Microscale Imaging of Thermal Conductivity Suppression at Grain Boundaries

Grain boundaries have a central importance in materials science. They can critically control thermal and elactrical transport, determining the performance of energy and electronic materials. In thermoelectric materials, 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.<br/><br/>In this work, we image the thermal conductivity of individual 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 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, 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 microstructure works. This development can improve our understanding of carrier-defect interactions, advancing the engineering of materials for thermoelectrics.