Abstract:

Opaque thermal barrier materials play a pivotal role in thermal radiation shielding of turbine blades, since the intensity of thermal radiation rapidly increases with the increase of operating temperature of gas turbines and has become a new and major concern for the durability of metallic blades. The conventional thermal barrier coating (TBC) materials such as YSZ and Gd₂Zr₂O₇, however, are almost translucent to thermal radiation and are unable to protect the blades at such harsh environment. Although searching for new thermal barrier materials is significant, it is still a challenge to make the current TBC materials opaque without significantly modifying the composition or other physical properties. To cope with this challenge, GdMnO₃ is incorporated as an absorptive second phase into Gd₂Zr₂O₇ in this work, which is originally translucent (absorption coefficient 10¹-10² m^-1) in the near-infrared wavelengths. Intriguingly, with less than 5 wt.% GdMnO₃, the Gd₂Zr₂O₇/GdMnO₃ becomes opaque to thermal radiation and successfully refrains the rise of thermal conductivity at high temperatures. Meanwhile, the lattice thermal conductivity and mechanical properties are almost unchanged. The small polaron mechanism is confirmed for GdMnO₃, leading to a high absorption coefficient (> 10⁶ m^-1) for near-infrared radiation. To understand the underling mechanism, a theoretical model is built to estimate the absorption coefficient of the Gd₂Zr₂O₇/GdMnO₃ composites (> 10⁴ m^-1). This paper proposes a powerful strategy to design thermal-radiation-shielding TBCs through incorporating minor second-phase particles with high-absorption mechanism, such as polaron excitation.

Key words: Thermal barrier coatings, Thermal radiation, Radiative heat transfer, Emissivity, Small polaron