Microstructures and Physical Properties of Colored Layer of Cu-4 mass%Au Alloy (Shakudou) Fabricated by Japanese Traditional Technique

Abstract

  The microstructures and physical properties of Niiro-colored Cu-4 mass%Au alloy (shakudou in Japanese) have been investigated. Specimens are fabricated by using a traditional Japanese metal-craft technique. An aqueous solution containing cupric sulfate, alum and artificial verdigris is used to color the alloy sheets. Coloring is performed at 373 K(100°C). The fine structures are observed using a transmission electron microscope. The matrix of the colored layer consists of crystalline and amorphous Cu₂O and the crystal size is 30~80 nm. The relation Cu₂O{111}//Cu-Au{111} is observed between the grown Cu₂O and the matrix Cu-Au alloy. The Au fine particles with 2-5 nm diameters are distributed in the Cu₂O layer. Many impurities of Cl and S introduced from the coloring solution are detected from the top of the colored layer by AES analysis. The reflectance of the colored specimen is approximately 5% of the uncolored alloy and a broad reflection peak near 450 nm is observed. The threshold voltage of Cu₂O/Cu-Au alloy contact for the colored specimen is 0.2-0.3 V. This is approximately 1/3 of that of thermally oxidized Cu₂O /Cu-Au alloy contact. The coloring is mainly due to light absorption and reflection by the fine Au particles distributed in the Cu₂O layer.