Thermal Transport Studies of Individual Grain Boundaries within Polycrystalline Materials At the nanoscale, interfaces can strongly restrict heat transfer by scattering the heat carriers, which are mainly phonons in nonmetallic materials. Such interfacial phonon scattering and its resulting interfacial thermal resistance are important to many applications, ranging from the thermal management of nanoelectronic and optical devices, to effective thermal insulation materials, to thermoelectric energy conversion. However, the fundamental understanding of how phonons interact with an interface is still limited after decades of research, especially when the complexities of a real interface are considered. In particular, the thermal resistance of a single grain boundary (GB) has not been directly measured for a polycrystalline bulk material or thin film. Existing thermal studies can only extract an averaged GB thermal resistance by fitting the temperature-dependent thermal conductivity of the whole material. To address this critical issue, this project combines thermal measurements and atomistic simulations to reveal the detailed phonon transport across individual GBs. For general interfaces, the knowledge gained from this project will provide important guidance for tailoring the interfacial phonon transport by varying the interfacial atomic and nanoscale structures. The integrated educational plan aims to involve undergraduate and high-school students, especially those from underrepresented groups, in cutting-edge energy research. Innovative outreach activities also include developing a challenge for middle-school students in the Mathematics, Engineering, Science Achievement program, and demonstrating the importance of nanotechnology research to the general public through museum exhibitions. The objective of the proposed research is to better understand the phonon transport across an individual GB within polycrystalline bulk materials and thin films. The investigations focus on GBs formed by two widely used techniques for materials synthesis, i.e., chemical vapor deposition (CVD) for thin films and hot press for nanostructured bulk materials. Thermal resistance measurements are carried out on a single GB, using nanofabricated thermal sensors to measure the steady-state temperature jump across this GB under a given heat flow. This will provide unprecedented experimental data for phonon transport across single GBs within these polycrystalline materials. As a comparable case of a real GB within hot-pressed bulk materials, planar film-wafer interfaces by hot press are also measured for varied crystal misorientations across the interface. All thermal measurements can be directly compared to predictions based on atomistic Green?s function (AGF) simulations that employ the exact interfacial atomic structure (e.g., dislocations, crystal orientation, roughness, nanoscale strain as atomic displacement) revealed by different microscopy techniques. The integration of individual GB measurements and AGF simulations reaches beyond previous AGF studies that often use guessed interfacial atomic structures and are seldom validated by experiments. Fundamentally, the proposed study will elucidate the relationship between the synthesis condition, interfacial atomic structure, and the corresponding GB thermal transport. The success of this project will significantly advance the thermal studies for many important applications, such as nanoelectronic devices using CVD films and film-wafer bonding, polycrystalline thin-film solar cells, structural and optical bulk materials by hot press, thermoelectric materials, and thermal barrier coatings.