Microstructural stability of coatings with high solar absorptance for ceramic particles during thermocyclic exposure for concentrated solar thermal applications

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.