Anne-Christine Baudouin

Frequency selective microwave absorption induced by controlled orientation of graphene-like nanoplatelets in thin polymer films

Highly ordered polycarbonate films containing parallel graphite nanoplatelets have been produced by squeezing the corresponding random nanocomposites in the melt. Orientation of the conductive fillers is observed in the plane of the film, i.e., perpendicular to the squeezing direction. It only appears above a critical concentration of 15 wt. % and results from a confinement effect. Oriented samples show a resonance-like sharp increase of the conductivity at a given frequency in the microwave region, with the possibility to control the value of this frequency and the resulting absorption by changing the nanoplatelets concentration. Above this frequency, the oriented polymer nanocomposites show a high level of electromagnetic absorption, which opens the possibility to tailor materials with effective electromagnetic interference shielding by absorption in selected frequency ranges. The in-plane stacking of conductive nanoplatelets separated by insulating polymer induces their strong capacitive coupling to the signal propagating in the plane of the polymer film. As a result, the equivalent circuit of this propagation becomes a resonant system composed of capacitors, inductors, and resistors, which agrees well with the experimental results.

Foamed Nanocomposites for EMI Shielding Applications

INTRODUCTION : The addition of nanoparticles having specific properties inside a matrix with different properties creates a novel material that exhibits hybrid and even new properties. The nanocomposites presented in this paper combine the properties of foamed polymers (inexpensive, lightweight, easy to mould into any desired shape, etc.) with those of carbon nanotubes (CNTs). The addition of any conductive nanoparticles to an otherwise insulating matrix leads to a significant increase of the electrical conductivity. But CNTs have a very high aspect ratio; a much lower content of CNTs is therefore required to get the same conductivity increase as the one obtained with more compact nanoparticles. This is especially interesting for EMI shielding materials since, as will be explained in further details in this chapter, it is desirable for such materials to have a high conductivity but a low dielectric constant, in order to minimize the electromagnetic power outside the shield casing but also to minimize the power reflected back inside the casing, as is explained in section 2. In particular, two parameters of interest when comparing shielding materials are detailed and discussed. The polymer/CNTs nanocomposites were fabricated and characterized using a two-step diagnostic method. They were first characterized in their solid form, i.e. before the foaming process and the most interesting polymer matrices (with embedded CNTs) could be selected. This way, only the promising blends were foamed, therefore avoiding the unnecessary fabrication of a number of foams. These selected blends were foamed and then characterized. The samples, both solid and foamed, are described and their fabrication processes are briefly explained in section 3 while the characterization methods are shown in section 4. A simple electrical model is given and explained in section 5 and an optimized topology for the foams is also proposed in the second part of the same section. The measurement results for the solids and for the mono-layered and multi-layered foams are summarized and discussed in section 6. They are then compared to results obtained using the electrical model presented in the previous section and they are also correlated to rheological characterizations.