Study of Thermoelectric Properties of Lead Telluride Based Alloys and Two-Phase Compounds

Abstract

The growing need of energy worldwide has lead to an increasing demand for alternative sources of power generation. Thermoelectric materials are one of the ‘green energy sources’ which convert directly heat into electricity, and vice–versa. The efficiency of this conversion is dependent on ‘figure of merit’ (z T), which depends on the material’s Seebeck coefficient (S), electrical resistivity (ρ) and thermal conductivity (κ) through the relation z T=S2T/ρκ, where T is the temperature. High values of z T lead to high efficiency, and therefore, z T must be maximized. Lead telluride is well–established thermoelectric material in the temperature range 350 K and 850 K. The aim of this thesis is to improve the z T of the material by adopting two different approaches – (i) doping/alloying and (ii) introducing additional interfaces in bulk i.e. having two phase PbTe. In this thesis, first an introduction about the thermoelectric phenomenon is given, along with the material parameters on which z T depends. A survey of literature associated with PbTe is done and the current status of thermoelectric devices is summarized briefly. This is followed by a description of the synthesis procedure and the measurement techniques adopted in this work. The first approach is the conventional alloying and doping of the material by which carrier concentration of the material is controlled so that maximum power factor Sρ2 is achieved and a simultaneous reduction of thermal conductivity takes place by mass fluctuation scattering. Under this, two systems have been studied. The first system is PbTe1−ySey alloys doped with In (nominal composition: Pb0.999In0.001Te1−ySey, y=0.01, 0.05, 0.10, 0.20, 0.25, 0.30). The compounds were single phase and polycrystalline. Lattice constants obtained from Rietveld refinement of X–ray diffraction (XRD) data showed that Vegard’s law was followed, indicating solid solution formation between PbTe and PbSe. Compositional analysis showed lower indium content than the nominal composition. Temperature dependent Seebeck coefficient showed all the samples to be n–type while Pisarenko plots showed that indium did not act as a resonant dopant. Electrical resistivity increased with temperature, while mobility vs T fitting showed a mixed scattering mechanism of acoustic phonon and ionized impurity scattering. Thermal conductivity followed a T1 dependence, which indicated acoustic phonon scattering. At high temperature, slight bipolar effect was observed, which showed the importance of control-ling carrier concentration for good thermoelectric properties. A z T of 0.66 was achieved at 800 K. The second alloy studied under this approach was Mn doped Pb1−ySnyTe alloy (nominal composition Pb0.96−yMn0.04SnyTe (y=0.56, 0.64, 0.72, 0.80)). All the samples followed Vegard’s law, showing formation of complete solid solution between PbTe and SnTe. Microstructure analysis showed grain size distribution of <1 µm to more than 10 µm. Seebeck coefficient showed all samples were p-type and the role of two valence band conduction in p–type PbTe based materials. Electrical resistivity showed a de-crease possibly due to (i) large carrier concentration or (ii) increased mobility due to Mn2+ ions. Thermal conductivity decreased systematically with decreasing Sn content. Bipolar effect was observed at high temperatures. Accordingly, the highest z T of 0.82 at 720 K was obtained for the sample with Sn (y=0.56) content due to optimum carrier concentration and maximum disorder. The second approach of having additional interfaces in bulk focuses on reducing thermal conductivity by scattering phonons. Under this approach, three systems were studied. The first system is PbTe with bismuth (Bi) secondary phase. The XRD and Ra-man studies showed that bismuth was not a dopant in PbTe, while micrographs showed micrometer–sized Bi secondary phase dispersed in bulk of PbTe. Reduction in Seebeck coefficient showed possible hole donation across PbTe–Bi interfaces, while electrical re-sistivity and thermal conductivity showed that the role of electrons at the interfaces was more important than phonons for the present bismuth concentrations. For the parent PbTe, z T of 0.8 at 725 K was reached, which, however decreased for bismuth added samples. The second system studied under the two phase approach was indium (In) added PbTe. Indium was not found to act as dopant in PbTe, while micrometer sized indium phase was found in PbTe bulk. A decrease in the electronic thermal conductivity ac-companied by a simultaneous increase of the electrical resistivity and Seebeck coefficient throughout the measurement range indicated increased scattering of electrons at PbTe-In interfaces. Higher values of the lattice thermal conductivity showed that the PbTe–In interfaces were ineffective at scattering phonons, which was initially expected due to the lattice mismatch between PbTe and In. For PbTe with 3 at. % In phase, z T value of 0.78 at 723 K was achieved. Under the two phase approach, as a comparative study, PbTe with both micrometer sized Bi and In phases together was prepared, in which no improvement in z T was found. A comparison of both the approaches showed that the alloying approach is better than the two–phase approach. This is because micrometer sized secondary phase scatter the electrons more than the phonons, leading to adverse effect on the transport coef-ficients, and hence, on z T. Alloying, on the other hand, is more beneficial in reducing thermal conductivity by mass fluctuation scattering, along with a simultaneous reduction of electrical resistivity.