Fabrication of bulk magnetic composite materials consisting of controlled nano-structures

点击翻译按钮获取中文摘要

With aiming at developing new bulk magnetic materials having both high saturation magnetization density and high magnetic permeability for high-frequency electric applications, fabrication of MgFe_2O_4/Fe-Ni alloy bulk nano-composites, in which Fe-Ni grains (〜5μm in diameter) are fully surrounded with thin MgFe_2O_4layer (〜0.3μm in thick), has been tried: the former metal grains have high saturation magnetization density and the latter has high electric resistivity.The first stage, the best chemical compositions of MgFe_2O_4 have been investigated. Mg(Fe_<1-x>Mn_x)_2O_4 solid solution ferrites were prepared using both conventional solid reactions and sol-gel process, and their magnetic properties were measured precisely ; x=0.2 composition gave the best data. The second stage, plastic deformation of Mg(Fe_<1-x>Mn_x)_2O_4 ferrites were evaluated at temperature 1200 to 1350℃ in air.Finally MgFe_2O_4/Fe-Ni alloy bulk composites has been tried by adopting high-pressure sintering two powder … More s: 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℃. The latter was fine MgO+a-Fe_2O_3 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℃/30MPa/3min) or HIP (950°-1250℃/196MPa/2h). Phase identification by XRD and SEM observation revealed that dense microstructure consisting of two phases (spinel MgFe_2O_4 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 Wusteit phase of Mg_<1-x>Fe_xO 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 B_s of 1.20 T, and permeabilityμ〜15-30 at 1 MHz, and a Curie point above 460℃. Less