Dielectric Contribution Research Articles

Piezoelectric and Dielectric Properties for Doped Lead Zirconate Titanate Ceramics under Strong Electric Field

Piezoelectric and dielectric responses were investigated for doped lead zirconate titanate (PZT) ceramics under strong electric field (1.6 MV/m). Displacements and polarization changes were measured and the effects of dopants were discussed. Doping of 2% Nb significantly increased the displacement of (Pb0.875Sr0.125)(Zr0.55Ti0.45)O3 ceramics. Doping of 2% Fe suppressed 90° domain rotation and decreased displacement. Results were discussed by dividing displacement and polarization change into two components, i.e., a piezoelectric or dielectric contribution and a 90° domain rotation contribution.

Domain wall dielectric response in the phase transition region in some ferroelectrics

Abstract Properties of two dielectric contributions (I and II) observed in the phase transition region, are presented for a displacive ferroelectric TSCC and for a order-disorder ferroelectric TGS. Order clusters are discussed to be origin of both contributions.

Soft-mode dielectric anomaly of a chiral smectic-B liquid crystal with sign reversal in the electroclinic response.

The static dielectric permittivity of the smectic-${\mathit{B}}^{\mathrm{*}}$ phase is studied as a function of temperature. The field-induced tilt angle, which is due to the electroclinic response, is measured as a function of temperature. The investigated phase precedes the chiral smectic-${\mathit{A}}^{\mathrm{*}}$ phase upon heating. In the ${\mathit{A}}^{\mathrm{*}}$ phase, the soft-mode dielectric contribution increases with decreasing temperature but does not diverge at the transition, a behavior similar to that of a first-order phase transition. Below the ${\mathit{A}}^{\mathrm{*}\mathrm{\ensuremath{-}}}$${\mathit{B}}^{\mathrm{*}}$ transition temperature ${\mathit{T}}_{\mathit{C}}$, the dielectric permittivity shows a sharp drop to a value considerably lower than that of the adjacent ${\mathit{A}}^{\mathrm{*}}$ phase. On further cooling one would expect a monotonic decrease of the soft-mode contribution. However, in this compound, at a certain temperature ${\mathit{T}}_{\mathrm{rev}}$, in the ${\mathit{B}}^{\mathrm{*}}$ phase, the dielectric permittivity starts to increase again with decreasing temperature. This anomaly in the soft-mode dielectric response is attributed to a change of sign in the electroclinic coefficient, which has been observed by an electro-optic experiment. An attempt to fit the measured dielectric permittivity to a modified expression for the dielectric function that contains a temperature-dependent coupling coefficient is presented.

Dielectric dispersions in pentakis methyl ammonium bismuthate single crystals. II. (CH3NH3)5Bi2Cl11

For pt.I see ibid., vol.4, p.2687 (1992). The dielectric dispersion of the permittivity epsilon c* in (CH3NH3)5Bi2Cl11 single crystals has been studied in the frequency range from 1 MHz to 20 GHz in the temperature range 150-400 K and from 18 to 40 GHz in the temperature range 300-360 K. Near the high-temperature ferroelectric phase transition (308 K) the dielectric behaviour is determined by the properties of two relaxators; a low-frequency relaxator in the megahertz region showing a critical slowing down and a high-frequency relaxator in the gigahertz region which is thermally activated. The contribution of the high-frequency relaxator vanishes near the temperature of the second phase transition at about 170 K where another contribution of a third relaxator with an extremely small strength appears. The origins of the observed dielectric contributions with respect to the molecular motion of the CH3NH3+ cations is discussed.

Interfacial electrostatic potential of polyions as probed by acid‐base indicator dyes covalently attached to the main chain

AbstractPolyanions with various charge densities with a solvatochromic acid‐base indicator dye attached to the main chain were prepared by the terpolymerization of acrylic acid, acrylamide, and a small mole fraction of 1‐(2‐methacryloyloxyethyl)‐4‐(2‐(4‐hydroxyphenyl)ethenyl)pyridinium bromide. The apparent pK value for the polymer‐bound indicator dye (pKabs) was determined by the spectroscopic pH titration. The interfacial electrostatic potential (ψ) was calculated from pKobs. Dielectric contribution to the acid‐base equilibrium was taken into account for the calculation. The electrostatic potentials thus obtained were compared with those calculated from the nonlinearized Poisson‐Boltzmann equation for a cylinder model. The indicator dye moieties are suggested to be reporting the mean field potential at a distance of about 10 Å from the axis of the polyion cylinder. The observed values as a function of the charge density were also discussed in connection with Manning's theory.

Dielectric studies of the soft mode and Goldstone mode in ferroelectric liquid crystals

Abstract Different contributions to the dielectric permittivity in ferroelectric liquid crystals are discussed, with emphasis on the soft mode and the Goldstone mode and their location in the dielectric spectrum. Experimentally, the complex dielectric permittivity has been studied as a function of temperature and frequency in the range 5 Hz - 13 MHz for three different ferroelectric liquid crystal materials. The main problems encountered in dielectric measurements at low and high frequency are discussed in some detail. The soft mode dielectric behaviour has been studied as a function of temperature, frequency and bias electric field. The applicability of the Curie-Weiss law for the soft mode dielectric contribution in the A* phase was analyzed. In the C* phase the temperature dependence of the dielectric contribution of the Goldstone mode has been measured. By applying a bias electric field, we have been able to study the soft mode dielectric behaviour also deep into the C* phase. In the A* and C* phases the relaxation frequency and the inverse of the dielectric strength of the soft mode vary linearly with |T-TC|, which can be used to determine the A*-C* transition temperature to a good accuracy. The ratio of the slopes of the soft mode relaxation frequency vs temperature line and the inverse of dielectric strength vs temperature line in the C* and A* phases has been calculated for the three compounds and compared with solid ferroelectric systems. The soft mode rotational viscosity in the A* phase has been calculated for one of the compounds.

Signal-to-noise improvement in mid-field MRI surface coils: A degree in plumbing?

From a series of standard SE imaging sequences, performed on a Bruker 0.28 T imaging system, with the assistance of a healthy volunteer, the image signal-to-noise ( S N ) ratio obtained from a 23-cm square surface coil has been shown to increase by up to 38% as the tube gauge is increased from 4 to 18 mm. The reason did not lie solely in the much improved Q factor of the unloaded coils. Despite a more than twofold increase in the unloaded Q factor, the loaded coil Q values only increased by 8%. It would appear, however, that the resistive, dielectric and inductive noise components are all reduced, and hence contribute to the observed improved S N . The reduction in pure ohmic losses accounts for a quarter of the improved S N , while the reduced inductive and dielectric losses provide the remaining three quarters. No independent quantification of the two latter noise sources was attempted, although a reduced dielectric contribution is confirmed qualitatively by a reduction in the negative frequency shift of the resonance frequency as a function of increasing coil gauge when the coil is loaded.

Structural and energetic aspects of defects in high-T c oxides from atomistic lattice simulations

This paper reports recent atomistic lattice calculations of specific aspects of point defects related to high-Tc superconductivity. They are Jahn–Teller effects in La2CuO4, the defect chemistry of potential electron superconductors and the dielectric contribution to bipolaron formation in La2CuO4 and Nd2CuO4. It is shown that Jahn–Teller effects can be introduced empirically into defect calculations and that in the case of La2CuO4 these effects favour polaron formation by 0.2–0.4 eV. From an examination of the stability of holes and defect electrons in a range of ternary cuprates it is shown why La2CuO4 leads only to hole conductors and Nd2CuO4 and Pr2CuO4 only to electron conductors. Calculations show important differences between the interaction of Cu and O holes in La2CuO4. In both cases a confining potential is found for interplanar small polaron pairs and an attractive interaction for intraplanar O hole pairs in the small-polaron limit. The interaction of defect electrons in Nd2CuO4 shows neither of these characteristics.

Molecular Orientational Structures in Ferroelectric, Ferrielectric and Antiferroelectric Smectic Liquid Crystal Phases as Studied by Conoscope Observation

Molecular orientational structures in MHPOBC were studied by means of conoscope observation. Contrary to a ferroelectric response of the conoscope to an electric field in the smectic C* phase, the conoscopic figure in the antiferroelectric smectic CA* phase does not shift its center and is biaxial with its optic plane perpendicular to the field direction. This conoscope change is only due to a dielectric contribution, indicating the existence of the inherent threshold in the electric field induced transition to the ferroelectric phase. In between the ferroelectric and antiferroelectric phases, a kind of ferrielectric phase was clearly distinguished in the field response of the conoscopic figure; the shift of the conoscope center perpendicular to the applied field and the biaxial optic plane parallel to the field suggest a novel molecular orientational structure.

Dielectric and anharmonic behaviour of mixed alkali halides

In the present paper an analysis of the dielectric behaviour and anharmonic contribution to the dielectric constants has been made of KCl−KBr mixed ionic crystals. The temperature and volume derivatives of static (e0), electronic (e∞) dielectric constants and energy gap parameter (Eg) have been calculated using the Szigeti and Havinga and Bosman dielectric theories. The anharmonic contributions have been estimated in terms of temperature derivatives of dielectric constants at constant volumes. The modified form of Clausius-Mossotti theory of dielectric polarization has been used for the mixed crystal under study. The calculated quantities have been compared with the available experimental data. A good agreement has been obtained.

Modelling the effect of pressure on the rates of ionic and polar reactions

The rate kP of ionic and polar reactions in solutions is known to be sensitive to the applied pressure P. In the present work, structural and dielectric contributions to kP are expressed explicitly as functions of P. The pressure dependence of the volume and the dielectric constant of the solvent are described by the Tait equation. The model predicts the relation ln (kP/k0)=–νP/B–µ ln(1 +P/B) where ν, µ and B are adjustable parameters. This expression fits remarkably well the kinetic data of 11 ionic and polar reactions up to 45 kbar. The resulting activation volume is ΔV[graphic omitted]P=ΔV[graphic omitted]∞+(ΔV[graphic omitted]0–ΔV[graphic omitted]∞)/(1 +P/B) where the limiting values of ΔV[graphic omitted]P at zero and infinite pressures are expressed in terms of intrinsic volume changes in the reactants and in the surrounding solvent, and by the strength of the electrostatic interaction. The results suggest that for ionic and polar reactions, the structural and the dielectric contributions to the activation volume can be separated objectively, using a single solvent. The meaning and the significance of the various parameters are discussed.

Microwave study of the soft ferroelectric mode in Sn2P2S6crystals

Abstract This paper presents the results of the investigation of dielectric dispersion in semiconductive ferroelectric Sn2P2S6 crystals over the frequency range 1 kHz to 78.5 GHz. The main dielectric dispersion in Sn2P2S6 is caused by the soft ferroelectric Bu mode and in the vicinity of the Curie point it occurs in the millimeter region. The frequency of the soft mode in the paraelectric phase varies according to vs = 35 (T-Tc )½ GHz on approaching the Curie point. The soft mode is strongly overdamped. Close to Tc the relative damping is y/vs = 14. The dielectric contribution of the soft mode is equal to the static dielectric permittivity and it explains its whole temperature-dependence.

Green function theory of charge transfer processes in solution

By using a Green function Q to characterize the linear response of a dielectric body to electric charges, we obtain a theory for the solvent dielectric contribution to relaxation along the reaction coordinate RC(t) in an electron transfer process. For an electron transfer reaction model, in which the ions are embedded in a dielectric continuum, the theory gives, at t=0, the reorganization free energy derived by Marcus in 1956. For the same model the characteristic time τQ associated with RC(t) is evaluated in terms of the dielectric function εω of the medium. How the rate constant ket for an electron transfer process depends on τQ is illustrated for both high-barrier and low-barrier cases by approximating RC(t) as a Smoluchowski process on a potential surface. Applying the theory to a molecular model (charged hard sphere ions in a dipolar hard sphere solvent), treated in the mean spherical approximation for the response at any frequency (Wolynes, 1987), indicates that the effects of the molecular structure of the solvent on τQ are large even if the ion is several times larger than a solvent molecule.

Radiofrequency penetration and absorption in the human body: limitations to high-field whole-body nuclear magnetic resonance imaging.

This study presents experimental results about the effective depth of penetration and about the radiofrequency (rf) power absorption in humans as a function of frequency. The frequency range investigated covers 10 up to 220 MHz. For the main part, the results were derived from bench measurements of the quality factor Q, and of the resonance frequency shift due to the loading of the coil. Different types of head-, body-, and surface coils were investigated loaded with volunteers or metallic phantoms. For spin-echo imaging at 2 T (85 MHz), the local specific absorption rate (SAR) was found to be approximately equal to 0.05 W/kg using a pi pulse of 1-ms duration and pulse repetition time TR = 1 s. Measurements of the quality factor Q as a function of frequency show that the SAR depends upon the frequency f according to approximately f2.15. The effective depth of rf penetration as derived drops from 17 cm at 85 MHz to 7 cm at 220 MHz. Head imaging with B1 penetrating from practically all sides into the object should be possible up to 220 MHz (5 T) with SAR values staying within the local limit of 2 W/kg as set by the FDA. Whole-body imaging of large subjects as well as surface coil imaging is depth limited above 100-MHz frequency. Perturbation methods are applied in order to separate the total rf power deposition in the patient into dielectric and magnetic contributions. The observed effects due to interactions of rf magnetic fields with biological tissue contradict predictions based on homogeneous tissue models. A refined tissue model with regions of high electrical conductivity, subdivided by quasi-insulating adipose layers, provides a rationale for a better understanding of the underlying processes. At frequencies below 100 MHz, the rf power deposition in patients is apparently more evenly distributed over the exposed body volume than currently assumed.

Variations des caracteristiques dielectriques des resines epoxydes au cours de leur reticulation sous microondes

AbstractThe dielectric loss factor of epoxy resin cured by amine is measured all along the curing, performed under a microwave field (2,45 GHz). A linear hypothesis based on the dielectric contributions of all known constituents allows to show a polar intermediate state; the formation of this state is reversible. An interpretation taking into account hydrogen bonds is proposed. The evolution of dielectric constants during curing brings evidences that microwaves are a peculiarly convenient means of energy transfer to chemical processes as cross‐linking reactions.

Dielectric Huang scattering from point defects in ionic crystals

Abstract We discuss the intensity of the scattering of X-rays or neutrons from point defects in ionic crystals. It is well known that the long-range elastic distortion field surrounding a defect causes the intensity to diverge as the scattering wave-vector approaches a reciprocal lattice vector, an effect known as Huang scattering. We show that the dielectric polarization field surrounding a defect with a net charge in an ionic crystal gives rise to a divergence of the same kind and we derive a formula for the intensity, which includes this dielectric contribution. Published experimental analyses of Huang scattering in ionic crystals have not taken account of this contribution, and may need to be reconsidered.

Surface Polariton Mode Densities

Abstract Starting from the Green functions of the electromagnetic (EM) field in two adjacent dielectric half-spaces, the surface and bulk contributions to the EM-density of states are discussed. The mode densities determine the thermodynamic properties of the radiation field.

Dielectric relaxation contribution to dispersion of junction admittance

Significant frequency dispersion is observed in capacitance vs voltage measurements on solar cell structures prepared on high resistivity semiconductors such as a-Si and CdS. An equivalent circuit analysis of both Schottky barrier and Mott barrier devices shows that the equivalent parallel plate capacitance measured in an experiment corresponds to the distance from the interface to a position in the semiconductor bulk where the dielectric relaxation time is τ(x 1) = 1 ω with ω the angular frequency. Comparison of measured C - V - f curves with reported for junctions in the dark and under ∼ 1 3 AM1 illumination.

Electrical Conductivity and Relaxation in Ice Crystals with Known Impurity Content

AbstractThree-terminal dielectric bridge measurements (in the range 20 Hz to 100 kHz between — 5°C and —90 to — 120°C) have been made of ice doped with (a) conductivity-enhancing ionic impurities (HCl, HF, NaCl, KF, NH4F) and (b) conductivity-depressing solutes (NH4OH, NH4Cl, NH5CO3, NaHCO3). Blocking electrodes were used for the first group. The true ice parameters were extracted from linearized plots of the Debye equations. Chlorides and fluorides showed very similar characteristics in their spectra and static conductivity. The results suggest that static conductivity is controlled by extrinsic protons. On the other hand, bases, or solutes that impart a positive freezing potential to the ice, suppress extrinsic protons. In this case, the static conductivity was not, or only weakly, temperature dependent and lower than in the first group. A conductivity cross-over was observed in neither case. The dielectric conductivity contribution is strongly dependent on impurity concentration but apparently less affected than the static conductivity by the nature of the solute. The principal relaxation time is reduced by most solutes, exceptions are pure (bicarbonate-free) bases, sodium bicarbonate, and carbon dioxide.

Electrical Conductivity and Relaxation in Ice Crystals with Known Impurity Content

Abstract Three-terminal dielectric bridge measurements (in the range 20 Hz to 100 kHz between — 5°C and —90 to — 120°C) have been made of ice doped with (a) conductivity-enhancing ionic impurities (HCl, HF, NaCl, KF, NH4F) and (b) conductivity-depressing solutes (NH4OH, NH4Cl, NH5CO3, NaHCO3). Blocking electrodes were used for the first group. The true ice parameters were extracted from linearized plots of the Debye equations. Chlorides and fluorides showed very similar characteristics in their spectra and static conductivity. The results suggest that static conductivity is controlled by extrinsic protons. On the other hand, bases, or solutes that impart a positive freezing potential to the ice, suppress extrinsic protons. In this case, the static conductivity was not, or only weakly, temperature dependent and lower than in the first group. A conductivity cross-over was observed in neither case. The dielectric conductivity contribution is strongly dependent on impurity concentration but apparently less affected than the static conductivity by the nature of the solute. The principal relaxation time is reduced by most solutes, exceptions are pure (bicarbonate-free) bases, sodium bicarbonate, and carbon dioxide.