Magnetic shielding property for cylinder with circular,square, and equilateral triangle holes

The shielding property of cylinder with circular, square, and equilateral triangle holes was investigated by finite element analysis(FEA). The hole area(S_(hole)) plays an important role in magnetic circuit on the surface of cylinder. When Sholeis less than the critical area(S_H), cylinder with three shapes of holes obtained the same remanent magnetization inside,indicating that the shielding property is unaffected by the shape of the hole. Hence, high-permeability material is the major path of the magnetic field. On the condition of S_(hole) S_H, the sequence of the shielding property is equilateral triangle square circular, resulting from magnetoresistance of leakage flux in air dielectric. Besides, the anisotropy of shielding property caused by hole structural differences of the cylinder is evaluated. We find that a good shielding effectiveness is gained in the radial direction, compared with the axis direction. This research focuses on providing a theoretical support for the design of magnetic shield and improvement on the magnetic shielding ability.     

Total transmission and total reflection by zero index metamaterials with defects

We theoretically investigate microwave transmission through a zero-index metamaterial loaded with dielectric defects. The metamaterial is impedance matched to free space, with the permittivity and permeability tending towards zero over a given frequency range. By simply varying the radii and permittivities of the defects, total transmission or reflection of the impinging electromagnetic wave can be achieved. The proposed defect structure can offer advances in shielding or cloaking technologies without restricting the object's viewpoint. Active control of the observed exotic transmission and reflection signatures can occur by incorporating tunable refractive index materials such as liquid crystals and BaSrTiO3.     

Filling of a Plane Slit Volume with a Glow Discharge in a Transverse Magnetic Field and Its Effect on the Discharge Contraction

We consider a dc glow discharge in a plane slit volume with electrodes in the slit plane in a magnetic field transverse to the current, which has been studied experimentally. As in the experiment, the discharge is artificially confined at one of the dielectric boundaries of the volume and propagates to the opposite dielectric boundary until it is stabilized. It is shown using a 2D calculation of the nonstationary process that the discharge in a magnetic field occupies a noticeably larger volume (with a lower current density at the electrodes) than in zero magnetic field. The effect of the magnetic field is also manifested in that it hampers the contraction of the discharge, substantially elevating the threshold current of the diffuse discharge. The discharge contraction is calculated in the approximation of a homogeneous positive column along the current right to the attainment of the stationary state. In calculations with a magnetic field, hysteresis appears in transitions from the diffuse to the contracted state and back.     

A microstrip transmission line volume coil for human head MR imaging at 4T

A high-frequency RF volume coil based on the use of microstrip transmission line (MTL) has been developed for in vivo 1H MR applications on the human head at 4T. This coil is characterized by major advantages: (i) completely distributed coil circuit, (ii) high-quality factor (Q), (iii) simple coil structure, and (iv) better sensitivity and less signal-intensity variation in the MR image of the human head compared with an RF shielded birdcage coil of similar coil size. The proposed MTL volume coil does not require additional RF shielding for preventing Q degradation from radiation losses due to the unique MTL structure; thus, it provides a maximal useable space inside the volume coil when compared with most volume coils available at high fields with the same overall coil size. The intrinsic B(1) distribution of the MTL volume coil effectively compensates for the dielectric resonance effect at 4T and improves the signal homogeneity in human head MR images in the transaxial planes. The results of this study demonstrate that the MTL volume coil design provides an efficient and simple solution to RF volume coil design for human MR studies at high fields.     

Dielectric properties and EMI shielding efficiency of polyaniline and ethylene 1-octene based semi-conducting composites

The preparation of polyaniline (PAni) was carried out by the oxidative emulsion polymerization of aniline and the semi-conducting composites were prepared by mixing it with a polyolefinic thermoplastic elastomer ethylene 1-octene copolymer (EN). Different electrical properties and electromagnetic interference shielding efficiency (EMI SE) of these composites were measured. The results revealed that the incorporation of PAni in EN increases the conductivity, dielectric constant, dielectric loss and EMI SE. These composites exhibit pressure dependent dielectric properties and may act as pressure sensor. There are increase in AC conductivity and decrease in dielectric constant with the increase in applied pressure on composites. A model correlation between EMI SE and AC conductivity at same frequency for the composites having maximum 40% of PAni was obtained through extrapolation and linear regression analysis, which shows that EMI SE has linear relationship with AC conductivity. Because of their semi-conductive behavior these composites can find application as antistatic materials and electromagnetic interference (EMI) shielding material.     

Electromagnetic shielding mechanisms using soft magnetic stainless steel fiber enabled polyester textiles

This work studied the effects of conductivity, magnetic loss, and complex permittivity when using blended textiles (SSF/PET) of polyester fibers (PET) with stainless steel fibers (SSF) on electromagnetic wave shielding mechanisms at electromagnetic wave frequencies ranging from 30 MHz to 1500 MHz. The 316L stainless steel fiber used in this study had 38 vol% γ austenite and 62 vol% α′ martensite crystalline phases, which was characterized by an x-ray diffractometer. Due to the magnetic and dielectric loss of soft metallic magnetic stainless steel fiber enabled polyester textiles, the relationship between the reflection/absorption/transmission behaviors of the electromagnetic wave and the electrical/magnetic/dielectric properties of the SSF and SSF/PET fabrics was analyzed. Our results showed that the electromagnetic interference shielding of the SSF/PET textiles show an absorption-dominant mechanism, which attributed to the dielectric loss and the magnetic loss at a lower frequency and attributed to the magnetic loss at a higher frequency, respectively.     

Electromagnetic shielding effectiveness of lightweight and flexible ultrathin shungite plates

Carbon-containing shielding materials, especially with absorbing properties, received a lot of attention from the scientific community as having flexibility, lightness and corrosion resistance. Reducing the thickness of such materials (usually a few millimeters) is also an urgent task. In this paper, we offered results of studying the reflective and absorbing properties of shielding materials from natural graphene-containing shungite. Flexible shungite plates with a thickness of 10–20 μm showed good reflection and absorption similar shielding performance to a thicker 2–3 mm synthetic polymer/carbon based composites. Such characteristics are associated with the fact that shungites are unique natural composites with a conductive carbon matrix and micro-sized dielectric mineral inclusions. Shungite could be considered as a raw to manufacture shielding materials, and as a model for more complete understanding of interaction of radiation with graphene-containing conducting solids in order to improve the shielding performances.     

Transverse optic mode Gruneisen parameter and dielectric properties of alkali halides

The well known Szigeti relations for the dielectric theory of ionic crystals have been critically examined by investigating the volume dependence of dielectric behaviour of alkali halides. It has been predicted that the strain derivative of effective ionic charge parameter should be nearly equal to zero. Values of transverse optic mode Gruneisen parameter for NaCl structure alkali halides have been estimated.     

Simulation of the electromagnetic characters of a Faraday-shielded antenna in a helicon wave plasma source

In this article, a structure that employs a Faraday shield between the Shoji antenna and the dielectric tube, which aims to reduce the dielectric wall sputtering, is investigated for the helicon wave plasma (HWP) sources. Faraday shield is usually used between the antenna and the reaction chamber to reduce the radiation from the high electromagnetic field generation, as well as to prevent the sputtering of the antenna material from polluting the reaction chamber during the discharge. Here, the influence of the Faraday shield on the longitudinal and radial electric field of the antenna is analysed through COMSOL Multiphysics^TM. Significant attenuations of both the longitudinal and radial fields are observed in the presence of the shield. In addition, by comparing the electric field distribution under two different shielding parameters, it is found that the shielding effects are not the same. Therefore, a detailed study of two kinds of design (pitch and gap) for the shield was carried out. The results show that the pitch has a little impact on the overall shielding effect when the gap is unaltered. The best shielding performance appears when we set the pitch at T of 8 and 10. In addition, the shielding effect also becomes worse as the gap increases while the pitch remains unchanged. A relatively good shielding effect can be produced by setting the gap to the value of 4–8 mm (a gap/pitch ratio of 2/15–4/15). This work provides a theoretical basis for designing a Faraday shield structure between the antenna and the dielectric tube, which is helpful to realize stable and controllable HWP discharges.     

The 38 K transition in TTF-TCNQ viewed as a percolation phenomenon

The d.c. conductivity, static dielectric constant and thermopower of TTF-TCNQ have been calculated for temperatures below 54 K within a self-consistent effective medium approximation. TTF-TCNQ is assumed to consist of small regions which are semiconducting and others which are highly conducting. The conducting regions have a large negative dielectric constant, a large d.c. conductivity and a negative thermopower; the semiconducting portions have a large positive dielectric constant, an activated d.c. conductivity and a large positive thermopower. The volume fraction of conductor increases from zero at T = 0 to unity at 54 K. The model is the first to reproduce the observed metal-insulator transition (e.g. the d.c. conductivity anomaly) near 38 K. The dielectric constant is calculated to rise initially with temperature and then become negative as the temperature increases above 38 K. The thermopower changes sign below 54 K and increases very rapidly at 38 K.     

高浓度纤维增强材料介电特性计算方法

针对复合材料的微观结构非均匀和各向异性特点带来的数值方法计算慢、内存消耗大的问题,利用均匀化方法计算纤维增强复合材料的等效电磁参数.采用了纤维低体积添加比至高体积添加比的迭代方法,同时提出了一个描述材料微观结构的修正的特征长度,将现有的均匀化方法推广至非准静态(微波频段)条件下高纤维浓度情况.提出的修正的均匀化模型可直接用于反射系数、屏蔽效能等计算,其屏蔽效能与实际微观结构复合材料的数值仿真结果进行了对比,验证了提出的等效电磁参数计算公式的有效性和频率适用范围.     

Steplike transmission of light through a metal-dielectric multilayer structure due to an intensity-dependent sign of the effective dielectric constant

We numerically study light propagation through a specially designed nonlinear nanoscale metal-dielectric multilayer structure with a linear effective dielectric constant just below zero. The calculated dependence of the output intensity on the input intensity shows a steplike behavior. It rests upon an intensity-dependent change of the effective dielectric constant from negative (low-transmission state) to positive (high-transmission state) values, corresponding to a transition of the optical properties from metalliclike to dielectriclike. The study of the transient behavior of the structure demonstrates a switching time of around 1 ps.     

Theoretical analysis of whispering-gallery mode dielectric resonator in MM-wave MIC

The whispering-gallery mode (WGM) disk type dielectric resonator used in millimeter wave microstrip integrated circuits has been computed rigorously by the radial mode matching method. The dependence of resonant frequencies of WGM's on the size of shielding box and Q factors of WGM's relating to leakage loss in the case of parallel-plate shielding structure have been analyzed.     

Length-Scale Dependence of Hydration Free Energy: Effect of Solute Charge

Size and curvature are important determinants of particle wettability, in addition to surface chemistry and texture. Hydration free energy of a nonpolar solute scales with volume for small solutes and with surface area for larger ones. If the solute acquires a surface charge, the scaling regimes can be affected, with size-dependence of the charge playing a critical role. For isolated particles grown at fixed surface charge density, the Born approximation gives scaling of hydration free energy with volume. We consider a distinctly different but practically important scenario, where the charged solute and surrounding counterions are dissolved together. For this process, our molecular simulations demonstrate the electrostatic contribution to the solvation free energy, calculated per unit area of the solute, to be virtually independent of solute size. We explain this behavior in terms of counterion shielding effect on the curvature-dependent solute energy in the dehydrated state, an effect closely balanced by the influence of dielectric screening in water. As a result, for moderate surface charge densities of the solute, the net electrostatic contribution is dominated by counterion solvation, and scales with solute surface area independently of the ionic strength in the solution.     

The effective dielectric constant of a dispersion of clustering spheres

If one measures the dielectric constant of a water in oil microemulsion the large increase of this dielectric constant both as function of the temperature as well as a function of the volume fraction of water suggests that the water droplets will bind together in clusters. In order to analyse this suggestion in more detail we derive a systematic expansion of the Clausius-Mossotti function in terms of integrals over products of excess cluster polarizabilities and correlation functions. It is found that the excess polarizability of a cluster of spherical droplets is zero except when the spheres almost touch each other. Crucial for this property is the fact that the water spheres are conducting or if they are not conducting have a dielectric constant much larger than the dielectric constant of oil. The large increase of the dielectric constant is a consequence of the increase of the number of bound clusters. An explicit formula is given for the resulting temperature and volume fraction dependence of the dielectric constant.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday     

Total Reflection and Cloaking by Triangular Defects Embedded in Zero Index Metamaterials

In this work, we investigate wave propagation through a zero index metamaterial (ZIM) waveguide embedded with triangular dielectric defects. We provide a theoretical guidance on how to achieve total reflection and total transmission (i.e., cloaking) by adjusting the defect sizes and/or permittivities of the defects. Our work provides a systematical way in manipulating wave propagation through ZIM in addition to the widely studied dielectric defects with cylindrical and rectangular geometries.     

Ice shielding in the large scale GENIUS experiment for double beta decay and dark matter search

We suggest here the use of ice as shielding material in the large scale GENIUS experiment for the ultimate sensitive double beta decay and dark matter search. The idea is to pack a working volume of several tons of liquid nitrogens, which contains the “naked” Ge detectors, inside an ice shielding. Very thin plastic foil would be used in order to prevent leakage of the liquid nitrogen. Due to the excellent advantages of ice shielding (high purity and low cost, self-supporting ability, thermo-isolation and optical properties, safety) this could be another possible way of realization of the GENIUS project. Received: 30 July 1998     

Double‐shell metamaterial coatings for plasmonic cloaking

We study a double‐shell cloak for hiding objects with the dimensions comparable with the radiation wavelength. We demonstrate that the structure consisting of a dielectric layer and a layer of an epsilon‐near‐zero material can suppress sub‐ stantially the scattering from a sphere and at the same time shield its interiors. The double‐layer coating allows to cloak different objects with various material and geometrical parameters. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Apparently Negative Electric Polarization in Shaped Graded Dielectric Materials

By using a first-principles approach, we investigate the pathway of electric displacement fields in shaped graded dielectric materials existing in the form of cloaks with various shapes. We reveal a type of apparently negative electric polarization （ANEP）, which is due to a symmetric oscillation of the paired electric permittivities, satisfying a sum rule. The ANEP does not occur for a spherical cloak, but appears up to maximum as a/b （the ratio between the long and short principal axis of the spheroidal cloak） is about 5/2, and eventually disappears as a/b becomes large enough corresponding to a rod-like shape. Further, the cloaking efficiency is calculated for different geometrical shapes and demonstrated to closely relate to the ANEP. The possibility of experiments is discussed. This work has relevance to dielectric shielding based on shaped graded dielectric materials.     

Electrostatic and optical resonances of arrays of cylinders

The solutions of the electrostatic potential problem for the square and hexagonal arrays of circular cylinders with zero applied field (homogeneous or resonant solutions) are studied. We show that for non-touching cylinders, the set of resonances is discrete except in the neighbourhood of one point, at which the dielectric constant of the array has an essential singularity. For arrays of touching cylinders, the set is well represented by a continuous distribution. This representation enables the derivation of the asymptotic form of the expansion for the dielectric constant of the array when the dielectric constant of the cylinders is large. The known value of the first term in the expansion enables us to derive the second term. The physical characteristics of the resonant solutions are studied. Metals achieve values of dielectric constant which are close to the resonant values (real and negative) for certain wavelengths. Curves are given which enable the prediction of those wavelengths at which the optical resonances of both arrays occur, for any area fraction and composition of a columnar cerment film.