Showing papers on "Dielectric loss published in 1993"

Dielectric Properties of Wood and Wood-Based Materials

Abstract: 1 Interaction between the Electromagnetic Field and Wood. Main Features of Dielectric Properties of Wood.- 1.1 Polarization of Wood.- 1.2 Dielectric Parameters of Wood.- 1.3 Some Principal Formulas Reflecting Interactions Between an Electromagnetic Field and a Material.- 2 Wood Composition and Dielectric Properties of Its Components..- 2.1 Physical Model of Wood as a Multicomponent Dielectric.- 2.2 Dielectric Parameters of the Cell Wall Substance and Its Elements.- 2.3 Dielectric Characteristics of Air, Free and Bound Water, and Ice.- 3 Measurement of the Dielectric Properties of Wood.- 3.1 Measurement of the Dielectric Constant and Loss Tangent at Low Frequencies.- 3.2 Measurements of the Dielectric Constant and Loss Tangent at High Frequencies.- 3.3 Measurement of the Dielectric Constant and Loss Tangent at Super High Frequencies.- 3.4 Preparation of Samples.- 4 Dielectric Properties of Oven-Dry Wood.- 4.1 Influence of Anisotropy, Density, and Frequency on the Dielectric Constant, Loss Tangent, and Loss Factor.- 4.2 Influence of Temperature on Dielectric Properties.- 5 Dielectric Properties of Moist Wood.- 5.1 The Dielectric Constant of Moist Wood.- 5.2 The Dielectric Loss Tangent of Moist Wood.- 5.3 The Dielectric Loss Factor of Moist Wood.- 5.4 Mechanism of the Influence of Moisture Content on the Dielectric Properties of Wood.- 6 Effect of Different Kinds of Treatment on the Dielectric Properties of Wood.- 6.1 Dielectric Properties of Compressed Wood.- 6.2 Dielectric Characteristics of Modified Wood.- 6.2.1 Wood Treated with Ammonia.- 6.2.2 Dielectric Properties of Wood Impregnated with Various Substances.- 6.3 Influence of Gamma-Radiation on the Dielectric Properties of Wood.- 7 Dielectric Properties of Bark.- 8 Dielectric Properties of Wood-Based Materials.- 8.1 Cellulose, Paper and Board.- 8.1.1 Influence of Density Value, Moisture Content, and Temperature on the Dielectric Properties of the Material.- 8.1.2 Influence of the Structure of Paper and Board on Their Dielectric Properties.- 8.1.3 Influence of Organic and Mineral Inclusions on the Dielectric Properties of Cellulose-Based Materials.- 8.2 Fiberboards.- 8.3 Particleboards.- 8.4 Materials Containing Wood, Cellulose, Paper (as a Filler), and Cellulose Derivatives.- 9 Recommendations for the Determination of the Dielectric Parameters of Wood and Wood-Based Materials and for Their Use in Calculations.- 9.1 Dielectric Parameters and Density of Oven-Dry Wood.- 9.2 Averaging the Dielectric Parameters Values.- 9.3 Choice of Frequency on Microwave Treatment of Wood.- Appendices.- References.

Chemical structure and intermolecular cooperativity: dielectric relaxation results

Abstract: The effect of chemical structure on the segmental relaxation behavior is examined for a wide range of polymers. Both the time and temperature dependence of the glass transition dispersion in the dielectric loss spectrum are shown to be correlated with the degree to which local structure engenders steric constraints on the relaxation from neighboring nonbonded segments. The polymers studied exhibit a range of segmental relaxation behavior. For polymers with smooth, compact, symmetrical chain backbones, segmental relaxation times have a near Arrhenius temperature dependence, representing the limiting-type behavior associated with relatively unconstrained ('Debye") relaxation. Polymers having lees flexible backbones andl or sterically-hindering pendant groups exhibit broad segmental dispersions and temperature dependencies near the extreme fragile edge; these characteristics reflect segmental relaxation that is strongly intermolecularly cooperative. The data for polymers parallel observations made on small-molecule glass-forming liquids, whose chemical structure similarly governs the strength of the intermolecular coupling, and thus the time and temperature dependence of segmental relaxation.

Models for breakdown-resistant dielectric and ferroelectric ceramics

Abstract: Models for dielectric breakdown are proposed and analysed, with emphasis on concepts leading to breakdown-resistant materials. The Griffith energy balance is extended to cracks under combined electrical and mechanical loading, and to conductive tubular channels. Breakdown strength for a perfect crystal is estimated by an analogue of the Frenkel model. In a crystal subjected to an electric field the equilibrium displacement of the electron clouds is described by a curve with periodicity of the lattice constant. A theory of breakdown-resistant laminates is proposed on the basis of charge relocation, facilitated by breakdown of the weak layers and the interfaces. A process by which a conducting path grows like a crack in ferroelectric ceramics is discussed, followed by an outline of fields around conducting cracks in piezoelectric ceramics.

Structural and electrical characteristics of SrTiO3 thin films for dynamic random access memory applications

Abstract: Polycrystalline SrTiO3 thin films having perovskite structure were prepared by the metallo‐organic solution deposition technique on platinum coated silicon and bare silicon substrates. Crack free and crystalline films with uniform composition and thickness were fabricated by spinning and post deposition rapid thermal annealing at a low temperature of 550 °C for 60 s. The films exhibited good structural, dielectric, and insulating properties. The dielectric constant was found to depend on film thickness and annealing temperature. The small signal dielectric constant and dissipation factor at a frequency of 100 kHz were 225 and 0.008, respectively, for a 0.8‐μm‐thick film annealed at 700 °C for 60 s. The frequency dependence of the dielectric constant and the dielectric loss was also studied. The measurement of the current‐voltage (I‐V) characteristics on films in metal‐insulator‐metal configuration indicated the conduction process to be bulk limited. The I‐V characteristics were ohmic at low fields and spa...

Leaky-wave propagation and radiation for a narrow-beam multiple-layer dielectric structure

Abstract: Previous work has demonstrated that very narrow beam radiation patterns can be obtained from a simple source embedded within multiple dielectric layers of appropriate thicknesses above a ground plane. The configuration consists of dielectric layers having permittivities epsilon /sub 1/ and epsilon /sub 2/ stacked in an alternating arrangement, with epsilon /sub 2/> epsilon /sub 1/. This narrow-beam effect can be attributed to weakly attenuated leaky waves that exist on the structure. Simple asymptotic formulas for the propagation and attenuation constants are derived. The formulas show how the beamwidth varies with the number of layers and the material constants. The exact radiation pattern is compared with the leaky-wave pattern for a specific case to demonstrate the role of the leaky waves in determining the total pattern. >

Structure property relationships in core-shell BaTiO3—LiF ceramics

Abstract: A sintering, microstructural development and dielectric property study of BaTiO3–LiF ceramics was performed to assess the potential application of low-fired multilayer capacitors. Not only does LiF allow for sintering below 1000 °C, it also allows for the manipulation of dielectric properties and interfaces within BaTiO3–LiF ceramics. Using mixing laws, a model of the dielectric properties of the core-shell microstructures is presented that agrees well with the observed experimental data.

Basic ideas of microwave processing of polymers

Abstract: The objective of this effort has been to investigate the relationship between polymer structure and microwave absorptivity. Dielectric loss factor, e″, loss tangent, tan δ, and oscillator strength, (eS −- e∞), were used to evaluate potential material processability under applied microwave radiation. Numerous polymeric materials varying in chemical and physical structures were irradiated in a low power (≤ 100W) electric field at 2.45 GHz. Electromagnetic radiation was applied as either traveling or resonant wave modes in cylindrical and rectangular waveguides. In general, heatability was found to be a direct function of the dielectric loss dispersion dependence on temperature and frequency. The dielectric loss factor obtained at low frequency measurements was found to be directly proportional to the heatability of polymers. A WLF plot was used to predict the shift of dielectric loss maxima into or out of the microwave frequency range.

Dielectric spectroscopy of MgTiO3-based ceramics in the 109?1014Hz region

Abstract: Magnesium titanate (MgTiO3) powder was prepared by a chemical route (Pechini method) and different dopants were added to prepare several compositions. These pure and doped compositions were sintered in air and dense ceramics were obtained. The pure MgTiO3 samples were also subjected to different heat treatments during sintering. Complex permittivity spectra of ceramic samples were determined by various techniques in the 109–1014 Hz range. These techniques included infrared spectroscopy in transmission and reflectivity modes and microwave dielectric measurements. Extrapolation to microwave frequencies from infrared data, according to the proportionality ɛ″(v) ∝ v, agrees quite well with the microwave data measured at 8 GHz and it is a useful procedure to estimate intrinsic microwave losses. Fast cooling from high temperatures of MgTiO3 samples increased dielectric loss, probably due to a structural disorder. Dopants have two types of effect depending on whether they form a distinct second phase or a solid solution with MgTiO3. In this last case intrinsic losses are strongly affected.

Electrophoretic Deposition of Ferroelectric Barium Titanate Thick Films and Their Dielectric Properties

Abstract: Barium titanate thick films 10-70 μm thick were obtained using a unique electrophoretic deposition technique, which utilizes a mixed ethanol-acetylacetone suspension as the dispersive medium The capacitance of the BaTiO 3 films was measured as 25-70 nF, at temperatures of up to 200°C, giving a dielectric constant of 4000-12000 and dielectric loss (tan δ) of 013-051 In the sintered films thus deposited, (111) orientation was observed, and the degree of orientation was found to increase with decrease of the thickness of the films In so-oriented films, the Curie point was confirmed to be near 120°C

Dielectric loss function of Si and SiO2 from quantitative analysis of REELS spectra

Abstract: A recently proposed model for quantitative analysis of reflection electron energy-loss spectra (REELS) has been applied to evaluate the dielectric loss function of Si and SiO 2 in the 4-100 eV energy range, and to determine inelastic scattering properties for these materials for low-energy electrons (500-10 000 eV). Appropriate trial energy-loss functions (i.e Im{1/e}) are used and the best loss function is found from the criterion that a satisfactory quantitative agreement is obtained between the simulated and experimental inelastic scattering cross-sections at several primary electron energies. The fact that the energy-loss functions determined in this work agree remarkably well with optical data gives some confidence in the applied procedure

Electric and magnetic properties of a thermoplastic elastomer incorporated with ferromagnetic powders

Abstract: The electric and magnetic properties of a composite material consisting of a thermoplastic elastomer incorporated with iron powder and nickel-iron alloy powder are investigated. The volume fraction of the powder filters ranges from 0.1 to 0.5. The composites are characterized in terms of their volume and surface resistivities, permittivities and electric loss factors and magnetic permeabilities 0 and magnetic loss factors. With the addition of the ferromagnetic powders, the volume and surface resistivity values of the composites decrease by more than seven orders of magnitude, the permittivity values increase tenfold and magnetic permeability values increase by a factor of about six. The ferromagnetic powders are more effective in contributing the electric and magnetic properties the composite at higher loading levels. The frequency variations of AC electric and magnetic properties are also measured in the range 20 Hz to 1 MHz. >

Dielectric materials for use as output window in high-power klystrons

Abstract: The breakdown of RF windows used in high-power klystrons is one of the most serious problems in the development of klystrons. The durability of several dielectric materials used for RF windows is discussed in terms of secondary electron emission (SEE), cathodoluminescence, and dielectric loss. High-power tests of these materials with TiN coatings, thus having low SEE, were also carried out using a traveling wave resonant ring. The results show that alumina ceramics are superior to sapphire and aluminum nitride. The origin of breakdown is investigated, and the requirements for RF window materials are presented. >

The nature of the spectral gap between bound and leaky solutions when dielectric loss is present in printed-circuit lines

Abstract: For lossless printed-circuit transmission lines, it is known that a spectral gap is present at the transition in frequency between bound and leaky modal solutions, and that the solutions are nonphysical within that gap. This paper examines the corresponding modal behavior when the dielectric substrate material is lossy and reports several new interesting and important results. The new features become particularly clear when the data are displayed on the steepest-descent plane, which itself changes when loss is present and contains a new narrow spectral region that vanishes when the dielectric becomes lossless

Dielectric relaxations in SrTiO3 doped with La2O3 and MnO2 at low temperatures

Abstract: In addition to the dielectric relaxation around 170 K in the cubic structure of Sr1−3x/2LaxTiO3, a similar relaxation was observed at about 70 K in the tetragonal structure with an activation energy in the range 0.13–0.16 eV which increases as x in Sr1−3x/2LaxTiO3 varies from 0.60–3.00 at%. This relaxation is explained by Skanavi's model and is discussed in terms of thermal motions of Ti4+ between potential minima produced by lattice distortions in the tetragonal structure. In order to provide a direct evidence for the suggestion that the dielectric relaxation around 170 K in the cubic is due to thermal motions of Ti4+, dielectric properties on manganese-doped specimens, Sr1−3x/2LaxMnyTi1−y),O3 with x=1.40×10−2 and y=0.1×10−2, were investigated because Mn4+ substitutes for Ti4+. As well as the relaxation due to Ti4+, this specimen exhibits another peak due to Mn4+ with an activation energy somewhat smaller than that of Ti4+. The activation energies and the relative intensity of these relaxation processes are explained by the difference in ionic radii of Mn4+ and Ti4+ and the difference in formation energies of a strontium vacancy adjacent to Mn4+ and that to Ti4+, which were calculated theoretically using a shell model.

Rigorous boundary integral equation solution for general isotropic and uniaxial anisotropic dielectric waveguides in multilayered media including losses, gain and leakage

Abstract: Bounded and leaky eigenmodes of arbitrary shaped polygonal dielectric waveguides embedded in a multilayered medium are determined based on a rigorous full-wave analysis. The dielectric waveguides consist of isotropic or uniaxial anisotropic material. Losses and gain inside the layers and the waveguides are allowed. The eigenmodes are determined with a boundary integral equation technique in conjunction with the method of moments. Results for the propagation constants are presented for a number of waveguides and, where possible, compared with published data. Special attention is devoted to the transition from a dielectric waveguide to a perfectly conducting waveguide when the loss tangent of the waveguide material changes from zero to infinity. >

Successive paraelectric‐antiferroelectric‐ferroelectric phase transitions in highly ordered perovskite lead ytterbium tantalate

Abstract: Dielectric properties such as the permittivity, dielectric loss tangent and spontaneous polarization of highly ordered perovskite Pb(Yb1/2Ta1/2)O3 (PYT) compound which were prepared through the wolframite phase oxide were investigated. A dielectric anomaly which is characterized by a slight dielectric dispersion was found at a temperature Tm (e.g., Tm; 177 and 183 °C at 1 and 100 kHz, respectively). At temperatures below Tm, polarization P‐electric field E hysteresis loops were observed, and then ferroelectricity in antiferroelectric ordered perovskite PYT was confirmed. Highly ordered PYT has two successive paraelectric‐antiferroelectric‐ferroelectric phase transitions, similar to highly ordered perovskite lead cobalt tungstate (PCW).

Dielectric Loss in a Piezoelectric Ceramic Transducer under High-Power Operation; Increase of Dielectric Loss and Its Influence on Transducer Efficiency

Abstract: Dielectric loss in a piezoelectric ceramic transducer is related to the vibration, and increases markedly under high-power operation. When PZT ceramics is used over the vibration velocity of about 0.25 (m/s), the dielectric loss becomes larger than the mechanical vibration loss. In this paper, the values of the resistances indicating the dielectric loss and mechanical vibration loss in the equivalent circuit are shown. If an equivalent electric circuit including these resistances is utilized, efficiency of the transducers operating at a high-power level may be calculated with more accuracy. By calculation of the efficiency, the maximum efficiency was obtained at the anti-resonance frequency, not at the resonance frequency. These facts cannot be explained by the ordinary equivalent circuit which does not involve dielectric loss related to the vibration.

Electrical and infrared dielectrical properties of silica aerogels and of silica-aerogel-based composites

Abstract: In this contribution we have studied the key electrical parameters of silica aerogels and of silica-aerogel-based composites, namely the dielectric constants ɛ, the dielectric losses tan δ (at 1 kHz), and the breakdown fields E b (at 50 Hz). For low-density bulk silica aerogels we find ɛ=1.25 and tan δ=0.0005. E b is about 500 kV/cm in quasi-homogeneous fields, and of the order of MV/cm in strongly inhomogeneous fields. The dielectric constants of partially densified aerogels increase linearly with density; their dielectric losses are relatively large and their breakdown fields are comparativiely low. The same results are found for aerogels in the form of settled materials, i.e. aerogel granules and powders in air. Acrylate-based aerogel composites with volume fractions larger than 70% have low dielectric constants but their losses are at least 10 times higher than those of low-density aerogels. These materials sustain high local fields in the MV/cm region, while in quasihomogeneous fields, breakdown occurs at about 100 kV/cm. Based on the present results and the interplay with other physical properties (low mechanical resistance, low thermal conductivity, adsorption of water, etc.), silica aerogels and silica aerogel-acrylate-based composites are predicted to have a low potential for electrical insulation.

Electromagnetic emission from two-dimensional plasmons in a semiconductor-dielectric structure with metal grating: rigorous theory

Abstract: The spectrum of electromagnetic (EM) radiation from two-dimensional(2D) plasmons in a semiconductor-dielectric structure with metal grating has been calculated in a strict electromagnetic approach. It is shown that radiation frequency and linewidth vary over a broad range of magnitude as a function of the grating parameters. The radiative decay linewidth is found to be two orders of magnitude larger than expected from the perturbation theory. Comparison of the results with previously published experimental data on 2D plasmons in GaAs heterostructures makes us conclude that the main contribution to the experimental decay linewidth arises from dielectric losses in the layers of the structure.

Formula for dynamic power loss in ferrite cores taking into account displacement current

Abstract: A formula for dynamic power loss taking into account the displacement current effect is introduced. The calculations based on the formula and the variation of core parameters, i.e., frequency dependence of resistivity, permeability, and permittivity, are compared values measured from 10 kHz to 10 MHz. In this frequency range the resistivity has a more dominant influence on the power loss than the other parameters. >

In vivo measurement of the low-frequency dielectric spectra of frog skeletal muscle.

Abstract: Capacitance, conductance and dielectric loss spectra are obtained, in vivo, for a number of electrode separations in the gastrocnemius muscle of a frog. At each frequency the reciprocals of these parameters are plotted versus electrode separation. From the slopes of the resulting lines the complex permittivity and the conductivity of the muscle can be determined, with electrode effects eliminated. The sequence of power-law responses which is found is consistent with the fractal model proposed by Dissado. The electrical properties measured in vivo with needle electrodes are similar to those measured with surface electrodes for frequencies between 1 kHz and 1 MHz.

Electrothermal microactuators based on dielectric loss heating

Abstract: A method for microactuation based on excitation using the heat generated as a result of dielectric loss of materials at high frequencies is presented. This concept has been realized by fabricating electrothermal microactuators, composed of a layer of the copolymer of vinylidene fluoride and trifluoroethylene (PVDF-TrFE), on top of polyimide, using a fully integrated, self-aligned process. These actuators show larger deflection per unit temperature rise than conventional resistively heated electrothermal microactuators due to the properties of the materials used. In addition, they are easy to fabricate, can be remotely excited, and are capable of underwater operation. The actuation mechanism has been verified and a simplified model as well as a finite-element model have been developed. The measured values are in good agreement with those predicted by the model. >

Influence of Rare Earth Ions on BaO-TiO2-Rare Earth Oxide Ceramics for Microwave Applications

Abstract: The dielectric ceramic materials, 0.143BaO-0.714TiO2-0.143 rare earth oxides (REOx), have unique dielectric properties at microwave frequencies, which depend on the rare earth element (RE=Pr, Nd, Sm). In order to study the effect of the rare earth ions on dielectric properties, infrared reflection spectra of these ceramics were measured from 50 to 4000 cm-1 at room temperature. These data were analyzed according to the model proposed by Gervais and Piriou. Also, the fourth-order dielectric constants of these materials were estimated for the purpose of investigating the relationship between the dielectric loss and the anharmonic terms in the potential energy.

Fabrication of an insulated metal substrate (IMS), having an insulating layer with a high dielectric constant

Abstract: A copper-based insulated metal substrate (IMS) having an insulating layer with a high dielectric constant has been developed, using an epoxy compound filled with barium titanate filler. A wide variety of polymers and inorganic filters as well as coupling agents were tested for their ability to be fabricated into an insulating layer having a dielectric constant of 26 and a dielectric loss of 0.04 at 400 MHz. The temperature dependence of the cured dielectric was also studied. This copper-based IMS easily allowed for the addition of solder through holes. Reduction in the size of the microstrip line in an UHF power amplifier was attempted. An area less than 1/3 the original size of the conventional glass epoxy printed circuit is expected with this IMS. >

Recent Developments in the Microwave Processing of Ceramics

Abstract: Highly absorbing ceramics can be heated in a standard multimode microwave oven. However, most of the microwave energy is dissipated in their ceramic surfaces and heat is transported slowly throughout the sample volume by the usual thermal conduction mechanisms. As with conventional methods, this process leads to non-uniform heating. In contrast, ceramics that are fairly transparent to microwave energy are difficult to heat in standard microwave ovens. When they can be heated, reverse temperature gradients often develop. Reverse temperature gradients can be beneficial in some processes, e.g., combustion synthesis, chemical vapor infiltration (CVI) and binder burnout, but in most ceramic processes, temperature gradients are undesirable. These characteristics have presumably been partially responsible for the ceramic industry's failure to examine microwave processing more closely.The ability of dipoles to respond to the oscillating electric field and their subsequent inability to keep up with rapid reversals in the field result in dielectric losses that are manifested as heat. While some of the mechanisms by which electromagnetic radiation interacts with ceramics are well known, interaction mechanisms in the microwave frequency range are less well understood. Since several mechanisms, including electrical conductivity, can operate simultaneously, the total energy dissipation and heat generation depends on the effective loss factor. The effective loss factor (e″eff) is described bywhere e′ is the dielectric constant and is a measure of the extent of polarization, and tan δ indicates how responsive the dipoles are to the oscillating field.