PbTe PbS Research Articles

Vacancy-induced heterogeneity for regulating thermoelectrics in n-type PbTe

The fact that the thermoelectric performance is far inferior to that of p-type PbTe has inspired many strategies to develop n-type PbTe thermoelectrics. Alloying PbS in n-type PbTe effectively changes the shape of a valley to trigger a heavier conduction band for improving the Seebeck coefficient, while the resulting small orbital overlap inevitably leads to phase separation hindering electron transport. The effect of vacancies on the solubility of sulfur in n-type PbTe is ambiguous; especially, the heterostructure due to phase separation in high-content PbS-alloyed PbTe also requires sufficient modification to optimize the electroacoustic transport. This motivates the current work on the introduction of vacancies by charge-balancing doping via Sb2Te3 and discovers striking new insight that the introduced vacancies can induce a new heterostructure of Pb2Sb2S5 and suppress the aggregation of Sb and PbS in high-solubility n-type PbTe–PbS. The modification of the band structure and optimization of the electron transport give rise to a prominent enhancement in electronic performance. Furthermore, the Debye–Callaway model validates the dramatic contribution of vacancy aggregation and heterostructures to lattice thermal conductivity. As a result, the synergistic modulation of electroacoustic characteristics achieves a significant improvement in both the maximum zT and the near-room-temperature zT. Understanding such unique findings is critical for applicability to other thermoelectric materials.

Evolvement of microstructure and lattice thermal conductivity in Na doped PbTe–PbS pseudo−binary system

PbTe–PbS pseudo−binary is a renowned thermoelectric system largely due to its phase separation driven low thermal conductivity. To understand the thermal conductivity evolvement as well as its correlation with microstructural features (especially the nano-scale precipitates), systematic transmission electron microscope studies were performed in this work on a series of PbTe–PbS samples with gradually varying PbS or Na content. We confirmed the co-existence of two groups of precipitates in these samples, i.e. INPs (incipient nano-precipitates) ∼5 nm and PbS-rich ones ranging from 10 to a few hundreds of nano-meters. We also studied their individual size and concentration evolvement with varying PbS and Na. The effect of evolving microstructures on phonon transport was discussed and understood in a modified Callaway model, which conclusively led to the conclusion that significant impedance on heat conduction could only be realized in the case of nano-precipitates with a proper size and number density distribution.

Enhanced Thermoelectric Performance in PbTe–PbS Nanocomposites

Abstract In this study, we investigate the changes occurring in the microstructure and thermoelectric properties of PbTe0.9S0.1 alloy (PbTe–PbS) under varying thermal treatment conditions. Transmission electron microscopy (TEM) studies were utilized to reveal the distribution of nano-precipitates in the annealed alloy. PbTe0.9S0.1 alloy with PbS phase precipitates displayed a significant reduction in the thermal conductivity and an enhancement in the Seebeck coefficient. The 47% reduction in lattice thermal conductivity at the measurement temperature of 300°C in the alloy annealed at 400°C was attributed to the phonon scattering occurring due to the presence of nano-precipitates. The annealed alloy displayed higher electrical conductivity at room temperature than that of the alloy without annealing. The maximum figure of merit, ZT, was found to be 0.76 at 300°C in the alloy annealed at 400°C.

Superior thermoelectric performance in PbTe–PbS pseudo-binary: extremely low thermal conductivity and modulated carrier concentration

Na+diffuses and re-dissolves into matrix grains, resulting in carrier concentration modulation and thereby superior high temperature TE performances.

Thermoelectric performance of tellurium-reduced quaternary p-type lead–chalcogenide composites

A long-standing technological challenge to the widespread application of thermoelectric generators is obtaining high-performance thermoelectric materials from abundant elements. Intensive study on PbTe alloys has resulted in a high figure of merit for the single-phase ternary PbTe–PbSe system through band structure engineering, and the low thermal conductivity achieved due to nanostructuring leads to high thermoelectric performance for ternary PbTe–PbS compounds. Recently, the single-phase p-type quaternary PbTe–PbSe–PbS alloys have been shown to provide thermoelectric performance superior to the binary and ternary lead chalcogenides. This occurs via tuning of the band structure and from an extraordinary low thermal conductivity resulting from high-contrast atomic mass solute atoms. Here, we present the thermoelectric efficiency of nanostructured p-type quaternary PbTe–PbSe–PbS composites and compare the results with corresponding single-phase quaternary lead chalcogenide alloys. We demonstrate that the very low lattice thermal conductivity achieved is attributed to phonon scattering at high-contrast atomic mass solute atoms rather than from the contribution of secondary phases. This results in a thermoelectric efficiency of ∼1.4 over a wide temperature range (650–850K) in a p-type quaternary (PbTe)0.65(PbSe)0.1(PbS)0.25 composite that is lower than that of single-phase (PbTe)0.85(PbSe)0.1(PbS)0.05 alloy without secondary phases.

Thermoelectric Properties and Microstructure Studies of Spinodally Decomposed PbTe<SUB>0.38</SUB>S<SUB>0.62</SUB> Alloy

PbTe is a premiere mid-range temperature thermoelectric material and recent studies have proven nanostructing as an effective approach to reduce thermal conductivity of alloys. Whereas, little attention has been given to long-term thermal stability of secondary phases and microstructural evolution. Interestingly, replacing Te with S in PbTe provides an opportunity to form nanostructures in the bulk material through spinodal decomposition, which appears in binary phase diagrams of the PbTe–PbS system. Herein, the critical composition of PbTe_(0.38)S_(0.62) alloy is fabricated to n-type by chlorine doping. Thermoelectric transport properties of the alloy are investigated in the 300–850 K temperature range and the maximum zT achieved at 800 K is 0.75 with a predicted zT ∼ 0.85 at 750 K from single parabolic band model. The microstructure of the sintered samples was studied by FEG-SEM for both the as sintered and post transport properties measurement. The experimental results are compared with estimates from the parallel and series models for heterogeneous composites of single phase PbTe and PbS. The Seebeck coefficient is in agreement with the models predictions, but the resistivity is higher and the thermal conductivity is much lower than predicted values. We propose that this is attributed to the phonon and electron scattering on solute atoms in solid solutions and at interfaces.

Chemical composition tuning in quaternary p-type Pb-chalcogenides – a promising strategy for enhanced thermoelectric performance

Recently a significant improvement in the thermoelectric performance of p-type ternary PbTe-PbSe and PbTe-PbS systems has been realized through alternating the electronic band structure and introducing nano-scale precipitates to bulk materials respectively. However, the quaternary system of PbTe-PbSe-PbS has received less attention. In the current work, we have excluded phase complexity by fabricating single phase sodium doped PbTe, alloyed with PbS up to its solubility limit which is extended to larger concentrations than in the ternary system of PbTe-PbS due to the presence of PbSe. We have presented a thermoelectric efficiency of approximately 1.6 which is superior to ternary PbTe-PbSe and PbTe-PbS at similar carrier concentrations and the binary PbTe, PbSe and PbS alloys. The quaternary system shows a larger Seebeck coefficient than the ternary PbTe-PbSe alloy, indicative of a wider band gap, valence band energy offset and heavier carriers effective mass. In addition, the existence of PbS in the alloy further reduces the lattice thermal conductivity originated from phonon scattering on solute atoms with high contrast atomic mass. Single phase quaternary PbTe-PbSe-PbS alloys are promising thermoelectric materials that provide high performance through adjusting the electronic band structure by regulating chemical composition.

Fracture mode, microstructure and temperature-dependent elastic moduli for thermoelectric composites of PbTe–PbS with SiC nanoparticle additions

Twenty-six (Pb0.95Sn0.05Te)0.92(PbS)0.08–0.055% PbI2–SiC nanoparticle (SiCnp) composite thermoelectric specimens were either hot pressed or pulsed electric current sintered (PECS). Bloating (a thermally induced increase in porosity, P, for as-densified specimens) was observed during annealing at temperatures >603 K for hot-pressed specimens and PECS-processed specimens from wet milled powders, but in contrast seven out of seven specimens densified by PECS from dry milled powders showed no observable bloating following annealing at temperatures up to 936 K. In this study, bloating in the specimens was accessed via thermal annealing induced changes in (i) porosity measured by scanning electron microscopy on fractured specimen surfaces, (ii) specimen volume and (iii) elastic moduli. The moduli were measured by resonant ultrasound spectroscopy. SiCnp additions (1–3.5 vol.%) changed the fracture mode from intergranular to transgranular, inhibited grain growth, and limited bloating in the wet milled PECS specimens. Inhibition of bloating likely occurs due to cleaning of contamination from powder particle surfaces via PECS processing which has been reported previously in the literature.

Core–Shell Nanoparticles As Building Blocks for the Bottom-Up Production of Functional Nanocomposites: PbTe–PbS Thermoelectric Properties

The bottom-up assembly of nanocrystals provides access to a three-dimensional composition control at the nanoscale not attainable by any other technology. In particular, colloidal nanoheterostructures, with intrinsic multiphase organization, are especially appealing building blocks for the bottom-up production of nanocomposites. In the present work, we use PbTe-PbS as the model material system and thermoelectricity as the paradigmatic application to investigate the potential of the bottom-up assembly of core-shell nanoparticles to produce functional nanocomposites. With this goal in mind, a rapid, high-yield and scalable colloidal synthetic route to prepare grams of PbTe@PbS core-shell nanoparticles with unprecedented narrow size distributions and exceptional composition control is detailed. PbTe@PbS nanoparticles were used as building blocks for the bottom-up production of PbTe-PbS nanocomposites with tuned composition. In such PbTe-PbS nanocomposites, synergistic nanocrystal doping effects result in up to 10-fold higher electrical conductivities than in pure PbTe and PbS nanomaterials. At the same time, the acoustic impedance mismatch between PbTe and PbS phases and a partial phase alloying provide PbTe-PbS nanocomposites with strongly reduced thermal conductivities. As a result, record thermoelectric figures of merit (ZT) of ∼1.1 were obtained from undoped PbTe and PbS phases at 710 K. These high ZT values prove the potential of the proposed processes to produce efficient functional nanomaterials with programmable properties.

Analysis of Phase Separation in High Performance PbTe–PbS Thermoelectric Materials

AbstractPhase immiscibility in PbTe–based thermoelectric materials is an effective means of top‐down synthesis of nanostructured composites exhibiting low lattice thermal conductivities. PbTe1‐x Sx thermoelectric materials can be synthesized as metastable solid solution alloys through rapid quenching. Subsequent post‐annealing induces phase separation at the nanometer scale, producing nanostructures that increase phonon scattering and reduce lattice thermal conductivity. However, there has yet to be any study investigating in detail the local chemical structure of both the solid solution and nanostructured variants of this material system. Herein, quenched and annealed (i.e., solid solution and phase‐separated) samples of PbTe–PbS are analyzed by in situ high‐resolution synchrotron powder X‐ray diffraction, solid‐state 125Te nuclear magnetic resonance (NMR), and infrared (IR) spectroscopy analysis. For high concentrations of PbS in PbTe, e.g., x >16%, NMR and IR analyses reveal that rapidly quenched samples exhibit incipient phase separation that is not detected by state‐of‐the‐art synchrotron X‐ray diffraction, providing an example of a PbTe thermoelectric “alloy” that is in fact phase inhomogeneous. Thermally‐induced PbS phase separation in PbTe–PbS occurs close to 200 °C for all compositions studied, and the solubility of the PbS phase in PbTe at elevated temperatures >500 °C is reported. The findings of this study suggest that there may be a large number of thermoelectric alloy systems that are phase inhomogeneous or nanostructured despite adherence to Vegard's Law of alloys, highlighting the importance of careful chemical characterization to differentiate between thermoelectric alloys and composites.

Enhanced thermoelectric properties of (PbTe)0.88(PbS)0.12 composites by Bi doping

N-type (BixPb1−xTe)0.88(PbS)0.12 composites with x=0, 0.001, 0.003 and 0.005 have been prepared by melting the elements in sealed quartz tubes followed by ball milling and spark plasma sintering. The XRD results indicate that all samples are composites containing both major NaCl-type structure PbTe and minor NaCl-type structure PbS. The Hall measurements show that the electron carrier concentrations increase, from 4.5×1018cm−3 for x=0 to 2.8×1019cm−3 for x=0.003 at 298K, and electrical resistivity reduces with the increase of doped Bi. The absolute Seebeck coefficients decrease, from 1.96×10−4V/K for x=0 to 4.31×10−5V/K for x=0.005 at 298K, and the thermal conductivity increases, from 0.65W/mK for x=0.001 to 1.63W/mK for x=0.005 with Bi doping. However, both the power factor and the dimensionless thermoelectric figure of merit ZT can be enhanced by proper Bi doping. The maximum ZT value of 1.20 has been obtained in the sample (Bi0.001Pb0.999Te)0.88(PbS)0.12 at 573K.

High Performance Na-doped PbTe–PbS Thermoelectric Materials: Electronic Density of States Modification and Shape-Controlled Nanostructures

Thermoelectric heat-to-power generation is an attractive option for robust and environmentally friendly renewable energy production. Historically, the performance of thermoelectric materials has been limited by low efficiencies, related to the thermoelectric figure-of-merit ZT. Nanostructuring thermoelectric materials have shown to enhance ZT primarily via increasing phonon scattering, beneficially reducing lattice thermal conductivity. Conversely, density-of-states (DOS) engineering has also enhanced electronic transport properties. However, successfully joining the two approaches has proved elusive. Herein, we report a thermoelectric materials system whereby we can control both nanostructure formations to effectively reduce thermal conductivity, while concurrently modifying the electronic structure to significantly enhance thermoelectric power factor. We report that the thermoelectric system PbTe-PbS 12% doped with 2% Na produces shape-controlled cubic PbS nanostructures, which help reduce lattice thermal conductivity, while altering the solubility of PbS within the PbTe matrix beneficially modifies the DOS that allow for enhancements in thermoelectric power factor. These concomitant and synergistic effects result in a maximum ZT for 2% Na-doped PbTe-PbS 12% of 1.8 at 800 K.

Room temperature Young's modulus, shear modulus, Poisson's ratio and hardness of PbTe–PbS thermoelectric materials

Two-phase PbTe–PbS materials, in which PbS is a nanostructured phase, are promising thermoelectric materials for the direct conversion of heat energy into electricity. In this study, a Vickers indentation mean hardness of 1.18 ± 0.09 GPa was measured for hot pressed specimens Pb0.95Sn0.05Te–PbS 8% while the mean hardness of cast specimens was 0.68 ± 0.07 GPa. The mean fracture toughness of the not pressed specimens was estimated as 0.35 ± 0.04 MPa m1/2 via Vickers indentation. Resonant Ultrasound Spectroscopy (RUS) measurements on hot pressed specimens gave mean values of Young's modulus, shear modulus and Poisson's ratio of 53.1 GPa, 21.4 GPa and 0.245, respectively while for the cast specimens the Young's and shear moduli were about 10% lower than for the hot pressed, with a mean value of Poisson's ratio of 0.245. The differences between the hardness and elastic moduli values for the cast and hot pressed specimens are discussed.

Anisotropic Microwave Absorption in High-Tclike Semiconductor Superconducting Superlattices (001) PbTe-PbS

The anisotropic microwave absorption in the presence of alternative magnetic field has been studied for the first time in superconducting superlattices based on PbTe—PbS, which are layered anisotropic systems similar to high Tc superconductors. A new method of study has been used. The microwave response was detected under broad-band conditions and compared with the results of synchronous detection. All the features which have been observed in high T, materials are clearly seen here. PACS numbers: 72.20.My, 74.50.-}-r

HTSC-like superlattices PbTePbS microwave spectroscopy

Abstract For the first time magnetic field nonresonant microwave absorption at 9.4 GHz in the thin film superconductive superlattices (001) PbTePbS (Tc=5.5 K) was investigated. These superlattices may be considered as structural HTSC models. It was found that absorption in the superlattices has similar peculiarities to ones in HTSC thin films. The influence of modulating magnetic field on the conditions of formation of various hysteretic absorption signals was analysed.

Superconductivity of Non-Strained PbTe-PbS and Strained PbTe-SnTe Superlattices

A comparative study of structural and superconductive properties of semiconductor epitaxial superlattices PbTe-SnTe and PbTe—PbS grown on (001) KCl has been carried out. It has been found that the superconductivity of the PbTe—SnTe superlattices is caused by the stretching strain in the SnTe layers and it may be connected with the relative positions of L+8 and terms of PbTe and SnTe, respectively in the heterojunction. In contrast to the PbTe—SnTe superlattices, the superconductivity of the PbTe—PbS superlattices is found to be associated with regular misfit dislocation grids which are generated at the interfaces.

De Haas-van Alphen Effect in p-Type PbTe and n-Type PbS

A study of the de Haas-van Alphen oscillations in the magnetic susceptibility has been carried out in p-type PbTe and n-type PbS. Measurements have been made on oriented single crystal samples with carrier concentrations of the order of 1018/cm3 in pulsed magnetic fields up to 125 kgauss. The results for p-type PbTe indicate that the valence bands have a maximum at k=0, and four equivalent maxima at the {111} Brillouin zone faces. The 〈111〉 ellipsoids have a longitudinal mass to transverse mass ratio of 6.4±0.3. From the temperature dependence of the amplitude of the oscillations we obtain a value of 0.043±0.006 m0 for the transverse effective mass. The data indicate that the (000) maximum and the (111) maxima lie within 0.002±0.002 ev of each other. The data also indicate an effective broadening temperature of about 8°K which is attributed to inhomogeneities in the carrier concentrations in the samples investigated. Preliminary results on n-type PbS show a single isotropic Fermi surface cross section with a cyclotron mass of 0.14±0.04m0. The direct and indirect optical interband transitions are discussed in the light of these results.