To be officially considered as admitted to candidacy in the Ph.D. program, applicants must pass a qualifying exam. The qualification exams are the first of three major steps in the Mechanical Engineering Ph.D. program at the University of Utah. Students select two subject areas which support their Ph.D. research. The topics cover fundamental material with the expectation of a graduate-level understanding of the subjects. Taufiq took his qualification exams in the subject areas of thermodynamics and heat transfer.

The investigation of thermal transport across extremely small vacuum gap distances is of both practical and fundamental significance. Previous theoretical studies have predicted that in the near-field, or when the emitter-receiver separation is less than the thermal wavelength, thermal radiation can exceed Planck’s blackbody limit by up to several orders of magnitude due to radiation tunneling of evanescent electromagnetic waves. This enhancement has been verified by several recent experimental efforts providing exciting opportunities for the development of thermophotovoltaic energy conversion, passive radiative cooling, and nanoscale thermal management. While experimental findings for gap distances above 10 nm have observed good agreement with near-field thermal radiation theory within the fluctuational electrodynamics (FE) framework, the underlying physics of thermal transport in the sub-10-nm (i.e., extreme near-field) gap regime is still in significant debate. The aforementioned knowledge gap poses a fundamental question: How does thermal radiation transition to phonon heat conduction at contact? To investigate this question, the finite dipole model is combined with FE to elucidate the magnitude of extreme near-field thermal radiation. Then, an experimental setup based on a custom-built high-vacuum scanning probe microscope and platinum nanoheaters is developed to enable heat transfer measurements across sub-10-nm vacuum gaps. By implementing this experimental setup, thermal transport between a silicon tip and platinum nanoheater separated by single-digit nanometer vacuum gaps is found to be more than three orders of magnitude larger than FE predictions. The atomistic Green’s function for phonon transport illustrates that acoustic phonon tunneling links the measured gap and contact thermal transport mechanisms. The results of this dissertation suggest a direct correlation between the tip-substrate force and acoustic phonon transmission, such that interfacial heat transfer may be engineered using external force stimuli.

Congratulations Dr. Jarzembski!!!

Photon interactions with electrons have led to devices engineered to improve industries in communication, energy, and sensing. Among the different types of devices, nanoantennas are particularly useful in these industries due to their spectral versatility achieved through a prudent selection of material, shape, dimension, and periodicity. Similarly, the exploration of thermal interactions with electrons has produced advances in energy harvesting, most notably in the form of thermophotovoltaics, thermoelectrics, and thermionics. Of the three, thermionics contains the most potential for high-temperature applications, such as concentrated solar collection. However, significant knowledge gaps remain in designing hybrid resonant photonic/electronic devices and in the modeling and setup of thermionically induced electron and photon emission, particularly across nanometer distances. As such, this dissertation is divided into three main topics: (1) the numerical study of hybrid photonic/plasmonic structures, (2) comprehensive electron and photon transport modeling of thermionic energy converters and (3) the design of a nanoscale thermionic experiment.

This dissertation begins with a literature review of each topic followed by a numerical model of an array of gold rectangular nanoantennas placed atop a novel 3D photonic crystal. Using Lumerical’s Finite-Difference Time-Domain Solutions software, the model identified conditions under which photonic/plasmonic coupling yielded an order of magnitude increase in concentrated, localized fields. Next, photon and thermal perturbations on electrons are presented with emphasis on sub-micron vacuum spacing between TE electrodes. Using MATLAB, a comprehensive TE model of photon and electron transport is presented which accounts for the effects of applied voltage, gap distance, space-charge, image-charge, electron tunneling, photon tunneling, surface roughness, and hybrid cycles. As a follow-up work, the necessary tools and components to experimentally validate TE power generation are designed and set up. The outcome of these studies provides a strong platform on which to expand scientific understanding of photonic and thermal interactions with electrons.

Congratulations Dr. Jensen!!!