Dielectric Spectroscopy of Ferronematics Based on 6CHBT Liquid Crystal

In our study, the dielectric behaviour of the rod-like liquid crystal (6CHBT) doped with magnetic nanoparticles of spherical shape was investigated by means of dielectric spectroscopy in the frequency range from 20 Hz to 2 MHz. A low frequency dielectric dispersion in the nematic and isotropic phases of the pure liquid crystal (LC) has been assigned to the space charge polarization. After doping the host LC with the magnetic nano particles, a nearly Debye-like relaxation process was observed with the temperature dependence obeying the Arrhenius law. Considering a possible electric double layer formation on the particle surfaces, the detected relaxation process in the doped LC can be associated with the electric double layer polarization. The experimental results point out that in the measured frequency range the space charge and interfacial effects constitute the main dielectric response. Any anchoring effects were not observed and are therefore expected to appear in higher frequencies.     

Multilayered model in optics and quantum mechanics

Three types of dispersion equations are analyzed that describe the eigenvalues of the effective refractive index of a multilayer plane optical waveguide and the energy eigenvalues of a quantum particle placed in a piecewise constant potential field. The first equation (D1) is derived by setting to zero the determinant of the system of linear equations produced by matching the solutions in the layers. The second equation (D2) is obtained using the well-known method of characteristic matrices. The third equation has been obtained in the general case by the author and is known as a multilayer equation. Simple relations between the three equations are established. It is shown that the set of roots of D2 exactly coincides with the set of eigenvalues of the multilayer problem, while the roots of D1 and the multilayer equation contain those equal to the refractive index in the optical case (or to the potential in the quantum case) in internal layers of the system, which may be superfluous. Examples are presented.     

β-MnO2纳米棒的电磁特性和多极化驰豫损耗

采用低温水热合成法,以(NH₄)₂S₂O₈作氧化剂,以MnSO₄·H₂O为锰源,在高压反应釜中加热120 ℃并保压48 h制备了β-MnO₂纳米棒。利XRD,SEM,TEM和振动样品磁强计(VSM)对产物进行表征,并就产物的电磁特性和多极化损耗机制进行研究。结果表明,纳米粉体产物是直径为50～70 nm,长度约为几个微米并具有四方晶体结构的β-MnO₂纳米棒,同时产物表现出超顺磁性特征。β-MnO₂纳米棒具有优异的电磁波损耗特性,厚度为1 mm的试样其最大反射损耗在18 GHz能达到-8.1 dB,优于-5 dB的有效吸收带处于15.7～18 GHz宽频段内;2 mm厚的试样在9.8 GHz其反射损耗达到-25 dB,并且优于-5 dB的有效吸收带处于8～12.8 GHz。     

Structural properties and electrical characteristics of high-k Dy2O3 gate dielectrics

This paper describes the structural properties and electrical characteristics of thin Dy₂O₃ dielectrics deposited on silicon substrates by means of reactive sputtering. The structural and morphological features of these films after postdeposition annealing were studied by X-ray diffraction and X-ray photoelectron spectroscopy. It is found that Dy₂O₃ dielectrics annealed at 700 °C exhibit a thinner capacitance equivalent thickness and better electrical properties, including the interface trap density and the hysteresis in the capacitance-voltage curves. Under constant current stress, the Weibull slope of the charge-to-breakdown of the 700 °C-annealed films is about 1.6. These results are attributed to the formation of well-crystallized Dy₂O₃ structure and the reduction of the interfacial SiO₂ layer.     

Structural,optical and microwave dielectric properties of Ba1−xSrxWO4 ceramics prepared by solid state reaction route

In this paper, Barium Strontium Tungstate (Ba_1−xSr_x)WO₄ crystals with (x = 0; 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0) were prepared by standard wet milling ceramic preparation method. These crystals were structurally characterized by X-ray diffraction (XRD), Fourier transform Raman (FT-Raman) and Fourier transform infrared (FT-IR) spectroscopic techniques. The shape, growth and average crystal size distribution of these crystals were investigated by a scanning electron microscope (SEM). Their optical properties were investigated by ultraviolet visible (UV–vis) absorption and photoluminescence (PL) measurements. XRD patterns, Rietveld refinements data, FT-Raman and FT-IR spectroscopies indicate that all the crystals present a scheelite-type tetragonal structure without deleterious phases. FT-Raman spectra exhibited 6 Raman active modes in range from 100 to 1000 cm⁻¹, while the FT-IR spectra presented 2 infrared active modes in range from 500 to 1000 cm⁻¹. SEM micrographs showed well sintered BaWO₄ grains, while the substitution of Sr induced modifications in the shape and reduction in the grain size. UV–vis absorption measurements evidenced an increase in the values of the optical band gap (from 4.36 to 4.53 eV) with the increase of Sr into BaWO₄ lattice. Dielectric constant, temperature coefficient of resonant frequency (τ_f), quality factors were measured with Hakki–Coleman technique. The value of τ_f found −43.68 ppm/°C for BaWO₄ which increased to −21.40 ppm/°C for the SrWO₄.     

THEORY OF DIELECTRIC ELASTOMERS

In response to a stimulus, a soft material deforms, and the deformation provides a function. We call such a material a soft active material （SAM）. This review focuses on one class of soft active materials： dielectric elastomers. When a membrane of a dielectric elastomer is subject to a voltage through its thickness, the membrane reduces thickness and expands area, possibly straining over 100%. The dielectric elastomers are being developed as transducers for broad applications, including soft robots, adaptive optics, Braille displays, and electric generators. This paper reviews the theory of dielectric elastomers, developed within continuum mechanics and thermodynamics, and motivated by molecular pictures and empirical observations. The theory couples large deformation and electric potential, and describes nonlinear and nonequilibrium behavior, such as electromechanical instability and viscoelasticity. The theory enables the finite element method to simulate transducers of realistic configurations, predicts the efficiency of electromechanical energy conversion, and suggests alternative routes to achieve giant voltage-induced deformation. It is hoped that the theory will aid in the creation of materials and devices.     

Effective responses of nonlinear elliptic cylindrical composites

The higher-order dielectric responses of composites composed of weakly nonlinear elliptic cylindrical inclusions randomly embedded in linear media are investigated. The general formulae for effective DC coefficients are newly derived by the average field method up to the ninth order and then applied to analyze the effects of inclusion shapes on the effective response. The results are directly transformed to the quasi-static AC response of the composites under the sinusoidal external electric field with fundamental and third harmonic frequencies. The more general relationships between the effective DC and AC coefficients are established up to the seventh order and third harmonic, which are applicable to all weakly nonlinear isotropic composites.