Synthesis and characterization of magnetic hollow nanocomposite spheres

We used ionic poly((methyl methacrylate)-co-(methacrylic acid)) (PMA-MAA) copolymer spheres (500 nm) as a core template, synthesized using a soap-free emulsion polymerization method, to prepare monodispersed hollow nanocomposite spheres (HNCS). Ceramic type (CoFe2O4/SiO2) HNCS and metallic type (Co-Fe alloy/SiO2) HNCS were prepared by calcining PMA-MAA/CoFe2O4/SiO2 hybrid core/shell composite spheres in air and a reducing gas, respectively. The crystalline phase involved in ceramic HNCS had a cubic spinel ferrite structure with an average crystallite size between 3.4 and 10.5 nm, while the phase embedded in the metallic HNCS revealed a body-centered cubic structure with an average crystallite size of 28.7 nm. Scanning electron microscope and transmission electron microscope photographs show that the hollow spheres were uniform and had a shell thickness of about 40 nm. Based on an energy-dispersive X-ray line scan composition analysis of a cross-section of hollow spheres it was found that the SiO2 not only coated the surface of the spheres, but was also distributed over the shell of the hollow spheres. The HNCS exhibited hysteresis loops of a ferromagnetic nature, and coercivity and saturation magnetization varied with the size of the magnetic nanoparticles embedded in hollow spheres. The magnetic properties of HNCS were also evaluated as a function of temperature. Crown Copyright (C) 2011 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. All rights reserved.