Papers by Author: Hideo Nakajima

Abstract: Porous metals with long cylindrical pores aligned in one direction were fabricated by unidirectional solidification using pressurized gas (hydrogen) method (PGM) and thermal decomposition method (TDM). The pores are evolved from insoluble gas when the molten metal dissolving the gas is solidified. In the conventional PGM, the hydrogen pressurized in a high-pressure chamber is used as the dissolving gas. However, the use of high-pressure hydrogen is not desirable because of inflammable and explosive gas, in particular, for scaling up to mass production of lotus metals. In order to overcome this shortcoming, the thermal decomposition method was developed as an alternative simple fabrication method. Gas-forming compounds were added into the molten metal to fabricate lotus metals. The porosity and pore size were controlled by the amount of gas-forming compounds. TDM was applied to fabricate porous copper and aluminium

Abstract: Deformation behavior of lotus-type porous copper with long cylindrical pores aligned in one direction through equal-channel angular extrusion (ECAE) process was investigated using a die with channel angle of 150º. Although the density slightly increased after every pass, the porous structure remains in the process. The Vickers hardness and the compressive yield strength of lotus copper increased through the ECAE process. The compressive yield strength after 3 passes increased up to 10 times larger than that before processing. The deformation of lotus copper takes place by buckling and the shearing of the cell walls. The increase in hardness is considered to be caused by work hardening.

Abstract: The structural stability of hollow Cu2O and NiO nanoparticles, which were obtained via oxidation of Cu and Ni nanoparticles in air, was studied by transmission electron microscopy (TEM). Hollow Cu2O and NiO were observed to have shrunk at 473 and 623 K in annealing under 5.0×10-5 Pa, respectively, where the reduction reactions from oxides to metals started. As a result of shrinking associated with reduction, hollow oxides turned into solid metal nanoparticles after annealing at higher temperatures for a long time. In addition, hollow oxides shrunk and collapsed through high-temperature oxidation. It was found that shrinking of hollow oxides during oxidation occurs at temperature where the diffusion coefficients of slower diffusing species reach around 10-22 m2s-1. It seems that the hollow oxide nanoparticles tend to shrink and collapse at high temperatures because the hollow structures are energetically unstable.

Abstract: The formation of hollow metal oxide nanoparticles through oxidation process has been studied by transmission electron microscopy for Cu, Zn, Al, Pb and Ni in order to clarify the detailed formation mechanism of hollow oxide nanoparticles and their governing factors. For Cu, Zn, Al and Ni nanoparticles, hollow oxide nanoparticles are obtained as a result of vacancy aggregation in the oxidation processes. These results arise from faster outward diffusion of metal ions through the oxide layer in the oxidation processes. On the other hand, Pb nanoparticles turn to solid PbO, because the diffusivity difference DPb < DO in PbO leads to no formation of vacancy clusters.

Abstract: The formation of hollow zinc oxide has been studied by oxidation and subsequent thermal treatment of nanometer-sized zinc particles using in-situ TEM. The zinc particles produced under UHV condition were exposed to air at room temperature for 0.6 ks, which resulted in the formation of oxide layer with thickness of 3 nm. Subsequent heating inside UHV chamber of TEM induced the evaporation of the inner zinc, which resulted in the formation of hollow zinc oxide. The produced hollow zinc oxide had the wurtzite structure. Based upon the vapor pressure of the inner zinc, it seems reasonable to consider that the internal zinc vapor leaks away through the interface between the oxide layer and the amorphous carbon film used as a supporting substrate.

Abstract: A porous alumina with cylindrical pores was fabricated by unidirectional solidification under hydrogen gas flow (0.1 MPa) using alumina feed rods doped with silica, calcia or sodium oxide. The additives in the feed rods strongly affect the formation of porous structure during the solidification. The porosity increases with increasing silica content. The increase of porosity is enhanced by further addition of sodium oxide. The addition of calcia to the feed rod is effective on the homogenization of pore distribution. The porous alumina with 1mol%CaO and 20mol%SiO2 additives showed 50% porosity and homogeneous pores distribution.

Abstract: Oxidation behavior of Cu nanoparticles in the formation process of hollow Cu2O spheres was investigated by TEM. The thickness of Cu2O layers on Cu nanoparticles oxidized at 323 K in air was measured as a function of oxidation time. At the initial stage of oxidation until the oxide film with 2.5 nm in thickness is formed, the thickness of oxide films on Cu nanoparticles with the diameter of 10, 20 and 35 nm shows a nearly equal value regardless of diameter of Cu. After the formation of 2.5 nm layer, however, the growth rate of the oxide films on smaller nanoparticles becomes slower than that on larger nanoparticles. This result suggests that the voids formed at the Cu/Cu2O interface prevent Cu atoms from diffusing outward across the interface because the volume ratio of voids to inner Cu in smaller nanoparticles is much larger than that in larger nanoparticles.

Abstract: We studied the fatigue strength of lotus-type porous magnesium with cylindrical pores aligned unidirectionally, which was fabricated through unidirectional solidification in pressurized hydrogen atmospheres. The fatigue strength shows anisotropy; the fatigue strength in the direction parallel to the longitudinal axis of pores is higher than that in the perpendicular direction. Not only anisotropic pores but also fiber texture grown along the pore direction contributes to the anisotropy in the fatigue strength.

Abstract: A porous Ti-48.0at.%Al (Ti-rich TiAl) crystal, in which lotus-type long cylindrical pores were aligned and (γ/α2) two-phase lamellar structure was simultaneously developed, was fabricated by floating zone method under the pressure of hydrogen and helium mixed gas. Plastic deformation behavior and microstructure of the Ti-rich TiAl crystal with lotus-type aligned pores were investigated by focusing on the elongated pore direction. The as-grown and annealed crystals show a well-developed lamellar structure and no texture accompanied by 52% porosity and a mean pore diameter of 380 μm. Yield stress strongly depends on the loading direction against the elongated pore. When loading directions are parallel and perpendicular to the pore direction, yield stresses obey K=1 and 2.5, respectively, in equation of σ=σ0(1-p)K, where σ is the yield stress with pores, σ0 is the yield stress without pores and p is porosity. This reflects macroscopically homogeneous and locally heterogeneous plastic deformation between pores, respectively.

Abstract: Lotus-type porous metals, whose pores are aligned in one direction by unidirectional solidification, have a unique combination of properties. These are expected as revolutionary engineering materials with anisotropy of the properties. For the industrial use of the lotus-type porous metals, a reliable joining technology is required. We already reported the melting property of a few lotus-type porous metals by laser welding. These results indicated that these materials possessed anisotropy of melting property with the pore direction perpendicular and parallel to the specimen surface, especially remarkable anisotropy was obtained for the copper specimen owing to the difference of the laser energy absorption to the specimen surface. In this report, the three-dimensional heat transfer analyses, which take into account the difference of the laser energy absorption comparing with the anisotropy of thermal conductivity inherent to lotus-type porous metals, were performed by commercial code with user-defined subroutine. Predicted profile of weld fusion zone is in good agreement with the cross-sectional view obtained by experiments.