Near-Field Radiative Thermal Transport and Energy Conversion

Thermal radiation occurs when electromagnetic energy is emitted from one body and absorbed by another body. The net energy transferred between bodies, called radiative heat transfer, is well-understood when the distance between them is large compared to the wavelength of the electromagnetic waves. However, a question of fundamental interest is: what happens when the distance between the radiating bodies is smaller than or comparable to the wavelength of the radiation? That is, what happens when the bodies are brought into the “near-field?” Countless theoretical treatments now exist in the literature indicating that the radiative heat transfer can increase by orders of magnitude when the spacing between bodies is reduced to tens or hundreds of nanometers, and these predictions are largely supported by a handful of experimental studies. Moreover, computational work suggests that near-field radiation between parallel plates can have important, novel applications. However, their realization has thus far been prohibited by the technical difficulty in positioning parallel plates across nanoscale gaps. My first research objective was to measure near-field radiative heat transfer between parallel plates separated by less than a single micrometer, a goal which had eluded researchers for nearly half a century. Using a pair of microscale devices and a custom-built nanopositioner, we systematically demonstrated heat flux enhancements of 100-fold compared to the far-field by decreasing the inter-plate distance between parallel silica plates from 10 μm to approximately 60 nm. I then modified this approach to utilize a single planar microdevice situated across a vacuum gap from a macroscopic planar surface. By using devices with lesser curvature and higher mechanical stiffness, I reduced the minimum attainable gap size between silica plates to approximately 25 nanometers and measured a near-field heat flow 1,200 times higher than that of the far field, representing a significant improvement over the previous demonstration. Most importantly, replacing one of the microdevices with a macroscopic surface enabled a greater degree of flexibility in materials processing, opening up new opportunities for novel measurements. My second objective was to use this new technique to demonstrate novel near-field-enabled thermal diode using a doped silicon microdevice and an extended vanadium dioxide thin film. Because the emissive and absorptive properties of vanadium dioxide change dramatically when it undergoes an insulator-metal transition at 68 degrees Celsius, the radiative heat flow can change depending on the direction of the temperature difference. For a vacuum gap size of approximately 140 nanometers, I measured that the heat flow from metallic vanadium dioxide to doped silicon exceeds the heat flow from doped silicon to insulating vanadium dioxide by a factor of approximately two. Computational modeling showed that this rectification could be further improved by decreasing the thickness of the vanadium dioxide film. Finally, I demonstrated the first near-field power output enhancement in a thermophotovoltaic system. For a doped silicon emitter at 655 kelvin radiating at an indium arsenide-based cell, I measured a 40-fold increase in the electrical output power from the cell by reducing the vacuum gap spacing from 10 micrometers to approximately 60 nanometers. Additional experiments were carried out with a cell having a different bandgap energy, and its performance was compared to the first cell. Moreover, a detailed mathematical model was developed to identify ways to improve the device efficiency in the future. These studies represent an important milestone in near-field-enabled energy conversion.PhDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/146129/1/fiorino_1.pd