Iron is the most abundant metal on earth and is widely used by humans as iron ores, pigments, catalysts, in thermites and haemoglobin. Presently, there are 16 known different types of iron oxides. Iron oxides have many beneficial uses and are an integral part for the human race development. This way, the control of the redox processes and mutual transformation of iron oxides, are necessary for the design of a simple model that could help interpret the related behaviour; in particular, the alpha-, beta- and gamma- iron oxide phases. Specifically, magnetite has been identified to offer unique physical and chemical properties. Thereby, it has become an important and indispensable material for many industrial applications. Unfortunately, certain limitations on the application conditions are imposed by the redox stability issue associated with it. Therefore, fine control of the iron oxidation states represents a challenge for materials engineering and it is imperative they be resolved.
Recently, a team of researchers at University of Aveiro in Portugal: Nuno Ferreira and colleagues assessed the use of laser treatment as a tool for imposing strongly non-equilibrium conditions for processing iron oxides. They were hoping to achieve the successful growth of magnetite/hematite fibres with a core/shell structure using the floating zone method under air and certain oxygen pressure conditions. Their work is currently published in the research journal, Dalton Transactions.
In brief, the research method employed entailed the exploration of the relevant redox processes in iron oxides, processed under highly nonequilibrium laser floating zone conditions under atmospheres with different oxygen activities. In particular, under the aforementioned conditions, processing of ferric oxide green precursors was undertaken, resulting in the formation of fibres having a magnetite core and a ferric oxide shell geometry. The researchers then characterized the obtained fibres using XRD, SEM/EBSD, optical microscopy, electrical and magnetic measurements.
The Portuguese research team uncovered that lower hematite content and shell thickness for the fibres could processed under more oxidizing conditions. In addition, the combined structural and microstructural studies, supported by the analysis of the existing literature data, revealed that the redox processes during the laser floating zone process could be rather determined by kinetics of melt crystallization, nuclei formation and heat transfer than by the oxygen content in the gas phase. Moreover, the EBSD results clearly evidenced distinct microstructure and grain orientation in the shells, formed under different atmospheres.
In summary, the study by University of Aveiro scientists presented the successful growth of magnetite core and hematite shell structure fibres using the floating zone technique. In general, the highly oriented shell microstructure was formed during the growth under an atmosphere with lower oxygen content. Altogether, the results of the magnetic studies were in good agreement with structural and microstructural results.
N. M. Ferreira, M. C. Ferro, M. A. Valente, J. R. Frade, F. M. Costa, A. V. Kovalevsky. Unusual redox behaviour of the magnetite hematite core–shell structures processed by the laser floating zone method. Dalton Transactions, 2018, volume 47, page 5646.Go To Dalton Transactions