Functional magnetic nanocomposites for EMI shielding

Microwave heat treatment of iron oxide nanoparticles coated with conductive polymers such as polypyrrole is a simple method to increase the conductivity of the polymer via conversion of it from an amorphous material to a graphitic structure. The combination of magnetic and conductive composite nanoparticles can be used to develop electromagnetic interference (EMI) shielding materials, particularly effective in the high frequency range. However, heat treatment often decreases the magnetization of iron oxide nanoparticles by oxidation to a non-magnetic phase. We have found we can prevent this effect, if maghemite (γ-Fe₂O₃) nanoparticles are first coated with silica via the sol-gel method. The influence of microwave heat treatment on structure formation and the magnetic behavior of these nanoparticles were subsequently investigated by FT-IR, TGA, XRD, TEM, and VSM. The particles showed core-shell structures with a maghemite core and silica shell. The saturation magnetization (Ms) of silica-coated maghemite nanoparticles after heating in a standard household microwave oven was almost seven times higher than bare maghemite nanoparticles that were similarly heat-treated. The significant reduction of Ms for bare γ-Fe₂O₃ nanoparticles after microwave treatment was due to a phase change to α-Fe₂O₃ (hematite). Conversely, silica-coated nanoparticles underwent a phase change to Fe₃O₄ (magnetite) and Fe₂SiO₄ (fayalite), which accounted for only a slight reduction in Ms, compared to the pre-heat treated particles.