Optical characterization of alumina darkened with boron carbide inclusions for solar energy applications

Dark colored Al₂O₃-B₄C composites with 5–50 vol% B₄C additions were consolidated by hot pressing. The optical properties were investigated to evaluate new potential applications as solar receivers in concentrating solar power. The thermal emittance was estimated from 400 to 1700 K, as well as the opto-thermal efficiency when these materials are used as solar radiation absorbers in concentrating solar plants, for three exemplifying values of solar concentration ratio C, representative of different solar plants architectures. Efficiency values for the composites was increased by four times with respect to conventional white Al₂O₃ (for instance, at 800 K and C = 3000 we found 0.88 versus 0.20, at 1300 K 0.83 vs. 0.19) and remarkably, were also higher that the most advanced solar absorber currently in use in solar plants, namely SiC (efficiency 0.84 or 0.81 in the mentioned cases). Complementarily, the effect of B₄C on compaction behavior, microstructure and mechanical properties of the composites was also investigated. The addition of 50 vol% B₄C significantly increased hardness and toughness compared to pure alumina, which could be of interest for structural applications, among others.