Abstract
The rapid advancements in modern technology have largely been driven by the miniaturization of devices and improvements in materials engineering at the nanoscale. The intentional introduction of atomic-scale compositional disorder through doping and alloying has enabled a direct process to tune electrical and mechanical properties of materials. Furthermore, random solid solutions and high-entropy materials have demonstrated that the random configuration of atoms, configurational disorder, can lead to enhanced mechanical properties and improved thermodynamic stability. From a thermal transport perspective, compositional and configurational disorder at the atomic scale can significantly limit the ability for electrons and phonons to carry heat, resulting in a reduced thermal conductivity. While significant advances have been made in modeling the thermal conductivity of crystals, disorder beyond simple perturbations, especially in the case of amorphous solids which lack atomic periodicity, proves challenging to capture within the framework of these models, motivating the need for experimental study. Advancing the understanding of how disorder affects thermal conductivity under a common framework, this dissertation fills the void of current understanding in highly configurationally disordered crystals as well as compositionally disordered amorphous thin films.
Three experimental techniques are used to measure thermal properties of materials: time-domain thermoreflectance, frequency-domain thermoreflectance, and a newly developed steady-state thermoreflectance. These non-contact, optical pump-probe techniques ensure the capability to measure both thin films and bulk materials. After establishing the advances made in these experiments, this dissertation reports the thermal conductivity of configurationally disordered thin film entropy-stabilized oxides to demonstrate that the thermal conductivity decreases with increasing configurational entropy. Probing the local structure of these materials reveals that local ionic charge disorder enables amorphous-like thermal properties in these crystalline materials without diminishing elastic properties. Next, four new classes of bulk high-entropy ceramics -- high-entropy oxides, carbides, borides, and silicides -- are investigated to show that high configurational entropy again reduces the thermal conductivity of these materials relative to their constituent components. Using a thermal conductivity imaging technique, it is shown that grain boundaries and secondary phases can further reduce the local thermal conductivity. Finally, thermal transport in amorphous thin films is investigated. It is shown that film thickness can limit the thermal conductivity of amorphous silicon to reveal that propagating vibrational modes can significantly contribute to the thermal conductivity, an attribute typically associated with crystalline solids. The introduction of compositional disorder in amorphous solids is studied through hydrogenated amorphous silicon nitride to reveal that the atomic bond coordination dictates thermal conductivity and suggests a means for tuning thermal conductivity through hydrogenation and thermal annealing.
By considering the extreme cases of configurational, compositional, and structural disorder, this dissertation provides evidence that crystalline materials can behave thermally as if amorphous, while amorphous films can possess crystalline-like thermal properties. Taken together, this suggests a common framework of how disorder affects the thermal conductivity in all materials.
