Phase Transformations in Iron Oxide under the Action of Microwave Radiation
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ICAL SCIENCE OF MATERIALS
Phase Transformations in Iron Oxide under the Action of Microwave Radiation O. N. Kanyginaa, V. L. Berdinskiia, M. M. Filyaka, A. G. Chetverikovaa,*, V. N. Makarova, and M. V. Ovechkina aOrenburg
State University, Orenburg, 460018 Russia *e-mail: [email protected]
Received September 4, 2019; revised December 6, 2019; accepted January 24, 2020
Abstract—It has been shown that microwave radiation induces structural and, as a consequence, magnetic phase transformations in iron oxide α-Fe2O3 (hematite). After microwave irradiation of fine-dispersed partially amorphized Fe2O3 particles for 10 min in wet air, the percentage of crystalline hematite drops by 40% but the total amount of crystalline components increases owing to the formation of a new ferromagnetic modification, α-Fe2O3 maghemite. The initial antiferromagnetic and final ferromagnetic Fe2O3 samples are powders consisting of spherical particles with nearly equisized (40–60 nm) domains of X-ray coherent scattering. DOI: 10.1134/S1063784220080095
INTRODUCTION Microwaves acting on a disperse system as a whole generate a variety of local interactions inside it, which, in combination, cause a macroscopic effect. The physicochemical features of structuring in oxide particles are greatly influenced by their morphology and chemical composition, type of crystal lattice, structural defects, ion-exchange processes, etc. Many researchers worldwide are now concentrating largely on structural transformations in oxide materials exposed to a magnetic field. While heat transfer from the surface of a sample into its volume in standard synthesis methods takes place by means of heat conduction and convection, the energy of microwave field is absorbed volumetrically, as a result of which the sample heats up uniformly. In this case, the heating rate is not limited by heat conduction and, basically, one can form a required structure throughout the volume. In many studies [1–3] comparing conventional and microwave methods it was noted that the productivity of the process significantly rises and energy cost drops in the latter case. Owing to these advantages, material processing in microwave fields becomes an efficient tool for improving the properties of oxide materials that prevents material contamination by by-products. Mechanisms of microwave–oxide interaction have been poorly understood to date, since appropriate investigations are usually conducted selectively. Specifically, it was found [4] that oxides constituting clayey materials experience phase transformations under the action of microwave radiation: alumina exhibits polymorphic transformations, as a result of
which it partially transforms into α-corundum. Of the four modifications of silica contained in natural clay, two of them, β-quartz and β-crystobalite, remain in air and only β-quartz remains in a wet medium after microwave irradiation. Iron oxides offer a variety of magnetic properties, which considerably depend on the shape and size of particles. These parameters to a great extent are gover
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