Vertically Aligned and Interconnected Boron Nitride Nanosheets for

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Vertically Aligned and Interconnected Boron Nitride Nanosheets for Advanced Flexible Nanocomposite Thermal Interface Materials Jin Chen, Xingyi Huang, Bin Sun, Yuxin Wang, Yingke Zhu, and Pingkai Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08061 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 22, 2017

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ACS Applied Materials & Interfaces

Vertically Aligned and Interconnected Boron Nitride Nanosheets for Advanced Flexible Nanocomposite Thermal Interface Materials Jin Chen,† Xingyi Huang,†, * Bin Sun,†, ‡ Yuxin Wang,† Yingke Zhu† and Pingkai Jiang† †

Department of Polymer Science and Engineering, Shanghai Key Laboratory of Electrical

Insulation and Thermal Ageing, Shanghai Jiao Tong University, Shanghai 200240, China; ‡

College of Physics, Qingdao University, Qingdao 266071, China

KEYWORDS: Electrospun; Boron Nitride Nanosheets; Thermal conductivity; Nanocomposites; PDMS

ABSTRACT: The continuous evolution toward semiconductor technology in the “more-thanMoore” era and rapidly increasing power density of modern electronic devices call for advanced thermal interface materials (TIMs). Here we report a novel strategy to construct flexible polymer nanocomposite TIMs for advanced thermal management applications. Firstly, aligned polyvinyl alcohol (PVA) supported and interconnected 2D boron nitride nanosheets (BNNSs) composite fiber membranes were fabricated by electrospinning. Then the nanocomposite TIMs were constructed by rolling the PVA/BNNS composite fiber membranes to form cylinders and

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subsequently vacuum-assisted impregnation of polydimethylsiloxane (PDMS) into the porous cylinders. The nanocomposite TIMs not only exhibit super-high through-plane thermal conductivity enhancement of about 10 times at a low BNNS loading of 15.6 vol% in comparison with the pristine PDMS, but also show excellent electrical insulating property (i.e., high volume electrical resistivity). The outstanding thermal management capability of the nanocomposite TIMs was practically confirmed by capturing the surface temperature variations of a working LED chip integrated with the nanocomposite TIMs.

INTRODUCTION Currently, effective heat dissipation has become a critical limiting factor for the breakthrough in many fields, such as integrated electronic devices, light emitting diodes (LEDs), energy conversion and storage, aerospace industry and military.1-6 Thermal interface materials (TIMs), serving as heat transfer medium between contacting surfaces of heat source and heat sink, are the key component to guarantee stable and reliable heat dissipation from devices.7, 8 Taking the LED cooling as an example, ideal TIMs requires the combination of high thermal conductivity, excellent electrical insulation, flexibility, easy processing and good design freedom.9, 10 In this case, the conventional thermally conductive materials such as metal and ceramics cannot fulfill these requirements. Polymers have most of these characteristics, but most of them have inadequate thermal conductivity (usually less than 0.4 W/(m·K)).11, 12 Therefore, a rational way to develop high performance TIMs is introducing the thermally conductive but electrically insulating fillers into a polymer.13-19 Nevertheless, the thermal conductivity enhancement is usually at the cost of significant deterioration of flexibility and processing because of low thermal conductivity enhancement efficiency. Therefore, the key to resolve this challenge is realizing high-efficiency thermal conductivity enhancement at low filler loading.


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Significant efforts have been made in developing high thermal conductivity composites with low filler addition, such as filler functionalization20, 21, filler orientation22, filler agglomerates23 and filler self-assembly24. However, the high-efficiency thermal conductivity enhancement at low filler loadings is still a large challenge because of the incomplete formation of percolation pathway and the existence of large interfacial thermal resistance. It is expected that constructing simultaneously oriented and interconnected filler structure is crucial for enhancing the thermal management capability of TIMs.25-27 Electrospinning, a simple and straightforward method to prepare nano/microfibers from solutions or melts under a high electric field, has attracted extensive attention in recent decades.28, 29 Electrospinning makes it possible to construct long, continuous and aligned polymeric, ceramic, and more importantly composite fibers30, which possess favorable foreground applications in various fields including tissue engineering31, 32, medicine33, 34, filtrations35, sensing36, transparent electrodes37, lithium-ion batteries38, catalysis39, and nanogenerators40, etc. Despite these outstanding potential, the applications of electrospun nano/microfibers in the preparation of flexible TIM composites have been not deeply explored. Until now, many efforts15-17 have been devoted to prepare high-thermal-conductivity polymer composites via increase the loading of thermally conductive fillers. However, the high filler loading causes poor processing performance and significantly deteriorated mechanical property of the composites, which limits their potential applications. In fact, it is still a great challenge to prepare thermally conductive polymer materials via a low loading of filler. Hexagonal boron nitride (h-BN), having a similar hexagonal crystal structure of graphite, is a promising thermally conductive filler because of their intrinsic high thermal conductivity especially when it was exfoliated into two-dimensional (2D) BN nanosheets (BNNSs).41-44 The integrated 2D morphology and appropriate size of BNNSs can reduce the filler-filler thermal resistance via


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large area contact between BNNSs.45-47 More importantly, BNNSs have excellent electrical insulating properties, including high electrical resistivity and low dielectric loss. Taking these merits of h-BN into account, it is believed that high thermal conductivity can be expected by constructing aligned and continuously interconnected BNNS structure in the composites with a low loading of filler. Herein we report a novel strategy to construct flexible polymer nanocomposites with highly through-plane thermal conductivity. Firstly, the polyvinyl alcohol (PVA)/BNNS composite fibers were fabricated by electrospinning and subsequently the fibers were rolled to form a porous and vertically oriented PVA/BNNS cylinder. Second, the PDMS/PVA/BNNS nanocomposites were prepared by vacuum-assisted impregnation of the corresponding cylinders by PDMS. At a low BNNS loading of 15.6 vol%, the nanocomposites exhibit thermal conductivity enhancement of about 10 times in comparison with the pristine PDMS. In addition, the PDMS nanocomposites show excellent electrically insulating. As a TIM of LEDs, the PDMS/PVA/BNNS nanocomposites exhibit outstanding thermal management capability. RESULTS AND DISCUESSION Preparation of the PDMS/PVA/BNNS nanocomposites. BNNSs exfoliated from commercial available h-BN are chosen for electrospinning due to their unique combination of high thermal conductivity, electrical insulation, high mechanical strength, and more importantly, 2D structure nature. The entire fabrication pathway of the PVA/BNNS fibers and the corresponding PDMS nanocomposites was presented in Figure 1. Firstly, PVA and BNNSs were dissolved in deionized water/ethanol mixture to form PVA/BNNS precursor solution. The hydroxyl groups and amino groups on the edges and inner plate of BNNSs make it possible to be easily dispersed in the PVA solution and produce strong hydrogen bonding interaction with the PVA fiber during


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electrospinning.41, 42 In order to generate electrospun fibers with excellent orientation, a rotating drum covered with aluminum foil was used as the collector. After the electrospinning process, the membrane of oriented BNNS/PVA nanofibers was generated at a high rotation rate due to the match of centrifugal and electrical forces during spinning, and was subsequently tailored into narrow strips perpendicular to the direction of rotation. The fiber strips were tightly rolled so that the ultimate PVA/BNNS cylinder still retained the vertical orientation structure. Finally, the PDMS/PVA/BNNS nanocomposites were prepared by vacuum-assisted impregnation of the PVA/BNNS cylinders by PDMS. The optical photograph of PDMS/PVA/BNNS nanocomposites was showed in Figure S1 and S2. The PVA/BNNS cylinders were entirely immersed into the PDMS matrix and the remaining PDMS matrix is totally transparency. In addition, no dissociative PVA/BNNS fibers or BNNSs can be observed in the remaining PDMS matrix, illustrating that the integrated structure of the PVA/BNNS cylinders was entirely retained in the PDMS/PVA/BNNS nanocomposites. Figure S2 demonstrated the excellent flexibility of the PDMS/PVA/BNNS nanocomposites.


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Figure 1. Scheme illustrating the preparation process of PDMS/PVA/BNNS nanocomposites. Figure 2a and 2b presents the exfoliated BNNS morphology characterized by Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM). One can see that the exfoliated h-BN comprises of numerous separated flat or flexible thin sheets. The edge of 7 layers BNNS can be seen from inset of Figure 3b, indicating that the BNNS has a thickness about 2.3 nm. The majority of the BNNSs are 2–3 nm in thickness and 1–2 µm in lateral size.46 Figure 2c presents a SEM image of pure PVA fibers, which have a typical orientation structure comprising of smooth and well defined morphology. The fibers have a narrow diameter range of 300-700 nm. Beads or agglomerated fibers can’t be observed in the obtained PVA fiber membrane. Such a fact also illustrates that PVA can be completely dissolved in the


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water/ethanol solution to form a uniform precursor. Figures 2d-f show the typical SEM images of the PVA/BNNS fiber membranes. The fibers exhibited obvious aligned structure while no PVA beads or agglomerated BNNSs can be found. It is conceivable that BNNSs were uniformly dispersed in the PVA solution. It should be noted that the number and the density of BNNSs on PVA fibers shows an apparent increase as the BNNS concentration increases from 66.7 wt% (Figure 2d) to 75 wt% (Figure 2e). Moreover, the TEM image of single PVA/BNNS-3 fiber (Figure S3) and the SEM image shown in Figure 2f revealed that the BNNSs were highly interconnected and well-stacked on the surface of the PVA fibers, looking like the fallen domino. During the electrospinning process, the BNNSs were self-assembled onto the PVA fiber surface via strong hydrogen bonding interaction and well stacked one by one along the aligned direction of the fibers, where the in-plane plates of BNNSs contact with each other. Large contact area is achieved between BNNSs, which is expected to prominently reduce the thermal contact resistance when heat transfers along the PVA/BNNS fibers. After impregnation by PDMS, the pores and crack of the PVA/BNNS cylinders were fully filled with PDMS, as showed in Figures 2g-I and Figure S4. It is worth noting that the PVA/BNNS fibers exhibit good adhesion with PDMS, no obvious interfacial debonding is observed according to the sections transversal and parallel to fiber direction of the fractured PDMS/PVA/BNNS nanocomposites. The thickness of each PVA/BNNS layer in the cylinders is about 200-300 µm. Moreover, the aligned and interconnected BNNSs structure (i.e., PVA/BNNS cylinder) is well remained in the PDMS matrix. These features would be beneficial to enhance the through-plane thermal conductivity of the PDMS/PVA/BNNS nanocomposites.


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Figure 2. (a) SEM and (b) TEM images of BNNSs; SEM image of (c) aligned PVA fibers, (d) PVA/BNNS-2 fibers, here the BNNS concentration is 66.7 wt%; (e, f) PVA/BNNS-3 fibers, here the BNNS concentration is 75 wt%; (g, h) SEM image of PDMS/PVA/BNNS-2 with section (g) transversal to fiber direction and (h) parallel to fiber direction; (i) SEM image of PDMS/PVA/BNNS-3 with cross-section parallel to fiber direction Thermal and Electrical properties of the PDMS/PVA/BNNS nanocomposites. The thermal conductivity of the nanocomposites in both in-plane and through-plane direction is shown in Figure 3a as a function of BNNS concentration. Thermal conductivities of the pure PDMS and PDMS nanocomposites with randomly distributed BNNSs were also provided for comparison. All the nanocomposites showed enhanced thermal conductivity with increasing BNNS


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concentration. It is apparent that both the PDMS/PVA/BNNS-2 and PDMS/PVA/BNNS-3 nanocomposites showed highly anisotropic heat conduction performance. The vertical orientation of PVA/BNNS-2 and PVA/BNNS-3 in PDMS results in a dramatic enhancement of through-plane thermal conductivity in comparison with the in-plane thermal conductivity and randomly distributed BNNS nanocomposites. The aligned and interconnected BNNSs chains generate “directed percolated” thermal conductive pathway array throughout the nanocomposites with tiny interface thermal resistance (Figure 3b). However, in the other direction or randomly distributed nanocomposites, the thermal conductivity enhancement is suppressed due to the large filler/matrix thermal resistance and the poor heat conduction performance of PDMS. The thermal conductivity of the PDMS is only about 0.18 W/m·K at room temperature. Nevertheless, the PDMS/PVA/BNNS-3 nanocomposites with 15.6 vol% (i.e., 28.7 wt%) BNNSs exhibit high through-plane thermal conductivity of 1.94 W/(m·K) at room temperature (about 978% higher than that of the pure PDMS), which is a little higher than the through-plane thermal conductivity of PDMS/PVA/BNNS-2 nanocomposites at similar BNNS loading based on the fitting line. The reason is that in a single PVA/BNNS-3 fiber most BNNSs are interconnected like “domino” to form an integrated thermal pathway, as showed in Figure 2e. The oriented fibers support the thermal pathway of BNNSs, where phonons tend to transmit between BNNSs because of their contacting with each other. The BNNS-BNNS interface thermal resistance in the oriented and interconnected BNNSs composites can be considered to be much smaller than the BNNS-matrix interface thermal resistance in random dispersed BNNSs composites.27 However,

in a

PVA/BNNS-2 fiber many BNNSs are isolated by the polymer (figure 2d), which prevented the formation of thermal percolation networks. In this case, large interfacial resistance can be formed when phonons transmit between BNNSs and PVA. That is to see, the low thermal conductivity of


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PVA limits thermal conductivity increase of composites. That is, increasing BNNS concentration in a single fiber can enhance the thermal conductivity more effectively in the alignment direction. Figure 3c shows temperature-dependent thermal conductivity of the aligned PDMS/PVA/BNNS nanocomposites and randomly distributed nanocomposites with 23.3 wt% of BNNSs. The thermal conductivities of the samples were not noticeably altered as the temperature increases from 25 to 125 °C, indicating that our vertical orientated nanocomposites exhibit stable thermal conductivities in a broad temperature range, which is promising as TIMs for electronic devices and LED lighting.


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Figure 3. (a) Thermal conductivity at room temperature and (b) Schematic illustration of the enhanced through-plane heat transfer of the nanocomposites; (c) Temperature dependent thermal conductivity of PDMS/PVA/BNNS nanocomposites; (d) Thermal conductivity enhancement of the PDMS/PVA/BNNS nanocomposites and other BN based composites reported in previous work; (e) Thermal conductivity of PDMS/PVA/BNNS-3 nanocomposites with 15.6 vol% of BNNS upon multiple heating and cooling cycles. In order to demonstrate the superiority of the vertical aligned and interconnected BNNS structure in enhancing the thermal conductivity of composites, Figure 3d summarizes previously reported thermal conductivity enhancement (i.e., the percent of thermal conductive increase of composites to the corresponding polymer matrix) of BN based polymer composites20, 48-56. The vertical aligned and interconnected BNNS composite exhibits the highest thermal conductivity enhancement among those reported BN based polymer composites. This result suggests that the aforementioned structure has the better capability to improve heat transfer performance of polymer composites due to its high through-plane thermal conductivity. Figure 3e shows the thermal conductivity variation of PDMS/PVA/BNNS-3 nanocomposites with 15.6 vol% of BNNS upon multiple heating and cooling cycles alternating between 25 and 125 °C. The


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thermal conductivity exhibits a slight change (± 4%) within the 20 cycles, suggesting stable capability of heat conduction for TIMs in this temperature range. Such a small variation of thermal conductivity with temperature would be beneficial for the long-term device operation. Thermal stability is also an important parameter to evaluate the thermal performance of the composites, which basically illustrates the maximum operating temperature of PDMS based nanocomposites. Figure S5 presents the TGA curves of PDMS/BNNS, PDMS/PVA/BNNS-2 nanocomposite and PVA/BNNS-2 fiber. The PDMS/PVA/BNNS-2 nanocomposite showed better thermal stability than PVA/BNNS-2 fiber. More importantly, the residual mass fraction of PVA/BNNS-2 fiber is identical with the calculated value of BNNS loading. Since the samples used for the TGA measurement were randomly selected, these results also illustrated the uniform distribution of the PVA/BNNS fibers in the PDMS/PVA/BNNS nanocomposites. To understand the mechanism of thermal conduction of BNNS based composites, the interfacial thermal resistance in random dispersed and vertical oriented BNNS based composites were calculated using the effective medium theory (EMT) and Foygel’s theory, respectively. In PDMS/BNNS composite, thermal conductivity (K) follow EMT model57:

= and

=

(1)

×

(2)

where Km (0.18 W/(m·K)) is thermal conductivity of the PDMS, and Kp (300 W/(m·K)) is thermal conductivity of the filler; Vf is the volume fraction of BNNSs; R1 is the interfacial thermal resistance between PDMS and filler; d (3 nm) is the thickness of BNNS. In


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PDMS/BNNS composites, the calculated data of R1 is 1.9 ×10-5 m2·K/W. The EMT model provides a reasonable interfacial thermal resistance for random dispersed BNNSs composite. However, it is unsuitable to fit the vertically aligned and interconnected BNNS based composites, where the interfacial thermal resistance is mainly between BNNSs layers. Here, Foygel’s theory58 is adopted to analysis the interface thermal resistance.

= − " #$

'" =

%&

(3)

(4)

×(×[* &],-

where K0 is pre-exponential factor in relation to the contact between BNNSs as well as the topology of the percolation cluster; Vc(a) is the critical volume fraction of BNNSs, which depends on the morphology and aspect ratio (a) of BNNSs; t(a) is the conductivity exponent that is dependent on the aspect ratio of filler; Rc is the contact resistance between BNNSs; L is the length of BNNSs. In our previous work46, BNNSs are counted to have a majority of about 1 µm. In PDMS/PVA/BNNS-3 nanocomposite, by fitting the through-plane thermal conductivity, the values of K0, Vc(a) and t(a) are respectively obtained to be 21, 0.007 and 1.2. Then the calculated data of Rc is 1.8 ×107 K/W. We assume that 5% of a BNNS layer area contact with other BNNSs and contributes to the heat conduction, then the active interface area between BNNSs can be estimated as 5 × 10-14 m2. Based on the above data, the interfacial thermal resistance (R2) in PDMS/PVA/BNNS-3 nanocomposite is computed to be 9 ×10-7 m2·K/W. It is worth noting that R1 is two orders of magnitude higher than R2 of PDMS/PVA/BNNS-3 nanocomposite. These theoretical results are consistent with the experimental results of thermal


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conductivity and can be used to account for high thermal conductivity in the through plane of the PDMS/PVA/BNNS-3 nanocomposite. For electrical insulation applications, a high enough electrical resistivity of the composites is of crucial importance. Figure 4 presents the electrical conductivity of the PDMS nanocomposites at 0.1Hz, which can be roughly considered as the DC electrical conductivity because of their weak frequency dependence. The introduction of PVA fibers into PDMS results in an electrical conductivity increase of two orders of magnitude, which should be attributed to the low electrical resistivity of PVA and the enhanced mobility of charge carriers along the PVA fibers. The frequency dependent electrical conductivity was showed in Figure S6. One can see that the introduction of PVA fibers into PDMS results in a distinct increase and a weak frequency dependence of electrical conductivity, which is consistent with the increased leakage current in the PDMS/PVA composites. The addition of BNNSs, however, results in a slight decrease of the electrical conductivity of PDMS, which should be attributed to retardant effect of BNNSs on the mobility of charge carriers.3 In PDMS/PVA based composites, the retardant effect of BNNSs on the mobility of charge carriers become more apparent, ultimately resulting in significant decrease of electrical conductivities of the PDMS/PVA/BNNS composites, making the PDMS/PVA based composites have comparable electrical conductivity with pure PDMS.


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Figure 4. The electric conductivity of the PDMS, PDMS/PVA, PDMS/BNNS and PDMS/PVA/BNNS composites both in-plane (=) and through-plane (⊥) direction at 0.1 Hz.

Thermal management capability of the epoxy/3D-C-BNNS nanocomposites. In order to practically

evaluate

the

thermal

management

performance,

the

PDMS/PVA/BNNS

nanocomposite was used as TIM of a 10 W LED chip. The surface temperature variations of the lighted LED chip with time were recorded by an infrared thermal imager. PDMS, PDMS/BNNS and PDMS/PVA/BNNS-3 nanocomposites with the same size were respectively used as TIMs between the LED chip and a heat sink. The surface temperature variations of three 10 W LED chips without TIM and heat sink operated in the same condition is showed in Figure S7. Almost same temperature rise curves (± 1

o

C) were observed, demonstrating the excellent

reproducibility in the surface temperature of the LED chips operated in the same condition. Figure 5 showed the infrared thermal images (Figure 5a) and the corresponding plots of temperature against time (Figure 5b) of the LED chip integrated with TIM and heat sink. The overall process including the above three samples was recorded in Video 1 in Supporting


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Information. It can be seen that during the running process, the PDMS/PVA/BNNS-3 composites display much lower temperature rise in comparison with the pristine PDMS or the PDMS/BNNS composite based heat sink. For the PDMS/PVA/BNNS composite, the equilibrium temperature is only around 63 oC, which is respectively droped by 22 and 33 oC in comparison with the PDMS/BNNS composite and the pristine PDMS. This result illustrates that the PDMS/PVA/BNNS composites have much better thermal management capability, which is in good agreement with order of thermal conductivity of the three samples. The improved thermal management capability can guarantee timely dissipating of internally generated heat, resulting increased lifetime and efficiency of the LEDs eventually.

Figure 5. (a) Infrared thermal images of the LED chips integrated with TIMs and heat sink; (b) schematic diagram of the structure in a LED chip; (c) optical photograph of the LED chip; (d) The surface temperature variations of the LED chips against time.


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In a LED device (Figure 6), the thermal resistance of TIM (RTIM) is crucial for operating temperature, but the interface thermal resistance between the TIM and LED chip (R1) and between the TIM and heat sink (R2) is also worth attention, which may influence the eventual surface temperature test results of LEDs. In order to reduce the impact of the interface thermal interface, a thin layer of thermal conductive silver glue was used in the interface between the TIM and the LED chip and between the TIM and heat sink. In order to understand the reduction in the surface temperature of the LED, a simplified thermal resistance network model for the LED device was showed in Figure 6. T1 and T2 are the temperatures of working LED chip and heat sink, respectively. The thermal resistance of TIM and the interface thermal resistance of thermal conductive silver glue between the TIM and the LED chip or the heat sink were estimated in supporting information. The calculated of RTIM values are 3.39 K/W, 11.29 K/W and 33.77 K/W in PDMS/PVA/BNNS, PDMS/BNNS and PDMS, respectively. Since the same thermal conductive silver glue was used and the contact area between the TIM and the LED chip or the heat sink is the same, the sum of R1 and R2 can be considered to be very similar for different LED devices. Here, the calculated sum of R1 and R2 is 1.79 K/W in PDMS/PVA/BNNS, which is much lower than those of the calculated RTIM values of PDMS/PVA/BNNS, PDMS/BNNS and PDMS. Therefore, it can be concluded that the large reduction of surface temperature of the LED mainly originate from the reduced thermal resistance of the PDMS/PVA/BNNS composites.


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Figure 6. A simplified thermal resistance network model for a LED device.

CONCLUSIONS In summary, flexible PDMS/PVA/BNNS nanocomposite with ultra-high through-plane thermal conductivity were successfully prepared by constructing simultaneously oriented and interconnected structure of BNNSs via electrospinning. The nanocomposites not only show strong anisotropy with significantly enhanced through-plane thermal conductivity, but also are still highly electrically insulating. At a low BNNS loading of 15.6 vol%, the nanocomposite exhibit about 10 times of through-plane thermal conductivity enhancement of PDMS. For practical applications, the equilibrium temperature of the LED chip integrated with the nanocomposite TIM is about 33 oC lower in comparison with the pristine PDMS, displaying strong potential of thermal management applications for high-power electronic devices. EXPERIMENTAL Materials. The PVA (99.7%) and isopropanol (99%) were purchased from Adamas Reagent Ltd (China) and used as received. The BN powders with an average diameter of 3 µm were purchased from ESK Ceramics GmbH & Co. (Germany). PDMS (Sylgard 184) was supplied by Dow Corning Co., Ltd (China).


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Exfoliation of h-BN. The liquid phase exfoliation of h-BN proceeded according a previous work46. Briefly, 3.5 g h-BN powder was added into a 200 ml mixture solvent of isopropanol and deionized water (1/1). The solution was sonicated in FS-450N model sonication bath (Shanghai ShengXi ultrasonic instrument co., LTD) for 4 h with the frequency of 200 kHz. The resulting dispersions were centrifuged using TGL-15B supercentrifuge (Shanghai MeiYingPu instrument manufacturing co., China) at 4000 r.p.m. for 10 min to remove non-exfoliated h-BN. The supernatants were collected and centrifuged at 9000 r.p.m. for 30 min to collect the exfoliated BNNSs. Preparation of BNNS/PVA nanofiber membrane. The precursor solution was prepared by dissolving PVA in deionized water first at a concentration of 17.5 wt%, BNNSs in ethanol at a concentration of 17.5 wt% respectively, and then mixing them with a certain ratio. After stirring thoroughly for 6 h, the solution was left for half an hour before electrospinning. During electrospinning, the spinning voltage was 16 kV, and the work distance between the spinneret and the collector was 16 cm. The flow rate controlled by a syringe pump was of 1 ml per hour. The electrospinning proceeded at 23-27 V, and the ambient humidity was from 40 to 50% relative humidity (RH). After electrospinning, the BNNS/PVA nanofiber membrane was peeled out from aluminium foil and placed into 70 oC vacuum oven for 12 h drying. The PVA nanofibres were prepared using the similar process. The obtained nanofiber membrane was tailored into 15 mm narrow PVA/BNNS strips. As shown in Fig. 2, the strip was rolled up perpendicular to direction of fiber orientation, then next strip was interlinked with the previous one. A whole PVA/BNNS cylinder was formed until the diameter of cylinder was about 15 mm. The weight ratios of the BNNS to PVA in PVA/BNNS cylinders was controlled in 3/1 and 2/1, which were respectively designated as PVA/BNNS-3


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and PVA/BNNS-2, and the corresponding PDMS composites were designated as PDMS/PVA/BNNS-3 and PDMS/PVA/BNNS-2 Preparation

of

the PDMS/PVA/BNNS

nanocomposites.

The PDMS/PVA/BNNS

nanocomposites were fabricated using vacuum-assisted impregnation of PDMS. Firstly, the PDMS prepolymer and ethyl acetate uniformly mixed at room temperature. The weight ratio of component A, B and ethyl acetate was controlled in 10/1/1. The PVA/BNNS cylinder was then immersed completely into the mixture. After infiltration for 1 h, the samples were transferred into a vacuum oven at ambient temperature for 6 h to remove the air. Finally, the composites were cured at 100 °C for 1 h. The PDMS nanocomposites with randomly distributed BNNSs were also prepared by straightforwardly dispersing the same ratio of BNNSs into the PDMS matrix with identical curing process. Characterization. The morphology of the BNNSs, PVA fibers, PVA/BNNS fibers and PDMS/PVA/BNNS nanocomposites was observed by a field emission SEM (Nova NanoSEM 450, FEI, USA). The cross-sections of nanocomposites were prepared by fracturing the samples in liquid nitrogen. The morphology of the BNNSs was also observed by a TEM (JEM-2010, JEOL, Japan) at an acceleration voltage of 200 kV. The TEM samples were prepared by dropping a few drops of the BNNSs solution on a carbon-coated cooper grids and air-dried before measurement. The optical photos of the PDMS and nanocomposites were taken by digital camera (A6000, Sony). Thermal conductivity (K) was measured through the laser flash technique (LFA 467 HT HyperFlash@, NanoFlash, Netzsch). Thermal conductivity λ (W/(m·K)) was calculated as a multiplication of density (ρ, g/cm), specific heat (Cp, J/(g·K) and thermal diffusivity (D, mm2/s). Namely, λ = ρ × Cp × D. The density was assessed by the water displacement method, the specific heat and thermal diffusivity were measured by LFA 467. The


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composites with the same size (13 mm in diameter and 0.8 mm thickness) were placed between 10W LED chips and heat sinks, then LED chips work simultaneously at ambient temperature. The time dependent temperature of LED chips was recorded by an infra-red thermograph (FLIR E30). The surface temperature of different LED chips operated in the same condition was captured by an thermal infrared imager (FOTRIC 226). The frequency dependent electrical conductivity was measured by using a Novocontrol Alpha-N high resolution dielectric analyzer (GmbH Concept 40) with the frequency range 10−1-106 Hz at room temperature. A layer of gold was evaporated on both sides of the samples to serve as electrodes. Thermogravimetric analysis (TGA) of nanocomposites was performed using NETZSCH TG209 F3 at the range of 50 to 250 °C under nitrogen atmosphere with a heating rate of 20 °C min−1. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: The optical photograph of PDMS/PVA/BNNS nanocomposites before cutting; the optical photograph of flexible PDMS/PVA/BNNS nanocomposite; TEM images of PVA/BNNS-3; SEM image of PDMS/PVA/BNNS nanocomposite, Inset is simulated location in composites; TGA curves of PDMS/BNNS, PDMS/PVA/BNNS-2 composite and PVA/BNNS-2 fiber; frequency dependence of conductivity of the composites. (PDF) The surface temperature variation against time (Figure 5b) of the LED chip integrated with various TIM and heat sink. (AVI) AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This research was supported by the Special Fund of the National Priority Basic Research of China (no. 2014CB239503) and National Natural Science Foundation of China (nos. 51522703, 51477096).

REFERENCES (1)

Ji, H. X.; Sellan, D. P.; Pettes, M. T.; Kong, X. H.; Ji, J. Y.; Shi, L.; Ruoff, R. S.

Enhanced Thermal Conductivity of Phase Change Materials with Ultrathin-Graphite Foams for Thermal Energy Storage Energ. Environ. Sci. 2014, 7, 1185-1192. (2)

Xin, G.; Yao, T.; Sun, H.; Scott, S. M.; Shao, D.; Wang, G.; Lian, J. Highly Thermally

Conductive and Mechanically Strong Graphene Fibers Science 2015, 349, 1083-1087. (3)

Li, Q.; Chen, L.; Gadinski, M. R.; Zhang, S. H.; Zhang, G. Z.; Li, H. Y.; Haque, A.; Chen,

L. Q.; Jackson, T.; Wang, Q. Flexible High-Temperature Dielectric Materials From Polymer Nanocomposites Nature 2015, 523, 576-580. (4)

Chen, H.; Ginzburg, V. V.; Yang, J.; Yang, Y.; Liu, W.; Huang, Y.; Du, L.; Chen, B.

Thermal Conductivity of Polymer-Based Composites: Fundamentals and Applications Prog. Polym. Sci. 2016, 59, 41-85.


22

Page 23 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(5)

Zeng, X.; Sun, J.; Yao, Y.; Sun, R.; Xu, J.-B.; Wong, C.-P. A Combination of Boron

Nitride Nanotubes and Cellulose Nanofibers for the Preparation of a Nanocomposite with High Thermal Conductivity ACS Nano 2017, DOI: 10.1021/acsnano.7b02359. (6)

Zhao, B.; Jiang, L.; Zeng, X.; Zhang, K.; Yuen, M. M. F.; Xu, J.-B.; Fu, X.-Z.; Sun, R.;

Wong, C.-P. A Highly Thermally Conductive Electrode for Lithium Ion Batteries J. Mater. Chem. A 2016, 4, 14595-14604. (7)

Loeblein, M.; Tsang, S. H.; Pawlik, M.; Phua, E. J. R.; Yong, H.; Zhang, X. W.; Gan, C.

L.; Teo, E. H. T. High-Density 3D-Boron Nitride and 3D-Graphene for High-Performance NanoThermal Interface Material ACS Nano 2017, 11, 2033-2044. (8)

Suh, D.; Moon, C. M.; Kim, D.; Baik, S. Ultrahigh Thermal Conductivity of Interface

Materials by Silver-Functionalized Carbon Nanotube Phonon Conduits Adv. Mater. 2016, 28, 7220-7227. (9)

Cho, E.-C.; Huang, J.-H.; Li, C.-P.; Chang-Jian, C.-W.; Lee, K.-C.; Hsiao, Y.-S.; Huang,

J.-H. Graphene-Based Thermoplastic Composites and their Application for LED Thermal Management Carbon 2016, 102, 66-73. (10) Jiang, H.; Wang, Z.; Geng, H.; Song, X.; Zeng, H.; Zhi, C. Highly Flexible and SelfHealable Thermal Interface Material Based on Boron Nitride Nanosheets and a Dual CrossLinked Hydrogel ACS Appl. Mater. Interfaces 2017, 9, 10078-10084. (11) Han, Z.; Fina, A. Thermal Conductivity of Carbon Nanotubes and Their Polymer Nanocomposites: A Review Prog. Polym. Sci. 2011, 36, 914-944. (12) Huang, X.; Zhi, C.; Jiang, P.; Golberg, D.; Bando, Y.; Tanaka, T. Polyhedral Oligosilsesquioxane-Modified Boron Nitride Nanotube Based Epoxy Nanocomposites: An Ideal Dielectric Material with High Thermal Conductivity Adv. Funct. Mater. 2013, 23, 1824-1831.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

(13) Kuang, Z. Q.; Chen, Y. L.; Lu, Y. L.; Liu, L.; Hu, S.; Wen, S. P.; Mao, Y. Y.; Zhang, L. Q.

Fabrication

of

Highly

Oriented

Hexagonal

Boron

Nitride

Nanosheet/Elastomer

Nanocomposites with High Thermal Conductivity Small 2015, 11, 1655-1659. (14) Wang, X.; Weng, Q.; Wang, X.; Li, X.; Zhang, J.; Liu, F.; Jiang, X.; Guo, H.; Xu, N.; Golberg, D.; Bando, Y. Biomass-Directed Synthesis of 20 g High-Quality Boron Nitride Nanosheets for Thermoconductive Polymeric Composites ACS Nano 2014, 8, 9081-9088. (15) Dai, W.; Yu, J.; Liu, Z.; Wang, Y.; Song, Y.; Lyu, J.; Bai, H.; Nishimura, K.; Jiang, N. Enhanced Thermal Conductivity and Retained Electrical Insulation for Polyimide Composites with SiC Nanowires Grown on Graphene Hybrid Fillers Composites, Part A 2015, 76, 73-81. (16) Burger, N.; Laachachi, A.; Ferriol, M.; Lutz, M.; Toniazzo, V.; Ruch, D. Review of Thermal Conductivity in Composites: Mechanisms, Parameters and Theory Prog. Polym. Sci. 2016, 61, 1-28. (17) Zhao, H.; Yue, Y.; Zhang, Y.; Li, L.; Guo, L. Ternary Artificial Nacre Reinforced by Ultrathin Amorphous Alumina with Exceptional Mechanical Properties Adv. Mater. 2016, 28, 2037-2042. (18) Yao, Y.; Zeng, X.; Sun, R.; Xu, J.-b.; Wong, C.-P. Highly Thermally Conductive Composite Papers Prepared Based on the Thought of Bioinspired Engineering ACS Appl. Mater. Interfaces 2016, 8, 15645-15653. (19) Fu, L.; Wang, T.; Yu, J. H.; Dai, W.; Sun, H. Y.; Liu, Z. D.; Sun, R.; Jiang, N.; Yu, A. M.; Lin, C. T. An Ultrathin High-Performance Heat Spreader Fabricated with Hydroxylated Boron Nitride Nanosheets 2D Materials 2017, 4, 025047-025053. (20) Morishita,

T.;

Okamoto,

H.

Facile

Exfoliation

and

Noncovalent

Superacid

Functionalization of Boron Nitride Nanosheets and Their Use for Highly Thermally Conductive


24

Page 25 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and Electrically Insulating Polymer Nanocomposites ACS Appl. Mater. Interfaces 2016, 8, 27064-27073. (21) Yang, J.; Qi, G.-Q.; Tang, L.-S.; Bao, R.-Y.; Bai, L.; Liu, Z.-Y.; Yang, W.; Xie, B.-H.; Yang, M.-B. Novel Photodriven Composite Phase Change Materials with Bioinspired Modification of BN for Solar-Thermal Energy Conversion and Storage J. Mater. Chem. A 2016, 4, 9625-9634. (22) Shen, H.; Guo, J.; Wang, H.; Zhao, N.; Xu, J. Bioinspired Modification of h-BN for High Thermal Conductive Composite Films with Aligned Structure ACS Appl. Mater. Interfaces 2015, 7, 5701-5708. (23) Tanimoto, M.; Yamagata, T.; Miyata, K.; Ando, S. Anisotropic Thermal Diffusivity of Hexagonal Boron Nitride-Filled Polyimide Films: Effects of Filler Particle Size, Aggregation, Orientation, and Polymer Chain Rigidity ACS Appl. Mater. Interfaces 2013, 5, 4374-4382. (24) Choi, J. R.; Yu, S.; Jung, H.; Hwang, S. K.; Kim, R. H.; Song, G.; Cho, S. H.; Bae, I.; Hong, S. M.; Koo, C. M.; Park, C. Self-Assembled Block Copolymer Micelles with SilverCarbon Nanotube Hybrid Fillers for High Performance Thermal Conduction Nanoscale 2015, 7, 1888-1895. (25) Zhang, J.; Shi, G.; Jiang, C.; Ju, S.; Jiang, D. 3D Bridged Carbon Nanoring/Graphene Hybrid Paper as a High-Performance Lateral Heat Spreader Small 2015, 11, 6197-6204. (26) Zhang, Y.-F.; Han, D.; Zhao, Y.-H.; Bai, S.-L. High-Performance Thermal Interface Materials Consisting of Vertically Aligned Graphene Film and Polymer Carbon 2016, 109, 552557.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

(27) Zeng, X.; Yao, Y.; Gong, Z.; Wang, F.; Sun, R.; Xu, J.; Wong, C. P. Ice-Templated Assembly Strategy to Construct 3D Boron Nitride Nanosheet Networks in Polymer Composites for Thermal Conductivity Improvement Small 2015, 11, 6205-6213. (28) Sun, B.; Long, Y. Z.; Zhang, H. D.; Li, M. M.; Duvail, J. L.; Jiang, X. Y.; Yin, H. L. Advances in Three-Dimensional Nanofibrous Macrostructures via Electrospinning Prog. Polym. Sci. 2014, 39, 862-890. (29) Si, Y.; Yu, J.; Tang, X.; Ge, J.; Ding, B. Ultralight Nanofibre-Assembled Cellular Aerogels with Superelasticity and Multifunctionality Nat. Commun. 2014, 5, 5802-5810. (30) Wang, X.; Ding, B.; Sun, G.; Wang, M.; Yu, J. Electro-Spinning/Netting: A Strategy for the Fabrication of Three-Dimensional Polymer Nano-Fiber/Nets Prog. Mater. Sci. 2013, 58, 1173-1243. (31) Wu, S.; Duan, B.; Liu, P.; Zhang, C.; Qin, X.-H.; Butcher, J. T. Fabrication of Aligned Nanofiber Polymer Yarn Networks for Anisotropic Soft Tissue Scaffolds ACS Appl. Mater. Interfaces 2016, 8, 16950-16960. (32) Moroni, L.; Schotel, R.; Hamann, D.; de Wijn, J. R.; van Blitterswijk, C. A. 3D FiberDeposited Electrospun Integrated Scaffolds Enhance Cartilage Tissue Formation Adv. Funct. Mater. 2008, 18, 53-60. (33) Yohe, S. T.; Colson, Y. L.; Grinstaff, M. W. Superhydrophobic Materials for Tunable Drug Release: Using Displacement of Air to Control Delivery Rates J. Am. Chem. Soc. 2012, 134, 2016-2019. (34) Carlson, A. L.; Bennett, N. K.; Francis, N. L.; Halikere, A.; Clarke, S.; Moore, J. C.; Hart, R. P.; Paradiso, K.; Wernig, M.; Kohn, J.; Pang, Z. P.; Moghe, P. V. Generation and


26

Page 27 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Transplantation of Reprogrammed Human Neurons in the Brain Using 3D Microtopographic Scaffolds Nat. Commun. 2016, 7, 10862-10871. (35) Si, Y.; Fu, Q.; Wang, X.; Zhu, J.; Yu, J.; Sun, G.; Ding, B. Superelastic and Superhydrophobic Nanofiber-Assembled Cellular Aerogels for Effective Separation of Oil/Water Emulsions ACS Nano 2015, 9, 3791-3779. (36) Sun, B.; Long, Y. Z.; Liu, S. L.; Huang, Y. Y.; Ma, J.; Zhang, H. D.; Shen, G.; Xu, S. Fabrication of Curled Conducting Polymer Microfibrous Arrays via a Novel Electrospinning Method For Stretchable Strain Sensors Nanoscale 2013, 5, 7041-7045. (37) An, S.; Jo, H. S.; Kim, D.-Y.; Yarin, A. L.; Yoon, S. S.; Lee, H. J.; Ju, B.-K.; Al-Deyab, S. S.; Ahn, J.-H.; Qin, Y.; Swihart, M. T.; Yarin, A. L. Self-Junctioned Copper Nanofiber Transparent Flexible Conducting Film via Electrospinning and Electroplating Adv. Mater. 2016, 28, 7149-7154. (38) Dufficy, M. K.; Khan, S. A.; Fedkiw, P. S. Hierarchical Graphene-Containing Carbon Nanofibers for Lithium-Ion Battery Anodes ACS Appl. Mater. Interfaces 2016, 8, 1327-1336. (39) Son, H. Y.; Ryu, J. H.; Lee, H.; Nam, Y. S. Bioinspired Templating Synthesis of MetalPolymer Hybrid Nanostructures within 3D Electrospun Nanofibers ACS Appl. Mater. Interfaces 2013, 5, 6381-6390. (40) Li, T.; Xu, Y.; Willander, M.; Xing, F.; Cao, X.; Wang, N.; Wang, Z. L. Lightweight Triboelectric Nanogenerator for Energy Harvesting and Sensing Tiny Mechanical Motion Adv. Funct. Mater. 2016, 26, 4370-4376. (41) Zhu, H. L.; Li, Y. Y.; Fang, Z. Q.; Xu, J. J.; Cao, F. Y.; Wan, J. Y.; Preston, C.; Yang, B.; Hu, L. B. Highly Thermally Conductive Papers with Percolative Layered Boron Nitride Nanosheets ACS Nano 2014, 8, 3606-3613.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

(42) Xiao, F.; Naficy, S.; Casillas, G.; Khan, M. H.; Katkus, T.; Jiang, L.; Liu, H.; Li, H.; Huang, Z. Edge-Hydroxylated Boron Nitride Nanosheets as an Effective Additive to Improve the Thermal Response of Hydrogels Adv. Mater. 2015, 27, 7196-7203. (43) Weng, Q.; Wang, X.; Wang, X.; Bando, Y.; Golberg, D. Functionalized Hexagonal Boron Bitride Banomaterials: Emerging Properties and Applications Chem. Soc. Rev. 2016, 45, 3989-4012. (44) Yang, Z.; zhou, l.; Luo, W.; Wan, J.; Dai, J.; Han, X.; Fu, K.; Henderson, D.; Yang, B.; Hu, L. Thermally Conductive, Dielectric PCM-Boron Nitride Nanosheets Composite for Efficient Electronic System Thermal Management Nanoscale 2016, 8, 19326-19333. (45) Lei, W.; Mochalin, V. N.; Liu, D.; Qin, S.; Gogotsi, Y.; Chen, Y. Boron Nitride Colloidal Solutions, Ultralight Aerogels and Freestanding Membranes Through One-Step Exfoliation and Functionalization Nat. Commun. 2015, 6, 8849-8856. (46) Chen, J.; Huang, X.; Zhu, Y.; Jiang, P. Cellulose Nanofiber Supported 3D Interconnected BN Nanosheets for Epoxy Nanocomposites with Ultrahigh Thermal Management Capability Adv. Funct. Mater. 2016, 27, 1604754-1604762. (47) Zeng, X. L.; Ye, L.; Yu, S. H.; Li, H.; Sun, R.; Xu, J. B.; Wong, C. P. Artificial NacreLike Papers Based on Noncovalent Functionalized Boron Nitride Nanosheets with Excellent Mechanical and Thermally Conductive Properties Nanoscale 2015, 7, 6774-6781. (48) Yang, J.; Tang, L.-S.; Bao, R.-Y.; Bai, L.; Liu, Z.-Y.; Yang, W.; Xie, B.-H.; Yang, M.-B. An Ice-Templated Assembly Strategy to Construct Graphene Oxide/Boron Nitride Hybrid Porous Scaffolds in Phase Change Materials with Enhanced Thermal Conductivity and Shape Stability for Light-Thermal-Electric Energy Conversion J. Mater. Chem. A 2016, 4, 1884118851.


28

Page 29 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(49) Cui, X.; Ding, P.; Zhuang, N.; Song, N.; Tang, S.; Cui, X.; Shi, L. Thermal Conductive and Mechanical Properties of Polymeric Composites Based on Solution-Exfoliated Boron Nitride and Graphene Nanosheets: A Morphology-Promoted Synergistic Effect ACS Appl. Mater. Interfaces 2015, 7, 19068-19075. (50) Shen, H.; Cai, C.; Guo, J.; Qian, Z.; Zhao, N.; Xu, J. Fabrication of Oriented hBN Scaffolds for Thermal Interface Materials RSC Adv. 2016, 6, 16489-16494. (51) Wu, H. C.; Kessler, M. R. Multifunctional Cyanate Ester Nanocomposites Reinforced by Hexagonal Boron Nitride after Noncovalent Biomimetic Functionalization ACS Appl. Mater. Interfaces 2015, 7, 5915-5926. (52) Xu, S.; Liu, H.; Li, Q.; Mu, Q.; Wen, H. Influence of Magnetic Alignment and Layered Structure of BN&Fe/EP on Thermal Conducting Performance J. Mater. Chem. C 2016, 4, 872878. (53) Wang, H.; Tazebay, A. S.; Yang, G.; Lin, H. T.; Choi, W.; Yu, C. Highly Deformable Thermal Interface Materials Enabled by Covalently-Bonded Carbon Nanotubes Carbon 2016, 106, 152-157. (54) Lin, Z. Y.; Liu, Y.; Raghavan, S.; Moon, K. S.; Sitaraman, S. K.; Wong, C. P. Magnetic Alignment of Hexagonal Boron Nitride Platelets in Polymer Matrix: Toward High Performance Anisotropic Polymer Composites for Electronic Encapsulation ACS Appl. Mater. Interfaces 2013, 5, 7633-7640. (55) Jin, W.; Yuan, L.; Liang, G.; Gu, A. Multifunctional Cyclotriphosphazene/Hexagonal Boron Nitride Hybrids and their Flame Retarding Bismaleimide Resins with High Thermal Conductivity and Thermal Stability ACS Appl. Mater. Interfaces 2014, 6, 14931-14944.


29


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(56) Li, Q.; Zhang, G. Z.; Liu, F. H.; Han, K.; Gadinski, M. R.; Xiong, C. X.; Wang, Q. Solution-Processed Ferroelectric Terpolymer Nanocomposites with High Breakdown Strength and Energy Density Utilizing Boron Nitride Nanosheets Energ. Environ. Sci. 2015, 8, 922-931. (57) Nan, C. W., Birringer, R., Clarke, D. R., & Gleiter, H. Effective Thermal Conductivity of Particulate Composites with Interfacial Thermal Resistance J. Mater. Chem. A 1997, 81, 66926699. (58) Foygel, M., Morris, R. D., Anez, D., French, S., & Sobolev, V. L. Theoretical and Computational Studies of Carbon Nanotube Composites and Suspensions: Electrical and Thermal Conductivity. Phys. Rev. C 2005, 71, 104201-104208.

ToC FIGURE


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Vertically Aligned and Interconnected Boron Nitride Nanosheets for

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