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epoxy resin composites
Materials Science and Engineering B 127 (2006) 207–211
Microwave absorbing properties of activated carbon-fiber felt screens (vertical-arranged carbon fibers)/epoxy resin composites Naiqin Zhao ∗ , Tianchun Zou, Chunsheng Shi, Jiajun Li, Weikai Guo School of Material Science and Engineering, Tianjin University, Tianjin 300072, PR China Received 27 June 2005; received in revised form 15 September 2005; accepted 23 October 2005
Abstract Microwave absorbing properties of the composites containing inductive activated carbon-fiber felt screens (IACFFSs) and vertical-arranged carbon fibers (VACFs) have been investigated, respectively. It is found that the reflection properties of the composites containing IACFFSs are greatly affected by the element configurations of IACFFSs. With the interval between strips and the width of the strips in the IACFFS decreased, the absorbing properties of the composites are improved. The composite obtains a reflection loss below −10 dB in 8–18 GHz, which covers the frequency region of the radar, when the interval between strips and the width of the strips are 7 mm and 5 mm, respectively. The space between the adjacent carbon fibers is critical for the absorbing characteristics of the composites containing VACFs. The composite achieves a reflection loss below −10 dB over 7.6 GHz when the space is 4 mm. By applying the block design method, the composite was divided into four sub-regions, which were centrosymmetrical and included two IACFFSs and two VACF regions. The composite achieves a reflection loss below −20 dB in 11.8–18 GHz and the minimum value reaches −30 dB when the interval between strips, the width of the strips and the fiber space are 10 mm, 5 mm and 8 mm, respectively. © 2005 Elsevier B.V. All rights reserved. Keywords: Microwave absorbing materials; Inductive activated carbon-fiber felt screen; Vertical-arranged carbon fiber; Block design
1. Introduction In recent years, microwave absorption materials have attracted considerable attention because it is an essential part of a stealthy defense system for all military platforms, whether as aircraft, sea or land vehicles. They are also used to eliminate electromagnetic interference (EMI), which is a specific kind of environmental pollution due to the explosive growth in the utilization of electrical and electronic devices in industrial and commercial applications [1–4]. Some studies showed that the input impedance of the absorbing materials could be increased rapidly by inserting Frequency Selective Surfaces (FSSs) into them [5–6]. If properly used, FSS incorporated with other kinds of microwave absorbing materials can effectively improve the absorption performances of those materials [7–8]. In this work, activated carbon-fiber felt is used to replace the traditional material of FSS, which is
∗
Corresponding author. Tel.: +86 22 27407527; fax: +86 22 27405874. E-mail address: [email protected] (N. Zhao).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.10.026
metal. The alteration of the material is based on the following considerations: (i) in the GHz range, electrical properties of activated carbon-fiber felt are similar to that of metal; (ii) activated carbon-fiber felt has the structure of fabrics and the fibers composed of it has the irregular-shaped cross-sections, which are very advantageous for the absorption of electromagnetic wave [9]; (iii) activated carbon-fiber felt is lighter than metal. The carbon fiber (in continuous or discontinuous forms), which is one of the absorbents, has been widely employed in microwave absorbing materials in reducing backscattering from objects or radar targets, EMI suppressors and paints [10–13]. However, to the best of our knowledge, there are no reported experimental results on the absorbing effect of the carbon fibers in the vertical-arranged manner. When microwave absorbing material is divided into several sub-regions, the electromagnetic parameters of the materials in each sub-region control the amplitude and phase of propagating electromagnetic waves independently. If we adjusted these amplitudes and phases elaborately, the final synthesized vector (the final reflectivity) would be minimized. So the
N. Zhao et al. / Materials Science and Engineering B 127 (2006) 207–211
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block design is a very effective means to improve the absorbing performance and expand the operation frequency band [14]. In this paper, the absorbing properties of microwave absorbing materials containing inductive activated carbonfiber felt screens (IACFFSs) and vertical-arranged carbon fibers (VACFs) embedded in epoxy resin are studied, respectively. To combine the advantages of the IACFFSs and VACFs, we apply the sub-division technique to design the absorber. The composites consisting of four sub-regions (two IACFFSs and two VACF regions) are prepared, and the effects of these sub-regions on the reflection properties are also investigated. 2. Experimental 2.1. Materials The epoxy resin (E-44, Resin company in Wuxi, PRC) cured with the polyamide resin (203# , Chemical plant in Tianjin, PRC) was adopted as the matrix of the samples. The epoxy resin has the permittivity of 3.0–3.4 and the dielectric loss tangent of 0.01–0.03. The polyamide resin has the permittivity of 3.0–4.0 and the electrical resistivity of 1011 –1012 m. Viscose-based activated carbon-fiber felt (made by Metallography Lab, Tianjin University, PRC) with average thickness of 1.2 mm and specific surface area of 823 m2 g−1 was used as the material of FSS. The carbon fibers used in this work were poly-acrylonitrile (PAN)-based fibers (1 K), and the average diameter of the filaments is ∼9 m. The carbon fiber has the electrical resistivity of ∼0.05 m, the tensile strength of ∼3.73 GPa and the density of ∼1.76 g cm−3 .
Fig. 2. Geometry of the IACFFS.
2.3. Measurements of microwave absorption The absorption characteristics of the samples were measured over the frequency range of 8–18 GHz in a calibrated Naval Research Laboratory (NRL) arch using a HP8757E network analyzer [16]. The sample under test was positioned on an aluminium panel (180 mm × 180 mm). 3. Results and discussion
2.2. Preparation of composites The epoxy resin was blended uniformly with the polyamide resin in the mass ratio of 2:1. After vacuum deformation the mixture was cast into a conventional semi-overflowing die. The IACFFSs were put in the middle of the composites. The carbon fibers embedded in the composite were perpendicular to the surface of the sample and arrayed in the equidistance. When we prepared the absorber based on the block design, each subregion is fabricated independently. A piece of fiberglass cloth with the same size as the die was laid on the top of the sample to make the impedance of the samples well matched with the impedance of air [15]. Molding was carried out in a hydraulic press at 10 MPa pressure and 80 ◦ C for 2 h, obtaining specimens of 180 mm × 180 mm with thickness of 4 mm for reflectivity measurements. The configuration of the sample is shown in Fig. 1.
3.1. Absorption properties of the composites containing IACFFSs The IACFFS made of periodic grids is shown in Fig. 2, a being the interval between carbon-fiber felt strips and b being the width of the strips. The parameters of IACFFSs embedded in the samples are listed in Table 1. Fig. 3 shows the frequency dependence of the reflection loss (R.L.) of the composites containing IACFFSs, where a/b is taken as a parameter, b being constant. It can be seen that the values of a/b influence the R.L. of the composites greatly. With the reduction of a/b, the minimum R.L. decreases except for a/b = 1.4, and Table 1 Element configurations of the IACFFSs embedded in the samples Serial numbers
b (mm)
a/b
1#
5 5 5 5 7 3
4 3 2 1.4 3 3
2# 3# 4# 5# 6# Fig. 1. Vertical section of the sample.
N. Zhao et al. / Materials Science and Engineering B 127 (2006) 207–211
Fig. 3. Effect of a/b on microwave absorbing properties of the composites containing IACFFSs.
the bandwidth below −10 dB increases. For all the samples, the maximal absorption locates in the frequency range of 9–15 GHz. Also, it is observed that when a/b = 2, the R.L. of the composite is below −10 dB in the range of 8.4–18 GHz, and the minimum value is −30.6 dB at 12.4 GHz. While a/b = 1.4, the absorption curve illustrates the bandwidth less than −10 dB is over 10 GHz and it covers the frequency range of the radar wave. Fig. 4 shows the absorption curves of the samples containing IACFFSs, where b is taken as a parameter, a/b being constant. It can be seen that with b reduced, the absorption performances of the composites are improved. The maximal absorption locates in the frequency range of 11–14 GHz. Sample 6# presents better absorption characteristics as compared with others, the R.L. of which below −10 dB is obtained in the frequency range of 8.9–17.3 GHz, and the minimum value reaches −28.3 dB. The cross-sections of the carbon fibers in the felt are irregularshaped and their surface grooves are quite obvious as shown in Fig. 5. The activated carbon-fiber felt composed of these fibers has the structure of three-dimensional fabrics. The structure is like that of microwave anechoic chamber, where there are many small pyramids [9]. When incident wave arrives at the activated carbon-fiber felt, it is multi-reflected and gradually attenuated.
209
Fig. 4. Effect of b on microwave absorbing properties of the composites containing IACFFSs.
According to the electromagnetic field theory, one element in the inductive IACFFS can be considered as a rectangular waveguide. In this case, the material of the waveguide wall is the activated carbon-fiber felt, and the medium within the waveguide is the resin. Because the walls are not perfect conductors, some attenuation of electromagnetic fields occurs as they propagate in the waveguide. By using the expressions of the fields for the TE10 mode, the average power loss per unit length is expressed as [17] Pl =
Rs H02 [λ2 (l + 2w) + 4l3 (1 − λ2 /4w2 )] 2λ2
(1)
where Rs is the surface resistance, H0 the constant and determined by the wave source, λ the wave length, l and w are the length and width of the rectangular waveguide, respectively. Eq. (1) indicates that the average power loss rises with the reduction of λ. Moreover, if the frequency of the incident electromagnetic wave f is less than the cutoff frequency of the TM10 mode fc10 , no wave propagation will take place in the waveguide [17]. Therefore, the composites containing IACFFSs show bad microwave absorption properties at low frequency.
Fig. 5. SEM photos of viscose-based activated carbon-fiber felt.
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Fig. 6. Construction of the composites containing VACFs.
The rectangular waveguides in the IACFFS are coherent wave sources, and the incident field is partly attenuated by wave interference. When b (a/b) is constant, with the reduction of a/b (b) values, the number of waveguides (coherent wave sources) in one screen is increased, and so the absorbing properties of the composites containing IACFFSs are improved. 3.2. Absorption properties of the composites containing VACFs Fig. 6 shows the construction of the composite containing VACFs. The carbon fibers with the length of 4 mm were embedded in the epoxy resin matrix and perpendicular to the surface of the sample. The microwave absorption characteristics of the samples with different fiber spaces (d = 10 mm, 8 mm, 6 mm and 4 mm) are shown in Fig. 7. As can be seen from Fig. 7, the absorption curves of samples 8# , 9# and 10# have a similar trend. In the frequency range of 9–14 GHz, the R.L. of three samples fluctuates around −10 dB. When the frequency is higher than 14 GHz, the R.L. is sharply going down and the minimum values appear at the frequency about 16.3 GHz. In 17.5–18 GHz, the R.L. is below −10 dB and approximately the constant, which means the R.L. may be still lower than −10 dB in the nearby frequency region more than 18 GHz. When the space between the fibers is 4 mm, the R.L. below −10 dB is obtained in the frequency range of 11.8–18 GHz and the minimum value reaches −22 dB. For the carbon fibers have low electrical resistivity and are connected with the metal backing, the sample can be considered
Fig. 7. Effect of d on the microwave absorbing properties of the composites containing VACFs.
as a monopole vibrator antenna array. The antenna array captures the specific frequency region (which is called the operating band of the antenna) of electromagnetic wave and converts it into high-frequency currents in the carbon fibers. Therefore, the electromagnetic energy is partly transformed into heat energy. This may be the reason that the hollows occur in the absorption curves of samples 7# –10# . In addition, because the diameter of the carbon fiber is less than the wavelength of the incident wave, the electromagnetic wave would be scattered and partly attenuated when it arrives at the fibers [17]. 3.3. Absorption properties of the composites based on the block design method In order to combine the merits of two kinds of composites discussed above and obtain the materials with better absorption properties we apply the sub-division technique to fabricate the absorber. Fig. 8 shows the configuration of the composite based on the block design method. The sample was divided into four sub-regions which are centrosymmetric and consists of two IACFFSs and two VACF regions. Fig. 9 shows the absorbing curves of the samples 11# and 12# . The IACFFSs embedded in them are identical (the interval between strips a = 10 mm, the width of the strips b = 5 mm), and the fiber space is different (d = 8 mm and 6 mm, respectively). From Fig. 9, it can be found that compared with the composites only containing IACFFSs or VACFs, the absorption curves of the composites based on the block design method have three main characteristics. They are listed as follows: (i) At high frequencies, the absorption performance of the composites only containing the IACFFSs or the VACFs
Fig. 8. Construction of the composite based on the block design method.
N. Zhao et al. / Materials Science and Engineering B 127 (2006) 207–211
Fig. 9. Microwave absorption properties of the composites based on the block design method.
falls or has the trend to fall. However, the R.L. of the composite based on the block design method is still less than −15 dB at high frequencies, which suggests that the R.L. is lower than −10 dB in the nearby frequency region more than 18 GHz. (ii) The absorption curves of samples 11# and 12# fluctuate at high frequencies. The reason is that the absorbing effect of the composites includes two parts. One is the IACFFSs, the other is the VACFs. When they absorb the incident wave simultaneously, the R.L. should show a state of fluctuation. (iii) For sample 11# , the R.L. below −20 dB is obtained in the frequency range of 11.8–18 GHz, and the minimum value is –30 dB at 16.8 GHz. The flat-plate microwave absorbing material is divided into several sub-regions (assuming two sub-regions), and the R.L. of the two sub-regions is given by R1 = R1x + jR1y ;
R2 = R2x + jR2y
(2)
where R1 and R2 are the R.L. of sub-region I and sub-region II, respectively. Rx and Ry are the real part and imaginary part, respectively. The total R.L. of the absorbing material is given by [14] R = Rx + jRy ≈ (S1 R1x + S2 R2x ) + j(S1 R1y + S2 R2y )
(3)
where S1 is the ratio of the area of sub-region I to that of the absorbing material, and S2 is the ratio of the area of sub-region II to that of the absorbing material. From the expression, it can be seen that the total R.L. of the flat-plate absorbing material is the vector sum of the R.L. of all the sub-regions. Therefore, the block design method can improve the performances of the absorbing materials efficiently. 4. Conclusions 1. The composites containing IACFFSs exhibit good absorption performances in the frequency range of 8–18 GHz. It is
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noted that microwave absorption properties of the composites are strongly dependent on the element configurations of IACFFSs. In this study, with proper element configuration, the composite containing the IACFFS presents a reflection loss below −10 dB in the frequency range of 8–18 GHz. 2. The space between the adjacent fibers is critical for the absorbing characteristics of the composites containing VACFs. When this space is 4 mm, the composite achieves a reflection loss below −10 dB in the frequency range of 10.4–18 GHz and the minimum value is −22 dB. 3. By applying the block design method to fabricate the composites, the performances of the absorbing materials are improved effectively. The composite is divided into four subregions, which were centrosymmetrical and included two IACFFSs and two VACF regions. It achieves a reflection loss below −20 dB in the frequency range of 11.8–18 GHz and the minimum value reaches −30 dB when the interval between strips, the width of strips and the fiber space are 10 mm, 5 mm and 8 mm, respectively. Acknowledgement The work is supported by Natural Science Foundation of Tianjin of China (No. 013616911). References [1] A.N. Yusoff, M.H. Abdullah, J. Magn. Magn. Mater. 269 (2004) 271–280. [2] J.H. Oh, K.S. Oh, C.G. Kim, C.S. Hong, Composites: Part B 35 (2004) 49–56. [3] I.S. Seo, W.S. Chin, D.G. Lee, Compos. Struct. 66 (2004) 533–542. [4] S.Q. Zhang, C.G. Huang, Z.Y. Zhou, Z. Li, Mater. Sci. Eng. B 90 (2002) 38–41. [5] S.W. Lee, G. Zarrillo, C.L. Law, IEEE Trans. Antennas Prop. AP-30 (1982) 904–909. [6] A. Tennant, B. Chambers, IEEE Microw. Wireless Compon. Lett. 14 (2004) 46–47. [7] Y. Sha, K.A. Jose, C.P. Neo, V.K. Varadan, Microw. Opt. Technol. Lett. 32 (2002) 245–249. [8] S. Chakravarty, R. Mittra, N.R. Williams, IEEE Trans. Antennas Prop. 50 (2002) 284–296. [9] D.L. Zhao, Z.M. Shen, W.D. Chi, New Carbon Mater. 16 (2001) 66–72 (in Chinese). [10] K.L. Chen, Optimization and engineering of microwave absorbers, Ph.D. dissertation, Dep. Eng. Sci. Mech., Pennsylvania State University, State College, 1998. [11] C.P. Neo, V.K. Varadan, IEEE Trans. Electromagn. Compat. 46 (2004) 102–106. [12] C.I. Su, J.Y. Li, C.L. Wang, Textile Res. J. 75 (2005) 154–156. [13] D.D.L. Chung, Carbon 39 (2001) 279–285. [14] X.Y. Wang, J. Qian, R.X. Wu, H.F. Zhu, H.P. Wan, J. Nanjing Univ. (Natural Sci.) 37 (2001) 625–629 (in Chinese). [15] W.K. Guo, Effects of carbon fiber arrangements on microwave absorbing properties of structural composites and mechanics discussions, M.S. dissertation, School of Material Science and Engineering, Tianjin University, 2004 (in Chinese). [16] E.F. Knott, J.F. Schaeffer, M.T. Tuley, Radar Cross Section, Artech House Inc, New Jersey, 1985. [17] Z.S. Quan, Electromagnetic Field Theory, University of Electronic Science and Technology of China Press, Chengdu, 1995 (in Chinese).
Microwave absorbing properties of activated carbon-fiber felt screens (vertical-arranged carbon fibers)/epoxy resin composites Naiqin Zhao ∗ , Tianchun Zou, Chunsheng Shi, Jiajun Li, Weikai Guo School of Material Science and Engineering, Tianjin University, Tianjin 300072, PR China Received 27 June 2005; received in revised form 15 September 2005; accepted 23 October 2005
Abstract Microwave absorbing properties of the composites containing inductive activated carbon-fiber felt screens (IACFFSs) and vertical-arranged carbon fibers (VACFs) have been investigated, respectively. It is found that the reflection properties of the composites containing IACFFSs are greatly affected by the element configurations of IACFFSs. With the interval between strips and the width of the strips in the IACFFS decreased, the absorbing properties of the composites are improved. The composite obtains a reflection loss below −10 dB in 8–18 GHz, which covers the frequency region of the radar, when the interval between strips and the width of the strips are 7 mm and 5 mm, respectively. The space between the adjacent carbon fibers is critical for the absorbing characteristics of the composites containing VACFs. The composite achieves a reflection loss below −10 dB over 7.6 GHz when the space is 4 mm. By applying the block design method, the composite was divided into four sub-regions, which were centrosymmetrical and included two IACFFSs and two VACF regions. The composite achieves a reflection loss below −20 dB in 11.8–18 GHz and the minimum value reaches −30 dB when the interval between strips, the width of the strips and the fiber space are 10 mm, 5 mm and 8 mm, respectively. © 2005 Elsevier B.V. All rights reserved. Keywords: Microwave absorbing materials; Inductive activated carbon-fiber felt screen; Vertical-arranged carbon fiber; Block design
1. Introduction In recent years, microwave absorption materials have attracted considerable attention because it is an essential part of a stealthy defense system for all military platforms, whether as aircraft, sea or land vehicles. They are also used to eliminate electromagnetic interference (EMI), which is a specific kind of environmental pollution due to the explosive growth in the utilization of electrical and electronic devices in industrial and commercial applications [1–4]. Some studies showed that the input impedance of the absorbing materials could be increased rapidly by inserting Frequency Selective Surfaces (FSSs) into them [5–6]. If properly used, FSS incorporated with other kinds of microwave absorbing materials can effectively improve the absorption performances of those materials [7–8]. In this work, activated carbon-fiber felt is used to replace the traditional material of FSS, which is
∗
Corresponding author. Tel.: +86 22 27407527; fax: +86 22 27405874. E-mail address: [email protected] (N. Zhao).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.10.026
metal. The alteration of the material is based on the following considerations: (i) in the GHz range, electrical properties of activated carbon-fiber felt are similar to that of metal; (ii) activated carbon-fiber felt has the structure of fabrics and the fibers composed of it has the irregular-shaped cross-sections, which are very advantageous for the absorption of electromagnetic wave [9]; (iii) activated carbon-fiber felt is lighter than metal. The carbon fiber (in continuous or discontinuous forms), which is one of the absorbents, has been widely employed in microwave absorbing materials in reducing backscattering from objects or radar targets, EMI suppressors and paints [10–13]. However, to the best of our knowledge, there are no reported experimental results on the absorbing effect of the carbon fibers in the vertical-arranged manner. When microwave absorbing material is divided into several sub-regions, the electromagnetic parameters of the materials in each sub-region control the amplitude and phase of propagating electromagnetic waves independently. If we adjusted these amplitudes and phases elaborately, the final synthesized vector (the final reflectivity) would be minimized. So the
N. Zhao et al. / Materials Science and Engineering B 127 (2006) 207–211
208
block design is a very effective means to improve the absorbing performance and expand the operation frequency band [14]. In this paper, the absorbing properties of microwave absorbing materials containing inductive activated carbonfiber felt screens (IACFFSs) and vertical-arranged carbon fibers (VACFs) embedded in epoxy resin are studied, respectively. To combine the advantages of the IACFFSs and VACFs, we apply the sub-division technique to design the absorber. The composites consisting of four sub-regions (two IACFFSs and two VACF regions) are prepared, and the effects of these sub-regions on the reflection properties are also investigated. 2. Experimental 2.1. Materials The epoxy resin (E-44, Resin company in Wuxi, PRC) cured with the polyamide resin (203# , Chemical plant in Tianjin, PRC) was adopted as the matrix of the samples. The epoxy resin has the permittivity of 3.0–3.4 and the dielectric loss tangent of 0.01–0.03. The polyamide resin has the permittivity of 3.0–4.0 and the electrical resistivity of 1011 –1012 m. Viscose-based activated carbon-fiber felt (made by Metallography Lab, Tianjin University, PRC) with average thickness of 1.2 mm and specific surface area of 823 m2 g−1 was used as the material of FSS. The carbon fibers used in this work were poly-acrylonitrile (PAN)-based fibers (1 K), and the average diameter of the filaments is ∼9 m. The carbon fiber has the electrical resistivity of ∼0.05 m, the tensile strength of ∼3.73 GPa and the density of ∼1.76 g cm−3 .
Fig. 2. Geometry of the IACFFS.
2.3. Measurements of microwave absorption The absorption characteristics of the samples were measured over the frequency range of 8–18 GHz in a calibrated Naval Research Laboratory (NRL) arch using a HP8757E network analyzer [16]. The sample under test was positioned on an aluminium panel (180 mm × 180 mm). 3. Results and discussion
2.2. Preparation of composites The epoxy resin was blended uniformly with the polyamide resin in the mass ratio of 2:1. After vacuum deformation the mixture was cast into a conventional semi-overflowing die. The IACFFSs were put in the middle of the composites. The carbon fibers embedded in the composite were perpendicular to the surface of the sample and arrayed in the equidistance. When we prepared the absorber based on the block design, each subregion is fabricated independently. A piece of fiberglass cloth with the same size as the die was laid on the top of the sample to make the impedance of the samples well matched with the impedance of air [15]. Molding was carried out in a hydraulic press at 10 MPa pressure and 80 ◦ C for 2 h, obtaining specimens of 180 mm × 180 mm with thickness of 4 mm for reflectivity measurements. The configuration of the sample is shown in Fig. 1.
3.1. Absorption properties of the composites containing IACFFSs The IACFFS made of periodic grids is shown in Fig. 2, a being the interval between carbon-fiber felt strips and b being the width of the strips. The parameters of IACFFSs embedded in the samples are listed in Table 1. Fig. 3 shows the frequency dependence of the reflection loss (R.L.) of the composites containing IACFFSs, where a/b is taken as a parameter, b being constant. It can be seen that the values of a/b influence the R.L. of the composites greatly. With the reduction of a/b, the minimum R.L. decreases except for a/b = 1.4, and Table 1 Element configurations of the IACFFSs embedded in the samples Serial numbers
b (mm)
a/b
1#
5 5 5 5 7 3
4 3 2 1.4 3 3
2# 3# 4# 5# 6# Fig. 1. Vertical section of the sample.
N. Zhao et al. / Materials Science and Engineering B 127 (2006) 207–211
Fig. 3. Effect of a/b on microwave absorbing properties of the composites containing IACFFSs.
the bandwidth below −10 dB increases. For all the samples, the maximal absorption locates in the frequency range of 9–15 GHz. Also, it is observed that when a/b = 2, the R.L. of the composite is below −10 dB in the range of 8.4–18 GHz, and the minimum value is −30.6 dB at 12.4 GHz. While a/b = 1.4, the absorption curve illustrates the bandwidth less than −10 dB is over 10 GHz and it covers the frequency range of the radar wave. Fig. 4 shows the absorption curves of the samples containing IACFFSs, where b is taken as a parameter, a/b being constant. It can be seen that with b reduced, the absorption performances of the composites are improved. The maximal absorption locates in the frequency range of 11–14 GHz. Sample 6# presents better absorption characteristics as compared with others, the R.L. of which below −10 dB is obtained in the frequency range of 8.9–17.3 GHz, and the minimum value reaches −28.3 dB. The cross-sections of the carbon fibers in the felt are irregularshaped and their surface grooves are quite obvious as shown in Fig. 5. The activated carbon-fiber felt composed of these fibers has the structure of three-dimensional fabrics. The structure is like that of microwave anechoic chamber, where there are many small pyramids [9]. When incident wave arrives at the activated carbon-fiber felt, it is multi-reflected and gradually attenuated.
209
Fig. 4. Effect of b on microwave absorbing properties of the composites containing IACFFSs.
According to the electromagnetic field theory, one element in the inductive IACFFS can be considered as a rectangular waveguide. In this case, the material of the waveguide wall is the activated carbon-fiber felt, and the medium within the waveguide is the resin. Because the walls are not perfect conductors, some attenuation of electromagnetic fields occurs as they propagate in the waveguide. By using the expressions of the fields for the TE10 mode, the average power loss per unit length is expressed as [17] Pl =
Rs H02 [λ2 (l + 2w) + 4l3 (1 − λ2 /4w2 )] 2λ2
(1)
where Rs is the surface resistance, H0 the constant and determined by the wave source, λ the wave length, l and w are the length and width of the rectangular waveguide, respectively. Eq. (1) indicates that the average power loss rises with the reduction of λ. Moreover, if the frequency of the incident electromagnetic wave f is less than the cutoff frequency of the TM10 mode fc10 , no wave propagation will take place in the waveguide [17]. Therefore, the composites containing IACFFSs show bad microwave absorption properties at low frequency.
Fig. 5. SEM photos of viscose-based activated carbon-fiber felt.
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N. Zhao et al. / Materials Science and Engineering B 127 (2006) 207–211
Fig. 6. Construction of the composites containing VACFs.
The rectangular waveguides in the IACFFS are coherent wave sources, and the incident field is partly attenuated by wave interference. When b (a/b) is constant, with the reduction of a/b (b) values, the number of waveguides (coherent wave sources) in one screen is increased, and so the absorbing properties of the composites containing IACFFSs are improved. 3.2. Absorption properties of the composites containing VACFs Fig. 6 shows the construction of the composite containing VACFs. The carbon fibers with the length of 4 mm were embedded in the epoxy resin matrix and perpendicular to the surface of the sample. The microwave absorption characteristics of the samples with different fiber spaces (d = 10 mm, 8 mm, 6 mm and 4 mm) are shown in Fig. 7. As can be seen from Fig. 7, the absorption curves of samples 8# , 9# and 10# have a similar trend. In the frequency range of 9–14 GHz, the R.L. of three samples fluctuates around −10 dB. When the frequency is higher than 14 GHz, the R.L. is sharply going down and the minimum values appear at the frequency about 16.3 GHz. In 17.5–18 GHz, the R.L. is below −10 dB and approximately the constant, which means the R.L. may be still lower than −10 dB in the nearby frequency region more than 18 GHz. When the space between the fibers is 4 mm, the R.L. below −10 dB is obtained in the frequency range of 11.8–18 GHz and the minimum value reaches −22 dB. For the carbon fibers have low electrical resistivity and are connected with the metal backing, the sample can be considered
Fig. 7. Effect of d on the microwave absorbing properties of the composites containing VACFs.
as a monopole vibrator antenna array. The antenna array captures the specific frequency region (which is called the operating band of the antenna) of electromagnetic wave and converts it into high-frequency currents in the carbon fibers. Therefore, the electromagnetic energy is partly transformed into heat energy. This may be the reason that the hollows occur in the absorption curves of samples 7# –10# . In addition, because the diameter of the carbon fiber is less than the wavelength of the incident wave, the electromagnetic wave would be scattered and partly attenuated when it arrives at the fibers [17]. 3.3. Absorption properties of the composites based on the block design method In order to combine the merits of two kinds of composites discussed above and obtain the materials with better absorption properties we apply the sub-division technique to fabricate the absorber. Fig. 8 shows the configuration of the composite based on the block design method. The sample was divided into four sub-regions which are centrosymmetric and consists of two IACFFSs and two VACF regions. Fig. 9 shows the absorbing curves of the samples 11# and 12# . The IACFFSs embedded in them are identical (the interval between strips a = 10 mm, the width of the strips b = 5 mm), and the fiber space is different (d = 8 mm and 6 mm, respectively). From Fig. 9, it can be found that compared with the composites only containing IACFFSs or VACFs, the absorption curves of the composites based on the block design method have three main characteristics. They are listed as follows: (i) At high frequencies, the absorption performance of the composites only containing the IACFFSs or the VACFs
Fig. 8. Construction of the composite based on the block design method.
N. Zhao et al. / Materials Science and Engineering B 127 (2006) 207–211
Fig. 9. Microwave absorption properties of the composites based on the block design method.
falls or has the trend to fall. However, the R.L. of the composite based on the block design method is still less than −15 dB at high frequencies, which suggests that the R.L. is lower than −10 dB in the nearby frequency region more than 18 GHz. (ii) The absorption curves of samples 11# and 12# fluctuate at high frequencies. The reason is that the absorbing effect of the composites includes two parts. One is the IACFFSs, the other is the VACFs. When they absorb the incident wave simultaneously, the R.L. should show a state of fluctuation. (iii) For sample 11# , the R.L. below −20 dB is obtained in the frequency range of 11.8–18 GHz, and the minimum value is –30 dB at 16.8 GHz. The flat-plate microwave absorbing material is divided into several sub-regions (assuming two sub-regions), and the R.L. of the two sub-regions is given by R1 = R1x + jR1y ;
R2 = R2x + jR2y
(2)
where R1 and R2 are the R.L. of sub-region I and sub-region II, respectively. Rx and Ry are the real part and imaginary part, respectively. The total R.L. of the absorbing material is given by [14] R = Rx + jRy ≈ (S1 R1x + S2 R2x ) + j(S1 R1y + S2 R2y )
(3)
where S1 is the ratio of the area of sub-region I to that of the absorbing material, and S2 is the ratio of the area of sub-region II to that of the absorbing material. From the expression, it can be seen that the total R.L. of the flat-plate absorbing material is the vector sum of the R.L. of all the sub-regions. Therefore, the block design method can improve the performances of the absorbing materials efficiently. 4. Conclusions 1. The composites containing IACFFSs exhibit good absorption performances in the frequency range of 8–18 GHz. It is
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noted that microwave absorption properties of the composites are strongly dependent on the element configurations of IACFFSs. In this study, with proper element configuration, the composite containing the IACFFS presents a reflection loss below −10 dB in the frequency range of 8–18 GHz. 2. The space between the adjacent fibers is critical for the absorbing characteristics of the composites containing VACFs. When this space is 4 mm, the composite achieves a reflection loss below −10 dB in the frequency range of 10.4–18 GHz and the minimum value is −22 dB. 3. By applying the block design method to fabricate the composites, the performances of the absorbing materials are improved effectively. The composite is divided into four subregions, which were centrosymmetrical and included two IACFFSs and two VACF regions. It achieves a reflection loss below −20 dB in the frequency range of 11.8–18 GHz and the minimum value reaches −30 dB when the interval between strips, the width of strips and the fiber space are 10 mm, 5 mm and 8 mm, respectively. Acknowledgement The work is supported by Natural Science Foundation of Tianjin of China (No. 013616911). References [1] A.N. Yusoff, M.H. Abdullah, J. Magn. Magn. Mater. 269 (2004) 271–280. [2] J.H. Oh, K.S. Oh, C.G. Kim, C.S. Hong, Composites: Part B 35 (2004) 49–56. [3] I.S. Seo, W.S. Chin, D.G. Lee, Compos. Struct. 66 (2004) 533–542. [4] S.Q. Zhang, C.G. Huang, Z.Y. Zhou, Z. Li, Mater. Sci. Eng. B 90 (2002) 38–41. [5] S.W. Lee, G. Zarrillo, C.L. Law, IEEE Trans. Antennas Prop. AP-30 (1982) 904–909. [6] A. Tennant, B. Chambers, IEEE Microw. Wireless Compon. Lett. 14 (2004) 46–47. [7] Y. Sha, K.A. Jose, C.P. Neo, V.K. Varadan, Microw. Opt. Technol. Lett. 32 (2002) 245–249. [8] S. Chakravarty, R. Mittra, N.R. Williams, IEEE Trans. Antennas Prop. 50 (2002) 284–296. [9] D.L. Zhao, Z.M. Shen, W.D. Chi, New Carbon Mater. 16 (2001) 66–72 (in Chinese). [10] K.L. Chen, Optimization and engineering of microwave absorbers, Ph.D. dissertation, Dep. Eng. Sci. Mech., Pennsylvania State University, State College, 1998. [11] C.P. Neo, V.K. Varadan, IEEE Trans. Electromagn. Compat. 46 (2004) 102–106. [12] C.I. Su, J.Y. Li, C.L. Wang, Textile Res. J. 75 (2005) 154–156. [13] D.D.L. Chung, Carbon 39 (2001) 279–285. [14] X.Y. Wang, J. Qian, R.X. Wu, H.F. Zhu, H.P. Wan, J. Nanjing Univ. (Natural Sci.) 37 (2001) 625–629 (in Chinese). [15] W.K. Guo, Effects of carbon fiber arrangements on microwave absorbing properties of structural composites and mechanics discussions, M.S. dissertation, School of Material Science and Engineering, Tianjin University, 2004 (in Chinese). [16] E.F. Knott, J.F. Schaeffer, M.T. Tuley, Radar Cross Section, Artech House Inc, New Jersey, 1985. [17] Z.S. Quan, Electromagnetic Field Theory, University of Electronic Science and Technology of China Press, Chengdu, 1995 (in Chinese).