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 for measuring hemispherical total emissivity of electrica lly conducting materials at high temperatures (above 1500 K) using a f eedback-controlled pulse-heating technique has been developed. The tec hnique is based on rapid resistive self-heating of a solid cylindrical specimen in vacuum up to a preset high temperature in a short time (a bout 200 ms) and then keeping the specimen at that temperature under s teady state conditions for a brief period (about 500 ms) before switch ing off the current through the specimen. The specimen is maintained a t constant temperature with a feedback control system which controls t he current through the specimen. The computer-controlled feedback syst em operates a solid-state switch (composed of field-effect transistors ). The sensing signal for the feedback is provided by a high-speed opt ical pyrometer. Hemispherical total emissivity is determined at the te mperature plateau from the data on current through the specimen, the v oltage drop across the middle portion of the specimen, and the specime n temperature using the steady-state heat balance equation based on th e Stefan-Boltzmann law. The true temperature of the specimen is determ ined from the measured radiance temperature and the normal spectral em issivity; the latter is obtained from laser polarimetric measurements. The experimental quantities are measured and recorded every 0.2 ms wi th a 12-bit digital oscilloscope. To demonstrate the feasibility of th e technique, experiments were conducted on a tantalum specimen in the temperature range 2000 to 2800 K. The results on hemispherical total e missivity are presented and are compared with the data given in the li terature.