Effect of ball milling time on microstructure and magnetic properties of Fe-W nanocrystalline composite synthesised by hydrogen reduction and sintering

Abstract

Sintered nanocrystalline Fe–W composites were synthesised by isothermal reduction of a Fe2O3–WO3 powder mixture in pure hydrogen at 900°C. Before reduction, a 1:1 mixture of the powders was ball milled thoroughly for 3, 9 or 15 h, then compacted and dried at 90°C for 24 h. The reduced powders were re-compacted and sintered at 1000°C for 2 h in hydrogen. The reduction behaviour was studied by thermogravimetric analysis. The microstructure, phase composition, pore distribution and magnetic properties of the initial oxide mixtures and of the reduced and sintered products were characterised. Complete reduction was achieved in all cases, to give a nanocrystalline homogenous composite with Fe of 13 nm crystallite size and W of 38 nm. A dense crystalline structure was observed within sintered samples having a total porosity of 52–54%. The magnetic coercivity (H c) of the sintered Fe–W specimens increased with increasing milling time, whereas magnetisation (M s) decreased.