Parallel Dislocation Networks and Cottrell Atmospheres Reduce Thermal Conductivity of PbTe Thermoelectrics

Abstract

Dislocations play an important role in thermal transport by scattering phonons. Nevertheless, for materials with intrinsically low thermal conductivity, such as thermoelectrics, classical models require exceedingly high numbers of dislocations (>1012 cm–2) to further impede thermal transport. In this work, a significant reduction in thermal conductivity of Na0.025Eu0.03Pb0.945Te is demonstrated at a moderate dislocation density of 1 × 1010 cm–2. Further characteristics of dislocations, including their arrangement, orientation, and local chemistry are shown to be crucial to their phonon‐scattering effect and are characterized by correlative microscopy techniques. Electron channeling contrast imaging reveals a uniform distribution of dislocations within individual grains, with parallel lines along four <111> directions. Transmission electron microscopy (TEM) shows the parallel networks are edge‐type and share the same Burgers vectors within each group. Atom probe tomography reveals the enrichment of dopant Na at dislocation cores, forming Cottrell atmospheres. The dislocation network is demonstrated to be stable during in situ heating in the TEM. Using the Callaway transport model, it is demonstrated that both parallel arrangement of dislocations and Cottrell atmospheres make dislocations more efficient in phonon scattering. These two mechanisms provide new avenues to lower the thermal conductivity in materials for thermal‐insulating applications.

Boron Strengthened GeTe‐Based Alloys for Robust Thermoelectric Devices with High Output Power Density

“…We thus employed atom probe tomography (APT) to reveal the distribution and concentration of constituent elements at the atomic scale. [ 70–72 ] Figure 2 a shows a homogeneous distribution of Ge, Sb, and Te in 3D space. The composition profile along the z ‐axis further proves the homogeneity of composition and an average bulk composition close to the nominal Ge 0.9 Sb 0.1 Te (Figure 2b).…”

Dislocations Stabilized by Point Defects Increase Brittleness in PbTe

“…Dopant aggregates (which do not necessarily constitute a secondary phase) appear with a number density of 1.6 × 10 17 cm −3 in the Te‐rich sample versus 4.1 × 10 16 cm −3 in the Pb‐rich sample. Past works on Pb chalcogenides doped with embrittling Ag, Cu, Na, and/or Eu dopants find similar dopant aggregation [ 29–32,64 ] and similar features are responsible for age hardening in structural Al‐based and reactor alloys. [ 65,66 ] While APT can not identify the non‐hardening n‐type dopant iodine (I and Te are indiscernible in APT), Bi and La appear to distribute homogenously in PbTe.…”

“…Cd, Cu, Eu, Ga, Ag, and/or Na additions measurably added strain and dislocations to Pb chalcogenides subjected to plastic deformation by ball milling or hot pressing. [12,[29][30][31][32][33]44] These dopants enable higher dislocation densities than plastic deformation alone, as evidenced by an increase in maximum strain from Na/ Eu doping in ball milled PbTe. [44] Figure 4 demonstrates that Na has a tendency to increase dislocation density in PbTe without any additional processing.…”

“…In many cases, interactions between dopant elements and dislocations are directly identified using microscopy, signifying a mechanism by which dislocation mobility is limited and internal strain is increased. [29][30][31][32] Given that plastic deformation in a crystal relies on dislocations traveling through its lattice, such defect engineering strategies should have obvious and significant effects on brittleness. Indeed, Pb chalcogenides with extensive defect engineering are often measured to have high hardness-an indicator of embrittlement.…”

Synergistic Texturing and Bi/Sb‐Te Antisite Doping Secure High Thermoelectric Performance in Bi0.5Sb1.5Te3‐Based Thin Films

“…[6] Band engineering strategies, such as band convergence and resonant state engineering, can effectively increase S. [7,8] In terms of suppressing κ l , various scattering centers have been introduced into thermoelectric materials to strengthening phonon scattering at full wavelength scale (ranging from microscale to atomic scale) and miniaturize κ l . [9] Most typically, dense point defects introduced by doping/alloying, [10] corresponding dense dislocations, [11] and strain fields, [12] can effectively scatter short-wavelength phonons, and dramatically reduce κ l . Dense grain boundaries and interfaces introduced by nanostructuring and nanoprecipitates can strengthen mid-to long-wavelength phonon scattering.…”