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Characteristics of an electromagnetic wave absorbing composite structure with a conducting polymer electromagnetic bandgap (EBG) in the X-band
Composites Science and Technology 68 (2008) 2485–2489
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Characteristics of an electromagnetic wave absorbing composite structure with a conducting polymer electromagnetic bandgap (EBG) in the X-band Won-Jun Lee a, Jong-Woo Lee b,1, Chun-Gon Kim a,* a School of Mechanical, Aerospace and System Engineering, Department of Aerospace Engineering, Korea Advanced Institute of Science and Technology, 335 Gwahangno, Yuseong-gu, Daejeon 305-701, Republic of Korea b DPI Solutions Corporation. Ltd., 6 Sinseong-dong, Yuseong-gu, Daejeon 305-345, Republic of Korea
a r t i c l e
i n f o
Article history: Received 17 December 2007 Received in revised form 17 April 2008 Accepted 2 May 2008 Available online 15 May 2008 Keywords: A. Functional composites A. Glass fibers B. Electrical properties B. Mechanical properties C. Modelling
a b s t r a c t The object of this study is to design radar absorbing structures (RASs) with a conducting polymer (CP) electromagnetic bandgap (EBG) for application in the X-band. Glass/epoxy plain-weave composites with specific stiffness and strength containing an EBG pattern layer were fabricated. An intrinsic conducting polymer paste with poly(ethylenedioxy)thiophene (PEDOT) was fabricated and applied for the EBG pattern printing. The unit cell of the EBG pattern layer was square, and the variables such as thickness, conductivity, size, and arrangement, were determined. The microwave absorbing characteristics of the RASs were verified in the X-band under 10 dB reflection loss (90% absorption), and the influence of the EBG pattern layer in the composite structures was investigated using the interlaminar shear strength (ILSS) test. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Radar cross section (RCS) reduction technology for military aircraft has become essential for viability in contemporary warfare. Stealth technology is categorized into the shape design of aircrafts, radar absorbing materials (RAMs), and radar absorbing structures (RASs). The principle of shape design is to reduce the electromagnetic (EM) waves backscattered in the radar source direction. Shape design technology can be used in a wideband frequency range, but shape inhibits improvements in aerodynamic performance in general. RAMs and RASs have been developed to absorb EM energy and thereby minimize the reflected waves [1,2]. Considering that an F-117 with a straight shape has a larger RCS than an F-22, development of RAMs and RASs is essential for stealth technology in the future. Generally, RAMs are fabricated in sheets consisting of insulating polymer and lossy materials such as ferrites, permalloys, carbon black, carbon nanotubes, and so on. RAMs have the advantage of direct application to the surface of existing structures; however, they increase the total weight of the structure and have poor mechanical properties. Thus, RAMs cannot be used as load bearing structures [3–5].
* Corresponding author. Tel.: +82 42 869 3719; fax: +82 42 869 3710. E-mail address: [email protected] (C.-G. Kim). 1 Tel.: +82 42 865 6914. 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.05.006
A RAS composed of fiber reinforced composites and lossy fillers in the matrix can be used as a load bearing structure with electromagnetic (EM) wave absorbing characteristics [6]. The EM properties of the composites can be tailored by the content of the lossy fillers, and the design of multi-layered RASs with wideband absorbing characteristics is possible. However, the homogeneous dispersion of lossy fillers for repetition and thickness control of the prepreg containing various fillers remains unsolved [7]. The electromagnetic bandgap (EBG) that consists of a simple arrangement of unit patterns with highly conductive materials shows EM wave filter characteristics such as the frequency selective surface (FSS). When the conductivity of the materials for the EBG pattern changes, EM wave absorbing characteristics occur [8,9]. Applying an EBG pattern layer in fiber reinforced composite structures can be effective in designing load bearing RASs with wideband EM wave absorbing characteristics. Conducting polymer (CP) is a promising material for EBG patterns due to its controllable high electric conductivity [10]. The CP paste, which shows a wide range of electric conductivity, can be adopted as the EBG pattern material with advantages such as electric conductivity control, simple and effective surface coating, compatibility with other polymers, and so on. The intrinsic conducting polymer paste designed for EBG pattern printing has been developed and applied in laminated composites. In this study, a glass fiber reinforced composite structure with a CP EBG pattern layer was designed, and the reflection loss of the
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RAS for normal incident EM waves in the X-band (8.2–12.4 GHz) was measured. Based on the square unit cell of the EBG pattern with CP printing on the glass fabric prepreg, the RAS for a wideband frequency range, including the X-band, was designed and fabricated. The practical applicability of this process was confirmed by the excellent agreement between the computational analysis and the measured reflection loss of the RAS. 2. CP EBG pattern Layer 2.1. Conducting polymer paste Intrinsic conducting polymer paste containing PEDOT (poly(ethylenedioxy)thiophene) with a binder was synthesized. The content of PEDOT (Bayer Baytron) was 0.6 wt% and the binder was 2.0 wt%; the viscosity of the paste was 5000 cps for the screen printing. To apply the EM wave shielding and absorption, the specific conductivity of the CP paste was calculated by measuring the surface resistance and coating thickness. The maximum conductivity of the CP paste was 1300 S/m and the conductivity could be controlled from 0 to 1300 S/m as the addition of PEDOT changed from 0 wt% to 0.6 wt%. As the CP paste for the EBG pattern printing was applied to the glass fiber reinforced composite prepreg surface, cure monitoring was conducted. Fig. 1 shows the cure monitoring of the CP paste coated on the surface of the glass fiber reinforced composite. Based on the specific surface area coated by the CP paste, electrodes connected to a computer measured the resistance change of the
coating during the cure condition. The oven temperature was 120 °C, which is the curing temperature of the glass fiber reinforced composite structure. In the early stages, an infinite resistance was recorded, but within the initial 30 min, the resistance was fixed within a specific range and maintained for more than 18 h. Based on these results, the minimum curing time was 30 min and the electric stability of the CP paste coating in the cure temperature was confirmed. The coating thickness was controlled by a bar coater, and the pattern was fabricated using a screen printer with a patterned mask. Considering the wt% of the solid in the CP paste, the thickness of the remaining solid could be calculated. 2.2. Electromagnetic bandgap (EBG) The electromagnetic bandgap that adopts a surface resistance layer is similar in principle to the Salisbury and Jaumann absorber, however EBG absorbers are made from layers that not only contain a resistive component but are reactive as well. This is accomplished by using periodic surfaces made from lossy materials [11]. Fig. 2a shows an equivalent circuit of a single EBG layer and (b) shows the resonance characteristics of EBG layer. The resistive (R) component results from the lossy element material,
lane nd p Grou
L
Power source
Thermal Chamber
C
Circuit CP CP coated coated film film
eet
G sh
B CP E
d A A
R
d
V V
yg
ya+yg
Initial 30 min. Equivalent circuit
50
(a) equivalent circuit of EBG layer
Resistance (kohm)
40
30
20
10
0
0.2
0.4
0.6
0.8
15
16
Time (hour) Fig. 1. Monitoring of CP paste cure condition.
17
18
(b) resonance characteristics of EBG layer[12] Fig. 2. Characteristics of EBG layer.
W.-J. Lee et al. / Composites Science and Technology 68 (2008) 2485–2489
while the inductance (L) is associated with the straight section of the element and the capacitance (C) with the gaps between the elements. At the center frequency (f0), the distance between the ground plane and the EBG layer is approximately k/4, while the RLC-series circuit resonates as a simple Salisbury screen. However, at a lower frequency (fL), the input admittance (Yg) toward the ground plane becomes inductive, while the RLC layer (Ya) becomes capacitive. The total admittance of the EBG absorber is the sum of Yg and Ya. The reactive parts of Yg and Ya cancel each other out to a large degree and lead to a small reflection of EM waves. The same is true for higher frequencies (fH). If the real part of the EBG layer admittance is equal to (Y0), the bandwidth enlarges. Moreover, the EBG absorber can be fabricated using a multi-layered EBG pattern, which can make an ultra-wideband EM wave absorber [12]. 2.3. Characteristics of CP EBG Based on the principle of the EBG absorber, RLC design and matching can generate EM wave absorbing characteristics. As the resistance (R) of the EBG pattern can be controlled using the CP paste conductivity and coating thickness, the inductance (L) and capacitance (C) were set at a specific range and the resistance (R) was controlled. In this study, a simple EBG pattern with a square unit cell is considered based on the grid model [13,14]
"
2 # A 1 A p d 1 ln cscð Y ind ¼ ðjÞðt t Þ þ Þ C 2 k 2A d k t ¼ 1 0:41 A A 1
and ð1Þ
A, C, and k indicate the size of the unit cell, and the wavelength is d = (AC)/2. The complete transmission condition is:
k d ¼ 1 0:41 A A
ð2Þ
After designing the metallic EBG characteristics using a simple square pattern, the effect of the resistance change was observed via computer simulation. Without changing the geometry (L and C), only a change in the resistance (R) could generate the resonance frequency in the specific range and control the transmission and reflection characteristics as shown in Fig. 3.
2.4. CP EBG RAS design For the design of RASs with a CP EBG pattern in the composite structure, the dielectric properties of the composite material should be considered. As the glass epoxy plain-weave composite can be a load bearing composite structure with low permittivity, it is suitable for RASs. The back plate that prevents the transmission of incident EM waves consists of a carbon fiber reinforced composite. The permittivity of the glass epoxy plain-weave composite prepreg (K 618, Hankuk Fiber Co. Ltd.) used in the RAS design was measured. The cured composites were cut precisely to the dimensions of an X-band rectangular waveguide, 22.86 mm 10.16 mm, and were then inserted into the waveguide. The permittivity was calculated using the phase change of transmission coefficient (S21) in the composites [15]. The phase change should be more than 90° in a specimen, because the specimens with low loss are required to be thicker than other specimens. However, the high thickness caused the magnitude of S21 to be reduced to the measurement limit, resulting in a measurement error. Thus, each specimen had a range of thicknesses that enabled precise permittivity calculation. The thickness of the glass epoxy plain-weave composite specimen was approximately 15 mm. The air gap between the waveguide and the specimen affected the signal of S21 resulting in the gap being perfectly sealed with silver paste. Permittivity was calculated using the S-parameter and as a result, the real part of permittivity was approximately 4.2, and the imaginary part was below 0.02. Based on the permittivity of the glass epoxy plainweave composite structure, the CP EBG patterned RAS was designed. From a previous study on a rectangular EBG pattern, the square unit cell was selected. Variables such as square size, distance of unit cell, conductivity of CP paste, location of EBG layer in the composite structure, and so on were controlled to ensure application in the X-band. Computational analysis with a commercial tool (CST-MWS) was performed. The boundary conditions were set to generate a TEM (transverse electromagnetic) mode plane wave, and the back plate was a PEC (perfect electric conductor). A parameter sweep was conducted to produce the RAS for X-band (under 10 dB) with given variables. The requirements were: a. RAS should be a wideband absorber covering the X-band. b. Reflection loss (S11) should be under 10 dB in the X-band. c. Total thickness of the RAS should be less than 4 mm. The variables for the CP EBG pattern were:
5
a. Conductivity of the CP paste should be between 0 and 1300 S/m. b. Coating thickness of the CP EBG pattern should be between 5 and 20 lm. c. Geometry of EBG pattern: the length of the square unit cell. d. Location of the single CP EBG layer in the glass epoxy plainweave composite.
0 -5
Reflection loss (dB)
2487
-10
1 S/m 325 S/m 650 S/m 975 S/m 1300 S/m
-15 -20 -25 -30 -35 -40 -45 0
5
10
15
20
25
Frequency (GHz) Fig. 3. EM wave absorbing characteristics with resistance change.
30
Finally, the size of the unit cell was 10 mm 10 mm, and the CP EBG pattern was 6 mm 6 mm 0.016 mm. The unit cell modeling of the CP EBG RAS is shown in Fig. 4. The thickness of the front layer was 1.5 mm and that of the rear was 2.4 mm; the CP EBG pattern was inserted between these two layers. The back plate was covered with aluminum and the total thickness was below 4 mm. Based on the design and simulation of the RAS, specimens were fabricated. The geometry of a typical specimen was 150 mm 150 mm 3.9 mm, and the surface of the CP EBG layer (inner layer) is shown in Fig. 4.
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CP layer
Average width : 5.0 mm / thickness : 6.8 mm / length : 45 mm 55 50
40
20 15 10 5
(39.3 MPa)
25
(44.1 MPa)
30
Glass fabric with CP layer
35
Glass fabric
Shear strength (MPa)
45
0 Fig. 5. ILSS test setup and result.
ture, the CP EBG printed composite structure can be used as an efficient RAS. 3.2. Measurement of reflection loss Specimens of the glass fiber reinforced composite structure with a CP EBG layer were fabricated, and the reflection loss was measured in the X-band. The free space technique system that measures the reflection loss of the TEM waves is shown in Fig. 6. The system Fig. 4. CP EBG layer inside of the specimen.
Network Analyzer HP 8510C
3. Fabrication of a composite RAS with a CP EBG Layer
GPIB
Personal Computer
3.1. Interlaminar shear strength (ILSS) test As the CP printed layer in the laminated composite can cause delamination and crack initiation, an interlaminar shear strength test (ASTM D 2344) was conducted to verify the effect of the CP printing on the laminated composite structure. The CP paste was printed on the surface of a single ply using a bar coater and the coating thickness was 5 lm. The total thickness of the specimens was determined based on the thickness of a single ply. The thickness per ply was calculated considering the compaction effect: 48 plies were used and the CP printed layer was inserted in the middle of the specimens. Fig. 5 shows the experimental setup and the result of the ILSS test. Three point bending induced cracks and delamination in the CP printed layers of each specimen. The shear strength without the CP layer was 44.1 MPa, but the sample with the CP layer was 39.3 MPa. The effect of the CP layer on the structural composite resulted in a shear strength reduction of approximately 10%, even though the CP was based on the urethane binder and the coating thickness was thin. Considering that the RAS with a glass epoxy plain-weave composite can be used widely as a secondary struc-
System Bus
Synthesized Sweeper S-parameter Test set
Coaxial cable
Port 1
Mode transitions antenna
Micrometer positioner
Port 2
Plexiglas sample holder
Test sample
Aluminum base
Coaxial cable
Mode transitions antenna
Aluminum table
Fig. 6. Free space technique for the measurement of reflection loss.
W.-J. Lee et al. / Composites Science and Technology 68 (2008) 2485–2489
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4. Conclusion This study examined the feasibility of a CP EBG layer for composite RASs. Intrinsic conducting polymer paste was fabricated for the EBG pattern printing on the glass epoxy plain-weave composite layer, and a square EBG pattern was applied for multi-layered composite RASs. Controlling variables such as square size and the distance of the unit cell in the EBG pattern could adjust the inductance (L) and capacitance (C) of the EBG pattern. Moreover, controlling the conductivity of the CP paste can change the resistance (R) of the EBG pattern, and this can be an effective matching method to generate harmonic peaks for the target frequency range. As a result, a wideband RAS including the X-band was designed and fabricated. The designed RAS covers whole the X-band with more than 90% absorption of incidence EM wave. The effect of the CP EBG printing on the glass epoxy plainweave composite structure was considered and an interlaminar shear strength test was conducted. The results show the feasibility of the CP EBG printing on the laminated composite which can be used as a structural component without dramatic degradation of the RAS. A study in progress aims to broaden the absorbing bandwidth of RASs composed of CP EBG layered composite structures with thin thicknesses. Acknowledgements Authors in KAIST appreciate the support of the Defense Acquisition Program Administration and the Agency for Defense Development for this study completed under contract UD070041AD. References Fig. 7. EM wave absorbing characteristics of CP EBG RAS.
Table 1 Comparison between analysis and experiment
Simulation (using designed thickness) Experiment
Thickness (mm)
Reflection loss (8.2 GHz)
Reflection loss (12.4 GHz)
3.9
18.7 dB
10.7 dB
3.86
17.4 dB
9.7 dB
consists of a pair of spot-focusing horn lens antennas, a sample holder, a HP 8510 C network analyzer, and a computer for data acquisition. The size of the aluminum table was 1.83 m 1.83 m and the standard sizes of the sample specimens were 150 mm 150 mm. This system used spot-focusing horn lens antennas to minimize the diffraction effects, and the TRL (through-reflect-line) calibration technique and time-domain gating of the HP 8510 C network analyzer to minimize multiple reflections [16]. The analytical and experimental reflection losses and the resonance frequency of the RASs are shown in Fig. 7 and Table 1. As the surface resistance control of the patterned layer could make dual peak in the target frequency range, the peak control with parameter sweep could adjust the RAS characteristics to the design goal. The results were found to be in good agreement with the predictions, except the slope and dB in a specific frequency range. The reflection loss over 10 dB indicates that the EM energy absorption was 90%. Based on the analytical result in the wideband range from 0 to 30 GHz, the first resonance peak was before the X-band and the second peak was after the X-band range.
[1] Pinho MS, Gregori ML, Nunes RCR, Soares BG. Performance of radar absorbing materials by waveguide measurements for X- and Ku-Band frequencies. Eur Polym J 2002;38:2321–7. [2] Park YJ, Lee CG, A study on ferrite-rubber composite for C–X band electromagnetic wave absorber. In: Proceedings of the 3th conference on national defense materials; 2000, p. C178–85. [3] Sandler J, Shaffer MSP, Prasse T, Bauhofer W, Schulte K, Windle AH. Development of a dispersion process for carbon nanotubes in an epoxy matrix and resulting electrical properties. Polymer 1999;40:5967–71. [4] Chung DDL. Electromagnetic interference shielding effectiveness of carbon materials. Carbon 2001;39:279–85. [5] Kaynak A, Polat A, Yilmazer U. Some microwave and mechanical properties of carbon fiber/polypropylene and carbon black/polypropylene composites. Mater Res Bull 1996;31(10):1195–206. [6] Oh JH. Design of radar absorbing structures using glass/epoxy composite containing carbon black in X-band frequency ranges. Compos Part B Eng 2004;35(1):49–56. [7] Kim M.S., A Study on the Mechanical Properties of MWNT-added Glass/Eopxy Fabric composites. Master thesis, KAIST, 2005. [8] Kern DJ, Werner DH. A genetic algorithm approach to the design of ultra-thin electromagnetic bandgap absorbers. Microwave Opt Technol Lett 2003;38(1): 61–4. [9] Gao Qiang, Yin Yan, Yan Dun-Bao, Yuan Nai-Chang. A novel radar absorbing material based on EBG structure. Microwave Opt Technol Lett 2005;47(3): 228–30. [10] Wong PTC, Chamber B, Anderson AP, Wright PV. Large area conducting polymer composites and their use in microwave absorbing material. Electron Lett 1992;28(17):1651–3. [11] Liang T, Li L, Bossard JA, Werner DH, Mayer TS, Reconfigurable ultra-thin EBG absorbers using conducting polymers. In: Antennas and propagation society international symposium, 2005 IEEE, vol. 2B; 2005, p. 204–7. [12] Munk BA. Frequency selective surfaces: theory and design, A. WileyInterscience Publication; 2000. [13] Lee SE, Oh KS, Kim CG. Electromagnetic characteristics of a frequency selective fabric composites. Electron Lett 2006;42(8):439–41. [14] Wu TK. Frequency selective surface and grid array. Wiley-Interscience; 1995. [15] Lee HS, Measurement of radar cross section of a coated cylinder with two plates, MS thesis (KAIST); 2002. [16] Seo IS, Chin WS, Lee DG. Characterization of electromagnetic properties of polymeric composite materials with free space method. Compos Struct 2004;66:533–42.
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Characteristics of an electromagnetic wave absorbing composite structure with a conducting polymer electromagnetic bandgap (EBG) in the X-band Won-Jun Lee a, Jong-Woo Lee b,1, Chun-Gon Kim a,* a School of Mechanical, Aerospace and System Engineering, Department of Aerospace Engineering, Korea Advanced Institute of Science and Technology, 335 Gwahangno, Yuseong-gu, Daejeon 305-701, Republic of Korea b DPI Solutions Corporation. Ltd., 6 Sinseong-dong, Yuseong-gu, Daejeon 305-345, Republic of Korea
a r t i c l e
i n f o
Article history: Received 17 December 2007 Received in revised form 17 April 2008 Accepted 2 May 2008 Available online 15 May 2008 Keywords: A. Functional composites A. Glass fibers B. Electrical properties B. Mechanical properties C. Modelling
a b s t r a c t The object of this study is to design radar absorbing structures (RASs) with a conducting polymer (CP) electromagnetic bandgap (EBG) for application in the X-band. Glass/epoxy plain-weave composites with specific stiffness and strength containing an EBG pattern layer were fabricated. An intrinsic conducting polymer paste with poly(ethylenedioxy)thiophene (PEDOT) was fabricated and applied for the EBG pattern printing. The unit cell of the EBG pattern layer was square, and the variables such as thickness, conductivity, size, and arrangement, were determined. The microwave absorbing characteristics of the RASs were verified in the X-band under 10 dB reflection loss (90% absorption), and the influence of the EBG pattern layer in the composite structures was investigated using the interlaminar shear strength (ILSS) test. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Radar cross section (RCS) reduction technology for military aircraft has become essential for viability in contemporary warfare. Stealth technology is categorized into the shape design of aircrafts, radar absorbing materials (RAMs), and radar absorbing structures (RASs). The principle of shape design is to reduce the electromagnetic (EM) waves backscattered in the radar source direction. Shape design technology can be used in a wideband frequency range, but shape inhibits improvements in aerodynamic performance in general. RAMs and RASs have been developed to absorb EM energy and thereby minimize the reflected waves [1,2]. Considering that an F-117 with a straight shape has a larger RCS than an F-22, development of RAMs and RASs is essential for stealth technology in the future. Generally, RAMs are fabricated in sheets consisting of insulating polymer and lossy materials such as ferrites, permalloys, carbon black, carbon nanotubes, and so on. RAMs have the advantage of direct application to the surface of existing structures; however, they increase the total weight of the structure and have poor mechanical properties. Thus, RAMs cannot be used as load bearing structures [3–5].
* Corresponding author. Tel.: +82 42 869 3719; fax: +82 42 869 3710. E-mail address: [email protected] (C.-G. Kim). 1 Tel.: +82 42 865 6914. 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.05.006
A RAS composed of fiber reinforced composites and lossy fillers in the matrix can be used as a load bearing structure with electromagnetic (EM) wave absorbing characteristics [6]. The EM properties of the composites can be tailored by the content of the lossy fillers, and the design of multi-layered RASs with wideband absorbing characteristics is possible. However, the homogeneous dispersion of lossy fillers for repetition and thickness control of the prepreg containing various fillers remains unsolved [7]. The electromagnetic bandgap (EBG) that consists of a simple arrangement of unit patterns with highly conductive materials shows EM wave filter characteristics such as the frequency selective surface (FSS). When the conductivity of the materials for the EBG pattern changes, EM wave absorbing characteristics occur [8,9]. Applying an EBG pattern layer in fiber reinforced composite structures can be effective in designing load bearing RASs with wideband EM wave absorbing characteristics. Conducting polymer (CP) is a promising material for EBG patterns due to its controllable high electric conductivity [10]. The CP paste, which shows a wide range of electric conductivity, can be adopted as the EBG pattern material with advantages such as electric conductivity control, simple and effective surface coating, compatibility with other polymers, and so on. The intrinsic conducting polymer paste designed for EBG pattern printing has been developed and applied in laminated composites. In this study, a glass fiber reinforced composite structure with a CP EBG pattern layer was designed, and the reflection loss of the
2486
W.-J. Lee et al. / Composites Science and Technology 68 (2008) 2485–2489
RAS for normal incident EM waves in the X-band (8.2–12.4 GHz) was measured. Based on the square unit cell of the EBG pattern with CP printing on the glass fabric prepreg, the RAS for a wideband frequency range, including the X-band, was designed and fabricated. The practical applicability of this process was confirmed by the excellent agreement between the computational analysis and the measured reflection loss of the RAS. 2. CP EBG pattern Layer 2.1. Conducting polymer paste Intrinsic conducting polymer paste containing PEDOT (poly(ethylenedioxy)thiophene) with a binder was synthesized. The content of PEDOT (Bayer Baytron) was 0.6 wt% and the binder was 2.0 wt%; the viscosity of the paste was 5000 cps for the screen printing. To apply the EM wave shielding and absorption, the specific conductivity of the CP paste was calculated by measuring the surface resistance and coating thickness. The maximum conductivity of the CP paste was 1300 S/m and the conductivity could be controlled from 0 to 1300 S/m as the addition of PEDOT changed from 0 wt% to 0.6 wt%. As the CP paste for the EBG pattern printing was applied to the glass fiber reinforced composite prepreg surface, cure monitoring was conducted. Fig. 1 shows the cure monitoring of the CP paste coated on the surface of the glass fiber reinforced composite. Based on the specific surface area coated by the CP paste, electrodes connected to a computer measured the resistance change of the
coating during the cure condition. The oven temperature was 120 °C, which is the curing temperature of the glass fiber reinforced composite structure. In the early stages, an infinite resistance was recorded, but within the initial 30 min, the resistance was fixed within a specific range and maintained for more than 18 h. Based on these results, the minimum curing time was 30 min and the electric stability of the CP paste coating in the cure temperature was confirmed. The coating thickness was controlled by a bar coater, and the pattern was fabricated using a screen printer with a patterned mask. Considering the wt% of the solid in the CP paste, the thickness of the remaining solid could be calculated. 2.2. Electromagnetic bandgap (EBG) The electromagnetic bandgap that adopts a surface resistance layer is similar in principle to the Salisbury and Jaumann absorber, however EBG absorbers are made from layers that not only contain a resistive component but are reactive as well. This is accomplished by using periodic surfaces made from lossy materials [11]. Fig. 2a shows an equivalent circuit of a single EBG layer and (b) shows the resonance characteristics of EBG layer. The resistive (R) component results from the lossy element material,
lane nd p Grou
L
Power source
Thermal Chamber
C
Circuit CP CP coated coated film film
eet
G sh
B CP E
d A A
R
d
V V
yg
ya+yg
Initial 30 min. Equivalent circuit
50
(a) equivalent circuit of EBG layer
Resistance (kohm)
40
30
20
10
0
0.2
0.4
0.6
0.8
15
16
Time (hour) Fig. 1. Monitoring of CP paste cure condition.
17
18
(b) resonance characteristics of EBG layer[12] Fig. 2. Characteristics of EBG layer.
W.-J. Lee et al. / Composites Science and Technology 68 (2008) 2485–2489
while the inductance (L) is associated with the straight section of the element and the capacitance (C) with the gaps between the elements. At the center frequency (f0), the distance between the ground plane and the EBG layer is approximately k/4, while the RLC-series circuit resonates as a simple Salisbury screen. However, at a lower frequency (fL), the input admittance (Yg) toward the ground plane becomes inductive, while the RLC layer (Ya) becomes capacitive. The total admittance of the EBG absorber is the sum of Yg and Ya. The reactive parts of Yg and Ya cancel each other out to a large degree and lead to a small reflection of EM waves. The same is true for higher frequencies (fH). If the real part of the EBG layer admittance is equal to (Y0), the bandwidth enlarges. Moreover, the EBG absorber can be fabricated using a multi-layered EBG pattern, which can make an ultra-wideband EM wave absorber [12]. 2.3. Characteristics of CP EBG Based on the principle of the EBG absorber, RLC design and matching can generate EM wave absorbing characteristics. As the resistance (R) of the EBG pattern can be controlled using the CP paste conductivity and coating thickness, the inductance (L) and capacitance (C) were set at a specific range and the resistance (R) was controlled. In this study, a simple EBG pattern with a square unit cell is considered based on the grid model [13,14]
"
2 # A 1 A p d 1 ln cscð Y ind ¼ ðjÞðt t Þ þ Þ C 2 k 2A d k t ¼ 1 0:41 A A 1
and ð1Þ
A, C, and k indicate the size of the unit cell, and the wavelength is d = (AC)/2. The complete transmission condition is:
k d ¼ 1 0:41 A A
ð2Þ
After designing the metallic EBG characteristics using a simple square pattern, the effect of the resistance change was observed via computer simulation. Without changing the geometry (L and C), only a change in the resistance (R) could generate the resonance frequency in the specific range and control the transmission and reflection characteristics as shown in Fig. 3.
2.4. CP EBG RAS design For the design of RASs with a CP EBG pattern in the composite structure, the dielectric properties of the composite material should be considered. As the glass epoxy plain-weave composite can be a load bearing composite structure with low permittivity, it is suitable for RASs. The back plate that prevents the transmission of incident EM waves consists of a carbon fiber reinforced composite. The permittivity of the glass epoxy plain-weave composite prepreg (K 618, Hankuk Fiber Co. Ltd.) used in the RAS design was measured. The cured composites were cut precisely to the dimensions of an X-band rectangular waveguide, 22.86 mm 10.16 mm, and were then inserted into the waveguide. The permittivity was calculated using the phase change of transmission coefficient (S21) in the composites [15]. The phase change should be more than 90° in a specimen, because the specimens with low loss are required to be thicker than other specimens. However, the high thickness caused the magnitude of S21 to be reduced to the measurement limit, resulting in a measurement error. Thus, each specimen had a range of thicknesses that enabled precise permittivity calculation. The thickness of the glass epoxy plain-weave composite specimen was approximately 15 mm. The air gap between the waveguide and the specimen affected the signal of S21 resulting in the gap being perfectly sealed with silver paste. Permittivity was calculated using the S-parameter and as a result, the real part of permittivity was approximately 4.2, and the imaginary part was below 0.02. Based on the permittivity of the glass epoxy plainweave composite structure, the CP EBG patterned RAS was designed. From a previous study on a rectangular EBG pattern, the square unit cell was selected. Variables such as square size, distance of unit cell, conductivity of CP paste, location of EBG layer in the composite structure, and so on were controlled to ensure application in the X-band. Computational analysis with a commercial tool (CST-MWS) was performed. The boundary conditions were set to generate a TEM (transverse electromagnetic) mode plane wave, and the back plate was a PEC (perfect electric conductor). A parameter sweep was conducted to produce the RAS for X-band (under 10 dB) with given variables. The requirements were: a. RAS should be a wideband absorber covering the X-band. b. Reflection loss (S11) should be under 10 dB in the X-band. c. Total thickness of the RAS should be less than 4 mm. The variables for the CP EBG pattern were:
5
a. Conductivity of the CP paste should be between 0 and 1300 S/m. b. Coating thickness of the CP EBG pattern should be between 5 and 20 lm. c. Geometry of EBG pattern: the length of the square unit cell. d. Location of the single CP EBG layer in the glass epoxy plainweave composite.
0 -5
Reflection loss (dB)
2487
-10
1 S/m 325 S/m 650 S/m 975 S/m 1300 S/m
-15 -20 -25 -30 -35 -40 -45 0
5
10
15
20
25
Frequency (GHz) Fig. 3. EM wave absorbing characteristics with resistance change.
30
Finally, the size of the unit cell was 10 mm 10 mm, and the CP EBG pattern was 6 mm 6 mm 0.016 mm. The unit cell modeling of the CP EBG RAS is shown in Fig. 4. The thickness of the front layer was 1.5 mm and that of the rear was 2.4 mm; the CP EBG pattern was inserted between these two layers. The back plate was covered with aluminum and the total thickness was below 4 mm. Based on the design and simulation of the RAS, specimens were fabricated. The geometry of a typical specimen was 150 mm 150 mm 3.9 mm, and the surface of the CP EBG layer (inner layer) is shown in Fig. 4.
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CP layer
Average width : 5.0 mm / thickness : 6.8 mm / length : 45 mm 55 50
40
20 15 10 5
(39.3 MPa)
25
(44.1 MPa)
30
Glass fabric with CP layer
35
Glass fabric
Shear strength (MPa)
45
0 Fig. 5. ILSS test setup and result.
ture, the CP EBG printed composite structure can be used as an efficient RAS. 3.2. Measurement of reflection loss Specimens of the glass fiber reinforced composite structure with a CP EBG layer were fabricated, and the reflection loss was measured in the X-band. The free space technique system that measures the reflection loss of the TEM waves is shown in Fig. 6. The system Fig. 4. CP EBG layer inside of the specimen.
Network Analyzer HP 8510C
3. Fabrication of a composite RAS with a CP EBG Layer
GPIB
Personal Computer
3.1. Interlaminar shear strength (ILSS) test As the CP printed layer in the laminated composite can cause delamination and crack initiation, an interlaminar shear strength test (ASTM D 2344) was conducted to verify the effect of the CP printing on the laminated composite structure. The CP paste was printed on the surface of a single ply using a bar coater and the coating thickness was 5 lm. The total thickness of the specimens was determined based on the thickness of a single ply. The thickness per ply was calculated considering the compaction effect: 48 plies were used and the CP printed layer was inserted in the middle of the specimens. Fig. 5 shows the experimental setup and the result of the ILSS test. Three point bending induced cracks and delamination in the CP printed layers of each specimen. The shear strength without the CP layer was 44.1 MPa, but the sample with the CP layer was 39.3 MPa. The effect of the CP layer on the structural composite resulted in a shear strength reduction of approximately 10%, even though the CP was based on the urethane binder and the coating thickness was thin. Considering that the RAS with a glass epoxy plain-weave composite can be used widely as a secondary struc-
System Bus
Synthesized Sweeper S-parameter Test set
Coaxial cable
Port 1
Mode transitions antenna
Micrometer positioner
Port 2
Plexiglas sample holder
Test sample
Aluminum base
Coaxial cable
Mode transitions antenna
Aluminum table
Fig. 6. Free space technique for the measurement of reflection loss.
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4. Conclusion This study examined the feasibility of a CP EBG layer for composite RASs. Intrinsic conducting polymer paste was fabricated for the EBG pattern printing on the glass epoxy plain-weave composite layer, and a square EBG pattern was applied for multi-layered composite RASs. Controlling variables such as square size and the distance of the unit cell in the EBG pattern could adjust the inductance (L) and capacitance (C) of the EBG pattern. Moreover, controlling the conductivity of the CP paste can change the resistance (R) of the EBG pattern, and this can be an effective matching method to generate harmonic peaks for the target frequency range. As a result, a wideband RAS including the X-band was designed and fabricated. The designed RAS covers whole the X-band with more than 90% absorption of incidence EM wave. The effect of the CP EBG printing on the glass epoxy plainweave composite structure was considered and an interlaminar shear strength test was conducted. The results show the feasibility of the CP EBG printing on the laminated composite which can be used as a structural component without dramatic degradation of the RAS. A study in progress aims to broaden the absorbing bandwidth of RASs composed of CP EBG layered composite structures with thin thicknesses. Acknowledgements Authors in KAIST appreciate the support of the Defense Acquisition Program Administration and the Agency for Defense Development for this study completed under contract UD070041AD. References Fig. 7. EM wave absorbing characteristics of CP EBG RAS.
Table 1 Comparison between analysis and experiment
Simulation (using designed thickness) Experiment
Thickness (mm)
Reflection loss (8.2 GHz)
Reflection loss (12.4 GHz)
3.9
18.7 dB
10.7 dB
3.86
17.4 dB
9.7 dB
consists of a pair of spot-focusing horn lens antennas, a sample holder, a HP 8510 C network analyzer, and a computer for data acquisition. The size of the aluminum table was 1.83 m 1.83 m and the standard sizes of the sample specimens were 150 mm 150 mm. This system used spot-focusing horn lens antennas to minimize the diffraction effects, and the TRL (through-reflect-line) calibration technique and time-domain gating of the HP 8510 C network analyzer to minimize multiple reflections [16]. The analytical and experimental reflection losses and the resonance frequency of the RASs are shown in Fig. 7 and Table 1. As the surface resistance control of the patterned layer could make dual peak in the target frequency range, the peak control with parameter sweep could adjust the RAS characteristics to the design goal. The results were found to be in good agreement with the predictions, except the slope and dB in a specific frequency range. The reflection loss over 10 dB indicates that the EM energy absorption was 90%. Based on the analytical result in the wideband range from 0 to 30 GHz, the first resonance peak was before the X-band and the second peak was after the X-band range.
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