Thermoanalytical techniquesExcellent tools for the characterization of ferrite/SiO2 nanocomposites and their precursors

Thermal analysis and FTIR spectroscopy were used to investigate the formation and thermal decomposition of several manganese ferrite precursors, obtained in the interaction between a mixture of manganese(II) nitrate and iron(III) nitrate (molar ratio 1:2) and different polyols/polyvinyl alcohol, 1,2-propanediol, and ethylene glycol, naked or embedded in silica gel. All prepared precursors consisted in mixture of Mn(II) and Fe(III) carboxylates, obtained through the polyol oxidation by nitrates ions. Similar manganese ferrite precursors were obtained in silica gel also. By the thermal decomposition of the naked precursors, manganese ferrite was obtained at 300 and 400 °C, as fine nanoparticles, with diameters up to 15 nm. Above 400 °C, the manganese ferrite has begun to decompose, due to the Mn(II) oxidation to Mn(III). At 700 °C, only crystalline phases containing Mn(III) were evidenced in the XRD patterns. In case of the precursors embedded in silica gel, their thermal decomposition took place at higher temperatures. Manganese ferrite was partially stabilized by the silica matrix. Thus, manganese ferrite was present in silica matrix, even at 1000 °C, with bixbyite (FeMnO3) impurities. Pure manganese ferrite embedded in silica matrix was obtained in all cases, after annealing the powders obtained at 400 °C, in argon atmosphere, at 1000 °C. The magnetic behavior of the manganese ferrite/silica nanocomposites obtained at 1000 °C in air and in argon was superparamagnetic, with maximum magnetization values of 25 emu g−1.