A combined transient and brief steady-state technique for measuring hemispherical total emissivity of electrical conductors at high temperatures: Application to tantalum

A new method Ik~r measuring hemispherical total emissivity of electrically conducting materials at high temperatures (above 1500 K) using a feedbackcontrolled pulse-heating techniqt, c has been developed. The technique is based on rapid resistive self heating of a solid cylindrical specimen in vacuum up to a preset high temperature in a sho]t time labout 200 ms) and then keeping the specimen at that temperature trader steady-state conditions for a brief period labour 500 nrsl bet\we switching oil the current through the specimen. The specimen is maintained at constant temperature with a feedback control system which controls the current through the specimen. The computer-controlled [~.edback system operates a solid-state switch (composed of lield-cffcct transistors). The sensing signal I\w the feedback is provided by a high-speed optical pyrometer. Hemispherical total emissivity is determined at the temperature plateau from the data on current through the specimen, the voltage drop across the middle portion of" the specimen, and the specimen temperature using the steady-state heat balance equation based on the Stefan Boltzmann law. The truc temperature of the specimen is determined from the measured radiance temperature and the normal spectral emissivity: the latter is obtained from laser polarimctric measurements. The experimental quantities are measured and recorded every 0.2 ms with a 12-bit digital oscilloscope. To demonstrate the [ʹcasibility of the technique, experiments were conducted on a tantalum specimen in the temperature range 2000 to 2800 K. The results on hemispherical total emissivity are preseInted and are compared with the data given in the literature.