Plastic and nonmetallic article shaping or treating: processes – Vacuum treatment of work
Reexamination Certificate
2000-03-29
2001-07-10
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Vacuum treatment of work
C264S232000, C264S234000, C264S299000, C264S319000, C264S621000
Reexamination Certificate
active
06258305
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates generally to the field of aerogels, and more particularly to a method of using aerogels to form net-shaped materials and perform net-shaping.
Aerogels are unique solids with up to 99% porosity. Such large porosities confer a number of useful properties on aerogels, including high surface area, low refractive index, low dielectric constant, low thermal-loss coefficient, and low sound velocity. To date, however, the potential of aerogels has not generally been realized in these applications because conventional supercritical aerogel processing is energy intensive and often dangerous. Silica aerogels, with thermal conductivities as low as 0.02 W/mK, have potential utility in superinsulation systems.
Traditionally, aerogels are made by processes whereby the liquid contained within the continuous network of pores of a gelatinous solid is replaced by air. Typically, this is achieved by supercritical solvent extraction, i.e., by placing the gel in an autoclave where the temperature and pressure is increased above the critical point of the liquid phase. This process was initially proposed by Kistler (Kistler, U.S. Pat. No. 2,249,767) to avoid the shrinkage and cracking of porous materials (water filled) due to capillary forces generated during simple evaporative drying. Improvements to Kistler's process were developed. Notably, Nicolaon and Teichner (Nicolaon et al., U.S. Pat. No. 3,672,833) supercritically dried silica gels under conditions exceeding the critical point (240° C., 78.5 atm) of the methanol solvent contained within the pores of a gel. Tewari and Hunt (U.S. Pat. No. 4,610,863) developed a process whereby the initial pore fluid (alcohol) is exchanged for carbon dioxide (31° C., 72.9 atm), thus reducing the temperature required for processing and enhancing process safety by the elimination of flammable solvents at high pressure.
In another advance in aerogel processing, Deshpande et al. (Deshpande et al., U.S. Pat. No. 5,565,142; incorporated herein by reference) describe a means for surface modification of the wet precursor gel to change the contact angle of the fluid meniscus in the pores during drying to avoid shrinkage of the gel. In another advance in aerogel processing, Brinker et al. (Brinker et al., U.S. Pat. No. 5,948,482; incorporated herein by reference) describe a low temperature/pressure (LTP) process to form thin films, eliminating the need for supercritical processing by chemical derivatization of the wet gel surface, followed by simple drying under ambient temperature and pressure conditions. The chemical surface treatment causes the drying shrinkage of the thin films to be reversible: during drying the gel thin film shrinks, then re-expands to recreate the porosity and volume of the wet gel state.
Because aerogels are made by sol-gel processing, their microstructure can be tailored to optimize properties desired for specific applications. Various precursors, including metal alkoxides, colloidal suspensions, and a combination of both under several mechanisms of gelation may be used to synthesize gels. Aerogels can also be made from wet precursor gels that contain both inorganic and organic components or from organic gels. For the composite gels, the organic and inorganic phases can be mixed on different length scales such that the organic component resides solely on the internal pore surface, is incorporated into the spanning gel structure, or forms a separate gel structure from the inorganic phase.
Some applications, such as insulation with a cavity of a complex shape, require materials that can form to the shape of the mold or cavity and provide desired properties. In some applications, foams are suitable materials for such uses. Aerogels, with their high porosity and low thermal conductivities, could be used for such applications. However, aerogels have not been used because the inherent limitations of conventional supercritical routes to aerogels, such as high pressure autoclave processing and difficult processing of large or complex shapes, have contributed to high processing costs and thus have severely restricted successful commercial development of aerogel processes for these type of applications.
Virtually all existing aerogel processes for the fabrication of bulk aerogel materials, including supercritical processing and low temperature/pressure (LTP) processing, depend upon expensive molding and machining techniques to fabricate parts with controlled geometries. For example, to obtain shaped articles using conventional processes, one must cast the sol into a suitable mold and process the gel in a pressure chamber (such as an autoclave) that is large enough to contain the molded shape. These processes are suitable, albeit expensive, for small simple shapes; however, they are unsuitable for complex shapes or for applications that demand cost-effective manufacturing. For most applications, current techniques impose severe restrictions to facile manufacturing; for example, molding technology requires precision mold machining, casting technology, mold material compatibility, custom mold design, effective mold release agents, and controlled part shrinkage while cost-effective machining of aerogels is difficult due to their fragility. Some of these limitations can be overcome by the use of granular aerogel materials, which are more manufacturable but are unsuitable for many applications requiring bulk shapes because of the large voids between granules.
Useful would be a method of preparing net-shape aerogel materials for a range of applications that avoids the disadvantages and limitations inherent in conventional supercritical processing and exploits and improves upon the advantages of recent low temperature/pressure (LTP) processing.
SUMMARY OF THE INVENTION
According to the present invention, a method of net-shaping using aerogel materials is provided by forming a sol, aging the sol to form a gel, the gel having a fluid component, forming the gel into a final shaped gel material, derivatizing the final shaped gel material to render the material unreactive toward further condensation, removing a portion of the fluid component of the final shaped gel material to form a partially-dried medium, placing the partially-dried medium into a cavity, with the volume of the medium being less than the volume of the cavity, and removing a portion of the fluid component of the medium. This removal causes the volume of the medium to increase, thereby forming a net-shaped material. The steps of forming a gel and a final shaped gel material can take place in any order after the formation of the sol. The sol comprises at least one metal oxide, metalloid oxide, hydroxide, alkoxide, oxohydroxide, or oxoalkoxide. The aging occurs by a method selected from the group consisting of heating the sol, adding an acid to the sol, and adding a base to the sol. The step of derivatizing the final shaped gel material is performed by adding a derivatizing agent, where the derivatizing agent can be an organofunctional silane, an alcohol amine, a carboxylic acid, and a &bgr;-diketonate, such as trimethylchlorosilane and hexamethyidisilazane. The fluid component can be removed by heating, applying a vacuum, or both. The final shaped gel material can be a variety of materials, including a powder, bulk material, and granules. Upon adding a solvent to the net-shaped material, the material is again reduced in volume, thereby facilitating removal of the material from said cavity.
In another embodiment, a method is provided to first form a sol, the sol is aged to form a gel, the gel is then derivatized to render the material unreactive toward further condensation and the gel is formed into a final shaped gel material, thereby creating newly-exposed surfaces. These surfaces can be optionally be derivatized again. A portion of the fluid component of the final shaped gel material is removed to form a partially-dried medium, with the partially-dried medium having the property of increasing in volume upon subsequent drying. This medium can be s
Ashey Carol S.
Brinker C. Jeffrey
Harris Thomas M.
Reed Scott T.
Sriram Chunangad S.
Fiorilla Christopher A.
Klavetter Elmer A.
Sandia Corporation
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