<p>Understanding of thermal transport in small scales gains more importance
with increasing demand in microelectronics and advancing fabrication
technologies. In addition, scarce in energy sources adds more pressure with
increasing expectations on research in energy conversion devices and renewable
energies. In parallel to these, new phenomena observable only in small scales
are discovered with the research, bringing more opportunities for engineers to
solve real-world problems by applying the discoveries and more questions to
answer. Thermal radiation as a thermal transport phenomenon is the epicenter of
this research. Recent developments such as near-field radiative heat transfer
exceeding blackbody radiation or control of radiative cooling via biasing grows
the attraction on thermal radiation because these examples challenge our
long-lasting understanding of nature. Exploring nature further in the small
scale may help us meet the expectations mentioned above.</p>
<p> </p>
<p>In this thesis work, first, we carry out analyses on radiative heat transfer of natural
hyperbolic material, calcite, and compare to that of a polar material SiC. Our
study reveals that the high-
modes within the hyperbolic bands are
responsible for the substantial enhancement in near field radiation. Comparison
of calcite with SiC illustrates the significance of the high-
modes in calcite vs. surface polariton modes
in SiC in their contributions to near-field radiation enhancement, for
temperature differences ranging from 1 K to 400 K. We also noticed that the
contributions of high-
modes in calcite to near-field radiation is
comparable to that of surface polaritons in SiC. The results of these analyses
will be helpful in the search of hyperbolic materials that can enhance near
field radiative transfer.</p>
<p> </p>
<p>Second, we demonstrate an experimental
technique to measure near-field radiative heat transfer between two parallel
plates at gap distances ranging from a few nanometers to far-field. A
differential measurement circuit based on resistive thermometry to measure the
defined temperatures are explained. To predict the defined temperatures, a
computational method is utilized. We also detail an alignment technique that
consists of a coarse and fine alignment in the relevant gap regions. This
technique presents a method with high precision for gap measurement, dynamic
gap control, and reliable sensitivity for extreme near-field measurements.
Finally, we report experimental results that
shows 18,000 times enhancement in radiative heat transfer between two parallel
plates.</p>
<p> </p>
<p>Third, we analyze near-field radiative transfer due to hyperbolic phonon
polaritons, driven by temperature gradient inside the bulk materials. We
develop a mesoscale many-body scattering approach to account for the role of
hyperbolic phonon polaritons in radiative transfer in the bulk and across a
vacuum gap. Our study points out the equivalency between the bulk-generated
mode and the surface mode in the absence of a temperature gradient in the
material, and hence provide a unified framework for near-field radiative
transfer by hyperbolic phonon polaritons. The results also elucidate
contributions of the bulk-generated mode and the bulk temperature profile in
the enhanced near-field radiative transfer.</p>
<p> </p>
<p>Forth, we study radiative heat transfer in
hyperbolic material, hyperbolic boron nitride (hBN), and show a major
contribution to energy transport arising from phonon polaritons supported in
Reststrahlen bands. This contribution increases spectral radiative transfer by
six orders of magnitude inside Reststrahlen bands compared to that outside
Reststrahlen bands. The equivalent radiative thermal conductivity increases
with temperature increase, and the radiative thermal conductivity can be of the
same order of the phonon thermal conductivity. Experimental measurements are
discussed. We showed the radiative contribution can account for as much as 27 % of the total thermal transport at 600 K.
Hence, in hBN the radiative thermal transport can be comparable to thermal
conduction by phonons. We also demonstrate contribution of polaritons to
thermal transport in MoO<sub>3</sub>. To calculate radiative heat transfer in
three principal coordinates separately, we modify and apply the derived
many-body model. Our analysis shows that radiative thermal conductivity in both
in- and out-of-plane directions increases with temperature and contribution to
energy transport by polaritons exceeds that by phonons.</p>
<p> </p>
Fifth, we build an experimental setup to examine
near-field properties of materials using an external thermal source. The nanospectroscopy
setup combines near-field microscopy technique, near-field scanning optical
microscopy (NSOM), and Fourier-transform infrared (FTIR) spectroscopy. We
further explain challenges in building a nanospectroscopy setup using a weak
thermal source and coupling two techniques. This method enables us to investigate
spectral thermal radiation and local dielectric properties in nanoscale.
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/15062487 |
| Date | 27 July 2021 |
| Creators | Hakan Salihoglu (11191989) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/THERMAL_RADIATION_BETWEEN_AND_THROUGH_NATURAL_HYPERBOLIC_MATERIALS/15062487 |
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