Hydrogen-poor Phase Research Articles

The self-healing of defects induced by the hydriding phase transformation in palladium nanoparticles

Nanosizing can dramatically alter material properties by enhancing surface thermodynamic contributions, shortening diffusion lengths, and increasing the number of catalytically active sites per unit volume. These mechanisms have been used to explain the improved properties of catalysts, battery materials, plasmonic materials, etc. Here we show that Pd nanoparticles also have the ability to self-heal defects in their crystal structures. Using Bragg coherent diffractive imaging, we image dislocations nucleated deep in a Pd nanoparticle during the forward hydriding phase transformation that heal during the reverse transformation, despite the region surrounding the dislocations remaining in the hydrogen-poor phase. We show that defective Pd nanoparticles exhibit sloped isotherms, indicating that defects act as additional barriers to the phase transformation. Our results resolve the formation and healing of structural defects during phase transformations at the single nanoparticle level and offer an additional perspective as to how and why nanoparticles differ from their bulk counterparts.

Imaging the Hydrogen Absorption Dynamics of Individual Grains in Polycrystalline Palladium Thin Films in 3D.

Defects such as dislocations and grain boundaries often control the properties of polycrystalline materials. In nanocrystalline materials, investigating this structure-function relationship while preserving the sample remains challenging because of the short length scales and buried interfaces involved. Here we use Bragg coherent diffractive imaging to investigate the role of structural inhomogeneity on the hydriding phase transformation dynamics of individual Pd grains in polycrystalline films in three-dimensional detail. In contrast to previous reports on single- and polycrystalline nanoparticles, we observe no evidence of a hydrogen-rich surface layer and consequently no size dependence in the hydriding phase transformation pressure over a 125-325 nm size range. We do observe interesting grain boundary dynamics, including reversible rotations of grain lattices while the material remains in the hydrogen-poor phase. The mobility of the grain boundaries, combined with the lack of a hydrogen-rich surface layer, suggests that the grain boundaries are acting as fast diffusion sites for the hydrogen atoms. Such hydrogen-enhanced plasticity in the hydrogen-poor phase provides insight into the switch from the size-dependent behavior of single-crystal nanoparticles to the lower transformation pressures of polycrystalline materials and may play a role in hydrogen embrittlement.

Three-dimensional imaging of dislocation dynamics during the hydriding phase transformation

Crystallographic imperfections significantly alter material properties and their response to external stimuli, including solute-induced phase transformations. Despite recent progress in imaging defects using electron and X-ray techniques, insitu three-dimensional imaging of defect dynamics remains challenging. Here, we use Bragg coherent diffractive imaging to image defects during the hydriding phase transformation of palladium nanocrystals. During constant-pressure experiments we observe that the phase transformation begins after dislocation nucleation close to the phase boundary in particles larger than 300 nm. The three-dimensional phase morphology suggests that the hydrogen-rich phase is more similar to a spherical cap on the hydrogen-poor phase than to the core-shell model commonly assumed. We substantiate this using three-dimensional phase field modelling, demonstrating how phase morphology affects the critical size for dislocation nucleation. Our results reveal how particle size and phase morphology affects transformations in the PdH system.

Avalanching strain dynamics during the hydriding phase transformation in individual palladium nanoparticles.

Phase transitions in reactive environments are crucially important in energy and information storage, catalysis and sensors. Nanostructuring active particles can yield faster charging/discharging kinetics, increased lifespan and record catalytic activities. However, establishing the causal link between structure and function is challenging for nanoparticles, as ensemble measurements convolve intrinsic single-particle properties with sample diversity. Here we study the hydriding phase transformation in individual palladium nanocubes in situ using coherent X-ray diffractive imaging. The phase transformation dynamics, which involve the nucleation and propagation of a hydrogen-rich region, are dependent on absolute time (aging) and involve intermittent dynamics (avalanching). A hydrogen-rich surface layer dominates the crystal strain in the hydrogen-poor phase, while strain inversion occurs at the cube corners in the hydrogen-rich phase. A three-dimensional phase-field model is used to interpret the experimental results. Our experimental and theoretical approach provides a general framework for designing and optimizing phase transformations for single nanocrystals in reactive environments.

Structural and Mössbauer spectroscopy characterization of bulk and nanostructured TiFe0.5 Ni0.5/graphite compounds and their hydrides

The structural and hyperfine properties of bulk TiFe0.5Ni0.5 intermetallic and ball-milled TiFe0.5Ni0.5/graphite compounds and their hydrides have been studied. The bulk and nanostructured TiFe0.5Ni0.5 compounds crystallize in the cubic crystal structure of CsCl (B2). After hydrogenation, the formation of hydrogen-poor phase (∝-phase) and hydride phase (β-phase) have been determined for the bulk compound. However, the formation of the ∝-phase and the hydrogen-richest phase (γ-phase) and other secondary phases have been determined for the ball-milled TiFe0.5Ni0.5/graphite sample. It has been determined that the ball-milled TiFe0.5Ni0.5/graphite sample presents a large amount of the γ-phase which indicates that the presence of graphite nearby nanostructured intermetallic grains enhances the absorption of hydrogen. Mossbauer results are consistent with the structural results. Meanwhile, no significant changes in the isomer shift (IS) value has been determined for the α-phase with respect to the intermetallic compound, a strong increase in the IS value has been determined for the β- and γ-phases with respect to the ∝-phase. That increase indicates a decrease of the s-electron density at the Fe nuclei due to the charge transfer from the metal to the nearby hydrogen atoms.

Disproportionation reaction of LaH2 at high pressure and low temperature

Structural change of LaH2 has been investigated at high pressure and low temperature by synchrotron radiation X-ray diffraction. LaH2 has an fcc metal lattice with interstitial hydrogen atoms located at tetrahedral sites. The fcc-LaH2 decomposes into two phases, which have different hydrogen compositions such as hydrogen-poor LaH and hydrogen -rich LaH2+δ phases, at 11 GPa at room temperature. In spite of largely suppressed diffusivity of the interstitial hydrogen atoms at low temperature, we have found the disproportionation reaction around 14 GPa at 200 K. Difference in the molar volume between each phases suggests monohydride LaH is formed as the disproportionation product even at low temperature. The volume fraction of the hydrogen-poor phase relative to the hydrogen-rich one at 200 K rapidly rises against pressure and reaches to ~0.45 at ~17 GPa.

In situ neutron diffraction study on Pd-doped Mg 0.65Sc 0.35 electrode material

The behavior of a negative composite electrode made of Pd-doped Mg 0.65Sc 0.35 active material has been studied dynamically by in situ neutron diffraction during a complete charge–discharge electrochemical cycle starting from a virgin alloy. From the analysis of the collected diffraction patterns, phase identification, phase amounts, structural changes and cell volume evolutions have been determined as a function of the electrochemical state of (dis)charge. For the first charge, the active material shows a structural transformation from body-centered cubic (bcc) to face-centered cubic (fcc). During the following discharge, a two-phase behavior with equilibrium between a hydrogen-poor fcc phase and a hydrogen-rich one is observed. Complete discharge beyond the hydrogen-poor phase was not reached and a good electrochemical reversibility is expected between the hydrogen-poor and the hydrogen-rich fcc phases for this electrode material.

粉末製造技術とその評価 Ｎｂ‐Ｚｒ‐Ｆｅ合金の水素吸収による粉化現象

In order to find a new energy-saving-powder manufacturing process, the pulverization due to hydrogenation of Nb-Zr-Fe alloys with 35 different compositions has been studied. The hydrogenation was performed under the hydrogen atmosphere of 0.1 MPa and room temperature in an arc melting chamber without exposure to air after arcmelting.Nb-Zr-Fe alloys absorb a large amount of hydrogen from 0.1 to 1.8 wt%H and pulverize into powders. This pulverization is closely related to the composition of alloys.The composition which is easy to pulverize is concentrated to the ranges of 15 at%Fe in Nb-Zr-Fe alloys and 15-30 at%Fe with fixed to Nb/Zr??1/1 in Nb-Zr-Fe alloys. For an example, hydrogen absorbed powder of 45Nb-40Zr-15Fe alloy has wide size distribution from 5 to 200 micron with flake figure and with many fine cracks.The hydrogen in powder of 45Nb-40Zr-15Fe alloy is stably stable until 473 K and dehydrogenation temperature is from 473 K to 773 K.It is estimated that hydrogenation is induced by proper amount of iron element in Nb-Zr-Fe alloys and a hydrogen rich phase and a hydrogen poor phase are formed in an alloy ingot. Forming of two phases by absorbing hydrogen causes to pulverization of Nb-Zr-Fe alloys.

Hydrogen transport through LaNi4.7Al0.3Hx porous electrode in the coexistence of two hydride phases (α- and β-phases): current transient analysis

The hydrogen transport through a LaNi4.7Al0.3Hx porous electrode in the coexistence of two hydrogen-poor phase α-LaNi4.7Al0.3Hx and hydrogen-rich phase β-LaNi4.7Al0.3Hx has been investigated during the hydrogen discharging from the electrode by analysing current decay transients supplemented by a cyclic polarization curve and open-circuit potential transients. From the occurrence of three-staged current decay transients, the β-LaNi4.7Al0.3Hx patches were sporadically embedded in the α-phase matrix during the hydrogen charging into the electrode at the overpotentials between −0.01 and −0.03 V (RHE) and the β-LaNi4.7Al0.3Hx layer was completely embedded in matrix below −0.03 V (RHE). The hydrogen transport through the spherical symmetric LaNi4.7Al0.3Hx particle proceeded by the same mechanism of the “up-hill diffusion” coupled with interface-controlled phase boundary movement, as did the hydrogen transport through the plane symmetric Pd foil electrode. The current decay transient technique employed in this work proved to be a more informative approach to evaluation of the charge-discharge performance of the metal hydride electrode in practical use of the nickel/metal hydride secondary battery.