Flower resting on a slab of aerogel over a flaming bunsen burner.

Science of Silica Aerogels

Physical Properties

Note: Most of the properties listed here are significantly affected by the conditions used to prepare the aerogel and any subsequent post-processing.

Property Value Comments
Apparent Density 0.003-0.35 g/cm3 Most common density is ~0.1g/cm3.
Internal Surface Area 600-1000 m2/g As determined by nitrogen adsorption/desorption.
% Solids 0.13-15% Typically 5% (95% free space).
Mean Pore Diameter ~20 nm As determined by nitrogen adsorption/desorption (varies with density).
Primary Particle Diameter 2-5 nm Determined by electron microscopy.
Index of Refraction 1.0-1.05 Very low for a solid material.
Thermal Tolerance to 500 C Shrinkage begins slowly at 500 C, increases with inc. temperature. Melting point is >1200 C.
Coefficient of Thermal Expansion 2.0-4.0 x 10-6 Determined using ultrasonic methods.
Poisson's Ratio 0.2 Independent of density. Similar to dense silica.
Young's Modulus 106-107 N/m2 Very small (<104x) compared to dense silica.
Tensile Strength 16 kPa For density = 0.1 g/cm3.
Fracture Toughness ~0.8 kPa*m1/2 For density = 0.1 g/cm3. Determined by 3-point bending.
Dielectric Constant ~1.1 For density = 0.1 g/cm3. Very low for a solid material.
Sound Velocity Through the Medium 100 m/sec For density = 0.07 g/cm3. One of the lowest velocities for a solid material.

Optical Spectrum

Thermal Conductivity