Improved Performance of Ceramic Solar Absorber Particles Coated with Black Oxide Pigment Deposited by Resonant Acoustic Mixing and Reaction Sintering

Spherical particles based on bauxite-type raw materials, commonly referred to as proppants, are state-of-the-art for particle receivers of concentrated solar power plants. Particles are heated in a fluidized reactor by focused sunlight and are transported to a heat-exchanger or a storage tank. Therefore, key properties for absorber particles are high solar absorptance and mechanical endurance. Due to their relatively poor content of color-giving transition-metal cations, bauxite-derived proppants show limited solar absorptance, which is even deteriorating by long-term heat exposure. A deep-black Cu, Mn, Fe- pigment with a spinel structure was employed to coat standard proppants in order to improve solar absorptance. The coating process was performed by high-energy, high-speed mixing of proppants and small quantities of spinel powders in a resonant acoustic mixer. A continuous powder coating is achieved by electrostatic attraction between the proppant surface and the coating particles. Consolidation and strong attachment of the coating is achieved by the subsequent sintering beyond the spinel melting temperature. Chemical reaction and bonding between spinel coating and proppant lead to the incorporation of Al, Mg and Ti into the spinel structure. Coated bauxite proppants exhibit a significantly improved, long-term stable solar absorption accompanied by a promising abrasion resistance. The presented coating methodology is considered to be scalable to industrial production.