Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
2002-08-16
2004-08-03
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C501S095200, C501S087000, C428S293400, C428S293700, C428S478800, C442S077000, C442S140000, C442S178000, C264S640000, C264S643000
Reexamination Certificate
active
06770584
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a hybrid insulation material composed of aerogel and rigid ceramic fiber materials and methods for their production, including ceramic fiber insulating materials impregnated with aerogel and other nanoporous materials. More specifically, the invention relates to low-conductivity ceramic fiber insulating materials for use on reusable launch vehicles.
BACKGROUND OF THE INVENTION
Reusable launch vehicles (RLVs), such as the space shuttle, repeatedly travel into or beyond the Earth's upper atmosphere and then return to the Earth's surface. During flight, the RLVs experience extreme temperatures, ranging from −250° F. while in orbit to over 3000° F. upon reentry to the atmosphere. Because of the extreme temperatures, the vehicle and its contents must be protected by a thermal protection system. The thermal protection system is an outer covering of insulation, the purpose of which is to prevent the body of the vehicle from reaching a certain maximum temperature. For the space shuttle, the maximum temperature is about 450° F., the temperature at which the aluminum structure of the shuttle begins to weaken.
Thermal protection systems for RLVs are constructed from a large number, usually several thousand, of insulative rigid tiles and blankets. The tiles, which are used mostly on the lower surface due to its smoother surface, function to insulate the vehicle from the environment and to radiate and reflect heat from the vehicle. In addition to protecting the vehicle from environmental heat sources, the insulative tiles also provide protection from localized heating from such sources such as the vehicle's main engine, rocket boosters and directional thrusters.
RLVs such as the space shuttle typically utilize a variety of tiles to cover the lower surface of the vehicle. Different areas of the vehicle encounter different heat profiles and different physical stresses during flight. Therefore, a variety of tiles having different compositions, densities, and coatings are placed at different positions of the vehicle depending on if such positions are leeward or windward, upper or lower surfaces, etc. The most predominate tiles used today on lower surface are Lockheed Insulation (LI) and Alumina Enhanced Thermal Barrier (AETB) are used on the base heat shield due to its relatively higher thermal conductivity.
The Lockheed Insulation materials are comprised of high purity amorphous silica fiber. To produce the Lockheed Insulation, a slurry of the silica fibers having a diameter of 1 to 3 &mgr;m is formed in deionized water with a V-blender. The slurry is mixed with ammonia and stabilized colloidal silica solution after which it is placed in a casting tower where it is dewatered and slightly pressed to remove a portion of the water. The partially dried slurry is heated to a temperature of 250° F. to remove the remaining residual water. The dried silica composition is then fired to a temperature of up to 2300° F., which causes the colloidal silica to sinter the silica fibers to one another. The resulting insulative material is a low density mass of randomly arranged fused silica fibers. By selectively pressing the silica fiber slurry and subjecting to different firing temperatures, various densities of the resulting dry silica material may be produced. The Lockheed Insulation tiles are marketed under the trade names LI-900™, LI-1500™ and LI-2200™, having densities of 9 lb/ft
3
, 15 lb/ft
3
and 22 lb/ft
3
, respectively.
The Alumina Enhanced Thermal Barrier (AETB) consists of about 68 percent silica fiber, about 12 percent Nextel fiber (a combination of alumina, silica, and borate), about 20 percent alumina fiber, and about 2 percent silicon carbide. The fiber diameter ranges from 1 to 3 &mgr;m for silica and alumina fibers, and from 5 to 10 micron for Nextel fibers. The processing is very similar to the Lockheed Insulation. Colloidal silica is not added to the AETB material before firing. Instead, high temperatures experienced during firing cause the borate within the Nextel fiber to form boron oxide, which fuses to the fibers and sinters the ceramic fibers to one another. The AETB material is commonly marketed in the forms of AETB-8™, AETB-12™, AETB-16™, and AETB-20™ tiles, having densities of 8 lb/ft
3
, 12 lb/ft
3
, 16 lb/ft
3
and 20 lb/ft
3
respectively.
Because of its extraordinary low thermal conductivity, LI-900™ insulation tiles are used on the lower surface of most RLVS. The pure silica fiber skeleton of LI-900™ tiles is capable of remaining in tact up to temperatures of 2500° F., which exceeds the maximum temperature (2300° F.) experienced by RLVs during reentry into the Earth's atmosphere. LI-900™ insulation, however, suffers from two main disadvantages. First, it suffers from severe shrinkage after exposure to temperatures above 2500° F. and for long periods of time. Shrinkage along the mold line of the RLV leads to widening gaps between the insulating tiles as well as surface recession and thus increases heating at the inner mold line. Second, LI-900™ and other Lockheed Insulations are not compatible with the tough coating, TUFI (toughened unipiece fibrous insulation) which is needed for improved surface durability. Application of TUFI coating results in slumping of the pure silica insulation. Because of incompatibility with the tough coating, LI-900™ materials are easily susceptible to damages during flight or servicing of the RLV.
Unlike LI-900™ insulation, the AETB material is compatible with the TUFI coating. As a result, the AETB is a much more durable tile which requires less frequent replacement. AETB, however, is more thermally conductive than the Lockheed Insulation materials. As a result of the increased thermal conductivity, the AETB material is unable to protect the RLV substructure from temperatures experienced during reentry. Therefore, AETB may not be used on much of the lower surface of the RLVs.
What is needed is a ceramic fiber insulative material having the same or lower thermal conductivity found in LI-900™ insulation while exhibiting the durability, strength and dimensional stability of AETB tile material.
SUMMARY OF THE INVENTION
The present invention is an insulating material for use in extreme temperatures having a variety of applications, but designed for the protection of reusable launch vehicles (RLVs). The insulating material is a unique combination of a substrate of sintered ceramic fibers which form a low density, highly porous material and an aerogel or other nanoporous material which impregnates at least a portion of the porous ceramic substrate. The resulting insulation has very low thermal conductivity (lower than a LI900 tile). Additionally, the insulation exhibits sufficient tensile strength, good dimensional stability, and good compatibility with the TUFI coating to withstand damage typically suffered during flight and servicing of the RLV.
The basis of the invention is the combination of a porous ceramic tile substrate with a low density nanoporous material such as silica- or alumina-based aerogel. The porous tile substrate of one embodiment includes 60 to 80 wt % silica (SiO
2
) fibers, 20 to 40 wt % alumina (Al
2
O
3
) fibers, and with 0.1 to 1.0 wt % boron-containing constituent as the sintering powders. The silica-based or alumina-based nanoporous material typically has a density of about 1.0 lb/ft
3
to about 10 lb/ft
3
.
The boron-containing constituent contained in the tile substrate provides boron-containing by-products which act to fuse and sinter the silica and alumina fibers of the substrate when heated. No supplemental binder is required during production of the insulative material
A preferred embodiment of the tile substrate composition is 65 wt % to 75 wt % silica fibers, 25 wt % to 35 wt % alumina fibers, and 0.1 wt % to 0.5 wt % boron-containing constituent. A particularly preferred tile substrate composition is 67 wt % silica fibers, 32.75 wt % alumina fibers, and 0.25 wt % boron-containing powders such as boron carbide (B
4
C).
The tile substrate materi
Barney Andrea O.
Droege Michael
Heng Vann
Oka Kris Shigeko
Santos Maryann
Group Karl
The Boeing Company
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