Fabrication and electromagnetic characteristics of microwave absorbers containing carbon nanofibers and NiFe particles
Introduction
The absorption and the interference shielding of electromagnetic (EM) wave have been very important issues for both commercial and military applications. Electromagnetic interference (EMI) shielding problems are increasing in electronic and military communication owing to electronic device sensitivity and component density, so that EMI shielding research has received increasing attention in recent years. In addition, EM wave absorption technologies are at the heart of stealth systems. Reduction of military vehicle detectability, often expressed as the radar cross section (RCS), affects not only mission success rates but also survival rates in the hostile territory. Much effort has been devoted to the development of better radar absorbing materials (RAMs) and radar absorbing structures (RASs) using fiber reinforced polymeric composite materials. In general, EM wave absorption characteristics of materials depend on their dielectric properties (permittivity), magnetic properties (permeability), thickness, and frequency [1], [2], [3], [4].
Previously developed dielectric and magnetic RAMs are not without their problems. Dielectric absorbers employing conductive nano-fillers require heavy matching thicknesses and narrow absorbing bandwidths (BWs). At the same time, magnetic absorbers employing metallic or ferrimagnetic micron-sized materials suffer from heavy weight and poor characteristics in frequency ranges higher than gigahertz due to the Snoek limit [5]. The enhanced EM wave absorbers must be light, thin, and capable of wide bandwidth absorption. Previous works on single-layered X-band microwave absorbers in terms of matching thickness, 10 dB absorbing bandwidths, and mixing ratios can be found in Table 1 [4], [6], [7], [8], [9], [10], [11], [12]. Dielectric type absorbers showed 10 dB BW of about 3 GHz with the very low filler contents and thickness less than 3.0 mm. Magnetic type absorbers had 10 dB BW over about 4 GHz with the very high filler contents and thickness less than 2.5 mm. On the other hand, hybrid-type absorbers employing a single filler, and mixed-type absorbers employing two fillers for both dielectric and magnetic characteristics are possible candidate materials for overcoming the narrow absorption of dielectric RAMs and heavy weight of magnetic RAMs. However, complex chemical processes and uniform dispersion techniques, in addition to other manufacturing requirements, are required for hybrid-types and mixed RAMs.
This research focuses on the use of both dielectric and magnetic lossy materials for the development of composite microwave absorbers that are both thin and capable of broadband absorption. Carbon nanofibers (CNFs) and submicron sized NiFe particles were used as dielectric and magnetic lossy materials, respectively. This paper addresses the fabrication of RAMs, measurement of complex permittivities and permeabilities in the range of 2–18 GHz, design of single-layered X-band (8.2–12.4 GHz) and Ku-band (12.0–18.0 GHz) RAMs, and performance evaluation of several selected specimens.
Section snippets
Lossy materials
Owing to their outstanding physical and electrical properties, carbon nanoparticles such as carbon nanotubes (CNTs) and carbon nanofibers (CNFs) are promising and popular raw materials. Composites containing carbon nanoparticles may have various merits such as high tensile strength, elastic modulus, thermal conductivity and electrical conductivity according to fabrication processes and content ratios [13], [14].
In this study, vapor-grown CNFs (Showa Denko, Japan) were used as dielectric lossy
Fabrication of composites
First, an epoxy resin pre-mixture containing hardener and lossy materials was prepared. It was sufficiently dispersed using a homogenizer at about 5000 rpm for 20 min in a double boiler kept at an internal temperature of 60 °C. Deaeration (in a deaerator under vacuum for 20 min at 80 °C) removed any dissolved air and gas formed during the mixing process. The curing process was then performed in an autoclave for 2 h at 120 °C at atmospheric pressure. Finally, the fabricated specimens were cut into
Principle and numerical formulas of electromagnetic wave absorbers
Fig. 7a shows the single-layered electromagnetic wave absorber with the thickness d. According to the transmission line theory, the input impedance at the surface of the absorber in Fig. 7a iswhere the impedance of absorber material is and μr are the complex relative permittivity and permeability) and the propagation constant γ is . As the boundary back layer consists of the perfect electric conductor (PEC), the
Conclusions
This research focused on the design, fabrication, and performance evaluation of single-layered microwave absorbers employing both dielectric and magnetic lossy materials. Carbon nanofibers (CNFs) were used as dielectric lossy materials, and NiFe particles were used as magnetic lossy materials. Twelve specimens with various weights of added fillers and compound ratios were fabricated and classified into dielectric, magnetic, and mixed types. The mixed specimens were successfully fabricated
Acknowledgement
This study was performed as a part of basic research project of KIMS and supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by MOCIE in Republic of Korea.
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