Aerogels are a class of ceramic materials fabricated from a sol-gel by carefully evacuating the solvent to leave a fragile polymer network which is 90-99% air by volume. Due to its structural and material properties, aerogels exhibit some remarkable properties. Silica aerogels have been fabricated with bulk densities in the range of 0.003-0.35 g/cm3, index of refractions from 1.0-1.05, thermal conductivities from 0.008-0.017 W/mK, and tensile strengths of 16 kPa or higher. They are one of the most insulating and least dense materials.

Aerogels have been used to make highly insulating materials such as blankets and weather jackets (Aspen Systems Inc. ). NASA, in conjunction with JPL, has sent aerogels to collect micrometeorite particles in space (Stardust , 1999). They also have the potential to be used as sensors for detecting chemical species (Hrubesh, 1998). There are many other potential applications yet to be discovered. Unfortunately the complexity and the time scale involved in the fabrication process limits its practicality. The solvent exchange is a complicated process and generally takes over 24 hours to complete, making large-scale implementation a challenging task.
Aerogels originate as sol gels. A sol gel is a colloidal suspension of particles in a network formed through a polymerization reaction. To make an aerogel the sol-gel solvent must be replaced by air. Aerogels can be made from any number of metallic, non-metalic precursor such as silicon, aluminum, titanium etc. This work is focused on the manufacture and performance of silica aerogels.

For each reaction, a precursor chemical and a hydrolyzing agent are necessary and an acid or base catalyst is typically required. The most common precursors are metal alkoxides, chosen because of their readiness to react with water as a hydrolyzing agent. For an alkoxide the byproducts will be an alcohol like ethanol or methanol. A simplified reaction is presented below:

(C2H5O)4Si + 2H2O --> SiO2 + 4C2H5OH (1)

where (C2H5O)4Si is Tetraethylorthosilicate, H2O is water and C2H5OH is ethanol. By controlling the concentrations and types of chemicals, properties such as density, thermal conductivity, and porosity can be controlled.

When the reaction takes place it begins to form a highly cross linked structure that consists of repeated monomer units of SiO2. This can result in a structure that consists of clumps of connected units or a single crystal that spans the whole container. When the sol gel reaction is complete, if the specimen is left to dry naturally it will form a xerogel (high density aerogel) and if it is processed at elevated temperature and pressure it can form aerogel. The distinction between an aerogel and xerogel rests on their densities, aerogel are typically 90-99% air while xerogel are 60-90% air.

The low density of an aerogel is due to the evacuation process that removes the alcohol from the gel and replaces it with air. This procedure requires techniques to overcome the surface tension that exists between the solid network and the solution. This can be done using a surfactant to reduce the surface tension (Lev, 1995) or removing the liquid-gas interface at cryogenic temperature or at supercritical temperatures where the solution has both liquid and gaseous properties (Poco, 1996). Another technique is to use a carbon dioxide super critical extraction in which the solvent, methanol, is replaced with carbon dioxide and then brought to supercritical state and evacuating the gel, leaving air in the pore spaces (see Wagh, 1999).

Ben Gauthier '02 worked with Professor Ann M. Anderson for his senior project last year and developed a new aerogel fabrication technique (see Gauthier, 2002). The technique is a one-step, precursor to aerogel, method that uses a mold and hydraulic hotpress to reach supercrical temperatures. This process has been identified as "Fast Supercritical Extraction Technique for Simplified Aerogel Fabrication". The aerogels obtained using this technique have not been characterized nor has the effect of chemistry on these properties been tested. The objective of this project is to characterize the thermal and structural properties of the aerogels and to use a design of experiments approach to study the effects of chemistry on the process and properties. Characterization will involve measuring the density, thermal conductivity and porosity of silica aerogels. The effects of chemistry have been studied by Venkateswara et al. (1997) who varied the solvent concentration and observed the resulting aerogels for cracks and electromagnetic transmissions for window applications. They evacuated the aerogels using nitrogen gas extraction technique where nitrogen gas was repeatedly flushed through the gels.

The project details are outlined below. The project schedule provides a description of what will be accomplished within a span of 20 weeks. The budget section gives an estimate of money that will be spent in order to carry out the experiments.