Dielectric loss enhancement induced by the microstructure of CoFe_(2)O_(4) foam to realize broadband electromagnetic wave absorption

CoFe_(2)O_(4)has been widely used for electromagnetic wave absorption owing to its high Snoek limit,high anisotropy,and suitable saturation magnetization;however,its inherent shortcomings,including low dielectric loss,high density,and magnetic agglomeration,limit its application as an ideal absorbent.This study investigated a microstructure regulation strategy to mitigate the inherent disadvantages of pristine CoFe_(2)O_(4)synthesized via a sol–gel auto-combustion method.A series of CoFe_(2)O_(4)foams(S0.5,S1.0,and S1.5,corresponding to foams with citric acid(CA)-to-Fe(NO_(3))_(3)·9H_(2)O molar ratios of 0.5,1.0,and 1.5,respectively)with two-dimensional(2D)curved surfaces were obtained through the adjustment of CA-to-Fe^(3+)ratio,and the electromagnetic parameters were adjusted through morphology regulation.Owing to the appropriate impedance matching and conductance loss provided by moderate complex permittivity,the effective absorption bandwidth(EAB)of S0.5 was as high as 7.3 GHz,exceeding those of most CoFe_(2)O_(4)-based absorbents.Moreover,the EAB of S1.5 reached 5.0 GHz(8.9–13.9 GHz),covering most of the X band,owing to the intense polarization provided by lattice defects and the heterogeneous interface.The three-dimensional(3D)foam structure circumvented the high density and magnetic agglomeration issues of CoFe_(2)O_(4)nanoparticles,and the good conductivity of 2D curved surfaces could effectively elevate the complex permittivity to ameliorate the dielectric loss of pure CoFe_(2)O_(4).This study provides a novel idea for the theoretical design and practical production of lightweight and broadband pure ferrites.

Ultrafine Vacancy-Rich Nb_(2)O_(5)Semiconductors Confined in Carbon Nanosheets Boost Dielectric Polarization for High-Attenuation Microwave Absorption

The integration of nano-semiconductors into electromagnetic wave absorption materials is a highly desirable strategy for intensifying dielectric polarization loss;achieving high-attenuation microwave absorption and realizing in-depth comprehension of dielectric loss mechanisms remain challenges.Herein,ultrafine oxygen vacancy-rich Nb_(2)O_(5)semiconductors are confined in carbon nanosheets(ov-Nb_(2)O_(5)/CNS)to boost dielectric polarization and achieve high attenuation.The polarization relaxation,electromagnetic response,and impedance matching of the ov-Nb_(2)O_(5)/CNS are significantly facilitated by the Nb_(2)O_(5)semiconductors with rich oxygen vacancies,which consequently realizes an extremely high attenuation performance of-80.8 dB(>99.999999%wave absorption)at 2.76 mm.As a dielectric polarization center,abundant Nb_(2)O_(5)–carbon heterointerfaces can intensify interfacial polarization loss to strengthen dielectric polarization,and the presence of oxygen vacancies endows Nb_(2)O_(5)semiconductors with abundant charge separation sites to reinforce electric dipole polarization.Moreover,the three-dimensional reconstruction of the absorber using microcomputer tomography technology provides insight into the intensification of the unique lamellar morphology regarding multiple reflection and scattering dissipation characteristics.Additionally,ov-Nb_(2)O_(5)/CNS demonstrates excellent application potential by curing into a microwave-absorbing,machinable,and heat-dissipating plate.This work provides insight into the dielectric polarization loss mechanisms of nano-semiconductor/carbon composites and inspires the design of high-performance microwave absorption materials.

Dielectric properties of spark plasma sintered AlN/SiC composite ceramics

In this study, we have investigated how the dielectric loss tangent and permittivity of AlN ceramics are affected by factors such as powder mixing methods, milling time, sintering temperature, and the addition of a second conductive phase. All ceramic samples were pre-pared by spark plasma sintering （SPS） under a pressure of 30 MPa. AlN composite ceramics sintered with 30wt%-40wt%SiC at 1600℃ for 5 min exhibited the best dielectric loss tangent, which is greater than 0.3. In addition to AlN and β-SiC, the samples also contained 2H-SiC and Fe5Si3, as detected by X-ray difraction （XRD）. The relative densities of the sintered ceramics were higher than 93%. Experimental results indicate that nano-SiC has a strong capability of absorbing electromagnetic waves. The dielectric constant and dielectric loss of AlN-SiC ce-ramics with the same content of SiC decreased as the frequency of electromagnetic waves increased from 1 kHz to 1 MHz.

Tracking Regulatory Mechanism of Trace Fe on Graphene Electromagnetic Wave Absorption

Polarization and conductance losses are the fundamental dielectric attenuation mechanisms for graphene-based absorbers, but it is not fully understood in revealing the loss mechanism of affect graphene itself. For the first time, the reduced graphene oxide(RGO) based absorbers are developed with regulatory absorption properties and the absorption mechanism of RGO is mainly originated from the carrier injection behavior of trace metal Fe nanosheets on graphene. Accordingly, the minimum reflection loss(RLmin) of Fe/RGO-2composite reaches-53.38 dB(2.45 mm), and the effective absorption bandwidth achieves 7.52 GHz(2.62 mm) with lower filling loading of 2 wt%. Using off-axis electron hologram testing combined with simulation calculation and carrier transport property experiments, we demonstrate here the carrier injection behavior from Fe to graphene at the interface and the induced charge accumulation and rearrangement, resulting in the increased interfacial and dipole polarization and the conductance loss. This work has confirmed that regulating the dielectric property of graphene itself by adding trace metals can not only ensure good impedance matching, but also fully exploit the dielectric loss ability of graphene at low filler content,which opens up an efficient way for designing lightweight absorbers and may be extended to other types materials.

Absorption properties and mechanism of lightweight and broadband electromagnetic wave-absorbing porous carbon by the swelling treatment

Bioderived carbon materials have garnered considerable interest in the fields of microwave absorption and shielding due to their reproducibility and environmental friendliness.In this study,KOH was evenly distributed on biomass Tremella using the swelling induction method,leading to the preparation of a three-dimensional network-structured hierarchical porous carbon(HPC)through carbonization.The achieved microwave absorption intensity is robust at-47.34 dB with a thin thickness of 2.1 mm.Notably,the widest effective absorption bandwidth,reaching 7.0 GHz(11–18 GHz),is attained at a matching thickness of 2.2 mm.The exceptional broadband and reflection loss performance are attributed to the 3D porous networks,interface effects,carbon network defects,and dipole relaxation.HPC has outstanding absorption characteristics due to its excellent impedance matching and high attenuation constant.The uniform pore structures considerably optimize the impedance-matching performance of the material,while the abundance of interfaces and defects enhances the dielectric loss,thereby improving the attenuation constant.Furthermore,the impact of carbonization temperature and swelling rate on microwave absorption performance was systematically investigated.This research presents a strategy for preparing absorbing materials using biomass-derived HPC,showcasing considerable potential in the field of electromagnetic wave absorption.

The Influence of Free Water Content on Dielectric Properties of Alkali Active Slag Cement Paste

The dielectric performance of alkali activated slag （AAS） cement paste was investigated in the frequency range of 1 to 1000 MHz. The experimental results showed the unstable dielectric properties of harden paste were mostly influenced by the fraction of free water in paste or absorbed water from ambient, but not including hydration water and microstructure. The free water was completely eliminated by heat treatment at 105 ℃ about 4 hours, and then its dielectric loss was depressed; but with the exposure time in air increasing, the free water adsorption in ambient air made the dielectric property of harden cement paste to be bad. The temperature and relative humidity of environment was the key factors of free water adsorption; hence, if the influence of free water on dielectric constant was measured or eliminated, the cement-based materials may be applied in humidity sensitive materials or dielectric materials domains.

Identification of the Intrinsic Dielectric Properties of Metal Single Atoms for Electromagnetic Wave Absorption

Atomically dispersed metals on N-doped carbon supports(M-N_(xCs)) have great potential applications in various fields.However,a precise understanding of the definitive relationship between the configuration of metal single atoms and the dielectric loss properties of M-N_(xCs) at the atomic-level is still lacking.Herein,we report a general approach to synthesize a series of three-dimensional(3D)honeycomb-like M-N_xC(M=Mn,Fe,Co,Cu,or Ni) containing metal single atoms.Experimental results indicate that 3D M-N_(xCs) exhibit a greatly enhanced dielectric loss compared with that of the NC matrix.Theoretical calculations demonstrate that the density of states of the d orbitals near the Fermi level is significantly increased and additional electrical dipoles are induced due to the destruction of the symmetry of the local microstructure,which enhances conductive loss and dipolar polarization loss of 3D M-N_(xCs),respectively.Consequently,these 3D M-N_(xCs) exhibit excellent electromagnetic wave absorption properties,outperforming the most commonly reported absorbers.This study systematically explains the mechanism of dielectric loss at the atomic level for the first time and is of significance to the rational design of high-efficiency electromagnetic wave absorbing materials containing metal single atoms.

Effect of Gadolinium Substitution on Dielectric Properties of Bismuth Ferrite

Multiferroic Bi1-xGdxFeO3(x =0, 0.05, 0.1, 0.15, 0.2) ceramics were prepared by conventional solid state reaction method. X-ray diffraction was carried out to characterize the crystal structure and to detect any possible impurities existing in these ceramics. Frequency dependence of dielectric properties of Bi1-xGdxFeO3 samples at room temperature was measured in a frequency range from 100 Hz to 1 MHz using an HP4294A precision impedance analyzer. For all the samples studied, the dielectric constant and dielectric loss decreases with increasing frequency in the range between 100 Hz and 1 MHz, as can be expected from a typical orientational dielectric relaxation process. There is no indication of any dips over the whole frequency range studied, which is in direct contrast with that reported previously. It is found that both dielectric constant and dielectric loss are strongly dependent on the Gd3+ content. The effect of introducing Gd3+ is to increase the dielectric constant and to decrease the dielectric loss for slightly doped sample Bi0.95Gd0.05FeO3: the dielectric constant of the sample at 1 kHz reaches 600, six times bigger than that for pure BiFeO3. Complicated dielectric behaviors are observed at higher doping levels. Furthermore, the substitution of rare earth Gd for Bi helps to eliminate the impurity phase in BiFeO3 ceramics. There is strong evidence that both lattice constants a and c of the unit cell become smaller as the Gd3 + content is increased. The dielectric constant and loss and their frequency responses can be varied dramatically by substitution of Gd.

Small Rb^+ Doping in CaCu_3Ti_4O_(12)——A Possible Approach to Reduce Dielectric Loss

Ca1-xRbxCu3Ti4O12 （x=0, 0.01, 0.02 and 0.03） ceramics were synthesized by the sol-gel method. Doping Rb＋ reduces dielectric loss, which reaches minimum when x=0.02. By measuring properties of electrical conduction, larger leakage current density and height of grain-boundary Sehottky potential barrier （φB） were found in the doped samples, and φB became maximum when x=0.02. These results are attributed to the increase in the amount of oxygen vacancies and the formation of Cu-rich/Ti-poor grain-boundary layers, and it can be concluded that the dielectric loss in CCTO ceramic can be reduced by manipulating the composition and electrical properties of grain boundary.

Study on Ba_(6-3x)Sm_(8+2x)Ti_(18)O_(54) Microwave Dielectric Ceramics

The effects of different x values on the dielectric properties in the Ba6-3xSm8+2xTi18O54 microwave dielectric ceramics(0.3≤x≤0.7) were studied. The results showed that the major crystal phase was BaSm2Ti4O12 and the sintering temperature of the ceramics was 1340 ℃ when the x value was 0.67. The dielectric properties were better: εr≈79, tanδ≈1×10-4(1 MHz); εr≈80, Q·f=9230 GHz(3.97 GHz), τf=-17 ppm·℃-1.

Comparative Study of Dielectric and Magnetic Properties of Selected 3D Reticulated Ceramics and Their Same Composition Ceramic Disks

3-dimensional reticulated ceramics (3DRCs) and their same composition ceramic disks(SCCDs) were fabricated by sol-gel method, with the composition of SrO-6Fe203(30%), SiC(35%) and Ti02(35%), sintered at 1200C in N2. The dielectric and magnetic parameters of such 3DRCs and their SCCDs were measured respectively in a temperature range from room temperature to 800癈 and in a frequency range from 2.6 GHz to 18 GHz. The results showed that the dielectric and magnetic loss of 3DRCs were obviously larger than those of their SCCDs in a wide range of temperature and the whole range of measuring frequency. The increase of dielectric loss of SDRCs was much higher than that of magnetic loss compared to their SCCDs, which was found due to the 3D net structure extrinsic characteristics.

Comprehensive studies on dielectric properties of p-methoxy benzylidene p-decyl aniline with function of temperature and frequency in planar geometry: A potential nematic liquid crystal for display devices

The dielectric properties of the nematic mesophase, p-methoxy benzylidene p-decyl aniline(MBDA), measured in planar geometry with a function of frequency and temperature are investigated in detail. The complex dielectric permittivity(ε' and ε'') is also studied at a bias voltage of 10 V for planar aligned sample cell of nematic mesophase. The dielectric permittivity with bias voltage attains a higher(> 2 times) value than that without bias voltage at a temperature of 56℃,which is due to the fact that the linking group of nematic molecules is internally interacted with an applied bias voltage.This is supported by observing an enhanced dielectric permittivity of nematic liquid crystal(LC) in the presence of bias voltage, which can be fully explained as the increasing of the corresponding dipole moment. The dielectric relaxation behaviors of nematic LC are also demonstrated for planar aligned sample cell. The remarkable results are observed that the relaxation frequency shifts into low frequency region with the increase of the bias voltage applied to the planar aligned sample cells. The dielectric relaxation spectra are fitted by Cole–Cole nonlinear curve fitting for nematic mesophase in order to determine the dielectric strength.

Temperature-frequency dependence and mechanism of dielectric properties for γ-Y_2Si_2O_7

This paper reports that single-phase γ-Y2Si2O7 is prepared via a sufficient blending and cold-pressed sintering technique from Y2O3 powder and SiO2 nanopowder. It studies the dielectric properties of γ-Y2Si2O7 as a function of the temperature and frequency. The γ-Y2Si2O7 exhibits low dielectric loss and non-Debye relaxation behaviour from 25 to 1400℃ in the range of 7.3 18 GHz. The mechanism for polarization relaxation of the as-prepared γ-Y2Si2O7 differing from that of SiO2 is explained. Such particular dielectric properties could potentially make specific attraction for extensive practical applications.

Dielectric loss of half-doped manganite La_(0.5)Ca_(0.5)MnO_3

The dielectric loss tanfi of half-doped manganite Lao.sCao.sMnO3 is investigated using Green＇s function technique. The Lao.sCao.sMnO3 is described by the Kondo-lattice model in the double exchange limit, taking into account the Jahn- Teller distortion and the super-exchange interaction between the localized electrons. It is found that the intensity of tans decreases with increasing [e]TI, V, and U. It is also observed that the transition temperature Tp rises as l eJTI and U increase. It is worth noting that Tp remains unchanged and the strength of tans increases with increasing g. The calculated dielectric loss results are explained theoretically, and these behaviors are in qualitative agreement with the experimental results.

The Dielectric Properties and Preparation Method of BaTiO_3/PVDF 0-3 Composites

A series of 0-3 composites of the polyvinylidene fluoride (PVDF) and BaTiO 3 was prepared.BaTiO 3 was modified with titanate coupling agent.The dielectric properties and the interfacial interaction of composites by different preparation methods were examined and compared.The result shows that the relative dielectric constant ε of the composite prepared in solution has a maximum value at about 70% weight fraction of BaTiO 3 and the dielectric loss tanδ increases rapidly when the fraction exceeds 70%.For the composite prepared in melt,the relative dielectric constant ε almost reaches a maximum value at about 60% weight fraction of BaTiO 3 and the dielectric loss is comparatively lower.The dielectric properties of composites are improved by using a coupling agent.

Dielectric Properties of Nanocrystalline TiO_(2) Prepared by the Stearic Acid Gel Method

Nanocrystalline TiO 2 was prepared by means of a stearic acid gel method. The product was investigated by differential thermal analysis, thermogravimetric analysis, X-ray powder diffraction and transmission electronic microscopy(TEM). The results of the dielectric properties tested indicate that the dielectric constant of anatase-form nanocrystalline TiO 2 is much higher than that of the coarse crystal. The dielectric constant of the nanocrystalline material is not related with the amplitude of an external AC field in a certain range.

Dielectric Properties of Fishbone Kutum and Composite Materials with Its Participation

This paper presents the results of studying the frequency dependence of permittivity and dielectric loss of fishbone (Fb) Kutum and polyethylene composites filled with nanoparticles fishbone Kutum and the influence of aluminum(Al) nanoparticles with dimensions 80-nm on the dielectric properties of polymer composites. Studies were carried out at the frequency range of 0 - 1 MHz. It was found that the variation of the volume of filler content of fishbone and aluminum nanoparticles might be prepared composite materials with the required dielectric parameters.

Structural and Dielectric Properties of (Bi2O3Fe2O3)0.4(Nb2O5)0.6 for Different Sintering Temperature

In this research work, (Bi2O3Fe2O3)0.4(Nb2O5)0.6 was made by the solid state reaction method. Samples were sintered at four different temperatures (850°C, 925°C, 1000°C and 1150°C) to study the effect of sintering temperature on the various properties of the samples. X-ray diffraction analysis confirmed that single phase Bi1.721δ0.089Fe1.056Nb1.134O7 was found when sintering temperature increased. At the same time, larger grain size was found when sintering temperature increased. From variation of dielectric loss with respect to frequency, a small peak was found when sample was sintered at higher temperature (1150°C). Dielectric constant of the sample decreases with the increase of frequency for all the samples. With the variation of temperature, DC resistivity of the samples showed that resistivity decreases with the increase of measuring temperature which indicates semiconducting nature.

Electrical and dielectric properties of Na_(1/2)La_(1/2)Cu_3Ti_4O_(12) ceramics prepared by high energy ball-milling and conventional sintering

We report on the measurements of the electrical and dielectric properties of Na1/2La1/2Cu3Ti4O12 （NLCTO） ce- ramics prepared by high energy ball-milling and conventional sintering without any calcination steps. The x-ray powder diffraction analysis shows that pure perovskite-like CCTO phase is obtained after sintering at 1025 ℃-1075 ℃. Higher sintering temperatures result in multi-phase ceramics due to thermal decomposition. Scanning electron microscope observations reveal that the grain size is in a range of - 3 μm-5 μm for these ceramics. Impedance spectroscopy measurements performed in a wide frequency range （1 Hz-10 MHz） and at various temperatures （120 K-470 K） are used to study the dielectric and electrical properties of NLCTO ceramics. A good compromise between high ε′ （5.7 ×10^3 and 4.1 × 10^3 at 1.1 kHz and 96 kHz, respectively） and low tan δ （0.161 and 0.126 at 1.1 kHz and 96 kHz, respectively） is obtained for the ceramic sintered at 1050 ℃. The observed high dielectric constant behavior is explained in terms of the internal barrier layer capacitance effect.

Reduced graphene oxide aerogel decorated with Mo_(2)C nanoparticles toward multifunctional properties of hydrophobicity,thermal insulation and microwave absorption

Reduced graphene oxide(rGO)aerogels are emerging as very attractive scaffolds for high-performance electromagnetic wave absorption materials(EWAMs)due to their intrinsic conductive networks and intricate interior microstructure,as well as good compatibility with other electromagnetic(EM)components.Herein,we realized the decoration of rGO aerogel with Mo_(2)C nanoparticles by sequential hydrothermal assembly,freeze-drying,and high-temperature pyrolysis.Results show that Mo_(2)C nanoparticle loading can be easily controlled by the ammonium molybdate to glucose molar ratio.The hydrophobicity and thermal insulation of the rGO aerogel are effectively improved upon the introduction of Mo_(2)C nanoparticles,and more importantly,these nanoparticles regulate the EM properties of the rGO aerogel to a large extent.Although more Mo_(2)C nanoparticles may decrease the overall attenuation ability of the rGO aerogel,they bring much better impedance matching.At a molar ratio of 1:1,a desirable balance between attenuation ability and impedance matching is observed.In this context,the Mo_(2)C/r GO aerogel displays strong reflection loss and broad response bandwidth,even with a small applied thickness(1.7 mm)and low filler loading(9.0wt%).The positive effects of Mo_(2)C nanoparticles on multifunctional properties may render Mo_(2)C/r GO aerogels promising candidates for high-performance EWAMs under harsh conditions.