Bulk PbTe Research Articles

Optical-intensity modulators with PbTe thermoelectric nanopowders for ultrafast photonics

As a new optical material with huge potential application, nanomaterials have attracted much attention in the field of ultrafast photonics due to their proper band gap, multiple structure and stable chemical property. Although there have been many reports on light intensity modulators in ultrafast photonics fabricated by nanomaterials in recent years, few work has been done to analyze the role of nanomaterials at different scales with light-matter interaction. It is found that bonding of different scales of PbTe to the tapered fiber surface by a controlled variable method confirms that nanopowder PbTe has better light intensity modulation capability compared to bulk PbTe. At last, an optical-intensity modulator based on nano PbTe was utilized to achieve 830 fs laser pulse in fiber ring laser. These studies indicate that PbTe shows great promise for ultrafast photonic applications, and effectively promoting the development of semiconductor crystal-based optical devices.

Doping‐Induced Viscoelasticity in PbTe Thermoelectric Inks for 3D Printing of Power‐Generating Tubes

AbstractThermoelectric (TE) technologies offer promising means to enhance fossil energy efficiencies by generating electricity from waste heat from industrial or automobile exhaust gases. For these applications, thermoelectric modules should be designed from the perspective of system integration for efficient heat transfer, system simplification, and low processing cost. However, typical thermoelectric modules manufactured by traditional processes do not fulfil such requirements, especially for exhaust pipes. Hence, a 3D‐printing method for PbTe thermoelectric materials is reported to design high‐performance power‐generating TE tubes. The electronic doping‐induced surface charges in PbTe particles are shown to significantly improve the viscoelasticities of inks without additives, thereby enabling precise shape and dimension engineering of 3D bulk PbTe with figures of merit of 1.4 for p‐type and 1.2 for n‐type materials. The performance of the power‐generating TE tube fabricated from 3D‐printed PbTe tubes is demonstrated experimentally and computationally as an effective strategy to design system‐adaptive high‐performance thermoelectric generators.

Low thermal conductivity and enhanced Seebeck coefficient in Antimony and Selenium co-doped PbTe Nanopowders

We report a new method to synthesize Sb and Se co-doped PbTe nanopowders by a simple aqueous chemical method without the assistance of any surfactant or template. X-ray diffraction studies revealed that the prepared nanopowders are polycrystalline in nature and crystallize in the face-centered-cubic (fcc) PbTe structure. SEM and TEM studies reveal that Sb and Se co-doped PbTe nanopowders consist of an interconnected network of spherical nanoparticles with diameters around 12 nm. Seebeck coefficient of the undoped PbTe (436 µV/K at 301 K) and co-doped PbTe (627 µV/K at 301 K) is enhanced compared to that of the bulk PbTe (265 µV/K at 298 K) and the thermal conductivity is drastically decreased [undoped PbTe (0.265 W/m-K) and co-doped PbTe (0.525 W/m-K) at 378 K] compared with the value for the bulk PbTe (2.2 W/m-K at 300 K). The dimensionless figure of merit ZT attains its maximum value 0.055 at 378 K for undoped and 1.09 × 10−5 at 352 K for co-doped PbTe.

Ultra-low lattice thermal conductivity and giant phonon–electric field coupling in hafnium dichalcogenide monolayers

Phonons in crystalline solids are of utmost importance in governing its lattice thermal conductivity (kL). In this work, kL in hafnium (Hf) dichalcogenide monolayers has been investigated based on ab initio DFT coupled to linearized Boltzmann transport equation together with single-mode relaxation-time approximation. Ultra-low kL found in HfS2 (2.19 W m−1 K−1), HfSe2 (1.23 W m−1 K−1) and HfSSe (1.78 W m−1 K−1) monolayers at 300 K, is comparable to that of the state-of-art bulk thermoelectric materials, such as, Bi2Te3 (1.6 W m−1 K−1), PbTe (2.2 W m−1 K−1) and SnSe (2.6 W m−1 K−1). Gigantic longitudinal-transverse optical (LO-TO) splitting of up to 147.7 cm−1 is noticed at the Brillouin zone-centre (Γ-point), which is much higher than that in MoS2 single layer (∼2 cm−1). It is driven by the colossal phonon–electric field coupling arising from the domination of ionic character in the interatomic bonds and Born effective or dynamical charges as high as 7.4e on the Hf ions, which is seven times that on Mo in MoS2 single layer. Enhancement in kL occurs in HfS2 (2.19 to 4.1 W m−1 K−1), HfSe2 (1.23 to 1.7 W m−1 K−1) and HfSSe (1.78 to 2.2 W m−1 K−1) upon the incorporation of the non-analytic correction term. Furthermore, the mode Grüneisen parameter is calculated to be as high as ∼2.0, at room temperature, indicating a strong anharmonicity. Moreover, the contribution of optical phonons to kL is found to be ∼12%, which is significantly high than that in single-layer MoS2. Large atomic mass of Hf (178.5 u), small phonon group velocities (4–5 km s−1), low Debye temperature (∼166 K), low bond and elastic stiffness (Young’s modulus ∼75 N m−1), small phonon lifetimes (∼6 ps), low specific heat capacity (∼17 J K−1 mol−1) and strong anharmonicity are collectively found to be the factors responsible for such a low kL. These findings would be immensely helpful in designing thermoelectric interconnects at the nanoscale and 2D material-based energy harvesters.

The high-energy milling process as a synergistic approach to minimize the thermal conductivity of PbTe nanostructures

The mechanochemical synthesis of PbTe nanostructures via the high-energy milling (HEM) process is proposed, as a synergistic approach, to minimize the thermal conductivity of this semiconductor material at room temperature. In this study, the effect of process control agent (PCA) addition on the microstructure and thermal properties of the as-milled PbTe nanostructures was also investigated. The synthesis design is based on a solid-state technique which allows enhancing the grain boundary scattering and consequently, decreases the thermal conductivity in the PbTe nanostructures. Moreover, the challenging part is the formation of coarse particles with embedded quantum dots and nanoparticles, which are expected to scatter long- and mid-wavelength phonons. In this sense, via the HEM-process, it is feasible to activate other phonon scattering modes by introducing rough-surfaced grains and nanoroughness to the particle surfaces. Overall, the thermal conductivity is decreased in the PbTe nanostructures by two means; namely, adding scattering processes and phonon group velocity, which were heightened due to the PCA added during milling. To elucidate the role of these components of the phonon thermal conductivity, atomic force microscopy and high-resolution transmission electron microscopy were used. On the other hand, photoacoustic (PA) and photopyroelectric (PP) techniques were applied to study the thermal properties of the PbTe nanostructures obtained by the HEM-process. The total thermal conductivity values of the PbTe nanostructures obtained by the PA and PP techniques were at least a half lower (1.06 W m−1 K−1) than that of bulk PbTe value counterpart (∼2.13 W m−1 K−1) and lower than those from PbTe nanomaterials (ranging from 1.5 to 2.2 W m−1 K−1) obtained via other means. The thermal conductivity values of the as-milled PbTe nanostructures were closely related to the carrier concentration experimental findings. We inferred that this strategy can be widely applicable to enhance existing thermoelectric compounds into their nanoforms by a scalable and a simple low-cost route.

Feasibility of a high stable PbTe:In semiconductor for thermoelectric energy applications

High-efficiency thermoelectric conversion is achieved by using materials with a maximum figure of merit Z = S2σ/κ, where S is the Seebeck coefficient, and σ and κ are the electrical conductivity and thermal conductivity, respectively, over a wide temperature range. Lead telluride alloys were some of the first materials investigated and commercialized for generators; however, their full potential for thermoelectrics has only recently been revealed to be greater than commonly believed. The maximal value of Z, as a function of electron density, is attained only for a specific location of the Fermi level EF relative to the conduction band edge EC. A systematic study of structural, microstructural, and thermoelectric properties of bulk PbTe doped with indium is presented. Samples were prepared by the pulsed electric current sintering technique. The high dimensionless figure of merit ZT ≈ 0.8 over 200–500 °C temperature range for PbTe doped with 0.05–0.1 at. % of In was obtained. Moreover, ZT is practically the same for Pb0.9995In0.0005Te and Pb0.99In0.01Te compounds at high temperature. Therefore, indium dopant in PbTe stabilizes the optimal location of the Fermi level. The effect of the negative process of indium diffusion into the matrix during the long service time of the TE generator could be avoided by doping heavily with indium the hot side of n-type functionally graded PbTe:In leg.

Solid-State Bonding of Bulk PbTe to Nickel Electrode for Thermoelectric Modules

The efficiency of thermoelectric generators is defined by the thermoelectric performance of materials, as expressed by the thermoelectric figure-of-merit, and their contacts with electrodes. Lead chalcogenide thermoelectric materials and, in particular, PbTe perform well in the 500–900 K temperature range. Here, we have successfully bonded bulk PbTe to Ni electrode to generate a diffusion barrier, avoiding continuous reaction of the thermoelectric legs and conducting electrodes at the operating temperature. We have modified the commonly used spark plasma sintering assembly method to join the Ni electrode to bulk PbTe by driving the total supplied electrical current through the Ni and PbTe solid interfaces. This permits the formation of a thin diffusion layer, roughly 4.5 μm in thickness, which is solely comprised of nickel telluride. This new technique toward the bonding of PbTe with the electrode is beneficial for thermoelectric materials, since high temperatures have proven to be damaging to the quality...

Symmetry-dependent topological phase transitions in PbTe layers

By stacking PbTe layers there is a non-monotonic topological phase transition as a function of the number of monolayers. Based on first principles calculations we find that the proper stacked crystal symmetry determines the topological nature of the slab. While a single PbTe monolayer has a nontrivial phase, external pressure can induce topological phase transition in bulk PbTe. Between these two limits, where finite size effects are inherent, we verified that, by applying an external pressure, odd stacking layers can be tuned easily to a topological phase, while even stacking keeps a larger band gap, avoiding band inversion. The quite distinct behavior for odd/even layer is due to the symmetry of the finite stacking. Odd layers preserve the bulk symmorphic symmetry with strong surface hybridization, while even layers belong to a nonsymmorphic group symmetry. Nonsymmorphism induces extra degeneracy reducing the hybridization, thus protecting band inversion, postponing topological phase transitions.

Enhanced thermoelectric performance of Se-doped PbTe bulk materials via nanostructuring and multi-scale hierarchical architecture

The combination of feasible bottom-up hydrothermal synthesis is demonstrated to notably enhance thermoelectric performance of the PbTe matrix with different Se-doped contents. The instinct electrical properties are improved by annealing with reducing gas to achieve relatively high power factor. Furthermore, multi-scale hierarchical architecture is established together with the point defects and dislocations introduced by doping Se and the formation of nanoscale participates, couple with increased boundaries and a large grain size scale from 20 nm to 1.5 μm and, resulting in scattering an even broader frequency spectrum of phonon mean free path to reduce the lattice thermal conductivity effectively. Consequently, it plays an essential synergistic optimization to achieve a maximum ZT value of 0.98 at 625 K from the as-prepared n-type PbTe1-xSex (x = 0.06) alloys. This study provides a facile and low-cost method to fabricate multi-scale hierarchical architecture materials on a large scale with significant enhancement of thermoelectric performance.

Properties of heavily impurity-doped PbSnTe liquid-phase epitaxial layers grown by the temperature difference method under controlled Te vapor pressure

We propose the use of heavily impurity-doped Pb1-xSnxTe/PbTe epitaxial layers grown via the temperature difference method under controlled vapor pressure (TDM-CVP) liquid-phase epitaxy (LPE) for the preparation of IV–VI compounds for mid- to far-infrared optical device applications. A flat surface morphology and the distribution of a constant Sn concentration for 0.05≤x≤0.33 were observed in the epitaxial layers using electron-probe microanalysis. The segregation coefficient of Sn in Pb1-xSnxTe grown via TDM-CVP LPE (Tg=640°C) was xSSn／xLSn=0.28. The appearance of the Fermi level pinning and persistent photoconductivity effects in In-doped PbSnTe were also proposed; we estimated that the activation energies of these processes were 2.8 and 39.7meV, respectively, based on the In-doped Pb1-xSnxTe carrier profile as a function of ambient temperature. In Hall mobility measurements, Sn was assumed to be a main scattering center in the Pb1-xSnxTe epitaxial crystals. The impurity effect was also observed in Pb1-xSnxTe epitaxial growth, similar to the effects observed for Tl-doped PbTe bulk crystals. We concluded that the heavily doped Pb1-xSnxTe crystals grown via TDM-CVP LPE can be used to fabricate high-performance mid- to far-infrared optical devices.

Thermoelectric properties of In and I doped PbTe

A systematic study of structural, microstructural, and thermoelectric properties of bulk PbTe doped with indium (In) alone and co-doped with both indium and iodine (I) has been done. X-ray diffraction results showed all the samples to be of single phase. Scanning electron microscopy (SEM) results revealed the particle sizes to be in the range of micrometers, while high resolution transmission electron microscopy was used to investigate distinct microstructural features such as interfaces, grain boundaries, and strain field domains. Hall measurement at 300 K revealed the carrier concentration ∼1019 cm−3 showing the degenerate nature which was further seen in the electrical resistivity of samples, which increased with rising temperature. Seebeck coefficient indicated that all samples were n–type semiconductors with electrons as the majority carriers throughout the temperature range. A maximum power factor ∼25 μW cm−1 K−2 for all In doped samples and Pb0.998In0.003Te1.000I0.003 was observed at 700 K. Doping leads to a reduction in the total thermal conductivity due to enhanced phonon scattering by mass fluctuations and distinct microstructure features such as interfaces, grain boundaries, and strain field domains. The highest zT of 1.12 at 773 K for In doped samples and a zT of 1.1 at 770 K for In and I co-doped samples were obtained.

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.

Enhanced thermoelectric properties of topological crystalline insulator PbSnTe nanowires grown by vapor transport

Bulk PbTe and alloy compounds thereof are well-known thermoelectric materials for electric power generation. Among these alloys, PbSnTe hosts unique topological surface states that may have improved thermoelectric properties. Here we report on the vapor-transport growth and thermoelectric study of high-quality single-crystalline PbTe and PbSnTe nanowires. The nanowires were grown along the direction with dominant {100} facets; the chemical compositions of the wires depend strongly on the substrate position in the growth reactor. We measured the thermopower and electrical and thermal conductivities of individual nanowires to determine the thermoelectric figure of merit ZT. Compared to bulk samples, the PbSnTe nanowires showed both improved thermopower and suppressed thermal conductivity, enhancing the ZTs to ~0.018 and ~0.035 at room temperature. The enhanced thermopower may result from the unique topological surface states; the suppression of thermal conductivity may relate to increased phonon-surface scattering. Compared to PbTe nanowires, the PbSnTe wires have lower thermopower but significantly higher electrical conductivities. This study highlights nanostructuring in combination with alloying as an important approach to enhancing the figure of merit ZT of thermoelectric materials.

Thermoelectric properties of spark plasma sintered lead telluride nanocubes

Abstract

Annealing Temperature Effect on Properties of Chemically Deposited PbTe Films and Bulk

Abstract The PbTe films were deposited onto glass substrate (microscopic slices) by a chemical bath method (CBD) at room temperature. The deposited films are dense, smooth, and uniform with silver gray metallic luster structure. Using XRD, it found that the structure of PbTe possesses stable face centered cubic (fcc) phase. The grain size of the PbTe bulk increased within the range of 33– 57 nm with annealing temperature increasing. AFM micrographs of surface of the prepared film are observed that horizontal distance in the rang (230– 395) nm. The band gaps of the PbTe are determined from UV-Vis spectrophotometer and are found to be within the range ( 0.39 - 0.95) eV. Energy band gab of PbTe which determined from FT -IR spectrophotometer is (0.36ev). The activation energy varied from 0.35- 1.72 eV for films and from 0.11-0.34 eV for bulk with annealing temperature variations from 373-573K. Films and bulk exhibit p-type conduction and resistivity in the range (75×10-4 Ω. cm - 146×10-4 Ω.cm). The carriers density and Hall mobility in PbTe bulk were in the rang 5.8 ×1023 m-3 and 4.004 m2/Vs.

Electrodeposition of PbTe thin films: electrochemical behavior and effect of reverse pulse potential

This paper reports on the voltammetric behavior of gold substrates in acidic nitrate electrolytes containing HTeO2+ and Pb2+. Particular anodic behavior is highlighted and studied by anodic stripping experiments, demonstrating that the presence of PbTe dendrites induces an additional anodic peak prior to the dissolution of bulk PbTe. The diffusion coefficients of Pb2+ and HTeO2+ were measured in order to adjust their respective concentration in the electrolyte. Diffusion of Pb2+ being slightly faster than HTeO2+, optimal concentrations were calculated to be 4.1mM Pb2+ for 5mM HTeO2+. For steady state potentiostatic deposition, Pb is underpotentially deposited onto overpotentially deposited Te0 from -175mV and its content in the deposit slightly increases with the applied overpotential. However, increasing the overpotential also induces dendritic morphologies. In contrast, reverse pulse deposition can lead to the formation of spherical assemblies of hollow trigonal nanorods with high specific area. However, it strongly decreases the Pb content in the film.

Optical phonons in PbTe/CdTe multilayer heterostructures

The infrared reflection spectra of PbTe/CdTe multilayer nanostructures grown by molecular-beam epitaxy are measured in the frequency range of 20–5000 cm−1 at room temperature. The thicknesses and high-frequency dielectric constants of the PbTe and CdTe layers and the frequencies of the transverse optical (TO) phonons in these structures are determined from dispersion analysis of the spectra. It is found that the samples under study are characterized by two TO phonon frequencies, equal to 28 and 47 cm−1. The first frequency is close to that of TO phonons in bulk PbTe, and the second is assigned to the optical mode in structurally distorted interface layers. The Raman-scattering spectra upon excitation with the radiation of an Ar+ laser at 514.5 nm are measured at room and liquid-nitrogen temperatures. The weak line at 106 cm−1 observed in these spectra is attributed to longitudinal optical phonons in the interface layers.

Thermoelectric properties of IV–VI-based heterostructures and superlattices

Doping in a manner that introduces anisotropy in order to reduce thermal conductivity is a significant focus in thermoelectric research today. By solving the semiclassical Boltzmann transport equations in the constant scattering time (τ) approximation, in conjunction with ab initio electronic structure calculations, within Density Functional Theory, we compare the Seebeck coefficient (S) and figure of merit (ZT) of bulk PbTe to PbTe/SnTe/PbTe heterostructures and PbTe doping superlattices (SLs) with periodically doped planes. Bismuth and Thallium were used as the n- and p-type impurities, respectively. The effects of carrier concentration are considered via chemical potential variation in a rigid band approximation. The impurity bands near the Fermi level in the electronic structure of PbTe SLs are of Tl s- and Bi p-character, and this feature is independent of the doping concentration or the distance between impurity planes. We observe the impurity bands to have a metallic nature in the directions perpendicular to the doping planes, yet no improvement on the values of ZT is found when compared to bulk PbTe. For the PbTe/SnTe/PbTe heterostructures, the calculated S presents good agreement with recent experimental data, and an anisotropic behavior is observed for low carrier concentrations (n<1018cm−3). A large value of ZT|| (parallel to the growth direction) of 3.0 is predicted for n=4.7×1018cm−3 and T=700K, whereas ZTp (perpendicular to the growth direction) is found to peak at 1.5 for n=1.7×1017cm−3. Both electrical conductivity enhancement and thermal conductivity reduction are analyzed.

Bonding and high-temperature reliability of NiFeMo alloy/n-type PbTe joints for thermoelectric module applications

PbTe is an extremely important thermoelectric (TE) material, due to its high TE conversion efficiency. Consequently, our effort focuses on developing PbTe-based TE modules, which requires developing novel approaches for bonding metallic contacts to PbTe. In this study, Fe, Mo, and NiFeMo alloy foils were directly bonded to n-type PbTe using a rapid hot press at 600, 700, or 800 °C under a pressure of 40 MPa and for various holding times. We find that in the case of Fe and Mo, it is difficult to form a metallurgically bonded high strength joint with PbTe. However, we find that NiFeMo alloy does effectively bond to PbTe at 700 °C, but not at 600 °C. Significant liquid Pb, which might be due to the reaction of PbTe with Ni, is found that penetrates along the NiFeMo grain boundaries near NiFeMo/PbTe joints during bonding at 700 °C where the extent of liquid Pb penetration can be controlled with the time of bonding. Furthermore, the Seebeck coefficient of bulk PbTe with NiFeMo contacts is similar to that without NiFeMo contacts. Finally, the accelerated thermal aging of NiFeMo/PbTe elements at 600 °C for 240 h shows that the failure mechanism of NiFeMo/PbTe joints under operating conditions is the continued formation and penetration of eutectic liquid NiFeMo–PbTe and liquid Pb along the NiFeMo grain boundaries.

Reduction of Lattice Thermal Conductivity in PbTe Induced by Artificially Generated Pores

Highly dense pore structure was generated by simple sequential routes using NaCl and PVA as porogens in conventional PbTe thermoelectric materials, and the effect of pores on thermal transport properties was investigated. Compared with the pristine PbTe, the lattice thermal conductivity values of pore-generated PbTe polycrystalline bulks were significantly reduced due to the enhanced phonon scattering by mismatched phonon modes in the presence of pores (200 nm–2 μm) in the PbTe matrix. We obtained extremely low lattice thermal conductivity (~0.56 W m−1 K−1at 773 K) in pore-embedded PbTe bulk after sonication for the elimination of NaCl residue.