Thermal Transport Mechanisms Across Solid-State Phase Transformations

The primary objective of this dissertation is to provide insight into how thermal transport mechanisms change as a result of solid-state phase transformations. In this respect, depending on the nature of the change in the atomic or electronic structure, various forms of solid-state phase transformations could take place. For instance, phase transformations could occur as a result of structural changes in the material either from amorphous-to-crystalline phases such as chalcogenide-based phase change materials, or crystalline-to-crystalline phases such as ferroelectric and antiferroelectric materials. Alternatively, the phase transformation could occur as a result of a purely electronic change such as metal-insulator transition or threshold switching. Regardless of the phase transformation mechanism, all of these processes lead to substantial changes in the electrical, optical, and thermal properties of materials.
In this dissertation, I strive to improve our understanding of these reversible phase transformations and the degree to which each mechanism could alter the transport of energy carriers in materials from a nanoscale thermal science perspective. Specifically, using conventional optical pump-probe thermometry techniques, this dissertation aims to interrogate the changes in thermal properties such as thermal conductivity, thermal boundary conductance, specific heat, and sound velocity upon phase transformations. For this purpose, I target four prevalent classes of phase transformation mechanisms: (i) metallic phase transformation, i.e. amorphous-to-crystalline with metal-insulator transition, (ii) non-metallic phase transformation, i.e. amorphous-to-crystalline without metal-insulator transition, (iii) antiferroelectric-to-ferroelectric, and (iv) antiferroelectric-to-paraelectric. I explore phase transformation across a wide range of materials, from chalcogenide-based phase change materials such as Ge2Sb2Te4 to antiferroelectric perovskites such as PbZrO3. This work not only has far-reaching implications in both foundational physics and engineering but also due to the ubiquity of phase transformation phenomena in the industry, it has direct applications in the improvement of future nanoscale electronics, photonics, thermoelectrics, and thermal circuits.