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 article info 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 abstract 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 pat- tern printing. The unit cell of the EBG pattern layer was square, and the variables such as thickness, con- ductivity, 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 air- craft 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 electromag- netic (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 perfor- mance in general. RAMs and RASs have been developed to absorb EM energy and thereby minimize the reflected waves [1,2]. Consid- ering 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 tech- nology 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]. A RAS composed of fiber reinforced composites and lossy fill- ers 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 thick- ness control of the prepreg containing various fillers remains un- solved [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 selec- tive 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 pat- terns 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 con- ducting 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 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.05.006 * Corresponding author. Tel.: +82 42 869 3719; fax: +82 42 869 3710. E-mail address: cgkim@kaist.ac.kr (C.-G. Kim). 1 Tel.: +82 42 865 6914. Composites Science and Technology 68 (2008) 2485–2489 Contents lists available at ScienceDirect Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech