Tuning Thermal Transport in Thin Films
Decades of research have enabled new understanding of thermal transport at the nanoscale. Leveraging this new understanding to tune heat conduction in thin films (TFs) is of significant interest for both fundamental and applications. This work explores tuning thermal conduction in TFs by structuring, strain engineering, and annealing. Specifically, three approaches are interrogated: 1) nanostructuring to achieve significant in-plane thermal conduction anisotropy in TFs; 2) strain engineering of thermal conductivity (k) of a basic structure in flexible electronics, Au-on-polyimide films, and a representative 2D material, graphene; and 3) high temperature annealing of reduced graphene oxide (RGO) films to enhance k. Nanostructuring is well known as an efficient method to modulate thermal conductivity. Extending beyond previous studies, this work investigates the directional dependence of the thermal conductivity introduced by anisotropic boundaries. Specifically, thickness modulators are introduced in TFs to structurally impact the thermal conduction anisotropy. Simulations, based on the phonon Boltzmann Transport Equation (BTE), demonstrate the ability to tune the in-plane thermal anisotropy ratio across an order of magnitude via modulating the thickness of the thin films. To predict the thermal conductivity of nanostructures, simplified Monte Carlo (MC) methods have been developed considering the expensive computational cost of solving the full BTE. We reevaluate the simplified MC methods and the applicability of these methods is evaluated. Beyond structural engineering, this dissertation explores strain engineering of thermal conductivity in nanostructured materials including flexible TFs and 2D materials. Past experimental study on the impact of strain on thermal conductivity is very limited due to the challenges of measuring k of thin films under controlled strain. This work develops fully suspended devices on flexible substrate for strain control and evaluation of strain-dependent k using a new electrothermal measurement method. By extending conventional electrothermal approaches, the new method allows accurate thermal conductivity measurements with minimal assumptions. Finally, this dissertation investigates annealing as an effective method to tune thermal transport in RGO films. Both the electrical and thermal conductivity increases significantly as the annealing (or reduction) temperature increases. The measured electrical and thermal conductivity are analyzed using a 3D Mott variable range hopping model and a thermal conductivity integral model, respectively. Further, the application of RGO films for high temperature thermoelectrics and extreme temperature sensing is discussed based on the measured electrical and thermal conductivity across a wide temperature range (10 K - 3000 K). Key contributions of this dissertation include new understanding of engineering thermal conduction in TFs and characterization of thermal conductivity in strained TFs. The high in-plane thermal anisotropy ratio by nanostructuring is promising for directing heat flow in modern applications. Tuning thermal conductivity by strain control is of significant interest for modern devices with stress/strain such as flexible electronics and other devices with extreme thermomechanical stresses. For materials with an extremely high melting point, annealing at extreme temperatures by Joule heating suspended films enables additional modulation of the thermal conductivity. In summary, this dissertation enhances the understanding of tuning thermal transport with structuring, strain, and annealing through experimental and computational efforts.
Marconnet, Purdue University.
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