Mechanochemical Activation of Titanium-Magnetite Mixtures

A. A. CRISTÓBAL; E. F. AGLIETTI; J. M. PORTO LÓPEZ; F. R. SIVES; R. C. MERCADER

Rio de Janeiro, Brasil

Congreso; LACAME2006; 2006

Mechanochemical processes involving reactions between metals and crystalline oxides have become of interest in the last decades because in addition to their potential technological applications in structural, magnetic or electric materials through the development of metastable and non-crystalline materials with controlled properties-- they add to the understanding of the natural occurring processes that lead to the formation of minerals and soils. Mechanochemical activation also allows the controlled development of displacement reactions, as a means of synthesis of novel nanocrystalline materials. On the other hand, natural series of minerals in the solid solution series between magnetite (Fe304) and Ulvöspinel (Fe2Ti04) and their intermediate members, titano-magnetites, also display differences in the degree of oxidation and their cation distribution among the spinel structural sites. The controlled studies of how the distribution of cations takes place can help toward building a model for the nature of their magnetism, and, since they are the primary carriers of rock and soil magnetism, are therefore intensively investigated in many experimental and theoretical studies. Toward a better understanding of the thermal, physical-chemical, magnetic and hyperfine behavior, we have set about to study a series of titanium and magnetite mixtures with diverse activation conditions. We have studied the development of new phases other than the initial Ti and Fe3O4 ones by x-ray diffraction (XRD), scanning electron microscopy, differential thermal analysis and Mössbauer spectrometry. The results show that as a result of the mechanochemical activation produced by high-energy ball-milling of the mixtures in Ar atmosphere at room temperature, the Ti atoms reduces the Fe ions in the Fe3O4 spinel, partly to Fe2+ and partly to metallic Fe. Fe ions are partly substituted for Ti in the structure, leading to a stoichiometry of the type Fe3+(IV)Fe2+(VI)1+xFe3+(VI)1-2xTi4+(VI)xO4, where we have assumed that Ti4+ incorporates mainly into octahedral (VI) sites and that part of the Fe2+ ions (x) is produced by reduction of the octahedral Fe3+(VI) ions. By Mössbauer spectrometry and XRD, we have been able to follow the evolution of the changes in the contents of the main phases, α-Fe, γ-Fe, Ilmenite and Ulvöspinel. The divergence between the values obtained by both techniques is discussed according to the different materials properties on which the techniques are based.