Effects of Gd2O3 and Yb2O3 on the microstructure and performances of O'-Sialon/Si3N4 ceramics for concentrated solar power

O'-Sialon/Si₃N₄ composite ceramics for solar absorber were prepared in situ through pressureless sintering from Si₃N₄ and low pure Al₂O₃ with different rare-earth oxides (i.e., Yb₂O₃ and Gd₂O₃). This study investigates the effects of Yb₂O₃ and Gd₂O₃ on phase composition, microstructure, densification, oxidation resistance and solar absorptance. The results revealed that Yb₂O₃ and Gd₂O₃, which were applied as flux materials, significantly reduced the sintering temperature and improved the densification, oxidation resistance of the composite ceramics. Moreover, the introduction of the two sintering aids promoted the formation of O'-Sialon through the liquid-phase sintering mechanism. The samples doped with Gd₂O₃ exhibited more O'-Sialon content, lower porosity, and better oxidation resistance compared with those doped with Yb₂O₃. Especially, sample A2 (6 wt% Gd₂O₃ additional) sintered at 1600 °C exhibited the best comprehensive properties for 10.10% water absorption, 23.29% porosity, 105.57 MPa bending strength, 75.16% solar absorption. Dense oxide layers were generated on sample surfaces after oxidation at 1300 °C, which protected the ceramics samples from further oxidation. However, the two additives had characteristic reflection peaks in the ultraviolet–visible region (300–400 nm) and were also blamed for high reflectivity in the near–infrared region, which resulted in the decrease in absorption.