Enveloping Fe–12%Al atomized powders in selectively-oxidized insulating films for soft magnetic composite (SMC) cores

Fe-12 wt%Al gas atomized powders were enveloped in a uniform Al2O3-containing oxide film by heat-treatment of a mixture of Fe–Al and Mg(OH)2 powders at 900 °C for 1 h in static hydrogen atmosphere. In this novel method using a semi-closed system without adding any oxidants, water vapor generated by decomposition of Mg(OH)2 was used as an oxidant to selectively oxidize aluminum in the Fe-12 wt%Al alloy. Most of the generated water vapor was consumed to form the Al2O3 film and the thickness of the insulating film could be controlled in the tens of nanometers range by controlling the amount of added Mg(OH)2 in the mixture. Fe–12%Al powders enveloped in around 50 nm thick oxide films could be applied to make reasonably high-performance soft magnetic composite cores, because the high temperature heat-treatment up to 900 °C was possible and hence easily relieved the strain energy generated when shaping the cores.