Dielectric anomalies in bismuth: Electronic relaxation times at far-infrared frequencies

Two dielectric anomalies in bismuth at far-infrared frequencies in the range 30–85 cm^–1 have been studied. Scattering rates derived from a linewidth analysis show basically a linear increase as a function of frequency. However, deviations from a linear dependence at fields near the extreme quantum limit are seen.     

Dielectric relaxation with dipolar screening

The phenomenon of dipolar screening does not appear to have received sufficient attention in the past, in a manner comparable to charged particle screening, and yet it is shown that it is capable of explaining some forms of dielectric response to which no other evident explanation exists. While reorientations of mathematical dipoles of zero “length” do not entail any charge displacements, physical dipoles of finite length, l, do produce charge displacements and therefore screening. A theory of dipolar screening is presented and it is shown that relatively “dense” systems give complete screening so that neighbouring dipoles do not “see” one another, while in more dilute systems many dipoles fall within a screening radius, Rs, of any one dipole. Dipoles within Rs tend to adopt energetically favoured configuration and the entire system “seizes up” in a “domain”, thus reducing the number of dipoles which can be reoriented at finite frequencies. This revised version was published online in November 2006 with corrections to the Cover Date.     

Dielectric relaxation in polychlorotrifluoroethylene

The effect of increasing lamellar thickness in bulk polychlorotrifluoroethylene (PCTFE) by various methods (annealing and increasing time and temperature of crystallisation) on the parameters of the and relaxations has been studied.For the a relaxation consistent increase in temperature of maximum loss and activation energy with increasing lamellar thickness confirms the strong suggestion of earlier work [6] on oriented specimens and shows that the relaxation arises mainly in the interior of the lamellae with little contribution from the chain folds.No systematic changes in these parameters with increasing lamellar thickness was found for the relaxation and this is explained by the presence of _c and _a components in crystalline and amorphous regions respectively. The component _a (as well as the _c one) shows a marked anisotropy in magnitude with respect to the draw direction in oriented specimens relative to the direction of the applied electric field. This implies a lining up of the chain molecules parallel to one another in amorphous regions.The non-existence of analogous short chain compounds and experimental scatter prevented detailed checking of the consistency of the data for the relaxation with the theory for the _c-C _c model [5].     

Dielectric relaxation of relaxor ferroelectric P(VDF-TrFE-CFE) terpolymer over broad frequency range

Dielectric properties of a relaxor ferroelectric polymer, poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) [P(VDF-TrFE-CFE)] terpolymer, were investigated over a broad range of frequency (from 0.1 kHz to 1 GHz) and a broad range of temperature (-20degC to 76degC). Time-temperature superposition was used to extrapolate the dielectric constant to high frequencies (~1 GHz) from low frequency data (<1 MHz). The consistency between the directly measured and the extrapolated data indicate that the time-temperature superposition can be applied at temperature ranging from the glass transition to the broad ferroelectric-paraelectric transition peak of relaxor, indicating that the glass transition is still the dominating relaxation process at room temperature for the ferroelectric relaxor. Compared with the dielectric properties of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer, the terpolymer shows a higher dielectric constant even at 1 GHz, which is considered to originate from the random defects modification converting the long-chain polar-molecular conformation to short-range molecular microstructures and enhancing the molecular motions in both polar and nonpolar nanodomains.     

Tunneling with very long relaxation times in glasses,organic materials,and Nb-Ti-H (D)

The long-time (t=10–200 h) heat release from glasses, from organic materials, and from Nb-Ti-H (D) was measured at 30T70 mK. For Suprasil W glass, Dimethyl-Siloxan, Stycast 1266, Stycast 2850 FT, Vespel, and for Nb-Ti-H (D) with various Ti and D concentrations, we found . Typical values are = 0.05 nW/g for the organic materials and for Nb-Ti-H (D) and = 0.005 nW/g for the glass att=100 h after cooldown from room temperature. For charging temperaturesT _i <5 K, we find the predicted dependence (investigated for Suprasil W glass and for Nb-Ti-D). The observed time and temperature dependences agree with predictions of the conventional two-level tunneling model for amorphous materials even at these very long times. No heat release was observed for Teflon, graphite, and Al₂O₃.     

Dielectric relaxation of vegetable-based polyurethane

Vegetable-based polyurethane (PU) was prepared in the thin film form by spin coating. This polymer is synthesised from castor oil, which can be extracted from the seeds of a native plant in Brazil called mamona. This polymer is biocompatible and is being used as material for artificial bone. The PU was characterised by dielectric spectroscopy in a wide range of frequency (10^–5 Hz to 10⁵ Hz) and by thermally stimulated discharge current (TSDC) measurements. The glass transition temperature (T _g = 39°C) was determined and using the initial rise method the activation energy was found to be 1.58 eV.     

Generalized thermo-viscoelastic plane waves with two relaxation times

The model of the two-dimensional generalized thermo-viscoelasticity with two relaxation times (Green and Lindsay theory) is established. The normal mode analysis is used to obtain the exact expressions for the temperature distribution, thermal stresses and the displacement components. The resulting formulation is applied to three different concrete problems. The first deals with a thick plate subjected to a time-dependent heat source on each face. The second concerns to the case of a heated punch moving across the surface of a semi-infinite thermo-viscoelastic half-space subjected to appropriate boundary conditions and the third problem deals with a plate with thermo-isolated surfaces subjected to a time-dependent compression. Numerical results are given and illustrated for each problem. Comparisons are made with the results predicted by the coupled theory.     

Dielectric relaxation of NdMnO3 nanoparticles

The neodymium manganate (NdMnO₃) nanoparticles are synthesized by the sol–gel process. The phase formation and particle size of the sample are determined by X-ray diffraction analysis and transmission electron microscopy. The band gap of the material is obtained by UV–visible absorption spectroscopy using Tauc relation. Dielectric properties of the sample have been investigated in the frequency range from 42 Hz to 1 MHz and in the temperature range from 303 K to 573 K. The dielectric relaxation peaks are observed in the frequency dependent dielectric loss spectra. The Cole–Cole model is used to explain the dielectric relaxation mechanism of the material. The complex impedance plane plot confirms the existence of both the grain and grain-boundary contribution to the relaxation. The temperature dependence of both grain and grain-boundary resistances follow the Arrhenius law with the activation energy of 0.427 and 0.431 eV respectively. The frequency-dependent conductivity spectra follow the power law.     

不同纳米填料填充聚偏氟乙烯基复合材料的介电松弛行为

以绝缘性BaTiO₃、半导性SiC和具有导电性的纳米石墨片（GNP）为填料,采用溶液法制备了聚偏氟乙烯（PVDF）基复合材料,着重研究了具有不同性质的填料对PVDF基复合材料介电行为的影响。结果表明：随着不同纳米填料用量的增加,PVDF基复合材料的介电常数都有增大的趋势,尤其是添加半导性SiC和导电性GNP对PVDF基复合材料介电性能改善的效果最明显,其影响程度可由ε'-ε"曲线获知;当SiC和GNP含量高于渗流阈值后,其高频松弛峰趋于平直;采用介电模量的形式可以很好地描述材料在频率依赖下的松弛行为,其松弛激活能随着填料用量的增大而降低,表明填料的加入促进了PVDF基复合材料的极化。     

The coupled magnetothermoelastic problem in elastic half-space with two relaxation times

This paper deals with the magnetothermoelastic disturbances in a perfectly electrically conducting elastic half-space, in contact with a vacuum, due to a thermal shock applied on the plane boundary of the half-space. The theory of thermoelasticity proposed by Green and Lindsay is used to account for the interaction between the elastic and thermal fields. The solution achieved is in analytical form, which reduces to that of Kaliski and Nowacki when the coupling between the temperature and strain fields and the relaxation time of Green and Lindsay theory are neglected.     

Guided thermoelastic waves in functionally graded plates with two relaxation times

Functionally graded material (FGM) is a promising heat insulation material. Wave propagation in FGM structures has received much attention for the purpose of non-destructive testing and evaluation. Few literatures dealt with the thermoelastic wave in FGM structures although the thermal effect would cause attenuations of elastic waves. In this paper, guided thermoelastic waves in FGM plates subjected to stress-free, isothermal boundary conditions are investigated in the context of the Green–Lindsay (GL) generalized thermoelastic theories (with two relaxation times). Coupled wave equations and heat conduction equation are solved by the Legendre polynomial approach. Dispersion curves for a pure elastic graded plate are calculated to make a comparison with the published data. For the thermoelastic graded plate, dispersion curves of thermal modes and elastic modes are illustrated simultaneously. Attenuation curves for graded plates with different relaxation times are compared. The influences of different material gradient shapes are discussed. Two homogeneous thermoelastic plates with different volume fractions are obtained to show their differences from graded plates. Finally, thermoelastic wave dispersion curves for a homogeneous plate and a graded plate are calculated in the context of the classical coupled thermoelastic theory (CT) to show its differences and similarities to the generalized theory.     

Stress relaxation in ferroelectric materials

Stress relaxation takes place in BaTiO₃ type ferroelectric materials due to the motion of non 180° domain boundaries, which are the twin boundaries. The nature of stress relaxation taking place due to plastic deformation in crystalline solids and that due to twin boundaries in ferro electric materials is discussed. The usefulness of the stress relaxation data for the study of domain wall motions in this type of ferroelectrics is pointed out.