Thermal and Optical Characterization of Rare Earth Compounds for Protective Barrier Coatings

Rare earth compounds, such as oxides and zirconates, exhibit promising thermal and optical properties essential for next-generation protective barrier coatings in ultrahigh temperature (1500K+) and hypersonic applications. These materials are characterized by low thermal conductivity, excellent oxidation and corrosion resistance, and high thermomechanical and thermochemical stability. In these phonon-dominated systems, the behavior of vibrational heat carriers largely governs thermal and radiative transport, though these properties have rarely been experimentally studied at operating temperatures.

In this work, we conduct a series of thermal and optical studies using pump-probe thermoreflectance and spectroscopic ellipsometry to investigate the thermal and optical properties of rare earth compounds at high temperatures. Utilizing an infrared variable-angle spectroscopic ellipsometer (IR-VASE), we extract dielectric functions for single- and multi-component rare earth sesquioxides, zirconates, and tantalates. By analyzing these optical properties, we estimate the lifetimes of optical phonons and observe modal redshifting to identify scattering mechanisms related to multi-cation doping. These trends are then correlated with the thermal conductivity of these materials, as measured by time-domain thermoreflectance (TDTR). Furthermore, using the extracted dielectric functions, we calculate temperature-dependent trends in the spectral emissivity of single- and multi-cation rare earth compounds at temperatures up to 1000°C, providing insight into the temperature-driven phonon dynamics. Understanding these trends is critical for unraveling key design considerations for next-generation thermal barrier coatings.