Structure, cation distribution, and properties of nanocrystalline titanomagnetites obtained by mechanosynthesis: Comparison with soft chemistry

Nanocrystalline Fe-based spinels with composition Fe2.5Ti0.5 O-4 were synthesized using two different routes: soft chemistry and high-energy ball milling, In the first case, two steps were involved: precipitation in an aqueous solution followed by thermal annealing under a reducing mixture of N-2/H-2/H2O gases. In the second case, the spinel phase was directly formed in the mill at room temperature and under argon atmosphere from Fe, Fe2O3, and TiO2 in stoichiometric proportions. The as-prepared powders are characterized by X-ray diffraction, scanning and transmission electron microscopy, surface area measurement, and Mossbauer spectrometry. In both cases, the crystallite's size is about 15 nm, but whereas in the case of mechanosynthesis, the ball-milled powders consist of aggregates,those obtained by soft chemistry are very well dispersed. In contrast to the soft chemistry route, both lattice defects and cation site inversion are induced by high energy ball milling, as evidenced by X-ray diffraction and thermogravimetric analysis. Finally, the particle coercivity is studied and discussed according to particle size and the degree of oxidation of Fe cations inferred from thermogravimetry. (C) 1998 Academic Press.