Synthesis and characterization of absorption-enhanced alumina solid particle materials for direct irradiation solar-thermal conversion

Based on solar-thermal application more than 1000 °C, a novel combination of sol-gel synthesis and stepwise calcination to prepare metal-ion doped alumina composite particles as heat transfer media were proposed, enhancing solar energy absorption and lowering preparation temperature. Investigations focused on the elemental distribution, phase composition, oxidation states, optical properties and thermal stability of the composites, which were doped with Cr, Co, Cu, Fe, and Mn metal ions in a 100:5 molar ratio. The first-principles calculations elucidate the mechanism behind the impact of transition metal ions on alumina's optical properties. Results show that introducing a small quantity of metal atoms results in new impurity energy levels and electron states around EF=0. Specifically, the band gap energy of Cu- and Mn-Al2O3 decreases from 6.017 eV to 4.464 eV and 2.170 eV, respectively. Correspondingly, the absorptivity of these composite particles increases from around 20% to 76.6%. And Tauc analysis indicates a decrease in the optical band gap of the particle materials from 5.37 eV to a range of 2.88-4.90 eV. Additionally, doping with Mn, Fe, and Cu accelerates the in-situ formation of α-Al2O3 crystals, with a phase transition temperature below 1000 °C and a crystallinity exceeding 93%. Thermal stability tests
show that the particles are highly stable, exhibiting mere 0.49% mass change between 30 °C and 1200 °C. The stepwise inert annealing strategy and metal-iron doping positively influence the optical performance of alumina, high-temperature stability and α-Al2O3 phase transition temperature. These attributes make a favorable choice for solar-thermal applications in direct irradiation.