Abstract
The microstructures and optical properties of niiro-colored Cu-20 mass%Ag eutectic alloy (shibuichi in Japanese) have been investigated. Specimens are fabricated by a traditional Japanese metal-craft technique. An aqueous solution containing cupric sulfate, alum and artificial verdigris is used to color the alloy sheet at 373 K (100°C). A transmission electron microscope, X-ray diffractometer, EPMA, XPS, AES and spectrophotometer are used to determine the physical properties of the coloring layer. The αCu areas in the eutectic structure is preferentially strongly colored bluish black. Although the color change of αAg areas is less, the areas around αAg are a light yellowish orange and brown. The colored layer consists of crystalline and amorphous Cu2O, and Ag nanograins of various shapes distributed in the Cu2O matrix. The large Ag grains are nonoxidized αAg grains in a Cu-Ag eutectic structure. The Ag nanograins precipitate when the αCu grains are oxidized. The elements Cl and S are detected in the colored surface by EPMA and the compounds CuS, CuCl2 and CuSO4 are detected by XPS. Ag2S is detected near Ag grains by lattice image analysis. The reflectance of the specimen decreases with increasing coloring time. In conclusion, the Cu2O film and Ag nanograins in Cu2O mainly contribute to the color change.
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