Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
1998-11-02
2001-05-01
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C501S087000, C501S099000, C428S293400, C428S312600, C428S368000
Reexamination Certificate
active
06225248
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to high temperature, lightweight ceramic insulation such as porous carbon tile comprising carbon, silicon and oxygen. More particularly, the invention relates to a lightweight, ceramic carbon insulation comprising carbon, silicon, and oxygen which is capable of retaining its shape and strength when exposed to an oxidizing environment at temperatures as high as 1700° C. and to the method of preparation which comprises combining carbon substrates with a reaction product derived from the reaction of di- and trifunctional silanes to form a gel and, subsequently heating or pyrolyzing the gel and the carbon substrate, in an inert atmosphere, to form the high temperature, lightweight ceramic insulation.
Space vehicles such as the space shuttle, which leave and reenter the earth's atmosphere, require exterior thermal insulation. The operation of the space shuttle requires the development of lightweight and thermally efficient exterior insulation capable of withstanding a variety of environments. During reentry into the earth's atmosphere, the insulation must maintain the vehicle's exterior structure below 175° C. while experiencing substantial aeroconvective thermal environments which heat the surface of the insulation to temperatures in excess of 1,000° C. In space, the thermal protection must insulate the vehicle from the cold (e.g., −70° C.) experienced while in orbit. In addition to thermal and aeroconvective environments, the insulation must be able to withstand the mechanical stress associated with launch vibrations, acoustics, structural movement of the vehicle's surface, and landing impacts.
For example, lightweight ceramic state-of-the art thermal insulation tiles, as developed by Lockheed (LI-900) and NASA/Ames Research Center (AETB, AIM, FRCI, etc.), are limited to use-temperatures of about 1300° C. in an oxidizing environment. For applications which experience temperatures above 1300° C., a dense ceramic material must be used which adds a substantial weight penalty. Presently, thermal insulation used for protecting space vehicles includes both rigid and flexible ceramic insulation with a carbon composite being used on the leading edges of the vehicle. However, these ceramic carbon composites must be very porous in order to maintain their weight at a reasonably low level. This could be accomplished by using the ultra-high temperature, lightweight, ceramic carbon insulation of this invention.
DESCRIPTION OF THE PRIOR ART
In general, low-density insulations are required to thermally protect the structure of the space shuttle from the high temperatures normally encountered during atmospheric entry. The material developed for the space shuttle was a rigidized fibrous insulation, called reusable surface insulation (RSI). Its density and conductivity were optimized (minimum conductivity and weight) to keep the thermal protection system weight as low as possible, consistent with adequate mechanical properties to increase the resultant payload capability of the vehicle.
A characteristic of a successful insulation is high thermal shock resistance, which is required to survive the rapid temperature changes and high thermal gradients normally incurred during entry. The temperature limitations of prior materials and the desirability of improving their mechanical properties are reasons for developing alternative materials. There is also a need to develop alternative insulation systems for advanced earth-entry vehicles. These needs are relative to the state-of-the-art materials and include improved mechanical properties, higher temperature capability, equivalent thermal shock resistance, low thermal conductivity, and adequate morphological stability.
Presently, composite insulating materials intended for use on orbital reentry vehicles, such as the Space Shuttle, consist of a coating in combination with low-density insulation substrates. Examples of these composites and their use, incorporated herein by reference, are provided in Leiser et al., U.S. Pat. No. 4,148,962, issued Apr. 10, 1979; Fletcher et al., U.S. Pat. No. 3,953,646, issued Apr. 27, 1976; Fletcher et al., U.S. Pat. No. 3,955,034, issued May 4, 1976; and Johnson et al., U.S. Pat. No. 4,612,240, issued Sep. 16, 1986.
More specifically, details regarding ceramic insulations are disclosed, for example, in various other U.S. patents. Leiser et al., U.S. Pat. No. 5,618,766, issued Apr. 8, 1997, discloses lightweight ceramic compositions comprising a porous carbon preform. The carbon preform contains a tetralkoxy silane, a dialkoxy silane, and a trialkyl borate. Riccitiello et al., U.S. Pat. No. 4,713,275, issued Dec. 15, 1987, relates to a ceramic tile for use in a thermal protection system, employing a ceramic cloth having additional ceramic material deposited therein.
Jouffreau, U.S. Pat. No. 4,804,571, issued Feb. 14, 1989, relates to a thermal protection system for reentry vehicles or high speed aircraft including multiple refractory tiles of varying thickness defined by thermal requirements at the point of installation. Seibold et al., U.S. Pat. No. 4,100,322, issued Jul. 11, 1978, relates to a high thermal capacity fiber-resin-carbon composites having a polymer resin filler. The composite is prepared by impregnating a woven fabric of carbon or graphite yarn with a resin, curing the resin, pyrolyzing the impregnated fabric, and re-impregnating the resulting fiber-porous carbon char composite with resin.
Owens et al., U.S. Pat. No. 4,605,594, issued Aug. 12, 1986, relates to a ceramic article including a woven ceramic cloth having a non-porous core and a porous periphery prepared by treating with an acid. The porous periphery can be infiltrated with materials such as a metal, a metal oxide, a catalyst and an elastomer. The articles can be used as fiber optic elements, catalyst supports and heat resistant tiles for aerospace purposes. Gardner et al., U.S. Pat. No. 5,154,787, issued Oct. 13, 1992, describes a method of manufacturing prepreg mats. A prepreg strand formed of inorganic fibers impregnated with a thermoplastic binder or a ceramic matrix powder is heated, cooled and compacted to fuse into a uniform, dense prepreg. Geltman, U.S. Pat. No. 3,533,813, issued October 1970, relates to a low density, nonstructural ceramic employing a porous ceramic support in combination with organic fillers. The process includes burning off the organics to form pores within the ceramic. This process reduces the mass of the composite, thereby reducing its density while maintaining inherent strength.
SUMMARY OF THE INVENTION
This invention relates to lightweight, high-temperature, ceramic insulation, e.g., a carbon tile comprising carbon, silicon, and oxygen derived from the reaction of an organodialkoxy silane and an organotrialkoxy silane to form a sol-gel in the presence of a porous carbon substrate. More particularly, the invention relates to an oxidation resistant, ceramic carbon substrate containing carbon, silicon, and oxygen, and to the method of preparing a ceramic carbon insulation, e.g., a carbon tile capable of retaining its shape and strength when exposed to an oxidizing environment at temperatures in excess of 1200° C. The method of this invention comprises coating or impregnating a porous carbon substrate with a reaction product derived from the reaction of dialkoxy and trialkoxy silanes such as di- and tri-functional silane to form a gel, in situ, or in the presence of the porous carbon substrate followed by drying the carbon substrate and subsequently heating or pyrolyzing the infiltrated carbon substrate, in an inert atmosphere, to form the ceramic carbon insulation.
The preferred di- and tri-functional alkoxides include the silicon alkoxides having di- and tri-oxygen functionality in which the silicon alkoxide has two and three Si—O bonds, respectively. In some instances, the trialkoxy silane can be replaced with up to about 50% by weight with a tetralkoxy silane. Preferably, the tetralkoxy silane can replace the trialkoxy
Chen Timothy S.
Hsu Ming-Ta S.
Leiser Daniel B.
Group Karl
Mannix John G.
Padilla Robert M.
The United States of America as represented by the Administrator
Wong Carla M.
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