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F Fujara

Publications and source records attributed to F Fujara.

17 recordsLinked to original sources
PubMed2006

Deuteron spin lattice relaxation in amorphous ices.

Temperature-dependent deuteron spin lattice relaxation times T(1) have been obtained from water in its three amorphous states at ambient pressure: low density amorphous (LDA), high density amorphous (HDA), and very high density amorphous (VHDA). It is found that in all of these states the magnetization recovery is essentially monoexponential and that T(1) of LDA is significantly longer than that of the higher density forms. Thus, T(1) can be used as a monitor parameter to study the kinetics of the transitions from HDA to LDA and from VHDA to LDA. During the transformation of VHDA to LDA an intermediate state is formed, which, according to its T(1) at low temperature, is clearly determined to be HDA-like. However, and most significantly, the transition from VHDA to this HDA-like state and further on to LDA occurs at temperatures significantly above the kinetic stability limit of native HDA produced at 77 K. These findings contribute to the current discussion on the nature of HDA and VHDA by strengthening the view that the annealing of VHDA at ambient pressure produces a relaxed HDA-like state.

Journal Article↗
PubMed2001

Magnet design with high B(0) homogeneity for fast-field-cycling NMR applications.

The design, construction, and performance of a low-inductance solenoidal coil with high B(0) homogeneity for fast-field-cycling NMR is presented. It consists of six concentric layers. The conductor width is varied to minimize the B(0) inhomogeneity in the volume of the sample. This is done using an algorithm which takes the real shape of the conductor directly into account. The calculated coil geometry can be manufactured easily using standard computerized numeric control equipment, which keeps the costs low. The coil is liquid cooled and produces a B(0) field of 0.95 T at 800 A. The field inhomogeneity in a cylindrical volume (diameter 5 mm, length 10 mm) is about 10 ppm, and the inductance is 190 microH. Switching times below 200 micros can be achieved. During 6 months of operation the coil has shown good stability and reliability.

Electromagnetic Fields↗
PubMed2000

Crystal-like high frequency phonons in the amorphous phases of solid water.

The high frequency dynamics of low-density (LDA) and high-density (HDA) amorphous ice and of cubic ice ( I(c)) has been measured by inelastic x-ray scattering in the 1-15 nm(-1) momentum transfer ( Q) range. Sharp phononlike excitations are observed, and the longitudinal acoustic branch is identified up to Q = 8 nm(-1) in LDA and I(c) and up to 5 nm(-1) in HDA. The narrow width of these excitations is in sharp contrast to the broad features observed in all amorphous systems studied so far. The "crystal-like" behavior of amorphous ices, therefore, implies a considerable reduction in the number of decay channels available to soundlike excitations which is interpreted as a sign of low local disorder.

Journal Article↗
PubMed2000

Ice XII in its second regime of metastability

We present neutron powder diffraction results which give unambiguous evidence for the formation of the recently identified new crystalline ice phase [2], labeled ice XII, in completely different conditions. Ice XII is produced here by compressing hexagonal ice I(h) at T = 77, 100, 140, and 160 K up to 1.8 GPa. It can be maintained at ambient pressure in the temperature range 1.5<T<135 K. Accurate structural properties are obtained from high-resolution diffraction carried out at T = 1.5 K. At T = 140 and 160 K additionally ice III/IX is formed. The increasing amount of ice III/IX with increasing temperature gives an upper limit of T approximately 150 K for the successful formation of ice XII with the presented procedure.

Journal Article↗
PubMed2000

Relating structure and translational dynamics in aqueous dispersions of monoolein.

The temperature dependence of the molecular diffusion in monoolein/water systems is investigated at several levels of hydration. Using the proton/deuteron selectivity, field gradient NMR allows the simultaneous determination of the diffusion constants of both, lipid and water molecules in the various lamellar and non-lamellar phases. Due to the mesoscopic structure of the monoolein/water phases, the diffusion coefficients are interpreted as 'reduced' or 'effective' diffusion coefficients, and are related to the microscopic molecular displacements by a so-called 'obstruction factor'. Changes in the microscopic structure at the phase transition from the bicontinuous cubic phases to the inverse hexagonal phase are reflected in the obstruction factor of the monoolein diffusion coefficients. The reduction of the water diffusion coefficients is too high to be explained by an obstruction factor only, implying a mechanism of molecular motion, which strongly differs from that of bulk water. Experiments on samples prepared with isotopic labeled water (2H(2)O and H(2)(17)O) indicate a chemical exchange of protons between the water molecules and the lipid headgroups on a millisecond timescale.

Calorimetry, Differential Scanning↗
PubMed2000

NMR study of translational and rotational dynamics in monoolein-water mesophases: obstruction and hydration effects

Using a variety of NMR methods (magnetic field gradient echo decays, 2H one- and two-dimensional spectra, spin-lattice relaxation rates), both translational and rotational dynamics of the constituents of monoolein MO/H(2)O mesophases have been studied. The experiments lead to the following conclusions. The translational dynamics of the lipid molecules is essentially dominated by obstruction effects due to the topologies of the diverse mesophases. On the other hand, water dynamics-in the regime of small water concentrations-is strongly influenced by hydration of the lipid head groups. Hydration is seen in diffusion data, in spectra and in spin-lattice relaxation of the water molecules. This work represents an involved extension of our recently published work [Chem. Phys. Lipids 106, 115 (2000)].

Journal Article↗
PubMed1998

2D Deuteron Exchange NMR and Stimulated Echoes: Molecular Dynamics of the Vacancy Diffusion in Polycrystalline Benzene

Since in crystalline benzene the unit cell consists of four differently oriented molecules, the vacancy diffusion is expected to go along with a corresponding molecular reorientation. Therefore, 2H NMR is able to yield detailed information about the time scale and local geometry of this jump diffusion process. We have carried out 2H NMR stimulated echo and 2D experiments. These data, their fit to calculated spectra, and a comparison between the (1D) stimulated echo treatment and the 2D analysis are presented. Copyright 1998 Academic Press. Copyright 1998 Academic Press

Journal Article↗
PubMed1998

2H NMR Time Domain Analysis of Ultraslow Reorientations in Supercooled Liquids

A method for evaluating 2H NMR stimulated echo experiments in the time domain is presented. It exhibits a high sensitivity to molecular reorientation mechanisms for small angles. Reorientations with jump angles below 25degrees can be resolved with a precision of approximately 1degrees so that, e.g., rotational diffusion and finite jump angle mechanisms become distinguishable. The method, applicable to isotropic reorientation models, is thought to extend the domain of 2D exchange spectroscopy where the best resolution is obtained in the large angle range of anisotropic reorientational mechanisms. Application to reorientation in the supercooled melt of ortho-terphenyl is presented. These data clearly show that the molecular reorientation of ortho-terphenyl molecules cannot be described by small angular step rotational diffusion. A better parametrization of the experimental data incorporates elementary jump angles in the range of approximately 10degrees. Copyright 1998 Academic Press. Copyright 1998 Academic Press

Journal Article↗
PubMed1998

Demonstrating the Spatial Resolution of Field Gradient NMR

In recent years, field gradient NMR has become a method of increasing importance in measuring very small dislocations of molecules. Rough estimations indicate that, by utilizing large field gradients, one should be able to detect motions down to the 10-nm scale. However, this limit has not yet been experimentally detected. In this paper, we present a method that allows the direct measurement of the spatial resolution of field gradient NMR. For our experimental setup, utilizing an extremely large static field gradient of about 180 T m-1, we find a lower limit of 7 nm, thus for the first time confirming the expected resolution. Furthermore, we discuss the effect of unwanted vibrations as a limiting factor of the method. Copyright 1998 Academic Press.

Journal Article↗
PubMed1994

Anisotropic diffusion in etched particle tracks studied by field gradient NMR.

Etched particle tracks produced after heavy ion irradiation of polymer foils are used as model systems to test the performance of NMR in a newly developed ultrahigh magnetic field gradient system. The stimulated NMR echo decay of molecules diffusing in the channels, formulated in terms of the self part of the intermediate scattering function, is anisotropic and yields the form factor of the channels.

Anisotropy↗