Improved Silica Aerogel Composite Materials
- Wednesday, 27 August 2008
Shrinkage and cracking are greatly reduced.
A family of aerogel-matrix composite materials having thermal-stability and mechanical-integrity properties better than those of neat aerogels has been developed. Aerogels are known to be excellent thermal- and acoustic-insulation materials because of their molecular-scale porosity, but heretofore, the use of aerogels has been inhibited by two factors:
- Their brittleness makes processing and handling difficult.
- They shrink during production and shrink more when heated to high temperatures during use. The shrinkage and the consequent cracking make it difficult to use them to encapsulate objects in thermal-insulation materials.
A material in the present family consists of a silica aerogel matrix reinforced with silica fibers and silica powder. The density of this composite material is typically only about 10 percent greater than the density of the corresponding neat aerogel. The underlying concept of aerogel-matrix composites is not new; the novelty of the present family of materials lies in formulations and processes that result in superior properties, which include (1) much less shrinkage during a supercritical-drying process employed in producing a typical aerogel, (2) much less shrinkage during exposure to high temperatures, and (3) as a result of the reduction in shrinkage, much less or even no cracking.
Synthesis of a composite aerogel of this type is based on a sol-gel process. The first step is to make a silica sol by refluxing and distilling a mixture of silicon alkoxide (tetramethyl ortho silicate, tetraethyl ortho silicate), a suitable solvent (methanol, ethanol), water, and nitric acid. The resultant concentrated sol is then diluted with acetonitrile. The second step is to prepare a solution for casting the composite aerogel: Fumed silica (325-mesh powder having specific surface area of about 200 m2/g) and silica powder (particle sizes between 1 and 2 μm) are suspended in acetonitrile and then the silica sol, water, and ammonium hydroxide base are added to the acetonitrile/powder suspension. The amount of each component can be adjusted to suit a specific application. After thus preparing the aerogel-casting solution, a piece of silica fiber felt (destined to become the fiber reinforcement in the composite) is placed in a mold. Then the aerogel-casting solution is poured into the mold, where it permeates the silica fiber felt (see Figure 1). After the solution has gelled, the casting is transferred to an autoclave filled with acetonitrile, wherein the casting is subjected to supercritical drying at a temperature of 295 °C and pressure of 5.5 MPa.
Heretofore, neat silica aerogels had been observed to undergo linear shrinkages between 5 and 10 percent upon supercritical drying. In tests of a composite of the present type, the incorporation of the silica fiber felt has been found to reduce the shrinkage to a negligible level (see Figure 2). The silica fiber felt seems to strengthen the aerogel and to serve as rigid framework that prevents shrinkage. It has been conjectured that the silica fiber felt divides the volume of the casting into small subvolumes, thereby confining strain to relatively small unit spaces (between fibers) instead of allowing strain to act over relatively large (millimeter to centimeter) lengths.
In other tests, a neat aerogel exhibited linear shrinkage of about 6 percent after exposure to a temperature of 1,000 °C in a vacuum for four hours, and an even greater shrinkage (about 50 percent) after four hours at 1,000 °C in air. In contrast, a composite aerogel of the present type exhibited no apparent shrinkage after 1 week at 1,000 °C in a vacuum, and a linear shrinkage of only about 2 percent after a week at 1,000 °C in air.
This work was done by Jong-Ah Paik, Jeffrey Sakamoto, and Steven Jones of Caltech for NASA’s Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
Refer to NPO-44287, volume and number of this NASA Tech Briefs issue, and the page number.
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