title: Thermal Properties of Colloidal Structured Materials
Creators: name: Neuhöfer, Anna M.
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: Thermal Conductivity
subject: Thermal Diffusivity
subject: Radiation
subject: Specific Heat Capacity
subject: Emulsifierfree Emulsion Polymerization
subject: Dispersion Polymerization
subject: Hollow Spheres
subject: 540

description: Thermal management plays a huge role in our everyday life. It is crucial in a wide range of applications, e.g., for developing insulating materials for buildings or aerospace components, and more sophisticated materials that can direct the heat flow for energy storage technologies. The requirements for the materials are increasing steadily, and new materials have to be developed. An important parameter is the size range of the device. Since electronic devices are getting smaller, the thermal management material needs to be functional at nano- and microscales. Therefore, it is necessary to develop a detailed understanding of the structure-property relationships of the thermal management material. In this thesis, the thermal properties of different nanoscopic materials and their superstructures have been examined. The main target was to understand and control the thermal transport of colloidal structures, focusing on their insulating properties. Solid and hollow silica and hollow titania particles have been used as colloidal building bricks. The first achievement of this thesis was the synthesis of titania hollow particles in the sub-micrometer size range. Three necessary steps have been elaborated: synthesis and functionalization of polystyrene template particles , growth of the shells - including an aging step , and the calcination parameters. Following these steps, it was possible to fabricate titania hollow particles in a simple, scalable, and reproducible way. Titania material is known for its optical properties since it strongly absorbs light in the wavelength region between 1-8 µm. Therefore, it is used in thermal insulation applications to suppress thermal radiation at elevated temperatures. In contrast, silica material is optically transparent in the region between 1-5 µm and consequently strongly affected by thermal radiation. Colloidal superstructures from solid and hollow silica particles in the sub-micrometer size range have been characterized using light-flash analysis. The transition from conduction-dominated to radiation-dominated thermal transport mechanism was observed and described in the temperature range from 25 to 925°C, using an extended existing analytical framework. The results provide a better understanding of the challenges of fabricating and analyzing efficient high-temperature insulation materials. At room temperature, silica hollow particle materials are highly insulating. To extend pre-existing knowledge about the structure-property relationship, the thermal properties of binary colloidal assemblies of 500 and 900 nm of solid and hollow silica particles have been studied. It was found that especially for hollow particles, the influence of the particle size exceeds the effect of binary mixtures, and a thermal conductivity of 20 mW/mK in vacuum was achieved. Thermal characterization often requires information about the material specific heat coefficient. Especially when new materials are thermally characterized, characterization via DSC is essential. With my knowledge about this technique, I contributed to several further projects. While the former projects were based on inorganic materials, also organic and organic-inorganic composite materials are characterized in the second part of this thesis. The first project was on ampholytic polymers. This unique class of polymers exhibits a hydrogen bond donor and acceptor group per repetition unit and therefore has a high functional group density. Four different ampholytic polymers have been investigated using infrared spectroscopy and the photoacoustic method. It was proven that the thermal conductivity increased with increasing hydrogen bond strength in the polymer. I supported this project by applying modulated DSC measurements to determine the glass transition temperatures of the polymers in the humid state. In the remaining two projects, the anisotropic thermal properties of two-dimensional Bragg stacks were investigated. Bragg stacks consist of aligned nanoplatelets with in-plane dimensions in the micrometer regime, while the thickness in cross-plane direction is only a few nanometers. As platelets, hectorite and graphene oxide have been used. The in-plane thermal conductivity was significantly higher than the cross-plane thermal conductivity for both materials. The in-plane thermal conductivity of the hectorite stacks was found to be higher as in the graphene oxide stacks. Consequently, the overall anisotropy ratio was 2- to 3-fold higher. In a more advanced study, hectorite platelets have been layered with polyvinylpyrrolidone (PVP) with extreme polymer confinement and perfect periodicity. Increasing PVP volume fractions led to basal spacing in the range from 1.9 to 3.8 nm. The dominating influence of the PVP-hectorite interfaces led to a drastic drop of the cross-plane thermal conductivity of 0.09 W/mK compared to the pure materials 0.17 W/mK for PVP and 5.71 W/mK for hectorite. Overall, the structural perfection of these samples led to an exceptionally high thermal conductivity anisotropy. I provided both projects with all necessary DSC measurements to determine the specific heat of all materials and composites. This thesis shows the beauty of “playing” with colloidal building bricks, extending the number of accessible materials and structures. Understanding the thermal structure-property relationship helps create tailored thermal management materials. In the future, this research may help solve thermal challenges, such as insulation applications and heat sinks in electronic devices.