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，即当这种材料的孔径变得比气体分子的平均自由程小时，热传递减少5-8倍(努森效应)。该项目的目标是制备空心二氧化硅胶体，并将其顺畅地组装成导热系数接近气凝胶性质的材料。与已建立的气凝胶合成不同，我们的概念使用定义的微型对象的组装作为构建块，以产生分层组织的结构，使高孔隙率与优于已建立的气凝胶的机械强度相结合。为此，我们将制定一个分两个阶段的流程。首先，我们研究了以亚微米尺寸的油滴为模板的中空二氧化硅微球的微乳液合成，二氧化硅沉积在其周围。我们假设外壳在仍处于分散状态时通过固结而机械增强，我们基于初步结果，即胶囊可以通过蒸发水和油来干燥，而不会受到毛细管力的破坏。在第二步中，在除去溶剂之前，将分散的空心球与倍半硅氧烷的水溶液混合，倍半硅氧烷是亲水的二氧化硅前体。倍半硅氧烷的加入有两个目的：一是空心球胶体变得粘稠，可以使水分散体胶凝；二是倍半硅氧烷控制空心球之间的气孔形成，使液体成分蒸发。多孔和中空二氧化硅胶体的模板合成是基于我们开发的二氧化硅前体化合物，这种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0.5mN/m以下，并允许自发形成微乳液。前体随后的水解会在油滴周围形成一层二氧化硅外壳。亲水性或两亲性倍半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，提供了新的可能性来控制干燥过程中的气孔形成，并辅之以溶剂的蒸发顺序(首先是酒精，其次是水，最后是油)。与应用相关的基准是：(1)控制二氧化硅胶体的孔隙率和孔径，以接近从干凝胶到平均孔径小于60 nm的气凝胶的转变(空气的克努森数为1)。(2)干燥过程中对毛细管力的稳定性。(3)通过模板油滴中包含的成分对凝胶进行疏水处理，以防止最终材料在潮湿环境中使用时吸水。(4)在第二步胶凝过程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剂，以获得机械强度，该强度可承受1公斤/平方厘米的负荷，而依从性很小且可恢复。

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.