Liming Yuan

Effects of Multi-walled Carbon Nanotubes on the Electromagnetic Absorbing Characteristics of Composites Filled with Carbonyl Iron Particles

The electromagnetic (EM) wave absorbing property of silicone rubber filled with carbonyl iron particles (CIPs) and multi-walled carbon nanotubes (MWCNTs) was examined. Absorbents including MWCNTs and spherical/flaky CIPs were added to silicone rubber using a two-roll mixer. The complex permittivity and complex permeability were measured over the frequency range of 1–18 GHz. The two EM parameters were verified and the uniform dispersion of MWCNTs and CIPs was confirmed by comparing the measured reflection loss (RL) with the calculated one. As the MWCNT weight percent increased, the RL of the spherical CIPs/silicone rubber composites changed insignificantly. It was attributed to the random distribution of spherical CIPs and less content of MWCNTs. On the contrary, for composites filled with flaky CIPs the absorption bandwidth increased at thickness 0.5 mm (RL value lower than −5 dB in 8–18 GHz) and the absorption ratio increased at lower frequency (minimum −35 dB at 3.5 GHz). This effect was attributed to the oriented distribution of flaky CIPs caused by interactions between the two absorbents. Therefore, mixing MWCNTs and flaky CIPs could achieve wider-band and higher-absorption ratio absorbing materials.

Microwave Absorption and Shielding Property of Composites with FeSiAl and Carbonous Materials as Filler

Silicone rubber composites filled with FeSiAl alloys and multi-walled carbon nanotubes (MWCNT)/graphite have been prepared for the first time by a coating process. The complex permittivity and permeability of the composites were measured with a vector network analyzer in a 1–4 GHz frequency range, and the DC electric conductivity was measured by a standard four-point contact method. These parameters were then used to calculate the reflection loss (RL) and shielding effectiveness (SE) of the composites. The results showed that the added MWCNT increased the permittivity and permeability of composites in the L-band, while the added graphite increased only the permittivity. The variation lies in the interactions between two carbonous absorbents. Addition of 1 wt% MWCNT enhanced the RL in the L-band (minimum −5.7 dB at 1 mm, −7.3 dB at 1.5 mm), while the addition of graphite did not. Addition of MWCNT as well as graphite reinforced the shielding property of the composites (maximum SE 13.3 dB at 1 mm, 18.3 dB at 1.5 mm) owing to the increase of conductivity. The addition of these carbonous materials could hold the promise of enforcing the absorption and shielding property of the absorbers.

Enhancement mechanism of the additional absorbent on the absorption of the absorbing composite using a type-based mixing rule

A silicone rubber composite filled with carbonyl iron particles and four different carbonous materials (carbon black, graphite, carbon fiber or multi-walled carbon nanotubes) was prepared using a two-roller mixture. The complex permittivity and permeability were measured using a vector network analyzer at the frequency of 2–18 GHz. Then a type-based mixing rule based on the dielectric absorbent and magnetic absorbent was proposed to reveal the enhancing mechanism on the permittivity and permeability. The enforcement effect lies in the decreased percolation threshold and the changing pending parameter as the carbonous materials were added. The reflection loss (RL) result showed the added carbonous materials enhanced the absorption in the lower frequency range, the RL decrement value being about 2 dB at 4–5 GHz with a thickness of 1 mm. All the added carbonous materials reinforced the shielding effectiveness (SE) of the composites. The maximum increment value of the SE was about 3.23 dB at 0.5 mm and 4.65 dB at 1 mm, respectively. The added carbonous materials could be effective additives for enforcing the absorption and shielding property of the absorbers.

Effects of particle sizes on the electromagnetic property of flaky FeSi composites

Flaky FeSi absorbents with different size ranges were fabricated by sintering after mechanical milling process. X-ray diffraction (XRD) was used to analyze the particle crystal grain structure. The complex permittivity and permeability of FeSi/paraffin composites were measured in frequency of 2–12 GHz using a vector network analyzer, and the DC electric conductivity was measured by the standard four-point contact method, then microwave reflection loss (RL) and shielding effectiveness (SE) were calculated. It was obtained that α-Fe appeared in the super-lattice diffraction peaks in XRD pattern. As the particles size decreased, the permittivity decreased due to the inferior microwave electrical conductivity and dielectric loss tangent, while the permeability increased due to the decrease of diameter-thickness ratio, which could be demonstrated in the comparison between the experiment and calculation results. When thickness is 1 mm, the composites with the smallest FeSi particles addition had a better absorbing property for the better impedance matching characteristic, and the minimum RL was −7.9 dB at 4.6 GHz. While the composites with larger FeSi particles addition had an excellent shielding property due to the higher permittivity, the SE value ranged from 15 dB to 30 dB at the frequency band.

The effective ellipsoid: a method for calculating the permittivity of composites with multilayer ellipsoids

Abstract In order to calculate the effective permittivity of a mixture with multilayer ellipsoids, this paper presents a self-consistent approximation (SCA) on the basis of the Bruggeman’s analytical model. The effective permittivity of a mixture with aligned multilayer ellipsoids is derived directly from the linear system of equations, which are built using the boundary condition of the electric field on the confocal ellipsoidal interface in the ellipsoidal coordinate system. Furthermore, for a mixture with multilayer ellipsoids oriented randomly, an effective ellipsoid is introduced to substitute the original multilayer ellipsoid, and the permittivity of the effective ellipsoid is derived by jointly solving the two linear systems of equations for the situation of the original multilayer ellipsoid and that of the effective ellipsoid, then the effective permittivity of the mixture can be calculated by the existing Maxwell-Garnett formula. After comparisons, it is revealed that there is a good agreement between this SCA method and existing theories.

A Three-Dimensional Numerical Analysis on the Effective Permittivity of Composites Including Ellipsoids

To investigate the effective permittivity of composites composed of ellipsoidal inclusions, three-dimension numerical models for ellipsoidal inclusions distributed randomly are built with the finite-element modeling software Comsol Multiphysics. After calculating the effective permittivity for different cases and comparing the results with analytical results from the Maxwell-Garnett mixing rule, we find that the finite-element method has an advantage in detecting details of the interaction among inclusions, which have some impacts on the effective permittivity and could not be accurately taken into account in the analytical model. The finite-element method is expected to solve more complex problems on electromagnetic computation.

Absorbing/Shielding Property of Graphite-FeSi by Mechanical Milling

Graphite-FeSi absorbents were fabricated by mechanical milling method. The complex permittivity and permeability were measured in frequency 1-4 GHz, and then reflection loss (RL) and shielding effectiveness (SE) were calculated. It was obtained that the graphite was bonded to the surface of FeSi by X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images. The permittivity and permeability could be enlarged as graphite added in the milling process. It was attributed to the excellent conductivity of graphite and interactions of the two particles. The graphite-FeSi composites had a better shielding property (maximum 25.93 dB) in 1-4 GHz as well as the absorbing property at 1GHz than FeSi composites.