Anomalous Electronic Transport in Dual-Nanostructured Lead Telluride

Asymmetrical Transport Distribution Function: Skewness as a Key to Enhance Thermoelectric Performance

How to achieve high thermoelectric figure of merit is still a scientific challenge. By solving the Boltzmann transport equation, thermoelectric properties can be written as integrals of a single function, the transport distribution function (TDF). In this work, the shape effects of transport distribution function in various typical functional forms on thermoelectric properties of materials are systematically investigated. It is found that the asymmetry of TDF, characterized by skewness, can be used to describe universally the trend of thermoelectric properties. By defining symmetric and asymmetric TDF functions, a novel skewness is then constructed for thermoelectric applications. It is demonstrated, by comparison with ab initio calculations and experiments, that the proposed thermoelectric skewness not only perfectly captures the main feature of conventional skewness but also is able to predict the thermoelectric power accurately. This comparison confirms the unique feature of our proposed thermoelectric skewness, as well as its special role of connection between the statistics of TDF and thermoelectric properties of materials. It is also found that the thermoelectric performance can be enhanced by increasing the asymmetry of TDF. Finally, it is also interesting to find that the thermoelectric transport properties based on typical quantum statistics (Fermi-Dirac distributions) can be well described by typical shape parameter (skewness) for classical statistics.

Exceptionally High Average Power Factor and Thermoelectric Figure of Merit in n-type PbSe by the Dual Incorporation of Cu and Te

Thermoelectric materials with high average power factor and thermoelectric figure of merit (ZT) has been a sought-after goal. Here, we report new n-type thermoelectric system Cu_xPbSe_0.99Te_0.01 (x = 0.0025, 0.004, and 0.005) exhibiting record-high average ZT ∼ 1.3 over 400-773 K ever reported for n-type polycrystalline materials including the state-of-the-art PbTe. We concurrently alloy Te to the PbSe lattice and introduce excess Cu to its interstitial voids. Their resulting strong attraction facilitates charge transfer from Cu atoms to the crystal matrix significantly. It follows the increased carrier concentration without damaging its mobility and the consequently improved electrical conductivity. This interaction also increases effective mass of electron in the conduction band according to DFT calculations, thereby raising the magnitude of Seebeck coefficient without diminishing electrical conductivity. Resultantly, Cu_0.005PbSe_0.99Te_0.01 attains an exceptionally high average power factor of ∼27 μW cm^-1 K^-2 from 400 to 773 K with a maximum of ∼30 μW cm^-1 K^-2 at 300 K, the highest among all n- and p-type PbSe-based materials. Its ∼23 μW cm^-1 K^-2 at 773 K is even higher than ∼21 μW cm^-1 K^-2 of the state-of-the-art n-type PbTe. Interstitial Cu atoms induce the formation of coherent nanostructures. They are highly mobile, displacing Pb atoms from the ideal octahedral center and severely distorting the local microstructure. This significantly depresses lattice thermal conductivity to ∼0.2 Wm^-1 K^-1 at 773 K below the theoretical lower bound. The multiple effects of the dual incorporation of Cu and Te synergistically boosts a ZT of Cu_0.005PbSe_0.99Te_0.01 to ∼1.7 at 773 K.

Extraordinary Thermoelectric Performance Realized in n-Type PbTe through Multiphase Nanostructure Engineering

Lead telluride has long been realized as an ideal p-type thermoelectric material at an intermediate temperature range; however, its commercial applications are largely restricted by its n-type counterpart that exhibits relatively inferior thermoelectric performance. This major limitation is largely solved here, where it is reported that a record-high ZT value of ≈1.83 can be achieved at 773 K in n-type PbTe-4%InSb composites. This significant enhancement in thermoelectric performance is attributed to the incorporation of InSb into the PbTe matrix resulting in multiphase nanostructures that can simultaneously modulate the electrical and thermal transport. On one hand, the multiphase energy barriers between nanophases and matrix can boost the power factor in the entire temperature range via significant enhancement of the Seebeck coefficient and moderately reducing the carrier mobility. On the other hand, the strengthened interface scattering at the intensive phase boundaries yields an extremely low lattice thermal conductivity. This strategy of constructing multiphase nanostructures can also be highly applicable in enhancing the performance of other state-of-the-art thermoelectric systems.

Enhanced Average Thermoelectric Figure of Merit of the PbTe-SrTe-MnTe Alloy

Thermoelectric properties of Na-doped PbTe-SrTe system have been improved by the addition of Mn. The substitution of Mn for Pb modifies the band structure of the PbTe-SrTe alloy, which enlarges the band gap and increases the valence band degeneracy. This leads to increased thermopowers and power factors near room temperature, and the electronic contribution to the total thermal conductivity is also substantially reduced due to increased resistivity. Moreover, alloying of MnTe within the PbTe matrix introduces low angle grain boundaries, and significantly reduces the lattice thermal conductivity due to the dislocation scattering. A thermoelectric figure of merit as high as 1.98 and an enhancement of the average thermoelectric figure of merit by 18% are achieved for the sample with 4 at% Mn with respect to the Mn-free sample, which can be mainly attributed to the synergistic effects of the band structure modification and the dislocation scattering on phonon transport, both induced by alloying with MnTe. Our experimental results demonstrate the promising potential of PbTe-SrTe-MnTe system for the application of waste heat recovery.

Sb induces both doping and precipitation for improving the thermoelectric performance of elemental Te

Eco-friendly Sb-doping leads to a zT of 0.9 in elemental Te.

Structural disorder, anisotropic micro-strain and cation vacancies in thermo-electric lead chalcogenides

Structural disorder, cation defects and anisotropic microstrain is quantified in the deceptively simple rock salt lead chalcogenides, PbX (X = S, Se, Te), based on high-resolution synchrotron powder X-ray diffraction analysis.

Boundary Engineering for the Thermoelectric Performance of Bulk Alloys Based on Bismuth Telluride

Thermoelectrics, which transports heat for refrigeration or converts heat into electricity directly, is a key technology for renewable energy harvesting and solid-state refrigeration. Despite its importance, the widespread use of thermoelectric devices is constrained because of the low efficiency of thermoelectric bulk alloys. However, boundary engineering has been demonstrated as one of the most effective ways to enhance the thermoelectric performance of conventional thermoelectric materials such as Bi2 Te3 , PbTe, and SiGe alloys because their thermal and electronic transport properties can be manipulated separately by this approach. We review our recent progress on the enhancement of the thermoelectric figure of merit through boundary engineering together with the processing technologies for boundary engineering developed most recently using Bi2 Te3 -based bulk alloys. A brief discussion of the principles and current status of boundary-engineered bulk alloys for the enhancement of the thermoelectric figure of merit is presented. We focus mainly on (1) the reduction of the thermal conductivity by grain boundary engineering and (2) the reduction of thermal conductivity without deterioration of the electrical conductivity by phase boundary engineering. We also discuss the next potential approach using two boundary engineering strategies for a breakthrough in the area of bulk thermoelectric alloys.

Assembly of thiometalate-based {Mo16 } and {Mo36 } composite clusters combining [Mo2O2S2 ](2+) cations and selenite anions

A new family of thiometalate-based composite molecular materials is synthesized and characterized. 1.6 and 1.9 nm-sized clusters are observed in the gas phase utilizing high-resolution ESI-MS. The diversity of the selenite anions as an inorganic ligand is demonstrated by the isolation of the highest nuclearity selenium-based oxothiometalate materials reported so far. The observed proton conductivity of the selenite based oxothiometalate species renders them as promising alternative materials for fuel-cell applications.

Enhancing thermopower and hole mobility in bulk p-type half-Heuslers using full-Heusler nanostructures

The concept of band structure engineering near the Fermi level through atomic-scale alteration of a bulk semiconductor crystal structure using coherently embedded intrinsic semiconducting quantum dots provides a unique opportunity to manipulate the transport behavior of the existing ensembles of carriers within the semiconducting matrix. Here we show that in situ growth of coherent nanometer-scale full-Heusler quantum dots (fH-QDs) within the p-type Ti(0.5)Hf(0.5)CoSb(0.9)Sn(0.1) half-Heusler (hH) matrix induces a drastic decrease of the effective hole density within the hH/fH-QD nanocomposites at 300 K followed by a sharp increase with rising temperature. This behavior is associated with the formation of staggered heterojunctions with a valence band (VB) offset energy, ΔE at the hH/fH phase boundaries. The energy barrier (ΔE) discriminates existing holes with respect to their energy by trapping low energy (LE) holes, while promoting the transport of high energy (HE) holes through the VB of the fH-QDs. This "hole culling" results in surprisingly large increases in the mobility and the effective mass of HE holes contributing to electronic conduction. The simultaneous reduction in the density and the increase in the effective mass of holes resulted in large enhancements of the thermopower whereas; the increase in the mobility minimizes the drop in the electrical conductivity.

Enhanced thermoelectric properties of Pb-doped BiCuSeO ceramics

A high-performance thermoelectric oxyselenide BiCuSeO ceramic with ZT > 1.1 at 823 K and higher average ZT value (ZTave ≈0.8) is obtained. The heavy doping element and nanostructures can effectively tune its electronic structure, hole concentration, and thermal conductivity, resulting in substantially enhanced mobility, power factor, and thus ZT value. This work provides a path to high-performance thermoelectric ceramics.

A promising mid-temperature thermoelectric material candidate: Pb/Sn-codoped In₄PbxSnySe₃

A facile polycrystalline Pb/Sn-codoped In4Pb0.01SnySe3 material is obtained, its thermoelectric performance is evaluated, and the intrinsic reasons have been studied. This material shows an excellent ZT value of 1.4 at 733 Kelvin, among the best values of the relative materials, thus making it a promising candidate for the mid-temperature thermoelectric application.

Effect of aluminum on the thermoelectric properties of nanostructured PbTe

In the present work, the effect of aluminum (Al) on the thermoelectric properties of PbTe is studied. Aluminum doped PbTe samples, fabricated by a ball milling and hot pressing, have Seebeck coefficients between -100 and -200 μV K-1 and electrical conductivities of (3.6-18) × 104 S m-1 at room temperature, which means that Al is an effective donor in PbTe. The first principle calculations clearly show an increase of the density of states close to the Fermi level in the conduction band due to Al doping, which averages up the energy and effective mass of electrons, resulting in enhancement of the Seebeck coefficient. The maximum figure-of-merit ZT of 1.2 is reached at 770 K in the Al0.03PbTe sample.

Band engineering of thermoelectric materials

Lead chalcogenides have long been used for space-based and thermoelectric remote power generation applications, but recent discoveries have revealed a much greater potential for these materials. This renaissance of interest combined with the need for increased energy efficiency has led to active consideration of thermoelectrics for practical waste heat recovery systems-such as the conversion of car exhaust heat into electricity. The simple high symmetry NaCl-type cubic structure, leads to several properties desirable for thermoelectricity, such as high valley degeneracy for high electrical conductivity and phonon anharmonicity for low thermal conductivity. The rich capabilities for both band structure and microstructure engineering enable a variety of approaches for achieving high thermoelectric performance in lead chalcogenides. This Review focuses on manipulation of the electronic and atomic structural features which makes up the thermoelectric quality factor. While these strategies are well demonstrated in lead chalcogenides, the principles used are equally applicable to most good thermoelectric materials that could enable improvement of thermoelectric devices from niche applications into the mainstream of energy technologies.

Strong phonon scattering by layer structured PbSnS(2) in PbTe based thermoelectric materials

The incorporation of PbSnS(2) in PbTe results in a tremendous reduction of the lattice thermal conductivity to 0.8 W/mK at room temperature, a reduction of almost 60% over bulk PbTe. Transmission electron microscopy reveals very high density displacement layers, misfit dislocations, and phase boundaries. Our thermal transport calculations based on modified Debye-Callaway model, well in agreement with the experimental measurements, reveal that the layer structured PbSnS(2) plays an important role in reducing the lattice thermal conductivity.