Development of a non-selective solar absorber coating material based on cristobalite and amorphous silica phases for solar thermal applications

Despite advancements in solar selective coatings, wet-deposited non-selective coatings remain the most used in high-temperature (≥400 $°C$) solar thermal systems; however, the available options are limited and have been relatively underexplored in previous research studies. This study reports the synthesis and performance analysis of a non-selective solar absorber coating based on cristobalite and amorphous silica, designed for applications exceeding 400 $°C$. The coatings were fabricated via a scalable spray-coating method, combining a binder and absorber pigment derived from colloidal nano-silica particles and methyltrimethoxysilane, and were deposited onto stainless steel substrates. Thermal characterization confirmed the coating's stability up to 550 $°C\text{.}$ A high solar absorptivity of 0.94 was reported, with a retained value of approximately 0.90 after 72 h of continuous annealing at 400 $°C$, demonstrating strong thermal aging resistance. Under 2000 W/m² irradiance, the coated surface achieved a 40.1% thermal gain compared to bare stainless steel. These results demonstrate the viability of utilizing abundant, low-cost, and environmentally benign materials to develop thermally stable, non-selective solar absorber coatings with strong optical performance and long-term durability for high-temperature applications. This study further introduces the novel application of cristobalite and amorphous silica—materials rarely explored in solar thermal coatings—revealing their untapped potential as thermally robust and cost-effective alternatives to conventional absorber materials.