Highly Anisotropic, Thermally Conductive, and Mechanically Strong

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Highly Anisotropic, Thermally Conductive, and Mechanically Strong Polymer Composites with Nacre-like Structure for Thermal Management Applications Yan-Fei Huang, Zhi-Guo Wang, Hua-Mo Yin, Jia-Zhuang Xu, Yuanwei Chen, Jun Lei, Lei Zhu, Feng Gong, and Zhong-Ming Li ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00514 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 3, 2018

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ACS Applied Nano Materials

Highly Anisotropic, Thermally Conductive, and Mechanically Strong Polymer Composites with Nacre-like Structure for Thermal Management Applications

Yan-Fei Huang,† Zhi-Guo Wang,† Hua-Mo Yin,† Jia-Zhuang Xu,†,* Yuanwei Chen,† Jun Lei,† Lei Zhu,‡ Feng Gong,§ and Zhong-Ming Li,†,*

†

College of Polymer Science and Engineering, State Key Laboratory of Polymer

Materials Engineering, Sichuan University, Chengdu 610065, People’s Republic of China ‡

Department of Macromolecular Science and Engineering, Case Western Reserve

University, Cleveland, Ohio 44106-7202, United States §

School of Energy Science and Engineering, University of Electronic Science and

Technology of China, Chengdu 611731, People’s Republic of China

* Corresponding author. Tel.: +86-28-8540-0211; Fax: +86-28-8540-5402. E-mail: [email protected] (J. Z. Xu); [email protected] (Z. M. Li)

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ABSTRACT The conventional approach to improve the thermal conductivity (TC) of polymers by blindly adding inorganic fillers suffers from the limited TC enhancement (< 3.0 W/mK) with isotropic TC and poor mechanical performance. Here, highly anisotropic, thermally conductive, and mechanically strong boron nitride (BN)/ultrahigh molecular weight polyethylene (UHMWPE) composites are prepared via a facile solid-phase extrusion (SPE) technology. The in-plane TC approaches 12.42 W/mK at the BN loading of 50 vol%, which is 242% higher than that of the high-pressure compressed counterpart (3.63 W/mK). The anisotropic index of TC reaches as high as ~1000%, allowing the heat transfer more readily along in-plane direction than through-plane direction as revealed by infrared imaging results. We attribute the increased TC to the unique nacre-like structure induced by the flow assisted alignment. BN platelets are highly oriented to form connected thermal conductive pathways for phonon transport along the basal plane. The interfacial thermal resistance is reduced by two orders of magnitude as deduced by theoretical calculation. More strikingly, compared to the controlled samples with randomly distributed structure, SPE composites exhibit significantly superior strength and toughness at equivalent filler content. These results demonstrate that the prepared SPE composites have high potential for the engineering application in heat dissipation fields. Keywords: anisotropic thermal conductivity, mechanical enhancement, polymer composites, nacre-like structure, structural manipulation, orientation

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INTRODUCTION Thermally conductive and electrically insulating polymer-based composites have

been extensively used for thermal management, which is essential for the performance, lifetime and reliability of modern electronics, automobiles, and energy storage devices.1-4 In many cases, high thermal conductivity (TC) of a material in a specific direction is highly desired since it helps to transfer unwanted heat directionally without interfering the neighboring components.5-9 However, most of the polymer-based thermal conductive composites show isotropic TC. In this regard, the proper alignment of the anisotropic fillers holds the key and several strategies have been

employed,

such

as

ice-template

assembly,10,11

magnetic

induced

arrangement,12,13 etc. Unfortunately, the anisotropy ratio of TC is very limited, which poses one of the major challenges faced in the advanced thermal conductive materials.

Figure 1. Schematic diagram of the design of nacre-like BN/polymer composites during SPE processing. 3



On the other hand, large amounts of filler are needed to sustainably increase the TC of the composites due to phonon scattering arising from the high thermal interface resistance between the filler and polymer matrix.14-16 It undesirably causes severe deterioration in the mechanical performance, restricting the broad applications of the target products. It is another challenge to guarantee the mechanical reliability of thermal conductive composites. To address these two challenges, we intended to construct a layered structure with highly oriented inorganic filler layers interpenetrated with a well-aligned polymer matrix. It is analogous to the “brick-and-mortar” microstructure evidenced in natural nacre, which is renowned for its outstanding integration of strength and toughness,17-20 and shows great potential to realize high anisotropic TC due to the oriented filler arrangement. Hexagonal boron nitride (h-BN) and ultrahigh molecular weight polyethylene (UHMWPE) were used to demonstrate the proof-of-concept and the nacre-like structure was constructed by a solid-phase extrusion (SPE) technology, which enables the monodirectional orientation of both matrix and fillers via the high-pressure extrusion below the melting point of the polymer matrix.21-23 As schematically shown in Figure 1, the joint effects of high pressure and strong shear drive both BN platelets and UHMWPE lamellae aligned along the extrusion direction. The anisotropic assembly of BN platelets acts as the “brick”, forming an oriented conductive framework for directional thermal transport. And the highly oriented UHMWPE lamellae serve as the “mortar” to bridge the adjacent BN platelets. 4


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The cross-plane transfer of the heat could be stunted to a large extent, on account of the fact that phonon propagation is more readily along the chain skeleton than between chains bonded by weak van der Waals interaction.24 On the other hand, oriented polymer lamellae are the typically structural motif for mechanical reinforcement.25,26 We find that the as-prepared nacre-like composites exhibit an in-plane TC up to 12.42 W/mK with an anisotropic ratio as high as 1000%. Furthermore, the tensile property of the SPE composites with nacre-like structure is prominently enhanced. In particularly, tensile strength and toughness of SPE UHMWPE/BN (85/15 vol%) composites are 112.0 MPa and 22.0 kJ/m2, respectively, much higher than the compression molding (CM, 22.5 MPa and 0.32 kJ/m2) and high-pressure molding (HP, 16 MPa and 0.18 kJ/m2) counterparts at the same BN loading. This novel strategy paves the way for the industrial fabrication of the thermal management composites with integrated performance.

EXPERIMENTAL SECTION Materials. UHMWPE with a molecular weight of ~1.0 × 106 g/mol and density

of 0.94 g/cm3 was kindly provided by Beijing Yanshan Petrochemical High-Tech Co., Ltd, China. BN platelets (trademark, BBN-10) with an average particle size of 10 µm, thickness of 30 nm, and density of 2.25 g/cm3 were supplied by Ya'an Bestry Performance Material’s Co, Ltd, China. Sample fabrication. UHMWPE granules were first mechanically mixed with different contents of BNs (0, 15, 30, and 50 vol%). For SPE samples, UHMWPE/BN 5



composites were first consolidated to a parison at 200 oC and 10 MPa for 20 min and then extruded at 130 MPa and 120 oC by a home-made SPE apparatus (Figure S1). Extrusion rate was slow enough to ensure sufficient time for the deformation of the parison. It is essential to state that the upper limit of BN fraction in SPE composites is 50 vol%, otherwise the mechanical performance is deteriorated sharply. For CM samples, UHMWPE/BN mixtures were compression molded at 200 oC and 10 MPa for 20 min. For HP samples, UHMWPE/BN mixtures were high-pressure molded at 200 oC and 130 MPa for 20 min. Volume content of BN in the composites was confirmed by thermal gravimetric analysis (TGA). It was consistent with the additive amount during the mixing process (Figure S2 and Table S1). Characterization. An Olympus BX51 Polarizing optical microscopy (POM, Olympus Co., Tokyo, Japan) equipped with a MicroPublisher 3.3 RTV CCD was employed to observe the filler distribution. Field-emission scanning electron microscopy (SEM, Nova NanoSEM450, FEI, USA) was used to detect the morphology of samples. Specimens were cryogenically fractured in liquid nitrogen, and etched at 25 oC for 48h. The etching reagent is 0.5% solution of potassium permanganate in a mixture of concentrated sulfuric acid (98%) and concentrated nitric acid (65%). After washing and drying completely, the etched surface was sputter coated with a thin layer of gold and observed at an operation voltage of 20 kV. More detail about the sample preparation for SEM observation was described in Supporting Information. Wide-angle X-ray diffraction (WAXD) measurement was carried out at beamline BL15U1 of the National Synchrotron Radiation Laboratory, Shanghai, 6


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China. A Mar165 CCD detector (2048 × 2048 pixels in an area of 80 × 80 µm2) was used to collect the signal pattern. The distance from the specimen to the detector was 178.5 mm. Orientation degree of UHMWPE and BN was calculated mathematically using Picken’s method for the (110) reflection of WAXD for PE,27-29 and (002) reflection of WAXD for BN (see Equations S1 and S2). Thermal diffusivities (α) along two different directions and specific heat capacity (Cp) were measured using a laser-flash conductometer (LFA 467, NETZSCH, Germany) at 25 oC. TC was calculated by K = α Cp ρ, where ρ is the density and was measured using a precision analysis scale (XS205, Mettler Toledo, Switzerland). At least three samples were tested for each group. The variation of surface temperature was recorded by an infrared thermography (IR-160P, RNO, USA), where the samples was put on a hot stage with the thermal equilibrium temperature of 100 oC. Tensile test of the dumbbell samples was performed on a universal test instrument (Model 5967, Instron Instruments, USA) according to ASTM D-638 at a cross-head speed of 10 mm/min. Tensile specimens with a cross sectional area of 2 × 6 mm2 and 500 mm in length were punched by a dumbbell-shaped knife along the extrusion direction. Tensile toughness was obtained by integrating the area under the stress-strain curves.

RESULTS AND DISCUSSION Morphology and structure. POM was utilized to observe the distribution of BN

platelets in the SPE composites. BN layers (the dark phase, Figure 2a1) assemble alternatively with UHMWPE layers (the bright phase). In clear contrast, BN platelets 7



show a random framework in HP (Figure 2b1) and CM (Figure S3a) counterparts. Such a structure difference is also evidenced by SEM (Figure 2a2, 2b2 and S3b). For SPE composites, BN platelets in the brick-like framework are well-aligned and stack face-to-face along the extrusion direction (Figure 2a3 and 2a3’). UHMWPE lamellae are highly oriented accompanied with BN platelets (Figure 2a4). It shares a striking resemblance to the microstructure of natural nacres, where two-dimensional aragonite microplates are highly aligned and the organic phase is alternately stacked together with aragonite to form a layered “brick-and-mortar” structure (the inset of Figure 2a1).17,18,30 Herein, the oriented BN platelets serve as “brick” and the aligned UHMWPE lamellae are regarded as “mortar”. While both BN and UHMWPE lamellae are randomly distributed in HP (Figure 2b3 and 2b4) and CM samples (Figure S3c and S3d). It is worth noting that the oriented lamellae are recognized as a favorable self-reinforced unit for mechanical enhancement of semi-crystalline polymers.25 In particular, the oriented UHMWPE lamellae are robustly interlocked with each other (Figure 2a4). Such an interlocked state creates stronger mechanical enhancement than normal oriented lamellae.31,32 Thus, a notable improvement in mechanical tolerance is anticipated in the SPE composites compared to CM and HP composites.

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Figure 2. POM images of (a1) SPE15 and (b1) HP15 (the suffixal number corresponds to the volume fraction of BN in the composites). SEM images of the cryo-fractured surface of (a2, a3, and a3’) SPE15 and (b2 and b3) HP15 before etching; a3 and a3’ show edge-view and top-view image of the extrusion plane of SPE15, respectively; 9



Almost all the BN platelets in a3’ show flat-on arraignment. In b3, BN platelets labeled by arrows show the edge-on arrangement, while the rest show flat-on arrangement. SEM images of the cryo-fractured surface of (a4) SPE15 and (b4) HP15 after etching. The white arrow is the flow direction.

Figure 3. 3D-WAXD images of (a) HP15 and (b) SPE15. Degree of orientation for (c) UHMWPE and (d) BNs in CM, HP and SPE composites with different BN loadings.

WAXD measurement was conducted to quantify the orientation of SPE composites. Different with the isotropic rings of (110) and (200) lattice planes of PE in CM (Figure S4a) and HP composites (Figure 3a and S4b), arc-like diffractions are visibly observed in all the SPE samples (Figure 3b and S4c). These signals of the oriented UHMWPE lamellae coincide well with the SEM results. Moreover, the (002) 10


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plane of BN platelets also shows high orientation (Figure 3b and S4c).33,34 Degree of orientation of UHMWPE lamellae and BN platelets was calculated according to Hermans’ orientation function (Equations S1 and S2). As plotted in Figure 3c, UHMWPE lamellae in SPE composites show degree of orientation of 0.63 – 0.97, whereas they are absolutely isotropic in CM and HP composites. The high orientation of UHMWPE lamellae is of great benefit for the mechanical enhancement. Meanwhile, BN platelets in SPE are also highly oriented with degree of orientation ranging from 0.72 to 0.89 (Figure 3d). These results substantially testify that a well-oriented structure is generated with the assistance of the powerful shear offered by SPE. It is worth to address that the SPE technology recruited here is simple, time-saving, and scalable to prepare the nacre-mimetic composites compared to previously reported techniques, such as self-assembly,35 synthetic mineralization,19 and layer-by-layer assembly.36 Thermal conductivities. The creation of nacre-like structure greatly improves the in-plane TC of SPE composites compared to the CM and HP counterparts. As shown in Figure 4a, the in-plane TC of SPE composites increases linearly with the BN content, suggesting that the additive BN platelets mostly involve into forming the thermal conduction pathways. Particularly, in-plane TC of SPE50 climbs up to 12.42 W/mK, 242% larger than that of HP50 (3.63 W/mK). Note that the TC of CM samples with the BN content higher than 30 vol% cannot be measured due to the poor structural integrity. These results demonstrate that appropriate use of the processing fields holds the key for the thermal conduction enhancement of polymer composites 11



with fixed components. Moreover, compared to other BN-based composites reported in previous works,13,33,37-42 the TC of our SPE samples is superior at a given filler loading (Figure 4b).

Figure 4. (a) TC of CM, HP and SPE composites with different BN loadings. (b) Comparison of TC in this work with other BN-based composites reported in literatures.13,33,37-42 (c) Enhanced TC (∆TC) of SPE composites along different directions compared to HP composites. (d) Comparison of anisotropic ratio in this 12


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work with those of other thermal composites reported in literatures.11,14,43-47 (e) Infrared thermal images and (f) surface temperature variation of SPE50 and HP50 as a function of heating time.

Anisotropic structure of SPE composites brings varied TC enhancement (∆TC) along the in-plane and through-plane directions. ∆TC along these two directions was obtained based on the following equations, ∆TC = TCSPE - TCHP. Interestingly, ∆TC exhibits a sharp increase in the in-plane direction and a gentle drop in the through-plane direction (Figure 4c). As for SPE0, the in-plane TC is increased by 2.64 W/mK while the through-plane TC is decreased by 0.15 W/mK. It is related to the high orientation of UHMWPE molecular chains.24 While for the SPE composites, the anisotropy of TC is more distinct. Taking SPE50 as an example, in-plane TC shows an augment of 9.46 W/mK compared to HP50, whereas through-plane TC falls by 1.44 W/mK. It is attributed to anisotropic thermal conduction pathways constructed by the nacre-like structure of SPE composites. Phonon transfer is much faster along the basal plane of BN platelets. To visualize this heat dissipation difference, an infrared imaging was used to record the temperature response during heating, as shown in Figure 4e. We can clearly see that the surface temperature of SPE50 rises much faster along the in-plane direction than the through-plane direction. As plotted in Figure 4f, the argument of the surface temperature with time follows the sequence: in-plane SPE50 > in-plane HP50 > through-plane SPE50. For instance, at the time of 34 s, the surface temperature of in-plane SPE50 is 46.8 oC, while it is 44.5 and 44.9 oC for 13



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through-plane SPE50 and in-plane HP50, respectively. The observed transient temperature response agrees well with the thermal conductive results, confirming that the capacity of thermal diffusion is much faster along the in-plane direction than the through-plane direction of SPE composites. The anisotropy ratio of TC was accessed based on the following equation: anisotropy ratio = (TCin-plane – TCthrough-plane)/TCthrough-plane. From Figure 4d, the anisotropy ratio of TC in SPE composites approaches ~1000%, which is at the advanced

level

in

comparison

to

other

anisotropic

thermal

conductive

composites.11,14,43-47 The SPE composites with highly anisotropic TC allow the heat transferring directionally without disturbing the neighboring electronic components, showing a bright prospect as the thermal conductive media for next-generation microelectronic devices.[5-7, 47-49] Thermal conductive modeling. To understand the intrinsic factors relevant to the TC improvement, we employed the effective medium theory (EMT) to investigate the TC of HP composites with isotropic conduction network. In this model, the TC of polymer/BN composites is described by the following equation11,50,51

 K − Km  3 + 2Vf  BN  Km   K = Km  K  3 − Vf (1 − α ) - m  KBN  

(1)

where K, KBN, and Km are the TC of HP composites, BN platelets (31 W/mK), and neat UHMWPE (0.48 W/m K, Figure 4a), respectively. Vf is the BN volume fraction, and α is a dimensionless parameter defined as, 14


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α =

Rc Km d

(2)

where d is the thickness of BN platelets (~ 30 nm). Rc is the interfacial thermal resistance between BN and UHMWPE. By matching the predicted TC with the measured TC, the EMT model yields Rc of 9.9 × 10-7 m2K/W for HP composites (Figure S5), which is comparable to both the experimental and theoretical Rc in the existing literatures.10,11,51 For the SPE composites with the highly anisotropic conduction network, the Rc is suggested to mainly originate from BN-BN interface rather than BN-UHMWPE interface due to the brick-and-mortar structure (Figure 2a1). The Foygel nonlinear model,10,11 instead of the EMT model, was applied to predict Rc of BN-BN interface, which is expressed by τ

V − Vc  K - Km = K0  f   1 − Vc 

(3)

where K0 is a pre-exponential factor ratio associated with the contribution of BN platelets. In-plane Km and through-plane Km of SPE0 are 3.12 and 0.33 W/mK, respectively (Figure 4a). Vc is the critical volume fraction of BN platelets (fitted from a tangent process of experimental data and labeled in Figure S6); τ is a conductivity exponent, dependent on the aspect ratio of BN platelets. By fitting the experimental data of SPE composites in these orthogonal directions (Figure S7), the values of K0 and τ are obtained (Table S2). Based on these results, the contact resistance (R) between adjacent BN platelets can be calculated by the following equation

R =

1

(4)

K0L (Vc )

τ

15



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where L is the plate size of BN (~ 10 µm). The calculated R is 5.1 × 104 and 5.9 × 105 K/W along the in-plane and through-plane directions, respectively. The Rc is finally gained according to the relation of Rc = R × S, with S being the overlapping area between adjacent BN platelets. Assuming that 1/100 of each BN surface participates in the heat conduction of the percolation network,52,53 the active interface between adjacent BN platelets could be estimated as 7.85 × 10-13 m2 according to:

S =

π L

2

(5)

  100  2 

Thus, Rc of BN-BN in the in-plane direction and through-plane direction of SPE composites is 4.0 × 10-8, and 4.6 × 10-6 m2K/W, respectively. It qualitatively explains why SPE composites exhibit high in-plane TC and strong anisotropic conductivity. On the basis of the above calculation, the mechanism behind the thermal enhancement is uncovered. For CM composites, BN platelets possess a random distribution with few connections to each other (Figure S3). The heat can only transfer in a filler-polymer-filler manner, resulting in severe phonon scattering and thus a low TC.54,55 Utilization of high pressure improves the contact of neighboring BN, reducing the phonon scattering to some extent. Thus, HP samples exhibit slightly higher TC than CM counterparts, particularly at high filler loadings (Figure 4a). In contrast, the highly oriented network of BN platelets forms in SPE composites, which markedly decreases the in-plane Rc by 10 times compared to HP composites. Meanwhile, the cluster of aligned BN promoted by SPE brings a great number of filler-filler connections, providing an unimpeded expressway for heat conduction 16


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(Figure 1a1). Besides, UHMWPE also shows high orientation in SPE composites (Figure 2a4, 3b and 3c), which causes the efficient phonon transport along the extrusion direction and a reduced TC in the transverse direction. Thereby, a fairly high in-plane TC is achieved in SPE composites (Figure 4a). Mechanical properties. Figure 5 depicts the mechanical properties of UHMWPE/BN composites as summarized from the stress-strain curves (Figure S8). For CM and HP composites, the addition of BN improves Young’s modulus (Figure 5a) but results in serious deterioration of tensile strength (Figure 5b) and tensile toughness (Figure 5c). This dilemma is very common for most polymer-based thermal conductive composites.52,56 In clear contrast, SPE composites exhibit tremendously robust tensile property compared to CM and HP composites. The Young’s modulus and ultimate tensile strength of SPE15 are 2.90 GPa and 112 MPa, outperforming HP15 by 103% and 600%, respectively. Even that BN content is as high as 50 vol%, the ultimate tensile strength of SPE50 is well maintained at a value of 25.7 MPa, close to the neat UHMWPE (25.9 MPa), and 260% higher than HP50. The increased strength arising from the self-reinforced lamellae compensates the loss caused by adding fillers. Meanwhile, tensile toughness is always higher for SPE composites in comparison to CM and HP composites at a given BN content. Intriguingly, the mechanical property of SPE composites is the highest among all the other PE/BN composites reported in literatures.52,56 In particular, the tensile strength (112.0 MPa) and toughness (22.0 MJ/m3) of SPE15 are comparable or even superior to those of natural nacre (80 – 135 MPa, 1.8 MJ/m3).17,57,58 17



Figure 5. (a) Young’s modulus, (b) ultimate tensile strength, and (c) tensile toughness of SPE, HP and CM composites with different BN loadings. (d) The schematic diagram of the crack of SPE composites under a stress.

The improved mechanical property of SPE composites is of crucial relevance to the nacre-like structure.17,18,30 A substantial fraction of tensile load could transfer from the UHMWPE mortar to the rigid BN bricks. It is in favor of the enhanced tensile strength for SPE composites in comparison to CM and HP composites. As the crack begins to propagate, the nacre-like structure distorts the crack path, which absorbs much energy, causes a ductile fracture surface and elevates tensile toughness (Figure 5d). Moreover, SPE composites with oriented UHMWPE lamellae was endowed with a rigid framework that can bear higher stress. This is also helpful to the mechanical tolerance. Especially, the interlocked crystalline state (Figure 2a4) enhances the 18


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interfacial adhesion between adjacent lamellae and provide a tendency to improve mechanical properties in all directions.25,59 As shown in Table S3, the yield strength of SPE0 in the transverse direction (30.5 MPa) is still higher than that of CM0 (22.5 MPa). The superior mechanical performance enables the application of SPE composites in some thermal management fields with harsh stress conditions.

CONCLUSION We have successfully fabricated a nacre-like architecture with highly oriented

BN and UHMWPE lamellae by means of a SPE technology. The obtained SPE composites possess a high anisotropic TC with the anisotropic index as high as ~1000%, allowing the heat transfer more readily along in-plane direction than through-plane direction as revealed by infrared imaging results. The in-plane TC approaches 12.42 W/mK for SPE50, 242% higher than that of HP50. We attribute the improved TC to the oriented UHMWPE lamellae, and more importantly, to the well-aligned BN and the interconnected filler network in UHMWPE matrix, which decreases the thermal interface resistance of SPE composites (4.0 × 10-8 m2K/W) by almost 10 times in comparison to HP composites (9.9 × 10-7 m2K/W). Meanwhile, the mechanical properties of SPE composites are significantly superior to those of CM and HP composites due to the formation of brick-and-mortar structure. The SPE composites with high TC and mechanical performance have great potential in thermal management fields to protect the electronic devices.

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ASSOCIATED CONTET

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ***. Schematic of the home-made SPE apparatus, TGA measurement, Sample preparation for SEM observation, Hermans’ orientation function, POM and SEM images of CM15, 3D-WAXD images for various CM, HP, and SPE samples, EMT model fitting results, Foygel model fitting results, Stress and strain curves of CM, HP, and SPE composites, mechanical strength of SPE0 in the transverse direction

AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected] (J. Z. Xu). *E-mail: [email protected] (Z. M. Li) ORCID Yan-Fei Huang: 0000-0001-9383-5063 Jia-Zhuang Xu: 0000-0001-9888-7014 Jun Lei: 0000-0001-6803-5216 Lei Zhu: 0000-0001-6570-9123 Feng Gong: 0000-0002-5204-1395 Zhong-Ming Li: 0000-0001-7203-1453 Notes 20


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The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support from the Program of

National Natural Science Foundation of China (51533004, 51528302, 51773136, 51721091), National High Technology Research and Development Program 863 (2015AA033605), Outstanding Young Scholars Research Fund of Sichuan University (2016SCU04A17), and State Key Laboratory of Polymer Materials Engineering (sklpme2016–3–08). We would like to express sincere thanks to the beamlines BL15U1 of the Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China) for the kind help on WAXD measurements. Y.-F. Huang acknowledges financial support from China Scholarship Council (201606240042).

REFERENCES

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TOC Graphic

Highly anisotropic, thermally conductive, and mechanically strong BN/UHMWPE composites with a nacre-like structure are prepared via a solid-phase extrusion (SPE) technology. The anisotropic index of thermal conductivity is as high as ~1000%. The in-plane TC approaches 12.42 W/mK, 242% higher than that of high pressure counterpart.

31


Highly Anisotropic, Thermally Conductive, and Mechanically Strong

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