Date of Award

Fall 12-16-2022

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Advisor

Sheila Edalatpour

Second Committee Member

Carl P. Tripp

Third Committee Member

Mathieu Francoeur

Additional Committee Members

Yingchao Yang

Olivier Putzeys

Abstract

Thermal emission observed at sub-wavelength distances from the thermal source is referred to as near-field thermal radiation. Thermal radiation in the near-field regime can exceed Planck’s blackbody limit by orders of magnitude and be quasi-monochromatic. Due to these unique properties, near-field thermal radiation is very promising for several thermal management and energy harvesting applications. Many of these applications, such as nanogap thermophotovoltaics and thermal rectification, require near-field spectra that are not found among natural materials. Artificial metamaterials, which are engineered at the sub-wavelength scale, have been theoretically proposed for tuning the spectrum of near-field thermal radiation. However, engineering the near-field spectra using metamaterials has not been experimentally demonstrated mostly due to the complexities associated with guiding the near-field evanescent waves to an FTIR spectrometer located in the far zone. Additionally, the possibility of tuning the near-field spectra by engineering materials at length scales much smaller than the thermal wavelength, i.e., atomic length scales, has not been explored theoretically or experimentally.

In this dissertation, a new technique is proposed and implemented for measuring the near-field thermal spectra. The proposed technique is verified against the theoretical predictions of near-field thermal radiation from natural materials. This technique is then utilized for measuring the near-field spectra thermally emitted by metamaterials made of silicon carbide nanopillars, and the tunability of the near-field thermal spectra by changing the dimensions of the nanopillars at the sub-wavelength scale is demonstrated. Using numerically-exact simulations, it is shown that the effective medium theory, commonly used for theoretical study of the near-field thermal spectra of nanopillar metamaterials, is not valid in the near-field regime. Additionally, the tunability of near-field spectra by using spherically-shaped sub-wavelength particles is theoretically investigated by developing analytical expressions for predicting the energy density emitted by spherical particles. Lastly, near-field thermal radiation from quantum dots, which have a length scale comparable to the atomic scales, is theoretically studied for the first time. It is shown that the near-field thermal radiation is highly impacted by the size-dependent quantum confinement effect that arises at the atomic length scales, thus providing a new mechanism for tuning the near-field thermal emission spectra.

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