Papers by Author: Soong Keun Hyun

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 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: 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.

Abstract: We studied the effective electrical conductivity of lotus-type porous nickel with cylindrical elongated pores by applying the extended effective-mean-field (extended EMF) theory that is based on Hatta-Taya’s theory and Bruggeman’s effective-medium approximation. We modeled lotus-type porous nickel as the composite materials with ellipsoidal void inclusion, and examined the effect of shape and orientation of the pores and applied-electric-field direction on the effective electrical conductivity. The extended EMF theory revealed that well-known Archie’s power law is quite useful for expressing the porosity dependence of the electrical conductivity of lotus-type porous metals even though the deviation of porous morphology exists.

Abstract: Lotus-type porous magnesium was fabricated by unidirectional solidification of the melt dissolving hydrogen in a high-pressure mixture gas of hydrogen and argon. The damping constant
of porous magnesium with various porosities was measured by the hanging excitation method. The damping constant was defined as α=log (xn/xn+1)/
T, where xn and xn+1 are the successive amplitude values of the damping wave, and
T is the damping time. The frequency-amplitude dependence curve was obtained by Fast Fourier Transform analysis. The damping time of the lotus-type porous magnesium was observed to be shortened greatly compared with non-porous metals and porous copper. Moreover, the damping constant of the lotus-type porous magnesium was calculated by the damping amplitude.

Abstract: Lotus-type porous materials whose long cylindrical pores are aligned in one direction were fabricated by unidirectional solidification from the melt dissolving gas. The pores were evolved by supersaturation of gas atoms when the liquid was solidified. Although such porous metals with high thermal conductivity were produced by casting process, the process could not produce porous metals with low thermal conductivity, which possess uniform pore size and porosity. In order to obtain uniform pore size and porosity in metals with low thermal conductivity, we invented a new “continuous zone melting technique”. Using this technique various metals, alloys, intermetallic compounds and semiconductors were fabricated. Mechanical properties of tensile strength on lotus-type porous metals are described; lotus-type porous iron fabricated using a pressurized nitrogen gas instead of hydrogen exhibits superior strength. The lotus materials can be applied to various functional materials. Some examples are also shown in this paper.

Abstract: Lotus-type porous magnesium with a large number of unidirectional cylindrical pores was fabricated by unidirectional solidification of melt dissolving hydrogen in a pressurized hydrogen atmosphere. The vibration-damping capacity of the lotus-type porous magnesium plate which has many open pores was measured in this work. The attenuation coefficients of the free vibration of lotus-type porous magnesium were measured by hammering-vibration-damping test, which revealed that the attenuation coefficients increase with increase in porosity; the damping capacity of lotus magnesium is higher than that of non-porous magnesium. The mechanism for high damping capacity was analyzed on the basis of the Fourier transform technique, which indicates that various vibration modes of high frequency are observed. The excited vibrations of high frequency enhance the damping capacity of lotus-type porous magnesium.

Abstract: Tension and compression fatigue property was investigated for lotus-type porous copper possessing cylindrical pores aligned in one direction. The cyclic stress was applied in the direction parallel and perpendicular to the longitudinal axis of the pores. It was found that the fatigue strength at finite life of lotus-type porous copper is lower than that of nonporous copper, and the strength in the direction parallel to the longitudinal axis of the pores is higher than that in the perpendicular direction.