Fabrication of Porous Copper with Directional Pores through Thermal Decomposition of Compounds
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UCTION
POROUS and foamed metals exhibit various characteristics such as an inherent low-density and large surface area, which differ from bulk metals. Hence, these metals should be applicable as light-weight materials, catalysts, electrodes, vibration and acoustic energy damping materials, impact energy absorption materials, etc.[1–3] Recently, a new type of porous metal, called lotus-type porous metal, has attracted much attention due to the long cylindrical pores aligned in one direction. Lotus-type porous metals are fabricated by a unidirectional solidification process in a pressurized gas atmosphere such as hydrogen.[4–7] The pores evolve from insoluble gas when the molten metal dissolving the gas is solidified. Lotus-type porous metals not only have the properties of conventional porous metals, but also have unique properties originating from their directional pores. In particular, these metals exhibit superior mechanical properties compared to conventional porous metals, which have nearly spherical, isotropical pore shapes.[8] Thus, porous metals are attracting considerable attention for various industrial applications.[7,9–11] Shapovalov[4] and the present authorsÕ group[5,7] have fabricated lotus-type porous metals with homogeneously distributed pores using a mold casting technique (Gasar method) in high pressure gas. Although this technique is a simple process, it is difficult to control the solidification velocity, which consequently prevents a uniform pore size and porosity for metals with a lower HIDEO NAKAJIMA, Professor, and TAKUYA IDE, Graduate Student, are with The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan. Contact e-mail: [email protected] Manuscript submitted May 21, 2007. Article published online January 9, 2008 390—VOLUME 39A, FEBRUARY 2008
thermal conductivity[12]. In order to improve this shortcoming, we have developed a continuous zone melting technique[13–15] and a continuous casting technique.[16–18] Because these techniques can control the solidification velocity, various types of lotus metals with homogeneously distributed long pores can be fabricated. Although our fabrication techniques are quite advanced, one large technical barrier remains; high pressure hydrogen gas must be used. Employing high pressure hydrogen gas has inherent risks because it may lead to inflammable and explosive accidents if oxygen is mixed. Therefore, a technique that does not require high pressure hydrogen to fabricate lotus metals is highly desirable. In order to overcome this difficulty, we propose an alternative, but simple method to fabricate such lotus-type porous metals by using a thermal decomposition method (TDM) of compounds containing gas elements in a nonhydrogen atmosphere under nearly atmospheric pressure. The present article reports the principle of TDM and the first versatile method to control the pore morphology, including pore size and porosity of lotus-type porous metals.
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EXPERIMENTAL PROCEDURES
Copper (99.99 pct pure) was melted by
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