Dielectric Relaxation Mechanism Research Articles

The molecular relaxation mechanisms in cork as studied by thermally stimulated discharge currents

The dielectric relaxation mechanisms present in cork have been investigated in the temperature range -100 to 100 °C using the technique of thermally stimulated discharge currents. A relaxation mechanism was detected which showed a compensation behaviour as observed for the α-relaxation (or glass transition relaxation) of synthetic polymers and which lead us to attribute to cork a glass transition-like temperature of 18 °C. One lower temperature mechanism was also observed, with low activation enthalpy and entropy, which is presumably originated by local motions (internal rotations) of polar molecular groups. An upper Tg relaxation of higher intensity was also detected which was attributed to large-scale non-cooperative motions of polymeric segments.

Process of polarization and analysis of the water adsorption on a zeolite

We report the study of the dielectric relaxation mechanisms in the zeolite Na-X, “Faujasite”, by means of the method of Thermally Stimulated Depolarization Currents (TSDC), and polarization conductivity versus frequency. We give particular interest to the effect of adsorption on the evolution of dielectric parameters. Two distinct relaxation processes attributed to adsorbed water were found by TSDC measurements. Moreover, the polarization conductivity gives two sets of values of the potential barrier height. The evolution of the polarization conductivity as a function of the rate p/p° is comparable to the curve showing the isothermal water adsorption.

Dielectric Spectroscopy of Nematic Liquid Crystal Confined in Random Porous Matrices

ABSTRACTWe performed dielectric spectroscopy measurements to study dynamics of dielectrically active modes of nematic liquid crystals (LC) 5CB with polar molecules confined in silica porous glasses with average pore sizes of 1000 Å (volume fraction of pores 40%) and 100 Å (27%). In the nematic phase of bulk 5CB there is only one mechanism of dielectric relaxation: rotation of polar molecules around short molecular axis with relaxation time τ ∼10−8s. The spatial confinement and the existence of a highly developed interphase have a strong influence on dielectric properties of LC. We found that at temperatures about 30°C below the bulk melting point in both porous matrices the dielectric behavior of confined liquid crystal is very different from the behavior expected for solid state. The dielectric modes were not frozen in both porous matrices and we observed four well defined relaxational processes with relaxation times τ1 ∼ 10−1s, τ2 ∼10−5s, τ3 ∼10−8s (bulk like) and τ3 ∼10−9s. These four processes do not vanish even at temperatures corresponding to deep bulk isotropic phase. The relaxation time of the first process (slow) shows glass-like behavior in a wide temperature range below bulk melting point.

Dielectric dispersion and relaxation mechanism of some rigid polar molecules and their mixtures at microwave frequencies

The dielectric absorption of some rigid polar molecules, viz., pyridine, chlorobenzene, benzophenone and their mixtures have been studied at microwave frequencies 9.1 GHz and 19.64 GHz and also at 10KHz and optical frequency at 30°C in dilute solutions of benzene. The permittivity and dielectric loss at different frequencies have been plotted against concentration (weight fraction). The slopes a″ have been plotted against a' in a complex plane. The plots are semi-circles for single polar molecules as well as for mixtures of polar molecules. The measured dielectric data have been used to obtain relaxation time (τ). The relaxation time of these substances is also calculated by the simple, reciprocal mixing rule and by the relations proposed by M.P.Madan [Can. J. Phys., 65 (1987) 1573].

Dielectric relaxation of polyaniline/polyvinylalcohol blends

Blends from conducting polyaniline and insulating polyvinylalcohol were prepared, and their dielectric properties were investigated at 21°C from 5 Hz to 4 GHz. The dielectric relaxation of these blends can be very well described by the Cole-Cole equation up to a weight fraction of 6% polyaniline. The dielectric relaxation mechanism is discussed, dc conductivity study reveals a percolation threshold of about 8% for the insulator-semiconductor transition.

Depolarization thermocurrent studies in poly(hydroxyethyl acrylate)/water hydrogels

AbstractDetailed investigations on the dielectric relaxation mechanisms in poly(hydroxyethyl acrylate) (PHEA), by means of the thermally stimulated depolarization currents (TSDC) method in the temperature range 77‐300 K are reported. There is particular interest in the dependence of the dielectric relaxation mechanisms on the water content h, h = 0 − 0.5 w/w, in an attempt to contribute to a better understanding of the physical structure of water in the PHEA hydrogels. We employ thermal sampling (TS) and partial heating (PH) techniques to experimentally analyze the observed complex relaxation processes, due to the secondary (βsw) and the main (α) relaxation, into approximately single responses and to determine the spectra of activation energies E(T) at different h values. Measurements with different electrode configurations reveal different aspects of the dynamics of the relaxation mechanisms and allow the distinction between dipolar and conductivity relaxation contributions. It is shown that by means of these techniques we can determine certain temperature characteristics for the α relaxation and investigate their dependence on water content. We discuss the relation of these characteristic temperatures to the calorimetric glass transition temperature Tg. © 1994 John Wiley & Sons, Inc.

Dielectric relaxation and hydrogen diffusion in amorphous silicon

Hydrogen diffusion is technologically critical to the processing of amorphous Si for solar cell applications. It is shown that this diffusion belongs to a broad class of dielectric relaxation mechanisms which were first studied by Kohlrausch in 1847. A microscopic theory of the Kohlrausch relaxation constant β K is also constructed. This theory explains the values of β observed in many electronic, molecular and polymeric relaxation processes. It is based on two novel concepts: Wiener sausages, from statistical mechanics, and the magic wand, from axiomatic set theory.

Dielectric spectroscopy on CiEj/water mixtures as a function of composition

AbstractAt 25°C the complex dielectric permittivity of mixtures of water with low‐weight poly (ethylene glycol) monoalkyl ethers (CiEj) has been measured as a function of frequency v (1 MHz ≤ v ≤ 55 GHz) and mole fraction x(0 ≤ x ≤ 1). Various relaxation spectral functions are fitted to the measured spectra. It is found that the dielectric relaxation of all liquids (i, j = 2,2; 4,2; 6,2; 4,3; 4,1) can be consistently described by a sum of a Debye term with discrete relaxation time and a Davidson‐Cole term with an underlying unsymmetrical relaxation time distribution. With the exception of the C4E3/H2O mixtures the Debye term vanishes at high content of CiEj molecules (x > 0.25). It is assumed that this term represents a water‐rich subphase while the Davidson‐Cole term reflects a microphase of low water content. With the C4H3/H2O system crankshaft‐like motions of the C4E3 molecule obviously also contribute to the Debye term which exists also in the spectrum of the pure organic liquid. The parameters of all spectra are discussed with respect to the composition of the microphase of low water content, to effects of dipole orientation correlation, and to the mechanism of dielectric relaxation in associated liquids.

Dielectric studies of molecular mobility in hydrated zeolites

We report on detailed investigations of the dielectric relaxation mechanisms in a Na-13X zeolite by means of the method of thermally-stimulated depolarization currents (TSDC) over a wide range of temperatures, 77–300 K, and water content, 0.020–0.566 w w . Interest is mainly focused on quantitatively characterizing the effects of water on the motion of ions and the reduction of the mobility of water molecules in the confined pore geometry. Five distinct relaxation processes were identified. Three of them were attributed to motions of ions, namely, in the order of increasing temperature, local jumps of ions in the supercages, a Maxwell-Wagner effect and d.c. conductivity. The remaining two processes, present in the samples only for water content higher than a critical one for the completion of the first hydration layer, about 0.30, were ascribed to the reorientation of water molecules in the supercages and the small β-cages, respectively. The reduction of the mobility of water molecules was much stronger in the β-cages than in the supercages.

Dielectric properties of organic solute/water mixtures. Hydrophobic hydration and relaxation: In honour of Professor Hermann Gerhard Hertz

Results from measurements of the dielectric spectrum of organic solute/water mixtures are summarized. A survey of hydrophobic hydration effects as emerging from the experimental data is given. Recent ideas of the mechanism of dielectric relaxation in hydrogen bonded liquids are discussed. Comparison is made with some aspects resulting from computer simulation studies.

Dielectric dispersion and relaxation mechanism of associative + non-associative (rigid) polar molecules at microwave frequencies

The dielectric behaviour of mixtures of chlorobenzene with methyl alcohol, ethyl alcohol, n-propyl alcohol and tert-butyl alcohol has been measured at microwave frequencies 8.98 GHz and 19.64 GHz and also at 10 kHz and optical frequency at 30°C in dilute solutions of benzene. The permittivity and dielectric loss at different frequencies have been plotted against concentration (weight fraction). The slopes a have been plotted against a in a complex plane. The plots are semicircles indicating a single relaxation mechanism in these mixtures.

On the mechanism of dielectric relaxation in aqueous DNA solutions

The complex dielectric response of calf thymus DNA in aqueous saline solutions has been measured from 1 MHz to 1 GHz. The results are presented in terms of the relaxation of the incremental contributions to the permittivity and conductivity from the condensed counterions surrounding the DNA molecules. Measurements of the low-frequency conductivity of the samples also lends support to the condensed counterion interpretation.

Tunnelling mechanism of dielectric relaxation

Tunnelling mechanism of dielectric relaxation

Flashover in wide-band-gap high-purity insulators: Methodology and mechanisms

The electron beam of a scanning electron microscope (SEM) is used to charge an unmetallized insulator (Al2O3, Y2O3, SiO2) in vacuum. The charging is found to be stable in time after the e-beam is switched off. The SEM is also used to measure the implanted charge by measuring the resulting electrostatic potential. The distribution of potential around the trapped charges is determined by the classical laws of electrostatics. The electrostatic energy stored in the polarized dielectric can thus be determined. The implanted charge can be removed by the introduction of carriers into the polarized sample by flooding the surface with electron beams of varying energies. Slow or rapid relaxation occurs depending on the operating conditions of the flood gun (energy, intensity, etc.) which introduces the electrons. When the relaxation kinetics are slow, the electrostatic charge decreases slowly as a function of time. On the other hand, a rapid relaxation of the dielectric leads to the appearance of a high-density plasma which spreads over the insulator surface, resulting in treeing on the insulator surface. Along the path of the arc, the transfer of energy to the lattice triggers the sublimation of the ceramic and its mechanical fracture through thermal shock. At particular flood gun setup conditions, we have observed the formation of parallel fracture lines along the ceramic surface, away from the treeing region. These results constitute the basis for a new approach to understanding flashover along ceramics-vacuum interfaces. The important step is the plasma initiation, which we interpret on the basis of dielectric relaxation mechanisms. The parameter of the insulator that determines its breakdown initiation is its complex dielectric constant. It is concluded that the insulator’s band gap, the nature and density of defects localized in the band gap, and the dipolar relaxation induced by a variation of the electric field and connected with the presence of defects determine the holdoff level of insulators.

Molecular mechanisms of dielectric relaxation of highly polar liquids

The dielectric spectra of highly absorbing simple liquids CH3F, CH3Cl, CH3l CHF3 caused by reorientations of polar molecules have been computed. The calculations were performed over a wide temperature range for the confined rotator (CR) model and for the hybrid confined rotator/extended diffusion model (HM). The mechanisms of absorption and dielectric losses result from periodic librations of molecules and from their orientation in the alternating local electric field Ec(t), respectively.The calculated dielectric spectra agree with experiment over the frequency range 0 < ν/cm–1 < 200, covering the Debye relaxation and Poley absorption regions of the spectra. With increasing temperature T, the libration amplitude β increases while the libration time τ(the correlation time of the angular velocity |Ω|) and the barrier height UΩ decrease. The maximum discrepancy between theory and experiment occurs at temperatures close to the critical value; in this case the HM gives a better description of the Debye frequency spectrum than the CR model. Estimates based on application of the resonance condition, the liquid cell model and the hard sphere model are proposed.The self-diffusion coefficient D(T) has been calculated; in this case the collision frequency of molecules 1/τv(V) was estimated for the hard-sphere model and it was assumed that the self-diffusion is caused by molecules whose translational velocity v is sufficient to surmount the barrier Uv which is dependent on T. It was shown that the barrier Uv is essentially less than UΩ and for a first approximation, is proportional to UΩ.

The dielectric relaxation mechanism and internal rotation in biphenyl derivatives

The dielectric relaxation mechanism of 2,2'- and 3,3'-dibromodiphenyl has been examined from measurements at different frequencies and temperatures. All dielectric measurement have been done on diluted solutions in p-xylene. A simple dielectric absorption band was observed in both cases. It was found that the Goulon-Rivail model provides a good description of the dielectric relaxation mechanism.

Dielectric relaxation mechanism and internal rotation in phenylpyridines

Dielectric relaxation measurements have been carried out on three isomers of 3- and 4-phenylpyridines in p-xylene in the temperature range 293–323 K and from static frequency up to 10 GHz. Analysis of the dielectric dispersion of 2-, 3- and 4-phenylpyridines shows that the internal motion should be taken into account in order to derive a model for the dielectric relaxation process in these types of molecules.

Tunnelling mechanism of dielectric relaxation

Abstract It has been shown previously that on considering dielectric relaxation at low temperatures tunnelling should be taken into account. In this report the kinetic equation describing electric polarization in solid dielectrics is presented with complete consideration of the thermoactivated transition and tunnelling of relaxators. Solving the equation under a series of simplifying conditions allows to calculate the dielectric relaxation time, complex dielectric susceptibility and the density of thermostimulated currents. It is shown on the example of ice that polarization in polar dielectrics with H-bonds is of quantum nature at low temperatures.

Dielectric relaxation mechanism for proton glass

Abstract Dielectric relaxation of proton glasses is controlled by 0-H…0 proton intrabond hopping responsible for ‘Takagi’ HPO4 and H3PO4 intrinsic defect diffusion. The diffusion path has a one-dimensional fractal topology. The defects diffuse in a potential which also has a fractal nature, giving a barrier height distribution leading to a wide spread in dielectric relaxation times at low temperature. Expressions for dielectric relaxation and ac susceptibility are derived, and their fit to experimental results is briefly discussed.

Dielectric Relaxation Analysis of Water Adsorption in Sol Gel Derived Silica Gel Monoliths

ABSTRACTThe polarization mechanism of the low frequency dielectric relaxation (R1) observed in water adsorbed in porous silica gel is investigated using monolithic samples. It is attributed to electrode polarization involving proton hopping conduction. The influence of the thickness of the adsorbed water layer on the relaxation mechanism is discussed.