Investigation of thermal and hydrogen effects on emissivity of refractory metals and carbides

Direct thermal-to-electric power-conversion systems operate at high temperatures to ensure high power density and proper heat transfer in such systems, critical to their stability. Radiation in this temperature regime is a main means of heat transfer and the control of the surface characteristics of radiative components is thus an important issue. High-temperature normal spectral emissivity of various high-temperature components was investigated. Materials examined include tungsten, a dispersion strengthened tungsten (WHfC), a chemical vapor deposition (CVD) coating (CVD-W) and five types of carbides (WC, TaC, NbC, ZrC and HfC). Spectral emissivities were measured using single- and dual-wavelength radiation thermometries. Emissivity of tungsten was found to be 0.5 and remained constant even after vacuum annealing up to 2723 K. The five carbides were exposed to high temperatures, both in vacuum and in hydrogen. Results show the process to be thermally activated and hydrogen to have little effect on the ceramics examined. Tungsten carbide was the least stable and it transformed into W2C upon annealing whereas NbC decomposed after similar exposure. Hafnium carbide and ZrC were found to be the most stable and possessed the highest emissivity (0.9) among the carbides investigated. The experimental observation was examined in terms of thermodynamics of the materials. The implications of emissivity on the thermal characteristics of a typical space nuclear power system were examined.