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- Bloch, J., et al.
(författare)
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Hydrogen-vanadium system in thin films : Effect of film thickness
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:24, s. 245428-
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Tidskriftsartikel (refereegranskat)abstract
- The absorption of hydrogen in thin V(001) films under pressures of 1-10(5) Pa H-2 and at temperatures between 350 and 530 K was studied as a function of film thickness between 50 and 5 nm using in situ electrical resistivity measurements. The critical temperatures for the order-disorder transitions taking place in the V-H system are decreased with decreasing film thickness. At 370 K, the high-concentration (epsilon-VH) phase disappears as the thickness of the film is reduced from 50 to 10 nm and the low-concentration (beta-V2H) phase follows when the film thickness is further decreased to 5 nm. The difference in solubility at 530 K of H in the alpha phase of V films down to 10 nm is small but it increases for lower temperatures. At low concentrations the heat of solution in the 10 nm film is somewhat lower than in the bulk, but around H/V = 0.07, the values of Delta H-H in the film approach those of the bulk. The values of Delta S-H(nc) of the film are rather close to those of the bulk. Significant difference is found in the pressure-resistivity isotherms above the maximum of the residual resistivity, Delta R-max. For the 10 nm film, in contrast to the 50 nm film, a minimum is located just beyond Delta R-max for the whole temperature range but the difference between the maximum and minimum starts to decrease above 420 K. This results in, by extrapolation, the vanishing of the maximum around 520 K. We suggest that this phenomenon is the result of an Oz -> T transition due to a final size effect. This transition means additional randomly distributed interstitial sites available for the H atoms in the lattice of the V film leading to a continuous increase in the residual resistivity with H concentration.
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| 2. |
- Bloch, J., et al.
(författare)
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Prediction and Hydrogen Acceleration of Ordering in Iron-Vanadium Alloys
- 2012
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 108:21, s. 215503-
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Tidskriftsartikel (refereegranskat)abstract
- Ab initio calculations of binary metallic systems often predict ordered compounds in contrast to empirical reports of solid solutions or disordered phases. These discrepancies are usually attributed to slow kinetics that retains metastable structures at low temperatures. The Fe-V system is an example of this phenomenon, in which we predict two ordered stable ground states, Fe3V and FeV3, whereas a disordered sigma phase is reported. We propose to overcome this difficulty by hydrogen absorption, which facilitates metal atom mobility through vacancy formation and separation between the two elements due to their opposite affinities towards it, thus accelerating transformation kinetics. Hydrogen also increases the relative stability of the ordered structures compared with that of the sigma phase without affecting the shape of the phase diagram. The hydrogen-induced formation of the ordered structures is expressed by a reversible decrease of the electrical resistivity with increasing hydrogen pressure. Such behavior has not been reported before in thin H absorbing films. Formation of the ordered structures is further substantiated by the kinetics of the resistivity changes upon variation of the hydrogen pressure, where two stages are distinguished: a fast initial stage and a much slower subsequent process in which the resistivity changes direction, associated with hydrogen dissolution and phase transformation, respectively.
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