Degradation of solar collector coatings: model calculations for nickel-pigmented aluminum oxide

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Abstract
We introduce a model of the optical properties of nickel pigmented aluminium oxide selective solar absorbers that can explain the degradation at elevated temperatures. The properties of the Ni-A1203 cermet layer are described by the Bruggeman effective medium theory. On top of this layer we assume one homogeneous and one graded aluminium oxide layer. Our optical model can qualitatively explain the change in opticai properties of the coatings upon heat treatments in air. The degradation mechanism appears to be oxidation of the nickel particles throughout the cermet layer. We also propose a novel theoretical model that can be used to account for optical degradation of solar collector coatings. The degradation is described in terms of an activation energy and a kinetic exponent. These quantities are determined for the Ni-A1203 coatings and are compared to data on the oxidation kinetics of nickel.

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