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

对建筑进行隔热是降低能耗的关键。在这方面，最近的研究集中在非纤维的、无定形的二氧化硅基绝缘材料上。通过调整合成参数，可以定制这些材料的微观结构和性能。然而，有两个相互矛盾的目标，一方面增加孔隙率会降低导热系数，另一方面也会降低机械阻力。显示双峰多孔结构的分层结构二氧化硅单体已经被定制以满足一个或另一个目标，但是裁剪材料与两个相反参数的优化组合还没有完成。对于这项任务，目前还没有合适的模型系统和系统研究孔隙率、孔径及其结构排列对这些类型材料的导热性和机械阻力的影响。因此，该项目的主要目标是首次系统地研究具有开孔多孔结构的自支撑sio2基单片材料的孔径、孔隙度、孔隙结构（单峰或双峰）与导热性以及机械阻力之间的复杂关系，并对这些特性进行建模，最后开发一种合成途径来实现所获得的见解。为了实现这一目标，使用不同的合成方法生产了三种不同的硅模型体系：(i)由多孔玻璃制成的单模态孔隙分布，（ii）具有系统变化孔径的气凝胶，（iii）具有双峰孔隙率的溶胶-凝胶单体。表征的实验方法包括结构分析、汞侵入、微层析成像（µXCT）、小角散射（SAXS）和电子显微镜（SEM, FIB-SEM， STEM）。热输运性质用热丝法量化。通过纳米压痕法和原位膨胀法吸附氮，以及各种力学测试，确定了不同结构水平上的力学特性。这些实验方法旨在为使用有限体积法进行模拟提供输入数据，以系统地研究结构与性能之间的关系。最后，对具有优化定制性能的模拟结构进行合成，以验证模拟结果。这是第一次在表征多孔结构，导热性和机械阻力方面的专家以及合成专家将在这个协同合作中联合在一起，研究二氧化硅整体结构和性能之间的基本关系。所获得的见解可用于开发创新的定制无机绝缘材料，此外还可以作为表征更复杂的多孔建筑材料（如混凝土和砖）的起点。

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.