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Flyweight 3D Graphene Scaffolds with Microinterface BarrierDerived Tunable Thermal Insulation and Flame Retardancy Qiangqiang Zhang,*,†,‡,§,∥,# Menglong Hao,∥,⊥,# Xiang Xu,§ Guoping Xiong,∥,⊥ Hui Li,§ and Timothy S. Fisher*,∥,⊥ †

School of Civil Engineering and Mechanics, Lanzhou University, Lanzhou 730000, P. R. China Key Laboratory of Mechanics on Disaster and Environment in Western China, Lanzhou University, The Ministry of Education of China, Lanzhou 730000, P. R. China § School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, P. R. China ∥ Birck Nanotechnology Center and ⊥School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907, United States ‡

S Supporting Information *

ABSTRACT: In this article, flyweight three-dimensional (3D) graphene scaffolds (GSs) have been demonstrated with a microinterface barrier-derived thermal insulation and flame retardancy characteristics. Such 3D GSs were fabricated by a modified hydrothermal method and a unidirectional freezecasting process with hierarchical porous microstructures. Because of high porosity (99.9%), significant phonon scattering, and strong π−π interaction at the interface barriers of multilayer graphene cellular walls, the GSs demonstrate a sequence of multifunctional properties simultaneously, such as lightweight density, thermal insulating characteristics, and outstanding mechanical robustness. At 100 °C, oxidized GSs exhibit a thermal conductivity of 0.0126 ± 0.0010 W/(m K) in vacuum. The thermal conductivity of oxidized GSs remains relatively unaffected despite large-scale deformation-induced densification of the microstructures, as compared to the behavior of reduced GSs (rGSs) whose thermal conductivity increases dramatically under compression. The contrasting behavior of oxidized GSs and rGSs appears to derive from large differences in the intersheet contact resistance and varying intrinsic thermal conductivity between reduced and oxidized graphene sheets. The oxidized GSs also exhibit excellent flame retardant behavior and mechanical robustness, with only 2% strength decay after flame treatment. In a broader context, this work demonstrates a useful strategy to design porous nanomaterials with a tunable heat conduction behavior through interface engineering at the nanoscale. KEYWORDS: microinterface barrier, thermal insulating, flame retardant, 3D graphene scaffolds, tunable heat conduction, interface engineering

1. INTRODUCTION With the ongoing depletion of fossil fuel resources, human society faces a daunting energy future.1,2 While alternative energy sources are being developed intensely, improving energy utilization efficiency will certainly be a substantial part of a sustainable overall energy ecosystem. Enormous amounts of fossil fuel are consumed for heating and air conditioning in the domestic sector.3,4 Good thermal insulation of buildings is crucial for conservation of energy and greenhouse gas emission. Thermal insulation materials are also important in many other applications, such as automotive, aerospace, domestic appliances, and petrochemical plants, to name a few.5 For example, excellent thermal insulation is a vital requirement for the skin of reentrant aerospace vehicles to protect the interior from the high surface temperatures generated when penetrating the earth’s atmosphere.6 The thermal insulating ability of a material is characterized by its thermal conductivity (κ, W/(m K)). Currently, the most © XXXX American Chemical Society

popular thermal insulators are fossil-fuel-derived porous materials, which can be synthesized or exist naturally.3,4,7−13 They typically possess fine porous microstructures with cells small enough to trap and stagnate air. For instance, common thermal insulators include expanded polystyrene foam (κ = 0.030−0.040 W/(m K)), polyurethane (κ = 0.020−0.030 W/(m K)), fiberglass (κ = 0.033−0.044 W/(m K)), cellulose nanofiberbased foam (κ = 0.018 W/(m K)), and vermiculite (κ = 0.033− 0.044 W/(m K)).3,4 However, high demands for scalable thermal insulation materials with even lower thermal conductivity and flyweight density still exist. Most successful thermal insulation materials are composed of one or more low κ constituents and have reported highly porous structures because the low density suppresses solid-state heat Received: February 4, 2017 Accepted: April 5, 2017 Published: April 5, 2017 A

DOI: 10.1021/acsami.7b01697 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Microstructure characterization of GSs with a density of 10 mg cm−3. (a) Optical images of as-prepared GSs. (b) SEM image showing the morphology of rGSs in a transverse cross-section. (c) A magnified view of (b) showing the honeycomb structure. (d) The cellular unit formed by π−π interaction among graphene sheets with a pore size of approximately 35 μm. (e) The longitudinal cross-sectional view of the cylindrical sample with wellarrayed graphene cellular walls. (f) The vertically oriented and wall-by-wall interconnected microstructure with an interspacing of 30 ± 5 μm.

3000 W/(m K) and are typically used in thermal spreading applications.27−35 GO sheets, however, exhibit significantly lower thermal conductivity due to lattice distortion and defects that produce phonon scattering.17,36 In this work, we demonstrate that with highly porous microstructures and properly engineered interface barriers, the thermal conductivity of 3D porous graphene scaffolds (GSs) constructed from GO sheets can be tuned to a very low level, making it attractive as a robust and scalable material for thermal insulation.

conduction while small pores confine air in a small space to prevent thermal convection.3,13,14 Although some porous nanostructures, such as silicon aerogels and cellulose nanofiber-based foams, have demonstrated a low κ of approximately 0.020 W/(m K), mechanical brittleness or processing complexity make them difficult to scale up to meet industrial requirements.3,11,12 For other naturally existing (e.g., wood chip, cork, and cellulose) or synthesized (e.g., fiberglass, polyurethane, and polystyrene) thermal insulation materials,3,4,7−13 apart from the relatively poor thermal insulation properties with κ in the range of 0.020−0.050 W/(m K), their sensitivity to ambient humidity, poor thermal stability, and low flame resistance preclude their applications in many circumstances.3,4 Previously reported results have demonstrated that utilization of two-dimensional (2D) precursors can effectively decrease the thermal conductivity of bulk materials by creating a plethora of phonon barriers.3,15,16 Graphene oxide (GO) sheets exhibit much lower thermal conductivity compared to that of pristine graphene due to the prevalence of oxygenic functional groups (e.g., epoxide, hydroxyl, and carboxyl) and defect-induced scattering of phonons.17 Because of the electrostatic attraction between parallel displaced graphene sheets, a strong π−π interaction exists between them, offering an effective means for GO sheets to self-assemble into strongly connected and wellarrayed three-dimensional (3D) macroscopic monoliths.7,18,19 Such 3D macroscopic graphene monoliths, including graphene petals,20 hydrogels,21 sponges,22 and aerogels,3,18,19,22−26 offer a promising combination of low density, high porosity, and mechanical robustness. They have demonstrated broad applicability in many scientific and engineering fields, including stretchable electronics, energy-storage devices, sensors, liquid absorbers, electrocatalysts, and biomedical scaffolds, to name a few.3,18,19,22−26 With regard to heat transfer, low-dimensional graphitic materials, including graphene and carbon nanotubes (CNTs), are considered to be among the best thermal conductors with pristine thermal conductivity of higher than

2. EXPERIMENTAL SECTION 2.1. Characterization and Instrumentation. The structure of GO was characterized by transmission electron microscopy (Tecnai G2 F30; FEI). Micrographs of GSs were characterized by the Hitachi S-4800 field emission scanning electron microscope (SEM). The in situ observations of microstructure evolution during compression were conducted on an SEM (HELIOS NanoLab 600i; FEI). The structure and chemical composition of GO, oxidized GSs, and reduced GSs (rGSs) were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The XRD analysis was carried out by an X-ray diffractometer (X’PERT PRO MPD; PANalytical, Netherlands) using Cu Kα radiation (1.540598 Å) with a 2θ ranging from 5 to 50°. The XPS (PHI 5700 ESCA System; Physical electronics Co.) utilized monochromatic Al Kα radiation as the X-ray source (hv = 1486.6 eV). The Raman spectra were recorded by a micro Raman spectrometer (laser source: 532 nm) in the Raman shift range of 1000−3000 cm−1. The thermogravimetric analysis (TGA) was performed with a simultaneous TGA/differential scanning calorimetry thermal analyzer (TGA/SDTA851e; Mettler-Toledo, Switzerland) at a heating rate of 10 °C/min with temperatures elevating from 25 to 800 °C under argon protection and in air conditions, respectively. The specific surface area (SSA) was measured using a Brunauer−Emmett− Teller (BET) analyzer (Micromeritics; TRISTAR II 3020). An electronic universal material testing machine (Instron 5569, U.K.) was used to characterize compressive deformation of oxidized GSs at a constant loading rate (1 mm/min) before and after flame treatment. The IR mapping is obtained with an IR camera (ThermoVision ExaminIR SC6400; FLIR Systems, Inc.). Thermal conductivities of both oxidized and rGS samples are measured with a one-dimensional (1D)B


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Figure 2. Characterization of the structure and chemical composition of GO, oxidize GSs, and rGSs, respectively. (a, b) Comparative XRD pattern and Raman spectra. (c) XPS survey spectra for all elements (C1s, N1s, and O1s). (d) XPS spectrum for C1s of rGSs. (e) Comparative TGA results. (f) The N2 adsorption/desorption isotherms using the BET method. %) for 24 h to remove residual impurities (EDA) and to adjust the temperature of icing. The as-dialyzed hydrogels were freeze-casted in a low-temperature chamber (−80 °C) for 24 h, and then 3D oxidized GSs were obtained by a subsequent drying process under vacuum conditions. Some oxidized GSs were further thermally annealed at 1000 °C for 1 h in a tube furnace under an argon atmosphere (200 sccm at a pressure of 14.5 psi) to obtain rGSs with a typical metallic luster. The finally obtained rGSs have very low densities ranging from 2 to 10 mg/cm3.

reference bar system modified from ASTM D-5470 with the schematics shown in Figure S1, which are detailed in the Supporting Information (SI). 2.2. Fabrication of GSs. A GO precursor with an average lateral diameter in the range of 40−50 μm was synthesized by a modified Hummers’ method using natural flake graphite (50 mesh; Nanjing Xianfeng Nanomaterials Tech. Co. Ltd., Nanjing, China), as reported in our previous publications.18,19,25,26 Briefly, a mixture of 20 μL of ethanediamine (EDA; Sigma-Aldrich) and 10 mL of the GO precursor (2−10 mg/mL) was ultrasonically dispersed for 0.5 h with a mild power in an ice bath and then transferred to a Teflon-lined hydrothermal synthesis autoclave to obtain a black cylindrical hydrogel after a 6 h static reaction at 120 °C. Before carrying out the freeze-drying process, the asformed graphene hydrogels were dialyzed in an ethanol solution (10 vol

3. RESULTS AND DISCUSSION 3.1. Microstructure and Chemical Composition Characterization. Highly porous cylindrical GS samples were fabricated by a modified hydrothermal method, followed by a unidirectional freeze-casting process. The final products exhibit a C


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effects of EDA with 9.5% of C connected with the N atom via CN covalent bonds during the hydrothermal process (Table 1). In addition, approximately 4% of the C was still grafted with the N atom within rGSs after thermal annealing at a high temperature of 1000 °C as a result of the chemical reaction of some oxygenic functional groups with amidogen (NH2) in EDA,18,37−40 which is further demonstrated in the deconvoluted C1s peak of rGSs at 285.4, as shown in Figure 2d. The thermal stabilities of GO, oxidized GSs, and rGSs were characterized by TGA in the temperature range of 25−800 °C, as shown in Figure 2e. Under argon protection, rGSs demonstrate the best thermal stability with a negligible weight loss at 800 °C. Comparatively, weight losses at 500 °C were 15.9 (GO) and 3.6% (oxidized GSs) due to partial removal of the oxygenic functional groups during thermal annealing. However, in air condition, rGSs presents relatively high weight drops due to thermal ablation under temperatures over 500 °C. Furthermore, the microstructures of GSs were characterized using the BET method. As shown in Figure 2f, the BET SSA and pore volume of GSs are 33.8 m2/g and 0.02 cm3/g, respectively, for the sample with a density of 8 mg/cm3. 3.2. Thermal Conductivity Measurements. For thermal conductivity measurements, the experimental setup comprised a 1D-reference bar system modified from ASTM D-5470 and is schematically illustrated in Figure S1.41,42 All thermal measurements were conducted in vacuum to preclude convection and oxidation effects. Ideally, heat transfer in the test column occurs through 1D heat conduction only, and a linear temperature profile is generated with slopes corresponding to different local thermal conductivities (κ ∝ ∇T−1). However, this assumption is not strictly valid when radiative heat loss to the surroundings becomes significant compared to heat conduction. For hightemperature measurements, this radiation-induced nonlinearity becomes more pronounced because thermal radiation scales with the fourth order of absolute temperature according to the Stefan−Boltzmann law (qw ∝ T4, where qw is the heat flux density (W/m2)). Hence, to calibrate the effect of radiation loss from the GSs samples and thus improve accuracy, a two-step data evaluation scheme was executed for all thermal conductivity measurements. Data evaluation schematics and other details of the measurement process are included in SI (see Figures S2 and S3). Notably, due to the vacuum environment and porous nature of the samples, the measured thermal conductivity has contributions from conduction and radiation, but not convection. The experimental results of thermal measurements are summarized in Figure 3, with all data collected at the average temperature of 100 °C. Except for rGSs (ρ > 8 mg/cm3), all GS samples exhibit good thermal insulation behaviors with thermal conductivities lower than that of dry air at 100 °C (0.0314 W/(m K)) (see Figure 3a). In particular, oxidized GSs with a density of 2 mg/cm3 exhibit a thermal conductivity of only 0.0126 mW/(m K). Comparatively, the thermal conductivities (κ*) of GSs obtained without considering radiation heat loss are 14.2−30% lower than the values reported here, confirming the necessity of the radiation-corrected data evaluation scheme (Figure S4). Within the tested range of samples, thermal conductivities of rGSs and original oxidized GSs appear to monotonically increase with the volume fraction (Figure 3a). This trend is reasonable given that these aerogels of different densities possess similar microstructures despite the difference in pore sizes and cellular wall thicknesses, as indicated by SEM images. A close look at the GS’s microstructure indicates anisotropic characteristics from

black color for oxidized GSs and a dark metallic luster for rGSs, as presented in Figure 1a. Figure 1b−d displays SEM images showing the morphologies at a transverse cross-section. It is apparent from these images that GSs present well-ordered 2D isotropic honeycomb-like microstructures with pore sizes of tens of micrometers (approximately 35 μm). The homogenous cellular pores were shaped by squeezing ice crystals during freezing, making it feasible to design and control the microstructure by adopting different freezing strategies with diverse temperatures and orientations.25,26 At the microscale, GO sheets were first self-assembled and stacked via a faciallinking interface based on the strong π−π interaction, with typical stacking lengths of 20−30 μm, and, subsequently, formed a mechanically robust monolith after being squeezed by the ice crystallization process. The lateral views shown in Figure 1e,f indicate well-arrayed leaf-like structures of the multilayer graphene sheets with a longitudinal orientation. The backbone cellular walls are cross-linked with small sub-branches (20−35 μm) of few-layered graphene sheets. Such microstructures not only lead to a mechanically stable and structurally self-supporting system, but also dictate the fundamental characteristics of heat conduction in the material. Raman spectroscopy, XRD, and XPS were used to characterize the structure and chemical composition of GO, oxidized GSs, and rGSs. As shown in Figure 2a, XRD peaks are present at 11.7, 23.7, and 26.3° for the GO, oxidized GSs, and rGSs, respectively, indicating a shift of the interlayer spacing from 7.65 (between (001) planes in GO) to 3.42 Å (between (002) planes in rGSs). The sharp peak at 26.3° for rGSs implies a high degree of graphitization and ordered stacking among the graphene sheets,18 whereas the broad peak near 23.7° for oxidized GSs suggests only partial reduction of the GO sheets during the hydrothermal assembling process with interlayer spacing enlarged to 3.80 Å. Figure 2b displays the typical Raman spectra of GO, oxidized GSs, and rGSs with the D-band, G-band, 2Dband, and S3-band (also known as the D + D′ band and an indication of a defect population37−40) centered at 1345, 1575, 2700, and 2920 cm−1, respectively, indicating the recovery of the pristine graphene structure.18 In addition, the peak intensity ratio (ID/IG) of the oxidized GSs decreases from 1.15 to 1.0 after thermal annealing (reduction reaction) at 1000 °C, indicating a considerable recovery of the conjugated domains (sp2) in the rGSs.18 To quantify the chemical composition evolution during the hydrothermal synthesis and thermal reduction process of GO to rGSs, the XPS analysis was conducted, with results shown in Figure 2c. The C1s, N1s, and O1s peaks are located at 285.3, 400.5, and 533.2 eV, respectively. As listed in Table 1, quantification of Table 1. Atomic Concentration of GO, Oxidized GSs, and rGSs in the XPS Spectra (%) elements

C1s

O1s

N1s

GO oxidized GSs rGSs

72.2 85.1 93.8

27.8 6.8 2.5

0 8.1 3.7

the XPS peak intensities suggests a significant increase in the atomic ratio of C/O, from 2.59:1 for GO to 37.4:1 for rGSs, implying that over 90% of the oxygenic functional groups were removed upon thermal annealing (see Figure 2c). The increased C/O ratio of 12.5:1 for oxidized GSs and the emerging peaks at 400.5 eV for N1s are attributed to the bridging/cross-linking D


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Figure 3. Thermal conductivity (κ) of GSs. (a) Relationship between κ and bulk density. (b) Temperature-dependent κ with a density of 6 mg/cm3. (c) Comparison among a spectrum of materials for thermal applications. (d) Change in thermal conductivity in a compressed state. (e) Data in (d) replotted to show the effect of compression on the total sample thermal resistance normalized to 1 cm initial height of the cylindrical samples.

Such low thermal conductivities of oxidized GSs can be attributed to their high porosity (99.44−99.91%), the intrinsically low thermal conductivity of GO sheets, and interfacial phonon scattering. Firstly, high porosity limits the cross-sectional area for heat conduction. Secondly, compared to that of the perfect pristine graphene with κ as high as 5000 W/(m K), GO sheets as basic elements building the GSs exhibit a much lower

two directions. All thermal conductivity results shown in Figure 3 are measured in the vertical direction. For comparison, we measured the horizontal thermal conductivity of a 10 mg/cm3 oxidized GS at 100 °C and found it to be 14% lower than in the vertical direction (Figure S5a). However, the leading trends of thermal conductivity changing with density and temperature will likely be valid for both directions. E


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Figure 4. In situ observation of the heat-transfer pathway evolution in the microstructure of GSs during mechanical compressing deformation with strain up to 60%. (a) Initial unloaded status. (b−f) Uniaxial compression with a strain of 10, 20, 35, 50, and 60%, respectively. (g) Schematic illustration of interfaces between GO sheets. (h) Schematic illustration of interfaces between reduced GO sheets.

thermal conductivity of ∼10 W/(m K) due to the prevalence of oxygenic functional groups and defect-induced scattering of phonons.17,27−35 Lastly, intersheet thermal resistance suppresses phonon transport, leading to a further decrease of κ. Because the cellular walls of oxidized GSs are assembled from single/fewlayer GO sheets, the number density of the interface barriers is quite large. The important role of the interfacial resistance in the porous material made from nano building blocks such as CNT pellets has been illustrated in several prior studies.8,10,14 This conclusion is also confirmed by the impacts of interface quantity on the sheet’s thermal conductivity, as shown in Figure S5b. Because GSs have been proposed as promising thermal insulation and flame retardant materials, their heat-transfer characteristics under elevated temperatures are therefore of great importance. The influence of temperature on κ is shown in Figure 3b, in which κ of rGSs and oxidized GSs monotonically increases with the temperature increasing to 500 °C. Recall that the measured thermal conductivity has conductive and radiative components. Three common mechanisms, namely, inelastic phonon scattering within the GO sheets, intersheet resistance, and thermal radiation, are hypothesized to account for this increasing trend of rGSs and oxidized GSs. Notably, however, oxidized GSs are not fully thermally stable at temperatures up to

500 °C as shown by the TGA results (Figure 2e). Partial thermal reduction during high-temperature testing also contributes to the increase in thermal conductivity. A control experiment has been performed to investigate this effect. A fresh 2 mg/cm3 oxidized GSs sample was annealed at 500 °C and remeasured at 100 °C. The related thermal conductivity increased up to 0.0135 W/(m K) (approximately 7% increase) compared to 0.0148 W/(m K) (approximately 17.5% increase) for that of rGS annealed at 1000 °C (Figure S6). Hence, we conclude that the effect of partial thermal reduction on the thermal conductivity of oxidized GSs is rather small for temperatures below 500 °C. Within reduced GO sheets, heat conduction via phonons (as opposed to electrons) is generally believed to be dominant.17 Although free electrons become more abundant with the increase of the average kinetic energy, their contribution to thermal conductivity is less than 3% as estimated by the Wiedemann− Franz law43 (κe = 2.44 × 10−8σT, where κe is the electronic contribution to thermal conductivity, σ is the electrical conductivity of rGSs and is about 60 S/cm at room temperature,18 and T is the temperature in Kelvin). Further, the number of conduction modes by phonons is an order of magnitude higher than that by electrons for graphene.44 Increased inelastic phonon scattering events at high temperatures result in a shorter F


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Figure 5. Flame retardant and thermal insulation applications of oxidized GSs. (a) Mechanical robustness before and after flame treatment. (b) Snapshots of flame treatment and the mechanical compression process. (c) Raman spectra evolution during flame treatment. (d) Thermographic patterns of temperature distribution with samples of oxidized GSs and rGSs placed on heat plates. Emissivity is not locally calibrated for this imaging; therefore, temperature readings on parts other than the samples are not to be taken quantitatively.

corrosion resistance (metallic), poor thermal stability, and flame retardancy (polymer);3,4 (2) thermally high-conducting solid materials, such as dense metal Al, crystalline AlN, and pristine carbon nanomaterials (graphene, CNT);14,32 and (3) thermally superinsulating materials, including the GSs in this work. The thermal transport properties of GSs were further investigated under compressed states. Quite different heattransfer characteristics were observed for the two types of GSs (rGSs and oxidized GSs). In our experiments, GSs with a starting density of 4 mg/cm3 were gradually compressed to various effective densities up to around 40 mg/cm3 (corresponding to a 90% volume compression ratio) (see Figure 3d). The corresponding thermal conductivity and total thermal resistance changes are shown in Figure 3d,e. In order for direct comparison between rGS and oxidized GS samples with different heights, the resistances in Figure 3e are normalized to an initial height of 1 cm. Not surprisingly, the total thermal resistance of both types of GSs decreases as the samples become more compressed, as apparent in Figure 3e. The decrease is more pronounced in the initial stage and slowly saturates toward the end. These trends are expected as compression densifies the materials and shortens the thermal paths by making more contacts between the GO sheets. However, above approx. 50% compression strain (effective density of 8 mg/cm3), the thermal resistance of rGSs decreases much more drastically than that of oxidized GSs. This distinction

phonon mean free path and, therefore, lower intrinsic thermal conductivity of the GO sheets. As discussed above, the intersheet resistance also makes a significant contribution to the total thermal resistance. This boundary resistance typically decreases at higher temperatures mainly due to increased phonon irradiation at the interface barriers.45 This effect has been observed for CNT-based materials46 and thoroughly reviewed in ref 47. In addition, because of the highly porous nature of the scaffold, thermal radiation within the material could also be an important factor especially at elevated temperatures. Because each cellular wall only partially absorbs IR emission, radiative transport is not limited within individual pores, further enhancing its effectiveness. Rough quantification of the radiative transport is performed, with the details presented in SI, as shown in eq S3.48 The results suggest that radiative thermal conductivity is a significant factor at elevated temperatures, contributing more than half of the total thermal conductivity at temperatures above 250 °C (Figure S7). To put the measured properties of GSs in perspective, a stateof-the-art summary of polymer,3,4 carbon,10,13,14,30,33,34 ceramic,11,44,45,49 metal,50 and related porous solid materials is shown in Figure 3c. These materials can be divided into three distinguishing categories: (1) conventional foam structures, including metallic-, polymer-, and ceramic-based insulation materials (0.027 < κ < 10 W/(m K)) with a porosity ranging from 0 to 99%, and exhibiting brittleness (ceramic), low G


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are highly graphitized. Therefore, phonons face much lower interface barriers. 3.4. Flame Retardancy Measurements. The flame retardant results are summarized in Figure 5. In prior work, we have shown that a 3D porous graphene framework can be compressed into a thin “pancake,” and it can then recover its original macroscopic shape without noticeable fracture or collapse of the microstructure (Figure 5a).18,51 Here, we found that after flame treatment for 30 min, the oxidized GSs retain the same excellent elasticity and mechanical robustness with a reversible compressive strain of up to 95% (see Figure 5b), similar to those of the original oxidized GSs (see Video S1 and Figure 5a). The maximum strength and stiffness show only slight degeneration (2% decay of maximum stress). The structural robustness is also verified by the Raman spectra shown in Figure 5c. After flame treatment, D and G peaks are present at the original locations (1345 and 1575 cm−1, respectively) with a drop of the D-band intensity and slight strengthening of the 2D-band at 2700 cm−1, indicating that the GO sheets are partially thermally reduced. A fraction of the sample surface was burned to ashes; however, the internal structure is quite well retained. Oxidized GSs exhibit good flame retardancy (95.3% weight retention, Figure S8) primarily for two reasons. Firstly, the oxygenic functional groups on GO sheets significantly increase the activation energy of the carbon atoms, which retards combustion.3 Secondly, the low thermal conductivity effectively impedes heat transfer from the surface to the interior of the material. The thermal insulation performances of both rGSs and oxidized GSs in air are further verified using IR imaging. As shown in Figure 5d, heat is effectively blocked from flowing into the top stainless bar, indicating outstanding thermal insulation properties. The lateral views of the oxidized GSs show a large temperature gradient (dT/dx ≈ 60 °C/mm, qw = κ dT/dx ≈ 1.6 × 10−3 W/mm2) along the cylindrical sample in the axial direction with the sample heated by a hot plate at 500 °C (Figure 5d). Only a slight portion of the thermal energy flows through the sample, implying excellent thermal insulation characteristics of oxidized GSs in air as well. Owing to radiation and convection effects, however, this result cannot be directly used to quantify the thermal conductivity of the oxidized GSs in air. Notably, after the hot plate experiments in air, no apparent degradation can be seen. However, it is expected that oxidized GSs are partially reduced, whereas rGSs are slightly oxidized as supported by the TGA results in air with relatively high weight drops (shown in Figure 2e). For practical high-temperature applications, thermal reduction of oxidized GSs and oxidation of rGSs need to be carefully considered especially in harsh environments.

is illustrated more clearly in Figure 3d. Up to approx. 50% compression strain, the thermal conductivity of the oxidized GSs stays relatively unchanged, whereas that of rGSs increases only slightly. The decrease in the total thermal resistance is largely compensated by the concurrent reduction of the sample height, resulting in a fairly constant thermal conductivity. As the compression strain exceeds 50% (effective density above 8 mg/ cm3), the two materials drastically diverge in behavior. The thermal conductivity of the rGSs abruptly increases from 0.0.023 ± 0.003 to 0.050 ± 0.005 W/(m K) and continues increasing as the sample is further compressed. In contrast, the thermal conductivity of the oxidized GSs remains on a relatively flat curve and terminates at a value that is even lower than that in the initial uncompressed state. 3.3. In Situ Observation of Tunable Thermal Conduction. To elucidate the intriguing differences between both the GS samples, we performed in situ SEM imaging to study the microstructural evolution during compressive deformation. In Figure 4a−f, the vertical compression was applied on rGSs in the 1−1 direction, which is the same as the orientation of the thermal conductivity test on cylindrical samples; the heat is mainly transferred through the vertically oriented pathways of the multilayer graphene cellular walls. Two tag lines (AB and CD) are added in the micrographs to track the evolution of these thermal pathways (see Figure 4a). At the first stage (strain less than 20%), although some vertically arrayed cellular walls are compressed to lose elastic stability and deform out of plane, the pathways for heat conduction (marked as lines AB and CD) are individually bended without mutual interruption and remain their original geometric length of 600 μm (see Figure 4b,c). The motion of the phonons remains on the original pathways, leading to little fluctuation of thermal conductivity (0.018 to 0.019 W/ (m K)). Subsequently, a small portion of the bended cellular walls are folded to come in contact with the neighboring walls after the strain increases to 35%, and, therefore, the related heattransfer pathways are effectively shortened (AB decreases to 400 μm and CD remains 600 μm) (Figure 4d), which is synchronously accompanied by a steady decrease of the thermal resistance. Lastly, after the strain increases up to 50−60%, nearly all the bended cellular walls substantially appear to have a domino-effect-like compaction with multiple additional intersheet contacts generated (Figure 4e,f). Most of the heat-transfer pathways become shorter (AB decreases to 380 μm and CD to 410 μm) because more optimal routines are created. κ rapidly increases to around 0.050 W/(m K) as a result of a “thermal avalanche” triggered by the severe domino-effect-like folding of the (reduced) GO sheets and creation of a large number of shortened thermal paths. Although a similar shortening of thermal pathways is also observed in the microstructures of oxidized GSs during axial compression, a thermal avalanche phenomenon does not occur, as apparent in Figure 3c. This phenomenon is explained by the differing contact quality in rGSs and oxidized GSs. To take advantage of the shortened thermal paths under compression, phonons must cross multiple loosely touching interfaces between the graphene (oxide) cellular walls. As schematically illustrated in Figure 4g,h, the oxygenic functional groups intercalate the oxidized graphene sheets to hinder close interaction, with an interlayer spacing larger than 3.8 Å (see Figures 2a and 4g).18 In rGSs, however, the surfaces of GO sheets are substantially cleaned by the thermal annealing process and have become nearly free of the oxygenic functional groups (Figure 4h). Intersheet gaps are narrower, and graphene sheets

4. CONCLUSIONS In this work, a 3D GS was synthesized by a modified hydrothermal method and a unidirectional freeze-casting process with its thermal conductivity measured using a modified 1Dreference bar method. Because of a combination of high porosity, high interfacial resistance, and low intrinsic thermal conductivity of the multilayer graphene cellular walls, the effective thermal conductivity of GSs is several orders of magnitude lower than that of pristine graphene sheets, reaching a thermal conductivity as low as 0.0126 W/(m K) at 100 °C in vacuum for oxidized GSs with a density of 2 mg/cm3. The as-formed GSs demonstrate superior thermal insulation and flame retardant properties than most traditional polymer-based insulating foams with the H


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under its Scalable Nanomanufacturing program (Grant 1344654).

additional advantages of lightweight density and remarkable mechanical robustness. Under compression, an intriguing divergence between oxidized GSs and rGS is observed in terms of their thermal transport behaviors. Oxidized GSs retain their excellent thermal insulation property at all strains tested (up to approximately 90%). rGSs, however, experience a rather drastic drop in thermal resistance beyond a certain strain threshold. In situ SEM observations of microstructural evolution under compression provided vital information to understand this phenomenon. We hypothesize that oxidized GSs maintain low thermal conductivity in a compressed state because of the surface functional groups on the GO sheets, which hinder effective phonon transport across the intersheet barriers, whereas the rGSs experience the thermal avalanche due to the high contact quality of the additional generated interfaces. The good thermal insulation of the oxidized GSs even under a compacted state is a great advantage in certain applications, such as clothing and thermally insulating protective materials. On the basis of the results obtained in this study, we conclude that GSs are a very promising class of materials for thermal insulation at near or above room temperatures and flame retardant applications.

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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.7b01697. System schematic setups of the thermal conductivity test; two-step schematic illustration processes for experimental data evaluation of thermal conductivity; thermal conductivity comparison of GSs with and without consideration of the radiation effect; the comparison of thermal conductivity between vertical and horizontal directions, interface thermal resistance evaluation with different volume factions of graphene; the effects of partial thermal reduction on thermal conductivity; investigation of radiation-induced thermal conductivity variations; estimation of the radiative thermal conductivity; weight loss of oxidized GSs during flame treatment (PDF) Video showing flame treatment for oxidized GSs (AVI)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.Z.). *E-mail: tsfi[email protected] (T.S.F.). ORCID

Qiangqiang Zhang: 0000-0002-6082-6782 Author Contributions #

Q.Z. and M.H. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge support from China National Key Technology R&D Program (Grant 2011BAK02B02), the U.S. Air Force Office of Scientific Research under the MURI program on Nanofabrication of Tunable 3D Nanotube Architectures (PM: Dr. Joycelyn Harrison, Grant FA9550-12-1-0037), National Science Foundation/U.S. Department of Energy Partnership on Thermoelectric Devices for Vehicle Applications, and the US National Science Foundation I


Research Article


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