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Facile Preparation of Superelastic and Ultralow Dielectric Boron Nitride Nanosheet Aerogels via Freeze-Casting Process Xiaoliang Zeng,†,‡ Lei Ye,§ Shuhui Yu,† Rong Sun,*,† Jianbin Xu,*,§ and Ching-Ping Wong§ †

Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences. Shenzhen 518055, China Shenzhen College of Advanced Technology, University of Chinese Academy of Sciences, Shenzhen 518055, China § Department of Electronics Engineering, The Chinese University of Hong Kong, Hong Kong, China ‡

Downloaded by GEORGETOWN UNIV on August 22, 2015 | http://pubs.acs.org Publication Date (Web): August 21, 2015 | doi: 10.1021/acs.chemmater.5b00505

S Supporting Information *

ABSTRACT: As a structural analogue of graphene, boron nitride nanosheets (BNNSs) have attracted ever-growing research interest in the past few years, due to their remarkably mechanical, electrical, and thermal properties. The preparation of BNNS aerogels is considered to be one of the most effective approaches for their practical applications. However, it has remained a great challenge to fabricate BNNS aerogels with superelasticity by a facile method. Here, we report the preparation of BNNS aerogels via a facile method involving polymerassisted cross-linking and freeze-casting strategies. The resulting aerogels exhibit a well-ordered and anisotropic microstructure, leading to anisotropic superelasticity, high compressive strength, and excellent energy absorption ability. The unique microstructure also endows the aerogels with ultralow dielectric constant (1.24) and loss (∼0.003). The successful fabrication of such fascinating materials paves the way for application of BNNSs in energy-absorbing services, catalyst carrier, and environmental remediation, etc.

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INTRODUCTION Boron nitride nanosheets (BNNSs), so-called “white graphene”, which consist of a few layers of hexagonal boron nitride (h-BN) planes,1 have many unique properties compared with those of graphene, including a wide energy band gap,1 superb oxidation resistance,2 electrical-insulating property,3 and high thermal conductivity.4 The combination of the properties predestines BNNSs to be promising materials in some special applications, such as electronic devices,5−7 composite materials,8−10 environmental remediation,11−13 and thermal management.14−16 However, similar to graphene, a major challenge encountered in practical applications is that BNNSs easily form irreversible agglomeration due to π-stacking among BNNSs, which prevents the full realization of their outstanding properties, and thus greatly limits their applications. The integration of individual BNNSs into aerogels with three-dimensional (3D) porous structure is considered to be one of the most effective strategies to resolve the agglomeration issue. Furthermore, this strategy may result in hierarchical systems possessing synergistic effects from individual components, and therefore open up new applications, just as graphene aerogels.17−20 Up to now, BNNSs aerogels have been fabricated by various techniques, including chemical vapor deposition (CVD),21,22 supercritical drying,23 and template-free synthesis.12,24 Especially, Guo and colleagues pioneered an ultralight 3D BN aerogel with ultralow permittivity and superelasticity by CVD on a nickel foam template. 21 Interestingly, the BN aerogel can completely recover to its original shape after being strained up to 70% with dielectric constant within 1.12 times that of air. In spite of the great © XXXX American Chemical Society

promise this BN aerogel provides, the fabrication process involved the use of templates, high temperature, explosive and hazardous precursors, and sophisticated fabrication techniques. This shortcoming may limit its potential use in different applications. Hence, it still remains a technical challenge to fabricate BNNS aerogels with superelasticity and high compressive strength via a facile process. Herein, we report on a facile freeze-casting process to fabricate BNNSs aerogels. Compared with other techniques, freeze-casting is a relatively facial procedure for the controlled freezing of inorganic suspensions in water, and has been widely used to fabricate aerogels with controlled microstructure.25 This approach involves freeze-casting suspensions containing noncovalent functionalized BNNSs (NF-BNNSs) and polymer that acts as the cross-linking agent to link NF-BNNSs via covalent bonding. The synthesized BNNS aerogels show superelasticity, high compressive strength, and excellent energy absorption ability. Moreover, the unique hierarchical structure of the aerogels also results in ultralow dielectric constant (1.24) and loss (∼0.003). It is expected that the BNNSs aerogels will find possible applications in energy-absorbing services, catalyst carrier, and environmental remediation, etc.

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EXPERIMENTAL SECTION

Preparation of BNNSs Aerogels. The NF-BNNSs were first prepared using our previous method.26 The detailed process was Received: February 9, 2015 Revised: August 18, 2015

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DOI: 10.1021/acs.chemmater.5b00505 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials provided in the Supporting Information. In a typical procedure for the fabrication of a BNNS aerogel with density of 21 mg cm−3, the NFBNNSs solution (5.0 mL, 10.0 mg mL−1) was mixed with 1,4butanediol diglycidyl ether (BDGE, 25 mg) in a 10 mL cylindrical glass vial, which was then placed in ambient temperature for 24 h to obtain a BNNSs hydrogel. The hydrogel was then immersed in an ice bath to freeze for 24 h. After freeze-drying for 48 h, the BNNS aerogel was obtained. BNNS aerogels with a density ranging from 10.4 to 30 mg cm−3 were prepared under the same conditions except using different concentrations of NF-BNNSs and the ratio of NF-BNNSs to BDGE by weight remained consistent at 2:1. The detailed information was included in Table S1 in the Supporting Information. Characterization. The NF-BNNSs were characterized using atomic force microscopy (AFM, Bruker, Dimension ICON-PT), thermogravimetric analysis (TGA, TA Instrument, Q600), and Fourier transform infrared spectroscopy (FTIR, BRUKER, VERTEX 70), respectively. The BNNS aerogels were analyzed by FTIR, SEM, TGA, scanning electron microscope (SEM, FEI, Nova NanoSEM450) with an accelerating voltage at 5−10 kV, transmission electron microscope (TEM, FEI, Tecnai G2 F20 S-Twin), and X-ray diffraction (XRD, Bruker, D8 Advance), respectively. Apparent densities were calculated by weighing the aerogels and measuring their volumes. The mechanical properties of the BNNSs aerogels were measured by dynamic mechanical analyzer (DMA, TA Instruments, Q800) at a rate of 50% strain min−1. The dielectric constant and loss of the BNNSs aerogels with the density of 21 mg cm−3 were measured by Agilent 4294A impedance analyzer at various frequencies. A Keysight 16451B dielectric test fixture was used to analyze the change of dielectric constant with the strain.

Figure 1. Fabrication, formation mechanism, and characterization of BNNSs aerogel. (a) Illustration of the fabrication process of the BNNSs aerogels. (b) Scheme of the formation mechanism of BNNSs aerogels via ring opening of epoxy groups between BDGE and SC functionalized BNNSs. (c) FTIR spectra of the intermediates and BNNSs aerogel with the density of 21 mg cm−3. (d) TGA curve of BNNSs aerogel with the density of 21 mg cm−3. (e) XRD pattern for raw h-BN and BNNSs aerogel with the density of 21 mg cm−3.

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RESULTS AND DISCUSSION To prepare BNNS aerogels, BNNSs were obtained using a method in combination of low-energy ball milling and sonication technique. Sodium cholate (SC), which has demonstrated strong affinity with the basal plane of BNNSs via van der Waals interactions,27 was used to functionalize the BNNS surface by attaching hydroxyl groups onto the BNNSs. The morphology of the as-prepared NF-BNNSs was confirmed by AFM imaging (Figure S1, Supporting Information). The lateral width of the BNNSs is about 10 μm, which is at least 10 times larger than the exfoliated BN flakes as reported previously.28,29 The average topographic height is around 4 nm, corresponding to about 12 layers of NF-BNNSs (0.33 nm for single-layer BNNS). The SC content grafted onto the NFBNNSs is estimated to be 5 wt % by thermogravimetric analysis (TGA, Figure S2, Supporting Information). Figure 1a shows the proposed fabricating process of BNNSs aerogels involving polymer-assisted cross-linking and freeze-casting strategies. First, BDGE, used as cross-linking agent, was introduced into the NF-BNNSs solution to initiate the NF-BNNSs assembly into a 3D network (Figure 1a, step 1). After several hours, the NF-BNNSs solution became sticky and transformed into hydrogels due to the epoxy ring opening reaction between BDGE and hydroxyl on the surface of the NF-BNNSs (Figure 1b). It is interesting to note that the generation of BNNSs hydrogels can be only achieved by the mixture of NF-BNNSs and BDGE (Figure S3, Supporting Information). The BNNSs aerogels were then fabricated using a controlled freeze-casting technique (Figure 1a, step 2), which has been successfully adopted to prepare cellular foams30,31 and bioceramics.32,33 To enhance the interfacial strength and mechanical properties, the aerogels were then thermally cured at 110 °C for 4 h (Figure 1a, step 3). FTIR was used to verify the intermediates and final products. Compared with raw h-BN, the FTIR spectrum of NFBNNSs exhibits the additional −OH, C−H, and weakened C− O bands in 3450, 2962−2855, and 1000−1200 cm−1 spectral

regions, respectively, as shown in Figure 1c. The result indicates that the noncovalent functionalization of BNNSs occurs via van der Waals bonding, which is consistent with Coleman et al.’s work.34 In the spectrum of BNNS aerogel with the density of 21 mg cm−3, the absorption peaks at 2941 and 2861 cm−1 represent the stretching vibrations of the CH2 groups, and the characteristic peaks of h-BN at 1380 and 780 cm−1 still can be observed. Moreover, the peak of 3400 cm−1 ascribed to −OH groups in SC and the peak of 910 cm−1 attributed to epoxy groups in BDGE both disappear, indicating the reaction between SC and BDGE as expected. Figure 1d presents a typical TGA curve of the BNNSs aerogel with the density of 21 mg cm−3, which is used to characterize component and thermal stability of materials. A slight mass loss (∼2.0 wt %) below 230 °C appears likely due to the evaporation of adsorbed water. This results from the existence of hydrophilic SC in the BNNSs aerogel. Furthermore, there is a distinct mass loss (∼18.0 wt %) between 230 and 500 °C, which can be assigned to the decomposition of cured BDGE. This indicates that the amount of cured BDGE is about 18 wt % in the BNNS aerogel. In addition, the aerogel retains thermal stability below 230 °C, which meets the requirement for most applications, although it is still lower than that of BNNS aerogel prepared by a CVD method.21 XRD result suggests that the interlayer distance is larger than that of raw h-BN, implying that the BDGE can effectively prevent NF-BNNSs from agglomeration, as shown in Figure 1e. Because of the simplicity of the freeze-casting process, the BNNSs aerogels can be formed into arbitrary shapes with B


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Chemistry of Materials

Figure 2. Macroscopic and microscopic structures of the BNNSs aerogels. (a) Digital photograph of BNNSs aerogels with various shapes. (b) A BNNSs aerogel with the density of 21 mg cm3 standing on a flower-like dog’s tail. (c) Out-of-plane and (d) in-plane SEM micrographs of BNNSs aerogel. (e) A typical SEM image of honeycomb-like cell of the BNNSs aerogel. (f and g) TEM images of BNNSs at different magnifications, and the inset in part g is the corresponding SAED image.

Figure 3. Mechanical properties of BNNS aerogel with density 21.0 mg cm‑3. (a) A set of real-time images of a compressed sample showing the recovering process. (b) The out-of-plane compressive σ−ε curves of BNNS aerogel at different maximum strains of 20%, 40%, 60%, and 75%, respectively. (c) The out-of-plane compressive σ−ε curves of BNNSs aerogel at the maximum strain of 70% for 10 cycles. (d) The corresponding Young’s modulus, maximum stress, and energy loss coefficient for different cycles. (e) Fatigue resistance of BNNS aerogels at 70% strain, 1 Hz, for the 1st and 100th cycles. (f) The in-plane compressive σ−ε curves of BNNS aerogel at the maximum strain of 70% for 10 cycles.

further enlarging the instrumental size. Furthermore, owing to its low volume density (21.0 mg cm−3), the BNNSs aerogel can freely stand on the dog tail flower (Figure 2b). Figure 2c,d presents in-plane and out-of-plane views of SEM micrographs

desired densities, such as rods and cylinders, as shown in Figure 2a. In addition, BNNS aerogel with the largest diameter of 5.0 cm was prepared by our group. It is very likely that one could realize larger scale fabrication for industrial production by C


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Chemistry of Materials of a BNNS aerogel with density of 21.0 mg cm−3, respectively. Along the out-of-plane direction (perpendicular to the freezing direction), the aerogel exhibits a honeycomb-like structure with the cell size of 100 μm (Figure 2e). However, there does not exist honeycomb-like structure in the in-plane direction (parallel to the freezing direction). Most of the BNNSs are aligned in the direction of ice growth. In addition, all aerogels possess a high porosity, as high as ∼99% (Table S1, Supporting Information). TEM analysis reveals that the cell wall is constructed by few-layer BNNSs (∼10 layers), as shown in Figure 2g, which is consistent with AFM results (Figure S1, Supporting Information). The selected area electron diffraction (SAED) result (insert of Figure 2g) shows high crystallinity, indicating that the hexagonal structure of BNNSs is not destroyed through the fabrication process. The anisotropic microstructure of the BNNS aerogels is attributed to the anisotropic direction of ice growth. During the freeze-casting process, the BNNSs repel each other because of the insertion of ice crystals. Since the growth rate of ice crystals in different direction is highly anisotropic, the BNNSs are forced to align along the moving solidification front. Meanwhile, they become concentrated and then squeezed at the crystal boundaries, yielding a highly ordered layered assembly, as shown in Figure 2d. When the BNNS concentration reaches a critical value (6.0 mg mL−1 in our work), a formation instability perpendicular to the freezing direction occurs, leading to the honeycomb-like morphology in the BNNSs aerogels, as shown in Figure 2c. To assess the mechanical properties of the BNNS aerogels, we measured their compressive stress (σ) as a function of strain (ε). Interestingly, the as-formed aerogel with the density of 21.0 mg cm−3 shows a nearly complete recovery from strains exceeding 75%, as shown in Figure 3a and Movie S1 (Supporting Information). Figure 3b presents the corresponding out-of-plane (the stress was loaded perpendicularly to the honeycomb cell direction) σ−ε curves of BNNSs aerogel during the first compression cycle at preset ε maxima of 20%, 40%, 60%, and 75%, respectively. Three regimes of the deformation are observed during the loading process, including an initial Hookean region at ε < 15% with an Young’s modulus of 55.9 kPa, a plateau at 16% < ε < 55% with a reduced modulus of approximately 27 kPa, and a final densification, corresponding to rapid rise of stress as ε approaches 75%. The three characteristic regions are similar to that of representative closed-cell foams,35 in which the Hookean region, the flat stress plateau, and rapid stress increase region correspond to bending, elastic buckling of the cell wall, and densification of the cell, respectively. Furthermore, the Hookean region is much broader than that of a typical closed-cell cellular solid (≤5%).35 The elastic modulus normalized by the density is found to be ∼2.7 MPa cm3 g−1, which is much larger than the values for the aerogels based on polymers, graphene, BN, and so on. Moreover, the compressive strength at the plateau region (20−40 kPa) and the maximum compressive stress (60−70 kPa) at a strain of 70% are 2−3 orders of magnitude higher than that of the BNNS aerogel prepared by the CVD method.21 For the first cycle, we estimate the work done (W) to be 15.3 MJ cm−3 and the energy dissipation to be 9.6 MJ cm−3 in the compression process, yielding an energy loss coefficient of 0.63 (η, “characterization of energy loss coefficient”, Figure S4, Supporting Information). The energy dissipation likely results from the friction among BNNSs or movement of air through the cellular structure. Hysteresis curves of 10 loading− unloading cycles at very large ε (70%) at 1 Hz show no

significant plastic deformation or degradation in compressive strength (Figure 3c). To demonstrate the properties of BNNS aerogel in compression cycles, the maximum stress, W, and η were calculated, shown in Figure 3d as a function of cycle. It is apparent that it yields a high maximum stress of 70.0 kPa, W of 15.3 kJ m3, and η of 65% in the first cycle, and then both stabilized at 60 kPa, 12 kJ m3, and 60%, respectively, in the subsequent cycles. The maximum degradation in compressive strength of the BNNS aerogel is ∼20% after 100 cycles (Figure 3e). The sag factor, a relative ratio of stresses at two deformations of 65% and 25%, is a key criterion to evaluate the cushioning performance of foams. For BNNSs aerogel (at cycle 10, σ = 37.3 kPa at ε = 65% and σ = 8.97 kPa at ε = 25%), the sag factor desirably reaches 4.1, which is in agreement with that of foam-like carbon nanotube films. 36 The high compressive stress strength and sag factor make our BNNSs aerogels suitable for the applications requiring strong cushioning effects. According to the literature,21 the existing BNNS aerogels prepared by the CVD method are prone to permanent residual deformation that cannot recover when the compression exceeds 70% strain. Besides, previous studies suggest that the porous BN made of a monolayer or a few layers of BN exhibits brittle yield when subjected to a large deformation.11 However, our BNNS aerogel exhibits enhanced elasticity and no permanent residual deformation at strain exceeding 70%. This may be attributed to the combined effect of the unique well-ordered hierarchical structure, high mechanical module of BNNSs, and the fact that BDGE makes the cell walls stiffer by cross-linking (Figure 1c). In a microscopic view, the cells in the BNNS aerogel are organized in a honeycomb-like manner to maximize the elastic modulus and strength. At the lower level, the cell wall is composed of multilayers of face-to-face oriented BNNSs cross-linked by the BDGE, and the cell walls interlock with each other, resulting in the 3D network which exhibits excellent mechanic resilience. In addition, the anisotropic microstructure shown in Figure 2 implies anisotropic mechanic properties. Therefore, the in-plane compression (the stress loaded parallel to the honeycomb cell direction) was also carried out. As expected, in spite of the σ−ε curves that are similar to that of out-of-plane deformation, the maximum stress of the in-plane compression is much lower than that of the out-of-plane deformation, as shown in Figure 3f. The reason for the anisotropy of mechanic properties is that honeycomb cells are stronger and stiffer along the cell axis (out-of-plane direction) than those parallel to the cell axis (in-plane direction), which agrees well with the phenomenon observed in other aerogels and foams.31,37 To estimate the upper limit of mechanical properties and quantitatively capture the stress distribution in BNNSs aerogel, a mechanical model was developed using the commercial finite element software, ANSYS. The model is idealized to a level that the microstructure is represented by a regular cubic cell (Figure S5, Supporting Information). The details of the finite element analyses (FEA) are described in the Supporting Information (Figure S6). Similar to experimental results, the unit cell can be compressed to at least 80% strain and be able to completely recover to its original shape rapidly when the loading is removed (Movies S2 and S3, corresponding to stress and strain change when loading, Supporting Information). A comparison between finite element simulation and experimental compressive σ−ε curve is shown in Figure 4a. The simulated ε−σ curve is also similar to the experimental one, but the simulated stress D


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Figure 4. Finite-element analysis of the BNNSs aerogel with density 21 mg cm‑3. (a) Experimental and FEA simulation σ−ε curves. (b) von Mises stress distribution at the maximum strain (80%).

at the maximum strain of 80% is 3.6 MPa, which is about 50 times higher than the experimental value. This is probably due to the insufficient interfacial bonding between BNNSs and polymer, as well as imperfect structure in the experimental BNNSs aerogels. The von Mises stress and strain distribution within a unit cell is shown in Figure 4b. It is evident that the stress is mainly concentrated in the cell walls, which indicates that the cell walls are the main source to superelasticity of the BNNSs aerogels. The simulated results show that although the BNNSs aerogel exhibit high elasticity, the full potential of the mechanical properties of BNNSs has not yet to be exploited. Further improvements in the interfacial bonding and the controlled structure are expected to offer much higher mechanical performance. The effect of the density of BNNSs aerogels on the mechanical properties was also investigated, shown in Figure 5a. Although the ε−σ curves of BNNSs aerogels look similar in their shape, according to the various densities from 10.4 to 30 mg cm−3, the maximum stress and Young’s modulus in Hookean region show significant diversities, which increase from 14 and 6.3 kPa to 125 and 127 kPa, respectively (Figure S7a, Supporting Information). By plotting the compressive modulus (Y) versus density (ρ) for various aerogels in the Hookean region, the relationship between Y and ρ can be considered as Y ∼ ρ2.2 (Figure S7b, Supporting Information). This scaling law indicates that the BNNS aerogels have bending-dominated mechanical behavior, possessing highly efficient load transfer between cell walls.30,38 In contrast, most of the reported porous carbon and polymer aerogels follow Y ∼ ρ3, because of inefficient load transfer among their cell walls.39,40 The W at maximum strain (∼70−75%) increases from 3 to 25 kJ m−3, and the η slightly increases from 0.6 to 0.68, when the density increases from 10 to 30 mg cm−3 (Figure S7c, Supporting Information). The density-dependent mechanical properties are mainly related to the thickness of the cell walls. An increase in the density results in the increased thickness of the cell walls and the resistance to cell wall bending and cell collapse will rise, leading to a higher W, and η.

Figure 5. Mechanical properties of BNNSs aerogels with different densities. (a) The ε−σ curves of BNNSs aerogels at maximum strain of 70%. (b) Comparison of energy absorption ability of our BNNSs aerogels with the reported ones.

Since W and η are the two key factors of cellular aerogels, we compare W and η of our BNNS aerogel to those of the previously reported polymer, ceramic, graphene aerogels, and lattice (Figure 5b). The detailed data are listed in Table S2 in the Supporting Information. It should be noted that the W and η were obtained from the loading−unloading curves with at least three cycles after they stopped changing. Obviously, our BNNSs aerogels exhibit a better balance of W and η. The η values of the BNNSs aerogels are comparable with those of other graphene based or BN aerogels (Table S2, Supporting Information), but the W values are much higher than the reported ones. For example, our sample with a density of 21 mg cm−3 gives a W of 15.3 kJ m−3, which is much higher than that of the polymer foam (W of 0.8 kJ m−3) and a group of graphene aerogels (W of 0.8−6.4 kJ m−3). More importantly, the W of our BNNSs aerogel is 2 orders higher than that of the BNNSs aerogel (W of 0.015 kJ m−3) prepared by CVD. To the best of our knowledge, there exist only a few aerogels that could offer such high W and η simultaneously, namely, Al nanolattices and modified CNT aerogel. In addition, the unique structure of our BNNS aerogels combines the advantages of BNNSs and nature of the aerogel, resulting in not only excellent mechanical properties, but also an ultralow dielectric property. For example, the BNNSs aerogel with the density of 21.0 mg cm−3 exhibits ultradielectric constant (εr ∼ 1.24) and loss (tan δ ∼ 0.003) at the frequency range from 1.0 kHz to 1.0 MHz, as shown in Figure 6a. Moreover, the dielectric constant presents its stability even under external frequency disturbances. Figure 6b displays the dielectric constant (at 1.0 MHz) response to compression. The E


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thickness of the cell walls. Also, we have demonstrated that although the BNNSs aerogel exhibits the excellent superelastic property, the full potential of the mechanical properties of BNNSs has not yet to be exploited. In addition, the unique microstructure of the BNNSs aerogel renders the ultralow dielectric constant (1.24) and loss (∼0.003). The dielectric constant also exhibits positive compressive strain response, when the compressive strain is altered from 0% to 70%. The high performance of the BNNSs aerogel paves the way for its applications in the field of energy-absorbing services, catalyst carrier, and environmental remediation. Furthermore, this strategy may be very useful in assembling other twodimensional nanosheets into macroscopic ones.

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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/acs.chemmater.5b00505. Preparation of NF-BNNSs; characterization of energy loss coefficient; details of finite element modeling of BNNSs aerogel; AFM image, height profile and TGA result of NF-BNNSs; optical images of BDGE, NFBNNSs, and the mixture solution of BDGE and NFBNNSs; mechanical and microstructural properties of BNNSs aerogels with different densities; comparison of energy absorption and energy loss coefficient of our BNNSs aerogel with the reported aerogels (PDF) Aerogel strain recovery (MPG) Stress change when loading (AVI) Strain change when loading (AVI)

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Figure 6. Dielectric properties of the BNNSs aerogel with density 21 mg cm‑3. (a) Frequency dependence of dielectric constant and loss of the BNNSs aerogel. (b) The relationship between dielectric constant and compressive strain. Inset: Dielectric constant change when repeatedly compressed up to 70% strain for 10 cycles.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

dielectric constant increases slightly from 1.24 to 1.32 with the strain increased to ∼40% and increases remarkably when the strain is >40%. Because of the substantial deformation, the density of contact sites among the BNNSs as skeletons in the aerogel increases rapidly, thus leading to a remarkable increase in dielectric constant. However, even for the BNNSs aerogel at 70% strain, its dielectric constant is only 1.70. Furthermore, the response of dielectric constant of the BNNSs aerogel is highly repeatable under multiple cycles of compression, further demonstrating its remarkable structural resilience.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. X.Z. and L.Y. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Science Foundation of China (No.51377157), Guangdong and Shenzhen Innovative Research Team Program (No. 2011D052 and KYPT20121228160843692), Shenzhen Electronic Packaging Materials Engineering Laboratory (No.2012-372), Research Grants Council of Hong Kong, particularly, via Grant Nos. AoE/P-03/08, and CUHK Group Research Scheme. The authors thank Xiaoxing Zeng from Shenzhen University for completing the FEA work.

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CONCLUSION In summary, by employing the facile freeze-casting technique, we have successfully fabricated high-performance BNNSs aerogels with controlled microstructure, shape, and density. The obtained BNNSs aerogels show well-ordered, anisotropic, and porous microstructure, leading to superelasticity and high compressive strength and excellent energy absorption ability, superior to other reported aerogels, such as graphene aerogels, polymer and carbon nanotube aerogels, and BNNS aerogel prepared by other methods. The anisotropic microstructure also results in anisotropic mechanical properties in which the maximum stress of the in-plane compression is much lower than that of the out-of-plane deformation. The mechanical properties are enhanced with the density of BNNSs aerogels from 10 to 30 mg cm−3, which is attributed to increased

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ABBREVIATIONS BNNSs, boron nitride nanosheets; CVD, chemical vapor deposition; NF-BNNSs, noncovalent functionalized BNNSs; SC, sodium cholate; σ, stress; ε, strain; BDGE, 1,4-butanediol diglycidyl ether; W, work done; η, energy loss coefficient; FEA, finite element analyses; Y, compressive modulus; ρ, density; εr, dielectric constant F


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