Multistage nanostructures induced by precursor phase spontaneous partitioning lead to an excellent thermoelectric performance in Cu1.8S0.8Se0.2

Abstract

Altering the carrier concentration and decreasing the lattice thermal conductivity are essential strategies to further improve the intrinsic thermoelectric properties of thermoelectric materials. In this study, the carrier concentration of Cu_1.8S_0.8Se_0.2 blocks was optimized by controlling the degree of cubic digenite phase decomposition during the sintering process. The spontaneous partitioning of the precursor cubic digenite phase in the sintering process introduced many complex multistage nanostructures, including nanodots, local lattice distortions, nano-twins, and nano-inclusions. These multistage nanostructures or nano-defects are mainly in the size range of 1–20 nm, strongly scattering phonons (with widely ranging wavelengths and mean free paths) and significantly reducing the lattice thermal conductivity of the sintered block. The optimized power factor and reduced lattice thermal conductivity increased the figure of merit of the sintered Cu_1.8S_0.8Se_0.2 block (figure of merit = 0.76 at 450 °C) by 90% from that of pure Cu_1.8S (figure of merit = 0.4 at 450 °C) thermoelectric material. The high temperatures induced spontaneous partitioning of the cubic digenite phase, ensuring stability and coherence between the matrix and introduced nanostructures. The strategy of introducing nanostructures through spontaneous partitioning of the precursor phase is not only applicable to chalcogenide thermoelectric materials but also to other thermoelectric materials with inherent high-temperature phase transformations.