同轴传输反射法测量高损耗材料微波介电常数

同轴传输反射方法可以用来测量高损耗材料的微波介电常数。该方法将环形样品嵌入同轴线内,通过测量样品两端的散射系数来确定材料的微波介电常数。文中介绍了测量原理及测量系统,测量了一种石蜡基混合样品的微波介电常数,并通过改进的Bruggeman数学模型,推算出其中陶瓷材料的介电常数。     

Measurement of the reflection and transmission properties of conducting fabrics at milli-metric wave frequencies

There is increasing interest in conducting fabrics and their uses at radio frequencies and microwave frequencies. The reflection and transmission measurements of bobbinet and knitted materials from around 8 GHz into the milli-metric frequency range 110 GHz, where the material geometry is comparable to the wavelength of the wave are considered. Bobbinet materials were found to behave like lossy dielectrics and may be useful in the construction of thin lightweight screening and absorption planes. While the knitted materials, with very small mesh geometry, gave a reflection coefficient which was comparable to a metal foil     

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.     

Hydration monitoring of cement-based materials with resistivity and ultrasonic methods

Two test setups, the electrical resistivity and ultrasonic techniques, were used to monitor the hydration process of cement-based materials. In the electrical resistivity method, a non-contacting device was used. In the ultrasonic method, a wave was transmitted and measured by the embedded piezoelectric ultrasonic transducers, which had good coupling with the surrounding materials. The focus of the study was to detect the setting and hardening behaviors of cement paste during the first 7 days of hydration using the above techniques. Immediate after placing the cement paste into the mould, the measurement started and continued throughout the hydration process. The obtained resistivity and ultrasonic data were used to interpret the hydration process of the specimens. The correlation of two techniques was also studied. The results illustrated that both electrical resistivity and ultrasonic techniques were effective to accurately monitor the hydration of cement pastes. The resistivity method was able to study both the chemical reaction and physical change during hydration, while ultrasonic method was sensitive to physical change of cement only.     

Microwave transmission,reflection and dielectric properties of conducting and semiconducting polypyrrole films and powders

Microwave transmission, reflection and some dielectric properties of the conducting polymer, polypyrrole, are presented. Methods are discussed for determining microwave transmission and reflection of electrochemically synthesized and doped polypyrrole films with conductivities ranging from 0.1–5000 S m⁻¹. Polypyrrole films were placed between waveguides and irradiated with microwaves centred at frequencies 2.45 and 10 GHz with 0.1 GHz span. The results indicate that the conductivity of doped polypyrrole films has a significant effect on both transmission and reflection. Microwave opacity of polypyrrole varied with the synthesis conditions of the polymer. Samples with low conductivity exhibited high transmission whereas low transmission readings were observed with highly conducting films. Dielectric properties are also presented for frequencies from 100 to 10⁶ Hz in a temperature range of 90–280 K and at microwave frequencies of 2.45 and 10 GHz. These measurements indicate that the real and imaginary parts of the dielectric constant increase in magnitude with increasing doping level.     

纳米TiO2与水泥复合材料的吸波机理探讨

纳米TiO2作为微波吸收剂,掺入到水泥材料中,使水泥基复合材料具有一定的吸波功能.通过对掺有锐钛型TiO2、纳米锐钛型TiO2和纳米金红石型TiO2的水泥基复合材料的导电性能、电磁参数和反射率的分析比较,探讨了纳米TiO2与水泥复合材料的吸波机理.由于纳米TiO2的小尺寸效应、表面效应和量子尺寸效应,使纳米TiO2与水泥复合材料在高频段体现更好的吸波性能,当纳米TiO2掺量为5%时,在12.5～18GHz频率范围内复合材料的反射率基本上都＜-0dB,最小反射率达-16.34dB.     

Interpretation of reflection and transmission spectra for thin films: transmission

The optical behavior of a thin film, that is, peak positions and intensities, is discussed for transmission under a thin-film approximation. The infrared transmission spectra of thin films, both standing films and those on dielectric substrates, are simulated for s and p polarization at various angles of incidence. For spectral simulation, the matrix method is used in conjunction with noise-free complex refractive indices based on the dispersion theory. The peak positions in the simulated spectra are compared with transverse optic and longitudinal optic frequencies based on the macroscopic theory. The simulated peak intensities for the standing films are compared with the prediction based on the thin-film approximation. Furthermore, it is found from the spectral simulation for thin films on dielectric substrates that the peak intensity for a thin film may depend on the thickness and refractive index of the substrate.     

碳纤维增强水泥基复合材料的制备及热电性能研究

将不同含量(0.5%,1.0%,1.5%(质量分数))的碳纤维掺入到硫铝酸盐水泥基体中,制备了碳纤维增强水泥基复合材料。通过SEM、阿基米德排水测试法、四探针法等手段,研究了碳纤维含量对增强水泥基复合材料断面结构、抗弯强度、孔隙率、电导率、热导率和塞贝克系数的影响,并模拟太阳辐射进行了能量收集实验。结果表明,碳纤维均匀地分布在水泥基体中形成网格结构,碳纤维与水泥基体有很强的结合力。当碳纤维含量由0.5%(质量分数)增加到1.5%(质量分数)时,水泥基复合材料的抗压强度由71.36 MPa增加到106.51 MPa,增长了49.26%;孔隙率由0.8%增加到2.0%,增长了150.0%;电导率由0.0214 S/m增加到0.2408 S/m,增长了1025%;热导率由0.261 W/(m·K)减小到0.210 W/(m·K),减少了19.54%;塞贝克系数迅速增大,最大为1.22×10^4μV/K。当碳纤维含量为1.5%(质量分数)时,厚度为20 mm的水泥基复合材料每1 m^2可输出5~6μW的功率;在400 min辐照下,试样表面温度迅速达到70℃左右,1 m^2水泥基复合材料面板上收集到的能量高达8.1×10^-6 J。由此可知,碳纤维含量的增加,极大地提高了碳纤维增强水泥基复合材料的热电性能。     

碳纤维增强水泥基复合材料的吸波性能研究

利用弓形反射法(NRL)测试了碳纤维掺量分别为0.2%、0.4%、0.6%、0.8%和1.0%(质量分数)时碳纤维增强水泥基复合材料(CFRC)在低频段4～8GHz和高频段8～18GHz时的反射率,讨论了纤维掺量、频率、反射率之间的关系.结果发现,在纤维掺量相同条件下:低频段时,反射率＜-10dB,CFRC表现出吸波性;高频段且纤维掺量超过0.6%(质量分数)时,反射率＞-10dB,CFRC对电磁波表现出反射性.随着纤维掺量的增加,低频段时反射率先降低、后又有所回升,吸波性由弱变强、又变弱,纤维掺量为0.6%(质量分数)时出现最小反射率-15.0dB;高频段时反射率总体上呈上升趋势,材料对电磁波的反射性越来越强,纤维掺量为0.4%(质量分数)时出现最小反射率-19.4dB.     

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.     

Hardening and properties of cement-based materials incorporating heavy metal oxides

The subject of the study was the influence of oxides PbO, ZrO₂ and Cr₂O₃ immobilized in cement matrix. The obtained results show the positive effect of the present oxides in hydrating cement on compressive strength development and the quality of the formating pore structure of the resulting material. A surprising effect showing the paralysis of the positive effect of heavy metal oxide admixture has been shown by the increasing of the added quantity of PbO (from 1 to 4%) and the used w/c ratio (0·7 instead of 0·4). The increase of the quantity overcoming the optimum of the reaction product of the interaction between hydrating cement and PbO added for the cause of the paralysis is supposed. The explanation of the adverse effect and the identification of the reaction product in question needs more detailed study.     

反射型和透射型导模共振滤光片的设计

本文设计了中心波长为1315 nm的反射型和透射型导模共振滤光片,并利用严格耦合波法分析了其光谱特性。首先设计了中心波长1315 nm单层反射型导模共振滤光片,但由于覆盖层和基底层的折射率不匹配,使得远离中心波长处的反射率偏高,引入减反射层设计后,中心波长两侧的反射率明显降低。其次设计了中心波长1315 nm的透射型导模共振滤光片,分析了光栅层厚度对中心波长位置的影响。用严格耦合波法计算表明,设计的反射型和透射型导模共振滤光片具有优良的光谱性能。     

掺铁氧体和石墨水泥基复合材料吸收电磁波性能

研究了添加铁氧体和石墨水泥基复合材料(F/G/C)的吸波性能。通过改变铁氧体、石墨的配比,分析了影响该材料电磁吸收效能的主要因素,并探讨了其影响机制。结果表明:石墨添加量20 vol%时材料出现逾渗现象,逾渗区内介电损耗有较大值;0.5～4 GHz范围内影响吸收深度的最主要因素是石墨,影响吸收宽度的最主要因素是铁氧体,最佳水平组合为20 wt%60μm铁氧体、10 wt%250μm铁氧体、30 vol%石墨,吸收深度可达-32 dB且明显优于单组分复合;吸收主要机制是磁损耗,石墨的加入改善了材料空间波阻抗。     

A transmission electron microscopy study of interfaces and matrix homogeneity in ultra-high-performance cement-based materials

Ultra-high-performance cement-based materials produced under different conditions have been characterized by transmission electron microscopy (TEM), scanning transmission electron Microscopy (STEM), high resolution transmission microscopy (HRTM) and chemical analysis. In addition to cement, these materials contain large amounts of crushed quartz and amorphous submicrometre silica. A post-set heat treatment was also applied in some cases. An abrasive thinning method combined with grazing angle ion etching allowed the preparation of 100 nm thick specimens with wide observation surface areas while avoiding any water or CO₂ contact which may cause changes. Clinker, silica fume and crushed quartz reactivity as a function of the curing processes have been studied, as well as the interfacial zones with the hydrated matrices. The Ca/Si ratio spatial distribution in hydrated products has been analyzed and shown to undergo strong local fluctuations. Nevertheless, the composition fluctuations were less pronounced and the average Ca/Si ratio was lower than in silica-free cement paste. HRTM lattice imaging shows the coexistence of nanocrystalline phases and mesoscale ordered regions within an amorphous matrix. A d-spacings analysis of the nanocrystalline phase suggests a tobermorite-like structure for the calcium silicate hydrates, whereas the mesoscale order might reflect modulations in the water content.