Exploring Nanoscale Energy Conversion and Biometabolism Using Custom Calorimetric Tools
Mittapally, Rohith
2021
Abstract
Radiative heat transfer (RHT) between two bodies separated by gaps larger than the thermal wavelength (10 µm at 300 K) occurs through modes that are propagating across the medium separating them, where the total RHT between them is bound by the blackbody limit. RHT beyond this limit is possible with contributions from surface modes, such as evanescent modes or surface phonon/plasmon polaritons. The principal goal of this thesis is to study emerging questions related to radiative heat transport in nanoscale gaps with implications on thermophotovoltaic power generation, thermal management of electronics and calorimetric techniques. One important question in RHT is what the fundamental limits to near-field enhancements mediated by surface polaritons may be. Recent theoretical studies predicted materials with potential 5-fold enhancement over that of SiO2, a state-of-the-art polar material for RHT. By leveraging high-resolution, micro-fabricated calorimeters in a custom-built nanopositioner, I show in chapter 2 of this thesis that RHT rates up to 3-times larger than SiO2 can be obtained from MgF2 and Al2O3 that have stronger phonon-polariton resonances than SiO2. This represents the first experimental demonstration of enhanced RHT beyond that of SiO2 using dielectric materials and thus, should enable future studies with potential enhancements up to the theoretical limit. Next, in chapter 3 of this thesis I describe how near-field effects can be employed to achieve novel heat to electricity conversion technologies. Specifically, I explore how thermophotovoltaic technologies—where a hot emitter and a PV cell are employed to convert heat to electricity—can be enhanced by employing near-field effects. I explored this possibility by developing doped-Si microdevices with an integrated platinum heater that could be heated to 400º C. By placing this hot object at a few tens of nanometers away from a commercial photodiode, we demonstrated—for the first time—that ~ 40-times larger power outputs as compared to the far-field can be obtained. Next, I describe how this previous work was extended to explore how the performance of a near-field TPV system can be further enhanced in terms of the power density and efficiency. In order to achieve this, I fabricated silicon microdevices that can endure temperatures up to ~1000 º C. Then, by leveraging the nanopositioner, we placed the silicon heater at known distances away from a thin-film InGaAs PV cell. When the distance was reduced to 100 nm, the total system demonstrated a record-high efficiency of ~6.7% at a power density of 5000 W/m2. Additional improvements can enable several-fold gains in the performance and pave the way for realization of practical devices. Finally, I describe in chapter 4 how I built a calorimetric tool with 270 pW heat resolution based on calorimetric techniques developed in my RHT studies. This was accomplished by minimizing the thermal conductance of a commercial glass capillary tube down to 27 µW/K and improving the thermometry to an unprecedented 10 µK resolution. Using this tool, we measured metabolic heat outputs from wild-type C. elegans, a biological model organism. Our measurements on daf-2, a variant with an increased lifespan, reveal interesting metabolic shifts as compared to wild-type variant. Thus, we demonstrated for the first time that metabolic rate measurements on living systems can be performed at sub-nanowatt resolution in real time with 270 pW resolution.Deep Blue DOI
Subjects
Thermal Radiation at the nanoscale Thermophotovoltaic energy conversion at nanoscale Calorimetry for biological metabolic rate measurements Micro/nano-scale heat transfer
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Thesis
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