Effect of silicon carbide dispersion on the microwave absorbing properties of silicon carbide-epoxy composites in 2–40 GHz

Yaw-Shun Hong, Tzu-Hao Ting, Chih-Chia Chiang, Ken-Fa Cheng

 
Abstract
Wide-band, strong absorption with low density and thin matching thickness are essential for electromagnetic wave absorbers. In this study, silicon carbide powders were successfully synthesized by the method of preheating combustion synthesis in nitrogen atmosphere and introduced into epoxy resin to be microwave absorber. The spectroscopic characterization of the formation processes of silicon carbide was studied by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Microwave absorbing properties of the silicon carbide and thermal plastic resin were investigated by measuring reflection loss in the 2-18 and 18-40 GHz microwave frequency range using the free space method. It was found that the composite specimens of the silicon carbide and thermal plastic resin had the best microwave absorption due to the reflection losses between from -10 to -19.5 dB and from -3 to -9.1 dB at frequencies between 2-18 and 18-40 GHz.
Keywords: Microwave absorption; Silicon carbide; X-ray diffraction; Scanning electron microscopy
1. Introduction
During the past a few decades, the development of new microwave absorbing composites is being encouraged because these materials achieve better efficient ways for reducing the level of electromagnetic wave pollution generated by electronic and telecommunication systems. Recently many applications have been carried out on the microwave technology in the frequency range of 2–40 GHz [1-3]. To reduce the radar signature, many types of electromagnetic (EM) wave-absorbing materials have been designed to meet the requirements of both commercial and military affairs. The materials used as electromagnetic wave-absorbing materials can be classified as magnetic, dielectric or a hybrid, respectively. Actually, these classifications are based on the mechanism of the wave-material interaction, which varies based on the types of absorber centres used. Ideal microwave absorber should exhibit low-reflecting properties, strong reflection loss in broad bandwidth, low density and small thickness to facilitate their applications in many fields [4, 5].

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As we know, the composite materials generally represent the natural interface between two worlds of chemistry each with very significant contributions to components interact at a molecular level. Dielectric polymer-matrix materials can include two different compounds with complementary properties in a single material and can be combine to reinforce or modify each other in specific applications. Extensive studies have been carried out to develop new and highly efficient absorbents, and various absorbers (such as conductive metal powder, ferrites, carbon products, chiral materials, synthetic organic fibres, etc.) have been isolated or synthesised [6-9]. However, in these materials, most absorbers like traditional ferrite powders and carbon series are unable to be employed at higher temperatures due to lower Curie temperatures and oxidation problem, respectively [10-14]. It is becoming very urgent to look for new microwave absorbers making electromagnetic wave disappearance by interference, or satisfying the requirements of higher structural strength and temperature resistances in higher temperature environments.
Due to their physical and electronic properties, Silicon carbide (SiC) is an important carbide, studied as a structural ceramic for a long time and has attractive properties, such as excellent strength and chemical resistance at high temperatures, semi-conductivity, high thermal stability and thermal conductivity, make it an attractive material in high-temperature structural, electric and functional applications [15-20]. On the other hand, Silicon carbide (SiC) is one of the preferred and best characterised filler materials and is used in combination with polymers in military or civilian products [20-23].
Meanwhile, to the best of our knowledge, there are very few reported experimental results on the electromagnetic wave adsorption of silicon carbide between 2–18 and 18–40 GHz. Here, we present the microwave absorbing properties of the silicon carbide reinforced epoxy resin composites tested at 2–18 and 18–40 GHz using arch method, which was chosen to validate the absorbing efficiency of microwave absorbing material [24, 25]. The NRL (Naval Research Laboratory) arch free-space measurement method is a well-established measurement system for validating the absorbing efficiency of flat materials over broad frequency ranges. The NRL arch was widely used initially by the U.S. Navy for research testing purposes, and is a microwave measurement system that can measure the free space radar reflection coefficient. The reflection loss diagram showed that the powder silicon carbide-epoxy resin with 30-50 by weight ratio of silicon carbide to polymer is a good candidate material for use as a broad frequency microwave absorber.
The NRL Arch is the industry standard for measuring the free space radar reflection coefficient of flat radar absorbing materials (RAM). It was first developed by the U.S. Naval Research Lab, the NRL. The NRL Arch is a well–established, free‐space measurement system for testing the absorbing efficiency of flat materials over broad frequency ranges. It was originally designed at the United States Naval Research Laboratory (NRL) in 1945 for measuring angular‐dependent performance of broadband Radar Absorbing Materials (RAM).
2. Experimental
2.1 Preparation of silicon carbide
The silicon carbide powders were synthesized by the method of preheating combustion synthesis in nitrogen atmosphere, using silicon powder (＜45 μm, 99.9% purity, mass fraction) and carbon black (20-40 nm, 99.9% purity) as the raw materials. The molar ratio of silicon powder and carbon black was blended in a molar ratio of Si-50% C. The mixed powders were poured into a graphite crucible and initiated by pre-heating at 1350 ï‚°C with the heating rate of 40 ï‚°C / min in a 0.1 MPa nitrogen atmosphere inside a resistance. After the synthesis process, the product was heated at temperature 850 ï‚°C for 4 h in atmosphere condition to burn the excess carbon. The final cleanup to remove Si was carried out by leaching in HF, rinsing in distilled water and drying.
2.2 Preparation of silicon carbide-epoxy composites
The composite specimens were prepared by molding and curing the mixture of silicon carbide and a thermal-plastic epoxy resin to be silicon carbide-epoxy composites. The mixing ratio of specimen powders to epoxy resin was 30 %, 35 %, 40 %, 45 % and 50 % by weight and the corresponding samples are marked with S-1, S-2, S-3, S-4 and S-5, respectively. Molding was carried out in a hydraulic press at 5 Mpa pressure and 80 ï‚°C for 1.5 h, obtaining specimens of 180 mm × 180 mm with thickness of 2 mm for reflectivity measurements [26].
2.3 Experimental techniques
The characteristics of silicon carbide such as diameter and morphology were observed by scanning electron microscopy with EDX (SEM, HITACHI S-4800). The crystalline phases of the silicon carbide were analyzed by X-ray diffraction with Cu Kα radiation. The performance test of radar absorbing was evaluated by reflectivity using Arch method. Reflectivity R is ratio of radar-absorbing material (RAM) reflective power to metallic plate reflective power, which can be expressed as:
(1)
Where Pa is the reflective power of the sample and Pm is the reflective power of metallic plate.
In practice, we surveyed the ratio of the reflective power of the sample and the reflective power of metallic plate to the same reference signal that was in direct proportion to transmit, respectively.
, (2)
Where Pi is the reference signal. So
(3)
The Reflectivity was finally expressed with db as:
(4)
The schematic diagram of the experimental setup was shown in Fig. 1. The reflectivity of the samples were measured and compared with that from a plane metallic plate. Measurement was carried out using an HP8722ES network analyzer in the swept frequency range of 2–18 and 18–40 GHz. All samples were made 180 × 180 mm with thickness of 2 mm in order to cover the metallic plate for reflectivity measurements.
3. Results and discussion
3.1 Structure characterization
Figure 2 shows the scanning electron micrograph of the fresh silicon carbide. From this figure it is evident that majority of the silicon carbide particles are angular in nature. The surface composition of silicon carbide particles was distinctly determined with SEM-EDX spectrum (Fig. 2c). EDX analysis reveals that the SiC composed of the Si and C elements.
The XRD pattern for the silicon carbide samples is presented in Fig. 3. From the XRD patterns, it can be easily observed that β-SiC was formed by present major peaks located at 35.6 (111), 41.2 (200), 60.1 (220), 71.8 (311) and 75.1 (222), all of which are attributed to β-SiC (JCPDS no. 29-1129). So the prepared product is pure β-SiC powder. This result agrees well with the results obtained for β-SiC prepared by the literature methods [27-30].
 
3.2 Microwave absorbing properties in 2–18 GHz
The different content of produced silicon carbide powders may change the impedance matching condition of microwave-absorption. Thus, as shown in Fig. 4, the reflection loss (RL) varies with filler content of the silicon carbide-epoxy composite in the frequency range of 2–18 GHz. It can be seen that with increasing the addition of silicon carbide and a maximum reflection loss of -19.5 dB was obtained at 7 GHz with the thickness 2.0 mm. Meanwhile, the centers of the reflection loss peaks for silicon carbide-epoxy composites move gradually to the lower frequencies (from 7 GHz to 4 GHz for S-1, -2, -3, -4 and -5 composites, respectively), which may also be attributed to the enhanced silicon carbide content. These results are consistent considering that the mechanism of wave absorption is mainly due to heat dissipation effects (on the condition of same thickness) of silicon carbide satisfying the perfect absorption condition and, therefore, are strongly linked to the conductivity of medium.
Fig. 5(a) shows the three-dimensional of RL values for silicon carbide-epoxy composites in terms of volume fraction at frequencies between 2 GHz and 18 GHz. Silicon carbide-epoxy composites absorbers present the effective absorption (RL  
3.3 Microwave absorbing properties in 18–40 GHz
Fig. 6 shows the experimental absorption characteristics of silicon carbide-epoxy composites in terms of volume fraction at frequencies between 18.0 GHz and 40.0 GHz. The variation of matching frequency with silicon carbide volume fraction is shown in Fig. 6. As we can see, the silicon carbide-epoxy composites displayed poor microwave absorption performance. Among the silicon carbide-epoxy composites, the powder prepared using an silicon carbide content of 45 wt% (S-4) had a pronounced absorption band at 25.2 GHz with a reflection loss of -9.1 dB. The significant improvement was considered to be resulted from a better impedance matching due to the certain ratio of silicon carbide, which might be ascribed to the special structures in the silicon carbide-epoxy composites.
Fig. 7 (a, b) displayed the visual three dimensional and color-filling patterns of microwave absorption values of the silicon carbide-epoxy composites with different weight ratio of the silicon carbide. Obviously, S-1, S-2, S-3 and S-5 silicon carbide-epoxy composites absorbers present weak absorption (RL  
4. Conclusion
In summary, we have successfully prepared the silicon carbide via combustion method in nitrogen atmosphere. XRD and SEM studies have established formation of the silicon carbide material. Experimental results indicate that the silicon carbide-epoxy composites in 2–18 GHz exhibit better absorption performances than in 18–40 GHz. The shifts of the attenuation peak in microwave absorbing properties of composites are due to increasing the content of silicon carbide in all frequency range of 2–40 GHz. It was found that the optimum reflection loss could be obtained over a broad frequency region on the silicon carbide-epoxy composites. Microwave absorbing properties can be modulated simply by controlling weight ratio of silicon carbide on the samples for the required frequency bands. Due to the reflectivity performance and easy and low cost preparation routes, the silicon carbide has a promising potential for microwave absorber.
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