Thermoelectric generators (TEGs) make use of the Seebeck effect in semiconductors for the direct conversion of heat to electrical energy. The possible use of a device consisting of numerous TEG modules for waste heat recovery from an internal combustion (IC) engine could considerably help worldwide efforts towards energy saving. However, commercially available TEGs operate at temperatures much lower than the actual operating temperature range in the exhaust pipe of an automobile, which could cause structural failure of the thermoelectric elements. Furthermore, continuous thermal cycling could lead to reduced efficiency and lifetime of the TEG. In this work we investigate the long-term performance and stability of a commercially available TEG under temperature and power cycling. The module was subjected to sequential hotside heating (at 200°C) and cooling for long times (3000 h) in order to measure changes in the TEG's performance. A reduction in Seebeck coefficient and an increase in resistivity were observed. Alternating-current (AC) impedance measurements and scanning electron microscope (SEM) observations were performed on the module, and results are presented and discussed.
Phase 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.
We present a detailed study of the charge transport, optical reflectivity, and thermal transport properties of n-type PbSe crystals. A strong scattering, mobility-limiting mechanism was revealed to be at play at temperatures above 500 K. The mechanism is indicative of complex electron-phonon interactions that cannot be explained by conventional acoustical phonon scattering alone. We applied the first order non-parabolicity approximation to extract the density of states effective mass as a function of doping both at room temperature and at 700 K. The results are compared to those of a parabolic band model and in the light of doping dependent studies of the infrared optical reflectivity. The thermal conductivity behavior as a function of temperature shows strong deviation from the expected Debye-Peierls high temperature behavior (umklapp dominated) indicating an additional heat carrying channel, which we associate with optical phonon excitations. The correlation of the thermal conductivity observations to the high temperature carrier mobility behavior is discussed. The thermoelectric figure of merit exhibits a promising value of ∼ 0.8 at 700K at ∼ 1.5 × 10 19 cm −3 .
Recent advances in the field of thermoelectrics have shown embedding appropriate nanostructures can significantly suppress the lattice thermal conductivity and therefore enhance ZT. Here we report a new class of thermoelectric composites of PbTe-PbSnS 2 . PbSnS 2 is a naturally layered material (space group Pnma) comprised of Sn-Pb bilayers approximately 0.6 nm in thickness. High resolution transmission electron microscopy reveals the PbSnS 2 segregates into coherent lamellar structures 50-100 nm in thickness that extend 100 nm to 15 mm in length. Despite the relatively large size of the PbSnS 2 precipitates, we find that incorporation of PbSnS 2 in PbTe results in significant reduction in lattice thermal conductivity to 0.4-0.65 W m À1 K À1 over the temperature range 300-700 K, a reduction of 50-70% over bulk PbTe. As a result, a maximum ZT of 1.1 is obtained for ingot samples of the PbTe-PbSnS 2 6% composition. We provide extensive characterization of the physical, structural, and chemical properties of this materials system including powder X-ray diffraction, infrared reflectivity, scanning and transmission electron microscopy, and thermoelectric properties measurements. The synthesis method is simple and general, opening possibilities for similar systems to yield materials exhibiting low lattice thermal conductivity without it being necessary to embed nanoscale (5-20 nm) features.
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