Abstract
Thermoelectric materials that enable direct conversion between heat and electricity have been employed in commercial devices for refrigeration and power generation. Bismuth telluride based compounds possess superior thermoelectric efficiency in room temperature regime, showing promising applicability to low temperature waste heat recovery. A novel structure of bismuth telluride based thermoelectric material that contains two regions with different Seebeck coefficients were fabricated and studied. A systematic study including individual thermoelectric transport properties, fabrication of junction structures, spatial voltage effects and thermoelectric power efficiency of corresponding modules has been performed. The study is organized as two parts: the first part contains the basic properties and doping effects of n-type Bi-Te(Se) compounds, and the second part elaborates the thermoelectric effects of junction structured materials and modules. Firstly, a two-step annealing process is proposed to optimize the thermoelectric transport properties on cold-pressed Bi2Se0.21Te2.79 bulk, showing 30 % enhancement in figure of merit. Secondly, in addition to intrinsic Bi-Te(Se) compounds, the effects of doping silver/copper on thermoelectric transport properties are elaborated by the lattice defect mechanisms. The dual electronic nature of silver-related lattice defects is carefully identified with the formation of interstitial (Agi) and substitutional (AgBi’’) defects. The investigations of intrinsic base and extrinsic doping regions develop the concept of junction structured material with different Seebeck coefficients of two regions. There are three kinds of junction structured materials, including bulks with P+/P, N-/N, and P/N regions, which can be easily fabricated with and without dopants. Thirdly, the transverse voltage effects that are inherent in the novel structure have been well studied with good agreements between experimental and theoretically modeled results. Finally, the novel thermoelectric generation module that is assembled by junction structured materials shows better efficiency than the traditional Π-shape modules under certain dimensional criteria. The special arrangement of electrodes introduces a two-dimensional temperature gradient resulting in a larger voltage output during power generation, especially for thin thermoelectric modules. Introducing a horizontal temperature gradient to complement the insufficient vertical temperature gradient is expected to be the solution of improving the power efficiency of thin junction structured thermoelectric modules.