Dielectric Theory Research Articles

Wave Propagation in Flexoelectric Microstructured Solids

Wave propagation in elastic dielectrics with flexoelectricity, micro-inertia and strain gradient elasticity is investigated in this paper. Dispersion phenomenon, which does not exist in classical elastic dielectric theory, is observed in the flexoelectric microstructured solids. Analytical solutions for the phase velocity \(C_{p}\), group velocity \(C_{g}\) and their ratio \(\gamma = C_{g} / C_{p}\) are calculated for the case of harmonic decomposition. The magnitudes of the phase velocity and group velocity changed with the increasing of the wave number, while they are constant in the classical elastic dielectric theory. It is shown that the flexoelectricity, micro-inertia and microstructural effects are significant to predict the real behavior of longitudinal wave propagating in flexoelectric microstructured solids. Microstructural effects are not sufficient for dealing with realistic dispersion curves in flexoelectric solids, the micro-inertia and flexoelectricity are needed to obtain a physically acceptable value of the phase and group velocities.

Lightweight ferroferric oxide nanotubes with natural resonance property and design for broadband microwave absorption

Due to strong magnetism along with low density and low percolation threshold, hollow Fe3O4 nanostructures have important potential applications in absorbing materials. In this work, Fe3O4 nanotubes with both dielectric and magnetic losses, namely bi-loss features, were obtained through two-step chemical methods (hydrothermal method and activated carbon reduction). The Fe3O4 nanotubes show high dielectric loss due to the electronic relaxation polarization, and the concentration dependence of dielectric properties for Fe3O4 nanotubes composite can be well described by the effective dielectric theory. In comparison with bulk Fe3O4 with natural ferromagnetic resonance around 1.2 GHz, the as-prepared Fe3O4 nanotubes present a natural resonant peak at 4 GHz frequency, leading to the higher magnetic loss in the radar band (2–18 GHz). Therefore, the Fe3O4 nanotubes show better microwave absorption with minimum reflection loss up to −50.94 dB compared with other Fe3O4 nanostructures. Moreover, double loss peaks were observed in 70 and 80 wt% samples with thickness of 5 mm, making this material a good candidate for designing broadband metastructure absorber.

New instrumentation and cutting edge research

Stimulated by Ondrej Krivanek's contributions, the complex interaction between research and innovations in the instrumentation for electron microscopy is discussed. Specific attention is given to aberration correction and to spectroscopy in both the valence region and at the energies of localised phonons or bond vibrations. Current thinking about projection imaging and dielectric excitation theory may be challenged. More significantly in the new fields of investigation opened up to them, electron microscopists will need to build closer relations, particularly with the photonics and scanning tunnelling microscopy communities. Further improvements in instrumentation could usefully be directed towards imaging and spectroscopy at higher scattering angles as well as the incorporation of other facilities such as photon stimulation and secondary electron imaging.

Dielectric properties of lunar surface

Measurements of the dielectric characteristics of lunar soil samples are analyzed in the context of dielectric theory. It has been shown that the real component of the dielectric permittivity and the loss tangent of rocks greatly depend on the frequency of the interacting electromagnetic field and the soil temperature. It follows from the analysis that one should take into account diurnal variations in the lunar surface temperature when interpreting the radar-sounding results, especially for the gigahertz radio range.

Calculation of Surface Excitation Parameters by a Monte Carlo Method

Electron inelastic mean free path (IMFP) is an important parameter for surface chemical quantification by surface electron spectroscopy techniques. It can be obtained from analysis of elastic peak electron spectroscopy (EPES) spectra measured on samples and a Monte Carlo simulation method. To obtain IMFP parameters with high accuracy, the surface excitation effect on the measured EPES spectra has to be quantified as a surface excitation parameter (SEP), which can be calculated via a dielectric response theory. However, such calculated SEP does not include influence of elastic scattering of electrons inside samples during their incidence and emission processes, which should not be neglected simply in determining IMFP by an EPES method. In this work a Monte Carlo simulation method is employed to determine surface excitation parameter by taking account of the elastic scattering effect. The simulated SEPs for different primary energies are found to be in good agreement with the experiments particularly for larger incident or emission angles above 60° where the elastic scattering effect plays a more important role than those in smaller incident or emission angles. Based on these new SEPs, the IMFP measurement by EPES technique can provide more accurate data.

Effect of chemical interface damping and aggregation size of bare gold nanoparticles in NaCl on the plasmon resonance damping

The plasmon resonance properties of gold nanoparticles (AuNPs) in liquid ionic medium with different concentration was investigated using a UV-visible/NIR spectrophotometer. The optical absorption spectra of AuNPs suspended in NaCl solutions in the range of 400 – 800 nm were measured; specifically, 10 nm bare AuNPs separately suspended in NaCl solution with molar concentrations of 0.01M and 1M resulted in different shapes and widths in the plasmon resonance peak. The optical spectral resonances were analyzed using the Mie theory, Lorenz-Drude size-dependent dielectric functions and effective medium theory. Analysis of the spectral features by means of theoretical models showed that the damping of the plasmon resonance of AuNPs in ionic medium is size dependent. The experimental results indicated that the effect of chemical interface damping is more prominent compared with the inherent size effects of AuNPs.

Thermal annealing induced physical properties of electron beam vacuum evaporated CdZnTe thin films

The thermal annealing induced physical properties of electron beam vacuum evaporated CdZnTe thin films have been investigated in the present work. The films of thickness 300nm were deposited on indium tin oxide coated glass substrates followed by thermal annealing at 100°C, 200°C and 300°C. The pristine and annealed films were subjected to the X-ray diffraction, UV–Vis spectrophotometer, Fourier transform infrared (FTIR) spectroscopy, source meter and Energy-dispersive spectroscopy for structural, optical, electrical and elemental composition analysis respectively. The structural analysis revealed that the films have cubic oriented polycrystalline structure with preferred orientation (111). The optical absorbance is found to increase with annealing while optical energy band gap is observed to decrease. Different optical constants like absorption coefficient, refractive index, dielectric constants and energy loss functions have been calculated using Herve-Vandamme model, dielectric theory, Swanepoel model and discussed. The FTIR study indicates the absorption intensity is increased with annealing temperature. The elemental analysis revealed to the presence of Cd, Zn and Te elements in the deposited films. The current-voltage tests show that the electrical conductivity is increased with thermal annealing. The results revealed to a demanding behavior of annealed films for the solar cell applications.

Ionizing Doses Calculations for Low Energy Electrons in Silicon and Aluminum

The electron transport at low and very low energy (10 eV–2 keV) is investigated with a Monte Carlo (MC) code in silicon and aluminum. The elastic scattering with nuclei is described by Mott’s model of partial waves, whereas the inelastic collisions with electrons are described by the complex dielectric function theory. Comparisons of MC simulations with electron emission yields (EEY) and energy loss spectra experimentally measured in ultrahigh vacuum on Ar-etched samples are given. The practical ranges and the ionizing dose calculations are presented down to 10 eV for electrons in silicon and aluminum. The simulation results show a correlation between the EEY and the ionizing doses. At low energy, while the electrons stay in the first ~10 nm from the surface due to the elastic scattering, the EEY increases and the ratio of the ionizing dose over the incident energy decreases. Above 200 eV, when the electrons go deeper into the solid due to the inelastic scattering, the EEY decreases and the ionizing dose ratio increases.

Investigation on the viscoelastic behaviors of a circular dielectric elastomer membrane undergoing large deformation

To explore the time-dependent dissipative behaviors of a circular dielectric elastomer membrane subject to force and voltage, a viscoelastic model is formulated based on the nonlinear theory for dissipative dielectrics. The circular membrane is attached centrally to a light rigid disk and then connected to a fixed rigid ring. When subject to force and voltage, the membrane deforms into an out-of plane shape, undergoing large deformation. The governing equations to describe the large deformation are derived by using energy variational principle while the viscoelasticity of the membrane is describe by a two-unit spring-dashpot model. The evolutions of the considered variables and the deformed shape are illustrated graphically. In calculation, the effects of the voltage and the pre-stretch on the electromechanical behaviors of the membrane are examined and the results show that they significantly influence the electromechanical behaviors of the membrane. It is expected that the present model may provide some guidelines in the design and application of such dielectric elastomer transducers.

Dielectric response of filled high temperature vulcanized silicone rubber

The dielectric response of three kinds of filled high temperature vulcanized (HTV) silicone rubber - SiO 2 based nanocomposite with different alumina tri-hydrate (ATH) micro-filler contents are investigated over a wide range of frequency and temperature. The results are analyzed in terms of the many-body Dissado-Hill dielectric response theory. In all three kinds of silicone rubber, a quasi-DC (QDC) process, a Dissado-Hill loss peak process and a diffusion process are observed. Specifically, the ATH filler contributes a QDC electrical transport/hopping process, which at higher temperatures causes the formation of a diffusion layer at the electrodes. The nano-SiO 2 particles combined with rubber matrix give a Dissado-Hill loss peak dielectric relaxation. These two dispersion processes are shown to be associated with the interior interfaces of filled HTV silicone rubber. The effects of ATH filler content and temperature on the dielectric response are also analyzed in depth.

Physical model of dimensional regularization

We explicitly construct fractals of dimension 4-epsilon on which dimensional regularization approximates scalar-field-only quantum-field-theory amplitudes. The construction does not require fractals to be Lorentz-invariant in any sense, and we argue that there probably is no Lorentz-invariant fractal of dimension greater than 2. We derive dimensional regularization's power-law screening first for fractals obtained by removing voids from 3-dimensional Euclidean space. The derivation applies techniques from elementary dielectric theory. Surprisingly, fractal geometry by itself does not guarantee the appropriate power-law behavior; boundary conditions at fractal voids also play an important role. We then extend the derivation to 4-dimensional Minkowski space. We comment on generalization to nonscalar fields, and speculate about implications for quantum gravity.

On the sign of the relaxation activation energy for interfacial polarization in reduced graphene oxide–based nano-composites

The dielectric relaxation mechanism associated with the interfacial polarization in polyaniline/reduced graphene oxide (PANI/RGO) nano-composites is found to shift towards lower frequencies on increasing temperature. Accordingly, the effective activation energy value is negative. Basic concepts of the Sillars dielectric theory of a heterogeneous medium are revisited for a material consisting of conducting platelets dispersed in a semi-insulating matrix in order to explain the negative sign of the relaxation energy. A plausible explanation to this observation involves a thermally activated de-trapping mechanism through the effective potential barrier at the interfaces between RGO and PANI. This results in an enhancement of the density of charge carriers which contributes to dc conductivity at the expense of the density of charge carriers that relax within RGO inclusions. Subsequently, the intensity of the dielectric peak is suppressed on heating which results in a systematic modification of the shapes of the dc conductivity vs temperature curves.

Enhancement of optical and structural properties of vacuum evaporated CdTe thin films

The temperature induced optical and structural properties of thermally evaporated CdTe thin films have been investigated in the present work. The deposited films were annealed at 150 °C, 250 °C and 350 °C in air atmosphere and subjected to the UV–Vis spectrophotometer, XRD, source meter, SEM and EDAX for optical, structural, electrical, morphological and elemental analysis. The films are found to be crystallized in cubic zinc-blende structure with preferred orientation (111) and crystallinity is improved with annealing temperature. The optical absorbance is found to increase with annealing while energy band gap is observed to decrease. Different optical parameters like extinction coefficient, refractive index, relative density, dielectric constant and energy loss functions have been calculated using dielectric theory, Swanepoel model and Herve-Vandamme model. The surface morphology shows that the films are homogeneous, smooth and free from defects. The current-voltage characteristics show that the electrical conductivity is increased with annealing and resistivity is observed to decrease. The results confirm that films annealed at 350 °C may be used as absorber layer to fabricate CdTe/CdS solar cell devices.

Exact solutions for the electromechanical responses of a dielectric nano-ring

Based on an extended linear theory for dielectrics, this work presents exact solutions for the electromechanical responses of a dielectric nano-ring subjected to mechanical and electrical loads. By incorporating terms involving the strain gradient and the electric field gradient into the electric Gibbs free energy, both the direct and converse flexoelectric effects can be captured. The general solution to the differential governing equation is a linear combination of modified Bessel functions and the electromechanical fields are obtained by solving boundary value problems. The influences of the material surfaces and the electric circuit conditions on the electromechanical coupling behavior of the dielectric nano-ring have also been considered. It is found that the flexoelectricity and the surface effect on the electromechanical fields are substantial, and they are affected by the size of the ring structure. Moreover, the flexoelectric effect is sensitive to the material length scales introduced by the new theory and the electric circuit conditions. From simulation results, it is also suggested that nano-scaled electromechanical coupling devices could be built based on dielectric materials through flexoelectricity, which thereby opens up new perspectives for nano-technology.

Linear and nonlinear dielectric theory for a slab: The connections between the phenomenological coefficients and the susceptibilities.

The response of dielectric media to electromagnetic fields can be described by using either the response to a Maxwell field E or to an externally produced field E. The former response is measured by phenomenological (dielectric) coefficients and the latter by susceptibilities. With the purpose of clarifying some recent proposals, the connections between the linear (two-point) and first non-vanishing nonlinear (four-point) dielectric coefficients and the susceptibilities for media confined to a slab are examined using a general procedure developed sometime ago. Unlike the relations found for correlations between a local polarization density and the integrated polarization densities (total polarizations), the point-point connections give rise to non-vanishing cross correlations between polarization densities which are parallel and perpendicular to the slab surfaces. The cross correlations in the two-point connections vanish when one polarization density is integrated to form the correlations between a local polarization density and the total polarization thereby losing angular information. The integrated parallel and perpendicular correlations remain different. When the four-point connections are similarly integrated most, but not all, cross correlations vanish. The angular correlations induced by the slab surfaces render the use of point-point correlations that are valid for isotropic media invalid for use in the integrated slab densities. In addition, the nonlinear fluctuations in the perpendicular components are drastically reduced relative to those in the parallel components or in isotropic media.

Insights into the spurious long-range nature of local r-dependent non-local exchange-correlation kernels.

A systematic route to go beyond the exact exchange plus random phase approximation (RPA) is to include a physical exchange-correlation kernel in the adiabatic-connection fluctuation-dissipation theorem. In the previous study [D. Lu, J. Chem. Phys. 140, 18A520 (2014)], we found that non-local kernels with a screening length depending on the local Wigner-Seitz radius, rs(r), suffer an error associated with a spurious long-range repulsion in van der Waals bounded systems, which deteriorates the binding energy curve as compared to RPA. We analyze the source of the error and propose to replace rs(r) by a global, average rs in the kernel. Exemplary studies with the Corradini, del Sole, Onida, and Palummo kernel show that while this change does not affect the already outstanding performance in crystalline solids, using an average rs significantly reduces the spurious long-range tail in the exchange-correlation kernel in van der Waals bounded systems. When this method is combined with further corrections using local dielectric response theory, the binding energy of the Kr dimer is improved three times as compared to RPA.

Thermal evolution of physical properties of vacuum evaporated polycrystalline CdTe thin films for solar cells

This paper reports the change in optical, structural, morphological and electrical properties of CdTe thin films with post-deposition thermal treatment in air atmosphere. The polycrystalline cadmium telluride (CdTe) thin films of thickness 850 nm were deposited on glass and ITO coated glass substrates by thermal vacuum evaporation technique followed by annealing at different temperature 150, 250 and 350 °C. The physical properties have been investigated using characterization tools like UV–Vis spectrophotometer, X-ray diffraction, scanning electron microscopy coupled with EDS and source meter. The optical transmittance was observed to increase with post-deposition heat treatment which revealed the systematic reduction in optical energy band gap from 1.78 to 1.54 eV. The Swanepoel and Herve–Vandamme models as well as the dielectric theory were used to calculate various optical and dielectric constants. The thermal annealing enhances the crystallinity (zinc blende cubic structure) of films with preferred orientation (111). A number of crystallographic parameters were also evaluated and studied in depth with heat treatment. The surface morphology studies indicate that the films are homogeneous, densely packed, uniform and free from crystal defects. The current–voltage measurements exhibited ohmic behavior and the conductivity was found to increase with annealing temperature.

Design and calibration of a new compound sensor for water content and electrical conductivity in soilless substrates

Reliable and affordable compound sensors for measuring the water content and electrical conductivity simultaneously in soilless substrates are limited. In this study, a new compound sensor based on dielectric theory and four-electrode method is presented, and the size of the sensor is determined with the help of ANSYS software. A water content calibration test is designed in vinegar residue and coconut chaff with the same electrical conductivity. Test results show that the water content calibration equation is suitable for different substrates. Similarly, an electrical conductivity calibration equation is obtained in different concentrations of salt solutions. Considering the influence of electrical conductivity to water content measurement, a two-factor (water content and electrical conductivity) orthogonal test is designed in coconut chaff. Based on the analysis of test results, two compensation models – the multiplicative model and the additive model – are proposed. To evaluate the reliability of the models, a multiple linear regression analysis is employed. The results indicate that the additive model has a relatively higher reliability, i.e. the additive model can be regarded as the compensation model. After compensation, the relative errors of the measured values (including volumetric water content and electrical conductivity) are no more than 10%, and the compound sensor is suitable for different soilless substrates without recalibration.

Nanoscale Probing of Bandgap States on Oxide Particles Using Electron Energy-Loss Spectroscopy

Surface and near-surface electronic states were probed with nanometer spatial resolution in MgO and TiO2 anatase nanoparticles using ultra-high energy resolution electron energy-loss spectroscopy (EELS) coupled to a scanning transmission electron microscope (STEM). This combination allows the surface electronic structure determined with spectroscopy to be correlated with nanoparticle size, morphology, facet etc. By acquiring the spectra in aloof beam mode, radiation damage to the surface can be significantly reduced while maintaining the nanometer spatial resolution. MgO and TiO2 showed very different bandgap features associated with the surface/sub-surface layer of the nanoparticles. Spectral simulations based on dielectric theory and density of states models showed that a plateau feature found in the pre-bandgap region in the spectra from (100) surfaces of 60nm MgO nanocubes is consistent with a thin hydroxide surface layer. The spectroscopy shows that this hydroxide species gives rise to a broad filled surface state at 1.1eV above the MgO valence band. At the surfaces of TiO2 nanoparticles, pronounced peaks were observed in the bandgap region, which could not be well fitted to defect states. In this case, the high refractive index and large particle size may make Cherenkov or guided light modes the likely causes of the peaks.

Dielectrophoretic- and electrohydrodynamic-driven translational motion of a liquid column in transverse electric fields

Computer simulations are performed to study translational motion and deformation of a liquid column or jet, in a plane perpendicular to its axis, due to a transverse electric field. A front tracking/finite difference scheme is used in conjunction with the Taylor-Melcher leaky dielectric theory to solve the governing equations. The column is confined within a rectangular channel, wall-bounded in the vertical direction and periodic in the horizontal direction. It is shown that perfect dielectric columns move toward electrode wall of shorter initial distance, but the leaky dielectric columns may move toward or away from it, depending on the relative importance of the ratios (drop fluid to suspending fluid) of their electric permittivity and conductivity. Furthermore, the degree of interface deformation might increase or decrease with the initial separation distance from the shorter electrode wall due to the same factor. Scaling arguments are used to discern the correlation between the translational velocity and the initial separation distance from the electrodes.