New combination of composite nanoparticles for improved electromagnetic interference shielding

The scope of this research is to investigate generation and use of new “composite” nanoparticles, in particular conductive and magnetic nanoparticles, and study their potential to improve electromagnetic interference absorption for the high frequency range applications. EMI (electromagnetic interference) shielding is a method to prevent electromagnetic fields flowing between two locations by means of a barrier composed of functional materials. This dissertation presents research findings that lead to the improvement of microwave absorption properties of materials effective in the gigahertz range (0.1 - 18 GHz), by using nanocomposites of magnetic nanoparticles and conductive polymers. As a first step, dispersion of maghemite nanoparticles in epoxy resin was studied. Then, to increase the breadth of absorption, maghemite/polypyrrole composite nanoparticles were synthesized by an in-situ oxidative polymerization method, to increase microwave absorption over a wider frequency range. The microwave absorption results for the maghemite/polypyrrole composite nanoparticles showed, however, that the wideband absorption was still low. This was attributed to the low electrical conductivity of polypyrrole phase in the synthesized composite nanoparticles. Microwave plasma heat treatment of maghemite/polypyrrole composite nanoparticles was found to be a simple method to produce a highly electrically-conductive phase, as the heat treatment induces conversion of the amorphous conductive polymeric material to a significantly more electrically-conductive graphic structure. This conversion in maghemite/polypyrrole composite nanoparticles can be used to develop electromagnetic interference (EMI) shielding properties over a wide frequency range, particularly in the high frequency range (GHz). However, an undesirable side effect of such heat treatment is the decrease of magnetization of the iron oxide core particles due to oxidation to a non-magnetic phase. The results show that maghemite phase becomes converted to hematite (non maghemite) after such a microwave heat treatment. A facile route is developed in this work to prevent this heating effect on the maghemite. When the maghemite nanoparticles were coated with a nanometer thin silane layer, the magnetization value increased after microwave treatment because the cores particles were converted to magnetite. The proposed method can thus be used to increase the crystallinity of the magnetic composites via rapid heat treatment, whilst preventing any expected adverse effects on magnetic properties. This will be particularly useful in the case where we subsequently post-treated the silica- or silane-coated particles with polypyrrole, and the heat treatment serves to graphitize this layer and greatly increase its electrical conductivity. Incorporation of such particles into a polymer matrix forms nanocomposites with a broad spectral range of shielding, with the electrically-conductive phase relevant at higher frequencies, whilst the magnetic phase shields effectively at lower frequencies.