Giant dielectric constant in titania nanoparticles embedded in conducting polymer matrix

Complex impedance and dielectric permittivity of titania-polypyrrole nanocomposites have been investigated as a function of frequency and temperature at different compositions. A very large dielectric constant of about 13,000 at room temperature has been observed. The colossal dielectric constant is mainly dominated by interfacial polarization due to Maxwell-Wagner relaxation effect. Two completely separate groups of dielectric relaxation have been observed. The low frequency dielectric relaxation arises from surface defect states of titania nanoparticles. The broad peak at high frequency region is attributed to Maxwell-Wagner type polarization originating from the inhomogeneous property of nanocomposite. An abrupt change in grain boundary conductivity and dielectric relaxation associated with titania was observed at around 150 K. Anomalous behavior in conductivity and dielectric relaxation is qualitatively explained by band tail structure of titania nanoparticle.     

Compensation effect in styrene-acrylonitrile copolymer and acrylonitrile-butadiene-styrene terpolymer as explored by thermally stimulated depolarization measurements

Thermally stimulated depolarization currents of styrene-acrylonitrile and acrylonitrile-butadiene-styrene copolymers have been investigated in the high-temperature range. A peak associated with the glass transition temperature was observed which has the characteristic of a dipolar relaxation process. Parameters such as activation energy and relaxation time associated with the dipolar relaxation process have been calculated. The compensation effect is observed to be valid and the value of the compensation temperature differs slightly from one compound to other. The linear relationship between the activation energy and the logarithm of the preexponential factor of the relaxation time is discussed in terms of the compensation temperature and the chain environment.     

Thermal-softening properties of water-swollen wood: The relaxation process due to water soluble polysaccharides

To clarify the thermal-softening properties of water-swollen wood, the viscoelastic properties of wood specimens conditioned to various moisture contents and of water solutions of amorphous cellulose, hemicellulose and milled wood lignin, have been studied in the temperature range from –150 to 0°C at frequencies ranging between 0.5 and 10 Hz. A relaxation process around –40°C was observed in wood specimen with high moisture content. It was observed in wood specimens with lower moisture content at higher temperature. The value of apparent activation energy for the relaxation process was 50 kJ/mol. On the other hand, a similar relaxation process was observed in water solutions of amorphous cellulose and hemicellulose around –40°C, but it was not observed in water or water solution of milled wood lignin. The values of apparent activation energy for the relaxation process were 230–545 kJ/mol. Therefore, we considered that the relaxation process was due to the water-swollen polysaccharides and affected by the situations of water molecules.     

High-temperature relaxations

In 1947, Kê observed a large relaxation peak around 0.46T_m in polycrystalline aluminum. This peak being absent in single crystal, Kê concluded that this relaxation effect was due to grain boundary sliding (GBS). In the 1970s, Woirgard, in the same temperature range, observed a peak in single crystal. Later on Rivière, Esnouf, and No systematically studied different relaxation effects in single and polycrystals and they concluded that the Kê peaks, as well as the other relaxations observed in this temperature range, are due to dislocation motion, the mechanism being probably the climb of jogs. More recently, in Ni–Cr, Cao clearly observed the presence of a large relaxation peak in polycrystals, which is absent in single crystal. He showed that the peak is due to grain boundary sliding, the mechanism of which is the climb of dislocations in the grain boundaries. Does a relaxation effect due to grain boundary sliding exist in metals? The question is still open and will be discussed in this paper considering recent results of the Chinese, French, Spanish and Swiss school.     

Nuclear spin-lattice relaxation of isotopic impurities in solid hydrogen

We report calculations of the nuclear spin-lattice relaxation of isotopic impurities (HD) in solid hydrogen. The temperature-independent relaxation observed at low temperatures is attributed to the quantum tunneling of the impurities, and the pronounced minimum observed at intermediate temperatures is associated with scattering from thermally excited vacancies.     

GRP(CSM)三点弯曲梁松弛实验

本文对短玻璃纤维毡增强聚酯玻璃钢的三点弯曲梁试件的松弛行为进行了实验研究。并通过有限元分析与实验结果的比较,建立了一个初步的“变剪切模量G₁₃(t)”模型。      

固态高聚物的应力松弛行为

研究了高密度聚乙烯（HDPE）和聚丙烯（PP）变形过程的应变率敏感性和应力松弛行为，实验发现应力松弛行为与应变历史有关，加载过程中的应力松弛表现为应力随时间的增长而减小，卸载过程中应力松驰则表现出不同的现象，在卸载初始阶段，应力逐渐减小并趋于其平衡值，当卸载程度较大，其应力松弛表现为应力随时间逐渐增大并趋向平衡值。     

Doping Dependent Quasiparticle Relaxation Dynamics in?SmFeAsO1?x F x Single Crystals: Comparison of?Spin-Density Wave and Superconducting States

We use optical femtosecond spectroscopy to investigate the quasiparticle relaxation and low-energy electronic structure in undoped and near-optimally doped SmFeAsO_1?x F _x iron-pnictide superconductor single crystals (SC). In the undoped SC, a single relaxation process with a divergent-like relaxation time at the spin density wave (SDW) transition is observed. From the relaxation time in the normal state, significantly above T _SDW, the second moment of the Eliashberg function is determined to be ?????? ²????135?meV². Below T _SDW, the temperature dependence of the photoexcited reflectivity transients indicates the appearance of a bottleneck due to opening of a SDW gap with a BCS-like temperature dependence and the characteristic magnitude, 2??_SDW/k _B T _SDW=7??3, at?4.2?K. In the superconducting SC, multiple relaxation processes are present. In addition to the relaxation processes observed in the normal state, a distinct superconducting state relaxation component is observed consistent with the presence of BCS T-dependent gaps.     

Stress relaxation characteristics of warm frozen clay under triaxial conditions

Stress relaxation is one of the main concerns in geotechnical engineering, especially in permafrost regions with warm frozen soil. This paper investigated the mechanical behaviors of a warm frozen soil during relaxation process. A series of triaxial relaxation tests were carried out on the warm frozen soil samples under the temperature from − 1.0 °C to − 0.2 °C and confining pressure of 0.5 MPa to 3.0 MPa. Three stages were observed during the whole stress relaxation process, i.e. instantaneous relaxation, intense relaxation and slow relaxation. Relationship between deviatoric stress and time was described by a hyperbolic relationship. With relatively small pre-strain (3% and 4%), volumetric changes during stress relaxation contracted initially followed by expansion, but contraction dominates the relaxation process with the magnitude of pre-strain over 6%. Characteristic parameters such as the instantaneous stress relaxation, the critical relaxation duration and relaxation rate are defined and analyzed. It is found that the parameters are all dependent on soil temperature, confining pressure and magnitude of pre-strain.     

Lattice Relaxation in Solid 4He: Effect on Dynamics of 3He Impurities

We present a new simplified derivation of the effect of lattice relaxation that accompanies the quantum tunneling of ³He impurities in solid ⁴He on the nuclear spin-lattice relaxation of the ³He impurities for very low impurity concentrations. As a result of the larger zero point motion of the ³He impurity compared to the ⁴He atoms, a significant lattice distortion accompanies the impurity as it moves through the lattice and the dynamics of the impurity depends on both the interaction energy between two ³He atoms and on the relaxation of the lattice for the tunneling impurity. Using a phenomenological model for the lattice relaxation we compare the nuclear spin-lattice relaxation rates observed at low temperatures with the dependence on temperature expected for a ⁴He lattice relaxation comparable to that observed by Beamish et al. (Phys. Rev. Lett. 96:195304, 2006).     

Brownian relaxation of interacting magnetic nanoparticles in a colloid subjected to a pulsatile magnetic field

We have investigated and modeled the effect of interaction among magnetic particles and the magnitude and duration of external applied magnetic field on Brownian relaxation in a colloidal suspension. In the case of interacting magnetic particles, Brownian relaxation depends on the interparticle dipole-dipole interaction, which slows down the overall Brownian relaxation process of magnetic particles in the colloidal suspension. The individual magnetic particle experiences torque when a pulsatile magnetic field is applied. The torque due to the external field randomizes the particle rotation similar to that of the thermal energy. A faster Brownian relaxation is observed when individual magnetic particles are magnetized for a short duration. Magnetizing the magnetic particle for a longer duration suppress the rotational motion hence the effect of torque on Brownian relaxation.     

Internal magnetic relaxation in levitating superconductors

The effect of stopping the levitation force relaxation was observed upon reversal of the superconductor magnetization. A model of the internal magnetic relaxation in levitating superconductors is proposed, according to which both the magnetic moment and the force acting upon the sample are constant.     

Relaxation behavior of polymers probed by positron annihilation lifetime spectroscopy

Relaxation behaviors of polyethylene, polypropylene, and polycarbonate have been studied by positron annihilation lifetime spectroscopy (PALS). In PALS positron sources made of radioisotopes are used to inject positrons into polymer as a micro probe. The injected positron probes can induce radiation effect, which plays an important role in detecting the polymer relaxation behavior through electrons trapped in shallow potentials at low temperature. Monitoring the intensity (I₃) of ortho-positronium (o-Ps), transitions of γ and δ relaxation can be measured by PALS as a secondary effect. In this experiment, the change of I₃ below Tg is connected with the number of the trapped electrons, which can be excited from the shallow potential by the thermal motion of polymer structures and visible light irradiation. In the PALS measurements of non-irradiated PP samples, relaxation of methyl groups was observed as low as 50 K, which can be assigned as the δ relaxation. Relaxations of β and γ were also observed for the non-irradiated PP samples between 100–370 K. However for the 3 MGy γ-ray irradiated PP samples, only β relaxation was observed because the large radiation dose caused a large number of scissions of –CH₃ groups from main chains and the characteristics changed. For the irradiated samples, radiation hardening was observed. Electronic Publication     

Retarded magnetic relaxation in levitated superconductors

The phenomenon of the magnetic relaxation slowing down in levitated superconductors has been observed. It is suggested that the decrease in the magnetic relaxation rate is related to a magnetic bias feedback in the sample, which restores a nonequilibrium magnetic structure broken by the magnetic flux creep.     

The molecular relaxation mechanisms in cork as studied by thermally stimulated discharge currents

The dielectric relaxation mechanisms present in cork have been investigated in the temperature range -100 to 100 °C using the technique of thermally stimulated discharge currents. A relaxation mechanism was detected which showed a compensation behaviour as observed for the -relaxation (or glass transition relaxation) of synthetic polymers and which lead us to attribute to cork a glass transition-like temperature of 18 °C. One lower temperature mechanism was also observed, with low activation enthalpy and entropy, which is presumably originated by local motions (internal rotations) of polar molecular groups. An upper T _g relaxation of higher intensity was also detected which was attributed to large-scale non-cooperative motions of polymeric segments.     

电子辐照的P(VDF/TrFE)共聚物的铁电弛豫异常现象

本文研究了聚偏氟乙烯和三氟乙烯的铁电共聚物P(VDF/TrFE)的铁电弛豫现象,并讨论了铁电弛豫异常想象.比较了VDF不同含量的共聚物在辐照前后铁电性能的改变,电子辐照大大改变了聚合物样品的性能,使VDF含量52%和70%的样品出现了明显的铁电弛豫现象,但发现在VDF含量为80%的样品中存在一种铁电弛豫异常现象:铁电居里峰随着频率的升高而向低温端移动.研究样品的二氟乙烯(VDF)的摩尔含量在52%～80%之间.     

Stress relaxation of high impact polystyrene

The stress relaxation behaviour of high impact polystyrene has been correlated with the microstructural changes observed in tensile tests. The inhomogeneity of plastic deformation, manifested as stress whitening, has been measured using microhardness tests. This method has been found to be sensitive to the amount of crazing in the material. The stress relaxation behaviour changed at the onset of crazing, but did not change appreciably as the volume fraction of crazes increased. An analysis of the relaxation in terms of a site population model based on White's approach suggests the macroscopic stress relaxation is related to the crazes in the boundary regions between the stress whitened and unwhitened material.     

Quantum Tunneling of the Magnetization In Mn12-Ac

We present new experiments on the zero-field magnetic relaxation in a single crystal of Mn12-acetate. This study shows the important role played by the interactions between the spins and their environment. It suggests the existence of two relaxation regimes. Above 2.0 K, the relaxation time follows the Arrhenius law. The time decay is not strictly exponential due to dipolar interactions. At lower temperatures, the relaxation time depends weakly on the temperature. A square root decay is observed, as recently predicted by a theoretical model in which the tunneling is assisted by dipolar and hyperfine interactions.     

Effects of humidity changes on damping and stress relaxation in wood

Mechanical damping (tan ) and stress relaxation on pine veneer samples under stepwise humidity changes were investigated. The loss factor shows a transient peak every time the relative humidity (RH) of the surrounding atmosphere is changed. The effect appears to be associated with the diffusion of water molecules into or out of the material, the peaks being observed both when the sample is humidified (5%–85% RH) or subjected to drying. The results are supplemented by stress relaxation data obtained on similar specimens. Also in this case a higher relaxation rate is observed when the RH level around the sample is changed, the total stress decrease after several cycles being significantly larger than the corresponding value observed after the same period at the higher RH level.     

Vibrational Relaxation in Several Derivatives of Benzene

Acoustical spectroscopy at frequencies up to 10 GHz gives the possibility of the investigation of liquid substances, where the relaxation process observed is caused by energy transfer between translational and vibrational degrees of freedom. The compounds presented in this article belong to this group of liquids. The acoustic investigations in the group of benzene derivatives, particularly research of the dependencies of acoustic parameters and the structure of organic liquids, demonstrated some interesting regularities in the group of these compounds in gas and liquid states. In this article, the results of research on five cyclic liquids: bromo-, chloro-, fluoro-, iodo-, and nitrobenzene as well as toluene and aniline are discussed and compared to benzene. The acoustic relaxation observed in all these compounds was found to result from Kneser’s processes (vibrational relaxation). Based on investigations reported in this article, as well as by other authors, and taking into account experimental and literature data concerning a great number of compounds, one can draw a conclusion that almost all acoustic relaxation (Kneser-type) processes in liquids can be described using a single relaxation time. It also seems that all vibrational degrees of freedom of the molecule take part in this process. It is known that the appearance of differences in transition probabilities could be caused by additional attraction in interactions of molecules having dipole moments. Halogen derivatives have higher values of dipole moments than benzene. This difference could be responsible for the difference of transition probabilities and changes in the relaxation times. However, benzene derivatives with amino, nitro, and methyl groups and halides show the other type of relaxation.