THERMAL RADIATION BETWEEN AND THROUGH NATURAL HYPERBOLIC MATERIALS

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.

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.

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.

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.

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 MoO₃. 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.

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.