Fabrication of Lotus-Type Porous Cobalt and Silicon through Decomposition of Moisture

Porous cobalt and silicon with cylindrical pores were fabricated by unidirectional solidification in a mold, including moisture in argon atmosphere. The pores are evolved by hydrogen decomposed from the moisture during solidification. Hydrogen atoms dissolve into molten cobalt and silicon according to the reaction xM + yH₂O = M _x O _y  + yH₂ at the given temperature. Insoluble hydrogen evolves long pores oriented in one direction during the unidirectional solidification. However, no pores are observed in solidified copper. It is suggested that the formation of pores strongly depends on the magnitude of the dissociation constant of water at the casting temperatures for materials. The oxide clusters formed by metallic elements and oxygen may serve as nucleation sites for the pore evolution to yield fine pore distribution. Thus, the moisture decomposition method is superior to the conventional fabrication method, which requires high-pressure hydrogen gas.