Effect of the Processing Route on the Thermoelectric Performance of Nanostructured CuPb₁₈SbTe₂₀

Abstract

The quaternary AgPb18SbTe20 compound (abbreviated as LAST) is a prominent thermoelectric material with good performance. Endotaxially embedded nanoscale Ag-rich precipitates contribute significantly to decreased lattice thermal conductivity (latt) in LAST alloys. In this work, Ag in LAST alloys was completely replaced by the more economically available Cu. Herein, we conscientiously investigated the different routes of synthesizing CuPb18SbTe20 after vacuum-sealed tube melt processing, including: (i) slow cooling of the melt; (ii) quenching and annealing; (iii) consolidation by spark plasma sintering (SPS); and also by the state-of-the-art (iv) Flash-SPS. Irrespective of the method of synthesis, the electrical (σ) and thermal (tot) conductivities of CuPb18SbTe20 samples were akin to that of LAST alloys. Both the flash-SPSed and the slow cooled CuPb18SbTe20 samples with nanoscale dislocations and Cu-rich nanoprecipitates exhibited an ultra-low latt  0.58 W/mK at 723 K, comparable with that its Ag counterpart, regardless of differences in their size of the precipitates, type of precipitate-matrix interfaces and other nanoscopic architectures.The sample processed by flash sintering manifested higher figure of merit (zT  0.9 at 723 K), due to better optimization and trade-off between the transport properties by decreasing the carrier concentration and latt without degrading the carrier mobility. In spite of their comparable σ and tot, the zT of the Cu samples were low compared to the Ag samples due to their contrasting thermopower values. First-principles calculations attribute this variation in Seebeck to the dwindling of the energy gap (from 0.1 eV to 0.02 eV) between the valence and conduction bands in MPb18SbTe20 (M = Cu or Ag), when Cu replaces Ag. Materials and Methods ReagentsPb (Strem Chemicals, 99.999%), Sb (Alfa Aesar, 99.999%), Cu (Alfa Aesar, 99.999%) and Te (JGI, 99.999%) were used for synthesis without any further purification. SynthesisIn this work, several different processing routes were investigated, however, the first step (synthesis) was common to all of the processing routes. Samples of CuPb18SbTe20 were synthesized using the vacuumsealed tube melt processing. Stoichiometric amounts of the starting elements of Cu, Pb, Sb and Te were introduced into a fused silica tube. The tube was prepared by cleaning with hydrofluoric (HF) acid and distilled water, then dried under vacuum. The ampoules were sealed under a vacuum of 10 -6 Torr, then placed in a rocking furnace and slowly heated to 1223 K over a period of 12 hours, then held at that temperature for 15 hours. Four different batches of samples were prepared, the first two were produced directly from the molten material, followed by: (i) cooling the melt to room temperature over a period of 18 hours (samples denoted as 'SS'); (ii) rapidly quenching the tube in water, followed by annealing at 973 K for 8 hours (denoted as 'MQ'). The other two batches used the material produced by method (i), which was crushed and milled. The powders were then ...

Realizing a stable high thermoelectric zT ∼ 2 over a broad temperature range in Ge1−x−yGaxSbyTe via band engineering and hybrid flash-SPS processing

“…Just recently, we have shown how to improve the TE properties of PbTe-based materials by the so called 'Hybrid Flash-SPS' processing. 53 Motivated by those results, we have extended that process to Ga and Sb codoped GeTe. Indeed, Ge0.90Ga0.02Sb0.08Te was consolidated by 'Hybrid' Flash-SPS processing, where the powders were sintered at 893 K with a heating rate of  10,000 o C/min (heated from 293 -893 K in 3 seconds) under an axial pressure of 55 MPa.…”

“…68 But the 'Hybrid' Flash-SPS processing route is a variant of the originally developed Flash-SPS method, involving the use of a thin, low thermal inertia metal die to contain the TE powder during sintering. 52,53 The schematics of the experimental set-up and the current flow paths are available in SI ( Figure S1). Highly dense disk-shaped pellets were obtained with theoretical densities of  98% for Hybrid Flash-SPS.…”

“…Thus, the Ge0.88Ga0.02Sb0.10Te composition with a high thermopower and ultra-low total manifested a maximum zT  1.75 at 725 K, which was 80% higher than that of pristine GeTe. With the state-of-the art 'hybrid flash-SPS' processing, 52,53 this zT value was further improved to  2 at 725 K (more than 100% improvement when compared to undoped GeTe). Interestingly, this high value of zT is notably maintained over a broad temperature range (600 -775 K).…”

Experimental Investigation on the Thermal Performance of Pulsating Heat Pipe Heat Exchangers

“…Sources of low temperature waste heat mostly include heat loss from industrial products, equipment and processes, and heat discharged from combustion processes. Therefore, using heat exchangers to recover the low temperature waste heat would be the major approach rather than the thermoelectric units [2][3][4].…”

Preparation and Dielectric Properties of K1/2Na1/2NbO3 Ceramics Obtained from Mechanically Activated Powders

“…However, obtaining advanced nanostructured materials requires the use of more advanced technologies, such as spark plasma sintering (SPS) and hybrid flash spark plasma sintering. SPS involves direct joule heating of electrically conductive dies (usually graphite), whereas the more advanced hybrid spark SPS employs a thermal runaway to achieve ultrafast sintering with heating rates as high as 10,000 • C/min [11][12][13].…”