aramid nanofiber composite film

Journal Pre-proof Highly thermally conductive, ductile biomimetic boron nitride/aramid nanofiber composite film Guang Xiao, Jiangtao Di, Hao Li, Jianfeng Wang PII:

S0266-3538(19)33173-2

DOI:

https://doi.org/10.1016/j.compscitech.2020.108021

Reference:

CSTE 108021

To appear in:

Composites Science and Technology

Received Date: 13 November 2019 Revised Date:

15 January 2020

Accepted Date: 20 January 2020

Please cite this article as: Xiao G, Di J, Li H, Wang J, Highly thermally conductive, ductile biomimetic boron nitride/aramid nanofiber composite film, Composites Science and Technology (2020), doi: https:// doi.org/10.1016/j.compscitech.2020.108021. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Author statement

Guang Xiao: Investigation, Methodology, Data curation, Writing - Original Draft Jiangtao Di: Writing-review&editing Hao Li: Visualization Jianfeng

Wang:

Conceptualization,

Writing-review&editing

Supervision,

Funding

acquisition,

Highly thermally conductive, ductile biomimetic boron nitride/aramid nanofiber composite film Guang Xiao a, Jiangtao Di b, Hao Li c, Jianfeng Wang a,* a

College of Materials Science and Engineering, Hunan University, Changsha 410082,

China b

Key Lab of Nano-Devices and Applications, Suzhou Institute of Nano-Tech and

Nano-Bionics Chinese Academy of Sciences, Suzhou, 215123, China. c

College of Chemistry and Chemical Engineering, Hunan University, Changsha,

410082, China *Corresponding authors: [email protected]

Abstract: Combining a large volume of boron nitride platelets with a little polymer to fabricate ductile, thermally conductive composite materials for the thermal management of flexible electronic devices is a challenge. Herein, we imitate the hierarchical layered architecture of mollusk nacre to design high-loading boron nitride composite film. Boron nitride/aramid nanofiber composite films are prepared by a sol-gel-film transformation approach and show nacre-like hierarchical layered composite structure. High-strength aramid nanofibers form a three-dimensional framework as matrix, which hosts 40-70 wt% oriented BN platelets. Thanks to the high interconnectivity of aramid nanofiber in the three-dimensional framework, the composite films have large ductility, up to 18.3-49.4%. Meanwhile, the composite films have high thermal conductivity (47.4-122.5 W m-1 K-1) because high-loading 1

boron nitride platelets form a percolative, oriented heat conduction path. Moreover, the outstanding ductility and thermal conductivity are integrated with high volume resistivity (1014-1016 Ω cm) and thermal decomposition temperature (515 oC), enabling the structured boron nitride/aramid nanofiber composite films to be promising as flexible substrate for cooling electronic components.

Keywords: Aramid nanofiber; Boron nitride; Layered structure; Mechanical property; Thermal conductivity

2

1. Introduction Hexagonal boron nitride (BN) with inherent high thermal conductivity, excellent mechanical, electrically insulating and thermally stable properties attracts great attention as a complementary to graphite for thermal management application [1-3]. It has unique advantage for improving the heat dissipation of electronic devices in some fields where electrical insulation is compulsive [4,5]. Unfortunately, pure BN material is extremely brittle, which limits its practical application [6-8]. Combining BN platelet with polymer for constructing a composite structure is a universal strategy to improve its ductility [9,10]. The resultant ductility of composite materials depends strongly on the loading of BN platelet. Composite films containing low-loading BN platelet (< 10 wt%) have large ductility, but low thermal conductivity (< 10 W m-1 K-1) [11-15]. Even though the small amount of BN platelet is well organized to an oriented arrangement or a network structure, the thermal conduction ability is still unsatisfactory [16-21]. Recently, some efforts have made to develop composite films with high-loading BN platelets (> 40 wt%) [22,23]. These composite films show a substantial improvement of thermal conductivity, yet they are often brittle with low fracture strain (< 10%), mostly manifested as a linear tensile stress-strain curve [24-26]. Thus, increasing the weight loading of BN while achieving simultaneous high ductility remains a daunting challenge. Mollusk nacre, a natural biological composite material containing 95 vol% calcium carbonate microplatelets and 5 vol% biopolymers, evolves into a well-ordered hierarchical structure and realizes perfect unification of their structure 3

and mechanical properties [27,28]. Although natural nacre contains such high-loading inorganic platelets, it still shows somewhat ductile deformation behavior, characterized by tensile yielding and plastic elongation [29]. Microstructure observation reveals that the calcium carbonate microplatelets are embedded into an organic framework, forming a well-ordered hierarchical layered structure [30-32]. Recent work proves that the organic framework contains a robust three-dimensional (3D) chitin nanofiber network, which provides mechanical support for maintaining the integrity of the biological composite material before failure [33]. The 3D interconnectivity of high-strength chitin nanofibers makes stress distribution uniform and thus allows the sliding of oriented, high-loading calcium carbonate platelets to spread over a large volume of materials during deformation [34]. Mimicking the nanofiber framework in layered mollusk nacre offers a crumb of hope for fabricating high-loading ductile composite materials [35,36] Aramid nanofiber (ANF) extracted from synthetic aramid microfiber retains original crystallinity and high mechanical strength [37-39]. Some studies prove that ANF has great potential to reinforce polymer materials because the abundant polar amide groups dramatically improve nanofiber-to-matrix stress transfer [40,41]. Recent studies prove that ANF can form robust hydrogel with low solid content and aerogel with low density, in which the nanofibers entangle into porous network structure [42,43]. Strong hydrogen bond interaction formed between the nanofibers promotes nanofiber-to-nanofiber stress transfer. Additionally, it is reported that pure ANF film has excellent tensile strength and toughness, as high as 160 MPa and 8.3 MJ m-3 [44]. 4

These imply that the polymeric nanofiber would be a promising building block as matrix for constructing advanced organic-inorganic composite materials [45-49]. In this study, in order to address the brittleness issue of high-loading BN composites, we imitate the hierarchical layered structure of mollusk nacre and fabricate boron nitride/aramid nanofiber composite films by a sol-gel-film transformation approach. 40-70 wt% BN platelets are embedded into a 3D ANF framework, forming a natural-like hierarchical layered composite structure. Importantly, the composite films achieve large ductility (18.3-49.4%) and high thermal conductivity (47.4-122.5 W m-1 K-1) simultaneously. Moreover, good electrical insulation and thermal stability are integrated with these excellent mechanical properties. Their application as flexible conductive board for cooling electronic components is demonstrated. In addition, we realized continuous fabrication of the highly thermal conductive, ductile composite films. 2. Experimental 2.1 Materials BN powder (CAS: 7440-42-8, XFNANO Materials Tech Co., Ltd), AMF (Kevlar 29, Dongguan SOVETL Co., Ltd), potassium ethoxide (≥ 95%, Sigma Aldrich) and dimethyl sulfoxide (99.7%, Sinopharm Chemical Reagent Co., Ltd) were used as received. 2.2 Preparation of BN/ANF-X film AMF and potassium ethoxide were added into dimethyl sulfoxide (AMF: potassium ethoxide : dimethyl sulfoxide = 2 : 1.5 : 96.5). The mixture was 5

mechanically stirred for two days at 40 oC, forming a viscous, dark red ANF solution (2 wt%). The ANF solution was diluted to 0.3 wt% and then mixed with BN powder, followed by ultrasonic (400 W, 40 KHz) for 3 h. A desired amount of viscous 2 wt% ANF solution was added and stirred mechanically at room temperature for 2 h, forming a uniform sol. The sol was poured into a Petri dish and immersed into deionized water for 12 h, forming a white hydrogel. The hydrogel was rinsed repeatedly to fully remove the dimethyl sulfoxide and potassium ethoxide. Finally, the hydrogel was dried at room temperature and then at 80 ℃ for 24 h, forming a white BN/ANF-X film. By using this sol-gel-film transformation process, BN/ANF-X films with different ratios of BN to ANF-X (0/100, 40/60, 50/50, 60/40, 70/30) were prepared through altering the addition amount of BN in sol (Table S1). Their thickness was controlled to be about 45-55 µm by adjusting the amount of the poured sol in Petri dish. 2.3 Characterization Scanning electron microscopy (SEM) was performed using a field-emission scanning electron microscope (S-4800, Hitachi). Transmission electron microscopy (TEM) was performed using a JEOL JEM-2100 instrument at 200 kV. TGA was performed by using an HCT-4 integrated thermal analyzer with a heating rate of 10 °C min-1 in air. Tensile properties were measured on a Shimadzu AGS-X Tester. Sample length and loading speed were 5 mm and 1 mm min-1. Rheological test was carried out on an ARES-G2 rheometer (TA instruments) using a 25 mm parallel-plate at room temperature. The hydrogel samples were covered with a thin layer of silicon 6

oil to prevent evaporation of water. In dynamic frequency sweep experiment, the oscillatory strain was fixed to be 0.2 % and angular frequency was set from 0.6 to 60 rad s-1. In strain sweep experiments, angular frequency was fixed to be 6 rad s-1 and the oscillatory strain was set from 0.1% to 100%. Volume resistivity was measured with an insulation resistance tester (PC40B, Shanghai Anbiao Electronics Co., Ltd). Thermal conductivities were measured by a laser flash method (LFA 447 NanoFlash, Netzsch). The specific heat capacity was measured with a differential scanning calorimetry (Q20, TA). The specific heat capacity of ANF, BN/ANF-X 40/60, BN/ANF-X 50/50, BN/ANF-X 60/40, BN/ANF-X 70/30 and BN is 1.250, 1.205, 1.148, 1.056, 1.043 and 0.750 J g-1 K-1. The density of BN/ANF-X films is calculated based on the weight and volume of the films. The density of BN network, ρBN, is calculated to be the product of the density of BN/ANF-X film and the weight fraction of BN. The density of ANF framework, ρANF-X, is calculated to be the product of the density of BN/ANF-X film and the weight fraction of ANF. Infrared thermal images were taken with an infrared thermal imager (FLIR T620, FLIR Systems, Inc., USA). 3. Results and discussion The boron nitride/aramid nanofiber composite film is prepared by a sol-gel-film conversion approach, as shown in Fig. 1a. First, commercial aramid microfiber (AMF) was mechanically stirred in dimethyl sulfoxide/potassium ethoxide solution. Potassium ethoxide extracts hydrogen atoms from the amide group of aramid molecule, thereby reducing the hydrogen bond attraction between nanofibers within AMF. The de-protonated nanofiber with positive charge repulses mutually, resulting 7

in a highly stable, dark red ANF solution [37]. Its fibrillary morphology is observed by TEM image, as shown in Fig. 1b. Its diameter is determined to be in the range of 3-18 nm from statistical analysis of 200 nanofibers (Fig. S1). White BN powder with the same weight as ANF was dispersed into the ANF solution by mechanical stirring, forming an orange sol (Fig. 1c). The added BN has a flaky morphology with a diameter of 2-10 µm and a thickness of more than 50 nm. Next, the sol was poured into a Petri dish, followed by immersion into deionized water. Water re-protonates ANF and restores its electrical neutrality [42]. In appearance, the viscous sol is converted to a white hydrogel quickly. SEM images reveal that the re-protonated ANF entangles together and forms a 3D porous network (Fig. 1d). BN platelets are discretely distributed in the ANF network (Fig. 1e). The entangled ANF network endows the hydrogel with robust mechanical property, which is confirmed by rheological test (Fig. S2). Frequency sweep reveals that the storage modulus of the hydrogel is more than one order of magnitude higher than loss modulus, indicating that the entangled ANF form a permanent elastic network. Although the composite hydrogel contains 97.6% water, its storage modulus is up to 114 kPa. Strain sweep reveals that the storage modulus becomes lower than loss modulus at shear strain beyond 24%, indicating that the hydrogel is converted to quasi-liquid state at large shear strain. The corresponding shear stress-strain curve shows that shear stress levels off at strain beyond 24%, also indicating the hydrogel starts to flow irreversibly at large shear strain. Finally, the hydrogel was dried at room temperature and converted to a white 8

flexible film. Surface SEM image displays that many BN platelets lie down along film surface, due to excluded volume effect during water removal (Fig. 1f) [50]. Each BN platelet is covered with a layer of ANF network (Fig. 1g). Cross-sectional SEM image displays a hierarchical layered composite structure, similar to mollusk nacre (Fig. 1h, i) [30,31]. The BN platelets are oriented along film surface and stacked alternatively with ANF network layers together. Furthermore, the ANF layers are connective in film thickness direction, arising from the 3D entanglement of ANF in the preformed hydrogel. Therefore, we regard the connective ANF network layers within the film as a whole and call it as ANF framework. The film is denoted as BN/ANF-X 50/50, in which ANF-X and 50/50 represent the ANF framework and the weight ratio of BN to ANF-X, respectively. Its structure is illustrated in Fig. 1j. In addition, BN/ANF-X films with different weight ratios of BN to ANF-X (0/100, 40/60, 60/40, 70/30) were also prepared by altering the addition amount of BN powder in the sol.

9

Fig. 1. Preparation and structure of BN/ANF-X film. a) Sol-gel-film transformation process to prepare the film. ANF is exfoliated from AMF in dimethyl sulfoxide/potassium ethoxide and mixed with BN platelet, forming a uniform sol. The sol is converted into hydrogel by immersion in water, and then into film by drying. b) TEM image of the exfoliated ANF. c) SEM image of BN platelets. d, e) SEM images of freezing dried hydrogel, showing a porous ANF network and some discrete BN platelets. f, g) Surface SEM images of the BN/ANF-X 60/40 film, showing that BN platelet is covered by ANF network. h, i) Cross-sectional SEM images of BN/ANF-X 70/30 film, showing a layered composite structure with alternately stacking of BN platelet and branched ANF network layer. Dash lines mark the branched ANF network layer. j) Illustration for the hierarchical layered composite structure of BN/ANF-X film. 10

The mechanical properties of BN/ANF-X films with different weight ratios of BN to ANF-X are measured in tensile mode and compared with that of pure ANF film prepared by the same sol-gel-film transformation approach, as shown in Fig. S3. Their representative stress-strain curves are shown in Fig. 2a. Overall, these films exhibit similar deformation process that a linear elastic deformation occurs initially, followed by yielding and plastic elongation. Their modulus, yield strength and strain-to-failure are tabulated in Table S2. It is found that the modulus, yield strength and ultimate strength of BN/ANF-X films are obviously lower than those of pure ANF film. This indicates that the addition of BN platelets facilitates the yielding of 3D ANF framework matrix. Considering that the used BN platelet is thick without exfoliation (more than 50 nm) and has inert surface without modification, the stress transfer from the 3D ANF framework to BN platelets is not insufficient, thus decreasing the modulus and yielding strength. Importantly, the strain-to-failure of BN/ANF-X (sample 40/60 and 50/50) is obviously higher than pure ANF film. It implies that the increased ductility is achieved with the sacrifice of modulus and strength. The mechanical properties of BN/ANF-X films depend on the ratio of BN to ANF-X. With the increase of the ratio from 40/60 to 50/50, the modulus and yielding strength are slightly decreased (Fig. 2b), while the strain-to-failure and toughness are increased (Fig. 2c). It indicates that the increase of BN loading further promotes the yielding of 3D ANF framework matrix. After yielding, the interconnective 3D ANF framework can maintain the integrity of the macroscopic sample for plastic elongation, 11

thereby increasing the ductility and toughness. As a result, the BN/ANF-X 50/50 shows a large strain-to-failure of 49.4 ± 4.3% and a high toughness of 25.1 ± 1.6 MJ m-3. Considering that the BN loading is up to 50 wt%, the values of strain-to-failure and toughness are remarkable and rarely achieved in previous reports. Generally, BN/polymer composites with similar filler loading are brittle with an elongation at break being less than 10%. It indicates the 3D ANF framework as matrix has a prominent advantage in the ductility improvement of high-loading BN composite materials, in comparison to normal polymer as matrix. In contrast, when the ratio of BN to ANF-X surpasses 50/50, the strain-to-failure and toughness of the BN/ANF-X films decreases sharply (Fig. 2c). The strain-to-failure is decreased to 18.3 ± 0.2% for BN/ANF-X 70/30, and its toughness is decreased to 4.4 ± 0.3 MJ m-3. To clarify the sharp decrease in ductility and toughness, the microstructure of BN/ANF-X 70/30 is examined by SEM. It is observed that the ANF framework is incomplete, and some BN platelets are bare without being covered by ANF network (Fig. S4). The imperfect ANF framework probably causes uneven stress distribution, thus leading to the decrease in strain-to-failure and toughness. The remarkable ductility of BN/ANF-X 50/50 is closely related to the unique preparation approach (sol-gel-film transformation). As a control sample, a hybrid film of BN/ANF 50/50 was prepared by normal filtration approach. The BN/ANF 50/50 film exhibits a strain-to-failure of 12.5 ± 0.9%, much lower than that of BN/ANF-X 50/50 prepared by sol-gel-film transformation approach (Fig. S5). In addition, we also 12

use the two approaches to prepare pure ANF film and measure their tensile properties for comparison. Again, the ANF film prepared by sol-gel-film transformation approach has larger ductility than the film prepared by filtration approach (32.6 ± 1.8% Vs 9.7 ± 0.8%) (Fig. S6). These indicate that the 3D nanofiber framework evolved from pre-organized ANF network in the hydrogel plays an important role in resultant ductility of film.

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Fig. 2. Tensile properties of BN/ANF-X films. a) Tensile stress-strain curves of BN/ANF-X films with different weight ratios of BN to ANF-X. b) Modulus (E) and yield strength (σy) as a function of the weight ratio. c) Strain-to-failure (εb) and toughness (U) as a function of the weight ratio. The dash lines represent the values of pure ANF film. d) Stress-strain curves of BN/ANF-X 50/50 under an increasing tensile rate. e) Stress-strain cycle curve of BN/ANF-X 50/50 with an increasing elongation of 10%, 20% and 30%. f) Stress-strain cycle curve of BN/ANF-X 50/50 with a constant strain of 20% for 5 cycles. 13

The BN/ANF-X 50/50 with complete ANF framework is further examined by fast tensile test. A strip is stretched with a gauge length of 5 mm at an increasing rate from 1 to 200 mm min-1 (Fig. 2d). Its ultimate strength is increased gradually from 56 MPa to 81 MPa. Importantly, its strain-to-failure is basically constant with a value of about 50%. It implies that the ANF framework can deform instantaneously in response to exterior fast loading. Cyclic tensile test is carried out to further prove the excellent deformability of BN/ANF-X 50/50. Cyclic tension with a strain of 10%, 20% and 30% generates three hysteresis loops (Fig. 2e). The area of the hysteresis loops is increases with the tensile strain, indicating an increasing energy dissipation by the sliding of interior ANF and BN platelets. Cyclic tension with 20% strain for five cycles generates a permanent elongation of 14.5%, indicating that the sliding of interior ANF and BN platelets is irreversible (Fig. 2f). In order to reveal the fracture behavior, we observe the crack propagation and fracture surface morphology of the BN/ANF-X 50/50 by SEM. A notched strip suffers from slow tension to arrest a stable crack. Surface image shows that the crack presents a tortuous growth path, as show in Fig. 3a. Enlarged views behind the crack tip show that the crack bypasses stiff BN platelets, thereby leading to the tortuous path (Fig. 3b, c). The deflection of crack would relieve the stress concentration at the crack tip. As a result, the two crack planes next to the tip contact without being obviously separated (Fig. 3d). Additionally, it is observed that the ANF network layer ahead of crack tip deforms intensively. Both BN platelet-caused crack deflection and the deformation of 14

ANF network improve the energy absorption during the crack extension. The resultant fracture surface shows a stair-like morphology with the ANF network layer being pulled apart, which is similar with abalone nacre (Fig. 3e, f and Fig. S7).

Fig. 3. Fracture behavior of BN/ANF-X 50/50 observed from SEM images. a-d) Surface views of an arrested stable crack during failure. a) Overview of the whole crack. b, c) Enlarged views behind the crack tip. d) Enlarged view at the crack tip. e, f) Lateral views of fracture surface. The in-plane thermal conduction property of the BN/ANF-X films is measured by using a laser flash apparatus (Netzsch, LFA-447 NanoFlash). A laser pulse radiates the center of lower surface of the sample. Simultaneously, an infrared detector is used to detect the temperature rise at the position far from the center of upper surface. The representative temperature signal-time curve is shown in Fig. 4a. It is seen that the sample undergoes two stages of temperature rise. Once the laser pulse irradiates, sample temperature increases rapidly and then stabilizes at 15-80 ms, forming the first temperature rise stage. Afterwards, the sample temperature continues to increase 15

slowly and levels off beyond 550 ms, forming the second temperature rise stage. Such two stepwise temperature rise is also detected for graphene-containing layered composite film in previous report [51]. The first temperature rise is described to the high-loading BN platelets, which form an oriented percolative network for fast heat conduction. The second temperature rise arises from the ANF framework, which transfers heat slowly in relative to the BN network. The thermal diffusivity of the two networks is determined by careful mathematical fitting of the two temperature rises, as show in Fig. 4b. It is seen that the thermal diffusivity of BN network increases linearly from 124.5 mm2 s-1 for BN/ANF-X 40/60 to 174.3 mm2 s-1 for BN/ANF-X 70/30. The increase is probably due to an improved contact between BN platelets at high loading. Differently, the thermal diffusivity of ANF framework always maintains a low value of about 6.2 mm2 s-1 with little change. The thermal diffusivity value is reasonable because pure ANF film has a measured in-plane thermal diffusivity of 5.33 mm2 s-1 and a calculated in-plane thermal conductivity of 7.33 W m-1 K-1. The in-plane thermally conductivity of ANF is obviously higher than normal polymers (～ 0.20 W m-1 K-1). In the composite film, the thermal conductive ANF would generate low heat resistance between BN platelets, thus facilitating the formation of percolative BN network. We assume that the BN network and ANF framework contribute to the overall thermal conductivity of BN/ANF-X film independently [52,53]. The overall thermal conductivity can be calculated based on a parallel model, =

(1)

+ 16

where KBN and KANF-X are the thermal conductivities of BN percolative network and ANF framework, respectively. KBN is calculated based on the specific heat capacity of BN (CBN), thermal diffusivity (αBN) and density (ρBN) of the BN network [54], as follows =



(2)



Similarly, KANF-X is calculated based on the specific heat capacity of ANF (CANF), thermal diffusivity (αANF-X), and density (ρANF-X) of the ANF framework, as follows =



(3)



The calculated overall in-plane thermal conductivity is shown in Fig. 4c (red squares). The value depends strongly on the ratio of BN to ANF-X. With the ratio increasing from 40/60 to 70/30, the in-plane thermal conductivity is boosted from 47.4 W m-1 K-1 to 122.5 W m-1 K-1. For the BN/ANF-X 50/50 film with largest ductility and highest toughness, its in-plane thermal conductivity is 64.1 W m-1 K-1. Due to remarkable ductility, cyclic bending at a curvature of 1 mm-1 for 10000 cycles does not decrease its thermal conductivity (Fig. S8). The through-plane thermal conduction property of the BN/ANF-X films is also measured with the laser flash approach. A laser pulse radiates the center of lower surface of the sample. Simultaneously, the temperature rise at the center of upper surface was detected. Only one stage of temperature rise occurs, which is different from in-plane temperature evolution. This is due to that BN platelets are microscopically separated by ANF network layers and do not form a percolative network in through-plane direction (Fig. 1i). The through-plane thermal conductivity 17

is calculated based on the through-plane thermal diffusivity, specific heat capacity and density of the composite film. The calculated values increase from 2.0 W m-1 K-1 to 2.8 W m-1 K-1, slightly dependent on the weight ratio of BN to ANF-X (Fig. 4c). These values are much lower than those of in-plane thermal conductivity, indicating an anisotropic thermal conduction. The ratios of in-plane and through-plane thermal conductivities are in the range of 24 and 44. To clarify the level of thermal conduction, we compare the in-plane thermal conductivity of BN/ANF-X film with that of recently reported high-loading BN-based composites. The reported high-loading BN-based composites are divided into BN/polymer (Fig. 4d, yellow columns) [20,24,55-58] and BN/nanoparticle (Fig. 4d, blue columns) [23,25]. It is seen that the BN/ANF-X 50/50 is obviously superior to BN/polymer composites with similar filler loading because BN platelets separated by polymers have lower contact probability to form percolative network. The thermal conductivity of BN/ANF-X 50/50 is even comparable to that of 95 wt% BN/TPU composite. Moreover, the thermal conductivity surpasses the two BN/nanoparticle references (95 wt% BN/GO and 70 wt% BN/CNF), probably because of large-size BN platelets used in this work (2-10 µm) and relatively high thermal conductivity of ANF. More importantly, its ductility is 8-165 times higher than these high-loading BN-based references (Table S3). These comparisons confirm that the BN/ANF-X 50/50 integrates high thermal conduction ability with large deformability. The

thermal

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thermogravimetric analysis (TGA), as shown in Fig. 4e. Its initial decomposition 18

temperature is as high as 515 oC. The excellent thermal stability arises from the inherent nature of its component materials. BN platelet has no weight loss until 700 o

C, and ANF begins to decompose at 509 oC. In addition, the BN/ANF-X films have

good electrical insulation property. Their volume resistivity is in the range of 8.7 × 1014 and 1.1 × 1016 Ω cm, slightly dependent on the BN loading (Fig. 4f). The high resistivity is also attributed to the inherent nature of the BN and ANF components,

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Fig. 4. Thermally conductive and electrically insulating properties of BN/ANF-X films. a) Representative in-plane temperature evolution of BN/ANF-X film at different times after pulsed laser heating. b) Dependence of in-plane thermal diffusivity of BN percolative network and ANF framework on the weight ratio of BN to ANF-X. c) Dependence of in-plane and through-plane thermal conductivities of BN/ANF-X films on the weight ratio of BN to ANF-X. d) Comparison of thermal conductivity of the BN/ANF-X film with that of reported high-loading BN-based composites. e) TGA curves of BN, ANF and BN/ANF-X 60/40. f) Volume resistivity 19

of BN/ANF-X films with different ratios of BN to ANF-X.

With large ductility, excellent thermal conduction ability, high thermal stability and good electrical insulating property, the BN/ANF-X film has promising application in the thermal management of flexible electronic devices. We demonstrate its application as flexible substrate for cooling LED chip, as shown in Fig. 5. A 10 W LED chip is attached to the BN/ANF-X film and connected to a direct-current power of 10 V (Fig. 5d). The surface temperature of the LED chip is monitored with an infrared thermal imager until it levels off. The infrared thermal images are compared with those of the ANF-supported LED chip (Fig. 5a-c). Their temperature evolution curves with working time is shown in Fig. 5e. It is seen that BN/ANF-X film with excellent thermal conductivity has better ability to cool LED chip than ANF film. The hot-spot temperature of ANF film-supported LED chip is stabilized in 180 s to be 103 o

C. Differently, the LED chip supported by BN/ANF-X 50/50 and BN/ANF-X 70/30

reaches stable hot-spot temperature of 78 oC and 67 oC in 120 s, which is 25 oC and 36 oC lower than that of ANF film-supported LED chip. The lowered hot-spot temperature is significant, because intensive heat severely degrades the luminous efficiency and lifetime of LED devices [59]. For every 10 oC drop in temperature, the luminous efficiency would increase by about 5% and the lifetime of LED would increase by half.

20

Fig. 5. Application demonstration of BN/ANF-X films for cooling LED chip. a-c) Infrared thermal images of a LED chip attached to ANF, BN/ANF-X 50/50, BN/ANF-X 70/30 substrates. d) The photograph of the LED chip attached to BN/ANF-X 50/50. e) Temperature evolution curves of the LED chip with working time.

Finally, we extend the sol-gel-film transformation process to a continuous mode for fabricating BN/ANF-X tape. A setup is constructed, which mainly includes a syringe pump, die, water tank and conveyor (Fig. 6a). The structure of the die is shown in Fig. 6b. The viscous sol containing BN and ANF is extruded from syringe to the die with a rectangular gap (30 mm × 0.5 mm) at a speed of 2 ml min-1, and then to water tank. Once the sol contacts with water, it fast gelates because water exchanges with the dimethyl sulfoxide solvent and causes the re-protonation of ANF. In appearance, the orange sol was gradually changed to a continuous, white hydrogel. The hydrogel is shown in Fig. 6c. Finally, the hydrogel is transferred by the conveyor belt for drying, forming a continuous BN/ANF-X tape with a width of 25 mm and a 21

thickness of 9 µm. The width and thickness can be conveniently adjusted by altering the rectangular gap size of the die. The BN/ANF-X tape is ductile, which is demonstrated by twisting it with hands into a fiber without fracture (Fig. 6d).

Fig. 6. Continuous fabrication of BN/ANF-X tape. a) A designed setup, including syringe pump, die, water tank and conveyor. b) Structure of the die. c) A continuous BN/ANF hydrogel. d) A dried BN/ANF-X tape. Red arrow points to a fiber that is twisted from a BN/ANF-X 50/50 tape.

4. Conclusions In conclusion, BN/ANF-X films with 40-70% BN platelets are fabricated by a sol-gel-film transformation process. This process leads to a nacre-like layered composite structure, in which ANF forms a 3D framework and BN platelets are embedded into the framework along in-plane direction. As a result, the structured BN/ANF-X films show the integration of large ductility, high toughness, excellent thermal conductivity and good thermal stability, which is rarely achieved in previous 22

BN/polymer composite films. Additionally, their continuous fabrication is demonstrated based on a designed setup. The outstanding multi-property combination and continuous preparation process make the electrically insulating BN composite films promising as a flexible conductive board for cooling electronic components.

Acknowledgements The authors acknowledge the financial support from the High-level Innovative Talent Project in Hunan Province (2018RS3055), National Natural Science Foundation of China (51973054, 21603264, 21975281), CAS Pioneer Hundred Talents Program (J. Di) and the Fundamental Research Funds for the Central Universities.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: