Thermoelectric Properties of V-VI Semiconductor Alloys and Nanocomposites
Yelgel, Ovgu Ceyda
Date: 15 July 2013
University of Exeter
Doctor of Philosophy in Physics
Thermoelectric materials are materials which are capable of converting heat directly into electricity and vice versa. They have long been used in electric power generation and solid-state cooling. The performance of a thermoelectric device determined by the dimensionless figure of merit (ZT) of the material, defined as ZT = (S2 ...
Thermoelectric materials are materials which are capable of converting heat directly into electricity and vice versa. They have long been used in electric power generation and solid-state cooling. The performance of a thermoelectric device determined by the dimensionless figure of merit (ZT) of the material, defined as ZT = (S2 σ/κ)T, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the total thermal conductivity, and T is the absolute temperature. The total thermal conductivity consists of contribution from electrons, electron-hole pairs and phonons. Since the 1960s, the best thermoelectric material has been Bi2Te3 alloys, with a ZT of 1.0 at room temperature. In recent years, the idea of using nanotechnology has opened up the possibility of engineering materials at nanoscale dimensions to achieve higher values of ZT in other words to have more efficient thermoelectric devices. This thesis starts with a broad introduction to thermoelectricity including various thermoelectric effects and their applications. The state-of-the-art thermoelectric materials and the optimisation methods to enhance the value of ZT have also been reviewed. A systematic theoretical modelling of the thermoelectric properties of three dimensional bulk semiconductors has been presented in Chapter 2. Electronic properties (Fermi level, Seebeck coefficient, and electrical resistivity) and thermal conductivity contribution from carriers (donor electrons or acceptor holes) have been derived by using the nearly-free electron approximation and the Fermi-Dirac statistics. Other thermal conductivity contributions originated from electron-hole pairs and phonons have also been described in detail. In Chapter 3, this theoretical study is extended to two dimensional semiconducting quantum well structures bearing in mind that the Fermi level should change with the temperature as well as the quantum well width and additional interface scattering mechanisms (interface mass-mixing and interface dislocation scatterings) should be included for the definition of anharmonic scattering rate. Thermoelectric properties of n-type (Bi2Te3)0.85(Bi2Se3)0.15 single crystals doped with 0.1 wt.% CuBr and 0.2 wt.% SbI3 and p-type (Bi2Te3)x(Sb2Te3)1−x single crystals doped with 3 wt.% Te (0.18 ≤ x ≤ 0.26) have been explored in Chapter 4 and 5, respectively. It has been found that p-type Bi2Te3 based alloys showed higher values of ZT due to their larger power factor (S2σ) and smaller thermal conductivity values. These calculations have concluded that the influence of the composition range of semiconductor alloys together with its type and amount of dopant plays an important role in enhancing the ZT. In Chapter 6, a detailed theoretical investigation and comparision of the thermal conductivities of these single crystals have been reported including frequency dependence of the phonon thermal conductivity for different temperatures. In Chapter 7, based on temperature and well width dependent Fermi level, a full theory of thermoelectric properties has been investigated for n-type 0.1 wt.% CuBr doped Bi2Se3/Bi2Te3/Bi2Se3 and p-type 3 wt.% Te doped Sb2Te3/Bi2Te3/Sb2Te3 quantum well systems. Different values of well thicknesses have been considered for both types of quantum well systems to study the effect of confinement on all thermoelectric transport coefficients. It has been found that reducing the well thickness has a pronounced effect on enhancing the ZT. Compared to bulk single crystals studied in Chapter 4 and 5, significantly higher thermoelectric figure of merits have been estimated theoretically for both n- and p-type semiconducting quantum well systems. For the n-type Bi2Se3/Bi2Te3/Bi2Se3 quantum well system with taking 7 nm well width the maximum value of ZT has been estimated to be 0.97 at 350 K and for the p-type Sb2Te3/Bi2Te3/Sb2Te3 quantum well with well width 10 nm the highest value of the ZT has been found to be 1.945 at 440 K. Chapter 8 briefly recapitulates the results presented in this thesis and outlines possibilities for future work.
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