Microwave Electromagnetic Characteristics of a Microcoiled Carbon Fibers/paraffin Wax Composite in Ku Band
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The complex relative permittivity ⑀␥ and permeability ␥ of microcoiled carbon fibers (MCCFs) imbedded in paraffin wax were measured at Ku band frequencies (12.4–18 GHz). Both the real and imaginary parts of the complex relative permittivity of the MCCFs/paraffin wax composite decreased with increase of the frequency. The real part of the complex relative permeability of the composite increases with increase of the frequency, and the imaginary part is nonzero and nearly constant over the measured frequency range. The dielectric loss tangent (tan ␦⑀), the magnetic loss tangent (tan ␦m), and amplitude attenuation factor (␣) were determined as well. On the basis of the experimental results, the MCCFs/paraffin wax composite is mainly a kind of dielectric lossy material with small magnetic loss and diamagnetism in the Ku band.
I. INTRODUCTION
Electromagnetic response of substances is the result of complex mutual interactions between the electron system of the material and electromagnetic field. The electromagnetic wave absorption characteristics of a composite mainly depend on its material properties, particularly its dielectric properties (the permittivity, ⑀) and magnetic properties (the permeability, ). Dielectric composite absorbers depend on the ohmic loss of energy, which can be achieved by adding lossy fillers like carbon, graphite, or a metal particle/powder to a matrix, and magnetic composite absorbers are dependent on a kind of magnetic hysteresis effect, which is obtained when magnetic particles are added to a matrix.1 These lossy fillers and magnetic particles are usually prepared using advanced technologies and have to be lightweight and physically and chemically stable and of course have a high ability of absorbing electromagnetic wave radiation. Microcoiled carbon fibers (MCCFs), which are one kind of pyrolytic product prepared by pyrolyzing hydrocarbons such as acetylene in the presence of a nickel catalyst in a hydrogen, nitrogen, or argon atmosphere,2–8 are composed of carbon atoms with an sp2 hybrid orbital. They not only have high strength, low density, high electric conductivity, and other special functional properties but also are types of chiral materials.2,9–10 Scientists have confirmed that chiral inclusions imbedded in a matrix are very
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J. Mater. Res., Vol. 17, No. 5, May 2002 Downloaded: 18 Mar 2015
promising as electromagnetic wave absorbers.11–13 Motojima et al. explored their electromagnetic wave absorption properties by imbedding the MCCFs in a low dielectric constant, low-density matrix.14,15 However, little work has been reported on the microwave electromagnetic properties of MCCF composites, which is important for MCCFs to be used as electromagnetic shielding and absorbing materials. In the paper, MCCFs were prepared by catalytic pyrolysis of acetylene with nano-sized nickel powder catalyst using the substrate method2,16 and the complex relative permittivity ⑀␥ and permeabil
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