Porous Si₃N₄ ceramics are promising high-temperature wave transparent materials for use as radomes or antenna windows in hypersonic aircraft. However, a trade-off between the dielectric and thermomechanical properties is still challenging. Therefore, tailoring the microstructure and properties of porous Si₃N₄ is highly important. In this work, porous Si₃N₄ ceramics with uniform and fine structures were obtained via dual-solvent templating combined with the freeze-casting method. The as-prepared porous Si₃N₄ ceramic, with 56% porosity, possesses high mechanical properties, with flexural strength and compressive strength values of 95±14.8 and 132±4.5 MPa, respectively. The uniform spherical pore structure improved the mechanical properties, and the rod-shaped Si₃N₄ grains facilitated crack deflection. The decreased pore size effectively blocks phonon transport, leading to a low thermal conductivity of only 4.2 W/(K·m). Moreover, the porous Si₃N₄ ceramic maintains a small dielectric constant of 3.3, and the dielectric loss is stable between 1.0×10⁻³–4.0×10⁻³, which guarantees its potential application in high-temperature wave-transparent components. These results significantly advanced the development of high-performance wave-transparent materials used in hypersonic aircraft.

ZrP₂O₇ is a promising wave-transparent material due to its low dielectric constant and low dielectric loss, but its inherent phase transition characteristic at approximately 300 °C limits its high-temperature application. Therefore, suppressing the phase transition is necessary for ZrP₂O₇ to serve in extremely harsh environments. In this work, introducing Ti and Hf into ZrP₂O₇ causes significant lattice distortion and an increase in entropy, both of which synergistically limit the crystal structure transformation. In addition, enhanced phonon scattering by mismatch of atomic mass and local distortion leads to a reduction in the thermal conductivity. Lattice distortions also cause changes in both bond length and tilting angle, so that (Ti_1/3Zr_1/3Hf_1/3)P₂O₇ does not undergo sudden expansion as does ZrP₂O₇. (Ti_1/3Zr_1/3Hf_1/3)P₂O₇ maintains excellent dielectric properties, which highlights it as a promising high-temperature wave-transparent material.

Low thermal conductivity, compatible thermal expansion coefficient, and good calcium- magnesium-aluminosilicate (CMAS) corrosion resistance are critical requirements of environmental barrier coatings for silicon-based ceramics. Rare earth silicates have been recognized as one of the most promising environmental barrier coating candidates for good water vapor corrosion resistance. However, the relatively high thermal conductivity and high thermal expansion coefficient limit the practical application. Inspired by the high entropy effect, a novel rare earth monosilicate solid solution (Ho_0.25Lu_0.25Yb_0.25Eu_0.25)₂SiO₅ was designed to improve the overall performance. The as-synthesized (Ho_0.25Lu_0.25Yb_0.25Eu_0.25)₂SiO₅ shows very low thermal conductivity (1.07 W·m^-1·K^-1 at 600 ℃). Point defects including mass mismatch and oxygen vacancies mainly contribute to the good thermal insulation properties. The thermal expansion coefficient of (Ho_0.25Lu_0.25Yb_0.25Eu_0.25)₂SiO₅ can be decreased to (4.0-5.9)×10^-6 K^-1 due to severe lattice distortion and chemical bonding variation, which matches well with that of SiC ((4.5-5.5)×10^-6 K^-1). In addition, (Ho_0.25Lu_0.25Yb_0.25Eu_0.25)₂SiO₅ presents good resistance to CMAS corrosion. The improved performance of (Ho_0.25Lu_0.25Yb_0.25Eu_0.25)₂SiO₅ highlights it as a promising environmental barrier coating candidate.