To give you an idea of what makes aerogels special materials for so many uses, it is instructive to provide some structural and functional definitions.
Many chemical reactions that involve polymerization of molecules form dispersed nanoparticles in solution, particularly those that occur in three dimensions via cross-linking between growing solid chains. A gel precursor is a chemical or mixture of chemicals that can be activated toward molecular nucleation into small colloidal particles. A gel forms when a solution of these dispersed nanoparticle colloids (also called “sols”) are induced to form a semi-solid form via interparticle condensation. Sols can be stable for very long periods of time if the solution conditions are not conducive to interparticle condensation. When a catalyst is added to create the appropriate conditions for interparticle condensation, the viscosity of the solution can increase very rapidly as the semi-solid state is reached.
At the gel point, the liquid phase within a gel structure is not able to diffuse freely and will not flow or change shape if the volume is tilted on an axis. When the chemistry approach is designed correctly, the solid in the wet gel phase can define a high surface area network of pores that confine the liquid within the structure. Aspen Aerogels processes various materials to contain liquid filled pores that average about ten nanometers (a nanometer is 1/100,000th the diameter of a human hair), with a distribution of pore sizes typically between 1 and 100 nanometers. The general approach to generating nanoparticle sols and their subsequent gel phase is called the sol-gel process. The sol gel process has been commonly applied to metal oxide precursors where the metal is silicon, aluminum, or titanium. However, there are many examples of organic aerogels and mixed inorganic/organic aerogel structures.
An aerogel is directly derived from a wet gel in a process that replaces the entrained liquid phase with air. If the gel is formed from a water phase, the resulting semi-solid is called a hydrogel, and the water must be exchanged with organic solvent prior to drying. If the gel is formed within an alcohol phase, the resulting semi-solid is called an alcogel, and can be dried directly. This is accomplished by increasing the temperature and pressure of the solvent phase inside of the gel structure beyond its critical point. This “supercritical” extraction condition lowers the surface tension between the liquid and the solid pore surfaces so that depressurization of the system at temperatures above the critical temperature leaves the pore structure filled with gas.
A supercritical drying process avoids the tremendous pressures exerted when liquids evaporate from tiny pores. The crushing forces during evaporation are inversely proportional to the pore radii; thus a nanoporous material is easily crushed during ambient pressure drying. Some well-known chemical processing approaches chemically modify a gel surface structure to mitigate much of the damage caused by ambient pressure drying. Drying a gel at near ambient pressures is called a “xerogel” process, and produces aerogel materials that are typically denser than supercritically dried aerogel materials.
Aerogels represent a structural morphology (amorphous, open-celled nanofoams) rather than a particular chemical constituency. However, a great deal of study has been devoted to silica aerogels and their properties over the past 70 years. Silica aerogels were first discovered in 1931 by Kistler. His process used polymerization of silicic acid (Si(OH)4), which in turn was generated by acidic neutralization of sodium silicate in water (Equation 1).
Using Kistler’s method, the aqueous silica hydrogels were repeatedly rinsed with volumes of fresh anhydrous methanol to remove all but trace amounts of water. Kistler brought the contents of the gel past the critical point of methanol (240°C and 1600 psi; making the solvent system “supercritical”) in a high-temperature autoclave and slowly depressurized the system at a temperature that prevented recondensation of methanol within the porous silica gel structure. This method has numerous disadvantages, particularly in the toxicity of methanol, and in the handling of a flammable solvent at very high temperatures and pressures.
In the seven decades since Kistler’s seminal work, there have been significant advances made in both the use of new precursor materials, and in the removal of solvent from them. For instance, Teichner and others established in the 1960’s that silicon alkoxides (e.g. tetraethylorthosilicate or TEOS) are the preferred soluble silica source for formation of silica gels because the need for water/alcohol solvent exchange could be avoided.[2,3] Ethanol based processing using silicon ethoxide derivatives such as TEOS and polydiethylsilicate (PDEOS) have become the preferred precursors to silica aerogels over the last few decades. With these materials, water is added to liberate alcohol and silicic acid. The silicic acid is very sensitive to condensing with itself, and rapidly building sol particles. As the process continues, a three dimensional gel network is formed, filling the mold volume and entraining all of the liquid solvent. The gels can then be further strengthened and treated in an alcohol solution, avoiding the need for solvent exchange if desired.
In the past 20 years, the use of supercritical carbon dioxide as a solvent for drying of gels containing organic solvent has made the process safer and more economical. Hunt pioneered the use of supercritical carbon dioxide as the solvent medium for aerogel processing , further reducing hazards associated with venting of superheated methanol vapor.
From this foundation of research, Aspen Aerogels has advanced the chemistry and processing technology to the point where the phenomenal properties of aerogels are now available high-volume production.
Essentially, any liquid can be gelled when a continuous solid lattice structure is formed within the liquid phase, entraining the solvent within open pores. The key to making a nanoporous aerogel involves developing a balance between polymer chain growth and interchain cross-linking that will net the desired pore structure and high surface area. The modulus (stiffness) of the resulting gels must be high enough to resist the capillary forces generated during solvent removal. This property is strongly influenced by the number of links between neighboring chains per unit volume of gel. Many materials can be incorporated into the silica- or other metal oxide-matrix at the sol stage, including organic polymers that uniquely modify the aerogel physical properties. Many highly cross-linked polymer systems can also be induced to create gels in organic solvents, such as resorcinol-formaldehyde, melamine-formaldehyde, polyimides, polyurethanes, polyisocyanurates, and various unsaturated hydrocarbon materials.
The highly porous nature of an aerogel structure provides a huge amount of surface area per unit weight. For instance, a silica aerogel material with a density of about 100 kg/m3 (or about 1/10th that of water) can have surface areas of around 800 to 1500 m2/g depending on the precursors and process utilized to produce it. That is equivalent to roughly 3 to 5 football fields per gram of solid material – showing that an extraordinary amount of surface is folded in on itself within the aerogel structure. The percentage of open space within an aerogel structure is about 94% for a gel with a density of 100 kg/m3.
Other typical “extreme” properties of silica aerogel materials are:
- Aerogels have the lowest thermal conductivity values of any solid
- Aerogels are exceptional reflectors of audible sound, making excellent barrier materials; aerogels have very low sound velocity through structure (~100 m/s)
- Aerogels can be exotic energy absorbers, showing capability to capture high velocity dust particles in space that would penetrate thick steel
- High internal surface areas (up to 1500 m2/g)
- Ultra-low refractive index values for a solid (1.025), approaching that for air
- Ultra-low dielectric constants for a solid (can be < 1.1)