Radiative emissivity of metals and oxidized metals at high temperature

Abstract

Radiative emissivities of metals at high temperatures influence the energy balance and remote sensing in a wide range of manufacturing processes as well as research and development activities and thereby determine performance and even economic viability. Accurate and comprehensive measurements of metals thermal emissivity have always been a challenge due to numerous influential factors such as: spectral range, temperature, sample topology, oxidation, contamination, composition, etc. Therefore, the influence of optically thick and thin metal oxides on normal spectral and directional, as well as on the complex index of refraction, was studied. An experimental setup for emittance measurements in air at high temperature was developed during the course of study, and includes a Fourier Transformed Infrared spectrometer (FTIR) and a special design sample holder which allows full directional measurements. The optical system can operate over a very wide wavelength range from 1 to 20 ?m, with sample temperatures between 673 K and 973 K. Directional measurements were performed from normal to the sample surface to a 72° polar angle. X-ray diffraction (XRD), scanning electron microscopy (SEM), and Auger spectroscopy (AES) were employed to characterize the samples. Experimental data were used in conjunction with electro-magnetic theory to determine the complex index of refraction. The reported data show good agreement with Fresnel’s relation; uncertainty in the emissivity measurements was found to be less than 3.5%. In addition, the normal emissivity of high purity metals such nickel, titanium, and zirconium was studied in ultra high vacuum conditions. Their normal emissivities and determined index of refraction exhibit both free and bound electron effects. A second experimental device was constructed for this study by coupling the FTIR with an electromagnetic levitator (EML) where the sample is electromagnetically heated, leading to reduced chances of sample contamination and/or sample-holder interaction. The optical system operates over a broad spectral range from 1 to 16 ?m, with sample temperatures between 1273 K and 1650 K. The uncertainty in emissivity was found to be less than 4 %.