Microstructural aspects of the self-propagating high temperature synthesis of hexagonal barium ferrites in an external magnetic field

Abstract

Thermal initiation of a mix of BaO2, Fe2O3 and iron powder under a flow of oxygen inside a quartz tube induces a self-propagating reaction with velocity 2 mm s−1 and a temperature of ca. 975 °C. This leads to the formation of a fused multi-phasic product which on sintering at 1210 °C for 6 h produces phase pure BaFe12O19. Reactions were also studied in an applied field of 1.1 T. In this case the pre-ground starting mixture, was attracted to the center of the field and adopted an hour-glass configuration in cross section; initiation of this aligned green mixture under a flow of oxygen produced a propagation velocity of ca. 5 mm s−1 and a temperature of ca. 1150 °C. The product contained two phases, a fused shiny metallic-looking component predominantly from the center of the magnet and a matt red-black coloured component from the edges of the sample. The shiny and matt components were manually separated and analysed both before and after annealing at 1150 °C for 6 h. The effect of changes in reactant particle size of the iron powder, flow of oxygen and addition of an internal oxidising agent (NaClO4) were investigated. Reactions were also studied in which the green mixture was aligned by using two small NdFeB magnets, with the magnets being withdrawn prior to initiation. All the samples both before and after annealing were characterised by X-ray powder diffraction, infrared, vibrating sample magnetometry, scanning electron microscopy, energy dispersive analysis by X-rays, electron microprobe analysis and Mössbauer spectroscopy. The sintered ferrites showed comparable remanence (σr) and maximum magnetisation σmax to conventionally prepared samples, but significantly lower coercivity.