In the optimization of multilayer stacks for various optical filtering purposes not only the thicknesses of the thin films are to be optimized, but also the sequence of materials. Materials with very different optical properties, such as metals and dielectrics, may be combined. A genetic algorithm is introduced to search for the optimal sequence of materials along with their optical thicknesses. This procedure is applied to a heat mirror in combination with a blackbody absorber for thermal solar energy applications at elevated temperatures (250 °C). The heat mirror is based on silver films with antireflective dielectric layers. Seven dielectrics have been considered. For a five-layer stack the sequence (TiO(2)/Ag/TiO(2)/Ag/Y(2)O(3)) is found to be optimal.

Articles you may be interested inInvestigation of temperature dependent dielectric constant of a sputtered TiN thin film by spectroscopic ellipsometry

Si02 -TiNO-Cu absorbers were prepared with activated reactive evaporation (ARE). The deposition parameters for the ARE process were adjusted according to the results of the numerical optimizations by a genetic algorithm. We present spectral reflectance, calorimetric and grazing mcidence X-ray reflection (GXR) measurements. Best coatings for applications as selective absorber in the range of T = 100 . . . 200°C exhibit a solar absorptance of 0.94 and a near normal emittance of 0.044 at 100 °C. This emittance is correlated with a hemispherical emittance of 0.061 obtained from calorimetric measurements at 200 °C.First results on lifetime studies show that the coatings are thermally stable under vacuum up to 400 °C. The Si02 film passivates the absorber, a substantial slow down of degradation in dry air is observed. Our tests demonstrate that the coating will withstand break down in cooling fluid and vacuum if mounted in an evacuated collector. O-8194-1266-X/93/$6.OO SPIE Vol. 2017 / 345 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/15/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx SPIE Vol. 2017/3.55 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/15/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

Reliability and structural properties of selective TiNO-Cu tandem absorbers for solar applications at high temperatures were investigated. A tempering process which improves thermal emittance of the coatings was observed. Degradation is monitored by directional total thermal emittance. Four steps of degradation have been separated and confirmed by XRD, EDAX, REM and optical spectroscopy. Coatings show crystalline phases and transitions to amorphous due to degradation and tempering. They are susceptible to oxidation and to reactions with the substrate.

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