Radiator Designs and Experimental Platforms for Near-Field Radiative Heat Transfer

Abstract

Near-field radiative heat transfer (NFRHT) consists of evanescent electromagnetic coupling occurring between two bodies at sub-wavelength distances, allowing to increase the radiative exchange beyond conventional laws of thermal radiation. NFRHT has demonstrated great potential for applications such as energy conversion and heat transfer control. In particular, multiple works predict that performances of near-field thermophotovoltaic (NFTPV) modules, in which a hot radiator is positioned at sub-wavelength distances from a cold photovoltaic (PV) cell, could significantly surpass those of current solid-state heat-to-electricity conversion technologies. Despite its tremendous potential, experimental progress on NFRHT and NFTPV has been slow due to technical challenges and gaps in knowledge, three of which are at the core of this thesis. Firstly, only a limited number (i.e., fewer than 30) of NFRHT experimental platforms have been reported worldwide. The field therefore faces limited capabilities in characterizing novel materials for NFRHT. Secondly, NFTPV technology commands highly specialized PV cells. Unfortunately, most reported NFTPV platforms relied on basic PV cells fabricated in-house, or on commercially available photodetectors, resulting in modest performances. This leaves many promising PV materials, e.g., InAs, largely unexplored. Thirdly, theoretical research on new radiator materials for NFTPV applications has predominantly focused on one class of materials (i.e., plasmonic materials) that are difficult to experimentally investigate as their optical properties are contingent upon factors such as film annealing and deposition conditions. Therefore, the most suitable radiator material for NFTPV remains ambiguous. To address these three challenges, our first objective is to develop experimental methods for NFRHT, our second objective is to achieve NFTPV measurements using an optimal bandgap (i.e., InAs-based) PV cell, and our third objective is to theoretically design an optimized radiator for an InAs-based NFTPV system. Within the scope of our first objective, we experimentally demonstrate the potential of using nanomechanical resonators as a temperature sensor for NFRHT measurements. Our approach offers a high-precision flexible NFRHT platform that could facilitate the testing of many new interesting materials. We report measurements in the deep sub-wavelength regime (i.e., regime dominated by surface resonance coupling) without the need for custom-fabricated micro-devices. Moreover, we demonstrate that the use of nanomechanical resonators could allow fundamental advances on simultaneous measurement of NFRHT and Casimir forces. If we can distinguish the contributions from Casimir and NFRHT effects, nanomechanical resonators could provide a path to experimentally demonstrate the correction to the Casimir forces out of thermal equilibrium. For our second objective, we aim to conduct experimental research on the NFTPV effect using InAs-based PV cells. These cells, optimized for near-field thermal radiation, were custom-fabricated by PV cell experts (Prof. Karin Hinzer group, uOttawa, and Prof. Zbig Wasilewski group, University of Waterloo) in close collaboration with our group during this work. Unfortunately, the geometry of our radiator significantly limited the capabilities of our experimental platform, preventing NFTPV measurements. In fact, the large thermal mass radiator employed in our initial approach precluded high-temperature scans and greatly complicated the alignment process. In the future, we, therefore, recommend a revised approach relying on a localized heating element, enabling vibrational contact detection. Finally, regarding our third objective, using a one-dimensional fluctuational electrodynamics model, we theoretically explore new radiator materials for NFTPV, focusing specifically on reproducible crystalline materials. More precisely, we investigate spectral electromagnetic coupling of a near-field thermal radiator with a PV cell in order to enhance spectral efficiency and total output power. We find that when the radiator and PV cell are both made of InAs, nearly a threefold improvement of spectral efficiency is possible compared to a silicon radiator with the same InAs cell. This enhancement reduces subgap thermal transfer while maintaining power output. Through this work, we also uncover an overestimation of free carrier absorption in InAs dielectric function models. We propose a corrective model and demonstrate that it accurately represents the absorption at moderate doping levels but could be further refined for improved accuracy at higher doping levels.