Use of Silica Aerogels
Absorbing Kinetic Energy
When someone who has never seen a piece of silica aerogel holds some for the first time, the following chain of events usually results. The observer first notices the transparency and light weight of the aerogel and makes some sort of remark about these properties. Then the piece is held between two fingers and gently squeezed. The aerogel gives a little, and springs back. Then a little more force is applied and...pffttt, the piece shatters into a thousand pieces, most of which find a home deep in the carpeting, never to be seen again. Not surprisingly, researchers who have spent the time and effort required to make silica aerogels are usually very reluctant to hand them out to just anyone. Because of this, an application of silica aerogels that has been largely overlooked is their use as an absorber of kinetic energy (impacts) in safety and protective devices.
Energy Absorbing Materials
In very simplified terms, materials absorb kinetic energy by plastic deformation, elastic deformation, brittle fracture, or by the fluid dynamics of gases or liquids within the material. Materials used today for absorbing impacts are commonly organic foams, such as expanded polystyrene, polyurethanes, polyethers, or polyethylene. These typically show elastomeric or plastic behavior. Silica aerogels, being an inorganic solid, are inherently brittle. A brittle material would, at first, seem to be a poor choice for a cushioning material. However, as silica aerogels are usually very low density materials, the collapse of the solid network occurs gradually, spreading the force of impact out over a longer time. Additionally, as silica aerogels are an open-pored material, the gas contained within the bulk of the solid in forced outwards as the material collapses. In doing so, the gas must pass through the pore network of the aerogel. The frictional forces caused as a gas passes through a restricted opening are indirectly proportional to the square of the pore diameter. As silica aerogels have very narrow pores (~20-50 nm), gases rapidly passing through the material will absorb a considerable amount of energy. Therefore, the energy of an object impacting a silica aerogel is taken up by the aerogel by the collapse of its solid structure and the release of gas from within the material.
An effective material for use in safety devices will serve to minimize the force felt by the object (or person) to be protected. This is usually done by spreading the deceleration of the impacting object over a longer period of time. The graphic below shows load versus time for a silica aerogel sample, and two other materials. The samples were cubes 5 cm on a side and were crushed by an 8 lb. weight traveling at 11 ft/sec. The red curve represents a silica aerogel with a density of 0.1g/cm3, the yellow curve is expanded polystyrene, and the green is an elastomeric polypropylene foam. The plots show that both the aerogel and the polystyrene foam reduce the maximum load produced to a very low level. It may, therefore, seem that readily available polystyrene foam may be a more appropriate material than the more unusual silica aerogel.
The situation is not as straightforward as this. Many organic foams produce a significant amount of rebound when they are impacted. This transfers a portion of the energy absorbed by the material back into the object that impacted it (such as a human head). This rebound effect can often do further damage to the object being protected. The plot of deflection (distance moved by the impacting object) vs. time for silica aerogel and polystyrene shown below demonstrates the differences of these materials (Note: Deflection data are derived from measured load values and are for comparison purposes only). The polystyrene (yellow), which behaves elastically and plastically, is crushed by the impacting weight (positive deflection) but then springs back to a considerable fraction of its original volume. Conversely, the weight that impacts the silica aerogel (red) travels a certain distance into the material and then comes to a complete stop without bouncing. This is an important phenomenon to consider when developing materials for safety and protective devices.
The production and use of silica aerogels is environmentally benign. No significantly hazardous wastes are produced during their production. The disposal of silica aerogels is perfectly natural. In the environment, they quickly crush into a fine powder that is essentially identical to one of the most common substances on Earth, namely, sand. Additionally, silica aerogels are completely non-toxic and non-flammable. If they eventually find their way into widespread use as protective materials, they could eliminate a very large amount of unwanted plastic materials.
The attractive energy absorbing properties of silica aerogels may lead to their use in various applications. These may include personal protection in motor vehicles, protection of sensitive equipment such as aircraft flight data recorders, and protection of electronic equipment such as laptop computer hard drives.