Inhibiting the bipolar effect via band gap engineering to improve the thermoelectric performance in n-type Bi2-Sb Te3 for solid-state refrigeration

Abstractp‐Type elemental tellurium (Te) has been found to be a promising thermoelectric (TE) material due to its high band degeneracy near the valence band maximum, and has exhibited a high zT ≈ 1.0 above 600 K. However, when forming Te1−xSex solid solutions, the maximal zTs are reduced because of the severely decreased carrier concentration. It is demonstrated that Se alloying is beneficial for enhancing TE performance of elemental Te provided the carrier concentration is optimized. Through Se alloying, the lattice thermal conductivity is remarkably suppressed by the induced large mass and strain field fluctuation, while the power factor can be maintained at a relatively high value as a result of the moderate alloying scattering potential, the unchanged density‐of‐state effective mass, and the optimized carrier concentration in Te1−xSex alloys. Notably, a positive temperature dependence of carrier mobility is observed near room temperature in Te1−xSex, which is proven to be caused by grain boundary scattering. A maximal zT ≈ 1.05 at 625 K is realized in Te0.93Se0.04As0.03 alloys, about 9% higher than the Se‐free Te. The conversion efficiency between 300 and 625 K is also improved ≈18% via Se alloying.

High-efficiency thermoelectric (TE) technology is determined by the performance of TE materials. Doping is a routine approach in TEs to achieve optimized electrical properties and lowered thermal conductivity. However, how to choose appropriate dopants with desirable solution content to realize high TE figure-of-merit (zT) is very tough work. In this study, via the use of large mass and strain field fluctuations as indicators for low lattice thermal conductivity, the combination of (Mg, Bi) in GeTe is screened as very effective dopants for potentially high zTs. In experiments, a series of (Mg, Bi) co-doped GeTe compounds are prepared and the electrical and thermal transport properties are systematically investigated. Ultralow lattice thermal conductivity, about 0.3 W m-1 K-1 at 600 K, is obtained in Ge0.9 Mg0.04 Bi0.06 Te due to the introduced large mass and strain field fluctuations by (Mg, Bi) co-doping. In addition, (Mg, Bi) co-doping can introduce extra electrons for optimal carrier concentration and diminish the energy offset at the top of the valence band for high density-of-states effective mass. Via these synthetic effects, a superhigh zT of ≈2.5 at 700 K is achieved for Ge0.9 Mg0.04 Bi0.06 Te. This study sheds light on the rational design of effective dopants in other TE materials.

Topological insulator VxBi1.08-Sn0.02Sb0.9Te2S as a promising n-type thermoelectric material

CaMg2Bi2-based compounds, a kind of the representative compounds of Zintl phases, have uniquely inherent layered structure and hence are considered to be potential thermoelectric materials. Generally, alloying is a traditional and effective way to reduce the lattice thermal conductivity through the mass and strain field fluctuation between host and guest atoms. The cation sites have very few contributions to the band structure around the fermi level; thus, cation substitution may have negligible influence on the electric transport properties. What is more, widespread application of thermoelectric materials not only desires high ZT value but also calls for low-cost and environmentally benign constituent elements. Here, Ba substitution on cation site achieves a sharp reduction in lattice thermal conductivity through enhanced point defects scattering without the obvious sacrifice of high carrier mobility, and thus improves thermoelectric properties. Then, by combining further enhanced phonon scattering caused by isoelectronic substitution of Zn on the Mg site, an extraordinarily low lattice thermal conductivity of 0.51 W m−1 K−1 at 873 K is achieved in (Ca0.75Ba0.25)0.995Na0.005Mg1.95Zn0.05Bi1.98 alloy, approaching the amorphous limit. Such maintenance of high mobility and realization of ultralow lattice thermal conductivity synergistically result in broadly improvement of the quality factor β. Finally, a maximum ZT of 1.25 at 873 K and the corresponding ZTave up to 0.85 from 300 K to 873 K have been obtained for the same composition, meanwhile possessing temperature independent compatibility factor. To our knowledge, the current ZTave exceeds all the reported values in AMg2Bi2-based compounds so far. Furthermore, the low-cost and environment-friendly characteristic plus excellent thermoelectric performance also make the present Zintl phase CaMg2Bi2 more competitive in practical application.

Enhanced thermoelectric performance of Cr-doped ZnSb through lattice thermal conductivity reduction below the phonon glass limit

Lattice distortion and mass fluctuation are two long-believed potential mechanisms for the reduced lattice thermal conductivity in high-entropy ceramics (HECs). However, related studies remain unclear. Taking high-entropy diborides (HEBs) as the prototype, the lattice-distortion-driven reduced lattice thermal conductivity in HECs is uncovered, whereas the influence of mass fluctuation is neglectable. Specifically, two groups of HEBs are designed by regulating the long-believed mechanisms of lattice distortion and mass fluctuation based on machine-learning-potential-based molecular dynamics simulations. The theoretical and experimental results show that lattice distortion plays a pivotal role in modulating the lattice thermal conductivity of HEBs, while the influence of mass fluctuation is neglectable. Further studies find that the aggravation of lattice distortion enables the reduction of the lattice thermal conductivity through the decreased phonon velocity and Debye temperature resulting from the simultaneously enhanced scattering of strain field fluctuation and bond strength fluctuation. In addition, lattice distortion is found to lower the electronic thermal conductivity by competing with vacancies. The research unravels the long-standing mystery of the reduced lattice thermal conductivity in HECs and offers insightful guidance for developing HECs with ultra-low thermal conductivities.

GeTe and its derivatives emerging as a promising lead-free thermoelectric candidate have received extensive attention. Here, a new route was proposed that the minimization of κL in GeTe through considerable enhancement of acoustic phonon scattering by introducing ultrafine ferroelectric domain structure. We found that Bi and Ca dopants induce strong atomic strain disturbance in the GeTe matrix because of large differences in atom radius with host elements, leading to the formation of ultrafine ferroelectric domain structure. Furthermore, large strain field and mass fluctuation induced by Bi and Ca codoping result in further reduced κL by effectively shortening the phonon relaxation time. The co-existence of ultrafine ferroelectric domain structure, large strain field, and mass fluctuation contribute to an ultralow lattice thermal conductivity of 0.48 W m-1 K-1 at 823 K. Bi and Ca codoping significantly enhances the Seebeck coefficient and power factor through reducing the energy offset between light and heavy valence bands of GeTe. The modified band structure boosts the power factor up to 47 μW cm-1 K-2 in Ge0.85Bi0.09Ca0.06Te. Ultimately, a high ZT of ∼2.2 can be attained. This work demonstrates a new design paradigm for developing high-performance thermoelectric materials.

Significantly improved thermoelectric properties of Nb-doped ZrNiSn half-Heusler compounds

Effects of Ge doping on the thermoelectric properties of TiCoSb-based p-type half-Heusler compounds

The dearth of n-type sulfides with thermoelectric performance comparable to that of their p-type analogues presents a problem in the fabrication of all-sulfide devices. Chalcopyrite (CuFeS2) offers a rare example of an n-type sulfide. Chemical substitution has been used to enhance the thermoelectric performance of chalcopyrite through preparation of Cu1-xSnxFeS2 (0 ≤ x ≤ 0.1). Substitution induces a high level of mass and strain field fluctuation, leading to lattice softening and enhanced point-defect scattering. Together with dislocations and twinning identified by transmission electron microscopy, this provides a mechanism for scattering phonons with a wide range of mean free paths. Substituted materials retain a large density-of-states effective mass and, hence, a high Seebeck coefficient. Combined with a high charge-carrier mobility and, thus, high electrical conductivity, a 3-fold improvement in power factor is achieved. Density functional theory (DFT) calculations reveal that substitution leads to the creation of small polarons, involving localized Fe2+ states, as confirmed by X-ray photoelectron spectroscopy. Small polaron formation limits the increase in carrier concentration to values that are lower than expected on electron-counting grounds. An improved power factor, coupled with substantial reductions (up to 40%) in lattice thermal conductivity, increases the maximum figure-of-merit by 300%, to zT ≈ 0.3 at 673 K for Cu0.96Sn0.04FeS2.

The competition between phase stability and band gap in Yb-filled cobalt-free Fe4-Ni Sb12 skutterudites

Nanostructuring technology has been widely employed to reduce the thermal conductivity of thermoelectric materials because of the strong phonon-boundary scattering. Optimizing the carrier concentration can not only improve the electrical properties, but also affect the lattice thermal conductivity significantly due to the electron-phonon scattering. The lattice thermal conductivity of silicon nanostructures considering electron-phonon scattering is investigated for comparing the lattice thermal conductivity reductions resulting from nanostructuring technology and the carrier concentration optimization. We performed frequency-dependent simulations of thermal transport systematically in nanowires, solid thin films and nanoporous thin films by solving the phonon Boltzmann transport equation using the discrete ordinate method. All the phonon properties are based on the first-principles calculations. The results show that the lattice thermal conductivity reduction due to the electron-phonon scattering decreases as the feature size of nanostructures goes down and could be ignored at low feature sizes (50 nm for n-type nanowires and 20 nm for p-type nanowires and n-type solid thin films) or a high porosity (0.6 for n-type 500 nm-thick nanoporous thin films) even when the carrier concentration is as high as 1021 cm-3. Similarly, the size effect due to the phonon-boundary scattering also becomes less significant with the increase of carrier concentration. The findings provide a fundamental understanding of electron and phonon transports in nanostructures, which is important for the optimization of nanostructured thermoelectric materials.

0D Point defects, 1D dislocations and 2D interfaces act as effective sources for scattering phonons due to the fluctuations in atomic mass and lattice strain. Dislocations have been demonstrated recently to enable a significant reduction in lattice thermal conductivity of PbTe, and a co‐substitution of Eu and Na at Pb site was found to effectively introduce dense in‐grain dislocations in p‐PbTe. This motivates this work to focus on a further PbSe alloying in Na0.03Eu0.03Pb0.94Te for introducing point defects in addition to these dislocations, with participation of a further reduction in lattice thermal conductivity. The resultant extremely low lattice thermal conductivity (a minimum of ≈0.4 W m−1 K−1) in the entire temperature range leads to an eventual ≈30% enhancement in the average thermoelectric figure of merit zT in the working temperature range and a peak zT as high as ≈2.3 at 850 K in Na0.03Eu0.03Pb0.94Te0.9Se0.1.

AgCrSe2, which crystallizes in alternative layers of Ag+ and CrSe2− octahedral structure, has shown great potential as good thermoelectric material due to its unique ultralow lattice thermal conductivity. In this work, compound Cr2/3Te is alloyed with the matrix Ag0.97CrSe2 and the carrier concentration ranges within 1018–1020 cm−3, enabling a reliable assessment of transport properties based on single parabolic band model at room temperature. Moreover, homogeneous nanoprecipitate is observed in the matrix for high Cr2/3Te content samples, which leads to the scattering of main heat carrier of long-wavelength phonons, and thus a slight reduction of lattice thermal conductivity (∼0.3 W/m K) compared with intrinsic AgCrSe2. Combined with the optimized carrier concentration and the low lattice thermal conductivity, a figure of merit zT of 0.6 at 650 K is achieved, exceeding other reported AgCrSe2 systems, demonstrating the current Ag0.97CrSe2(Cr2/3Te)x materials as good potential thermoelectrics.

α-MgAgSb is taken as the p-type leg material for recently focused Mg-based thermoelectric devices because of the high thermoelectric performance near room temperature. However, the thermoelectric performance of α-MgAgSb is inhibited by the existence of the Ag-rich second phase. The ordinary methods like carrier concentration optimization and minimizing lattice thermal conductivity were nearly invalid because of the extremely low doping level for heteroatoms and intrinsically low lattice thermal conductivity. The crystal structure of α-MgAgSb can be viewed as Ag atom filled in half distorted hexahedron in the distorted rock salt skeleton formed by the Mg–Sb sublattice. In this work, by replacing the smaller Mg in the sublattice with Pb, the volume of the distorted hexahedron is effectively expanded to accommodate Ag atoms and then lead to the re-dissolution of Ag-rich second phase in the matrix. In addition, as Ag is the main source of low-frequency phonons, the enhanced lattice anharmonicity by Pb doping leads to stronger scattering of phonons in the distorted hexahedron and results in 20% reduction of lattice thermal conductivity in the temperature range of 300–500 K. Finally, the figure of merit zT is enhanced by ∼40% in the whole temperature range, demonstrating that lattice management is a promising method for the optimization of α-MgAgSb materials.