Date of Award

2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering and Applied Mechanics

Department

Mechanical, Industrial and Systems Engineering

First Advisor

Yi Zheng

Abstract

Micro/nano scale radiative transport phenomena present new opportunities to design spectrally selective radiative surfaces and configurations that enable an active control of radiative transfer. Several applications such as thermophotovoltaic energy technology, local thermal management and sensing can benefit from fundamental research in the field. An investigation into photonic metamaterials and near-field thermal radiation was conducted to explore various ways to achieve spectral control and modulation of radiative heat transfer. This was followed by designing novel metamaterials and configurations to consider their feasibility for specific applications of thermophotovoltaics (TPV) and nanoscale thermal management.

An experimental investigation was conducted to estimate optical and radiative properties of SU-8 thin films for various film thicknesses. Samples consisted SU-8 films of thickness ranging from 10 μm to 157 μm deposited on gold coated silicon substrates and were prepared using spin coating. Thickness dependent reflective properties were confirmed using Fourier Transform Infrared Spectrometer measurements. Dielectric function of SU-8 in the range 2 μm m to 15 μm was calculated using the reflectance spectra of the samples. Optical properties of SU-8 in mid-infrared (mid-IR) region were reported before and after UV treatment. Measurements imply a change in optical properties of SU-8 upon exposure to UV and heat treatment.

Using microscopic thin films is one of the simplest ways to achieve a change in emission spectra. A methodology was proposed to shift the wavelength selectivity in the desired location using thin films embedded with nanoparticles. For the media doped with nanoparticles, an effective dielectric function using the Maxwell- Garnett-Mie theory is employed to calculate emissivity and radiative heat transfer. Influence of parameters such as particle size and volume fraction was studied. It was also shown that wavelength selective behavior of such nanocomposite thin films can be related to their effective refractive indices.

A theoretical study to explore Mie-resonance metamaterials (nanocomposites) for possible use in TPV technology was conducted. Metamaterials were designed to achieve spectral matching of thermal emitter and photovoltaic (PV) cell. The emitter consists of a thin film of SiO2 on the top of tungsten layer deposited on a substrate. Both near-field and far-field configurations were considered. The methodology followed Maxwell-Garnett-Mie theory discussed earlier. The results showed that the proposed Mie-metamaterial thermal emitter can significantly improve the efficiency of TPV system. It was shown that, by changing volume fraction of nanoparticles and thickness of SiO2 it is possible to tune the near-field thermal radiation to obtain enhanced output power and high thermal efficiency.

Methods to achieve dynamic control of radiative heat transfer and thermal rectification characteristics were investigated using a phase-change material called vanadium dioxide (VO2). For a far-field configuration, a tri-layer structure consisting a thin film of KBr sandwiched between a thin film of VO2 and a reflecting layer of gold was proposed. The structure is highly reflective when VO2 is in insulating state (below 68 °C), while it is highly absorbing when VO2 is in its metallic state. Thermal rectification achieved by such a structure is greater than 11 a temperature bias of 20 K, which is the highest rectification ever predicted for far-field radiative diode configurations. A near-field thermal diode configuration was also considered. Possible configurations using bulk, thin film and gratings of VO2 were studied. It was shown that using 1-D rectangular or triangular grating of VO2, a high degree of contrast can be achieved in the tunneling of surface waves across the two interface of thermal diode. For minimal temperature difference of 20 K, rectification ratio as high as 16 was obtained and it is maximum in existing literature to date for comparable operating temperatures and separation.

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