Frequency Dielectric Response Research Articles

The effects on polyetheretherketone and polyethersulfone of electron and γ irradiation

In this study, the effects of ionizing radiation on the aromatic polymers polyethersulfone (PES) and polyetheretherketone (PEEK) were investigated. UV light, /spl gamma/ rays and a high-energy electron beam were used as radiation sources. A range of techniques has been utilized to investigate changes in PES and PEEK as a result of exposure to these sources. Electron spin resonance (ESR) measurements showed the creation of radicals in PES by UV or /spl gamma/ radiation. These radicals decayed, but some were still present 24 h after irradiation at room temperature. In PEEK, radicals were formed as a result of UV at room temperature and by UV or /spl gamma/ irradiation at 77 K. After irradiation at room temperature, these radicals decayed totally within 24 h. UV-visible absorption measurements of PES samples showed a dose related change at the absorption edge; no such change was observed for PEEK. Phosphorescence after /spl gamma/ irradiation increased in intensity for PES at low temperature but decreased at higher temperatures. For PEEK, no such changes were observed after irradiation. The differential scanning calorimeter (DSC) measurements showed changes in the crystallization, glass transition and melt-point temperatures of both PES and PEEK after electron irradiation. Electrical changes induced by irradiation were investigated using thermally stimulated discharge current (TSDC) and low frequency dielectric responses, derived from transient current (TC) measurements. Dielectric changes were observed for PES exposed to either /spl gamma/ rays or an electron beam. For PEEK only minor changes in the TSDC spectrum were observed after exposure to /spl gamma/ radiation. More significant changes were observed after exposure to the electron beam. Correlations were found between the production of radicals and the changes in the structural and electrical properties of the materials. Since significant changes were observed in the dielectric spectra of PEEK and PES at low frequencies, dielectric spectroscopy may provide a measure of aging in both insulation systems.

Low frequency dielectric response in spin density wave phase of bechgaard salts

We report the dielectric response (100mHz-1MHz) of the spin density wave (SDW) in (TMTSF) 2PF 6 and (TMTSF) 2AsF 6 between 1.4K–4.2K. The relaxation gradually slows down in PF 6, while in AsF 6 it shows a tendency of divergence hinting the appearance of a glassy transition. The latter correlates with the important increase of the electrical threshold field for the SDW sliding found in AsF 6, but not observed in PF 6 compound.

Frequency and wave-vector dependent dielectric function of water: Collective modes and relaxation spectra

The longitudinal frequency and wave-vector dependent complex dielectric response function χ(k,ω)=1−1/ε(k,ω) is calculated in a broad range of k values by means of molecular dynamics computer simulation for a central force model of water. Its imaginary part, i.e., Im{ε(k,ω)}/|ε(k,ω)|2, shows two main contributions in the region of small k values: Debye-like orientational relaxation in the lower frequency part of the spectrum and a damped librational resonance at the high frequency wing. The Debye relaxation time does not follow a de Gennes-like pattern: τ(k) goes through a maximum at k≈k*≈1.7 Å−1, while the static polar structure factor S(k) peaks at k≈3 Å−1. The resonance frequency ω(k) and the decay decrement γ(k) show a dispersion law, indicative of a decaying optical-like mode, the libron. With an approximate normal mode approach, we analyze the origin of this mode on a molecular level which shows that it is due to a damped propagation of molecular orientational vibrations through the network of hydrogen bonds. At high k the decay, due to dissipation of collective into single particle motions, dominates. The static dielectric function is calculated on the basis of the response function spectra via the Kramers–Kronig relation. In the small k region ε(k) decreases from the macroscopic value ε≈80 to a value ≈15, i.e. it exhibits a Lorentzian-type behavior. This behavior is shown to be determined by higher order multipole correlation functions. In the intermediate and high k range, our results on ε(k) and χ(k) are in excellent agreement with data extracted from experimental partial pair correlation functions: ε(k) exhibits two divergence points on the k axis with a range of negative values in between where a maximum in χ(k) is found with χmax(k)≫1, indicative of overscreening. Consequences of quantum corrections to χ(k) with respect to a purely classical calculation are discussed and consequences are shown for the interaction energy between hydrated ions.

Improved low frequency and microwave dielectric response in strontium titanate thin films grown by pulsed laser ablation

We have grown epitaxial strontium titanate films on lanthanum aluminate substrates at a range of oxygen pressures and substrate temperatures. The complex dielectric function was measured as a function of temperature and electric field bias using a microwave ring resonator and a flip-chip technique. The films having the highest dielectric constant were grown with an oxygen pressure of 600 mTorr and showed large grains in the plane of the film. The small-signal dielectric constant of these films could be changed by a factor of 4 by applying an electric field. The films with the highest dielectric constant showed increased losses, but an improved figure of merit for application to tunable circuits.

Dielectric response and its light-induced change in undopeda−Si:Hfilms below 13 MHz

The low frequency (<13 MHz) dielectric response and its light-induced change in undoped a-Si:H were investigated in detail. The dielectric constant epsilon (the real part) in this range decreases with illumination time: following a stretched exponential law similar to that found for other light-induced changes. The saturation relative change was about 0.1-0.2 % for the measured samples. The change is fading away either after repeated illumination-annealing training or by aging at room temperature. The present results indicate some rearrangement of the whole Si network caused by light soaking.

Light-Excited Structural Instability of a-Si:H

ABSTRACTWith the accumulation of experimental data, it has been recognized by many that the light-induced metastable change of a-Si:H, Staebler-Wronski effect (SWE), may be related to a structural instability of the whole a-Si:H network. However, direct evidence of such a structural change is still lacking. In the present paper, the efforts of our laboratory in this direction will be reviewed, including the light-induced changes of Si-H bond absorption, low frequency dielectric response, and an apparent photo-dilation effect.

Low Frequency Dielectric Response in the Smectic C* Phase of a FLC Mixture

Experimental studies of the dielectric response of a ferroelectric liquid crystal (FLC) mixture having low spontaneous polarization at different temperature and frequency have been done in smectic C* (Sm-C*) phase. The dielectric measurement were carried out on homogeneously aligned samples. In the Sm-C* phase the temperature dependent dielectric contribution of the Goldstone mode has been measured using two cells of thicknesses of 20μm and 100 μm. Dielectric strength and corresponding relaxation frequencies are determined From the experimental results it has been observed mat the variation of dielectric strength and relaxation frequency with temperature are similar for the two cell, but the dielectric strength obtained for 20μm cell is higher than that obtained for l00μm cell at all temperatures. On the other hand, the temperature dependence of relaxation frequency for both 20μm and l00μm cell is almost the same. The mean field coupling coefficients between tilt and polarization are obtained from a fit of extended microscopic model for the interpretation of experimental results. The Goldstone mode rotational viscosity has been calculated from dielectric data. The results have been discussed.

Polar properties of a mesoscopic ferroelectric

At the 1995 Williamsburg workshop we reported[3] on the condensation of polar nanoregions in KTa1−x Nb x O3 approximately 20 K above the actual ferroelectric transition for Nb concentrations ranging from 1.2% to 30%. We have now studied the novel polar properties to which these nanoregions give rise. These include slow relaxational dynamics manifested by a strong dispersion in the low frequency dielectric response, deviations from a Curie-Weiss-like behavior and the appearance of higher harmonics in the polarization response signaling the onset of non-linearities. The first non-linear coefficient of the susceptibility has been derived from dielectric measurements in the presence of dc bias fields. The measurements under external fields also reveal the appearance of piezoelectric resonances that are temperature dependent. These properties may be characteristic of other ferroelectric systems with polarization inhomogeneities on a mesoscopic scale.

Low and infralow frequency dielectric response of γ and n - irradiated PLZT ceramics

Abstract The effects of different kinds of irradiation on the low (LF)- and infralow frequency (ILF) dielectric properties of PLZT ceramics at concentrations La=7at%, 8at% and Zr/Ti- 65/35, respectively, were investigated and the temperature behaviour of polarization loops (PL) in these specimens were studied.

Study of high frequency dielectric response of AgNbxTa1−xO3mixed system for x = 0.65, 0.82 and 1

Abstract The quasi-elastic diffusion measured near the specific M2-M3 phase transition in the AgTa1−xNbxO3 system is described by a modified Debye relaxational function from which dielectric dispersion can be calculated in the 1 to 100 cm−1 frequency range. An alternative description of the low-frequency Raman spectra, using Barker's formula is proposed in order to explain the structural phase transition.

Densification and frequency dielectric response (102 to 109 Hz) of ferroelectric K(Ta0.5Nb0.5)O3 ceramics

The frequency dielectric response in K (Ta 0.5 Nb 0.5 )O 3 ceramics (KTN) is investigated over the temperature range 280 to 450 K and over a wide range of frequency (10 2 to 10 9 Hz). The KTN ceramics are sintered using Li2CO 3 as an addition, allowing liquid phase sintering. Dielectric measurements are performed on two samples : a fast fired sample and a conventionally fired one. The ferroelectric-paraelectric transition occurs at T c 380 K ; it is a first-order transition. Two dielectric dispersions are observed : a polydispersive low frequency one (10 2 to 10 5 Hz) and a high frequency relaxation (10 8 to 10 9 Hz). The latter is explained on the basis of correlation chains or correlation volumes in a double potential well model.

Electrophoretic Mobility and Dielectric Response Measurements on Electrokinetically Ideal Polystyrene Latex Particles

We measured the dielectric response and electrophoretic mobility of a polystyrene latex to test the classical theory of electrokinetics. Measurements of the low frequency dielectric response of the latex at various (low) volume fractions were made using the four-electrode signal-processing spectrometer of Myers and Saville (J. Colloid Interface Sci. 1989, 31, 448). Electrophoretic mobilities over a range of NaCl concentrations were determined with a Coulter DELSA440. Good agreement between the ζ-potentials derived from the two complementary measurements was found using the standard model of electrokinetics and the ζ-potential from the upper branch of the mobility-ζ-potential relation. To our knowledge this is the first time that classical electrokinetic behavior has been found with off-the-shelf latex particles.

Vogel-Fulcher relationship for the dielectric permittivity of relaxor ferroelectrics.

The low frequency dielectric response of relaxer ferroelectrics is analyzed. Following the conventional approach this response is considered as a sum of those of relaxators having an exponentially wide and smooth spectrum of the relaxation times. It is shown that the Vogel-Fulcher relationship for the positions of the temperature maximum for the real and imaginary parts of the dielectric permittivity can be obtained as a direct consequence of gradual broadening of the spectrum with decreasing temperature. It is shown that such a Vogel-Fulcher-type relationship does not necessarily imply ''freezing'' in the system.

Dielectric spectroscopic characterization of a weak‐acid polystyrene colloid

AbstractA commercially available aqueous polystyrene colloid with surface ‐COOH groups was purified by exhaustive dialysis. This was then investigated for dielectric frequency response in the low frequency domain (0.01‐100 Hz), as a function of pH, ionic strength and the chemical nature of the counterions in the electrical double layer. In previous work on strong‐acid (sulfonate) colloids it had been shown that a plot of the imaginary part of the complex impedance as a function of the real part gave semicircles whose radii were a direct function of the surface charge density. In this work, surface charge is a function of pH. The degree of counterion binding was a function of hydrated ion size, as determined from an analysis of titration data and ionic strength of the continuous phase, which in turn was determined from the phase angle and the characteristic relaxation frequency. Dielectric spectroscopy is thus a powerful non‐invasive tool for quantitative analysis of the properties of the electrical double layer in these systems.

The high frequency dielectric response of a ferroelectric liquid crystal

Abstract The temperature and frequency dependences of the complex dielectric susceptibility of a ferroelectric liquid crystal near the smectic C*-smectic A phase transition have been calculated using the classical and generalized Landau models. It is shown that although the dielectric response of the S*C phase consists generally of four modes (soft, Goldstone, and two high frequency polarization modes) only three bands appear in the dielectric loss spectrum of ferroelectric liquid crystals at the SA–S*C phase transition. These results are in agreement with recent experimental data.

Dielectriic Dispersion Measurements of Salt-Water Saturated Porous Glass Compared with Local Porosity Theory.

AbstractA recent study [1] of the dielectric frequency response of a two component composite performed on a single specimen shows that local porosity theory LPT [2] represents a substantial improvement compared with other theories predicting the complex dielectric dispersion [3,4,5]. The purpose of the present work is to extend this investigation to a systematic study on several specimens with different compositions.

Note on the interrelation between low frequency dielectric response and [formula omitted] noise in condensed matter systems

Condensed dielectrics following the fractional power law are shown to generate a thermal 1 f voltage noise spectrum. However, systems which do not belong to this class may also produce a wide range 1 f voltage noise as demonstrated by a classical model of lossy dielectrics. The 1 f thermal voltage noise should carefully be distinguished from the 1 f power spectrum due to parametric resistivity fluctuations.

The role of multipoles in the dielectric anomaly of porous media

The effective polarizability of a brine filled pore in the close vicinity of other pores is strongly enhanced by multipolar interactions. We propose that this enhancement may be an important ingredient to the anomalous low frequency dielectric response of porous rocks and ceramics.

The Dynamics of Polar Solvation

AbstractFluorescence upconversion experiments with subpicosecond resolution are described. The time dependent fluorescence spectra of several probe molecules in polar solvents are measured and used to construct the Stokes shift correlation function C(t). The correlation functions decay nonexponentially in contrast to the predictions of simple Debye/Onsager continuum theory. At high values of the static dielectric constants the solvation times are much slower than predicted from standard continuum theory. Generalizations of continuum theory to include non‐spherical shapes, non‐Debye dielectric response and saturation effects are discussed. The experimental data are qualitatively in accord with predictions based on a frequency and position dependent dielectric response.

Low Frequency Dielectric Response of Chiral Smectic C

Abstract The low frequency dielectric response of SmC* is analysed within the framework of the electrodynamic continuum theory. The electric field induced distortion of the structure is calculated. The macroscopic contributions in the dielectric susceptibility are determined in fields parallel as well as normal to the helix. These contributions originate in the presence of the spontaneous polarization, the helical structure and the electromechanical effect.