Mechanochemical Reaction of Various Acicular Magnetic Iron Oxide Powder

Abstract

The mechanochemical reactions by grinding acicular particles of γ-Fe₂O₃, Fe₃0₄, Co-modified γ-Fe₂O₃ and Co-doped γ-Fe₂O₃ were investigated. Changes in magnetic properties (coercivity, magnetization flux density, and squareness), morphology (length and axial ratio), crystallite size, specific surface area, degree of transition to α-Fe₂O₃, heat-stability, and open-pore size were measured, Influences of grinding atmosphere (temperature, humidity, and oxygen partial pressure) on the mechanochemical reaction of γ-Fe₂O₃ and Fe₃O₄ were also investigated.
In the first stage of grinding, acicular particles were cut in the longitudinal direction to have lower axial ratios, 3-4 (Figs.9, 10). The coercivity and the squareness of γ-Fe₂O₃ and Fe₃O₄, decreased at first, and then increased gradually (Fig.6). The magnetization flux density decreased with the formation of α-Fe₂O₃ (Figs.6, 7). On grinding the various magnetic materials in an atmosphere (relative humidity =50%) at 20°C: γ-Fe₂O₃ obtained by dehydration of γ-FeOOH was most easily converted into α-Fe₂O₃, and the transition into α-Fe₂O₃ occurred easily in the order of γ-Fe₂O₃, Fe₃O₄ and Co-doped γ-Fe₂O₃, whereas it did not almost occur in the case of Co-modified γ-Fe₂O₃ (Figs.5, 7, 13). Co-modified γ-Fe₂O₃ is protected from the mechanochemical reaction by the layer of cobalt ferrite at the surface of each particle. In regard to the grinding atmosphere, the presence of water in air depressed the transition, and that of oxygen accelerated it, whereas the temperature did not affect it (Fig.14). Generally, the grinding effect becomes larger with increasing the specific surface area. It was clarified by investigating the influences of grinding atmosphere and the effect of Co-Fe surface treatment that the mechanochemical transition occurs from the surface (Figs.12-15).