High-Thermal-Transport-Channel Construction within Flexible

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High Thermal Transport Channel Construction within Flexible Composites via Welding Boron Nitride Nanosheets Xiao Hou, Yapeng Chen, Le Lv, Wen Dai, Su Zhao, Zhongwei Wang, Li Fu, Cheng-Te Lin, Nan Jiang, and J. H. Yu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01939 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 5, 2019

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ACS Applied Nano Materials

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High Thermal Transport Channel Construction within Flexible

2

Composites via Welding Boron Nitride Nanosheets

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Xiao Hou,

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Lin,*† Nan Jiang, *† Jinhong Yu*†

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†

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Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering,

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Chinese Academy of Sciences, Ningbo, 315201, China.

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‡

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Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201,

†♯

Yapeng Chen,

†♯

Le Lv,

†

Wen Dai,

†

Su Zhao, ‡ Zhongwei Wang, ║ Li Fu, ⊥ Cheng-Te

Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine

Zhejiang Provincial Key Laboratory of Robotics and Intelligent Manufacturing Equipment Technology,

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China.

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║

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Qingdao, 266590, China.

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⊥

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310018, China.

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*Corresponding

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[email protected].

17

♯

Shandong University of Science and Technology, College of Materials Science and Engineering,

College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou,

author,

E-mail:

[email protected];

[email protected];

These authors contributed equally to this study.

18 19

Abstract: Efficient heat dissipation is a perquisite for further improving the integration of devices.

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However, the polymer composites are not satisfied heat dissipation. For that reason, high thermal transport

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channels were manufactured by direct freezing method, and further weld the BNNS by carbonization.

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Composites with high thermal conductivity (7.46 W m-1 K-1) were obtained by immersed in PDMS.

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Thermal conductivity enhancement of composites was reached about 3900% at 15.8 vol% loading of

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BNNS. Besides, the composites maintained structural flexibility of PDMS and allowed bending and

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twisting repeatedly. In addition, the PDMS composites exhibit excellent antistatic properties due to

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conductive network formed by residual carbon. Therefore, the dust can be avoided and the surface is kept

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clean. It provides a better choice for thermal management materials and meet the antistatic requirements

ACS Paragon Plus Environment

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of the devices.

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Key words: boron nitride nanosheets; thermal conductivity; antistatic; composites; PDMS

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1. Introduction

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With high integration and high power of devices, overheating become a key issue to resolve for the

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devices and equipment. Excessive heat accumulation would affect the lifetime and reliability of devices.

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1-5

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amorphous nature of polymer materials without effective thermal conductive pathways. Thus,

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constructing thermal transport channel for polymer composites was the key to improving the capacity of

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heat dissipation. The addition of inorganic fillers such as Al2O3, 6-8 SiC, 9-11 AlN, 12, 13 and BN 14, 15 is an

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effective method for enhancing thermal conductivity and maintaining insulation. However, even in high

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filler loading, there are not significantly enhanced in thermal conductivity.

Therefore, the heat transfer capacity of materials must be considered in application. The result of

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Boron nitride nanosheets (BNNS) is composed of hexagonal boron nitride unit, which was slightly

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similar to the structure of graphene. The intrinsic high thermal conductivity and low dielectric constant

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attracted the interest of researchers.

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arrangement, which guarantees the BNNS possess excellent thermal conductivity.

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composites were fabricated and showing outstanding thermal management performance according to

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previous reported.

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devices.

28-32

16-24

The high transfer of phonons benefits from regular atomic 25-27

Many polymer

But, the heat dissipated composites still long way to meet the requirements of

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Polydimethylsiloxane (PDMS) shows outstanding performance such as structural flexibility,

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compatibility, thermal and chemical stability, because of the unique molecular chain structure. That is

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important to be utilized in many fruitful applications.

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PDMS based composites restrict its application of thermal management. In addition, a certain problem

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exists in polymer materials, which the surface easy trapping dust, caused by accumulating charges on the

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surface of materials. 34-36 It would destroy the capability of devices and even causing fires and explosion,

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therefore antistatic properties of materials are seriously considered.

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33

However, inadequate thermal conductivity of

In this work, we focus on the high thermal conductive composites with highly flexible and excellent


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antistatic properties by welding BNNS construct effective phonon transmission channel in PDMS matrix.

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First, BNNS foam was prepared by traditional directional freezing technology and unique carbonization

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process, which play a crucial role in maintaining the physical structure and dissipate static charges. And

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then, high thermal conductivity composites were prepared by immersed in PDMS. The prepared 3D-

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BNNS/PDMS composites first achieved ultra-high thermal conductivity (7.46 W m-1 K-1) at low fillers

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addition in PDMS, and the thermal conductivity enhancement was reached 3900 %. Furthermore, the 3D-

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BNNS/PDMS composites maintain highly flexible and be endowed with excellent antistatic properties.

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There are potentials for thermal management applications of electrical equipment with antistatic

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requirements in many fields. The proposed constructing thermal transfer channels by welding BNNS can

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be considered as an excellent solution for heat dissipation.

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2. Experimental

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2.1 Materials

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Chitosan (deacetylation degree, 95%; viscosity≤200 mPa·s) were obtained from Aladdin Reagent Co.,

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Ltd, (Shanghai, China), and acetic acid (≥99.8%) were purchased from Sinopharm Chemical Reagent Co.,

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Ltd. (Shanghai, China). Boron nitride nanosheets (BNNS, with lateral size of 2-20 µm and thickness about

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5 nm) was purchased from Lida Chuangxing Materials Technology Co., Ltd. (Tianjin, China). PDMS

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(Sylgard 184) was obtained by Dow Corning Co., Ltd. (Shanghai, China).

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2.2 Fabrication of 3D-BNNS foam

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The schematic illustrating of prepared BNNS foam was shown in Fig. 1(a). The BNNS/Chitosan

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dispersion was prepared by mixing certain amount of BNNS with chitosan (1.0 wt%) and acetic acid (1.0

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wt%) aqueous solution. The acetic acid could improve the dispersion of chitosan, and low chitosan and

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acetic acid addition were chosen for decrease the impact. The anisotropic BNNS foam was fabricated by

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directional freezing of BNNS/Chitosan dispersion, then vacuum freeze dryer sublimate the ice.

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brief, the BNNS/Chitosan dispersion was poured on the surface of copper block which was surrounded by

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polyethylene foam, and the bottom contact with liquid-nitrogen. The model diagram was shown in Fig.

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S1. After BNNS/Chitosan dispersion was frozen completely, the sample was moved into freeze dryer

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more than 48 h at -50 °C with vacuum degree of 30 Pa. Digital pictures of the preparation process were


37, 38

In


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shown in Fig. 1(b), the anisotropy and ordered structure are visible to the naked eye. The sample was

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transferred into a quartz tube furnace for carbonization at 800 oC for 0.5 h.

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Fig. 1 (a) Schematic illustrating the preparation of BNNS foam and (b) digital pictures of BNNS

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foam preparation process.

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2.3 Preparation of 3D-BNNS/PDMS composites

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The 3D-BNNS/PDMS composites were prepared by immerse PDMS with vacuum-assisted. First,

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the prepolymer of PDMS and curing agent were controlled about 10:1 and the mixing uniformity

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at room temperature. Then, the BNNS foam was immersed into the mixture, and sample was

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moved air by vacuum oven for more than 8 h to remove the air. Finally, the sample was cured at

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80 oC for 5 h. The random-BNNS/PDMS composites were also prepared by mixing the BNNS

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with prepolymer/curing agent mixture and the curing process was same as previous.

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2.4 Characterization

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The microstructure images of BNNS and BNNS foam were obtained by scanning electron microscopy

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(SEM, Quanta FEG250, USA). The thickness of BNNS was measured by atom force micrographs (AFM,

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Veeco, USA). The spectra of Fourier transform infrared (FTIR) was obtained by Nicolet 6700 (Thermo,

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USA). The spectra of X-ray diffraction (XRD) was obtained by D8 Advanced (Bruker, Germany). The

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spectra of Raman were obtained by Reflex Raman System (Renishaw, UK). Thermal conductivity (λ, W


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m-1 K-1) was calculated using the equation below:

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λ = ρ ∙ CP ∙ α

(1)

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Where ρ, Cp and α refer to density (g cm-1), specific heat (J g-1 K-1) and thermal diffusivity (α, mm2 s-1)

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of the composites. The density was measured by water displacement. The specific heat and thermal

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diffusivity were measured using LFA 467 Nanoflash (Netzsch, Germany). The infrared thermal images

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were captured by infrared camera (Fluke, USA). Thermal gravimetric analysis (TGA) were taken by TGA

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209 F3 (Netzsch, Germany) in nitrogen atmosphere at heating rate of 10 oC min-1. The surface resistivity

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(ρs) and volume resistivity (ρv) was investigated by Keithley 6517B Electrometer (Tektronix, USA).

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3. Results and discussion

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3.1 Characterizations of BNNS

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Fig. 2 (a) SEM image and (b) the size distribution of BNNS; (c) The AFM image and (d) the

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thickness distribution of BNNS.

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As is shown in Fig. 2(a), the flake morphology of the BNNS was observed. And lateral sizes are

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approximately 7 μm, which were calculated according to the SEM. In order to accurately analyze the

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lateral sizes of raw materials, the size distribution from 120 pieces observed by SEM analysis was shown

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in Fig. 2(b). The statistical analysis indicates the majority of BNNS was in the range of 6-10 μm. Fig. 2(c)

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shows the AFM image and thickness curve of the BNNS, and the thickness was found to be approximately



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5.1 nm. In addition, the distribution of thickness from 120 pieces by analysis AFM were presented in Fig.

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2 (d), the majority of pieces was in the range of 3-7 nm.

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3.2 Microstructure of 3D-BNNS/PDMS composites

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The enhancement of thermal conductivity was resulted by the addition of BNNS and structure

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of BNNS foam. Depending on the welding of residual carbon, the foam maintained the ordered

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structure, which same as before carbonization (Fig. S2). So that, various BNNS foam was prepared

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by control addition. The morphology of vertical to the ice growth of BNNS foam were shown in

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Fig. 3 (a-d). It can be seen that the BNNS walls was built along the ice growth, and the adjacent

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walls was connected. As shown in Fig. 3 (a), highly ordered microstructure can be seen and the

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distance of adjacent BNNS wall was about 50 μm, when the addition of BNNS is 3.0 vol%. With

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the increased of BNNS, the BNNS foam could maintain the ordered microstructure, but the cells

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were thinner and the walls were thicker.

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Fig. 3 Cross-section SEM images of BNNS foam and 3D-BNNS/PDMS composites with different

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BNNS volume fractions: (a) (e) 3.0 vol%; (b) (f) 7.3 vol%; (c) (g) 10 vol%; (d) (h)15.8 vol%.

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Due to obtained BNNS foam was porous structure, the PDMS prepolymer/curing agent mixture

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can be introduced into these holes, easily. After a period of curing, the 3D-BNNS/PDMS

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composites can be obtained. The Fig. 3 (e-h) presents the typical cross-section SEM images of 3D-

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BNNS/PDMS composites. The ordered microstructures same as the BNNS foam can be observed,

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and no pores exist in the field of vision, indicates that the PDMS was complete infiltration. The


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EDS detection was shown in Fig. S3 reveals the distribution of N and C elements in composites.

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With the SEM images of composites, it gives further evidence of the composites maintain the

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microstructure of BNNS foam. For distribution of C elements in composites, indicating that

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residual carbon was coated on BNNS surface. The residual carbon plays a critical role in welding

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adjacent BNNS, but the foam could not be obtained by BNNS alone under the same condition. 39

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The FTIR, XRD and Raman spectra were investigated the BNNS and residual carbon after

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carbonization (shown in Fig.S4). In FTIR spectra of BNNS and 3D-BNNS (Fig. S4 (a)), the peak

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at 813 cm-1 and absorption at 1380 cm−1 are the characteristic peak of BN.

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absorption at 3200 cm−1 appeared in FTIR spectrum of BNNS, and disappeared after treated at

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high temperature. That should be the hydroxyl groups (-OH) vibration, caused by surface defects

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according our previous work.

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were investigated.

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indicates the carbonization process did not destroy the crystal structure of BNNS. According to

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the Raman spectra of 3D-BNNS and BNNS in Fig. S4 (c), amorphous carbon exists in 3D-BNNS.

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It can be considered as the residual carbon of chitosan after carbonization. It attached the surface

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of BNNS and welding adjacent BNNS to maintain the porous structure.

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3.3 Thermal management capability of 3D-BNNS/PDMS composites

42, 43

25, 41

40

It is note that the

As shown in Fig. S4 (b), BNNS powders characteristic peaks

The 3D-BNNS exhibits the same distinct characteristic peaks as BNNS,

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Fig. 4 (a) shows the through-plane thermal conductivity of 3D-BNNS/PDMS composites with

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various filler loading. Thermal conductivity of pure-PDMS and random-BNNS/PDMS composites

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with the same addition of BNNS were provided for comparison. Because of its amorphous

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structure and resulting serious the phonon scattering, the thermal conductivity of pure-PDMS is

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very poor (about 0.185 W m-1 K-1). That result was agreement with previous results. 28, 39,44 It can

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be seen that a significant enhancement of thermal conductivity resulted by 3D-BNNS structure

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compared with the random-BNNS composites. In other word, the high thermal transport channels

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were successfully constructed in polymer composites. The thermal conductivity reached 7.46 W m-1

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K-1 at room temperature, when the addition of BNNS was 15.8 vol%.



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Fig. 4 (a) Thermal conductivity of PDMS composites and (b) thermal conductivity enhancement;

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(c) The stability test of thermal conductivity for 3D-BNNS/PDMS composites with 15.8 vol%

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filler and pure-PDMS; (d)The reported data of PDMS based composites filled with below 30 vol%.

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In order to investigated the inherent factors to enhanced thermal conductivity of composites, Y.

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Agari equation was adopted to calculate the effective thermal conductive chains. 45 The Y. Agari

7

equation:

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lg Kc = ∅ ∙ C2 ∙ lg Kf + (1 - ∅) ∙ lg (C1 ∙ Km)

(2)

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Where the Kc, Kf and Km are the thermal conductivity of polymer composites, fillers and polymer

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matrix. In this article the Kf and Km is 390 W m-1 K-1 and 0.185 W m-1 K-1, respectively. 28, 46 The

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ϕ is the volume fraction of fillers. In the model of Y. Agari, the index C1 was measure the thermal

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conductivity caused by crystallinity and crystal size change of polymer matrix after filler added.

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However, this effect can be neglected because of PDMS’ disordered chains and the index C1 can

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be considered as 1 according to previous research.

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The ease of forming thermal conductive


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chains was indicated by C2. The higher value of C2, the much easier for conductive chains

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formation. According to the Y. Agari’s theory, the value of C2 is between 0 and 1. In this work,

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the value of C2 can be considered as 1, and the predicted thermal conductivity data by Y. Agari

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equation was shown in the Fig. 4 (a). The predicted thermal conductivity is slightly lower than the

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measurement of random-BNNS/PDMS composites. Modify the equation of Y. Agari, the value of C2

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can be considered as the measurement of effective thermal conductive chains at the same volume

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fraction. The higher C2 value indicates more effective thermal conductive chains. The calculated

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value of C2 was about 3.64, which uses the equation to fit the measurement data of 3D-

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BNNS/PDMS composites. The result of calculation much higher than the value of C2 in the

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random-BNNS/PDMS composites (about 1.48). It indicates that phonon scattering was reduced

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benefit from welding adjacent BNNS, more effective phonon transmission channels were built in

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composite. Due to these channels in composite, heat can transfer quickly (like a rabbit). In contrast,

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randomly dispersed BNNS cannot form effective phonon conductive channels, and the phonon

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will scatter at the interfaces between the BNNS and PDMS matrix, the heat would be dissipated

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in the internal of composites. Therefore, the random-BNNS/PDMS composite shows poor heat

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dissipation (like turtle). In addition, the thermal conductive comparison of 3D-BNNS/PDMS

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without carbonization was shown in Fig. S5. The thermal conductivity was significantly enhanced

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after carbonization.

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The thermal conductivity enhancement was shown in Fig. 4 (b), 3D-BNNS/PDMS composite was

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about 3900% enhanced in the pure-PDMS at 15.8 vol%. The enhancement of thermal conductivity

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was calculated by equation:

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η=

Kc - Km Km

(3)

23

Fig. 4 (c) shows the thermal conductive stability test of 3D-BNNS/PDMS composites (15.6 vol%) and

24

pure-PDMS. Obviously, the thermal conductivity has slightly change in 15 cycles, it shows that the

25

composites has stable heat dissipation ability in the temperature range from 25 to 125 oC. The stable

26

thermal conductivity in cooling and heating cycles would ensure materials are reliable and effective.

27

Thermal stability is an important performance in the materials application, which determined the



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maximum operating temperature of composites. The TGA data of various 3D-BNNS/PDMS composites

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was presented in Fig. S6. It reveals that the thermal stability of composites was enhanced with the

3

addition of BNNS. The parallel burning tests were shown in Fig. S7 and Movie S1. The 3D-

4

BNNS/PDMS composite displays fire retardant properties benefits from the improvement of

5

thermal stability. The tested composite can automatically extinguish and maintain the original

6

shape. However, pure-PDMS burns rapidly and becomes white dust.

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For illustrating the superiority of 3D-BNNS network by welding the BNNS, Fig. 4 (d) summarizes

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previous reported thermal conductivity of PDMS based composites (filler ≤ 30 vol%). 8, 29, 39, 44, 47-58 The

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3D-BNNS/PDMS composites by welding the BNNS exhibits the extremely thermal conductivity with

10

similar filler loading or even more. That suggests that built thermal transport channels by directional

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freezing and carbonization is an effective way to improve the thermal conductivity of PDMS based

12

composites.

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In order to demonstrate the capability of thermal conductivity composites, the temperature of

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surface variation in the heating and cooling was inspected by infrared thermal images during

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heating. The samples of pure-PDMS, random-BNNS/PDMS, 3D-BNNS/PDMS composites have

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same dimension and thickness (about 1.0 mm) for ensure accuracy, and placed on the surface of

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ceramic heater. The optical picture and thermal images of the heating process were shown in Fig.

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5 (a). One can see that the surface temperature of all composites continuously increased with time. Due

19

to the high thermal conductivity, 3D-BNNS/PDMS composites have highest surface temperature

20

with the same time. In Fig. 5 (b), the surface temperature of 3D-BNNS/PDMS composites is about

21

8 oC higher than random-BNNS/PDMS, and 10 oC higher than pure-PDMS. The result indicates

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that the 3D-BNNS/PDMS composites have outstanding thermal absorbing capability. To

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investigate heat dissipation performance, the surface temperature of pure-PDMS, random-

24

BNNS/PDMS, 3D-BNNS/PDMS composites was inspected in the cooling process. Fig. 5 (c) and

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(d) shows the various surface temperature and the infrared images of the cooling process. The samples

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were cooling down from different temperature, gradually. The surface temperature of 3D-BNNS/PDMS

27

composites distance with other materials gradually decreases, even though beginning at high


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1


temperature.

2 3

Fig. 5 (a) Optical and heating infrared thermal images; (b) Surface temperature curves with heating

4

process from room temperature; (c) Surface temperature curves in cooling time and (d) cooling

5

infrared thermal images of pure-PDMS, random-BNNS/PDMS, 3D-BNNS/PDMS composites

6

(sample size: 1.0 × 1.0 cm2).

7

3.4 Antistatic Performance of 3D-BNNS/PDMS composites

8

The surface resistivity (ρs) and volume resistivity (ρv) of pure PDMS, random-PDMS and 3D-

9

BNNS/PDMS composites were shown in the Fig. 6 (a) and (b). Due to the residual carbon in 3D-

10

BNNS/PDMS composites, the ρs and ρv are decreased, respectively. The ρv of the pure-PDMS was

11

1.85×1012 Ω·cm, corresponding to the range of insulator. However, the ρv was decreased below

12

109 Ω·cm after carbonization, which corresponding to the range of semiconductor.

13

reveals that a conductive network was formed by residual carbon in the PDMS matrix. It was noted

14

that excellent antistatic properties of composites endue with this slightly conductive network. The

15

conductive network decreased the surface resistance, and enhanced the antistatic performance.

16

The ρs with a lower than 1012 Ω could exhibit a certain degree of antistatic effect, and when the

17

surface resistivity is below 109 Ω, the substrate would show excellent antistatic properties. 36 It can


35

The result


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be seen that the ρs of 3D-BNNS/PDMS composites were increased with filler loading, and the ρs

2

was 9.42×108 Ω (at 15.8 vol%) less than 109 Ω. In addition, the comparision of ρs and ρv of 3D-

3

BNNS without carbonization and 3D-BNNS (carbonization) was shown in Fig. S8. The ρs and ρv

4

was decreased after carbonization. Therefore, the 3D-BNNS/PDMS composites exhibit excellent

5

antistatic property after carbonization process.

6 7

Fig. 6 (a) Surface resistivity, (b) volume resistivity, (c) digital image before electrostatic adsorption

8

and (d) after electrostatic adsorption, (e) schematic diagram of static dissipation of pure PDMS, random-

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BNNS/PDMS and 3D-BNNS/PDMS composites (sample size: 10 ×10 cm2).

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For characterization of the antistatic properties of composites more directly an ingenious

11

experiment was designed. The digital images of pure PDMS, random-PDMS and 3D-BNNS/PDMS

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composites were hung up (Fig. 6 (c)). Three samples were rubbed down with a towel and placed into the

13

box full of polystyrene microspheres, then gently pull to vertical and the polystyrene microspheres are

14

adsorbed on the surface. One can see that the sample of random-BNNS/PDMS composites adsorbed the

15

most polystyrene microspheres compared with the other samples, and the 3D-BNNS/PDMS composites


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only absorbed a few microspheres. As a result of high surface resistivity, larger number of charges

2

aggregate on the surface of random-BNNS/PDMS composites. In contrast, the 3D-BNNS/PDMS

3

composites can quickly dissipation static charge accumulation by the conductive network formed by

4

residual carbon. Fig. 6 (d) illuminate the mechanism of antistatic. In the pure-PDMS and random-

5

BNNS/PDMS composites, because amorphous nature of PDMS without effective conductive

6

pathway, number of static charges would aggregate on the surface after rubdown with a towel. Owning

7

to loading of boron nitride, this aggregation is more serious. Thanks to the amorphous carbon forming

8

conductive pathway, the static charge can be quickly dissipated, therefore the material surface should be

9

clean. Meanwhile, the 3D-BNNS/PDMS composites maintain ideal flexibility. It can maintain

10

structural integration after repeated bending and twisting (Fig. S9).

11

4. Conclusions

12

In summary, high thermal transport channels were constructed in PDMS composites by welding the

13

BNNS in carbonization process. The flexible PDMS composites had first achieved ultra-high thermal

14

conductivity of 7.46 W m-1 K-1 at low fillers loading (15.8 vol%) in PDMS, which about 3900% in

15

enhancement of thermal conductivity. The experiments data indicated that combination of directional

16

freezing and welding BNNS by carbonization can significantly enhance thermal conductivity of PDMS

17

based composites. Besides, the method would give the PDMS based composites excellent antistatic that

18

can be utilized in many industrial fields.

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Associated Content

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Supporting Information

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The model of directional freezing process; SEM-EDS of 3D-BNNS/PDMS composites (3.0 vol%);

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FTIR, XRD and Raman spectra of BNNS and 3D-BNNS; TGA curves of 3D-BNNS/PDMS composites;

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Digital images of pure-PDMS and 3D-BNNS/PDMS composite on flame; The digital images of 3D-

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BNNS/PDMS composites (15.8 vol%) with the state of twisting and bending states. The comprasion of

25

thermal conductvity, surface resistivity, volume resistivity of pure PDMS and 3D-BNNS/PDMS

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composites (before and after carbonization). Density of various composites. (PDF)

27

Process of pure-PDMS and 3D-BNNS/PDMS composite on flame. (AVI)



1

This material is available free of charge via the Internet at http://pubs.acs.org.

2

Author Information

3

Corresponding Authors

4

* E-mail: [email protected];

5

* E-mail: [email protected];

6

* E-mail: [email protected].

7 8 9

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Notes The authors declare no competing financial interest. Acknowledgments

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The authors are grateful for the financial support by the National Natural Science Foundation of China

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(51573201), NSFC-Zhejiang Joint Fund for the Integration of Industrialization and Informatization

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(U1709205), Public Welfare Project of Zhejiang Province (2016C31026), The Scientific Instrument

13

Developing Project of the Chinese Academy of Sciences (YZ201640), the Project of the Chinese Academy

14

of Sciences (KFZD-SW-409), and the Science and Technology Major Project of Ningbo (2016S1002 and

15

2016B10038). We also thank the Chinese Academy of Sciences for the Hundred Talents Program, the

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Chinese Central Government for the Thousand Young Talents Program, 3315 Program of Ningbo.

17

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