Effects of small polarons on the high-temperature thermal radiation shielding properties of La2(Zr0.7Ce0.3)2O7/LaMnO3 composite ceramics

Abstract

Rare-earth zirconate ceramic materials have a high melting point, high-temperature phase stability and good application prospects in thermal barrier coatings. However, owing to the continuous improvement in the working temperature, rare-earth zirconate ceramic materials are nearly transparent to thermal radiation at high temperatures, and an increase in their thermal conductivity considerably impairs their insulation performance. In this study, considering the poor shielding effects of rare-earth zirconate ceramic materials on thermal radiation at high temperatures, LaMnO₃ with small polarons was added to La₂(Zr_0.7Ce_0.3)₂O₇ as an absorptive second phase to prepare La₂(Zr_0.7Ce_0.3)₂O₇/LaMnO₃ composite ceramics. The crystal structure, microstructure and infrared emissivity of La₂(Zr_0.7Ce_0.3)₂O₇/LaMnO₃ were evaluated, and the effects of La₂(Zr_0.7Ce_0.3)₂O₇/LaMnO₃ on the thermal conductivity of high-temperature photons were studied. The La₂(Zr_0.7Ce_0.3)₂O₇/LaMnO₃ composite ceramics possessed a dual-phase structure comprising pyrochlore and perovskite, demonstrating uniform grain distribution and clear grain boundaries. At high temperatures, with a gradual increase in the second-phase (LaMnO₃) content, the average infrared emissivity of La₂(Zr_0.7Ce_0.3)₂O₇/LaMnO₃ in the wavelength range of 3–5 μm increased from 0.71 to 0.92, which reduced the impact of photon thermal conductivity on the overall thermal conductivity. To understand the high-absorption mechanism of the La₂(Zr_0.7Ce_0.3)₂O₇/LaMnO₃ composite ceramic materials, a LaMnO₃ small-polaron-hopping model was established. A linear relationship between temperature and conductivity of LaMnO₃ was established, which proved that LaMnO₃ exhibited the light absorption effect of small polarons and high absorption performance for near-infrared radiations at high temperatures.