Thermal stability of the solvothermal-synthesized MnFe sub(2)O sub(4) nanopowder

Manganese ferrite nanopowder was prepared by a new solvothermal method, using 1,2 propanediol as solvent and KOH as precipitant. The as-synthesized powder, by solvothermal treatment in autoclave at 195 degree C, for 12 h, consisted of fine manganese ferrite nanoparticles. The further thermal treatment of the initial manganese ferrite powder to higher temperature resulted in manganese ferrite decomposition due to Mn(II) oxidation to Mn(III), as observed by X-ray diffraction. FT-IR spectroscopy has evidenced that the oxidation takes place even at 400 degree C. The oxidation of Mn(II) to Mn(III) was studied by TG/DSC simultaneous thermal analysis. It was shown that Mn(II) oxidation takes place in a very small extent up to 400 degree C. The main oxidation step occurs around 600 degree C, when a clear mass gain is registered on TG curve, associated with a sharp exothermic effect on DSC curve. The exothermic effect is smaller in case of the powder annealed at 400 degree C, confirming the superficial oxidation of Mn(II) up to 400 degree C. In order to avoid Mn(II) oxidation, the powder obtained at 400 degree C was further annealed at 800 degree C in argon atmosphere, without degassing, when manganese ferrite MnFe sub(2)O sub(4) was obtained as major crystalline phase (69 %). All manganese ferrite powders showed a superparamagnetic behavior, with maximum magnetization of 51 emu g super(-1) in case of the as-synthesized powder, characteristic of magnetic ferrite nanopowders.