Ultralow-Cr2O3-doped CuAlO2 with high infrared emissivity and trace leachable Cr(VI) for energy-saving

Abstract

Cr₂O₃ doping has been reported to improve the high-temperature phase stability of CuAlO₂ up to 1600 °C, but the possible formation of hexavalent chromium species remains a concern. In this study, an ultralow-Cr-input strategy was proposed by regulating the Cr₂O₃ addition from 0 to 0.1 mol, and the formation, phase transformation and microstructure evolution of CuAlO₂ phase were discussed. The results show that only a small amount of CuAlO₂ was present at a Cr₂O₃ doping content of 0.01 mol. When the Cr₂O₃ content reached 0.025 mol, CuAlO₂ became the dominant phase, and further increasing the Cr₂O₃ content to 0.05 and 0.1 mol retained the same phase composition. Density functional theory calculations of the Cr-substitution formation energies were used as qualitative indicators. In addition, Cr₂O₃ doping induced lattice vibrational absorption and free-carrier transition absorption, thereby enhancing the infrared emissivity. The sample doped with 0.025 mol Cr₂O₃ exhibited the best overall infrared radiation performance, with an emissivity of 0.887 in the near-infrared region and 0.841 in the mid-infrared region at room temperature. At 800, 1000, and 1200 °C, the near- and mid-band emissivity remained above 0.7, demonstrating strong broadband infrared radiation characteristics. After thermal cycling 20/40 h at 1600 °C, CuAlO₂ remained the dominant phase and high emissivity was retained. The average leachable Cr(VI) concentrations were 0.8, 1.1, 1.3, and 1.8 ppm for the samples doped with 0.01, 0.025, 0.05 mol and 0.1 mol, all values were below the EU standard.