Abstract: Grain boundaries have a central importance for functional properties of materials. They can critically control thermal and elactrical transport, determining the performance of energy and electronic materials. In thermoelectric materials - promising for refrigerant-free cooling and waste heat energy harvesting - grain boundaries can be leveraged to suppress the thermal conductivity, but can also detrimentally suppress the carrier mobility. Grain boundaries are not all equal: they are associated to several degrees of freedom, and can come in multiple orientations, symmetries, and chemistries. Recent evidence suggests that some types of grain boundaries could be more beneficial than others for the thermoelectric performance. Despite the importance, we lack a clear understanding of how grain boundaries modify the microscale transport owing to the scarcity of local investigations. Usually the role of grain boundaries is inferred from bulk, effective measurements. However, understanding how grain boundaries impact transport locally is a crucial perspective to enable grain-boundary engineering for the next generation of high-performance thermoelectrics.

In this seminar, I will introduce our recent work on thermal conductivity imaging of grain boundaries via spatially-resolved frequency-domain thermoreflectance. Measurements with microscale resolution reveal a suppression in thermal conductivity at grain boundaries both in thermoelectric SnTe and photovoltaic multicrystalline silicon. In contrast to conventional thermal modeling, which assumes that all boundaries are perfect scatterers and lead to uniformly suppressed thermal conductivity, we observe a non-uniform suppression localized within a few microns of a boundary. Furthermore, not all grain boundaries behave the same: misorientation angle, symmetry, as well as interface roughness and morphology are found to strongly correlate with the effective thermal boundary resistance. Extracting transport properties from microscale imaging can provide comprehensive understanding of how individual microstructural components work. In particular, it can advance the study of grain boundary phases - i.e. two-dimensional phases stabilized at the boundary and that can be controlled via thermodynamics - by correlating how thier local chemistry and structure impact functional properties. This development can improve our understanding of carrier-defect interactions, enabling the rational engineering of interfaces and materials microstructures for superior performance in energy and electronics.

Bio: Eleonora Isotta is currently a Postdoctoral scholar in the Department of Materials Science and Engineering at Northwestern University. She joined in fall 2022 and has been working on thermal transport in semiconductors, developing methods to perform thermal imaging of defects, with a focus on grain boundaries, together with Prof. G. J. Snyder and Prof. O. Balogun. She received her B.S. in Environmental Engineering in 2015 and M.S. in Energy Engineering in 2017 from University of Trento. In 2021, she received a Ph.D. in Materials Science from University of Trento, with a dissertation on the interplay between crystal structure and transport properties in thermoelectric materials. She then pursued a Postdoc at Michigan State University working on thermal transport and elasticity in thermoelectric materials. Her current research interests involve thermal transport, microstructure engineering, and materials for thermoelectrics and electronics.