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.     

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 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.     

Dielectric relaxation in sulfur at very high pressures

The ac electrical properties of sulfur were measured at pressures from 20 to 50 GPa. The loss tangent vs. frequency data indicate dielectric anomalies in the pressure range of structural transformations: tanS rises by about one order of magnitude as the frequency decreases from 0.8 to 0.4 kHz and by a factor of more than 2 in the range 3–10 kHz.     

Dielectric and relaxation behaviour studies in acrylonitrile-methylacrylate copolymer

Poly(acrylonitrile-methylacrylate) copolymer is a semicrystalline material, as defined by X-ray diffraction, and is thermally stable up to 425 K as revealed by infrared investigations. The study of dielectric properties obtained from this copolymer allows us to follow the relaxation processes and showed the difference between three processes; -, - and -relaxations. The first two processes occur in the two phases, amorphous and crystalline, and the third process is associated with the glass transition temperature. The origins of these processes were attributed to local motion of the polymer backbone segments, to a dipole orientation of the chain side groups and to an ionic space-charge relaxation, respectively.     

Dielectric relaxation behavior of CdS nanoparticles and nanowires

The dielectric relaxation and scaling behavior of CdS nanoparticles and nanowires were investigated in the frequency range 10²–10⁶ Hz and in the temperature range 373–573 K by complex impedance spectroscopy and electric modulus spectroscopy. Studies on the complex permittivity revealed that the dielectric relaxation in CdS nanostructures deviates from Debye like behavior. A detailed study on the grain and grain boundary charge transport was carried out. The charge carrier transport in CdS nanostructures was identified to be hopping of polarons. From the combined analysis of the variation of imaginary part of electric modulus and complex impedance with frequency, it was found that at high temperatures localized conduction is dominant in CdS nanoparticles where as the long range hopping process is dominant with nanowires. It was also found that the scaling behavior of CdS nanoparticles varied considerably from that reported earlier.     

Dielectric losses in materials with limited range of relaxation times

Features of the spectra of dielectric materials with the range of relaxation times limited from below have been studied. It is established that the dielectric losses (and conductivity) exhibit relaxation at an average frequency that is significantly (up to several orders of magnitude) greater than the permittivity relaxation frequency. These effects are characteristic of both homogeneous materials with non-Debye spectra and inhomogeneous materials of the statistical mixture type near the percolation threshold. A giant enhancement of the permittivity and conductivity in these statistical mixtures takes place at various concentrations of components corresponding to the dielectric-conductor and conductor-dielectric phase transitions.     

Dielectric relaxation analysis for 8 mol% YSZ single crystal

In order to verify an effectiveness of a dielectric relaxation in the investigation of oxygen ionic conduction mechanism, dielectric and DC, AC measurements have been carried out for single crystalline 8 mol% YSZ. Total activation energies in the dielectric relaxation and the electrical conduction was almost same below T^c. Activation energies change at T^c. Disagreement of both energies above T^c have been improved by the modification of the dielectric relaxation analysis.     

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 of epoxy polymer in composites cured in an electric field

Results of a study of the effect of various fillers (discrete aluminum-oxide fibers and powdered barium titanate) on the molecular mobility of the epoxy polymer in composites cured under the action of an electric field are given. Institute of General and Inorganic Chemistry, Academy of Sciences of Belarus, Minsk, Belarus. Translated from Inzhenerno-Fisicheskii Zhurnal, Vol. 69, No. 4, pp. 589–597, July–August, 1996.     

等静压力诱导的PMN-PT单晶介电弛豫特性研究

研究了不同等静压力下的PMN-PT单晶的介电常数(εr)与温度(T)的关系,以及常温下介电常数(εr)及损耗(tanδ)与压力(p)之间的关系.发现了PMN-PT单晶在压力作用下出现明显的介电频率弥散现象.当等静压力增大到一定压力时,PMN-PT单晶的介电常数值迅速降低,介电频率弥散更加显著,且出现弛豫特性,介电损耗的弛豫特征更加明显.随着频率的增加,介电损耗峰对应的峰值压力向高压方向移动.其根本原因是压力诱导的介电-压力弛豫特性导致的.     

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。