| Description |
Radiative thermal transport at the nanoscale, also known as near-field thermal radiation, has recently attracted attention due to its fundamental importance and its significant impact on a wide range of applications from data storage to thermal management and energy conversion. Previous theoretical studies have predicted that when the geometric dimensions or distances between objects are smaller or comparable to the characteristic wavelength of thermal radiation (lT) defined by Wien's displacement law, radiative heat transfer can exceed Planck's blackbody limit by several orders of magnitude. By exploiting the near-field enhancement of thermal radiation to boost thermionic emission, a new energy harvesting concept is introduced. The proposed near-field enhanced thermionic energy conversion (NETEC) system is uniquely configured with a low-bandgap semiconductor cathode separated from a thermal emitter with a subwavelength gap distance, such that a significant amount of electrons can be photoexcited by near-field thermal radiation to contribute to the enhancement of thermionic current density. It is theoretically demonstrated that the NETEC system is capable of generating power at a remarkably lower temperature compared to the conventional thermionic system, and the energy conversion efficiency can reach to 40%. The near-field photoexcitation can increase the thermionic power output by more than ten times, making this hybrid system a compelling alternative for energy recycling. Although the enhancement of near-field thermal radiation was experimentally implied half a century ago, it was not until the last decade that the number of quantitative studies became more than a handful. Besides the inherent challenges in any heat transfer measurement, demonstration of near-field thermal radiation enhancement requires excellent control over the separation gap and parallel alignment between the macroscopic samples. Since most of the applications of near-field thermal radiation require planar structures due to their higher energy throughput, plate-plate configurations have been the main focus of several recent experimental studies. In order to overcome the challenges in controlling a subwavelength gap distance with good parallelism in plane-plane experiments, many of these efforts have been limited to samples with the minimum area (e.g., MEMS devices) or to a fixed separation gap (e.g., using spacers between the two plates). To address these bottlenecks, a robust experimental system is designed and developed with a well-engineered measurement technique to further investigate the potential of near-field thermal radiation between different materials, at lower gap distances and with high-temperature gradients. The enhancement of radiative heat transfer at the near-field regime is experimentally demonstrated by measurements performed between two 55 mm2 quartz samples for a minimum separation distance of 200 nm. Further, by improving the samples design and the experimental setup, gap distances of less than 100 nm have been achieved between two large plates made of silicon and silicon carbide. Lastly, the effect of vacuum pressure level on the near-field radiative transfer is investigated by precisely controlling the pressure inside the chamber. The outcomes of this dissertation will pave the way for experimental studies of nanoscale energy transport between planar structures and its applications for energy conversion. |
