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

摘要

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/ε ratio, which is the ratio of the solar absorptivity a to the total emissivity s) as a function of the RMS roughness. This ratio is decreasing with increasing RMS roughness rapidly up to around 1 urn and then tends to sta bilize for higher roughness. It suggests that increasing the roughness, starting from a flat surface, will pro gressively 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/ε ratio, but these structures are not stable at high tem perature and disappear due to a natural high temperature smoothing. Finally our results also show that the α/ε ratio of rhenium is lower than the one of tungsten.