Showing papers on "Dielectric published in 1982"

Semiconducting and other major properties of gallium arsenide

Abstract: This review provides numerical and graphical information about many (but by no means all) of the physical and electronic properties of GaAs that are useful to those engaged in experimental research and development on this material. The emphasis is on properties of GaAs itself, and the host of effects associated with the presence of specific impurities and defects is excluded from coverage. The geometry of the sphalerite lattice and of the first Brillouin zone of reciprocal space are used to pave the way for material concerning elastic moduli, speeds of sound, and phonon dispersion curves. A section on thermal properties includes material on the phase diagram and liquidus curve, thermal expansion coefficient as a function of temperature, specific heat and equivalent Debye temperature behavior, and thermal conduction. The discussion of optical properties focusses on dispersion of the dielectric constant from low frequencies [κ0(300)=12.85] through the reststrahlen range to the intrinsic edge, and on the ass...

Critical exponents of the dielectric constants in diffused-phase-transition crystals

Abstract: The critical exponent γ in the relation between the dielectric constant and temperature (1/e—1/em = C'-1x (T—Tm)γ) has been determined precisely for relaxor ferroelectrlics Pb(Mg1/3Nb2/3)O3, Pb(Zn1/3 Nb2/3)O3 and a related solid solution 0.88Pb (Zn1/3 Nb2/3)O3-0. I2PbT103, as well as for normal ferroelectrics BaTiO3 and K(Ta0. 55Nb0.45)O3. A high correlation of the γ value with the plase transition diffuseness has been found empirically. Moreover, this γ value is very close to another critical exponent γ* which is defined in the relation between the dielectric constant and hydrostatic pressure (1/e—1/em = C* (p—pm)γ*)

Dielectric and Piezoelectric Properties of 0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3 Single Crystals

Abstract: The dielectric, piezoelectric and elastic properties of 0.91(PbZn1/3Nb2/3)O3–0.09PbTiO3 single crystals have been investigated as functions of temperature and applied electric field. Two multiple phase transitions at 68°C and 178°C, the crystal changing from the rhombohedral ferroelectric phase to tetragonal ferroelectric and then to cubic paraelectric, have been observed. Both the transitions are of first-order, but both are slightly diffused. Significant increases in the dielectric, piezoelectric and elastic constants are observed at the lower transition point. In particular, the sample poled along the pseudo-cubic [001] axis reveals anomalously large piezoelectric and electro-mechanical coupling constants at room temperature in the rhombohedral phase (d[001]//=1500×10-12 C/N, k[001]//=0.92).

Diffuse Ferroelectric Phase Transitions in Ba(Ti1-yZry)O3 Ceramics

Abstract: Very high and broad Curie maxima are observed in ceramic Ba(Ti1-yZry)O3 mixed crystals, which are often used for the preparation of ceramic dielectrics The ferroelectric-to-paraelectric phase transition of ceramic Ba(Ti1-yZry)O3 was studied using dielectric measurements, quantitative DTA, X-ray diffraction, and determination of the remanent polarization At higher Zr concentrations, it was found that ferroelectric and paraelectric phases coexist in a wide temperature region Up toy =016, the phase transition remains first order The diffuse character is promoted by the small energy difference between the ferroelectric and paraelectric phases appearing at higher Zr content It can be best described by a normal distribution of Curie temperatures using the phenome-nological theory of Devonshire and the Newton-Raphson mathematical approximation

Piezoelectricity of a high‐content lead zirconate titanate/polymer composite

Abstract: A binary system consisting of polyvinylidene fluoride (PVDF) and lead zirconate titanate (PZT) powder was investigated to determine its dielectric constant, piezoelectric constant, and Young’s modulus. An expression was obtained for the piezoelectric constant, and expressions for the dielectric constant and Young’s modulus were also mentioned. The calculated values had good agreement with the observed values for the composite with a large PZT volume fraction. The dielectric constant and the piezoelectric constant were 152 and 48.3×10−12 C/N, respectively, for the composite with the PZT volume fraction of 0.67. Changing the mixing ratio of PVDF and the fluorine elastomer, Young’s modulus was varied without changing the piezoelectric constant.

Conference on Electrical Insulation and Dielectric Phenomena

Abstract: Whilst the discovery and development of manmade polymer materials dates from the pioneering works of Goodyear (vulcanized rubber) and Hyatt (celluloid plastics) in the mid-nienteenth century, and to Baekeland (Phenol-formaldehyde resins) at the beginning of this century, the remarkable growth of the synthetic fiber, rubber and plastics industries followed the preparative achievements of the I.C.I. group in Cheshire (polyethylene), Carothers at Du Pont, Wilmington (linear polyesters and nylons) and the German chemists (polyvinyl halides) in the 1930's, and the U.S. Government Synthetic Rubber Program during World War II. The ability to vary chemical structure and composition (e.g. for copolymers) and physical structure (by fillers, plasticizers, thermal and mechanical treatments, processing methods) made it possible to create materials which could be tailor-made for a particular application. Thus the industries grew to the dominant positions they hold today.

Charge accumulation and mobility in thin dielectric MOS transistors

Abstract: A study of the turn-on of very thin dielectric MOS devices from subthreshold to strong inversion is described. A functional form has been found for the derivative of channel charge with respect to gate voltage, the derivative of channel charge with respect to distance along the channel, and the electric field along the channel in this transition region. A method to extract electron mobility versus gate voltage independent of any arbitrarily defined threshold voltage has been shown. Measured data on the electron mobility vs gate voltage for 100A gate dielectric MOS devices are reported.

Ab Initio Force Constants of GaAs: A New Approach to Calculation of Phonons and Dielectric Properties

Abstract: It is shown that self-consistent calculations of the electronic charge density in large periodic cells containing a single displaced atom provide all the information needed for ab initio determination of force constants, phonon dispersion curves, effective charges, and the static dielectric constant. Results are presented for GaAs.

Accurate model for effective dielectric constant of microstrip with validity up to millimetre-wave frequencies

Abstract: A closed-form expression is presented for the effective dielectric constant of single microstrip lines which is valid with high accuracy up to mm-wave frequencies. The formula given has been designed for use in MIC and MMIC CAD programs as well as in the indirect measurement of substrate permittivities by resonance techniques.

Ba(Mg 1⁄3 Ta 2⁄3 )O 3 Ceramics with Temperature-Stable High Dielectric Constant and Low Microwave Loss

Abstract: A dense ceramic with an ordered perovskite structure with chemical formula Ba(Mg1/3Ta2/3)O3 is prepared, aiming at materials for a dielectric resonator with temperature-stable high dielectric contsant and low loss at microwave frequency. A small amount of Mn ion is doped to the sample to complete the sintering. The dielectric constant and unloaded Q are 25 and 16800 at 10.5 GHz respectively. The temperature coefficient of resonant frequency is estimated as 2.7 ppm/°C in the vicinity of room temperature. The value of Q we obtained is the highest among those reported so far on ceramics having a similar characteristic.

Dielectric properties of mammalian tissues from 0.1 to 100 MHz; a summary of recent data

Abstract: Dielectric permittivity and conductivity measurements are reported from various soft excised mammalian non-tumour tissues, at frequencies between 0.1 and 100 MHz, and at room and body temperatures. The data over this wide frequency range can be well represented by a Cole-Cole equation for either the complex conductivity or complex permittivity. A summary of fitted parameters for the tissue data is presented, which can be used to reconstruct the original data values. The data are compared to older data that are still frequently quoted. Finally, the contribution of cells and organelles to the dielectric permittivity of one tissue is estimated, to help elucidate the mechanisms that are responsible for the observed dielectric data.

Design of microstrip antennas covered with a dielectric layer

Abstract: The design of a microstrip antenna covered with a dielectric layer is presented. Due to loading, the resonant frequency of the antenna changes. The absolute value of the change increases with the operating frequency, the relative permittivity (except plasma), and the thickness of the dielectric layer. This change may cause degradation in performance due to the inherent narrow bandwidth of microstrip antennas if the effect of loading is not considered in the design. The curves presented here may be used to design microstrip antennas that may be subjected to icing or a plasma environment or coated with protective layers. Numerical and experimental results for the fractional change in the resonant frequency are round to be in good agreement.

Dielectric properties of donor-doped polycrystalline SrTiO3

Abstract: Various donor dopants such as Y3+ and Nb5+ were incorporated in SrTiO3 in amounts ⩽25 mol % during sintering in air at 1450° C for ⩽ 15 h Dense ceramic materials with grains of optimum uniformity and largest size were obtained when the cation stoichiometry was adjusted to allow for charge compensation of the donor ions by strontium vacancies For donor levels ≳ 03 mol%, the dielectric constant measured at 25° C and 1 kHz was up to two orders of magnitude higher than that of undoped strontium titanate The increase in permittivity, however, was dependent on grain size, was influenced by the method of electrode application, and was suppressed by the presence of ≲ 01 mol% Mn in the ceramic These observations, together with data obtained from electrical measurements at other temperatures and frequencies, were consistent with interpretation of the anomalously high dielectric constant as a boundary-layer effect resulting from semiconducting grains and weakly insulating grain-surfaces It is suggested that donor doping influences the electrical properties of SrTiO3 mainly by increasing the volatility of oxygen from the grains during sintering, and by decreasing the rate of re-oxidation during cooling

Electrostrictive effect in Pb(Mg1/3Nb2/3)O3-type materials

Abstract: Lead magnesium niobate Pb(Mg1/3Nb2/3)O3 (PMN) is a relaxor ferroelectric with a disordered perovskite structure. A statistical inhomogeneity in the distribution of Mg2+ and Nb5+ ions in the B-sites of an ABO3 perovskite lattice leads to a wide Curie temperature range in which the crystal exhibits an intimate mixture of paraelectric and ferroelectric microregions. Relaxor-type ferroelectrics such as PMN show somewhat unusual electrostrictive phenomena. Although the polarization-related electrostrictive coefficients Q (defined as x = QP2 ) are one order of magnitude smaller than those of simple perovskite compounds, large electrostrictive strains can be obtained because of their huge dielectric constants. A quadratic relation between the induced elastic strain and the electric polarization holds even below the average Curie temperature. The electrostrictive properties of PMN and related materials are reviewed in this article. Relaxor ferroelectrics are promising transducer materials for use as acti...

The dielectric relaxation time of supercooled water

Abstract: The complex dielectric constant of liquid water was measured at 9.61 GHz down to −18 °C by means of a two resonant cavities apparatus. The static dielectric constant of bulk samples was also measured at 27.5 MHz down to −16.5 °C using a resonant circuit technique. From the analysis of the experimental results it follows that water in the metastable region has practically a single dielectric relaxation time τ. An analysis of the dynamic properties of water using our results and available data in literature, is presented. The main result is that self‐diffusion DS, shear viscosity η, and τ below 0 °C are related to the same mechanism. For T≳0°C another mechanism affecting η rises.

Surface flashover of solid dielectric in vacuum

Abstract: The surface of a solid dielectric insulator becomes electrically charged when subjected to a high‐voltage stress in vacuum. A method for calculating the surface flashover voltage based on the assumption that the discharge occurs in a layer of desorbed gases from the insulator surface is proposed. The electric field strength required to cause surface flashover is calculated by taking into account the secondary electron emission characteristics of the dielectric material. The dependence of the surface flashover field on the insulating material is deduced. A dependence of the flashover voltage on the insulator length to a power law of 0.5 is theoretically predicted. The calculated surface flashover voltage is compared with the previously reported measurements and good agreement is obtained.

Dielectric enhancement due to electrochemical double layer: Thin double layer approximation

Abstract: The dielectric constant and conductivity of a dilute ensemble of immobile, spherical particles with fixed surface (zeta) potential Φo, immersed in an electrolytic solution, is obtained in the thin double layer approximation δ≪a, δ being the thickness of the double layer, and a the radius of the particles. Equations of motion for coions and counter‐ions are solved by the method of matched asymptotics. The equations of motions, linearized in the applied electric field Eo and with coefficients that are functions of the unperturbed potential (zeroth order in Eo), are solved to second order in (δ/a). The term giving enhancement in the real part of the effective dielectric constant of the ensemble e1e, is second order in δ/a; but the series converges if (δ/a)t2/(1–t2)≪1, where t=tanh(eΦo/kBT), e being the ionic charge, kB the Boltzmann constant, and T the absolute temperature. The static value of e1e, to this order, is e1e∼36fe1t2/(1–t2)2, where f is the volume fraction of particles, e1 the real part of the die...

Dielectric Waveguide Phase Shifter

Abstract: Using ferrite waveguide toroids and dielectric ribs (epsilon/sub r/ = 50), we have demonstrated that a dielectric waveguide (no metal walls) phase shifter (Delta Phi = 600°) can propagate with reasonable insertion loss (/spl ap/3 dB) and modest cross coupling (/spl ap/ 15 dB). With brass inserts, the cross coupling can be further reduced (> 20 dB) with some sacrifice of insertion loss and phase shift. The use of dielectric waveguide phase shifters should allow simpler, lower-cost phased arrays in the conventional frequency range (3 to 20 GHz). At millimeter wave frequencies there is the possibility of making a column of phase shifters from slabs of ferrite and dielectric using flat-grinding and cutting techniques.

Structural and Dielectric Investigation on the Nature of the Transition in a Copolymer of Vinylidene Fluoride and Trifluoroethylene (52/48 Mol %).

Abstract: : The effect of temperature on the structure and dielectric properties of a 52/48 mol % copolymer of vinylidene fluoride and trifluoroethylene has been investigated at temperatures up to 140 C. Undrawn or unpoled specimens contain an intimate mixture of two disordered crystalline phases, both of which undergo a large increase in d-spacing at 65-70 C, with eventual transformation to a single phase in which the chains assume a disordered 3/1-helical conformation above 90 C. The 70 C transition is accompanied by a dielectric anomaly. High electric fields applied at temperatures below 70 C induce a phase change to a single, well-ordered all-trans conformation, leading to remanent polarization with piezoelectric and pyroelectric coefficients comparable to those of poly(vinylidene fluoride). The changes in crystal phase and dipole orientation upon poling result in a reduction of the dielectric constant at room temperature, a shift of the dielectric anomaly to about 80 C, stability of the all-trans crystal phase to somewhat higher temperatures, and a discrete change in d-spacing to that of the disordered 3/1-helical conformation at the transition region. The loss of polarization in poled specimens at this ferroelectric-to-paraelectric transition is attributable primarily to the molecular change from the polar all-trans conformation to its non-polar, disordered 3/1-helical counterpart, as well as to the onset of rotational dipolar motions leading to the dielectric anomaly. (Author)

Ba(Zn 1⁄3 Nb 2⁄3 )O 3 –Sr(Zn 1⁄3 Nb 2⁄3 )O 3 Solid Solution Ceramics with Temperature-Stable High Dielectric Constant and Low Microwave Loss

Abstract: Solid solution ceramics in the Ba(Zn1/3Nb2/3)O3(BZN)–Sr(Zn1/3Nb2/3)O3 (SZN) system have been studied with a view to finding materials for use as dielectric resonators at microwave frequency. The relative dielectric constant and the unloaded Q at 10 GHz are respectively 41 and 5400 for BZN, and 40 and 2000 for SZN. The temperature coefficient of the resonant frequency is estimated as 30 ppm/°C for BZN and -38 ppm/°C for SZN. The dielectric constant of 0.3 BZN–0.7 SZN ceramic is nearly independent of temperature, which gives a very small estimated temperature coefficient of the resonant frequency of -5 ppm/°C.

On the effect of substrate thickness and permittivity on printed circuit dipole properties

Abstract: The effect of substrate thickness and relative permittivity on the radiation properties of printed circuit dipoles (PCD's) is investigated. A trade-off between substrate thickness and resonant input resistance, bandwidth, and radiation efficiency is presented for a polytetrafluorethylene (PTFE) glass random fiber substrate. It is found that for a fixed substrate thickness h , the resonant length and directivity decrease with increasing relative dielectric constant \epsilon_{r} . The E - and H -plane normalized power pattern is also examined as a function of \epsilon_{r} and h . It is shown that even for thin substrates, multiple-beam radiation can result for certain values of \epsilon_{r} by the excitation of surface waves. Multiple-beam patterns can also be obtained with increasing h for a given \epsilon_{r} . In fact as h increases it is determined that the resonant length, bandwidth, and resonant resistance approach the apparent value of a PCD on a dielectric halfspace.

Dielectric behaviour of mixed Li-Ni ferrites at low frequencies

Abstract: The dielectric constants and dielectric loss tangents of mixed Li-Ni ferrites of various compositions were measured at room temperature using a capacitance bridge in the frequency range 10 2 –10 5 Hz. A qualitative explanation for the composition and frequency dependence of the dielectric constant is given. The dielectric constant for these mixed ferrites is roughly inversely proportional to the square root of the resistivity.

Use of a liquid suspension of dielectric spheres as an artificial Kerr medium

Abstract: We present the derivation of scaling relationships that describe the behavior of liquid suspensions of dielectric spheres as nonlinear optical media and report results of four-wave mixing experiments that are in good agreement with these predictions.

Low-Frequency Dielectric Relaxations in Nematics and Dual-Frequency Addressing of Field Effects

Abstract: Novel low threshold nematic mixtures are presented exhibiting very low dielectric crossover frequencies fc ≃ 1 kHz at 20°C and unusually large, low- as well as high-frequency dielectric anisotropie...

Dielectric tapered rod antennas for millimeter-wave applications

Abstract: Tapered dielectric rod antennas of rectangular cross section are developed for possible application in millimeter-wave dielectric integrated circuits. Design principles for a maximum gain design are described, and an example is given. Experimental results for antennas designed for low sidelobes and for maximum gain are presented.