Developing high-efficiency multiphase thermoelectric materials

This thesis explores strategies for improving the efficiency of thermoelectric materials, with a particular focus on multiphase bismuth chalcogenide compounds. The introduction and literature review provide the necessary background on thermoelectricity and its applications, define key parameters such as the dimensionless figure of merit zT, and outline established methods for optimising single and multiphase thermoelectric materials. The literature review chapter delves into important concepts and mechanisms such as energy filtering, modulation doping, and phonon scattering in multiphase systems. The experimental methods chapter then outlines the different material synthesis techniques used, such as melting, ball milling, and spark plasma sintering, as well as the analytical approaches used to study the materials, including structural, electronic, and thermal transport characterisation. The subsequent results chapters examine the effects of incorporating magnetic dopants and secondary phases into bismuth sulphide, telluride, and selenide host systems. A notable finding was that magnetic co-doping of Bi2S3 with chromium and chlorine significantly increased the thermopower and power factor, attributed to a magnetic drag effect that increases the effective carrier mass. The addition of a Bi14Te13S8 secondary phase to Bi2Te3 matrix compounds was also investigated; the presence of this phase led to an energy filtering effect that improved the thermopower but also introduced additional phonon scattering at phase interfaces that reduced the lattice thermal conductivity. Further studies of sulphur-containing Bi2Te2.7Se0.3 revealed that sulphur inclusion dramatically alters the density of states and native defect concentrations in both single and multiphase samples. Interestingly, multiphase Bi2Te2.7Se0.3 samples exhibited complex electronic behaviour, suggesting possible impurity band formation at higher secondary phase contents. Further investigations Bi0.5Sb1.5Te3 with added CrSb compounds showed that small amounts of the magnetic secondary phase increased the thermopower via an increased effective mass, but higher CrSb contents degraded the performance due to reduced carrier mobility. Finally, iodine doping of single phase Bi14Te13S8, an important component of the multiphase materials studied, was found to effectively optimise the power factor and reduce the lattice thermal conductivity, culminating in an improved figure of merit zT. In summary, this work provides compelling evidence that strategies such as energy filtering, modulation doping, and phonon scattering can be successfully exploited to improve the efficiency of multiphase bismuth chalcogenide thermoelectric materials. The results provide valuable insights to guide ongoing research and development efforts towards higher performance thermoelectric materials for real-world applications.