Fabrication of Porous Iron with Directional Pores through Thermal Decomposition of Chromium Nitride
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INTRODUCTION
POROUS and cellular metals have attracted much attention because of their various characteristics different from those of nonporous metals, such as an inherent low density and a large surface area.[1] Although most of the porous metals have isotropic and spherical pores, lotus-type and Gasar porous metals have unique pore morphology with long cylindrical pores aligned in one direction. The porous metals can be fabricated by unidirectional solidification from the metallic melt dissolving gas through the gascrystallization reaction.[2] These pores are evolved by precipitation of insoluble gas due to the solubility gap between liquid and solid state during unidirectional solidification.[3–5] Lotus metals were usually fabricated by unidirectional solidification in the pressurized gas atmosphere (pressurized gas method, abbreviated as PGM), because the gas is dissolved into the melt. In this case, higher pressure is better to have higher porosity of the lotus metals according to the Sieverts’ law. However, there were two experimental difficulties. One is the need for a high pressure chamber, which is expensive and requires particular care in the handling procedures. Another is to use hydrogen gas, which is flammable and explosive gas when a small amount of oxygen is mixed. These are obstacles for mass production of lotus metals and alloys. Recently, Makaya and Fredriksson[6] produced porous Fe-base materials by dissolving CrN compound into a metallic melt of Fe-base alloy in an argon T. WADA, formerly Graduate Student, The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan, is Researcher, with the Mitsubishi Materials Co., Ltd.,Okegawa, Saitama 363-8510, Japan. T. IDE, Specially Appointed Assistant Professor, and H. NAKAJIMA, Professor, are with The Institute of Scientific and Industrial Research, Osaka University. Contact e-mail: [email protected] Manuscript submitted December 4, 2008. Article published online October 27, 2009 3204—VOLUME 40A, DECEMBER 2009
atmosphere. The decomposition of the compound leads to dissolution of a gas into the melt. 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 performed theirs. Nakajima and Ide[7] studied fabrication of lotus-type porous copper using titanium hydride in an argon atmosphere instead of a pressurized hydrogen atmosphere. This method is called the ‘‘thermal decomposition method’’ (TDM): gas atoms can dissolve in the melt from the compound containing a gas element through endothermic reaction. In order to fabricate the porous metals with unidirectional pores, unidirectional solidification was usually carried out using the mold casting technique. The bottom of the mold is cooled by circulated water and the melt is solidified from the bottom toward the top (Gasar process[5]). However, this technique cannot be applied to the met
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