THE EFFECT OF ONE- AND TWO-DIMENSIONAL NANOFILLERS ON THE THERMOELECTRIC PROPERTIES OF N-TYPE BISMUTH TELLURIDE NANOCOMPOSITES

Abstract

Bismuth telluride alloys are the most commercially mature thermoelectric materials, but their conversion efficiencies are still low due to the undeveloped n-type semiconductor material. In this work, the thermoelectric properties of n-type Bi2(Te,Se)3 alloys prepared by ball-milling and hot-pressing techniques were systematically optimized. The optimization routes were carried out for the pristine n-type composition, hot-pressing parameters, post-annealing of the hot-pressed discs, as well as the addition of different content of different 1D and 2D nanofillers. The chosen nanofillers were 2D graphene, 2D MXene, and 1D Eocene Midra shale. By the end of this thesis, several promising outcomes were revealed. The figure-of-merit value of the starting n-type Bi2(Te,Se)3 was 0.3 at room temperature with the optimized composition of Bi2Te2.55Se0.45. After optimizing the hot-pressing parameters and the addition of 0.05 wt.% graphene at the last 10 minutes of ball-milling, the figure-of-merit reached a value of 0.4 at the same temperature. It was noticed that the addition of 2D graphene improved the electrical conductivity remarkably by activating the tunneling behaviour of charge carriers. The second nanofiller, MXene, was then investigated. Optimization of 0.01 wt.% MXene nanofiller added in the last four hours of ball milling improved the figure-of-merit to 0.57 at room temperature by increasing the Seebeck coefficient though the energy filtering effect. Then, the addition of Eocene Midra shale as the third nanofiller was considered. The usage of 0.05 wt.% of freely available Eocene Midra shale added in the last 20 minutes showed, surprisingly, better results compared to graphene and MXene. Eocene Midra shale was able to provide the greatest improvements in the thermoelectric properties, and the figure-of-merit reached a value of 0.64 at room temperature. Despite all efforts in optimizing the figure-of-merit through nanofiller addition, a figure-of-merit value of 0.64 was still low to show improvements in conversion efficiencies. Thus, further optimization of annealing hot-pressed pellets was considered. Astonishingly, high figure-of-merit values of 1.1 and 1.3 were obtained for the pristine Bi2Te2.55Se0.45 alloy at room temperature and 160oC, respectively. The reported results in this thesis provide promising and low-cost techniques to prepare n-type Bi2(Te,Se)3 with high thermoelectric performance for commercial applications.