Compositions: ceramic – Ceramic compositions – Pore-forming
Reexamination Certificate
2000-01-24
2002-01-15
Marcantoni, Paul (Department: 1755)
Compositions: ceramic
Ceramic compositions
Pore-forming
C501S085000, C501S133000, C423S338000
Reexamination Certificate
active
06339034
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
This invention relates to an ultra high-temperature, lightweight ceramic insulation such as porous ceramic tile containing silicon, carbon and oxygen. More particularly, the invention relates to a lightweight, ceramic insulation containing silicon, carbon and oxygen capable of retaining its shape, strength and physical properties when exposed to an oxidizing environment at temperatures as high as 1700° C. More specifically, this invention relates to the sol-gel process of preparing ceramic insulation which comprises an insulation product derived from the reaction of multifunctional silanes to form a small-pore, wet siloxane gel followed by drying the gel at ambient pressures with limited shrinkage and, subsequently heating or pyrolyzing the dried gel, in an inert atmosphere, to form the high-temperature, lightweight ceramic insulation.
An important use for the insulation of this invention includes, for example, the space vehicles, such as the space shuttle which leaves and reenter the earth's atmosphere and therefore requires 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 surface of the vehicle, and the landing impact.
The use of state-of-the art thermal insulation, lightweight ceramic tiles, developed by Lockheed (LI-900), and NASA/Ames Research Center (AETB, AIM, FRCI, etc.), are all limited to temperatures of use at about 1300° C. in an oxidizing environment. For applications which experience temperatures above 1300° C., a dense ceramic insulation must be used which adds a substantial weight penalty. Presently, the thermal insulation used for protecting space vehicles includes both the 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 the weight at a reasonable low level. This can be accomplished by using the ultrahigh-temperature, stable, lightweight ceramic insulation of this invention.
2. 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 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 the high-thermal shock resistance, required to survive the rapid temperature changes and high thermal gradients normally incurred during entry. The temperature limitation of the prior materials and the desirability to improve the mechanical properties of these materials are the reasons to develop alternate materials. There is a need to develop alternate 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 are provided in U.S. Pat. No. 4,148,962, issued Apr. 10, 1979; U.S. Pat. No. 3,955,034, issued May 4, 1976, and U.S. Pat. No, 4,612,240, issued Sept. 16, 1986.
More specifically, details regarding prior ceramic insulations are disclosed in various other U.S. patents. For example, U.S. Pat. No. 5,618,766 discloses lightweight ceramic compositions comprising a porous carbon preform. The carbon preform contains a tetralkoxy silane, a dialkoxy silane and a trialkyl borate. Pat. No. 4,713,275 relates to a ceramic tile for use in a thermal protection system, employing a ceramic cloth having additional ceramic material deposited therein. Pat. No. 4,804,571 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. Pat. No. 4,100,322 relates to a high thermal capacity fiber-resin-carbon composite 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. Pat. No. 4,605,594 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 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. Pat. No. 3,533,813 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.
Further, it is generally known in the art that a gel consists of a tenuous solid skeleton immersed in liquid. When the liquid is supercritically extracted in an autoclave, the skeleton is called an aerogel, and the volume formerly occupied by liquid becomes porosity. If the wet gels are dried at ambient pressure, the gels shrink to a small dry gel with higher density. Silica aerogels are porous, and are very good insulation materials, prepared from wet gels (alcogels) by the supercritical drying method. U.S. Pat. No. 4,327,065 describes the method of preparing silica aerogel. The preparation is effected by means of hydrolysis of a tetraalkoxysilane in an alcohol, in the presence of a catalyst for the formation of an alcogel which is aged and washed with alcohol to remove water. The alcogel is thereafter treated in an autoclave by means of a temperature increase to above the critical point of the alcohol, isothermic pressure drop by means of the release of alcohol vapor, and then a temperature drop. U.S. Pat. No. 4,610,863 describes an improved supercritical drying process for forming silical aerogel. The improvement includes the additional step, after alcogels are formed, of substituting a solvent, such as CO
2
, for the alcohol in the alcogels, the solvent having a critical temperature less than the critical temperature of the alcohol. The resulting gels are dried at a supercritical temperature for the selected solvent, such as CO
2
.
SUMMARY OF THE INVENTION
In comparison to the teaching's of the prior art, this invention relates to lightweight, high-temperature ceramic insulation comprising atoms of silicon, carbon and oxygen derived from the reaction of at least one organodialkoxy silane and at least one tetraalkoxy silane to form a siloxane gel in the presence of a catalyst in a liquid medium such as alcohol. In addition, the reaction mixture can include effective amounts of at least o
Chen Timothy S.
Hsu Ming-Ta S.
HC Chem Research & Service Corp.
Marcantoni Paul
Tura James V.
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