A comparative study of the effective dielectric constant of a medium containing randomly distributed dielectric spheres embedded in a homogeneous background medium

The effective dielectric constant ∈_eff of a medium containing randomly distributed dielectric particles has been analysed by conventional methods: Foldy's approximation, the quasi-crystalline approximation (QCA) and the QCA with coherent potential. These conventional methods, however, have been indicated to become invalid for particles with a high dielectric constant; we have thus presented a new method that is valid for them. This paper compares ∈_eff of our method with those of the conventional methods by changing the volume fraction and the dielectric constant of spheres. As a result, our method is shown to be more powerful for the analysis of ∈_eff than the conventional methods.     

A dipole approximation for a dielectric mixture based on the equal field energy method

In this paper, a dipole-energy approximation for calculating the electric field distribution and saturation charge of spheres in an infinitely large dielectric mixture has been carried out. The approximation rests on the assumption that the field energy of mixture calculated using two different methods have the same value. One way is considering that the mixture in a uniform field E₀ as a uniform object of effective permittivity ?_eff from a macroscopic point of view, where ?_eff is seen as the average characteristic parameter of the object. The other way is assuming that the spheres in mixture are in the equivalent external field E₀′, and E₀′ related to the dielectric mismatch and the ratio of the sphere radius to the average distance between neighboring spheres has been obtained. Based on dipole-energy model, the approximate formulas for calculating the maximum field strength and saturation charge of spheres are derived separately.     

Anisotropic dielectric constant of TTF-TCNQ observed by dielectric resonance

Independent measurements at low temperatures of the anisotropic dielectric constant (∈₁) of TTF-TCNQ are reported. The measurements utilize dielectric resonance techniques to determine ∈^B₁ and ∈^a₁ on the same crystal. The results confirm the unusually large value for ∈^b₁(4.2 K) > 10³ and the anisotropic behavior ∈^b₁(4.2 K) > 10² found earlier in cavity perturbation studies of TTF-TCNQ.     

Determination of the effective dielectric function of silver powders produced by inert gas evaporation

Silver powder samples were prepared by evaporation of silver in an inert gas atmosphere. They were investigated by dc-conductivity-, reflectance- and photoacoustic (PAS) measurements. Model spectra based on the Bergman ansatz for the effective dielectric function ε_eff, the effective thermal and electrical conductivities λ_eff, and σ_eff, were fitted to the experiments. Good agreement has been found assuming a spongily packed, only weakly percolating topology and an increased damping of the free carriers in the small particles (classical size effect).     

Directional hemispherical radiative properties of random dielectric rough surfaces

In this paper the directional hemispherical reflectivity and transmissivity of one-dimensional, randomly rough, dielectric surfaces are determined by the use of the integral method. This method is derived from electromagnetic theory without any restrictive hypotheses. Since this exact approach is computationally very intensive, a geometric optics approximation method is also developed. Curves displaying radiative properties versus the correlation length for a constant mean square deviation of the surface from flatness are presented. In this respect, the influence on the validity of the approximate method of multiple scattering, the shadowing effect and the real index of refraction of the dielectric have been investigated. Transverse electric and transverse magnetic polarized incident plane waves are considered. For the latter, our interest is focused on the influence of roughness on the reflected and transmitted intensities for an angle of incidence close to the Brewster angle.     

Study of dielectric behaviour of Mn-Zn nano ferrites

Dielectric properties of Mn_0.4Zn_0.6Fe₂O₄ ferrites synthesised by co-precipitation method have been investigated as a function of frequency (up to 30 MHz) at different temperatures. Dispersion in dielectric constant has been observed between temperatures 450-500 K. DC resistivity was found to increase up to 100 times greater than those for the samples prepared by the conventional ceramic methods. Resistivity variation with temperature is also reported in the present work. The particle size is calculated using Scherrer equation for Lorentzian peak, which comes out between 9 and 19 nm. Possible mechanisms contributing to these processes have been discussed.     

Thermodynamic and dielectric properties of polar lattices

Monte Carlo results are presented for the free energy and dielectric constant of systems of permanent dipoles disposed on the sites of simple cubic and face-centred cubic lattices as functions of the parameter ? = μ²ρ/kT. Alternative schemes are considered for taking account of the long-range character of the dipolar interaction, based on an Ewald-type sum or a reaction-field approximation; use of such a procedure is essential if the system is to have the correct dielectric properties. Different methods of calculating the dielectric constant are also compared. It is shown that the two methods of treating the long-range contribution to the energy lead to similar results for the dielectric constant, but use of the Ewald method apparently introduces a small but systematic error. The results on both free energy and dielectric constant are discussed in the light of predictions of a number of analytical approaches. In the case of the simple cubic lattice the general trend in the computed dielectric constant as a function of ? is in closer accord with the solution of the mean spherical approximation than with a simple Padé approximant to the Clausius-Mossotti function.     

High-frequency dielectric constant of a two-dimensionally disordered model medium

An exact solution is derived for the two-dimensional two-phase model of a disordered medium proposed by Morozovskii and Snarskii [Ukr. Fiz. Zh. (Russ. Ed.) 28, 1230 (1983)] that arises because of hierarchical mixing of phases with different dielectric constants. The problem reduces to a nonlinear recurrence relation for the dielectric constants. It is found that the expressions for the dielectric constants at the nth stage of the iteration process can be expressed in terms of elementary functions. It is also found that in the absence of absorption the high-frequency dielectric constant of a composite material with different signs of the initial dielectric constants ε does not converge to a limit and is an oscillatory function of the frequency and n. Finally, for massive and thin-film samples, the local plasmon frequencies in the medium are established. Zh. éksp. Teor. Fiz. 114, 669–675 (August 1998)     

Effective dielectric functions of alkali halide composites and their spectral representation

In the electrostatic approximation inhomogeneous samples with characteristic scale of inhomogeneities much smaller than the wavelength of light can be characterized by an effective dielectric function _eff. This paper analyzes in which cases simple mixing formulas can be used to calculate _eff from the dielectric functions of the consituents. These considerations are compared with reflectance measurements of alkali halide composites in the far infrared. In many cases it turns out that it is essential to employ the general ansatz of the Bergman spectral representation in order to describe all effects due to the geometrical arrangement of the phases in a proper manner. It is necessary to adjust Bergman's spectral density to measurements. This can be done by use of a Monte-Carlolike algorithm. Further on we try to answer the question how to extract the dielectric function of one of the constituents from measurements of the effective dielectric function.     

Polymer thin-film transistors with high dielectric constant gate insulators

Field-effect transistors consisting of poly(3-hexylthiophene) have been fabricated with high dielectric constant SrBi₂Ta₂O₉ films working as the gate insulator. Significantly enhanced gate effects were observed in these devices compared to similar transistors with conventional SiO₂ gate dielectric. Our devices exhibited operating voltages around 10 V, as compared to about 100 V for devices employing SiO₂ as the gate dielectric. Moreover, inverters based on such polymer transistors were demonstrated with nice input–output characteristics. PACS 82.35.Cd     

Unique dielectric properties in polyaniline/poly(vinylidene fluoride) composites induced by temperature variation

To explore an effect of temperature on the dielectric properties in polyaniline/poly(vinylidene fluoride) (PANI/PVDF) composites, the dielectric properties of these composites with different volumetric fractions of PANI (?_PANI) were studied in a wide temperature range. An increase in the effective conductivity (σ_eff) and dielectric permittivity (?_eff) was observed with increasing temperature in all PANI/PVDF composites. Particularly, for the composite with ?_PANI = 0.01, less than the percolation threshold (?_C = 0.045), the increase in σ_eff and ?_eff was most significant. A tunneling effect could be responsible for the unique dielectric properties. The results provided us useful information related to the microstructure of composites, which was not reported previously. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Angular reflectance of suspended gold, aluminum and silver nanospheres on a gold film: Effects of concentration and size distribution

In this article, we describe a parametric study of the effects of the size distribution (SD) and the concentration of nanospheres in ethanol on the angular reflectance. Calculations are based on an effective medium approach in which the effective dielectric constant of the mixture is obtained using the Maxwell–Garnett formula. The detectable size limits of gold, aluminum, and silver nanospheres on a 50-nm-thick gold film are calculated to investigate the sensitivity of the reflectance to the SD and the concentration of the nanospheres. The following assumptions are made: (1) the total number of particles in the unit volume of suspension is constant, (2) the nanospheres in the suspension on a gold film have a SD with three different concentrations, and (3) there is no agglomeration and the particles have a log-normal SD, where the effective diameter, d _eff and the effective variance, ν _eff are given. The dependence of the reflectance on the d _eff, ν _eff, and the width of the SD are also investigated numerically. The angular variation of the reflectance as a function of the incident angle shows a strong dependence on the effective size of the metallic nanospheres. The results confirm that the size of the nanospheres (d _eff <100 nm) can be detected by reflected light from the bottom surface of a gold film with a reasonable sensitivity if a proper angle of incidence is chosen based on the type of metallic particles on a gold thin film at λ = 632 nm. We show that the optimum incident angle to characterize the size of nanospheres on a gold film is between 70° and 75° for a given concentration with a particular SD.     

离子导体在直流偏压作用下的介电特性

在我们以前的工作中,发现α-LilO₃在c向直流偏压作用下表观介电常数增加。并把这一现象归结为由离子导电所产生。本文进一步研究了离子导体KLiSO₄和NaCl单晶在直流偏压下的介电特性,也观察到了表观介电常数改变的现象,证实了上述结论的正确性。我们也研究了离子导体的这一特性与温度、频率和样品四周气氛的关系,讨论了它发生的条件。 关键词：     

Vibrational and dielectric properties of magnesium aluminate spinel: A first-principles study

The vibrational and dielectric properties of MgAl₂O₄ are investigated within the framework of density functional perturbation theory. Results of phonon frequencies at the Brillouin zone center, static dielectric constant, and electronic dielectric constant are reported. In comparison with experimental results, we find that the generalized gradient approximation potential results in more accurate phonon frequencies than local density approximation potential does. Dielectric, refractive index, extinction coefficient and infrared reflectance spectra of MgAl₂O₄ are given, and the figures suggest that MgAl₂O₄ presents good transmission properties in the spectrum range above 1000 cm−1 and below 300 cm−1.     

Effect of TiO2 crystalline composition on the dielectric properties of TiO2/P(VDF‐TrFE) composites

In order to study the effect of the TiO₂ particle crystalline composition (with different proportions of rutile and anatase crystals) on the dielectric properties of the composite, titanium dioxide (TiO₂) particles and TiO₂/poly(vinylidene fluoride‐co‐trifluoroethylene) [P(VDF‐TrFE)] composites were synthesized by a reflux method and the solution route, respectively. The results indicated that the optimum TiO₂ particle crystalline composition is anatase content of 37% and rutile content of 63% for dielectric‐constant modifier applications. Furthermore, a dielectric constant of 25.7 with dielectric loss of 0.17 at 100 Hz at room temperature were obtained in the composite with 40 wt% TiO₂ particles. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The structure of an induced electrorheological solid

We have extended a method devised by Rayleigh and calculate the effective dielectric constants (ϵ_eff) of face-centred-cubic (fcc), body-centred tetragonal (bct) and hexagonal close-packed (hcp) structures of spheres of a dielectric material (ϵ_p) in a dielectric matrix (ϵ_f) specially. The comparison of the (ϵ_eff − ϵ_f)/f_c of these three types of lattice shows that the body-centred tetragonal structure is favoured in energy, where f_c is the volume fraction of particles that are touching for a given lattice.     

Stability of a bubble in a dielectric liquid in an external electrostatic field

Critical instability conditions are found for a gas bubble in a liquid dielectric in a uniform external electrostatic field E ₀. It is shown that they depend both on the magnitude of E ₀ and on the properties of the liquid, as well as on the gas pressure in the bubble. In a linear approximation with respect to the square of the eccentricity of an equilibrium spheroidal form, the equilibrium eccentricity of the bubble exceeds the equilibrium eccentricity of a drop in the field E ₀. The gas pressure in the bubble lowers the critical electric field E ₀ for development of an instability in the bubble. Zh. Tekh. Fiz. 69, 43–48 (August 1999)     

Propagation of Gaussian beams through parabolic index optical waveguides with random dielectric constant gradient

Abstract

The propagation of gaussian beams through parabolic index optical waveguides having random irregularities in the dielectric constant gradient has been studied. For fundamental mode propagation, the perturbation approach has been employed and an analytic expression for the loss of power from the fundamental mode has been obtained. For an incident gaussian beam with arbitrary width, geometrical optics approximation has been used and an exact analytical expression for the average value of the beamwidth has been derived for a particular random process, namely, the dichotomic Markov process. The fluctuations in the beamwidth have also been calculated.     

Microscopic calculations of ferroelectric instability in perovskite crystals

First-principles calculations are performed relating to the stability of a series of perovskite crystals with respect to transition to the ferroelectric and the antiferroelectric state. The calculations employ the generalized Gordon-Kim method, in which the total charge density of an ionic crystal is represented as a superposition of the densities of the individual ions. In the spirit of the nonequilibrium thermodynamics of Leontovich the charge density of an individual ion is calculated in the presence of external auxiliary fields which deform this density. Multipole deformations up to quadrupole are taken into account. The actual magnitude of the deformation is found by minimizing the total energy of the crystal in the Thomas-Fermi-Dirac approximation. The calculated values of the ion shifts in the ferroelectric phase for BaTiO₃, and also the electron contribution to the dielectric constant ε _∞ and the dynamic Born effective charges Z ^eff are found to be in good agreement with the experimental data. The proposed method allows one to obtain an analytical expression for ε _∞, Z ^eff, and the dynamic vibration matrix. It is shown that these expressions formally coincide with the expressions arising in the phenomenological models of the polarized and deformed ion. Analysis of the expressions obtained confirms the validity of the classical theory of ferroelectrics of displacement type for perovskite crystals. Zh. éksp. Teor. Fiz. 114, 333–358 (July 1998)     

The transverse dielectric properties of KH2PO4

We have measured the transverse dielectric constant of KH₂PO₄ crystal from liquid N₂ to room temperatures. Extending the Blinc-de Gennes' model in conjunction with a cluster approximation leads to a new independent determination of the Slater-Takagi parameters and improves significantly the fit of the model to the data.