A survey of hemispherical total emissivity of the refractory metals in practical use

• Published in: Energy (Oxford) 2005-02, Vol.30 (2), p.535-543
• Main Authors: Otsuka, Akihiko, Hosono, Kazuo, Tanaka, Ryohei, Kitagawa, Kuniyuki, Arai, Norio
• Format: Article
• Language: English
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Exact sciences and technology; Optical properties and condensed-matter spectroscopy and other interactions of matter with particles and radiation; Optical properties of bulk materials and thin films; Physics
• ISSN: 0360-5442
• DOI: 10.1016/j.energy.2004.04.019

The applicability of the hemispherical total emissivity measurement for metals those have a thin protective layer (passive scale) on its surface was investigated at high temperatures (above 1000 °C) using a modified hot-filament method. In this method, the Joule heat and the temperature of the specimen were measured by two sets of electrodes (two sets of thermocouples) spot-welded at the middle portion of the specimen, in order to reduce the influence of heat loss from the water cooled electrodes at both ends of the ribbon specimen. The measured hemispherical total emissivity for type 304 stainless steel and molybdenum above 1000 °C showed a few times higher value compared with the data obtained from polished metals given in the literatures. This result indicated that the emissivity of the metals with protective layer including oxide scale must be carefully considered not only in measuring temperature with the radiometric method but also the designing various devices where radiation phenomena were critical. The measured hemispherical total emissivity above 1000 °C decreased due to the vaporization of the protective layer on the specimen surface during the measurements. Thus, the upper limit of the measuring temperature in this method is ruled by the stability of these surface products in a vacuum at high temperatures. Therefore, such pulse heating methods those enable the more rapid heating within nano- or picoseconds are strongly recommended for the measurements of the emissivity of the metals those having the protective layer.