Calorimetric Systems to Explore Radiative Thermal Transport and the Thermodynamics of Hydrogen (H2) Reactions for Energy Utilization

Abstract

Thermal radiation plays a crucial role in various technological applications, ranging from thermal energy conversion devices to the manufacturing of electronic devices. In this dissertation, I discuss how nanoscale thermal radiation can be controlled via thin films and how phase transitions in thin films can be employed to control the flow of heat. Moreover, I describe how novel calorimetric tools can be developed to probe the thermodynamics of chemical reactions with nanomaterials. In Chapter 1, prior to presenting my research, I delve into background material covering the quantification of heat flow in calorimetry, conceptual discussions regarding the blackbody limit, hydrogenation reactions of transition metals, and related topics. In Chapter 2, I outline how I developed a photonic thermal transistor comprising a source-drain device (hot and cold electrodes) and a vanadium oxide-based planar gate electrode, whose dielectric properties can be adjusted by changing its temperature. I demonstrate that when the gate, located near (< ~1 µm) the source-drain device, undergoes a metal-insulator transition, the radiative heat transfer between the source and drain changes by a factor of three. These findings are supported by detailed calculations that highlight the mechanism of thermal modulation. Furthermore, I show that this photonic thermal transistor features a much faster switching time (~500 ms as opposed to minutes) due to the small thermal mass of the nanoscale devices employed in the work. The advances described here are expected to open new opportunities for designing photonic thermal circuits or computing systems for advanced thermal management. In Chapter 3, I present my exploration of radiative heat transfer (RHT) between objects separated by nanometer-sized vacuum gaps to measure the thickness-dependence of RHT between planar nanofilms. This phenomenon is remarkably distinct from far-field thermal radiation and can exceed the blackbody radiation limit by orders of magnitude. I report direct measurements of the thickness-dependence of RHT between two planar magnesium fluoride nanofilms (thickness ranging from 20 to 500 nm) performed using microfabricated devices and a custom-developed nanopositioner. These results directly demonstrate, for the first time, that nanofilms have the capability to enhance thermal radiation by up to 800 times beyond the blackbody limit. Moreover, they prove to be as effective as bulk materials when the dimensions of the nanoscale gaps are smaller than the thickness of the nanofilm. In Chapter 4, I describe the construction of a calorimetric system with a resolution of <3 μW/√Hz and long-term stability of <4 μW/hour. The study of calorimetry involving reactions with nanomaterials is currently of significant interest, demanding high-resolution heat flow measurements and long-term thermal stability. Using the custom-built high-resolution calorimetric system I developed, I measure the heat output during the absorption of hydrogen in Pd nanoparticles. This calorimetric system enables the direct measurement of heat energy during the hydrogenation process, providing an opportunity to explore both the thermodynamics and kinetics associated with the reaction. Finally, in Chapter 5, I provide a summary of my work and discuss future research directions and ideas to advance the scope of my findings. This work paves the way for exploring hydride-based hydrogen storage systems by storing substantial amounts of hydrogen in hydride nanomaterials. These findings will open up the possibility of hydrogen utilization for fuel cells, which serve as a clean and sustainable energy source for various applications.