Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
2002-03-08
2004-07-20
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
C423S324000
Reexamination Certificate
active
06764667
ABSTRACT:
BACKGROUND OF THE INVENTION
Aerogel is a low-density, nanoporous solid material composed of 50 to >99% air by volume. Aerogel is typically prepared by supercritically extracting the liquid medium from gel in such a way that the gel's solid matrix is isolated without collapsing due to capillary forces. Aerogel typically shows densities ranging between 0.5 g/cm
3
to 0.001 g/cm
3
, yet at the same time can support between 500 and 4,000 times its weight in an applied force distributed across its surface area. Internal surface areas in aerogels range from 250 m
2
/g to 2,500 m
2
/g, giving them excellent insulative and impact-absorbing capabilities.
Aerogels are prepared by supercritically drying precursor gels called alcogels. A gel is a colloidal system in which a network of interconnected solid particles spans the volume of a liquid medium. In the most researched type of aerogel, silica aerogel, this gel is composed of a silica matrix spanning a solvent such as ethanol or acetone.
Modern aerogel is synthesized by the sol-gel process, in which a metal alkoxide is reacted with water in a polar organic solvent to yield a colloidal system of oligomeric silica clusters suspended in the solvent, or a sol, which, upon further polymerization, forms a gel.
Aerogel was discovered by Samuel Steven Kistler in the 1930s at the College of the Pacific (Stanford University)
1
. Aerogel was only researched extensively, however, in the 1980s when NASA investigated it as a possible medium for capturing micrometeoroids during space flights. Since then, aerogel production has been significantly improved, and has made aerogel popular in numerous applications industrially, commercially, and in research. NASA used aerogel on the Mars Pathfinder probe to survey the atmosphere of Mars, and has used it on several probes as micrometeor sponge.
1
NASA Jet Propulsion Laboratory.
Aerogel
. March 1996.
Despite its useful properties, aerogel's use has been greatly limited by the expense and difficulty of its manufacture. In addition, aerogel's blue color has prevented it from being usable in applications such as windowpanes and other transparent insulation applications.
Preparation of Alcogel
Silica aerogel, is typically formed by the reaction of a silicon alkoxide with water to form an alcogel through the sol-gel process. Alcogel can then be supercritically dried to leave behind the silica matrix that gave the precursor alcogel its rigidity. The resulting low-density solid material is aerogel. Examples of silicon alkoxides include tetramethyl orthosilicate and tetraethyl orthosilicate.
The formation of the alcogel is the step in which the physical and nanostructural properties of the aerogel are defined. Since the polymer matrix of which the aerogel will be composed is assembled during the gelation process, the chemistry of the alcogel solution can be adjusted to give specific properties to the derivative aerogel.
Silica alcogel will be used as the subject of discussion for chemical reactions, since it is of most relevance to the method of the invention.
The net reaction for the formation of the alcogel is as follows
2
:
nMe(OR)
4
+4nH
2
O→nMe(OH)
4
+4nROH Hydrolysis (1)
nMe(OH
4
)→nMeO
2
+2nH
2
O Condensation (2)
R=alkyl group (i.e. C
2
H
5
)
The metal alkoxide reacts with four moles of water yielding hydroxyl groups and four moles of alcohol. The hydroxyl groups then condense to produce a metal oxide and water. This happens in a three-step reaction mechanism
2
.
≡MeOR+HOH
≡MeOH+ROH Hydrolysis (1)
≡MeOR+HOMe
≡MeOMe≡+ROH Alcohol Condensation (2)
≡MeOH+HOSi
≡MeOMe≡+HOH Water Condensation (3)
2
Tillotson, Thomas M. and Hrubesh, Lawrence.
As the reactions occur, the solution forms a sol of silica clusters capped by alkyl and hydroxyl groups. These clusters can be made to interconnect to form a matrix throughout the liquid medium thus forming a gel. This process, known as the sol-gel process, allows for metal oxide to carefully form and interconnect by network-type bonds. After gelation occurs, the alcogel is aged and soaked in an organic solvent for several days to remove any unwanted water, catalyst, and alcohol.
Once an alcogel has been diffused with a pure solvent, preferably a non-alcoholic solvent such as toluene, it can be dried to produce an aerogel. Upon evaporation, like gelatin, a gel will condense into a hard glass-like material. As liquid is evaporated out of the silica matrix of an alcogel, hydroxyl groups lining the edges of the matrix interact by weak hydrogen bonding and stick together. As capillary action pulls the matrix inward, the hydrogen bonding causes the matrix to stick and collapse. There are ways to prevent the solid matrix from collapsing, however, such as supercritically extracting the solvent from the gel.
The supercritical extraction process requires that the gel's solvent be brought past its critical point—the temperature and pressure that once reached, the substance cannot condense into a liquid by adding further pressure. Such supercritical fluids have extremely rapid diffusion rates, and possess some properties similar to liquids, such as density, and other properties similar to gases, such as expansion
3
. Supercritical extraction allows for removal of the liquid solvent from the gel while still providing the physical support needed to prevent the solid matrix from collapsing. At critical point, the solvent in the gel can diffuse into the surroundings without disrupting the structure of the silica matrix be vented and replaced with air. Once the system is cooled and depressurized, the gel's solid matrix is left intact. The remaining solid is a network of amorphous particles linked in a superstrong matrix, in the shape of the original alcogel and with approximately the same volume.
3
Pardue, Harry and Bodner, George.
Chemistry: The Experimental Science
. New York, N.Y.: John Wiley and Sons, Inc. Copyright 1995.
One problem with the supercritical extraction is that organic solvents used in the preparation of the alcogels not only have high critical temperatures and pressures, but are extremely flammable at those conditions and are very dangerous to work with. Arlon Hunt of Lawrence Berkeley National Laboratory demonstrated that a solvent exchange with liquid CO
2
can be performed prior to extraction of the solvent from the gel to greatly reduce the danger associated with the extraction process. Carbon dioxide has the benefits of being non-flammable, miscible with organics, and having a low critical point of 31.1° C. at 75 atmospheres
4
. For the supercritical drying to succeed, alcogel must soak in liquid CO
2
long enough for the CO
2
to completely diffuse through the gel and take the place of the organic solvent. The CO
2
can then be brought to supercritical temperatures and pressures for solvent extraction, instead of the organic solvent.
4
Weast, Robert ed.
Handbook of Chemistry
&
Physics,
48
th Edition
. Cleveland, Ohio: Chemical Rubber Co. Copyright 1967.
Origin of Rayleigh Scattering in Aerogels and Ways to Reduce it
Nearly all silica aerogels exhibit a blue appearance despite being transparent. The blue color is a result of Rayleigh scattering of white light as it passes through the aerogel's nanopores
5
. These nanopores, sized from 5 to 150 nm across, are much smaller than the wavelengths of visible light, yet large enough to scatter the higher frequency colors. The larger of these pores scatter visible light more easily. The larger pores (>15 nm) cause most of the scattering in the aerogel. Shorter wavelengths are diffracted more than longer wavelengths, and therefore blue and violet light are diffracted the most. Although both blue and violet light are diffracted by these nanopores, only a blue color is perceived. This is because the human eye is more sensitive to blue light than any other color, and so violet light scattered by the aerogel is perceived to
Boyle Fredrickson Newholm Stein & Gratz S.C.
Johnson Edward M.
Silverman Stanley S.
LandOfFree
Method for the formation of aerogel precursor using rapid... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for the formation of aerogel precursor using rapid..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for the formation of aerogel precursor using rapid... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3202841