Development of nano silica based coating materials for solar absorber applications

While solar selective coatings (SSCs) offer improved performance for solar thermal applications, their deployment in high-temperature (≥400 ℃) environments are significantly constrained by the insufficient thermal durability of existing materials, coupled with the high costs and intricate fabrication techniques required for their production. Consequently, despite recent advancements of SSCs, non-selective coatings remain the predominant choice for high-temperature solar thermal applications. Nevertheless, only a limited subset of commercially available wet-coated non-selective paints currently satisfy the rigorous requirements imposed by such hightemperature environments. This situation highlights the urgent need for ongoing research aimed at developing solar absorber coating materials that are based on abundant and cost-effective raw materials, employ simple and scalable fabrication methods, and adhere to principles of environmental sustainability. Addressing these criteria is essential to facilitate the widespread implementation and long-term performance of solar thermal technologies operating at elevated temperatures. This study experimentally investigates the synthesis and performance of a novel non-selective solar absorber coating material. The coating was developed using silica and silicon-based precursors through a straightforward, low-complexity process. Thecoating was applied to AISI 304 stainless steel substrates using a spray-coating technique. Colloidal nano-silica particles and methyltrimethoxysilane (MTMS) serveas the raw materials for both the binder and the absorber phases. A spray-coating technique was employed to deposit the coating onto AISI 304 stainless steel substrates. For the optimized layer configuration selected in this study, a maximum solar absorptivity of 0.94 was achieved. However, it was found that coatings with highly dense surfaces could attain solar absorptivity values as high as ~0.96. Structural and chemical characteristics of the coating were analyzed using Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), and X-ray Diffraction (XRD). Cristobalite and amorphous silica phases were identified in the coating, representing a novel observation not previously documented in solar absorber coating research. Additionally, the thermal stability and thermal degradation behavior of the coating were evaluated using thermogravimetric analysis (TGA). The materialdemonstrated stability up to 550 ℃ without loss of adhesion and exhibited only a 2.2% reduction in solar absorptance. Thermal aging tests conducted at 400 ℃, for up to 72 hours yielded a performance degradation value of 0.026, which is approximately half of the acceptable limit. Furthermore, under an irradiance level of 2000 W/m2, the coated surface exhibited a 40.1% increase in steady-state temperature compared to theuncoated substrate. These findings, specifically the high operating temperature range and favorable thermal aging behavior suggest that the developed material is a promising candidate for high-temperature solar thermal applications.