Beta-manganese dioxide nanorods for sufficient high-temperature electromagnetic interference shielding in X-band

As the development of electronic and communication technology, electromagnetic interference (EMI) shielding and attenuation is an effective strategy to ensure the operation of the electronic devices. Among the materials for high-performance shielding in aerospace industry and related high-temperature working environment, the thermally stable metal oxide semiconductors with narrow band gap are promising candidates. In this work, beta-manganese dioxide (β-MnO₂) nanorods were synthesized by a hydrothermal method. The bulk materials of the β-MnO₂ were fabricated to evaluate the EMI shielding performance in the temperature range of 20–500 °C between 8.2 and 12.4 GHz (X-band). To understand the mechanisms of high-temperature EMI shielding, the contribution of reflection and absorption to EMI shielding was discussed based on temperature-dependent electrical properties and complex permittivity. Highly sufficient shielding effectiveness greater than 20 dB was observed over all the investigated range, suggesting β-MnO₂ nanorods as promising candidates for high-temperature EMI shielding. The results have also established a platform to develop high-temperature EMI shielding materials based on nanoscale semiconductors.     

Synthesis and study of electroactive nanoparticles and their polymer composites for novel applications

Polymer ceramic composites using a polymer binder, nanosized BaTiO₃ and metal particles were developed for radiation shielding in the microwave region. From X-ray Diffraction (XRD) the crystallinity and nanosize of BaTiO₃ was confirmed in the composite. Interesting changes in Differential Scanning Calorimetry (DSC) were observed before and after ball milling of BaTiO₃. Shielding Efficiency (SE) of microwave radiation has been measured from transmitted fraction (TF) of electromagnetic waves (EM) at different frequencies. The changes in TF were assigned to reflection and absorption of EM waves in different composites.     

Electrical conductivity and electromagnetic interference shielding characteristics of multiwalled carbon nanotube filled polyacrylate composite films

Multiwalled carbon nanotubes (MWCNTs) were homogeneously dispersed in pure acrylic emulsion by ultrasonication to prepare MWCNT/polyacrylate composites applied on building interior wall for electromagnetic interference (EMI) shielding applications. The structure and surface morphology of the MWCNTs and MWCNT/polyacrylate composites were studied by field emission scanning microscopy (FESEM) and transmission electron microscopy (TEM). The electrical conductivity at room temperature and EMI shielding effectiveness (SE) of the composite films on concrete substrate with different MWCNT loadings were investigated and the measurement of EMI SE was carried out in two different frequency ranges of 100-1000 MHz (radio frequency range) and 8.2-12.4 GHz (X-band). The experimental results show that a low mass concentration of MWCNTs could achieve a high conductivity and the EMI SE of the MWCNT/polyacrylate composite films has a strong dependence on MWCNTs content in both two frequency ranges. The SE is higher in X-band than that in radio frequency range. For the composite films with 10 wt.% MWCNTs, the EMI SE of experiment agrees well with that of theoretical prediction in far field.     

Electromagnetic interference shielding effectiveness of reinforced composite with graphene oxide-lead oxide hybrid nanosheets

ABSTRACT

The rapid development of telecommunication devices and related technologies improved the distribution of electromagnetic (EM) waves within the environment, which can adversely affect the performance of electronic systems and put the life of living species in serious danger; thus, practical alternatives are required to protect vulnerable sources from them. To address this demand, we developed reinforced composites with graphene oxide (GO)-lead oxide (GO-Pb₃O₄) hybrid 2D nanosheets at diverse filler loadings by employing the vacuum shock technique. Next, the developed fillers and composites were well characterized by the diverse analysis methods and their electromagnetic interference shielding effectiveness (EMI SE) was evaluated and compared with their X-ray shielding performance. The obtained results showed that the developed composites can perfectly attenuate X-ray waves and weaken the total power of applied EM waves (in X-band frequency (8.2–12.4 GHz)) more than 50%. In this regard, a 6 mm thick specimen containing 10?wt% GO-Pb₃O₄ attenuated emitted X-ray waves equal to 4.06, 4.83 and 3.91?mmAl at 40, 60 and 80?kVp energies, which shows about 124%, 124% and 121% increase, respectively, in the X-ray attenuation compared with neat epoxy resin. These results indicated that developed composites are perfect candidates to protect vulnerable sources from diverse EM sources, and despite the insulative nature of the GO-Pb₃O₄, these hybrid nanosheets can act as a potential barrier against EM waves.     

The electromagnetic shielding of Ni films deposited on cenosphere particles by magnetron sputtering method

Ni-coated cenosphere particles were successfully fabricated by an ultrasonic-assisted magnetron sputtering equipment. Their surface morphology and microstructure were analyzed using field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). FE-SEM results indicate that the Ni films coated by magnetron sputtering are uniform and compact. Ni film uniformity was related with the sputtering power and a large uniform film could be achieved at lower sputtering power. XRD results imply that the Ni film coated on cenospheres was a face-centered cubic (fcc) structure and the crystallization of film sample increases with increasing the sputtering power. The electromagnetic interference (EMI) shielding effectiveness (SE) of Ni-coated cenosphere particles were measured to be 4-27 dB over a frequency range 80-100 GHz, higher than those of uncoated cenosphere particles. The higher sputtering power and Ni film thickness are the higher EMI SE of the specimens. Ni-coated cenosphere particles are most promising alternative candidates for millimeter wave EMI shielding due to their lightweight, low cost, ease of processing, high floating time, good dispersion and tunable conductivities as compared with typical electromagnetic wave countermeasure materials.     

频率选择表面结构的电子系统K/Ka波段电磁屏蔽分析

电子设备和无线技术不断向K/Ka波段发展以及电子系统集成度的不断提高给电子系统的电磁屏蔽设计带来了严峻挑战。提出一种将频率选择表面（FSS）用于电子系统屏蔽的新方法,可以替代传统散热孔阵,在满足通风散热性能的同时确保电子系统在5G毫米波段的电磁屏蔽性能。基于金属腔中心点屏蔽效能和全局屏蔽效能,分析了FSS孔阵排布方式、电磁波极化与入射角度对金属外壳电磁屏蔽效能（SE）的影响。结果表明:FSS孔阵排布方式对金属腔屏蔽性能的影响较小,并且SE不受入射电磁波极化方式影响;含FSS通风孔阵的金属外壳在23.0～25.5 GHz范围内屏蔽效能约为30 dB,比含传统散热孔阵金属腔屏蔽效能提高15 dB。     

Structural investigations of the xTeO2-(1−x)GeO2 (x=0, 0.2, 0.4, 0.6, 0.8 and 1) tellurite glasses: A composition dependent Raman spectroscopic study

Raman spectra of xTeO₂-(1−x)GeO₂ (x=0, 0.2, 0.4, 0.6, 0.8 and 1) germanium tellurite glasses were measured and analyzed in an effort to follow the structural changes caused by mixing two typical glass formers. Systematic Raman intensity measurements have been performed in an effort to elucidate the composition induced structural changes and a possible mechanism accounting for these changes was proposed. The network structure of the glass is characterized by TeO₄ trigonal bipyramid mixed with TeO₃ trigonal pyramid units, while GeO₄ tetrahedral units are also present. Changing the GeO₂ content results in the conversion of the TeO₄ units to TeO₃ units with a neutral doubly bridged oxygen atom, while the existence of charged terminal oxygen atoms is questionable. The measured relative Raman intensities are semi-quantitatively correlated to the transformation of the TeO₄ trigonal bipyramid to TeO₃ trigonal pyramids.     

Charge transport properties of composites of multiwalled carbon nanotube with metal catalyst and polymer: application to electromagnetic interference shielding

We report charge transport properties such as d.c. conductivity (σ_DC) and its temperature dependence for composites of poly(methyl methacrylate) (PMMA) and multiwalled carbon nanotubes (MWCNTs). The MWCNTs were synthesized through chemical vapor deposition with Fe or Co as catalyst. The MWCNTs were homogeneously dispersed in PMMA matrix through sonication to prepare MWCNT–PMMA composite films. We controlled mass concentration of MWCNTs in the composites, and the thickness of MWCNT–PMMA composite films was 20–400 μm. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy were used to study structure and homogeneity of the composites. The σ_DC at room temperature of MWCNT–PMMA composites increased as mass concentration of MWCNTs increased, which followed percolation theory. Electromagnetic interference (EMI) shielding efficiency (SE) of MWCNT–PMMA composites was measured in the frequency range of 50 MHz–3.5 GHz. We observed the increase of EMI SE of MWCNT–PMMA composites with increasing the concentration of MWCNTs.     

Electromagnetic interference shielding characteristics of fabric complexes coated with conductive polypyrrole and thermally evaporated Ag

Through the chemical coating of polypyrrole (PPy) doped with naphthalene sulfonic acid (NSA) on electrically insulating poly (ethylene terephthalate) (PET) woven fabric, PPy–NSA/PET complexes were synthesized. By using the electrochemical coating of PPy doped anthraquinone-2-sulfonic acid (AQSA) on PPy–NSA/PET complexes, PPy–AQSA/PPy–NSA/PET complexes were synthesized. The silver (Ag) was thermally vacuum evaporated on the surface of PPy–AQSA/PPy–NSA/PET complexes (Ag|PPy–AQSA/PPy–NSA/PET). Electromagnetic interference (EMI) shielding efficiency (SE) and dc conductivity (σ_dc) of fabric complexes were measured for EMI shielding characteristics and theoretical simulation. The measurement of EMI SE in the frequency range from 50 MHz to 1.5 GHz was performed by using ASTM D4935-99 method. The EMI shielding characteristics such as transmittance, reflectance and absorbance were obtained from the S (scattering)-parameter analysis. We control the contribution of the absorbance or the reflectance to total EMI SE through the coating of conductive PPy and the evaporation Ag.     

Crystallization study of sol-gel un-doped and Pd-doped TiO2 materials

Sol-gel nanostructured titania materials have been reported to have applications in areas ranging from optics via solar energy to gas sensors. In order to enhance the photocatalytic activity, there are many studies regarding the doping of titanium dioxide (TiO₂) material with either non-metals (S, C, N, P) or metals (Ag, Pt, Nd, Fe). The present work has studied some un-doped and Pd-doped sol-gel TiO₂ materials (films and gels), with various surface morphologies and structures, obtained by simultaneous gelation of both precursors Ti(OEt)₄ and Pd(acac)₂. Their structural evaluation and crystallization behavior with thermal treatment were followed by DTA/TG analysis, infrared (IR) spectroscopy, Fourier transform infrared (FTIR), spectroellipsometry (SE), X-ray diffraction (XRD) and atomic force microscope (AFM). The influence of Pd on TiO₂ crystallization for both supported and un-supported materials was studied (lattice parameters, crystallite sizes, internal microstrains). The changes in the optical properties of the TiO₂-based vitreous materials were correlated with the changes of the structure. The hydrophilic properties of the films were also connected with their structure, composition and surface morphology.     

Preparation of LiCoO2 nanofibers by electrospinning technique

Using a sol-gel processing and electrospinning technique, extrathin fibers of PVA (polyvinyl alcohol)/lithium chloride/cobalt acetate composite were prepared. After calcinations of the above precursor fibers at 600°C, LiCoO₂ nanofibers with a diameter of 100-150 nm, were successfully obtained. Measurements of TG/DTA, IR, XRD, Raman, SEM, EDS, respectively, were performed to characterize the properties of the as-prepared materials. We observed a strong correlation between crystalline phase and morphology of the fibers and calcinations temperature.     

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.     

Crystallization and annealing effects of sputtered tin alloy films on electromagnetic interference shielding

Sn, Al and Cu not only possess electromagnetic interference (EMI) shield efficiency, but also have acceptable costs. In this study, sputtered Sn-Al thin films and Sn-Cu thin films were used to investigate the effect of the crystallization mechanism and film thickness on the electromagnetic interference (EMI) characteristics. The results show that Sn-xAl film increased the electromagnetic interference (EMI) shielding after annealing. For as-sputtered Sn-xCu films with higher Cu atomic concentration, the low frequency EMI shielding could not be improved. After annealing, the Sn-Cu thin film with lower Cu content possessed excellent EMI shielding at lower frequencies, but had an inverse tendency at higher frequencies. For both the Sn-xAl and Sn-xCu thin films after crystallization treatment, the sputtered films had higher electrical conductivity, however the EMI shielding was not enhanced significantly.     

Phase transition behavior and proton conduction mechanism in cesium hydrogen sulfate/silica composite

Proton conduction and crystal structure in CsHSO₄/SiO₂ composite composed of polycrystalline CsHSO₄ and mesoporous silica particles were investigated based on conductivity measurement and characterizations using Raman spectroscopy, XRD, and differential thermal analysis. The conductivity of pure CsHSO₄ abruptly changes at around 414 K (superprotonic phase transition), being accompanied with the structural transformation from a monoclinic phase to a tetragonal phase, while the conductivity of CsHSO₄/SiO₂ composite is significantly larger by over three orders of magnitude than that of pure CsHSO₄ below the critical temperature of the superprotonic phase transition (353-414 K). Raman spectroscopy and XRD indicate that this remarkable conductivity-enhancement in the composite is not due to the stabilization of the tetragonal phase (superprotonic phase) below its critical temperature. The line-broadening of the internal modes in the Raman spectra suggests that the rapid reorientational motion of the HSO₄⁻ ion, which leads to superprotonic conduction, is induced in the composite even below the critical temperature. The reorientational motion of the HSO₄⁻ ion below the critical temperature will occur at the interfacial phase which is structurally disordered and forms between CsHSO₄ and SiO₂ in the mesopores and/or on the surfaces of silica particles. Proton transfer will be accelerated via the interfacial conduction-pathway in the composite.     

Synthesis and structural characterization of a series of lanthanide stannate pyrochlores

A simple hydrothermal method has been employed to prepare a series of lanthanide stannate pyrochlores Ln₂Sn₂O₇ (Ln=Y, La, Pr-Yb) at a relatively low temperature of less than 200 °C successfully. On the basis of structural characterizations by X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR) absorption spectroscopy and Raman spectroscopy, it was found that the positions of bands in vibrational spectra are sensitive to the ionic radius of Ln³⁺, and the linear relationship can be seen between the frequency of Sn-O stretching mode and the lanthanide ionic radius in IR spectrum, as well as the frequency of O-Sn-O bending mode and the lanthanide ionic radius in Raman spectrum.     

Effects of rapid thermal annealing on the structural properties of TiO2 nanotubes

The structural properties of TiO₂ nanotubes with rapid thermal annealing (RTA) and traditional thermal annealing in O₂ were studied by X-ray diffraction (XRD) and Raman scattering measurements. From analyzing the line width of XRD and the correlation length of the Raman peak, we demonstrate that RTA can be an effective tool for amorphous-anatase transformation in TiO₂ nanotubes. The Raman peak redshifts and reduces its line width after thermal annealing and RTA, which may involves the reduction of oxygen-related defects.     

Synthesis and characterizations of microwave sintered ferrite powders and their composite films for practical applications

Phase pure single phase ferrite powders of (Ni_xR_1−x)_0.5Zn_0.5Fe₂O₄ (R=Mn, Co, Cu; x=0, 0.5) were manufactured using microwave sintering at 930 °C for 10 min in air atmosphere. The powders were characterized for their structure, microstructure, thermal, and magnetic properties. Selected powders were used as fillers to prepare their composite films using polymethyl methacrylate polymers as matrix. The composite films were prepared using the melt blending approach and were tested for their microstructure, thermal, and magnetic hysteresis loop as well as 3D magnetic field space mappings using an electromagnetic compatibility scanner. Among the studied ferrites, cobalt doped ferrites and their composites showed the best electromagnetic interference (EMI) shielding effectiveness value and have potential for practical EMI shielding applications.     

Densification mechanism of polyacrylonitrile-based carbon fiber during heat treatment

Polyacrylonitrile (PAN)-based carbon fibers were heat treated at various temperatures for varying durations to simulate the graphitization process in the manufacture of C/C composites. Densification of the resulting fibers was confirmed by density measurement. The composition and structure of the fibers were investigated by means of elemental analysis, X-ray diffraction and Raman spectroscopy. For specified isothermal heat treatment time, the structural parameters depended strongly on heat treatment temperature. The nitrogen content decreased with increased heat treatment temperature and extended time at constant temperature. Nitrogen loss was complete at temperatures above 1900 °C. The graphite crystallite size increased rapidly with increasing heat treatment temperature, and slowly with extended isothermal heat treatment time. At 2100 °C a more ordered graphitic structure appeared. Denitrogenation induced “puffing”, which made the fibers expand. Decrease in density in the heat treatment temperature range 1500-1900 °C originated from the abrupt evolution of nitrogen, and above 1900 °C the graphitization transition induced steadily increasing density. Densification of the carbon fibers was determined both by the rate of denitrogenation and the rearrangement of carbon atoms.     

Study of the effect of precipitant on the high-temperature phase stability of SrO-ZrO2 prepared by n-butanol soft-template method

SrO doped zirconia (20%) was synthesized by n-butanol soft-template method using both NaOH and ammonia solution as precipitants. The high-temperature phase stability was investigated following further heat treatment at 1000°C for 2 h. XRD and Raman spectra were used to characterize the crystal form of zirconia. In addition, TEM was used to characterize the dispersibility of SrO doped zirconia. The results indicated that the concentration of OH^? introduced into the ZrO₂ lattice was the main factor controlling the crystal form of nanosized zirconia. The NaOH solution precipitant could improve the dispersibility of SrO doped t-ZrO₂, and could also prevent the phase transformation of zirconia from t-ZrO₂ to m-ZrO₂ effectively.     

Study of phase transition in ZrO2 doped with Pb3O4

Electrical conductivity of ZrO₂ doped with Pb₃O₄ has been measured at different temperatures for different molar ratios (x=0, 0.01, 0.02, 0.03, 0.04, 0.05 and 0.06). The conductivity increases due to migration of vacancies, created by doping. The conductivity increases with increase in temperature till 180 °C and thereby decreases due to collapse of the fluorite framework. A second rise in conductivity at higher temperatures beyond 500-618 °C is due to phase transition of ZrO₂. DTA and X-ray powder diffraction were carried out for confirming doping effect and transition in ZrO₂.The addition of Pb₃O₄ to ZrO₂ shifted the phase transition of ZrO₂ due to the interaction between Pb₃O₄ and ZrO₂.