WO2017185009A1 - Procédés de fabrication d'aérogels de silice présentant des formes personnalisées à l'aide d'une lyophilisation - Google Patents

Procédés de fabrication d'aérogels de silice présentant des formes personnalisées à l'aide d'une lyophilisation Download PDF

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WO2017185009A1
WO2017185009A1 PCT/US2017/028903 US2017028903W WO2017185009A1 WO 2017185009 A1 WO2017185009 A1 WO 2017185009A1 US 2017028903 W US2017028903 W US 2017028903W WO 2017185009 A1 WO2017185009 A1 WO 2017185009A1
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solvent
wet gel
aerogel
aerogels
catalyst
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Massimo Bertino
Lauren White
Dalton ECHARD
Tyler SELDEN
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Virginia Commonwealth University
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Virginia Commonwealth University
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Priority to US16/095,158 priority Critical patent/US20190143290A1/en
Priority to EP17786738.9A priority patent/EP3445805A4/fr
Publication of WO2017185009A1 publication Critical patent/WO2017185009A1/fr
Anticipated expiration legal-status Critical
Priority to US16/939,769 priority patent/US11654409B2/en
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    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
    • C08G18/00—Polymeric products of isocyanates or isothiocyanates
    • C08G18/06—Polymeric products of isocyanates or isothiocyanates with compounds having active hydrogen
    • C08G18/28—Polymeric products of isocyanates or isothiocyanates with compounds having active hydrogen characterised by the compounds used containing active hydrogen
    • C08G18/40—High-molecular-weight compounds
    • C08G18/64—Macromolecular compounds not provided for by groups C08G18/42 - C08G18/63
    • C08G18/6469—Macromolecular compounds not provided for by groups C08G18/42 - C08G18/63 having silicon
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/113—Silicon oxides; Hydrates thereof
    • C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
    • C01B33/14—Colloidal silica, e.g. dispersions, gels, sols
    • C01B33/157—After-treatment of gels
    • C01B33/158—Purification; Drying; Dehydrating
    • C01B33/1585—Dehydration into aerogels
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
    • C08G18/00—Polymeric products of isocyanates or isothiocyanates
    • C08G18/06—Polymeric products of isocyanates or isothiocyanates with compounds having active hydrogen
    • C08G18/70—Polymeric products of isocyanates or isothiocyanates with compounds having active hydrogen characterised by the isocyanates or isothiocyanates used
    • C08G18/72—Polyisocyanates or polyisothiocyanates
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • C08J9/28—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof by elimination of a liquid phase from a macromolecular composition or article, e.g. drying of coagulum
    • C08J9/286—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof by elimination of a liquid phase from a macromolecular composition or article, e.g. drying of coagulum the liquid phase being a solvent for the monomers but not for the resulting macromolecular composition, i.e. macroporous or macroreticular polymers
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/60—Optical properties, e.g. expressed in CIELAB-values
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2201/00—Foams characterised by the foaming process
    • C08J2201/02—Foams characterised by the foaming process characterised by mechanical pre- or post-treatments
    • C08J2201/026—Crosslinking before of after foaming
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2201/00—Foams characterised by the foaming process
    • C08J2201/04—Foams characterised by the foaming process characterised by the elimination of a liquid or solid component, e.g. precipitation, leaching out, evaporation
    • C08J2201/048—Elimination of a frozen liquid phase
    • C08J2201/0482—Elimination of a frozen liquid phase the liquid phase being organic
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2205/00—Foams characterised by their properties
    • C08J2205/02—Foams characterised by their properties the finished foam itself being a gel or a gel being temporarily formed when processing the foamable composition
    • C08J2205/026—Aerogel, i.e. a supercritically dried gel
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2333/00—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical, or of salts, anhydrides, esters, amides, imides, or nitriles thereof; Derivatives of such polymers
    • C08J2333/04—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical, or of salts, anhydrides, esters, amides, imides, or nitriles thereof; Derivatives of such polymers esters
    • C08J2333/06—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical, or of salts, anhydrides, esters, amides, imides, or nitriles thereof; Derivatives of such polymers esters of esters containing only carbon, hydrogen, and oxygen, the oxygen atom being present only as part of the carboxyl radical
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2383/00—Characterised by the use of macromolecular compounds obtained by reactions forming in the main chain of the macromolecule a linkage containing silicon with or without sulfur, nitrogen, oxygen, or carbon only; Derivatives of such polymers
    • C08J2383/04—Polysiloxanes
    • C08J2383/06—Polysiloxanes containing silicon bound to oxygen-containing groups

Definitions

  • the present invention relates to the field of aerogel synthetic chemistry and processing. More particularly, the present invention in embodiments relates to fabrication of native and cross-linked aerogels in monolithic form by freeze-drying wet gels.
  • Aerogels are solid materials of extremely low density, produced by removing the liquid component from a conventional wet gel. They are ultra-light, highly porous and highly thermally insulating materials composed of a network of interconnected nanostructures. Their typical density is lower than 0.1 g/cm 3 , their surface area is in the 700- 1000 m 2 /g range and their thermal conductivity can be as low as 2.1 mW/mK (see N. Leventis, Accounts of Chemical Research., 2007, 40, 874 ("Leventis, 2007”) and A. C. Pierre, G. M. Pajonk, Chem. Rev., 2002, 102, 4243 ("Pierre, 2002”)).
  • aerogels are being considered for applications as varied as thermal and sound insulation for the aerospace industry, as absorbents for environmental remediation and as catalyst supports.
  • aerogels are also mechanically fragile and their use has been limited to niche applications such as thermal insulation for the Mars Rovers, as collectors of space and comet dust and as Cerenkov detectors (see Leventis, 2007 and Pierre, 2002).
  • other efforts in this area include those described by Leventis et al., such as in U.S. Patent Nos. 7,732,496 and 8,227,363 as well as in U.S. Patent Application No. 2011/0250428 Al, hereby incorporated by reference in their entireties.
  • Aerogels are fabricated starting from wet gels.
  • Wet gels are porous materials with the same porosity and surface area of aerogels.
  • the pores of wet gels are filled with solvent and precursors used for the synthesis.
  • the solvent is some alcohol (e.g. , methanol, ethanol, or propanol) and some water is added to catalyze the synthetic reaction.
  • the solvent typically cannot be evaporated without cracking the gel because of capillary forces. That is, the solvent adheres strongly to the pore walls and induces cracks and pore collapse when it evaporates.
  • a fluid with a low (ideally zero) surface tension is employed, which minimizes the capillary forces.
  • This solvent is typically a supercritical solvent.
  • Embodiments of the present invention provide methods for the production of both native silica and cross-linked aerogel monoliths that incorporate freeze-drying in lieu of supercritical solvent drying.
  • the solvent within the wet gel is frozen.
  • the gel monolith is then placed in a vacuum chamber where the solvent is removed by sublimation.
  • Advantages over supercritical drying include a reduction in hazard risks posed by drying at supercritical conditions as well as the ability to up-scale the process to accommodate large pieces of material without introducing risk.
  • a second advantage of the invention is a substantial reduction in capital expenditures.
  • Autoclaves used for supercritical drying require thick walls and pose liability issues because of the high pressures (on the order of 70 atmospheres) used in supercritical drying.
  • Freeze drying uses vacuum chamber(s) instead which are much cheaper to produce and pose minimal liability issues.
  • a supercritical drying autoclave costs 10 times more than a freeze drying vacuum chamber of the same capacity.
  • inexpensive and more sophisticated mold technologies which are not impervious to supercritical conditions, can be used to produce aerogel materials according to the freeze-drying method of the invention. This introduces a level of freedom never before available for the production of aerogel components.
  • the present invention provides a method for producing an aerogel which includes providing a first solution comprising an alkoxide, providing a second solution comprising a catalyst, mixing the first and second solutions to provide a gelation mixture or composition, and optionally pouring the solutions into a mold.
  • the mixing of the first and second solutions results in formation of a wet gel as a result of hydrolysis of the alkoxide and polymerization of the hydrolyzed alkoxide.
  • the wet gel is dried to form an aerogel.
  • the gel is dried by freeze-drying.
  • the present invention provides a method for producing an aerogel which includes mixing a first solution comprising an alkoxide, a photoinitiator, and a first acrylic monomer and a second solution comprising a catalyst and a second acrylic monomer and optionally pouring the solutions into a mold.
  • the mixing step results in the formation of a wet gel as a result of hydrolysis of the alkoxide and polymerization of the hydrolyzed alkoxide.
  • the method further includes exposing the wet gel to a source of visible light with sufficient intensity to catalyze cross-linking of the wet gel, and drying the wet gel.
  • the gel is dried by freeze drying.
  • the present invention provides a method for producing an aerogel which includes mixing an alkoxide and a catalyst together in an aqueous solution to provide a composition and optionally pouring the composition into a mold.
  • the mixing of alkoxide and catalyst together results in formation of a wet gel as a result of hydrolysis of the alkoxide and polymerization of the hydrolyzed alkoxide.
  • the method further includes performing a solvent exchange step to remove water from the wet gel, and freeze-drying the wet gel to remove solvent from the wet gel to form an aerogel.
  • the present invention provides a method for producing an aerogel which includes mixing an alkoxide, a catalyst, an acrylic monomer, a silica derivatizer and a polymerization initiator together to provide a composition, and optionally pouring the composition into a mold.
  • the mixing step results in the formation of a wet gel as a result of hydrolysis of the alkoxide and polymerization of the hydrolyzed alkoxide.
  • the method further includes exposing the wet gel to a stimulus with sufficient intensity to catalyze cross- linking of the wet gel by the polymerization initiator, and freeze-drying the cross-linked wet gel to form an aerogel.
  • the present invention provides methods of improving the transparency of aerogels through mechanisms such as altering pore size and pore size distribution, changing the size and/ or shape of skeletal aggregates, or removing light- scattering reagents.
  • FIG. 1A is a photograph showing a side view of a cross-linked silica aerogel fabricated by freeze drying.
  • FIG. IB is a photograph showing a top view of an aerogel fabricated by freeze drying, where gelation byproducts such as methanol were partially removed by evaporation prior to freezing and where the dried monolith is illuminated from below to show improved optical characteristics.
  • FIG. 1C is an image showing an aerogel sample fabricated as in FIG. 1A, after heating to 400 °C, showing that removal of the cross-linking polymer improves transparency.
  • FIG. ID is an image of a native aerogel patterned with a polymer honeycomb and compressed uniaxially, where areas between the polymer honeycomb have become transparent and where all samples were reinforced by acrylic polymers which yield opaque aerogels.
  • FIGS. 3A and 3B are graphs showing simulations of light scattering from aggregates of silica nanoparticles with different shapes. N indicates the number of silica nanoparticles in the aggregate. Note how the scattering lobe (and thus haze) is wider for the globular aggregate (FIG. 3B) than for the fractal aggregate (FIG. 3A).
  • N indicates the number of nanoparticles in the aggregate.
  • FIG. 4 is a diagram showing an example of cross-linking of silica aerogels.
  • the gelation solution contains a monomer which engages the moiety at the surface of silica nanoparticles when polymerization is initiated.
  • FIG. 5 is an image of a series of aerogels cross-linked with polyurethane at a temperature of 100 °C.
  • the cross-linker concentration increases from left (native aerogels) to right, and the transparency of the materials with the lowest cross-linker concentration resembles that of native aerogels. All samples had a diameter of 7 mm.
  • FIGS. 6 A and 6B are schematic diagrams showing the effect of conformal coating
  • FIG. 6A shows a fibrillar skeletal morphology.
  • the conformal coating increases aggregate size by approximately twice the coating's thickness.
  • FIG. 6B shows a Globular skeletal morphology. The conformal coating fills the gaps between the particles and gives rise to a large aggregate.
  • FIG. 7 is an image of a transparent aerogel produced by freeze drying.
  • the aerogel had been strengthened by prolonged aging, and a DCCA was used to control pore size distribution.
  • a DCCA was used to control pore size distribution.
  • FIGS. 8A-8C are images of flexible aerogels with FIG. 8A showing the transparency of different formulations and FIG. 8B showing demonstration of stress recovery. Adapted from K. Kanamori, M. Aizawa, K. Nakanishi, and T. Hanada, "New Transparent Methylsilsesquioxane Aerogels and Xerogels with Improved Mechanical Properties", Adv. Mater. 2007, 19, 1589-1593.
  • FIG. 8C shows flexible aerogel fabricated by the inventors by freeze drying using the precursor formulation reported in Kanamori et al. [00024] FIG.
  • FIG. 9 is a graph showing simulation of the U value of a glass pane coated with a coating of emissivity 0.1, attached to an aerogel pane with thermal conductivity of 30 mW/m-K.
  • the total thickness of the retrofit is kept to 1 ⁇ 4 inch.
  • FIGS. 10A-B are photographs of aerogel samples prepared according to embodiments described in this disclosure.
  • FIG. 10A shows a transparent sample with a thickness of 6 mm
  • FIG. 10B shows a translucent 3 mm disk prepared without an oven aging step.
  • FIG. 11 A-B are graphs showing pore size distributions calculated using the BJH approximation (FIG. 11 A) and providing an adsorption isotherm of transparent and translucent regions of the same sample (FIG. 11B), where the translucent region was in the core and had been frozen more slowly than the outer regions.
  • the present invention provides a method of fabrication of native and cross-linked silica aerogels in monolithic form by freeze-drying.
  • the silica aerogels may be fabricated into a variety of custom shapes.
  • the custom aerogel shapes may be used in a variety of thermal insulation applications or as window panes.
  • the processes used for native and cross-linked aerogels differ in one main aspect.
  • the synthesis is carried out using mostly water as the solvent for gelation. This water must be removed by solvent exchange.
  • the inventors prepared a composition (which may also be referred to as a gelation solution, gelation mixture, a mixture, or a solution) with a very limited amount of solvent, e.g. , water (about 4% by volume of solvent). This small amount of water was sufficient to induce gelation, but it did not induce cracks into freeze dried monoliths. For this technique, the wet gel is freeze dried starting from the parent solution.
  • freeze-dried native silica aerogels have been obtained with a surface area of -400 m /g.
  • cross-linked silica aerogels have been obtained with a surface area of -100 m 2 /g and a modulus of 50 MPa. All these values are in line with those of native and cross-linked aerogels fabricated by supercritical drying.
  • the aerogels may be produced in molds having a variety of shapes suitable for thermal insulation and/or window applications, or may be produced without using a mold. As the aerogels may be manufactured as monoliths with a volume of up to 100 cm , the gels may be cast into a variety of shapes suitable for a variety of applications.
  • native silica aerogels can be fabricated by derivatizing their surface with an organic moiety. This moiety makes the skeletal oxide structure flexible and able to withstand considerable stresses ⁇ see K. Kanamori, M. Aizawa, K. Nakanishi, T. Hanada, J. Sol-Gel Sci. Techn., 2008, 28, 172).
  • surfactants are also added to the gelation solution. These surfactants help control pore size distribution which, in turn, helps reducing freezing stresses (see G. W. Scherer, "Freezing gels", Journal of Non-Crystalline Solids 155 (1993) 1-25).
  • a method for producing a native aerogel comprising: (a) providing a first solution comprising an alkoxide; (b) providing a second solution comprising a catalyst; (c) mixing the first and second solutions to provide a gelation mixture; (d) optionally pouring the solutions (or gelation mixture) into a mold; wherein the mixing of the first and second solutions results in formation of a wet gel as a result of hydrolysis of the alkoxide and polymerization of the hydrolyzed alkoxide; and (e) after gelation, freeze- drying the wet gel to form an aerogel. Further, after gelation and before freeze-drying, solvent exchange may be applied to the gel to remove excess water. Also, embodiments may include addition of a surfactant to the first solution or the second solution or the gelation mixture.
  • porous matrices are synthesized through a modification of hydrolysis condensation of alkoxides in which addition of water is reduced, or minimized.
  • the reaction occurs in an organic solvent and water with a concentration of water of approximately 4.4% v/v or lower, such as an ethanol-water azeotrope mixture (as used in this specification, references to "ethanol” are intended to mean an ethanol-water azeotrope mixture); the water in the azeotrope slowly hydrolyzes the alkoxide.
  • Lower water concentrations can also be employed but they usually increase gelation time.
  • water can be present in the gelation solution in an amount ranging from about 0.05 % v/v to about 5 % v/v, such as from 0.1 % v/v to 4 % v/v, or from about 0.2 % v/v to 3 % v/v, or from 0.3 % v/v to 2 % v/v, or from 0.4 % v/v to 1.5 % v/v, or from about 0.5 % v/v to 1 % v/v, or from about 0.6 % v/v to 0.8 % v/v.
  • Water can also be provided by adding to the gelation solution hydrated metal salts.
  • a compound that reacts with the alkoxide or dissociates at high temperature and liberates water could be used, such as formic acid.
  • any mixture of water and an organic solvent can be used.
  • a mixture of acetone and water, or alcohol and water, or methanol and water, or butanol and water, or propanol and water can be used.
  • the porous matrix or aerogel is freeze dried. Suitable freeze-drying techniques are found in the art, including for example as disclosed by German Patent Application Publication No. DE10233703 Al and corresponding German Patent No. DE10233703 B4.
  • porous matrices are synthesized by hydrolyzing an alkoxide without adding any water beyond that present in the ethanol-water azeotrope.
  • First tetramethyl orthosilicate (TMOS) or polyethoxydisiloxane (PEDS) is dissolved into ethanol.
  • TMOS tetramethyl orthosilicate
  • PEDS polyethoxydisiloxane
  • a second solution is prepared which contains ethanol and an amine such as triethanolamine.
  • the two solutions are then mixed and poured into a mold and a gel forms typically within about an hour.
  • the gel is then removed from the mold and freeze dried to yield a porous material (aerogel).
  • the gel can be freeze dried in the mold.
  • the ethanol used in the procedure may be an ethanol-water mixture which contains no more than 4-5 % water by volume.
  • a base or acid is added during the hydrolysis step as a catalyst. Additional processing steps include a curing step performed overnight.
  • the first and second solutions are combined to provide a gelation mixture or composition and the alkoxide is present in an amount ranging from about 0.1 % v/v to 50% v/v based on total volume of the composition, and/or the composition comprises an amine present in an amount ranging from about 0.1 % v/v to 20 % v/v based on total volume of the gelation mixture, and/or the composition comprises an organic solvent present in an amount ranging from about 20 % v/v to 90 % v/v based on total volume of the gelation mixture.
  • the organic solvent can be present in these concentrations and can be chosen from one or more of an alcohol or a ketone, such as one or more of ethanol, methanol, butanol, propanol, acetone, or dimethylsulf oxide.
  • an alcohol or a ketone such as one or more of ethanol, methanol, butanol, propanol, acetone, or dimethylsulf oxide.
  • water is used as a solvent in the composition in lieu of any organic solvent.
  • native silica aerogels are prepared by combining urea, cetyl trimethylammonium bromide (CTAB), and Acetic Acid with water as the solvent and thoroughly mixing these reagents. Methyltrimethoxysilane is added to the solution to form a gelation composition or mixture and continually stirred for 30 minutes. This composition is then poured into a mold and placed in an oven at 60 X2 for 36 hours to allow for gelation and aging. Water is removed through solvent exchange with an organic solvent to prepare for the freeze-drying step. Once solvent exchange is completed so the water concentration is less than 5% of the volume solvent, the monoliths are frozen and then dried.
  • the gelation mixture or composition may comprise a ratio of alkoxide to solvent
  • the gelation mixture may comprise a ratio of catalyst to solvent at about 1 :99 to 99: 1 , such as from 2:98 to 98:2, or from 3 :97 to 97:3, or from 5:95 to 95:5, or from 10:90 to 90: 10, or from 20:80 to 80: 10, or from 30:70 to 70:30, or from 40:60 to 60:40, or 50:50, or any range within these ratio ranges.
  • the ratio of catalyst to alkoxide may be around 1 :99 to 99: 1 , such as from 2:98 to 98:2, or from 3:97 to 97:3, or from 5:95 to 95:5, or from 10:90 to 90: 10, or from 20:80 to 80: 10, or from 30:70 to 70:30, or from 40:60 to 60:40, or 50:50, 1 :2, 2: 1, 1 :3, 3: 1 , 1 :4, 4: 1 , 1 :5, 5: 1 , 1 :6, 6: 1, 1 : 10 or 10: 1 and so on, or any range within these ratio ranges.
  • Such ratios may be determined by weight or by volume.
  • the present invention provides a method of synthesizing cross-linked aerogels in a single step and in a single pot without requiring any solvent exchange.
  • embodiments of the invention allow for fabrication of custom aerogel parts with large dimensions, as well as high volume fabrication of aerogels.
  • the custom aerogel parts may be used in a variety of thermal insulation applications.
  • cross-linked silica aerogels are prepared. Their mechanical strength arises by cross-linking the skeletal oxide particles with a polymer (see U.S. Patent Nos. 8,277,676 and 8,227,363). The strength of these materials allows them to withstand the freezing stresses.
  • These gels may be fabricated using the inventors' previously reported methods ⁇ see International Patent Application Publication No. WO 2016019308) with or without any additional modifications.
  • a method for producing a cross-linked aerogel comprising a) mixing a first solution comprising an alkoxide with a second solution comprising a catalyst, an acrylic monomer, a silica derivatizer and a polymerization initiator and optionally pouring the solutions into a mold, wherein such mixing step results in the formation of a wet gel as a result of hydrolysis of the alkoxide and polymerization of the hydrolyzed alkoxide, b) exposing the wet gel to a stimulus with sufficient intensity to catalyze cross-linking of the wet gel by the polymerization initiator, and c) freeze-drying the cross-linked wet gel to form an aerogel.
  • the present invention in embodiments provides a method in which a porous monolith is synthesized, made mechanically robust by polymer cross-linking and dried by freeze-drying into an aerogel, without requiring any intermediate processing steps and/or solvent exchange.
  • a first solution is prepared by adding an alkoxide carrying a polymerizable moiety (such as vinyltrimethoxysilane (VMOS)) to a solution of ethanol and TMOS.
  • a second solution is prepared separately which contains ethanol, triethanolamine, a polymerization initiator and a monomer such as methylacrylate.
  • the solutions are then mixed and a gel is synthesized by mixing the two solutions.
  • the gel is then dried by freeze-drying.
  • the gel may be polymerized before the drying step through thermal initiation or photopolymerization .
  • cross-linked aerogels are fabricated by combining an alkoxide carrying a polymerizable moiety, e.g. vinyltrimethoxysilane (VMOS) or tetramethylorthosilane (TMOS), added to a primary, secondary or tertiary alcohol.
  • VMOS vinyltrimethoxysilane
  • TMOS tetramethylorthosilane
  • a second solution is prepared separately which includes the same or another alcohol, a catalyst such as triethanolamine, a monomer such as methylmethacrylate, a silica derivatizer such as trimethoxysilylpropyl methacrylate and a polymerization initiator.
  • a catalyst such as triethanolamine
  • a monomer such as methylmethacrylate
  • silica derivatizer such as trimethoxysilylpropyl methacrylate
  • a polymerization initiator e.g., a polymerization initiator.
  • water can be added to reach
  • the two solutions are then mixed and poured into a mold. Gelation occurs within one hour.
  • polymerization is initiated thermally if a thermal initiator is employed, or by photopolymerization when a photoinitiator is used. After polymerization, the monoliths are freeze dried.
  • the first solution may comprise a ratio of alkoxide to alcohol at about 1 :99 to 99: 1 , such as from 2:98 to 98:2, or from 3 :97 to 97:3, or from 5:95 to 95:5, or from 10:90 to 90: 10, or from 20:80 to 80: 10, or from 30:70 to 70:30, or from 40:60 to 60:40, or 50:50, or any range within these ratio ranges.
  • the second solution may comprise a ratio of catalyst to alcohol at about 1 :99 to 99: 1, such as from 2:98 to 98:2, or from 3 :97 to 97:3, or from 5:95 to 95:5, or from 10:90 to 90: 10, or from 20:80 to 80: 10, or from 30:70 to 70:30, or from 40:60 to 60:40, or 50:50, or any range within these ratio ranges.
  • the second solution may comprise a ratio of monomer to alcohol of 1 :99 to 99: 1 , such as from 2:98 to 98:2, or from 3 :97 to 97:3, or from 5:95 to 95:5, or from 10:90 to 90: 10, or from 20:80 to 80: 10, or from 30:70 to 70:30, or from 40:60 to 60:40, or 50:50, or any range within these ratio ranges.
  • the ratio of catalyst to alkoxide may be around 1 :99 to 99: 1, such as from 2:98 to 98:2, or from 3 :97 to 97:3, or from 5:95 to 95:5, or from 10:90 to 90: 10, or from 20:80 to 80: 10, or from 30:70 to 70:30, or from 40:60 to 60:40, or 50:50, 1 :2, 2: 1, 1 :3, 3: 1, 1 :4, 4: 1, 1 :5, 5: 1, 1 :6, 6: 1, 1 : 10 or 10: 1 and so on, or any range within these ratio ranges.
  • Such ratios may be determined by weight or by volume.
  • the native silica and cross-linked aerogel monoliths can be frozen by any freezing technique known in the art. If the gels are kept inside the original molds, freezing can be carried out by placing the gels into liquid nitrogen, into a cryogenic mixture (e.g. , dry-ice acetone bath), or inside a refrigerator. Any freezing method works, provided that the gels are brought to a temperature below the freezing temperature of the solvent used for the synthesis or solvent exchange. Use of tert-butanol as a solvent appears to be especially advantageous, since this solvent freezes just about at room temperature and therefore it does not require to be cooled to low temperatures (a household freezer is sufficient).
  • a cryogenic mixture e.g. , dry-ice acetone bath
  • any solvent with a low freezing temperature may work, such as for example ethanol, 1-butanol, dimethyl sulfoxide, and carbon tetrachloride.
  • the frozen gel is then dried using a conventional freeze dryer or, more simply, placed in a vacuum chamber which is pumped by a conventional rotary pump.
  • the part of the chamber hosting the sample must be kept at temperatures on the order of the freezing temperature of the solvent.
  • Additional embodiments include any method of increasing the transparency of an aerogel.
  • the methods may increase transparency through any mechanism, such as altering pore size and pore size distribution, changing the size and/or shape of skeletal aggregates, removing substances such as water or other reagents, and the like.
  • these methods may include but not be limited to the use of heat treatments, uniaxial compression, the use of polyurethane monomers for cross-linking instead of acrylic monomers, the use of organosilanes, the use of surfactants and drying control chemical additives, or adjusting the reaction conditions such as increasing the temperature during polymerization, increasing the amount of catalyst, reducing the concentration of monomer, or using polyfunctional monomers.
  • Embodiments of the invention also include aerogels with improved transparency as measured by the level of light transmission through a sample of the aerogel.
  • the surfactants may include quaternary ammonium salts such as cetrimonium bromide (CTAB), cetylpyridinium chloride (CPC), benzalkonium chloride (BAC), benzethonium chloride (BZT), dimethyldioctadecylammonium chloride, dioctadecyldimethylammonium bromide (DODAB), and the like.
  • CTAB cetrimonium bromide
  • CPC cetylpyridinium chloride
  • BAC benzalkonium chloride
  • BZT benzethonium chloride
  • dimethyldioctadecylammonium chloride dioctadecyldimethylammonium bromide
  • DODAB dioctadecyldimethylammonium bromide
  • anionic surfactants e.g. sulfates, sulfonates, phosphates, and carboxylates
  • the alkoxide may be a silicon alkoxide such as the organo-orthosilicates tetraethyl orthosilicate (TEOS), tetramethyl orthosilicate (TMOS), polyethoxydisiloxane (PEDS), methyltrimethoxysilane (MTMS), or vinyltrimethoxysilane (VMOS).
  • TEOS tetraethyl orthosilicate
  • TMOS tetramethyl orthosilicate
  • PEDS polyethoxydisiloxane
  • MTMS methyltrimethoxysilane
  • VMOS vinyltrimethoxysilane
  • Alkoxides of other transition metals, as well as chlorides, nitrates or acetylacetonates can also be employed.
  • sodium silicate and/or aluminum chloride could be used in particular.
  • silica any metal oxide and sulfide can also or alternatively be used.
  • the gelation reaction may be catalyzed by an acid (such as a metal salt) or base (such as an amine) or a catalyst containing fluoride.
  • a metal salt may be added as an acid catalyst.
  • aluminum chloride is used.
  • salts containing other acidic ions including Cr 3+ , Fe 3+ Bi 3+ , Be 2+ , NH 4 + are used.
  • acids such as hydrochloric acid, sulfuric acid, and nitric acid are used.
  • Base catalysts may include amines such as triethanolamine, diethanolmethylamine, dimethylethlyamine, or dimethanolmethylamine or hydroxides such as ammonium hydroxide. Other embodiments may use ammonium fluoride.
  • embodiments may employ an acrylate or acrylated monomer for the synthesis of cross-linked aerogels, and the cross-linking reaction may be catalyzed by photopolymerization or thermal polymerization.
  • acrylated monomers include HDD A (hexanediol diacrylate) and acrylated DPHA (dipentaerythritol hexaacrylate).
  • Other non- limiting examples of acrylates include methyl acrylate, ethyl acrylate, 2-chloroethyl vinyl ether, 2-ethylhexyl acrylate, hydroxyethyl methacrylate, butyl acrylate, and butyl methacrylate.
  • Photopolymerization of cross-linked aerogels may be initiated by including a photoinitiator in one of the precursor solutions.
  • photoinitiators include Eosin Y, Nile Red, Alizarine Red S, and Rhodamine B.
  • Other examples of photoinitiators are known (see Fouassier et al, Dyes as Photoinitiators or Photosensitizers of Polymerization Reactions, Materials 2010, 3, 5130-5142). Polymerization can be also induced thermally or by using thermal initiators.
  • Exemplary alcohols for use in the gelation precursor solutions or mixtures or in solvent exchange may include methanol, ethanol, butanol, tert-butanol, isopropyl alcohol, isobutanol, benzyl alcohol, and the like, including any one or more solvent having a low entropy of fusion, such as cyclohexane or tert-butanol (t-butanol).
  • the alcohols may be any primary, secondary, or tertiary alcohol capable of freeze-drying. Additional solvents may be ketones, such as acetone, or acetonitrile, or mixtures of alcohols and these solvents.
  • freeze-drying of wet gel has been previously reported by other groups (see L.F. Su, L. Miao, S. Tanemura, G. Xu, Sci. Technol. Adv. Mat. 2012, 13, 035003; A. Pons, L. Casas, E. Estop, E. Molins, K.D.M. Harris, M. Xu, J. Non-Cryst. Solids, 2012, 358, 461 ; S.R. Mukai, H. Nishihara, H. Tamon, Micropor. Mesopor. Mat., 2003, 63, 43; and E. Degn Egeberg, J.
  • the inventors produced wet gels with flexible reinforced silica skeletons.
  • the modified backbone withstands the freezing stresses.
  • the result was native and cross-linked aerogels in monolithic form. Disk-shaped samples with a diameter of up to 4 cm have been produced, having been limited by the maximum size allowed by the particular vacuum chambers used.
  • the materials can be handled with the same precautions used for handling native silica aerogels. If mechanically solicited, they may crack, but they do not turn into dust.
  • 0.88 g of urea, 0.1167 g of cetyl trimethylammonium bromide (CTAB), 0.019 ml of Acetic Acid and 2.842 ml of water as the solvent are thoroughly mixed.
  • 1.672 ml of methyltrimethoxysilane is added to the solution and continually stirred for 30 minutes. This mixture is then poured into a mold and placed in an oven at 60 "C for 36 hours to allow for gelation and aging. Wet gels synthesized in this way are not suitable for freeze-drying.
  • any freezing method works, provided that the gels are brought to a temperature below the freezing temperature of the solvent used for the synthesis.
  • Use of tert-butanol as a solvent appears to be especially advantageous, since this solvent freezes just about at room temperature and therefore it does not require to be cooled to low temperatures (a household freezer is sufficient).
  • the frozen gel is then dried using a conventional freeze dryer or, more simply, placed in a vacuum chamber which is pumped by a conventional rotary pump.
  • the part of the chamber hosting the sample must be kept at temperatures on the order of the freezing temperature of the solvent.
  • This fabrication method yields a porous material (aerogel) with a density of about 0.2 g/cm 3 and a surface area of up to 450 m /g.
  • Cross-linked aerogels are fabricated as follows. 0.5 ml of an alkoxide carrying a polymerizable moiety (see N. Leventis, C. Sotiriou-Leventis, G. Zhang, A.-M.M. Rawashdeh, Nano Lett., 2002, 2, 957; C. Wingfield, A. Baski, M.F. Bertino, N. Leventis, D.P. Mohite, H. Lu, Chem. Mater. , 2009, 21 , 2108; and C. Wingfield, L. Franzel, M.F. Bertino, N. Leventis, J. Mater. Chem. 2011, 21, 11737), e.g.
  • VMOS vinyltrimethoxysilane
  • TMOS tetramethylorthosilane
  • a second solution is prepared separately which consists of 1.0 ml of the same or another alcohol, 40 ⁇ of triethanolamine, between 0.2 and 1.5 ml of a monomer such as methylmethacrylate, between 0.05 and 0.3 ml of a silica derivatizer such as trimethoxysilylpropyl methacrylate and a polymerization initiator.
  • an alcohol such as ethanol, butanol or tert-butanol or other primary, secondary or tertiary alcohol.
  • a second solution is prepared separately which consists of 1.0 ml of the same or another alcohol, 40 ⁇ of triethanolamine, between 0.2 and 1.5 ml of a monomer such as methylmethacrylate, between 0.05 and 0.3 ml of a silica derivatizer such as trimethoxysilylpropyl methacrylate and a polymerization
  • This fabrication method yields a porous material (aerogel) with a density of about 0.25 g/cm 3 and a surface area of ⁇ 100 m 2 /g and a modulus of 50 MPa.
  • This same chemistry was used in the inventors' previous disclosure (see International Patent Application Publication No. WO 2016019308). One difference being that gels are now freeze-dried and not supercriticaily-dried.
  • the aerogels can be cast into molds of any arbitrary shape.
  • Molds and aerogels can be placed into a freeze dryer. After drying, the aerogels can be removed from the molds to yield aerogel components with custom shape. Fabrication of aerogels with custom shapes was reported in the inventors' original patent application and published recently (see International Patent Application Publication No. WO 2016019308 and L. S. White, D. R. Echard, M. F. Bertino, X. Gao, S. Donthula, N. Leventis, N. Shukla, J. Kosny, S. Saeed and K. Saoud, Transl. Mater. Res. 3 (2016)). This work differs from the previous one because it allows use of any molding material. Because of the high temperatures, supercritical solvent drying does not allow plastic molds, and certain metal molds can react with the hot solvents. Freeze-drying is a much more benign approach which has virtually no limitations on the type of molds.
  • FIGS. 1A-1D show results of methods employed according to this disclosure.
  • Aerogel monoliths can be produced by freeze drying (FIG. 1A), and transparency of the monoliths can be improved by refinement of the synthesis procedure (FIG. IB), by heat treatment (FIG. 1C) and/or by uniaxial compression (FIG. ID).
  • Reinforcement can be provided by polymer cross-linking, by increasing the density of the gels ⁇ i.e., using an excess of silica precursor) or by adding fibers to the gelation solution ⁇ see Journal of Non-Crystalline Solids 385 55-74 (2014)). All of these strategies are straightforward to implement and yield materials which are at least one order of magnitude stronger than native aerogels.
  • the critical size i.e. , the largest size that can be dried without fragmentation) depends on the bulk modulus of the material. Aerogels produced in the past by freeze drying ⁇ see K. anamori, M. Aizawa, K. Nakanishi, T.
  • FIGS. 2A and 2B compares native and aerogels cross-linked with polyurethane, dried supercritically. Polyurethane cross-linking yields smaller secondary particles than acrylic cross-linking (used in FIGS. 1A-1D), and, consequently, more transparent materials.
  • T-butanol solvents with a low entropy of fusion such as cyclohexane or tert-butanol (t-butanol).
  • T-butanol was employed for the fabrication of the sample in FIG. 1A.
  • water and methanol as synthesis byproducts. These solvents are detrimental to freeze drying. Removal of these solvents by evaporation prior to gelation or by solvent exchange after gelation yields aerogels with improved transparency (FIG. IB). Drying stresses can be further reduced by synthesizing gels with a narrow pore size distribution.
  • Pore size distributions can be controlled by adding to the gelation solution an amphiphilic surfactant (see Micropor. and Mesopor. Mater. 158, 247-252 (2012)) or a polar molecule that binds to silanol groups such as dimethylformamide ⁇ see Microporous Materials 12, 63-69 (1997)).
  • Amphiphilic surfactant see Micropor. and Mesopor. Mater. 158, 247-252 (2012)
  • a polar molecule that binds to silanol groups such as dimethylformamide ⁇ see Microporous Materials 12, 63-69 (1997).
  • Narrow pore size distributions typically greatly aid freeze drying. The solvent in the pores will freeze at the same temperature, solvent diffusion will be minimized and so the stresses.
  • FIGS. 2A and 2B transparent, mechanically strong materials can be produced by a careful selection of the cross-linking polymer using supercritical drying.
  • the transparency of the materials in FIGS. 2A and 2B can be increased by reducing the concentration of the crosslinker and by controlling pore size distribution (see Micropor. and Mesopor. Mater. 158, 247-252 (2012); see Microporous Materials 12, 63-69 (1997)).
  • Achieving a sufficient transparency would eliminate the need for densification and sandwiching, since the composites are mechanically strong.
  • Transparency can also be improved by heating and/or by uniaxial compression.
  • the comparatively poor properties of aerogels are due, for the most part, to light scattering.
  • the microscopic structure of aerogels consists of primary particles with a size ⁇ 5 nm which aggregate into secondary particles with a size of 20-30 nm.
  • Micropores are the pores between primary particles and have a size ⁇ 10 nm.
  • Mesopores are the pores between the secondary particles and have a size of tens of nanometers.
  • Organosilanes may also play a role in obtaining greater transparency.
  • Di- and tri- functional silicon alkoxides such as methyltrimethoxysilane (MTMS) and polyethoxydisiloxane (PEDS) have been shown in some investigations to lead to materials with increased transparency (see K. Kanamori, M. Aizawa, K. Nakanishi, T. Hanada, "Elastic organic-inorganic hybrid aerogels and xerogels", / Sol-Gel Sci Technol (2008) 48, 172-181 ; P.B. Wagh, R. Begag, G.M. Pajonk, A. V. Rao, D.
  • MTMS methyltrimethoxysilane
  • PEDS polyethoxydisiloxane
  • Haranatha "Comparison of some physical properties of silica aerogel monoliths synthesized by different precursors", Materials Chemistry and Physics 57 (1999) 214- 218; A. V. Rao, S. D. Bhagat, "Synthesis and physical properties of TEOS-based silica aerogels prepared by two step (acid-base) sol-gel process", Solid State Sciences 6 (2004) 945-952; I. Adachi, T. Sumiyoshi, K. Hayashi, N. Iida, R. Enomoto, K. Tsukada, R. Suda, S. Matsumoto, K. Natori, M. Yokoyama, H.
  • the surfactant prevents phase separation induced by the hydrophobic organic moiety (methyl) attached to the organosilane. Increased transparency by addition of surfactants is in overall agreement with the trend of other authors such as Husing ⁇ see N. Husing and U. Schubert, "Organofunctional Silica Aerogels", Journal of Sol-Gel Science and Technology 8, 807-812 (1997); N. Husing, U. Schubert, K. Misof and P. Fratzl, "Formation and Structure of Porous Gel Networks from Si(OMe)4 in the Presence of A(CH2) «Si(OR)3 (A) Functional Group)", Chem. Mater.
  • Organosilanes increase the flexibility of the aerogel skeleton (see K. Kanamori, M. Aizawa, K. Nakanishi, T. Hanada, "Elastic organic-inorganic hybrid aerogels and xerogels", J Sol-Gel Sci Technol (2008) 48, 172-181 ; and K. Kanamori, M. Aizawa, K. Nakanishi, and T. Hanada, "New Transparent Methylsilsesquioxane Aerogels and Xerogels with Improved Mechanical Properties", Adv. Mater. 2007, 19, 1589-1593), which is beneficial for freeze drying.
  • the derivatizing moiety can be introduced by exchanging the gelation solvent with a solution of the desired trifunctional alkoxide. However, processing steps could be saved by performing synthesis and derivatization in the same step.
  • Organosilane precursors can also be used alone or in combination with TEOS, TMOS and other alkoxides.
  • DCCA surfactants and drying control chemical additives
  • Kulkarni "Effect of glycerol additive on physical properties of hydrophobic silica aerogels", Materials Chemistry and Physics 77 (2002) 819-825; K. Kanamori, M. Aizawa, K. Nakanishi, and T. Hanada, “New Transparent Methylsilsesquioxane Aerogels and Xerogels with Improved Mechanical Properties", Adv. Mater. 2007, 19, 1589— 1593; and M. Nogami, S. Hotta, K. Kugimiya, H. Matsubara, “Synthesis and characterization of transparent silica-based aerogels using methyltrimethoxysilane precursor", J Sol-Gel Sci Technol (2010) 56, 107-113).
  • Narrow pore size distributions also help the freeze drying effort by leading to uniform freezing temperatures throughout the monolith and minimizing solvent diffusion and mechanical stresses, as discussed below in Section C.
  • Samples can be characterized with optical methods, but also and with scanning electron microscopy (SEM) and small-angle X-ray scattering (SAXS) (two alternative and complementary methods for structural investigation of the nanostructure of aerogels), and solid- state nuclear magnetic resonance (NMR) spectroscopy.
  • SEM scanning electron microscopy
  • SAXS small-angle X-ray scattering
  • NMR solid- state nuclear magnetic resonance
  • cross-linked aerogels For cross-linked aerogels, transparency has been seldom reported, and this is related to the cross-linking polymer and to the polymerization kinetics. As shown in FIG. 4, the cross-linking polymer attaches to the surface of the primary particles and yields a conformal coating of the oxide skeleton. It reinforces the aerogels but it also increases the size (and thus the scattering) of the skeletal aggregates. Thus, cross-linked aerogels are typically opaque.
  • FIGS. 2A, 2B, and 5 show that adequate choice of cross-linker
  • polyurethane and processing conditions (high temperatures) yields transparent materials.
  • a key for transparency is increasing the rate of the polymerization reaction to reduce the size of the polymer aggregates. Increased polymerization rates can be readily achieved with urethanes.
  • the gelation solution is typically exchanged with a solution of acetone and an isocyanate. The gel is then heated to about 60 °C (in a closed vessel to prevent solvent evaporation) to initiate polymerization ⁇ see M.A.B. Meador, L.A. Capadona, L. McCorkle, D. S. Papadopoulos, and N.
  • the polymerization temperature is determined by the low boiling temperature of the solvent (acetone).
  • acetone acetone
  • a solvent with a higher boiling point e.g. , propylene carbonate
  • higher temperatures accelerate the polymerization reaction and limit the size of polymer aggregates.
  • the transparent samples shown in FIGS. 2A, 2B, and 5 were synthesized at 100 °C. Likely, higher temperatures could further improve transparency.
  • Transparency could be further increased in two ways. Reducing the concentration of monomer in the gelation solution appears to be safest and most immediate solution.
  • the sample with best transparency (second left, top panel, FIG. 5) had also the lowest concentration of cross-linker. Yet, its mechanical properties were quite reasonable: the modulus was -15 MPa and the stress at break was ⁇ 1 MPa ⁇ see N. Leventis, C. Sotiriou-Leventis, G. Zhang, and A.-M. M. Rawashdeh Nano Letters, 2002, 2 (9), 957-960; M.A.B. Meador, L.A. Capadona, L. McCorkle, D. S. Papadopoulos, and N.
  • FIGS. 6A and 6B Another way of increasing transparency is to use materials with a fibrillar instead of a globular skeletal structure.
  • the cross-linking polymer builds a conformal coating of the oxide structure. If the starting structure is fibrillar as in FIG. 6A, the coating increases the diameter of the fibrils but it does not bridge between them. However, if the starting structure is more globular (FIG. 6B), the polymer can bridge between adjacent branches. The result provides an aggregate which is larger than the starting structure, and scattering is increased.
  • the native and the cross-linked samples reported in FIGS. 2A, 2B, and 5 can further be optimized for transparency, where light transmission was of about 65% in the sample of FIGS. 2A and 2B (which was optimized for strength, contained about 50% of polymer by weight and had a modulus of about 100 MPa), and of about 75% in the sample of FIG. 5 (which had a modulus of 15 MPa).
  • FIGS. 1A- 1D show that aerogel monoliths can be produced by freeze drying.
  • the fabrication of monoliths is typically limited by the size of the freeze dryer. Fabrication of aerogel panes with a minimum size of 15 x 15 cm and a thickness of up to 25 mm are possible with appropriately sized freeze dryers. This size is the minimum necessary to measure thermal conductivity with ASTM C518, which is the most common thermal test for insulation.
  • Aerogels according to the invention can be prepared with thicknesses ranging from 1 mm to 50 mm, for example, such as from 2-40 mm, or from 3-30 mm, or from 4-25 mm, or from 5-45 mm, or from 8-20 mm, such as from 15-28 mm, or from 18-32 mm, or from 9-24 mm, or from 12-26 mm, and so on.
  • Up-sizing to tens of cm needs to address stresses within the solid skeleton of porous materials induced by the freezing process. The stresses are caused by growth of large crystals inside pores and solvent diffusion between pores. Because of the larger surface-to-volume ratio, the solvent in small pores freezes at a lower temperature than the solvent in large pores.
  • Cross-linking has been realized as one way to strengthen the aerogels and prevent fragmentation. This indicates that considerable stresses arise within the monolith, even though t-butanol is being employed. Cross-linking likely allows up-scalable fabrication of monoliths.
  • the critical part size i.e., the largest size that can be dried without fragmentation
  • Aerogels produced in the past by freeze drying see E. Degn Egeberg, J.
  • a second strategy is use of urethane cross-linkers instead of the acrylics used in
  • FIGS. 1A-1D As shown in FIGS. 2A and 2B, urethane is a cross-linker that yields the strongest, yet transparent materials. Acrylics tend to yield opaque materials with a lower modulus than urethane cross-linkers (see L. S. White, M. F. Bertino, S. Saeed, K. Saoud, "Influence of silica derivatizer and monomer functionality and concentration on the mechanical properties of rapid synthesis cross-linked aerogels", Microporous and Mesoporous Materials, 217, 244-252 (2015)). An additional way of reducing freezing stresses is to synthesize materials with a narrow pore size distribution.
  • Flexible aerogels can be produced by careful tuning of density and concentration of cross-linking agent (see L. A. Capadona, M. A.B. Meador, A. Alunni, E. F. Fabrizio, P. Vassilaras, N. Leventis, "Flexible, low-density polymer crosslinked silica aerogels", Polymer 47 (2006) 5754-5761), by using a flexible cross-linker (see H. Guo, B. N. Nguyen, L. S. McCorkle, B. Shonkwiler and M. A. B.
  • the dried aerogel was opaque, likely because of residual water.
  • the synthetic procedure reported previously uses a high water concentration (50% by volume) in the gelation solution. Repeated washings with t-butanol are necessary to remove the water.
  • the test sample of FIG. 8C was washed twice in a lOx excess t-butanol, which was likely not sufficient to remove all the water. T-butanol expanded during freezing and yielded an opaque material. More experimentation is being carried out to determine the pore size distribution of the wet and dry gels and to determine the optimum number of washings.
  • Aerogels processed at 450 °C were used in one of the most successful aerogel window projects funded by the European Union (see K.I. Jensen, F. H. Kristiansen and J. M. Schultz, Public Final Report, Contract Number ENK6-CT-2002-00648, "Highly insulating and light transmitting aerogel glazing for super insulating windows", Nov. 2005).
  • temperatures > 400 °C sineresis occurs, which leads to larger pores and higher scattering. Transparency is recovered only at temperatures close to the sintering temperature, when the aerogel completely loses its porosity.
  • mild heat treatment could also be used to remove (partially or totally) the cross-linking polymer, and thus further increase transparency, as shown in FIG. 1C).
  • Optimum thermal conductivity is also important to minimize thickness, costs and light scattering. Hydrophobicity is important to prevent moisture penetration, and ultraviolet resistance must be ensured to prevent degradation of the materials. All these properties can be tweaked as appropriate.
  • Thermal conductivity is usually between 15 and 20 mW/m-K for native aerogels, around 30 mW/m-K for aerogels with a polymer content ⁇ 20% by weight, and of about 50 mW/m-K for aerogels with a polymer content > 50% by weight (see L. S. White, D. R. Echard, M. F. Bertino, X. Gao, S. Donthula, N. Leventis, N. Shukla, J. Kosny, S. Saeed and K. Saoud, "Fabrication of native silica, cross-linked, and hybrid aerogel monoliths with customized geometries", Transl. Mater. Res. 3 (2016); and N.
  • opacification can be carried out using Ti0 2 ⁇ see J. Wang, J. Kuhn, X.
  • a preferred thermal conductivity is 30 mW/m-K without opacification, and -25 mW/m-K with opacification.
  • a cross-linking solution of di-isocyanate, of a multifunctional isocyanate carrying an acrylic moiety, and of a highly hydrophobic monomer such as 2,3,4,5-pentafluorostyerene are commercially available and commonly used in industry, and they have been used in the past to fabricate hydrophobic aerogels (see U. F. Ilhan, E. F. Fabrizio, L. McCorkle, D. A. Scheiman, A. Dass, A. Palczer, M. A. B. Meador, J. C. Johnston and N. Leventis, "Hydrophobic monolithic aerogels by nanocasting polystyrene on amine-modified silica", /. Mater. Chem., 2006, 16, 3046-3054).
  • UV radiation to test for aging.
  • Silica, cross-linkers and hydrophobic derivatizers are not strongly affected by high temperatures. Moisture is likely not an issue, as long as the materials remain hydrophobic. UV light, instead, can degrade the organic components of the materials.
  • TiO is transparent in the visible, is a good UV absorber and it would also contribute to opacification.
  • Composites can be analyzed with standard analytical techniques (absorption spectroscopy, FT-IR, NMR, SEM, etc.) during the aging testing to determine the causes of deterioration and tweak the synthesis correspondingly.
  • the aerogels can have a light transmittance ranging from 10% up to 100%, such as from 15% to 90%, or from 20% to 95%, or from 25% to 80%, or from 30% to 85%, or from 40% to 75%, or from 60% to 78%, or from 70% to 98%, and so on.
  • cross-linked aerogels can be used as Category 1 products with minimal optimization. These materials are mechanically strong (10-100 MPa in modulus), can be glued without shearing (see L. S. White, D. R. Echard, M. F. Bertino, X. Gao, S. Donthula, N. Leventis, N. Shukla, J. osny, S. Saeed and K. Saoud, "Fabrication of native silica, cross-linked, and hybrid aerogel monoliths with customized geometries", Transl. Mater. Res. 3 (2016)) and would only need a polycarbonate sheet (1/32 inch thick) on the exterior surface for scratch protection and a low-e coating (0.1) to improve thermal insulation.
  • the required U-value would require a minimum pane thickness of 5/32 inch for a thermal conductivity of 30 mW/m-K, see FIG. 9.
  • a light transmittance of 80% for a 5/32 inch thick panel is equivalent to a light transmittance of 57% for a pane thickness of 10 mm.
  • This light transmittance is easily achieved by aerogels.
  • the panes would be sandwiched between glass panes, 3/64 inch thick to increase mechanical strength. This configuration is particularly attractive for mechanically weak aerogels.
  • the sandwich structure would require use of tempered glass to make composite as strong as a conventional glass pane.
  • Tempered glass in fact, has an ultimate strength ⁇ 8 times higher than that of soda lime glass.
  • techniques to minimize scattering and reflection at the interfaces should be employed, including determining optimum adhesives, and optimizing low-e coatings. Seal edge and desiccant technology is also a consideration to be integrated to fabricate a full-scale pane laminate.
  • Translucent and transparent aerogels can also be fabricated from compositions disclosed by the Group of N. Leventis in Chem. Mater. 2006, 18, 285-296.
  • gels can be fabricated using 3-aminopropyltriethoxysilane (APTES) and tetramethylorthosilicate as silica precursors. It is believed that APTES serves as derivatizer for the pore walls and a catalyst for the gelation reaction.
  • APTES 3-aminopropyltriethoxysilane
  • Samples can be prepared for example using 0.350 ml TMOS, 0.067 ml APTES, 0.626 ml Acetonitrile, and 0.105 ml H 2 0.
  • the sample can be placed in a leak-tight container filled with excess acetonitrile at a 5:1 ratio and set in an oven for four hours at 70 °C.
  • the acetonitrile can then be exchanged for fresh acetonitrile at a 5: 1 ratio, and then again after 3 hours.
  • the solvent can then be exchanged with a solution of an isocyanate (such as di-isocyanate or tri-isocyanate) in acetonitrile.
  • an isocyanate such as di-isocyanate or tri-isocyanate
  • a typical solution comprises 0.560 g di-isocyanate in 10 ml acetonitrile, however, other proportions are also possible.
  • the sample can be placed in a 5:1 excess acetonitrile and placed in an oven at 70 °C for 24 hours. During this time, the sample is cross-linked with di-isocyanate. After cross-linking with di-isocyanate is complete, the pore-filling acetonitrile solution can be exchanged with tert-butanol or another freeze-dryable solvent.
  • the samples are typically exchanged with a 5:1 wash of tert-butanol three times, with the first exchange taking two hours and the remaining washes three hours.
  • DMSO dimethylsulfoxide
  • solvents used in freeze drying are cyclohexane or dimethylsulfoxide (DMSO). These solvents are popular because they freeze near room temperature. Other solvents such as ethanol can be theoretically employed. However, most organic solvents require freezing to very low temperatures, which makes their use more impractical. It is preferred that the solvent used for freeze drying be kept “dry” (as free of water as possible (such as ⁇ 1% by volume or better)). However, use of solvents that form a eutectic or a glassy phase with water, such as for example DMSO, could be potentially employed.
  • DMSO dimethylsulfoxide
  • Samples exhibiting a preferred level of transparency or translucency can be obtained according to this procedure, which allows for very rapid gelation. Rapid gelation minimizes the size of light-scattering aggregates and improves transparency. Rapid gelation can also be attained by gelation at high temperatures, and/or by using excesses of acid or base catalysts, and/or by using di-isocyanates. Another preferred technique includes aging the sample for a minimum of four hours before cross-linking in an oven kept at 70°C, which is believed to reinforce the skeleton of the aerogel to withstand freezing stresses. Additionally, it has been found that samples that are frozen slowly have lower transparency than samples frozen quickly (for example, by using higher freezing temperatures).
  • Samples can be frozen by placing them in refrigerators, but are best refrigerated by placing them into a pre-cooled liquid, such as water-antifreeze mixtures, or organic solvents such as methanol.
  • a pre-cooled liquid such as water-antifreeze mixtures, or organic solvents such as methanol.
  • the higher density of the liquid ensures more rapid cooling. Best results are obtained when the temperature of the whole sample is brought below the freezing point of the sample in less than 4 minutes, but other velocities are possible.
  • FIGS. 10A-B A transparent sample with a thickness of 6 mm is shown in FIG. 10A.
  • a translucent 3 mm disk prepared without an oven aging step is shown in FIG. 10B.
  • the samples have a density of 0.457 grams per cubic centimeter, a surface area of about 350 m 2 /g and a mean pore radius of 5 nm.
  • more opaque (but still translucent) aerogels prepared according to other methods can result in samples having a density of 0.457 grams per cubic centimeter, a surface area between 200 and 250 m 2 /g, and a mean pore size of about 5 nm, with the lower surface area indicative of formation of light- scattering macropores in the opaque materials.
  • Adsorption isotherms for transparent and more cloudy materials are of type IV, as shown in FIGS. 11A-B.
  • pore size distributions shown are calculated using the BJH approximation (FIG. 11 A) and an adsorption isotherm of transparent and translucent regions of the same sample is shown (FIG. 1 IB), where the translucent region was in the core and had been frozen more slowly than the outer regions.

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Abstract

L'invention concerne un procédé de synthèse d'aérogels et d'aérogels réticulés qui incorpore une lyophilisation au lieu d'un séchage par un solvant supercritique. Les avantages par rapport au séchage supercritique comprennent une réduction des risques de danger posés par le séchage dans des conditions supercritiques ainsi que la possibilité de mettre à l'échelle le procédé pour recevoir de grandes pièces de matériau sans introduire de risque. De plus, des technologies de moule peu coûteuses et plus sophistiquées, qui ne sont pas insensibles aux conditions supercritiques, peuvent être utilisées pour produire des matériaux de type aérogel selon le procédé de lyophilisation de l'invention. Ceci introduit un niveau de liberté qui n'a jamais été disponible précédemment pour la production de constituants de type aérogel.
PCT/US2017/028903 2016-04-21 2017-04-21 Procédés de fabrication d'aérogels de silice présentant des formes personnalisées à l'aide d'une lyophilisation Ceased WO2017185009A1 (fr)

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CN111072037A (zh) * 2020-02-10 2020-04-28 洪永建 一种柔韧性较好的二氧化硅气凝胶的制备方法
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WO2020264070A1 (fr) * 2019-06-28 2020-12-30 Virginia Commonwealth University Fabrication d'aérogels et de composites d'aérogel obtenus par sublimation à pression ambiante de solvants congelés
US10889501B2 (en) 2016-02-24 2021-01-12 Massachusetts Institute Of Technology Solar thermal aerogel receiver and materials therefor
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CN115340098A (zh) * 2022-08-15 2022-11-15 中国科学院苏州纳米技术与纳米仿生研究所 高自粘结强度的氧化硅气凝胶材料、其制备方法及应用

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EP3990275A4 (fr) * 2019-06-28 2023-01-25 Virginia Commonwealth University Fabrication d'aérogels et de composites d'aérogel obtenus par sublimation à pression ambiante de solvants congelés
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