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Metals with fine surface asperity show higher efficiency of sunlight absorption than those with flat surfaces. In the present study, ultravioletvisibleinfrared light absorption by metals with porous surface layers was examined. A three-dimensionally interconnected porous structure was formed on the surface of copper and iron substrates by oxidizing and then reducing. Pore size and layer thickness were dependent on oxidization and reduction temperatures and times. It was found that metals with porous surface layers exhibited higher efficiencies of light absorption over a wide wavelength range than those with mirror-polished surfaces. High light absorption efficiencies were obtained for metal substrates with fine porous structures. In particular, a very fine porous structure was formed on the surface of iron substrate by oxidizing and reducing it at low temperatures, and its light absorption reached 7580% over a wide wavelength range corresponding to ultravioletvisible infrared light. This light absorption index is much higher than light absorption by conventional metal substrates with flat surface.

It has been found that metals and semiconductors with high surface asperity readily absorb sunlight. In our previous work, iron and copper substrates with porous surface layers were prepared by redox treatment. The metal substrates with porous surface layers absorbed more light of ultraviolet visible infrared wavelength than those with flat surfaces. In the present study, we attempt to enhance the light absorption properties of ceramic materials made from blast furnace slag, using needle shaped hydrate crystals formed on the surface of the slag compact by hydrothermal reaction. Under the hydrothermal conditions used, blast furnace slag particles react with highly pressurized water to form a hydrate glass phase containing H 2 O, and form needleor plate shaped hydrate crystals such as Tobermorite (5CaO・6SiO 2 ・5H 2 O) between original slag particles and on the surface of the slag compact. The slag compact with needle like crystals on its surface absorbed ultraviolet visible infrared light more poorly than that with a flat surface. However, after the surface of slag compact was coated with metal, the sample with needleor plate shaped hydrate crystals on its surface absorbed light significantly better than that with a flat surface, exceeding 90 over ultraviolet visible infrared wavelengths.

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