in the range 0.5–6 MPa was studied by the method of differential scanning calorimetry. It has been shown that at x = 0 and x = 0.05 the absorption (or desorption) proceeds via the formation of a stable intermediate hydride phase.

ChemInform Abstract: A SIMS Investigation of Hydrogen Penetration of Titanium Electrodes

Abstractcarried out using a spectrometer which is able to detect and image hydrogen confirms that during prolonged corrosion of Ti in acidic solutions a stable titanium hydride phase is built up which extends to a considerable depth (several microns) in the metal.

The thermodynamics of the ErFe 2H system using differential heat conduction calorimetry

A differential twin-cell heat flow calorimeter has been constructed for the measurement of the enthalpy changes of the reaction of hydrogen gas with intermetallic compounds as a function of their hydrogen contents. It was designed especially for use with those intermetallic compounds which form very stable hydride phases; therefore prior to the calorimetric studies it must be possible to evacuate the sample in situ at elevated temperatures to remove all of the hydrogen resulting from the activation procedure. The calorimeter was employed in this research for the determination of the thermodynamics of the reaction of H 2 with ErFe 2H x over the range of hydrogen contents x = 0.4. ErFe 2H is a multiplateau system; the enthalpy change for hydride formation corresponding to the plateau near x = 3 is more exothermic than that corresponding to the preceding plateau near x = 2. In addition to the calorimetric determination of the enthalpies of reaction, the free energies were determined simultaneously from the equilibrium hydrogen pressures. Using these free energy and enthalpy data the entropies were also obtained over a range of hydrogen contents.

Degradation of LaNi 5 by temperature-induced cycling

We studied the hydrogen absorption-desorption properties of LaNi 5 and (La 0.9Eu 0.1)Ni 4.6Mn 0.4 for 1500 thermally induced cycles at approximately 8 – 20 atm and found that the amount of reversibly absorbed hydrogen decreased markedly. In addition to the gas absorption measurements, X-ray measurements and Mössbauer spectroscopy studies of 151Eu in the intermetallic were performed. Combining the results of these measurements we established that the degradation occurs via the formation of a stable hydride phase rather than via oxidation by water or oxygen impurities. The degradation that we observe is thus an intrinsic process. We believe this to be the first report of intrinsic degradation of the gas-absorbing properties in LaNi 5. The results are discussed in the light of recent magnetization measurements on pressure-cycled LaNi 5.