Optical properties and thermal stability of Cu spinel oxide nanoparticle solar absorber coatings

Optimal coatings on receivers for concentrated solar power plants (CSP) not only need to have high solar absorptance, but should also possess superior stability in air at elevated temperatures. For next-generation CSP plants, the surface temperature of the receivers is expected to exceed 750 degrees C. In this work, we systematically studied optical properties and long term thermal stability of solar absorbing coatings (SACs) made from various Cu(II) containing spinel oxide nanoparticles, including CuCr2O4, Cu0.5Cr1.1Mn1.4O4, CuFeMnO4, and compared these properties to those of the state-of-the-art Pyromark 2500 coating. The solar absorptance of each sample was measured after isothermal annealing at 800 degrees C in air for durations of 100, 300, 1000, and 2000 h. We found that porous Cu0.5Cr1.1Mn1.4O4 had the highest solar absorptance at 97.1% before the thermal annealing and remained the highest throughout thermal testing, remained at 97.2% after 2000 h, whereas Pyromark 2500 exhibited considerable degradation in solar absorptance, from 96.4% to 94.6%. We analyzed the chemical composition, microstructures, and particle sizes of all the samples before and after the thermal annealing and discussed their implications on optical properties and thermal stability.