Fabrication of Lotus-Type Porous Aluminum through Thermal Decomposition Method
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TRODUCTION
THE porous and foamed metals exhibit various characteristics such as an inherent low density and large surface area, which differ from bulk metals. These metals are expected to be used as lightweight materials, catalyst carriers, electrodes, vibration, acoustic energy damping materials, impact energy absorption materials, etc.[1] However, these materials have the same problem deteriorating mechanical properties such as strength, stiffness, and fatigue, due to inhomogeneous pore number density distribution and pore size. Many researchers have made an effort to develop porous materials by powder metallurgy, melt route, and other methods.[2–4] Especially, lotus-type porous metals have attracted much attention because of the long cylindrical pores aligned in one direction. These materials are fabricated by the unidirectional solidification process in a pressurized gas atmosphere such as hydrogen (Gasar method).[5,6] The pores are evolved from insoluble gas when the molten metals dissolving the gas are solidified. The lotus-type porous metals have not only the properties of conventional porous metals but also the unique properties originating from their directional pores. In particular, these metals exhibit superior mechanical properties to the conventional porous metals, which have nearly spherical, isotropic pore shape.[7,8] The use of pressurized gas such as hydrogen, nitrogen, and oxygen was required for the fabrication process of lotus-type porous metal with unidirectional elongated S.Y. KIM, Research Associate, J.S. PARK, Specially Appointed Assistant Professor, and H. NAKAJIMA, Professor, are with The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan. Contact e-mail: nakajima@sanken. osaka-u.ac.jp Manuscript submitted August 20, 2008. Article published online February 20, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
pores.[9–11] Especially, hydrogen was the most useful to fabricate the lotus metals. However, hydrogen gas is explosive and inflammable when mixed with a small amount of oxygen, so that its use is not convenient for industrial mass production. A safety procedure is necessary to fabricate lotus-type porous metals. In order to avoid the difficulties associated with hydrogen, it is desirable to use an alternative technique without the use of hydrogen gas. Makaya and Fredriksson[12] produced porous Fe-base materials by dissolving CrN compound into a metallic melt of Fe-base alloy in an argon atmosphere. The decomposition of the compound leads to dissolution of a gas into the melt. Pores were subsequently formed during solidification due to the solubility gap between the liquid and solid phases. They solidified the melt in the crucible to produce porous metals with an isotropic pore structure. However, no investigations were carried out to produce porous metals with an anisotropic cylindrical pore structure until Nakajima and Ide’s work. Nakajima and Ide[13] studied fabrication of lotus-type porous copper using titanium hydride in argon atmosphere instead
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