Measurement of dielectric properties of high-absorption materials at microwave frequencies

A method is proposed for measuring the dielectric constant ε′ and dielectric loss ε″ of high-absorption dielectric materials by measuring the reflection of electromagnetic radiation with the aid of an adjustable quarter-wave matching plate of nonabsorbing material close to the dielectric surface. Translated from Izmeritel'naya Tekhnika, No. 5, pp. 45–47, May, 1999.     

Dielectric and electrical properties of ordinary Portland cement and slag cement in the early hydration period

The dielectric constant and electrical conductivity of ordinary Portland cement (OPC) with water-cement ratios (w/c) of 0.30, 0.35 and 0.40 were measured for the first 30 h hydration, using a microwave technique in the frequency range 8.2–12.4 GHz. It was found that both the dielectric constant and electrical conductivity of the cement paste are sensitive to the water-cement ratio, the higher the w/c value, the greater the dielectric constant and electrical conductivity, and the longer the hydration time. We also found that the higher the frequency the greater the electrical conductivity but the smaller the dielectric constant. The dielectric constant and electrical conductivity of high- and low-slag cement with water-solid ratio (w/s) of 0.40 were measured in the first 30 h after mixing. The changes in dielectric constant and electrical conductivity of low-slag cement with time are similar to that of OPC, but the high-slag cement shows very different dielectric and electrical properties compared with OPC and low-slag cement. The relationship between the dielectric and electrical properties of cement paste and cement hydration was also discussed.     

Electrical properties of films at microwave frequencies

A technique suitable for the measurement of electrical properties of films/sheets at microwave frequencies has been described. Groups of materials are identified for which the absence of any kind of electrodes in these microwave measurements can be utilised with advantage. Results are presented for a variety of materials such as metal films, superionic conductors, semiconductors and insulating polymers.     

Measurement of the dielectric constant of highly absorbing polar liquids at microwave frequencies

Nomograms are given for claculating the dielectric constant and dielectric loss of polar liquids by measuring the standing wave ratio for an infinitely thick layer of fluid and the layer thickness at which wave reflection is minimum. Translated from Izmeritel'naya Tekhnika, No. 2, pp. 46–47, February, 1996.     

Sintering behaviors,microstructure and dielectric properties of Ce0.5Nd0.5O1.75 ceramic at microwave frequencies

Journal of Materials Science: Materials in Electronics - A new temperature-stable Ce0.5Nd0.5O1.75 ceramics were synthesized by the conventional solid-state reaction method, and its sintering...     

Dielectric properties of conducting polymer composites at microwave frequencies

The dielectric properties of conducting polymer composites containing polypyrrole (PPy) crushed films, PPy powder, polyaniline (PAn) base and acid powders as the dispersants and silicone rubber and vinyl ester as matrix materials have been investigated in the frequency range 2-18 GHz. The dielectric parameters such as the real part, , and imaginary part, , of the permittivity and loss tangent, tan, increase with increasing conductivity and concentration of the dispersant. The geometrical shape of the dispersant governs the ability of conductive network formation which is indicated by a large drop in the resistivity of the composite. Also, dispersant/matrix interactions and physical properties of the matrix influence the agglomeration of the dispersant phase which, in turn, affects the dielectric properties of the composites. Flakes of PPy obtained by crushing highly conductive films and large PAn powder aggregates were unable to form a conducting network. The composites without a network of dispersant exhibit low dielectric parameters. On the other hand, high values of tan ranging from 0.7–1.1 were achieved for the PPy powder (15 parts)/silicone rubber composites where a conducting network was observed.     

A free-space method for measurement of dielectric constants andloss tangents at microwave frequencies

For measurements of dielectric properties of planar slabs of ceramic and composite materials, the authors have developed a free-space measurement system in the frequency range of 14.5-17.5 GHz. The key components of the measurement system are a pair of spot-focusing horn lens antennas, a network analyzer, and a computer. Because of the far-field focusing ability of horn lens antennas, the free-space measurements can be made at microwave frequencies in a relatively compact and simple measurement setup. The time-domain gating feature of the network analyzer and the thru, reflect, and line calibration technique were used to eliminate the effects of undesirable multiple reflections. The dielectric constants and loss tangents were measured for standard materials such as fused quartz, Teflon, and polyvinylchloride (PVC). Dielectric properties for Teflon and PVC were also measured in a waveguide medium for purposes of comparison with the free-space method     

Some properties of nickel-coated carbon fibre-polypropylene composite at microwave frequencies

Some properties of nickel-coated carbon fibre-polypropylene composite are considered at microwave frequencies in the range 8 to 12 GHz. The paper presents the measured values of the insertion and return losses and the equivalent input impedance in the above frequency band. Measurements were performed on composite material for different nickel-coated fibre concentrations of 0, 5, 10, 16 and 28 wt%. The filler effects shows a transition region and identifies the dependency of the electrical properties on these concentrations. Furthermore, shielding effectiveness is calculated for the given concentrations and it is found that its value could reach more than 30 dB over the whole frequency band for a specimen of 2.6 mm thickness and with 28 wt% concentration.     

Measurement of the magnetic properties of liquid magnetic materials at microwave frequencies

Equations are obtained which give a unique functional relation between the required magnetic properties of a liquid magnetic material and the informative parameters on the position and value of the extrema of the experimental dependence of the reflected signal on the thickness of the layer of material. Various methods and algorithms for solving them are considered, taking the degree of attenuation of the wave in the material into account. __________ Translated from Izmeritel’naya Tekhnika, No. 4, pp. 47–49, April, 2007.     

纳米炭黑水泥砂浆的导电性与电热特性研究

研究了纳米炭黑水泥砂浆的导电性和电热特性.研究表明,当炭黑掺量＜2.5%时,随着炭黑掺量的增加,电阻率下降明显;炭黑掺量为2.5%时,炭黑水泥砂浆电阻率可达到102Ω·cm;而炭黑掺量＞2.5%时,电阻率下降趋缓.在炭黑掺量为5%左右电阻率基本不变,电阻率在30～40Ω·cm之间.与石墨粉相比,纳米炭黑做导电相只需较小掺量,就可制备满足电热功能应用的导电混凝土.当炭黑掺量＞1%时,龄期对电阻率影响小,其电阻比较稳定.炭黑水泥砂浆板电功率稳定、电热功能重复性好.     

Free-space, broadband measurements of high-temperature, complexdielectric properties at microwave frequencies

A free-space microwave measurement system that is used for the high-temperature measurement of dielectric constants and loss tangents of homogeneous materials and that is applicable to composite materials as well is discussed. The system is capable of operating in the 5.85-40-GHz frequency range and ambient to 850°C temperature range. A computer is used to control and coordinate furnace temperature, network analyzer functions, and data storage. Dielectric constants and loss tangents of the materials are calculated from the measured values of S₂₁. The measurement system, including the high-temperature furnace and the calibration technique, is described. Dielectric constants and loss tangents are presented for fused quartz and boron nitride grade HP in the frequency range 13.0-17.4 GHz and the temperature range from ambient to 850°C     

基于谐振法测试微波介质材料的介电参数

介电常数是介质材料的主要性能参数之一,该文讨论了微波介质的介电参数的理论计算,分析了谐振法的模型,研制了基于谐振法测量微波介质材料介电参数的测量系统.通过对多个样品进行实测,结果表明与参考值比较吻合,评价了该测量系统测量结果不确定度,从而建立了微波频段内微波介质材料介电常数测量平台.     

Metal slits and liquid crystals at microwave frequencies

While the use of liquid crystals (LCs) over the visible region is ubiquitous in flat-screen displays, there has been little by the way of applications at other wavelengths. Specifically, notwithstanding the continuing permittivity anisotropy to be found at longer wavelengths, there has been virtually no development in the microwave domain. This is largely due to the fact that scaling up the micrometer-thick LC layers used for visible radiation to millimetre dimensions is seen as impractical. In this study, it is shown how, using thin slits in metal structures, a completely new generation of LC devices for use at microwaves may be realized. Such structures include slatted metal Fabry-Perot resonators, beam-steering devices, thin flexible voltage tunable filters and even cascade structures with strongly enhanced and reshaped microwave fields.     

Sintering and microwave dielectric properties of LTCC-zinc titanate multilayers

The effects of ZnO–B₂O₃–SiO₂ (ZBS) on the sintering behavior and microwave dielectric properties of ZnO–TiO₂ system were investigated as a function of ZBS content and sintering temperature. Densities of the specimens were enhanced with an increase of ZBS up to 2 wt.% and then decreased. X-ray diffractometry analyses results indicated that the phase stability region of the hexagonal ZnTiO₃ extended to lower temperatures as the amount of ZBS increased. The dielectric properties of ZnO–TiO₂ system with ZBS are strongly dependent on the sintering conditions, especially near the phase decomposition temperature. The sintering temperature of the specimens could be reduced to 900 °C without the degradation of the microwave dielectric properties. From 900 °C, the temperature compensation characteristics occurred as the phase composition changed from ZnTiO₃ to two phases: Zn₂TiO₄ and rutile. The dielectric constant (ɛ_r) increased and Q×f value decreased due to the phase decomposition. The ɛ_r value of 27, Q×f value of 19,396 (at 6 GHz) and τ_f value of 2 ppm/°C were obtained for ZnO–TiO₂ ceramics with 2.0 wt.% ZBS sintered at 900 °C for 3 h. The low-temperature sintering ceramics powders were suitable for the tape casting process. Also, the material is compatible with Ag electrodes and, therefore, is suitable for LTCC application.     

Local electrical and dielectric properties of nanocrystalline yttria-stabilized zirconia

Grain core and grain boundary electrical and dielectric properties of nanocrystalline yttria-stabilized zirconia (YSZ) were analyzed using a novel nano-Grain Composite Model (n-GCM). Partially sintered pellets with average grain sizes ranging from 10 to 73 nm were analyzed over a range of temperatures using AC impedance spectroscopy (AC-IS). Local grain core and grain boundary conductivities, grain boundary dielectric constants, and effective grain boundary space charge widths were determined from the fitted circuit parameters. Required grain core dielectric constant data were provided by AC-IS measurements of single crystal YSZ over a range of temperatures. The local grain core conductivity of all the nanocrystalline samples was slightly decreased with respect to that of microcrystalline YSZ. Conversely, the local grain boundary conductivity was enhanced up to an order of magnitude compared to microcrystalline YSZ. At the nanoscale, there was a noticeable increase in local grain boundary dielectric constant versus single crystal values at the same temperature.     

ZnO掺杂对CLST微波介质陶瓷烧结及介电性能的影响

研究了烧结助剂ZnO对CaO-Li2O-Sm2O3-TiO2(简写为CLST)系微波介质陶瓷的烧结特性及介电性能的影响.结果表明:ZnO的添加能有效地降低CLST陶瓷的烧结温度至1150℃.掺杂2wt%ZnO的CLST陶瓷取得了较好的介电性能:εr=84,tanδ=0.009,τf=-15ppm/℃.