The time evolution of three refractory metals molybdenum (Mo), tantalum (Ta), and tungsten (W) at 1100°C under vacuum environment were studied. The roughness, surface morphology, surface composition, emissivity in the range of 1-6 μm, and the proportion of 1-2 μm emissivity were observed under different polishing conditions. During the long-term annealing process, molybdenum with better surface polishing is the first to oxidize, and the surface MoO 2 increases with time. The formation of oxides makes the average emissivity increase significantly, and the proportion of 1-2 μm emissivity is below 30%. For polished tantalum, the surface undergoes carburization upon annealing, forming Ta 2 C, which also results in an increase in the average emissivity. The proportion of emissivity within the 1–2 μm wavelength range is below 40%. For the annealing of different polished tungsten, the average emissivity decreases slightly, but the proportion in the 1-2 μm portion increases to 40%. After 375 hours, there is no oxidation or carbonization on the surface. These results indicate that tungsten maintains excellent stability at long-term high-temperature, and the enhanced proportion of emissivity in 1-2 μm portion make it very suitable for use as emitters in the radioisotope thermophotovoltaic system.
Lei et al. (Sun,) studied this question.