Both electrical conductivity σ and Seebeck coefficient S are functions of carrier concentration being correlated with each other, and the value of power factor S²σ is generally limited to less than 0.01 W m⁻¹ K⁻². Here we report that, under the temperature gradient applied simultaneously to both parallel and perpendicular directions of measurement, a metallic copper selenide, Cu₂Se, shows two sign reversals and colossal values of S exceeding ±2 mV K⁻¹ in a narrow temperature range, 340 K < T < 400 K, where a structure phase transition takes place. The metallic behavior of σ possessing larger magnitude exceeding 600 S cm⁻¹ leads to a colossal value of S²σ = 2.3 W m^–1 K^–2. The small thermal conductivity less than 2 W m⁻¹ K⁻¹ results in a huge dimensionless figure of merit exceeding 400. This unusual behavior is brought about by the self-tuning carrier concentration effect in the low-temperature phase assisted by the high-temperature phase.

Skutterudite compounds such as Co antimonite (CoSb3) contain cage-like voids inside crystal structure, which can be completely or partially filled with various different atoms, including group 13 elements. The multiple filling approach is known as an effective way of reducing lattice thermal conductivity (κlat), which results in a high value of the thermoelectric dimensionless figure of merit (zT). In this work, enhanced zT was achieved for the Ga and In co-added CoSb3 samples with a preferable microstructure and the nominal composition (Ga0.8In0.2)xCo4Sb12 (x = 0.05−0.45). Although all added In atoms occupied exclusively the void sites, the Ga species filled both the void and Sb sites of CoSb3. Moreover, Ga atoms added in the quantities exceeding the solubility limit precipitated as GaSb nanoparticles. The sample with x = 0.45 was characterized by the largest filling factions of Ga and In as well as the unique microstructure, consisting of microscale grains of the skutterudite phase and corresponding amounts of the GaSb nanoparticles. The Ga and In co-added skutterudite samples with optimized chemical composition and microstructure maintained high carrier mobility and sufficiently low κlat values, resulting in zT > 1.1, one of the best values for the skutterudites filled with group 13 elements.

The properties of the interlayer thermal expansion of 6-layered reduced graphene oxide (rGO) were investigated by using X-ray diffraction (XRD). The interlayer distance of rGO increases linearly as the temperature increases from room temperature to 1300 K. This linear pattern is similar to that of graphite. However, the degree of interlayer thermal expansion of rGO is 22% higher than that of graphite in the temperature range from 288–300 to 1073 K, which is explained by the weak interlayer interaction of 6-layered rGO.