Radar absorbing materials, conducting polymers

Radar absorbing materials 2.2,1 Phenomenology of interaction 370 4.5 Influence of structural parameters of conducting polymers on their ... 

Nevertheless, several general papers have been published on stealth technology applied to aircraft such as LOCKHEED F 117 [5,6] or others [7,8,9]. The description of radar absorbing materials is restricted to iron or ferrite loaded paints or elastomers except the case of retinyl Schiff base salts [10,11]. This molecule attracted interest of many scientists in 1987. In fact this material which is a charge transfer salt derived from the A vitamin, never exhibited high-frequency dielectric constant due to its intrinsic conductivity, despite its beautiful orange colour Moreover, reference books as the one of Knott el al. [12] do not mention the use of conductive polymers for this purpose. 

As evidenced by these examples, conducting polymers stand out as radar absorbing materials in that they offer engineers unlimited design possibilities. 

We have intended to show in this chapter, how conductive polymers could provide a significant improvement in designing Radar Absorbing Materials. In fact, it is mainly the wide diversity of synthesis paths which is responsible for the variety of potential applications. 

These conducting polymers have some specific characteristics that make them far more interesting than traditional dielectric materials. Beyond their reproducible properties, the chemistry of conducting polymers offers a great variety of methods of synthesis. The insertion of conductivity into various materials (insulating polymer matrix, reinforcing fabrics, honeycomb structure) is now possible and leads to complex structures. They absorb radar waves and can match new environmental constraints (mechanical properties for example). 

Chambers and coworkers [72-74] also explored the properties of semiconductive sheets of polypyrrole treated paper and cloth, which they characterize as a parallel RC circuit. They too report that the capacitive part of the complex impedance of the sheets depends on the morphology of the poly pyrrole coating. Measured reflectivity plots for Salisbury screen (narrowband) and Jaumann (broadband) absorbers fabricated from sheets of polypyrrole-treated material are also presented in their article. The measured results agree well with their model calculations, indicating the potential utility of conductive polymer-treated fabrics in radar-absorbing structures. 

The thermal properties of fibre-reinforced composites are anisotropic. Expansion in the direction of the fibre is usually very small or negative, but the thermal conductivity of some carbon fibre composites in the fibre direction can be high. Most polymers and several types of fibre are good electrical insulators. Carbon and some ceramic fibres have a much lower resistivity. It is thus possible to use composites for manufacturing radomes as well as for electromagnetic screening materials and systems which absorb radar signals. 

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