Bismuth sulfide (Bi2S3) has garnered extensive attention for thermoelectric (TE) applications due to its earth abundance, nontoxicity, and ultralow lattice thermal conductivity. However, simultaneously improving its electrical and thermal properties remains challenging due to the strong coupling between these parameters. Herein, we propose a structural evolution strategy to synergistically optimize the electrical and thermal transport properties of Bi2S3-based TE compounds. By leveraging the distinct crystal structures of Bi2S3 and Bi2Se3, alloying Se at S sites successfully reconstructs the carrier transport channels within the Bi2S3 matrix, thereby facilitating carrier mobility. Additionally, the weakened chemical bonding leads to a significant increase in carrier concentration, approaching the optimal range. Furthermore, the structural evolution from particle-like to lamellar grains, coupled with the large mass difference between Se and S, induces lattice distortion that effectively reduces the lattice thermal conductivity. Combining the dramatically enhanced power factor and ultralow thermal conductivity, an optimized figure of merit (ZT) of 0.38 at 623 K is achieved in the Bi2SSe2 solid solution. This structural evolution strategy offers a promising avenue for enhancing the TE performance of binary compounds.
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The extensive utilization of thermoelectric (TE) conversion technology necessitates stricter performance requirements for bismuth telluride (Bi2Te3)-based commercial materials. Despite the numerous optimization methods available for Bi2Te3-based materials, each optimization method has a certain upper limitation, and combining multiple strategies can achieve the optimal thermoelectric figure of merit (zT). In this study, the thermoelectric properties of (Bi,Sb)2Te3 materials are enhanced through the combined use of the heavy element Pb to regulate carrier concentration and the In element to optimize the band structure. Notably, indium (In) can suppress p-type antisite defects, which generate abundant Te vacancies, and help regulate the carrier concentration to its optimal level. This co-doping strategy achieves optimal carrier concentration, thereby enhancing the power factor (PF = 4.57 × 103 μW·m−1·K−2), and generating abundant dislocations, the presence of the rich nano-second phase Sb2O3 contributes to reduced lattice thermal conductivity. Consequently, a peak zT value of 1.41 at 323 K and a high average zT value of 1.23 between 300 K and 500 K are achieved. Additionally, two pairs of thermoelectric modules, composed of p-type (Bi0.42Sb1.58)0.994(In, Pb)0.006Te3 and zone-melted n-type Bi2Te2.7Se0.3, demonstrate a conversion efficiency of 7.3% at a temperature difference of 250 K. This underscores the promising potential of these thermoelectric modules in commercialization. Thus, this study demonstrates the feasibility of combining multiple strategies and is expected to provide a potential reference for other thermoelectric systems.
Thermoelectric 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 Bi0.42Sb1.58Te3 (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.
The 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 (Bi0.42Sb1.58Te3 + 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 Kα radiation (λ = 1.5406 Å) 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 κtot = DρCp ,where D is the thermal diffusivity, Cp 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).
When 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.
In 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.
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The average zT determines the conversion efficiency, and the power factor plays an important role in average zT value. However, the inadequate electrical conductivity of SnSe materials seriously limits its application. Herein, the TaCl5-doped in polycrystalline SnSe0.95 materials synthesized using the melting method and combined with spark plasma sintering technology achieves a zT value of 1.64 at 773 K and a record zTave of 0.62 from 323 K to 773 K. The electrical conductivity increases due to the released electron carrier induced by effective TaCl5 doping. According to the DFT calculation, the energy band of TaCl5-doped samples is narrowed, which can enhance the electron transport. Besides, the Seebeck coefficient is maintained at an elevated level as a result of the incorporation of the heavy element Ta. Due to the significantly enhanced electrical conductivity and maintained high Seebeck coefficient, the power factor reaches to 622 μW·m−1·K−2 at 773 K for the SnSe0.95 + 1.75% (in mass) TaCl5 sample, which is almost 21 times higher than that of the pristine sample. Simultaneously, a high average power factor value of 334 μW·m−1·K−2 for the SnSe0.95 + 1.75% (in mass) TaCl5 sample from 323 to 773 K was obtained. It is surprisingly found that the Ta element plays another important role to improve the stability of SnSe0.95 by forming Ta2Sn3 and removing the low melting point Sn, which usually existed in n-type SnSe samples, resulting in the decreased lattice thermal conductivity. A low lattice thermal conductivity value of 0.24 W·m−1·K−1 was also obtained for the SnSe0.95 + 2.0% (in mass) TaCl5 sample at 773 K due to the multiscale defects. Consequently, the SnSe0.95 + 2.0% (in mass) TaCl5 sample obtains a peak zT value of 1.64 at 773 K and a record zTave of 0.62 from 323 to 773 K, and the theoretically calculated conversion efficiency reaches 11.2%, it can be utilized for power generation and/or cooling at a broad temperature range. This strategy of introducing high-valence halides with heavy element can optimize the thermoelectric performance for other material systems.
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CuPbBi5S9 compounds have been investigated as gladite for years. However, there have been no significant studies on their physical and chemical properties. This work demonstrates that upon alloying with moderate Cu, Pb, Bi, and S using an appropriate preparation method, quaternary CuPbBi5S9 compounds can exhibit excellent figure of merit ZT within the temperature range 298–723 K. A low average velocity, low Young's modulus and Debye temperature, and large Grüneisen parameter, determined experimentally, indicate strong lattice anharmonicity in CuPbBi5S9 crystals. Furthermore, density functional theory calculations (local vibration of low-frequency acoustic phonons) justify the low lattice thermal conductivity of CuPbBi5S9 compounds. Because of the low thermal conductivity (0.514 W m−1K−1) and a relatively high power factor (293 μW m−1K−2), a maximum ZT of 0.42 was achieved at 723 K for CuPbBi5S9 prepared by mechanical alloying combined with solid-state melting. Thus, CuPbBi5S9 materials are promising candidates for use as high-performance thermoelectric materials in the intermediate-temperature range.
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