Water absorption, membrane swelling, and self-diffusivity of water in 1100 equivalent weight Nafion were measured as functions of temperature and water activity. Free volume per water at 80 °C, determined from water uptake and volume expansion data, decreases with water content in the membrane from 12 cm3/mol at λ = 0.5 H2O/SO3 to 1.5 cm3/mol at λ = 4. The change in free volume with water content displays a transition at λ = 4. Limiting water self-diffusivity in Nafion was determined by pulsed gradient spin echo NMR at long delay times. The limiting self-diffusivity increases exponentially with water activity; the rate of increase of diffusivity with water content shows a transition at λ = 4. The tortuosity of the hydrophilic domains in Nafion decreased from 20 at low membrane water activity to 3 at λ = 4. It suggested a change in the connectivity of the hydrophilic domains absorbed water occurs at λ ∼ 4. The diffusivity results were employed to separate the contributions of diffusional and interfacial re...

Diffusion and interfacial transport of water in Nafion.

A study on crystal structure, bonding and hydriding properties of Ti–Fe–Ni intermetallics – Behind substitution of iron by nickel

Hydrogen reaction kinetics of Mg-based alloys synthesized by mechanical milling

Chapter 137 Hydrogen in rare-earth metals, including RH2+x phases

Parameters of interaction of hydrogen with magnesium powders and structure of powder magnesium alloys alloyed with different metals and oxides (such as Fe, Ni, Al, Cu, Ti, Pd, NiPd, V2O5, and VH2) prepared by mechanical activation under either argon or hydrogen atmosphere in a vibration mill have been studied. The mechanically activated alloys absorb to 7 wt% hydrogen at 300°C for 20 min. For most of the additions used, the effect of the grain size and type of addition on the rate of hydrogen absorption was found to manifest itself only at the stage of the formation of the MgH2 phase upon mechanical activation in the hydrogen atmosphere; virtually no effect is observed upon subsequent hydrogenation. The temperature of the hydrogen desorption also depends only slightly on the addition kind. The increase in the hydrogenation rate of the Mg-based alloys resulting from the mechanical activation was shown to be due to the formation of a specific structural state of the particle surface, which exhibits a high catalytic activity for the hydrogen sorption. A study of the mechanically activated alloys by proton nuclear magnetic resonance showed a substantial increase in the rate of proton spin-lattice relaxation as compared to that observed for MgH2 produced by direct hydrogenation. This can be due to the interaction of protons with paramagnetic centers formed at the imperfect surface of mechanically activated Mg particles.

Kinetics of interaction of Mg-based mechanically activated alloys with hydrogen

Measurements are reported of the temperature dependence of the proton spin-lattice and spin-spin relaxation times ${T}_{1}$ and ${T}_{2}$ in yttrium and lanthanum dihydrides containing controlled levels of gadolinium as low as 50 ppm. The results demonstrate unambiguously that paramagnetic ions in concentrations so low as to have heretofore been regarded as insignificant have marked effects on the magnitude, frequency dependence, and temperature dependence of ${T}_{1}$ and to a lesser extent on ${T}_{2}$, and on the electronic structure and hydrogen diffusion parameters derived therefrom. The ${\mathrm{Gd}}^{3+}$ ion contributes an additional spin-lattice relaxation rate ${T}_{1p}$, which in these hydrides arises entirely from the dipolar coupling between impurity and proton moments. Proton magnetization is transported to the relaxation centers by spin diffusion at low temperatures and by hydrogen-atom diffusion at intermediate and high temperatures. The rate ${R}_{1p}$ is directly proportional to Gd-ion concentration at both low and high temperatures, but in the atom diffusion regime ${R}_{1p}$ is 20-25 times greater than for spin diffusion. The impurity-induced relaxation is shown to have profound effects on the apparent nuclear-nuclear dipolar relaxation rate ${R}_{1d}$ associated with hydrogen diffusion. At impurity levels as low as 10 ppm Gd, a secondary minimum appears in the temperature dependence of ${T}_{1}$ which may be readily misinterpreted in terms of a second motional process with lower activation energy. Even lower impurity levels yield a characteristic "slope-change" effect, which may be construed as indicating a change in the activation energy for hydrogen diffusion. At low temperatures ${R}_{1p}$ interferes with the determination of the conduction-electron contribution ${R}_{1e}$ and the Korringa product ${T}_{1e}T$. Separation of ${R}_{1e}$ and ${R}_{1p}$ is complicated by the fact the ${R}_{1p}$ is not temperature independent as has typically been assumed. Methods of achieving this separation are discussed, and it is shown experimentally that this difficulty can be circumvented by replacing the major part of the hydrogen with deuterium, thereby inhibiting spin diffusion. Measurement of ${T}_{1}$ as a function of resonance frequency and of ${T}_{2}$ can also be of value in separating the various sources of relaxation.

Paramagnetic impurity effects in NMR determinations of hydrogen diffusion and electronic structure in metal hydrides. Gd 3 + in Y H 2 and La H 2.25

Nuclear-magnetic-resonance (NMR) experiments have been done on cerium hydride (CeH/sub x/) samples to search for correlations between NMR properties and known electrical conductivity changes as a function of hydrogen concentration and temperature. Data are presented for the /sup 1/H spin-lattice relaxation rate R/sub 1/ ( = 1/T/sub 1/) and some line shapes for 2.10< or =x< or =2.92 for temperatures from about 100 to 375 K. Although two /sup 1/H resonances are observed at some temperatures, proton spin-lattice relaxation is characterized by a single relaxation time at each x and T. To a good approximation R/sub 1/ = A/T+R, where A/T is attributed to direct dipolar coupling between protons and the electronic magnetic dipole moment of Ce/sup 3 +/, and R is an essentially temperature-independent term attributed to indirect (Ruderman-Kittel-Kasuya-Yosida (RKKY)) coupling to the Ce/sup 3 +/ moment. The A/T term is so large that for most experiments the proton-proton dipolar and proton--conduction-electron couplings are negligible. The x dependence of the constant A is consistent with the dipolar coupling. The constant R decreases in a steep manner as x is increased above xroughly-equal2.65 just below the regime 2.75

Proton spin-lattice relaxation mechanisms and the metal-insulator transition in cerium hydrides

We report determinations of the proton Korringa product T1eT in the dihydride phases of scandium, yttrium, lanthanum and lutetium based on samples prepared from the highest purity metals available in the Ames Laboratory and in addition in some cases utilizing the technique of partial deuteration to suppress further the paramagnetic impurity contribution to the spin-lattice relaxation rate. We conclude that the quantity (T 1e T) − 1 2 , which is essentially proportional to the electronic density of states N(EF), has the value 0.055 ± 0.002 s − 1 2 K − 1 2 at the dihydride composition in all systems. The dependence of ( T 1e T) − 1 2 on hydrogen concentration within the dihydride phase appears to be weak; however, in LaHx (T1eT)− 1 2 decreases substantially for x ⪢ 2 indicating a decrease in N(Ef) in this hydrogen concentration range. The implications of these results for relative s and d band contributions and hyperfine fields are discussed in the light of recent low temperature heat capacity measurements.

Conduction electron density of states and proton spin-lattice relaxation in the dihydrides of scandium, yttrium, lanthanum and lutetium

We report the results of a preliminary survey of the effects on the proton spin-lattice relaxation time T1 resulting from the presence of controlled low levels of the Kramers ions cerium, neodymium, gadolinium, dysprosium and erbium in YH2. The Ce3+ results in particular are presented in some detail because the cerium ion behaves very differently from the others, reflecting the fact that it is an extremely “fast relaxing” ion. From the measured proton T1 we conclude that the Ce3+ ion spin-lattice relaxation time τi is 1.65 × 10−12s at 77 K.

Paramagnetic impurity effects in nuclear magnetic resonance determinations of hydrogen diffusion and electronic structure in metal hydrides: Cerium in YH2

T. T. Phua

Papers

Paramagnetic impurity effects in NMR determinations of hydrogen diffusion and electronic structure in metal hydrides. Gd 3 + in Y H 2 and La H 2.25

Proton spin-lattice relaxation mechanisms and the metal-insulator transition in cerium hydrides

Conduction electron density of states and proton spin-lattice relaxation in the dihydrides of scandium, yttrium, lanthanum and lutetium

Paramagnetic impurity effects in nuclear magnetic resonance determinations of hydrogen diffusion and electronic structure in metal hydrides: Cerium in YH2