Insulated electrodes for eliminating conductivity in dielectric relaxation experiments

In a variety of samples, conductivity is a limiting factor regarding the resolution of dielectric loss peaks of interest. One approach to eliminating the signal that originates from conductivity is the use of insulating layers on one or both electrodes. For the typical case, it is shown that insulation layers add errors rather than improving the situation.     

Synthesis and dielectric characteristics of La0.5Bi0.5MnO3 ceramics

La_0.5Bi_0.5MnO₃ ceramics with a single phase were prepared by a solid-state reaction method, and their dielectric properties were characterized. Two dielectric relaxations with a giant dielectric constant were identified in the temperature range from 125 to 350 K. The electron hopping between Mn³⁺ and Mn⁴⁺ was found to be the origin of the dielectric relaxation at low temperatures (125–200 K) with an activation energy of 0.18 eV. The high temperature (200–350 K) dielectric relaxation can be attributed to the conduction.     

Effect of nickel nanofillers on the dielectric and magnetic properties of composites based on rubber in the X-band

Nickel–rubber nanocomposites were synthesized by incorporating ferromagnetic nickel nanoparticles in a natural rubber as well as neoprene rubber matrix. Complex dielectric permittivity and magnetic permeability of these composites were evaluated in the X-band microwave frequencies at room temperature using cavity perturbation technique. The dielectric loss in natural rubber is smaller compared to neoprene rubber. A steady increase in the dielectric permittivity is observed with increase in the content of nickel in both the composites. The magnetic permeability exhibits a steady decrease with increase in frequency and magnetic loss shows a relaxation at 8 GHz. The suitability of these composites as microwave absorbers is modeled based on the reflection loss which is dependant on the real and imaginary components of the complex dielectric permittivity and magnetic permeability.     

Segmental relaxation in semicrystalline polymers: A mean-field model for the distribution of relaxation times in confined regimes

The effect of confinement in the segmental relaxation of polymers is considered. On the basis of a thermodynamic model we discuss the emerging relevance of the fast degrees of freedom in stimulating the much slower segmental relaxation, as an effect of the constraints at the walls of the amorphous regions. In the case that confinement is due to the presence of crystalline domains, a quasi-Poissonian distribution of local constraining conditions is derived as a result of thermodynamic equilibrium. This implies that the average free-energy barrier F for conformational rearrangement is of the same order of the dispersion of the barrier heights, ( F) , around F . As an example, we apply the results to the analysis of the -relaxation as observed by dielectric broad-band spectroscopy in semicrystalline poly(ethylene terephthalate) cold-crystallized from either an isotropic or an oriented glass. It is found that in the latter case the regions of cooperative rearrangement are significantly larger than in the former.     

Dielectric and conductivity relaxation in mixtures of glycerol with LiCl

We report a thorough dielectric characterization of the α relaxation of glass-forming glycerol with varying additions of LiCl. Nine salt concentrations from 0.1 to 20mol% are investigated in a frequency range of 20Hz-3GHz and analyzed in the dielectric loss and modulus representation. Information on the dc conductivity, the dielectric relaxation time (from the loss) and the conductivity relaxation time (from the modulus) is provided. Overall, with increasing ion concentration, a transition from reorientationally to translationally dominated behavior is observed and the translational ion dynamics and the dipolar reorientational dynamics become successively coupled. This gives rise to the prospect that, by adding ions to dipolar glass formers, dielectric spectroscopy may directly couple to the translational degrees of freedom determining the glass transition, even in frequency regimes where usually strong decoupling is observed.     

Dielectric response of water confined in metal–organic frameworks

Dielectric response of water confined in metal–organic frameworks was investigated in broad temperature range from 140 to 410 K and from 20 Hz to 1 MHz using a capacitance bridge. Several dispersion regions of characteristic shape were found, caused by freezing–melting of adsorbed water molecules, which disappear after a prolonged heating at 410 K. Temperature dependencies of relaxation time of confined water molecules were obtained and are compared to those of water confined in MCM-41 mesoporous molecular sieves.     

Electro-optic and dielectric properties of Hafnium-doped congruent lithium niobate crystals

We report the optical and dielectric properties in hafnium (Hf)-doped lithium niobate (LN) crystals. We investigated samples of congruent composition with various doping concentration varying from 0 to 8 mol%. The clamped and unclamped values of the electro-optic coefficient r ₂₂ of Hf-doped LN and the corresponding dielectric permittivity as well, have been experimentally determined and compared with the results obtained in undoped congruent LN crystals. We show that the electro-optic and dielectric properties are only slightly affected by the introduction of hafnium ions, and therefore Hf-doped LN has the advantage of low photorefractive damage compared with the undoped congruent LN.     

Microstructures and impedance studies of Bi<Subscript>3.15</Subscript>Nd<Subscript>0.85</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> thin films

Microstructures and impedance characteristics of chemical-solution-derived Bi_3.15Nd_0.85Ti₃O₁₂ thin films were studied as functions of temperature. A dielectric anomaly was found at around 450°C, corresponding to the paraelectric to ferroelectric transition. Via complex impedance studies, grain and grain boundary contributions to the impedance were separated. The resistance of grain and grain boundaries is found governed by the same kind of space charge with an activation energy around 1.1 eV, close to that of oxygen vacancies in perovskite ferroelectrics. The low temperature ac conductance of BNdT thin films shows a frequency dispersion, which can also be ascribed to space charges mainly due to oxygen vacancies. The results were compared with SrBi₂Ta₂O₉ in terms of oxygen vacancy conductivity.     

Solid solutions of bismuth-based Aurivillius oxides: structural and dielectric characterization

Bismuth layered perovskite structures show interesting physical properties varying as a function of external parameters (temperature, frequency, electric and magnetic fields). When a magnetic ion is incorporated in some of these materials, some of the structures show simultaneous ferroelectricity and ferro/antiferromagnetism. Thus, they exhibit magnetoelectric properties under the influence of an external magnetic field. This paper compares the structural (XRD and SEM) and electrical properties of two eight-layered Aurivillius oxides.     

A path integral approach to effective non-linear medium

In this article, we propose a new method to compute the effective properties of non-linear disordered media. We use the fact that the effective constants can be defined through the minimum of an energy functional. We express this minimum in terms of a path integral allowing us to use many-body techniques. We obtain the perturbation expansion of the effective constants to second order in disorder, for any kind of non-linearity. We apply our method to the case of strong non-linearities (i.e. , where is fluctuating from point to point), and to the case of weak non-linearity (i.e. where and fluctuate from point to point). Our results are in agreement with previous ones, and could be easily extended to other types of non-linear problems in disordered systems. Received: 13 May 1998 / Accepted: 27 July 1998     

Electric-field dependence of dielectric properties of sol-gel derived Ba(Zr0.2Ti0.8)O3 ceramics

The effect of a dc bias field on the diffuse phase transition and nonlinear dielectric properties of sol-gel derived Ba(Zr_0.2Ti_0.8)O₃ (BZT) ceramics are investigated. Diffuse phase transitions were observed in BZT ceramics and the Curie–Weiss exponent (CWE) was γ∼2.0. The dielectric constant versus temperature characteristics and the γ in the modified Curie–Weiss law, ε ⁻¹=ε _m ⁻¹[1+(T−T _m ) ^γ /C₁](1≤γ≤2), as a function of the dc bias field was obtained for BZT ceramics. The results indicated that γ is a function of dc bias field, and the γ value decreased from 2.04 to 1.73 with dc bias field increasing from 0 to 20 kV/cm. The dielectric constant decreases with increasing dc bias field, indicating a field-induced phase transition. The dc bias field has a strong effect on the position of the dielectric peak and affects the magnitude of the dielectric properties over a rather wide temperature range. The peak temperature of the dielectric loss does not coincide with the dielectric peak and an obvious minimum value for the dielectric loss at the temperature of the dielectric peaks is observed. At room temperature, 300 K, the high tunability (K=80%), the low loss tangent (≈0.01) and the large FOM (74), clearly imply that these ceramics are promising materials for tunable capacitor-device applications.     

The\bar pp \to e^ + e^ - reaction at rest and the hierarchy of the annihilation ranges

A new procedure is devised to evaluate the proton form-factor at the ¯pp threshold from experimental data on ¯ppe ⁺ e ^– annihilation at rest. Comparison of the ¯ppe ⁺ e ^– and¯pp ^+– processes reveals that annihilation ranges for various channels are ranked in line with quark dynamics.This work has been funded in part by the German Federal Minister for Research and Technology (BMFT) under the contract number 06HD756     

Analysis of proton elastic scattering from16O and40Ca at 800 MeV within the α-particle model

Elastic scattering of proton from¹⁶O and⁴⁰Ca are analyzed within the -particle model. The results are in good agreement with the experimental data. The role displayed by the phase factor ofp- scattering amplitude is investigated.     

On the competiting role of mean-field and residual interactions in flow observables

Theoretical analyses of heavy-ion reactions are performed in the framework of the semi-classical Landau-Vlasov approach. The incident energies are investigated in the range from intermediate to low energy regimes, where transverse collective motion has been experimentally evidenced. The influence of the equation of state (E.O.S.) parameters on various collective observables is studied in relation with the action of the residual interactions. From the sensitivity to both aspects, and taking into account the experimental biases limitations, our investigation indicates that E.O.S. signatures should be more expected at energies below 100 MeV per nucleon.     

On the chaining dynamics of inclusions in SmC* free standing films

A simplified model of an inclusion represented by (+1)-wedge disclination and an accompanying hyperbolic defect ((−1)-wedge disclination) in smectic C* free standing films is used to describe the early stage of the ordering process of inclusions into chains. The elastic interaction between inclusions and their associated hyperbolic defects is used to discuss the dynamics observed experimentally during the inclusion chaining when inclusions are at distances much larger than their radii. This work was also supported by Grant No. 202/02/0840 of the Grant Agency of the Czech Republic and by the research project AV0Z1-010-914.     

Possible odderon effects in inclusive hadron production

We consider the possible contribution of odderon (reggeon with α _Odd(0)∼1 and negative signature) exchange to the differences in the inclusive spectra of particle and antiparticle in the central region. The energies and/or accuracy of the currently available experimental data do not allow for a clear conclusion on the presence of an odderon component, but the upcoming LHC data should finally elucidate the question of the existence of the odderon.     

The effects of chemical etching of porous silicon on Raman spectra

We have investigated the effects of chemical etching on Raman spectra of porous silicon. The as-anodized porous silicon consisted mainly of crystalline silicon, as indicated by the Raman spectra. The background in the spectrum was strong, indicating that the porous silicon surface was rough due to the presence of pores. When chemical etching was performed five times, the Raman spectrum revealed the presence of spherically shaped nanocrystalline silicon whose diameter was around 3.5 nm. Further chemical etching, however, extinguished the nanocrystallites, in addition to smoothing the surface morphology.     

Comparison of gold and silver dispersion laws suitable for FDTD simulations

We report optical planar waveguide formation and modal characterization in β-BaB₂O₄ crystals by Cu²⁺-ion implantation at an energy of 3.0 MeV and doses of ∼ 10¹⁴ ions/cm². The prism-coupling method was used to investigate the dark-mode property at wavelengths of 633 nm and 1539 nm. The refractive-index profile of the waveguide was reconstructed by an effective refractive index method. The modal analysis indicates that the fields of TM modes can be well restricted in the guiding region, which means the formation of a non-leaky waveguide in the crystal. The results show that the β-BaB₂O₄ waveguides may be used in the application of high efficiency frequency conversion. PACS 61.80.Jh; 42.70.Mp; 42.65.Wi