Journal of Materials Science & Technology  2019 , 35 (11): 2658-2664 https://doi.org/10.1016/j.jmst.2019.05.060

Orginal Article

Foam structure to improve microwave absorption properties of silicon carbide/carbon material

Wanchong Liab*, Chusen Lia, Lihai Linab, Yan Wangc, Jinsong Zhangab*

Corresponding authors:   *Corresponding authors at: Institute of Metal Research, Chinese Academy of Sciences, Shenyang110016, China.E-mail addresses: liwanchong@imr.ac.cn (W. Li), jshzhang@imr.ac.cn (J. Zhang).*Corresponding authors at: Institute of Metal Research, Chinese Academy of Sciences, Shenyang110016, China.E-mail addresses: liwanchong@imr.ac.cn (W. Li), jshzhang@imr.ac.cn (J. Zhang).

Received: 2018-10-18

Revised:  2018-12-7

Accepted:  2019-05-15

Online:  2019-11-05

Copyright:  2019 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

Foam structure materials are well known for their lightweight, efficient, and broadband microwave absorption properties compared to bulk material. However, little has been understood about the effect of a foam structure on the absorption performance of the foam material. In this study, the role of foam structure properties of the silicon carbide/carbon (SiC/C) foam material on microwave absorption is explored using experiment and simulation. We find that the foam structure of SiC/C foam material causes diffraction, multiple reflections, improves the interfacial polarization, and compatibilization. The absorption performance of SiC/C foam material is also studied. The -10 dB effective absorption bandwidth can be adjusted from 4.0 GHz to 18 GHz by tuning SiC/C foam material thickness to 3-7 mm. Therefore, the foam structure design is an effective way to improve the absorption performance of the SiC/C foam material.

Keywords: Foam materials ; Microwave absorption properties ; Structure effects

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Wanchong Li, Chusen Li, Lihai Lin, Yan Wang, Jinsong Zhang. Foam structure to improve microwave absorption properties of silicon carbide/carbon material[J]. Journal of Materials Science & Technology, 2019, 35(11): 2658-2664 https://doi.org/10.1016/j.jmst.2019.05.060

Introduction

Microwave absorbing materials are crucial to the stealth for defense purposes [[1], [2], [3]]. Usually, researchers used different composite material [[4], [5], [6], [7], [8], [9]] or material structure design [[10], [11], [12], [13], [14]] to increase the microwave absorption performance. The foam composite material as a typical structure design absorbing material, has attracted extensive attention [[15], [16], [17], [18], [19], [20], [21], [22], [23]].

Regarding microwave absorbing performance, most investigators have tried to use the advantage of foamed structure materials [16,[24], [25], [26], [27], [28], [29], [30], [31]]. In fact, foam structure can significantly increase both the electromagnetic loss and impedance matching. Optimizing the cell size and volume fraction of the porous structure materials could improve the microwave absorption performance further [26,[31], [32], [33], [34]]. Previously, few researchers paid attention to the effect of foam structure on the electromagnetic absorbing performance [26,31,[35], [36], [37], [38]]. However, research on the effects of foam structure on the electromagnetic performance have provided better methods for using structures to improve microwave absorption performance [26,31,39]. Now silicon carbide/carbon(SiC/C) material is widely investigated as the microwave absorbing material, due to the fact that SiC/C material meets absorbing materials needs of relatively high compressive strength, excellent corrosion resistance, good thermal stability, and low cost [[40], [41], [42], [43], [44], [45]].

Therefore, in this work, we investigate the role of the foam structure on the SiC/C foam material electromagnetic performance using a comparison between SiC/C foam material and SiC/C bulk material through experimentation and the simulation. Finally, we study the microwave absorption performance of the SiC/C foam material through simulation. Mechanical properties of the SiC/C foam material are also investigated.

Experimental and simulation

Sample preparation

SiC/C foam material is prepared by the template method and thermolysis method. The commercially available polyurethane (PU) foams (Dayetengfei Sponge Tech, Changzhou, china) are chosen as the template foams. The cell size of the PU foams is about 1.5 mm. The PU foams are impregnated with the resin slurry (phenolic resin 40 wt%, SiC powder 40 wt% and ethanol 20 wt%) at room temperature. The size of SiC particles is about 5 μm. The impregnated PU foam is fished out from the resin slurry carefully. Then we remove excess resin slurry from the impregnated PU foams by blowing method in order to form an interlinking open cell structure in the final product. We dry out impregnated PU foams at 60 °C for 2 h. And we repeat these steps until the volume fraction of the impregnated PU foams is 30%. The impregnated PU foams are cured at 100 °C in the presence air atmosphere for 24 h. The final impregnated PU foams are pyrolysed at 810 °C for 25 min to fabricate the SiC/C foam material with protection of inert gas by using horizontal pyrolysis furnace (Hengjin, Shenyang, china). The preparation of SiC/C foam material investigated here were also full described in our previous paper [15,45].

SiC/C bulk material is prepared by the powder mould method and thermolysis method. First step, we dry off the resin slurry (phenolic resin 40 wt%, SiC powder 40 wt% and ethanol 20 wt%) at room temperature for two days. Second step, we put the dry resin slurry into composites powder by using mechanical milling. At last, we put the composites powder into the square mould, and control the mechanical compaction force until the density of the bulk composites is same as the skeleton of the coating PU foams. The final bulk composites are pyrolysed at 800 °C for 40 min to fabricate the SiC/C bulk absorbing materials with protection of inert gas.

Characterization and dielectric measurement

The composites are machined into 1 cm cubes, cleaned ultrasonically with ethanol and dried. The density of composites is determined by liquid draining method. The conductivity of composites are measured by a numeric double bridge circuit instrument. The morphology and element contents of composites is examined by scanning electron microscopy (SEM, JSM-6301 F, JEOL, Japan) with a links systems energy dispersive spectrometer (EDS). Electron probe micro analyzer (EPMA, EPMA-1610, SHIMADZU, Japan) is used to analyze the elemental map of the SiC/C materials. X-ray tomography (XRT) technique is also employed to investigate the porous structure of the foam materials. And the laboratory-based system with an Xradia Versa XRM-500 system (Carl Zeiss X-ray Microscopy Inc., Pleasanton, USA) is used in the XRT test. The materials phase compositions is tested by using X-ray diffraction (XRD, D/Max2500PC, Rigaku, Japan). The electromagnetic parameters of SiC/C foam material are tested by a waveguide method using vector network analyzer (VNA Agilent 5230A, USA). The specimen dimensions of C1-band is 22.15 mm × 47.55 mm × 6 mm, C2-band is 15.80 mm × 34.85 mm × 6 mm, X-band is 10.16 mm × 22.86 mm × 4 mm and Ku-band is 7.89 mm × 15.79 mm × 3 mm. The electromagnetic parameters of SiC/C bulk material are measured by a coaxial cables method. The SiC/C bulk material is processed into a coaxial ring with a thickness is 3 mm. The outside diameter of the coaxial ring is 7 mm, the inside of the coaxial ring diameters is 3.04 mm.

The mechanical properties of the sample is tested by using a universal testing machine equipped (MMW-1A, China) with a 2000 N load cell. And the compression velocity is 1 mm/min.

The simulation of 3D-foam

The software High Frequency Structure Simulator (HFSS) is used to research the electromagnetic spreading regulation in the SiC/C foam material and SiC/C bulk material. As shown in Fig. 1(a), we chose the tetrakaidecahedron model for the SiC/C foam material [31,45,46]. The simulation model of SiC/C bulk material is shown in Fig. 1(b). The periodic boundary conditions, floquet port on the top faces boundary and the perfect electric conductor boundary as the ground plane are used on the simulation model [31,45].

Fig. 1.   Simulation models of (a) SiC/C foam material and (b) SiC/C bulk material.

Results and discussion

Preparation of SiC/C materials

Fig. 2(a) shows the commercial PU foams used in this article. The PU foams are used as supporting structure during the template method. Fig. 2(b) shows the impregnated PU foams, which were fabricated by the template method. The resin slurry is coated on the PU foams forming the open cell structure materials. Fig. 2(c) shows the SiC/C foam material, which were fabricated by the thermolysis method. Fig. 2(d) shows the SiC/C bulk material, which were fabricated by the powder mould method and precursor thermolysis method.

Fig. 2.   Optical images of (a) commercial PU foams, (b) impregnated PU foams, (c) SiC/C foam material and (d) SiC/C bulk material.

Density and electrical conductivity

In order to research the role of foam structure on the absorption performance, the base material of SiC/C foam material and SiC/C bulk material should be same. The density and electrical conductivity of SiC/C foam material and SiC/C bulk material are the key parameters for our study. The Table 1 shows the result of the density and electrical conductivity of the SiC/C materials, we can see that the density and electrical conductivity of the SiC/C foam skeleton material and the SiC/C bulk material are almost same.

Table 1   Density and electrical conductivity of the SiC/C material.

ParameterSiC/C bulk materialSiC/C foam material
SkeletonEntirety
Density (g/cm3)1.961.950.56
Electrical conductivity (s m-1)2.272.250.63

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Morphology and compositions of SiC/C material

Fig. 3 shows the images of SiC/C foam material and SiC/C bulk material. Fig. 3(a) and (c) shows the macrostructure and microstructure of the SiC/C bulk material by the pyrolysis process. Fig. 3 (b) shows the macrostructure of SiC/C foam material by the pyrolysis process. It can be clearly seen that 3-D foam structures with the cell size is about 1 mm. The PU foams template disappeared through the pyrolysis process. Fig. 3(d) shows the microstructure of the SiC/C foam material. The microstructure of the SiC/C bulk material is same to the SiC/C foam material. The EDS elemental analysis of SiC/C bulk material and foam material are shown in Fig. 3(e) and (f). It can be seen that SiC/C bulk material and foam material are all mainly composed of carbon and silicon elements. XRT could help analyze the structure characterization for materials [47,48]. Fig. 4 shows the XRT images of SiC/C foam material and we can see clearly the three-dimensional porous structure of SiC/C foam material.

Fig. 3.   Morphologies of (a, c) SiC/C bulk material, (b, d) SiC/C foam material, EDS spectrum of (e) SiC/C bulk material and (f) SiC/C foam material.

Fig. 4.   3D XRT image of SiC/C foam material.

The analysis using EPMA shows the precise map of these elements in the SiC/C bulk material and foam material (Fig. 5). As shown in Fig. 5, SiC/C bulk material and foam material are mostly made up with carbon and silicon elements. The map of silicon element can show the distribution of SiC in the SiC/C bulk material and foam material.

Fig. 5.   (a) Backscattered electron images (BEI) of SiC/C bulk material, EPMA of SiC/C foam material showing distribution of elements (b) carbon, (c) silicon, (d) BEI of SiC/C bulk material, EPMA of SiC/C foam material showing distribution of elements (e) carbon, (f) silicon.

The XRD patterns of the SiC/C foam material and bulk material are shown in Fig. 6. There are two weak diffraction peaks of graphitic carbon. The two weak and wide diffraction peaks of graphitic carbon shows that carbon of the SiC/C bulk material and SiC/C foam material have low degree of graphitization [32,45,49]. It is revealed that the composite materials mainly consist of SiC and amorphous structures carbon. Therefore, the SiC/C foam material and SiC/C bulk material include the same base material.

Fig. 6.   (a) XRD patterns of the SiC/C bulk material; (b) XRD patterns of the SiC/C foam material.

Electromagnetic performance of SiC/C material

The microwave absorbing properties of SiC/C materials is affected by electromagnetic parameters, structure, and work frequency [31,45]. To investigate the role of the structure of SiC/C foam material on the electromagnetic wave absorption performance, the electromagnetic parameters of SiC/C material are measured between 4 GHz and 18 GHz. The SiC/C material have no magnetism, we consider only the complex permittivity. The real part (ε′) of the complex permittivity represents the storage capability of the electric energy and the imaginary part (ε′′) of the complex permittivity represents the loss capability of the electric energy [32,50]. Fig. 7 shows the ε′ and ε′′ of the SiC/C foam material and the SiC/C bulk material in the frequency range of 4-18 GHz. As shown in Fig. 7, the ε′ and ε′′ values of SiC/C foam material exhibit a marked increase with frequency reducing from 18 GHz to 4 GHz. The ε′ values of SiC/C bulk material also exhibit a marked increase with frequency reducing from 18 GHz to 4 GHz, while the ε′′ values of SiC/C bulk material are relatively stable between 10 GHz and 18 GHz, and the ε′′ values of SiC/C bulk material has a slow downward trend with frequency reducing from 10 GHz to 4 GHz. From Fig. 7, it appears that ε′ of the SiC/C bulk material higher than the SiC/C foam material throughout the whole frequency and the ε′′ of the SiC/C bulk material higher than the ′′ of the SiC/C foam material in the band of 7-18 GHz, but the ε′′ of the SiC/C foam material higher than the ε′′ of the SiC/C bulk material in the band of 4-7 GHz. Porous effect of the SiC/C foam material such as interfacial polarization and multiple scattering may plays an important role in these phenomena [31,32,45]. The SiC/C foam material is a kind of porous heterogeneous material, while the influence of scale effect can be neglected if the cell size of the foam materials is much smaller than the incident wavelength, according to the Maxwell-Gernett theory [16,31,32]:

εe=[$\frac{(ε_2+2_{ε1})+2_q(ε_2-ε_1)}{(ε_2+2_{ε1})-q(ε_2-ε_1)}$]ε1

where εe is the effective complex permittivity, ε1 is the complex permittivity of solid materials, ε2 is the complex permittivity of the air, q shows the volume fraction of air in the composite foam material [31,32]. From the Fig. 8, we can see that the SiC/C foam material’s complex permittivity is not in line with the effective complex permittivity, and the effective ε′′ of the SiC/C foam material lower than ε′′ the SiC/C foam material in the frequency range of 4-18 GHz.

Fig. 7.   Frequency dependence of complex permittivity for SiC/C foam material and SiC/C bulk material.

Fig. 8.   The frequency dependence of complex permittivity and effective permittivity for SiC/C foam material.

There are two possible reasons for these phenomenon. On the one hand, the SiC/C foam materials have a 3-D network structure which could change the path of electromagnetic wave propagation [31,32]. We simulated the electromagnetic distribution of SiC/C materials by HFSS. Fig. 9 shows the distribution of electromagnetic filed vector and energy of the SiC/C foam and the SiC/C bulk material. From Fig. 9(a) and (b), the multiple reflections of electromagnetic waves can be clearly seen in SiC/C foam material, which could increase the loss path, improving the absorption performance. Fig. 9(c) and (d) shows the diffraction phenomenon of the electromagnetic wave for the SiC/C foam material. Diffraction can lead to some energy in horizontal transmission. This means that SiC/C foam material has a larger loss path than the SiC/C bulk material [31,51,52]. On the other, SiC/C foam material with a large specific surface area correspond with large solid-void interfaces of SiC/C foam material, which then give rise to the interfacial polarization and associated relaxation, according to the Maxwell-Wagner effect [31,32,52]. The foam structure of the SiC/C foam material could also provide the means of interfacial compatibilization [31,53,54].

Fig. 9.   Electromagnetic wave scatting of (a) SiC/C foam material, (b) SiC/C bulk material, phenomenon of energy regional distribution of (c) SiC/C foam material (d) SiC/C bulk material.

The SiC/C foam material contains air part, which could increase the impedance match between the air and the absorbing material. This foam structure could also improve the loss ability [31,32,34]. Fig. 10 shows the absorption properties of SiC/C foam material with different thicknesses. We can see that the absorption performance of SiC/C foam material could reach -20 dB at 14 GHz with a block density of only 0.56 g/cm3 and a thickness of only 3 mm. The band of reflectivity is less than-10 dB could be adjusted between 4-18 GHz by changing the SiC/C foam material thickness to 3-7 mm. This demonstrates the high efficiency absorption performance of the SiC/C foam material.

Fig. 10.   Reflection loss vs. frequency of SiC/C foam material with different absorber thicknesses.

In general, foam material is too weak to handle and coat radar absorbing material. Therefore, the strength of SiC/C foam material is important in practical application. Fig. 11 shows the compressive strength of the SiC/C foam material, which is 5.8 MPa. Compared with other foam absorbing materials, the SiC/C foam material has relatively high compressive strength [35,55]. Thus, the SiC/C foam material can meet the practical demands for engineering purposes.

Fig. 11.   Stress-strain curves of the SiC/C foam material.

It should be noted that only the weight ratio between SiC and C is kept constant in this study, while during the practical application weight ratio between SiC and C has a distinct effect on the microwave absorption performance and different weight ratios between SiC and C should be investigated in the future.

Conclusion

In summary, the role of the foam structure of SiC/C foam material for the utilization of microwave absorbing performance has been studied as comparison between SiC/C foam material and SiC/C bulk material. The foam structure of SiC/C foam material could cause the diffraction and multiple reflection which increases the loss path. The solid-void interfaces of SiC/C foam material is increased, which improves the interfacial polarization and associated relaxation. The SiC/C foam material could be adjusted flexibly for absorbing performance which offers great promise of being an ideal absorbing material. In the future research, we will use foam materials with different SiC and C weight ratios and different base foam composites, in order to create lightweight and highly efficient broadband radar absorbing materials.

Acknowledgements

The authors are highly grateful to L. Feng, X.L. Shi, S.T. Deng, J.W. Song and Y.L. Jiao for the beneficial discussion.


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