Abstract
Mechanisms of thermal transport vary drastically from the nanoscale to the macroscale. At the nanoscale, the characteristic length and time scales over which heat transfer occurs can be on the order of nanometers and picoseconds, respectively. For example, in nanoelectronics, thin adhesion layers (< 10 nm) will often be implemented to better increase adherence of Au contacts. The interfacial conductances of these regions, however, can pose as limiting resistors of the system, potentially limiting the maximum allowable heat to be transferred into the heat-sinking substrate. Macroscale analogies for these nanoscale structures can additionally be found throughout the aerospace industry. Specifically, coatings for gas turbine engine blades consist of thick, steam-resistive materials bonded to a high thermal conductivity, heat-sinking blade via a thin, insulating amorphous layer. The time and length scales over which heat transfer typically considered in these components are orders of magnitude larger than their nanoscopic counterparts, occurring on the order of seconds to hours and microns to millimeters. Engine efficiencies are dictated by the maximum allowable operating temperatures, thus necessitating a robust understanding of the thermal profiles found in each layer of the multilayer turbine blade coating.
In this dissertation, these nano- to macroscale phenomena are addressed through a series of experimental works using an advancement in pump/probe metrology that will provide insight into heat transfer mechanisms in composite coatings at these differing time and length scales. At the nanoscale, an emphasis is placed on the Au/TiOx system, which offers a plethora of information relevant to the nanoelectronics industry while providing an ideal test system to study how chemistry at heterogeneous material boundaries impacts thermal transport processes across orders of magnitude time and length scales. Specifically, this dissertation elucidates mechanisms of thermal transport in Ti adhesion layers that are oxygen defected. This includes the influence of said defects on the electron-phonon coupling factor in these layers via examining the picosecond pump/probe dynamics of time-domain thermoreflectance and implementing a two-temperature model analysis to understand mechanisms of heat flow between the two fundamental carriers of heat in substoichiometric TiOx. To study heat transport mechanisms across meso-to-macroscopic length scales, environmental barrier coating materials will provide this focus. This will include analysis of the thermal conductivity anisotropy of yttrium disilicate, a rare earth disilicate that is commonly implemented as a barrier coating material. Additionally, the spatio-temporal evolution of a steam-cycled ytterbium disilicate coating and its ultimate influence on the thermal profile experienced in the hot-section of a gas-turbine engine is presented, exemplifying the need for an understanding of the thermal properties of these evolving systems.
In examining each of these systems, significant advancements in pump/probe metrology are made. By further developing these techniques to examine the spatially-varying thermal characteristics of macroscale systems on the order of just a few micrometers, and by using advanced modeling to understand the excited carrier dynamics of thin films in regimes of non-equilibrium, this dissertation provides an advancement in the understanding of heat transport mechanisms in multilayer systems at mesoscopic length and time scales.
