Achieving a Collapsible, Strong, and Highly Thermally Conductive

Research Article www.acsami.org

Achieving a Collapsible, Strong, and Highly Thermally Conductive Film Based on Oriented Functionalized Boron Nitride Nanosheets and Cellulose Nanofiber Kai Wu,† Jinchao Fang,† Jinrui Ma,† Rui Huang,† Songgang Chai,‡ Feng Chen,*,† and Qiang Fu*,† †

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China ‡ Guangdong Shengyi Technology Limited Corporation, Dongguan 523039, China S Supporting Information *

ABSTRACT: Boron nitride nanosheet (BNNS) films receive wide attention in both academia and industry because of their high thermal conductivity (TC) and good electrical insulation capability. However, the brittleness and low strength of the BNNS film largely limit its application. Herein, functionalized BNNSs (f-BNNSs) with a well-maintained in-plane crystalline structure were first prepared utilizing urea in the aqueous solution via ball-milling for the purpose of improving their stability in water and enhancing the interaction with the polymer matrix. Then, a biodegradable and highly thermally conductive film with an orderly oriented structure based on cellulose nanofibers (CNFs) and fBNNSs was prepared just by simple vacuum-assisted filtration. The modification of the BNNS and the introduction of the CNF result in a better orientation of the f-BNNS, sufficient connection between f-BNNS themselves, and strong interaction between f-BNNS and CNF, which not only make the prepared composite film strong and tough but also possess higher in-plane TC. An increase of 70% in-plane TC, 63.2% tensile strength, and 77.8% elongation could be achieved for CNF/f-BNNS films, compared with that for CNF/BNNS films at the filler content of 70%. Although at such a high f-BNNS content, this composite film can be bended and folded. It is even more interesting to find that the in-plane TC could be greatly enhanced with the decrease of the thickness of the film, and a value of 30.25 W/m K can be achieved at the thickness of ∼30 μm for the film containing 70 wt % fBNNS. We believe that this highly thermally conductive film with good strength and toughness could have potential applications in next-generation highly powerful and collapsible electronic devices. KEYWORDS: boron nitride nanosheets, cellulose nanofiber, thermal conductivity, mechanical properties

1. INTRODUCTION With the rapid evolution of electronic devices toward miniaturization, high speed, and large power, intense heat generated by the components inevitably gives rise to thermal failure, performance degradation, and loss of service life, which makes the issue of heat dissipation become increasingly urgent.1−5 The present materials for thermal management in electronics are mainly isotropic polymer composites combined with thermally conductive fillers because of their appealing advantages such as excellent processability, good flexibility, and low cost.5−8 Compared to isotropic conductive materials, papery films with the anisotropic thermal conductivity (TC) and desirable mechanical properties, which could dissipate heat from the hot regions along the in-plane direction while preventing neighboring components from being influenced, are highly needed particularly for next-generation portable and collapsible electronic devices.9−14 Hexagonal boron nitride nanosheets (BNNSs), as graphene analogues, possess high TC, good electrical insulation capability, and low dielectric constant because electrons of © 2017 American Chemical Society

the BNNS are localized owing to the ionic characteristic of B− N bonds, whereas heat transmission is beneficial because of the high phonon velocity.15,16 These comprehensive performances enable BNNSs to be applied in the thermal management of highly powerful electronics that are not suitable for electrically conductive fillers such as graphene and carbon nanotubes. So far, the studies concerning the BNNS film or composites of the BNNS film are receiving wide attention both in academia and industry. For example, Yu et al. recently reported an ultrathin BNNS film through hydroxylation of the BNNS and a followed filtration process, which could exhibit an excellent in-plane TC as high as 58.3 W/m K.17 Zeng et al. reported a bioinspired BNNS/GO film, whose in-plane TC could achieve 29.8 W/m K owing to the well-designed BNNS/GO interface and special bionic structure.18 However, the brittleness and low strength of these films (fragile for BNNS film; tensile strength of ∼17 MPa Received: June 8, 2017 Accepted: August 16, 2017 Published: August 16, 2017 30035

DOI: 10.1021/acsami.7b08214 ACS Appl. Mater. Interfaces 2017, 9, 30035−30045

Research Article

ACS Applied Materials & Interfaces

Figure 1. Preparation process of f-BNNS, TEMPO-oxidized CNF, and CNF/f-BNNS composite film.

and elongation of ∼0.32% for BNNS/GO film) largely limit their application, particularly in flexible electronic devices. It is believed that combining the BNNS with a polymer is an effective way to enhance its mechanical properties. For example, it was reported that the combination of BNNS with poly(vinyl alcohol) (PVA) could obviously improve both tensile strength and elongation of the film while maintaining its in-plane TC as 6.9 W/m K because the long-chain PVA molecules serve as bridges to connect adjacent BNNS as well as enhance the filler−filler interface.19 Unfortunately, previously reported polymer/BNNS composite films with the in-plane TC below 20 W/m K are usually unsatisfactory particularly for higher power electronics. The primary causes are concluded as follows. First, the anisotropic characteristic of BNNSs makes phonon transmission much more advantageous along the maximally exposed (002) plane direction than other directions.15 However, previous studies with randomly dispersed BNNSs or partially oriented BNNSs could not take this anisotropic advantage to the fullest. Second, the strong van der Waals interaction between the BNNS and the chemically inert surface makes them challengeable to be homogeneously dispersed in polymer solution or polymer matrix especially at the high content, which leads to insufficient formation of BNNS network to reduce the thermal resistance along the inplane direction. Hence, the key point for the preparation of BNNS composite films with high in-plane TC as well as excellent mechanical properties is the order of orientation of the BNNS along the in-plane direction and formation of homogeneous BNNS network. Cellulose nanofibers (CNF), which is a naturally abundant, environmentally friendly, and biodegradable macromolecule, were previously reported to be able to disperse 2D BNNS in aqueous solution by means of both electrostatic repulsion and steric hindrance.20 With the help of this characteristic, Hu reported a good composite film based on CNFs and BNNSs for the purpose of achieving the homogeneous distribution.21 On the one hand, it was found that outstanding in-plane TC can be obtained with the increase of the BNNS content when the BNNS content was below 50 wt %. On the other hand, considering the mechanical properties, the tensile strength and flexibility inevitably sharply decreased with the increase of BNNS loading at high loading, which would limit its practical applications if the composite films need to experience repeated folding and bending because the assistant dispersed ability of

the CNF was restricted when gradually increasing BNNS loading, which caused more agglomerations in the aqueous solution as well as final composites and further destroyed the orientation of the BNNS along the in-plane direction during vacuum-assisted filtration. These factors are adverse for further enhancement of TC at the same time maintaining the desirable mechanical properties. Hence, achieving an orderly oriented BNNS structure along the in-plane direction as well as maintaining the homogeneous distribution to form continuous and dense BNNS network remains a challenge. Although functionalization of the BNNS is far less wellexplored compared to C-based materials, there are several strategies for successful BNNS modification to improve the stability in solvent, including hydroxylation, amination, and alkylation.17,22−27 However, the majority of these methods needs harsh experimental conditions or would cause damage to the crystalline structure. These generated defects on the plane of the BNNS were disadvantageous for phonon transmission, which is similar to that in graphene.17,28,29 Hence, not in-plane modification but edge-modification of BNNS is significant. Herein, urea was utilized to functionalize the exfoliated BNNS in the aqueous solution under a gentle and environmental condition to further enhance the aqueous stability of the BNNS as well as the interaction between CNFs and BNNSs. Only chemically reactive edges can be modified so that the in-plane crystalline structure can be well-maintained.22 Then, a biodegradable film with an orderly oriented structure based on CNFs and functionalized BNNSs (f-BNNSs) was prepared just by simple vacuum-assisted filtration. As a result, it is found that the in-plane TC of the CNF/f-BNNS film is much higher than that of the CNF/BNNS film, especially at the high filler content. For example, addition of 70% f-BNNS results in 70.4% higher in-plane TC than that of the CNF/70% BNNS film. More interestingly, the in-plane TC of CNF/70% f-BNNS film dramatically increases with the decrease of the thickness, ultimately achieving as high as 30.25 W/m K at the thickness of 30 μm. In comparison with the mechanical properties of the CNF/BNNS film, the CNF/f-BNNS film also exhibits 63.2% higher tensile strength and 77.8% higher elongation at the filler content of 70%. These excellent mechanical properties enable this CNF/f-BNNS film with such high loading even to be bended and folded. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses both demonstrated that this high TC and excellent mechanical properties are mainly 30036


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Figure 2. (a) TEM image of BNNS and their corresponding (b) thickness as well as (c) fast Fourier transform image.

Figure 3. XPS (a) B 1s core-level spectra and (b) N 1s core-level spectra of BNNS; XPS (c) B 1s core-level spectra and (d) N 1s core-level spectra of f-BNNS. hypochlorite (NaClO), N,N-dimethylformamide (DMF), and hydrogen peroxide (H2O2) were bought from Kermal Chemical Reagent Plant, China. Boron nitride (BN) powders were provided by SigmaAldrich (∼1 μm). Urea (AR, 99%) was supplied by Aladdin Reagent Limited Corporation, China. 2.2. Preparation of Exfoliated and f-BNNS. Exfoliated BNNSs were prepared from raw BN according to the previous study.31 In a typical experimental step, 1 g of BN powder (Sigma-Aldrich, 1 μm) was probe-sonicated in DMF for 10 h. Afterward, centrifugation with 3000 rpm was performed to remove the residual BN, and about 72 mg of the BNNS was collected. Then, the exfoliated BNNS was modified for obtaining f-BNNSs as follows. In a typical procedure, 500 mg of BNNS, 30 g of urea, and 12 mL of H2O were mixed together to be mushy; then, they were ball-milled at 500 rpm for 16 h. Finally, the collected powder was washed with H2O for four times to remove the residual urea. 2.3. Preparation of f-BNNS Composite Films. The CNF/fBNNS composite films with different f-BNNS contents were prepared

ascribed to the homogeneous f-BNNS dispersion, orderly oriented morphology, and stronger interfacial interaction between the CNF and the f-BNNS. Besides, this CNF/fBNNS film also exhibits a low dielectric constant of 4.86 at 1000 Hz and extraordinary insulating characteristic which is much better than neat CNFs. All these outstanding properties guarantee that this prepared film is expected to be applied in next-generation portable and collapsible electronic devices for heat dissipation.

2. EXPERIMENTAL SECTION 2.1. Materials. The raw cellulose materials (Celish MFC KY100-S) were obtained from Daicel Chemical Industries, Ltd., Japan. 2,2,6,6Tetramethyl-1-piperidinyloxy (TEMPO)-oxidized CNF mentioned in this research was fabricated according to the previous literature.30 TEMPO (98 wt %) was purchased from Sigma-Aldrich. Sodium 30037


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N−H, suggesting that −NH2 was grafted on the f-BNNS. These −NH2 are mainly divided into two parts. One may be from the reaction between urea and B−OH; another may be from the direct graft of −NH2 to BNNS.22 Hence, it can be considered that the f-BNNS utilized in this study is the aminated BNNS, whose −NH2 may largely enhance the aqueous stability of f-BNNSs as well as the interaction between CNFs and f-BNNSs. To further testify the existence of −NH2, second, Fourier transform infrared (FTIR) characterization was carried out. As shown in Figure S3, the asymmetric peak located at 3422 cm−1 indicates the combination of N−H stretching vibration and O−H stretching vibration, respectively, because of the existence of −NH2 and absorbed water.22 Third, thermogravimetric analysis (TGA) was applied to calculate the content of functional groups on f-BNNSs (Figure S3). About 1.8% weight loss is observed when the f-BNNS was heated from room temperature to 800 °C. Considering the created B− O owing to the hydrolysis at the defects, the content of −NH2 in f-BNNSs may be lower than 1.8 wt % in this study. Therefore, fourth, a water contact angle was characterized to demonstrate better interaction between the f-BNNS and water as well as f-BNNS and CNF. As shown in Figure 4, one can find

according to Figure 1. Typically, the homogeneously dispersed CNF/fBNNS aqueous solution was obtained through 30 min intense stirring and sonication. Then, the homogeneous solution was vacuum-filtrated using a membrane filter with a diameter of 5 cm and pore size of 0.25 μm. Finally, the wet CNF/f-BNNS films were all dried at ambient temperature to remove residual H2O. For comparison, the preparation process for obtaining the CNF film and CNF/BNNS films was similar to above. 2.4. Characterization. Transmission electron microscopy (TEM; JEF-2100F, JEOL) was tested at a voltage of 200 kV to observe the morphology of the BNNS. The XRD pattern of composite films was tested to analyze the crystalline structure and the orientation of BNNS or f-BNNS. X-ray photoelectron spectroscopy (XPS; Axis Ultra DLD, Kratos Co., UK) of BNNS and f-BNNS was performed using focused Al Kα radiation (15 kV). The TC of the composite films was calculated according the equation TC = α × ρ × C, where α, ρ, and C, respectively, correspond to the thermal diffusivity, density, and specific heat capacity of the CNF composites (Tables S1 and S2). The in-plane and out-of-plane thermal diffusivity of composite films (diameter of 25.4 mm) were measured at 25 °C with an LFA 467 analyzer (NETZSCH, Germany) via using different modules. As shown in Figure S1, for cross-plane TC, the laser can directly transmit through the film along the cross-plane direction. However, for in-plane TC, the specific module needs laser to first pass along the in-plane direction. Then, it can further transmit across the composite film and be detected by the equipment. The average of at least three measurements was calculated as final results. Each ρ of composite films was determined by the weight and dimension of samples. Also, C was determined according to the equation Ccomposites = CBNNS × Φ + CCNF × (1 − Φ), where Φ corresponds to the BNNS volume content, and CBNNS and CCNF were, respectively, tested as 0.78 and 1.35. The dispersed condition of BNNS or f-BNNS in CNF aqueous solution and the morphology as well as filler distribution in CNF composites were also characterized by SEM. The mechanical properties of composite films (5 mm × 20 mm and thickness of ∼90 μm) were measured on an Instron 5567 machine with 1 kN load cell at 25 °C with the stretching rate of 1 mm/min. The dielectric properties and the ac conductivity of composite films (diameter of 16 mm) were all tested by a broad frequency dielectric spectrometer Concept 50 (Novocontrol, Germany). Before characterization, silver paint was brushed on both sides of the samples.

3. RESULTS AND DISCUSSION 3.1. Characterizations of BNNS and f-BNNS. The thin layer structure of a few-layer BNNS utilized in this study was first characterized by TEM and AFM as follows. TEM image in Figure 2 and AFM image in Figure S2 both suggest the wafery and lamellar morphology of the BNNS whose lateral size is about several hundred micrometers and whose thickness is about 1−6 nm. Figure 2c which was obtained from the fast Fourier transformation displays a special sixfold symmetry owing to the special hexagonal crystalline structure of the BNNS.16,17 It indicates the purity and good hexagonal crystalline structure of the exfoliated product. Then, the above-mentioned BNNS were ball-milled in the aqueous solution with urea to obtain f-BNNSs. Herein, we will demonstrate the successful modification from four aspects. First, XPS is considered to be an effective and important method to detect the chemical composition of the materials. For BNNSs (Figure 3), B 1s core-level spectra display the weak peak of B−O, which corresponds to the small quantity of residual −OH on BNNSs.27 N 1s core-level spectra of BNNSs suggest that only N−B exists. After the modification through ball-milling, the peak of B−O obviously increases, indicating that B−O forms in the aqueous solution owing to the hydrolysis at some edge-defects.24,27 Moreover, N 1s corelevel spectra of the f-BNNS in Figure 3d display clear peak of

Figure 4. Results of water contact angle for (a) BNNS and (b) fBNNS.

that the f-BNNS displays the much smaller initial water contact angle (25.4°) than that of the BNNS (59.9°). Also, as time goes by, the water drop can be completely absorbed by the f-BNNS for only 1400 ms, whereas the BNNS cannot. This implies that, in comparison with BNNSs, f-BNNSs have much stronger interaction with water owing to the polar covalent bond of −NH2 that is located at the f-BNNS. Moreover, CNFs are highly hydrophilic because of the large amount of −OH and −COOH. Hence, f-BNNSs are believed to have much better interaction with the CNF in comparison with the BNNS. In addition to the functionalization, the dimension and crystalline structure of f-BNNS themselves are also significant for determining the final properties of the composite films. To calculate the dimension, at least 100 BNNSs or f-BNNSs in the SEM images (Figure S4) were selected to count the lateral size, and at least 40 BNNSs or f-BNNSs in the TEM images (Figure S5) were selected to count the thickness. As shown in Figure S6, the lateral size of the BNNS ranges from 100 to 600 nm and the average is about 260 nm. The thickness of the BNNS ranges from 1 to 6 nm, and the average is about 3 nm. After modification, f-BNNSs exhibit no obvious decrease in 30038


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Figure 5. (a) In-plane and cross-plane TC of CNF composite films (150 mg); (b) anisotropy of thermal conductivities (K/K⊥) for CNF composite films (150 mg); (c) in-plane TC of CNF/70 wt % f-BNNS film as a decrease of the thickness; and (d) photographs of CNF/70 wt % BNNS and CNF/70 wt % f-BNNS films with different thickness.

successful edge-aminated BNNSs without sacrificing the inplane crystalline structure and changing the BNNS dimension, which improves the stability of the f-BNNS in water as well as enhancing the interaction between f-BNNS and CNF. 3.2. Thermal Conductivity of CNF/f-BNNS Films. To demonstrate the role of functionalization of BNNS to the TC and mechanical properties of BNNS composite films, the CNF/ BNNS and CNF/f-BNNS films with different BNNS or fBNNS loading were prepared and compared. The photographs of composite films are provided in Figure S9. It is clear that the BNNS or f-BNNS composite film is white and opaque after introducing the BNNS or f-BNNS. The accurate mass fraction of the filler was also checked and calculated by TGA as about 0, 10, 30, 50, and 70% (Figure S10 and Table S3). Figure 5a exhibits the in-plane and cross-plane TC of composite films whose thickness was fixed as ∼90 μm. The cross-plane TC of CNF/BNNS and CNF/f-BNNS films both gradually increase with the increase of the filler content, but what is different is that the cross-plane TC of the CNF/f-BNNS film is always lower than the CNF/BNNS film. Lower cross-plane TC means better anisotropic heat dissipation capability of the CNF/fBNNS film, which can prevent neighboring components from being influenced while only dissipating heat from the hot regions along the in-plane direction. Compared to the crossplane TC, in-plane TC is critical for heat dissipation capability of the anisotropic thermally conductive materials. For in-plane TC, the pure CNF is as high as 1.6 W/m K, which is almost 5fold of that of conventional polymers. This is ascribed to the high crystallization in the CNF that is beneficial for phonon

dimension distribution or average size. It suggests that the ballmilled BNNS in the aqueous solution with urea does not cause great damage to their dimension, and the BNNS will not be further exfoliated to thinner sheets. Raman spectra shown in Figure S7 also can demonstrate this result. One can find that Gband frequencies of both BNNSs and f-BNNSs are consistent at 1367.8 cm−1 without any red shift or blue shift, also indicating no obvious change in thickness of f-BNNSs after ballmilling.23,32 In this case, the following comparison between CNF/BNNS and CNF/f-BNNS films will be not affected by the variation in filler dimension. In addition to the filler dimension, XRD was also performed to study the physical crystalline structure of the BNNS and f-BNNS. As illustrated in Figure S8, the intensity of (002) and (004) planes for f-BNNSs is almost consistent with that of BNNSs, manifesting that the in-plane crystalline structure is commendably maintained without any evident damage. Some studies previously reported that the edges of BNNSs is much chemically active than the surface; hence, the functional groups could be grafted at the edges instead of on the surface, if the reaction condition is not too intense.26,33 Deduced from the XRD results, −NH2 may not be located at the surface of the f-BNNS. Otherwise, defects will generate on the in-plane surface to break the (002) and (004) crystalline structure. Further lines of evidence need to be studied in the future, and they may be published for modification of the BNNS elsewhere. From what has been discussed above, the gentle and environmentally friendly modification method via ball-milling of the BNNS with urea in aqueous solution brings about 30039


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ACS Applied Materials & Interfaces Table 1. Comparison of the In-Plane Thermal Conductivities in BN-Based Films Previously Reporteda

a

composites

loading (wt %)

thermal conductivity (W/m K)

thickness (μm)

test method

references and year

PMMA/BNNS CNF/BNNS CNF/BNNS PI/h-BN CNF/BNNS PVA/BNNS CNF/BNNT PVA/BNNS GO/BNNS PVA/BN PVA/BN PVB/BN CNF/f-BNNS

80 10 90 75 50 50 25 94 95 30 63.6 68 70

14.7 11.3 30 17.5 145.7 13 21.39 6.9 29.8 4.41 11 14 30.25

300   20−50   15  80−100 100−500   30

thermowave analyzer steady-state method steady-state method scanning laser heating AC method steady-state method LFA LFA LFA LFA LFA LFA LFA LFA

201612 201635 201635 201336 201421 201637 201713 201519 201618 201338 201439 201439 this work

The symbol “” means that the information cannot be obtained from the literature.

Figure 6. Stress−strain curves of (a) CNF/BNNS and (b) CNF/f-BNNS films with different filler loading; (c) tensile strength; and (d) elongation of the CNF-based film with the increase of the filler content.

transmission as well as good alignment of the CNF in the film.13,34 After introducing 10% BNNS or f-BNNS, both of the in-plane TC sharply increase, and the values are very close. However, high addition of the filler leads to a different trend. At the higher filler loading, the in-plane TC of the CNF/f-BNNS film increases rapidly while that of the CNF/BNNS film improves slowly. Hence, the in-plane TC of the CNF/70% fBNNS film can ultimately achieve 12.79 W/m K, which is almost 1.7-fold of that for the CNF/70% BNNS film, suggesting great impact of functionalization to the in-plane TC of the BNNS composite film. As a consequence, the obtained CNF/70% f-BNNS film with the thickness of ∼90 μm

demonstrated an exceptionally stronger anisotropy of TC than that of the CNF/70% BNNS film, where the K/K⊥ can achieve ∼21 (Figure 5b). More interestingly, it is further found from Figure 5c that the in-plane TC sharply increases with the decrease of the thickness for the CNF/70% f-BNNS film, achieving as high as 30.25 W/m K at the thickness of ∼30 μm. We think that the sample preparation (vacuum-assisted filtration) can induce in-plane orientation owing to the tensile stress along the film plane. A thinner f-BNNS composite film means short filtrating time and large filtrating tensile stress. For example, composite films with the thickness of 90 μm need about 1 day for vacuum filtration, whereas 30 μm films only 30040


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Figure 7. (a) Dielectric constant, (b) dielectric loss, and (c) ac conductivity of th CNF/f-BNNS film.

spend 3 h. Hence, the thinner the film, higher is the orientation degree of the BNNS that the composite film obtains. Table 1 lists the in-plane TC for previously reported BN composite films. Clearly, most BN composite films have an inplane TC below 20 W/m K, which is unsatisfactory for highly powerful electronics. It is also found that some studies utilizing the steady-state method reported higher in-plane TC compared to other methods, such as 145.7 W/m K for the CNF/50 wt % BNNS film.21 Recently, a reported study where 100% BNNS film was fabricated could exhibit excellent in-plane TC as high as 58.3 W/m K (LFA method), which was still much lower in comparison with 145.7 W/m K.17 If abundant one-dimensional (1D) CNF was introduced into the system, the fibrous CNF must hinder the efficient connections between the BNNS, resulting in higher thermal resistance compared to that in the 100% BNNS film. Hence, we think that the in-plane TC of CNF/BNNS film may be lower than 58.3 W/m K if the LFA method is applied. The in-plane TC of the CNF/70% f-BNNS in our study is reasonable, and it is the highest in comparison with the most previous studies (LFA method). Therefore, on the one hand, we consider that the difference between our result (30.2 W/m K at 70 wt % BNNS loading) and 145.7 W/ m K at 50 wt % BNNS loading is mainly ascribed to the much different test method. Generally, LFA is the popular method especially for testing the TC of anisotropic materials. On the other hand, the lateral size as well as the thickness of the BNNS are also significant in determining the final in-plane TC of films. The lateral size of the BNNS utilized in that study was also different from ours. In addition, we also find that the BNNS composite films with the high in-plane TC in Table 1 can only be bended but not folded owing to their limited mechanical properties, which unquestionably restricts their practical application such as in wearable electronics or portable and collapsible devices. This disadvantage was also observed in our study for unmodified BNNS composite films. For CNF/70% BNNS films, a further decrease of thickness results in fragmentation of the film during the natural drying process at a certain weak pressure, whereas ultrathin CNF/70% f-BNNS films with the thickness of ∼30 μm can be even bended and folded without any fragmentation (Figure 5d). We think that the above-excellent in-plane TC and well-maintained mechanical properties are attributed to a better orientation and dispersion of f-BNNSs, sufficient connection between f-BNNS themselves, and good interaction between f-BNNSs and CNFs; it will be demonstrated and discussed in the following. This also implies the great impact of functionalization to the mechanical properties of the CNF-based film. Hence, in the following

section, the mechanical properties of CNF/BNNS and CNF/fBNNS films were also compared at a certain thickness. 3.3. Mechanical Properties of CNF/f-BNNS Films. Because thinner CNF/BNNS films tend to be split during the drying process, the CNF-based films with the thickness of ∼90 μm were characterized, and the results were compared. Figure 6 depicts the mechanical properties of CNF/BNNS and CNF/f-BNNS films with the different filler content. For CNF/ BNNS films, at the low filler content, the addition of BNNSs leads to obvious increase of elongation but a slight decrease of tensile strength because during the stretching process, oriented BNNS tend to slip and be pulled out from the composite film, which could largely enhance the elongation of the composite film, as shown in Figure S12. However, the high content of BNNSs especially after 30 wt % results in both sharply decreased tensile strength and elongation. After the BNNS was modified, the situation became different. The low content of fBNNSs (

Achieving a Collapsible, Strong, and Highly Thermally Conductive

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