Abstract

Barium ferrite (BaFe₁₂O₁₉), lithium ferrite (Li_0.5Fe_2.5O₄) and magnesium zinc ferrite (Mg_0.5Zn_0.5Fe₂O₄) have been prepared by self-propagating high-temperature synthesis (SHS) reactions from iron, iron(III) oxide and metal oxides and peroxides. The driving force for the reactions is the oxidation of iron powder. Reactions were carried out under an oxygen flow and with the addition of sodium perchlorate as an internal oxidising agent. Reactions were also carried out in the presence of an applied magnetic field of 1.1 T. Pre-organisation of the reactant mixture in an applied field led to increases in synthesis wave velocity (from 2 to 5 mm s⁻¹) and temperature. The reactions were studied by time resolved X-ray powder diffraction on Station 16.4 of the CLRC Daresbury Laboratory. A white beam of X-rays was used in combination with three fixed energy sensitive detectors. Spectra were acquired with scan intervals of 100–250 ms and an effective 2θ range of 10–60°. Transformation of reactants to products occurred on the order of 200 ms in all of the systems studied with the exception of the applied field barium ferrite synthesis where an intermediate of Fe₃O₄ was observed. Lattice expansion effects during SHS enabled the rates of cooling in the system to be investigated. All of the materials synthesised by SHS were examined both before and after annealing by X-ray powder diffraction, energy dispersive X-ray analysis (EDXA), scanning electron microscopy (SEM), FTIR, Mössbauer spectroscopy and vibrating sample magnetometry.