IntroductionThermoelectric materials have a promising prospect as they can directly convert thermal energy into electrical energy. Some thermoelectric materials are discovered in recent years, especially bismuth telluride (Bi₂Te₃)-based materials that are capable of large-scale commercialization. At present, the average zT value and conversion efficiency of Bi₂Te₃-based materials can be further enhaced. The low-valent cation doping is achieved via doping to optimize the electrical conductivity, while introducing defects as phonon scattering centers to effectively reduce the lattice thermal conductivity. This strategy is verified to be the most effective optimization method. In this paper, Na₂S was selected as a p-type dopant to dope Bi_0.42Sb_1.58Te₃ (BST) alloys, the conductivity was optimized via replacing the cation position of the BST matrix, and improving the solid solubility of Na and strengthening its mechanical properties by element S. The lattice distortion caused by Na₂S doping in the BST alloys and the nanopore structure generated by the volatile element Te in the matrix were investigated.MethodsThe original high-purity Bi powder (99.99%, in mass, the same below), Te powder (99.99%), Sb (99.99%), and Na₂S powder (Aladdin Co., China) were precisely weighed in an Ar atmosphere glove box based on their nominal composition (Bi_0.42Sb_1.58Te₃ + x% Na₂S, where x = 0, 0.2, 0.5, and 1.0). The weighed mixed powder was then placed in a quartz tube, evacuated to a vacuum degree of 10^–4 Pa, and sealed. The quartz tubes were pre-plated with carbon to avoid the possibility of Na corrosion. The mixed powder in the sealed quartz tube was heated in a vertical resistance furnace for melting at 1125 K for 12 h and kept for 16 h. The ingots obtained after melting were ground by a model QM-3SP2 planetary ball mill at 425 r/min for 6 h. Finally, the powder was sintered at 698 K and 50 MPa by spark plasma sintering to prepare the bulk samples.The phase structure of the samples was analyzed by X-ray diffraction (XRD, Rigaku Co., Japan) with Cu K_α radiation (λ = 1.540 6 A) in a diffraction angle range of 20°–60° with a step size of 0.02° (5 (°)/min). The microstructure of the samples was examined by scanning electron microscopy (SEM, ZEISS Co., Germany). The thermoelectric performance of the samples was analyzed via measuring their Seebeck coefficient, electrical conductivity, and power factor in a model ZEM-3M10 Seebeck coefficient/electric resistivity measuring system (Ulvac-Riko Co., Japan) under a thin helium atmosphere. The thermal diffusivity of the samples was measured by a model LFA457 laser flash instrument (NETZSCH Co., Germany), and the thermal conductivity was calculated based on κ_tot=DρC_p, where D is the thermal diffusivity, C_p is the specific heat capacity deduced via the Dulong-Petit limit, and ρ is the density of samples. The density was determined according to the Archimedes principle. The carrier concentration and mobility of the samples were measured at 295 K under an applied magnetic field of 1.5 T and an electrical current of 30 mA in a model PPMS-9T physical properties measurement system (Quantum Design Inc., Japan).Results and discussionWhen Na₂S is used as a dopant, the electrical conductivity of 0.5% Na₂S-doped BST sample increases from 598 S/cm of the pure sample to 749 S/cm at 300 K, and the lattice thermal conductivity of the sample decreases to 0.55 W/(m·K) at 300 K. The peak zT of 0.5% Na₂S-doped BST sample reaches 1.3 at 300 K due to the increase of power factor and the decrease of thermal conductivity, which is 49.4% higher than that of the undoped sample. The thermoelectric conversion efficiency of the single-arm device reaches 3% as ΔT=275 K due to its excellent thermoelectric figure of merit. In addition, the average hardness of the doped samples also increases to 1.09 GPa.ConclusionsIn this work, p-type BST thermoelectric materials were prepared by a solid-state method and a spark plasma sintering technology. Utilizing Na₂S as a dopant achieved a low-valent substitution at cation sites, and introduced holes, thus enhancing the carrier concentration and optimizing the electrical conductivity of BST matrix thermoelectric materials. The peak zT of 0.5% Na₂S-doped BST sample reached 1.3 at 300 K, which was 49.4% higher than that of the undoped sample. Furthermore, a large number of point defects enhanced phonon scattering and reduced the thermal conductivity of the material. In addition, element S could also enhance the solid solubility of Na in the BST matrix and the solid solution strengthening. The average hardness of the doped samples increased to 1.09 GPa.