In this work, resin composites of α-Fe and Co_0.5Fe_2.5O₄ mixed powders were prepared and their electromagnetic wave absorption properties were investigated. Two peaks were observed in the frequency dependence of relative permeability, and the magnetic resonance frequency (f_r) corresponding to the highest peak of the imaginary part of relative permeability (μ_rʺ) could be controlled from 0.91 to 13.3 GHz according to the mixing ratio of the magnetic powders. The results suggest that changes in the effective demagnetizing field H_d^eff with changes in the mixing ratio influenced the peak shift of permeability. A matching frequency with reflection loss of less than -20 dB could be tuned from 0.30 to 9.95 GHz by controlling mixing ratio of the α-Fe and Co_0.5Fe_2.5O₄ mixed powders and the thickness of the composites.

Dense [Fe-3mass%Si]/[(Ni-Mn-Zn) and (Mn-Zn) ferrites] composites have been fabricated by pulsed electric-current pressure sintering (PECPS), aiming at high saturation magnetization density B_s and high electrical resistivity ρ, which are required for many applications used under high frequency electro-magnetic field. To suppress the mutual diffusion between metal and oxides, the former had been coated with Ni (0.1 µm in thickness) as the protection film. And to avoid the brittle failure of oxide film formed on metal particles during compacting, two kinds of mixtures consisting of fine “(MnO₂ or MnO, NiO, ZnO, α-Fe₂O₃)” and” (MnO₂, ZnO, α-Fe₂O₃)” powders were adopted to synthesize both Ni_0.25Mn_0.25Zn_0.5Fe₂O₄ and Mn_0.577Zn_0.316Fe_2.071O₄ ferrites, respectively. The former ferrite was selected for better affinity to Ni film and relatively high ρ, and the latter for nearly zero magnetostriction constant λ_s and crystalline magnetic anisotropy constant K_I to attain high μ even under stress. Furthermore, by appropriating MnO for Ni-Mn-Zn ferrite source and utilizing high pressure powder compacting and PECPS, under 1 GPa and 200 MPa, respectively, dense metal/hybrid ferrite (core: Ni-Mn-Zn, intermediate: Mn-Zn ferrites) composites revealed B_s of 1.50 T and permeability µ around 45 at 1 kHz and 20 at 5 MHz.

An electronic fuel injection system of automobiles have been commercialized at the end of 1960’s, they have been adopted as environmental measures for many automobiles and motorcycles. In recent years, metal injection molding (MIM) has been adopted as a process for manufacturing a solenoid valve which is a component of an electronic fuel injector, and PB permalloy components have been commercialized. However, Ni, an element of PB permalloy, is expensive material, making it difficult to produce the fuel injector component at a low cost. As a solution to this problem, we studied the magnetic and material properties of Fe-Cr-Si and Fe-Cr-Si-Mo alloys by MIM process. These results revealed that the magnetic and mechanical properties were improved by reducing Cr content, and the corrosion resistance were improved by adding Mo even if the amount of Cr reduced. On the other hand, we found that the hardness of these alloys increased by adding Si and Mo. In this study, Fe-Cr-Si alloy specimens containing Si and Mo to increase hardness are manufactured by MIM process, and we investigated the magnetic and mechanical properties. These results revealed that Fe-12Cr-4.5Si-2Mo alloy is a material with excellent magnetic and mechanical properties.

In this paper, towards higher performances of powder metallurgical (P/M) products, three past analytical studies are introduced: 1) powder characteristics affecting the die filling, 2) densification process of partially alloyed steel powder during warm compaction and 3) the microscopic approach to investigate functions of grain boundaries in the sintered Mn-Zn ferrite using a scanning tunneling microscopy. It would be a pleasure for the author if these approaches would be helpful in the development of future PM materials/products.

From the viewpoints of grain refinement and preferred orientation, a mechanical alloying (MA) and hot-extrusion technique has been proposed to fabricate Bi₂Te₃-based bulk materials. In this paper, the MA and hot-extrusion process, microstructure and texture, thermoelectric and mechanical properties of extruded samples, and the effect of Cu-doping in n-type samples have been reviewed. The extruded samples exhibited fine-grained microstructure, and their basal planes were predominantly oriented parallel to the extrusion direction. The p-type Bi_0.4Sb_1.6Te₃ samples showed significant enhancements in both thermoelectric and mechanical properties. For example, the sample extruded at 400°C had a maximum value of ZT = 1.2 at room temperature and a much higher hardness than unidirectionally solidified samples. However, the n-type Bi₂Te_2.85Se_0.15 samples showed lower thermoelectric properties due to their higher carrier concentrations and smaller fractions of twin boundaries. Cu-doping in n-type samples resulted in a reduction in carrier concentration, and thus increases in related Seebeck coefficient and electrical resistivity and decrease in carrier thermal conductivity. The resulting ZT value of 0.86 for Cu_0.05Bi₂Te_2.85Se_0.15 sample was obtained.

Magnesium antimonide (Mg₃Sb₂) is a promising thermoelectric compound utilized for thermoelectric power generation. We have found out that the Mg₃Sb₂ compound can be made from Mg and Sb powder mixture via the combustion synthesizing reaction. In this study, we manufactured the compounds from the compacted bodies with a different Mg fraction between 60.0 and 75.0 at% in an argon gas flow at 650°C for 1 hour via the combustion synthesis process. Morphology, porosity, phases and thermoelectric properties (electrical resistivity and Seebeck coefficient at room temperature to 500°C) were investigated for the manufactured samples. All samples manufactured with different Mg fractions were mainly consisted of Mg₃Sb₂ and became foamed bodies with a porosity between 50 and 70%. The thermoelectric properties changed with the Mg fraction. The maximum power factor of 42.1 μW/mK² at 488°C was obtained in the foamed body with the Mg fraction of 64 at%. The dimensionless figure of merit (ZT) of the sample was also estimated using the thermal conductivity, which was calculated considering its large porosity. The maximum dimensionless figure of merit (ZT_max) would be 0.13 at 488°C.