Fabrication of mechanically strong honeycombs with aerogel cores

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Fabrication of mechanically strong honeycombs with aerogel cores. Lauren S White, Tyler Selden, Massimo F Bertino, Charles Cartin, Joseph Angello, Marina Schwan, Barbara Milow, and Lorenz Ratke Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04058 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 25, 2017

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Industrial & Engineering Chemistry Research

Fabrication of mechanically strong honeycombs with aerogel cores. Lauren S. White1, Tyler Selden1, Massimo F. Bertino1,*, Charles Cartin2, Joseph Angello2, Marina Schwan3, Barbara Milow3, and Lorenz Ratke3 1

Department of Physics, Virginia Commonwealth University, Richmond, VA 23284. Department of Mechanical Engineering, Virginia Commonwealth University, Richmond, VA 23284. 3 Institute of Materials Research, German Aerospace Center, DLR, 51170 Cologne, Germany. * Corresponding Author. Email: [email protected]. 2

Abstract Honeycomb aerogel composites were fabricated by reinforcing selected regions of a native aerogel matrix using photopolymerization. First, alcogels were synthesized by hydrolysis-condensation of a siloxane, and by adding a multifunctional acrylic monomer and a visible-light initiator to the gelation solution. Alcogels were then placed on a programmable translation stage and exposed to a laser. Polymerization, and mechanical reinforcement were induced in the exposed regions. After exposure alcogels were dried supercritically. Thermal conductivity and out-of-plane modulus of the resulting honeycombs could be varied between values typical of native aerogels (11 mW/mK and 0.75 MPa) and those of uniformly polymerized composites (65.8 mW/mK and 36.26 MPa) by varying the translation stage speed between 2 and 3 mm/s. The results were interpreted using a rule-of-mixtures model. The mechanical properties of the composites were also investigated using finite element analysis.

1. Introduction Metal oxide aerogels have numerous potential applications, especially in the field of thermal insulation. For example, silica aerogels have the lowest-known thermal conductivity among nonevacuated materials (on the order of 10-15 mW/mK). Besides thermal insulation, aerogels are being considered as catalyst supports 1, soundproofing and environmental remediation

2,3

. Of all these

applications, commercialization has been attained only in a few cases and mostly for niche markets. One of the main reasons behind the comparatively small market share of metal oxide aerogels is fragility. Oxide aerogels consist of a skeleton of oxide nanoparticles which are kept together by thin necks bridging between them 4. As a result, the modulus of elasticity of silica aerogels is on the order of kPa and the load at rupture in a three-point bending configuration is often so low that it can often not be measured by standard instrumentation 4,5. In pioneering work, the group of N. Leventis exchanged the 1 ACS Paragon Plus Environment

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gelation solution with a monomer capable of linking to the surface of the skeletal nanoparticles

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4,6

. For

example, metal oxide gels are terminated with -OH groups which can react with isocyanates. Pore walls can also be derivatized by adding an alkoxide carrying a polymerizable moiety, such as an acrylic group. When polymerization is triggered the monomer engages the surface moiety leading to a network of oxide nanoparticles cross-linked by polymer strands. Cross-linking increases the modulus of the resulting composites by 1-2 orders of magnitude. Most importantly, the polymer forms a conformal coating of the skeleton and surface area and porosity and density are not overly compromised. Typical cross-linked aerogels have a surface area of 200-300 m 2/g and a modulus of tens of MPa. Cross-linked aerogels (also called X-aerogels) can be regarded as a new class of materials, intermediate between oxide aerogels and organic-inorganic ceramic composites. They have extremely promising properties, the most intriguing being a specific energy absorption higher than Kevlar’s 6. X-aerogels, however, have a thermal conductivity that is intermediate between that of polyurethane foams (20-25 mW/mK) and that of expanded polystyrene (~30 mW/mK). The increased thermal conductivity is also a consequence of the homogeneity of X-aerogels. In these composites, the polymer reinforcement is ubiquitous and includes regions which may or may not require strengthening. This reasoning led to wonder if one could combine cross-linked and native aerogels within the same monolith. These hydbrid aerogels have been developed by our group and are a class of composites where regions of an otherwise native and fragile aerogel matrix are mechanically reinforced by polymer cross-linking previous work

7

7,8

. In

it was shown that square honeycombs of mechanically strong aerogels could be

fabricated by photolithography inside a native aerogel matrix. In principle, such honeycombs couple mechanical strength with thermal insulation.

Filling of a rigid honeycomb structure with a soft

material is a well-known, convenient and promising way of fabricating multifunctional composites which combine the mechanical strength and lightweight of honeycombs with the physical properties of the filling materials. For example, Nomex® (aramide) honeycombs filled with resorcinol-formaldehyde aerogels have a low thermal conductivity (~ 40 mW/mK) and a high out-of-plane modulus (~13 MPa) 9

. Simulations of a rigid acetal resin (Delrim®) honeycomb filled with a soft polymer (Sylgard® 186)

show that the composites are expected to be excellent acoustic insulators with a loss factor between 5% and 10% 10. The filler can also help improve the mechanical properties of honeycombs. For example, filling aluminum honeycombs with a syntactic foam increases the in-plane elastic modulus by up to 31% and energy absorption by up to 39% 11. These examples show that filled honeycombs have great potential for applications. However, there are some concerns regarding the fabrication of these

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materials and their overall performance. Specifically, most filled honeycombs require multiple fabrication steps. The most critical step is manufacture of the cores of the soft material. These cores must be machined with tight tolerances to prevent detrimental voids forming between the honeycomb and the filler. Machining issues are especially relevant for aerogel-filled honeycombs. These composites are a very promising class of filled honeycombs, since they combine mechanical strength with thermal and acoustic insulation. However, the fragility of aerogels makes machining of the cores extremely difficult or downright impossible. For this reason, aerogel-honeycomb composites are usually fabricated by pouring aerogel granules inside the load-bearing structure

12,13

. Alternatively, a

honeycomb can be used as a mold for a sol-gel process, followed by supercritical drying 9. Mechanical and long-term stability of filled honeycombs is another issue affecting these composites. Typically, honeycomb and filler are two different materials, which may de-bond. For example, delamination has been reported for the in-plane compression of Nomex-aerogel composites 9. Fabrication and delamination issues of aerogel-filled honeycombs are potentially alleviated in hybrid aerogels. Hybrid aerogels have several attractive features. Their fabrication requires few processing steps and it can be completed in a matter of hours 8. In addition the starting material is the same native aerogel, thus delamination may be less of an issue.

Here optimum fabrication and

processing parameters of honeycomb hybrid aerogels are determined. Characterization shows that the composites do not delaminate even when subjected to out-of-plane strains on the order of 50%. Further, the thermal conductivity of the honeycombs can be tuned between that of native aerogels and that of mechanically strong, cross-linked aerogels by varying the stage translation speed. This result is particularly relevant, since it shows that hybrid honeycomb aerogels are not only mechanically robust but also extremely good thermal insulators. The experimental work is accompanied by finite element simulations. These simulations show that most of the strain is concentrated within the cross-linked regions (i.e., the honeycomb walls). These regions, being mechanically strong, become the loadbearing part of the composite and prevent the fragile native aerogel to be subject to excessive strain.

2. Experimental 2.1.General Fabrication Principle. Hybrid aerogels consist of native and cross-linked aerogels and are fabricated by photolithographic methods as follows. First, a silica alcogel is synthesized



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folliowing standard procedures. The pore walls of the alcogel are derivatized with a polymerizable moiety (e.g., an acrylic group), and the pore-filling solvent is an ethanolic solution of a monomer and a photoinitiator. Polymerization is triggered in regions of the monolith exposed to light of suitable wavelength. Polymerization engages the surface moiety, cross-links the skeletal oxide nanoparticles and greatly enhances the aerogel’s mechanical strength 6. Since polymerization occurs only in exposed regions, monoliths can be fabricated that consist of mechanically strong, cross-linked regions embedded in an otherwise native matrix

7,8

. To fabricate honeycombs, the alcogel monolith is mounted

onto a programmable mechanical translation stage and exposed to a laser beam as shown in Figure 1. For our experiments we fabricated hexagonal patterns with side l = 3 mm and thickness t = 1 mm, as shown in Figure 2.

Figure 1: Schematic diagram of the set-up used to fabricate a honeycomb pattern of cross-linked aerogel inside an otherwise native matrix. An alcogel is mounted on a computerized translation stage and exposed to laser radiation (532 nm). L indicates a 1 m focal length lens used to focus the beam on the sample, see also Section 2.4.


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Figure 2: Schematic representation of the honeycomb geometry. The hexagonal pattern has a side l, a thickness t and a density ρxlwhich is assumed to be equal to the density of a homogeneously cross-linked aerogel. The core of the pattern is a native aerogel.

2.2. Reagents Tetramethyl orthosilicate (TMOS), trimethoxysilylpropyl methacrylate (MTMA), methyldiethanolamine (Amine), and Eosin Y were purchased from Acros Organics. Trimethylolpropane triacrylate (TMPTA), in a purity of 99%, was provided by Allnex SA, Belgium, the structure of which is shown in Figure 3. It has a density of 1.10 g ml -1 at 25°C and a molecular weight of 296.32 g mol -1. A

sample

of

2-Propenoic

acid,

1,10-[2-[[3-hydroxy-2,2-bis[[(1-oxo-2-propen-1-

yl)oxy]methyl]propoxy]methyl]-2-[[(1-oxo-2-propen-1-yl)oxy]methyl]-1,3-propanediyl]ester (DPHA), in a purity of 99%, was also provided by Allnex SA,Belgium, the structure of which is shown in Figure 3. It has a density of 1.17 g ml -1 at 25 °C and a molecular weight of 524 g mol -1. All reagents were used as-received. The ethanol-water azeotrope mixture (containing 4.4% water and 95.6% pure ethanol by volume) was used as gelation solvent and as supercritical fluid in the drying process. Unless stated otherwise, all references to ethanol in this manuscript refer to the ethanol-water azeotrope mixture. 2.3 Synthesis of alcogels Alcogels were synthesized by base-catalyzed hydrolysis of TMOS. As described in our previous work 14, reducing the water content of the gelation solution is key to the onepot fabrication of cross-linked aerogels. By using the water-ethanol azeotrope (4% water by weight) one can add polyfunctional (and very hydrophobic) monomers such as DPHA to the gelation solution without phase separation. For these experiments, we used monomers with high functionality, (tri-, and 5 ACS Paragon Plus Environment


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penta-acrylates, respectively). Increasing functionality increases reactivity, which in turn allows for the operation of the translation stage at higher speeds and reduces processing time. In addition, crosslinkers with higher functionality yield stronger materials

15

. The photonitiator consisted of Eosin Y

(which absorbs strongly in the green) and a tertiary amine as co-initiator. This type of initiator is wellknown in the polymer chemistry arena

16,17

and it was used several times in the past for the fabrication

of cross-linked aerogels 7,8,14,18. Determining the optimal Eosin Y concentration was paramount for these experiments. A too low Eosin Y concentration would reduce polymerization initiation, while an excessive concentration would prevent penetration of the beam through the entire depth of the monolith. A number of trial exposures was therefore carried out, which led to the Eosin Y concentrations reported below. In these trials, the Eosin concentration was varied between 4 times and 1/10 of the optimum value (reported below). Higher-than-optimum concentrations yielded patterns that did not completely penetrate through the samples or which were wedged. Lower-than optimum concentrations required very slow translation speeds to produce distinguishable patterns. To synthesize alcogels, stock solutions were prepared in advance with the following compositions: 

DPHA Stock Solution (DPHA SS): 16.6 mL (19.42 g) DPHA + 29.33 mL EtOH



TMPTA Stock Solution (TMPTA SS): 16.6 mL (18.26 g) TMPTA + 29.33 mL EtOH



Eosin Y Stock Solution (EtOH+): 150 mg Eosin Y + 45 mL EtOH (azeotrope) The gelation solution for DPHA samples was as follows: 9.186 mL DPHA SS, 0.474 mL

MTMA, 2.610 mL TMOS, 0.036 mL EtOH, 0.024 mL EtOH+, 0.386 mL Amine. The gelation solution for TMPTA samples was: 9.186 mL TMPTA SS, 0.474 mL MTMA, 2.610 mL TMOS, 0.060 mL EtOH+, 0.386 mL amine. The reagents were mixed and poured into a mold coated with silicone grease 6

. The mold material was teflon. For most experiments we used tubing with a wall thickness of 3 mm

and an inner diameter of 5 cm. The lower part of the tubing was sealed with paraffin film, which was also used to seal the top after the precursor solution was poured into the mold. Gelation occurred within 30 minutes. The gelation solution for the native silica sample was the same as the TMPTA sample solution. The sample, however, was not exposed to light. The monomer was washed out, unreacted, via solvent exchange, as described in Section 2.5.


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Figure 3: Structure of the acrylic monomers used in the experiments.

2.4. Exposure. To fabricate honeycomb patterns, alcogels were placed on an X-Y translation stage (Newport UTS150). The stage was programmed via Tool Command Language (TCL) scripts interpreted by a motion controller (Newport XPS). The translation speed of the controller was varied between 2.0 and 3.0 mm/s. The sample was exposed to light (532 nm) of a 2W laser (Coherent Verdi), focused on the sample via a long focal length lens (1 meter). The long focal length reduced the beam spot size from 3 mm to 1 mm without producing appreciable divergence through the thickness of the sample. For sample size used in the experiments (ca. 30 mm diameter), exposures were on the order of 10 minutes. To prevent solvent evaporation from the pores of the alcogel, a ~1 mm thick layer of ethanol was kept on the surface of the samples and constantly replenished during exposure. For control purposes, homogeneously cross-linked samples were also produced. For this, samples were extruded from the molds, placed in a leak-tight, transparent container and irradiated using an array of 1W, green LEDs 8. One native sample was also produced following the procedure indicated in Section 2.3. 2.5 Supercritical Drying. After exposure, the samples were removed from the molds and exchanged with an ethanol-water azeotrope mixture to remove unreacted precursors and gelation residues. Samples were bathed in 5x excess ethanol for 3 times over a period of 2 days. Solvent exchange was followed by supercritical drying. The supercritical dryer used to dry our samples was a Parr Instruments model 4602 pressure vessel with a capacity of 2 liters. The pressure vessel was equipped with a thermowell and it was heated by three ceramic heaters, each with a power of 800 W. Heating rates were controlled by varying the power delivered to the heaters with a variac. To prevent solvent evaporation from the alcogels before the supercritical point was reached, an excess volume of 7 ACS Paragon Plus Environment


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ethanol (azeotrope) was poured into the drying vessel. For a 2-liter vessel a minimum of about 500 ml of ethanol had to be added to prevent cracking and shrinking of the monoliths. The pressure vessel was heated to reach the supercritical temperature and pressure of the ethanol-water azeotrope mixture (P c = 7.31 MPa, Tc = 526 K), kept at supercritical conditions for 20 minutes and then vented. We recommend to read the temperature using a thermocouple placed inside the reaction vessel. Placing thermocouples outside the vessel can lead to undershooting the critical temperature (hence to subcritical drying), or to overshooting, which can affect the cross-linking polymer. As described in Refs. (14) and (15), supercritical drying is inherently hazardous, and state and local regulations should be followed. For example, gas should be vented into excess water and the pressure vessel should be equipped with a rupture disk (also venting into water). Most importantly, aluminum and, possibly, other metal alloys should not be inserted in the dryer, as they can give rise to runaway reactions 15. Stainless steel (AISI 316) appears to be stable under the conditions and fluid compositions reported in our experiments. 2.6. Characterization. The thermal conductivity was measured at room temperature and ambient pressure using a HotDisk 2500 S, (Hot Disk, Sweden), which is based on the Transient Plane Source technique. The instrument was equipped with a sensor with a radius of 6.403 mm. As recommended by manufacturer the diameter of sample is two times (in our case 2.5 times) larger than the diameter of the sensor, which is sandwiched between two samples. We are aware that the TPS technique is best suited for homogeneous samples. However, the small size of the samples (determined in turn by the diameter of the drying autoclave) prevented other techniques, such as the hot wire technique, to be be used. Prior to measurements the samples were heated in a vacuum oven (20 mbar) for 24h at 100°C. The measurements took place at 23-25°C and relative humidity around 44-51%. Standard deviations were calculated from 5 measurements of each sample, and were found to be below 2 mW/mK for each sample. Compression tests were performed at room temperature with compression machine (Latzke, Germany) and load cells of 5000 N, with 2 mm min-1 speed of compression. Since standard testing methods for aerogels do not exist, the compression test was based on the recommendations of ISO 844:2014. Young’s moduli were determined in the Hooke’s region, before the irreversible deformation occurs, typically at strain rates between 2.5 and 7.5. The envelope density of the samples was measured pycnometrically with a GeoPyc 1360 (Micromeritics, USA), which has an accuracy of +/- 1.1%, as stated by the manufacturer.


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3. Results and Discussion. 3a. Experiment Digital camera images of alcogels were taken after exposure and solvent exchange. The alcogels were sufficiently transparent to allow measurement size l and thickness t of the honeycomb pattern and also to confirm that patterns had penetrated through the monolith, as shown in Figure 4.

Figure 4: Digital camera images of alcogels following exposure and solvent exchange. a) top view. b) Perspective view, showing complete penetration of the honeycomb pattern through the alcogel thickness. Since the sample was imaged at an angle to show pattern penetration, part of the image is out of focus. c) Side view, also showing complete penetration and lack of wedging. Note: patterns are extremely difficult to image because of poor contrast and perspective issues. Vertical dotted lines have been added to identify the clearest pattern in the image. Dried samples were typically opaque or translucent as shown in Figure 5a). Out-of-plane compression experiments were carried out on multiple samples. In all cases samples could be compressed to a strain of up to 60% without cracking (chipping at the edges was, however, observed), as shown in Figure 5b). The core regions of compressed samples often became translucent or transparent, as shown in Figure 5c). A stress-strain curve of a sample cross-linked with DPHA at a velocity of 2.5 mm/s is reported in Figure 5c). It exhibits a nearly linear region for strains below about 9 ACS Paragon Plus Environment


10% (inset of Figure), followed by a densification-like behavior

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19

. The stress-strain curves for all

samples investigated were comparable to that presented in Figure 5c) and exhibited a linear region followed by densification. In some cases, cracks developed in the monoliths. These cracks were in random locations but they were seldom located at the boundary between cross-linked and native regions. This suggested that shearing of the native cores was not prominent.

Figure 5: a) Digital camera image of a dried honeycomb composite. b,c) The same sample, after compression. No cracks were noticed and the honeycomb cores became transparent. d) Stress-strain curve of a sample patterned at a speed of 2.5 mm/s. Inset: same, to better show the low-strain region. Solid circles in the inset: results from theoretical simulations, see Section 3.b.1 10 ACS Paragon Plus Environment

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Samples were prepared using tri- and penta-functional monomers to determine whether functionality and monomer reactivity affected the mechanical properties. Honeycomb fabrication was also carried out at three different speeds of the translation stage: 2.0, 2.5 amd 3 mm/s, respectively, to determine optimum exposure conditions. Density, out-of-plane modulus and thermal conductivity of the honeycombs are reported in Table 1. The values are compared to those of a native aerogel and of aerogels homogeneously cross-linked with TMPTA and DPHA, respectively.

Sample ID Density (g/cm3) (monomer; stage speed)

Density ρxl of honeycomb pattern (g/cm3)1

Out-of-Plane Modulus (MPa)

Thermal conductivity λexp (mW/mK)

Native

0.124

Not patterned

0.75

11.8

TMPTA, uniform 0.476 exposure

Not patterned

29.85

86.7

DPHA, uniform 0.483 exposure

Not patterned

36.26

65.8

TMPTA; 2.0 mm/s 0.168

Not measured

0.71

13.6

TMPTA; 3.5 mm/s Not measured

Not measured

0.99

24.3

DPHA; 2.0 mm/s

0.2677

0.49

11.47

50.2

DPHA; 2.5 mm/s

0.223

0.38

5.82

35.2

DPHA; 3.0 mm/s

0.192

0.30

2.74

35.7

Table 1: Physical properties of fabricated samples. 1ρxl was calculated using Eq.(1), see also related discussion. Uniform exposure refers to samples which had been illuminated uniformly to yield a homogeneous cross-linking. Two samples for each sample type were prepared and their densities, moduli and thermal conductivities coincided within 5%. The data reported in Table 1 shows the following trends. Native aerogels are mechanically weak (modulus

Fabrication of mechanically strong honeycombs with aerogel cores

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