• 한국분말재료학회지
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• 제20권2호
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• pp.125-128
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• 2013

Magnetic and dielectric properties of rubber composites are controlled by using two kinds of high-permeability metal particles with different electrical conductivity (Sendust, Permalloy), and their effect on microwave absorbance has been investigated, focusing on the quasi-microwave frequency band (0.8-2 GHz). Noise absorbing sheets are composite materials of magnetic flake particles of high aspect ratio dispersed in polymer matrix with various filler amount of 80-90 wt.%. The frequency dispersion and magnitude of complex permeability is almost the same for Sendust and Permalloy composite specimens. However, the complex permittivity of the Permalloy composite (${{\varepsilon}_r}^{\prime}{\simeq}250$, ${{\varepsilon}_r}^{{\prime}{\prime}}{\simeq}50$) is much greater than that of Sendust composite (${{\varepsilon}_r}^{\prime}{\simeq}70$, ${{\varepsilon}_r}^{{\prime}{\prime}}{\simeq}0$). Due to the large dielectric permittivity of Permalloy composite, the absorbing band is shifted to lower frequency region. However, the investigation of impedance matching reveals that the magnetic permeability is still small to satisfy the zero-reflected condition at the quasi-microwave frequency band, resulting in a small microwave absorbance lower than 10 dB.

• 한국세라믹학회지
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• 제32권1호
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• pp.25-30
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• 1995

The frequenc dependence of magnetic permeability($\mu$r) and dielectric constant($\varepsilon$r) in MHz-GHz frequency range and their relationships with microwave absorbing properties were investigated in sintered and composite Ni-Zn ferrites, respectively. It was confirmed that zero reflection condition was required the real parts of permeability and permittivity in sintered specimen, and the complex permeability, permittivity and dielectric loss tangent in composite specimen. The real part of permittivity varied with the replacement of nickel by manganese in sintered Ni-Zn ferrite. Therefore, we could control the matching frequency and matching thickness.

• Tribology and Lubricants
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• 제27권3호
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• pp.156-161
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• 2011

The rectangular waveguide technique can be used to measure the complex permittivity of dielectric material of various thickness and cross section. This paper presents the analysis system of engine oil permittivity at which deterioration of engine oil is measured at the X-band(8-12.5 GHz). The middle of the rectangular waveguide has engine oil case and is connected with VNA(Vector Network Analyzer) for the measurement of the transmission$(S_{21})$ and reflection$(S_11)$ and then the permittivity is extracted. The deterioration of engine oil is proved by the comparison with both the extracted data and reference data. As the additional research, This paper suggest that an accurate permittivity is considered by not only the wave guide length but the air gap between oil case and the waveguide.

• 전자공학회논문지A
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• 제29A권11호
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• pp.14-18
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• 1992

A gradient matrix method in conjunction with the transmission-reflection method to determine the complex permittivity and permeability of a microwave material is presented. This method does not incur the phase ambiguity due to an improper sample length, and is applicable to the measurement of low-loss materials of a half wavelength. A gradient matrix for a coaxial cable sample is derived, and the results are illustrated.

• 대한전자공학회:학술대회논문집
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• 대한전자공학회 2008년도 하계종합학술대회
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• pp.341-342
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• 2008

A conductivity of soot at microwave frequency is presented using a novel technique for complex permittivity of materials. The method overcomes limitations of conventional methods which are cavity perturbation and transmission/reflection method. Resonant frequencies and Q factors are measured and simulated for the cylindrical cavity, and they are compared to each other. Similar material property of both real material and simulation material produce similar values of resonant frequency and Q factor. The complex permittivity of material can be determined by simulating the cavity to change material property until the simulation results are nearly the same as the measurement results. Cylindrical cavity has been realized for measurement at 880 MHz, and conductivity of soot is measured. A sample was made by depositing the soot on the glass. The proposed method shows that the conductivity of soot is 11 S/m.

• Composites Research
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• 제16권2호
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• pp.48-53
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• 2003

본 논문에서는 카본 블랙의 함유율에 따른 유리섬유/에폭시 직조 복합재료 적층판의 유전율에 대한 실험 및 예측방법에 대한 연구를 수행하였다. 유전율 측정은 5 GHz∼18 GHz의 주파수 영역에서 수행하였으며, 복합재료의 유전율은 카본 블랙의 함유율과 주파수의 함수로 얻을 수 있었다. 카본 블랙의 함유율에 따른 복합재료의 유전율을 모사하는 혼합법칙을 얻기 위한 새로운 방법이 제시되었으며. 이 방법에서는 실험적으로 얻기 어려운 카본 블랙 자체의 유전율도 얻을 수 있다. 혼합법칙으로 계산된 복합재료 유전율 결과는 실험적으로 얻은 유전율을 비교적 잘 모사하는 결과를 얻을 수 있었다.

• Composites Research
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• 제22권3호
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• pp.44-54
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• 2009

본 논문에서는 퍼콜레이션 이론적 관점에 기초한 복소 유전율의 수치모델을 제시하고, 전기 전도성이 뛰어난 카본 블랙을 혼합한 유리섬유/에폭시 복합재료 적층판의 복소 유전율을 이용하여 이 깃을 검증하였다. 제시된 모델은 카본 블랙의 함유율이 퍼콜레이션 임계함유율 보다 높고, 주파수가 충분히 높아서 복합재료의 교류 전기전도도가 주파수에 비례하는 구간에서의 복소 유전율을 모사한다. 복합재료의 복소 유전율은 벡터회로망분석기와 7 mm 동축관을 이용하여 $0.5\;GHz\;{\sim}18\;GHz$ 대역에서 측정되었다. 제시된 모델은 퍼콜레이션 이론에서 유용하게 사용되는 축척(눈금잡이) 법칙의 함수형태와 실험을 통하여 구한 상수들로 구성되어 있으며, 복소 유전율을 주파수와 가본 블랙의 함유율의 함수로 나타내었다. 제시된 모델은 복소 유전율을 측정결과와의 비교를 통하여 검증되었다.

• 한국재료학회지
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• 제12권6호
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• pp.442-446
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• 2002

Design of microwave absorbers using high frequency properties of fiber reinforced composites are investigated. Two kinds of composite materials (glass and carbon) are used and their complex permittivity and permeability are measured by transmission/reflection technique using network analyzer. Low dielectric constant and nearly zero dielectric loss are determined in glass fiber composite. However, carbon fiber composites show the high dielectric constant and large conduction loss which is increased with anisotropy of fiber arrangement. It is, therefore, proposed that the glass and carbon fiber composites can be used as the impedance transformer (surface layer) and microwave reflector, respectively. By inserting the foam core or honeycomb core (which can be treated as an air layer) between glass and carbon fiber composites, microwave absorption above 10 dB (90% absorbance) in 4-12 GHz can be obtained. The proposed fiber composites laminates with sandwitch structure have high potential as lightweight and high strength microwave absorbers.

• 한국복합재료학회:학술대회논문집
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• 한국복합재료학회 2004년도 춘계학술발표대회 논문집
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• pp.155-160
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• 2004

This paper presents a study on the permittivities of the carbon black/epoxy composite at microwave frequency. The measurements were performed at the frequency band of $1 GHz\~18GHz$. The results show that the complex permittivities of composites depend strongly on the natures and concentrations of the carbon black dispersion. The frequency spectrums of dielectric constants and ac conductivities of composites show the good conformities with descriptions of the percolation theory. The carbon black concentration dependencies do not have conformities with the descriptions of percolation theory and there is no peculiar concentration like percolation threshold, on that concentration, the conductivity of composite jumps up. A new scheme, that is a branch of Lichtenecker-Rother formula, is proposed to obtain a mixing law to describe the complex permittivities of the composites as function frequency and concentration of carbon black.

• 대한화학회지
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• 제56권1호
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• pp.20-27
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• 2012

Complex permittivity spectra of Allyl Chloride (AC), 2-Butanone (2-BU) and their binary mixtures over the entire range of concentration were obtained using the Time Domain Reflectometry (TDR) technique in microwave frequency range at various temperatures. Static dielectric constant and relaxation time are obtained from complex permittivity spectra. Density ($\rho$) and refractive index ($n_D$) are also measured. These parameters are used to determine excess dielectric constant, excess inverse relaxation time, excess molar volume, excess molar refraction, polarity, Bruggeman factor and thermodynamic parameters viz. enthalpy of activation and entropy of activation. The values of static dielectric constant and relaxation time increases while density and refractive index decreases with the percentage of 2-Butanone in Allyl Chloride increases. Excess parameters were fitted to a Redlich-Kister equation.