The research objective of this EArly-Concept Grant for Exploratory Research (EAGER) is to explore a combinatorial nanoparticle synthesis/processing technique to manufacture a new type of thermoelectric material. A central issue in the thermoelectric community is to develop materials with a high figure-of-merit ZT. During the past decade, most research activities have focused on strategies for enhancing the peak ZT. However, the efficiency of thermoelectric devices depends on the average ZT over the working temperature range. Spatially-uniform thermoelectric materials exhibit a substantially lower average ZT than their peak value when operating over a large temperature difference. The University of Maryland proposes to explore a combinatorial nanoparticle synthesis/processing technique to manufacture a new type of thermoelectric material: functionally-graded two-component nanocomposites, in which the use of the 'functionally graded' principle in conjunction with ?two-component nanocomposites? is expected to enhance both average and peak figure-of-merit. This effort will experimentally examine the process-structure-property relationships in model composite systems where the nanocomponent size and composition and their spatial distribution can be controlled. The proposed research will be integrated with the educational mission of University of Maryland (UMD), College Park and provide a broad range of impact on student learning and community outreach. Apart from the education and training opportunities for the students directly supported in this project, underrepresented undergraduate students will be involved in this research through the Undergraduate Research Assistant Program. Research results will also be quickly disseminated to industry via the consortium of Center for Environmental Energy Engineering (CEEE), which has members from more than thirty companies.
Thermoelectric materials, which can turn waste heat energy into useful electrical energy, play an important role in a global sustainable energy solution. Each year billions of watts of electrical power are wasted in the United States along in waste heat from industrial processes, automobiles, and air conditioning. The relatively low efficiency of thermoelectric materials is a major barrier to widespread implementation of thermoelectric technology. Development of thermoelectric materials with a high figure-of-merit is currently a central issue in the thermoelectric community. During the past decade, most research activities have focused on strategies for enhancing the peak figure-of-merit in spatially-uniform materials. However, it is the average figure-of-merit over the working temperature range that determines the efficiency of thermoelectric devices. Spatially-uniform thermoelectric materials exhibit a substantially lower average figure-of-merit than their peak value when operating over a large temperature difference. Alternatively, functionally-graded materials provide new possibilities to improve the average figure-of-merit. This exploratory project has manufactured and characterized Si-Ge functionally-graded nanocomposites for thermoelectric application. High energy milling has been used to prepare nanoparticle composites through repeated cold welding, fracturing, and rewelding. The transfer of mechanical energy to the powder particles results in refinement of particle and grain size, as well as introduction of strain into the powder through generation of dislocations and other defects. A major concern in the processing of thermoelectric nanocomposite materials by mechanical milling is the nature and amount of impurities that get incorporated into the powder and contaminate it. The Si-Ge nanocomposites have been characterized with a combination of Transmission electron microscopy (TEM), Energy Dispersive X-ray Spectroscopy (EDS), X-ray diffraction. It was found that the Ge concentration varied from 0% to 20% from one end to the other of the Si-Ge nanocomposite pellet. The thermal conductivity of the SiGe nanocomposite samples was measured and compared with the state-of-the-art bulk SiGe material used in RTG applications and SiGe superlattices. The thermal conductivity of the prepared Si-Ge nanocomposites is 53% less than that of the RTG samples (4.9 W/m-K) and about 10 % lower than the cross-plane conductivity of the superlattices with similar chemical composition (2.6 W/m-K). These results indicate that Si-Ge functionally-graded nanocomposites are promising materials in thermoelectric application, but further study is needed to optimize their composition and microstructure for optimal thermoelectric properties. One Ph.D. has been graduated, partially supported by this project.
