Geometric and Plasmonic Effects in Radiative Heat Transfer

Abstract

Radiative heat transfer rates between a hot and a cold body can experience a significant increase when the distance between the objects is well below Wien's wavelength (~10 µm at 300 K). Recent advancements in nanofabrication have expanded the possibilities for exploring radiative heat transfer in nanoscale gaps towards real-world applications of near-field radiative heat transfer (NFRHT). Until now, studies on NFRHT have primarily concentrated on nanoscale gap measurements between two plane parallel surfaces or between a sphere and a plane. However, practical applications may necessitate the consideration of different designs, including curved surfaces. Further, it is also important to understand how NFRHT can be dynamically modulated using external stimuli such as electric fields. This dissertation presents an experiment that systematically explores NFRHT between two spherical surfaces. To perform the desired experiments, smooth silica spheres (~40 µm diameter) were integrated into custom-fabricated calorimetric probes to create a “receiver” device. Further, a second “emitter” device was also created, a planar silicon device onto which a smooth silica sphere was integrated. NFRHT between the two silica spheres was probed by integrating the emitter and receiver devices into a custom-built nanopositioner that enables both relative alignment of the two spheres as well as control of the gap size between them. In the first series of studies reported in Chapter 3 of this dissertation heat flow between the two spheres was measured when the Si surface was heated by ~10-30 K above the ambient temperature and the gap size between the spheres was systematically reduced from ~5 µm to 10s of nm. During this process a significant enhancement in radiative heat transfer was observed, increasing ~15 times. The peak measured thermal conductance, at the minimum gap size (80 nm), was 3.82 nW/K. This measured value is approximately two orders of magnitude lower than the predicted value for plate-plate heat transfer between two circular silica plates with a diameter of 40 µm, and about half of what was measured for heat transfer using the same receiver sphere and an effectively infinite silica plate. The measured, lower sphere-sphere near-field thermal conductances are expected and in excellent agreement with numerical simulations performed using fluctuational electrodynamics. In addition to the work described above, this dissertation explores if NFRHT can be actively controlled by electrostatically tuning the dielectric properties of a material (Chapter 4 of the dissertation). Specifically, experiments were performed to explore if graphene could be employed for tuning NFRHT. Towards this goal, an experiment was designed by combining the calorimetric probe described above with a planar, three-terminal graphene device, with back gate-tunable dielectric properties. These experiments characterized NFRHT between the sphere embedded in the calorimetric probe and the planar sample as a function of the applied gate voltage. Upon sweeping the back gate voltage, these preliminary experiments revealed a distinct, nanowatt-scale increase in the radiative thermal conductance. While these preliminary results are promising for tuning NFRHT, additional experiments showing a decrease with opposite back gate tuning are required to confirm these findings, as the current observations are potentially confounded by the increased capacitive force on the probe. Finally, I conclude the dissertation by describing possible approaches for mitigating some of the challenges encountered in this work and opportunities for future work.