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Temperature-Induced Ageing Mechanisms and Long-Term Stability of Solar Thermal Absorber Coatings

Research output: Book/ReportDoctoral thesisMonograph

Details

Original languageEnglish
Place of PublicationTampere
PublisherTampere University of Technology
Number of pages194
ISBN (Electronic)978-952-15-3329-7
ISBN (Print)978-952-15-3313-6
Publication statusPublished - 15 Aug 2014
Publication typeG4 Doctoral dissertation (monograph)

Publication series

NameTampere University of Techology. Publication
PublisherTampere University of Technology
Volume1222
ISSN (Print)1459-2045

Abstract

The ageing mechanisms and long-term stability of different solar thermal absorber coatings were investigated at elevated temperatures of 200-500 °C. The absorbers were aged by means of thermal accelerated ageing studies and short-period heat treatments up to 500 °C for two hours. The solar absorbers studied were various sputtered, evaporated, and electroplated industrial absorber surfaces on copper, nickel-coated copper, aluminium, anodized aluminium, and stainless steel substrates. The absorber coatings were based on chromium oxide, chromium oxy-nitride, titanium oxy- nitride, or titanium aluminium silicon oxy-nitride coating layers. The anti- reflection coatings were tin oxide or silicon oxide. The self-deposited, experimental absorber coatings were magnetron sputtered chromium oxide based or titanium aluminium silicon oxy-nitride coatings on copper with tantalum nitride or aluminium diffusion barriers between the substrate and the absorber coating. The ageing mechanisms and degradation of the absorbers were analysed by optical measurements (solar absorptance with a UV/Vis/NIR spectrophotometer and thermal emittance by FTIR spectrophotometry), microstructural analysis using a field-emission scanning electron microscope (FESEM) equipped with an energy dispersive X-ray spectrometer (EDS) and transmission electron microscope (TEM) with selected area electron diffraction (SAED), and composition by time-of-flight elastic recoil detection analysis (TOF-ERDA) before and after the ageing studies. The results clearly demonstrated the effect of the coating microstructure on the long-term stability of the absorber at elevated temperatures. Absorbers can suffer from an increase in the oxygen/metal ratio in the absorber coatings, diffusion of copper substrate atoms into the coating or through the coating to the surface, formation of copper oxide islands on the surface of the coating, formation of voids in the substrate surface, and chemical and structural changes in the coating or substrate during thermal ageing. The relation between optical degradation and ageing mechanisms was studied using optical modelling and simulation with CODE Coating Designer. The microstructure of the absorber coatings had a significant effect on stability at elevated temperatures. The dense sputtered chromium oxy-nitride coating on copper was clearly more stable than a similar coating material with large columns and open porosity between the columns.

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