Fibrous-ceramic/aerogel composite tiles have been invented to afford combinations of thermal-insulation and mechanical properties superior to those attainable by making tiles of fibrous ceramics alone or aerogels alone. These lightweight tiles can be tailored to a variety of applications that range from insulating cryogenic tanks to protecting spacecraft against reentry heating.
The advantages and disadvantages of fibrous ceramics and aerogels can be summarized as follows:
- Tiles made of ceramic fibers are known for mechanical strength, toughness, and machinability. Fibrous ceramic tiles are highly effective as thermal insulators in a vacuum. However, undesirably, the porosity of these materials makes them permeable by gases, so that in the presence of air or other gases, convection and gas-phase conduction contribute to the effective thermal conductivity of the tiles.
- Other disadvantages of the porosity and permeability of fibrous ceramic tiles arise because gases (e.g., water vapor or cryogenic gases) can condense in pores. This condensation contributes to weight, and in the case of cryogenic systems, the heat of condensation undesirably adds to the heat flowing to the objects that one seeks to keep cold. Moreover, there is a risk of explosion associated with vaporization of previously condensed gas upon reheating.
Aerogels offer low permeability, low density, and low thermal conductivity, but are mechanically fragile. The basic idea of the present invention is to exploit the best features of fibrous ceramic tiles and aerogels. In a composite tile according to the invention, the fibrous ceramic serves as a matrix that mechanically supports the aerogel, while the aerogel serves as a low-conductivity, low-permeability filling that closes what would otherwise be the open pores of the fibrous ceramic. Because the aerogel eliminates or at least suppresses permeation by gas, gas-phase conduction, and convection, the thermal conductivity of such a composite — even at normal atmospheric pressure — is not much greater than that of the fibrous ceramic alone in a vacuum.
In a typical application, a composite tile according to the invention is made from an open-pore rigid ceramic-fiber tile preform by impregnating the preform with an aerogel part way through its thickness (see figure). The details of the impregnation process depend on the specific ceramic and aerogel materials, the desired thickness of the aerogelimpregnated layer, and the desired density of the aerogel. In general, one prepares an aerogel-precursor solution by mixing two component solutions. The preform is partially infiltrated with the precursor solution. The gelation reaction occurs spontaneously between the components of the solution at room temperature. To complete the process, the aerogel is dried in one or more subprocesses that can include fluid extraction at supercritical temperature and pressure, heating to a temperature above ambient but below the sintering temperature of the aerogel, venting, and/or purging with dry air.
This work was done by Susan M. White and Daniel J. Rasky of Ames Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Materials category.
Inquiries concerning rights for the commercial use of this invention should be addressed to
the Patent Counsel
Ames Research Center
Refer to ARC-12070.