Ashoka Bali

Tetrahedrites as thermoelectric materials: an overview

Tetrahedrites are natural earth-abundant minerals consisting of environmentally-friendly elements of copper and sulphur. Recently, research has been focused on the natural and synthetic minerals of tetrahedrite materials for thermoelectric applications. The thermoelectric figure of merit zT of around unity at ∼723 K for many doped and natural tetrahedrite materials in the past 2–3 years was determined and this value is comparable to conventional p-type TE materials. In this review, a brief history of tetrahedrite materials is followed by information about its crystal structure and chemical bonding, electronic band structure and transport properties. Different synthesis approaches have been summarized. Also, this review outlines the effect of different doping elements on the thermoelectric properties of tetrahedrite materials, and the natural mineral tetrahedrite that can be used as thermoelectric materials.

Thermoelectric properties of PbTe with encapsulated bismuth secondary phase

Lead Telluride (PbTe) with bismuth secondary phase embedded in the bulk has been prepared by matrix encapsulation technique. X-Ray Diffraction results indicated crystalline PbTe, while Rietveld analysis showed that Bi did not substitute at either Pb or Te site, which was further confirmed by Raman and X-Ray Photoelectron Spectroscopy. Scanning Electron Microscopy showed the expected presence of a secondary phase, while Energy Dispersive Spectroscopy results showed a slight deficiency of tellurium in the PbTe matrix, which might have occurred during synthesis due to higher vapor pressure of Te. Transmission Electron Microscopy results did not show any nanometer sized Bi phase. Seebeck coefficient (S) and electrical conductivity (sigma) were measured from room temperature to 725 K. A decrease in S and sigma with increasing Bi content showed an increased scattering of electrons from PbTe-Bi interfaces, along with a possible electron acceptor role of Bi secondary phase. An overall decrease in the power factor was thus observed. Thermal conductivity, measured from 400K to 725K, was smaller at starting temperature with increasing Bi concentration, and almost comparable to that of PbTe at higher temperatures, indicating a more important role of electrons as compared to phonons at PbTe-Bi interfaces. Still, a reasonable zT of 0.8 at 725K was achieved for undoped PbTe, but no improvement was found for bismuth added samples with micrometer inclusions

Thermoelectric properties of indium doped PbTe1-ySey alloys

Lead telluride and its alloys are well known for their thermoelectric applications. Here, a systematic study of PbTe1-ySey alloys doped with indium has been done. The powder X-Ray diffraction combined with Rietveld analysis confirmed the polycrystalline single phase nature of the samples, while microstructural analysis with scanning electron microscope results showed densification of samples and presence of micrometer sized particles. The temperature dependent transport properties showed that in these alloys, indium neither pinned the Fermi level as it does in PbTe, nor acted as a resonant dopant as in SnTe. At high temperatures, bipolar effect was observed which restricted the zT to 0.66 at 800 K for the sample with 30% Se content.

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 ...

Improvement of electrical conductivity in Pb0.96-yMn0.04SnyTe alloys for high temperature thermoelectric applications

Lead–tin–telluride is a well-known thermoelectric material in the temperature range 350–750 K. Here, this alloy doped with manganese (Pb0.96−yMn0.04SnyTe) was prepared for different amounts of tin. X-ray diffraction showed a decrease of the lattice constant with increasing tin content, which indicated solid solution formation. Microstructural analysis showed a wide distribution of grain sizes from <1 μm to 10 μm and the presence of a SnTe rich phase. All the transport properties were measured in the range of 300−720 K. The Seebeck coefficient showed that all the samples were p-type indicating holes as dominant carriers in the measurement range. The magnitude increased systematically on reduction of the Sn content due to possible decreasing hole concentration. Electrical conductivity showed the degenerate nature of the samples. Large values of the electrical conductivity could have possibly resulted from a large hole concentration due to a high Sn content and secondly, due to increased mobility by sp–d orbital interaction between the Pb1−ySnyTe sublattice and the Mn2+ ions. High thermal conductivity was observed due to higher electronic contribution, which decreased systematically with decreasing Sn content. The highest zT = 0.82 at 720 K was obtained for the alloy with the lowest Sn content (y = 0.56) due to the optimum doping level.

Raman & infrared study of Bi filled Co4Sb12

CoSb3 skutterudites are established thermoelectric materials in the 500-800K temperature range. Undoped and Bi filled CoSb3 samples were synthesized by induction melting-annealing process and phase confirmation done by X-Ray diffraction. The role of bismuth as a filler in CoSb3 was investigated by Raman and far infrared reflectance study. It was found that bismuth strengthens Sb vibrations, and can potentially scatter Sb related acoustic phonons effectively. As a result substantial reduction in thermal conductivity may be possible with proper control of Bi filling.

Thin Film Thermoelectric Materials for Sensor Applications: An Overview

The modern world depends completely on electricity, the demand for which is increasing day by day. However, carbon-based fuels for power generation and transportation being currently used, e.g., coal, petroleum, etc., are expected to ultimately deplete, due to which there is an urgent need for non-conventional energy sources. At the same time, a large (~60 %) amount of heat is wasted from these industries and vehicles, which will increase further with economic growth and industrialization. Other renewable sources have their own limitations and many are region-specific. Thermoelectric (TE) materials are a means of utilization of waste heat on a small scale by directly converting it into electricity. The generated voltage also finds application in several types of sensors. In addition, these materials can be used for cooling, which is equivalent to direct conversion of electricity into heat, without using any harmful chlorofluorocarbon (CFC) gases. This chapter is intended to give an overview of thin film thermoelectric materials with a focus on their application as different sensors.

Elastic Properties Of Sulphur And Selenium Doped Ternary PbTe Alloys By First Principles

Lead telluride (PbTe) is an established thermoelectric material which can be alloyed with sulphur and selenium to further enhance the thermoelectric properties. Here, a first principles study of ternary alloys PbSxTe(1-x) and PbSexTe(1-x) (0 <= x <= 1) based on the Virtual Crystal Approximation (VCA) is presented for different ratios of the isoelectronic atoms in each series. Equilibrium lattice parameters and elastic constants have been calculated and compared with the reported data. Anisotropy parameter calculated from the stiffness constants showed a slight improvement in anisotropy of elastic properties of the alloys over undoped PbTe. Furthermore, the alloys satisfied the predicted stability criteria from the elastic constants, showing stable structures, which agreed with the previously reported experimental results.

Compositional and Morphological Study of Sb doped PbTe

Bulk samples of antimony doped (Sb = 0.5 and 1 at. %) lead telluride have been prepared by liquid matrix encapsulation technique. The surface morphology and composition has been studied by Scanning Electron Microscopy (SEM) and Electron Probe Micro Analysis (EPMA). Samples prepared by the above method were compared with the samples prepared by the conventional melt‐quench method. It was observed that the morphology is different for both the types of samples although similar chemical composition was observed in both the cases. Chemical analysis of the surface by XPS showed that Sb exists at the surface in elemental state.