Temperature and compositional dependencies of normal spectral emissivities at 632.8 nm for solid Cu-Ni alloys—Ellipsometric measurements and formulation of empirical prediction equation

Abstract

Normal spectral emissivities of solid Cu-Ni alloys were determined using an ellipsometer combined with an electric furnace for a wavelength of 632.8 nm in the temperature ranges between 800 K and the respective solidus temperatures. The emissivities of the alloys had slightly positive temperature coefficients and seemed to be on quadratic functions of the atomic fraction of nickel. Thermal radiation of transition metals in the visible and near-infrared regions is generally considered to stem from the excitation-relaxation processes of free electrons and inner shell electrons, i.e., electrons at the d-like levels in the Cu-Ni system. Thus, the contribution from the former to the emissivities has been calculated on the basis of the free-electron model with damping. The difference between measured and calculated values corresponds to the contribution from inner shell electrons, which has been found to depend strongly upon neither temperature nor chemical composition in the range of the nickel atomic fraction, X Ni>0.2. On the basis of these findings, an empirical prediction equation for the emissivity at 632.8 nm for solid Cu-Ni alloys has been proposed: this equation applies at temperatures higher than 800 K in the range of X Ni>0.2 and can predict emissivity with an uncertainty less than 13 pct.