Silica Aerogel
Silica aerogels can be fabricated in many shapes and sizes, as well as being enclosed in other structures.
What are aerogels?
Aerogels are a unique class of open-pore polymer foams that can be prepared from either organic or inorganic precursors. Aerogels are typically prepared using sol-gel chemistry, starting with liquids that form tiny sol particles, which eventually connect together in chain-like fashion to make a semi-solid gel. When the solvent is removed from the wet gel and replaced by air in a manner that does not collapse the delicate gel structure, a solid aerogel results. What makes the aerogels unique is their nano-sized structure; they consist of particles and pores both of which are smaller than wavelengths of visible light. This unique nano-structure is responsible for many of the unusual acoustic, mechanical, optical and thermal properties observed for aerogels.
Silica aerogel
Silica aerogels are the most common of the aerogel varieties. The main reasons for their popularity are probably their unusual wispy look (they have been described as looking like “frozen smoke”) and their highly publicized applications in space experiments where they are used to insulate electronics on the MARS rovers and they have been used to collect space-dust and comet particles returned to EARTH for analysis. Silica aerogel is renowned as the lightest and lowest-density solid material known, at barely more than the density of air. Silica aerogel has a porous, sponge-like structure in which greater than 98 percent of its volume is empty space (air). By comparison, silica aerogel is 1,000 times less dense than glass. Even though the silica aerogel is mostly air, one of its remarkable features is that it can support more than 10,000 times its own weight. Silica aerogel can have a very large surface area within its bulk, ranging from 500 to 1500 square meters per gram, depending on its density. This means that if flattened-out, one inch cube of the aerogel would have more surface area than an entire football field! Silica aerogel is also known for its low density, low thermal conductivity, low sound speed, low refractive index and high transparency
Aerogel properties
With the lowest density, highest thermal insulation, lowest refractive index, dielectric constant and sound speed, and highest surface area per unit volume of any solid, aerogels exhibit an amazing range of properties which are not observed in any other kind of material. The following table summarizes physical properties of typical silica aerogels
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Properties
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Value
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Density
|
0.1 (0.3 - 0.05) g/cm3
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Dielectric Constant
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1.02 - 1.48 (@20 Ghz)
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Surface Area, BET
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800 m2/g
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Percent Solids
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0.5 - 14%
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Mean Pore Diameter
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~20 nm
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Primary Particle Diameter
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2 - 5 nm
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Index of Refraction
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1.002 - 1.063
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Thermal Tolerance
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to 500°C
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Poisson's Ratio
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0.24
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Young's Modulus
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10MPa - 0.01MPa
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Tensile Strength
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16 kPa
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Fracture Toughness
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~0.8 kPa*m1/2
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Compressive Modulus
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0.3 MPa
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Coefficient of Thermal
Expansion (CTE) |
2 ppm/C° @20 - 80°C
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Electrical Resistivity
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1015 ohm-cm
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Thermal Conductivity in Air
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0.016 W/m/°K
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Thermal Conductivity in
Vacuum |
0.004 W/m/°K
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Sound Velocity Through
the Medium |
< 200 m/sec
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Transparency
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>90% visible wavelengths
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Aerogel Applications
The unique combination of properties makes aerogels useful in a range of applications and best candidates for many others in the future.
Aerogels have been used for thermal insulation, super capacitors, water deionizers, sensors for gas detection, pollution filters, dielectric coupling layers integrated circuits, absorbents for desiccation, insecticides, dangerous liquid storage vessels, catalysis, matching layers for acoustical transducers, sound insulation, kinetic energy absorbers, sound insulation, impact protection, crucibles for molten metals, nuclear particle detectors, and many more.
