WO2013055433A2

WO2013055433A2 - Coating of spheres and tubes with aerogels

Info

Publication number: WO2013055433A2
Application number: PCT/US2012/049853
Authority: WO
Inventors: Christoph DAWEDEIT; Juergen Biener; Alex V. Hamza; Sung Ho Kim; Joe Satcher; Christopher C. WALTON; Marcus A. WORSLEY; Kuang Jen Wu
Original assignee: Lawrence Livermore National Security, Llc
Priority date: 2011-08-08
Filing date: 2012-08-07
Publication date: 2013-04-18
Also published as: WO2013055433A3

Abstract

A method for preparing aerogel coatings in the interior surface of a tube or sphere using a device simulating a microgravity environment is described.

Description

COATING OF SPHERES AND TUBES WITH AEROGELS

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 61/521 ,310, filed August 8, 201 1 , which is incorporated in its entirety herein for all purposes.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

[0002] The United States Government has rights in this invention pursuant to Contract No.

DE-AC52-07NA27344 between the United States Department of Energy and Lawrence

Livermore National Security, LLC for the operation of Lawrence Livermore National

Laboratory.

BACKGROUND OF THE INVENTION

[0003] New approaches to ICF target fabrication need to be developed to build the increasingly complex targets necessary to take full advantage of the unique laboratory environment created by ICF experiments. Foam-lined indirect-drive ICF targets, the subject of this work, are related to spherical foam targets for direct-drive and fast-ignition fusion experiments that have been developed over the last 20 years. The function of the spherical foam shell in direct-drive fusion targets is to define the shape of the cryogenic deuterium-tritium (DT) fuel layer as first pointed out by Sacks and Darling, or to act as a surrogate to simulate the cryogenic fuel layer. This application can tolerate relatively thick (> 50 μιτι) foam shells and, in the case of surrogate targets, high-density foams ( 1 80-250 mg/cc). Targets containing these mechanically relatively robust foam shells can be fabricated by first fabricating unsupported foam shells using a triple-orifice droplet generator, followed by coating of the dried foam shells with a thin permeation barrier.

[0004] By contrast, the foam liner in the indirect-drive NIF targets described here will serve as a scaffold to bring dopants in direct contact with the DT fuel for diagnostics and nuclear physics experiments. This function requires thinner and lower-density foam shells in combination with a much thicker ablator shell (table 1 ). This makes the fabrication of these targets by established methods very difficult if not impossible. For example, the thinnest shells that have been fabricated with the triple-orifice-droplet generator technique had a wall thickness of -20 pm. These shells, however, had a diameter of only 500 μιτι and were made from a higher-density foam formulation. In addition, they were mechanically very sensitive, difficult to dry, and showed relatively large deviations from roundness.

Table 1: Comparison of ICF targets for direct/indirect drive experiments on OMEGA and NIF.

Indirect drive fusion

Direct drive targets for

Requirements application targets for

Omega

NIF

Shell diameter [mm] 0.8-0.9 2

Thickness of the foam shell [μιτι] 50-120 15-30

Foam density [mg/cc] 50-250 < 30

Mostly resorcinol-

Foam composition pure CH_X

formaldehyde (RF) based

Permeation barrier/ablator thickness [μηη] 1 -5 80-1 50

Glow discharge polymer

GDP, Be, High-density

Permeation barrier/ablator material (GDP), polyvinylphenol

Carbon (HDC) (PVP)

[0005] To avoid the challenges that arise from handling free-standing foam shells, we explored a new approach based on using prefabricated ablator shells as molds to cast concentric thin- walled low-density foam shells by sol-gel chemistry. Here, we describe the development of crucial components of this approach, including a new polymer-based sol-gel chemistry, filling of the ablator with the sol-gel precursor solution with nanol iter precision, and a coating technique that is capable of producing uniform coatings on the inside of spherical capsules.

BRIEF SUMMARY OF THE INVENTION

[0006] In one embodiment, the present invention provides a method for coating an interior surface of a container with an aerogel, including forming a reaction mixture in the container of a first monomer and a solvent; rotating the container to simulate a microgravity environment in the container's interior, under conditions sufficient to form a wet gel coating the container's interior surface; and contacting the wet gel with supercritical carbon dioxide under conditions suitable to form the aerogel, thereby forming the aerogel coating.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 shows a schematic fabrication process of indirect-drive foam-lined ICF targets: (a) filling of a prefabricated ablator shell with the desired amount of an aerogel precursor solution, (b) formation of a smooth and uniform gel layer by deterministic rotation of the capsule during

polymerization, and (c) doping and supercritical drying.

[0008] Figure 2 shows DCPD-based aerogels: (a) schematic diagram of the copolymerization of DCPD and NB with the ROMP reaction catalyzed by a Grubbs' ruthenium complex; (b-c) effect of NB addition on uniformity of nominally 180 μιτι thick gel layers formed in a rotating horizontal glass vial; (c) NB addition (10 wt.%) drastically improves the film homogeneity; (d) USAXS for a 30 mg/cc DCPD aerogel through stages of filling with liquid H₂: vacuum, at ~22K exposed to gaseous hydrogen, at 15K submerged in liquid ¾ as depicted in the inset, and then warmed and returned to a gaseous H₂ environment; (e) x-ray computed microtomography of identical 30 mg/cc DCPD samples, as prepared (top) and after wetting (bottom) with liquid hydrogen. Formation of cracks was not observed.

[0009] Figure 3 shows Capsule filling setup and filling calibration: (a) the filling setup consists of a small vacuum chamber that contains a vial with the aerogel precursor solution and a linear feed through to which the ablator shell is attached. To achieve high reproducibility with high accuracy, it can be helpful underpressurize the capsule before submerging it into the precursor solution; (b) injected liquid and corresponding layer thickness versus applied pressure differential. The injected volume, as derived from weight gain measurements, follows the linear behavior (dashed line) expected from the ideal gas law and is almost independent of the hole diameter.

[0010] Figure 4 shows foam layer fabrication: (a) coating setup and (b) motion of a random surface point of the rotating sphere; (c) radiograph of a 2-mm-HDC capsule (I.D.) coated with a P(DCPD-r-NB) wet gel layer (50 mg/cc DCPD, 20 wt.% NB, 0.2wt.% catalyst, filled at ΔΡ= 102.4 torr, rotated at 10/14.142 rpm for ~5 h), and (d) corresponding power mode spectrum of the inner wet gel surface. [0011] Figure 5 shows (a) Radiograph of a 2-mm-diamond capsule with an iodine-doped P(DCPD-r- NB) foam layer (25 mg/cc DCPD, 15 wt.%NB, 0.2wt.% catalyst, filled with Δρ=104.5 torr, and coated at 10/14.142 rpm for 17 h). The feature in the lower left corner of the shell is glue residue on the outside of the ablator shell; (b) two representative power spectra of the inner foam surface summarizing the currently achieved foam shell uniformity.

[0012] Figure 6 shows the ROMP process using dicyclopentadiene (DCPD) and an optionally substituted norbornene (NB) with Grubbs' first generation catalyst to prepare an aerogel of the present invention.

[0013] Figure 7 shows a plot of the gelation timescale as the catalyst concentration is increased for the DCPD/NB aerogel.

[0014] Figure 8 shows a plot of viscosity and strength for DCPD (50 mg/cm^J; 0.1 wt% catalyst).

[0015] Figure 9 shows the relationship between increasing norbornene concentration and viscosity and shear resistance for the DCPD/NB gel.

[0016] Figure 10 shows ultra-small-angle X-ray scattering for an empty shell, a filled shell and a shell coated with a DCPD aerogel at 30 mg/cm³ density, demonstrating that aerogels prepared in the container and in a beaker have the same void structure and density.

[0017] Figure 1 1 shows a two dimensional model of a rotating cylinder.

[0018] Figure 12 shows the non-concentricity (mode 1 ) is less than 3 microns for the aerogels of the present invention.

[0019] Figure 13 shows the roughness of the aerogel coating of the dried foam shell with a surface roughness of less than 10 nm.

[0020] Figure 14 shows DCPD/NB gels prepared with increasing amounts of norbornene, and demonstrates that higher concentrations of norbornene result in smaller feature sizes in the gel, as demonstrated by the decreasing opacity of the gel.

[0021] Figure 1 5 shows cycloolefin monomers of the present invention. [0022] Figure 16 shows a sphere with a silica aerogel coating prepared from tetramethoxy silane.

[0023] Figure 17 shows an aerogel coating in a vial prepared from resorcinol/formaldehyde. [0024] Figure 18 shows a cut-away of a target of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

I. GENERAL

[0025] The present invention provides a method for preparing substantially uniform aerogel coatings on the interior surface of a sphere, tube or other object, by rotating the object around at least one axis to simulate a microgravity environment. The aerogel coatings are substantially uniform and can have a roughness of less than 10 μιη.

II. DEFINITIONS

[0026] "Monomer" refers to a compound containing a polymerizable group such as a olefin. Representative monomers include cycloolefins, tetraalkoxysilanes, hydroxyaryls, aldehydes, and transition metal containing monomers.

[0027] "Alkoxy" refers to an alkyl group having an oxygen atom that connects the alkyl group to the point of attachment: alkyl-O-. As for alkyl group, alkoxy groups can have any suitable number of carbon atoms, such as C_{] -6}. Alkoxy groups include, for example, methoxy, ethoxy, propoxy, iso-propoxy, butoxy, 2-butoxy, iso-butoxy, sec-butoxy, tert-butoxy, pentoxy, hexoxy, etc.

[0028] "Aryl" refers to an aromatic ring system having any suitable number of ring atoms and any suitable number of rings. Aryl groups can include any suitable number of ring atoms, such as, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15 or 16 ring atoms, as well as from 6 to 10, 6 to 12, or 6 to 14 ring members. Aryl groups can be monocyclic, fused to form bicyclic or tricyclic groups, or linked by a bond to form a biaryl group. Representative aryl groups include phenyl, naphthyl and biphenyl. Other aryl groups include benzyl, having a methylene linking group. Some aryl groups have from 6 to 12 ring members, such as phenyl, naphthyl or biphenyl. Other aryl groups have from 6 to 10 ring members, such as phenyl or naphthyl. Some other aryl groups have 6 ring members, such as phenyl. Aryl groups can be substituted or unsubstituted. [0029] "Hydroxy-aryl" refers to an aryl groups as described above, substituted with one or more hydroxy groups. Representative hydroxy-aryl groups include, but are not limited to, phenol, resorcinol and phloroglucinol.

[0030] "Solvent" refers to water-miscible or -immiscible solvents capable of dissolving either or both of water-soluble and water-insoluble compounds. Exemplary organic solvents useful in the present invention include, but are not limited to, alcohols, acids, polyols and other water-miscible organic solvents such as methanol, ethanol, tetrahydrofuran, diethylether, benzene, toluene, ethyl acetate, acetone and hexanes, among others. Other solvents include water, glycols, dimethylformamide, dimethylacetamide, etc. [0031] "Aerogel" refers to a crosslinked polymer system, or gel, where the solvent or liquid used to form the gel has been removed and replaced with air. The aerogel can be prepared using a variety of polymers, such as organ ic or inorganic polymers. The aerogels are prepared by forming a crosslinked polymer matrix trapping solvent in the pores of the gel, a wet gel, and then drying the wet gel using supercritical carbon dioxide. [0032] "Supercritical carbon dioxide" refers to a fluid state of carbon dioxide where the pressure and temperature are maintained at or above the critical point of carbon dioxide of 304 (31 °C) and 7.39 MPa ( 1071 psi).

[0033] "Liquid carbon dioxide" refers to a fluid state of carbon dioxide where the pressure and temperature are maintained at or above the triple point of carbon dioxide of 216 K (-57°C) and 517 kPa (75 psi).

[0034] "Simulating a microgravity environment" refers to manipulating the container of the present invention such that the contents of the container, the reaction mixture, act as if they are in a microgravity environment. In the present invention, the microgravity environment is simulated by rotating the container about at least one axis of the container. [0035] "Roughness" refers to the texture of the surface of the aerogel coating on the interior surface of the container. Roughness is measured by the vertical deviation of the surface from the ideal surface, and can be represented by the measuring the average distance from the troughs of the valleys to the height of the peaks. For the aerogel coatings of the present invention, the roughness can be measured by first plotting the mode ( \/2nr) versus the mean power of layer thickness variation in the coating (μηι²), thereby generating the power number. The square root of the power number then provides the roughness value, the average of the depth of the trough to the height of the peak. A roughness number of 1 μηι is considered to be substantially uniform. Other roughness numbers are also considered to be substantially similar and can be calculated using other methods.

[0036] "Ring opening metathesis polymerization" (ROMP) refers to a process of cyclic olefin metathesis chain-growth polymerization using a catalyst such as RuCl₃. ROMP can use a variety of catalysts such as the Grubbs catalyst, a dichloro ruthenium catalyst. The Grubbs' catalyst includes the following catalysts, and analogs thereof:

[0037] "Metal" refers to elements of the periodic table that are metallic and that can be neutral, or negatively or positively charged as a result of having more or fewer electrons in the valence shell than is present for the neutral metallic element. Metals useful in the present invention include the alkali metals, alkali earth metals, transition metals and post-transition metals. Alkali metals include Li, Na, K, Rb and Cs. Alkaline earth metals include Be, Mg, Ca, Sr and Ba. Transition metals include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, La, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg and Ac. Post-transition metals include Al, Ga, In, Tl, Ge, Sn, Pb, Sb, Bi, and Po. One of skill in the art will appreciate that the metals described above can each adopt several different oxidation states, all of which are useful in the present invention. In some instances, the most stable oxidation state is formed, but other oxidation states are useful in the present invention.

[0038] "Dopant" refers to a component added in low concentration to alter the electrical or optical properties of the substance. Dopants useful in aerogel coatings of the present invention include metals, halogens, and inorganic particles. III. METHOD FOR PREPARING AN AEROGEL COATING

[0039] The present invention provides a method for coating an interior surface of a container with an aerogel, including forming a reaction mixture in the container of a first monomer and a solvent; rotating the container to simulate a microgravity environment in the container's interior, under conditions sufficient to form a wet gel coating the container's interior surface; and contacting the wet gel with supercritical carbon dioxide under conditions suitable to form the aerogel, thereby forming the aerogel coating.

[0040] The aerogel can be any suitable aerogel prepared from materials described below. The aerogel coating can have any suitable thickness. For example the aerogel coating can have a thickness from about 0.1 μιτι to about 1 mm, or from about 1 μπι to about 500 μηι, or from about 1 μηι to about 100 pm, or from about 5 μιη to about 50 μιτι, or from about 10 μπι to about 25 μιη. In some embodiments, wherein the aerogel coating has a thickness of from about 1 to about 500 μηι. In some embodiments, the aerogel coating has a thickness of from about 5 to about 50 μιη.

[0041] The aerogel coating can have any suitable density. For example, the aerogel coating can have a density of from about 1 to about 1000 mg/cm³, or from about 1 to about 500 mg/cm^J, or from about 1 to about 100 mg/cm^J, or from about 1 to about 50 mg/cm \ or from about I to about 25 mg/cm^J, or from about 5 to about 25 mg/cm³. In some embodiments, the aerogel coating has a density of from about 0.1 to about 100 mg/cm^J. In some embodiments, the aerogel coating has a density of from about 5 to about 25 mg/cm³. [0042] The aerogel coating can have any suitable roughness. Roughness is a measure of the vertical deviation of the actual surface from the ideal surface, and can be represented by the measuring the average distance from the troughs of the valleys to the height of the peaks. For the aerogel coatings of the present invention, the roughness can be measured by first plotting the mode (\/2nr) versus the mean power of layer thickness variation in the coating (μηι ), thereby generating the power number. The square root of the power number then provides the roughness value, the average of the depth of the trough to the height of the peak. A roughness number of 1 μπι is considered to be substantially uniform. Other roughness numbers are also considered to be substantially similar and can be calculated using other methods. For example, the aerogel coatings of the present invention can have a roughness of less than 100 μπι, 90, 80, 70, 60, 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2 or 1 μιτι. In some embodiments, the aerogel coating has a roughness of less than about 10 μηι. In some embodiments, the aerogel coating has a roughness of less than about 2 μηι. In some embodiments, the aerogel coating is substantially uniform.

[0043] The container of the present invention can be any su itable container having a curved interior surface. Representative containers include tubes, cylinders, ellipsoids and spheres. The containers can be of any suitable material, including, but not limited to, carbon-based, metallic, silica, polymeric, or combinations thereof. For example, the container can be high density carbon. The container can also be any suitable size and dimension. For example, the container can have a diameter of from about 0.1 mm to about 10 cm, or from about 0.1 mm to about 1 cm, or from about 0.1 mm to about 500 mm, or from about 0. 1 mm to about 100 mm, or from about 0.1 mm to about 50 mm, or from about 0.1 mm to about 10 mm, or from about 1 mm to about 10 mm. In some embodiments, the container has a diameter of from about 0.1 mm to about 10 cm.

[0044] The method of the present invention simulates a microgravity environment during the preparation of the aerogel. The simulated microgravity environment enables the preparation of a substantially uniform aerogel coating. Because gelation occurs in the solution layer flowing against gravitational acceleration, overcoming the effect of gravitation would require rotational speeds in excess of 1000 rpm, thus the unavoidable film thickness non-uniformity is provided by: 2gh³ wherein h is the average film thickness, R is the radius, ω is the rotational velocity and v is the viscosity.

[0045] The microgravity environment can be simulated using a variety of devices, such as the device shown in Figure 4a, using two perpendicular and independently driven rotating frames in combination with computer controlled software to provide a deterministic, continuous random change in orientation relative to the gravity vector. Rotation about axis 1 (col) and axis 2 (co2) can be independent and operate at a variety of speeds. In some embodiments, the container is rotated about a single axis. In some embodiments, the container is rotated about two axes. In some embodiments, the container is rotated about two perpendicular axes. [0046] The aerogels of the present invention can be prepared from any suitable material. For example, the monomers used in the polymerization process to make the aerogels can be any suitable monomer, such as organic materials, transition metal-containing monomers, inorganic monomers, etc. Suitable organic monomers and monomer combinations include cycloolefins suitable for ring opening metathesis polymerization (ROMP), and combinations of monomers such as polyhydroxy-aryls and formaldehyde. Cycloolefins suitable in the method of the present invention include cycloalkyl groups containing one or more olefins. The cyclooalkyl groups of the present invention refer to a partially unsaturated, monocyclic, fused bicyclic or bridged polycyclic ring assembly containing from 5 to 20 ring atoms, or the number of atoms indicated. The cycloolefins can have 1 , 2, 3 or more olefins. Representative cycloolefins include, but are not limited to, cyclobutene, cyclopentene, norbornene, norbornadiene, cyclopentadiene, cyclooctene, cyclooctadiene, cyclododecatriene, and dicyclopentadiene, and analogs thereof. Other cycloolefins useful in the methods of the present invention are those described in Figure 15 (l ,4-di(bicyclo[2.2.1 ]hept-5-en-2-yl)benzene and l ,3,5-tri((bicyclo[2.2.1 ]hept-5-en-2- yl)methoxymethyl)-2,4,6-trimethylbenzene). [0047] Other organic monomers useful in the methods of the present invention include the a hydroxy-aryl in combination with an aldehyde such as formaldehyde. Hydroxy-aryls useful in the methods of the present invention include, but are not limited to, phenol, resorcinol, phloroglucinol, and analogs thereof. Aldehydes useful in the methods of the present invention include, but are not limited to, formaldehyde, acetaldehyde, propionaldehyde, among others. In some embodiments, the hydroxy-aryl can be resorcinol and the aldehyde can be formaldehyde. [0048] Inorganic monomers useful in the methods of the present invention include tetra-alkoxy silanes such as tetramethyl silane (TMOS) and tetraethoxy silane (TEOS). These monomers, when combined with a suitable solvent such as methanol or water, and optionally in combination with ammonium hydroxide, form silica gels. [0049] Other monomers useful in the methods of the present invention include transition metal containing monomers. Suitable transition metals include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, La, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg and Ac.

[0050] When the monomers include the cycloolefins described above, the polymerization process can be carried out using ring opening metathesis polymerization (ROMP). To make the aerogel via ROMP, at least one of the monomers used in the polymerization can contain at least two polymerizable olefins, such as dicyclopentadiene, norbornadiene, or the monomers shown in Figure 15. Other cycloolefin monomers can also be used, and in some embodiments, a combination of monomers is useful. For example, the combination of a cycloolefin with two olefins, such as dicyclopentadiene, and a cycloolefin with a single olefin, such as norbomene, are particularly useful in the methods of the present invention. Other combinations of cycloolefin monomers are also useful in the present invention. When a combination of monomers is used, the two monomers can be present in any suitable relative amount. For example, the ratio of the first monomer to the second monomer can be 1 : 100, 1 :50, 1 :25, 1 :20, 1 : 15: 1 : 10, 1 :9, 1 :8, 1 :7, 1 :6, 1 :5, 1 :4, 1 :3, 1 :2, 1 : 1 , 2: 1 , 3: 1 , 4: 1 , 5 : 1 , 6: 1 , 7:1 , 8: 1 , 9: 1 , 10: 1 , 15: 1 , 20: 1 , 25: 1 , 50: 1 , or 100: 1 , based on weight or moles.

[0051] When the monomer used in the method of the present invention is a cycloolefin that is polymerized via ROMP, a transition metal catalyzed can be present to catalyze the

polymerization. Suitable catalysts include ruthenium based catalysts such as RuC^, the Grubbs' first generation catalyst, the Grubbs' second generation catalyst, and the Hoveyda-Grubbs catalyst (see descriptions above). The catalyst can be present in any suitable amount based on the weight of the monomers present. For example, the catalyst can be present in an amount of at least 0.01 %, 0.02, 0.03, 0.4, 0.05, 0.1 , 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 , 5, or 10% by weight.

[0052] The methods of the present invention can also include any suitable solvent. When the polymerization method is ROMP, the solvent can be an organic solvent such as toluene, benzene, or dimethyl benzene. When the polymerization method uses TMOS or TEOS as the monomer, the solvent can be an alcohol, such as methanol or ethanol, or water.

[0053] In some embodiments, the first monomer can be dicyclopentadiene or norbornadiene, wherein the composition also includes a second monomer that can be norbornene, cyclobutene, cyclopentene, cyclooctene, cyclooctadiene or cyclododecatriene, and a catalyst suitable for use in Ring Opening Metathesis Polymerization (ROMP). In some embodiments, the catalyst is a Grubbs' catalyst. In some embodiments, the catalyst can be the first generation Grubbs catalyst, the second generation Grubbs catalyst, or the Hoveyda-Grubbs catalyst.

[0054] When multiple monomers are present, in some embodiments, the ratio of the first monomer to the second monomer is greater than about 5: 1 (wt/wt). In some embodiments, the ratio of the first monomer to the second monomer is greater than about 10: 1 (wt/wt). In some embodiments, the ratio of the first monomer to the second monomer is greater than about 20: 1 (wt/wt).

[0055] The method of the present invention can be carried out under any suitable reaction conditions. For example, any suitable time, temperature or concentration of reactants can be used. In some embodiments, the reaction mixture is heated at about 80°C when the monomer is a cycloolefin.

[0056] In some embodiments, the first monomer can be a tetraalkoxy silane, the solvent can be water, such that the aerogel is a silica aerogel. In some embodiments, the first monomer can include a transition metal.

[0057] The method of the present invention also includes drying the wet gel to form the aerogel, i.e., removing the solvent in the pores of the gel. The drying of the wet gel can be accomplished by any method known to one of skill in the art. For example, the wet gel can be dried using supercritical carbon dioxide, where the carbon dioxide replaces the solvent in the pores of the wet gel, and then evaporates as the pressure is slowly decreased, thereby replacing the solvent with air and forming the aerogel. The supercritical carbon dioxide is formed by maintaining the pressure and temperature above the critical point for carbon dioxide (304 (31 °C) and 7.39 MPa ( 1071 psi)). For example, the step of contacting the wet gel with supercritical carbon dioxide can be at any temperature of at least 31 °C in combination with a pressure of at least 1071 psi, including, but not limited to, a temperature of at least 35, 40, 45, 50, 55 or at least 60°C. Moreover, the pressure can be any suitable pressure of at least 1071 psi in combination with a temperature of at least 31 °C, including, but not limited to, a pressure of at least 1 100, 1200, 1300, 1400, 1500, 1600, 1700, or at least 1800 psi. In some embodiments, the step of contacting the wet gel with the supercritical carbon dioxide is performed at a temperature of about 50°C and a pressure of about 1600 psi.

[0058] The drying process can also include an intermediate step of replacing the solvent in the wet gel with liquid carbon dioxide that is not in the supercritical phase. Liquid carbon dioxide is obtained when the temperature and pressure are maintained above the triple point of carbon dioxide (216 (-57°C) and 517 kPa (75 psi)). For example, liquid carbon dioxide can be obtained at any temperature of at least -57°C in combination with a pressure of at least 75 psi, including, but not limited to, a temperature of at least -40, -30, -20, - 10, 0, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, or at least 20°C. Moreover, the pressure can be any suitable pressure of at least 75 psi in combination with a temperature of at least -57°C, including, but not limited to, a pressure of at least 75, 100, 1 50, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800, 900, or at least 1000 psi. In some embodiments, the method of the present invention includes contacting the wet gel with liquid carbon dioxide prior to contacting the wet gel with supercritical carbon dioxide. In some embodiments, the step of contacting the wet gel with liquid carbon dioxide is performed at a temperature of about 12°C and a pressure of about 900 psi. [0059] The aerogel coatings of the present invention can also be doped. Suitable dopants include, but are not limited to, transition metals, particles such as inorganic particles, carbon nanotubes, and halogens such as bromine and iodine. In some embodiments, the method of the present invention also includes contacting the aerogel coating with a dopant to dope the aerogel coating. In some embodiments, the dopant is iodine. IV. TARGETS

[0060] The methods of the present invention can be used to prepare aerogel-lined inertia] confinement fusion (ICF) targets. The targets prepared by the methods of the present invention can be characterized by an aerogel coating with a thickness of 25 μπη and a density of 25 mg/cm³. In some embodiments, the present invention provides a target having a container having an interior surface, and an aerogel coating the interior surface wherein the aerogel has a thickness of from about 10 μιτι to about 200 μιη, a density of less than about 50 mg/cm³, and a roughness of less than about 10 μιτι. In some embodiments, the present invention provides a target prepared by the method of the present invention.

V. EXAMPLES

Example 1. Preparation of Aerogel Coating via Ring Opening Metathesis Polymerization

[0061] Dicyclopentadiene (DCPD, C,₀Hi₂, Aldrich), norbornene (NB, C₇H,₀, 99%, Aldrich), and Grubbs' 1 ^st generation catalyst, bis(tricyclohexylphosphine)benzylidene ruthenium (IV) dichloride (+97%, Aldrich) were used as received without further purification. As supplied, the DCPD monomer is predominantly endo isomer and contains butylated hydroxytoluene as stabilizer. Toluene (anhydrous, 99.8%, Aldrich) was bubbled and degassed with nitrogen prior of use.

Filling of Container

[0062] The foam shell thickness is controlled by the volume of the precursor solution filled into the ablator. To achieve the required μιη-scale thickness control, we developed a pressure- differential filling method that allows us to reproducibly inject 0.1 -1 μΐ of the precursor solution (for 2-mm-diameter shells, this translates into a layer thickness of 10-1 30 μιη) into the ablator. The filling setup is described in detail in figure 3a. It consists of a small vacuum chamber that contains a vial filled with the precursor solution and a linear feedthrough to which the ablator shell is attached. First, the chamber is slightly underpressurized (20 - 250 torr below atmospheric pressure, depending on the desired foam layer thickness) while the capsule is not in contact with the precursor solution. This prevents the formation of air-bubbles attached to the fill hole that is critical for achieving the required reproducibility. The under-pressurized capsule is then submerged in the precursor solution, and filled by re-pressurizing the chamber to atmospheric pressure. Concentration changes by loss of toluene or DCPD NB through evaporation are minimized by keeping the time between evacuation and re-pressurization as short as possible. The injected volume, as derived from weight gain measurements with the density of toluene under ambient conditions (0.867 g/cc), increases linearly with the applied pressure differential (with -0.01 μΐ precision) and is almost independent of the hole diameter (figure 3b). The linear behavior can be fit by an expression easily derived from the ideal gas law: AV= (V_s/p₀)Ap where AV is the injected volume, Ap is the applied pressure differential, and V_s and p₀ are the shell volume (4.2 μΐ) and standard pressure (760 torr), respectively.

Gel Formation

[0063] Poly(DCPD-random-NB) or P(DCPD-r-NB) copolymer aerogels were prepared from ring opening metathesis polymerization (ROMP) of DCPD and NB in toluene using a Grubbs' 1 ^st generation catalyst. The density was controlled by adjusting the concentrations of the DCPD/NB monomer solutions.

[0064] Coating the inside of a spherical capsule with a smooth and homogenous film requires precise control over rotation speed and direction. Initial film formation experiments that used the tumbling motion of a capsule in a rotating vial were promising but not reproducible. We therefore decided to build a system that provides a deterministic, continuous change in orientation relative to the gravity vector, thus simulating a true microgravity environment. Such an environment can be realized with two perpendicular and independently driven rotating frames in combination with computer controlled software (figure 4a,b). The capsule, filled with a precise amount of the gel precursor solution, is mounted in the center of rotation, which is where both rotation axes intersect. The capsule is then rotated until gelation is complete, typically for 4- 24 h, depending on the aerogel density and the DCPD/NB/catalyst ratio. Independent gelation experiments, performed under stationary conditions in glass vials, have revealed that for a given catalyst concentration, the gel time increases with decreasing aerogel density and DCPD/NB ratio.

[0065] Concentricity, sphericity, and roughness of the resulting gel-layer were assessed by x- ray imaging (figure 4c,d). The sphericity of the foam shells is determined by the sphericity of the diamond molds that have only nm scale deviations. The foam shell thickness non-uniformity is typically dom inated by the non-concentricity (mode 1 thickness non-uniformity) of the inner and outer foam shell surface. All non-uniformities with mode numbers higher than two are well below 1 μηι. To improve the mode 1 uniformity, we are currently developing a computational fluid dynamics model that wil l al low us to identify the best combinations of rotational velocity and viscosity for uniform coatings. [0066] To prevent densification of the foam by solvent evaporation through the fill hole during the prolonged coating process, the capsule is placed in a small container that is either filled with saturated vapor of toluene (used as the solvent in the aerogel precursor solutions) or with liquid water. The miscibilit of toluene in water is very low (0.5633 ml/1000 ml ¾0), and covering the fill hole with water thus effectively blocks the loss of toluene. To avoid periodic patterns of the rotation irrational ratios of the rotational frequencies of the two independently driven frames (typically different by a factor of Λ/2) were selected. The critical rotational frequency that is required to coat the inside of the rotating capsule with a smooth, uniform gel layer was estimated analytically and by a computational fluid dynamics model (details will be published elsewhere). In short, the analysis has revealed that the critical rotational velocity scales with the square of the film thickness, and the inverse of the aerogel precursor solution viscosity. For a 2-mm-diameter shell and a film thickness of 30 μιτι, the critical rotational velocity decreases from several hundred to a few rotations per minute as the viscosity of the aerogel precursor solution increases by - 10 starting at the viscosity of toluene (0.56 cP), as the precursor solution approaches the gel point. Clearly, the viscosity of the aerogel precursor solution at the gel point is of crucial importance for the formation of smooth, uniform films inside the capsule.

[0067] High density carbon (HDC) shells (2 mm ID) with 30-50 μηι diameter fill holes (Diamond Materials GmbH, Germany) were used to study the coating behavior of various foam compositions and densities. To facilitate characterization, we used relatively thin-walled (20-30 μιτι) and transparent micro-crystalline HDC shells. All coating experiments were performed in a custom-made positioning machine with a cardanic frame (Microgravity Science 2009, 21 , 287). The cardan ic frame is made of titanium and allows for rotation up to 20 rpm around two independently driven axes. The system is equipped with computer controlled motors (maxon motors, Switzerland) and can be placed in furnace with a maximum temperature of 80 °C. Drying

[0068] The gel-coated ablator shells then underwent solvent exchange in liquid CO₂ at 12°C and 900 psi in a Polaron critical point dryer to replace the toluene in the gel pores with liquid CO2. Once the toluene was exchanged, the temperature and pressure in the critical point dryer were increased to reach the supercritical regime (~50°C and 1600 psi). The pressure was then allowed to slowly decrease to atmospheric pressure while keeping the temperature constant. [0069] The ability of DCPD foams to survive wetting with liquid hydrogen was assessed by ultra-small angle x-ray scattering (USAXS, Advanced Photon Source, sector 32ID) and x-ray microtomography (Beamline 8.3.2 at the Advanced Light Source). The uniformity of wet gel/aerogel coatings was assessed by analyzing x-ray micrographs. The position of inner and outer surfaces of the wet gel/foam layer was detected by identifying sharp intensity gradients along a series of radial density profiles extracted from raw images. The weaker contrast of the inner foam surface required averaging over about 10-20 neighboring intensity profiles over a small angular range (about +/- 1 degree). The wet gel/foam layer thickness was then taken as the radius difference between the two gradient maxima, and power mode spectra were obtained by discrete FFT of the thickness versus angle data.

Doping

[0070] The thin foam shell in fusion application targets will be used as a scaffold to bring high-atomic-number (high-Z) dopants in close contact with the DT fuel. Doping with high-Z elements also allows us to non-destructively characterize the low-density aerogel shell inside the thick, high-density ablator using x-ray imaging. Doping can be achieved by either adding functionalized monomers to the polymer solution or by doping of the final gel/aerogel layer. To test the first approach, we added an iodine-functional ized NB monomer, C9H 12I2 (full synthetic details for this new monomer will be published elsewhere). The second approach was realized by iodine addition to the remaining C=C double bonds of the DCPD NB polymer. Iodination was achieved by either placing the ablator shell coated with the wet gel layer for several days in an iodine-toluene solution or by storing the dried shell in an iodine vapor environment. Both approaches were successful, with liquid phase doping resulting in somewhat higher doping levels as judged by the higher contrast in radiographs. We have also demonstrated that the dried foam shell can be uniformly doped using atomic layer deposition. Final ly, the wet gel layer was supercritical ly dried by direct solvent exchange with carbon dioxide. A representative radiograph of an iodine-doped foam shell inside a diamond ablator is shown in figure 5.

Example 2. Preparation of Silica Aerogel Coating

[0071] The silica aerogel coatings of the present invention can be preapred by combining l Og TMOS, 22g MeOH, 3.6g H2O, 20μΙ_^ NI I4OI I, adding the reaction mixture to the container as described above, and then forming the aerogel coating according to the procedures outlined in Journal of Non-Crystalline Solids 1992, 145, 44. The resulting aerogel coating is shown in Figure 16.

Example 3. Preparation of Resorcinol/Formaldehvde Aerogel Coating

[0072] The resorcinol/formaldehyde aerogel coating was prepared by combining 0.6261 g resorcinol, 0.901 9 g formaldehyde, 0.0032 catalyst, and 27.903 g water, adding the reaction mixture to the container as described above, and then forming the aerogel coating according to the procedures outlined in Journal of Materials Science 1989, 24, 3221 . The resulting aerogel coating is shown in Figure 17.

[0073] Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, one of ski l l in the art will appreciate that certain changes and modifications may be practiced within the scope of the appended claims. In addition, each reference provided herein is incorporated by reference in its entirety to the same extent as if each reference was individually incorporated by reference. Where a conflict exists between the instant application and a reference provided herein, the instant application shall dominate.

Claims

WHAT IS CLAIMED IS : 1 . A method for coating an interior surface of a container with an aerogel, comprising:

forming a reaction m ixture in the container comprising a first monomer and a solvent; rotating the container to simulate a microgravity environment in the container's interior, under conditions sufficient to form a wet gel coating the container's interior surface; and

contacting the wet gel with supercritical carbon dioxide under conditions suitable to form the aerogel, thereby form ing the aerogel coating.

2. The method of claim 1 , wherein the container is rotated about two perpendicular axes.

3. The method of claim 1 , wherein the step of contacting the wet gel with the supercritical carbon dioxide is performed at a temperature of about 50°C and a pressure of about 1 600 psi.

4. The method of claim 1 , further comprising contacting the wet gel with liquid carbon dioxide prior to contacting the wet gel with supercritical carbon dioxide.

5. The method of claim 4, wherein the step of contacting the wet gel with liquid carbon dioxide is performed at a temperature of about 12°C and a pressure of about 900 psi.

6. The method of claim 1 , wherein the aerogel coating has a thickness of from about 1 to about 500 μηι.

7. The method of claim 6, wherein the aerogel coating has a thickness of from about 5 to about 50 μηι.

8. The method of claim 1 , wherein the aerogel coating has a density of from about 0.1 to about 100 mg/cm³.

9. The method of claim 8, wherein the aerogel coating has a density of from about 5 to about 25 mg/cm³.

10. The method of claim 1 , wherein the aerogel coating has a roughness of less than about 10 μιτι.

1 1. The method of claim 1 , wherein the aerogel coating has a roughness of less than about 2 μηι.

12. The method of claim 1 , wherein the aerogel coating is substantially uniform.

13. The method of claim 1 , wherein the container has a diameter of from about 0.1 mm to about 10 cm.

14. The method of claim 1 , wherein the first monomer is selected from the group consisting of dicyclopentadiene and norbornadiene, and wherein the composition further comprises:

a second monomer selected from the group consisting of norbornene, cyclobutene, cyclopentene, cyclooctene, cyclooctadiene and cyclododecatriene, and a catalyst suitable for use in Ring Opening Metathesis Polymerization (ROMP).

15. The method of claim 14, wherein the catalyst is a Grubbs' catalyst.

16. The method of claim 14, wherein the catalyst is selected from the group consisting of the first generation Grubbs catalyst, the second generation Grubbs catalyst, and the Hoveyda-Grubbs catalyst.

1 7. The method of claim 14, wherein the ratio of the first monomer to the second monomer is greater than about 5 : 1 (wt/wt).

1 8. The method of claim 14, wherein the ratio of the first monomer to the second monomer is greater than about 1 0: 1 (wt/wt).

19. The method of claim 14, wherein the ratio of the first monomer to the second monomer is greater than about 20: 1 (wt/wt).

20. The method of claim 14, wherein the reaction mixture is heated at about 80°C.

21. The method of claim 1 , wherein the first monomer comprises a tetraalkoxy silane, and the solvent comprises water, such that the aerogel is a silica aerogel.

22. The method of claim 1 , wherein the first monomer comprises a transition metal.

23. The method of claim 1 , further comprising contacting the aerogel coating with a dopant to dope the aerogel coating.

24. The method of claim 23, wherein the dopant is iodine.