Non-Stoichiometric Ferroelectrics and their Associated Thermoelectric Properties

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NON-TECHNICAL: The research in this project opens up new research strategies for sustainable high performance thermoelectrics that could have major societal impacts on energy savings, CO2 reduction and the environment. This investigation explores the interface between two unusual classes of functional materials: ferroelectrics and thermoelectrics, for low cost ¡V high efficiency energy harvesting. There are major economic advantages with a lower cost thermoelectric materials solution for generators; as an example, every automobile, household furnace, and factory exhaust chimney could have heat exchangers equipped with thermoelectric devices and these new enabling thermoelectric materials and strategies. There is an important educational outreach activity with local schools (Park Forest and Mount Nittany Middle Schools), engaging students in grades 6-8 in material science. Under this project, a materials classification exercise is being designed to test previous perceptions and prejudices of the children towards selecting metals and insulators through ¡§show and touch. The same materials are being introduced to introductory materials classes at Penn State University to see how opinions differ from those of the younger children. The theoretical basis of this concept underpins the technical part of the advances used in transforming ferroelectric materials to thermoelectrics. Based on preliminary data, the core intellectual property for Penn State University in the ferroelectric-thermoelectrics area has already been protected. Across this project, activities range from local education to global energy-environment-economic benefits in terms of its impact. TECHNICAL: This investigation explores the interface between two unusual classes of functional materials: ferroelectrics and thermoelectrics. The aim of this work is to provide a broader understanding of highly non-stoichiometric ferroelectric materials near the critical electronic carrier concentration separating semiconducting and metallic conduction, the so-called Mott criterion. Initial observations of thermoelectric properties in non-stoichiometric perovskite BaTiO3-d and 001 tungsten bronze (Sr,Ba)Nb2O6-d ferroelectric materials shows attractive properties, and this project will establish: (1) general structure-thermoelectric property data for various ferroelectric compositions and structures, (2) quantification and modeling of electrical conductivity, Seebeck coefficient, and thermal conductivity, identifying the materials physics controlling the transport, (3) insights into the "best" ferroelectric-thermoelectrics, (4) the relation between the nature of the ferroelectric phase transition behavior and the semiconductor-metallic transition in different materials, and (5) analysis of the optical band edge behavior, quantifying phonon¡Velectron coupling, and structural modifications in the highly non-stoichiometric ferroelectrics across phase transitions. Tetrahertz spectroscopy measurements of the non-stoichiometric ferroelectrics produced under this project are used to establish detailed understanding of the phonon dynamics in the transition region and could open up a new sub-field by challenging the understanding of phase transitions in ferroelectric materials under these unique conditions. The research collaboration with the Czech Institute of Physics will expose the Penn State University students involved in this investigation to the importance of the global network of science and collaboration with groups with unique and world-class facilities and expertise.