HYDROTHERMALLY PREPARED NANOCRYSTALLINE Mn-Zn FERRITES: SYNTHESIS AND CHARACTERIZATION

摘要

Nanosized manganese-zinc ferrite powders with cubic spinel crystal structure and a significantly high surface area have been synthesized from metal chloride precursors through a hydrothermal precipitation route using aqueous ammonia followed by freeze drying of the products. Pressed compacts made from those powders have been subsequently sintered under controlled oxygen partial pressure conditions. The ferrite formation was found to be quite sensitive to the procedures adopted for preparing the hydroxide slurry prior to hydrothermal treatment. The chlorine ion concentration in the solution and the pH of the precipitation is shown to play a crucial role in retaining the initial stoichiometry of the solution in the nanoparticles. The produced powders were examined by X-ray diffraction for identification of the crystalline phases present, by Scanning and Transmission Electron Microscopy for identification of their morphological structure and properties, nitrogen sorption for determination of the BET surface area and by thermogravimetric and Differential Scanning Calorimetry for identification of the oxidation/reduction behavior upon firing. All powder characterization results point out to the formation of nanosized manganese-zinc ferrite crystals ranging from 5 to 30 nm with a relatively high surface area (～ 75 m~2/g) when compared to this of conventionally prepared powders by the mixed oxide methods (～5-7 m~2/g). Compacts made from those powders have been sintered to various temperatures using manganese-zinc ferrite equilibrium oxygen partial pressures atmospheres. It appeared that for the same final densities the sintering temperatures are significantly lower when compared to those of conventionally prepared ferrite powders. Moreover, the achievement of very fine grained microstructures and simultaneous high densities becomes possible. Ring shaped fired specimens of the previously mentioned nanocrystalline powders have been characterized for their electromagnetic performance. They exhibit low electromagnetic power losses and resonance frequencies in the MHz range. As can be concluded from the processing and characterization results, in terms of industrial applications, hydrothermally prepared nanocrystalline manganese-zinc ferrites may offer the following advantages: a) lower manufacturing cost of inductive electromagnetic components since the energy consuming prefiring step can be eliminated and the firing temperatures can be significantly reduced. b) Manufacturing of high frequency inductive electromagnetic components because of their fine grained microstructure, extending therefore the frequency application range of manganese-zinc ferrites and at the same time meeting the market trends for high frequency operation and miniaturization of inductive components.