General Figures of Merit ZQ for Thermoelectric Generators Under Constant Heat-In Flux Boundary.

The thermoelectric figure of merit ZT bridges the efficiency and material parameters for a thermoelectric device operating under constant temperature of the hot- and cold-source thermal boundary (Type-I TB). However, many application scenarios fall under the constant heat-in flux (qh ) and constant cold-source temperature (Tc ) thermal boundary (Type-II TB), for which a figure of merit is absent for more than half a century. This study aims to fill this gap and propose a figure of merit ZQD for the thermoelectric devices under the Type-II TB condition, defined as Z Q D = ( Z T c Z T c + 1 ) ( h κ ) ( q h T c ) $Z{Q}_{\mathrm{D}} = ( {\frac{{Z{T}_{\mathrm{c}}}}{{Z{T}_{\mathrm{c}} + 1}}} )( {\frac{h}{\kappa }} )( {\frac{{{q}_{\mathrm{h}}}}{{{T}_{\mathrm{c}}}}} )$ , where Z, h, and κ are the traditional figure of merit, leg height, and thermal conductivity, respectively. The effectiveness of ZQD is verified through both numerical calculations and experiments, which are more accurate and practical than ZT. Furthermore, a system-level figure of merit ZQS is suggested after considering the external thermal resistance. Finally, optimization strategies for thermoelectric systems based on ZQS are proposed, showing a 30% enhancement in the efficiency. ZQD and ZQS are expected to be widely used in the thermoelectric field.

Wide-temperature-range thermoelectric n-type Mg3(Sb,Bi)2 with high average and peak zT values.

Mg3(Sb,Bi)2 is a promising thermoelectric material suited for electronic cooling, but there is still room to optimize its low-temperature performance. This work realizes >200% enhancement in room-temperature zT by incorporating metallic inclusions (Nb or Ta) into the Mg3(Sb,Bi)2-based matrix. The electrical conductivity is boosted in the range of 300-450 K, whereas the corresponding Seebeck coefficients remain unchanged, leading to an exceptionally high room-temperature power factor >30 µW cm-1 K-2; such an unusual effect originates mainly from the modified interfacial barriers. The reduced interfacial barriers are conducive to carrier transport at low and high temperatures. Furthermore, benefiting from the reduced lattice thermal conductivity, a record-high average zT > 1.5 and a maximum zT of 2.04 at 798 K are achieved, resulting in a high thermoelectric conversion efficiency of 15%. This work demonstrates an efficient nanocomposite strategy to enhance the wide-temperature-range thermoelectric performance of n-type Mg3(Sb,Bi)2, broadening their potential for practical applications.

Bi-Deficiency Leading to High-Performance in Mg3 (Sb,Bi)2 -Based Thermoelectric Materials.

Mg3 (Sb,Bi)2 is a potential nearly-room temperature thermoelectric compound composed of earth-abundant elements. However, complex defect tuning and exceptional microstructural control are required. Prior studies have confirmed the detrimental effect of Mg vacancies (VMg ) in Mg3 (Sb,Bi)2 . This study proposes an approach to mitigating the negative scattering effect of VMg by Bi deficiency, synergistically modulating the electrical and thermal transport properties to enhance the thermoelectric performance. Positron annihilation spectrometry and Cs -corrected scanning transmission electron microscopy analyses indicated that the VMg tends to coalesce due to the introduced Bi vacancies (VBi ). The defects created by Bi deficiency effectively weaken the scattering of electrons from the intrinsic VMg and enhance phonon scattering. A peak zT of 1.82 at 773 K and high conversion efficiency of 11.3% at ∆T = 473 K are achieved in the optimized composition of Mg3 (Sb,Bi)2 by tuning the defect combination. This work demonstrates a feasible and effective approach to improving the performance of Mg3 (Sb,Bi)2 as an emerging thermoelectric material.

Large Transverse and Longitudinal Magneto-Thermoelectric Effect in Polycrystalline Nodal-Line Semimetal Mg3 Bi2.

Topological semimetals provide new opportunities for exploring novel thermoelectric phenomena, owing to their exotic and nontrivial electronic structure topology around the Fermi surface. Herein, the discovery of large transverse and longitudinal magneto-thermoelectric (MTE) effects in Mg3 Bi2 is reported and predicted to be a type-II nodal-line semimetal in the absence of spin-orbit coupling (SOC). The maximum transverse power factor is 2182 µW m-1 K-2 at 13.5 K and 6 Tesla. The longitudinal power factor reaches up to 3043 µW m-1 K-2 , which is 20 times higher than that in a zero-strength magnetic field and is also comparable to state-of-the-art MTE materials. By compensating the Mg loss in Mg-rich conditions for tuning the carrier concentration close to intrinsic state, the sample fabricated in this study exhibits a large linear non-saturating magnetoresistance of 940% under a field of 14 Tesla. Using density functional calculations, the authors attribute the underlying mechanism to the parent linear-dispersed nodal-line electronic structure without SOC and the anisotropic Fermi surface shape with SOC, highlighting the essential role of high carrier mobility and open electron orbits in the moment space. This work offers a new avenue toward highly efficient MTE materials through defect engineering in polycrystalline topological semimetals.

Solid-State Janus Nanoprecipitation Enables Amorphous-Like Heat Conduction in Crystalline Mg3 Sb2 -Based Thermoelectric Materials.

Solid-state precipitation can be used to tailor material properties, ranging from ferromagnets and catalysts to mechanical strengthening and energy storage. Thermoelectric properties can be modified by precipitation to enhance phonon scattering while retaining charge-carrier transmission. Here, unconventional Janus-type nanoprecipitates are uncovered in Mg3 Sb1.5 Bi0.5 formed by side-by-side Bi- and Ge-rich appendages, in contrast to separate nanoprecipitate formation. These Janus nanoprecipitates result from local comelting of Bi and Ge during sintering, enabling an amorphous-like lattice thermal conductivity. A precipitate size effect on phonon scattering is observed due to the balance between alloy-disorder and nanoprecipitate scattering. The thermoelectric figure-of-merit ZT reaches 0.6 near room temperature and 1.6 at 773 K. The Janus nanoprecipitation can be introduced into other materials and may act as a general property-tailoring mechanism.

Enhanced Thermoelectric Performance by Strong Phonon Scattering at the Heterogeneous Interfaces of the Mg2Sn/Mg3Sb2 High-Content Nanocomposite.

Nano approaches are practical strategies to boost the thermoelectric figure of merit due to the strong phonon scattering from the grain boundaries and nanoinclusions. Here, we have reported a strong phonon scattering at the heterogeneous interfaces of Mg2Sn/Mg3Sb2 high-content nanocomposites (HCnCs). As a result, a significantly reduced lattice thermal conductivity of 1.09 W m-1 K-1 was observed in the equimolar Mg2Sn/Mg3Sb2 HCnC, 80% lower than pure Mg2Sn and 25% lower than pure Mg3Sb2. As a result, a high ZT ∼ 1.13 at 773 K was achieved in the Mg2Sn/Mg3Sb2 HCnC. Furthermore, various defects, including solid solutions, nanoinclusions, and misfit dislocations, were observed in both the Mg3Sb2 phase and the Mg2Sn phase through the microstructure characterization.

The Electronic Transport Channel Protection and Tuning in Real Space to Boost the Thermoelectric Performance of Mg3+δ Sb2-y Bi y near Room Temperature.

The optimization of thermoelectric materials involves the decoupling of the transport of electrons and phonons. In this work, an increased Mg1-Mg2 distance, together with the carrier conduction network protection, has been shown as an effective strategy to increase the weighted mobility (U = µm ∗3/2) and hence thermoelectric power factor of Mg3+δ Sb2-y Bi y family near room temperature. Mg3+δ Sb0.5Bi1.5 has a high carrier mobility of 247 cm2 V-1 s-1 and a record power factor of 3470 µW m-1 K-2 at room temperature. Considering both efficiency and power density, Mg3+δ Sb1.0Bi1.0 with a high average ZT of 1.13 and an average power factor of 3184 µW m-1 K-2 in the temperature range of 50-250°C would be a strong candidate to replace the conventional n-type thermoelectric material Bi2Te2.7Se0.3. The protection of the transport channel through Mg sublattice means alloying on Sb sublattice has little effect on electron while it significantly reduces phonon thermal conductivity, providing us an approach to decouple electron and phonon transport for better thermoelectric materials.

Giant thermopower of ionic gelatin near room temperature.

Harvesting heat from the environment into electricity has the potential to power Internet-of-things (IoT) sensors, freeing them from cables or batteries and thus making them especially useful for wearable devices. We demonstrate a giant positive thermopower of 17.0 millivolts per degree Kelvin in a flexible, quasi-solid-state, ionic thermoelectric material using synergistic thermodiffusion and thermogalvanic effects. The ionic thermoelectric material is a gelatin matrix modulated with ion providers (KCl, NaCl, and KNO3) for thermodiffusion effect and a redox couple [Fe(CN)6 4-/Fe(CN)6 3-] for thermogalvanic effect. A proof-of-concept wearable device consisting of 25 unipolar elements generated more than 2 volts and a peak power of 5 microwatts using body heat. This ionic gelatin shows promise for environmental heat-to-electric energy conversion using ions as energy carriers.