Analysis of Phase Separation in High Performance PbTe–PbS Thermoelectric Materials

Abstract

Phase immiscibility in PbTe–based thermoelectric materials is an effective means of top‐down synthesis of nanostructured composites exhibiting low lattice thermal conductivities. PbTe1‐x Sx thermoelectric materials can be synthesized as metastable solid solution alloys through rapid quenching. Subsequent post‐annealing induces phase separation at the nanometer scale, producing nanostructures that increase phonon scattering and reduce lattice thermal conductivity. However, there has yet to be any study investigating in detail the local chemical structure of both the solid solution and nanostructured variants of this material system. Herein, quenched and annealed (i.e., solid solution and phase‐separated) samples of PbTe–PbS are analyzed by in situ high‐resolution synchrotron powder X‐ray diffraction, solid‐state 125Te nuclear magnetic resonance (NMR), and infrared (IR) spectroscopy analysis. For high concentrations of PbS in PbTe, e.g., x >16%, NMR and IR analyses reveal that rapidly quenched samples exhibit incipient phase separation that is not detected by state‐of‐the‐art synchrotron X‐ray diffraction, providing an example of a PbTe thermoelectric “alloy” that is in fact phase inhomogeneous. Thermally‐induced PbS phase separation in PbTe–PbS occurs close to 200 °C for all compositions studied, and the solubility of the PbS phase in PbTe at elevated temperatures >500 °C is reported. The findings of this study suggest that there may be a large number of thermoelectric alloy systems that are phase inhomogeneous or nanostructured despite adherence to Vegard's Law of alloys, highlighting the importance of careful chemical characterization to differentiate between thermoelectric alloys and composites.

Hydrothermal synthesis and thermoelectric properties of PbS

“…Thermoelectric materials capable of converting energy between heat and electricity are currently attracting significant attention as a part of a search for sustainable alternative energy sources [1][2][3][4][5][6]. The efficiency of thermoelectric devices is strongly associated with the dimensionless figure of merit ZT defined as:…”

“…The rocksalt-structured lead-based chalcogenides, have been extensively studied as promising thermoelectric materials [1,[6][7][8][9]. Especially lead telluride (PbTe), as one of the most * E-mail: whylxs@126.com † E-mail: taichao80@hotmail.com frequently investigated materials in this system, remains the top-performing thermoelectric at midtemperature range (600 K to 900 K) [10].…”

Chemical composition tuning in quaternary p-type Pb-chalcogenides – a promising strategy for enhanced thermoelectric performance

“…4,18 In contrast, higher zT values of PbTe-PbS [19][20][21][22] compared to the binary systems are credited to the reduction in lattice thermal conductivity, which originates from phonon scattering at the interfaces of secondary phases, as PbS shows very limited solubility in the PbTe matrix. 23 Here, we present a strategy to design single-phase quaternary alloys with high thermoelectric efficiency. We have extended the PbS solubility limit in the PbTe matrix by simultaneous alloying with PbSe and PbS.…”

Enhanced Thermoelectric Performance in PbTe–PbS Nanocomposites

“…The nucleation and growth region is generally found at the outer extremes of the composition range where free energy curve has positive curvature suggesting even small variation in composition may result in enhanced free energy (Girard et al 2013). When the system is cooled rapidly, it assumes the metastable phase.…”

“…PbTe-PbS is one of the well-known binary systems, which exhibits the miscibility gap vulnerable to thermodynamic phase separation through metastable nucleation and growth or spinodal decomposition process (Girard et al 2013;Liu and Chang 1994), achieving two different nanostructures as schematically shown in Figure 1. The nucleation and growth region is generally found at the outer extremes of the composition range where free energy curve has positive curvature suggesting even small variation in composition may result in enhanced free energy (Girard et al 2013).…”