Effect of niobium valence on the mechanochemical activation of niobium oxides-hematite magnetic ceramic nanoparticles

Three types of nanostructured systems: xNbO center dot(1-x)alpha-Fe2O3, xNbO(2)center dot(1-x)alpha-Fe2O3, and xNb(2)O(5)center dot(1-x)alpha-Fe2O3 were synthesized by ball milling at different molar concentrations (x=0.1, 0.3, 0.5, and 0.7). The effect of Nb valence and milling time on mechanochemical activation of these systems were studied by X-ray diffraction and the Mossbauer spectroscopy measurements. In general, Nb-substituted hematite was obtained at lower molar concentrations for all Nb oxides. For the NbO-Fe2O3 system the favorable substitution of Fe2+ for Nb2+ in the octahedral sites in the NbO lattice was observed after 12 h milling for x=0.7. In the case of the NbO2-Fe2O3 and Nb2O5-Fe2O3 systems a formation of orthorhombic FeNbO4 compound was observed, in which Fe3+ cations were detected. For the highest concentration of NbO2 (x=0.7) iron was completely incorporated into the FeNbO4 phase after 12 h milling. The molar concentrations of x=0.3 and 0.5 were the most favorable for the formation of ternary FeNbO4 compound in the Nb2O5-Fe2O3 system. Influence of ball milling on thermal behavior of the powders was investigated by simultaneous DSC-TG measurements up to 800 degrees C. (C) 2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved.