Fabrication of Lotus-Type Porous Cobalt and Silicon through Decomposition of Moisture
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DUCTION
POROUS metals with highly oriented pores, so-called ‘‘lotus-type porous metals’’ (hereafter, referred to as ‘‘lotus metals’’) can be fabricated through unidirectional solidification under pressurized hydrogen or nitrogen atmosphere.[1–4] Such metals exhibit superior mechanical strength to foamed metals and sintered porous metals.[5–7] Moreover, lotus metals possess various unique properties such as sound absorption, vibration damping, shock absorption, and anisotropy in mechanical, thermal, and electrical properties.[1] Usually, lotus metals can be produced by unidirectional solidification from melt dissolving hydrogen. The pores are evolved by agglomeration of insoluble hydrogen when the melt is solidified in the pressurized hydrogen atmosphere. However, Suematsu et al. fabricated lotus nickel by unidirectional solidification in argon atmosphere, although argon is insoluble even in molten metals.[8] It was suggested that the pore evolution is attributed to hydrogen dissolved in the melt. The hydrogen is generated from the moisture, which is contained in the water glass layer coated on the side wall of the mold. The moisture in the molten nickel may decompose into hydrogen and oxide according to the following reaction:[9] Ni þ H2 O ¼ NiO þ H2
½1
The results of the previous study and thermodynamic data suggest that the formation of pores strongly
H. ONISHI, Graduate, S. UENO and S.K. HYUN, Specially Appointed Associate Professors, 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 March 23, 2008. Article published online November 27, 2008 438—VOLUME 40A, FEBRUARY 2009
depends on the dissociation constant of water at the casting temperature. Because the free energy changes of Eq. [1] are known to be positive values, the dissociation P constant PHH2O is very small.[10,11] It is worthwhile to 2 investigate in detail whether the feasibility of pore formation in other materials is also closely related with the magnitude of the dissociation constant of water in the moist atmosphere. In the present work, the feasibility to fabricate lotus silicon, cobalt, and copper was investigated by unidirectional solidification using moisture contained in the mold. Furthermore, the formation mechanism of the pores through dissociation of moisture is discussed with P the dissociation constant PHH2O . 2
II.
EXPERIMENTAL PROCEDURE
Figure 1 shows the schematic drawing of the mold casting apparatus. This apparatus consists of melting and solidification parts, both of which are installed in a high-pressure chamber. Hydrogen or argon gas was introduced in the chamber at an elevated pressure. The metal in the crucible was melted by an induction coil and the molten metal was poured into the mold. The bottom of the mold was cooled by a water chiller, while the lateral wall was made of molybdenum whose surface was coated with a mixture of alumina, water glass (54.5 pct H2O, 31.2 pct Si
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