Mechanochemical treatment of alpha-Fe2O3 powder in air atmosphere
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Powder of alpha-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 alpha-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 alpha-Fe2O3. Before nucleation of the Fe3O4 phase, the crystallites of the alpha-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 fre...ezing of highly localized sites of a short lifetime. Excitation on a time scale of similar to 10(-5) s corresponds to a temperature rise of the order of (1-2) x 10(3) K. Decay of the excited state occurs rapidly at a mean cooling rate higher than 10(6) K s(-1). (C) 1998 Elsevier Science S.A. All rights reserved.