Hydrogen diffusion in ultrafine-grained palladium: Roles of dislocations and grain boundaries

Iwaoka, Hideaki; Arita, Makoto; Horita, Zenji

doi:10.1016/j.actamat.2016.01.069

Cited by 72 publications

(25 citation statements)

References 62 publications

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“…Much attention has focused on clarifying the mechanism of the high diffusivity of H in nanostructured Pd, such as nanocrystalline grain boundaries [6][7][8], nanoparticles [9,10], and dislocation cores [11,12]. Kofu et al [9] recently examined the diffusion dynamics of H in 8-nm Pd nanoparticles using quasielastic neutron scattering and observed a fast relaxation process with the small activation energy of 0.12 eV, in addition to a conventional relaxation process (0.24 eV) inside the PdH 0.47 nanoparticles.…”

Section: Introductionmentioning

confidence: 99%

Mechanism of fast lattice diffusion of hydrogen in palladium: Interplay of quantum fluctuations and lattice strain

2018

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Understanding the underlying mechanism of the nanostructure-mediated high diffusivity of H in Pd is of recent scientific interest and also crucial for industrial applications. Here, we present a decisive scenario explaining the emergence of the fast lattice-diffusion mode of interstitial H in face-centered cubic Pd, based on the quantum mechanical natures of both electrons and nuclei under finite strains. Ab initio path-integral molecular dynamics was applied to predict the temperature-and strain-dependent free energy profiles for H migration in Pd over a temperature range of 150-600 K and under hydrostatic tensile strains of 0.0%-2.4%; such strain conditions are likely to occur in real systems, especially around the elastic fields induced by nanostructured defects. The simulated results revealed that, for preferential H location at octahedral sites, as in unstrained Pd, the activation barrier for H migration (Q) was drastically increased with decreasing temperature owing to nuclear quantum effects. In contrast, as tetrahedral sites increased in stability with lattice expansion, nuclear quantum effects became less prominent and ceased impeding H migration. This implies that the nature of the diffusion mechanism gradually changes from quantum-to classical-like as the strain is increased. For H atoms in Pd at the hydrostatic strain of ∼2.4%, we determined that the mechanism promoted fast lattice diffusion (Q = 0.11 eV) of approximately 20 times the rate of conventional H diffusion (Q = 0.23 eV) in unstrained Pd at a room temperature of 300 K.

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Section: Introductionmentioning

confidence: 99%

Mechanism of fast lattice diffusion of hydrogen in palladium: Interplay of quantum fluctuations and lattice strain

2018

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“…The evidence exhibited a more sensitive value of diffusion coefficient with charging current density in SPD-processed metals, both in the absorption step and in the desorption step, compared with that in coarse counterparts. The results also indicated a higher value of diffusion coefficient as well as a lower activation energy in the SPD-processed matrix [48].…”

Section: Grain Boundariesmentioning

confidence: 64%

“…Grain boundaries also offered different hydrogen solute site energies which contributed to the enhancement in hydrogen diffusivity along the grain boundary. Recently, Iwaoka et al investigated the diffusion behavior of hydrogen in SPD-processed UFG palladium by electrochemical permeation tests [48]. The evidence exhibited a more sensitive value of diffusion coefficient with charging current density in SPD-processed metals, both in the absorption step and in the desorption step, compared with that in coarse counterparts.…”

Section: Grain Boundariesmentioning

confidence: 99%

A Critical Review of Mg-Based Hydrogen Storage Materials Processed by Equal Channel Angular Pressing

Lisha

Jiang

et al. 2017

Metals

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Abstract:As a kind of cost-efficient hydrogen storage materials with high hydrogen capacity and light weight, Mg-based alloys have attracted much attention. This review introduces an effective technique in producing bulk ultrafine-grained (UFG) Mg alloys and promoting its hydrogen storage property, namely, equal-channel angular pressing (ECAP). This paper briefly describes the technical principle of ECAP and reviews the research progress on hydrogen storage properties of ECAP-processed Mg alloys. Special attention is given to their hydrogen storage behaviors including hydrogen storage dynamics, capacity, and cycling stability. Finally, it analyzes the factors that affect the hydrogen storage properties of ECAP-processed Mg alloys, such as the grain sizes, lattice defects, catalysts, and textures introduced by ECAP process.

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“…It is orders higher by comparison with regular solids with defects or dislocations at similar temperatures. [11][12][13] By carefully inspecting the atomic structure and MD trajectories, we failed to find any well-defined dislocations or grain boundaries. Instead, it looks like all atoms participate in the diffusion process concertedly, though not simultaneously.…”

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confidence: 96%

Prediction of a Mobile Solid State in Dense Hydrogen under High Pressures

Geng

Sun

2016

J. Phys. Chem. Lett.

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The solid state of matter is usually characterized by the structural rigidity and spontaneous resistance to the change in its shape against external forces. The particles in a typical solid are tightly bound to each other, and vibrate around their equilibrium sites. In a crystalline state, particles distribute in a regular geometric lattice, thus giving long-range positional ordering.1 Amorphous or glassy states are also considered as solid, except there particles distribute irregularly. Usually a crystalline state must be a solid, and to crystallize is always equivalent to make a solid. The liquid state, on the other hand, is flowing, thus being homogeneous and isotropic. It is absence of any long-range spatial ordering, which is a consequence of the free mobility of particles at both microscopic and macroscopic scales in this state. with regular solids with defects or dislocations at similar temperatures. 11-13 By carefully inspecting the atomic structure and MD trajectories, we failed to find any well-defined dislocations or grain boundaries. Instead, it looks like all atoms participate in the diffusion process concertedly, though not simultaneously. Thus it is a bulk behavior, rather than migration confined to dislocations or boundaries. Because we know that bulk flowing is an intrinsic characteristic of liquid, this raises the speculation that in this phase the protons could be considered as a flow as in a liquid.This observation is consistent with previous simulations, where it was taken as a normal liquid. 14 However, we noticed that though the calculated radial distribution function (RDF) gives a feature very similar to a typical liquid, a distorted lattice pattern still presents through the long-time averaged proton density distribution (the inset in the left panel of Fig. 1). This is at odds with the assumption that it is in an isotropic liquid state. Furthermore, the projected RDFs 15 (right panel in Fig. 1) unveil a strong anisotropy and long-range positional ordering. This is quite different from our understanding about a typical liquid, in which the particle density distribution must not have any local or long-range features and the system must be isotropic. The unexpected emergence of anisotropy implies that the matter still have some crystalline 6 features, which is a strong indicator for a solid state. For these reasons, it seems plausible to tentatively interpret it as a novel matter state in which the solid and liquid features coexist harmonically in one pure phase.On the other hand, the observed structure pattern and anisotropy are maintained over a wide temperature range. As shown in Fig. 1 that was calculated with AI-PIMD simulations at 100 K, the increase of temperature does not change the structure pattern very much, and the accumulated RDF and projected RDFs are almost the same as that at 50 K. But interestingly, the rMSD (thus the self-diffusion mobility) is greatly enhanced. In other words, in this phase the atomic diffusivity can be enhanced without at the expense of the solid framew...

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Hydrogen diffusion in ultrafine-grained palladium: Roles of dislocations and grain boundaries

Cited by 72 publications

References 62 publications

Mechanism of fast lattice diffusion of hydrogen in palladium: Interplay of quantum fluctuations and lattice strain

Mechanism of fast lattice diffusion of hydrogen in palladium: Interplay of quantum fluctuations and lattice strain

A Critical Review of Mg-Based Hydrogen Storage Materials Processed by Equal Channel Angular Pressing

Prediction of a Mobile Solid State in Dense Hydrogen under High Pressures

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