Hollow Silica Colloids as Building Blocks for Highly Porous Water Borne Aerogels with Improved Mechanical Stability

气凝胶的制造仍然是一个挑战。它们的重要性源于这样一个事实，即当这些材料的孔径小于气体分子的平均自由程时，热传递减少了5到8倍（Knudsen效应）。该项目旨在制备中空二氧化硅胶体，并将其不拥挤的组装成具有接近气凝胶性质的导热性的材料。与现有的气凝胶合成方法不同，我们的概念是使用已定义的微物体的组装作为构建块，以产生分层组织的结构，使高孔隙率与机械强度优于现有气凝胶的组合成为可能。为此，我们将开发一个两阶段流程。首先，我们研究了以亚微米大小的油滴为模板的空心二氧化硅球的微乳液合成，二氧化硅沉积在其周围。我们假设外壳在分散状态下通过固结而机械地加强，并且我们建立在初步结果的基础上，即胶囊可以通过水和油的蒸发而不被毛细力破坏而干燥。在第二步中，在去除溶剂之前，将分散的空心球体与硅氧烷的水溶液混合，硅氧烷是亲水性硅前体。添加硅氧烷有两个目的：使空心球体胶体具有粘性并使水分散体凝胶化；在液体组分蒸发时，硅氧烷控制空心球体之间的孔隙形成。多孔和空心二氧化硅胶体的模板合成是基于我们开发的二氧化硅前驱体化合物，这些前驱体化合物最初用作表面活性剂，将油和水之间的界面张力降低到0.5 mN/m以下，并允许自发形成微乳液。前驱体随后的水解产生围绕油滴的硅壳。亲水或两亲硅氧烷作为二氧化硅的前体是俄罗斯合作伙伴的原创开发，并提供了在干燥过程中控制孔隙形成的新可能性，这是由溶剂蒸发的顺序（首先是酒精，其次是水，最后是油）补充的。与应用相关的基准是：(1)控制二氧化硅胶体的孔隙度和孔径，以接近从干凝胶到平均孔径小于60 nm的气凝胶的过渡（空气的Knudsen 1）。(2)干燥过程中抗毛细力的稳定性。(3)模板油滴中所含成分对凝胶的疏水性，以防止在潮湿大气中使用时最终材料对水的吸附。（iv）在第二步凝胶化中添加强化成分的可能性，如剥离的层状硅酸盐、纳米纤维和有机凝胶，以达到可以承受1 kg/cm2负载的机械强度，并且可以恢复顺应性。

Fabrication of aerogels remains a challenge. Their importance originates from the fact that heat transport reduces 5 to 8 times when the pore sizes of such materials become smaller than the mean free path of the gas molecules (Knudsen effect). This project aims on preparation of hollow silica colloids and their uncongested assembly to a material with a thermal conductivity that approaches the properties of an aerogel. Other than established syntheses of aerogels, our concept uses assembly of defined micro-objects as building blocks to yield a hierarchically organized structure that enables the combination of high porosities with mechanical strength superior to established aerogels. For this purpose, we will develop a two-stage process. First, we attend to a micro-emulsion synthesis of hollow silica spheres with sub-micron sized oil droplets as a template around which silica deposits. We hypothesize that the shell strengthens mechanically by consolidation while still in the dispersion, and we build on preliminary results that capsules can be dried by evaporation of the water and oil without destruction by capillary forces. In a second step, and before removal of the solvent, the dispersed hollow spheres are mixed with an aqueous solution of silsesquioxanes, which are hydrophilic silica precursors. The addition of the silsesquioxanes serves two objectives: The hollow sphere colloids become sticky and can gel the aqueous dispersion and the silsesquioxanes control pore formation between the hollow spheres upon evaporation of the liquid components. The template synthesis of porous and hollow silica colloids is based on our development of silica precursor compounds that serve initially as surfactants to reduce the interfacial tension between oil and water to values below 0,5 mN/m and which allow spontaneous formation of a micro emulsion. Subsequent hydrolysis of the precursor yields a silica shell around the oil droplets. Hydrophilic or amphiphilic silsesquioxanes as a precursor for silica are an original development of the Russian partner and offer new possibilities to control pore formation during drying which is complemented by the order of the evaporation of the solvents (alcohol first, water second, oil last). Application relevant benchmarks are: (1) control of the porosity and pore sizes of the silica colloids in order to approach the transition from xerogels to aerogels with average pore diameters less than 60 nm (Knudsen number 1 for air). (2) Stability against capillary forces during drying. (3) Hydrophobisation of the gels by components contained in the templating oil droplets in order to prevent sorption of water by the final materials during use in humid atmosphere. (iv) The possibility to add fortifying components such as exfoliated layered silicates, nanofibers and organic gelators in the second gelation step in order to achieve mechanical strength that can withstand a load of 1 kg/cm2 at little and recoverable compliance.