Influence of roughness and composition on the total emissivity of tungsten, rhenium and tungsten–25% rhenium alloy at high temperature

Abstract

Abstract Measurements of the total directional and hemispherical emissivity of tungsten, rhenium and tungsten with 25% rhenium samples of various surface states was carried out in two wavelength ranges, 0.6–2.8 and 0.6–40 μm, from 1300 up to 2500 K in high vacuum. A clear trend was identified for the ratio of the 0.6–2.8 μm to the 0.6–40 μm hemispherical emissivity (assumed to be close to the true α/e ratio, which is the ratio of the solar absorptivity α to the total emissivity e) as a function of the RMS roughness. This ratio is decreasing with increasing RMS roughness rapidly up to around 1 μm and then tends to stabilize for higher roughness. It suggests that increasing the roughness, starting from a flat surface, will progressively give more weight to the long wavelength range emissivities in comparison to the short wavelength ones. Beside the RMS roughness, a sharp surface structure on a finer scale found on some samples also led to an additional decrease of the α/e ratio, but these structures are not stable at high temperature and disappear due to a natural high temperature smoothing. Finally our results also show that the α/e ratio of rhenium is lower than the one of tungsten.