Self-Assembly of Porous Boron Nitride Microfibers into Ultralight

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Self-assembly of Porous Boron Nitride Microfibers into Ultralight Multifunctional Foams of Large Sizes Jing Lin, Xiaohai Yuan, Gen Li, Yang Huang, Weijia Wang, Xin He, Chao Yu, Yi Fang, Zhenya Liu, and Chengchun Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16198 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 8, 2017

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ACS Applied Materials & Interfaces

Self-assembly of Porous Boron Nitride Microfibers into Ultralight Multifunctional Foams of Large Sizes Jing Lin†,‡, Xiaohai Yuan†,‡, Gen Li†,‡, Yang Huang†,‡,*, Weijia Wang†,‡, Xin He†,‡, Chao Yu†,‡, Yi Fang†,‡, Zhenya Liu†,‡, Chengchun Tang†,‡ †

School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130,

P. R. China ‡

Hebei Key Laboratory of Boron Nitride Micro and Nano Materials, Hebei University of

Technology, Tianjin 300130, P. R. China * E-mail: [email protected] (Y. H.)

KEYWORDS: boron nitride, foam, ultrasonic-assisted self-assembly, large sizes, absorption, thermal insulation

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ABSTRACT

As a kind of macroscopic BN architectures, ultralight BN cellular materials with high porosity and great resilience would have broad range of applications in energy and environmental areas. However, creating such BN cellular materials in a large size has still been proven challenging. Here we report on the unique self-assembly of one-dimensional (1D) porous BN microfibers into an integral three-dimensional (3D) BN foam with open-cell cellular architectures. An ultrasonicassisted self-assembly, freeze-drying and high-temperature pyrolysis process has been developed for the preparation of cellular BN foam with large size and desired shape. The developed BN foam has low density, high porosity (~99.3%), great resilience and excellent hydrophobiclipophilic nature. The foam also exhibits excellent absorption capacities for a wide range of organic solvents and oils (wt.% of ~5130-7820%), as well as a high recovery efficiency (~94%). Moreover, the unique hierarchical porous structure enables the foam demonstrating a very low thermal conductivity (~0.035 W·K-1m-1). The excellent thermal insulation performance, superior mechanical property, and superb chemical and thermal stability enable the developed BN foam as an integrating multifunctional material in a broad range of high-end applications.


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INTRODUCTION Ultralight cellular material with three-dimensional (3D) porous network structure is a kind of macroscopic architectures with high porosity.1 As a representative of ultralight cellular materials, carbon-based aerogels and/or foams are attractive due to their high absorption capacities and excellent mechanical properties, thus may have great potential applications in energy and environmental areas.2-7 However, the inherent limits on the poor thermal stability and oxidation resistance of carbon-based cellular materials present challenges in their practical applications, especially when used in harsh environment. Recently, hexagonal boron nitride (hBN) cellular materials have attracted increased attention due to hBN’s high thermal stability, chemical inertness, electrical insulation and superb mechanical properties.8-14 Especially, the thermal and chemical stabilities at high temperature make BN cellular material a reliable candidate for their practical applications. So far various methods have been developed for the synthesis of ultralight BN cellular materials.15-24 For example, A. Zettl et al. synthesized BN aerogel using graphene aerogel as a template via carbonthermal reduction of boron oxide and simultaneous nitridation.15 W. Guo et al. grew BN foam on a nickel foam template by CVD method using borazane as a precursor. The BN foam with 3D hollow-tube-like networks showed ultralow permittivity and superelasticity.16 However, the concerns risen by template approaches are their relatively complex or high demanding synthesis conditions, such as high reaction temperatures (~1800°C) and/or template removal procedures. Assembling of micro/nanomaterials into monolithic materials while maintaining the unique properties of individual building blocks has gained great attention as it represents an effective approach for building novel architectures with advanced properties.25 For example, two-


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dimensional (2D) BN nanosheets as an effective particulate stabilizer can be self-assembled to form inverse Pickering emulsions.26 Moreover, by using 2D BN nanosheets as building blocks, BN cellular materials can be also successfully constructed. Y. Chen et al. synthesized ultralight BN aerogels by cryodrying of colloidal few-layer BN nanosheets dispersions.17 X. Zeng et al. prepared BN nanosheets aerogels via a polymer-assisted cross-linking and freeze-casting process.18 Using these strategies, 2D BN nanosheets with excellent flexibility can serve as basic units, resulting in the formation of 3D cellular architectures with excellent mechanical properties. However, the remaining issues in the large-scale preparation of BN nanosheets units become the bottleneck for the development of 3D BN cellular architectures in large sizes. From the viewpoint of practical applications, it is highly desirable to develop a facile sustainable method for the synthesis of large-size BN cellular materials. Inspired

by

the

micro/nanostructures,27,

strong 28

and

flexible

properties

of

one-dimensional

(1D)

BN

1D BN micro/nanostructures with high aspect ratios hold great

promise as exceptional nanoscale building blocks for constructing macroscopic BN cellular architectures.29, 30 Compared with BN cellular architectures using 2D BN nanosheets as units, the 3D architectures constructed by 1D BN micro/nanostructures will produce higher internal connectivity, which is beneficial for the permeation of gas or liquid during absorption process, especially those fluid with high viscosity. In this paper, we present a facile method for creating ultralight 3D BN foams with a hierarchical cellular structure that consists of 1D porous BN microfibers. The premise for our design is using 1D porous BN microfibers with high specific areas and large pore volumes as building blocks to construct 3D resilient cellular foams with desirable shapes and large sizes. Through the combination of an ultrasonic-assisted selfassembly, freeze-drying and pyrolysis procedure, numerous 1D porous BN microfibers serve as


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skeletons to create the open-cell cellular architectures with hierarchical porosity. The developed 3D BN cellular foams exhibit integrated multifunctional properties of low density, great resilience and super-high absorption capacities for a wide range of organic solvents and oils. Moreover, the BN foam shows very low thermal conductivity due to their high porosity structure. Compared with conventional 3D porous foam-like materials, the present BN foam offers additional advantages such as thermal insulation, thermal stability and resistance to harsh environment, suggesting their promising applications in aerospace and building fields.

EXPERIMENTAL SECTION Synthesis of BN foam. In a typical synthesis, boric acid (H3BO3) and melamine (C3N6H6) with a molar ratio of 3:1 was dissolved in 0.5 L of distilled water. Then the reaction mixtures were heated at 90 °C for 4 h to get a mixed H2O-H3BO3-C3N6H6 solution. Then the hot aqueous solution was cooled to 70 °C under simultaneously ultrasonic treatment until a white bulk melaminc diborate (C3N6H6·2H3BO3, M·2B) agglomerate was obtained. Subsequently, the mixture was frozen and dried using a vacuum freeze dryer to get the precursor, i.e. a white bulk melaminc diborate foam. After that, the melaminc diborate foam was heated to 1100 °C for 4 h in a flow of NH3 (200 mL/min). Finally white BN foam was obtained. Material characterization. X-ray diffraction (XRD) patterns of the samples were collected using a BRUKER D8 FOCUS. The morphology of the samples was observed using HITACHI S4800 scanning electron miscroscope (SEM) under high vacuum at 5 kV. Transmission electron microscopy (TEM) experiments were performed on a Tecnai F20 electron microscope (Philips, Netherlands) with an acceleration voltage of 200 kV. Fourier transform infrared (FTIR) spectra were recorded on a VECTOR22. X-ray photoelectron spectroscopy (XPS) was measured by


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ESCALAB 250Xi. The nitrogen adsorption-desorption isotherms were measured at 77 K on a Autosorb iQ from Quantachrome Corporation. The macropores of the foam were characterized using mercury porosimetry (Micromeritics' AutoPore IV 9500 Series). Thermogravimetry (TG) and differential thermal analysis (DTA) were measured on a SDTQ-600 thermal analyzer from room temperature to 1200 °C at a heating rate of 10 °C/min under nitrogen. An advanced goniometer (DAS30, KRUSS) was used to measure the contact angle. The microcomputer control electron universal testing machines (CMT6104, SNAS) was employed for mechanical test at room temperature. Thermal conductivity analysis was conducted on thermal conductivity instrument (XIATECH TC3000E). Oil and organic solvent absorption capacity measurements. Various kinds of organic solvents and oils with different densities were used for measurement, including ethanol, 1,4-dioxane, N, N-dimethylformamide, styrene, N-heptane, triethylamine, ethylene glycol, chlorobenzene, tetraethylorthosilicate and gear oil. The BN foams were placed inside the organic solvents. After the absorption was complete, the foams were picked out for measurements. The foam weights after and before absorption were recorded for calculating the absorption capacity (wt.%, the ratio of the final weight after full absorption to the initial weight of foam). Weight measurements were performed quickly to avoid evaporation of absorbed organic liquids. After fully saturated absorption, the foams were rapidly burned in air for recovery.

RESULTS AND DISCUSSION The 3D BN foam can be directly synthesized by a template-free method through the combination of an ultrasonic-assisted self-assembly, freeze-drying and pyrolysis procedure (Figure 1a). Briefly, an aqueous boric acid-melamine solution was readily prepared by


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dissolving of boric acid and melamine in hot distilled water. Then the hot aqueous solution was cooled under high-frequency ultrasonication to obtain a white B, N-contained molecular crystal, i.e. melamine diborate (C3N6H6·2H3BO3).31, 32 After that, the as-prepared solid-liquid mixture was frozen and dried under a freeze-drying process to get a white highly porous foam architected by melamine diborate fibers as a precursor (Supporting Information, Figure S1). After hightemperature pyrolysis, the foam-like melamine diborate precursor was converted into 3D BN foam finally. The shape of the melamine diborate foam can be simply controlled by using desired container as a mold, while the BN foam after pyrolysis still keeps the original shape from the precursor, as shown in Figure 1b. Within the foam, 1D porous BN microfibers serve as skeletons which interweave with each other, forming an open-cell isotropic network, as illustrated in Figure 1c and 1d. The density of the BN foam is measured as ~15 mg/cm3, yielding a high porosity of ~99.3%. Due to the ultralow density, the BN foam can be easily placed on dandelion and petals of flowers (Figure 1e and 1f). It should be noted that BN foam synthesized in this method has good processibility, and can be manufactured in a large-scale (Supporting Information, Figure S2). We have successfully prepared a foam with large size of ~20 cm * 5 cm * 8 cm (Figure 1g and 1h). A typical low-magnification SEM image clearly reveals the microscopic structure of the BN foam (Figure 1i). It consists of numerous 1D microfibers with high purity. The microfibers have lengths of several hundreds of micrometers, displaying very high aspect ratios. Enlarged SEM images show that most of the fibers have discrete interfaces and are interwoven with each other to form 3D BN network (Figure 1j and 1k). The cross areas are also displayed in Figure S3 (Supporting Information). Interestingly, each microfiber exhibits a porous structure, as revealed by TEM image shown in Figure 1l.


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Figure 1. (a) Schematic illustration of the synthetic steps for 3D BN foam. (b) Photograph of the product. (c, d) Schematic showing that the 1D porous fibers assemble into 3D cellular foam. (e, f) A BN foam can be easily placed on dandelion and petals of flowers, respectively. (g, h) Photos of as-prepared BN foam with a large size. (i-k) SEM images showing the microscopic structures of BN foam at various magnifications (l) Typical TEM image of the fiber, displaying a porous nature.

The microscopic structure of the fibers was further characterized by detailed TEM analysis. Figure 2a shows a typical TEM image of an individual microfiber with uniform diameter of ~1


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µm. There are many straight long holes distributed inside the fiber, exhibiting a unique perforated appearance (Figure 2c). The corresponding SAED pattern (Figure 2b) shows characteristic (110) (100) and (002) diffraction rings of layered BN. Enlarged TEM image (Figure 2d) further indicates the high porosity of the fiber. The pores connect together to form long channels. High-resolution (HRTEM) image (Figure 2e) shows many disordered BN layers, indicating polycrystalline characteristic of the microfiber. Several parallel fringes can be clearly observed, corresponding to the stacked hBN layers (Figure 2f). XRD pattern shows a diffraction peak which can be indexed to the (0002) peak of hBN. The calculated interplanar distance of BN layer is slightly larger than that of bulk hBN, revealing a turbostratic BN phase of the microfibers. FTIR spectrum (Figure 2h) shows two absorption bands located at ~1388 and ~808 cm-1, which can be attributed to the B-N and B-N-B stretching vibration modes, respectively. In addition, the peaks at ~3420, ~3250, and ~2900 cm-1 are ascribed to B-OH, B-NH2, and –C=O stretching vibrations. The appearance of these vibrations can be related to impurity incorporation and defect existing in the BN fibers.

31, 33

Electron energy loss spectrum (EELS) confirms the

existence of B, N and a small amount of C and O elements in the microfiber (Supporting Information, Figure S4). The atomic ratio of B/N was calculated to be ~1:1. The chemical state and compositions were further verified by XPS analysis (Supporting Information, Figure S5). The results indicate the C and O-related impurities existing in the fiber, which are consistent with the FTIR and EELS results. Then Brunauer-Emmett-Teller (BET) analysis was performed to study the specific surface area and pore size distribution (PSD) of the foam. The measured N2 adsorption/desorption isotherms (Figure 2i) is an intermediate between typical I and typical IV isotherms according to the IUPAC nomenclature, and the existing hysteresis loop belongs to H4-type loop. The isotherms


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indicate that the BN foam has hierarchical porosity containing both micropores and mesopores. It is noteworthy that, there is a rapid increase of N2 adsorption at relative low pressure range, implying the presence of abundant micropores in the product. The BET specific surface area has been calculated to be ~773 m2/g. Besides, the PSD of the BN foam is shown in inset of Figure 2i. It is a bimodal distribution with the main characteristic pore sizes of ~1.1, ~2.3 and ~6.8 nm. In order to characterize the macropores of the foam, mercury porosimetry measurements were performed. The result shows the BN foam containing of abundant macropores with a broad PSD (Supporting Information, Figure S6). The high specifc surface area and high porosity imply a high absorption capacity of the foam for organic solvents and oils.

Figure 2. (a) Typical TEM image of an individual BN microfiber. (b) The corresponding SAED pattern. (c) Sketch of the microfiber showing straight long holes distributed inside the fiber. (d) Enlarged TEM image further exhibiting the pore structure of the fiber. (e, f) HRTEM images of the porous BN microfiber. (g) XRD pattern of the foam. (h) FTIR spectrum of the product. (i)


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Nitrogen adsorption-desorption isotherms of BN foam. (Insert) The corresponding pore size distribution.

The present BN foams were directly made by the conversion from a novel foam-like melamine diborate precursor. The ultrasonic-assisted process played a key factor for the self-assembly of melamine diborate microfibers into 3D architectures. Here a growth model has been proposed. During the ultrasonic-assisted cooling process, molecular crystal melamine diborate precipitated and recrystallized as 1D microfibers with high aspect ratios via the reaction: 2H3BO3 + C3N6H6 → C3N6H6·2H3BO3.31 As the melamine diborate microfibers growing up, they became highly crossed with each other to form an agglomerate in the solution. Once dried by cryodesiccation, the self-supported ultralight architectures made of melamine diborate microfibers were obtained. After a high-temperature pyrolysis treating process, the melamine diborate precursor decomposed and in-situ converted into porous BN microfibers, leading to the formation of a BN foam. The final foam kept identical microfiber-assembled cellular network with the foam-like melamine diborate precursor. The importance of ultrasonic-assisted treatment is reflected in two points. On one hand, the ultrasonic-assisted treatment can enhance the nucleation, growth and interweaving of melamine diborate crystal fibers, resulting in the formation of melamine diborate foam precursors in very large sizes and high porisity. On the other hand, it can also improve the uniformity of melamine diborate nuclei in the solution, which leads to the homogeneity of foam precursors in a large-scale. Because of the unique assembly process in our methodology, the BN foam with large size and desired shapes can be readily accessible. The mechanical properties of the BN foam were studied by compression test. As shown in Figure 3a, BN foam with density of ~15 mg/cm3 exhibits recoverable deformation when


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released from compression. Then we measured its compressive stress (σ) as a function of strain (ε). The σ-ε curves reveal great resilience of the BN foam up to 50% strain during 10 loadingunloading cycles (Figure 3b). For the first compression cycle, three deformation regimes can be observed: initial Hookean region at ε < 10% with an Young’s modulus of ~ 43 kPa, a stress plateau at 10% < ε < 20%, and a abrupt increasing stress regime. The deformation in Hookean region is nearly linear elastic, corresponding to the bending of fibrous cell walls. The relatively flat plateau is attributed to the post buckling deformation of the fibrous cell walls, and the rapid stress increase region is due to the densification of the cells. At large strain, the interconnected cellular pores would be densely packed and the fibrous cell walls begin to touch each other. Besides, similar to other resilient cellular materials, 18, 20, 34, 35 hysteresis loops can be observed in all σ-ε curves, indicating energy dissipation. The dissipation can be attributed to the buckling and kinking of microstructures, as well as the friction and adhesion between the long fibrous cell walls. Under unloading, unrecoverable residual deformations take place after 50% strains. This is probably due to the twisted and fractured fibers happened during the compression cycles. However, the stress does not completely vanish until the strain returns to zero, indicating an unrecoverable residual deformation of ~6%. Then the energy loss coefficients (∆U/U) at ~50% strain were calculated and summarized in Figure 3c. The ∆U/U for the first cycle is calculated to be ~77% and tends to stabilize at ~52% after 10 cycles, which is higher than those of other fibrous aerogels.34 Besides, the maximum stress (σM) and the elastic modulus display similar tendency with no significant decrease after the first cyclic compression. The high energy loss coefficient of the BN foam implys their potential applications in cushioning material.


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Figure 3. Compression test on the BN foam. (a) Photographs of the BN foam under a compressing and releasing cycle. (b) Stress-strain (σ-ε) curves of compression test during 10 loading-unloading cycles. (c) Summary of the energy loss coefficients (∆U/U), the maximum stress (σM) and the Young’s modulus (E) during compressive cycles at a strain of ~50%.

The 3D BN foam demonstrates unique robust selective wettability, as revealed in Figure 4a. We measured two different dynamic contact angles (CAs) with water and silicone oil, respectively. The BN foam exhibits hydrophobicity with a water CA of 150°, and superlipophilicity with an oil CA of 0°. Interestingly, the foam exhibits low adhesion to the water droplet with no permeation after 5 min’s contact, while the oil droplet permeates into the foam immediately after dropping. We believe that the surface roughness of BN foam with many crosslinked fibers is the main cause to create its hydrophobicity.36 On the other hand, the highly abundant porous spaces result in the superlipophilic nature of the foam. In order to study the oil absorption capacity of the BN foam, we put a block of BN foam into a beaker containing of a mixture of silicone oil (dye with a red agent) and water. As shown in


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Figure 4b, the foam block floats on the oil/water surface at the initial stage of the experiment. After 40 s, the red-dyed oil has been absorbed completely and the bottom of foam has become red. Moreover, due to its excellent hydrophobic-lipophilic nature and low density, the foam still floats on the surface of water and keeps its original shape, indicating that the foam can be easily collected after absorption. The BN foam exhibits super-high absorption capacities for a wide range of organic solvents and oils, as shown in Figure 4c. The absorption capacities of BN foam (wt.%, the ratio of the final weight after full absorption to the initial weight of foam) for various kinds of solvents and oils achieve ~5130-7820% dependent on the liquid density (Supporting Information, Figure S7). Especially, the absorption capacity for chlorobenzene is as high as ~7820%, indicating that the BN foam can absorb chlorobenzene up to ~78 times of its own weight. The excellent absorption capacities not only surpass those of BN-based cellular materials such as BN nanosheet aerogels,24 BN porous monoliths,21 tubular BN cellular-network foams,20 but also rival those of many other carbon-based foam-like materials, such as graphene foams,37 carbon aerogles,3 suggesting excellent absorption performance of the newly prepared BN foam.


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Figure 4. (a) Photograph of water and oil droplets on the surface of BN foam, showing excellent hydrophobic-lipophilic nature of the foam. (b) Photograph of the oil absorption test for BN foam. A block of foam was put into a cuvette containing of a mixture of silicon oil (dye with a red agent) and water. (c) The absorption capacities of the BN foam (wt.%, the ratio of the final weight after full absorption to the initial weight of foam) for various kinds of solvents and oils. (d) Absorption capacities of the regenerated foam after ~20 cycles.

We believe the high absorption capacities and fast absorption rate of the as-prepared BN foam can be attributed to the following reasons. On one hand, different from the previously reported 2D BN nanosheets-assembled aerogels/foams, the present 3D architectures constructed by 1D microfibers possess a unique open-cell cellular network, which can produce very high internal connectivity and porosity.20 The high porosity (~99.3%) and internal connectivity are extremely beneficial for the permeation and diffusion of gas and/or liquids, especially of those fluids with


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high viscosity. On the other hand, the building blocks of this foam, i.e. porous BN microfibers, possess high surface areas and large volumes of micropores and mesopores.31,

32

Once the

surfaces of microfibers are in contact with oil molecules, high-density capillarity filling within the abundant micro-/mesopores takes places, result in the fast absorption. Due to the excellent thermal stability of BN foam (Supporting Information, Figure S8), the foam after saturated absorption can be readily recycled by a simple burning process in air. We chose the oil-saturated BN foam (with waste diesel) for recycling test. As shown in Figure 4d, the regenerated foam still keeps very high absorption capacity after ~20 cycles (remains ~94% efficiency) without destroying their structures, suggesting the BN foam as an reliable absorbent for potential applications in oil spill and pollution treatments. Notably, the BN foam is a good thermal insulator with low thermal conductivity. As shown in Figure 5, the measured thermal conductivity of as-prepared BN foam is as low as ~0.035 W·K1

m-1, which is comparable to other previously reported conventional ultralight aerogels, such as

monolithic silica aerogels (~0.036 W·K-1m-1),38 alumina aerogels (~0.031 W·K-1m-1),39 SiC foams (~0.02 W·K-1m-1),40 and CNT sponges (~0.15 W·K-1m-1),4 suggesting its excellent thermal insulation performance. Although bulk hBN is a kind of good thermal conductor, the present BN foam shows very low thermal conductivity due to their unique highly porous structures. The BN foam has very low density and hierarchical porous structure with high specific areas. The small solid fraction and enriched channels make the contribution of solid heat transfer very low. Moreover, BN foam exhibits high porosity containing abundant micropores and mesopores. The small nanometer-sized pores lead to the suppression of gas molecule motion inside the pores, which results in the sufficiently low contribution of gaseous heat transfer. Therefore, the present foam shows good thermal insulation. Interestingly, the BN foam under compression at ~80%


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strains still displays very low thermal conductivity of ~0.087 W·K-1m-1. The gradually increase of thermal conductivity at different compression strains is due to the increase of contact areas between various 1D BN microfibers, which leads to the enhancement of solid heat transfer. Considering the additional advantages such as excellent thermal stability and resistance to harsh environment of the present BN foam, it could be highly valuable for applications in aerospace and building fields.

Figure 5. Thermal conductivities of as-prepared BN foam measured at different compressive strains.

CONCLUSION In conclusion, we have demonstrated a template-free strategy for the scalable synthesis of ultralight BN cellular foams through the combination of an ultrasonic-assisted self-assembly, freeze-drying and pyrolysis process. For the first time, porous BN microfibers with high aspect ratios cross-assembled into 3D cellular foam with high porosity (~99.3%) and great resilience. Because of the unique ultrasonic-assisted assembly process in our methodology, the BN foam with desirable shapes and large sizes can be readily accessible. Due to the open-cell network constructed by 1D porous microfibers, the present BN foam exhibits excellent absorption


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capacities for a wide range of organic solvents and oils (wt.% of ~5130-7820%), as well as a high recovery efficiency (~94%) after 20 cycles regeneration. The hierarchical porous structure with high specific areas enables BN foam showing a very low thermal conductivity of ~0.035 W·K-1m-1. The developed multifunctional BN foam may have a high potential for applications in wastewater treatment, air purification, catalyst carrier, energy storage, aerospace and building fields, etc. We also envision that the synthetic approach should be a very promising strategy which can be easily expanded to other 3D cellular materials.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. SEM and photo images of the M·2B precursors (PDF) Photos of BN foam cuttings form a bulk one (PDF) Enlarged SEM image of the BN foam (PDF) EEL spectrum of the BN foam (PDF) XPS spectra of the BN foam (PDF) The pore size distribution of BN foam measured by mercury porosimetry (PDF) The absorption capacities and the liquid densities for various solvents and oils (PDF) TG-DTA analyses of the BN foams (in air) (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y. H.)


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ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51772075, 51572068, 51402086), the Natural Science Foundation of Hebei Province (E2016202122) and the Hundred Talents Program of Hebei Province (E2014100011).

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Self-Assembly of Porous Boron Nitride Microfibers into Ultralight

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