Dielectric loss

About: Dielectric loss is a research topic. Over the lifetime, 20296 publications have been published within this topic receiving 349254 citations.

Prussian blue analogues derived magnetic FeCo alloy/carbon composites with tunable chemical composition and enhanced microwave absorption.

Abstract: A series of magnetic FeCo alloy/carbon composites have been successfully prepared through in situ pyrolysis of Prussian blue analogues (PBAs) with different Fe/Co ratios. The Fe/Co ratio can affect the crystalline phase, particle size, and magnetic property of the FeCo alloy particles, as well as the relative graphitization degree of the carbon frameworks. As a result, the electromagnetic functions of these composites will be highly associated with the Fe/Co ratio, where high Co content is beneficial to the formation of strong dielectric loss and moderate Co content can facilitate the magnetic loss. When Fe/Co ratio reaches 1:1, the as-obtained composite (sample S4) displays excellent reflection loss characteristics with powerful absorption in a very broad frequency range (over −10 dB in 3.2–18.0 GHz), which is superior to those of single magnetic metal (Fe or Co)/carbon composite derived from PBAs, as well as many previously reported FeCo alloy/carbon composites. Electromagnetic analysis reveals that the excellent microwave absorption of sample S4 benefits from its preferable matching of characteristic impedance and good attenuation ability toward incident electromagnetic waves. These results provide new insight into the fabrication of carbon-based magnetic composites with enhanced microwave absorption by rationally manipulating the chemical composition of magnetic components.

A simple hydrothermal process to grow MoS2 nanosheets with excellent dielectric loss and microwave absorption performance

Abstract: In this study, two-dimensional MoS2 nanosheets synthesized by a hydrothermal method were firstly investigated for microwave absorbing performance. The obtained MoS2 nanosheets are highly desirable as an electromagnetic wave (EM) absorber because of its larger interfacial polarization and high dielectric loss. Our results show that the real and imaginary parts of permittivity of MoS2 prepared at 180 °C are higher than those of other samples. A broad bandwidth absorption at a thin thickness can be obtained between 2 and 18 GHz. The microwave reflection loss (RL) of MoS2 nanosheets prepared at 180 °C reaches as high as −47.8 dB at 12.8 GHz due to its high electrical conductivity and the polarization effect. It can also be found that MoS2 exhibits an effective electromagnetic wave absorption bandwidth of 5.2 GHz (<−10 dB) at the thicknesses of 1.9 and 2.0 mm. The results showed that the MoS2 nanosheets can be a candidate for microwave absorption with a broad effective absorption bandwidth at thin thicknesses.

Thermal conductivity epoxy resin composites filled with boron nitride

Abstract: Boron nitride (BN) micro particles modified by silane coupling agent, γ-aminopropyl triethoxy silane (KH550), are employed to prepare BN/epoxy resin (EP) thermal conductivity composites. The thermal conductivity coefficient of the composites with 60% mass fraction of modified BN is 1.052 W/mK, five times higher than that of native EP (0.202 W/mK). The mechanical properties of the composites are optimal with 10 wt% BN. The thermal decomposition temperature, dielectric constant, and dielectric loss increase with the addition of BN. For a given BN loading, the surface modification of BN by KH550 exhibits a positive effect on the thermal conductivity and mechanical properties of the BN/EP composites. Copyright © 2011 John Wiley & Sons, Ltd.

Planar microwave integrated phase-shifter design with high purity ferroelectric material

Abstract: Ferroelectric materials (FEM's) are very attractive because their dielectric constant can be modulated under the effect of an externally applied electric field perpendicular to the direction of propagation of a microwave signal. FEM may be particularly useful for the development of a new family of planar phase shifters which operate up to X-band. The use of FEM in the microwave frequency range has been limited in the past due to the high losses of these materials; tan /spl delta/=0.3 at 3 GHz is typical for commercial BaTiO/sub 3/ (BTO) and due to the high electric field necessary to bias the structure in order to obtain substantial dielectric constant change. In this paper, a significant reduction in material losses is demonstrated. This is achieved by using a new sol-gel technique to produce barium modified strontium titanium oxide [Ba/sub 1-x/Sr/sub x/TiO/sub 3/ (BST)], which has ferroelectric properties at room temperature. Also demonstrated is how the use of thin ceramics reduces the required bias voltage below 250 V, with almost no power consumption required to induce a change in the dielectric constant. A phase shift of 165/spl deg/ was obtained at 2.4 GHz, with an insertion loss below 3 dB by using a bias voltage of 250 V. Due to the planar geometry and light weight of the device, it can be fully integrated in planar microwave structures.

Oriented Polarization Tuning Broadband Absorption from Flexible Hierarchical ZnO Arrays Vertically Supported on Carbon Cloth

Abstract: A novel strategy is used to design large-scale polarized carbon-based dielectric composites with sufficient interaction to electromagnetic waves. Highly uniform polar zinc oxide arrays are vertically grown on a flexible conductive carbon cloth substrate (CC@ZnO) via an in situ orientation growth process. Anion regulation is found to be a key factor to the morphology of hierarchical ZnO arrays including single-rod, cluster and tetrapod-shaped. As a typical dielectric loss hybrid composite, the electromagnetic parameters of the CC@ZnO system and charge density distribution in polarized ZnO rods confirm that the 3D intertwined carbon cloth is used as a conductive network to provide ballistic electron transportation. Moreover, the defect-rich ZnO arrays are well in contact with the CC substrate, favoring interface polarization, multiscattering, as well as impedance matching. Surprisingly, the efficient absorption bandwidth of the CC@ZnO-1 composite can reach 10.6 GHz, covering all X and Ku bands. The oriented ZnO possesses oxygen vacancies and exposure to a large amount of intrinsic polar surfaces, encouraging the polarization behavior under microwave frequency. Optimized CC@ZnO materials exhibit fast electron transportation, strong microwave energy dissipation, and superior wide absorption. The results suggest that the CC@ZnO composites have promising potential as flexible, tuning, and broadband microwave absorbers.