On the structural and physical properties of Earth-abundant sulphides for thermoelectric applications

The work presented in this thesis focuses on improving the thermoelectric performance of cost-efficient, widely available, non-toxic transition-metal sulphides. Three families of materials, titanium-disulphide derivatives, tetrahedrites and bornites, were investigated owing to their promising electrical and/or thermal properties. Alongside thermoelectric property measurements, structural characterisation and measurements of various physical properties were carried out in order to explain the relationship between structural and physical features. A series of composition CoxTiS2 (0 ≤ x ≤ 0.75) was synthesised and structural characterisation was carried out using a combination of powder X-ray diffraction and neutron diffraction analysis. Electrical and thermal transport property measurements, thermal analysis, magnetic susceptibility measurements and muon spectroscopy analysis were carried out to complete the investigation. The intercalation of cobalt within the van der Waals’ gap of TiS2 leads to the formation of three superstructure types, monoclinic M5S8 (x = 0.25), trigonal M2S3 (x = 1/3) and monoclinic M3S4 (x = 0.5). At intercalation levels x ≤ 0.20 and x ≥ 2/3, Co atoms randomly occupy octahedral sites. In the case of x ≥ 2/3, Co atoms also occupy tetrahedral sites. The thermoelectric properties of CoxTiS2 (0 ≤ x ≤ 2/3) have been systematically investigated. A figure of merit of 0.3 is obtained for CoxTiS2 (0.04 ≤ x ≤ 0.08) at 573 K, a 25 % improvement over pristine TiS2 and one of the highest figures of merit reported for a n-type sulphide at such a low temperature. A series with composition MoxTi1-xS2 (0 ≤ x ≤ 0.09) was synthesised and consolidated by hot-pressing and spark plasma sintering. The ball milling conditions used and the SPS processing lead to a power factor of S2ρ-1 ≈ 2.1 mW m-1 K-2 at 323 K, which is the highest reported for TiS2. The reproducibility of the electrical properties of TiS2 for application at temperatures that do not exceed 473 K was confirmed by a heat-soak experiment. Nanoparticles of TiS2 with controlled morphologies were synthesised using a scalable solution-based route. Measurements of the Seebeck coefficient were carried out on TiS2 nanoflakes and a stoichiometry of Ti1+xS2 where x lies within the range 0.02 ≤ x ≤ 0.03 was determined. The thermoelectric properties of TiS2 nanocomposites with TiO2 nanoparticles and carbon nanotubes (CNTs) were investigated. A 12 % increase in the figure of merit over that of pristine TiS2 at 573 K, ZT = 0.28, is observed for the nanocomposite with 0.5 vol% of nano-TiO2. Two families of copper sulphides were investigated for their promising p-type thermoelectric properties, tetrahedrites, including Cu12-xMnxSb4S13 (x = 0; 1) and Cu12+ySb4S13 (x = 0; 0.3; 1; 1.5 and 2), and manganese-doped bornite, Cu5Fe1-xMnxS4 (0 ≤ x ≤ 0.10). A combination of powder X-ray and neutron diffraction analysis of Cu12-xMnxSb4S13 (x = 0; 1) located the Mn atoms on the Cu(1) site in tetrahedral position. Manganese substitution leads to a substantial reduction in thermal conductivity and the figure of merit is improved with ZT = 0.56 at 573 K. Structural analysis using powder X-ray diffraction and measurements of the thermoelectric properties of a series of copper-enriched tetrahedrite Cu12+ySb4S13 (y = 0.3; 1; 1.5; 2) were carried out. At room temperature, Cu12+ySb4S13 exists as a mixture of a Cu-poor and a Cu-rich phase and a change of behaviour in the electrical and thermal properties are observed over the temperature range 380 ≤ T / K ≤ 410. An increase in the Seebeck coefficient and a simultaneous decrease in the thermal conductivity are responsible for a large improvement in the figure of merit with ZT = 0.62 at 573 K for Cu12+ySb4S13 (y = 1.5 and 2). A manganese-doped bornite series of composition Cu5Fe1-xMnxS4 (0 ≤ x ≤ 0.10) was successfully synthesised using mechanical alloying. This synthetic method offers a simplified route to large scale production as well as better thermoelectric performance (ZT = 0.55 at 543 K) compared with samples prepared by solid-state reaction (ZT ≈ 0.4 at 573 K). Structural investigation was carried out using a combination of powder X-ray diffraction and SEM/EDX analysis and the solubility limit of Cu5Fe1-xMnxS4 was determined to be x = 0.1. Repeated electrical measurements and thermogravimetric analysis were carried out in order to investigate the stability of the samples and the reproducibility of the electrical properties as a function of time and temperature

Tuning Ca3Co4O9 thermal and transport properties by TiC nanoparticles addition

Ca3Co4O9 + xwt.% TiC (x = 0, 0.25, 0.5, 0.75, and 1.0) polycrystalline thermoelectric ceramics have been prepared through the classical ceramic route. XRD characterization has demonstrated that all samples are mainly composed by the Ca3Co4O9 phase, while microstructural observation has shown that no reaction between both components has been produced. Moreover, TiC particles are well distributed for small additions, and start to agglomerate from 0.75 wt.% content. Density measurements showed that nearly no changes have been produced by TiC addition and nearly the whole porosity appears as open one. Furthermore, electrical resistivity decreases up to 0.75 wt.% addition, increasing for higher content. On the other hand, Seebeck coefficient has been maintained unchanged in all samples. In spite of an irregular behaviour of thermal conductivity with temperature, it tends to decrease when the temperature is raised, displaying the lowest values for the 0.25 wt.% samples. These data led to an increase of about 40% in ZT values at 800 °C for samples with 0.25 wt.% addition, when compared with the pure ones. Finally, linear thermal expansion coefficient is decreased when TiC content is increased, which can be exploited to fit the thermal expansion coefficients of all the components used to build a power generation thermoelectric module

Electron doping and phonon scattering in Ti1+xS2 thermoelectric compounds

Bulk polycrystalline samples in the series Ti1+xS2 (x = 0 to 0.05) were prepared using high temperature synthesis from the elements and spark plasma sintering. X-ray structure analysis shows that the lattice constant c expands as titanium intercalates between TiS2 slabs. For x=0, a Seebeck coefficient close to -300 μV/K is observed for the first time in TiS2 compounds. The decrease in electrical resistivity and Seebeck coefficient that occurs upon Ti intercalation (Ti off stoichiometry) supports the view that charge carrier transfer to the Ti 3d band takes place and the carrier concentration increases. At the same time, the thermal conductivity is reduced by phonon scattering due to structural disorder induced by Ti intercalation. Optimum ZT values of 0.14 and 0.48 at 300K and 700K, respectively, are obtained for x=0.025