Evolution of the Infrared Emissivity of Ni During Thermal Oxidation until Oxide Layer Opacity

The mid-infrared (3-22 mu m) emissivity of high-purity Ni has been studied in its pure state, during an isothermal oxidation in air at 730 degrees C and in the fully oxidized state. Measurements in pure Ni were performed in Ar between 200 and 800 degrees C and a change of slope in the temperature dependence of the total normal emissivity around its Curie temperature (354 degrees C) was observed. An oxidation process was carried out at 730 degrees C for 33 days, when the emissivity stopped evolving and the results were representative of NiO. During the first stages, the emissivity evolved forming the usual interference patterns of semi-transparent films. A mixture of oscillatory and monotonic behaviours of the emissivity as a function of wavelength and oxide layer thickness was found, which manifests as a non-trivial evolution of the total normal emissivity, different than that reported in previous studies. Finally, the emissivity of NiO was measured from below its Neel temperature (252 degrees C) to 850 degrees C. It showed the typical shape of a ceramic material with an extra vibrational mode due to two-phonon processes and an additional absorption band around 5 pm in the antiferromagnetic phase produced by magnons. The temperature dependence of its total normal emissivity differs significantly from that of a lightly oxidized nickel sample from the literature. Overall, the influence of the surface characteristics on the thermal radiative properties of oxidized Ni is thoroughly discussed and highlights the importance of accounting for all possible sources of infrared emissivity evolution in order to make accurate radiative heat transfer calculations.