Showing papers on "Relative permittivity published in 1999"

Dielectric relaxation of aqueous NaCl solutions

Abstract: The complex dielectric permittivity of aqueous sodium chloride solutions has been determined in the frequency range 0.2 ≤ v(GHz) ≤ 20 with a commercial dielectric measurement system based on a vector network analyzer. NaCl solutions 0.1 ≤ m (mol kg-1) ≤ 5 (mass fraction 0.005 ≤ w ≤ 0.23) were investigated at 5, 20, 25, and 35°C. An improved calibration procedure of the dielectric measurement system for conducting samples was developed. The complex permittivity spectra have been represented by a Cole-Cole relaxation time distribution. Where possible, the obtained fitting parameters, static permittivity ∈ and relaxation time τ, and distribution parameter a, are compared with literature data to assess the performance of the instrument, which was found to be comparable to that of time domain and waveguide systems. Effective solvation numbers were deduced from the effect of NaCl concentration on ∈. The data suggest that in addition to the irrotational bonding of water molecules by Na+ ions, kinetic depolarization under slip boundary conditions determines the solution permittivity. A three-state model is proposed to describe the concentration dependence of τ. © Copyright 1999 by the American Chemical Society.

Complex permittivity of some ultralow loss dielectric crystals at cryogenic temperatures

Abstract: Whispering gallery modes were used for very accurate permittivity and dielectric loss measurements of ultralow loss isotropic and uniaxially anisotropic single crystals. Several materials including sapphire, YAG, quartz, and SrLaAlO4 were measured. The total absolute uncertainty in the real part of permittivity tensor components was estimated to be ±0.1%, limited principally by the uncertainty in sample dimensions. Imaginary parts of permittivities were measured with uncertainties of about 10%, limited by the accuracy of Q-factor measurements of whispering gallery modes. It has been observed that, for most crystals, dielectric losses can be approximated by a power function of absolute temperature only in limited temperature ranges. At temperatures between 4-50 K, losses are often affected by impurities, which are always present in real crystals.

Complex permittivity determination from propagation constant measurements

Abstract: The authors present a new transmission line method for measuring the complex permittivity of dielectric materials using propagation constant measurements. In contrast to previous methods, a network analyzer calibration is unnecessary since calibrated scattering parameters are not required. They use measurements in X-band waveguide to show that this technique compares well with the transmission/reflection and cylindrical cavity methods.

Temperature effects on soil bulk dielectric permittivity measured by time domain reflectometry: A physical model

Abstract: Near-surface measurements of soil water content (θ) using time domain reflectometry (TDR) may exhibit anomalous behavior in the presence of diurnal temperature (T) fluctuations. Experimental results obtained in a companion paper led to the hypothesis that the observed bulk dielectric permittivity (ϵb) is determined by an interplay between two competing phenomena: (1) the reduction in the dielectric permittivity of bulk water with increased T; and (2) the increase in TDR-measured ϵb with increased T due to release of bound water. In this study we develop a physically based model for the temperature dependency of TDR-measured soil bulk dielectric permittivity and propose practical correction factors. The model considers the modified properties of water near solid surfaces to define a layer of rotationally hindered water (within the TDR frequency bandwidth) having a temperature dependent thickness. Changes in measured ϵb(T) are thus attributed to variations in the thickness of the rotationally hindered layer which has a lower dielectric permittivity than free water and hence is less “visible” to travel-time-based TDR waveform analyses. The model is sensitive to the soil specific surface area and the water content, both of which determine the ratio of bound to bulk soil water. Comparisons with experimental data covering a wide range of soils, water contents, and temperatures showed good agreement. Further studies are needed to evaluate some of the model's critical parameters such as the cutoff frequency below which water is considered bound. A temperature correction approximation is based on analytical expressions for TDR-measured bulk dielectric permitivity and requires estimates of soil specific surface area and bulk density, which may be estimated from soil texture. The thermodielectric sensitivity of TDR-measured bulk dielectric permittivity and water content may serve as a basis for estimating soil specific surface area.

Use of whispering-gallery modes for complex permittivity determinations of ultra-low-loss dielectric materials

Abstract: Whispering-gallery modes are used for very accurate permittivity, dielectric loss, and temperature coefficient of permittivity measurements for both isotropic and uniaxially anisotropic dielectric materials. The relationship between resonant frequencies, dimensions of the resonant structure, and permittivity of the sample under test is calculated with a radial mode-matching technique. The relative accuracy of these computations is better then 10/sup -4/. The influence of conductor losses on dielectric loss tangent determination is treated for both whispering-gallery-mode and TE/sub 01/spl delta//-mode dielectric-resonator techniques. Two permittivity tensor components of sapphire and their temperature dependence were measured from 4.2 to 300 K. The total uncertainty in permittivity when use is made of whispering-gallery modes was estimated to be less than 0.05%. The uncertainty was limited principally by uncertainty in sample dimensions. Experimental and calculated resonant frequencies of several whispering-gallery modes differed by no more than 0.01%. The dielectric loss tangent of sapphire parallel and perpendicular to its anisotropy axis was calculated to be less than 10/sup -9/ at 4.2 K. The permittivity and dielectric loss tangent of a commercially available low-loss high-permittivity ceramic material has also been measured at S- and C-band frequencies using a large number of whispering-gallery modes.

Full-wave analysis of a split-cylinder resonator for nondestructive permittivity measurements

Abstract: This paper presents a full-wave analysis of the split-cylinder resonator. We outline a model where the fringing fields are rigorously accounted for and the resonance condition is derived. Using this model, a method for nondestructively measuring the complex permittivity of materials is examined. Measurements of the complex permittivity for low-loss dielectric materials using the split-cylinder resonator agree well with measurements made in a cylindrical cavity. An uncertainty analysis for the complex permittivity is also provided.

Dielectric resonance phenomena in ultra high field MRI.

Abstract: Purpose: Dielectric resonances have previously been advanced as a significant cause of image degradation at higher fields. In this work, a study of dielectric resonances in ultra high field MRI is presented to explore the real importance of dielectric resonances in the human brain in this setting. Method: Gradient-recalled echo images were acquired using a transverse electromagnetic resonator at 1.5, 4.7, and 8 T. Images were obtained from the human head and from phantoms filled with pure water, saline, and mineral oil. In addition, an exact theoretical analysis of dielectric resonances is presented for a spherical phantom and for a model of the human head. Results: Theoretical results demonstrate that distilled water can sustain dielectric resonances in head-sized spheres near 200 and 360 MHz, but the presence of significant conductivity suppresses these resonances. These findings are confirmed experimentally with proton images of water and saline (0.05 and 0.125 M NaCl). For lossy phantoms, coupling between the source and phantom overwhelms the dielectric resonance. Because of their low relative permittivity, mineral oil phantoms with 20 cm diameter do not exhibit dielectric resonances below -900 MHz. Significant dielectric resonances were not observed in human head images obtained at 1.5, 4.7, and 8 T. Conclusion: It appears that dielectric resonances do not have any real importance in determining image quality for MR studies of the human head. It is advanced that RF coils are much more likely to be the source of any inhomogeneity. Index Terms: Magnetic resonance imaging, artifacts-Magnetic resonance imaging, ultra high frequency-Dielectric resonance-Image quality.

Imaging of microwave permittivity, tunability, and damage recovery in (Ba, Sr)TiO3 thin films

Abstract: We describe the use of a near-field scanning microwave microscope to quantitatively image the dielectric permittivity and tunability of thin-film dielectric samples on a length scale of 1 μm. We demonstrate this technique with permittivity images and local hysteresis loops of a 370-nm-thick Ba0.6Sr0.4TiO3 thin film at 7.2 GHz. We also observe the role of annealing in the recovery of dielectric tunability in a damaged region of the thin film. We can measure changes in relative permittivity er as small as 2 at er=500, and changes in dielectric tunability der/dV as small as 0.03 V−1.

Imaging of Microwave Permittivity, Tunability, and Damage Recovery in (Ba,Sr)TiO3 Thin Films

Abstract: We describe the use of a near-field scanning microwave microscope to quantitatively image the dielectric permittivity and tunability of thin-film dielectric samples on a length scale of 1 micron. We demonstrate this technique with permittivity images and local hysteresis loops of a 370 nm thick barium strontium titanate thin film at 7.2 GHz. We also observe the role of annealing in the recovery of dielectric tunability in a damaged region of the thin film. We can measure changes in relative permittivity as small as 2 at 500, and changes in dielectric tunability as small as 0.03 V$^{-1}$.

The dielectric response of a polymeric three-component composite

Abstract: The dielectric behaviour of a composite system of epoxy resin filled with Kevlar fibres and Aluminum powder is investigated in the frequency range 20 Hz to 10 MHz and the temperature interval 10 to 150°C. Dielectric permittivity is increasing with filler content and temperature, being always higher in the low than in the high frequency range. Dielectric permittivity and loss of the composites is mostly affected by interfacial polarization arising from inhomogeneites at interfaces introduced by the fillers. Equations based on the extension of the logarithmic law of mixtures are formulated and their applicability tested with the experimental data obtained.

Oxygen-vacancy-related dielectric anomalies in La:SrTiO3

Abstract: The influence of post-sintering annealing on the dielectric relaxation behavior was studied in (Sr1−1.5xLax)TiO3 in the temperature range of 1.5–300 K. The results show that the dielectric permittivity peaks with frequency dispersion can be gradually and absolutely eliminated by annealing in an oxidizing atmosphere, and recovered by annealing in a reducing atmosphere; this implies that these anomalies are closely related to oxygen vacancies.

Permittivity determination using amplitudes of transmission and reflection coefficients at microwave frequency

Abstract: A new method is proposed in this paper to determine the permittivity of materials. The general consideration is to use only the amplitudes of the transmission and reflection coefficients to do the permittivity determination. According to the analysis, the permittivity can be uniquely determined by measuring these two amplitudes when a sample is prepared with large enough attenuation that the multiple reflections between the two surfaces of the sample can be neglected. The validity of the method was proven by experiments. Using the method, the instantaneous and noncontacting measurements of the amplitudes can be realized. Thus, the dynamic measurement of the permittivity becomes possible.

Preparation and microwave characterization of baxsr1-xtio3 ceramics

Abstract: BaxSr1-xTiO3 ceramics have been synthesized by a conventional solid-state reaction for x=0.2, 0.4, 0.6, 0.8. The phase constitution was examined by X-ray diffraction. The microwave dielectric properties of BaxSr1-xTiO3, such as the real part of the relative dielectric constant and loss factor, were measured by the postresonator method on the TE011 resonant mode for the different compositions. The real part of the relative dielectric constant varies from 200 to 900, with a loss factor from 10-3 to 10-2 at a resonant frequency of approximated 2 GHz.

Zirconium titanate microwave dielectrics prepared via polymeric precursor route

Abstract: Zirconium titanate based materials are widely used as dielectric resonators in telecommunications. The performance of the devices depends on compositional and microstructural features. The polymeric precursor route enables the preparation of fine zirconium titanate powders at low processing temperatures. ZrTiO4 powders containing different amount of Hf (0·5% and 40 ppm) were prepared and sintered without additives at 1600°C. The resulting materials were single-phase and had densities 96–98% theoretical. The dielectric Q value at 5 GHz was 5640 and the relative permittivity 36. The presence of Hf does not significantly affect the microwave dielectric response.

Coating fluid for forming low-permittivity silica-based coating film and substrate with low-permittivity coating film

Abstract: A coating fluid capable of forming a silica-based coating film having a relative permittivity as low as 3 or below and a low density and excellent in resistance to oxygen plasma and suitability for other processings; and a substrate having a coating film having such properties. The coating fluid is characterized by comprising a polymer composition comprising (i) a polysiloxane which is a product of the reaction of fine silica particles with a hydrolyzate of at least one alkoxysilane represented by following general formula (I) and (ii) a readily decomposable resin; XnSi(OR)4-n (wherein X represents hydrogen, flourine, C1-8 alkyl, flouroalkyl, aryl, or vinyl; R represents hydrogen, C1-8 alkyl, aryl, or vinyl; and n is an integer of 0 to 3).

Dielectric and structural characteristics of perovskites and related materials as a function of tolerance factor

Abstract: The temperature coefficient of the dielectric permittivity (τϵ) and resonant frequency (τf) of perovskites and related materials is important in the application of these ceramics to microwave filters and resonators. Recent work has directly related changes in τϵ, and therefore τf, to the onset of octahedral tilting in Ba- and Sr-based complex perovskites. Whether a compound is tilted at room temperature or not depends largely on the tolerance factor, t. The relationship between t and τϵ is reviewed in perovskites and the effect of changing tolerance factor on τϵin the perovskite-related solid solution series Ba6-3xNd8+2xTi18O54 is discussed. Transmission electron microscopy is used to demonstrate examples of phase transformations in the above systems

Analysis of asymmetric coplanar waveguide with conductor backing

Abstract: This paper presents an analysis of the asymmetric conductor-backed coplanar waveguide by means of conformal mapping techniques. The effective permittivity, characteristic impedance, and electric-field strength are given. The field patterns are also plotted.

Cavity perturbation technique for the measurement of permittivity tensor of uniaxially anisotropic dielectrics

Abstract: Based on microwave resonant perturbation theory, a bimodal TE/sub 112/ cylindrical cavity is developed for the measurement of permittivity tensor of uniaxially anisotropic dielectrics. The two perpendicular TE/sub 112/ degenerate modes in the cavity are used to measure the two complex components in the permittivity tensor of a uniaxial sample, respectively. In the measurement of each component, only the corresponding mode resonates in the bimodal cavity, and the real and imaginary parts of the complex component are deduced from the changes of the resonant frequency and the quality factor of the cavity due to sample insertion. In the present technique, the quality factors of the cavity before and after sample insertion are measured at the same frequency, so the uncertainties caused by the variation of the total stored energy in the cavity due to sample insertion are eliminated. Experimental results show that the present technique is accurate and reliable for the measurement of the permittivity tensor of uniaxial media. As an example, the dielectric properties of /spl alpha/-SiO/sub 2/ samples with different crystal orientations are characterized using the present technique.

Capacitance extraction of 3-D conductor systems in dielectric media with high-permittivity ratios

Abstract: We describe a perturbation formulation for the problem of computing electrostatic capacitances of multiple conductors embedded in multiple dielectric materials. Unlike the commonly used equivalent-charge formulation (ECF), this new approach insures that the capacitances are computed accurately even when the permittivity ratios of the dielectric materials are very large. Computational results from a three-dimensional multipole-accelerated algorithm based on this approach are presented. The results show that the accuracy of this new approach is nearly independent of the permittivity ratios and superior to the ECF for realistic interconnect structures.

Dielectric Properties of Al2O3-Doped Barium Strontium Titanate for Application in Phased Array Antennas

Abstract: Effects of Al2O3 doping on the dielectric properties of barium strontium titanate for application in phased array antennas have been studied. The grain size of the specimen is maximum for barium strontium titanate doped with 1 wt% Al2O3. The density and the real part of the relative dielectric constant decreases with increasing Al2O3 content. The loss factor is minimum for barium strontium titanate doped with 2 wt% Al2O3 content. The real part of the dielectric constant and the loss factor with frequency of all the specimens is stable in the measurement frequency range from 1 MHz to 40 MHz. The real part of the dielectric constant of the specimens decreases with increasing applied DC field. The loss factor of the specimens is influenced much less by the DC field than the real part of the dielectric constant. The tunability is about 24% for barium strontium titanate doped with 1 wt% Al2O3 content.

Dielectric Properties of Fruits and Vegetables as a Function of Temperature and Moisture Content Dielectric Properties of Fruits and Vegetables as a Function of Temperature and Moisture Content

Abstract: The relative permittivity, e’ and relative loss factor, e” of apple, chervil, mushroom, parsley, and strawberry were measured at 2800 MHz, with a cylindrical TM012 perturbation cavity. The measurements were made over a range of moistures and temperatures. The results show that the relative loss factor has a maximum value at an intermediate moisture content for all the fruits and vegetables investigated. The relative permittivity decreases with increasing temperature at a high moisture content, but at intermediate moisture contents e’ increases with temperature. Equations were developed for interpolation of the relative permittivity and the loss factor.

Effective permittivity of dielectric honeycombs

Abstract: The effective permittivity of a dielectric honeycomb can be represented by a diagonal permittivity tensor. In the paper, an empirical equation is derived using the FD-TD (finite difference-time domain) method, which defines the diagonal components of the complex permittivity in terms of the honeycomb material permittivity and the fraction of honeycomb material per unit cell. The impact is investigated of electrically large honeycomb cells on effective permittivity failure. The manner in which the effective permittivity fails proves to favour a honeycomb orientation such that the direction of an incident plane wave's Poynting vector is parallel with the honeycomb's axis. The consequences are considered of an invalid effective permittivity on radar cross-section and planar reflectivity.

Linear response of regular asphalts to external harmonic fields

Abstract: Dielectric permittivity and mechanical complex compliance of regular asphalts, at T=323 K, are studied. Both the loss compliance and the loss of permittivity functions can be decomposed into two parts. The first part represents the viscous deformation and the ohmic conductivity, respectively. The second part then describes a “pure” response of the material to external harmonic fields. By this decomposition one can observe a relatively strong transition of regular asphalts into their relaxed state. A simple fractional model of the response function can be applied to both mechanical and dielectric transitions.

Evaluating properties of oil using dielectric spectroscopy

Abstract: A working fluid of a mechanical system is analyzed by measuring the permittivity of the working fluid as the temperature of the working fluid is varied over a range. Within this temperature range, a special temperature is identified, at which the rate of change of the permittivity over temperature is at a maximum. Subsequently, the viscosity, acid content, moisture content and density of the working fluid are determined from the special temperature, rate of change of permittivity at the special temperature, permittivity of the special temperature, and rate of change of permittivity above the special temperature, respectively. To ensure accurate determination of the special temperature and other parameter values, a curve-fitting technique is used, in which measurements of permittivity over the temperature range are fitted to a mathematical model of the expected curve, and this mathematical model is used to identify the various parameter values. Apparatus for performing this analysis method may be in the form of a desk-top oil analyzer for use in an on-site application, or an integrated analysis device suitable for mounting directly to the mechanical system, e.g., in the form of an adapter insertable between the oil filter and oil filter mounting. The working fluid being analyzed may be oil, hydraulic fluid, or any other fluid used in a mechanical system which is subject to contamination and degradation.