Highly Conductive and Fracture-Resistant Epoxy Composite Based on

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Highly Conductive and Fracture-Resistant Epoxy Composite Based on Non-oxidized Graphene Flake Aerogel Jin Kim,† Ne Myo Han,‡ Jungmo Kim,† Jinho Lee,† Jang-Kyo Kim,*,‡ and Seokwoo Jeon*,† †

ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 10/20/18. For personal use only.

Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea ‡ Department of Mechanical and Aerospace Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong S Supporting Information *

ABSTRACT: Graphene aerogel (GA) has shown great promise as reinforcement of polymeric composites with exceptional electrical and mechanical characteristics. Although there has been significant progress in controlling the structure of GAs, no studies have appeared on the enhanced properties of GAs by employing high-quality precursor graphene flakes (GFs). However, the assembly of high-quality GFs is particularly challenging due to their highly hydrophobic and agglomerative nature in aqueous media, and of the few methods available to synthesize high-quality GFs, most produce flakes with very small lateral sizes. Herein, we report the fabrication of highly crystalline GAs using large nonoxidized graphene flakes (NOGFs) prepared by a novel graphite intercalation compound-based method. Bidirectional freeze casting is utilized for aligning NOGFs in two orthogonal directions, vertically and laterally, where the NOGF walls individually function as effective conductive pathways. The as-prepared nonoxidized graphene aerogel (NOGA) exhibits a defect concentration as low as 1.4% of impurity oxygen with an excellent electrical conductivity of 202.9 S/m at a low density of 5.7 mg/cm3. The corresponding NOGA−epoxy composite shows a remarkable electrical conductivity of 122.6 S/m and a fracture toughness of 1.74 MPa·m1/2 at a low filler content of 0.45 vol %. KEYWORDS: nonoxidized graphene flake, graphene aerogel, epoxy composite, electrical composite, fracture toughness

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random structure.15 When incorporated into an epoxy matrix, the aligned GA composite delivered an ultralow electrical percolation of 0.007 vol %.16 The aligned structure has been achieved using different approaches such as three-dimensional printing,17 chemical reduction,18 direct templating,19 and directional ice-templating.15,16,20−24 Among these methods, the directional ice-templating offers the most versatility due to the facile control of ice crystal growth. By modifying freezecasting methods and rates, the microstructure of the GA could be easily tuned.25 For example, the pore size of the GA could be adjusted from 10 to 800 μm by controlling the freezecasting temperature from −196 to −10 °C.26 The ice crystal also greatly restricts volume shrinkage of the GA, which is caused by the inherent graphene agglomeration. Moreover, the GA itself could be fabricated in various shapes and sizes by simply altering the container.24 The main disadvantage of this method is the lack of a self-assembly process that chemically binds the graphene sheets together, resulting in an aerogel with relatively lower strength. This could, however, be addressed by

INTRODUCTION Multifunctional materials with ultralight weight are of high demand for many engineering applications, such as aerospace structures,1 construction,2,3 electronics,4 and energy storage.4,5 To fulfill such needs, graphene flakes (GFs) have been widely utilized as reinforcements in polymer composites owing to their low density, large surface area, and excellent mechanical, electrical, and thermal properties.6−10 GFs, however, tend to agglomerate in the polymer matrix during fabrication, leading to limited improvements in the desired properties of the resulting composites. A potential solution to achieving uniform dispersion is to rationally assemble GFs into a porous structure, which ensures stable percolation while maintaining graphene’s inherent properties.11−13 Graphene aerogels (GAs) are a new class of interconnected, 3D cellular materials that are ultralight but robust. They have attracted significant attention because of their fascinating properties, such as high mechanical strength and electrical conductivity, thermal resistance and flexibility, as well as the facile and cheap fabrication process compared to chemical vapor deposition (CVD)-grown graphene foams.14 Recent studies report on the benefits and optimization of aligning the aerogel structure. For example, Yang et al. showed that the aligned GA had 10 times higher resilience than that with a © XXXX American Chemical Society

Received: August 6, 2018 Accepted: October 1, 2018 Published: October 1, 2018 A

DOI: 10.1021/acsami.8b13415 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. Fabrication of NOGA. (a) NOGF/PVA solution used for precursor of NOGA. Bidirectional freeze-casting technique was utilized for aligning NOGFs in the vertical and lateral directions, induced by dual temperature gradients. (b) Schematic of NOGA structure. Scanning electron microscopy (SEM) images of (c) side view and (d) top view of a representative NOGA showing long-range alignment in two orthogonal directions.

fabricate GAs with a relatively high density and an irregular pore structure because of the small size of GFs and the nondirectional nature of the synthesis method used.36 Herein, we report the fabrication of highly aligned GAs composed of NOGFs, and the mechanical and electrical properties of their epoxy composites. The NOGFs were exfoliated from a novel ternary graphite intercalation compound (GIC), where GFs maintained high crystallinity and large lateral sizes.37 The NOGFs were assembled into a highly aligned porous structure using a bidirectional freezecasting process,15 with the aid of a small amount of poly(vinyl alcohol) (PVA) as a glue to hold the structure together. The prepared NOGF aerogel (NOGA) exhibited a low defect concentration with oxide content as low as 1.4% after thermal reduction. The NOGA had a remarkable electrical conductivity of 202.9 S/m, which is, to the best of our knowledge, the highest value ever achieved by GF-based aerogels at a similar density. The corresponding epoxy composite delivered an

physically connecting the graphene sheets, such as by introducing polymer binders.27 GAs with porous structure have been achieved mainly by assembling graphene oxide (GO) sheets with abundant surface functional groups, which allows for effective aqueous dispersion and assembly.11,13 However, these functional groups also act as crystallographic defects, resulting in significantly reduced mechanical and transport properties of GO sheets and the assembled structure even after an intensive reduction process.28−30 Therefore, the utilization of nonoxidized graphene flakes (NOGFs) with a negligible defect concentration has great potential to further improve the characteristics of GAs.31−35 Using defectless or less-defective GFs, however, can create a serious challenge due to their hydrophobicity and susceptibility to agglomeration in aqueous media. Moreover, NOGFs have been fabricated mostly in small sizes, making them unfavorable as the building block to assemble aerogels.24 A room-temperature freeze gelation method was used to B


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Figure 2. Structure of NOGA. (a−d) SEM images of cross section parallel to the ice growth direction of NOGAs prepared from different precursor concentrations. The inset in (d) shows an enlarged image of NOGF bridge and walls. (e) Average density and wall distance of the aerogels prepared from different precursor concentrations. (f) Nitrogen adsorption/desorption isotherms and adsorption pore size distribution (inset) of NOGAs at two different concentrations.

S2), with morphology particularly suitable for efficient assembly of aerogels.24,40−42 The exfoliated NOGFs were dispersed in an aqueous PVA solution. PVA is a widely used as binder to form porous materials due to its high solubility in water and adequate viscosity to hold the structure together.43 To align the NOGFs in two orthogonal directions, the bidirectional freeze-casting technique was utilized by freezing the precursor solution on a metal stage cooled by liquid nitrogen. The technique is a simple and efficient route to align nanoparticles in a long range induced by dual temperature gradients, which arise from the carefully controlled ice crystal growth and particle ejection direction.15,16 The aerogel assembled was then sintered at 700 °C to decompose the PVA binder and obtain a stable, aligned structure composed only of NOGFs (Figure 1b). Because the NOGA would contain minimum structural defects, it is expected to possess

excellent electrical conductivity of 122.6 S/m and a robust fracture toughness of 1.74 MPa·m1/2 at a filler component of 0.45 vol %, which is comparable to that of epoxy composites containing graphene foams grown by CVD.38

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RESULT AND DISCUSSION The schematic in Figure 1a illustrates the fabrication process of NOGA. The NOGFs used as the building blocks of GAs were fabricated using the ternary phase intercalation method. Because this method does not involve any oxidation process, NOGFs could be exfoliated in high crystallinity as presented by the clear hexagonal symmetry in the selective area electron diffraction (SAED) patterns (Figure S1).39 Expanded graphite was used to achieve large lateral sizes and exfoliation efficiency. As a result, NOGFs with an average lateral size of 4.66 μm and mostly few layers thick were obtained after exfoliation (Figure C


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Figure 3. Characterization of NOGA. (a) Raman spectra and (b) XPS survey of NOGF precursor, NOGA before and after the sintering process. (c) C 1s peak of NOGF, NOGA before and after sintering process. The peaks are deconvoluted in four minor peaks, where 285, 286.4, 297, and 289.1 eV correspond to the C−C, C−O, CO, and O−CO groups, respectively. (d) TGA of NOGA before and after the sintering process and pure PVA used as binder.

better properties than those of GO-based aerogels.32,34 Figure 1c,d depicts an as-prepared NOGA with NOGF walls almost perfectly aligned in two directions, vertically as well as laterally, at regular distances over a long range. The microstructure of the NOGA as well as the control of its morphology were further studied by preparing precursor solutions with different NOGF concentrations: 2, 3, 4, and 6 mg/mL while all other processing conditions were kept the same. As shown in the representative scanning electron microscopy (SEM) images (Figure 2a−d), all of the products consisted of highly aligned NOGFs walls, which were densely connected by transverse bridges (see inset of Figure 2d). The NOGF walls got denser with increasing NOGF concentration, which is also reflected by the measurements of wall distance and the density of aerogels. Figure 2e shows that the density of aerogel consistently increased while the wall distance decreased with increasing NOGF concentration toward saturation. This is due to the higher nucleation density of ice crystals with increasing NOGF concentration in the solution, resulting in the formation of more NOGF walls. However, saturation would be envisaged to occur when the concentration exceeds a certain limit because the ice crystals would either diminish or merge together, instead of forming very narrow, stable ice crystals.15 Therefore, NOGAs fabricated using NOGF concentrations exceeding the saturation point would have negligible changes in density and structure. The degree of alignment was quantitatively evaluated by measuring the angles of NOGF walls with respect to the freezing direction, using Image Pro Plus (Media Cybernetics, Inc.). The orientation distributions, shown in Figure S3, indicate that the NOGAs in all concentrations of precursor

solution were composed mostly of NOGF walls oriented vertically along the ice growth direction. A higher concentration favors better alignment of NOGF walls, while there is minor collapse of alignment in 6 mg/mL.16,24 This may also be due to unstable ice crystal growth in higher NOGF concentrations. For more detailed analysis, the specific surface area and the pore volume of the 2 and 6 mg/mL NOGAs were measured using the nitrogen absorption/desorption isotherms, as shown in Figure 2f. The concentration of NOGFs was the dominant factor determining both the specific surface area and the pore volume of aerogels, namely, 79.38 and 126.63 m2/g, and 0.21 and 0.29 cm3/g for the 2 and 6 mg/mL aerogels, respectively. These values for the NOGAs are both lower than the reported values of GO-based aerogel.16,24 One possible reason is the lack of functional groups, where micropores are formed in GO-based aerogels. Another reason is attributed to the dimensional difference between NOGFs and GO flakes. While GOs are reported to be exfoliated in average lateral size larger than 100 μm in monolayer thickness, NOGFs are exfoliated, as explained above, with an average size of 5 μm in a few-layer thickness. Because important mechanical and functional properties of nanocomposites depend largely on the surface area of nanofillers, the aerogel’s morphology could be tuned for specific applications, as the aforementioned findings imply. The quality of NOGAs was assessed by characterizing their defect concentrations and oxygen contents. The defect ratios of the precursor NOGFs and NOGAs were estimated by measuring the ID/IG ratio through Raman spectroscopy (Figure 3a).12,41 The Raman data show that the NOGFs and NOGAs before and after the sintering process all had the D


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Figure 4. Electrical conductivities of NOGA and the corresponding epoxy composite. (a) Electrical conductivity of NOGA in orthogonal directions plotted as a function of NOGF concentration. (b) Comparison of electrical conductivity between the current study and other representative composites as a function of aerogel density. (c) Electrical conductivity of NOGA−epoxy composites in orthogonal directions at different NOGF content. (d) Polarized Raman spectra of 0.24 vol % NOGA−epoxy composites.

overall ID/IG ratios below 0.1, revealing negligible damage to the NOGFs quality after aerogel fabrication. Moreover, these values are significantly lower than the reported values of GO flakes and GO aerogels, confirming superior quality of both NOGFs and NOGAs fabricated in this study.13,16,18,24 The slight increase in the D peak is thought to originate from the edges of stacked NOGFs after assembly into an aerogel. The X-ray photoelectron spectroscopy (XPS) survey spectra in Figure 3b show that the initial oxygen content of NOGFs was 6.2%. After freeze drying to form an NOGA, the oxygen content drastically increased to 27.3%. The C 1s peak of NOGA in Figure 3c indicates that the increase in oxygen content likely originated from the extra C−O bond at 286.4 eV due to the addition of PVA binder used to form stable NOGAs. After the sintering process, the spectrum presented a drastic decrease in oxygen content to 1.4% as a consequence of the successful removal of not only the PVA binder but also the poly(vinyl pyrrolidone) (PVP) surfactant. The C 1s peak also confirms this by showing a sharp decrease in oxygen-related peaks, including the C−O peak. The thermogravimetric analysis (TGA) result in Figure 3d indicates that PVA experienced a drastic weight loss at 200−300 °C and a minor one at 400−500 °C due to the decomposition of polymeric chains. The TGA data of NOGA before sintering also present a weight loss in the same temperature range, confirming that the oxygen content indeed originated from the bound PVA molecules. Complete removal of PVA was further proven by the minor weight loss of sintered NOGA at ∼550 °C, where the pristine graphene flakes start decomposing.33,34 As a result of the highly aligned structure and the excellent graphene quality, the NOGA shows extraordinary electrical

conductivity. At a concentration of 6 mg/mL, the NOGA exhibits remarkable electrical conductivity of 202.9 S/m when measured parallel to the alignment (Figure 4a). Moreover, with rising NOGFs concentration, the electrical conductivity is expected to continuously increase due to the formation of denser NOGF walls, which individually act as effective conductive pathways. The electrical conductivity measured transverse to the alignment, on the other hand, was much lower than that in the parallel direction, especially at high NOGF concentrations. Such an anisotropic behavior proves that the aligned nature could form an efficient unidirectional conductive pathway.16 Compared to the values of conductive porous structures reported in other studies (Figure 4b), the current NOGAs are proven to have the highest electrical conductivities at given densities, with the exception of CVDgrown graphene foams. Unlike other porous structures constructed from nanomaterials, the CVD-grown graphene foams should have better electrical conductivities because they comprise a cellular monolith with continuously interconnected graphene network.38,44 Nonetheless, among the assembled conductive aerogels, including those based on reduced GO,13,45 carbon nanotubes,45 and even pristine graphene with a random structure,36 the current NOGAs exhibit the highest conductivity, proving the significance of high-quality NOGFs used as the precursor material and the highly aligned structure. The NOGAs were subsequently infiltrated with epoxy under vacuum to form composites, and their electrical conductivities in the orthogonal direction are summarized in Figure 4c. The NOGA−epoxy composites also display an increase in electrical conductivity with rising NOGF content. An excellent electrical conductivity of 122.6 S/m was achieved E


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Figure 5. Mechanical properties of NOGA−epoxy composite. (a) Flexural modulus and strength and (b) fracture toughness of NOGA−epoxy composite as a function of NOGF content. (c) Comparison of enhancement in fracture toughness between NOGA and carbon-based nanocomposites in other studies.

Figure 6. Toughening mechanism of NOGA−epoxy composite. Representative SEM images of fracture surfaces of NOGA−epoxy composites with (a, c) 0.16 vol % and (b, e) 0.45 vol % NOGAs. The white arrows indicate crack propagation direction. The scale bars of (c) and (e) represent 20 μm. Schematics of crack propagation in composites with different NOGA contents, representing (d) 0.16 vol % and (f) 0.45 vol % NOGA contents.

other reported values (Table S1),18,24,38,44,46−49 the conductivities of the current composites are among the highest because of not only the inherently high electrical conductivity

in the direction parallel to the alignment at NOGF content of 0.27 vol %, which is approximately 12 orders of magnitude enhancement from that of neat epoxy. When compared to F


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due to the larger number of NOGF walls and longer tortuous crack paths, as demonstrated by the schematic in Figure 6d,f. Moreover, additional fracture energy could be absorbed due to interfacial debonding between the epoxy matrix and NOGA, slippage between the adjacent NOGF sheets when the crack meets the NOGF walls, contributing to much enhanced toughening of the composite (Figure S7).37,60,61 Having low filler-to-matrix interaction between the NOGA and the epoxy matrix, interfacial debonding could be observed throughout the composite. However, at a higher NOGF content of 0.57 vol %, the fracture toughness started to decease due possibly to agglomeration of NOGF walls, causing uneven stress concentrations across the composite with less effective crack deflection and interfacial debonding (Figure S8).

of the NOGA itself, but also the well preserved alignment of the aerogel structure in the epoxy matrix. The preservation of the structure is evident from the anisotropic behavior of Raman spectra, as shown in Figure 4d. The Raman spectrum obtained parallel to the aligned direction shows a significantly higher peak intensity than in the transverse direction owing to the signal resonance when NOGFs are parallel to the incident laser.16,24 Finally, the mechanical properties of the NOGA−epoxy composites, in terms of flexural modulus and fracture toughness, were characterized to evaluate the significance of the high quality and the aligned structure of NOGAs. Both measurements were carried out by applying load parallel to the NOGF wall alignment. Figure 5a summarizes the flexural modulus and strength. It is shown that the flexural modulus improves with increasing NOGF content, which is also evident by the decrease in fracture strain, as shown in Figure S4. Flexural strength, where the composite also fractures due to the brittle nature of the material, shows a similar tendency but a small decrease at high filler loading. At a filler loading of 0.34 vol %, the flexural modulus is enhanced by 25.4% and flexural strength by 10.2% compared to those of the neat epoxy. According to the rule of mixtures, the NOGA−epoxy composite is not expected to have a substantial increase in flexural properties due to the low filler loading. Moreover, the lack of surface functional groups on NOGFs, as shown in the XPS, would limit both the interfacial bond and load transfer between the NOGA and the epoxy matrix.24 The extensive debonding between them presented in the next section may originate from the weak interfacial interactions. The fracture toughness (Figure 5b), on the other hand, exhibits a sharper enhancement. The composite delivered an excellent 42.1% increase in fracture toughness at a very low loading of 0.16 vol %, with a maximum increase of 76.1% at 0.45 vol%, which is highly competitive compared to composites reinforced with other nanofillers (Figure 5c).18,24,38,50−54 Such remarkable improvements in fracture resistance at low filler loadings are partially attributed to the excellent mechanical properties of NOGFs, originating from the highly preserved crystallinity and graphitic structure. In addition, the fracture toughness is expected to be enhanced significantly due to effective crack propagation hindrance by the horizontally aligned NOGF walls. The detailed toughening mechanisms were studied by analyzing the crack propagation behavior and the fracture surfaces, as presented in Figure 6. The neat epoxy without reinforcement shows a clean, featureless surface with very weak resistance against crack propagation (Figure S5). The NOGA− epoxy composite, by contrast, exhibits a jagged crack pattern (Figure S6) and a characteristic rough fracture surface due to the presence of horizontally aligned NOGF walls, which were more prominent at higher filler contents (Figure 6a,b).18,55 The distances between NOGF walls in the composites are consistent with the average wall distances of the NOGAs observed in Figure 2e. Such NOGF walls would form effective tortuous crack paths where crack blunting and either crack bifurication or crack deflection along the NOGF boundaries occurred when the cracks had to overcome the interconnected NOGF walls (Figure 6c,e).18,56 These crack hindrances resulted in a tension−shear mixed-mode fracture phenomenon with largely increased fracture surface area and energy absorption of the composite.24,38,57−59 The composites with higher NOGF contents would experience higher toughening

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CONCLUSIONS In conclusion, we have demonstrated the fabrication of an aerogel solely composed of NOGFs, and how its quality and vertically aligned microstructure could synergistically enhance the electrical conductivity and mechanical properties of an epoxy composite. By utilizing a bidirectional freeze-casting technique, a graphene-based aerogel with a high degree of alignment in two orthogonal directions was successfully fabricated. Moreover, the aerogel exhibited an extremely low defect concentration due to oxidation because it contained NOGFs exfoliated directly from ternary phase GIC without involving any oxidation. As a result, the aerogel possessed an extraordinary electric conductivity of 202.9 S/m at a density of 5.7 mg/cm3, which is the highest value reported for conductive aerogels at the given density. The epoxy composites were fabricated by impregnating epoxy resin into the NOGA through vacuum infiltration, which delivered excellent electrical conductivities and mechanical properties, also reflecting the aerogel’s outstanding characteristics. The composites exhibited an electrical conductivity of 122.6 S/m at 0.57 vol % and a fracture toughness of 1.74 MPa·m1/2 at 0.45 vol %. Both of these values are remarkable compared to other reported ones, including those of epoxy composites containing CVD-grown graphene foams. Such remarkable properties of polymeric composites demonstrate that NOGA is promising for replacing GO-based aerogels especially in applications such as automotive, aerospace, and electrodes for supercapacitors and batteries that require high conductivity, mechanical strength, and ultralight weight.

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METHODS

Fabrication of NOGFs. Graphite powder (Asbury Graphite Mills) was first immersed in a sulfuric acid−nitric acid solution (3:1 weight ratio) for 24 h at room temperature. The graphite powder was extracted and thoroughly washed with deionized (DI) water and dried at 60 °C for another 24 h. The graphite powder was expanded under 800 W of microwave for 1 min using a microwave oven. Potassium naphthalenide solution (1 M) was prepared by dissolving bulk potassium metal and naphthalene in anhydrous tetrahydrofuran (THF). The expanded graphite was completely immersed in the potassium naphthalenide solution for at least 12 h to fabricate GIC. After the intercalation process, the GIC was washed with anhydrous THF to remove residual potassium ion and naphthalene. The rinsed GIC was immersed in PVP−dimethyl sulfoxide (2 mg/mL) and sonicated for 30 min in a bath sonicator (Branson). The NOGF solution was centrifuged at 1500 rpm for 30 min, and the supernatant was collected to remove thick graphite flakes. G


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ACS Applied Materials & Interfaces Fabrication of NOGF Aerogel. NOGFs in various weights were redispersed in PVA (MW = 31 000−50 000)−DI water solution (1 mg/mL, 10 mL) under probe sonication for 3 min. The precursor solution was then poured into a poly(lactic acid) (PLA) mold with one end having 20° wedge. Bidirectional freeze casting was conducted by placing the mold on a metal stage, which was cooled by liquid nitrogen. Figure 1b shows the schematic of freeze casting apparatus. The NOGF product was collected and freeze-dried for 48 h. Finally, the aerogel was annealed at 700 °C in an Ar atmosphere for 2 h to remove PVA. Fabrication of NOGA−epoxy Composite. Epoxy resin (LY1556, supplied by Huntsman Advanced Materials) and curing agent triethylenetetramine were mixed at a weight ratio of 100:12 and infiltrated in the NOGA under vacuum for 2 h. The mixture was then heated at 80 °C for 30 min followed by curing at 110 °C for 2 h. Characterization. SEM (S-4800, Hitachi) with field emission source at 10 keV was used to examine the structure of the NOGAs and the fracture surface of NOGA−epoxy composites. TEM analysis was performed with F2 F20 (Tecnai) at acceleration voltage of 200 keV. A nitrogen adsorption device (Coulter SA3100) was used to measure the adsorption/desorption isotherms of the NOGAs at 77 K. Raman spectroscopy was carried out on a Senterra system (Bruker). For thickness measurements of the NOGFs, atomic force microscopy (AFM) (SPA400, SII, Japan) was used. XPS was performed on a Sigma Probe system (Thermo VG Scientific) with Al Kα radiation source. TGA was performed on a TG 209 F3 (Netzsch-Gerätebau GmbH) at a heating rate of 5 °C/min in Ar atmosphere. The fourpoint probe system (Scientific Equipment & Services) was used for measuring the electrical conductivities of NOGAs and NOGA−epoxy composites. Mechanical Analysis of NOGF Aerogel Epoxy Composite. For measuring the flexural properties, a universal testing machine (MTS Insight 1 SL) was used according to the specification ASTM D790. The composite samples were cut into 28 mm (l) × 3 mm (w) × 1.6 mm (t), which were loaded in three-point bending with a support span of 24 mm at a cross-head speed of 2.0 mm/min. The mode 1 fracture toughness was measured according to the specification, ASTM D5045, on a universal testing machine (MTS Alliance RT/05). The measurement was carried out in three-point bending at a cross-head speed of 10.0 mm/min with single-edge notched bend specimens of 20 mm (l) × 2.1 mm (w) × 4.2 mm (t) and total crack length of 2 mm. The fracture toughness, KIC, was calculated using the following equations KIC =

Pmaxf (x) BW

1/2

f (x) = 6x1/2

, x=

a W

ORCID

Jang-Kyo Kim: 0000-0002-5390-8763 Seokwoo Jeon: 0000-0002-5338-0671 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Fabrication of aerogels and composites and the analysis of their properties were carried out when the first author (J.K.) was a visiting PhD student at the Hong Kong University of Science and Technology. This work was supported by Future Planning as Global Frontier (CASE-2013M3A6A5073173) and Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF2016M3A7B4900118).

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(1) Monteverde, F.; Scatteia, L. Resistance to Thermal Shock and to Oxidation of Metal Diborides−SiC Ceramics for Aerospace Application. J. Am. Ceram. Soc. 2007, 90, 1130−1138. (2) Bonderer, L. J.; Studart, A. R.; Gauckler, L. J. Bioinspired Design and Assembly of Platelet Reinforced Polymer Films. Science 2008, 319, 1069−1073. (3) Studart, A. R. Towards High-performance Bioinspired Composites. Adv. Mater. 2012, 24, 5024−5044. (4) Eda, G.; Chhowalla, M. Graphene-based Composite Thin Films for Electronics. Nano Lett. 2009, 9, 814−818. (5) Barber, P.; Balasubramanian, S.; Anguchamy, Y.; Gong, S.; Wibowo, A.; Gao, H.; Ploehn, H. J.; Zur Loye, H.-C. Polymer Composite and Nanocomposite Dielectric Materials for Pulse Power Energy Storage. Materials 2009, 2, 1697−1733. (6) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (7) Bolotin, K. I.; Sikes, K.; Jiang, Z.; Klima, M.; Fudenberg, G.; Hone, J.; Kim, P.; Stormer, H. Ultrahigh Electron Mobility in Suspended Graphene. Solid State Commun. 2008, 146, 351−355. (8) Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Superior Thermal Conductivity of SingleLayer Graphene. Nano Lett. 2008, 8, 902−907. (9) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Measurement of The Elastic Properties and Intrinsic Strength of Monolayer Graphene. Science 2008, 321, 385−388. (10) Allen, M. J.; Tung, V. C.; Kaner, R. B. Honeycomb Carbon: A Review of Graphene. Chem. Rev. 2010, 110, 132−145. (11) Xu, Y.; Sheng, K.; Li, C.; Shi, G. Self-assembled Graphene Hydrogel via a One-Step Hydrothermal Process. ACS Nano 2010, 4, 4324−4330. (12) Worsley, M. A.; Pham, T. T.; Yan, A.; Shin, S. J.; Lee, J. R.; Bagge-Hansen, M.; Mickelson, W.; Zettl, A. Synthesis and Characterization of Highly Crystalline Graphene Aerogels. ACS Nano 2014, 8, 11013−11022. (13) Worsley, M. A.; Pauzauskie, P. J.; Olson, T. Y.; Biener, J.; Satcher, J. H., Jr.; Baumann, T. F. Synthesis of Graphene Aerogel with High Electrical Conductivity. J. Am. Chem. Soc. 2010, 132, 14067− 14069. (14) Gorgolis, G.; Galiotis, C. Graphene Aerogels: A Review. 2D Mater. 2017, 4, No. 032001. (15) Yang, M.; Zhao, N.; Cui, Y.; Gao, W.; Zhao, Q.; Gao, C.; Bai, H.; Xie, T. Biomimetic Architectured Graphene Aerogel with

(1)

[1.99 − x(1 − x)(2.15 − 3.93x + 2.7x 2)] (1 + 2x)(1 − x)3/2

(2)

where Pmax is the maximum load and B, W, and a are width, height, and the initial crack length of the specimen, respectively. At least five samples were tested in all mechanical analysis

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b13415.

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REFERENCES

Additional information regarding characterization of NOGFs, NOGA filler orientation analysis, additional toughening mechanism, and comparison of fracture toughness to other reports (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.-K.K.). *E-mail: [email protected] (S.J.). H


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