Controlling Thermoelectric Properties of Complex Oxide Ceramics by Integrated Design of Grain Boundaries and Interfaces

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NON-TECHNICAL DESCRIPTION: Thermoelectric devices possess the ability to directly transform temperature gradients into electrical power and generate electricity from waste heat for various industry, automobile and space applications. Thermoelectric technology could be more efficient in most applications if the high-performance thermoelectric materials were made of non-toxic and earth-abundant elements. The emerging layered cobalt oxide materials are promising candidates for thermoelectric applications due to their thermal stability, natural abundance, lightweight, and non-toxicity. The energy conversion efficiency, for calcium cobaltite, in single crystal form shows excellent performance that approaches the well-developed conventional thermoelectric materials. However, the performance of polycrystalline ceramics remains low and only ~30-60 % of that found for the single crystals. The objective of this project is to modify polycrystalline calcium cobaltite through designing the interfaces between crystalline grains, aiming to significantly improve their thermoelectric performance even over the single crystals. Besides the direct impact on the development of thermoelectric oxide, the essential understanding of crystal interface engineering gained from this research will be instrumental to many other ceramic systems. This project provides the training to young scientists and graduate students in many disciplines, especially the newly launched Materials Science and Engineering program at West Virginia University. Integration of research and education activities in cutting-edge research in functional ceramics and energy harvesting are further strengthened through the involvement of undergraduate students from underrepresented groups including women.TECHNICAL DETAILS: High-performance thermoelectric materials need to have high electrical conductivity, high Seebeck coefficient, and low thermal conductivity. The low energy conversion efficiency of polycrystalline calcium cobaltite with incommensurate character is caused by the low electrical conductivity and low Seebeck coefficient. In this project, polycrystalline calcium cobaltite crystal texture and grain boundary density are both controlled by intragranular doping and especially the appropriate dopant segregation or depletion at the grain boundaries. While dopants segregating at the grain boundaries promote crystal texture and facilitate large carrier mobility and high electrical conductivity, the dopants segregation acts as carrier filter to decrease the carrier concentration and simultaneously increase the Seebeck coefficient. Furthermore, this project aims to reduce the thermal conductivity of ceramics by interface scattering through the insertion of the approximate secondary phases. The effect of grain boundaries on the mechanical properties has been extensively investigated in many materials. However, the understanding of the impact of grain boundaries on both the electrical and thermal transport properties of most thermoelectric materials is currently very limited. Successful completion of this project is expected to elucidate the underlying atomic structure origin and thermodynamic mechanisms that drive the formation of the crystal boundaries with dopant segregation or depletion, to understand the carrier transport and scattering along the designed boundaries/interfaces, and to ultimately utilize such knowledge to tune the physical properties of electroceramics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.