Fangqiu Zu

Ag-Segregation to Dislocations in PbTe-Based Thermoelectric Materials

Dislocations have been considered to be an efficient source for scattering midfrequency phonons, contributing to the enhancement of thermoelectric performance. The structure of dislocations can be resolved by electron microscopy whereas their chemical composition and decoration state are scarcely known. Here, we correlate transmission Kikuchi diffraction and (scanning) transmission electron microscopy in conjunction with atom probe tomography to investigate the local structure and chemical composition of dislocations in a thermoelectric Ag-doped PbTe compound. Our investigations indicate that Ag atoms segregate to dislocations with a 10-fold excess of Ag compared with its average concentration in the matrix. Yet the Ag concentration along the dislocation line is not constant but fluctuates from ∼0.8 to ∼10 atom % with a period of about 5 nm. Thermal conductivity is evaluated applying laser flash analysis, and is correlated with theoretical calculations based on the Debye-Callaway model, demonstrating that these Ag-decorated dislocations yield stronger phonon scatterings. These findings reduce the knowledge gap regarding the composition of dislocations needed for theoretical calculations of phonon scattering and pave the way for extending the concept of defect engineering to thermoelectric materials.

Enhanced thermoelectric properties of n-type Bi2Te2.7Se0.3 semiconductor by manipulating its parent liquid state

To enhance the properties of bulk thermoelectric (TE) materials, preparation methods have been intensively developed with advanced techniques, e.g., zone-melting, Bridgman or Czochralski unidirectional methods, hot pressing or spark plasma sintering, that follow various complicated synthesis steps. However, these innovative methods remain less competitive than conventional techniques for large-scale production and application. Here, we report that, by manipulating the liquid state of the Bi2Te2.7Se0.3 alloy doped with KI, n-type bulk specimens with desired TE performance can be simply solidified by air cooling and without costly equipments. The specimens solidified from the parent melt experienced temperature-induced liquid–liquid structural transition, which is indicated by the resistivity behavior of the liquid, the presence of a refined matrix and eutectics, and the observation of more nanoparticles and a higher density of lattice defects. It was confirmed these refined multiscale refined structures lead to a “phonon glass electron crystal” effect, i.e., a much lower lattice thermal conductivity but a much higher power factor (PF). The present method, which synergistically obtains higher PF and lower thermal conductivity, could also be applied to other TE compounds.

Simultaneous optimization of Seebeck, electrical and thermal conductivity in free-solidified Bi 0.4 Sb 1.6 Te 3 alloy via liquid-state manipulation

Abstract(BiSb)2Te3-based alloy is one of the best p-type thermoelectric (TE) materials near room temperature. However, it is challenging to improve its ZT value due to the interrelated Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ). In this study, the synergistic optimization of S, σ, and κ has been easily achieved in Bi0.4Sb1.6Te3 alloy by liquid-state manipulation (LSM). Specifically, more Te-rich eutectic strips are observed in the LSM sample, which would increase carrier density (p) and thus improve σ. Meanwhile, via LSM, the raised effective mass m* could compensate the effect of increased p on S and thus an enhanced S is obtained. Furthermore, the larger amount of nanoparticles, higher density of lattice distortions, and dislocations in the LSM sample would contribute to scattering phonons and a lower κ is attained. As a result, the highest ZT of 0.7 at 352xa0K is attained which is 40% higher than that of traditional melted Bi0.4Sb1.6Te3 alloy.

The Effect of SbI3 Doping on the Structure and Electrical Properties of n-Type Bi1.8Sb0.2Te2.85Se0.15 Alloy Prepared by the Free Growth Method

Thermoelectric technology is regarded as one of the most promising direct power generation techniques via thermoelectric materials. However, the batch production and scale-up application are hindered because of the high-cost and poor performance. In this work, we adopt the free growth method to synthesize a series of the bulk materials of SbI3-doped Bi1.8Sb0.2Te2.85Se0.15 alloys. The structural and component investigations as well as the electrical properties characterization are carried out. The results show that SbI3 promotes the formation of Te-rich regions in the matrix. In addition, the synergistically optimized electrical conductivity and Seebeck coefficient are attained by controlling the SbI3 doping concentration. Thus, the sample with 0.30xa0wt.% SbI3 displays a highly increased power factor of ∼xa013.57xa0μWxa0cm−1xa0K−2, which is nearly 21 times higher than that of the undoped one. Moreover, the free growth method is reproducible, convenient and economical. Therefore, it has great potential as a promising technology for the batch synthesis.

Dependence of Solidification for Bi 2 Te 3−x Se x Alloys on Their Liquid States

The resistivity versus temperature (ρ-T) behaviours of liquid n-type Bi2Te3−xSex (xu2009=u20090.3, 0.45 and 0.6) alloys are explored up to 1050u2009°C. A clear hump is observed on all ρ-T curves of the three studied Bi2Te3−xSex melts during the heating process, which suggests that a temperature-induced liquid-liquid structural transition takes place in the melts. Based on this information, the solidification behaviours and microstructures of the alloys with different liquid states are investigated. The samples that experienced liquid structural transition show that the nucleation and growth undercooling degrees are conspicuously enlarged and the solidification time is shortened. As a result, the solidified lamellae are refined and homogenized, the prevalence of low-angle grain boundaries between these lamellae is increased, and the Vicker Hardness is enhanced. Atom probe tomography analyses prove that there is no segregation or nanoprecipitation within the grains, but the Te-rich eutectic structure and the evolution of composition near the Te-matrix phase boundary are investigated in a sample that experienced liquid structural transition. Our work implies that the solidification behaviours of Bi2Te3−xSex alloys are strongly related to their parent liquid states, providing an alternative approach to tailor the thermoelectric and mechanical properties even when only a simple solidification process is performed.

Enhanced Thermoelectric Properties of Ag-Modified Bi0.5Sb1.5Te3 Composites by a Facile Electroless Plating Method

A large-scale and facile electroless plating Ag method has been developed to fabricate high-performing Ag/Bi0.5Sb1.5Te3 (Ag/BST) composites. Ag can be doped into BST and also forms Ag2Te secondary phase in BST, leading to a low lattice thermal conductivity of 0.34 Wm-1K-1. Consequently, a peak zT of 1.07 and average zT of 1.02 are achieved in 0.03wt% Ag/BST. The average zT value is enhanced by 100% in the temperature interval from 300 to 500 K compared with that of Ag-free BST. This work provides a facile and large-scale method to fabricate the high performance Bi2Te3-based alloy for applying in the low-temperature power generation.

Effects of Electroless Plating with Cu Content on Thermoelectric and Mechanical Properties of p-type Bi0.5Sb1.5Te3 Bulk Alloys

Bi0.5Sb1.5Te3/Cu core/shell powders were prepared by electroless plating and hydrogen reduction, and then sintered into bulk by spark plasma sintering. After electroless plating, with increasing the Cu content, the electrical conductivity keeps enhancing significantly. The highest electrical conductivity reaches 3341S/cm at room temperature in Bi0.5Sb1.5Te3 with 0.67wt% Cu bulk sample. Moreover, the lowest lattice thermal conductivity reaches 0.32 W/m·K at 572.2 K in Bi0.5Sb1.5Te3 with 0.67wt% Cu bulk sample, which is caused by the scattering of the rich-copper particles with different dimensions and massive grain boundaries. According to the results, the ZT values of all Bi0.5Sb1.5Te3/Cu bulk samples have improved in a high temperature range. In Bi0.5Sb1.5Te3 with 0.15wt% Cu bulk sample, the highest ZT value at 573.4 K is 0.81. When the Cu content increases to 0.67wt%, the highest ZT value reaches 0.85 at 622.2 K. Meanwhile, the microhardness increases with increasing the Cu content.

Thermoelectric Performance of Sb2Te3-Based Alloys is Improved by Introducing PN Junctions

Interface engineering has been demonstrated to be an effective strategy for enhancing the thermoelectric (TE) performance of materials. However, a very typical interface in semiconductors, that is, the PN junction (PNJ), is scarcely adopted by the thermoelectrical community because of the coexistence of holes and electrons. Interestingly, our explorative results provide a definitively positive case that appropriate PNJs are able to enhance the TE performance of p-type Sb2Te3-based alloys. Specifically, owing to the formation of the charge-depletion layer and built-in electric field, the carrier concentration and transport can be optimized and thus the power factor is improved and the electronic thermal conductivity is decreased. Meanwhile, PNJs provide scattering centers for phonons, leading to a reduced lattice thermal conductivity. Consequently, the p-type (Bi2Te3)0.15-(Sb2Te3)0.85 composites comprising PNJs achieve a ∼131% improvement of the ZT value compared with the pure Sb2Te3. The increased ZT demonstrates the feasibility of improving the TE properties by introducing PNJs, which will open a new and effective avenue for designing TE alloys with high performance.