MSE Seminar Series: David Cahill
Friday, October 7, 2011
Room 2110 Chemical and Nuclear Engineering Bldg.
301 405 5240
Ultralow Thermal conductivity and the Thermal Conductance of Interfaces
Department of Materials Science and Engineering, and Materials Research Lab
University of Illinois at Urbana
For many years, conventional wisdom has held that the lowest possible thermal conductivities are found in electrically-insulating glasses and certain classes of strongly disordered crystals. The thermal conductivity of these materials can be successfully predicted by a simple model, i.e., the minimum thermal conductivity, based on Einsteins 1911 theory of heat conduction. We have discovered recently, however, that anisotropic solids that combine order and disorder in the random stacking of two-dimensional crystalline sheets have a thermal conductivity that is many times smaller than the predicted minimum: the conductivity of thin films of disordered layered WSe2 is only a factor of 2 larger than air. The cause of this ultralow thermal conductivity is not fully understood but may be explained by a low thermal conductance for the exchange of vibrational energy between each sheet. Our experiments on the thermal conductivity of thin films and the thermal conductance of interfaces are enabled by our recent advances in time-domain thermoreflectance (TDTR). With TDTR, we can rapidly and routinely measure the thermal conductivity of almost any bulk or thin film material that has smooth surface with a spatial resolution of a few microns. High sensitivity to the near surface region enables studies that use ion beam processing to modify materials. TDTR measurements at high pressures within a diamond anvil cell allow us to tune the strength of anharmonic interfacial bonds and directly observe the role of interfacial bonding on the transport of thermal energy at interfaces.