Hemispherical total emissivity of niobium, molybdenum, and tungsten at high temperatures using a combined transient and brief steady-state technique

The hemispherical total emissivity of three refractory metals, niobium, mol ybdenum, and tungsten, was measured with a new method using a combined tran sient and brief steady-state technique. The technique is based on rapid res istive self-heating of a solid cylindrical specimen in vacuum up to a prese t high temperature in a short time (about 200 ms) and then keeping the spec imen at that temperature under steady-stale conditions for a brief period ( about 500 ms) before switching off the current through the specimen. Hemisp herical total emissivity is determined at the temperature plateau from the data on current through the specimen, the voltage drop across the middle po rtion of the specimen, and the specimen temperature using the steady-stale heat balance equation based on the Stefan-Boltzmann law. Temperature of the specimen is determined from the measured surface radiance temperature and the normal spectral emissivity; the latter is obtained from laser polarimet ric measurements. Experimental results on the hemispherical total emissivit y of niobium (2000 to 2600 K), molybdenum (2000 to 2700 K), and tungsten (2 000 to 3400 K) are reported.