Structure-property relationships of hierarchically structured silica monoliths as a model system for innovative inorganic thermal insulating materials

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Insulating buildings thermally is a key to reduce the energy consumption. In this regard, recent research is focusing on non-fibrous, amorphous SiO2-based insulating materials. The microstructure and therefore also the properties of these materials can be tailored by adjusting the synthesis parameters. However, there are two conflicting goals, as increasing porosity decreases thermal conductivity on the one hand, but also decreases mechanical resistance. Hierarchically structured silica monoliths showing bimodal porous structures have been already tailored to fulfill either one goal or the other, but tailoring materials with an optimized combination of both opposing parameters has not been done yet. For this task suitable model systems and systematic studies of the influence of porosity, pore size and their structural arrangement on thermal conductivity and mechanical resistance of these types of material are yet missing.The main goal of this project is therefore to systematically investigate for the first time the complex relationships between pore size, porosity, pore structure (monomodal or bimodal) and thermal conductivity as well as mechanical resistance of self-supporting SiO2-based monolithic materials with open-celled porous structure, to model these properties and finally to develop a synthetic route to implement the insights gained. For that to happen, different synthetic methods are used to produce three different silica model systems: (i) monoliths made of porous glass showing monomodal pore distribution, (ii) aerogels with systematically varied pore size and (iii) sol-gel monoliths with bimodal porosity. The experimental methods planned for characterization are for structural analysis mercury intrusion, micro-tomography (µXCT), small angle scattering (SAXS) and electron microscopy (SEM, FIB-SEM, STEM). The thermal transport properties are quantified by the hot wire method. The mechanical characteristics on different structural levels are determined by nanoindentation and nitrogen sorption with in-situ dilatometry, as well as by various mechanical testing. These experimental methods are designed to provide the input data for simulations using the finite-volume method to systematically investigate the relationship between structures and properties. Finally, the simulated structures with optimized tailored properties will be synthesized to validate the results of the simulations. For the first time experts in characterizing the porous structure, the thermal conductivity and the mechanical resistance as well as experts for the synthesis will join together within this synergetic collaboration to study the fundamental relationship between structure and properties of silica monoliths. The insights gained can be used to develop innovative tailored inorganic insulating materials and in addition can act as a starting point to characterize more complex porous construction materials such as concrete and bricks.