This project, supported by the Solid State and Materials Chemistry program focuses on the synthesis of new Zintl phases, incorporating transition metals in order to target a high density of states (DOS) at the Fermi level (through partially filled d orbitals) to obtain high Seebeck coefficient (Power factor) and therefore high zT. Nanostructuring will be employed to further reduce the thermal conductivity. Phases will be synthesized via flux and metallurgical routes. Utilizing a suite of physical characterization techniques including single crystal and powder X-ray diffraction, SEM and TEM, elemental wave dispersive microprobe and ICP-MS analysis, parallel electrical and magnetic measurements, the structure and phase composition will be correlated with electronic and thermal transport properties. The ultimate goal of the project is to discover new materials with high zT for the direct conversion of waste heat into electricity. Building upon recent results of demonstrated high zT in Zintl phases, this proposal closely links synthetic strategies with characterization of physical and chemical properties, thus providing efficient feedback to guide improvement of these thermoelectric systems.
NON-TECHNICAL SUMMARY: Thermoelectric devices for power generation convert thermal energy directly into electrical energy, require minimal maintenance, and can be operated over a large temperature range (room temperature to 1000 degrees C). Thermoelectric materials are described by a figure of merit, zT, which relates to how well a material converts heat flow to electricity - the higher the value, the greater the efficiency. Recent discoveries of Zintl phases with zT of 1.0 or greater highlight the importance of continued exploration of new materials. A Zintl phase is a compound which contains both cations such as alkali, alkaline earth, and rare earth elements which donate electrons to polyanionic units composed of main group elements such as those from groups 13, 14, and 15 of the periodic table. These types of compounds naturally provide low thermal conductivity which is a requirement for thermoelectric devices. With higher zT, the potential for solid-state power generation from waste heat recovery provides a framework for investigating new Zintl phase materials with stability and optimal properties at high temperatures (>300 degrees C). This project supports minorities and women at the initial stages (high school to college and college to graduate school), along with training graduate student for scientific careers. Students develop scientific, social, and professional skills through hands-on training, exploration, and dissemination of research to the broader scientific community. The topics under study include materials synthesis and property measurements, fundamental training in materials synthesis and structure-property correlations, providing an important foundation for the development of thermal to electrical energy conversion and the advancement of multidisciplinary research aligned with technology. The research will be presented at national and international meetings and the findings published in peer-review journals. Additionally, the PI supports students through the ACS SEED program (high school), MURPPS (undergraduate), and AGEP (graduate) programs focused on increasing underrepresented groups in science on campus. The ChemWiki will be employed to enhance student learning through writing and critical evaluation.
The synthesis and identification of new compounds that can lead to enhancements in existing technologies, or serve as the basis of revolutionary new technologies, is essential for developing new and improved technologies. Zintl phases are ideal for providing a systematic approach to optimize these compounds for their electronic and magnetic properties. Close collaboration between synthesis, structure and property measurements has lead to new discoveries of materials with useful properties for technologies such as thermoelectric generators. Thermoelectric generators have the potential to provide clean, reliable electricity through waste heat recovery. As current generator efficiencies are insufficient for widespread application, new and optimized thermoelectric materials hold the key as the primary enabling component. Generator efficiency depends on both the Carnot efficiency and the material thermoelectric figure of merit zT, where improvements have been challenging because of the interdependent nature of the properties required for good performance. However, Zintl phases have many of the required components for good performance. In particular, the ionic bonding, allows for judicious use of metal ions to donate electrons to the more electronegative anionic network. This allows for control of both carrier concentration and mobility in compounds with low lattice conductivity. New intermetallics have been prepared and studied along with a new facile route to the iron arsenide superconductor. New members of the Ca9Mn4Bi9 family were synthesized with the formula of Eu9Cd4-xCM2+x-y◊ySb9 (CM = coinage metal: Cu, Ag, and Au). We demonstrated the versatility of this low thermal conductivity material and showed that Eu was present as both Eu2+ and Eu3+, consistent with simple Zintl counting of electrons. Mössbauer spectroscopy showed that the Eu environments were complex with distinct magnetic sublattices. Eu11Cd6Sb11-xAsx (x < 3) was synthesized and shown to be a low thermal conductivity material. The As substitution is site selective and each site has distinct electronic consequences. The thermoelectric properties indicate that this phase may be promising for room temperature applications and this system is being pursued further. Eu3GasP4, a new Zintl phase, was prepared and shown to have a large magnetoresistance at low field (~30% at 2 Tesla). Magnetoresistance is important for many technologies and this result suggested that the Eu–P bonding is important for this behavior. New Clathrate structures with abundant elements were prepared and studies. Ba8AlxSi46-x (x = 8, 10, 12, 14, and 15) were studied and the Al substitution in the structure was shown to have an important influence on the rattling of the Ba cation in the cages. The rattling of the cation is known to be important for the low lattice thermal conductivity in these types of compounds. We developed a new facile route to the iron arsenide superconductor, Ba1-xKxFe2As2. A number of new intermetallics were synthesized and their structure, electronic and magnetic properties characterized. This research was presented at national and international meetings and published in peer reviewed scientific publications. This proposal educated graduate and undergraduate students in the area of solid state and materials chemistry with an aim torwards materials for energy applications, such as thermoelectrics for power generation, magnetoresistance fo memory technologies. New scientific training in state-of-the-art research for students at three critical transition points were emphasized: (i) high school to college, (ii) college to graduate school, and (iii) graduate school to scientific career. The topics under study were materials synthesis and property measurement to provide structure-property correlations and insight to developing better materials through directed investigations. This work provides an important foundation for the development of thermal to electrical energy conversion and the advancement of multidisciplinary research aligned with technology. Graduate and undergraduate students were provided with a number of opportunities for education such as a cyber course on the fundamentals of thermoelectrics.Graduate students visited Oark Ridge National Laboratory to collect neutron data and interact with staff scientists. Graduate students participate in outreach with the Chemistry Club, tutoring, and mentoring undergraduate students from MURPPS, Mentorships for Undergraduate Research Participants in the Physical and Mathematical Sciences, and high school students, sponsored by the American Chemical Society project SEED, summer experience for the economically disadvantaged, in research. Students attended weekly meetings and present research in a supportive atmosphere. Undergraduate students presented research at the UC Davis Undergraduate Research Symposium and the LaRock Chemistry Conference, along with the American Chemical Society meeting. Graduate and undergraduate students benefitted from interactions with national labs such as Oak Ridge, Los Alamos, and the Jet Propulsion Laboratory. Research was presented at national and international meetings such as the American Chemical Society National meetings and the International Conference on Thermoelectrics (ITC). This proposal funded 8 graduate students (4 women and 1 underrepresented minority) and 4 undergraduates (1 women and 3 underrepresented minorities). This proposal included both national and international collaborations, utilizing facilities such as the neutron facilities at Oak Ridge National Laboratory.
