微波遥感典型地物介电常数实地测量

在微波遥感定量化研究中,要求对遥感目标的介电特性有较准确的了解.为此采用自由空间法在野外实地测量典型目标的介电常数.给出了基本原理,定标方法及在X波段对黑土,草地,和冰所测的介电常数。分析比较了测量结果。     

NY-1高功率微波辐射源功率密度的简易测量

微波的生物热效应能使蛋白质变性.蛋白质变性所需要的辐照时间取决于入射的功率密度,所以高功率微波功率密度的测量是研究微波生物效应的基础.利用纸水混合介质的介电常数,计算出微波波阻抗,利用介质和空气界面上的反射系数和介质中衰减系数以及蒸发的水的质量,从而求出入射的微波功率密度.     

Valence state of europium and samarium in Ln2Hf2O7 (Ln = Eu,Sm) based oxygen ion conductors

Ln₂(Hf_2-xLn_x)O_7-x/2 (Ln = Sm, Eu; x = 0.1) pyrochlores have been prepared via mechanical activation of oxide mixtures, followed by heat treatment for 4h at 1450 and 1600 °C, respectively. According to the ESR data, the Eu cations on the Hf site in the Hf_1-xEu_xO₆ octahedra in pyrochlore Eu₂(Hf_2-xEu_x)O_7-x/2 (x = 0.1) are most readily oxidized and reduced. Oxidation at 840 °C for 24h in air reduces the total conductivity of the Ln₂(Hf_2-xLn_x)O_7-x/2 (Ln = Sm, Eu; x = 0.1) by a factor of 2.5–6, due to the decrease in the concentrations of oxygen vacancies and Ln²⁺ ions as a result of the oxidation. The anomalous low-frequency behavior of the permittivity of the Eu₂(Hf_2-xEu_x)O_7-x/2 (x = 0.1) at ~800 °C can be understood in terms of the changes in the oxygen sublattice of the pyrochlore structure as a result of the oxidation of divalent europium and partial filling of oxygen vacancies at this temperature.     

Nano-SiC-decorated Y2Si2O7 ceramic for enhancing electromagnetic waves absorption performance

To improve the electromagnetic (EM) wave absorption performance of rare earth silicate in harsh environments, this work synthesized dense SiC–Y₂Si₂O₇ composite ceramics with excellent EM wave absorption properties by using the polymer permeation pyrolysis (PIP) process, which introduced carbon and SiC into a porous Y₂Si₂O₇ matrix to form novel composite ceramics. SiC–Y₂Si₂O₇ composite ceramics with different numbers of PIP cycles were tested and analysed. The results show that the as-prepared composites exhibit different microstructures, porosities, dielectric properties and EM wave absorption properties. On the whole, the SiC–Y₂Si₂O₇ composite ceramics (with a SiC/C content of 29.88 wt%) show superior microwave absorption properties. The minimum reflection loss (RL_min) reaches ?16.1 dB when the thickness is 3.9 mm at 9.8 GHz. Moreover, the effective absorption bandwidth (EAB) included a broad frequency from 8.2 GHz to 12.4 GHz as the absorbent thickness varied from 3.15 mm to 4.6 mm. In addition, the EM wave absorption mechanism was analysed profoundly, which ascribed to the multiple mediums of nanocrystalline, amorphous phases and turbostratic carbon distributed in the Y₂Si₂O₇ matrix. Therefore, SiC–Y₂Si₂O₇ composite ceramics with high-efficiency EM wave absorption performance promise to be a novel wave absorbing material for applications in harsh environments.     

Nondestructive characterization of Salisbury screen and Jaumann absorbers using a clamped rectangular waveguide geometry

A nondestructive technique to characterize Salisbury screen and Jaumann absorbers is presented. The proposed method utilizes two flanged rectangular waveguides to unambiguously determine the permittivities of two-layer dielectric absorbers. The derivation of the theoretical reflection and transmission coefficients, necessary to determine material under test permittivities, is presented. The derivation makes use of Love’s equivalence principle and the continuity of transverse magnetic fields to formulate a system of coupled magnetic field integral equations. These integral equations are solved using the Method of Methods to yield theoretical scattering parameters. The unknown permittivities are then found using nonlinear least squares. To validate the proposed nondestructive technique, measurement results of three two-layer dielectric absorbers are presented and analyzed. In addition, an extensive error analysis is performed on the extracted permittivity values. The results of the proposed method are found to be in good agreement with the results returned by traditional, destructive waveguide transmission/reflection approaches.     

Determining the permittivity profile inside reinforced concrete using capacitive probes

Non-destructive techniques are the future of structure health assessment and monitoring. The dielectric permittivity is sensitive to the water and ionic content of concrete and it can be measured by capacitive probes. In this paper, an inversion method to obtain the permittivity profile inside reinforced concrete from surface capacitive measurements is presented. The inversion process is applied to synthetic models using two forms of parametrizations (discrete and continuous). In addition, the effect of data noise on the inversion is studied. At last, water saturation profiles inside a concrete slab are obtained by means of inversion of experimental capacitive data.     

Ac electrical behavior of a novel aromatic electro-optic polyimide

AC electrical behavior of a novel aromatic electro-optic polyimide was investigated in the temperature range 25°C to 300°C and covers frequency range from 1 Hz to 10 6 Hz. Three electrical quantities, ac impedance, conductivity and permittivity were reported. The experimental results show that the relative permittivity of aromatic electro-optic polyimide is temperature independent below 200°C, indicating that the chains below this temperature are nearly rigid. Above this temperature a relaxation process was observed with an activation energy 35 KJ/mole associated with a restricted rotational motion of the side chain chromophore.     

Analysis of MWCNT/epoxy composites at microwave frequency: reproducibility investigation

A wide-band microwave characterization of nanocomposites based on commercial multiwalled carbon nanotubes (MWCNTs) and epoxy resin is presented. The sample preparation method is discussed in detail. Field emission scanning electron microscopy is used for morphological sample analysis of nanocomposites and MWCNTs. The complex permittivity is measured in a wide frequency band (3 to 18 GHz) using a commercial dielectric probe (Agilent 85070D) and a network analyzer (E8361A). A statistical analysis based on one-way analysis of variance (ANOVA) technique is performed. The aim of this statistical analysis is to investigate the influence of concentration of nanoparticles inside the polymer matrix on the complex permittivity. This can be significantly different in nanocomposites even if the samples have similar electrical properties.     

Ho-doped BaTiO3: Polymorphism, phase equilibria and dielectric properties of BaTi1−xHoxO3−x/2: 0 ≤ x ≤ 0.17

An extensive range of Ho-doped BaTiO₃ solid solution forms in which Ho substitutes for Ti with creation of oxygen vacancies. The effect of Ho substitution is to destabilise thermodynamically the high-temperature hexagonal polymorph of BaTiO₃. Nevertheless, at high Ho contents, the hexagonal polymorph forms as a kinetically stable intermediate before transforming to the thermodynamically stable cubic polymorph; its formation represents an example of Ostwald's rule of successive reactions. Samples fired at 1400 °C and cooled in air are insulating and transform from ferroelectric to relaxor ferroelectric behaviour with increasing x.     

Characterization of Si-SiOx nanocomposite layers by comparative analysis of computer simulated and experimental infra-red transmission spectra

The infra-red (IR) transmission spectra of SiO_x (x ≤ 2) layers containing crystalline or amorphous Si nanoparticles deposited on p-Si substrates is simulated in the range 300-1500 cm^− 1. To that purpose the average dielectric function of the nanocomposites is calculated by means of the Bruggeman effective medium approximation. The IR spectra of the system (film and substrate) are computed. The results are compared with experimental IR spectra measured on Si-SiO_x nanocomposite layers with identical composition, fabricated by thermal evaporation of SiO in vacuum followed by thermal annealing at 700 °C or 1030 °C. A good correspondence between theory and experiment is found from where valuable information about important characteristics of the investigated nanocomposites is obtained, such as matrix density, homogeneity and composition of the layers.