SrO(SrTiO₃)₂ (Hereinafter referred to as Sr₃Ti₂O₇) is a layered perovskite-like structural material, which can be considered as a SrTiO₃ superlattice interspersed with SrO layers. Compared to SrTiO₃, Sr₃Ti₂O₇ exhibits higher asymmetry and more significant dipole moment changes, thus providing a broader space for infrared radiation performance regulation. Also, the interface between SrO layer and SrTiO₃ layer enhances phonon scattering, significantly reducing the thermal conductivity of electron-doped Sr₃Ti₂O₇, compared to cubic SrTiO₃. For the high-temperature stability and environmental friendliness of Sr₃Ti₂O₇, there is a motivation to explore the relationship between its thermoelectric transport properties and infrared radiation properties to develop novel multifunctional materials. However, little researches on the intrinsic relationship between these two properties in Sr₃Ti₂O₇ have been reported. Exploring the relationship between thermoelectric and infrared radiation properties and further studying the combination of thermoelectric materials with infrared materials can enhance the conversion efficiency and functional applications of thermoelectric materials, driving technological development and innovation, and open new avenues for applications in environmental monitoring, energy management, and efficient detection across multiple fields.

SrCO₃, TiO₂, Nb₂O₃, and Gd₂O₃ were mixed as raw materials with an appropriate amount of alcohol according to a specific molar ratio. The mixed powder was ground in a ball mill for 1.5 h. Afterwards, the prepared raw powder in a mold was pressed by an electric press at 3.0 MPa for 2.0 min, and then demold. The pressed pellets were tightly wrapped with a plastic wrap and pressed by cold isostatic pressing at 39.5-44.5 MPa for 8.0 min. Afterwards, the sample wrapped in carbon paper was placed in an alumina crucible. The sample in carbon powder was heated in a muffle furnace at 1200 ℃ for 5.0 h. The sintered pellets were ground into a powder. The powder was pressed in an electric press at 5.0 MPa for 1.5 min, and then by cold isostatic pressing again at 39.5-44.5 MPa for 8.0 min. The pellets wrapped in carbon paper were buried in carbon powder and heated in the muffle furnace at 1500 ℃ for 10.0 h to obtain the final sample.

The XRD patterns of (Sr_0.8Gd_0.2)₃(Ti_1-xNb_x)₂O₇ (0.01≤x≤0.05) show that the samples are similar to the standard material of Sr₃Ti₂O₇, with a small amount of Gd₂O₃ phase. The most intense peak shifts between the planes (105) and (110), indicating that Nb doping affects the phase composition. As Nb doping increases, the XRD peak positions between 31° and 33° shift to lower angles due to the lattice distortion caused by Nb⁵⁺ with the radius of 0.690 Å substituting for Ti⁴⁺ with the radius of 0.605 Å. The SEM image reveals that a small amount of Nb doping (1%, in mole) improves the density, but higher doping levels increase the porosity and reduce the density.

The electrical conductivity of the sample decreases with increasing temperature. The maximum electrical conductivity for the sample doped with 1% (in mole) Nb can be obtained, which can be enhanced by 503% at 340 K and 95% at 973 K, compared to undoped samples. The Seebeck coefficient of Gd20Nb1 significantly reduces, while the power factors of Gd20Nb1 and Gd20Nb2 are similar at high temperatures. The thermal diffusivity, lattice thermal conductivity, and total thermal conductivity all decrease with increasing temperature. The sample Gd20Nb1 exhibits the maximum thermal conductivity, while the sample Gd20Nb5 reaches the minimum value of 1.4 W·m^-1·K^-1 at 973 K. The zT value is maximum at 0.19 for the sample doped with 2% (in mole) Nb, which is primarily affected by the power factor. A lower thermal diffusivity of the sample Gd20Nb2 resulted in a lower thermal conductivity, compared to the sample Gd20Nb1, achieving the maximum zT value.

The Nb-doped samples show an intense infrared absorption in the range of 8-14 μm, as the infrared emissivity increases with temperature due to the enhanced carrier scattering. At 973 K, the infrared emissivity of the sample Gd20Nb1 reaches 95% in the range of 8-14 μm, indicating potential applications in high-temperature protective coatings. This demonstrates a promising potential for simultaneous tuning of thermoelectric and infrared properties, thus offering some possibilities for Sr₃Ti₂O₇-based materials in radiation heat shielding and other fields.

Gd/Nb co-doped Sr₃Ti₂O₇ ceramics were synthesized by a solid-state method. The influence of Nb doping concentration on the thermoelectric transport and infrared radiation properties as well as the interrelationship between these properties were analyzed. The results showed that a small amount of Nb doping significantly enhanced the electrical conductivity of the material, and the zT value reached an optimal 0.19 at 973 K when the Nb doping concentration reached 2% (in mole). Furthermore, the Nb-doped samples exhibited an intense absorption in the 8-14 μm range, and their infrared emissivity increased with the increase of the power factor and zT value. At 973 K, the sample Gd20Nb1 achieved the maximum average infrared emissivity of ~95% in the 8-14 μm atmospheric window. The sample Gd20Nb5 had the minimum thermal conductivity of ~1.4 W·m^-1·K^-1. The results revealed the the synergistic effects of Sr₃Ti₂O₇-based ceramics on the thermoelectric and infrared radiation properties, demonstrating their application potential in radiant heat protection and high infrared emissivity materials. This study could provide important references for the further optimization and design of novel thermoelectric and infrared materials.