Showing papers on "Dielectric published in 1989"

Dielectric properties of tissues and biological materials: a critical review.

Abstract: We critically review bulk electrical properties of tissues and other biological materials, from DC to 20 GHz, with emphasis on the underlying mechanisms responsible for the properties. We summarize the classical principles behind dielectric relaxation and critically review recent developments in this field. Special topics include a summary of the significant recent advances in theories of counterion polarization effects, dielectric properties of cancer vs. normal tissues, properties of low-water-content tissues, and macroscopic field-coupling considerations. Finally, the dielectric properties of tissues are summarized as empirical correlations with tissue water content in other compositional variables; in addition, a comprehensive table is presented of dielectric properties. The bulk electrical properties of tissues are needed for many bioengineering applications of electric fields or currents, and they provide insight into the basic mechanisms that govern the interaction of electric fields with tissue.

Ferroelectric properties of vinylidene fluoride copolymers

Abstract: Ferroelectric properties of copolymers of vinylidene fluoride with trifluoroethylene and tetrafluoroethylene are described with special interest in their polarization reversal and phase transition behavior. The ferroelectric phase consists of all-trans molecules packed in a parallel fashion while molecules adopt irregular TT, TG, T[Gbar] conformations in the paraelectric phase. In the ferroelectric phase, polarization reversal occurs at very high fields (> 100 MV/m) as a result of eventual 180° rotations of individual chain molecules around their axes. The switching time ranges from sec to nsec depending upon the strength of the applied field according to an exponential law with a particularly large activation field (∼ 1 GV/m). The value of the observed remnant polarization is consistent with prediction from a simple dipole sum implying a minor contribution from the Coulomb interaction. The ferroelectric-to-paraelectric transition appears most clearly for copolymers containing 50-80 mol% vinylide...

Short-channel effect in fully depleted SOI MOSFETs

Abstract: The short-channel effect in fully depleted silicon-on-insulator MOSFETs has been studied by a two-dimensional analytical model and by computer simulation. The calculated values agree well with the simulation results. It is found that the vertical field through the depleted film strongly influences the lateral field across the source and drain regions. The short-channel effect can be significantly reduced by decreasing the silicon film thickness. >

Continuum Mechanics of Electromagnetic Solids

Abstract: 1. Essential Properties of Electromagnetic Solids. 2. Elements of Continuum Mechanics. 3. General Equations of Nonlinear Electromagnetic Continua. 4. Elastic Dielectrics and Piezoelectricity. 5. Elastic Conductors. 6. Elastic Ferromagnets. 7. Elastic Ionic Crystals, Ferroelectrics and Ceramics. Appendices. Index.

A free-space method for measurement of dielectric constants and loss tangents at microwave frequencies

Abstract: For measurements of dielectric properties of planar slabs of ceramic and composite materials, the authors have developed a free-space measurement system in the frequency range of 14.5-17.5 GHz. The key components of the measurement system are a pair of spot-focusing horn lens antennas, a network analyzer, and a computer. Because of the far-field focusing ability of horn lens antennas, the free-space measurements can be made at microwave frequencies in a relatively compact and simple measurement setup. The time-domain gating feature of the network analyzer and the thru, reflect, and line calibration technique were used to eliminate the effects of undesirable multiple reflections. The dielectric constants and loss tangents were measured for standard materials such as fused quartz, Teflon, and polyvinylchloride (PVC). Dielectric properties for Teflon and PVC were also measured in a waveguide medium for purposes of comparison with the free-space method. >

Photonic band structure: The face-centered-cubic case.

Abstract: We employ the concepts of band theory to describe the behavior of electromagnetic waves in three dimensionally periodic face-centered-cubic (fcc) dielectric structures. This can produce a ``photonic band gap'' in which optical modes, spontaneous emission, and zero-point fluctuations are all absent. In the course of a broad experimental survey, we have found that most fcc dielectric structures have ``semimetallic'' band structure. Nevertheless, we have identified one particular dielectric ``crystal'' which actually has a ``photonic band gap.'' This dielectric structure, consisting of 86% empty space, requires a refractive index contrast greater than 3 to 1, which happens to be readily obtainable in semiconductor materials.

Dielectric and pyroelectric properties in the Pb(Mg1/3Nb2/3)O3-PbTiO3 system

Abstract: Dielectric and pyroelectric properties of relaxor ferroelectric in the (1-x) Pb(Mg1/3Nb2/3)O3-xPbTiO3 solid solution series have been investigated. The dielectric constant (K) and loss of poled and unpoled ceramic samples were determined. The pyroelectric coefficient and the spontaneous polarization were measured by the static Byer-Roundy method as a function of temperature and for various compositions in the binary system PMN-PT. An important contribution to the dielectric and pyroelectric behavior for the solid solutions compositions close to the morphotropic phase boundary (x ∼ 0.4) between pseudocubic and tetragonal regions existing in the grains of the ceramic were observed. Anomalous dielectric and pyroelectric behaviors occurred for compositions x = 0.275 ∼ 0.35 mole% PT in the vicinity of morphotropic phase boundary.

Method of making dense flash eeprom semiconductor memory structures

Abstract: An improved electrically erasable and programmable read only memory (EEprom) structure and processes of making it which results in a denser integrated circuit, improved operation and extended lifetime. In order to eliminate certain ill effects resulting from tolerances which must be allowed for registration of masks used in successive steps in forming the semiconductor structures, spacers are formed with reference to the position of existing elements in order to form floating gates and define small areas of these gates where, in a controlled fashion, a tunnel erase dielectric is formed. Alternatively, a polysilicon strip conductor is separated into separate control gates by a series of etching steps that includes an anisotropic etch of boundary oxide layers to define the area of the control gates that are coupled to the erase gate through an erase dielectric. In either case, the polysilicon layer strip can alternatively be separated by growing oxide thereon until it is completely consumed. A technique for forming a pure oxide dielectric layer of uniform thickness includes depositing a thin layer of an undoped polysilicon material and then oxidizing its surface until substantially the entire undoped polysilicon layer is consumed and made part of the resulting oxide layer. Overlapping doped regions are provided in the substrate by an ion implantation mask that adds spacers to the mask aperture to change its size between implants.

Relation between charge transfer absorption and fluorescence spectra and the inverted region

Abstract: The theoretical expression for the plot of a charge transfer (CT) band intensity vs frequency is known to parallel that for the plot of electron-transfer rate constant k^(ET) vs –ΔG^0. We use this parallelism to explore a recent model in the literature on the possible role, if any, of partial dielectric saturation of the solvent on the “inverted region” of electron-transfer reactions. A simple approximate expression is obtained for the full width at half-maximum for the k^(ET) and spectral plots and is tested using recent numerical results for calculated k^(ET) vs –ΔG^0 curves. Studies of experimental widths of CT bands are used, thereby, to test ideas on the possible presence and effects of partial dielectric saturation and on the observation of the inverted region. Further uses of charge-transfer spectra, when both the absorption and fluorescence data are available, are also described, including the determination of the “0 → 0 transition energy” ΔE_( 0 → 0) for systems displaying little or no vibrational structure.

Principles of dielectrics

Abstract: Elementary considerations Dielectrics in static fields Dielectrics in time-dependent fields Heterogeneous dielectrics Equilibrium polarization fluctuations The spectral properties of equilibrium fluctuations Basic principles of the calculation of the relative permittivity Electromechanical effects Dielectric saturation in dipolar liquids The electromagnetic field in matter Appendices References Index

Composite structures for the enhancement of nonlinear-optical susceptibility

Abstract: Calculations of the nonlinear-optical behavior are developed for model composites of nanospheres with a metallic core and nonlinear shell or with a nonlinear core and metallic shell suspended in a nonlinear medium. Optical phase conjugation is shown to be enhanced from each nonlinear region because the optical field can be concentrated in both the interior and the exterior neighborhoods of the particle and magnified at the surface-mediated plasmon resonance. For the model composite with a metallic core, a limited range of resonance tunability can be achieved by adjustment of shell thickness; the frequency range is dependent on the dielectric dispersion of the metal. For the composite with a metallic shell instead of a metallic core, this restriction is reduced so that tunability from ultraviolet to infrared can be attained. Enhancement of the phase-conjugate signal is calculated for the electrostrictive mechanism dominant in the microsecond time scale and for the electronic mechanism dominant in the picosecond time scale. Calculations based on the dielectric functions for gold and for aluminum indicate that phase-conjugate reflectivity enhancements of 108 can be achieved. The imaginary components of the composite dielectric functions are shown to limit the magnitude of the field enhancement at the surface-plasmon resonance and determine the absorption and figure of merit of the composite.

Response times and voltages for PDLC light shutters

Abstract: The response times and operating voltages of light shutters formed from polymer dispersed liquid crystals (PDLCs) have been studied experimentally and the results compared with calculations based on non-sperhically shaped nematic droplet models. The experiments were performed on light shutters with elongated and uniformly aligned droplets where the relaxation time and voltage response were measured. It is shown that the droplet shape can be a dominant factor, particularly for the relaxation time, and the data are compared with equations derived in terms of the aspect ratio of the droplet l = a/b, where a and b are the lengths of the semi-major and semi-minor axes, respectively, of the elongated droplet. It is further demonstrated that the electric field inside the droplet can be considerably smaller than the applied field, due to the conductivity and dielectric properties of the polymer and liquid crystal materials. These data are used to obtain values for the ratio of the conductivities of the p...

Time domain reflection methods for dielectric measurements to 10 GHz

Abstract: Total reflection methods and instrumentation for their use are described for measurements of dielectric permittivity and loss at frequencies to 10 GHz or more. Several cell designs are shown, together with analyses of their performance. Procedures are given for correcting effects of wave propagation in the cells and residual reflections in the cells by bilinear analysis with calibrations using dielectrics of known permittivity. Representative results are presented for highly polar liquids, dilute solutions of polar molecules in nonpolar solvents, electrolyte solutions, and ionic glasses with appreciable ohmic conduction.

Broadband stacked dielectric resonator antennas

Abstract: The input impedance of stacked cylindrical dielectric resonator antennas is investigated experimentally. The dielectric resonators are made of different materials. The bandwidth of 25% has been observed for a standing wave ratio better than 2.

Thermodynamic theory of the lead zirconate-titanate solid solution system, part III: Curie constant and sixth-order polarization interaction dielectric stiffness coefficients

Abstract: Vaiues of the Curie constant (C) and sixth-order polarization interaction dielectric stiffness coefficients (α112 and α123) are needed for the development of a thermodynamic theory for the entire lead zirconate-titanate (PZT) solid solution system. Low-temperature dielectric data measured on pure homogeneous polycrystalline PZT samples were used to determine values of these coefficients at several compositions across the phase diagram. Equations were then fitted to these data to determine the compositional dependence of the coefficients. The Curie constant was found to form a peak in the middle of the phase diagram at the PZT 50/50 composition.

Frequency-selective reflection and transmission by a periodic dielectric layer

Abstract: The feasibility of using a periodic dielectric layer, composed of alternating bars having dielectric constants epsilon /sub 1/ and epsilon /sub 2/, as a dichroic surface at millimeter-wave frequencies is examined For oblique incidence, it is found that total transmission and total reflection can be obtained at different frequencies for proper choices of epsilon /sub 1/ and epsilon /sub 2/ and the geometric parameters The frequencies of total reflection and transmission can be estimated from wave phenomena occurring in a layer of uniform dielectric constant equal to the average value in the periodic layers For some of the frequencies of total transmission, the bandwidth for 90% transmission is found to be 40% The bandwidth for 90% reflection is always found to be much narrower, the greatest value obtained being 25% >

Field‐Forced Antiferroelectric‐to‐Ferroelectric Switching in Modified Lead Zirconate Titanate Stannate Ceramics

Abstract: Electric-field-forced antiferroelectric-to-ferroelectric phase transitions in several compositions of modified lead zirconate titanate stannate antiferroelectric ceramics are studied for ultra-high-field-induced strain actuator applications. A maximum field-induced longitudinal strain of 0.85% and volume expansion of 0.95% are observed in the ceramic composition Pb0.97La0.02(Zr0.66Ti0.09Sn0.25)O3 at room temperature. Switching from the antiferroelectric form to the ferroelectric form is controlled by the nucleation of the ferroelectric phase from the antiferroelectric phase. A switching time of <1 μs is observed under the applied field above 30 kV/cm. The polarization and strains associated with the field-forced phase transition decrease with increasing switching cycle, a so-called fatigue effect. Two types of fatigue effects are observed in these ceramic compositions. In one, the fatigue effects only proceed to a limited extent and the properties may be restored by annealing above the Curie temperature, while in the other, the fatigue effects proceed to a large extent and the properties cannot be restored completely by heat treatment. Hydrostatic pressure increases the transition field and the switching time. But when the applied electric field is larger than the transition field, the induced polarization and strain are not sensitive to increasing hydrostatic pressure until the transition field approaches the applied field.

Electrical Properties of Biopolymers and Membranes

Abstract: Origin of dipole moment polarization of groups of polar molecules dynamic aspect of electrical polarization dielectric properties of polymer molecules dielectric properties of proteins, peptides and amino acids theory of interfacial polarization dielectric properties of nucleic acids water in biological systems non-linear dielectric properties electrical properties of biological membranes.

Excitonic and nonlinear-optical properties of dielectric quantum-well structures

Abstract: Excitonic and nonlinear-optical properties of dielectric quantum-well (DQW) structures are investigated theoretically. A DQW is a quantum well sandwiched by barrier materials with a smaller dielectric constant and a larger band gap than the well material. The fundamental physics determining the excitonic properties in a DQW, i.e., exciton binding energy, exciton oscillator strength, and nonlinear-optical response, are clarified. The most important mechanisms for enhancing the excitonic properties are quantum-confinement effect, mass-confinement effect, and dielectric-confinement effect. Quantum confinement increases the spatial overlap between an electron and a hole as a result of the potential well confinement, and it enhances oscillator strength. Mass confinement is based on the penetration of the carrier wave function into barrier layers with a heavier effective mass than the well layer. It increases the exciton reduced mass and hence the exciton binding energy. Dielectric confinement arises from the reduction of the effective dielectric constant of the whole system due to the penetration of the electric field into the barrier medium having a smaller dielectric constant than the well and enhances the Coulomb interaction between the electron and hole. On the basis of these analyses, the general guiding principles are established for designing DQW structures with optimum excitonic properties. Various practical examples of DQW's are examined with respect to the lattice-constant matching, the difference in the dielectric constant, and the difference in the carrier effective masses. ZnSe is found to be one of the most promising candidates for the barrier material of the GaAs DQW.

Sequential surface chemical reaction limited growth of high quality Al2O3 dielectrics

Abstract: Sequential surface reactions of trimethylaluminum and water vapor have been used to deposit Al2O3 on Si(100) surfaces. The self‐limiting nature of the surface reactions allows precise control of the thickness of the deposited layers and gives rise to films with highly conformal step coverage. High quality dielectrics have been deposited at temperatures as low as 100 °C. Resistivities of 1017 Ω cm, breakdown strengths of 8×106 V/cm, and interface‐state densities of 1011 states/eV cm2 have already been achieved and they suggest possible applications as a gate insulator or a dielectric passivation layer.

Computer simulations of the dielectric properties of water: Studies of the simple point charge and transferrable intermolecular potential models

Abstract: A series of very long molecular dynamics simulations has been completed for the rigid simple point charge (SPC) and transferrable intermolecular potential 4P (TIP4P) water models with reaction field boundary conditions. The dielectric constant corresponding to these models was evaluated in two ways: (1) by calculating the fluctuations in the mean square dipole moment of the system in the absence of an applied field and (2) by evaluating the polarization response of the system to an applied field. Consistent values for the dielectric constant are obtained by the two methods. For the TIP4P water model, the dielectric constant e0 is calculated to be ∼50 at 293 K, in agreement with previously published results. For the rigid SPC model, e0 is calculated to be ∼68 at 300 K and ∼59 at 350 K. The calculated dielectric constant is shown to be very sensitive to the way in which the truncation of long‐range forces is treated, although the short‐range liquid properties are insensitive to the truncation procedure. The effects on the dielectric response of system size, cutoff radius, and the smoothing of long range interactions at the cutoff boundary are examined. It is found that the introduction of a smoothing function causes perturbations of the water–water angular orientation correlation functions which can result in large errors in the calculated dielectric constant.

Thin-layer electromagnetic absorber design

Abstract: The design of effective broadband, thin-layer, electromagnetic absorbers for operation in the VHF/UHF regime has led to creation of design aids for custom-tailoring magnetic materials for this application. Theoretical analysis has implicitly determined the material configuration and electromagnetic parameter values of a single homogeneous layer of dielectric/magnetic material that will produce zero specular reflection. Based on this analysis, an easy-to-use explicit graphic aid for determining these properties has been produced. This not only simplifies the design process, but also provides an overall view of the interrelated numerical values of material properties required to implement specular electromagnetic absorber designs. >

Corona poling and real‐time second‐harmonic generation study of a novel covalently functionalized amorphous nonlinear optical polymer

Abstract: Thin films for optical second‐harmonic generation (SHG) were prepared from a newly designed and synthesized amorphous polymer that incorporated a high density of active nonlinear optical groups (p‐nitroaniline as attached side groups). For alignment of the nonlinear groups a very high electric field was applied by a corona discharge to the polymer films above Tg (125 °C). The subsequent freezing process resulted in a polymer film initially exhibiting a very high second‐order nonlinear coefficient, d33=31 pm/V, measured by the Maker‐fringe technique, plus excellent thermal, mechanical, and optical properties. The dynamics of polar alignment and decay, studied by in situ poling (or depoling) and SHG measurements, indicated a multiple exponential behavior with the average relaxation time somewhat longer than expected from extrapolation of the dielectric relaxations data according to the Williams–Landel–Ferry equation. The frozen‐in SHG behavior at room temperature (∼100 °C below Tg) relaxed after 5 days to a...

Screening of positrons in semiconductors and insulators

Abstract: Theoretical models are presented for the enhancement of the electron density at a positron in a semiconductor or insulator host. The model better suited for typical semiconductors is based on the many-body theory for the screening of a positron in electron gas. The starting point of the model for insulators is the atomic polarizability. The common parameter in both models is the high-frequency dielectric constant. Moreover, the enhancement depends on the ambient electron density in the semiconductor model and on the unit-cell volume in the insulator model. With use of the models developed, positron lifetimes in perfect semiconductor and insulator crystals have been calculated. In the calculations, three-dimensional electron densities and electrostatic potentials are obtained by atomic superposition and the fully three-dimensional positron wave functions are solved by a relaxation method. The calculated positron lifetimes agree with the experimental ones within a few picoseconds. Moreover, we have used the model to predict lifetimes of positrons trapped by lattice defects such as vacancies and vacancy clusters.

Dielectric waveguide to TEM transmission line signal launcher

Abstract: Mode converting signal launchers for efficient coupling signals between dielectric insular and image waveguides and TEM mode transmission lines. The launcher includes a conductive ground plane and an elongated high permittivity dielectric waveguide adjacent to the ground plane. A TEM mode transmission line provides an elongated conductive strip fixed adjacent to the dielectric waveguide such that a coupling region is formed having a length of at least two times the wavelength in the dielectric waveguide.

Structure and ferroelectric properties of P(VDF-TrFE) copolymers

Abstract: P(VDF-TrFE) copolymers with approximately 30 mol% of TrFE content exhibit above room temperature a ferroelectric transition of first order type with a large thermal hysteresis. We present recent investigations of the ferroelectric phase and of the phase transition using several techniques: second harmonic generation of light, high resolution X-ray diffraction, quasi-elastic neutron scattering, small angle X-ray scattering, Brillouin scattering, far infrared absorption and submillimeter dielectric spectroscopy. These results provide information on both the small crystallites and the amorphous inter-crystalline regions and show that temperature cycling throughout the Curie transition is able to modify the superstructure and to enhance the degree of crystallinity of the polymer. Accordingly, low frequency dielectric measurements, thermal expansion results and Brillouin spectroscopic investigations can be discussed in taking into account the composite structure of the semi-crystalline material.