Preparation of agglomeration-free hematite particles coated with silica and their reduction behavior in hydrogen.

To prepare silica-coated hematite particles without agglomeration, the effects of solid fraction, ion content in solution, and designed layer thickness on agglomeration and dispersion behavior after silica coating were examined. Since the ion concentration remained high in suspension after the hematite particles were prepared, these particles formed aggregates by the compression of an electric double layer on the hematite and silica layer produced a solid bridge between primary hematite particles. Silica bridge formation and agglomeration were almost completely prevented by decreasing the ion concentration and solid fraction of the hematite particles. Furthermore, the effects of the silica-layer thickness and structure on the reduction of hematite to iron under hydrogen gas flow and the iron core stability under air were discussed. When the solid fraction was low in suspension to prevent agglomeration during coating, a densely packed structure of nanoparticles formed by heterogeneous nucleation was observed on the silica-layer surface. Since this structure could not completely prevent oxide diffusion, the layer thickness was increased to 40 nm to obtain a stable iron core under air. Although a dense uniform layer was produced at a high solid fraction during coating, its thickness was reduced to 20 nm to completely reduce hematite to iron.