High-performance bulk thermoelectrics with all-scale hierarchical architectures

Abstract

With about two-thirds of all used energy being lost as waste heat, there is a compelling need for high-performance thermoelectric materials that can directly and reversibly convert heat to electrical energy. However, the practical realization of thermoelectric materials is limited by their hitherto low figure of merit, ZT, which governs the Carnot efficiency according to the second law of thermodynamics. The recent successful strategy of nanostructuring to reduce thermal conductivity has achieved record-high ZT values in the range 1.5-1.8 at 750-900 kelvin, but still falls short of the generally desired threshold value of 2. Nanostructures in bulk thermoelectrics allow effective phonon scattering of a significant portion of the phonon spectrum, but phonons with long mean free paths remain largely unaffected. Here we show that heat-carrying phonons with long mean free paths can be scattered by controlling and fine-tuning the mesoscale architecture of nanostructured thermoelectric materials. Thus, by considering sources of scattering on all relevant length scales in a hierarchical fashion--from atomic-scale lattice disorder and nanoscale endotaxial precipitates to mesoscale grain boundaries--we achieve the maximum reduction in lattice thermal conductivity and a large enhancement in the thermoelectric performance of PbTe. By taking such a panoscopic approach to the scattering of heat-carrying phonons across integrated length scales, we go beyond nanostructuring and demonstrate a ZT value of ∼2.2 at 915 kelvin in p-type PbTe endotaxially nanostructured with SrTe at a concentration of 4 mole per cent and mesostructured with powder processing and spark plasma sintering. This increase in ZT beyond the threshold of 2 highlights the role of, and need for, multiscale hierarchical architecture in controlling phonon scattering in bulk thermoelectrics, and offers a realistic prospect of the recovery of a significant portion of waste heat.

Low Sound Velocity Contributing to the High Thermoelectric Performance of Ag8SnSe6

“…This should lead to its electronic performance not as promising as conventional thermoelectrics, which can be seen from the temperature‐dependent Seebeck coefficient and resistivity as shown in Figure 4 a,b, respectively. The resulting maximum thermoelectric power factor ( S 2 / ρ ) of ≈6 μW cm −1 K −2 is much lower than 20–40 μW cm −1 K −2 that is normally seen in conventional thermoelectrics 6, 10, 37, 45, 46, 47, 48…”

“…One successful strategy for improving zT is to enhance the power factor S 2 / ρ through band engineering,2, 3, 4, 5, 6, 7 provided the carrier concentration is optimized 8. The other effective strategy is typified by minimizing the only one independent material property, the lattice thermal conductivity ( κ L ), through nanostructuring,9, 10, 11, 12, 13, 14, 15 liquid phonons,16, 17 and lattice anharmonicity 18, 19…”

Heavy Doping by Bromine to Improve the Thermoelectric Properties of n‐type Polycrystalline SnSe

“…Numerous approaches have been explored to enhance TE performance, including increased power factor ( S 2 σ) by band engineering (resonant doping, band convergence, and band flattening , etc. )3, 4, 5 and decreased thermal conductivity by defect engineering,6 and nanoengineering 7, 8, 9. An alternative way to achieve high ZT values is to seek new classes of TE materials with intrinsically low thermal conductivity, such as clathrates, skutterudites, sulfur‐based compounds, Ag 9 GaSe 6 , and SnSe etc 10, 11, 12, 13, 14…”

Ultrahigh Power Factor in Thermoelectric System Nb0.95M0.05FeSb (M = Hf, Zr, and Ti)

“…Comparison of temperature‐dependent PF among Nb 0.95 M 0.05 FeSb (M = Hf, Zr, Ti) and other high‐performance thermoelectric materials 10, 12, 29, 33, 35, 56, 57, 58, 59…”