NORMAL SPECTRAL EMITTANCE OF CRYSTALLINE TRANSITION-METAL CARBIDES

We report on experiments to determine the normal spectral emittance of crystalline forms of five transition metal carbides: hafnium carbide, niobium carbide, tantalum carbide, titanium carbide, and zirconium carbide. These emittance measurements were taken at the commonly used pyrometer wavelength of 0.65-mu-m. The specimens were cylindrical with 0.10-cm-diameter blackbody holes drilled axially to a depth of 1.0 cm. The crystalline specimens were prepared from sintered stock by floating zone arc refinement and then centerless ground to the desired diameter. Measurements were made in vacuum in the temperature range 1200 < T < 2400 K. The emittance of HfC tends to increase slightly with temperature (epsilon-HfC = 0.4322 + 1.065 x 10(-4) T) while those of NbC, TaC, TiC, and ZrC decrease (epsilon-NbC = 0.4913 - 6.6 x 10(-5) T, epsilon-TaC = 0.4662 - 5.084 x 10(-5) T, epsilon-TiC = 0.8192 - 1.66 x 10(-4) T, and epsilon-ZrC = 0.715 - 1.174 x 10(-4) T). Bulk stoichiometries of the samples were C/Hf = 0.86 +/- 0.03, C/Nb = 0.833 +/- 0.014, C/Ta = 0.79 +/- 0.05, C/Ti = 0.955 +/- 0.005, and C/Zr = 0.917 +/- 0.015. As a check on the accuracy of the method and procedure, normal spectral emittance measurements were taken on molybdenum yielding values consistent with those accepted. The normal spectral emittance was determined to be isotropic with respect to crystal orientation within the uncertainty of our measurements.