Radioelectric properties

Nevertheless, despite the great number of conductive fillers, the radioelectric properties of all these composites are generally similar. Differences are due to the... 

Figure 8.7. General principles of a device for characterisation of the radioelectric properties of materials (SUT Sample Under Test).

The radioelectrical property values are. then calculated using (8.10). 

This model allows us to describe the evolution of the polyaniline radioelectrical properties for different doping levels. What is interesting in this approach is that physical parameters are taken into account. Moreover, Structural characteristics, such as coherence length, can be recovered. Finally, the calculations we carried out confirm that charge carriers are localised over some benzene rings, the three-dimensional aspect of the transport phenomenon occurs after only about 10 periods. 

The blend produced by in situ growth presents a low percolation threshold and therefore the same rheologic properties as the matrix. In addition, this technique Creates blends with a very high reproducibility of radioelectric properties due to the growing process by itself... 

Interesting radioelectric properties have been discovered with this class of materials either from the point of view of level of radioelectric properties (typical values can attain 15-30 at 4 GHz) or under the form of absorption properties, since these films can be assembled by co-vulcanisation (especially in the case of polychoropene based films). In that case, they lead to flexible absorbing sheets which present levels of attenuation of [10] dB between 5 and 18 GHz by covulcanisation of different layers in order to produce a conductivity gradient [180]. 

The radioelectric properties of the resulting films strongly depend on the water content of the monomer solution. Indeed, the diffiision of water significantly affects the reproducibility of processes since PVA is water soluble. Two dominating factors have been identified ... 

Amongst others, the advantage of such an absorbant lies in the excellent structural properties conferred by the composite material (glass textile reinforcement coated with polyaniline-F epoxy matrix). Besides, this composite can be combined with other materials with which its mechanical and radioelectric properties are compatible (foams, honeycombs...), For example, a bandwidth at — lOdB between 5 and 20 GHz was obtained by adding only 4-5 kg/m to the starting composite material. 

The radioelectric properties of two composites based on polyaniline impregnated glass textiles with different conductivities have been characterised. Changes of (fi. e") with frequency are shown in Figure 8.41. A dependence quite different from those already presented is observed. Indeed, for both conductivities, log e" decreases linearly with log/ with a slope close to unity, e.g., 0.7 and 0.8. This behaviour, independent of conductivity, is representative of a purely resistive material. The electrical equivalent circuit was found to be a resistance in parallel with a capacitance [84,86]. 

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