Mechanochemical treatment of α-Fe2O3 powder in air atmosphere

Powder of α-Fe2O3 was mechanochemically treated in a planetary ball mill in an air atmosphere. Structural changes were followed by X-ray diffraction analysis, magnetization measurements and differential scanning calorimetry after various milling times. It was found that complete transformation of α-Fe2O3 to Fe3O4 is possible during milling in an air atmosphere under appropriate milling conditions. Presumably, the decrease in the oxygen partial pressure during milling has a critical influence on promoting the dissociation of α-Fe2O3. Before nucleation of the Fe3O4 phase, the crystallites of the α-Fe2O3 phase are reduced to a minimal size accompanied by the introduction of atomic-level strain. Local modeling of a collision event, coupled with a classical thermodynamic assessment of the Fe2O3-Fe3O4 system, were used to rationalize the experimental results. It is proposed that the mechanochemical reactions proceed at the moment of impact by a process of energization and freezing of highly localized sites of a short lifetime. Excitation on a time scale of ∼10−5 s corresponds to a temperature rise of the order of (1–2)×103 K. Decay of the excited state occurs rapidly at a mean cooling rate higher than 106 K s−1.