Date of Award

Summer 8-16-2024

Level of Access Assigned by Author

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Advisor

Sheila Edalatpour

Second Committee Member

Zhihe Jin

Third Committee Member

Nuri Emanetoglu

Additional Committee Members

Mohammad Ghashami

Olivier Putzeys

Abstract

Thermal radiation at observation distances greater than the dominant wavelength of thermal radiation (~10 μm at room temperature) is referred to as the far-field regime. Otherwise, thermal radiation is in the near-field regime. Far-field and near-field thermal radiation have significant applications in thermal management, energy-conversion devices such as thermophotovoltaic power generation, spectroscopy, imaging, infrared cloaking, temperature measurements, and optoelectronics, to name only a few. Usually, these applications require thermal radiative properties that are not found among natural materials. Metamaterials made of micro/nanoscale structures offer a great opportunity for designing thermal emitters with improved thermal radiation properties. This dissertation investigates the engineering of far-field and near-field thermal radiation using metamaterials. In the far-field regime, two classes of metamaterials, namely polaritonic metamaterials made of micro/nanostructures of polaritonic materials such as silicon carbide (SiC) and Mie-resonance based metamaterials made of micro/nanostructures of dielectric materials, have been studied. The first study elucidates the impact on far-field thermal radiation from SiC nanopillars when the interparticle spacing between nanopillars is reduced to the nanometer scale, leading to near-field interactions between adjacent nanopillars. Nanopillars of SiC support localized surface phonon modes in their Reststrahlen band which can be capitalized for enhancement of thermal radiation in this spectral range. It is found that the increased volume of thermal emitters as well as the spectral overlap of the localized surface phonon modes with the hybrid waveguide-surface-phonon-polariton mode in arrays with an optimized interparticle spacing of 300 nm enhance the spectral emissivity of silicon carbide to values as high as 1 across a wide range of angles. Usually, Reststrahlen band of dielectric materials such as SiC spans a limited spectral range and does not always overlap with wavelength of peak thermal radiation. Dielectric media can also emit Mie resonances, which are not limited to a spectral range, and can potentially be used for broadband enhancement of emissivity. Thus, in the second far-field study, we design a Mie resonance-based metamaterial that increases thermal radiation from a flat, unpatterned 6H-SiC substrate at the normal direction from 67% to 91% of a blackbody, while demonstrating enhanced emissivity in a wide angular range. We demonstrate that the Mie resonances of SiC microcuboids can be precisely tuned to occur outside the Reststrahlen band of SiC, aligning them with the peak thermal radiation wavelength. For both far-field studies, enhancement of thermal radiation is experimentally demonstrated. Mie resonance-based metamaterials are also promising for tuning the near-field spectra. Using numerically-exact simulations, the tunability of spectrum of near-field radiative heat flux between Mie resonance-based metamaterials is theoretically investigated. The origins of the peaks in the near-field spectra are identified and the effect of the refractive index, extinction coefficient, shape, and interspacing of Mie resonators, as well as the separation distance of the metamaterials on the spectral locations of peaks in near-field heat flux are studied.

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