ADDITIVE MANUFACTURING OF SILICA AEROGEL OBJECTS

Abstract

An ink composition for additive manufacture of silica aerogel objects essentially consists of a gellable silica sol containing an admixture of 30 to 70 vol.% of a mesoporous silica powder in a base solvent. The mesoporous silica powder has a particle size range of 0.001 to 1 mm and a tap density of 30 to 200 kg/m3 and comprises at least 10% by weight of silica aerogel powder. The composition has a yield stress in the range of 30 to 3000 Pa and a viscosity of 5 to 150 Pa.Math.s at a shear rate of 50 s−1. Furthermore, the composition has shear thinning properties defined as a reduction in viscosity by a factor between 10 and 103 for an increase in shear rate by a factor of 104 to 105. A method of additive manufacturing of a three-dimensional silica aerogel object by direct ink writing comprises providing such ink composition, forcing the same through a convergent nozzle, thereby forming a jet of the ink composition which is directed in such manner as to form a three-dimensional object by additive manufacturing. After initiating and carrying out gelation of the gellable silica sol constituting said object, a drying step yields the desired three-dimensional silica aerogel object.

Claims

1. An ink composition for additive manufacture of silica aerogel objects, comprising: a gellable silica sol containing an admixture of 30 to 70 vol.-% of a mesoporous silica powder in a base solvent, the silica sol having an equivalent silica content of 0.02 to 0.12 g/cm.sup.3 SiO.sub.2 equivalents, the mesoporous silica powder having a particle size range of 0.001 to 1 mm and a tap density of 30 to 200 kg/m.sup.3 and comprising at least 10% by weight of silica aerogel powder, the composition being substantially free from trapped air, the composition having a yield stress in the range of 30 to 3000 Pa, the composition having a viscosity of 5 to 150 Pa.Math.s at a shear rate of 50 s.sup.−1, the composition further comprising at least one additive selected from a surfactant, and a viscosity modifier, and the composition having shear thinning properties defined as a reduction in viscosity by a factor between 10 and 10.sup.3 for an increase in shear rate by a factor of 10.sup.4 to 10.sup.5.

2. The ink composition according to claim 1, further comprising functional micro- or nanoparticles.

3. The ink composition according to claim 1, wherein the mesoporous silica powder contains at least 30% by weight of silica aerogel powder.

4. The ink composition according to claim 1, wherein the gellable silica sol has an equivalent silica content of 0.05 to 0.08 g/cm.sup.3 SiO.sub.2 equivalents.

5. The ink composition according to claim 1, wherein the silica aerogel powder has an average particle size range of 0.010 to 0.200 mm.

6. The ink composition according to claim 1, wherein the silica aerogel powder is hydrophobic.

7. The ink composition according to claim 1, wherein the silica aerogel powder is hydrophilic.

8. The ink composition according to claim 1, wherein the base solvent is a polar solvent.

9. The ink composition according to claim 1, wherein the base solvent is a non-polar solvent.

10. The ink composition according to claim 1, wherein the base solvent is an alcohol solvent.

11. The ink composition according to claim 1, wherein the gellable silica sol is obtained from a silica precursor selected from the group consisting of waterglass, ion-exchanged waterglass, silicic acid, sodium silicate, tetraethoxysilane, tetramethoxysilane, methyltrimethoxysilane, methyltriethoxysilane and polyethoxydisiloxane.

12. The ink composition according to claim 1, wherein the viscosity modifier is a polymeric viscosity modifier.

13. A method of preparing the ink composition of claim 1, comprising: dispersing the 30 to 70 vol.-% of the mesoporous silica powder with the tap density in the range of 30-200 kg/m.sup.3 and a minimum silica aerogel content of 10% by mass into the gellable silica sol containing the base solvent, wherein the sol has the equivalent silica content of 0.02 to 0.12 g/cm.sup.3 SiO.sub.2 equivalents, adding the at least one additive selected from the group consisting of a surfactant, and a viscosity modifier, mechanically mixing until a homogeneous mixture with shear thinning properties is obtained, the mixture exhibiting the reduction in viscosity by a factor between 10 and 10.sup.3 for the increase in shear rate by a factor of 10.sup.4 to 10.sup.5, and removing trapped air via vacuum treatment, ultrasonication or centrifugation or a combination thereof, thereby obtaining said ink composition.

14. A method of additive manufacturing of a three-dimensional silica aerogel object by direct ink writing, comprising: a) providing an ink composition as defined in claim 1; b) forcing the ink composition through a convergent nozzle, thereby forming a jet of the ink composition; c) directing the jet according to a predefined directional sequence to form a three-dimensional object by additive manufacturing, the object comprising said gellable silica sol containing an admixture of 30 to 70 vol.-% of mesoporous silica powder in said base solvent; d) initiating and carrying out gelation of the gellable silica sol constituting said object; e) optionally carrying out a solvent exchange, thereby replacing the base solvent by a replacement solvent; f) optionally carrying out a surface modification; g) carrying out a drying, thereby obtaining said three-dimensional silica aerogel object.

15. The additive manufacturing method according to claim 14, wherein gelation is initiated through the addition of a solidification agent, particularly an acid or a base, through gas phase or solution phase addition.

16. The additive manufacturing method according to claim 14, wherein surface modification is achieved by adding a silica hydrophobizing agent, particularly a hydrophobizing agent selected from hexamethyldisiloxane, hexamethyldisilazane and trimethylchlorosilane.

17. A three-dimensional silica aerogel object formed by an additive manufacturing method according to claim 14, the object having a BET surface area of at least 500 m.sup.2/g and a BJH pore volume of at least 1.5 cm.sup.3/g, the object comprising silica aerogel particles embedded in a silica aerogel matrix, wherein the silica aerogel particles have a higher density than the silica aerogel matrix.

18. The ink composition according to claim 1, wherein the composition essentially consists of the gellable silica sol containing the admixture of 30 to 70 vol.-% of the mesoporous silica powder in the base solvent, and is substantially free from trapped air, has a yield stress in the range of 30 to 3000 Pa, has a viscosity of 5 to 150 Pa.Math.s at a shear rate of 50 s.sup.−1, comprises at least one additive selected from a surfactant, and a viscosity modifier, and has shear thinning properties defined as a reduction in viscosity by a factor between 10 and 10.sup.3 for an increase in shear rate by a factor of 10.sup.4 to 10.sup.5.

19. The ink composition according to claim 3, wherein the mesoporous silica powder contains at least 50% by weight of silica aerogel powder, at least 65% by weight of silica aerogel powder, or 100% by weight of silica aerogel powder.

20. The ink composition according to claim 8, wherein the polar solvent is selected from the group consisting of water, acetone and ethyl acetate.

21. The ink composition according to claim 9, wherein the non-polar solvent is selected from the group consisting of heptane, octane and nonane.

22. The ink composition according to claim 10, wherein the alcohol solvent is selected from the group consisting of ethanol, isopropanol, butanol and pentanol.

23. The ink composition according to claim 12, wherein the polymeric viscosity modifier is selected from chitosan, hydroxypropyl cellulose, polyethylene glycol, methoxypolyethylene glycol, microfibrillated cellulose, nanofibrillated cellulose and cellulose.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0073] The above mentioned and other features and objects of this invention and the manner of achieving them will become more apparent and this invention itself will be better understood by reference to the preceding description of various embodiments of this invention taken in conjunction with the accompanying drawings, wherein are shown:

[0074] FIG. 1 a schematic illustration of the principle of this invention, with [0075] (a) the step of ink-writing, [0076] (b) the step of gelation/fixation, [0077] (c) the step of solvent exchange, and [0078] (d) the step of gel drying;

[0079] FIG. 2 (a) apparent viscosity as a function of applied shear rate, and [0080] (b) storage modulus (G′) and loss modulus (G″) as a function of applied shear stress (b), [0081] for ink 003 (round symbols) and ink 006 (square symbols);

[0082] FIG. 3 (a) a ten-layer honeycomb, and [0083] (b) a 50 layers multi-wall object, [0084] made from silica aerogel that were 3D printed from a 410 μm conical nozzle;

[0085] FIG. 4 (a) apparent viscosity as a function of applied shear rate, and [0086] (b) storage modulus (G′) and loss modulus (G″) as a function of applied shear stress (b), [0087] of an MnO.sub.2 doped silica aerogel containing ink; and

[0088] FIG. 5 a three-dimensional silica aerogel object formed by additive manufacturing, with electron micrographs showing [0089] (a) an outer surface of a printed filament, with magnification 300×; [0090] (b) an enlarged view, with magnification 3000×, of the marked region in (a), showing interlocked aer-ogel particles (darker grey) embedded in a low-density aerogel matrix (lighter grey); [0091] (c) and (d) further enlarged regions, with magnifications 10000× and 25000×, respectively, wherein SP1 and SP2 denote high-denisty and low-density silica particles, respectively.

DETAILED DESCRIPTION OF THE INVENTION

[0092] The method of additive manufacturing of a three-dimensional silica aerogel object by direct ink writing is illustrated in FIG. 1. As shown in FIG. 1a, a device 2 for carrying out the method comprises an ink reservoir 4 containing an ink composition 6. The latter is forced through a convergent nozzle 8, thereby forming a jet of the ink composition. Using an appropriate and basically known positioning system, the ink jet is directed in such manner as to form a three-dimensional object 10 by additive manufacturing. In a next step, as shown in FIG. 1b, the object 10 is transferred into a closed vessel 12 containing a gaseous gelation catalyst 14. Initially, the object 10 essentially consists of a gellable silica sol, but under the influence of the gelation catalyst the object is turned into a gel form. Optionally, as shown in FIG. 1c, a solvent exchange step is carried out whereby the base solvent present in the gel is replaced by a replacement solvent 16. After an optional surface modification step, a drying step is carried out as shown in FIG. 1d. For this purpose, the gelled and optionally solvent exchanged object is placed into an autoclave 18 equipped with a vent 20. After an appropriate drying time, a three-dimensional silica aerogel object 22 has formed.

Examples 1-9: Ink Preparations

[0093] Examples 1-9 describe detailed examples of ink preparations suitable for direct ink writing of silica aerogel objects using following starting materials: [0094] Ethyl silicate with a silicon dioxide equivalent content of approximately 40-42 wt. %, obtained as Dynasylan® 40 from Evonik Resource Efficiency GmbH; [0095] 1-pentanol 98%, supplied by ABCR Schweiz AG; [0096] Ultrapure water (double distilled, >18 MΩ.Math.cm); [0097] Nitric acid (HNO.sub.3) 70%, supplied by Sigma-Aldrich; [0098] Poly(propylene glycol) bis(2-aminopropyl ether) (PPGNH, average Mn˜4,000), supplied by Sigma-Aldrich; [0099] Hydrochloric acid (HCl) 37%, obtained from Sigma-Aldrich; [0100] Ammonia solution (28-30%), supplied by Sigma-Aldrich; [0101] Hydrophobic (trimethylsilyl modified) silica aerogel particles (amorphous, 5-20 μm), obtained as ENOVA® IC3100 from Cabot Aerogel GmbH.

[0102] Polyethoxydisiloxane (PEDS) precursor (sol concentrate) preparation. 173 ml ethyl silicate with a silicon dioxide equivalent content of approximately 40-42 wt. % was mixed with 189 ml isopropanol (or ethanol or 1-pentanol) and 13.5 ml ultrapure water at 35° C., after stirring at 250 r.p.m. for 10 mins, the solution was cooled down to 25° C., with a continuous stirring at 250 r.p.m., aqueous 0.06 M HNO.sub.3 solution was added dropwise at a rate of 0.45 ml/min by using a syringe pump (LaboTechSystems LTS AG). The as-prepared silica sol concentrate precursor was kept at 4° C. for 24 hours before use.

[0103] Silica ink preparation. The ink was prepared first by mixing 7.3 g 1-pentanol with 0.7 g poly(propylene glycol) bis(2-aminopropyl ether) (PPGNH) at room temperature (25° C.) and stirring for 5 mins. Next 12 M HCl (37%) was added to adjust the pH value, and then 4 g polyethoxydisiloxane sol concentrate precursor was added to the base solvent mixture, and was mixed thoroughly at 500 rpm for 5 min. After that, a desired amount of hydrophobic (trimethylsilyl modified) silica aerogel particles (SAP) (amorphous, 5-20 μm) was added to achieve required rheological properties for direct ink writing, the amount of aerogel particles is specified in Table 1. The blend was then mixed in a planetary speedmixer or centrifuge for 7 minutes at 3000-3500 rpm.

TABLE-US-00001 TABLE 1 Composition of gellable silica inks in grams Compositions [grams] 1-Pentanol PPGNH PEDS SAP Example 1 7.3 0.7 4 1.14 Example 2 7.3 0.7 4 1.27 Example 3 7.3 0.7 4 1.41 Example 4 7.3 0.7 4 1.52 Example 5 7.3 0.7 4 2.0

Modified Ink Compositions, Example 6

[0104] An ink was prepared in an identical fashion as described in the above procedure for examples 1 through 5, with the difference that 7.5 ml of a 1:3 by volume mixture of isopropanol and nonane was used as a base solvent instead of pentanol.

Modified Ink Compositions, Example 7

[0105] An ink was prepared in an identical fashion as described in the above procedure for examples 1 through 5, with the only difference that 0.8 g hydroxypropyl cellulose (HPC) was used as a viscosity modifier instead of PPGNH.

Modified Ink Compositions, Example 8

[0106] An ink was prepared in an identical fashion as described in the above procedure for examples 1 through 5, with the difference that the sol was prepared from a 8:41 by volume mixture of Methyltriethoxysilane (MTES) and Tetraethoxysilane (TEOS) by co hydrolysis in isopropanol and that the base solvent system was replaced by a 1:7 mixture of isopropanol and nonane (instead of pentanol).

Modified Ink Compositions, Example 9

[0107] An ink was prepared in an identical fashion as described for example 3 in the above procedure, with the only difference that silica aerogel powder was replaced by a 15:85% by mass mixture of hydrophobic (trimethylsilyl modified) silica aerogel particles (amorphous, 5-20 μm) and hydrophobic fumed silica (fumed silica treated with HDMS, obtained as Aerosil R-812, Evonik industries).

[0108] Direct ink writing, Examples 10 and 11. Two as prepared silica inks were tested for their rheological properties before printing. A typical rheological characterization of example/ink 001 and example/ink 004 is shown below in FIG. 2. The as prepared gellable ink was loaded into empty, capped syringe barrels and centrifuged at 2,500-5,000 r.p.m. for 3 min to remove trapped air bubbles. The ink syringes were mounted into a Bioplotter printer (EnvisionTEC). Conical nozzles with inner diameter 200-410 μm (Smoothflow Tapered Tip, Hilgenberg) were used for printing the different inks. Printing patterns were generated by a CAD drawing software and converted into G-code and custom scripts to command the x-y-z motion of the printer head. Various replicate objects were then printed onto a glass substrate in an open laboratory atmosphere.

[0109] Initiation of the gelation, postprocessing and drying. The printed objects were placed inside a closed polystyrene box, containing a tray with 10 ml of a 5.5 M ammonia solution, however there was no contact of said ammonia solution with the printed objects. The NH.sub.3 gas atmosphere then leads to leads a change of pH inside the printed objects and gelation of the silica sol phase. After the gelation and resulting solidification, the printed objects were covered with ethanol. The silica gel phase silica phase then hydrophobized by soaking the objects in a dilute solution of hexamethyldisilazane in ethanol with an EtOH/HMDZ molar ratio of 17:1 at RT for 24 hours. As obtained printed and hydrophobized gel objects were finally dried from supercritical carbon dioxide (CO.sub.2).

[0110] Final printed aerogel replicate objects obtained in this way show a bulk density of 0.18±0.02 g cm.sup.−3 and a BET surface area of 751 m.sup.2 g.sup.−1 and a BJH pore volume of 3.16 cm.sup.3 g.sup.−1

Example 12

[0111] The compatible additives could be added into the silica ink to bring new functions, Example 12 demonstrates an ink formulation with the addition of MnO.sub.2 microspheres, the 35 wt. % silica aerogel particles in the ink 006 were replaced by the ramsdellite MnO.sub.2 microsphere, and the ink preparation follows the same procedure developed for Examples 1-9.

[0112] The printed objects show bulk density of 0.20±0.02 g cm.sup.−3, and a BET surface area of 658 m.sup.2 g.sup.−1 and a BJH pore volume of 3.53 cm.sup.3 g.sup.−1.

Example 13

[0113] Example 13 describes an example of 3D printing a hydrophilic aerogel object using the following starting materials. [0114] Tetraethyl orthosilicate (TEOS, 98%), supplied by Sigma-Aldrich; [0115] Low molecular weight (LMW) chitosan, supplied by Sigma-Aldrich; [0116] HCl (37%), obtained from Sigma-Aldrich; [0117] Acetic acid (≥99%), obtained from Sigma-Aldrich; [0118] Hydrophobic (trimethylsilyl modified) silica aerogel particles (amorphous, 5-20 μm).

[0119] Silica aerogel particles were treated in a tube furnace under air atmosphere at 640° C. for 6 hours. Then grinded up to around 10-50 μm diameter hydrophilic particles.

[0120] At room temperature, 0.5 g low molecular weight (LMW) chitosan was dissolved in 20 ml H.sub.2O with an assist of 0.5 ml acetic acid. After 4 hours, 1.5 ml TEOS was added, and the solution was stirred vigorously for another 4 hours, then the hydrophilic silica particles were added, the blend was speed mixed at 3,000 rpm for 5 mins, then 3,500 rpm for 2 mins.

[0121] The direct ink writing process follows the same procedure developed for Examples 1-9.

[0122] The printed objects were placed in a closed polystyrene box, and 1 M ammonia ethanol solution was added into the box to cover the printed samples. After the solidification, the objects were washed 3 times in 2 days with ethanol. The gels were finally supercritical dried from carbon dioxide (CO.sub.2).

[0123] The printed objects show a bulk density of 0.21±0.03 g cm.sup.−3 and a BET surface area of 203 m.sup.2 g.sup.−1 and a BJH pore volume of 0.71 cm.sup.3 g.sup.−1.

Example 14

[0124] Example 14 describes an example of 3D printing a hydrophilic aerogel object using the following starting materials. [0125] Sodium silicate solution (26.5% w/w SiO.sub.2, molar ratio Na.sub.2O:SiO.sub.2: 1:3.1, pH=11.5), supplied by Sigma-Aldrich; [0126] Polyethylene glycol (PEG solution, MW 4000-10000), supplied by Sigma-Aldrich; [0127] HCl (37%), obtained from Sigma-Aldrich; [0128] Hydrophobic (trimethylsilyl modified) silica aerogel particles (amorphous, 5-20 μm).

[0129] Silica aerogel particles were treated in the furnace at 640° C. for 6 hours to render them hydrophilic and then ground into particle form with a typical diameter in the range from 10-50 μm.

[0130] At room temperature, 8.5 ml water glass was diluted with 41.5 ml H.sub.2O to prepare an aqueous water glass solution, and the waterglass solution was passed through an Amberlyst 15 ion exchange resin in its protonated form to generate silicic acid sol. 10 ml Ion exchanged waterglass was mixed first with 0.75 ml PEG solution. The pH was adjusted by 0.03 ml HCl to around 2-3, and then hydrophilic silica particles were added. The aerogel particle suspension/sol slurry was speed mixed at 3,000 rpm for 5 mins, then 3,500 rpm for 2 mins.

[0131] The direct ink writing, solidification and drying process follows the same procedure developed for Example 13.

Example 15

[0132] Two identical square planar boards (55×55×7 mm.sup.3) were printed from ink in example 2 with a conical nozzle of diameter 1200 μm. After gelation and drying processes, the board samples were placed in a custom-built guarded hot plate device for thermal conductivity measurement (guarded zone: 50×50 mm.sup.2, measuring zone: 25×25 mm.sup.2, 50% RH, 25° C.), the setup was designed for small specimens of low thermal conductivity materials. And the thermal conductivity is 15.9±0.4 mW m.sup.−1K.sup.−1.