Microstructural stability of coatings with high solar absorptance for ceramic particles during thermocyclic exposure for concentrated solar thermal applications
C. Oskay, M. Bik, C. Grimme, E. White, S. Auler, M. Kerbstadt, G. Alkan, P. Mechnich, G. San Vicente, A. Morales, F. Sutter, D. Benitez, A.C. González Alves, N. Benameur, S. Merlin, M. Galetz
Solar Energy 301 (2025), 113961, DOI: 10.1016/j.solener.2025.113961
Hemispherical reflectance spectra of (a) DLR, (b) CIE and (c) DFI coated particles in the as-deposited or heat-treated condition and after thermocyclic exposure at 1000 °C; (d) solar-weighted absorptance of investigated coatings as a function of thermocyclic exposure duration (error bars showing the measurement uncertainty originating from the repeatability of individual measurements (0.2 % in [42]) and 0.5 % from the calibration standard [41]). Please note the varying y-axis in the respective hemispherical spectra for coatings.
Cross-sectional BSE images of (a) DLR, (b) CIE, and (c) DFI coatings deposited on substrate particles after thermocyclic exposure (3 h cycles) at 1000 °C for 3000 h. Yellow circles mark representative spots selected for WDX compositional analysis (refer to Table 4). The regions used for co-localized Raman confocal imaging (dashed yellow frame) and Raman spectroscopy (red dashed frame, embedded within the yellow frame in (b) and (c)) are also indicated. Corresponding EPMA elemental distribution maps are shown below each BSE image.
Raman confocal imaging of the DFI coating after thermocyclic exposure at 1000 °C for 3000 h.Reprinted from Solar Energy witrh permission from Elsevier according to the Creative Commons lincense.
Spinel-based coatings were deposited on Fe2O3-rich ceramic particles to investigate their microstructural stability and optical properties during thermocyclic oxidation at 1000 °C for up to 3000 h. The solar absorptance over time remained stable at a very high level (around 97 %) for the Cu-Cr and Cu-Mn-Co spinel coatings, whereas the Cu-Mn-Fe spinel coating showed a significant increase in its solar absorptance during early stages of exposure and thereafter maintained a stable behavior around 95 %. The microstructural investigations, coupling EPMA analysis with phase mapping via Raman spectroscopy, revealed the formation of complex microstructures consisting of different spinel phases for the coatings due to high temperature interdiffusion between the spinels and the substrate, respectively. Such exchange reactions continued during thermocyclic exposure, but they did not affect the highly promising long-term solar absorptance of the coatings for particle-based CST applications.
