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: precipitati on in an aqueous solution followed by thermal annealing under a reduci ng 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 at mosphere from Fe, Fe2O3, and TiO2 in stoichiometric proportions. The a s-prepared powders are characterized by X-ray diffraction, scanning an d transmission electron microscopy, surface area measurement, and Moss bauer spectrometry. In both cases, the crystallite's size is about 15 nm, but whereas in the case of mechanosynthesis, the ball-milled powde rs consist of aggregates,those obtained by soft chemistry are very wel l dispersed. In contrast to the soft chemistry route, both lattice def ects and cation site inversion are induced by high energy ball milling , as evidenced by X-ray diffraction and thermogravimetric analysis. Fi nally, the particle coercivity is studied and discussed according to p article size and the degree of oxidation of Fe cations inferred from t hermogravimetry. (C) 1998 Academic Press.