A Promising Radiation Thermal Protection Coating Based on Lamellar Porous Ca-Cr co-Doped Y₃NbO₇ Ceramic

Dissipation of heat efficiently from a hot object via radiation while minimizing the inward heat conduction is the key requirement of radiation thermal protection. In this study, a Ca-Cr co-doped Y₃NbO₇ coating with lamellar porous structure is fabricated, which shows an ultra-low thermal conductivity (<0.7 W m⁻¹ K⁻¹) and near-unity emissivity (>0.9) across a broad wavelength range of ≈1–24 µm. This record high emissivity to thermal conductivity ratio (≈1.3) is experimentally and theoretically revealed from a multi-scale perspective. The diffusoin-mediated thermal conduction feature of niobates combined with lamellar porous structure of the coating reduces its thermal conductivity to an impressive 0.5 W m⁻¹ K⁻¹ at 25 °C, surpassing the theoretical amorphous limitation of 0.72 W m⁻¹ K⁻¹. Experiments and FDTD calculation results demonstrate that the intrinsic emissivity dips at shallow extinction wavelengths (1 and 8 µm) and strong phonon-polariton resonances wavelengths (>13 µm) can be effectively compensated by the multiple scattering/absorption and gradual modulation of conical shape/effective refractive index induced by surface micro-protrusion structures, respectively. Furthermore, the coating exhibits robust mechanical and thermal stability with a high bonding strength (18.3 MPa) and thermal expansion coefficient (≈11 × 10⁻⁶ K⁻¹ at 1200 °C) comparable to YSZ, showing great potential in the radiation thermal protection field.