Aerogels: a flyweight amongst high-tech materials

In order to reach his goal, Kistler had to develop a method for drying gels without reducing the volume of the resulting structure. Not an easy task, but this much can be revealed: “Our efforts were successful”, writes Kistler in 1931 in the renowned scientific magazine Nature [1] about the results of his experiments, followed by a more extensive paper in the Journal of Physical Chemistry [2].

Kistler used sodium silicate, which he mixed with water and thus created a solution (water glass). After adding hydrochloric acid as precipitating agent, silicic acid particles were precipitated (precipitation reaction), which dispersed in the solution in an uncoordinated manner according to Brownian motion and also bumped into each other. […] Gradual adhesion led to particles aggregating over time and within approximately one day resulted in a gel with a web-like structure. Sodium chloride and excess hydrochloric acid were rinsed from the gel with water (aquagel), followed by adding alcohol (alcogel). This step is necessary, since water would destroy the gel structure during the following process. Slow evaporation of the alcohol leads to the formation of menisci due to surface forces. These ‘dig’ into the gel and create a tunnel-like structure, resulting in the shrinking of the gel and a porous structure with a pore content of only 50 percent. This exactly, however, was to be avoided. For the drying process Kistler therefore used an autoclave (a gas-tight sealable pressure container, editor’s note: this also includes pressure cookers) and raised temperature and pressure above the critical point, creating a supercritical fluid. This procedure is called supercritical evaporation. The phase boundary between gas and liquid was thereby lifted and surface forces, which otherwise would have led to the formation of menisci, no longer existed. The supercritical fluid was then discharged from the autoclave, drying the product and thus creating the aerogel. The aerogel retained the size and shape of the original gel. Kistler’s silicate aerogels had a density of 30-300 kg/m³ and a porosity between 86 and 98 percent. However, Kistler’s production method was lengthy and labor-intensive, especially the solvent exchange step before the alcohol evaporation [5].

In 1968, at the University of Lyon, Stanislaus Teichner developed and in 1986 improved the Sol-Gel-process, which became and for a long time remained the standard method for producing aerogels. In this method, toxic tetra-methyl-ortho-silicate (TMOS) is slowly hydrolyzed with a specific amount of water and an added catalyst into ortho-silicic acid and methanol. Water separates from the silicic acid, leading to the formation of silicon oxide (SiO2) tetrahedrons, cross-linking with each other to form a gel. The resulting alcogel is dried according to the Kistler method in the pressure cooker, during which the methanol reaches critical values of 239.4 °C und 80.9 bar. The characteristics of the developing aerogel, its structure and density, can be controlled by the choice of the catalyst, the pH value, and the proportions of the involved substances, especially methanol [5, 6].