The First Successful Application Of Silica Aerogels In Space

Silica Aerogel is an incredibly light and amorphous material with a high specific heat capacity. Its thermal insulation is of great interest for aerospace applications such as air ducts and insulation materials in spacecrafts, planetary vehicles and habitats. In addition, its inorganic nature makes it an effective firewall against fire, which would eliminate the need for toxic and flammable organic coatings currently used in the vicinity of the exhaust pipe of aircraft engines.

A number of techniques are available to evaluate the mechanical properties of Silica Aerogel in Insulation, including ultrasonic methods, three-point bending and uniaxial compression testing. However, the fractal network structure of aerogels results in brittle materials with a very low loading strength. Therefore, many research efforts are concentrated on developing a technique to mechanically strengthen the aerogels.

One of the most promising approaches is based on in situ network framework reinforcement, which involves cross-linking polymers into the microstructure of the aerogels. X-aerogels containing diisocyanates and other polymers show superior mechanical properties to monolithic silica aerogels, which are characterized by a low specific strength of around ten times lower than steel [41].

The first successful application of silica aerogels in space was the Orbital Debris Collector (ODC) that was deployed on the Mir environmental effects package in 1996. The ODC consisted of 0.63 m2 of porous, low-density silica aerogel arranged in two identical trays. One of the trays was configured to point into the ram direction, while the other was oriented in the opposite direction to capture man-made and natural hypervelocity debris in low Earth orbit.

It is important to note that the ODC was not designed to yield reliable dynamic information on each captured particle. Nevertheless, the unique ability of aerogel to trap and preserve the debris allowed for an extensive analysis of the particles impact track, which provided valuable data on the chronology and characteristics of the extraterrestrial debris.

The physics behind this extraordinary capability of the ODC is that the high surface area and large pores in the silica aerogel allow for the capture of particles through multiple mechanisms, including solid conduction, gaseous conduction (the movement of atoms and molecules within the porous structure) and radiation.

Another disadvantage of the ODC was that its performance was compromised by the syneresis phenomenon, a gel-like behavior induced by the continual hydrolysis and condensation reactions during its preparation and aging process. The syneresis effect is accompanied by significant gel shrinkage, as shown by Woignier and colleagues in Figure 1.

Moreover, the high cost of the supercritical drying process that is employed in the current commercial production of silica aerogels limits its broad use. Thus, the development of a preparation method that employs atmospheric pressure drying to reduce the production costs is of great significance for the future wide-spread application of silica aerogels in aerospace applications. This will also enable the creation of a variety of new types of aerospace materials with tailor-made physical properties, such as multifunctional nanoparticles or atomically engineered zeolite structures.