Calorimetric measurements of the total hemispherical emittance of selective surfaces at high temperatures

An apparatus is presented for measuring the hemispherical emittance (epsilon) in a temperature range of 100 - 400 degree(s)C. It is designed especially for samples with low values of (epsilon) between 0.015 and 0.15. The error margin lies around 6% of the measured value. A polished stainless steel sample (PTB, Braunschweig, Germany) serves as reference reproducing the PTB-results within 2%. Measurements of solar selective absorbers, a TiNxOy-Cu tandem absorber, a multilayer cermet absorber, and an Al2O3-Ni-Al absorber have been carried out. The values of (epsilon) range between 0.02 and 0.15 for 100 degree(s)C and between 0.035 and 0.3 for 400 degree(s)C. TiNxOy-Cu achieves the lowest values in both cases. The effects of temperature on the optical constants in the near IR of TiNxOy-Cu tandem layers are discussed. The results of measurements indicate a shift from dielectric to metallic behavior with rising temperature.

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