碳纳米管/聚苯胺纳米复合管的制备及其微波介电特性研究

用原位乳液聚合法在碳纳米管表面包覆聚苯胺，制备出了碳纳米管/聚苯胺一维纳米复合管.复合管的直径为50—80 nm，聚苯胺包覆层的厚度为20—30 nm，聚苯胺在碳纳米管表面以层状和枝晶状两种形态生长.研究了碳纳米管/聚苯胺复合管在2—18 GHz的微波介电特性.与纯碳纳米管相比，碳纳米管/聚苯胺复合管的介电常数的实部ε′和虚部ε″在2—18 GHz随频率变化较小，在低频波段介电常数值较小，作为微波吸收剂容易实现与自由空间的阻抗匹配，而且它的介电损耗角正切(tanδ=ε″/ε′)较高，是一种很好的微波吸收剂. 关键词： 碳纳米管 聚苯胺 微波介电特性 微波吸收剂     

Microwave radiation absorbers based on corrugated composites with carbon fibers

A complex analysis of the dependence of the absorption coefficient of polymer composites with nonmagnetic carbon inclusions on the real and imaginary parts of the complex permittivity, as well as on the material thickness is performed in frequency range 26–37 GHz. The composites containing 0.2 wt % of carbon fibers have been obtained. It has been experimentally found that the corrugation of the composite surface substantially increases the absorbability (from 63 to 92% at a frequency of 30 GHz and a thickness of 4.50 mm) upon a decrease in the sample mass (by 28%). A method has been proposed for calculating the absorptance of corrugated composites in the microwave range.     

On the application of homogenization formalisms to active dielectric composite materials

The Maxwell Garnett and Bruggeman formalisms were applied to estimate the effective permittivity dyadic of active dielectric composite materials. The active nature of the homogenized composite materials (HCMs) arises from one of the component materials which takes the form of InAs/GaAs quantum dots. Provided that the real parts of the permittivities of the component materials have the same sign, the Maxwell Garnett and Bruggeman formalisms give physically plausible estimates of the HCM permittivity dyadic that are in close agreement. However, if the real parts of the permittivities of the component materials have different signs then there are substantial differences between the Bruggeman and Maxwell Garnett estimates. These differences are slightly less pronounced if the relative permittivity of the metallic component material is described by the extended version of the Drude formula appropriate to very small particles. However, these differences become enormous - with the Bruggeman estimate being physically implausible - as the imaginary parts of the permittivities of the component materials tend to zero.     

Microwave radiative transfer in scattering atmosphere: Effects of complex permittivity of ice spherical crystals

This paper presents the effects of ice particle's complex permittivity uncertainties on the scattering properties and upwelling brightness temperatures of cloudy atmospheres at the Advanced Microwave Sounding Unit-B (AMSU-B) channel frequencies of 89, 150 and 183 GHz. We investigated the mean deviations of ice particle's optical efficiencies and asymmetry parameters due to the uncertainties in the real and imaginary parts of its complex permittivities. We assumed that the true values of ice particle's permittivity are, respectively, within ±20% for the imaginary part and ±5% for the real part of the permittivity values given by the model of Hufford. Microwave radiative transfer calculations were performed to estimate the absolute errors of brightness temperatures due to uncertainties in ice particle's permittivities. Ice particles were taken to be spherical and their diameters were chosen in the range of 40-4000 μm. Gamma-size distribution was employed in computing volume scattering properties and the effective diameters were 70, 100 and 150 μm with an effective variance being 0.25. We found that ±20% uncertainty in the imaginary part of ice particle's permittivity resulted in only about 10% mean deviations in the absorption efficiencies at the three AMSU-B channel frequencies. However, an uncertainty of ±5% in the real part resulted in more than 15% mean deviations in both scattering and extinction efficiencies, especially significant at the frequency of 183 GHz. The absolute variations of the emerging brightness temperature from the cloudy atmosphere due to uncertainties in the permittivity were found to be more than 1 K, which is already significant compared with the sensitivities achieved with today's technology for millimeter wave radiometers.     

Effect of nickel nanofillers on the dielectric and magnetic properties of composites based on rubber in the X-band

Nickel–rubber nanocomposites were synthesized by incorporating ferromagnetic nickel nanoparticles in a natural rubber as well as neoprene rubber matrix. Complex dielectric permittivity and magnetic permeability of these composites were evaluated in the X-band microwave frequencies at room temperature using cavity perturbation technique. The dielectric loss in natural rubber is smaller compared to neoprene rubber. A steady increase in the dielectric permittivity is observed with increase in the content of nickel in both the composites. The magnetic permeability exhibits a steady decrease with increase in frequency and magnetic loss shows a relaxation at 8 GHz. The suitability of these composites as microwave absorbers is modeled based on the reflection loss which is dependant on the real and imaginary components of the complex dielectric permittivity and magnetic permeability.     

Complex dielectric permittivities of the Ag2O-Ag2CO3 system at microwave frequencies and temperatures between 22 °C and 189 °C

The silver oxide-silver carbonate system comprises a thermally regenerable chemisorbent for carbon dioxide, which has been employed to revitalize CO₂-laden air within space suits. To evaluate the compatibility of these materials with thermal regeneration via dielectric heating, complex permittivities were studied over the microwave-frequency span between 4 and 26 GHz using a vector network analyzer based measurement system. Complex permittivities were determined over temperatures from 22 °C to 189 °C. The resulting values for the real and imaginary components of the complex permittivity, corresponding to polarization and energy-loss terms, respectively, were uniformly low for both the oxide and the carbonate under all test conditions. These results suggest that efficient dielectric heating of these materials is improbable. Received: 29 October 2001 / Accepted: 7 January 2002 / Published online: 3 May 2002     

Microwave permittivity and dielectric relaxation of a high surface area activated carbon

Carbonaceous materials are amenable to microwave heating to varying degrees. The primary indicator of susceptibility is the complex permittivity (^*), of which, the real component correlates with polarization, and the imaginary term represents dielectric loss. For a given material, the complex permittivity is dependent upon both frequency and temperature. Here we report the complex permittivity of a high surface area coconut shell activated carbon which is commonly used in analytical chemistry and a wide variety of industrial separations. Associated polarization-relaxation phenomena are also characterized. Broadband measurements were made using a high temperature compatible open-ended coaxial dielectric probe at frequencies between 0.2 and 26 GHz, and across the temperature region between 24 °C and 191 °C. PACS 77.22.-d     

导电炭黑／硅橡胶复合材料介电常数与压应力的关系

以填充了导电炭黑的导电硅橡胶和绝缘硅橡胶为主体材料,分别采用质量比1：9;3：7配比进行混炼、制备,研究了压力下复合材料的介电特性．实验结果表明：在恒压下样品介电常数实部ε’随外加电场频率的增加而减小;在恒压、同频率时导电硅胶含量大的样品介电常数大;在同频率时,增大压应力,样品介电常数实部ε’都会增加,导电硅胶含量大,增幅大．     

On the Bergman-Milton bounds for the homogenization of dielectric composite materials

The Bergman-Milton bounds provide limits on the effective permittivity of a composite material comprising two isotropic dielectric materials. These provide tight bounds for composites arising from many conventional materials. We reconsider the Bergman-Milton bounds in light of the recent emergence of metamaterials, in which unconventional parameter regimes for relative permittivities are encountered. Specifically, it is demonstrated that: (a) for nondissipative materials the bounds may be unlimited if the constituent materials have relative permittivities of opposite signs; (b) for weakly dissipative materials characterized by relative permittivities with real parts of opposite signs, the bounds may be exceedingly large.     

Microstructure and electromagnetic characteristics of BaTiO3/Ni hybrid particles prepared by electroless plating

To improve the microwave absorption ability, hybrid particles containing both dielectric loss of BaTiO₃ and magnetic loss of Ni were fabricated via electroless Ni plating on BaTiO₃ particles. A continuous Ni coating was successfully covered on the surface of the BaTiO₃. The effect of the Ni content on complex permittivity, complex permeability, and microwave absorption properties of BaTiO₃/Ni hybrid particles was investigated. The real (?′) and imaginary (?″) parts of complex permittivity as well as imaginary part of complex permeability (μ″) were found to increase with an increase in Ni content, while the variation of the real part of complex permeability (μ′) with Ni content was non-linear. The microwave absorption performances could readily be tuned base on the changing Ni content of the hybrid particles. The optimal absorption performances were attained when the content of Ni reached 38.9 wt% in hybrid particles.