Abnormal relaxation kinetics in D-mannitol glass confined by nanoporous alumina

The relaxation kinetics and phase transformations of the confined D-mannitol(DM) in nanoporous alumina are studied in-situ using a high-precision nano-calorimeter. We find that the crystallization behavior can be suppressed when it is confined in nanopores smaller than 50 nm. The confined DM glass has a much smaller fragility(～76) than free DM glass(～125), confirming the enhanced glass-forming ability. It is intriguing that during isothermal annealing both the confined and free DM glasses relaxation kinetics experience two relaxation stages that have distinct activation energies. The relaxation activation energy of the confined glass is about 25%-29% smaller than the free glass, which is attributed to the reduced dimensionality. The abnormal kinetics observed in the confined DM glass open a new avenue for preparing stable glasses.     

Spin-lattice relaxation in cuprate superconductors

This presentation gives a personal review of nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) spin-lattice relaxation studies in cuprate superconductors mainly dealing with the YBa₂Cu₄O₈ compound with many examples from the Zürich laboratory. The studies were performed in both the normal and the superconducting state with various NMR isotopes (e.g.,¹⁷O,^63,65Cu,^135,137Ba). The relatively broad signals were mostly obtained by a phase-alternating add-subtract spin-echo technique. We will discuss the general behavior of spin-lattice relaxation in the normal state and the calculation of the dynamic spin including an approach (on the basis of thet-J model) to calculate the relaxation for plane copper, oxygen, and yttrium. An application of the Luttingerliquid model to the relaxation of chain copper in YBa₂Cu₃O₇ and YBa₂Cu₄O₈ is also given. We then will deal with characteristic features of the YBa₂Cu₄O₈ structure: the spin gap, an electronic crossover in the normal state, the single-spin fluid model, and the d-wave pairing.     

Spin-lattice relaxation measurements in lithium acetate revisited

The Zeeman spin-lattice relaxation is measured in a single crystal and in powdered samples of lithium acetate (CH₃COOLi·2D₂O). Measurements have also been performed in powdered samples of lithium acetate which contain 50% and 95% of CD₃-groups besides the CH₃-groups. A non-exponential behaviour, i.e. time dependence, of the Zeeman spin-lattice relaxation is clearly observed at temperatures above the T₁-minimum in all samples. The results are explained within the framework of the symmetry-restricted spin-diffusion model of Emid and Wind.     

The “corset effect” of spin-lattice relaxation in polymer melts confined in nanoporous media

Linear polyethylene oxides with molecular weightsM _w of 1665 and 10170 confined in pores with variable diameters in a solid methacrylate matrix were studied by proton field-cycling nuclear magnetic resonance relaxometry. The pore diameter was varied in the range of 9–57 nm. In all cases, the spin-lattice relaxation time shows a frequency dependence close toT ₁∞ v^3/4 in the range ofv=3·10^?1-2·10¹ MHz as predicted by the tube-reptation model. This protonT ₁ dispersion essentially reproduces that found in a previous deuteron study (R. Kimmich, R.-O. Seitter, U. Beginn, M. Möller, N. Fatkullin: Chem. Phys. Lett. 307, 147, 1999). As a feature particularly characteristic for reptation, this finding suggests that reptation is the dominating chain dynamics mechanism under pore confinement in the corresponding time range. The absolute values of the spin-lattice relaxation times indicate that the diameter of the effective tubes in which reptation occurs is much smaller than the pore diameters on the time scale of spin-lattice relaxation experimens. An estimation leads to a valued ^*～0.5 nm. The impenetrability of the solid pore walls, the uncrossability of polymer chains (“excluded volume”) and the low value of the compressibility in polymer melts create the “corset effect” which reduces the lateral motions of polymer chains to a microscopic scale of only a few tenths of a nanometer.     

The relaxation dynamics of a confined glassy simple liquid

We use molecular-dynamics computer simulations to study the relaxation dynamics of a confined simple liquid. Two types of confining walls are considered: A rough wall and a smooth wall. The simulation is set up in such a way that the static properties of the confined system are identical to the ones of the bulk. Nevertheless, we find that upon cooling the relaxation dynamics of the confined systems differ strongly from the one of the bulk. In particular, we find that close to the rough/smooth wall this dynamics is slowed down/accelerated by orders of magnitude. Using these results we are able to extract a dynamical length scale of the system and we show that this length shows an Arrhenius dependence.Received: 1 January 2003, Published online: 14 October 2003PACS: 64.70.Pf Glass transitions - 68.15. + e Liquid thin films - 02.70.Ns Molecular dynamics and particle methods     

Quadrupole and knight shift relaxation of excited Ge nuclei in liquid gallium

A simple method based on perturbed angular correlations was applied to investigate relaxation phenomena of the 20 ms state of ⁷¹Ge in liquid gallium in the temperature range from 40°C to 1050°C.     

Spin-lattice relaxation of Fe nuclei in yttrium iron garnet

Pulse measurements of T₁ for ⁵⁷Fe nuclei in very pure, single crystals of YIG are reported. The temperature was varied from 2° to 292°K, and the externally applied field ranged from 0 to 6000 Oe. The temperature variation of T₁ is quite strong, being three orders of magnitude in the range 2°–40°K. At constant temperature, T₁ changed approximately one order of magnitude between saturation field and 6000 Oe. The data are compared with the results of a calculation by Beeman and Pincus, in which a second-order Raman process and the three-magnon process are assumed to predominate below 50°K. Agreement is only qualitative, the experimental values of T₁ being larger than predicted. At 4.2°K in zero field, it is found that a polycrystalline sample containing particles of ≈ 5 × 10⁻⁴ cm dia. has a value of 1/T₁ which is some two orders of magnitude larger than for a macroscopic crystal. The presence of a relaxation mechanism associated with surface effects is suggested.     

Peculiarities of gallium crystallization in confined geometry

The freezing and melting phase transitions for gallium embedded into a porous glass with a pore size of about 8 nm were studied using acoustic, NMR, and x-ray techniques. It was shown that the broadened solidification and melting transitions upon deep cooling up to complete freezing at 165 K were due to the formation of β-Ga within pores. The offset of confined β-Ga melting was lowered by about 21 K compared to the bulk β-Ga melting point. Both melting and freezing in pores were irreversible. The fulfillment of some special thermal conditions led to gallium crystallization into other modifications. The role of heterogeneous crystallization in freezing of confined gallium is discussed.     

Spin-lattice relaxation in solid 3He in weak magnetic fields

Spin-lattice relaxation times T ₁ were measured for solid ³He at temperatures of 0.22 to 0.73 K in a 44-Oe magnetic field. An increase in T ₁ at temperatures higher than approximately 0.4 K was related to switching on the vacancy mechanism of atomic mobility in the crystal. At a melting curve minimum, in the region of predominance of exchange motions of atoms in the crystal, measurements of T ₁ were performed in magnetic fields of 2 to 71 Oe. The data obtained in fields higher than 5 Oe were in agreement with the Cowan-Fardis theory.     

19F Spin-lattice relaxation and stable radicals in radiation-modified fluoropolymers

Fluorine-19 spin-lattice relaxation of electron-beam-irradiated poly(tetrafluoroethylene) (PTFE) has been investigated in the temperature range from 250 to 315 K. As shown before, in the initial step, radicals are produced by the electron-beam irradiation and chain scission takes place. The concentrations of radicals and chain end groups after irradiation of PTFE strongly depend on the irradiation conditions. Radicals like other paramagnetic species decrease the spin-lattice relaxation times. In addition, decreased polymer chain lengths shift theT ₁ minimum to lower temperatures. Tetrafluorosuccinic acid in solution was used as a model system and paramagnetic copper sulphate CuSO₄ added to quantify the effect on the relaxation times. The shift of the minima inT ₁ versus temperature in PTFE are compared with the chain length determined from high-resolution solid-state nuclear magnetic resonance spectra and with the concentration of paramagnetic species.     

Spin-lattice relaxation times of rare earth ions in binary nitrates

The Van Vleck method is used with the point-charge model to calculate the spin-lattice relaxation times of the Ce³⁺ ion in binary nitrates. Normal coordinates are found for the icosahedral XY₁₂ complex containing the paramagnetic ion and its nearest neighborhood. The theoretical transition probabilities for the purely ionic model turn out to be slightly above the experimental results, so covalent effects are important.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 8, pp. 31–37, August, 1969.The author thanks L. K. Aminov for supervision of and assistance in this study.     

Spin-lattice relaxation of 25Al and 28P in Pt

The spin-lattice relaxation times T ₁ for short-lived β emitters ²⁵Al(I?=?5/2, T _1/2?= 7.2 s) and ²⁸P(I?=?3, T _1/2?= 270 ms) in Pt were measured by means of the β-NMR technique. As a result, T ₁[²⁵Al in Pt] = (1.1 $^{+\ 0.7}_{-\ 0.3})$ s and T ₁[²⁸P in Pt] >0.5 s were obtained at temperatures of 17 and 20 K, respectively. The Knight shifts were estimated from the Korringa relation, which were evaluated by comparing to the first principle calculations.     

Spin-lattice relaxation and dynamic polarization of protons in ethyleneglycol-methanol samples

We report measurements of nuclear spin-lattice relaxation and dynamic polarization of protons in ethyleneglycol-alcohol mixtures at 9 kG and 4·2 K and 1·3 K. Comparison of experimental results with a proposed model of spin-lattice relaxation indicates that the role of dipolar and exchange reservoirs of paramagnetic ions in nuclear relaxation is not necessarily negligible, particularly in the presence of high concentration of paramagnetic impurities. The influence of methanol is to enhance the dynamic polarization of protons compared to pure ethyleneglycol, in the most favourable cases by almost 50% in our experimental conditions.     

Anomalously slow relaxation of a nonwetting liquid in the disordered confinement of a nanoporous medium

The time evolution of the water–disordered nanoporous medium Libersorb 23 (L23) system has been studied after complete filling at elevated pressure followed by full release of overpressure. It is established that relaxation of the L23 rapidly flows out during the overpressure relief time, following the variation in pressure. At a temperature below that of the dispersion transition (T < T _d = 284 K), e.g., at T = 277 K, the degree of filling θ decreases from 1 to 0.8 within 10 s. The degree of filling varies with time according to the power law θ ~ t ^–α with the exponent α < 0.1 over a period of t ~ 10⁵ s. This process corresponds to slow relaxation of a metastable state of a nonwetting liquid in a porous medium. At times t > 10⁵ s, the metastable state exhibits decay, manifested as the transition to a power dependence of θ(t) with a larger exponent. The relaxation of the metastable state of nonwetting liquid in a disordered porous medium is described in the mean field approximation as a continuous sequence of metastable states with a barrier decreasing upon a decrease in the degree of filling. Using this approach, it is possible to qualitatively explain the observed relaxation process and crossover transition to the stage described by θ(t) with a larger exponent.     

Freezing of fluids confined in a disordered nanoporous structure

Freezing of a simple fluid in a disordered nanoporous carbon is studied using molecular simulations. Only partial crystallization occurs, and the confined phase is composed of crystalline and amorphous nanodomains. This freezing behavior departs strongly from that for nanopores of simple geometry. We present a method for analyzing the freezing in such disordered materials in terms of a transition in the average size and number of crystalline clusters. The results provide a basis for the interpretation of experiments on freezing in such materials, particularly 1H-NMR and scattering experiments.     

Spin-lattice relaxation and magnetization transfer in intracranial tumors in vivo: Effects of Gd-DTPA on relaxation parameters

Spin-lattice relaxation time T₁ and relaxation parameters in magnetization transfer (MT) imaging were measured in 11 intracranial tumors before and after injection of Gd-DTPA at 0.1 T by using the inversion recovery method and the saturation transfer technique, respectively. Preinjection T₁ relaxation times of the tumors were longer than those of white matter, but after Gd-enhancement the relaxation times of most tumors were in the same range as those of white matter. Gd-DTPA shortened the apparent relaxation time in the presence of off-resonance saturation pulse (T_1α) due to marked shortening of the relaxation time of mobile water (T_1w). Gd-DTPA decreased the magnetization transfer contrast (MTC) but did not influence on the magnetization transfer rate (R_wm). The parameters MTC and R_wm differed clearly between Gd-enhanced tumors and normal brain, whereas the relaxation time T_1α was in many Gd-enhanced tumors in the same range as in normal brain.     

Spin-lattice relaxation times and nuclear overhauser enhancement effect for P metabolites in model solutions at two frequencies: Implications for in vivo spectroscopy

The relaxation time T₁ values and nuclear Overhauser enhancement factor for ³¹P signal were determined in model solutions of metabolites ATP, PCr and Pi, and AMP at two frequencies and in H₂O and ²H₂O solutions. The data were analyzed to resolve the contribution of different relaxation mechanisms. A knowledge of NOE is important in the light of recent applications of double resonance methods to enhance the sensitivity of in vivo ³¹P spectroscopy. The results show that chemical shift anisotropy is the dominant mechanism for ³¹P in ATP at the high field, whereas the dipolar interaction mechanism is the main feature for the ³¹P relaxation of PCr and Pi. The dipolar mechanism responsible for NOE originates from interactions of solvent water with ³¹P moiety. Implications for in vivo spectroscopy are indicated.     

Spin-lattice relaxation as a function of chain position in perdeuterated potassium palmitate

Using the quadrupolar echo Fourier transform technique (Davis et al., Chem. Phys. Lett.42, 390 (1976)), T₁ has been measured, in a sample of 70% potassium palmitate (d₃₁)/30% H₂O, as a function of chain position and temperature in the L_α liquid-crystalline phase. At temperatures near the transition from the liquid-crystal to the gel phase, T₁ is roughly constant for the first eight or nine chain positions but increases rapidly on moving further out along the chain. As the temperature is increased the relaxation times for all positions increase but those nearer the head group increase more rapidly, resulting, of temperatures above ∼100°C, in a minimum in the plot of T₁ versus chain position at the center of the chain (the 8 or 9 position). These results are in general accord with the temperature dependence of chain disorder (Davis and Jeffrey, Chem. Phys. Lipids20, 87 (1977) and offer some suggestions on the kind of model required to explain relaxation in these systems.