Nanostructured Bulk of Doped Strontium Titanium Oxide and Bismuth Telluride for Thermoelectric Applications

Abstract

Enhancement of thermoelectric performance is of key importance in the practical application of thermoelectric materials. Recently, the use of nanostructured bulk materials has been proposed to exploit the reduction in the thermal conductivity and consequently increase in the dimensionless figure-of-merit ZT. In this dissertation, I will discuss synthesis of nanocrystals, their compaction to nanostructured bulk materials, and the thermoelectric property characterization. Amongst many thermoelectric candidate materials, I will focus on nanostructured bulk materials of La-doped SrTiO3 and K-doped Bi2Te3. First, I will report on the synthesis of La-doped SrTiO3 nanoparticles with controlled La doping levels and their enhanced thermoelectric properties. The nanoparticles were sintered using a spark plasma sintering process to fabricate nanostructured bulk materials. The ZT value reaches a maximum of ~0.37 at 973 K, which is ~25% higher than that of the single-crystal bulk material. The increased ZT is due to the reduction of the thermal conductivity while maintaining the electrical conductivity and Seebeck coefficient. In addition, the nanostructured bulk La-doped SrTiO3 exhibits thermal stability even after heat treatment at 973 K, demonstrating their suitability for high-temperature applications. Second, I will report on the preparation of new nanostructured bulk materials of bismuth telluride having non-equilibrium phase compositions. Furthermore, potassium cations, which act as unconventional electron donors, were successfully doped into the bulk bismuth telluride. According to the results of scanning transmission electron microscope with electron energy loss spectroscopy studies and density functional theory calculations, K cations occupy the interlayer and interstitial sites. The resulting compounds with high tellurium and potassium concentrations show increased electrical conductivity and the Seebeck coefficient. Consequently, a high power factor of ~43 μW cm–1 K–2 and the ZT >1.1 at 350 K, which are among the highest values observed for n-type bismuth telluride materials, are achieved in the thermoelectric measurements.