25 November 1992 Effects of roughness on TiNxOy-Cu selective absorbers
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Abstract
Smooth TiNxOy-Cu absorbers show low hemispherical emittancies (epsilon) < 0.05 at 250 degree(s)C and poor absorptances of < 0.08, when choosing optimal thickness and composition. Thin TiNxOy films were deposited on rough copper substrates in order to increase the absorptance. Specular and diffuse reflectance measurements of these coatings are compared with measurements on smooth films. An increase of 0.05 ... 0.1 in absorptance can be achieved. With moderate roughnesses (delta) 01 approximately equals 50 ... 60 nm, the absorptance can be increased to values above 0.85 without increasing the emittance significantly. Thin films of approximately equals 30 nm and moderate roughness (delta) 01 equals 55 nm should be favored against thick films because the emittance is lower at high temperature. A theoretical model is presented, explaining the experiments for not too rough interfaces. This model is suitable to be used for optimizations, provided the temperature dependence of optical constants is taken into account. Coatings on very rough substrates (delta) > 150 nm can achieve absorptances of more than 0.9 but accompanied with emittancies greater than 0.1. The interference peak at short wavelengths, typical for TiNxOy-Cu, cannot be suppressed completely; (alpha) is limited by this interference peak. For high temperature applications roughnesses greater than 60 to 80 nm are of no use.
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Michel P. Lazarov, Michel P. Lazarov, A. Brunotte, A. Brunotte, Thomas Eisenhammer, Thomas Eisenhammer, R. Sizmann, R. Sizmann, Wolfgang Graf, Wolfgang Graf, Volker Wittwer, Volker Wittwer, } "Effects of roughness on TiNxOy-Cu selective absorbers", Proc. SPIE 1727, Optical Materials Technology for Energy Efficiency and Solar Energy Conversion XI: Selective Materials, Concentrators and Reflectors, Transparent Insulation and Superwindows, (25 November 1992); doi: 10.1117/12.130499; https://doi.org/10.1117/12.130499
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