The hydrogen diffusivity D in C14-type HfCr2H0.4 has been measured by means of the pulsed-field-gradient nuclear magnetic resonance over the temperature range 141–428K. The temperature dependence of D in this range is found to deviate strongly from the Arrhenius behavior. This is consistent with a quantum nature of H diffusion in HfCr2Hx. Measurements of the proton spin–spin relaxation rate R2 in HfCr2Hx (x=0.4, 0.6 and 0.74) have revealed the anomalous R2 peak near 230K for all the samples studied. The experimental results indicate that this peak originates from slow fluctuations of the proton resonance frequency. Such fluctuations are expected for protons diffusing through a spatially inhomogeneous medium on the length scale of about 500Å. The origin of the inhomogeneity suggested by the R2 data in HfCr2Hx remains to be elucidated.

Combined NMR cryoporometry, relaxometry and diffusometry were applied to characterize porous carbon materials. Pore space characterization in NMR cryoporometry is based on the measurement of melting point depression of the confined liquids, whereas NMR relaxometry and diffusometry explore the random motion of the molecules under confinement by the pore space. We demonstrate compatibility between the evidence of classical sorption experiments and NMR cryoporometry on pore size distribution. There is a distribution in both the nuclear magnetic relaxation rates and diffusion coefficients. These distributions have to be referred to heterogeneities in the pore space. Since they can only be observed if their influence is not averaged out on the diffusion paths covered by the molecules during the respective measurements, the spatial extension of the regions with structural differences (as evidenced by the differences in diffusion and nuclear magnetic relaxation of the probe molecules) may be estimated to be at least of the order of 20μm.

The diffusion of n-decane, n-heptadecane and their binary mixtures of two different compositions confined in novel mesoporous aluminosilicates was investigated by the Pulsed Field Gradient (PFG) NMR technique. The temperature range studied was between −100 and 23°C, comprising the bulk melting points of n-heptadecane at 23°C and n-decane at −30°C. The melting curves of all liquids measured by cryo-NMR allowed discrimination of the temperature ranges where non-frozen liquid existed in the pores. The study of n-decane and n-heptadecane mixtures of two different compositions by cryo-NMR does not reveal any preferential adsorption of one of the mixture components in the investigated material. In both the bulk and the pores, the diffusion coefficients follow Arrhenius dependencies with essentially identical activation energies. The diffusion coefficients of the liquids inside the pores are reduced by more than an order of magnitude.

Hydrogen diffusion coefficients D in disordered b.c.c. systems VyTa1−yH0.07 (y=0.25, 0.50 and 0.75) have been measured by means of the pulsed-field-gradient nuclear magnetic resonance over the temperature range 190–435K. The hydrogen diffusivities are found to depend strongly on the alloy composition, the room-temperature value of D in the sample with y=0.75 being an order of magnitude higher than those in the samples with y=0.25 and 0.50. For all the samples studied, the temperature dependences of H diffusivity follow the Arrhenius law with the activation energies of 0.186eV (y=0.25, 306–410K), 0.182eV (y=0.50, 295–401K) and 0.102eV (y=0.75, 190–435K). These values are in reasonable agreement with those obtained from the proton spin-lattice relaxation measurements reported earlier. The jump lengths of H atoms have been estimated by comparing the measured H diffusivities with the proton spin-lattice relaxation data.

Properties of SBA-15 materials modified by the impregnation of vanadium silicalite-1 zeolite-like nanoparticles (SBA-VS) have been studied. The influence of the amount and concentration of impregnated nanoparticles on the structural characteristics were studied by N2-sorption measurements, TEM and HRSTEM. Increasing the amount of nanoparticles on the SBA-15 material has large consequences on the ratio of open to narrowed mesopores, the micro- and mesopore volume and the vanadium content. The influence of the impregnated nanoparticles on mobility of guest molecules in the different SBA and SBA-VS materials was studied with help of pulsed field gradient nuclear magnetic resonance. Strong effects on molecular translational diffusion produced by loading were observed. Interpretation is given in terms of structure changes and molecular exchange between the meso- and macropores.

In situ studies of the transition from NaAlH4 to Na3AlH6 are performed by proton NMR for samples doped with TiCl3 and with Ti-13-nanoclusters. The local hydrogen dynamics in the different compounds is studied by the nuclear spin-lattice relaxation. For the Ti-doped NaAlH4 samples a double-exponential recovery of the nuclear magnetization is observed, indicating two fractions of hydrogen with different mobilities. In Na3AlH6 a transition from hindered rotation of the AlH6 groups to full isotropic reorientation of these groups is observed in the temperature range 200-260 K. (c) 2005 Elsevier B.V. All rights reserved.

Received 18 November 2004DOI:https://doi.org/10.1103/PhysRevB.70.219901©2004 American Physical Society

The proton magnetic resonance spectra of yttrium and lutetium trihydride, YH3 and LuH3, respectively, show an unusual doublet structure qualitatively similar to the Pake doublet that results from the mutual dipolar interaction of closely spaced, isolated proton pairs. However, both the magnitude of the splittings, roughly 70 kHz, and the second moment of the spectra, roughly 200 (Oe)(2), greatly exceed classical dipolar theoretical expectations based on known nearest-neighbor spacings. On the other hand, the YH3 doublet narrows to a single line above 300 K, consistent with the temperature dependence of the spin-lattice relaxation rate, whereas the LuH3 doublet shows no narrowing up to 454 K. In addition, the peaks themselves are close to Lorentzian in shape, indicative of strong indirect exchange narrowing. These features of the spectra indicate that substantial contributions from electron-mediated pseudodipolar and indirect exchange interactions exceed the classical dipolar interaction.

We present a study of the molecular dynamics in an octylcyanobiphenyl (8CB)–Aerosil complex above the bulk isotropization temperature. Using proton nuclear magnetic relaxation experiments in the laboratory frame (T1−1) and in the rotating-frame (T1ρ−1), we found a notable increase of the relaxation rates in the kHz frequency range as compared to the bulk 8CB liquid crystal at the same temperature. The field-cycling technique was used for the laboratory frame experiments while a conventional apparatus was used for the rotating frame method. The observed behavior is analyzed with the aid of Monte Carlo simulations on the basis of a two-phase fast-exchange model distinguishing surface-ordered and bulk phases. Two processes affecting the low frequency relaxation could be identified: reorientation mediated by translational displacements, accounting for molecular reorientations, and exchange losses of molecules from the surface to the bulk.

A new parameter for NMR mapping is suggested on the basis of the mean squared dipolar fluctuation (MSDF). The MSDF characterizes the relaxation mechanism due to ultra-slow dipolar fluctuations in liquids subject to local anisotropy of molecular motions. These fluctuations can be monitored on the time scale exceeding a few microseconds. In rubber materials, the MSDF is a function of the density of chemical cross-links strongly affecting (anisotropic) mesh chain fluctuations. Experimentally, the MSDF is determined from the attenuation curves of the quotient of the amplitudes of the stimulated and the primary echoes produced by the three 90 degrees radio-frequency pulse sequence. In order to evaluate the MSDF maps, the latter sequence was combined with the standard scheme of the magnetic field gradients providing a spatial resolution. The pixel values of the MSDF are "visualized" using grey shades related to the equidistant intervals covering the whole range of the measured values. The MSDF maps are demonstrated for the two composite samples. The first sample consists of a water filled tube in the middle part surrounded by high molecular mass polyisoprene (PI) in the outer part. The relaxation weighted spin density image of this sample is dominated by a water signal with PI producing a much weaker intensity. The MSDF map, on the contrary, enhances the relative intensity of the outer, PI, part while scaling the middle, water, part down to the level of noise. The second sample consists of the four rubber pieces with different cross-link density. This sample thus models an inhomogeneous rubber object. The MSDF map produces clear contrast for the relevant regions. The advantages of employing this kind of NMR mapping for a characterization of materials are discussed.

We present a novel NMR approach to the determination of crosslink densities in rubber materials. The method is based on the dipolar correlation effect (DCE) on the stimulated echo examined in a series of rubber samples and linear polyisoprene. The parameter evaluated from the echo attenuation curves is the mean-squared dipolar fluctuation associated with anisotropic reorientations of macromolecular backbones. The contributions to the DCE of the constraints due to excluded volume effects and chemical crosslinks are estimated. A strong dependence of the mean-squared dipolar fluctuation on the crosslink density of rubber combined with the simplicity of performing the measurements with inexpensive low-field instruments suggests that the ME is a useful tool for routine applications. The potential and problems of performing DCE measurements in low-magnetic-field conditions are discussed in detail. (C) 2001 John Wiley & Sons, Inc.

Local order and molecular dynamics of liquids near surfaces strongly deviate from the behavior in the bulk. This in particular refers to liquid crystals above the bulk isotropization temperature. Transverse relaxation data of 5CB examined in porous glasses with different pore sizes are reported. A strong pore size effect was found. For the interpretation, a simple diffusion-adsorption computer simulation was carried out. Molecules can diffuse from the isotropic bulk part of the pore fluid to the ordered surface layer and vice versa. The residual dipolar correlation function is characterized by a slowly decaying tail owing to repeated returns of molecules to the surface. At each return the molecular orientation correlation is recovered as far as the surface sites visited have orientations correlated to the initial site. That is, molecular orientation is controlled by the “reorientation mediated by translational displacement” process considered in previous papers.

Chain dynamics in a series of natural rubbers with different cross-link densities was studied using the dipolar correlation effect (DCE) on the stimulated echo. In dry samples, amplitudes of the stimulated echo were shown to be affected by attenuation mechanisms due to the DCE and spin exchange which occurs between the protons of CH and CH3 (and/or CH2) groups of polyisoprene chains. The estimated spin exchange time was 0.04 s. The DCE free of undulations due to spin exchange was examined using a special modification of the radio frequency pulse sequence with additional π-pulses inserted in the free-evolution intervals. Modulations of stimulated echo amplitudes could completely be suppressed. A strong dependence of the DCE on the cross-link density was observed. Attenuation curves of the dipolar-correlation quotient, that is, the quotient of the stimulated and the primary echo amplitudes, were fitted using analytical expressions. The mean-squared fluctuation of the dipolar coupling constant, 〈δΩd2〉, and correlation times were determined. In dry samples, values of 〈δΩd2〉 were shown to scale with the number of segments between cross-links (N) as ∝N−1.3±0.2. The attenuation of echo amplitudes due to the DCE was insensitive to the increase of a carbon black content by approximately a factor of two. In swollen samples, attenuations of the stimulated echo were free of modulations by spin exchange thus permitting measurements of the DCE with a standard three-90° pulse sequence. Contrary to dry samples, the attenuation curves contained slowly decaying components with relative intensities ⩽ 10%. These components were subtracted from the total signal before the dipolar-correlation quotient was evaluated. 〈δΩd2〉 scales with N with nearly the same exponent as in dry rubber. The DCE of dry and swollen rubber is suggested as a method for the determination of cross-link densities.

A numerical procedure is presented which permits one to derive a formal distribution of collective fluctuation modes from experimental field-cycling NMR-relaxometry data of an ordered system. The purpose is to distinguish true order-fluctuation modes from local reorientation mechanisms. The evaluation scheme is demonstrated using simulated as well as experimental data. Applications serving the elucidation and characterization of modified or limited director fluctuation modes as they occur with liquid crystals in pores or with lyotropic systems are discussed. Test experiments have been carried out with a potassium laurate system.