Enhanced Thermal Protection of Iron Oxide Nanoparticle by Insulating Nanoporous Char Layer: Effect of Core Size and Char Layer Properties

摘要

We report an enhanced thermal insulation of Fe3O4 nanoparticle by nanoporous char layer formed during thermal decomposition of Fe3O4@carboxymethyl cellulose. The role of char layer properties, decomposition pathways at different Fe3+/Fe2+ ratio, and the initial core size on thermal stability are evaluated using XRD, HRTEM, SAXS, TGA, FTIR, XPS, and laser Raman scattering. In situ high temperature XRD results show that the Fe3O4 core is stable up to 1000 degrees C due to the presence of a porous char shell. The presence of nanopores in the char layer of thickness similar to 7 nm was evident in HRTEM images. The percentage of magnetite increases from 82 to 100% as the temperature is increased from 30 to 1000 degrees C. The XPS results show that the relative concentration of Fe2+:Fe3+ is increased from 1:4 to 1:2 on increasing the annealing temperature from 550 to 1000 degrees C. For smaller particle size, a slower growth rate of magnetite core is observed, due to the effective blocking of the active growth sites. The mechanism of thermal protection of iron oxide core is explained using existing theoretical models. The increase in saturation magnetization of the nanocomposites after high temperature annealing has great significance for practical applications.