High Electromagnetic Wave Absorption Performance of Silicon

Jan 8, 2010 - The electromagnetic (EM) wave absorption properties of 2-mm-thick ... in the range of 2-40 GHz using a free-space antenna-based system...
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J. Phys. Chem. C 2010, 114, 1947–1952

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High Electromagnetic Wave Absorption Performance of Silicon Carbide Nanowires in the Gigahertz Range Sheng-Cheng Chiu, Hsin-Chih Yu, and Yuan-Yao Li* Department of Chemical Engineering, National Chung Cheng UniVersity, Chia-Yi 621, Taiwan, R. O. C ReceiVed: June 1, 2009; ReVised Manuscript ReceiVed: December 8, 2009

The electromagnetic (EM) wave absorption properties of 2-mm-thick silicon carbide nanowire (SiCNW)-epoxy composites were studied in the range of 2-40 GHz using a free-space antenna-based system. The 35 wt % SiCNW composites exhibited dual-frequency EM wave absorptions of -31.7 and -9.8 dB at 8.3 and 27 GHz, respectively. The minimum reflection loss of -32.4 dB was achieved at 31.1 GHz for the composites containing 25 wt % SiCNW. A study of the loss mechanism of EM wave absorption suggests the combination of the electric conductance loss caused by network-like SiCNWs in the resin matrix and the relaxation polarization loss in the interfaces of SiCNWs and the epoxy resin. Introduction Electromagnetic interference (EMI) has drawn a significant amount of attention because of the rapid development of wireless communications and high-frequency circuit devices in commercial, industrial, and military applications.1-4 Studies have focused on effective electromagnetic (EM) wave absorbing materials that have a wide absorption frequency, high absorption capability, low weight, good thermal stability, and antioxidation capability. EM wave absorption capability depends on a material’s nature, shape, and size.5-7 According to the EM energy conversion principle, the relative complex permittivity (εr ) εr′ - jεr′′), the relative complex permeability (µr ) ur′ jur′′), and the proper matching of the complex permittivity and permeability determine the reflection and attenuation characteristics of EM waves.8 Nanometer materials are better EM wave absorbing materials than conventional ferrites. These materials include Ni/Ag core-shell nanoparticles;6 carbon nanocoils;9,10 carbon fibers/carbonyl iron;11 carbon nanotubes (CNTs) filled with Fe, Co, or Ag nanowires (NWs) and ferromagnetic alloy NWs;12-15 Fe NWs;16 Ni NWs;17 ZnO nanostructures;18-20 and Fe/carbon core-shell nanocapsules.21 Silicon carbide nanowires (SiCNWs) are a good EM wave absorbing material because of their high thermal stability, chemical resistivity, good mechanical strength and hardness, and semiconducting properties. SiCNWs can be fabricated using several techniques, including the CNT-confined reaction method,22-24 chemical vapor deposition (CVD),25,26 and the arcdischarge method.27-29 In our previous work,30 we synthesized SiCNWs in large quantities using a one-step process that combined the CVD method to form CNTs and the CNTconfined reaction method to form SiCNWs. According to the literature, SiC powder31 or Ni-Co-P-coated SiC powder32 also exhibit good wave absorption ability. The reflection loss of a Ni-Co-P-coated SiC micrometer particle is almost -30 dB. However, EM absorption is mainly attributed to the magnetization of metals. In addition, SiC short fibers33 or SiC-based ceramic woven fabrics34 were investigated and demonstrated to be good candidate materials for EM wave absorption applications. The average of reflection loss of SiC-based ceramic * Corresponding author. Tel.: +886-5-2720411. Fax: +886-5-2721206. E-mail: [email protected].

micrometer fibers was about -10 dB at 17 GHz.34 Compared to SiC microparticles,31,32 short fibers,33 and SiC-based ceramic woven fabrics,34 SiCNWs might improve the performance of EM wave absorption because of their high surface-to-volume ratio, quantum size effects, and the network structure effect of the SiCNWs in the composite. In this study, SiCNWs were used as an EM wave absorbing material. SiCNW-epoxy composites were used to measure EM wave absorption in the gigahertz range (2-40 GHz, microwave range), which applies to mobile telephones, intelligent transport systems, local area network systems, microwave darkrooms, and radar stealth systems. The EM wave absorption mechanisms of SiCNW-epoxy resin composites were also investigated and are discussed herein. Experimental Procedure The SiCNW composite consists of SiCNW powder and epoxy resin. SiCNWs were synthesized using as-produced CNT/ Si@SiO2 heated at 1300 °C in an argon atmosphere following the growth of CNTs on an Fe-Ni-coated Si@SiO2 core-shell substrate at 650 °C.30 To purify the SiCNWs, the powder was placed in a 4 N NaOH solution at 60 °C for 5 h. The material was then filtered, washed with distilled water several times, and then dried at 60 °C overnight. The epoxy (Epon828, Shell Chemical), which is transparent to EM waves, was used as a matrix. The SiCNW composite was well-mixed by a mixing/ degassing machine (Thinky ARE250) with the desired weight ratio of SiCNWs to epoxy. The well-mixed composite was then molded and cured at 60 °C for 10 h to form a 15 × 15 cm2 SiCNW resin composite with a thickness of 2 mm. The SiCNWs were examined using a field-emission scanning electron microscope (FE-SEM, Hitachi S-4800) and a fieldemission transmission electron microscope (FE-TEM, Philips Tecnai F20). The crystallographic data were collected using X-ray diffraction (XRD, Rigaku Miniflex system using a Cu target, Cu KR ) 1.5418 Å). A free-space antenna-based system equipped with a vector network analyzer (HP8722ES) was used to measure the reflection loss (RL) of the composite sheet, which was backed by a reference metal plate of the same size. The RL values were obtained using the reflection level of the metal plate, whereas the relative complex permeability (εr) and

10.1021/jp905127t  2010 American Chemical Society Published on Web 01/08/2010

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Figure 1. (a) SEM image of SiCNWs, (b) TEM image of a single SiCNW, and (c) XRD pattern of SiCNWs.

permittivity (µr) were calculated from the scattering parameters measured by the vector network analyzer. RL, permittivity (µr), and permeability (εr) can be expressed as follows8,12,35,36

RL ) 20 log

|

|

Zin - Z0 ) 20 log |T| Zin + Z0

Zin ) Z0(µr /εr)1/2 tanh[j(2πfd/c)(µrεr)1/2]

(1)

(2)

where Zin is the input impedance of the absorber, Z0 is the impedance of free space, T is the reflection coefficient, f is the frequency of the EM wave, d is the thickness of the absorber, and c is the velocity of light. Here, an RL value of -10 dB is comparable to 90% EM wave absorption. In general, materials with RL values of less than -10 dB absorption are considered as suitable EM wave absorbers. Results and Discussion Figure 1a shows an SEM image of the SiCNWs synthesized using the combination of the CVD method and the CNTconfined reaction method.30 The characterization shows that the SiCNWs had diameters of 30-70 nm and lengths of 4-10 µm. A TEM image of a single SiC nanowire with a diameter of about 70 nm is shown in Figure 1b. The inset in Figure 1b shows the selected-area electron diffraction (SAED) pattern of the nanowire. The SAED pattern confirms that the nanowire is a single-crystalline β-SiC structure and indicates that the growth direction of the nanowire was along the [111] orientation. In Figure 1c, the XRD spectrum of the SiCNWs shows five clear strong peaks that represent cubic-structure β-SiC.

Figure 2. Fracture surfaces of (a) resin and (b) 35 wt % SiCNW composite sheet (SiCNWs with resin) and (c) elemental mapping of a composite sheet.

Parts a and b of Figure 2 show SEM images of fractured cross sections of the epoxy resin and the 35 wt % SiCNWs composite sheet, respectively. The randomly dispersed SiCNWs shown in Figure 2b reveal that the SiCNWs formed a network in the epoxy resin matrix. The corresponding silicon elemental map (green dots in Figure 2c) shows that the silicon was dispersed uniformly in the epoxy resin. Well-dispersed SiCNWs in the matrix ensure accurate and repeatable EM wave absorption measurements. Figure 3 shows RL in the frequency range of 2-18 GHz for epoxy resin; composites containing 15, 25, and 35 wt % SiCNWs; and composites containing 15 wt % SiC microparticles. The epoxy resin sample used as a reference sheet exhibited no EM wave absorption, and the composite with 15 wt % SiC microparticles had weak EM wave absorption of -1.91 dB at 16.2 GHz. The results show that epoxy resin and

High EM Wave Absorption of SiC Nanowires

Figure 3. Frequency dependences of RL for epoxy resin; composite with 15 wt % microparticles; and composites with 15, 25, and 35 wt % SiCNWs at 2-18 GHz.

the SiC microparticle composite have no significant effect on EM wave absorption. Similar results were obtained for composites with 25-50 wt % SiC microparticles in the resin. In contrast, SiCNW composites had good EM wave absorption in the range of 2-18 GHz. It was found that the EM wave absorption depends on the amount of SiCNWs in the composites. The RL peaks shifted to a lower frequency when the concentration of SiCNWs in the composite was increased from 15 to 35 wt %. The shift of the minimum RL value might due to an increase in the population density of the SiCNWs per unit volume in the composite.37 The results suggest that the adsorption frequency can be adjusted by controlling the SiCNW content of the composite. Wideband absorption can be achieved by combining SiCNW-epoxy layers with different SiCNW contents. As illustrated in Figure 3, the minimum RL value for the composite containing 35 wt % SiCNW was -31.7 dB at 8.3 GHz. RL values of less than -10 dB (over 90% EM wave absorption) were obtained in the range of 7.1-9.6 GHz. To investigate the EM wave absorption mechanism of the SiCNW composites, the relative complex permittivities and permeabilities of the samples were considered. Figure 4 shows the frequency dependences of the real part (εr′) and imaginary part (εr′′) of the complex permittivity (Figure 4a,b) and the real part (ur′) and imaginary part (ur′) of the complex permeability (Figure 4c,d), as well as the dielectric loss tangent (tan δE) and the magnetic loss tangent (tan δM) (Figure 4e,f). The dielectric and magnetic loss tangents can be expressed as tan δE ) εr′′/εr′ and tan δM ) ur′′/ur′, respectively. As shown in Figure 4a, the initial values of εr′ at 2 GHz were 8.56, 12.45, and 36.87 for SiCNW concentrations in the composites of 15, 25, and 35 wt %, respectively. In addition, the εr′ curve for the 35 wt % SiCNW composite had broad multiresonance peaks over the range of 2-18 GHz. It is known that the real permittivity is an expression of the polarizability of a material, which consists of dipolar polarization and electric polarization at microwave frequencies.38 However, dipolar polarization in this study was not a significant mechanism because the SiCNWs were coated with epoxy resin. The real permittivity mainly increases due to the increase of electric polarization. This can be explained by considering the fact that large amounts of SiCNWs increase the electric polarization and electric conductivity of the complex network structure in the SiCNW-epoxy composites. Figure 4b shows that the values of εr′′ increased slightly with little variation in the 8-18 GHz range when the concentration of the fillers in the resin matrix was increased. A significant peak with a shoulder appeared at 17.28 GHz when the

J. Phys. Chem. C, Vol. 114, No. 4, 2010 1949 concentration of the SiCNWs in the composite was 35 wt %. This imaginary permittivity was relatively high because of the intrinsic dielectric properties and the structure of the SiCNWs.39,40 Parts c and d of Figure 4 show the frequency dependencies of the real and imaginary parts, respectively, of the complex permeability of SiCNW composites. It was observed that the ur′ values decreased slightly with increasing concentration of fillers in the composite whereas ur′ remained almost constant at 0 in the range of 2-18 GHz. Moreover, the ur′′ values were negative at some frequencies, which is meaningless from the physics point of view. The negative values of complex permittivity and permeability might be due to noise. Nevertheless, the values of permittivity and permeability were negative at some frequencies in some studies, which is an indication of lefthanded materials (LHMs). Therefore, the negative values of permittivity and permeability imply that SiCNWs have potential as LHMs. Compared to the values of complex permittivity, the complex permeability values of both ur′ and ur′′ are relatively low. This indicates that the main contributor to RL is complex permittivity. Parts e and f of Figure 4 show the dielectric and magnetic loss tangents of composites with various concentrations of SiCNWs as functions of frequency. Figure 4e shows that the values of dielectric loss increased with increasing concentration of fillers in the composites in the frequency range of 10-18 GHz. In particular, the δE value of the 35 wt % SiCNW composite at 16.96 GHz was 2.79. The values of tan δM are almost all near or less than zero, as shown in Figure 4f, which means that there was no significant magnetic loss for any of the SiCNW composites. The EM wave absorption properties of SiCNWs result mainly from dielectric loss rather than magnetic loss. According to the literature,41 loss mechanisms of δE are due to leak conductance at relatively low frequencies. Relaxation polarization loss and electric conductance loss are major factors at microwave frequencies. Therefore, the EM wave absorption properties of SiCNW composites in the range of 2-18 GHz can be attributed to relaxation polarization loss and electric conductance loss. The high surface-to-volume ratio of the nanostructures that leads to a large amount of interfacial electric polarization at the interfaces between SiCNWs and the epoxy resin might also increase the absorption in the gigahertz frequency range. Chen et al. reported that the strong EM wave absorption of ZnO NW composites is due to the charge multipoles that form along the boundaries between the polyester and the nanowire surfaces.18 Therefore, the rough surfaces of the SiCNWs in our work might form a large number of electric polarizations that increase absorption losses. The above results suggest that SiCNWs increase EM absorption in terms of complex permittivity because of their dielectric nature and the network of nanowires in the matrix. Figure 5 shows a comparison of RL in the frequency range of 18-40 GHz for epoxy resin; 15, 25, and 35 wt % SiCNW composites; and a composite containing 15 wt % SiC microparticles. It was also found that the EM wave absorptions of pure epoxy and the composite containing 15 wt % SiC microparticles were relatively poor. The peak value of RL shifted to lower frequencies with increasing SiCNW concentration. However, no RL peak was observed for the 15 wt % SiCNW composite in the frequency range of 18-40 GHz. A minimum RL value of -32.4 dB was observed at 31.1 GHz for the composite containing 25 wt % SiCNW; the bandwidth for RL of less than -10 dB was 4.2 GHz in the range of

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Figure 4. Frequency dependences of (a) real and (b) imaginary parts of the complex permittivity and (c) real and (d) imaginary parts of the complex permeability and (e) dielectric and (f) magnetic tangent losses of SiCNW-epoxy composites at 2-18 GHz.

Figure 5. Frequency dependences of RL for epoxy resin; composites with 15 wt % microparticles; and composites with 15, 25, and 35 wt % SiCNWs at 18-40 GHz.

29.1-33.3 GHz. The RL results show that SiCNWs have excellent EM wave absorption capabilities.

Figure 6 shows the frequency dependencies of the real parts (εr′, ur′) and imaginary parts (εr′′, ur′) of the relative permittivity and permeability in the range of 18-40 GHz for composites containing 15, 25, and 35 wt % SiCNW. The εr′ values shown in Figure 6a significantly increased in the range of 18-28 GHz and remained approximately constant in the range of 28-40 GHz. The εr′′ values shown in Figure 6b slightly increased with increasing SiCNW concentration in the composites. Parts c and d of Figure 6 show the real and imaginary parts, respectively, of the relative complex permeability. As a result, both the ur′ and ur′′ values remain below 1. In a comparison between the complex permittivity (εr′ and εr′′) and the complex permeability (ur′ and ur′′), the values of εr′ and εr′′ were much larger, indicating that the dielectric loss was significant. In addition, the complex permittivity results show that the corresponding RL value of the 35 wt % SiCNW composite should be higher than that of the 25 wt % SiCNWs composite. However, the 25 wt % SiCNWs composite had a minimum RL of -32.4 dB at 31.1 GHz, whereas the 35 wt % SiCNWs composite had a minimum RL of -9.8 dB at 27 GHz, as shown in Figure 5.

High EM Wave Absorption of SiC Nanowires

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Figure 6. Frequency dependences of (a) real and (b) imaginary parts of the complex permittivity and (c) real and (d) imaginary parts of the complex permeability of SiCNW-epoxy composites at 18-40 GHz.

TABLE 1: Densities and Tensile Strengths of Epoxy Resin and SiCNW Composites

Figure 7. Frequency dependences of RL for 25 wt % SiCNW composites with different thicknesses at 18-40 GHz.

Fan et al. reported that the electric field of short-distance resonance multipoles resulted in reflection rather than adsorption with an increase of CNT concentration in the composite.42 This might be one of the reasons that complex permittivity increased with increasing SiCNW concentration in the composite, resulting in the reduction of the RL value at lower frequencies in the range of 18-40 GHz. This result suggests that it is possible to achieve better EM wave absorption by optimizing the SiCNW concentration in the range of 15-35 wt %. Figure 7 shows the RL of 25 wt % SiCNW composites for various thicknesses (1-3 mm) at 18-40 GHz. The minimum RL value shifts to lower frequencies with increasing thickness of the composite: The 1 mm-thick composite did not exhibit minimum RL values in the range of 18-40 GHz. The values of minimum RL for the 2- and 3-mm-thick composites are

filler content

density (g/cm3)

tensile strength (kN/m2)

0 (epoxy only) 15 wt % SiCNWs 25 wt % SiCNWs 35 wt % SiCNWs 25 wt % SiC particles

1.08 1.13 1.30 1.48 1.28

1.65 4.67 5.81 7.14 2.08

-32.35 and -31.6 dB at 31.09 and 21.36 GHz, respectively. This implies that RL below -30 dB can be easily obtained in the range of 18-40 GHz by controlling the SiCNW composite thickness. The RL value and wideband absorption properties of SiCNW composites can be tuned by properly designing the multilayered structure. The epoxy resin and all composites (15-35 wt %) were flexible sheets. The density and the mechanical properties of the epoxy resin and the composites are listed in Table 1. The results show that the densities of the composites, which are in the range of 1.13-1.48 g/cm3, increase with increasing concentration of SiCNWs. It was observed that the density of SiCNW composites is lower than that of pure SiCNWs (3.21 g/cm3) and higher than that of pure epoxy resin (1.08 g/cm3). In addition, it was found that the tensile strength of the SiCNW composites is in the range of 4.67-7.14 kN/m2. The tensile strength increased from 1.65 to 7.14 kN/m2 when the SiCNW content in the composites was increased from 0 to 35 wt %. The tensile strength of the 25 wt % SiCNW composite was 252% and 179% higher than those of pure epoxy resin and the composite containing 25 wt % SiC microparticles, respectively. This result is attributed to the improvement of the adhesion and

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good dispersion of SiCNWs in the epoxy resin. As a result, SiCNW composites have good workability and mechanical properties for practical applications. Conclusion The EM wave absorption characteristics of epoxy resin composites containing SiCNWs were investigated. The results show that the 35 wt % SiCNW composite exhibited dualfrequency EM wave absorption behavior in the ranges of 2-18 and 18-40 GHz. A minimum RL of -32.4 dB was observed at 31.1 GHz for the composite containing 25 wt % SiCNW. The excellent EM wave absorption properties mainly result from the dielectric network structure of the nanowires in the matrix and from electric polarization. SiCNWs are good EM wave absorbers because they have a low weight, a high surface-tovolume ratio, good thermal stability, antioxidation capability, and good mechanical stiffness. Acknowledgment. The authors thank the Chung-Shan Institute of Science and Technology, Taiwan, for their financial and technical support of this research under Contract 98-2623E-194-001-D. Dr. Jen-Sung Hsu and Mr. Zhuang are acknowledged for their advice and help in EM absorption measurements. References and Notes (1) Naito, Y.; Suetake, K. IEEE Trans. MicrowaVe Theory Techn. 1971, MT19, 65. (2) Wallace, J. L. IEEE Trans. Magn. 1993, 29, 4209. (3) Toneguzzo, P.; Viau, G.; Acher, O.; Fievet-Vincent, F.; Fievet, F. AdV. Mater. 1998, 10, 1032. (4) Butera, A.; Zhou, J. N.; Barnard, J. A. J. Appl. Phys. 2000, 87, 5627. (5) Zhang, X. F.; Dong, X. L.; Huang, H.; Liu, Y. Y.; Wang, W. N.; Zhu, X. G.; Lv, B.; Lei, J. P.; Lee, C. G. Appl. Phys. Lett. 2006, 89, 053115. (6) Lee, C. C.; Chen, D. H. Appl. Phys. Lett. 2007, 90 (19), 193102. (7) Deng, Y. D.; Zhao, L.; Shen, B.; Liu, L.; Hua, W. B. J. Appl. Phys. 2006, 100, 014304. (8) Liu, J. R.; Itoh, M.; Machida, K. Appl. Phys. Lett. 2006, 88, 062503. (9) Zhao, D. L.; Shen, Z. M. Mater. Lett. 2008, 62, 3704. (10) Tang, N. J.; Zhong, W.; Au, C. T.; Yang, Y.; Han, M. G.; Lin, K. J.; Du, Y. W. J. Phys. Chem. C 2008, 112, 19316. (11) Youh, M. J.; Hsu, J. S.; Huang, C. F.; Chiu, S. C.; Wu, H. C.; Jiang, M. T.; Liu, C. T.; Yu, H. C.; Li, Y. Y. A carbonyl iron/carbon fiber material for electromagnetic wave absorption, manuscript submitted. (12) Che, R. C.; Peng, L. M.; Duan, X. F.; Chen, Q.; Liang, X. L. AdV. Mater. 2004, 16, 401. (13) Lin, H. Y.; Zhu, H.; Guo, H. F.; Yu, L. F. Mater. Res. Bull. 2008, 43, 2697. (14) Zhao, D. L.; Li, X.; Shen, Z. M. Mater. Sci. Eng. B 2008, 150, 105.

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