title: Conductive and Radiative Heat Transport: Contributions from Experimental and Theoretical Methods
Creators: name: Herrmann, Kai
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: Thermal Transport
subject: Thermal Characterization
subject: Polymeric Thin Films
subject: Passive Daytime Radiative Cooling
subject: 540

description: This thesis is dedicated to mainly two aspects of heat transport in current research materials. The first aspect is conductive heat transport which, in the investigated non-metals, mostly takes place through vibrations. The second aspect is radiative heat transport which is of particular interest at high temperatures or special applications like passive radiative cooling. First, the photoacoustic technique was theoretically optimized and verified experimentally for the cross-plane conductive thermal characterization of polymeric thin films. With this measurement method, a temperature oscillation is optically excited, which can be measured with a microphone utilizing a resulting pressure oscillation. Depending on the sample's thermal properties, the pressure signal contains information about the surface temperature, which has a characteristic phase shift and amplitude relative to the excitation. Non-linear data analysis can, therefore, be used to determine the properties of the sample by fitting the measurement data to an appropriate multi-layer model. The optimized measurement setup could subsequently be used for the thermal characterization of current research materials in addition to the general proof of applicability. The first systems were P3HT thin films with different molecular weights, prepared from three different solvents, where possible relations between the microstructural optoelectronic properties extracted from deconvolution of absorption spectra and the thermal conductivity were explored. A variation of the optoelectronic properties, mainly regarding molecular weight, was apparent, while no direct influence of the solvent was discernible. In contrast, the thermal conductivities of all examined films demonstrated an insignificant variability. We, therefore, concluded that mainly the amorphous phase determines the thermal transport properties in these semi-crystalline thin films, as these represent a bottleneck for thermal transport. The second systems were fully amorphous ampholytic polymers that exhibited different donor and acceptor groups to form hydrogen bonds. In addition to measurements depending on the humidity and/or water absorption of the polymers, a correlation between the strength of the hydrogen bonds and the thermal conductivity was observed. The strength of the hydrogen bonds was determined by deconvolution of the characteristic carbonyl peak of the IR absorption. A direct correlation between the hydrogen bond strength and the thermal conductivity was observed for the thin films investigated. Thirdly, anisotropic hybrid Bragg stacks of highly ordered fluorohectorite clay layers that alternate with one or two poly(ethylene glycol) layers were explored. For in-plane thermal characterization, lock-in thermography was used to investigate the anisotropy. The mechanical properties were examined using Brillouin light spectroscopy, indicating an almost ideal reinforcement of the hybrid material and exceptionally high Young's moduli. Furthermore, 2D hybrid perovskites were studied compared to the widely used 3D methylammonium lead iodide. The influence of the organic cations on the thermal and electronic properties was investigated using light flash analysis, first-principles and molecular dynamics calculations, ultraviolet photoelectron spectroscopy and Raman measurements. As a result, an atomistic understanding of the effects of dimensional reduction on properties relevant to electronic and thermal transport was developed. While conduction is the dominant transport mechanism in the materials outlined above, radiative transport is the second subject to be investigated. Particulate silica materials are an exciting and at ambient thermally highly insulating material class that was investigated up to 925 °C by light flash analysis. Performing multibody optical simulations and a newly developed model, a transition of the main transport channel from conduction at ambient to radiation at high temperatures was unraveled. Therefore, the materials partially lose their extraordinary insulating properties at high temperatures. The final subject area, passive cooling, is a unique application of radiative heat transport. Although thermal radiation is relatively weak at ambient temperatures, outer space can be used as an ultimate heat sink when carefully designing the emitter material to take advantage of the atmospheric transparency window. First, an approach was developed to determine the optimum emitter thickness to maximize the cooling power at ambient conditions or reach the lowest equilibrium temperature. Furthermore, an in-house setup was designed, which allows reproducible cooling power measurements and the variation of several environmental parameters in contrast to rooftop measurements. In summary, the conductive heat transport in thin films and free-standing samples was investigated in addition to the methodological development. Various structure-property relationships were elucidated, which improved the understanding of thermal transport on small length scales and provides guidance for future material development. Furthermore, through the analysis of radiative heat transport, it could be shown that particulate silica materials, which are highly insulating at room temperature, lose part of their insulating properties at high temperatures. Finally, in the field of passive radiative cooling, it was possible to show how the emitter thickness can be optimized, and a highly reproducible and variable in-house measurement setup was developed.