Papers by Author: Hideo Nakajima

Abstract: We have been investigated application of the lotus type porous metal that is manufactured by the unidirectional solidification of the melt in pressurized gas atmosphere such as hydrogen or nitrogen for an air bearing development. In this study, wear properties of porous copper having cylindrical pores have been investigated. Generally, wear test is influenced by a shape, a wear test method, atmosphere and a way of lubrication. Wear test used pin on disk type's wear test.

Abstract: We investigated the pore morphology in lotus-type porous copper fabricated by continuous casting technique as a function of transference velocity range from 1 to 100 mm･min-1 under hydrogen gas pressure of 1.0 MPa. Lotus-type porous copper with long cylindrical pores aligned in one direction parallel to the transference direction was fabricated, which posses a sufficient uniformity of the porosity and pore size. The pores formed at transference velocity of 1 mm･min-1 were larger than other condition. Necks were observed in these pores, whose formation may be attributed to bubbling in the melt. The pore size decreased with increasing transference velocity, while the porosity was not varied much by transference velocity.

Abstract: Lotus-type porous NiAl and Ni3Al intermetallic compounds, possessing cylindrical pores aligned in the direction parallel to the solidification direction, were fabricated by using a unidirectional solidification technique in a pressurized hydrogen atmosphere of 2.5MPa. The porosity of lotus NiAl is 24.2 %, and the porosity of lotus Ni3Al is 3.2%; the porosity of the porous NiAl is larger than that of Ni3Al. This is because the solubility gap of hydrogen between liquid and solid phases of NiAl is larger than that of Ni3Al.

Abstract: Formation of hollow structure through oxidation of Al nanoparticles was studied by applying transmission electron microscopy. Al nanoparticles 6~8 nm in diameter were observed to become hollow particles after having been exposed to air at 295 K for a few minutes. An analysis of the Debye-Sherrer rings in the selected area diffraction patterns before and after oxidation showed that hollow oxide nanoparticles are amorphous. The formation mechanisms of hollow oxide are discussed based on the low-temperature oxidation mechanism of Al and on the comparison with our previous results of hollow ZnO formation via oxidation of Zn nanoparticles.

Abstract: Lotus-type porous Ni- (15, 28 and 31) at.% Al alloys whose long cylindrical pores are aligned in one direction were fabricated by continuous zone melting technique under high-pressure gas of hydrogen of 2.5 MPa. A part of 5-10 mm in length of the rod in the vicinity of the coil was melted by high frequency induction heating, and was moved downwards by electric motors at a constant velocity of 330 μms-1 to 500 μms-1 for unidirectional solidification. The pores are formed as a result of precipitation from the supersaturated hydrogen gas when the liquid metals dissolved with gas atoms is solidified. The porosity and the pore size decrease with increasing aluminum content. An increase of solidification velocity from 330 μms-1 to 500 μms-1 leads to a decrease of pore diameter and an increase of pore number in the porous Ni-28at%Al.

Abstract: Lotus-type porous Cu-5at.%Al alloy whose elongated pores are aligned in one direction was fabricated by unidirectional solidification in pressurized hydrogen gas atmosphere. The porosity of the Cu-Al alloy was higher than that of pure copper under the same fabrication conditions because of difference in hydrogen solubility. The pore structure was not round shape because the directional pore growth was interrupted with dendrite arms formed during the solidification; the pores grew to detour the obstacle of the dendrite arms.

Abstract: Lotus-type porous metals with low thermal conductivity are fabricated by continuous zone melting technique, which possess directional elongated pores. The porous metals have been able to be fabricated through the conventional casting method by utilizing the solubility gap between solid and liquid in pressurized gas atmosphere. However, there is a shortcoming that the pores are coarsened in the part farther from the chill plate in the ingot. In order to overcome such a shortcoming, we developed the continuous zone melting technique and successfully produced the lotus-type porous metals with even low thermal conductivity such as stainless steel and superalloys. Furthermore, from the viewpoint of mass production with low cost, we invented novel ”continuous casting technique”. The molten metals dissolving gas are solidified continuously by passing through the mold cooled with chiller and thus, lotus-type porous metal plate as long as one meter was produced for short time. Sufficient uniformity of the porosity and pore size was obtained in such long porous ingots. This technique is prospective method for commercial mass production.

Abstract: The diffusion coefficients of 44Ti, 63Ni and 59Fe in γ-TiAl single crystals have been measured by ion-beam sputter-sectioning technique, while those of In have been measured using ion implantation technique and secondary ion mass spectroscopy (SIMS) in order to clarify the diffusion anisotropy: the diffusion perpendicular and parallel to the [001] axis. The diffusion of Ti and In perpendicular to the [001] axis is faster than that parallel to the [001] axis. However, the diffusion anisotropies of Fe and Ni show opposite trend to those of Ti and In, namely the diffusion parallel to the [001] axis is faster than that perpendicular to the axis. The predominant process of diffusion perpendicular to the [001] axis has been discussed from a viewpoint of activation energy using the expression of the diffusion coefficients in L10-ordered alloys.

Abstract: A new fabrication method of porous ceramics using unidirectional solidification was proposed in this paper and the role of silica additive on the formation of porous alumina structure was discussed. A porous alumina was formed by unidirectional solidification process that was conducted using floating zone melt method in hydrogen atmosphere at normal pressure. The effect of silica additive on the porosity and the pore size of the solidified samples was examined. The porosity of the solidified samples increased with increase of silica contents. On the other hand, the pore size decreased with increase of silica content. The average pore size of 10% and 20% silica added samples were 0.396 mm and 0.184 mm, respectively.

Abstract: Lotus-type porous iron and stainless steel (SUS304L) whose long cylindrical pores are aligned in one direction were fabricated by unidirectional solidification in a pressurized hydrogen or nitrogen gas atmosphere. Pores are formed as a result of precipitation from the supersaturated gases when the liquid metal dissolved with gas atoms is solidified. The ultimate tensile and yield strengths of the porous iron produced in nitrogen atmosphere are about two times higher than in a hydrogen atmosphere. Such superior strength is attributed to solid-solution hardening due to solute nitrogen atoms in iron matrix.