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 (κ_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, and consolidation by (iii) spark plasma sintering (SPS) and also (iv) by the state-of-the-art flash SPS. Irrespective of the method of synthesis, the electrical (σ) and thermal (κ_tot) conductivities of the CuPb₁₈SbTe₂₀ samples were akin to those of LAST alloys. Both the flash-SPSed and slow-cooled CuPb₁₈SbTe₂₀ samples with nanoscale dislocations and Cu-rich nanoprecipitates exhibited an ultralow κ_latt ∼ 0.58 W/m·K at 723 K, comparable with that of its Ag counterpart, regardless of the differences in the size of the precipitates, type of precipitate-matrix interfaces, and other nanoscopic architectures. The sample processed by flash SPS manifested higher figure of merit ( zT ∼ 0.9 at 723 K) because of better optimization and a trade-off between the transport properties by decreasing the carrier concentration and κ_latt without degrading the carrier mobility. In spite of their comparable σ and κ_tot, zT of the Cu samples is low compared to that of the Ag samples because of their contrasting thermopower values. First-principles calculations attribute this variation in the Seebeck coefficient to dwindling of the energy gap (from 0.1 to 0.02 eV) between the valence and conduction bands in MPb₁₈SbTe₂₀ (M = Cu or Ag) when Cu replaces Ag.