Topologically-Enhanced Thermoelectric Properties in Bi₂Te₃-Based Compounds: Effects of Grain Size and Misorientation

Abstract

Topological insulators (TIs) and thermoelectric (TE) materials seem to belong to distinct physical realms; however, in practice, they both share common characteristics. Introducing concepts from TIs into TE materials to enhance their performance and achieve better understanding of electronic transport requires extensive research. Particularly, grain size, misorientation, and grain boundary (GB) character are of utmost importance to attain effective charge carrier transport in TE polycrystals; these factors, however, have not been thoroughly explored. Herein, we investigate the correlation between grain size, misorientation, and lattice strain in Bi 2 Te 3 and its TI signature, aiming to improve its TE performance. We reveal an unusual behavior showing that electron mobility increases upon the increase of grain size, reaching at a maximum value of 495 cm 2 /V•s for an optimum grain size of 600 nm and most-frequent GB misorientation angle of 60°and then decreases with increasing grain size. It is also indicated that the combined effects of grain size reduction and point defects induce lattice strain in the Bi 2 Te 3 -matrix that is essential to trigger the TI contribution to TE transport. This trend is corroborated by first-principles calculations showing that compressive strains form multiple valleys in the valence band and opens the TI band gap. Such a combination of physical phenomena in a well-known TE material is unique and can promote our understanding of the nature of TE transport with implications for TE energy conversion.

Development of Nanostructured Bi2Te3 with High Thermoelectric Performance by Scalable Synthesis and Microstructure Manipulations

“…It is clearly indicated that the Te precipitates are most effective in reducing the lattice thermal conductivity, which is achieved by the specific synthesis method of BT2. Again, we highlight that the fitting made between the Callaway model (blue and violet solid lines in Figure c) and the experimental data points (blue triangles and red circles) for BT2 and BCT1, respectively, is valid for the low-temperature regime, where BP effects are negligible …”

“…Boundary scattering dominates over intrinsic scattering (often referred to as the “Casimir limit”), which results from the unique microstructure of Te nanoparticles decorating the Bi 2 Te 3 grains. We note that evaluation of the above values of κ L is valid for adequately low temperatures, i.e., room temperature, where bipolar (BP) effects are negligible . Here, κ  κ e + κ L + κ BP .…”

“…Similar conclusions were reported by Gayner et al for Bi 2 Te 3 of different grain sizes. 25 We, therefore, expect that any anisotropy effect associated with the HP process on the measured TE properties should be marginal compared to the effects of synthesis routes, which are investigated herein. Here, ε k the calculated kth energy band.…”

“…We note that evaluation of the above values of κ L is valid for adequately low temperatures, i.e., room temperature, where bipolar (BP) effects are negligible. 25 Here, κ � κ e + κ L + κ BP . BP effects become significant with increasing temperature; for this reason, the trend of the data points for BT1, BT2, and BCT1 in Figure 6c is decreasing with temperature until 325−400 °C, then increases due to BP effects.…”

“…Again, we highlight that the fitting made between the Callaway model (blue and violet solid lines in Figure 6c) and the experimental data points (blue triangles and red circles) for BT2 and BCT1, respectively, is valid for the low-temperature regime, where BP effects are negligible. 25 3.5. Thermoelectric Performance.…”