Aesthetically Enhanced Silica Aerogel Via Incorporation of Laser Etching and Dyes

Safety glasses or goggles should be worn when preparing the aerogel precursor solutions, working with the hot press, and using the laser engraving system. Laboratory gloves should be worn when cleaning and preparing the mold, preparing the chemical reagent solution, pouring the solution into the mold in the hot press and handling the aerogel. Read Safety Data Sheets (SDS) for all chemicals, including solvents, prior to working with them.Tetramethyl orthosilicate (TMOS), methanol and concentrated ammonia, and solutions containing these reagents, must be handled within a fume hood. Dyes can be toxic and/or carcinogenic, so it is important to employ appropriate personal protective equipment (see the SDS). As noted in our previous protocol¹⁵, a safety shield should be installed around the hot press; the hot press should be properly vented and ignition sources should be removed. Before using the laser engraver, ensure that the vacuum exhaust system is operational.

1. Obtain or fabricate an aerogel monolith

NOTE: Methods for making a 10 cm x 11 cm x 1.5 cm aerogel monolith in a contained metal mold via a rapid supercritical extraction method (RSCE)^15,16,17,18 are described here. This RSCE process removes the solvent mixture from the pores of the silica matrix without causing structural collapse. Because the precursor mixture fills the mold, this method involves supercritical extraction of a significantly smaller volume of alcohol (in this case, methanol) than other high-temperature alcohol supercritical extraction methods. Aerogels produced using this method have densities of approximately 0.09 g/mL and surface areas of about 500 m²/g. For etching, the monolith can be of any size large enough to etch on and prepared via any appropriate method (i.e., CO₂ supercritical extraction, freeze drying, ambient drying). For dyed aerogels, these other methods may not be as suitable because the dye can leach out during solvent exchange steps. If using a monolith obtained from another source, skip to step 2.

1. Prepare the mold
   NOTE: All solution preparations should be performed in a fume hood wearing gloves and safety goggles.
   1. Obtain a three-part (4140 alloy) steel mold consisting of a top, middle, and bottom part with outer dimensions of 15.24 cm x 14 cm and a 10 cm x 11 cm cavity in the center (see Figure 1). The top part of the mold has fourteen 0.08-cm vent holes, seven on each side. This mold assembly will produce a 10 cm x 11 cm x 1.5 cm aerogel.
      NOTE: A different size mold may be used; however, the parameters will need to be adjusted, as described in Roth, Anderson, and Carroll²⁰.
   2. Use diluted soap and a rough textured sponge to scrub and clean the top, middle, and bottom part of the mold. Dry all parts of the mold using a clean paper towel.
   3. Pour 20 mL of acetone into a 50 mL or larger beaker. Dip a disposable cleaning wipe into the acetone and wipe the mold using a new cleaning wipe for each part. Repeat until the cleaning wipe appears clean after wiping.
   4. Lightly sand all surfaces with 2,000-grit sandpaper until the mold is smooth to the touch and any residue from previous uses has been removed. Pay extra attention to the inside of the middle mold where the aerogel is formed.
   5. Flow compressed air through the vent holes in the top mold part to clear them.
   6. Squeeze out approximately 2.4 mL of high-vacuum grease and manually apply a thick, even, 1-2 mm layer of grease to the entire (26 mm) top-connecting surface of the bottom mold (see Figure 1).
   7. Squeeze out approximately 1.0 mL of high-vacuum grease and manually apply a thick, even 1-2 mm layer of grease to the outer half (13 mm) of the bottom-connecting surface of the top mold (see Figure 1).
   8. Squeeze out approximately 0.5 mL of high-vacuum grease and manually apply a thin (less than 0.5 mm), even layer of grease to the inside surfaces of the top and bottom mold (those surfaces that will contact the precursor solution and resulting aerogel, see Figure 1).
   9. Wipe away excess grease with a disposable cleaning wipe until the surface feels smooth and no stickiness from the grease is felt.
   10. Squeeze out approximately 0.5 mL of high vacuum grease and manually apply a thin (less than 0.5 mm), even layer of grease to the inside surface of the middle mold (see Figure 1). Do not wipe away excess grease.
   11. Place the middle mold part on top of the bottom mold part. Use a rubber hammer covered with disposable cleaning wipes (to protect the mold surface) and gently hammer the middle part into the bottom part until all sides are evenly sealed.
   12. Using two 0.0005" (0.0127 mm) thick 16 cm x 15 cm pieces of stainless steel foil, and a 0.0625" (1.59 mm) thick 16 cm x 15 cm piece of flexible graphite sheet, make a bottom gasket consisting of the graphite sandwiched between two layers of stainless steel foil. Make a similar gasket for the top of the mold.
   13. Place the bottom gasket on the lower hot press platen and then place the assembled middle and bottom mold pieces on top of the gasket (see Figure 2). Ensure that the mold assembly is placed in the center of the hot press platen and use the hot press to apply a 90 kN force to the mold for approximately 5 min to seal the two pieces.
   14. Remove the mold from the hot press. Use a disposable cleaning wipe to remove excess grease that may have squeezed out between the middle and bottom pieces. Ensure that no debris is on the inside surface of the mold.
2. Prepare aerogel precursor mixture
   NOTE: This recipe is for a TMOS-based silica aerogel that can be made in the mold described above in section 1.1. Any suitable silica aerogel recipe can be used so long as the precursor recipe gelation takes more than 15 min but less than 120 min at room temperature (see, for example, Estok et al.¹⁹ for a suitable tetraethyl orthosilicate-based RSCE recipe). Aerogels can be prepared in native (step 1.2.1) or dyed form (step 1.2.2). All solution preparation work is performed in a fume hood using gloves and safety goggles.
   1. Native aerogels
      1. Gather the following reagents: TMOS, methanol, deionized water, and 1.5 M ammonia.
      2. Use an analytical balance to measure 34.28 g of TMOS into a clean 250 mL beaker. Pour the measured TMOS into a clean 600 mL beaker and cover with paraffin film.
      3. Use an analytical balance to measure 85.76 g of methanol into another 250 mL beaker. Pour the measured methanol into the 600 mL beaker containing TMOS and cover with paraffin film.
      4. Measure 14.14 g of deionized water into a 50 mL beaker using an analytical balance. Use a micropipette to add 1.05 mL of 1.5 M ammonia to the water in the beaker. Stir gently.
      5. Pour the water and ammonia mixture into the 600 mL beaker with the remaining reagents and cover with paraffin film. Place the beaker in a sonicator and sonicate for 5 min.
   2. Dye-doped aerogels
      ​NOTE: If a different procedure is used that involves solvent exchanges, a considerable amount of dye will be washed out during the exchanges; consequently, the colors of the resulting aerogels will not be as vibrant as those presented here.
      1. Gather the following reagents: tetramethyl orthosilicate (TMOS), methanol, deionized water, 1.5 M ammonia and a suitable dye.
      2. Use an analytical balance to measure 34.28 g of TMOS into a clean 250 mL beaker. Pour the measured TMOS into a clean 600 mL beaker and cover with paraffin film.
      3. Use an analytical balance to measure 42.88 g of methanol into a 250 mL beaker. Pour the measured methanol into the 600 mL beaker containing TMOS and cover with paraffin film. Use an analytical balance to measure another 42.88 g of methanol into the 250 mL beaker.
      4. Use an analytical balance to measure 0.050 g of fluorescein (to make a yellow-tinged aerogel) or 0.042 g of rhodamine B (to make a pink-tinged aerogel) or 0.067 g of Rhodamine 6 G (to make an orange-tinged aerogel) into a 10 mL beaker. Add the dye to the 250 mL beaker containing the methanol and gently mix until dissolved.
         NOTE: These instructions are for aerogels used in the example mosaic design; the dye concentration can be altered to change the depth of color in the resulting aerogel (see Table 1).
      5. Pour the dye solution into the 600 mL beaker containing TMOS and cover with paraffin film.
      6. Measure 14.14 g of deionized water into a 50 mL beaker using an analytical balance. Use a micropipette to add 1.05 mL of 1.5 M ammonia to the water in the beaker.
      7. Pour the water and ammonia mixture into the 600 mL beaker with the remaining reagents and cover with paraffin film. Place the beaker in a sonicator and sonicate for 5 min.
3. Perform rapid supercritical extraction
   NOTE: This procedure uses a 30-ton programmable hot press equipped with a safety shield. Gloves and safety goggles should be worn.
   1. Program the hot press extraction program with the parameters shown in Table 2. Parameters are set to prepare a 10 cm x 11 cm x 1.5 cm aerogel in the mold described in step 1.1.1. If a different size mold is used, the parameters will need to be adjusted, as described in Roth, Anderson, and Carroll²⁰.
   2. Place the middle/bottom mold assembly back on top of the bottom gasket in the hot press. Ensure that the mold is placed in the center of the hot press platen (see Figure 2).
   3. Pour the aerogel precursor solution (native or dye-containing) into the mold until the solution is ~2 mm from the top. This will ensure that the mold is completely filled with the precursor solution when the top piece of the mold is added. There will be approximately 10 mL of mixture remaining in the beaker, which can be discarded or allowed to gel at room temperature.
   4. Carefully place the top part of the mold in position on the middle/bottom mold assembly. Excess solution may come out of the vent holes on the top of the mold as it is placed on the middle mold. Wipe up the solution with a disposable cleaning wipe.
   5. Place disposable cleaning wipes on top of the mold to protect the mold surface. Use a rubber hammer to lightly tap the top mold until it is evenly sealed on each side.
   6. Place the top gasket on top of the assembled mold; close the safety shield and start the hot-press program. The precursor mixture gels as the system heats up. The entire process will take 10.25 h to complete for this size aerogel.
4. Remove aerogel monolith from mold
   ​NOTE: Gloves should be worn when handling the aerogel monolith.
   1. When the extraction process is complete, open the safety shield, remove the mold, and place it on a clean working surface.
   2. Insert a flat-head screwdriver into the cavity between the top and middle mold (see Figure 1). Place a gloved hand on the backside of the mold and push down on the screwdriver to separate the top and middle mold parts.
   3. Once the seal is broken, repeat step 1.4.2, going around the edges of the mold while pushing the screwdriver down to release the top mold part. Place the gloved hand wherever necessary to hold down the mold while opening it.
   4. When all sides of the top mold are free from the middle mold, remove the top mold. Place the top mold to the side.
   5. Obtain a lidded container large enough to hold the aerogel; remove the lid and place the bottom part of the container upside down on top of the middle mold with the container and mold cavity aligned. Flip the mold upside down; the aerogel should drop gently into the container.
   6. Put the lid back on the container to protect the aerogel. The aerogel can be stored indefinitely before performing any etching or cutting.

2. Prepare laser engraver print file

NOTE: It is possible to print text, patterns, and images on the aerogel. Any suitable drawing program can be used. Images are interpreted in grayscale. The laser engraver will ablate the aerogel surface in locations where there is text or a pattern and varies the laser pulse density to achieve gray scale values. Etching occurs in locations where the printed image is non-white. Etching does not occur where the image is white. Separate instructions are included for text, pattern, or image files. All three can be combined in one file if desired⁶.

1. Text files
   1. Open up the drawing application and start a new document. Add the desired text of any size, linewidth, and style directly to the document.
   2. Save the file.
2. Pattern files
   1. Open up the drawing application and start a new document.
   2. Add lines and shapes directly to the document using the desired linewidth.
   3. To design a mosaic pattern that will be cut from (instead of etched onto) the aerogel monolith, use shapes and lines in the toolbox and set all line widths to hairline. See Figure 3 for an example of a mosaic pattern.
   4. Save the file.
3. Image files
   1. Select an image and use any image-processing program to edit.
   2. Use image processing software to remove non-white sections that are not to be printed from the image. See Figure 4 for an example of this.
      NOTE: Etching occurs in any non-white location.
   3. Convert the image to grayscale for a visual indication of what the etched image will look like and adjust the contrast between image hues until satisfied that sufficient contrast exists to show the desired features (see Figure 4).
      ​NOTE: The level of contrast needed will depend on the amount of detail in the image that the user desires to etch onto the aerogel. The drawing program should provide guidance, but the user may need to experiment with different contrast levels to achieve the desired outcome.
   4. Open up the drawing application and start a new document. Upload an image to drawing program.
   5. Save the file.

3. Etching procedure

NOTE: The following instructions are for a 50 W CO₂ laser engraver/cutter but can be modified to use with other systems. This system adjusts speed and power properties on a percent basis from 0% to 100%. Relevant laser engraver properties are included in Table 3. A vacuum exhaust system should be used to vent the laser engraver. Use gloves when handling the aerogel monolith.

1. Turn on the laser engraver, vacuum exhaust system, and the attached computer.
2. Measure the size of the aerogel monolith surface that will be etched (in the example above, the size is 10 cm x 11 cm).
3. Start the drawing program and open the previously saved file (from step 2.1, 2.2, or 2.3). Set the document's dimension/piece size to correspond to the measured aerogel monolith size.
4. Open the lid of the laser engraver. Using a gloved hand, place the aerogel (native or dyed) on the laser engraver platform as shown in Figure 5. Align the aerogel in the top-left corner so that the aerogel touches the top and left rulers.
5. Take the V-shaped magnet manual focus gauge attached to the laser and flip it upside down. Press Focus on the laser engraver.
   NOTE: Because of the transparency of the silica aerogel monolith, it is necessary to manually set the focus parameters for etching. Do not use Auto Focus.
6. Place a disposable cleaning wipe on top of the aerogel monolith to protect it. Using the up arrow on the laser engraver control panel, move the laser engraver platform until the bottom part of the manual focus gauge just touches the aerogel.
7. Remove the disposable cleaning wipe and return the gauge to its original position. Close the laser engraver lid.
8. In the drawing program, click File and then Print. Choose the drawing program as the print location and open the Properties window.
9. Adjust the properties by selecting the Raster mode: a DPI of 600, a Speed of 100% (208 cm/s), and a Power of 55% (27.5 W). Confirm that the piece size matches the measured aerogel monolith size. Click Apply and then Print.
10. On the front panel of the laser engraver, click Job and select the corresponding file name. Click Go.
11. When the laser engraver finishes, click Focus and use the down arrow on the laser front control panel to lower the base. Using a gloved hand, gently remove the aerogel from the laser engraver platform and place it back in the container.
12. Purge the job from the laser engraver by clicking on the Trash button. Turn off the laser engraver and vacuum.

4. Cutting procedure

1. Turn on the laser engraver, vacuum exhaust system, and the attached computer.
2. Measure the size of the aerogel monolith surface that will be cut (in the example above, the size is 10 cm x 11 cm).
3. For general cutting, open the drawing program and start a new document. Enter the dimensions for the document/piece size to correlate with the measured aerogel monolith size.
4. Use the tools in the drawing program to create the shape or line that will be cut using a "hairline" line width. Locate the shape/line to match the desired cut location on the aerogel.
5. For mosaic patterns, import the previously saved file (from step 2.2) and adjust the size to match that of the aerogel monolith.
6. Obtain a 0.0005" (0.0127 mm) thick sheet of stainless steel foil large enough to cover the base of the aerogel monolith. Using a cleaning wipe, clean the stainless steel with acetone.
7. Open the lid of the laser engraver, place the stainless steel foil on the laser engraver platform to prevent residue on the platform from discoloring the aerogel during cutting and place the aerogel monolith on top of the foil. Align the aerogel and stainless steel foil in the top left corner with the aerogel touching the top and left rulers.
8. Follow steps 3.5-3.8 from the etching procedure above.
9. Adjust printing properties. Select the Vector mode: a DPI of 600, a Speed of 3% (0.27 cm/s), Power of 90% (45 W), and Frequency of 1,000 Hz. Make sure the piece size matches the measured aerogel size. The depth of the cut will vary with laser speed. See Table 4 and Figure 6.
10. Follow steps 3.10-3.12 from the etching procedure.
11. Small pieces of ablated aerogel will be left on the face of the monolith that was in contact with the laser, as shown in Figure 7. To remove the particles, use a foam brush and gently wipe away the pieces.

5. Making aerogel mosaics

1. To yield a tri-color mosaic, prepare three different monoliths of the same thickness but with different dyes. (It is also possible to yield mosaics with three different shades, using different monoliths of the same thickness but with varying concentrations of the same dye, or to include native aerogel with dyed aerogel in mosaic patterns.)
2. Use the cutting procedure in section 4 with the mosaic design from section 2.2 to cut the mosaic patterns into three different colored aerogels of the same thickness.
3. Place the cut colored aerogels on a flat, clean surface.
4. Gently disassemble each single-colored aerogel and separate the components of the cut design using tweezers or a sharp knife to ease separation and prevent breakage.
5. Gently brush the sides of each shape with a foam brush to remove the excess white particles left by the laser cutting procedure.
6. Interchange the same shapes with different colors to produce multicolored mosaics (Figure 8) and assemble the cut shapes by compressing them together to form a complete mosaic-like tile, which can be placed within a glass frame.