Showing papers on "Lead telluride published in 2020"

Highly efficient n-type PbTe developed by advanced electronic structure engineering

Abstract: Lead telluride is one of the most attractive state-of-the-art thermoelectric (TE) materials. Therefore, any improvement of its average thermoelectric figure of merit (ZT)av over a broad temperature range is a great challenge. Here, we show that advanced electronic structure engineering using the In quasi-local level with simultaneously optimized EF leads to a significant enhancement of energy conversion. Electronic structure calculations confirmed the formation of a half-occupied indium quasi-local level in PbTe, which, together with I-implemented electrons, provides the optimal chemical potential in terms of energy conversion, near the conduction band edge over the entire temperature range. As a result, an extremely high average thermoelectric figure of merit (ZT)av of up to ∼1.05 in the temperature range of 298 K to 773 K was achieved. Thermoelectric efficiency η was enhanced up to 14.2% for an n-type PbTe-based leg, which is one of the record-high values for a single-phase TE material. The performed theoretical analyses and experimental data prove that the large (ZT)av found in our system comes from the electronic structure and chemical potential engineering, offering a novel route for improving the efficiency of thermoelectric devices.

Trace bismuth and iodine co-doping enhanced thermoelectric performance of PbTe alloys

Abstract: Lead telluride (PbTe) is an excellent thermoelectric material in the intermediate temperature zone and has been applied to deep space exploration, waste heat recovery and other fields. However, the low thermoelectric conversion efficiency of the n-type PbTe alloys limits its applications. Here, the thermoelectric performances have been enhanced in n-type PbTe alloys through trace bismuth (Bi) and iodine (I) co-doping. The Pb1-xBixTe1-xIx (x = 0.00%, 0.05%, 0.10%, 0.20% and 0.50%) alloys are synthesized in the single phase compounds by a stepwise synthesis method. The carrier concentration has reached an optimal concentration range within the order of 1019 cm-3. The highest absolute Seebeck coefficient of 244 μV/K is obtained for 0.05% doped alloy at 730 K. The highest absolute Seebeck coefficient leads to high power factor for 0.05% doped, especially in low- and middle-temperature range. The highest power factor ~ 25 μW/K2cm has been obtained at 329 K. Complex micro-scale grain boundaries and point defects strongly increase the phonon scattering and then lead to the lowest lattice thermal conductivity of 0.64 W/mK at 674 K for x = 0.50%, which is 26% lower than that of pristine PbTe. As a result, the highest figure of merit, zT ~ 0.9 has been determined in 0.20% doped samples at 725 K. Moreover, the highest average figure of merit, zTave ~ 0.7 has been achieved in 0.05% doped samples in the 323 - 723 K temperature range, which is about two or three times higher than reported for single Bi or I doped PbTe samples.

Transport and thermoelectric performance of n-type PbTe films

Abstract: In the present work, a systematic study of structural, transport, and thermoelectric properties of n-PbTe films on a mica substrate has been carried out. A wide range of film samples with hole concentration from n = 8 × 1016 cm−3 to n = 3 × 1018 cm−3 were obtained by variation of composition of the stoichiometric PbTe and by additional doping with donor iodine impurity. Based on measurements obtained by the four coefficients method, the direct estimation of the density of states effective mass and the scattering mechanism of the investigated n-PbTe films has been carried out. The high thermoelectric figure of merit Z ≈ 1.3 × 10−3 K−1 at T = 300 K has been obtained. Such a high thermoelectric figure of merit Z for n-PbTe films opens a possibility for the fabrication of the Film Thermoelectric Modules (FTEM), which could be a candidate for application at micro-scale thermoelectric converters.

Point defects in PbCdTe solid solutions

Abstract: The defect subsystem of PbCdTe crystals was analyzed using the method of thermodynamic potential. The predominant type of point defects and their effect on the concentration of free charge carriers have been estimated. It was established that the electroneutral substitution defect CdPb is the dominant point defect in the investigated system. Furthermore, the carrier concentration of a solid solution is determined by the concentration of single-ionized interstitial cadmium atoms and single- and double-ionized vacancies of lead. The donor effect of cadmium in lead telluride is rather weak due to the efficient self-compensation processes. Consequently, n- or p-type conductivity materials with the optimal composition for thermoelectric application carrier concentrations (1018–1019 cm−3) could be obtained by introducing the stoichiometric metal/chalcogenide component of the base material (Pb/Te) or by the implementation of additional donor/acceptor impurities.The defect subsystem of PbCdTe crystals was analyzed using the method of thermodynamic potential. The predominant type of point defects and their effect on the concentration of free charge carriers have been estimated. It was established that the electroneutral substitution defect CdPb is the dominant point defect in the investigated system. Furthermore, the carrier concentration of a solid solution is determined by the concentration of single-ionized interstitial cadmium atoms and single- and double-ionized vacancies of lead. The donor effect of cadmium in lead telluride is rather weak due to the efficient self-compensation processes. Consequently, n- or p-type conductivity materials with the optimal composition for thermoelectric application carrier concentrations (1018–1019 cm−3) could be obtained by introducing the stoichiometric metal/chalcogenide component of the base material (Pb/Te) or by the implementation of additional donor/acceptor impurities.

Physicochemical Properties and Thermoelectric Studies of Electrochemically Deposited Lead Telluride Films

Abstract: In recent years, various works have been done for the improvement of thermoelectric (TE) figure of merit of materials. Among the studied TE materials, PbTe is a typical intermediate-temperature range TE material which has undergone extensive developments and achieved excellent high TE performance. Intensive research has provided PbTe-based TE materials better TE performance by manipulating charge and phonon transports, optimizing carrier density to tune the Fermi level, tailoring band structure to enhance effective mass, and designing all-scale hierarchical architectures to suppress phonon propagation giving the higher figure merit. In the present work, PbTe films were deposited on a pure copper substrate at room temperature using the three-electrode electrodeposition process, and various deposition parameters like mass of the material deposited on the substrate and deposition potential of the synthesized PbTe films were optimized. The synthesized film showed excellent stoichiometric and hydrophilic nature by contact angle measurements. The oxidation and reduction peaks during the cathodic and anodic scans of the electrolyte material were given by cyclic voltammetry (CV). Fourier transform infrared spectroscopy (FT-IR) provided the existence of different modes of PbTe in the film. The thermo-emf and thermo-power of the synthesized PbTe film were determined.

Preparation and Thermoelectric Properties of Microcrystalline Lead Telluride

Abstract: We have worked out conditions for the preparation of microcrystalline n-type lead telluride-based materials doped with lead iodide and investigated their microstructure and thermoelectric properties. The materials were prepared by hot-pressing powders produced by grinding an ingot to a particle size on the order of hundreds of microns in a planetary mill and to a particle size under hundreds of nanometers (mechanical activation) and by melt spinning. Fracture surfaces of the hot-pressed samples were examined on an optical and a scanning electron microscope. All of the samples had a nonuniform microstructure, with both small and larger grains present. In the samples prepared from the powders produced by mechanical activation, nanograins were detected. We have measured the Seebeck coefficient, electrical conductivity, and thermal conductivity of the samples at room temperature and in the range 300–800 K and evaluated their lattice thermal conductivity and thermoelectric figure of merit, ZT. Their lattice thermal conductivity was shown to decrease with decreasing grain size. The highest thermoelectric figure of merit, (ZT)max = 1.32 at 630 K, was offered by the materials produced from the mechanically activated powder.

Cation interstitial diffusion in lead telluride and cadmium telluride studied by means of neural network potential based molecular dynamics simulations

Abstract: Using a recently developed approach to represent ab initio based force fields by a neural network potential, we perform molecular dynamics simulations of lead telluride (PbTe) and cadmium telluride (CdTe) crystals. In particular, we study the diffusion of a single cation interstitial in these two systems. Our simulations indicate that the interstitials migrate via two distinct mechanisms: through hops between interstitial sites and through exchanges with lattice atoms. We extract activation energies for both of these mechanisms and show how the temperature dependence of the total diffusion coefficient deviates from Arrhenius behaviour. The accuracy of the neural network approach is estimated by comparing the results for three different independently trained potentials.

The Critical Point of Average Grain Size in Phonon Thermal Conductivity of Fine-Grained Undoped Lead Telluride

Abstract: Undoped PbTe was melted at 1123 K, ball milled (BM) at rotation speeds from 90 to 180 rpm and hot pressed (HP) at 147 MPa and 650 K. Milling at 120 rpm produced the minimum phonon thermal conductivity of 1.29 W m−1 K−1 and average grain size of 0.80 µm. Phonon thermal conductivity was constant from coarse grain size to fine grain size of 1 µm and decreased suddenly at 0.80 µm. This tendency of phonon thermal conductivity corresponded to theoretical calculations with grain boundary scattering. However, the observed critical point of 1 µm was much larger than the calculated value of 0.03 µm. There was a significant inverse relationship between phonon thermal conductivity and FWHM of X-ray diffraction peaks. The low phonon thermal conductivity was associated with not only grain boundary scattering but high internal strain.

Detection of the Thermovoltaic Effect in a Heterostructure Based on Lead Telluride

Abstract: The thermovoltaic effect was detected in a heterostructure based on a semiconductor material used in medium-temperature thermoelectric converters (PbTe). A voltage of ~0.015 V was generated at constant temperature T = 390 K. At T = 732 K, the voltage was 0.11 V.

Study of CaF2- Doped PbTe Thin Films Grown by Molecular Beam Epitaxy

Abstract: We present here a study on the electrical and structural properties of p-type PbTe films doped with CaF2. The layers were grown by molecular beam epitaxy on freshly cleaved (111) BaF2 substrates. The doping level was monitored by the CaF2 solid source cell temperature (TCaF2), which varied from 500 to 1150 °C. The films with low doping level, TCaF2 ≤ 1010 °C, exhibited flat surfaces with crystalline quality close to the undoped PbTe sample. In contrast, samples with high levels of doping (TCaF2 > 1010 °C) presented CaF2 agglomerates on the surface and a worse crystal quality. The hole density at 77 K versus TCaF2 oscillated between 1.3 × 1017 and 3.6 × 1017 cm-3 and did not exhibit a systematic behavior as the fluoride supply is raised. The results indicate that CaF2 is not an effective p-type dopant for PbTe, due to the abscence of a resonant level close to the valence band or to compensation of extrinsic dopant levels.

A Facile Chemical Synthesis of PbTe Nanostructures at Room Temperature.

Abstract: Thermoelectric (TE) materials are possible solutions of the current problems in the energy sector to overcome environmental pollution, increasing energy demand and the decline of natural resources. Thermoelectric materials are a promising alternative for the conversion of waste heat to electricity. Nanocrystalline PbTe powder was synthesized by a simple chemical method at room temperature and systematically investigated at various durations as samples A1–A5. Fourier Transform infrared spectroscopy (FTIR), x-ray diffraction (XRD), microstructural analysis by scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) confirmed the composition of the samples. TE parameters as thermo-emf of samples A1–A5 and electrical conductivity were measured. The cyclic voltammetry gives a band gap of 0.25 eV, which is in agreement with the optical band gap of the material. The A4 sample has an average crystal size of 36 nm with preferred orientation in (200) verifying the cubic morphology. The obtained TE parameters are beneficial for the non-uniform TE materials which might be due to strong current boundary scattering and extremely low thermal conductivity of the samples.

Preparation of PbTe Thin Films for High-Sensitive Mid-IR Photodetectors by PECVD

Abstract: Lead telluride (PbTe) possesses a good performance as a thermoelectric material due to both a low thermal conductivity and its electrical properties. It has peak thermoelectric characteristics at high temperature and is widely used in spacecraft power applications and as a waveguide-integrated detector monolithically integrated on a silicon substrate and operating at room temperature. In this work PbTe thin films were prepared via direct plasma-chemical interaction of lead and tellurium vapors. Argon of high purity was also used as a career gas for precursors transport to the plasma zone and as a plasma feed gas. The process was carried out at the low pressure (0.01 Torr) in inductively coupled non-equilibrium RF (40.68 MHz) plasma discharge. Optical emission spectroscopy (OES) was used to identify the exited species and to assume the possible mechanisms of plasma-chemical reactions. The stoichiometry, structure and morphology of the surface of the materials obtained was also studied by deferent analytical techniques dependently on the conditions of the plasma process.