Composites

Strong and Flexible Aerogels


There are now many different types of aerogels that are flexible and high-strength, some of which are so mechanically robust they can actually be used for structural applications!


Although it’s true that a typical silica aerogel could hold up to 2000 times its weight in applied force, this only holds if the force is gently and uniformly applied. Also, keep in mind that aerogels are also very light, and 2000 times the weight of an aerogel still might not be very much. Additionally, most aerogels as-produced are extremely brittle and friable (that is, they tend to fragment and pulverize). As a result, structural applications of aerogels were for a long time totally impractical.

But never fear! There are several ways aerogels can be made strong and even flexible, enough that aerogels can now be used as structural elements.

There are four general ways to enhance the mechanical properties of aerogels:

• Liquid-phase crosslinking
• Vapor-phase crosslinking
• Fiber reinforcing, and
• Reduced bonding

X-AEROGELS:

Particles of oxides, such as silica, are frequently mixed into plastics to make plastics with different properties. This process is called “doping”, in which the oxide particles are called a “filler”.

One day, Prof. Nicholas Leventis at the University of Missouri-Rolla (now the Missouri University of Science and Technology) wondered, “If you can dope a polymer with a filler, can you dope a filler with a polymer?”

So thinking about cohesive forms of fillers used for doping polymers, he thought of silica aerogels, which are effectively macroscopic assemblies of silica nanoparticles.


Starting with a preformed wet silica gel of the type used for making silica aerogels, Leventis soaked the gel in solutions containing diisocyanates-crosslinking agents used to make polyurethane varnish-and then heated the gels to get the diisocyanates to bond. Upon supercritical drying, a silica aerogel with remarkably improved mechanical properties resulted-an aerogel that can actually bend not unlike stiff rubber! Try doing this with an ordinary silica aerogel and you’ll be left with lots of little broken pieces.


Diisocyanates are linear molecules with two ends that can react with hydroxyl groups to form carbamate bonds. The hydroxyl groups lining the skeleton of silica gels are perfect candidates for reacting with diisocyanates, and since diisocyanates have two reactive ends, they can bond to the aerogel skeleton twice. The result-each diisocyanate
molecules acts like a nano-sized piece of Scotch^® tape bonded to the surface of the aerogel skeleton, resulting in a conformal polymer skin that ties together the spherical silica nanoparticles that make up the aerogel. This conformal polymer skin makes the resulting aerogel much stronger than a typical silica aerogel and allows the structure to flex without breaking-sort of like the marshmallow coating on a Rice Krispies^® treat!

Metal Oxide Aerogels


Metal oxide aerogels are the inorganic cousins of the more common silica aerogel–each type with its own unique properties. These aerogels are important as they can act as catalysts for various chemical transformations, matrices for explosives, precursors for other materials (such as carbon nanotube catalysts), can be magnetic, and are often quite colorful.

Up until the 1990′s, metal oxide aerogels had been historically much more difficult to synthesize than silica aerogels, largely because there were no good synthetic routes for making metal oxide gels. This is mostly due to the difficulty associated with handling metal alkoxide compounds (as they hydrolyze readily), or in some cases simply because of there aren’t any or any good metal alkoxides for a particular metal.


In chemistry, many metal oxides are referred to by changing the “-ium” suffix of the metal to “-ia”. Thus, aluminum oxide is often called alumina, chromium oxide is often called chromia, zirconium oxide is often called zirconia, etc. METAL OXIDES ARE NOT METALS. This is a common misunderstanding among the general public and a frequent oversight in the media. Just as rust is a form of iron oxide (which doesn’t have a special name, incidentally) and is very different from iron metal, or how silica (glass) is very different from silicon (a semiconductor), metal oxides are chemically different from their parent metals.

One of the most notable differences between metal oxide aerogels and silica aerogels is that many metal oxide aerogels are often brilliantly colored. It is important to remember that the bluish cast characteristic of a silica aerogel is a result of Rayleigh scattering by nanoparticles which make up the aerogel backbone, and that silica itself is not blue. Similarly, aerogels of metal oxides which are white as powders and clear in their crystalline form (such as alumina, titania , and zirconia among others) look like silica aerogels–transparent with bluish Rayleigh scattering, and perhaps occasionally somewhat cloudy white. However, many metal oxides (such as chromium oxide and iron oxide) exhibit bright coloring as both powders and crystals, and are in fact used in dyes, paints, and glazes because of this. As a result, aerogels of these oxides are also brightly colored–iron oxide aerogels are Martian red, for example. Below are some of the colors associated with different metal oxide aerogels.


Silica, Alumina, Titania, Zirconia: Clear with Rayleigh scaterring blue or white
Iron Oxide: Rust red or yellow, opaque
Chromia: Deep green or deep blue, opque
Vandia: Olive green, opaque
Neodymium Oxide: Purple, transparent
Samarium Oxide: Yellow, transparent
Holmium Oxide, Erbium Oxide: Pink, transparent




Organic and Carbon Aerogels

ORGANIC:


Organic aerogels have been around as long as any aerogel-in fact, the first aerogel Samuel Kistler is believed to have prepared was aerogel made from jelly (which is composed of the organic heteropolysaccharide pectin). Kistler also prepared aerogels of gelatin and rubbers, both of which are composed of organic polymers.


Basically, an organic aerogel is any aerogel with a framework primarily comprised of organic polymers. Generally, organic aerogels have very different properties from inorganic aerogels such as silica aerogel and metal oxide aerogels. They are generally less friable and less fragile than inorganic aerogels, instead squishing when compressed. The term “organic aerogel” can refer to one of many different kinds of aerogels, each with properties arising from the polymer which makes up the aerogel’s framework.


Organic aerogels can be made from resorcinol formaldehyde, phenol formaldehyde, melamine formaldehyde, cresol formaldehyde, phenol furfuryl alcohol, polyacrylamides, polyacrylonitriles, polyacrylates, polycyanurates, polyfurfural alcohol, polyimides, polystyrenes, polyurethanes, polyvinyl alcohol dialdehyde, epoxies, agar agar, agarose, and many others (see C. S. Ashley, C. J. Brinker and D. M. Smith, Journal of Non-Crystalline Solids, Volume 285, 2001).


Although organic aerogels have been around since the first aerogels were prepared, they were, for the most part, overlooked until the 1980′s when Lawrence Livermore National Laboratory scientists began producing organic aerogels made of phenolic resins. The bulk of this work was done by scientists Dr. Rick Pekala and Dr. Joe Satcher, who synthesized the first resorcinol-formaldehyde polymer aerogels (or RF aerogels for short).-essentially, aerogels composed of the same material as the plastic “Bakelite”. Depending on their density, RF aerogels range from light orange to deep red to black in color and range from translucent to opaque. Low density organic aerogels (<0.020 g cm^-3) are generally irreversibly squishy, similar in feel to green floral potting foam. High density organic aerogels (>0.5 g cm^-3) can be extremely robust and very hard to squeeze, almost like a car seat cushion.

CARBON:

Not long after RF aerogels were developed, Livermore scientists discovered that by heating them to temperatures of several hundred degrees Celsius in an inert atmosphere (such as nitrogen or argon), the polymer which makes up the aerogel can be dehydrated (or “pyrolyzed”) to leave behind an aerogel made of carbon! As the name might suggest, carbon aerogels are totally black and not transparent. To the touch they are very similar in consistency to activated charcoal like the type you’d use for an aquarium filter. Today, organic aerogels made from polymers of not only resorcinol but also melamine, phlorglucinol, and acetic acid are used to prepare carbon aerogels.


At the nanoscale, carbon aerogels are composed of nanoparticles of carbon with diameters approximately 1-2 nm. Like other aerogels, carbon aerogels are primarily mesoporous with a mean pore diameter of approximately 7-10 nm typical. Most
carbon aerogels have a surface area ranging from 500-800 m² g^-1 however this is highly dependent on density and whether or not other stuff has been introduced (intentionally or unintentionally) into the aerogel. The surface area of a carbon aerogel can be easily increased post-production by placing it under a flow of steam or hydrogen at elevated temperatures (400°C-1000°C). At these temperatures, water and hydrogen will react with carbon in the aerogel to form gaseous products and eat micropores (pores <2 nm in diameter) throughout the interior of the aerogel, thereby increasing their surface area up to 2 500 m² g^-1.
Carbon aerogels are exciting materials because they not only have surface areas ranging from about 500 to 2 500 m² g^-1 but they can also be electrically conductive! The combination of these two properties makes carbon aerogels valuable for applications which benefit from high-surface-area electrodes, such as supercapacitors, fuel cells, and desalination systems.