Fabrication and Evaluation of Composite Magnetic Materials

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The present study is based on two parts : one deals with the development of low-temperature sintering Mn-Zn ferrite with additives and the other is concerned with metal/oxide composite materials. Highly sinterable Mn-Zn ferrite powder consisting of submicrometer-size particles (〜0.18μm) with structural distortion has been prepared by grinding a coprecipitated powder with a media agitation mill. Dense pure Mn-Zn ferrites (>98% of theoretical) can be fabricated by heating at 900℃ in a nitrogen atmosphere. They show magnetic and electric properties in which their permeability μ and electric resistivity ρ are 520-635 and 0.8-1.6Ω・m, respectively. These properties have been further improved by adding a small amount of lithium borosilicate glass ; especially, high values of μ=1015 and ρ=474Ω・m are achieved in ferrites (900℃/12h) with 0.1wt% glass addition. The microstructures consist of large grains (〜35μm) with high resistive layers at grain boundaries.Dense composite materials (98.6% of theoretical) consisting of Super-Sendust (86.5Fe-6Si-4Al-3.5Ni wt%) and MgFe_2O_4 (75/25 vol%) have been fabricated by spark plasma sintering in air for 5 min at 850℃ and 130 MPa. They exhibit a high saturation magnetization density B_5 of 1.2 T and high electrical resistivity ρ of 1×10^<-2>Ω・m. Permeabilities μ>1000 at 1 kHz are obtained. Fine ferrite grains (〜0.6 μm) are located at the boundaries of the large spherical grains of Super Sendust (〜30 μm).Furthermore, continuously graded Super・Sendust/Mg・ferrite materials have been fabricated and their properties are under investigation.