Selective emitters are crucial as the key component determining the energy conversion efficiency of Radioisotope Thermophotovoltaic (RTPV) systems. Developing selective emitter materials with high selective emissivity, high spectral efficiency and excellent high-temperature stability can effectively improve the energy conversion efficiency and service life of RTPV systems. In order to adjust the selective emissivity and spectral efficiency, a series of rare earth tantalate selective emitters (Er(Ta_1-xNb_x)O₄ (0≤x≤0.2)) matching GaSb battery were prepared by high-temperature solid-state reaction and pressureless sintering method. The as-prepared Er(Ta_1-xNb_x)O₄ (0≤x≤0.2) ceramics exhibit high emissivity (49%-93%) in the selective band (1.40-1.60 μm), high spectral efficiency (59.46%-62.12%) and excellent high-temperature stability at 1400 ℃. On the one hand, doping Nb⁵⁺ into the B-site changes the crystal local structure symmetry around Er³⁺, which promotes the f-f transition of Er³⁺ and enhances the selective emission performance. On the other hand, doping Nb⁵⁺ ions into the B-site can alter the band gap and oxygen vacancy concentration to suppress non-selective emissivity. Increasing the selective emissivity and reducing the non-selective emissivity is beneficial for improving the spectral efficiency of selective emitters. Hence, the selective emissivity and spectral efficiency of Er(Ta_1-xNb_x)O₄ (0≤x≤0.2) can be effectively enhanced through compositional design, providing a new strategy for developing selective emitter materials for RTPV applications.

In response to the development of the concepts of “carbon neutrality” and “carbon peak”, it is critical to developing materials with high near-infrared (NIR) solar reflectivity and high emissivity in the atmospheric transparency window (ATW; 8–13 μm) to advance zero energy consumption radiative cooling technology. To regulate emission and reflection properties, a series of high-entropy rare earth stannate ceramics (HE-RE₂Sn₂O₇: (Y_0.2La_0.2Nd_0.2Eu_0.2Gd_0.2)₂Sn₂O₇, (Y_0.2La_0.2Sm_0.2Eu_0.2Lu_0.2)₂Sn₂O₇, and (Y_0.2La_0.2Gd_0.2Yb_0.2Lu_0.2)₂Sn₂O₇) with severe lattice distortion were prepared using a solid phase reaction followed by a pressureless sintering method for the first time. Lattice distortion is accomplished by introducing rare earth elements with different cation radii and mass. The as-synthesized HE-RE₂Sn₂O₇ ceramics possess high ATW emissivity (91.38%–95.41%), high NIR solar reflectivity (92.74%–97.62%), low thermal conductivity (1.080–1.619 W·m⁻¹·K⁻¹), and excellent chemical stability. On the one hand, the lattice distortion intensifies the asymmetry of the structural unit to cause a notable alteration in the electric dipole moment, ultimately enlarging the ATW emissivity. On the other hand, by selecting difficult excitation elements, HE-RE₂Sn₂O₇, which has a wide band gap (E_g), exhibits high NIR solar reflectivity. Hence, the multi-component design can effectively enhance radiative cooling ability of HE-RE₂Sn₂O₇ and provide a novel strategy for developing radiative cooling materials.