Showing papers on "Dielectric published in 1968"
TL;DR: In this paper, an improved expression of the dielectric function is given, which includes explicitly, in an approximate way, the short-range correlations arising from both Coulomb and exchange effects by being a functional of the structure factor.
Abstract: The dielectric function of a degenerate electron gas in the random-phase approximation, and the one proposed by Hubbard, which takes exchange effects into account, have been extensively used in the study of metallic properties. However, both dielectric functions lead to an overestimate of the short-range correlations between particles. This is manifest from the fact that the pair-correlation function is negative for small interparticle separations over the whole range of metallic densities, and implies an overestimate of the correlation energy. An improved expression of the dielectric function is given, which includes explicitly, in an approximate way, the short-range correlations arising from both Coulomb and exchange effects by being a functional of the structure factor. The structure factor and the dielectric function can then be determined in a self-consistent manner. The numerical solution of the self-consistent scheme yields a pair-correlation function which is positive for all values of the density up to ${r}_{s}=4$. For ${r}_{s}g4$, it is very slightly negative at small separations, but it is so small that it can be considered to be zero for all practical purposes. New estimates of the correlation energy are given for the entire metallic density range, and are smaller than the earlier estimates. These results are used to recalculate the cohesive energy of the alkali metals. A discussion of the plasmon dispersion relation, the compressibility, and the liquid-solid transition, both for the electron system and for an astrophysically interesting system of protons over a background of electrons, is also given.
761 citations
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TL;DR: In this paper, expressions for the conductor loss in microstrip transmission lines are derived for rutile and alumina substrates, taking into account the finite thickness of the strip conductor and apply to the mixed dielectric system.
Abstract: Expressions are derived for the conductor loss in microstrip transmission lines. The formulas take into account the finite thickness of the strip conductor and apply to the mixed dielectric system. Good agreement with experimental data is obtained for rutile and alumina substrates.
543 citations
TL;DR: In this article, the authors derived the coupling coefficient between adjacent dielectric-disk resonators within a metal waveguide below cutoff, and compared theoretical and experimental coupling coefficient values in each of the three bandpass configurations treated in this paper.
Abstract: This paper is concerned with dielectric disks used as resonators in microwave bandpass filters. For many years it has been known that modes of resonance occur in isolated dielectric bodies having air boundaries, and that very compact high-Q resonators can be achieved when /spl epsiv/ /sub r/, is high and tan /spl delta/ is low. High-purity TiO/sub 2/ ceramic material, for example, has an /spl epsiv/ /sub r/ about 100, tan /spl delta/ about 0.0001, and Q/sub u/ about 10 000. Practical applications of dielectric resonators have previously been limited by insufficient design information. Formulas are derived for the coupling coefficient between adjacent dielectric-disk resonators within a metal waveguide below cutoff. This metal enclosure is necessary for shielding and to prevent radiation loss. Comparisons between theoretical and experimental coupling coefficient values show very good agreement in each of the three bandpass configurations treated in this paper. Techniques of loop and probe coupling to the end resonators of a multiresonator bandpass filter are discussed and methods of supporting the resonators are suggested. The conclusion is made that microwave dielectric resonators offer important size reductions compared to conventional resonators of similar high Q, but that the center-frequency change of the dielectric resonators as a function of temperature is excessive for many applications. Temperature stabilization is one solution, but preferably a material should be developed having electrical characteristics similar to TiO/sub 2/ ceramic, but with at least an order of magnitude improvement in temperature sensitivity.
477 citations
TL;DR: In this paper, the dielectric constant, spontaneous polarization, optical absorption, refractive index, and birefringence data are reported for melt-grown single-domain BaTiO 3.
345 citations
TL;DR: In this article, a theoretical analysis of microwave propagation on microstrip, with particular reference to the case of coupled pairs of microstrip lines, is presented in graphical form for substrate dielectric constants of 1, 9, and 16.
Abstract: A theoretical analysis is presented of microwave propagation on microstrip, with particular reference to the case of coupled pairs of microstrip lines. Data on this type of transmission line are needed for the design of directional couplers, filters, and other components in microwave integrated circuits. The inhomogeneous medium, consisting of the dielectric substrate and the vacuum above it, is treated in a rigorous manner through the use of a "dielectric Green's function" which expresses the discontinuity of the fields at the dielectric-vacuum interface. Results are presented in graphical form for substrate dielectric constants of 1, 9, and 16, and a range of values of width and spacing of the strips. Numerical tables for these and other cases are also available. The tables present capacitance, characteristic impedance, and velocity of propagation of the even and odd normal modes. The method lends itself to the treatment of other geometries which are of practical interest, such as "thick" strips, presence of an unsymmetrically located upper ground plane, etc.
334 citations
TL;DR: In this paper, a simple model for the static electronic dielectric constants of zinc-blende and wurtzite crystals in terms of the diamond-type crystals was proposed, which suggests a natural definition of electronegativity in hybridized valence states.
Abstract: A simple model is proposed for the static electronic dielectric constants of zinc-blende and wurtzite crystals in terms of the dielectric constants of diamond-type crystals. The model suggests a natural definition of electronegativity in ($s{p}^{3}$)-hybridized valence states.
311 citations
TL;DR: In this article, the authors extended the classical theory of piezoelectricity to accommodate an electro-mechanical interaction in centrosymmetric (including isotropic) materials and a surface energy of deformation and polarization.
309 citations
TL;DR: In this paper, it was shown that the reversible switching is not due to a phase change of the dielectric, but rather due to an anodized titanium dioxide (TiO 2 ).
Abstract: Thin films of anodized titanium dioxide can be made to switch between three distinct conductivity states. Some electrical properties which characterize these states are given in the temperature range 4·2°K to 500°K. It is concluded that the reversible switching is not due to a phase change of the dielectric.
308 citations
TL;DR: In this article, an argon ion laser was used as the exciting source and an in-tandem grating spectrometer was used to disperse the scattered light, which provided rather complete data including the polarization characteristics of the individual lines and peaks.
Abstract: The Raman spectrum of single‐crystal SrTiO3 has been investigated as a function of temperature down to about 25°K. The lattice dynamics of this crystal have been extensively studied in recent years primarily because of its unusual dielectric and acoustic properties. Both nominally pure and impure samples were examined since these properties seem to be sensitive to small amounts of impurities. An argon‐ion laser served as the exciting source and an in‐tandem grating spectrometer was used to disperse the scattered light. These instrumental techniques provided rather complete data including the polarization characteristics of the individual lines and peaks. The above data, together with the known phonon dispersion curves of SrTiO3, made it possible to obtain a reliable interpretation of the Raman spectrum. At room temperature we find the Raman spectrum to be entirely second order, in agreement with the selection rules for the cubic perovskite structure. Further, the phonons contributing to the second‐order s...
271 citations
TL;DR: In this article, the loss tangent and permittivity of vacuum- baked dielectric films of anodized aluminium oxide and evaporated silicon oxide, in the frequency range 10 -2 -10 7 Hz and temperature range 77-600 °K, were reported.
231 citations
TL;DR: In this article, the dielectric constants of the materials were determined at a frequency of 0.53 GHz and at temperatures up to 800°C, and the results leave little doubt that BiFeO3 is ferroelectric or antiferroelectric.
Abstract: Solid solutions of BiFeO3 with PbTiO3, PbTi0.5Zr0.5O3, and PbZrO3 were prepared. The crystallographic data on these solutions, which are basically perovskitic, are given. The dielectric constants of the materials were determined at a frequency of 0.53 GHz and at temperatures up to 800°C. Dielectric Curie points were found in solutions containing up to 90 mole % BiFeO3. These results leave little doubt that BiFeO3 is ferroelectric or antiferroelectric. The extrapolated Curie point for BiFeO3 is above 850°C. BiFeO3 appears more likely to be ferroelectric than antiferroelectric, but the distinction between the two classifications may not be sharp.
TL;DR: In this article, an a posteriori theory for the structural energy of covalent crystals is developed based on ionic pseudopotentials and valence dielectric screening, which is an example of the "bootstrap" approach, applied here to treat the effect of the ground-state energy of the valence electron gas.
Abstract: An a posteriori theory is developed for the structural energy of covalent crystals. The microscopic theory is based on ionic pseudopotentials and valence dielectric screening. The theory explains the difference between empirical pseudopotential form factors derived from the optical spectra of semiconductors and the metallic form factors calculated from free-ion term values by Animalu and Heine. A byproduct of the theory, which utilizes Penn's model isotropic semiconductor dielectric function, is a relation between the covalent bonding charge and the macroscopic dielectric constant. In self-consistent form the theory is an example of the "bootstrap" approach, applied here to treat the effect of covalent bonding on the ground-state energy of the valence electron gas. It is argued that the axiomatic character of the covalent theory is to be expected on symmetry grounds, and it is shown that the theory is superior to a nonlinear multiple-scattering theory based on the free-electron dielectric function. The extension of the theory to III---V and II---VI semiconductors is described briefly. The theory may be used to calculate elastic and macroscopic dielectric properties of covalent crystals starting only from ionic pseudopotential form factors.
TL;DR: In this article, the dielectric properties of heterogeneous mixtures are treated, and especially the properties of mixtures containing water, in order to elucidate many of the phenomena which contribute to the general behaviour of many systems when brought into a microwave field.
Abstract: Since most of the systems to which microwaves are applied are heterogeneous, the dielectric properties of heterogeneous mixtures are treated, and especially the properties of mixtures containing water, This approach elucidates many of the phenomena which contribute to the general behaviour of many systems when they are brought into a microwave field. Some possible applications and some implications of the theory are treated, heterogeneous mixtures and especially of mixtures containing water, together with some implications and applications of the theory are treated.
TL;DR: In this article, the pyroelectric and specific heat measurements were obtained to within 5% accuracy, and the specific heat data are entirely consistent with these measurements. But the experimental technique enabled all these measurements to be made simultaneously or consecutively on the same sample.
Abstract: Measurements of the dielectric constant $\ensuremath{\kappa}$, specific heat ${c}_{p}$, and pyroelectric coefficient $\frac{d{P}_{s}}{\mathrm{dT}}$ of LiTa${\mathrm{O}}_{3}$ have been made over the temperature range from 25 to 700\ifmmode^\circ\else\textdegree\fi{}C. The experimental technique enabled all these measurements to be made simultaneously or consecutively on the same sample. In each of these measurements pronounced anomalies were observed at the ferroelectric Curie temperature ${T}_{C}=618\ifmmode^\circ\else\textdegree\fi{}$C, clearly indicating a transition of the second order. From the pyroelectric and specific-heat measurements, the temperature dependence of the spontaneous polarization has been obtained to within 5% accuracy. At temperatures $T$ where $\frac{T}{{T}_{C}}\ensuremath{\gtrsim}0.9$ it was found that ${{P}_{s}}^{2}\ensuremath{\propto}(T\ensuremath{-}{T}_{C})$ and $\frac{1}{\ensuremath{\kappa}}\ensuremath{\propto}(T\ensuremath{-}{T}_{C})$, in accordance with thermodynamic theory, and the specific-heat data are entirely consistent with these measurements.
TL;DR: These results are extended to cover: unequal optical thicknesses in plates and spaces; ( b ) oblique incidence; ( c ) layers of materials of other refractive indices above and below the stack itself; and ( d ) stacks consisting of repeats of more complex units.
Abstract: 1. A convenient method is presented for calculating the reflectance of a stack of dielectric layers consisting of a series of identical repeats of any particular sequence of layers. The method is closely related to that published by Lord Rayleigh in 1917. 2. In this method, two quadratic equations are formed from the thicknesses and refractive indices of the layers composing a single repeat unit. The reflectance is obtained by substituting the solutions of these equations into an explicit formula. 3. Particularly simple formulae result for the case of a stack of p plates, optical thickness λo/2π, uniformly spaced in an infinite medium with spaces of the same optical thickness. If r is the amplitude reflexion coefficient at a single interface, the reflectance of the whole stack is as follows: ( a ) when cos 2 φ 2 , reflectance = 1/1+4m 2 (r 2 -cos 2 φ)/r 2 (1-m 2 ) 2 , where m=(1-√r 2 -cos 2 φ/sin 2 φ/1+√r 2 -cos 2 φ/sin 2 φ) p ; ( b ) when cos 2 φ > r 2 reflectance = 1/1+cos 2 φ-r 2 /r 2 sin 2 pθ, Where cosθ = cos2φ-r 2 /1-r 2 ; ( c )when the number of repeats in the stack is large ( p →∞), reflexion is complete so long as cos 2 φ 2 . Outside this range the reflections is 1-√(1-r 2 cos 2 φ). 4. These results are extended to cover: ( a ) unequal optical thicknesses in plates and spaces; ( b ) oblique incidence; ( c ) layers of materials of other refractive indices above and below the stack itself; and ( d ) stacks consisting of repeats of more complex units.
TL;DR: In this paper, a self-consistent model was proposed to represent the Ohmic conduction process in the bulk of the fluids, with surface shears induced by the free interfacial charges placed in dynamic equilibrium by viscous stresses.
Abstract: It is well known that electromechanical polarization surface waves propagate along the lines of electric field intensity imposed tangential to the interface between perfectly insulating fluids. These waves have a stiffening effect on electrohydrodynamic equilibria that is analogous to that of the Alfven surface waves on hydromagnetic equilibria. An investigation is presented of the dynamical effects of charge relaxation on these waves. A self‐consistent model represents the relaxation in terms of an Ohmic conduction process in the bulk of the fluids, with surface shears induced by the free interfacial charges placed in dynamic equilibrium by viscous stresses. The dominant effect of the charge relaxation is to produce overstability. Experiments are described where this instability appears as a spontaneous oscillation of the interface with wavenumbers directed along the lines of electric field intensity. Detailed analytical results are given for liquid‐gas and liquid‐liquid interfaces. The field required to produce incipient instability and the propagation direction of the observed instability are satisfactorily predicted. It is found that in this liquid‐vapor case, relatively simple explicit expressions can be given for the incipient instability conditions. A discussion is given of the significance of this work for the dielectrophoretic orientation of liquids in the zero‐gravity environments of space.
TL;DR: In this paper, the authors analyzed the optical constants, dielectric constants, and energy-loss functions over the energy range 8 to 28·5 eV for anodized AL 2 O 3, films for incident photon energies from 5 to 18 eV.
TL;DR: In this article, the locations of the poles and zeros of a dielectric tensor function in the complex frequency plane have been investigated and the connections between these poles and zero locations, the frequencies and damping constants of optical modes, and the Lyddane-Sachs-Teller relation have been discussed.
Abstract: The dielectric dispersion function of a medium can be defined by the locations of the poles and zeros of that function in the complex-frequency plane. In this form the dispersion function is not restricted by special characteristics of any specific physical model from which dispersion might be derived. The locations of these poles and zeros are subject to several more or less fundamental physical restrictions which are described in this paper. The connections between the locations of the poles and zeros, the frequencies and damping constants of certain optical modes, and the Lyddane-Sachs-Teller relation are discussed. Adjustments of locations of poles and zeros, consistent with the physical restrictions, were performed to obtain least-squares fits of reststrahlen data from Pr${\mathrm{Cl}}_{3}$ and La${\mathrm{Cl}}_{3}$ using a small number of poles and zeros. In this way, approximate dispersion functions for the two dielectric tensor components of each crystal were obtained at room temperature and at lower temperatures.
TL;DR: In this article, an arrangement of the hydrogen atoms in ice IX is proposed, and the amplitude of the orientational polarization decreases continuously through the transformation region although the relaxation time is close to the value extrapolated from previous measurements in the disordered phase.
Abstract: The dielectric properties of ice III in the frequency range 10−1 to 105 cps have been measured down to − 160°C. There is a gradual transition from the orientationally disordered III to an orientationally ordered and probably antiferroelectric phase, which is designated IX, starting at about − 65°C and reaching completion at about − 108°C. An arrangement of the hydrogen atoms in ice IX is proposed. The amplitude of the orientational polarization decreases continuously through the transformation region although the relaxation time is close to the value extrapolated from previous measurements in the disordered phase. The limiting high‐frequency dielectric constant of the dispersion decreases with decreasing temperature. The cause of this behavior, which is unusual for molecular crystals, is undoubtedly that the polarization of the lattice vibrations contributes a large part of the high‐frequency dielectric constant. This contribution decreases with decreasing temperature because the decreasing anharmonic interaction increases the absorption frequencies. The high‐frequency dielectric constant decreases by about 5.7% at the III‐IX transition although there is little volume change; this decrease might be a valuable way of detecting order‐disorder transitions, particularly if they proceed slowly. The infrared polarizabilities of the various phases are briefly discussed. Ice IX could not be warmed into III because it always transforms to the more stable ice II.
TL;DR: The results give first experimental evidence for three effects of major significance: the system exhibits dielectric relaxation due to a chemical rate process, there is a considerable electric field effect of the helix–coil transition and conformation changes in biological systems could be potentially caused by direct action of an electric field.
Abstract: Dielectric relaxation of poly(γ-benzyl L-glutamate) in solution has been studied in the 5 kcps-10 Mcps range for various values of the helix content. The results give first experimental evidence for three effects of major significance. (1) The system exhibits dielectric relaxation due to a chemical rate process (namely helix formation). This confirms recent theoretical predictions. (2) The mean relaxation time τ* of the helix–coil transition could be evaluated as a function of the degree of transition. The results are in excellent agreement with a previously developed theory. At the midpoint of transition it is found τ*max = 5 × 10−7 sec. The elementary process of helical growth turns out to be practically diffusion-controlled (with a rate constant of hydrogen bond formation of 1.3 × 1010 sec−1). (3) There is a considerable electric field effect of the helix–coil transition. This indicates that conformation changes in biological systems could be potentially caused by direct action of an electric field.
TL;DR: In this paper, the spontaneous polarization in gadolinium molybdate was shown to arise from an elastic instability which gives rise to a spontaneous strain in the original piezoelectric paraelectric phase.
Abstract: Ferroelectric properties in gadolinium molybdate are shown to originate from a new mechanism completely different from that in conventional ferroelectric materials. In this crystal the spontaneous polarization results from an elastic instability which gives rise to a spontaneous strain in the original piezoelectric paraelectric phase. The dielectric permittivity of the clamped crystal ($\ensuremath{\epsilon}_{33}^{x}$) is low and independent of temperature, while the elastic constant ($c_{66}^{E}$) shows a strong temperature dependence with a marked anomaly at the Curie point.
TL;DR: In this article, a semiclassical formula for the energy-loss probability of fast electrons penetrating a thin foil with a dielectric constant is derived, which is discussed for the special cases of the free electron gas and the occurence of Cerenkov-radiation in substances with high Dielectric constants.
Abstract: Regarding the retardation of the electromagnetic field, a semiclassical formula for the energy-loss-probability of fast electrons penetrating a thin foil with a dielectric constantɛ=ɛ(Ω) is derived. This formula is discussed for the special cases of the free electron gas and the occurence of Cerenkov-radiation in substances with high dielectric constants. Numerical evaluations have been made for these cases. Significant deviations from the formula without retardation are found, which are confirmed by the experiments.
TL;DR: In this article, the vibrations of PO4−3 ions in single crystals of hexagonal Ca10(PO4)6F2 have been observed by means of polarized Raman scattering and polarized infrared reflectance.
Abstract: The vibrations of PO4−3 ions in single crystals of hexagonal Ca10(PO4)6F2 have been observed by means of polarized Raman scattering and polarized infrared reflectance. The T2(ν3) and T2(ν4) Raman vibration frequencies are each observed to split in accord with the predictions of the factor group of the unit cell. The optically active vibrational modes are also observed, and the anisotropy of the extraordinary ray frequencies have been measured. The observations are analyzed in terms of dipole–dipole interionic forces using Ewald dipole summation techniques to account for the entire phosphate lattice. Qualitative agreement between the dipole model and experiment is achieved. The roles of short‐range forces and background dielectric shielding are discussed.
TL;DR: The piezoelectric effect in wood, i.e. the occurrence of electric polarization under mechanical stress and also of mechanical strain in an electric field, was explained by considering the uniaxial orientation of cellulose crystallites in fibers and their monoclinic symmetry as discussed by the authors.
Abstract: The piezoelectric effect in wood, i.e. the occurrence of electric polarization under mechanical stress and also of mechanical strain in an electric field, was accounted for by considering the uniaxial orientation of cellulose crystallites in fibers and their monoclinic symmetry. A shear stress in one plane, including the grain direction, produced electrical polarization perpendicular to it. The value of the piezoelectric modulus for wood was approximately one twentieth of that of a quartz crystal. The chemical treatments which transform the lattice structure from cellulose I to II or III, increased the piezoelectric modulus. However, gamma-ray irradiation up to a dose sufficiently high to decrease the molecular weight had only little influence on the piezoelectric modulus. The variation with temperature of the phase angle between sinusoidal stress and polarization showed a maximum of advanced phase around room temperature and a maximum of delayed phase at about-100°C. Dielectric and viscoelastic measurements indicated that the former was caused by the dielectric loss due to water at a temperature above freezing and the latter by the viscoelastic loss due to local vibrations of cellulose molecules. The piezoelectric polarization in wood can be utilized in technical problems such as the measurement of shock velocity in timber. The physiological meaning of the piezoelectrical effect in plants has not been investigated.
TL;DR: In this article, a systematic investigation of orientational relaxation in liquids has been initiated employing the technique of Rayleigh scattering, and the experimental results are interpreted based on an approximate theory which is developed and justified.
Abstract: A systematic investigation of orientational relaxation in liquids has been initiated employing the technique of Rayleigh scattering. The experimental results are interpreted based on an approximate theory which is developed and justified. Scattering spectra from 15 different liquids, including the homologous alkyl bromides and polyethylene glycols as well as glycerol, 1,3‐butanediol, and n‐octyl alcohol, were obtained as a function of temperature using a 6328‐A Ne–He laser source and a Fabry–Perot interferometer. All spectra were found to have approximately Lorentzian form symmetric about the source frequency, which implies molecular reorientation is a relaxational process. Also, the width of these Lorentzian lines (which is reciprocally related to the orientational relaxation time) increased with increasing temperature in such a way that an effective orientational activation enthalpy was determined. In both the alkyl bromide and polyethylene glycol series, increased molecular size resulted in increased relaxation time, which is explained using both activation‐enthalpy and free‐volume approaches. The measured orientational relaxation times which reflect motion of the induced dipole moments are compared with dielectric relaxation times associated with the permanent moments and mechanical relaxation times determined ultrasonically. This comparison provides additional information about the detailed nature of the reorientation mechanism. For example since orientational relaxation is shown to be governed by rotation of ellipsoidal‐shaped molecules about their minor axes, it is concluded that mechanical motions are more nearly about major axes. In addition, it is possible to determine whether reorientation is a diffusion, jump, or an intermediate process. Experimental results indicate molecules in n‐octyl alcohol and probably in the alkyl bromides reorient by a diffusional process and that glycerol molecules reorient by rather large jumps, while 1,3‐butanediol is an intermediate case. These results are in excellent agreement with predictions based upon the McCuffie–Litovitz explanation for the distribution of dielectric relaxation times.
TL;DR: In this article, a short list of the best materials for low loss and high capacitance capacitors is drawn up for thin-film dielectrics in thin-filtered capacitors.
TL;DR: In this paper, the excitation of plasmons by energetic electrons, and their subsequent decay, in small spheres was studied. And they estimated that the electron decay light from electron bombardment of small, randomly distributed metal spheres suspended in a dielectric medium may be emitted with intensity comparable with that found in experiments on plasma decay from metallic slabs.
Abstract: We have studied the excitation of plasmons by energetic electrons, and their subsequent decay, in small spheres. We estimate that plasma decay light from electron bombardment of small, randomly distributed metal spheres suspended in a dielectric medium may be emitted with intensity comparable with that found in experiments on plasma decay light from metallic slabs.
TL;DR: In this paper, the dielectric constants of Bi2/3TiO3, PbZn1/3Nb2/ 3O3 or their derivatives have been obtained and the aging rate of frequency constant does not exceed 0.1%/time decade.
Abstract: PbTiO3 ceramics with high density and high resistivity have been prepared by adding less than 5 mol % of Bi2/3TiO3, PbZn1/3Nb2/3O3 or their derivatives. These ceramics have dielectric constants of about 200, and can be poled at 200°C under a d. c. field of 40 to 70 kV/cm. The piezoelectric coupling factors k15, k33 and k31 amount to 0.43, 0.35 and 0.068, respectively. The value of k33 is nearly constant up to the Curie temperature of about 500°C. The aging rate of frequency constant does not exceed 0.1%/time decade. The present ceramics can be used as stable piezoelectric elements in high temperature and high frequency.
TL;DR: In this article, the free volume concept is used to describe the temperature and the pressure dependence of the relaxation time for the micro-Brownian motion of amorphous chain segments, while the β relaxation due to local-mode motion of the frozen main chain shows weak dependence on temperature and pressure.
Abstract: The dielectric properties and the d. c. conductivity of poly(vinyl chloride), poly(vinyl acetate), polychlorotrifluoroethylene, and poly(ethylene terephthalate) were measured at temperatures above and below the glass transition temperature and at various pressures up to 3000 atm. The α relaxation associated with the micro-Brownian motion of amorphous chain segments depends strongly upon temperatures and pressure, while the β relaxation due to local-mode motion of the frozen main chain shows weak dependence on temperature and pressure. It is found that the free volume concept is valid for description of the temperature and the pressure dependence of the relaxation time for the α process. Activation energy and volume for both relaxation processes are determined from the experimental data. A simple relation between activation energy and volume for the α relaxation and pressure dependence of the glass transition temperature are derived. Temperature and pressure dependence of the d. c. conductivity in the rubbery state are notably different from those in the glassy state. Ionic conduction appears to be dominant in these polymers.
TL;DR: In this paper, single crystals of Pb(Fe1/2Ta 1/2)O3 were grown from ternary melts with excess PbO. The crystals, having cubic perovskite structure at room temperature, undergo a ferroelectric phase transition at about 243°K, changing to rhombohedral structure.
Abstract: Single crystals of Pb(Fe1/2Ta1/2)O3 were grown from ternary melts with excess PbO. The crystals, having cubic perovskite structure at room temperature, undergo a ferroelectric phase transition at about 243°K, changing to rhombohedral structure. The dielectric constant passes through a maximum of nearly 9500 at 1 KHz. The spontaneous polarization along [100] direction is ca 28 µC/cm2 at liquid nitrogen temperature. A magnetic anomaly at about 180°K is related to the onset of antiferromgnetic ordering.