Shigetoshi Koike

Superdiffusion of hydrogen in vanadium measured by the permeation method

Abstract The gas permeation method using a quadrupole mass spectrometer was employed to observe the superdiffusion phenomenon of hydrogen directly under the application of external stress in polycrystalline vanadium coated with an electroplated film of palladium. The effective diffusion coefficients measured under an applied stress above 18 MPa at 404 and 426 K steeply increase to 6–10 times larger values than the normal ones at zero applied stress. In addition, information on the behavior of hydrogen in the surperdiffusion state obtained from the finding that the solubility of hydrogen decreases by a factor of about 1 3 , presents new evidence that superdiffusion takes places by the hopping motion of 4 T hydrogen atoms.

Displaced-T Site Occupancy of Hydrogen in Nb Alloyed with a High Concentration of Mo

The site occupancy of hydrogen in 52 at. % Nb–48 at. % Mo alloys is investigated at room temperature for hydrogen concentrations 0.028 and 0.055 in the hydrogen–metal-atom ratio ( C H =[H]/[M]) by the channelling method utilizing a nuclear reaction 1 H( 11 B,α)αα with a 11 B beam. A large portion of H atoms are located at sites displaced from tetrahedral ( T ) sites towards their nearest neighbour octahedral ( O ) sites by about 0.3 A (displaced- T sites; d - T ), which are similar to the sites observed in V under uniaxial stress and completely different from the sites observed in the Nb alloys containing undersized Mo atoms up to 26 at. %. For C H =0.028 and 0.055, 30–40% and 15–25% of H atoms are located at T sites, respectively.

Change of site occupancy of hydrogen in Nb on alloying with undersized Mo

The site occupancy of hydrogen in Nb–20 at.%Mo alloys is investigated at room temperature for H concentrations ( C H ) 2.6, 4.2, and 8.0 at.% by a channelling method using a nuclear reaction 1 H( 11 B, α)αα. Different from previous results for lower Mo concentrations, a large portion of H atoms are at tetrahedral ( T ) sites and, in addition, the octahedral ( O ) site occupancy is newly observed. The hydrogen distribution changes with C H . At C H =2.6 and 4.2 at.%, the 20–30% of H atoms are located at O sites, while at C H =8.0 at.% the fraction of the O -site occupancy remarkably decreases to 0–10%.

Site Change of Hydrogen in Niobium on Alloying with Oversized Ta Atoms

In order to clarify a difference in hydrogen interaction with oversized solute atoms and with undersized solute atoms in bcc metals in the low solute concentration region, the site occupancy of hydrogen in Nb alloyed with 5 at. % of oversized Ta atoms has been studied at room temperature for hydrogen concentrations of 0.018 and 0.025 at the hydrogen-to-metal-atom ratio ( C H =[H]/[M]) by the channelling method utilizing a nuclear reaction 1 H( 11 B,α)αα with a 11 B beam of an energy of 2.03 MeV. Clearly different from the result on hydrogen in Nb alloyed with undersized Mo atoms, in both specimens H atoms are distributed over tetrahedral ( T ) sites and the displaced- T sites ( d - T sites) which are displaced from T sites by about 0.25 A towards their nearest neighbour octahedral ( O ) sites. The T site is more favourable for hydrogen occupancy, but the number of available T sites is limited, and excess H atoms occupy the d - T sites. Therefore, in contrast to a strong attractive interaction between hydr...

Irradiation-Induced Site Change of Hydrogen in Niobium

The effect of ion-irradiation on the state of hydrogen in Nb doped with hydrogen of a concentration of [H]/[M] = 0.023 (hydrogen-to-metal-atom ratio) from the gas phase has been investigated at room temperature by the channelling method utilizing a nuclear reaction 1 H( 11 B,α)αα with a 11 B beam of an energy of about 2 MeV. It has been demonstrated that, before irradiation, hydrogen is located at a tetrahedral ( T ) site, while, after irradiation at room temperature with the 11 B beam in the off-channelling direction up to a dose of about 1.4 ×10 16 /cm 2 , hydrogen is displaced from a T site by 0.45–0.55 A towards its nearest neighbour lattice point. It is concluded that this irradiation-induced site change of hydrogen is due to trapping of hydrogen by irradiation-introduced monovacancies. Hydrogen is located at a site displaced from one of the nearest neighbour T sites of a vacancy by 0.45–0.55 A towards that vacancy.

On the Superdiffusion of Hydrogen in Va-Metals

Experimental procedures for the determination of superdiffusion coefficients of hydrogen in V a -metals (V, Nb, Ta) under external tensile stress parallel to are described in detail. Results are analyzed in the light of the lattice activated tunneling theory. The activation energy E a and the tunneling matrix element | J | are obtained experimentally as follows: E a [meV]: 21 for V, 15 for Nb, 12 for Ta, and | J |[meV]: 0.44 for V, 0.085 for Nb, 0.065 for Ta. The superdiffusion mechanism of hydrogen in these metals is theoretically discussed.

Site Occupancy of Hydrogen in a Distorted bcc Lattice in Niobium Alloyed with Mo Atoms

In order to study the atomistic state of hydrogen in a bcc lattice, the site occupancies of hydrogen dissolved in Nb–Mo alloys containing 39, 48, and 60 at. % Mo atoms have been investigated at room temperature by the channelling method utilizing a nuclear reaction of 1 H( 11 B,α)αα with a 11 B + beam of about 2 MeV. In addition, the lattice parameters and half-widths of the X-ray reflection lines of the non-hydrogen-doped specimens have been measured at room temperature. Nb–Mo alloys form a solid solution over the entire Mo concentration range, maintaining a bcc crystal structure. From systematic studies including previous experiments on such alloys with lower Mo concentrations, it has been observed that the site occupancy of hydrogen changes very sensitively with Mo concentration. At low Mo concentrations, hydrogen is trapped by Mo atoms and occupies the trapped site ( T tr ), which is displaced by about 0.6 A from a tetrahedral ( T ) site attached to a Mo atom towards that Mo atom. At approximately 20 ...

Formation of Hydrogen-Associated Vacancy Clusters in Nb as Investigated from Site Occupancy Change of Hydrogen

In order to study hydrogen-defect interactions, the effects of ion irradiation and annealing on the site occupancy of hydrogen dissolved in Nb, NbH 0.023 , have been investigated at room temperature by the channelling method utilizing a nuclear reaction 1 H( 11 B,α)αα with a 2 MeV 11 B + beam. By irradiation at room temperature up to a dose of 1.4×10 16 /cm 2 of 2 MeV 11 B ions, the lattice location of hydrogen changes from the original tetrahedral ( T ) site ( T -1 state) to the T tr site, which is displaced from a T site by 0.45–0.55 A towards its nearest-neighbour lattice point. This site occupancy remains the same even for an approximately three times higher irradiation dose. On the subsequent annealing at 523 K for 1 h, the state of hydrogen changes from the T tr -site occupancy to the occupancies of (55–70)% of H atoms at T sites ( T -2 state) and (30–45)% at random ( R ) sites. The irradiation-induced site change from the T site to the T tr site is due to the trapping of hydrogen by vacancies, i.e....

Site Occupancies of Hydrogen in Nb Alloyed with Oversized Ta Atoms or Undersized Mo Atoms

Site occupancies of hydrogen in Nb alloyed with 2 or 5 at. % of oversized Ta atoms have been investigated at room temperature for hydrogen concentrations of 0.01 and 0.029 at the hydrogen-to-metal-atom ratio ( C H =[H]/[M]), Nb 0.98 Ta 0.02 H 0.01 , Nb 0.98 Ta 0.02 H 0.029 , and Nb 0.95 Ta 0.05 H 0.01 , by the channelling method utilizing a nuclear reaction 1 H( 11 B,α) 8 Be with a 11 B beam of about 2 MeV. It has been clearly demonstrated that hydrogen occupies also tetrahedral ( T ) sites, in addition to the displaced T sites ( d - T sites), which were observed at higher hydrogen concentrations in the Nb 0.95 Ta 0.05 alloys. The d - T site is a site displaced from a T site by about 0.25 A towards its nearest neighbour octahedral ( O ) site. The T site is more favourable, but the concentration of available T sites is limited, and excess H atoms occupy the d - T sites. This site occupancy is different from that in Nb alloyed with similar concentration of undersized Mo atoms, where hydrogen preferentially ...

Site Change of Hydrogen in Nb due to Interaction with Oxygen

To locate hydrogen in Nb alloyed with 0.3 at. % oxygen, which is the alloy system for the tunnelling motion of hydrogen to be observed, channelling experiments have been performed at room temperature utilizing a nuclear reaction 1 H( 11 B,α)αα with a 11 B beam of about 2 MeV. It has been demonstrated that for a hydrogen concentration of C H = 0.003 ( C H = [ H ]/[ M ]; hydrogen-to-metal-atom ratio), H atoms are located at sites displaced by 0.5–0.6 A from tetrahedral ( T ) sites towards their nearest-neighbour lattice points, while for C H = 0.019, most of the H atoms are located at T sites and the remains are at the same sites as those observed for C H = 0.003. This result indicates the trapping of hydrogen by oxygen and the formation of O–H pairs. The number of H atoms trapped by one O atom is approximately unity. As locations of the trapped hydrogen involved in the tunnelling motion, three types of displaced sites, which are described above, with the same separation of about 0.82 A between neighbouring...