Relaxation (NMR)

About: Relaxation (NMR) is a research topic. Over the lifetime, 29342 publications have been published within this topic receiving 689851 citations.

Dielectric relaxation and dielectric response mechanism in (Li, Ti)-doped NiO ceramics

Abstract: Giant dielectric permittivity (Li, Ti)-doped NiO (LTNO) ceramics are prepared by a simple PVA sol–gel method. The dielectric properties are investigated as a function of frequency (102–106 Hz) at different temperatures (233–473 K). The concentration of Li has a remarkable effect on the dielectric properties of the LTNO ceramics. The modified Cole–Cole equation, including the conductivity term, is used to describe the experimental dielectric spectra of a high permittivity response with excellent agreement over a wide range of frequencies (103–106 Hz) and temperatures (233–313 K). A frequency dielectric dispersion phenomenon in an LTNO ceramic is also analyzed by impedance spectroscopy. A separation of the grain and grain boundary properties is achieved using an equivalent circuit model. The grain and grain boundary conduction and the dielectric relaxation time of the Li0.05Ti0.02Ni0.93O follows the Arrhenius law associated with estimated activation energies of 0.216, 0.369 and 0.391 eV, respectively. Through the analysis by the modified relaxation model and impedance spectroscopy, it is strongly believed that the high dielectric permittivity response of the LTNO is not only contributed by the space charge polarization (Maxwell–Wagner polarization) mechanism at low frequency regions, but also by the defect-dipole polarization mechanism at high frequency regions.

Analysis of electrical relaxation in glasses and melts with large concentrations of mobile ions

Abstract: Various analyses of or fits to electrical relaxation data for a model Li 2 OAl 2 O 3 2SiO 2 glass s were tried. These included three-parameter fits using Kohlrausch-Williams-Watts (KWW) and Cole-Davidson (CD) distributions of electric field relaxation times, along with a four-parameter σ ′ ( ω n ) fit using the distribution implicitin the Jonscher expression for the frequency dependence of the real part of the complex conductivity, σ ′ = σ + Aω n . In the frequency range 10 −2 ≤ ω τ > ≤10 2 , where is the mean electric field relaxation time, the KWW firt was best, the σ ′ ( ω n ) fit second best and the CD fit the worst. Because the σ ′ ( ω n ) fit predicts qualitattively incorrect relaxation behavior at low frequencies, it is suggested that this method of data analysis not be used. It was noted that the dependence of σ ′ on ω 1 observed for ionically conducting solids at very high frequencies or very low temperatures make a nearly negligible contribution to the relaxation of the electric field.

Relationship between local structure and relaxation in out-of-equilibrium glassy systems.

Abstract: The dynamical glass transition is typically taken to be the temperature at which a glassy liquid is no longer able to equilibrate on experimental timescales. Consequently, the physical properties of these systems just above or below the dynamical glass transition, such as viscosity, can change by many orders of magnitude over long periods of time following external perturbation. During this progress toward equilibrium, glassy systems exhibit a history dependence that has complicated their study. In previous work, we bridged the gap between structure and dynamics in glassy liquids above their dynamical glass transition temperatures by introducing a scalar field called “softness,” a quantity obtained using machine-learning methods. Softness is designed to capture the hidden patterns in relative particle positions that correlate strongly with dynamical rearrangements of particle positions. Here we show that the out-of-equilibrium behavior of a model glass-forming system can be understood in terms of softness. To do this we first demonstrate that the evolution of behavior following a temperature quench is a primarily structural phenomenon: The structure changes considerably, but the relationship between structure and dynamics remains invariant. We then show that the relaxation time can be robustly computed from structure as quantified by softness, with the same relation holding both in equilibrium and as the system ages. Together, these results show that the history dependence of the relaxation time in glasses requires knowledge only of the softness in addition to the usual state variables.

Coalescence of sessile drops

Abstract: We present an experimental and theoretical description of the kinetics of coalescence of two water drops on a plane solid surface. The case of partial wetting is considered. The drops are in an atmosphere of nitrogen saturated with water where they grow by condensation and eventually touch each other and coalesce. A new convex composite drop is rapidly formed that then exponentially and slowly relaxes to an equilibrium hemispherical cap. The characteristic relaxation time is proportional to the drop radius R* at final equilibrium. This relaxation time appears to be nearly 107 times larger than the bulk capillary relaxation time tb = R*η/σ, where σ is the gas–liquid surface tension and η is the liquid shear viscosity.In order to explain this extremely large relaxation time, we consider a model that involves an Arrhenius kinetic factor resulting from a liquid–vapour phase change in the vicinity of the contact line. The model results in a large relaxation time of order tb exp(L/[Rscr ]T) where L is the molar latent heat of vaporization, [Rscr ] is the gas constant and T is the temperature. We model the late time relaxation for a near spherical cap and find an exponential relaxation whose typical time scale agrees reasonably well with the experiment.

The effect of entanglements on the rheological behavior of polybutadiene critical gels

Abstract: We investigated the stress relaxation behavior of critical gels originating from six nearly monodisperse, highly entangled polybutadiene melts of different molecular weight from 18000 to 97 000 g/mole. The polymers were vulcanized by a hydrosilation reaction which takes place nearly exclusively at the pendant 1,2-vinyl sites distributed randomly along the polybutadiene chain. The BSW spectrum represents the relaxation of the initial uncrosslinked precursor. A characteristic parameter is the longest relaxation time of the precursor. Crosslinking increases this longest time even further. Surprisingly, the relaxation spectrum of the precursor is not altered much by the crosslinking except for an additional long time behavior. At the gel point (critical gel), this long time behavior is self-similar. It follows the typical power law as described by the Chambon-Winter gel equation, G(t) = St −n , in the terminal zone. The critical relaxation exponent was found to be close to n = 0.5 over a range of stoichiometric ratios and for all precursor molecular weights analyzed. A new scaling relationship was found between the gel stiffness, S, and the precursor molecular weight of the form: S ∼ M , where exponent z from the zero shear viscosity-molecular weight relationship, η0 ∼ M , is commonly found to be z = 3.3 – 3.6.