| dc.description.abstract | Non-renewable energy sources, such as natural gas, coal, and oil, supply over 80% of the world’s energy demands. The earth’s atmosphere is heavily polluted with CO2 generated from fossil fuels and natural gases, raising the global temperature and creating the greenhouse effect. Utilizing renewable energy sources is the first step toward lessening this issue. Direct heat-to-electricity conversion can be accomplished by using thermoelectric generators (TEGs). A TEG can produce electricity directly from heat derived from radioisotopes, cars, industries, or even human bodies. Electricity can also power a TEG to function as a solid-state refrigerator. In recent years, tetrahedrite, a mineral sulfide with the chemical formula Cu12Sb4S13, has garnered global interest as a p-type thermoelectric material. Tetrahedrite (Cu12Sb4S13) has intrinsically low lattice thermal conductivity, which varies around 0.7 – 0.5 W m-1 K-1 in the 323 – 723 K temperature range. The electrical resistivity of tetrahedrite increases from 0.01 – 0.016 m𝛺 m as the temperature increases from 323 – 723 K. Tetrahedrites have a moderate Seebeck coefficient (~90 𝜇V K-1 at 300 K), which limits their thermoelectric conversion efficiency even with their favorable electrical resistivity and thermal conductivity. This thesis used the double substitution approach to improve the Seebeck coefficient, hence the power factor, and simultaneously reduce thermal conductivity. Furthermore, Ag addition in Cu-deficient tetrahedrite was performed to reduce the lattice part of the thermal conductivity.
This thesis’s first chapter provides an overview of a brief history and development of thermoelectric research before going into the three thermoelectric effects. Concerning the dimensionless figure-of-merit (zT), a material’s thermoelectric performance and conversion efficiency are reviewed. A summary of the several transport characteristics that decide a thermoelectric material’s zT is then given, including thermal conductivity, electrical conductivity, and Seebeck coefficient. A thorough review of the literature on the tetrahedrite (Cu12Sb4S13) material is provided, along with an explanation of the motivation for the works completed for the thesis. Chapter 2 presents the material synthesis technique, various characterization approaches, and transport properties measuring procedures.
In Chapter 3, Ag-added Cu deficient tetrahedrites, Agx-added Cu12-xSb4S13, were studied. The analysis of the transport coefficient revealed that the predominant scattering mechanism in the Ag-added samples is acoustic phonon scattering, and the transport coefficient’s minimal temperature dependency suggested that the samples had a robust electron-phonon interaction. Ag addition in tetrahedrite was effective in scattering acoustic phonons, which decreased the lattice thermal conductivity with little impact on the power factor. The Debye-Callaway model was used to fit the observed lattice thermal conductivity, assuming the point defect (PD), Umklapp (U), and electron-phonon (EP) interaction. It was revealed that the enhanced anharmonicity induced by Ag-addition is the primary cause of reduced lattice thermal conductivity. The above argument was further supported by evidence from Raman spectroscopy, which suggested that Ag addition would weaken the Sb-S bond. As a result, the lattice thermal conductivity was reduced to around 0.27 W m-1 K-1 and was achieved with Ag0.025 added Cu11.975Sb4S13 composition. A comparatively high power factor of ~1.3 mW m-1 K-2 was found for the same composition. The sample Ag0.025 added Cu11.975Sb4S13 had the highest thermoelectric figure of merit of 0.87 at 738 K because it had the lowest total thermal conductivity, at ~1.09 W m-1 K-1.
It is well known that resonant Se 4p states can be generated around the Fermi energy (EF) by introducing Se2- at the S(1)2- site, increasing the density of states (DOS). Se substitution is expected to not change the charge carrier concentration since Se2- and S2- are isoelectronic. However, the Selenium substitution in tetrahedrite improves the Seebeck coefficient. In chapters 4 and 5, Te substitution at Sb 8c and Gd substitution at Cu 12d were performed, respectively, while maintaining Se substitution as the common for both cases. In Chapter 4, samples with composition Cu12Sb3.9Te0.1S13-xSex (x = 0, 0.1, 0.5, 0.75, and 1) were prepared. XRD and EPMA confirmed the formation of the tetrahedrite phase. The Raman spectra showed that the Sb-S bond weakened as x (Se concentration) increased, the Sb-S/Se bending/stretching modes redshifted, and the low sound velocity in the x = 1 sample was confirmed from resonant ultrasound spectroscopy results. Low lattice thermal conductivity in the samples was made possible by the Sb-S/Se bonds being weaker. In the measured temperature range, samples x = 0.5 and 1 exhibited the lowest lattice thermal conductivity, ranging from 0.45 to 0.34 W m-1 K-1. For the samples x = 0.5 and 1, a high-power factor of 1.3 and 1.35 mW m-1 K-2 at 673 K were obtained, respectively. The double substituted Cu12Sb3.9Te0.1S12.5Se0.5 (x = 0.5) sample achieved the highest figure-of-merit, zT of ~0.84. In Chapter 5, samples with nominal composition Cu11.95Gd0.05Sb4S13-xSex (where x = 0, 0.2, 0.4, 0.6, and 0.8) were prepared. A successful tetrahedrite phase development was revealed by structural investigation using X-ray diffraction, and a successful substitution of selenium was suggested by the systematic increase of lattice parameters with the selenium concentration (x). With an increase of Se content in the samples, the Sb-S/Se bond was weakening, as revealed by the Raman spectroscopy. The XPS analysis verified the +3 and -2 oxidation states of Gd and Se, respectively. Gd3+, a higher valence substituent at the Cu+/Cu2+ tetrahedral site, aided reduced carrier concentration. Conversely, by producing resonant energy states close to the Fermi level, the isoelectronic substitution of Se2- for S2- enhanced thermopower and power factor. Thus, for the sample x = 0.4, a maximum power factor of ~1.35 mW m-1 K-2 at ~729 K was achieved. At 729 K, the sample with x = 0.4 also showed the lowest thermal conductivity at about 1.18 W m-1 K-1. As the Callaway model indicates, the combined effects of point defect and Umklapp scattering are responsible for this sample’s (x = 0.4) decreased lattice thermal component of conductivity. In the x = 0.4 sample, the concurrent reduction of thermal conductivity and carrier concentration optimization led to a comparatively elevated zT of ~0.83.
The summary of all the chapters is discussed in Chapter 6. A brief discussion about how the double substitution and addition approaches helped improve the figure-of-merit of tetrahedrite is presented. The future research plan to enhance thermoelectric efficiency further in tetrahedrite (Cu12S4S13) materials is also discussed in Chapter 6. | en_US |
