Effect of substitutional Cr on hydrogen diffusion and thermal stability for the BCT monohydride phase of the V–H system studied by 1H NMR

Enhancement of hydrogen diffusion in the body-centered tetragonal monohydride phase of the V–H system by substitutional Al studied by proton nuclear magnetic resonance

“…[32] and V 1Àx Cr x H 0.68 (x = 0.03, 0.05, 0.10) from Ref. [33]. The lattice parameter along the a-axis, a, increased and that along the c-axis, c, decreased with the Al content.…”

“…The density of states at the Fermi energy, N(E F ), has a relation with T 1e T [34]. We elucidated that the increase in T 1e T of the b phase of V 1Àx Mo x -H and V 1-x Cr x -H (x 6 0.10) corresponds to the reduction in N(E F ) and positive shift of the enthalpy for hydride formation [33]. In other words, the increase in T 1e T obtained by NMR corresponds to the increase in P eq in the P-C isotherm at a given temperature.…”

“…Solid state nuclear magnetic resonance (NMR) is one of the most powerful techniques to obtain information about hydrogen in metal hydrides. In our previous work, the site occupation of hydrogen in the b phase of V 1Àx Mo x -H [32] and V 1Àx Cr x -H [33] (x 6 0.10) was studied by means of proton NMR ( 1 H NMR). The temperature dependence of the 1 H spin-lattice relaxation times, T 1 , indicated a shift of hydrogen occupation from the O z sites to the T sites by the addition of Mo, although an obvious shift was not found by the addition of Cr.…”

Improving the Cyclic Stability of V–Ti–Mn bcc Alloys Using Interstitial Elements

“…1) While most of hydrogen absorbing metals and intermetallic compounds absorb H/M (the number ratio of absorbed hydrogen to metal atoms) ³1 of hydrogen (for example LaNi 5 , ZrMn 2 , TiMn 2 and TiFe), numerous V-based body-centered cubic (bcc) alloys show the total hydrogen storage capacity of H/M ³2. However, they can only desorb around a half of absorbed hydrogen at ambient conditions and often show poor cyclic stability, that is to say, the reversible hydrogen storage capacity gradually decreases as the hydrogen absorption and desorption process is repeated.…”

“…2,3) Note that difference in a bcc and a bct structure is that one axis of a bct unit cell is slightly longer than other two (a = b º c and 1 < c/a¯1.1). 4,5) If the bct distortion of a monohydride phase is an important factor for excellent cyclic stability, it is worth investigating the cyclic stability of V 0.37 Ti 0.33 Mn 0.3 6) which forms a monohydride phase with a large bct distortion (c/a ³ 1.3).…”