Functional Magnetic Nanocomposites for EMI Shielding

摘要

Microwave heat treatment of iron oxide nanoparticles coated with conductive polymerssuch as polypyrrole is a simple method to increase the conductivity of the polymer viaconversion of it from an amorphous material to a graphitic structure. The combination ofmagnetic and conductive composite nanoparticles can be used to develop electromagneticinterference (EMI) shielding materials, particularly effective in the high frequency range.However, heat treatment often decreases the magnetization of iron oxide nanoparticles byoxidation to a non-magnetic phase. We have found we can prevent this effect, if maghemite(γ-Fe_2O_3) nanoparticles are first coated with silica via the sol-gel method. The influence ofmicrowave heat treatment on structure formation and the magnetic behavior of thesenanoparticles were subsequently investigated by FT-IR, TGA, XRD, TEM, and VSM. Theparticles showed core-shell structures with a maghemite core and silica shell. Thesaturation magnetization (Ms) of silica-coated maghemite nanoparticles after heating in astandard household microwave oven was almost seven times higher than bare maghemitenanoparticles that were similarly heat-treated. The significant reduction of Ms for bare γ-Fe_2O_3 nanoparticles after microwave treatment was due to a phase change to α-Fe_2O_3(hematite). Conversely, silica-coated nanoparticles underwent a phase change to Fe_3O_4(magnetite) and Fe2SiO4 (fayalite), which accounted for only a slight reduction in Ms,compared to the pre-heat treated particles.