Poly- and nearly monocrystalline hematite particles having diameters of around 2 and 0.1 μm, respectively, were prepared by the gel-sol method and coated with a uniform silica layer by the sol-gel method. The core in the silica shell was reduced to iron without agglomerate formation between the particles by using a hydrogen stream. The microstructure and morphology of these cores and the silica layers were examined by high-resolution transmission electron microscopy, and electron and X-ray diffraction analysis. In hematite particles, around 2 μm in diameter, the reduced products were mostly α-Fe, but partially magnetite. In hematite particles, around 0.1 μm in diameter, only α-Fe was observed. Most of the raw hematite and iron particles produced were monocrystalline, and part of core grew hexagonal prism-shaped monocrystalline particles. In the case of the growth of a crystal to a hexagonal prism, a nanometer-scaled space at the interface between the iron crystal core and the silica layer was discovered.
Yonemochi, Y., Iijima, M., Tsukada, M., Jiang, H., Kauppinen, E. I., Kimata, M., Hasegawa, M., & Kamiya, H. (2005). Microstructure of iron particles reduced from silica-coated hematite in hydrogen. Advanced Powder Technology, 16(6), 621-637. https://doi.org/10.1163/156855205774483325