PbTe-based Materials Research Articles

Micro- and Macromechanical Properties of Thermoelectric Lead Chalcogenides.

Both n- and p-type lead telluride (PbTe)-based thermoelectric (TE) materials display high TE efficiency, but the low fracture strength may limit their commercial applications. To find ways to improve these macroscopic mechanical properties, we report here the ideal strength and deformation mechanism of PbTe using density functional theory calculations. This provides structure-property relationships at the atomic scale that can be applied to estimate macroscopic mechanical properties such as fracture toughness. Among all the shear and tensile paths that are examined here, we find that the lowest ideal strength of PbTe is 3.46 GPa along the (001)/⟨100⟩ slip system. This leads to an estimated fracture toughness of 0.28 MPa m1/2 based on its ideal stress-strain relation, which is in good agreement with our experimental measurement of 0.59 MPa m1/2. We find that softening and breaking of the ionic Pb-Te bond leads to the structural collapse. To improve the mechanical strength of PbTe, we suggest strengthening the structural stiffness of the ionic Pb-Te framework through an alloying strategy, such as alloying PbTe with isotypic PbSe or PbS. This point defect strategy has a great potential to develop high-performance PbTe-based materials with robust mechanical properties, which may also be applied to other materials and applications.

(Invited) Soft Mode Phase Transition and the Lattice Thermal Conductivity of PbTe-Based Materials

Exploiting the fascinating properties of materials near soft mode phase transitions is an emerging concept in the quest to increase thermoelectric efficiency [1,2]. The underlying idea is that soft phonons lead to intrinsically low thermal conductivity, while possibly preserving electronic transport properties. Using first principles simulations, we show that driving PbTe near the transition from the rocksalt to a rhombohedral structure will significantly reduce its lattice thermal conductivity. We illustrate this concept by applying biaxial tensile (001) strain to both PbTe and its alloy with rocksalt PbSe, and also by alloying PbTe with rhombohedral GeTe [3]. Moreover, we investigate in detail how tuning the proximity to the soft optical mode phase transition via chemical composition affects the lattice thermal conductivity of (Pb,Ge)Te alloys [4]. We show that the anharmonic contribution to the lattice thermal conductivity is minimized at the phase transition due to the maximized acoustic-optical interaction. Furthermore, mass disorder shifts the conductivity minimum towards the composition at which the scattering due to mass disorder is maximized. The total lattice thermal conductivity and its anharmonic contribution vary continuously between the rocksalt and rhombohedral phases as expected for the second-order phase transition. Our results show that alloys with soft optical mode transitions are promising materials for achieving low thermal conductivity and possibly high thermoelectric efficiency. [1] D. T. Morelli, V. Jovovic, and J. P. Heremans, Phys. Rev. Lett. 101, 035901 (2008). [2] L.-D. Zhao et al, Nature 508, 373 (2014). [3] R. M. Murphy, E. D. Murray, S. Fahy, and I. Savic, Phys. Rev. B 93, 104304 (2016). [4] R. M. Murphy, E. D. Murray, S. Fahy, and I. Savic, Phys. Rev. B 95, 144302 (2017).

Thermal stability of melt grown Tl-doped PbTeSe material for thermoelectric applications

Although the thermoelectric performance of PbTe-based materials, processed by hot-pressing, has recently been improved, there are concerns about its thermal stability. On the other hand, during the process of melt crystal growth the material is formed in a quasi-equilibrium condition, which results in homogeneous solid structure in a lower-energy state and, consequently, more thermally stable and mechanically robust. To test the thermal stability of the melt-grown crystal, a Tl-doped PbTe0.85 Se0.15 ingot has been grown from the melt by directional solidification. Four adjacent disc-shape samples were sliced perpendicular to the growth axis from the grown boule. The thermoelectric characterizations together with microstructure examination and mechanical hardness measurements were performed on each of the three as-grown samples as well as on the fourth sample after it has been annealed at 455°C for 570h. The characterization results, showing that the annealing has essentially no effects on the thermoelectric properties, metallurgical microstructure as well as mechanical hardness of the annealed sample, have demonstrated that structurally homogeneous materials processed by melt growth are more suitable than those processed by other methods, such as hot press or quench-annealing, for long-term thermoelectric applications at elevated temperatures.

Enhanced thermoelectric performance of PbTe bulk materials with figure of merit zT >2 by multi-functional alloying

Forming solid solutions in PbTe based materials can simultaneously reduce lattice thermal conductivity and engineer the band structure to enhance the electrical properties. In this paper, quaternary alloys of Pb1−xMgxTe0.8Se0.2 were designed to improve the figure of merit zT. The significant roles of MgTe in enhancing electrical properties and reducing thermal conductivity of PbTe0.8Se0.2 were investigated. A maximum zT of ∼2.2 at 820 K was achieved in PbTe0.8Se0.2 with 8% MgTe. Subsequently, a large dimension bulk (∼200 g, Φ42 mm × 18 mm) was fabricated and its homogeneity and the repeatability of high zT values were determined. The results show that high zT ∼2.0 can also be achieved even in such a large sample. These results highlight the multi-functional roles of quaternary alloying with Mg and Se, and demonstrate the realistic prospect of large-scale commercial fabrication in high performance PbTe-based thermoelectric materials.

Broadband phonon scattering in PbTe-based materials driven near ferroelectric phase transition by strain or alloying

The major obstacle in the design of materials with low lattice thermal conductivity is the difficulty in efficiently scattering phonons across the entire frequency spectrum. Using first principles calculations, we show that driving PbTe materials to the brink of the ferroelectric phase transition could be a powerful strategy to solve this problem. We illustrate this concept by applying tensile [001] strain to PbTe and its alloys with another rock-salt IV-VI material, PbSe; and by alloying PbTe with a rhombohedral IV-VI material, GeTe. This induces extremely soft optical modes at the zone center, which increase anharmonic acoustic-optical coupling and decrease phonon lifetimes at all frequencies. We predict that PbTe, Pb(Se,Te) and (Pb,Ge)Te alloys driven close to the phase transition in the described manner will have considerably lower lattice thermal conductivity than that of PbTe (by a factor of 2-3). The proposed concept may open new opportunities for the development of more efficient thermoelectric materials.

Power generation from nanostructured PbTe-based thermoelectrics: comprehensive development from materials to modules

In this work, we demonstrate the use of high performance nanostructured PbTe-based materials in high conversion efficiency thermoelectric modules.

Interfacial Reaction Between Nb Foil and n-Type PbTe Thermoelectric Materials During Thermoelectric Contact Fabrication

PbTe is a high-conversion-efficiency thermoelectric (TE) material that is commonly used in space exploration applications. Integration of PbTe in TE devices has a significant impact on the conversion efficiency and reliability of TE devices. Hence, our effort focuses on developing novel approaches for bonding metallic contacts to PbTe to improve device performance and reliability. In this study, pure Nb foil was directly bonded to PbTe-based TE materials to fabricate the hot-side contacts of TE elements using a rapid hot-press. The materials were sintered at 700°C under pressure of 40 MPa for various holding times. We found that a reaction layer of needle-like Nb_3Te_4 mixed with Pb forms at the interface of the Nb/PbTe joints and that Pb is distributed in the gaps of the Nb_3Te_4 grains. We analyze the resulting microstructure and finally calculate the time exponent of the growth kinetics of the Nb_3Te_4 layer. Fracture surface analysis showed that the Nb/PbTe joint fractures at the interface between Nb and Nb_3Te_4 and within the PbTe matrix, indicating that the bonding between Nb and Nb_3Te_4 is weak.

New promising bulk thermoelectrics: intermetallics, pnictides and chalcogenides

The need of alternative “green” energy sources has recently renewed the interest in thermoelectric (TE) materials, which can directly convert heat to electricity or, conversely, electric current to cooling. The thermoelectric performance of a material can be estimated by the so-called figure of merit, zT = σ α 2 T/λ (α the Seebeck coefficient, σ α 2 the power factor, σ and λ the electrical and thermal conductivity, respectively), that depends only on the material. In the middle 1990s the “phonon glass and electron crystal” concept was developed, which, together with a better understanding of the parameters that affect zT and the use of new synthesis methods and characterization techniques, has led to the discovery of improved bulk thermoelectric materials that start being implemented in applications. During last decades, special focus has been made on skutterudites, clathrates, half-Heusler alloys, Si1−x Ge x-, Bi2Te3- and PbTe-based materials. However, many other materials, in particular based on intermetallics, pnictides, chalcogenides, oxides, etc. are now emerging as potential advanced bulk thermoelectrics. Herein we discuss the current understanding in this field, with special emphasis on the strategies to reduce the lattice part of the thermal conductivity and maximize the power factor, and review those new potential thermoelectric bulk materials, in particular based on intermetallics, pnictides and chalcogenides. A final chapter, discussing different shaping techniques leading to bulk materials (eventually from nanostructured TE materials), is also included.

Electronic inhomogeneity inn- andp-type PbTe detected by125Te NMR

${}^{125}$Te nuclear magnetic resonance spectra and spin-lattice relaxation of $n$- and $p$-type PbTe, self-doping narrow band-gap semiconductors, have been studied and compared to those of $p$-type GeTe. Spin-lattice relaxation in GeTe can be fit by one component, while that in both PbTe samples must be fit by at least two components, showing electronically homogeneous and inhomogeneous materials, respectively. For PbTe-based materials, the spin-lattice relaxation rate 1/${T}_{1}$ increases linearly with carrier concentration. The data for GeTe fall on the same line and allow us to extend this plot to higher concentrations. Long and short ${T}_{1}$ components in both PbTe samples reflect ``low,'' \ensuremath{\sim}10${}^{17}$ cm${}^{\ensuremath{-}3}$, and ``high,'' \ensuremath{\sim}10${}^{18}$ cm${}^{\ensuremath{-}3}$, carrier concentration components. Carrier concentrations in both $n$- and $p$-type PbTe samples obtained from the Hall and Seebeck effects generally match the ``high'' carrier concentration component, and to some extent, ignore the ``low'' one. This demonstrates that the Hall and Seebeck effects may have a limited ability for the determination of carrier concentration in complex thermoelectric PbTe-based and other multicomponent materials.

Preparation of PbTe-based materials through thermal decomposition of lead acetate

This paper examines the possibility of producing microstructured lead telluride with a high grain-boundary density through the thermal decomposition of lead acetate in the presence of tellurium under a reducing atmosphere, followed by cold compaction of the powder and hot pressing of the green compacts. The formation of fine lead telluride particles was followed using differential thermal analysis and thermogravimetry. Electron microscopy, X-ray microanalysis, and specific surface area measurements were used to assess the conductivity and size of the powder particles and the structure and composition of the grains in the hot-pressed samples in relation to the preparation conditions and starting-mixture composition.

The effects of Ag-doping on thermoelectric properties of p-type Pb0.5Sn0.5Te compound

A series of Ag-doped p-type Agx(Pb0.5Sn0.5)1-xTe compounds is prepared by melting followed by slow-cooling process, and the phase compositions, microstructures and thermoelectric properties are also systematically investigated. The introduction of Ag in Pb/Sn site effectively increases the hole density which is much lower than the theoretically predicated value in the approximation of complete substitution and single acceptor of Ag, in spite of the fact that all samples show finely single phase for the 5% Ag-doped sample. This implies that part of Ag atoms enter into the interstitial sites acting as electron donor to reduce the hole density. With the increase of Ag content, the electrical conductivity increases gradually and the Seebeck coefficient shows an opposite variation tendency, mainly owing to the variation of hole density. Interestingly, the anomalous crossover of Seebeck coefficient at about 450 K indicates the transition of dominating valence valley from light-band to heavy-band while temperature is higher than 450 K. Consequently, due to the optimization of hole density and the domination of heavy band with large effective mass, 1% Ag-doped sample obtains a highest power factor of 2.1 mWm-1K-2 at 750 K, which results in a highest ZT of 1.05 combined with the suppressed lattice thermal conductivity via intensifying point defect phonon scattering. This high ZT is ～ 50% higher than that of Ag-free sample and also higher than commercial p-type PbTe material. Further, the 50% substitution of toxic and heavy Pb by Sn is beneficial for the practical application and environmental sustainability of PbTe-based materials.

Electrical, Thermal, and Mechanical Characterization of Novel Segmented-Leg Thermoelectric Modules

In this paper we report on the electrical, thermal, and mechanical characterization of segmented-leg PbTe-based thermoelectric modules. This work featured a thermoelectric module measurement system that was constructed and used to measure 47-couple segmented thermoelectric power generation modules fabricated by Tellurex Corporation using n-type Bi2Te3−x Se x to Ag0.86Pb19+x SbTe20 legs and p-type Bi x Sb2−x Te3 to Ag0.9Pb9Sn9Sb0.6Te20 legs. The modules were measured under vacuum with hot-side and cold-side temperatures of approximately 670 K and 312 K, respectively. In addition, the measurements on the PbTe-based materials are compared with measurements performed on Bi2Te3 reference modules. Efficiency values as high as 6.56% were measured on these modules. In addition to the measurement system description and the measurement results on these modules, infrared images of the modules that were used to help identify nonuniformities are also presented.

Transmission electron microscopy study of Pb-depleted disks in PbTe-based alloys

Abstract

High Thermoelectric Performance in PbTe Due to Large Nanoscale Ag2Te Precipitates and La Doping

Thermoelectrics are being rapidly developed for waste heat recovery applications, particularly in automobiles, to reduce carbon emissions. PbTe-based materials with small (<20 nm) nanoscale features have been previously shown to have high thermoelectric figure-of-merit, zT, largely arising from low lattice thermal conductivity particularly at low temperatures. Separating the various phonon scattering mechanisms and the electronic contribution to the thermal conductivity is a serious challenge to understanding, and further optimizing, these nanocomposites. Here we show that relatively large nanometer-scale (50–200 nm) Ag2Te precipitates in PbTe can be controlled according to the equilibrium phase diagram and these materials show intrinsic semiconductor behavior with high electrical resistivity, enabling direct measurement of the phonon thermal conductivity. This study provides direct evidence that even large nanometer-scale microstructures reduce thermal conductivity below that of a macro-scale composite of saturated alloys with Kapitza-type interfacial thermal resistance at the same overall composition. Carrier concentration control is achieved with lanthanum doping, enabling independent control of the electronic properties and microstructure. These materials exhibit lattice thermal conductivity which approaches the theoretical minimum above ∼650 K, even lower than that found with small nanoparticles. Optimally La-doped n-type PbTe-Ag2Te nanocomposites exhibit zT > 1.5 at 775 K.

Thermoelectric properties of HPHT sintered In-doped Pb 0.5Sn 0.5Te

The thermoelectric properties of Pb 0.5Sn 0.5Te doped with In at 1.0, 2.0, and 3.0×10 19/cm 3 and sintered at a high pressure and high temperature (HPHT) of 4.0 GPa and 800 or 900 °C, respectively, have been studied. All samples show p-type semiconducting behavior with positive thermopower. We find that HPHT sintering of conventionally synthesized materials improves their thermoelectric properties. The highest power factor is obtained for In doping of 2.0×10 19/cm 3 with 13.5 μW/cm K 2 at 230 °C. The corresponding figure of merit is 1.43×10 −3/K. This represents a twofold improvement in thermoelectric figure of merit, compared to the conventionally sintered materials reported in the literature. When exposed to 400 °C for 10 days, samples sintered at 900 °C exhibit more stable thermoelectric properties, while the properties of those sintered at 800 °C deteriorated. These results demonstrate that HPHT sintering is a viable and controllable way of tuning the thermoelectric properties of PbTe-based materials.

Thermal and electrical transport properties of VA-element doped Pb9.6M0.2Te10−xSex (M = Sb, Bi) thermoelectric materials

The VA-element doped Pb9.6M0.2Te10−xSex (M = Sb, Bi, x = 6, 7, 8) alloys have been synthesized and their thermoelectric (TE) properties were systematically investigated. The power factors of Sb- and Bi-doped samples reached ∼1.4 mW m−1 K−2 and 0.8–1.0 mW m−1 K−2, respectively, while the thermal conductivities decreased to 0.75 W m−1 K−1 and 0.8 W m−1 K−1 as the temperature was increased to 700 K. Due to the rather low thermal conductivity, a highest ZT of ∼1.05 has been attained for the Sb-doped sample, indicating that the VA element is another effective dopant besides the classical dopant iodine. The results show that the introduction of VA element is beneficial to improve TE performance of PbTe-based materials.

N-type Lead-Chalcogenide Thermoelectric Materials Alloyed with Tin

AbstractN-type conduction was observed in epitaxial Pb1-xSnxSe and Pb1-xSnxTe thin films grown by MBE with an increasing charge carrier concentration as well as an increasing thermopower value related to an increasing tin content reaching up to 12at%. The lattice thermal conductivity decreases as expected due to alloy scattering but the carrier mobility is almost stable. All these effects increase strongly the thermoelectric properties of Pb1-xSnxSe and Pb1-xSnxTe. The n-type conduction arises from a metal excess in the stoichiometry range. The same type of conduction has also been found in bulk samples. Bulk single crystals of Pb1-xSnxSe and Pb1-xSnxTe are grown by the unseeded vapor growth technique. A rather constant mobility and a decreasing thermal conductivity with increasing tin content are measured in the n-type bulk samples. A figure of merit of ZT=0.9 was measured at 325°C for a Pb0,965Sn0,035Se sample with a carrier concentration of about 9.3E18 cm-3, i.e. already better than known n-type PbTe-based materials for this temperature range.

High performance n-type PbTe-based materials for thermoelectric applications

Lead telluride-based compounds are known for their favorable thermoelectric properties in the 50–600 °C temperature range. The transport properties of homogeneous, cold-compacted and sintered PbI 2-doped PbTe samples were measured and compared to those of cast samples with similar dopant concentrations. Such a comparison is mandatory in order to determine the potential of graded samples, prepared by powder metallurgy, for thermoelectric applications. The present work focuses on improving the thermoelectric efficiency of PbTe-based materials by functional grading and by taking advantage of the specific features of indium as dopant element. The potential for achieving high thermoelectric efficiency in In-doped n-type PbTe-based thermoelectric compounds is described, and is compared to that of PbI 2-doped PbTe. Design, synthesis and characterization procedures are reported for fabricating In- and PbI 2-doped PbTe compounds with desired composition profiles.