Showing papers on "Permittivity published in 2007"

Permittivity of pure water, at standard atmospheric pressure, over the frequency range 0-25 THz and the temperature range 0-100 °C

Abstract: All the currently available experimental permittivity data for pure water are used to derive an interpolation function that precisely represents e(ν,t,) at standard atmospheric pressure, for frequencies and temperatures in the ranges 0⩽ν⩽25THz and 0⩽t⩽100°C. The permittivity data is represented in terms of relaxations and resonances processes. There are three relaxations in the microwave region and two resonances in the far infrared. The temperature dependence of the relaxation and resonance parameters are determined. For example, at 25°C the three relaxation frequencies are 18.56GHz, 167.83GHz, 1.944THz and the two resonance frequencies are 4.03 and 14.48THz.

Microwave absorbing performances of multiwalled carbon nanotube composites with negative permeability

Abstract: The permittivity and permeability behaviors of composites made from the multiwalled carbon nanotubes with magnetic impurity Ni and the wax have been studied in 3–18GHz. The unusual permittivity dispersion behaviors have been explained based on the Cole-Cole model and the conductivity contribution model. Permeability is found to have negative imaginary parts within 3–11GHz. The composites are found to show good microwave absorbing performances (reflection loss <−20dB): matching thickness is 1.5mm and absorbing frequency band is 11.6–12.4GHz, and the absorbing performance can be explained by the “geometrical effect.”

Microstructure and microwave absorption properties of carbon-coated iron nanocapsules

Abstract: Carbon-coated Fe [Fe(C)] nanocapsules were synthesized by a modified arc-discharge method, and their microstructure and electromagnetic (EM) properties (2–18 GHz) were investigated by means of transmission electron microscopy, Raman spectroscopy and a network analyser. The reflection loss R of less than −20 dB was obtained in the frequency range 3.2–18 GHz. A minimum reflection loss of −43.5 dB was reached at 9.6 GHz with an absorber thickness of 3.1 mm. The in-depth study of relative complex permittivity and permeability reveals that the excellent microwave absorption properties are a consequence of a proper EM match in microstructure, a strong natural resonance, as well as multi-polarization mechanisms, etc.

A Consistent Molecular Treatment of Dielectric Phenomena

Abstract: Revue des etudes sur la constante dielectrique et la permittivite des molecules a l'etat liquide. Etudes theoriques

Electric energy density of dielectric nanocomposites

Abstract: Dielectric materials with large electric energy density are actively pursued for many applications. The authors analyze the effective permittivity, breakdown strength, and electric energy density of dielectric nanocomposites using an effective medium approximation, modeling the nanocomposite as a three-phase material by the double-inclusion method. The addition of nanoparticles enhances the permittivity but reduces the breakdown strength, making the potential gain in electric energy density small. In addition, the interfacial interaction shifts the “percolation” threshold toward lower volume fraction of nanoparticles. The analysis suggests that the microstructure of nanocomposites must be carefully controlled to maintain high dielectric strength and therefore realize enhanced electric energy density.

Capacitance wire-mesh sensor for fast measurement of phase fraction distributions

Abstract: We introduce a new wire-mesh sensor based on capacitance (permittivity) measurements. The sensor can be used to measure transient phase fraction distributions in a flow cross-section, such as in a pipe or other vessel, and is able to discriminate fluids having different relative permittivity (dielectric constant) values in a multiphase flow. We designed and manufactured a prototype sensor which comprises two planes of 16 wires each. The wires are evenly distributed across the measuring cross-section, and measurement is performed at the wire crossings. Time resolution of the prototype sensor is 625 frames per second. Sensor and measuring electronics were evaluated showing good stability and accuracy in the capacitance measurement. The wire-mesh sensor was tested in a silicone oil/water two-phase bubbly flow.

Complex permittivity of sodium chloride solutions at microwave frequencies

Abstract: The complex permittivity of aqueous solutions at 20 °C has been measured at concentrations between 0.001 and 5 mol/L and over a frequency range 0.13–20 GHz. The results were combined with literature values to derive empirical equations to predict the dielectric behavior of sodium chloride solutions between 0 and 5 mol/L and 5°C–35 °C. Bioelectromagnetics 28:264–274, 2007. © 2007 Wiley-Liss, Inc.

Ferroelectric to relaxor crossover and dielectric phase diagram in the BaTiO3–BaSnO3 system

Abstract: The (1−x)BaTiO3–xBaSnO3 (0⩽x⩽0.30) perovskite solid solution ceramics were prepared by solid state reaction and studied by dielectric spectroscopy. The complex dielectric permittivity was measured as a function of frequency (0.1Hz–100kHz) in the temperature (T) range of 123–573K. The transition from the high-temperature paraelectric state where the dielectric constant obeys the Curie-Weiss law to the ergodic cluster state is found to occur at the same temperature of 485K in all the compositions of x⩾0.04 and at lower temperatures in those with a smaller x. For 0⩽x⩽xc=0.19, the temperature of the dielectric peak Tm, corresponding to the diffuse transition from the ergodic polar cluster state to the ferroelectric state, decreases with increasing x and does not depend on frequency. The diffuseness of the peak gradually increases. For x>xc, the permittivity exhibits relaxor behavior with the frequency-dependent Tm satisfying the Vogel-Fulcher law. The temperature variation of the permittivity on the high-temp...

Dielectric relaxations and formation mechanism of giant dielectric constant step in CaCu3Ti4O12 ceramics

Abstract: The dielectric relaxation behaviors of CaCu3Ti4O12 ceramics were evaluated together with the mixed-valence structure, and the formation mechanism of a giant dielectric constant step was discussed. The giant dielectric constant step was bounded by two dielectric relaxations in low and high temperature ranges. The low-temperature dielectric relaxation was intrinsic, and it was very similar to the electronic ferroelectricity, while the high temperature relaxorlike dielectric peak was assigned to be the result of defect ordering since it could be suppressed by O2 annealing. Both the low and high temperature dielectric relaxations were the thermal activated relaxation process following the Arrhenius law. Moreover, it was supposed that the giant dielectric constant step resulted from the competing balance between the low and high temperature dielectric relaxations.

Electrical Energy Density and Discharge Characteristics of a Poly(vinylidene fluoride-chlorotrifluoroethylene)Copolymer

Abstract: The high electric displacement (D>0.1 C/m2) and breakdown field (600 MV/m) in polyvinylidene fluoride based polymers suggest high electrical energy density in this class of polymers. By defect modifications which reduce or eliminate the remnant polarization in the polymer, a high electrical energy density can indeed be obtained. This paper shows that in properly prepared P(VDF-CTFE) copolymer film capacitors, an electrical energy density ~25 J/cm3 can be obtained with a breakdown field higher than 600 MV/m. The dielectric and discharge behavior of the polymer films were investigated. The results reveal that there are strong frequency dispersions in both the dielectric and discharge behavior. The dielectric constant decreases with frequency and the discharged energy density is also reduced at shorted discharge time (~1 mus) due to increased ESR for fast discharge. The results indicate the potential of this class of polymers for high energy density capacitors and suggest the need for further tuning of the polymer compositions to reduce the frequency dispersion.

Dielectric abnormities of complex perovskite Ba(Fe1∕2Nb1∕2)O3 ceramics over broad temperature and frequency range

Abstract: Dielectric characteristics of Ba(Fe1∕2Nb1∕2)O3 ceramics were investigated over a broad temperature and a frequency range. Two dielectric relaxations following Arrhenius law were observed at 150–400 and 406–650K. An extremely high relaxorlike dielectric peak (202 270 at 5Hz, 91 930 at 1kHz, and 37 030 at 100kHz) with very strong frequency dispersion was observed at 406–650K, and it was not relaxor ferroelectric but an oxygen defect induced dielectric abnormity. The significant drop of dielectric constant at 146–400K was also frequency dependent, and it was nearly a Debye relaxation. Between these two dielectric relaxations, there was a frequency dependent dielectric constant plateau over a wide temperature interval.

Liquid crystal clad near-infrared metamaterials with tunable negative-zero-positive refractive indices

Abstract: Near-infrared metamaterials that possess a reconfigurable index of refraction from negative through zero to positive values are presented. Reconfigurability is achieved by cladding thin layers of liquid crystal both as a superstrate and a substrate on an established negative-index metamaterial, and adjusting the permittivity of the liquid crystal. Numerical results show that the index of refraction for the proposed structure can be changed over the range from -1 to +1.8 by tuning the liquid crystal permittivity from 2 to 6 at a wavelength of 1.4 μm.

Significantly enhanced low-frequency dielectric permittivity in the BaTiO3/poly (vinylidene fluoride) nanocomposite

Abstract: To disclose an effect of ceramic particle size on dielectric properties of BaTiO3/ poly(vinylidene fluoride) (BT/PVDF) composites at low frequency, the dielectric properties of the BT/PVDF composites loaded with different diameter BT particles at the same volume concentration were studied at wide frequency ranges from 10−1to107Hz. A significant low-frequency dielectric permittivity increase in the BT/PVDF nanocomposite was noticed, which was not reported previously. The authors considered that the marked aggrandizement of interface and dipole polarization at low frequency is charged with the significant increase in dielectric permittivity.

Effect of composition on coupled electric, magnetic, and dielectric properties of two phase particulate magnetoelectric composite

Abstract: The particulate composite materials of ferrite-ferroelectric ceramics viz. nickel-cobalt-copper ferrite (i.e., Ni0.94Co0.01Cu0.05Fe2O4) and barium titanate were synthesized by the double sintering ceramic technique. The presence of constituent phases in the composites was confirmed by x-ray diffraction studies. The average grain size was calculated by using a scanning electron micrograph. The electrical properties such as dc resistivity and thermo-emf were measured as a function of temperature and volume fraction of constituent phases. The ac conductivity was calculated from dielectric data in the frequency range from 100Hzto1MHz. It is concluded that the conduction in the present composites is due to small polarons. The relative dielectric constant measured as a function of applied frequency varies with the variation in the dc resistivity and molar fraction of constituent phases. It shows dispersion in the lower frequency range. The hysteresis behavior was studied to understand the magnetic properties su...

Human skin permittivity determined by millimeter wave reflection measurements.

Abstract: Millimeter wave reflection from the human skin was studied in the frequency range of 37–74 GHz in steps of 1 GHz. The forearm and palm data were used to model the skin with thin and thick stratum corneum (SC), respectively. To fit the reflection data, a homogeneous unilayer and three multilayer skin models were tested. Skin permittivity in the mm-wave frequency range resulted from the permittivity of cutaneous free water which was described by the Debye equation. The permittivity increment found from fitting to the experimental data was used for determination of the complex permittivity and water content of skin layers. Our approach, first tested in pure water and gelatin gels with different water contents, gave good agreement with literature data. The homogeneous skin model fitted the forearm data well. Permittivity of the forearm skin obtained with this model was close to the skin permittivity reported by others. To fit reflection from the palmar skin with a thick SC, a skin model containing at least two layers was required. Multilayer models provided better fitting to both the forearm and palmar skin reflection data. The fitting parameters obtained with different models were consistent with each other. Bioelectromagnetics 28:331–339, 2007. © 2007 Wiley-Liss, Inc.

Magnetodielectric effect in double perovskite La2CoMnO6 thin films

Abstract: We report on the magnetodielectric properties of well-ordered epitaxial La2CoMnO6 films. The temperature dependence of the dielectric constant is measured in the 10–105Hz frequency range under applied magnetic fields up to 50kOe. As temperature is lowered, the dielectric constant decreases in the ferromagnetic regime and approaches a plateau below 100K. A significant enhancement in the dielectric constant under applied magnetic field is observed only in the ferromagnetic phase regime culminating around the transition temperature, while it is absent in the paramagnetic phase regime. Using Arrott plots, we also demonstrate that these films exhibit a second-order phase transition at their Curie temperature. The observed magnetodielectric characteristics of the films are interpreted using the phenomenological theory.

Finite size effects on the electrical properties of sol–gel synthesized CoFe2O4 powders: deviation from Maxwell–Wagner theory and evidence of surface polarization effects

Abstract: Fine particles of cobalt ferrite were synthesized by the sol?gel method. Subsequent heat treatment at different temperatures yielded cobalt ferrites having different grain sizes. X-ray diffraction studies were carried out to elucidate the structure of all the samples. Dielectric permittivity and ac conductivity of all the samples were evaluated as a function of frequency, temperature and grain size. The variation of permittivity and ac conductivity with frequency reveals that the dispersion is due to Maxwell?Wagner type interfacial polarization in general, with a noted variation from the expected behaviour for the cold synthesized samples. High permittivity and conductivity for small grains were explained on the basis of the correlated barrier-hopping model.

Temperature dependence of the complete material coefficients matrix of soft and hard doped piezoelectric lead zirconate titanate ceramics

Abstract: We have used resonance methods to determine the variation of all the independent piezoelectric, elastic, and dielectric material coefficients, as well as the corresponding electromechanical coupling factors, of soft and hard doped piezoelectric lead zirconate titanate (PZT) ceramics with compositions near the morphotropic phase boundary, as a function of temperature ranging between −165 and 195°C. The material coefficients were obtained by analyzing the fundamental resonance of the impedance or admittance spectra as a function of frequency for several sample resonance geometries. The piezoelectric coefficients d33, −d31, and d15, as well as the dielectric permittivity coefficients e11T and e33T, generally increased with temperature for both soft and hard PZT samples. However, the elastic compliance coefficients s11E, −s12E, s33E, and s55E exhibited abnormal variations seen as broad peaks over parts of the tested temperature range. Additionally, thermal hystereses were observed in all the studied material ...

Plasmonic nanoclusters: a path towards negative-index metafluids

Abstract: We introduce the concept of metafluids-liquid metamaterials based on clusters of metallic nanoparticles which we will term Artificial Plasmonic Molecules (APMs). APMs comprising four nanoparticles in a tetrahedral arrangement have isotropic electric and magnetic responses and are analyzed using the plasmon hybridization (PH) method, an electrostatic eigenvalue equation, and vectorial finite element frequency domain (FEFD) electromagnetic simulations. With the aid of group theory, we identify the resonances that provide the strongest electric and magnetic response and study them as a function of separation between spherical nanoparticles. It is demonstrated that a colloidal solution of plasmonic tetrahedral nanoclusters can act as an optical medium with very large, small, or even negative effective permittivity, epsilon(eff), and substantial effective magnetic susceptibility, Chi(eff) = mu(eff) -1, in the visible or near infrared bands. We suggest paths for increasing the magnetic response, decreasing the damping, and developing a metafluid with simultaneously negative epsilon(eff) and mu(eff).

Enhanced electromagnetic wave absorption properties of Fe nanowires in gigaherz range

Abstract: Fe nanowires with 70–200nm in diameter and 20–50μm in length were synthesized by a chemical vapor deposition method for electromagnetic wave absorption application. The frequency dependences of relative permittivity (er) and permeability (μr) were strongly dependent on the diameter of Fe wires. Compared with micrometer wires or flakelike samples, nanowires exhibited a magnetic resonance (μr″) peak in the range of 1–18GHz, suggesting that nanowires have significant effect for reducing the eddy current loss, therefore, the resin compacts of 29vol% Fe nanowires with thicknesses of 1.3–4.0mm provided good electromagnetic wave absorption performances in the range of 5.6–18GHz.

Reference liquids for the calibration of dielectric sensors and measurement instruments

Abstract: A comparative study of microwave complex dielectric permittivity data from different laboratories is provided for liquids. Four liquids—water, cyclohexane, methanol and dimethyl sulphoxide—are recommended as reference materials because their parameter sets from different measurements result in almost identical predictions of their dielectric properties in the frequency range up to 10 GHz. Within the limits of experimental error this agreement includes that of the extrapolated low frequency permittivity with the recently determined static permittivity of the liquids. Parameters for water are given for the frequency range 0–60 °C. For cyclohexane, which does not display relaxation behaviour up to the submillimetre frequency range, the frequency-independent permittivity is represented between 10 and 50 °C. Relaxation and static permittivity data for methanol and dimethyl sulphoxide are presented at 25 °C.

Dipolar correlations and the dielectric permittivity of water.

Abstract: The static dielectric properties of liquid and solid water are investigated within linear response theory in the context of ab initio molecular dynamics. Using maximally localized Wannier functions to treat the macroscopic polarization we formulate a first-principles, parameter-free, generalization of Kirkwood's phenomenological theory. Our calculated static permittivity is in good agreement with experiment. Two effects of the hydrogen bonds, i.e., a significant increase of the average local moment and a local alignment of the molecular dipoles, contribute in almost equal measure to the unusually large dielectric constant of water.

Reflectarray Element Based on Aperture-Coupled Patches With Slots and Lines of Variable Length

Abstract: A phase-shifter element for printed reflectarrays with more than 4 cycles of phase delay is presented in this work. The proposed reflectarray element allows very linear phase curves and is based on the combination of two previously studied and demonstrated concepts: variable length slots on a ground plane and patches coupled through a slot to variable length lines. In the former, the slots act as inductive loading on the patches depending on their length. With the second concept, the effect of nonconstant path delay is alleviated by the introduction of true-time delay (TTD) lines. The mixed effect allows a much larger range of phase delay to be reached and at the same time to improve the linearity of the phase delay behavior. Two ways of increasing the phase delay range are shown: the increase in length using U-shaped delay lines and the use of high permittivity substrates

Broadband dielectric spectroscopy on single-crystalline and ceramic CaCu3Ti4O12

Abstract: The authors present dielectric measurements of the colossal dielectric constant material CaCu3Ti4O12 extending up to 1.3GHz also covering so far only rarely investigated single-crystalline samples. Special emphasis is put on the second relaxation reported in several works on polycrystals, which the authors detect also in single crystals. For polycrystalline samples, the authors provide a recipe to achieve values of the dielectric constant as high as in single crystals.

Liquid Crystal Tunable mm Wave Frequency Selective Surface

Abstract: A frequency selective surface (FSS) which exploits the dielectric anisotropy of liquid crystals to generate an electronically tunable bandpass filter response at D Band (110-170 GHz) is presented. The device consists of two printed arrays of slot elements which are separated by a 130-mum thick layer of liquid crystals. A 3% shift in the filter passband occurs when the substrate permittivity is increased by applying a control signal of 10 V. Measured results show that the insertion loss increases from -3.7 dB to -10.4 dB at resonance (134 GHz), thus demonstrating the potential to create a FSS which can be switched between a transmitting and a reflecting structure.