Millimeter-Wave Transmittance Measurements on Ferrites Near Ferromagnetic Resonance

This paper presents complex dielectric permittivity and magnetic permeability measurements of pure and diluted barium ferrites in the millimeter-wave frequency range. A broadband quasi-optical millimeter-wave spectroscopy with backward-wave oscillators as high-power tunable sources of coherent radiation has been utilized. The real and imaginary parts of the dielectric permittivity and magnetic permeability as functions of frequency are computed from the transmittance spectra. A magneto-optical approach in millimeter waves has been successfully employed for the separation of the dielectric and magnetic effects and the simultaneous determination of the magnetic and dielectric parameters of barium ferrite materials near ferromagnetic resonance.     

Computer study of methane adsorption by water clusters

Methane adsorption by water clusters including 50 molecules was studied by the molecular dynamics (MD) technique. The calculated frequency dependence of the real and imaginary parts of the dielectric permittivity points to the prevalence of a high-frequency mode in the spectra of these parameters after methane is adsorbed by water clusters. Such changes are also observed in the spectrum of the IR radiation absorption coefficient; however, the average value of this coefficient remains nearly the same. Methane adsorption causes a considerable increase in the reflection coefficient of the disperse water medium and the integrated power of IR radiation emission.     

Characterization and dielectric properties of β-SiC nanofibres

SiC nanofibres produced by chemical vapour reaction technique are investigated using scanning and transmission electron microscopy. The nanofibres have been found to have a crystalline core of β-SiC sheathed with thorn-like turbostratic carbon or amorphous Si/O/C, respectively. For this material, real and imaginary part of relative permittivity is measured in a frequency range of 1–18 GHz at room temperature. The results reveal that the permittivity and dielectric loss in the SiC nanofibres are a magnitude higher compared with sub-microcrystalline SiC powder. Composition and nanostructure are held responsible for the difference in dielectric properties. The mechanisms of dielectric loss in the SiC nanofibres are discussed based on interfacial polarization, lattice defects in the SiC nanofibre cores and conduction loss of turbostratic carbon in the thorn-like sheath of SiC nanofibres.     

Cavity perturbation technique for the measurement of permittivitytensor of uniaxially anisotropic dielectrics

Based on microwave resonant perturbation theory, a bimodal TE₁₁₂ cylindrical cavity is developed for the measurement of permittivity tensor of uniaxially anisotropic dielectrics. The two perpendicular TE₁₁₂ degenerate modes in the cavity are used to measure the two complex components in the permittivity tensor of a uniaxial sample, respectively. In the measurement of each component, only the corresponding mode resonates in the bimodal cavity, and the real and imaginary parts of the complex component are deduced from the changes of the resonant frequency and the quality factor of the cavity due to sample insertion. In the present technique, the quality factors of the cavity before and after sample insertion are measured at the same frequency, so the uncertainties caused by the variation of the total stored energy in the cavity due to sample insertion are eliminated. Experimental results show that the present technique is accurate and reliable for the measurement of the permittivity tensor of uniaxial media. As an example, the dielectric properties of α-SiO₂ samples with different crystal orientations are characterized using the present technique     

Microwave synthesis of Al-doped SiC powders and study of their dielectric properties

To improve the dielectric properties of β-SiC powders, microwave synthesis was applied to produce SiC powders doped with different amounts of Al from fine powders of Si, C and Al under Ar atmosphere. The dielectric properties of the as-synthesized Al-doped SiC powders were investigated, and the mechanism of dielectric loss by doping has been discussed. The presence of Al influenced the formation of secondary phases (α-SiC and Al₄SiC₄) and the microstructure of the resultant powders. The produced powders form Al-SiC solid-solutions, which seemingly favor defect polarization loss effect in the high frequency region. This is consistent with the measurements of dielectric properties, which showed that doping of SiC with Al causes increase of permittivity, both real and imaginary parts, and loss tangent, within 8.2-12.4 GHz. The results show that SiC doped with 30% Al has the highest real part ?′ and imaginary part ?″ of permittivity and also loss tangent.     

碳化硅/石墨涂层复合材料的介电性探讨

研究了碳化硅和石墨含量对碳化硅/石墨双层复合材料介电常数实部、虚部、损耗角正切的影响。鉴于该材料多用于工程领域,测试了该复合材料的弯曲、剪切、拉伸等力学性能。结果表明:在低频段,该材料介电性能良好,碳化硅含量、石墨含量均对涂层复合材料的实部、虚部和损耗角正切影响较大;当碳化硅含量为24%,石墨含量为60%时,涂层复合材料的介电常数实部、虚部最大,其极化和损耗能力最强。另外,该复合材料具备良好的力学性能,最大弯曲强力为48.3N,最大剪切强力为57.5N,最大拉伸强力为618N。     

定向分布碳纤维复合材料介电性能研究

以碳纤维为填充物, 环氧树脂为基体, 制备了碳纤维/环氧树脂介电复合材料. 介绍了两种分布方式对复合材料介电性能的影响, 分别研究了两种分布方式的介电常数随碳纤维含量和长度的变化规律. 在2.6–8.2 GHz频率范围内, 轴向介电常数是径向介电常数的数倍; 实部和虚部都随着碳纤维含量的增加而增大; 碳纤维长度也对介电性能的各向异性影响显著. 双层微波传输带模型可以合理地解释这些规律.     

Measurement and modeling of dielectric properties of Pb(Zr,Ti)O3 ferroelectric thin films

In this study, the real and imaginary parts of the complex permittivity of lead zirconate titanate ferroelectric thin films are studied in the frequency range of 100 Hz to 100 MHz. The permittivity is well fitted by the Cole-Cole model. The variation of the relaxation time with the temperature is described by the Arrhenius law and an activation energy of 0.38 eV is found. Because of its nonlinear character, the dielectric response of the ferroelectric sample depends on the amplitude of the applied ac electric field. The permittivity is composed of three different contributions: the first is due to intrinsic lattice, the second is due to domain wall vibrations, and the third is due to domain wall jumps between pinning centers. This last contribution depends on the electric field, so it is important to control the field amplitude to obtain the desired values of permittivity and tunability.     

Mechanical,dielectric and microwave absorption properties of carbon black (CB) incorporated SiO<Subscript>2f</Subscript>/PI composites

Structural and functional integrated absorbing materials have received extensive attention in recent years. Single-layer absorbing material composites in X and Ku bands (8.2–18 GHz) were prepared by using different proportion of carbon black (CB) incorporated quartz glass fiber reinforced polyimide resin (SiO_2f/PI). The morphology, mechanical, dielectric and microwave absorbing properties of the composites were investigated. Different contents of CB with different distributions were observed according to the fractural morphology of the composites. When the CB content is up to 8 wt%, the flexural strength of the composite reaches a maximum value of 705?±?5 MPa. The real and imaginary parts of complex permittivity both show an increasing trend with the CB content increase. Meanwhile, the real and imaginary parts decrease with the increase of frequency, exhibiting frequency-dependent dielectric response. The reflection loss of the composite was calculated based on the transmission line theory, which is in a good agreement with the experiment.     

Investigation on dielectric response of PMN ceramics around paraelectric–ferroelectric diffuse phase transition

Abstract

The dielectric properties of relaxor ferroelectric lead magnesium niobate Pb(Mg_1/3Nb_2/3)O₃ (PMN) ceramics have been investigated. These studies were conducted taking into account the characterisations of the complex dielectric permittivity (real and imaginary parts) as a function of temperature and frequency in a wide interval. On the other hand, the dc 'bias' electrical field dependence of the real dielectric permittivity (C–V characterisations) has been analysed around the paraelectric–ferroelectric diffuse phase transition. Results obtained at room temperature (paraelectric phase) showed a typical behaviour of conventional dielectric materials, while measurements performed at 200 K (ferroelectric phase) evidenced an anomalous behaviour for frequencies up to 100 kHz. Such observed anomalies were characterised by two asymmetrical peaks on sweep-up and sweep-down, at ? 15 and + 18 kV cm^?1 respectively.     

Effect of ionizing radiation in ultrasmall doses on dielectric properties of CdZnTe crystals with anomalously high polarizability

The effect of ultrasmall exposure doses (10–45 R) of gamma-radiation on both real and imaginary parts of the complex dielectric permittivity of CdZnTe crystals in the low-frequency range has been discovered. It is shown that the observed effect is related to (i) changes in the system of intrinsic structural defects, which cause a deviation of the crystal composition from stoichiometry and (ii) growth-related large-scale fluctuations of the electric potential.     

Structural and analytical characterization of Ag(Br,I) nanocrystals by CRYO-AEM techniques

The morphology, crystalline and electronic structures and elemental compositions of AgX (X = Br,I) nanocrystals of 20–185 nm in diameter have been evaluated by cryo-energy-filtering transmission electron microscopy (EFTEM)/electron spectroscopic diffraction (ESD)/ electron energy-loss spectroscopy (EELS) and cryo-energy-dispersive X-ray (EDX) analysis in the scanning transmission (STEM) mode. A low-loss fine structure in EEL spectra between 4 and 26 eV was attributed to excitons and plasmons possibly superimposed with interband transitions and many-electron effects. Contrast tuning under energy filtering was used to image electron excitations in the AgX nanoparticles. Intensity oscillations caused by size-confined coupling of surface and volume losses were observed. The real and imaginary parts, ε₁ and ε₂, of dielectric permittivity were determined using Kramers-Kronig relations. The exciton peaks were assigned based on calculations of electronic band structure of AgBr. Combined silver and halide distributions were obtained by electron spectroscopic imaging (three-window method) and by STEM/EDX including high-magnification X-ray mapping of the individual nanocrystals.     

多壁碳纳米管/环氧有机硅树脂吸波涂层的介电和吸波性能研究

研究了不同直径和含量多壁碳纳米管填充环氧有机硅树脂吸波涂层在2~18GHz频率范围内的介电和吸波性能.可以得到吸波涂层的介电常数随着碳纳米管含量的增加而增大.当碳纳米管含量相同时,吸波涂层介电常数随着碳纳米管直径的增加而增大.当碳纳米管含量大于5wt%时,吸波涂层的介电常数在低频急剧增加,且随频率增大而减少,出现频散效应.反射率测试结果表明:当涂层中多壁碳纳米管含量为10wt%、厚度为2mm时,吸波涂层的最大吸收峰随碳纳米管直径的增大向低频移动.多壁碳纳米管填充环氧有机硅树脂吸波涂层的吸波性能在7~14GHz范围内可达到-10dB,具有较好的吸波效果.     

Millimeter- and Submillimeter-Wave Dielectric Measurements of Household Powders Using Fourier Transform Spectroscopy

This paper presents the first step in characterizing and detecting common household powders as hoax materials by investigating their broadband dielectric characteristics. Pesticide is also studied due to its similarities to anthrax. A modified two-beam polarizing interferometer and custom-made sample holders were utilized to obtain the refractive index (RFI), absorption coefficient, complex real and imaginary permittivity, and loss tangent of flour, dry milk, cornstarch, pesticide, baking soda, and talc as a continuous function of frequency in the range of 400-1200 GHz. Dispersive Fourier transform spectroscopy was employed to provide phase and amplitude information, leading to the accurate and direct acquisition of the complex RFI and complex dielectric permittivity spectra. It is the first time that such measurements were carried out over such an extended frequency range for common powders to reveal marked variation in absorption and RFI values for different powders. The powders exhibit high-absorption characteristics, and the RFI values lie between 2.4 and 3.4 in this spectral range. A unique resonance signature for talc was detected at 1135 GHz with an associated anomalous dispersion. Parameters such as grain size and orientation were analyzed for their effect on the dielectric properties of powders. Relationships between density, permittivity, and RFI are explored to provide the ability to extrapolate results for other densities. The RFI and the real part of the dielectric permittivity were found to be the most reliable for identification and detection purposes.     

A simple apparatus to plot the variations of the dielectricpermittivity versus temperature in the range of frequencies 10 Hz-100kHz

An apparatus able to plot the variations of the real and imaginary parts of the dielectric permittivity of materials versus temperature in the range of frequencies from 10 Hz to 100 kHz is presented. The circuit is based on a simple operational amplifier followed by detection, low-pass filters, and amplifiers. The resulting two DC voltages, which are proportional to ϵ' and ϵ", are sent to a plotter whose x input is driven by the temperature of the sampled     

Electrical Relaxation in Mixed Alkali Bi2O3-K2O-Li2O-Fe2O3 Glasses

A new glass system(Bi2O3)50(Fe2O3)10(Li2O)x(K2O)40-x, where x changes in steps of 5 mole fraction between 0 and 40, was selected to study the electrical relaxation and the mixed alkali effect(MAE)phenomena.Measurements of ac conductivity σac, dielectric permittivity ε' and loss factor tanδ in the frequency range of 0.12～102 kHz and in the temperature range of 300～650 K were carried out. The temperature dependence of the ac conductivity shows a slow increasing rate at low temperature and high frequency and a rapid increase at high temperature and low frequency. At constant temperature, the ac conductivity is found to be proportional to ωs, where s is the frequency exponent, which is less than 1. Analysis of the conductivity data and the frequency exponent shows that the overlapping large polaron tunnelling(OLPT)model of ions is the most favorable mechanism for the ac conduction in the present glass system. The ac response, the dc conductivity and dielectric relaxation have the same activation energy and they originate from the same basic transport mechanism. The results of the dielectric permittivity show no maximum peak in the temperature and frequency range studied. This absence of maximum peak is an indication of non-ferroelectric behavior of all the studied samples. The MAE has been detected in the ac conductivity, which is the same as the classical MAE in the dc conductivity. The electrical parameters such as dielectric permittivity ε' and real dielectric modulus M'show a typical minimum deviation from linearity by about two orders of magnitude. The loss factor tanδ and the imaginary dielectric modulus M" are insignificantly dependent on composition even at the same transition temperature Tg.     

Dielectric dispersion characteristics of sand contaminated by heavy metal, landfill leachate and BTEX (02-104B)

Experiments have been performed to investigate the dielectric properties of contaminated sand. The separate real and imaginary parts of a dielectric constant were investigated in the frequency range of 75 kHz to 12 MHz. The contaminated soils exhibit different complex dielectric dispersion from the uncontaminated soils. The variation in the real dielectric constant can be explained by a polarization mechanism while that in the imaginary dielectric constant by ionic conductivity loss mechanism. The differences of the dielectric behavior with contaminant types suggest that the monitoring of complex dielectric constant has the potential to classify contaminants. The additional analysis for the imaginary part of the dielectric constant can be recommended to obtain the clear information about the state of ionic contaminants in subsurface.     

Dielectric properties of BaBi2Nb2O9 ceramics

The crystal structure and dielectric properties as a function of temperature for Ba-based with Bi-layered structure BaBi₂Nb₂O₉ (BBN) ceramics were investigated. The obtained results confirmed the relaxor ferroelectric behavior of the studied ceramics, including a strong frequency dispersion of the permittivity maximum and a visible shift of its temperature with frequency. Analysis of the real and imaginary part of permittivity allowed us to determine the values of Burn’s temperature and of the freezing temperature characterizing the relaxor ferroelectrics. The physical processes, responsible for the relaxor behavior of the studied ceramics are discussed. The additional low frequency dielectric dispersion at high temperatures in the paraelectric phase range was also observed. Correlation between this dispersion and the thermally stimulated depolarization current was ascertained.     

Dielectric relaxation related to single-ionized oxygen vacancies in (Pb1−xLax)(Zr0.90Ti0.10)1−x/4O3 ceramics

The dielectric relaxation phenomenon has been studied in lanthanum modified lead zirconate titanate ceramics in the high temperature paraelectric phase. The high temperature dielectric response revealed an anomalous behavior, which is characterized by an increase of the real component of the dielectric permittivity with the increase of the temperature. At the same time, a similar behavior, with very high values, has been observed in the imaginary component of the dielectric permittivity, which can be associated with conduction effects related to the conductivity losses. The frequency and temperature behavior of the complex dielectric permittivity has been analyzed considering the semi-empirical complex Cole-Cole equation. The activation energy value, obtained from the Arrhenius’ dependence for the relaxation time, was found to decreases with the increase of the lanthanum concentration and has been associated with single-ionized oxygen vacancies. The short-range hopping of oxygen vacancies is discussed as the main cause of the dielectric relaxation.     

Complex permittivity measurements of ferroelectrics employing composite dielectric resonator technique

Composite cylindrical TE(0n1) mode dielectric resonator has been used for the complex permittivity measurements of ferroelectrics at frequency about 8.8 GHz. Rigorous equations have been derived that allowed us to find a relationship between measured resonance frequency and Q-factor and the complex permittivity. It has been shown that the choice of appropriate diameter of a sample together with rigorous complex angular frequency analysis allows precise measurements of various ferroelectric. Proposed technique can be used for materials having both real and imaginary part of permittivity as large as a few thousand. Variable temperature measurements were performed on a PbMg(1/3)Nb(2/3)O3 (PMN) ceramic sample, and the measured complex permittivity have shown good agreement with the results of measurements obtained on the same sample at lower frequencies (0.1-1.8 GHz).