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Highly Thermally Conductive Composite Papers Prepared Based on the Thought of Bioinspired Engineering Yimin Yao, Xiaoliang Zeng, Rong Sun, Jian-bin Xu, and Ching-Ping Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04636 • Publication Date (Web): 02 Jun 2016 Downloaded from http://pubs.acs.org on June 7, 2016

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ACS Applied Materials & Interfaces

Highly Thermally Conductive Composite Papers Prepared Based on the Thought of Bioinspired Engineering Yimin Yao, †,‡,§ Xiaoliang Zeng, †,‡,§ Rong Sun, *, ‡ Jian-Bin Xu, ║ Ching-Ping Wong‡,║, ┴ ‡

Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen, China.

§

Shenzhen College of Advanced Technology, University of Chinese Academy of Sciences,

Shenzhen, China. ║

Department of Electronics Engineering, The Chinese University of Hong Kong, Hong Kong,

China. ┴

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA

30332, United States. *

Address corresponding to [email protected]

ABSTRACT

The rapid development of modern electronics and three-dimensional integration sets stringent requirements for efficient heat removal of thermal-management materials to ensure the long lifetime of the electronics. However, conventional polymer composites that have been used

ACS Paragon Plus Environment

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widely as thermal-management materials, suffer from undesired thermal conductivity lower than 10 W m−1K−1. In this work, we report a novel thermally conductive composite paper based on the thought of bioinspired engineering. The advantage of the bioinspired papers over conventional composites lies in that they possess a very high in-plane thermal conductivity up to 21.7 W m−1K−1, along with good mechanical properties and high electrical insulation. We attribute the high thermal conductivity to the improved interfacial interaction between assembled components through the introduction of silver nanoparticles, and the oriented structure based on boron nitride nanosheets and silicon carbide nanowires. This thought based on bioinspired engineering provides a creative opportunity for design and fabrication of novel thermally conductive materials, and this kind of composite paper has potential applications in powerful integrated microelectronics. KEYWORDS: bioinspired engineering, thermal conductivity, composite paper, boron nitride, silicon carbide


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1. INTRODUCTION The continuing miniaturization of size and increased power densities in modern electronics are setting higher and stricter requirements for thermal management. Considering that the power density dissipated by electronics reaches more than 1000 W cm−2,1 it is commonly agreed that the limiting factor for the further advance of these devices is not the hardware itself, but rather the development of effective thermal-management materials.2 Conventional polymer composites have been currently used as thermal-management materials, but in general, they suffer from undesired thermal conductivity lower than 10 W m−1K−1, which hinders their practical application.3-5 Learning from biological material systems sometimes inspires the development of superior engineering solutions than that which conventional engineering approaches can offer. In the development of modern thermal-management materials, bioinspired engineering thus has become an emerging and futuristic thought.1, 6-15 For instance, the excellent thermal conductivity of spider silk makes people realize the importance of aligned micro-/nanostructures in governing thermal transport, and thus highly thermally conductive polyethylene nanofibers were fabricated.16-19 Previous studies of thermal-management materials have demonstrated the important roles of interface and well-designed orientation, in which the former reduces the interfacial thermal resistance, while the latter provides pathways for phonon conduction.4 At this point, natural nacre may provide another extensively attractive biological model for excellent thermal conduction due to its high-interaction interface between inorganic platelets and organic matrix together with the orderly layered structure.20-25. In our previous work, inspired by the microstructure of natural nacre, we have fabricated boron nitride nanosheets (BNNS)/polyvinyl alcohol (PVA) paper, which exhibits a relatively high thermal conductivity (6.9 W m−1K−1) due


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to the orderly orientation of BNNSs.26 Unfortunately, the thermal conductivity is still undesirable due to the weak interaction between BNNSs, which leads to large interfacial thermal resistance. It indicates that besides the orderly layered structure, the design of interface is urgently needed for further enhancement of thermal conductivity. In this study, under the guidance of bioinspired engineering thought, we designed a novel kind of composite paper with delicate interface and well-organized orientation. The composite paper was fabricated by a facile paper-making process based on two-dimensional (2D) BNNSs decorated with silver nanoparticles (AgNP), one-dimensional (1D) silicon carbide nanowires (SiCNW) decorated with AgNPs, and PVA. Specifically, AgNPs are introduced to mimic the mineral bridges in natural nacre between microplatelets from nanometer scale.23, 25 AgNPs are expected to pierce the PVA layer and link the BNNSs together,27, 28 resulting in the formation of more thermally conductive pathways inside the composite paper. In this way, energy dissipation through the network linked by AgNPs pervades through a larger area of the interface, rather than only at the BNNS/BNNS interfaces. Moreover, the high intrinsic thermal conductivity of AgNPs also helps to improve the thermal conduction of the composite paper. On the other hand, 2D BNNSs, 1D SiCNWs, and PVA are mimicking the 2D aragonite plates, 1D nanofibrillar chitin, and protein in ternary natural nacre, respectively,22 where the 2D BNNSs and 1D SiCNWs network layers are alternately stacked and held together by PVA. The BNNS possessing wide energy band gap and high thermal conductivity is a perfect filler to fabricate electrically insulating composite papers with excellent thermal properties, where CNTs and graphene are not possessed of.26, 29, 30 The SiCNWs are chosen to further optimize the macroscopic network for thermal conduction. As a result, a flexible, tough and electrical insulating paper with an in-plane thermal conductivity of up to 21.7 W m−1K−1 is achieved, which is much higher than most of the


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reported oriented BN-containing composites or films. The efficient heat removal of our composite paper mainly origins from: (1) improved interfacial interaction between assembled components introduced by AgNPs, and (2) the oriented structure based on BNNSs and SiCNWs. In addition, the bioinspired thought also contributes to good mechanical properties. The tensile strength and Young’s modulus reaches 39.80 MPa, 11.50 GPa, respectively, which give rise to the tensile toughness of around 0.21 MJ m−3. Furthermore, the composite paper exhibits a high volume resistivity up to 8.29×1013 Ω cm. This thought reminds us the importance of learning from those superior thermal-management mechanisms of naturally evolved biological systems, and opens a new avenue for design and fabrication of high-performance thermal-management materials in the near future. 2. EXPERIMENTAL SECTION 2.1 Materials The h-BN powders with the average size of 10 µm were purchased from Denka Co.,Ltd., Japan. SiCNWs were offered by Changsha Sinet Advanced Materials Co.,Ltd, China. Coupling agent 3aminopropyltriethoxysilane (KH-550), silver nitrate (AgNO3), N-N’dimethylformamide (DMF), polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), dimethylbenzene, and sodium dodecyl benzene sulfonate (SDBS) were purchased from Sinopharm Chemical Reagent Co. Ltd, China. All reagents were analytical grade, and used as received. 2.2 Preparation of BNNSs and BNNS–Ag hybrid BNNSs were prepared by sonication-assisted solvent exfoliation. The initial 2.0 g of h-BN powders and 500 mL of DMF were mixed into a 500 ml beaker for further sonication. The time


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of sonication is optimized to be 48 h according to our previous report.31 The solution was then centrifuged at 1500 rpm for 15 minutes to remove aggregates, and finally 0.27 mg mL−1 of BNNS solution was obtained. BNNS–Ag hybrid was synthesized by reduction of Ag+ on the surface of BNNSs and subsequent collected by vacuum-assisted filtration. In a typical process, a 520 mL of BNNS suspension (0.27 mg mL−1) was transferred into a flask containing 80 mg of PVP heated to 62 °C by a water bath under stirring, and then 40 mL of mg mL−1 AgNO3 aqueous solution was added into the flask within one hour. The color of the solution turned to be golden yellow from white upon the formation of AgNPs. The obtained BNNS–Ag hybrid was collected by vacuum-assisted filtration using membrane with a 0.45 um pore size to remove dissociative AgNPs, followed by redispered in deionized water under stirring and sonication. 2.3 Preparation of SiCNW–Ag hybrid 0.5 g of SiCNWs, 0.05 g of SDBS and 200 mL of deionized water were put into a 500 mL sealed tank for ball-milling upon 300 rpm for 12 h. The solution was then centrifuged at 2000 rpm for 10 min to remove aggregates, and finally a 0.20 mg mL−1 of SiCNW solution was obtained. A certain volume of SiCNW solution was filtrated to get SiCNWs for further surface modification by silane coupling agent. The obtained SiCNWs and KH-550 coupling agent were added into 500 mL flask containing 300 mL of dimethylbenzene, which was equipped with a mechanical stirrer, and the resulting mixture was then stirred at 135 °C for 12 h. After cooling down to room temperature, the mixture was filtrated, and the gray wet cake was quickly washed by fresh ethanol. The product was redispersed in DMF under sonication. The following procedures were the same as the preparation of BNNS-Ag hybrid. 2.4 Fabrication of BNNS–Ag/SiCNW–Ag/PVA papers


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In a typical experiment, the obtained BNNS–Ag and SiCNW–Ag dispersions were successively added to the PVA solution at different weight ratios. The ratio is on the basic of the weight of BNNSs or SiCNWs, where AgNPs are considered as extra components and their weight are not involved into the calculation. According to our previous report on bioinspired BNNS/PVA paper,26 we controlled the mass fraction of PVA to be 5 wt%, resulting in integrated high thermal conductivity and good mechanical property. The mixtures were stirred and sonicated for 20 and 5 min, respectively, to form a homogeneous dispersion. With vacuum-assisted filtration, the homogeneous dispersions were assembled to papers. After filtration, specimens were dried in oven until they could be peeled off the membrane. The fabricating procedures of BNNS– Ag/PVA papers are the same as the fabrication of BNNS–Ag/SiCNW–Ag/PVA papers. 2.5 Characterization The morphology and microstructures of the samples were examined by a FEI Nova NanoSEM 450 field emission scanning electron microscopy (FE-SEM) and a FEI Tecnai G2 F20 transmission electron microscope (TEM). Dynamic light scattering (DLS) was utilized to determine the mean size of BNNSs in the aqueous suspension obtained by using a Mastersizer 3000 Laser Diffraction Particle Size Analyzer. X-ray photoelectron spectrometer (XPS) analysis was carried out on a Kratos Axis Ultra DLD with Al Kα radiation. The crystalline structure of BNNS–Ag and SiCNW–Ag were investigated by X-ray power diffraction (Rigaku, D/max2500Pc) with Cu Ka radiation (λ=1.5418 Å). The measurement of volume resistivity was carried out with a Keithley-6517B high-resistance meter having an applied AC voltage of 1000 V. The in-plane thermal conductivities of the papers were measured using LFA 467 NanoFlash apparatus (NETZSCH), calculated by


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‫ܥ · ߩ · ߮ = ܭ‬௣ (1) where, φ is the thermal diffusivity, ρ is the density of composite paper obtained according to ρ=m·V−1, where m and V are the mass and volume of the sample, respectively. Cp is the specific heat capacity obtained by differential scanning calorimetry (DSC, TA Q20) with the sapphire method. The through-plane thermal conductivities were measured by LW-9389 TIM Thermal Resistance and Conductivity Measurement Apparatus (Long Win Science & Technology, Taiwan) which is based on ASTM D 5470-06 Standard. The through-plane thermal conductivity λ was calculated by ொ·௅

ߣ = − ஺·௱் (2) herein, Q is the heat flux, L is the thickness of the sample, A is the area of the sample, and ∆T is the temperature difference between temperature sensors of the hot meter bar. Static uniaxial inplane tensile tests were conducted with a dynamic mechanical analyzer (DMA Q800, TA Instruments). The tensile strength was extracted from the curve just before failure. Young’s modulus (E) was obtained from the slope of the linear fit in the initial section of the curve (0.1% strain). The tensile toughness was calculated by integrating the area underneath the stress–strain curve. 3. RESULTS AND DISCUSSION


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Figure 1. Characterization of BNNS and BNNS–Ag. (a) Low-magnification TEM image of a slice of the BNNS. The inset is a HR-TEM image of folded edge of a BNNS. (b) SEM image of BNNS–Ag. Insets are digital photographs of BNNS and BNNS–Ag dispersions, respectively. (c) TEM image of BNNS–Ag. (d) XPS spectra of BNNS and BNNS–Ag. (e) Ag3d XPS spectrum of BNNS–Ag. (f) XRD patterns of BNNS and BNNS–Ag. Figure 1a shows a typical slice of the BNNS with the lateral size in the range of 500 nm–2 µm. The layer numbers of over 20 were counted from the folded edge using HR-TEM, corresponding to the thickness of approximately 7.0 nm. DLS (Figure S1, Supporting Information) analysis shows a mean BNNS diameter of about 1.5 µm. After decoration of AgNPs, the color of the BNNS dispersion turns to be golden yellow from milky white (inset of Figure 1b). The obtained BNNS–Ag hybrids possess good dispersity in water due to the abundant hydrophilic groups of PVP. The SEM and TEM images (Figure 1b,c) of BNNS-Ag show that most BNNSs are decorated with several AgNPs having a diameter between 20 and 40 nm. XPS detection of BNNS–Ag hybrid reveals that besides the characteristic peaks of BNNS (Figure 1d), the


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elemental scan of Ag 3d5/2 and Ag 3d3/2 core level peaks was centered at 367.6 eV and 373.6 eV, respectively (Figure 1e). Compared with the reported values of Ag 3d5/2 (367.9 eV) and Ag 3d3/2 (373.9 eV),32 both the peaks were shifted to lower binding energies by 0.3 eV, indicating interactions between Ag and BNNS. During vigorous sonication of h-BN, highly reactive defects such as vacancy defects, topological defects, and exposed edges were introduced on the surfaces of BNNSs.33-39 Thus, the exposed edges and vacancy defects of BNNSs acquire more functional groups such as hydroxyl groups (–OH) and amino groups (–NH2).30, 33, 40, 41 It is speculated that Ag+ would be absorbed on the surface of BNNSs through the complexation between –NH2 and Ag+ and the electrostatic attraction between –OH and Ag+, leading to the in-situ formation of primary Ag ‘seeds’.42-47 XRD patterns in Figure 1f also suggest the successful formation of AgNPs. Besides the characteristic peaks of h-BN, the peaks at 38.2°, 64.4°, 77.5° and 81.5° are attributed to the (111), (220), (331) and (222) crystal face of face-centered cubic (fcc) Ag, respectively.27,

44, 46, 48

The elemental contribution of BNNS–Ag hybrid further confirms the

existence of Ag element (Figure S2, Supporting Information).


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Figure 2. Characterization of SiCNW and SiCNW–Ag. (a) Low-magnification TEM image of a stick of the SiCNW. The inset is a SEM image of SiCNWs. (b) SEM image of SiCNW–Ag. Insets are digital photographs of dispersions of SiCNW and SiCNW–Ag, respectively. (c) TEM image of SiCNW–Ag. (d) XPS spectra of SiCNW and SiCNW–Ag. (e) Ag3d XPS spectrum of SiCNW–Ag. (f) XRD patterns of SiCNW and SiCNW–Ag. Figure 2a shows that the diameter of SiCNW is around 300 nm, with an aspect ratio of more than 30. The color of uniform SiCNW dispersion in water turns to be dark brown after the reduction of Ag+ (Figure 2b) due to larger diameter of AgNPs (50–80 nm). Figures 2b and 2c shows that a number of AgNPs have been attached tightly onto surface of SiCNWs. XPS spectra (Figure 2d,e) clearly reveal the presence of silver atoms instead of silver ions with binding energies of Ag 3d5/2 (367.7 eV) and 3d3/2 (373.8 eV), respectively. Small shifts of binding energies are observed for the interactions between Ag and SiCNW. Additional peaks at 29.6°, 44.5°, 64.8°, and 81.2° in Figure 2f represent the four main crystallographic planes of fcc Ag.27, 48-50

The existence of Ag is also confirmed by the result of elemental distribution (Figure S3,

Supporting Information).


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Figure 3. Schematic illustration of proposed fabrication procedure of BNNS–Ag/SiCNW– Ag/PVA composite papers. Figure 3 provides a schematic illustration toward the fabrication of BNNS–Ag/SiCNW– Ag/PVA composite papers. In a typical process, the BNNS–Ag and SiCNW–Ag dispersions were added into the PVA aqueous solution, and were filtrated slowly to form free-standing composite papers. The uniform solution is critical to obtain a uniform freestanding nanocomposite film with good orientation. The cross-section morphology would become disorderly if the components are poorly dispersed (Figure S4, Supporting Information). The typical hierarchical layered structure of natural nacre (Figure 4a) is shown in Figure 4d, where hard aragonite microplatelets alternately stacks together to form highly ordered structure. Figures 4b and 4c show good mechanical flexibility of the prepared BNNS–Ag/PVA and BNNS–


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Ag/SiCNW–Ag/PVA composite papers. After the addition of SiCNW–Ag, the colour of the paper turns to be as same as that of SiCNW–Ag dispersion. Figures 4e and 4f show that 2D BNNS layers are stacked alternately, showing a highly aligned layered arrangement. The enlarged SEM image (Figure 4g) of the red square in Figure 4f shows the existence of 1D SiCNWs. Elemental area-scanning further confirms the existence of Si and Ag elements (Figure 4h–j), and qualitatively reveals the presence of 4.6 wt% Ag (Figure 4k).

Figure 4. Characterization of natural nacre and BNNS–Ag/SiCNW–Ag/PVA composite paper. (a–c) Digital images of natural nacre (a), BNNS–Ag/PVA (b), and BNNS–Ag/SiCNW–Ag/PVA (c) composite papers, respectively. (d–f) Cross-section morphologies of natural nacre (d), BNNS–Ag/PVA (e), and BNNS–Ag/SiCNW–Ag/PVA (f) composite papers, respectively, which show the similar structures with unique arrangement of 2D platelets. (g) Enlarged SEM image of


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the red square in (f). (h) EDS layered image of (g). (i, j) Elemental mappings of Ag and Si. (k) Elemental distribution of the composite paper.

Figure 5.Thermal conductivities of prepared composite papers and corresponding model of thermal conduction. (a) In-plane thermal conductivities of 95 wt% BNNS–Ag/5 wt% PVA composite papers with BNNS–Ag generated by different mole ratios of BNNS to AgNO3 (CBNNS:CAgNO3=1:1, 2:1, 3:1, 5:1, 10:1, and 15:1, respectively).

means the prepared paper is

not free-standing. (b) Effects of mole ratios of SiCNW to AgNO3 and SiCNW length distribution on the in-plane thermal conductivities of 90 wt% BNNS–Ag/5 wt% SiCNW–Ag/5 wt% PVA composite papers. (c) In-plane thermal conductivities of BNNS–Ag/SiCNW–Ag/5 wt% PVA


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composite papers with different weight ratios of BNNS–Ag to SiCNW–Ag. (d) Comparison of conductivity of our composite paper with other reported oriented BN-containing composites or films along the orientation. (e) Proposed model of AgNPs and SiCNWs for thermal conduction. To understand the role of AgNPs, the mole ratios of BNNS to AgNO3 were controlled to be in the range of 1:1 to 15:1 to generate different amounts and diameters of AgNPs on BNNSs (Figure S5, Supporting Information). It should be noticed that BNNSs with larger AgNPs could not be assembled into a free-standing paper. We speculate that larger AgNPs would broaden the distance between adjacent BNNSs, leading to incompact assembly and reduce the contact area of the components. Therefore, an obvious degeneration of mechanical property was observed in BNNS–Ag/PVA with small mole ratio of BNNS to AgNO3 (< 5:1). Figure 5a indicates that mole ratio of 10:1 is a critical value to achieve the maximum function of AgNPs with a higher thermal conductivity of 13.8 W m−1K−1. However, the improvement introduced by AgNPs is still limited with thermal conductivity below 15 W m−1K−1. We attribute it to the lack of effective linking among separated BNNSs. The SiCNW–Ag hybrids are thus introduced to further consummate the degree of thermally conductive network. To achieve the maximum enhancement of thermal conductivity, we further explored the optimization of the synthesis of SiCNW–Ag including mole ratios of SiCNW to AgNO3 (Figure 5b), length distribution of SiCNWs (Figure 5b), and the weight ratios of BNNS–Ag to SiCNW– Ag (Figure 5c). Three mole ratios of SiCNW to AgNO3 (4:1, 8:1, and 16:1) were chosen to synthesize SiCNW-Ag (Figure S6, Supporting Information). Mole ratio of 4:1 generate excessive AgNPs as the DMF filtrate appears yellow after removing the SiCNW–Ag, indicating existence of free AgNPs. As shown in Figure 5b, the composite paper containing SiCNW–Ag generated by mole ratio of 4:1 shows higher thermal conductivity (16.6 W m−1K−1) than those of 8:1 (11.8 W


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m−1K−1) and 16:1 (11.2 W m−1K−1), demonstrating more AgNPs might form more linking among assembled components. In addition, the thermal conductivity of the bioinspired paper was also sensitive to the length distribution of SiCNW. Figure S7 shows the SiCNW length distribution derived from three different ball-milling treatment time under the same rotation rate. Long time of ball-milling exhibits obvious shortening effect of SiCNW length as discussed in Supporting Information. Figure 5b shows a higher thermal conductivity when using relatively longer SiCNWs compared with the shorter ones, illustrating that shorter SiCNWs might introduce more interfaces with extra interfacial thermal resistance. Five different weight ratios of BNNS–Ag to SiCNW–Ag (95:0, 90:5, 85:10, 75:20, and 65:30) were selected to optimize the content of SiCNW–Ag in the composite papers. When the weight ratio of BNNS–Ag to SiCNW–Ag is 85:10, a maximum in-plane thermal conductivity of 21.7 W m−1K−1 is achieved as shown in Figure 5c. Figure 5d shows comparison of thermal conductivity of our composite paper with other reported oriented BN-containing composites or films along the orientation, with the detailed data listed in Table S1 (Supporting Information). Obviously, the thermal conductivity of our composite paper is one of the highest values ever reported for BN-containing composites or films. To understand how the weight ratio affects the thermal conductivity, the cross-section morphology of BNNS–Ag/SiCNW–Ag/PVA papers with different weight ratios of BNNS–Ag to SiCNW–Ag were characterized by SEM (Figure 6), along with the corresponding models. The BNNS–Ag hybrids are not adequately linked by SiCNW–Ag when the weight ratios of BNNS– Ag to SiCNW–Ag are over 85:10 (Figure 6a,b), rendering less improvement in thermal conductivity. Weight ratio of 85:10 leads to a relatively ideal situation, where BNNS–Ag and SiCNW–Ag maintain good orientation and BNNS-Ag are adequately linked by SiCNW–Ag, as shown in Figure 6c. When the weight ratio is below 85:10, the remained orientated structure has


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been completely destroyed (Figure 6d,e), in which SiCNW–Ag hybrids appear randomly distributed rather than horizontally arranged, because of the lack of sufficient oppression from BNNS–Ag. Therefore, the serious phonon scattering caused by the disordered arrangement of assembled components greatly reduces their thermal conductivity.4 This indicates that both the interfacial design based on AgNPs, and the orderly alignment based on BNNS and SiCNW corporately contribute to the high thermal conductivity of our bioinspired composite paper, as the model shown in Figure 5e.

Figure 6. Cross-section morphologies of BNNS–Ag/SiCNW–Ag/5 wt% PVA composite papers upon different weight ratios of BNNS–Ag to SiCNW–Ag along with the corresponding models. The ratio for (a)–(e) is 95:0, 90:5, 85:10, 75:20, and 65:30, respectively.


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Apart from in-plane thermal conductivity, the through-plane thermal conductivity of prepared composite papers was also characterized (Figure S8, Supporting Information). The value for BNNS/PVA paper merely reaches 0.73 W m−1K−1, which is much smaller than that of in-plane thermal conductivity (6.16 W m−1K−1). Addition of AgNPs and SiCNW–Ag further enhances the through-plane thermal conductivity to 1.09 and 1.36 W m−1K−1, respectively. We mainly attribute the significant differences between in-plane and through-plane thermal conductivity to the anisotropy of 2D BNNSs, as previously reported.29 Mechanical properties of thermal-management materials are of great importance in particular for applications in portable and flexible microelectronic. We previously reported the BNNS/GO complex with high in-plane thermal conductivity. 31 However, the tensile strength of BNNS/GO complex is only 16.3 MPa. In this work, we enhance the mechanical strength of the papers by addition of SiCNWs and AgNPs, inspired by the toughening mechanism of natural nacre. 22, 23 The stress–strain curves of BNNS/PVA, BNNS–Ag/PVA, and BNNS–Ag/SiCNW–Ag/PVA papers were measured (Figure 7), and the detailed data are listed in Table S2 (Supporting Information). The tensile strength (39.80 MPa), Young’s modulus (11.50 GPa), and tensile toughness (0.21 MJ m−3) of BNNS–Ag/SiCNW–Ag/PVA are higher than those of other papers. The enhancement of the mechanical performance is mainly originated from two aspects. First, the nano-asperities consisted of AgNPs make adjoining interfaces more difficult to slip upon loading in tension as parallel to the BNNSs, resulting in formation of dilatation bands, instead of a dominant brittle crack. 51 According to previous report, such bands are the source of the tensile inelasticity.52, 53 The multiplicity of bands needed to realize the observed “ductility” is linked to the initial strain hardening, enabled by asperity “climb”. This leads to efficient energy dissipation by frictional sliding during platelet pull-out as the asperities are the principal source of the shear


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resistance.24, 52, 53 Second, the bridging SiCNW–Ag hybrids can generate obvious resistance to the sliding of adjacent BNNS–Ag hybrids (strain hardening). The enhanced stress is transferred to next layer of BNNS–Ag and activates the potential sliding of adjacent multiple BNNS–Ag hybrids. With crack extension, such bridging and activation of multiple potential sliding sites are accumulative in a step fashion until the paper fractures, causing absorption of large energy.22 In this way, energy dissipation through cooperative sliding of AgNPs and SiCNW pervades through a relatively large volume, rather than only at the fracture surface, leading to improved mechanical properties.

Figure 7. Mechanical properties of BNNS/PVA, BNNS–Ag/PVA, and BNNS–Ag/SiCNW– Ag/PVA composite papers. Stress–strain curves (a), strength (b), modulus (c), and toughness (d) of BNNS/PVA, BNNS–Ag/PVA, and BNNS–Ag/SiCNW–Ag/PVA composite papers, respectively.


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Figure 8. Volume resistivities of BNNS/PVA and BNNS–Ag/SiCNW–Ag/PVA composite papers. Considering the addition of AgNPs and SiCNWs might reverse the electrical insulation of BNNS/PVA paper, the volume resistivity of BNNS–Ag/SiCNW–Ag/PVA composite paper was measured (Figure 8). The BNNSs/PVA paper possesses a high volume resistivity over 1016 Ω cm. After incorporation of AgNPs and SiCNWs, the value decreases to 8.29×1013 Ω cm, which is still far beyond the critical resistance for electrical insulation (109 Ω cm). We speculate that the electrically conductive pathways are not formed in the composite paper, because BNNSs with large band gap could be electrical barriers. In a separate study, we attempt to investigate the thermal conductivity of BNNS/CNF paper, following the procedures reported by Hu’s group.30 They reported that a exceptionally high thermal conductivity of 145 W m−1K−1 for BNNS/CNF composite paper can be achieved, similar to the thermal conducting aluminum alloys. However, our results show that the BNNS/CNF paper with 50 wt% BNNSs exhibits a much lower thermal conductivity of 7.20 W m−1K−1 (Figure S9, Supporting Information). The value is 20 times lower than the result found in Ref,30 and falls within the typical thermal conductivity range of BN-related composites (< 10 W m−1K−1). We do not yet know the exact reason for the huge difference. However, we speculate


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that it is likely due to the different measurement technology. Hu et al. obtained a very high thermal conductivity by steady-state method, which is theoretically improper. The steady-state method is based on the Fourier’s law, where the thermal conductivity is calculated depending on the temperature difference between temperature sensors of the hot meter bar. Sample with high thermal conductivity possibly couldn’t form valid temperature difference between hot and cold end, resulting in huge errors when calculating the thermal conductivity. This is why steady-state method is only valid for samples with low thermal conductivity (< 20 W m−1K−1), which is commonly known in academe and industries. 4. CONCLUSIONS Under the guidance of bioinspired engineering thought, we report a novel composite paper with high thermal conductivity. We have demonstrated that the design of interface by AgNPs can improve the contact areas and interactions between assembling components, while BNNSs and SiCNWs form the oriented structure, which provides efficient thermally conductive network. The resulted paper exhibits a high in-plane thermal conductivity of 21.7 W m−1K−1, much higher than other reported oriented BN-containing composites or films. Meanwhile, the paper possesses good mechanical properties and satisfactory electrical insulation. These findings provide us a new concept for the design and constructing of high-performance thermal-management materials for applications in electronics. In the longer term, we believe that constant exploration of biological systems will continuously supply fresh inspiration for the advanced thermalmanagement materials. ASSOCIATED CONTENT *Supporting Information


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DLS results of BNNSs, elemental distributions of BNNS–Ag and SiCNW–Ag hybrids, SEM images of BNNS–Ag hybrids generated by different CBNNS:CAgNO3, cross-section morphology of the resultant paper prepared by poorly dispersed components, SEM images of SiCNW–Ag hybrids generated by different CSiCNW:CAgNO3, variation of SiCNW length distribution with ballmilling time, through-plane conductivities of BNNS/PVA, BNNS–Ag/PVA, and BNNS– Ag/SiCNW–Ag/PVA composite papers, in-plane conductivity vs BN content in BNNS/CNF paper, comparison of in-plane thermal conductivity of our composite paper and other reported oriented BN-containing composites or films along the orientation, and summary of the mechanical properties of the prepared papers. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *Address corresponding to: [email protected] Author Contributions †

Yimin Yao and Xiaoliang Zeng contributed equally. The manuscript was written through

contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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The authors would like to acknowledge the financial support from Guangdong and Shenzhen Innovative Research Team Program (No. 2011D052 and KYPT20121228160843692). J. B. Xu would like to thank the National Science Foundation of China for the support, particularly, via Grant No 61229401. REFERENCES 1. Tao, P.; Shang, W.; Song, C.; Shen, Q.; Zhang, F.; Luo, Z.; Yi, N.; Zhang, D.; Deng, T., Bioinspired Engineering of Thermal Materials. Adv. Mater. 2015, 27, 428–463. 2. McGlen, R. J.; Jachuck, R.; Lin, S., Integrated Thermal Management Techniques for High Power Electronic Devices. Appl. Therm. Eng. 2004, 24, 1143–1156. 3. Meziani, M. J.; Song, W. L.; Wang, P.; Lu, F.; Hou, Z.; Anderson, A.; Maimaiti, H.; Sun, Y. P., Boron Nitride Nanomaterials for Thermal Management Applications. ChemPhysChem 2015, 16, 1339–1346. 4. Warzoha, R. J.; Fleischer, A. S., Heat Flow at Nanoparticle Interfaces. Nano Energy 2014, 6, 137–158. 5. Huang, X.; Jiang, P.; Tanaka, T., A Review of Dielectric Polymer Composites with High Thermal Conductivity. IEEE Electr. Insul. Mag. 2011, 27, 8–16. 6. Koch, K.; Bhushan, B.; Barthlott, W., Multifunctional Surface Structures of Plants: An Inspiration for Biomimetics. Prog. Mater Sci. 2009, 54, 137–178. 7. Parker, A. R.; Lawrence, C. R., Water Capture by a Desert Beetle. Nature 2001, 414, 33–34. 8. Ju, J.; Bai, H.; Zheng, Y.; Zhao, T.; Fang, R.; Jiang, L., A Multi-Structural and Multi-Functional Integrated Fog Collection System in Cactus. Nat. Commun. 2012, 3, 1247. 9. Gracheva, E. O.; Ingolia, N. T.; Kelly, Y. M.; Cordero-Morales, J. F.; Hollopeter, G.; Chesler, A. T.; Sanchez, E. E.; Perez, J. C.; Weissman, J. S.; Julius, D., Molecular Basis of Infrared Detection by Snakes. Nature 2010, 464, 1006–1011. 10. Nicole, L.; Rozes, L.; Sanchez, C., Integrative Approaches to Hybrid Multifunctional Materials: From Multidisciplinary Research to Applied Technologies. Adv. Mater. 2010, 22, 3208–3214. 11. Le Ferrand, H.; Bouville, F.; Niebel, T. P.; Studart, A. R., Magnetically Assisted Slip Casting of Bioinspired Heterogeneous Composites. Nat. Mater. 2015, 14, 1172–1179. 12. Mo, X.; Wu, Y.; Zhang, J.; Hang, T.; Li, M., Bioinspired Multifunctional Au Nanostructures with Switchable Adhesion. Langmuir 2015, 31, 10850–10858. 13. Liang, W.; Zhu, L.; Li, W.; Yang, X.; Xu, C.; Liu, H., Bioinspired Composite Coating with Extreme Underwater Superoleophobicity and Good Stability for Wax Prevention in the Petroleum Industry. Langmuir 2015, 31, 11058–11066. 14. Cho, J. H.; Vasagar, V.; Shanmuganathan, K.; Jones, A. R.; Nazarenko, S.; Ellison, C. J., Bioinspired Catecholic Flame Retardant Nanocoating for Flexible Polyurethane Foams. Chem. Mater. 2015, 27, 6784–6790. 15. Chen, W.-H.; Lei, Q.; Yang, C.-X.; Jia, H.-Z.; Luo, G.-F.; Wang, X.-Y.; Liu, G.; Cheng, S.-X.; Zhang, X.-Z., Bioinspired Nano-Prodrug with Enhanced Tumor Targeting and Increased Therapeutic Efficiency. Small 2015, 11, 5230–5242. 16. Shao, Z. Z.; Vollrath, F., Materials: Surprising Strength of Silkworm Silk. Nature 2002, 418, 741– 741. 17. Huang, X.; Liu, G.; Wang, X., New Secrets of Spider Silk: Exceptionally High Thermal Conductivity and Its Abnormal Change under Stretching. Adv. Mater. 2012, 24, 1482–1486. 18. Henry, A.; Chen, G., High Thermal Conductivity of Single Polyethylene Chains Using Molecular Dynamics Simulations. Phys. Rev. Lett. 2008, 101, 235502.


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