Hydrothermal Synthesis of Manganese Zinc Ferrite Powders from Oxides

Manganese zinc (MnZn) ferrite powders were prepared via the hydrothermal treatment of a homogeneous mixture of the raw oxides (i.e., Fe₂O₃, ZnO, and Mn₃O₄ or MnO) at temperatures of 220°-320°C in air or an inert atmosphere. The final results of the hydrothermal reactions between the raw oxides were fine powders with a heterogeneous phase composition. In addition to lower concentrations of the residual reactants (Fe₂O₃, Mn₃O₄), two types of spinel-structure-based reaction products-ferrite ((Mn²⁺,Zn)Fe₂O₄) and manganate ((Zn,Mn²⁺)Mn₂³⁺O₄)-were detected after the synthesis. The composition of the ferrite products, as well as the ratio of ferrite products to manganate products, were mainly functions of the oxidation state of the manganese that was present during treatment. The oxidation state of manganese during reaction was dependent on the valence of the manganese in the starting manganese oxide and on the atmosphere in the autoclave during reaction. When the hydrothermal reaction was conducted in air, almost-pure zinc ferrite was identified, whereas during reaction in an inert atmosphere, MnZn ferrite was formed. The kinetics of the hydrothermal reactions also were dependent on the oxidation state of manganese, as well as the temperature and specific surface area of the starting Fe₂O₃.