Papers by Author: Hideo Nakajima

Abstract: A Ti coating was deposited on not only surface but also inner surface (pore wall) of a lotus-type porous copper with porosity of 43.3% and pore size of 603.5μm by arc air plating for modification. The structure, microhardness and compressive properties of the porous copper were investigated, and compared to those after annealing under 600°C and 800°C for 6 hours. It is indicated that Ti coating with thickness from 4.6μm to 12.5μm has been deposited on the pore wall for the porous copper successfully. It is found that the HV_0.01 increases from 89.32 to 348.19 for the porous copper after the deposition, which is attributed to the higher hardness of the Ti coating. In case of 0.2% yield stress, it keeps almost the same for the porous copper with the deposition of the Ti coating, while increases from 21.22 to 23.79MPa after annealing, which may stem from the improved adhesion of the coating with pore wall by diffusion. It is believed and much possible for metal coating to improve the mechanical properties of lotus-type porous metal.

Abstract: The tensile deformation of lotus-type porous copper with cylindrical pores oriented in one direction was investigated. Deformation was occured homogeneously in the copper matrix for loadings parallel to the orientation direction of pores (pore direction), while deformation was localized in the matrix around pores for loadings perpendicular to the pore direction. In the case of parallel loading the decrease in cross section of tensile specimen was smaller than that of nonporous copper, because of the constant-volume law (i.e. incompressibility condition) for deformation was not applicable to the deformation of pores. In the case of perpendicular loading, the deformed regions were disconnected and constant-volume law holds only in the matrix around the pores, and thus, the cross section hardly decreases during the tensile deformation.

Abstract: Formation behavior of nanovoids during the annealing of amorphous Al₂O₃ and WO₃ was studied by transmission electron microscopy. The density and size of the voids in Al₂O₃ and WO₃ increase with increasing annealing temperature from 973 to 1123 K and from 573 to 673 K, respectively. It is suggested that the formation of nanovoids during annealing is attributed to the large difference in density between as-deposited amorphous and crystalline oxides.

Abstract: Lotus-type porous aluminum with cylindrical pores oriented in one direction was deformed by Equal Channel Angular Extrusion (ECAE) through a 150° die with sequential 180° rotations, and the pore morphology and Vickers hardness after the extrusion were investigated. The Vickers hardness increases with increasing number of passes in the extrusions both parallel and perpendicular to the pore direction, accompanied by the decrease of porosity. The densification occurs more easily in the perpendicular extrusions than in the parallel extrusions, and the large deformation by the densification gives rise to the large increase in the Vickers hardness for the perpendicular extrusions.

Abstract: Lotus-type porous iron was fabricated by continuous zone melting technique through thermal decomposition of chromium nitride(Cr1.18N). Nitrogen dissolves into the molten iron through thermal decomposition of Cr1.18N. When the molten iron is solidified in one direction, insoluble nitrogen forms the directional gas pores aligned along the solidification direction. The porosity increases with increasing transfer velocity. For most of lotus metals fabricated by pressurized gas method, the porosity does not change with the transfer velocity owing to constant gas solubility in liquid and solid phase. On the other hand, the porosity of lotus metal fabricated by thermal decomposition method depends on the transfer velocity. This difference is attributed to the decomposition behavior of gas compound dependent upon the heating rate.

Abstract: The formation process of oxide nanotube via metal oxidation reaction was studied by transmission electron microscopy for Cu, Fe, and Ni nanowires. Cu2O and Fe3O4 nanotubes were formed after the oxidation of Cu and Fe nanowires with a diameter of 55 nm in air at 423 and 573 K for 3.6 ks, respectively. Both Cu2O and Fe3O4 nanotubes had a cylindrical interior pore with uniform diameter. On the other hand, Ni nanowires became bamboo-like structures of NiO with separate interior pores after oxidation at 673 K for 7.2 ks. The formation of the interior pores in Cu2O and Fe3O4 nanotubes and NiO bamboo-like structures can be explained by the rapid outward diffusion of metal ions through the oxide layers and the clustering of excess vacancies.

Abstract: Lotus-type porous metal with many straight pores is attractive as a heat sink because larger heat transfer capacity is obtained due to the small diameter of the pores. The heat transfer capacity of the lotus-type porous copper heat sink was calculated using the model with the pores of uniform diameters. However, actual lotus-type porous metals have a distribution of pore diameter. In the present work, we investigated the lotus-type porous copper fin model by considering size distribution of the lotus-type porous copper fin. Prediction of the heat transfer characteristics for the lotus-type porous copper heat sink shows a good agreement with the experimental data.

Abstract: Lotus-type porous carbon steel (lotus carbon steel) plates were fabricated by continuous casting technique in a pressurized nitrogen atmosphere. The experiments were done both with adding 0.3wt% of NiO powder in molten carbon steel in a ceramic crucible and without NiO powder. The lotus carbon steel fabricated without NiO powder under nitrogen pressures of 0.1 and 0.5 MPa had single pores which grew independently. On the other hand, the lotus carbon steel fabricated with adding NiO powder had pores which coalesced each other and became in irregular shapes. Under nitrogen pressure of 2.5 MPa, the pores formed with adding NiO powder were smaller than that formed without NiO powder. The porosity increased by adding NiO powder in every pressurized nitrogen atmosphere. From these results, NiO powder in molten carbon steel is considered to act as nucleation sites for pores at the solid-liquid interface and to increase of the pore number.

Abstract: Lotus-type porous aluminum was fabricated by continuous casting technique in mixture gas of hydrogen and argon at various transfer velocities in order to understand formation process of pores. The porosity and pore diameter decrease with increasing transfer velocity. The transfer velocity dependence of the porosity in lotus aluminum is different from that in other lotus metals such as stainless steel and copper. It is considered that the difference is attributed to lower solubility in aluminum than that in other metals.

Abstract: Porous Al-5mass%Ti alloys were fabricated using a continuous casting technique in a hydrogen atmosphere, and the effects of transfer velocity (V) and the peritectic solidification process on the pore morphology and matrix microstructure were examined. In the case of V = 0.5 mm/min, columnar microstructure and directional pores grow along the transfer direction. The Al3Ti phases are formed in localized regions of matrix part, and however, they do not suppress the growth of directional pores in the other regions. In the case of 5.0 mm/min, because needle-like Al3Ti phases grow along the transfer direction, directional pores can grow between them. On the other hand, in the case of 10.0 mm/min, spherical pores surrounded by equiaxed peritectic microstructure and homogeneously distributed Al3Ti phases are formed, because the primary α -Al and Al3Ti phases probably prevent the growth of directional pores.