This research project focuses on the fabrication, characterization, and understanding of new high-efficiency thermoelectric materials for thermal energy harvesting and conversion. High-efficiency thermoelectric materials are used in thermoelectric devices designed to convert thermal energy into electricity. The conversion of solar thermal energy, waste heat, geothermal energy and other thermal energies into electrical energy will play an important role in the endeavor to develop advanced renewable energy technologies. In this project, a novel approach will be developed to enhance the energy conversion efficiency of thermoelectric materials by utilizing C60 fullerene based solid materials, C60/Bi2Te3 and C60/TiO2 nanocomposites. One of the novel properties of C60 fullerene is its super-low thermal conductivity, which is required for high-efficiency thermoelectric devices. The proposed nanocomposites are expected to benefit from the superior thermoelectric properties of the constituents, in addition to the size effect on the thermal transport. It is anticipated that new highly efficient thermoelectric materials will be identified at the end of the project period. The project will also provide research training and mentoring for graduate and undergraduate students, including minority students, by intimately engaging them in the research activities. A unique intellectual merit of the proposed work lies in the combined research efforts, including advanced material fabrication, characterization, and computational materials research for novel thermoelectric materials. The integrated research efforts will enable the scientists to understand the material properties, identify new materials and determine the optimal fabrication conditions. The success of this project will have a broader impact on the national needs for clean and renewable energy technologies. Harvesting thermal energy (solar, waste heat and geothermal, etc.) via thermoelectric conversion offers cleaner, more-efficient alternatives to the combustion of fossil fuels, and enables the reduction of greenhouse emissions. The success of the project will represent an important step toward achieving energy security and independence for our nation. The project is potentially transformative because the nanocomposite materials to be investigated possess combined properties of low thermal conductivity of C60 and high Seebeck and electrical conductivity of Bi2Te3 and TiO2. This will lead to a completely new group of thermoelectric materials with high figure of merit. This project is jointly funded by the Thermal Transport Processes (TTP) Program, of the Chemical, Bioengineering, Environmental, and Transport Systems (CBET) Division within the Directorate for Engineering (ENG), and by the Experimental Program to Stimulate Competitive Research (EPSCoR), of the Office of Integrative Activities (OIA).
We have observed anisotropy in the thermal conductivity κ in the directions parallel and perpendicular to the (101) defect planes and determined the origin for the discrepancy in the anisotropy of reduced single crystal TiO2 obtained at low and high temperatures. The anisotropy undergoes a cross-over as the temperature increases as the phonon scattering mechanism changes from the defect dominated to phonon-phonon type. Single crystal TiO2 was found to exhibit very large positive Seebeck coefficient S at low temperature due to the phonon drag effect of the holes. The thermoelectric power factor is 170 μW/K2cm at 10 K. C60 doped with P, Co, Al and Bi show extremely low thermal conductivity, typically in the range of 0.1 - 0.3 W/Km at room temperature. Large Seebeck coefficient (103 μV/K) was found in P doped C60. Bi doped C60 exhibits a crossover from Mott variable range hopping to intergranular tunneling at relatively high Bi concentration. TEM and Raman spectroscopy studies have revealed interesting microstructures that may shed light on the origin of enhanced Seebeck coefficient as well as much reduced resistivity of the C60 doped BiSbTe samples. We have obtained evidence that the sign of the superexchange coupling J2 between next-nearest neighboring Eu2+ magnetic moments in EuO is of antiferromagnetic nature (J2 < 0). The investigation of a series of oxygen-deficient EuO thin films provided strong evidence that the doped electrons form magnetic polarons with the nearby Eu2+ 4f spins, which is responsible for the enhanced Curie temperature observed near 140 K and the "double doom" feature in the magnetization in reduce EuO1-x. This project has resulted in 23 refereed publications and 11 presentations. In exploring a new method for the synthesis of nanocomposite thermoelectric samples, we have designed and fabricated a third generation hot pressure cell that can be used to consolidate nano-powders into pellets. This pressure cell can be used to anneal bulk solid pellets, under the working conditions of high temperature, high pressure, in vacuum or in argon/hydrogen gas environment. Thermoelectric materials, such as Bi0.46Te0.54, (Bi0.25Sb0.75)2Te3, are processed from ball milling (in about 20 to 30 hours) to form nano-size (10-40 nm) nano-powders. The powders are then cold pressed into a pellet. The pellet together with the pressure cell is placed into the furnace for annealing under controlled atmosphere.
