Normal spectral emissivity of near-stoichiometric single-crystal nickel oxide in the red and in the green

The normal spectral emissivity of an indirectly heated wafer of single-crystal NiO has been measured for λ = 0.645 μm and λ = 0.546 μm. The NiO was characterized by a room-temperature resistivity of 7 × 1010 Ω cm, and the measurements were made using a thermal equivalent of an electrical potentiometer consisting of an incandescent NiO surface as the ``unknown'', a tantalum blackbody furnace as the ``standard'', and a thermoprobe as a ``comparison device'' with which the surface temperatures of the ``unknown'' and the ``standard'' may be compared. This arrangement is useful for the determination of emissivities of substances which are not readily fashioned into blackbodies. It is found that NiO is a very good emitter having an emissivity which ranges from about 0.94 to 0.90 as the temperature is varied from 1100 to 1500°K; and that the emissivity in the red is almost identical to the emissivity in the green. These results permit the speculation that NiO may well radiate as an almost black graybody over the red-green range within the temperature interval stated. The effects of the temperature gradients normal to the surface, which occur as a consequence of the indirect method of heating the wafer, have been examined in detail, but the influence of these nonisothermal conditions was found to be negligible when compared to the limits of error except around the highest temperatures used for the measurements in the green. The intrinsic isothermal emissivities of NiO in the red and in the green were found to be ε0.645 μm = − 691 (5) × 10−8 T + 1.001 (7) and ε0.546 μm = − 959 (2) × 10−8T + 1.033 (1), respectively.