AerogelMystifying Blue Smoke

At first sight, aerogel resembles a hologram. Anexcellent insulator, aerogel has the lowest densityof any known solid — one form of this extraordi-nary substance is actually 99.8 percent air and0.2 percent silica dioxide (by volume). Aerogelshave open-pore structures similar to honeycomb,but in fact they are low-density, solid materialswith extremely fine microstructures. Typically sili-con-based like ordinary glass, or carbon-based likecommon organic synthetics, aerogels possessunique physical properties (see table). The uniquecharacteristics of aerogels are being applied tomeet new technological demands.

Aerogel was discovered in the late 1930s bychemist Samuel S. Kistler. Since then, numerousattempts have been made to further understandand develop it. At NASA’s Jet Propulsion Labora-tory (JPL), aerogel technology has found two pri-mary applications. The first is the capture of dustparticles from comet Wild 2 (pronounced “Vilt 2”)by the Stardust sample return mission. The par-ticles gradually decelerate as they bore deeper intothe threadlike silica network of Stardust’s gradient-density aerogel, so that the samples sustain mini-mal damage while being collected for return toEarth for scientific study in 2006.

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Property Silica Aerogel Silica Glass

Density (kgm3)

Refractive Index at632.8 nm

OpticalTransmittanceat 632.8 nm

Coefficient ofThermal Expansion1/C at 20–80 deg C

ThermalConductivity(W/mK) at 25 deg C

Sound Velocity (m/s)

Acoustic Impedance(kg/m2/s)

Electric Resistivity(ohm-cm)

Dielectric Constantat 3–40 GHz

Specific Surface Area(m2/g)

5 – 200

500 – 800

1.002 – 1.046

90%

~2 x 10–6

0.016 – 0.03

70 – 1300

104

1 x 1018

1.008 – 2.27

2300

0.1

1.514 – 1.644

99%

10 x 10–6

1.2

5000 – 6000

107

1 x 1015

4.0 – 6.75

JPL’s second application of aerogel is spacecraft in-sulation. Because aerogel is mostly air, an effectivethermal insulator is contained within its poroussilica network. This presents an excellent thermalbarrier to protect the spacecraft against the ex-treme cold of deep space. The Mars Pathfindermission used aerogel to protect the electronicsof the Sojourner rover against the frigid Martianenvironment during Sojourner’s 1997 travels onthe red planet. Each of the twin Mars ExplorationRovers, scheduled to land on Mars in early 2004,employs aerogel for thermal insulation of the bat-tery, electronics, and computer in the chassis, orwarm electronics box.

JPL is currently investigating a variety of futureapplications for aerogel. Discussions and experi-ments are ongoing regarding its use as a thermalinsulation material for light aircraft and spacecraft.Aerogel’s large surface area makes it a potentialbasis for chemical sensors. Aerogel may also finda home in areas such as sporting equipment andarchitectural design through corporate and aca-demic contacts.

For more information about aerogel, visithttp://stardust.jpl.nasa.gov

JPL 400-1119 9/03

Development Timeline

1930 1940 1980 1997 1999 2004 2004 2006 2008Aerogel discovered. Aerogel first

marketed commercially.Scientific usesof aerogel.

Sojourner insulation. Stardustlaunch.

Mars ExplorationRovers.

Stardust encounterwith comet Wild 2.

Stardust samplereturns to Earth.

Future uses.