Testing theories for thermal transport using high pressure

This dissertation focuses on experimental studies of thermal transport in various materials, such as heat transfer in crystals and amorphous polymers, and across interfaces, using an ultrafast pump-probe method, time-domain thermoreflectance (TDTR), combined with gem anvil cell techniques. I demonstrated that pressure tuning of physical properties of materials is an elegant approach to test the validity of theories for thermal transport. Pressure dependence of the cross-plane thermal conductivity ??(P) of a layered muscovite mica crystal was measured by TDTR combined with diamond anvil cell techniques. Under a simple relaxation time approximation, most of the ??(P) of muscovite mica can be described by the pressure dependence of the cross-plane sound velocity, indicating that the cross-plane sound velocity plays an important role in the thermal transport in a layered crystal. The validity of the minimum thermal conductivity model for amorphous polymers was verified by the good agreement between my measurements of the pressure dependent thermal conductivity of poly(methyl methacrylate) (PMMA) and the model prediction. The thermal energy exchange between non-propagating vibrational modes is the dominant mechanism of thermal transport in amorphous polymers. I also used high pressure to demonstrate the importance of interface stiffness on the interfacial thermal transport. By measuring the pressure dependence of thermal conductance G(P) of clean and modified Al/SiC interfaces, I found that G(P) of a clean interface with high interface stiffness is weakly dependent on pressure and can be well accounted for by the diffuse mismatch model (DMM). By contrast, G(P) of modified interfaces with low interface stiffness initially increase rapidly with pressure; as the interface stiffness is increased to be comparable to the stiffness of chemical bonds, G(P) saturate at the value for the clean interface and value predicted by the DMM. In order to extend the TDTR measurements to high pressures and high temperatures, I studied the pressure dependent thermoreflectance and piezo-optical coefficient of metal film transducers???Al, Ta, and Au(Pd) alloy (???5 at. % Pd) at a laser wavelength of 785 nm. The thermoreflectance of Ta and Au(Pd) are comparable to that of Al at ambient conditions and independent of pressure in the range 0<P