Facile microwave-assisted synthesis of magnetic ferrite: Rapid interfacial reaction underlying intensifying mechanism

Abstract

Microwave-assisted (MW-assisted) heating is clean and intensively used to synthesize materials owing to the high-efficiency volumetric and selective heating modes. In the current paper, a design and facile synthesis of magnetic ferrite ceramics by microwave-assisted heating of MnO2 and Fe2O3 was conducted to reduce the synthetic temperature of conventional heating. The microwave absorptivity and temperature rise characteristics of the MnO2–Fe2O3 system were first assessed for the material design of MnFe2O4, and then the phase transformation, microstructure evolution, and magnetism change of the MnO2–Fe2O3 system heated under conventional and MW-assisted heating were compared. Investigations on electromagnetic properties and heating behaviors of MnO2–Fe2O3 mixture suggested that a suitable cylinder sample size for microwave heating was designed to be 10 mm in diameter and 10 mm in length based on the dielectric loss and magnetic hysteresis loss of microwave penetration at different depths. The intensifying mechanism for microwave heating was associated with the rapid interfacial reaction between MnO2 and Fe2O3. Under microwave irradiation at 935 °C, some perfectly crystallized MnFe2O4 was preferentially formed at the interface of MnO2 and Fe2O3, and these MnFe2O4 were evenly distributed in the cylinder samples. Similar to the “self-propagating” effect, the in-situ generation of MnFe2O4 with good energy conversion ability evenly distributed everywhere of the cylinder would facilitate the reaction of MnO2 and Fe2O3 under microwave irradiation based on the distinctive selective and volumetric heating. Compared with conventional heating at 1200–1250 °C, ferrite synthetic temperatures via MW-assisted heating decreased to 900–1000 °C, and the low-temperature synthesis of ferrite also had a good electromagnetic performance. Eventually, the clean and value-added utilization of low-grade ferruginous manganese ores via MW-assisted heating was validated for the preparation of manganese ferrite from minerals under reduced roasting temperatures.