Chao-Feng Wu

Electrical and thermal transport properties of spark plasma sintered n-type Bi2Te3−xSex alloys: the combined effect of point defect and Se content

Performance enhancement can be realized in p-type Bi2Te3 but hardly in n-type Se-modified Bi2Te3 alloys fabricated by powder processing as compared with conventional ingots. To reveal the reasons and investigate the optimal Se content in fine-grained n-type Bi2Te3−xSex materials processed by mechanical alloying (MA) and spark plasma sintering (SPS), the amount of Se was varied in a wide range and electrical transport properties were investigated and discussed in association with the results of Hall measurements. Thermal transport properties were also studied with Raman spectra. It is revealed that different concentrations of point defects are induced by changing the Se content x from 0 to 1.0 in Bi2Te3−xSex. The point defects and their interaction (including donor-like effects) not only largely change the carrier concentration and mobility, but also enhance the phonon scattering that would lead to a decrease of thermal conductivity. Consequently, the optimal composition is confirmed as Bi2Te2.2Se0.8 with a corresponding carrier concentration of 4.5 × 1019 cm−3 and a maximum ZT value of 0.82 at 473 K, approximately 35% increased than that obtained at optimal Bi2Te2.7Se0.3 conventionally used for ingots. This work indicates that the point defects as well as their interaction during MA and SPS shift the optimal Se content from traditional x = 0.3 to x = 0.8 in n-type Bi2Te3−xSex, demonstrating the importance of comprehensive control of point defects by adjusting the Se content for different processes.

PbTe-based thermoelectric nanocomposites with reduced thermal conductivity by SiC nanodispersion

For further thermoelectric performance enhancement by the nanocomposite effect, a small amount (<2 vol. %) of 30 nm SiC particles were added into a compositionally optimized AgPbmSbTem+2 thermoelectric alloy fabricated by mechanical alloying and spark plasma sintering. Although the energy filtering effect is not available in the present composite due to the mismatched interface between SiC and the matrix, a small amount of SiC dispersions were revealed to be effective to reduce the thermal conductivity via enhancing phonon scattering. A high figure of merit up to 1.54 at 723 K was obtained in the AgPbmSbTem+2 matrix composite containing 1 vol. % SiC nanoparticles.

Thermoelectric SnS and SnS-SnSe solid solutions prepared by mechanical alloying and spark plasma sintering: Anisotropic thermoelectric properties

P–type SnS compound and SnS1−xSex solid solutions were prepared by mechanical alloying followed by spark plasma sintering (SPS) and their thermoelectric properties were then studied in different compositions (x = 0.0, 0.2, 0.5, 0.8) along the directions parallel (//) and perpendicular (⊥) to the SPS–pressurizing direction in the temperature range 323–823 Κ. SnS compound and SnS1−xSex solid solutions exhibited anisotropic thermoelectric performance and showed higher power factor and thermal conductivity along the direction ⊥ than the // one. The thermal conductivity decreased with increasing contents of Se and fell to 0.36 W m−1 K−1 at 823 K for the composition SnS0.5Se0.5. With increasing selenium content (x) the formation of solid solutions substantially improved the electrical conductivity due to the increased carrier concentration. Hence, the optimized power factor and reduced thermal conductivity resulted in a maximum ZT value of 0.64 at 823 K for SnS0.2Se0.8 along the parallel direction.

Is Cu3SbSe3 a promising thermoelectric material

Cu3SbSe3, a compound with an ultralow thermal conductivity, has been predicted as a promising thermoelectric material, but relevant experimental results are inadequate. In this work we studied the high-temperature thermoelectric properties of this ternary chalcogenide. An extremely low thermal conductivity was observed and a glass-like behavior was seen above an order–disorder transition. Possible mechanisms causing such an ultralow thermal conductivity were discussed concerning the disorder of Cu atoms. With a large band gap of ∼0.95 eV obtained by an optical absorption edge measurement, Cu3SbSe3 was found to be a nondegenerate p-type semiconductor, different from previous reports. A maximum zT of ∼0.25 was obtained at 650 K for Cu3SbSe3, which is much higher than the previously reported values, but this compound was considered inferior to Cu3SbSe4 in its thermoelectric performance by comparing some key physical parameters.

Enhancing average ZT in pristine PbSe by over-stoichiometric Pb addition

PbSe is an inexpensive alternative for PbTe as a mid-temperature thermoelectric material, but few investigations have been reported about its intrinsic properties despite recent efforts on doping techniques. In this work, pristine PbSe bulk materials were synthesized by a process combining mechanical alloying and spark plasma sintering, which is increasingly used for processing thermoelectric materials, and their electrical and thermal transport properties as well as thermoelectric performance were investigated in a wide temperature range. A maximum ZT ∼0.83 was obtained at 673 K in nominal composition PbSe + 3 or 4 at. % Pb, leading to nearly 50% enhancement from reported n-type pristine PbSe, mainly benefitting from the improved electrical performance. Furthermore, the potential thermoelectric efficiency was also improved due to the enhanced low-temperature performance, showing a high average ZT of 0.6 that is even comparable to that of commercial n-type Bi2Te3 materials.

Doping of thermoelectric PbSe with chemically inert secondary phase nanoparticles

For thermoelectrics, as well as many other applications of semiconductors, optimizing the carrier density is of great importance. This is normally done by atomic substitution. In this study, we present a result where systematic doping could arise from the least expected nanoparticles when composites are formed. We achieved this in a composite of PbSe and nanoparticle SiC. High carrier concentrations of up to 2 × 1019 cm−3 were achieved with SiC nanoparticle load up to 5 vol%. We believe this is due to interfaces that stabilized more defects than is allowed in bulk PbSe. Eventually, the nanoparticle-induced doping effect led to a decent thermoelectric performance with zT close to one at 800 K, comparable to optimized PbSe using conventional dopants. Our result indicates the good potential of this uncommon mechanism in some semiconductors for carrier concentration tuning where conventional doping has been found difficult.

Solvothermally synthesized SnS nanorods with high carrier mobility leading to thermoelectric enhancement

Rod-shaped SnS nanocrystals were synthesized by a facile solvothermal method, which were then compacted by a cold isostatic pressing and rapid annealing process. Two-order higher carrier mobility was achieved in the corresponding bulk samples, leading to over 200% enhancement of thermoelectric figure of merit.

Powder metallurgically synthesized Cu12Sb4S13 tetrahedrites: phase transition and high thermoelectricity

Cu12Sb4S13 tetrahedrite with intrinsically low lattice thermal conductivity has been identified as a promising thermoelectric material with earth-abundant and environmental-friendly resource, but as a natural mineral its synthesis process has not been established. This work studied a powder metallurgical process combining mechanical alloying (MA) and spark plasma sintering (SPS) to synthesize Cu12Sb4S13−x (x = 0, 0.1, 0.2, 0.3 and 0.4) compounds. It is found that single-phased Cu12Sb4S13−x bulks could be synthesized by the MA-SPS process, but tended to become powdered naturally in air at room temperature. Interestingly, this weathering-like phenomenon could be effectively suppressed when the MA-SPS process was repeated. Consequently, a high ZT value of up to 0.65 at 723 K was achieved at a nominal composition of Cu12Sb4S12.7, which is close to the best value of 0.70 obtained in Cu12Sb4S13 tetrahedrites prepared by the melting method, although the present process is more simple and cost-effective. In addition, in this study temperature-dependent phase transitions were investigated to explore the reasons for the weathering-like phenomenon observed in synthetic Cu12Sb4S13 tetrahedrites.

Enhanced thermoelectric performance of Cu 12 Sb 4 S 13− δ tetrahedrite via nickel doping

Cu12Sb4S13 tetrahedrite has received great attention as an earth-abundant and environmental-friendly thermoelectric material. This work aims to uncover the thermoelectric performance-enhancing effect and the mechanism of nickel doping on tetrahedrite. A series of Cu12−xNixSb4S13−δ (x = 0.5, 0.7, 1.0, 1.5 and 2.0) compounds were synthesized by mechanical alloying combined with spark plasma sintering. It is found that the thermal conductivity sharply reduces with increasing Ni content over the entire temperature range, < 0.9 W m−1 K−1, accompanied with an enhanced thermoelectric power factor. The model predicted that the reduced lattice thermal conductivity is attributed to mid-frequency phonon scattering, caused by precipitates and dislocations resulting from Ni doping. Consequently, a high ZT value up to 0.95 at 723 K was achieved for Cu11NiSb4S13−δ, corresponding to a ∼46% increase over non-doped Cu12Sb4S13−δ. Furthermore, the cyclic measurement showed that the Ni-doped tetrahedrites displayed high chemical stability.摘要Cu12Sb4S13是一种储量丰富、 环境友好的天然矿物, 被热电领域普遍关注. 本研究旨在揭示Ni掺杂提高黝铜矿材料热电性能的机理. 采用机械合金化(MA)结合放电等离子体烧结(SPS)的方法制备出Cu12−xNixSb4S13−δ (x = 0.5, 0.7, 1.0, 1.5, 2.0)样品. 实验结果表明, 在测量温度范围内(323–723 K), 随着Ni含量的增加, 样品的热导率急剧下降(< 0.9 W m−1 K−1), 同时热电功率因子逐渐增加. 理论模型计算表明, 晶格热导率的降低主要来源于Ni掺杂引起的析出相及位错对中频声子的强散射作用. 由于较低的热导率和较高的功率因子, Cu11NiSb4S13−δ 样品在723 K时获得最高ZT值0.95, 相对于未掺杂样品, 其热电性能提高了46%. 同时, 热循环测试表明, 通过Ni掺杂提高了黝铜矿热电材料的化学稳定性.

Nanoporous PbSe–SiO2 Thermoelectric Composites

Abstract Nanoporous architecture has long been predicted theoretically for its proficiency in suppressing thermal conduction, but less concerned as a practical approach for better thermoelectric materials hitherto probably due to its technical challenges. This article demonstrates a study on nanoporous PbSe–SiO2 composites fabricated by a facile method of mechanical alloying assisted by subsequent wet‐milling and then spark plasma sintering. Owing to the formation of random nanopores and additional interface scattering, the lattice thermal conductivity is limited to a value as low as 0.56 W m−1 K−1 at above 600 K, almost the same low level achieved by introducing nanoscale precipitates. Besides, the room‐temperature electrical transport is found to be dominated by the grain‐boundary potential barrier scattering, whose effect fades away with increasing temperatures. Consequently, a maximum ZT of 1.15 at 823 K is achieved in the PbSe + 0.7 vol% SiO2 composition with >20% increase in average ZT, indicating the great potential of nanoporous structuring toward high thermoelectric conversion efficiency.