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COMPOSITE MATERIALS AS BROAD BAND MICROWAVE ABSORBERS

S.Tarkuc, F. Özyurt, A. Yıldırım, L. Toppare Department of Chemistry, Middle East Technical University

Issue 7 - January 2008

Developments in high frequency electronic systems gave rise to growth of telecommunication devices such as cellular phone, wireless communication network and robot systems.
Electromagnetic waves emitted by these systems and devices may put other devices out of action and also create a danger for human beings.
These problems have been overcome by the use of electromagnetic wave absorbing materials. The importance of these materials accrued with their
ability to decrease the detectability of the military target in defense systems. The etectability of a target is measured in terms of the radar cross section(RSC). The radar cross section can also be reduced by shaping, active loading and passive loading except for distributed loading. Although shaping is very
important, its application on present military equipments is difficult.Distributed loading involves covering the surface with a radar absorbing material (RAM).
Stealth technology is a required specification
for modern weapons due to recent developments in electronic warfare technology. This technology covers covers a range of techniques used for aircrafts, ships and missiles in order to
make them less visible to radar, infrared and other detection methods. RAMs are an essential part of a stealthy defence system for all military platforms, whether aircraft, sea or land
vehicles. To this end, RAMs not only have to exhibit a low reflection coefficient over a wide frequency range, but also have to be thin and light weight, especially for aircraft applications. Moreover, for simplicity and ease of implementation, RAMs can be employed as either a single or multilayered structure.
Research on electromagnetic wave absorbers started in the 1930’s with the use of carbon black as a lossy resistive material and titanium dioxide for its high permittivity to reduce the
thickness. The historical development of RAMs went on with Dallenbach layer, Salisbury screen and Jaumann absorber. Today, the absorber design
process has been improved by optimization of these systems with desired magnetic and dielectric properties of the new materials. Ideally the designer wants to have control over permeability permittivity and loss constants, but
the main problem for the design of an absorber lays in the choice of the material. The importance of ferrite has been known. Ferrites are a very important class of magnetic materials and have been considered as highly valuable electronic materials for more than half a century. During the last two decades, there has been great interest in their application as radar absorbing materials
(RAMs) whose properties are influenced by the relative complex permeability and permittivity. The hexagonal type ferrites are suitable for RAM
due to large value of high value of magnetization
and planar anisotropic behavior, as well as good dielectric properties at microwave frequencies.
Up to now, the most commonly used
absorbers are magnetic materials (such as ferrite particles) and dielectric materials (such as carbon black particles). However,the expansion of applications was limited due to their thickness and weight. Conducting polymers
appear as potential radar absorbing materials to overcome these problems. Several materials produced using conducting polymers such as polypyrrole, polyaniline and their composites,
showed a very narrow absorption with less than –10 dB reflectivity. A hybrid dielectric/magnetic material was also made using carbonyl iron or ferrite however, extra bandwidth was not
realized for these materials.
The presence of a broad band absorber with low weight and thickness is rare in the literature. Also chemicals used to develop these compounds are supplied by companies and other compounds
are synthesized with expensive methods. Hence, cost of the developed absorber has increased. Weight, thickness, synthetic pathway and cost
should be taken into consideration. To close this gap and also improve national
war technology, we begun our studies in May, 2006. Both air and seaplatforms
absorbers were successfully developed in a year.
When the application platform and its dynamic are taken into account, absorber design should be developed separately for each platform (sea, air and land). Both thickness and weight are the parameters that create limitation of use of some absorbers for airplatforms. Application of absorber on airplane and missile has been enhanced by the development of ferrite-free absorbers. Although ferrite is a good magnetic
absorber, its weight inhibits its use as a magnetic absorber. Formulated ferrite free absorber is a multi-layer system with 3 mm thickness. One of the layers contains a conducting polymer. Uses of the conducting polymer enhance the application of this absorber in real life due to adjustable conductivity and easy synthesis of the polymer. The formulated ferrite-free absorber showed a broad absorption with higher than -15 dB average reflectivity at 8-14 GHz. Specifications of this material comply with international standards; MIL-A-17161D and ASTM-D5568. The development of stealthy sea platforms is easier than that of the airplanes.
Instead of conductive polymers, conducting particles are used to develop the battleship absorber. The known absorbing property of ferrite is used to design battleship absorber
since no weight problem exists for sea platforms. M-type ferrite (homemade) is used as a magnetic particle for this absorber. Unlike synthetic pathways in the literature, this synthesis is achieved without the use of organic solvents. Hence, both environmental pollution and risks in human healthy are eliminated. This absorber achieved a reduction in the reflectivity
of –20 dB over 8-18 GHz. Our strategies to design absorbers based on that magnetic and dielectric material should be used in separate
layers. Moreover, the materials may undergo high thermomechanical stress and should thus be able to work at temperatures above 600± 0C. Hence,
both magnetic and dielectric materials are embedded in appropriate insulating matrix to gain enough resistance against environmental conditions. Insulating matrix acts as a protective layer against oxygen uptake and moisture diffusion into the layers. These
absorbers not only resistive to oxygen and moisture, but also they have excellent durability against flame and friction. The advantages gained by reduction of radar cross section with radar absorbing materials, were increased using domestic chemicals (magnetic and
conducting materials) during the
development of these materials. Use of sources available in the nation is the fundamental concept of our research carried out at the Department of Chemistry, METU.