Exploring Nanoscale Energy Conversion and Biometabolism Using Custom Calorimetric Tools

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.PhDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/171350/1/mrohith_1.pd

Sub-nanowatt resolution direct calorimetry for probing real-time metabolic activity of individual C. elegans worms

Calorimetry is widely used for metabolic studies, but measurements of single cells and small organisms are limited by the sensitivity of current techniques. Here the authors develop a sensitive platform for performing time-resolved metabolic measurements of single C. elegans worms from larval to adult stages

Enhancement and saturation of near-field radiative heat transfer in nanogaps between metallic surfaces

Near-field radiative heat transfer (NFRHT) between planar metallic surfaces was computationally explored over five decades ago by Polder and van Hove [Phys. Rev. B 4, 3303 (1971)]. These studies predicted that, as the gap size (d) between the surfaces decreased, the radiative heat flux first increases by several orders of magnitude until d is ∼ 100 nm after which the heat flux saturates. However, despite both the fundamental and practical importance of these predictions, the combined enhancement and saturation of NFRHT at small gaps in metallic surfaces remains experimentally unverified. Here, we probe NFRHT between planar metallic (Pt, Au) surfaces and show that RHT rates can exceed the far-field rate by over a thousand times when d is reduced to ∼ 25 nm. More importantly, we show that for small values of d RHT saturates due to the dominant contributions from transverse electric evanescent modes. Our results are in excellent agreement with the predictions of fluctuational electrodynamics and are expected to inform the development of technologies such as near-field thermophotovoltaics, radiative heat-assisted magnetic recording, and nanolithograph

A Thermal Diode Based on Nanoscale Thermal Radiation

International audienceIn this work we demonstrate thermal rectification at the nanoscale between doped Si and VO2 surfaces. Specifically, we show that the metal–insulator transition of VO2 makes it possible to achieve large differences in the heat flow between Si and VO2 when the direction of the temperature gradient is reversed. We further show that this rectification increases at nanoscale separations, with a maximum rectification coefficient exceeding 50% at ∼140 nm gaps and a temperature difference of 70 K. Our modeling indicates that this high rectification coefficient arises due to broadband enhancement of heat transfer between metallic VO2 and doped Si surfaces, as compared to narrower-band exchange that occurs when VO2 is in its insulating state. This work demonstrates the feasibility of accomplishing near-field-based rectification of heat, which is a key component for creating nanoscale radiation-based information processing devices and thermal management approaches