Mechanical and Civil Engineering Seminar

Operating temperature, and hence heat dissipation, impacts the performance of energy conversion technologies ranging from solar cells to thermoelectric devices.  Thermal transport in advanced materials for these conversion technologies cannot be described simply by Fourier's law when the mean free paths of energy carriers are commensurate to feature sizes. Instead, interfaces and non-diffusive transport can govern macroscopic quantities like thermal conductivity.  The objective of my research group is to experimentally measure these effects, to better design and understand these materials. I will begin with an introduction to the nanoscale heat carriers, and I will then present two projects where we collaborated with computational modelers to reveal novel thermal transport properties in new and old materials.  (1) Nanocrystal arrays (NCAs) are self assembled organic-inorganic hybrid materials with novel electronic and optical properties.  Our measurements reveal that thermal transport in NCAs is mediated by the surface chemistry defined interfaces between neighboring nanocrystals. (2) Deeper understanding of phonon physics in NCAs will be based on a new laser-based technique that can directly measure the mean free path spectra of energy carriers.  Our initial studies of silicon reveal that ~40% of its thermal conductivity results from energy carriers with mean free paths longer than 1 mm, in stark contrast to historical predictions.