What Is an Aerogel?

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Aerogels are materials that are incredibly light, anywhere from a thousandth to half the density of water. There’s even an aerogel that is only 3 times heavier than air! The only reason it doesn’t float is because the air in its pores weighs it down. So what makes aerogels special?

What’s an aerogel?

Imagine a block of Jell-O. Have you ever considered how it sticks together so well, despite being mostly water? Gels like Jell-O have a solid framework holding everything together, with most of its mass and volume coming from water. If we were to dry out the block of Jell-O, we would be left with just the structure. An aerogel is like this, but harder. “Aerogel” refers to a geometry of a material and is not its own substance. Numerous types of aerogels exist, including those with organic polymers, metals, and metal oxides. These materials exhibit many fascinating properties.

Properties of Aerogels

  1. Porous
  2. High specific surface area
  3. Extremely low density
  4. Thermal insulator
  5. Electrical insulator
  6. Sound insulating
  7. Low optical index of refraction
  8. Brittle

Details

  1. Pore diameters can range from 1 to 100 nanometers (nm; billionths of a meter). Most aerogels are mesoporous, meaning that they contain pores ranging from 2 to 50 nm in diameter. Since these materials are so porous, they contain mostly air.
  2. The pores give aerogels a very high surface area. A special type of carbon aerogel has a specific surface area (per unit mass) of 3200 square meters per gram (or 1.5 football fields in the space of a paperclip)!
  3. Aerogels contain mostly air, so they have low densities ranging from 0.0011 to about 0.5 g/cm3 (for reference, the density of air is about 0.0012 g/cm3).
  4. Since they have high surface areas, heat can dissipate easily. This makes them useful for spacecraft.
  5. They conduct electricity about as well as a vacuum (dielectric constants for a vacuum and aerogel are 1 and 1.008, respectively). I.e., they essentially conduct nothing.
  6. Sound travels at different speeds depending on the medium through which it travels. Sound travels more slowly in water than air, for example. Through aerogels, it travels about a fifth as fast as it does through air (340 meters per second in air, 70 in aerogels).
  7. Light also travels at different speeds depending on the medium through which it passes. Everything has a property called the index of refraction (n). The higher n is, the slower light travels. We see this as the light “bending” in a material. In aerogels, light travels nearly as fast as it does in air (n for air is 1.0002; n for aerogels is 1.002).
  8. If a load is applied gently, it can support up to 2,000 times its own weight. However, if you poke it, it’ll dent. Aerogels are fragile and can be snapped easily, even by the pressure of one’s fingers pinching them. X-aerogels, however, are much stronger and more flexible than regular aerogels.

X-Aerogels

These are specially prepared aerogels made by cross-linking polymers. X-aerogels are much stronger and flexible than their regular aerogel counterparts. They are more dangerous and costly to make, but there are some advantages to using X-aerogels over aerogels.

Properties of X-Aerogels

  1. Flexible
  2. Waterproof
  3. Less insulating/less surface area
  4. Impact resistant/increased strength
  5. Increased stiffness
  6. Denser than aerogels
  7. Translucent to opaque

Details

  1. Since these are not as brittle as aerogels, they can bend without fracturing.
  2. Can deflect more water due to increased specific surface area.
  3. These have about half as much surface area as aerogels, which reduces their thermal insulating properties. They are about as insulating as styrofoam.
  4. Cross-linking polymers increases toughness, or amount of energy the material can absorb without fracturing. Have you ever wondered how the shuttle didn’t burn up when it re-entered our atmosphere? Enter: aerogels! These amazing materials have been around for decades, but what exactly are they like? Since they can absorb more energy, they can withstand impacts more easily.
  5. This does not contradict its increased flexibility. As a whole, the pieces of X-aerogel can bend, but the material itself is denser and thus more stiff.
  6. Density increases to anywhere from 0.33 to 0.5 the density of water.
  7. Cross-linking and increased surface area contribute to decrease of clarity.

Applications of aerogels

An “X” will be in parentheses next to the application when referring solely to X-aerogels. Anything else will include both types of aerogels.

  • Insulation
  • Soundproofing
  • Rocketry
  • Aircraft
  • Artificial skin
  • Non-deflatable tires (X)
  • Armor (x)
  • Membranes for battery and fuel cells (X)
  • Optical sensors

For more on aerogels, check out aerogel.org. To find out how they are made, check out this article by HowStuffWorks.

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