Abstract
Fabrication of MgFe2O4/Fe-Ni alloy bulk composites, in which Fe-Ni grains (∼5 μm in diameter) are fully surrounded with thin MgFe2O4 layer (∼0.3 μm in thick), has been tried by adopting high-pressure sintering for two powders: "Composite Powder I" and "Composite Powder II". The former was prepared as follows; fine MgO particle-coated Fe-Ni powders were heat-treated under various oxygen partial pressures at 600° and 700°C. The latter was fine MgO+α-Fe2O3 mixed powder particles-coated Fe-Ni powders. For the composite powders, two sintering processes were tried; i.e., pulsed-electric current pressure sintering PECPS (800°C/30 MPa/3 min) and HIP (950°-1250°C/196 MPa/2 h). Phase identification by XRD and SEM observations revealed that dense microstructure consisting of two phases (spinel MgFe2O4 and Fe-Ni alloy) was attained from the combination of composite powders II, PECPS and a re-oxidation process. However, other process resulted in formation of Wüsteit phase of Mg1-xFexO and FeO in addition to Fe-Ni due to highly reductive atmosphere during the sintering. Thus obtained composite materials consisting two phases showed good magnetic properties at room temperature as expected from their compositions and microstructures: saturation magnetization density Bs of 1.20 T, and permeability μ∼15-30 at 1 MHz, and a Curie point above 460°C.
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