Insulating polymer nanocomposites with high-thermal-conduction routes via linear densely packed boron nitride nanosheets

Composites Science and Technology 129 (2016) 205e213

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Insulating polymer nanocomposites with high-thermal-conduction routes via linear densely packed boron nitride nanosheets Hong-Baek Cho a, b, Tadachika Nakayama a, **, Hisayuki Suematsu a, Tsuneo Suzuki a, Weihua Jiang a, Koichi Niihara a, Eunpil Song b, Nu Si A. Eom b, Seil Kim b, Yong-Ho Choa b, * a b

Extreme Energy-Density Research Institute, Nagaoka University of Technology, Nagaoka, Niigata 940-2188, Japan Department of Fusion Chemical Engineering, Hanyang University, Ansan, Gyeonggi 426-791, South Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 October 2015 Received in revised form 26 April 2016 Accepted 30 April 2016 Available online 1 May 2016

Electrically insulating polymeric nanocomposites with high thermal conductivity have great potential for use as thermal-management materials in increasingly high-power-density electronics and optoelectronics. Conventional composite materials require a large amount, over 70 vol%, of electrically conducting fillers such as carbon allotropes to attain thermal conductivities of 1e5 W/mK, [Balandin, 2011] [1] which restricts the utility of these materials to applications that require both electrical and thermal conductivities. Here, we introduce a strategy to achieve the strongest enhancement of thermal conductivity to date at a low level of filler loading (
15 vol%) in insulating polymer nanocomposites with hexagonal boron nitride (BN) nanosheets. The combination of electric-field switching and the application of fillers with various aspect ratios enables the rearrangement of the BN nanofillers into linear densely packed BN structures (LDPBNs). Flexible nanocomposite films with LDPBNs exhibit electrical resistivity greater than 1.50  106 MU cm and a thermal conductivity of 1.56 W/mK, a dramatic enhancement over that of pristine polysiloxane with the same BN loading (0.4 W/mK). Our strategy of electric-field-induced BN nanosheet assembly offers insight into the possibility of solving thermal-management problems using ideal thermal interface materials, thus enabling improved next-generation integrated circuits and nanoelectronics. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Nanocomposites Polymer-matrix composites (PMCs) Electrical properties Thermal properties Anisotropy

1. Introduction Polymer-based nanocomposites have recently attracted significant interest because the ability to achieve a refined structure with a proper orientation of hard inorganic particles enables the control of the deformation, failure, heat resistance, and thermal properties of a polymer [2,3]. Organic-inorganic composite materials that contain small amounts of insulating nanoceramics could potentially be used as thermal interface materials (TIMs), which require high thermal conductivity and electrical resistivity. Because of the rapidly increasing power density in electronics such as nextgeneration integrated circuits, 3-dimensional integrated circuits, and ultra-high-power-density transistors, the efficient removal of

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T. Nakayama), choa15@hanyang. ac.kr (Y.-H. Choa). http://dx.doi.org/10.1016/j.compscitech.2016.04.033 0266-3538/© 2016 Elsevier Ltd. All rights reserved.

heat is becoming crucial to the performance and reliability of these devices. [6] However, the very low thermal conductivity of polymers and the thermal boundary resistance (TBR) [7] between the polymer and the filler are the two major obstacles to effective thermal conduction in polymer-based composites. Although polymers are excellent electrical insulators, and their flexible nature offers easy workability, they are the worst heat-conducting bulk solids, and phonon-phonon interactions in isotropic polymers cannot achieve thermal conductivities in excess of ~0.3 W/mK. [8] Because a high TBR is the result of an interface that constitutes an interruption in the regular crystalline lattice in which phonons propagate, [9] a potentially effective approach is to orient the filler along the direction of thermal flow. [10] Boron nitride (BN) ceramics are excellent conductors of heat, comparable to aluminum nitride, as well as electrical insulators, and they exhibit thermal conductivities that are among the highest of all electrical insulators. [11] Furthermore, hexagonal BN (h-BN) with a graphite-like layered structure exhibits anisotropic thermal

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conductivity; its in-plane thermal conductivity is 600 W/mK, which is 20 times higher than its out-of-plane value of 30 W/mK [12]. A polymer nanocomposite whose in-plane direction is perpendicularly oriented in the polymer matrix can attain a higher thermal conductivity at a low filler content [13,14]. The alignment of nanosheets, or 2-D fillers, via reorientation in the polymer matrix is a crucial technique for such composites, and shear forces [15], magnetic forces [16], and electric fields [15,17] have been widely used to tune the anisotropic orientations of nanosheets in polymer matrices. Recently, the shear-induced extrusion of a mixture of epoxy resin and BN nanosheets has been demonstrated to cause the longitudinal axes of the nanosheets to become highly oriented in the polymer matrix, resulting in a thermal conductivity of up to 38 W/mK (85 vol% BN) [18]. This approach permits the dramatic improvement of the thermal properties of such composites, but their practical applications are limited because of material redundancy, economic inefficiency, and low workability. It has been demonstrated that reorienting and relocating filler particles such that end-to-end attachment occurs in the polymer matrix is an effective means of enhancing the thermal conductivity using a relatively small amount of thermally conducting fillers, less than 20 vol% [19e22]. In previous studies, to facilitate the conduction of heat along 1-D or 2-D thermally conducting fillers instead of the polymer [23,24], the anisotropy and distribution of the fillers has typically been controlled by applying the first two of the four steps depicted in Fig. 1. Orienting the longitudinal direction of the thermally conducting filler particles parallel to the direction of the heat flux can effectively increase the thermal conductivity of a composite (step 1). The use of 1-D aligned nanotubes in a polymer matrix can allow the conditions the percolation transition of conductivity to be satisfied at a remarkably low volume fraction compared with the use of pristine powder (step 2) [25]. The linear structure produced by the end-to-end attachment of filler particles constitutes an easier heat-conduction route through the filler particles, avoiding the route through the polymer. This structure can be further developed into a more dense arrangement of the filler particles (step 3) through the application of ON-OFF cycles of an applied electric field [26] using conducting carbon nanotube (CNT) fillers prepared via surface functionalization with

Fig. 1. Schematic illustration of the development of 1-D and 2-D fillers in polymers induced by various rotation sources: step 1) Orientation parallel to the longitudinal direction, step 2) End-to-end attachment of fillers (linear structure), step 3) Denser localization of fillers to form a linear structure, and step 4) A longer route for thermal conduction.

tetraoctylammonium. In our previous work, the fabrication of filament-like linear structures of BN nanosheets (steps 2 & 3) was accomplished by controlling the polymer viscosity and the electric field [27]. The stretched assemblies of BN nanosheets (5 vol%) formed through end-to-end attachments exhibited an elevated thermal conductivity compared with samples with an anisotropic orientation of BNs. The facilitation of longer filler assemblies to establish thermal-conduction pathways in polymers using the lowest possible filler contents (steps 3 & 4) may be the most ideal approach to fabricating highly thermally conductive composite materials for use as TIMs; however, these technologies remain an unexploited field. Here, we report a straightforward route for the direct assembly of highly thermally conductive BN nanosheets into linear densely packed bundles in a nanocomposite film, in which the structural and thickness variations and the filler-to-filler gaps are controlled using applied electric fields. The combination of electrostatic and Coulombic attraction present in an assembly of BN nanosheets can drive the end-to-end assembly of different linear bundle structures. Our robust strategy of constructing thermal-conduction routes using LDPBNs allows for a sharp increase in the thermal conductivity of insulating nanocomposites at noticeably lower filler contents. 2. Material and methods 2.1. Materials and sample preparation for ordered polysiloxane/BN nanocomposites Polysiloxane/BN-nanosheet nanocomposite films were prepared by introducing BN nanosheets into a poly (dimethylsiloxane) elastomer [28,29]. Three types of hexagonal boron nitride (BN) nanosheets of commercial origin (Denka Co., Ltd, Japan), ranging from 10 to 20 mm in diameter and from 2 to 10 nm in thickness, were used (See Fig. 2): BN-HGP (particle diameter D90 ¼ 10.6 mm, aspect ratio ¼ 141.3) and BN-GP (D90 ¼ 17.1 mm, aspect ratio ¼ 228.0) were used as sheet-type fillers, and BN-SP2 (D90 ¼ 14.2 mm) was used as a powder-type filler with agglomerated nanosheets [14]. The inset image reveals that the BN nanosheets, BN-HGP, had a planar graphite-like structure with smooth surfaces and curved edges and a layered structure with a thickness of less than 10 nm that was composed of several BN layers. The amount of BN nanosheets that was added was 5, 10, or 15 vol%. The samples were prepared using a commercially available silicone elastomer (YE5822) manufactured by Momentive Performance Materials Inc., New York, USA. Two liquid components of YE5822 with different viscosities were used: YE5822(A), with a viscosity of 1.2 Pa s and Mwav of 21,000, and YE5822(B), with a viscosity of 0.2 Pa s and Mwav of 16,000. An indium-tin-oxide (ITO)coated glass slide (2.5  7.5  1.0 mm3, Sigma-Aldrich), with a surface resistivity of 8e12 U/sq, was used as an electrode for the application of the electric field. First, 3 g of silicone YE5820(A) was sonicated for 5 min; a mixture of 0.3 g of silicone YE5822(B) and 0.416 g of BN (5 vol%) was then introduced into the sonicated silicone YE5820(A), and the resulting mixture was further sonicated for 10 min. The mixture was stirred using a high-speed mixer at 1500 rpm for 5 min to produce a homogeneous dispersion, which was then cast onto a polyimide spacer (1.5 mm  1.5 mm  120 mm), which was placed between the two electrodes (Fig. 3) and subjected to a 1.0 kV AC (50 Hz), DC, or switching DC electric field for 16 h to orient and relocate the BN nanosheets in the mixtures. The polarity of the DC electric field was changed at 4-h intervals during the experiment. Finally, the prepared composites were dried for 0.5 h at 80  C to ensure complete curing.

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Fig. 2. SEM micrograph of three types of BN nanosheets (scale bar: 1 mm); BN-HGP, BN-GP and BN-SP2. The inset presents the corresponding TEM micrograph (scale bar, 20 nm).

Fig. 3. Schematic diagrams of the experimental setups for (a) the application of the electric fields and (b) the preparation of the composite films for surface analysis.

2.2. Characterization and evaluation of film properties The anisotropic alignment of the BN nanosheets in the polymer films was analyzed using X-ray diffraction (XRD; Rigaku RINT 2500, Rigaku Corp., Tokyo, Japan). The degree of anisotropy of the BN nanosheets perpendicular to the film surface was estimated by comparing the intensity ratios between the c axis at 2q ¼ 26.76 and the a axis at 2q ¼ 41.60 :

a axis  100% a axis þ c axis

(1)

The peaks at 2q ¼ 26.76 and 41.60 correspond to diffraction from the (002) and (100) planes of BN, respectively. The linear distribution of BN nanosheets in the polymer matrix was observed using a digital microscope (DM; Keyence VHX-9000, KEYENCE Corp., Ilinoi, USA) followed by cross-sectioning of the polymer/BN composite films, whose thicknesses ranged from 100 to 110 mm. The surface morphologies of the composites were examined using SEM (Jeol JSM-6700F, JEOL Ltd., Tokyo, Japan). The measurements of the thermal and electrical conductivities have been described elsewhere [30].

3. Results and discussion 3.1. Fabrication and modulation of LDPBNs in polysiloxane using various applied electric fields Fig. 4 presents surface and cross-sectional optical micrographs of polysiloxane/BN nanocomposites fabricated using various electric-field conditions. Composites fabricated without the

application of an electric field contained homogeneously distributed BN nanosheets (Fig. 4(a)). Upon application of an electric field, the nanosheets relocated, gathering to the sides of both film surfaces to form a linear, stretched structure. The bundle structures were anchored to the regions on the surfaces of the composite films in which the electrodes were located during film preparation (Fig. 4(bed)). Bridge-like structures of linearly aligned BN nanosheets (LDPBNs) formed in the polymer matrices, and the population and thickness of the fabricated LDPBNs differed depending on the applied electric field. The largest populations of LDPBNs formed in the polymer when the composites were prepared under an AC electric field, and their thicknesses ranged from 15 to 20 mm. These thicknesses were thinner than those of the films grown under the other two types of applied fields, which ranged from 40 to 70 mm. Cross-sectional observations (Fig. 4(b)) indicated that the LDPBNs were interconnected with each other, forming networks. By contrast, after the application of a DC electric field, the population of fabricated LDPBNs decreased; however, the individual thicknesses of the LDPBNs increased. Although there was no obvious difference in the thicknesses of the LDPBNs that resulted from the two different DC applications, surface observations indicated that LDPBNs with different populations of BN nanosheets were formed with and without the switching of the DC electric field. When a constant DC electric field was applied, both the number of localized domains of BN nanosheets and the population of BN nanosheets between localized BN domains were larger than when a switching DC electric field was applied. When the DC electric field was switched, both the number of BN nanosheets located between localized domains of BN nanosheets and the number of groups of localized LDPBNs decreased (see Fig. 4(c) and (d)). This finding indicates that under the switching field, the BN nanosheets that remained in the furrows between the localized groups of LDPBNs moved and joined the LDPBNs that had formed during the initial single DC application. Repeated switching further intensified the convergence of the BN nanosheets toward the LDPBNs in the polymer matrix. The anisotropic orientation and relocation of nanofillers in a suspension caused by an external torque force is highly dependent on the size and shape of the nanoparticles [31]. When the orientation of nanosheets is controlled in a viscous polymer via field inducement, the fillers encounter higher restraining forces, such as viscoelastic and shear forces, than in the case of nanorods or nanotubes [32]. When electrically conductive graphite nanosheets (GNs) and a prepolymer mixture are placed in an electric field, a field-induced torque T orients the GNs against the viscous drag of the polymer matrix in the direction of the electric field [31]. This torque is calculated as follows:

V ðs1  s2 Þ2 T ¼ ε0 ε2 2 s1 s2

!  E2 sin 2 q

(2)

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Fig. 4. Digital micrographs of nanocomposite films with linear densely packed assemblies of BN nanosheets (LDPBNs) fabricated under electric fields (BN, HGP type; content, 5 vol%; scale bar, 20 mm); (a) without any electric field, (b) AC electric field, (c) DC electric field, and (d) switching DC electric field.

The field-induced torque acting on a disk-like dielectric such as BN is described by Ref. [33].

V ðε1  ε2 Þ2 T ¼ ε0 2 ε1

!  E2 $sin 2 q;

(3)

where V is the volume of the BN flake [in Equation (2), V corresponds to a single graphite nanosheet], q is the angle between the electric field and the flake axis, ε0 is the permittivity of free space, and ε1, ε2, d1, and d2 are the relative dielectric constants and conductivities of the flake and the resin matrix, respectively [Equations (2) and (3)]. Because the relative dielectric constant of h-BN (ε1 z 5.06e6.85) [34] is lower than that of conductive GNs (ε1 z 12e15) [35], BN requires a higher electric field to control the anisotropy. However, when nanosheets are oriented within a polymer in a composite film of micrometer-scale thickness, the available electric field is limited by the breakdown voltage of the polymer, which is lower than that of BN. In this study, the prepared polysiloxane/BN composites, which were 240e255 mm in thickness, were electrically insulating up to 10.42 kV/mm.

The orientational anisotropy of the BN nanosheets in the polymer matrix was identified via XRD, as presented in Fig. 5. The peaks at 2q ¼ 26.76 and 41.60 correspond to the diffraction from the (002) and (100) planes of BN, respectively. As the relative number of BN nanosheets aligned perpendicular to the film plane increases, the intensity of the (002) peak decreases and the intensity of the (100) peak increases. When the BN nanosheets were introduced into the polymer without the application of an electric field, the (002) peak was dominant. The intensity of the (100) peak increased and the intensity of the (002) peak decreased under the application of an electric field. The application of a DC electric field increased the BN intensity ratio to 49.3%, and this ratio was further enhanced to 69.8% under AC application. Thus, the enhanced intensity ratio under the application of various electric fields confirmed that the initially randomly distributed BN nanosheets became aligned perpendicular to the plane of the composite film, establishing LDPBN bundles. Figs. 6 and 7 present cross-sectional SEM micrographs of composites prepared under DC and AC electric fields. Fig. 6 shows a single LDPBN bundle anchoring both film surfaces, 53 mm thick and

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BN N(002)

209

without electric field w f A AC D DC sw witching DC

B BN(100)

Intensity

Inttensity ratio=4.94 4%

BN(101))

Inttensity ratio=69.8 8% Inttensity ratio=49.3 3% Inttensity ratio=32.2 2% 25

30

35

40

45

2 theta (d deg) Fig. 5. X-ray diffraction patterns of polysiloxane/BN-nanosheet composite films under various electric fields (5 vol% BN, HGP type). The intensity ratio was calculated as follows: intensity ratio (%) ¼ [a axis/(a axis þ c axis)]  100(%).

Fig. 7. Cross-sectional SEM images of LDPBNs prepared under an AC electric field (BN, HGP type; scale bar, 10 mm).

network structure, which is consistent with the situation observed

Fig. 6. Cross-sectional SEM images of LDPBNs prepared under a DC electric field (BN, HGP type; scale bar, 10 mm): (a) overall view, (b) magnified image of the region indicated on the left-hand side of (a), (c) magnified image of the region indicated at the center of (a), and (d) magnified image of the region indicated on the right-hand side of (a).

250 mm long, and a filament-like LDPBN structure (rectangular region, Fig. 6(a)). Magnified views of three individual regions (Fig. 6(b)e(d)) reveal BN nanosheets aligned in the LDPBN. The BN nanosheets were embedded in the polymer facing in different directions, but their longitudinal axes were perpendicular to the film surface. In some domains, such as those indicated by the ellipses in Fig. 6(b) and (c), there were linear groups of BN nanosheets that either overlapped or were attached end-to-end. The composite fabricated under the application of an AC electric field (Fig. 7) contained a larger population of LDPBNs than did the composite formed under a DC field, but the LDPBNs were thinner in the AC case. The LDPBNs were anchored to the film surfaces and formed a

in Fig. 6(b). Fig. 6(b) and (d) reveal that the BN nanosheets located in the center of the LDPBN were aligned perpendicular to the film surface, i.e., parallel to the electric field. Nanosheets that were located near the surfaces of the composite were also aligned perpendicular to the surface, but their longitudinal axes were slightly slanted. This behavior was not observed at the surfaces of the composite fabricated under an AC field (Fig. 7). This phenomenon is typically observed when dielectric fillers are subject to Coulombic attraction under a uniform electric field, as occurs during DC application, and explains why the XRD peak-intensity ratio for the composite fabricated under a DC electric field was lower than in that fabricated under an AC field.

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3.2. Thermal and electrical properties Fig. 8 illustrates the dependence of the thermal conductivity on the BN content and filler type used in the composite films and the electric-field conditions during film fabrication. When an electric field was applied (Fig. 8(a)), the composites exhibited a noticeable increase in thermal conductivity. For 5 vol% BN, the composite prepared under a DC field exhibited a thermal conductivity of 0.33 W/mK, which was 1.87 times that of the corresponding composite prepared without an electric field (0.18 W/mK). Over the entire range of BN content, the composites prepared under switching DC fields exhibited the highest thermal conductivities, followed by those prepared under AC fields. Finally, for 15 vol% BN, a switching DC electric field yielded an enhancement of the thermal conductivity to 0.73 W/mK, which is 1.97 times that of the corresponding composite prepared without an electric field. Varying the type and dimensions of the BN nanosheets used also affected the thermal conductivity, as illustrated in Fig. 8(b). When BN-SP2, an agglomerated powder of BN nanosheets, was used, the

a) Thermal conductivity (W/mK)

0.8

BN-HGP-switching DC BN-HGP-DC BN-HGP-AC BN-HGP-without electric field

0.6

0.4

0.2

0.0 0

5

10

15

BN content (vol%) 1.8

b) BN-HGP-without electric field BN-GP-without electric field BN-SP2-without electric field BN-HGP-switching DC BN-GP-switching DC BN-SP2-switching DC

1.5

1.2

Resistivity (MΩ.cm)

Thermal conductivity(W/mK)

resulting composites exhibited the lowest thermal conductivities among the three types of BN nanosheets. Adding BN-GP (D90 ¼ 17.1 mm), which was larger than BN-HGP (D90 ¼ 10.6 mm) by a factor of 1.61, resulted in the highest thermal conductivities over the entire range of BN content. BN-GP resulted in the highest thermal conductivity achieved for any composite, 1.56 W/mK (15 vol% BN-GP), which was higher than that of the most thermally conductive composite fabricated using BN-HGP by a factor of 2.15. It is notable that the thermal conductivity of the composite was increased by a factor of 2.15 simply by using BN nanosheets that were 1.61 times larger. Furthermore, even though the size of BNSP2 was also greater than that of BN-HGP, the corresponding composite fabricated with BN-SP2 exhibited a thermal conductivity of only 0.43 W/mK, which represented the lowest enhancement relative to the randomly distributed equivalent (0.29 W/mK). It has been reported that CNTs with aspect ratios lower than 500 cause the thermal conductivities of nanocomposites to increase in proportion to their aspect ratio [36]. The orientation of flake-like BN nanosheets is highly susceptible to electric fields because of the wide band gap of BN [37], and nanosheets are more susceptible to viscoelasticity and shear forces [38] than are CNTs. Regardless, as long as the reorientation and relocation of flake-like BN can be controlled in a given system, LDPBNs composed of BN-GP (aspect ratio ¼ 228.0) have a higher probability of establishing thermalconducting routes in the polymer than do BN-HGP (aspect ratio ¼ 141.3) LDPBNs. Thus, the typical heat dissipation in the polymer attributable to the TBR [7] could be significantly reduced by the improvement in thermal conductivity that can be achieved using BN-GP LDPBNs. It may seem strange that the final thermal conductivity of a composite with oriented BN-HGP (D90 ¼ 10.6 mm) is higher than that of a composite with oriented BN-SP2 (D90 ¼ 14.2 mm), which is larger in size. This finding may be related to the difference in anisotropic alignment of the BN in the LDPBNs. Because the SP2-type BN nanosheets consist of BN nanosheet particles agglomerated in random directions, effective thermal conduction does not occur along the longitudinal LDPBN direction, as it does in composites fabricated with BN-HGP, which exhibits high anisotropy of its BN nanosheets, as verified by the XRD results presented in Fig. 5. The effect of the type of BN can be significant because hexagonal BN exhibits thermal anisotropy that depends on the direction of heat flow in the flake. The electrical resistivities of the composites were measured to compare the

0.9

0.6

0.3

6

10

BN-HGP-without electric field BN-GP-without electric field BN-HGP-AC BN-HGP-switching DC BN-GP-switching DC

0.0 0

5

10

15

BN content (Vol %) Fig. 8. Thermal conductivities of polysiloxane/BN-nanosheet nanocomposite films for (a) various electric-field conditions and (b) various BN fillers under a switching DC field: BN-HGP, D90 ¼ 10.6 (nanosheet); BN-GP, D90 ¼ 17.1 (nanosheet); and BN-SP2, D90 ¼ 14.2 (powder).

5

10

0

5

10

15

BN content (Vol%) Fig. 9. Resistivities of polysiloxane/BN-nanosheet nanocomposite films prepared under various applied electric fields.

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effects of structural and thickness variations in the LDPBNs, and the results are presented in Fig. 9. When the composites were fabricated under the application of an AC electric field, the population of LDPBNs was the highest; however, thinner bundles were formed compared with fabrication under the application of a switching DC field, which produced the thickest bundles. This result indicates that the LDPBN thickness has a more important effect on the thermal conductivity than does the number of LDPBNs. By contrast, structural differences in the LDPBNs do not affect the resistivity of the composite. In the absence of BN nanosheets, the resistivity of the polysiloxane was 1.62  106 MU cm. Varying the preparation conditions, such as changing the type of BN nanosheets without applying an electric field, changing the applied electric fields, and applying a switched DC field for different types of BN nanosheets, caused the resistivity to vary approximately from 1.50 to 2.64  106 MU cm; however, no discernible trend was observed. Therefore, the addition of BN and the LDPBN structure do not have any particular effect on the electrical properties of the composites. The wide band gap of BN, which ranges from 5.5 to 6.4 eV depending on the polymorph, may be the primary contributing factor, and the present results indicate that the polysiloxane/BN nanocomposites maintained their electrically insulating behavior even after the fabrication of LDPBNs in the polymer. 3.3. Mechanisms The application of electric fields enabled the fabrication and tuning of structural variations in the LDPBNs and the anisotropic orientation of BNs, of which the linear assemblies that developed under the application of a switching DC electric field offered the greatest enhancement of the thermal conductivity of the composites. It has been reported that during the orientation of GNs in epoxy resin, the GN flakes become aligned parallel to the electric flux to minimize the electrostatic energy and overcome the free energy of the system to achieve a stable configuration [39]. This finding is consistent with our previous research, which demonstrated that BN nanosheets are polarized by electric-field inducement and that the charge density increases at the longitudinal edges of the nanosheets [13]. The edges of each adjacent BN nanosheet attach to each other because of the attraction that arises during their electrophoretic movement in a prepolymer mixture, a liquid mixture of silicone elastomer and BN nanosheets. If a highly anisotropic orientation of BN nanosheets is desired, BN nanosheets of shorter longitudinal diameter [40] and a polymer matrix with a higher dielectric constant [see equation (3)] would be preferred because the rotation and electrophoretic movement of the inclusions would be enhanced by virtue of the reduced hindrance of the polymer matrix, such as that caused by shear forces. In this study, an electric field of less than 4 kV/mm, which is below the breakdown voltage of polysiloxane (12 kV/mm), was applied at room temperature (25  C), and the viscosity was controlled throughout the specified duration of field induction, which was 16 h [41]. These conditions enabled the controlled orientation and formation of LDPBNs with a longer longitudinal diameter (BN-GP type) than that of the BN-HGP type and resulted in an increased thermal conductivity of the composite through the mechanisms described in Fig. 1 (step 4). Under the application of a DC electric field, BN nanosheets reorient in the direction of the electric field because of their polarization, forming a filament-like or bridge-like LDPBN structure. Their electrophoretic movement toward the positive electrode and their end-to-end attachment are motivated by Coulombic attraction between the oppositely polarized ends of the BN nanosheets. When the direction of the electric field is reversed, the nanosheets begin to move toward the opposite sides of the film via electrophoresis and have additional opportunities to

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interact with adjacent nanosheets and with the LDPBNs that have already formed into bridge-like structures during the previous unidirectional DC application. When smaller amounts of BN nanosheets are present under the unidirectional application of a DC electric field, the number of filament-like LDPBN structures [13] that are localized near the positive electrode but have not formed a bridge-like structure (as observed in the rectangular region in Fig. 6(a)) is high because fewer inclusions have a lower probability of interacting with each other within a given volume of polymer. This structure will then transforms into a bridge-like structure and anchor to the film surfaces because of the switching of the DC electric field, and some of the groups of BN nanosheets that have initially become localized near the surface of the composite without forming a linear structure will approach and join the formation of LDPBNs under the repeated application of DC switching, resulting in the formation of thicker LDPBNs with denser populations of BN nanosheets. The resulting LDPBNs can enhance the thermal conductivity of the composite by allowing heat to diffuse along a route composed of BN nanosheets and by condensing the passages for heat diffusion through the polymer, as illustrated in Fig. 10. The considerable differences in thermal conductivity between polysiloxane [8] and BN and between the c (t) and a (k) axes of a BN nanosheet may explain the marked increase in thermal conductivity that can be achieved through the controlled orientation and assembly of BN in the polymer compared with polysiloxane without the incorporation of fillers. Furthermore, BN nanosheets exhibit 20-fold greater thermal conductivity when heat is transferred parallel to the a axis (k) rather than the c axis (t) [12]. An AC electric field facilitates the formation of the largest number of LDPBN bundles, which anchor to the film surfaces and form interconnections with neighboring LDPBNs to produce a networked structure. The rapid alternation of the electric field induced by the use of a high frequency (50 Hz) enables filler-tofiller interaction through Coulombic attraction before the BN nanosheets become completely aligned parallel to the electric filed. This process leads to the formation of linear assemblies among bridge-like LDPBNs and results in a high-density network of LDPBNs (Fig. 7). However, some BN nanosheets are more widely distributed in the polymer, and they form LDPBNs with wider interparticle gaps than in the case of fabrication under DC or switching DC electric fields. This phenomenon becomes more pronounced as the filler content increases. Compared with the application of an AC electric field, the application of a switching DC field yields narrower filler-to-filler gaps, resulting in a thickening of the LDPBNs and the significant enhancement of the thermal conductivity. The incorporation of conducting carbonaceous fillers such as CNTs and GNs into a polymer through self-assembly has been regarded as one of the most promising methods of enhancing the electrical conductivity of composites because the electricalconduction routes established by the filler particles enable effective charge transfer [42]. Although the recent demand for highly thermally conductive carbonaceous materials for use as composite fillers is increasing for the fabrication of TIMs for modern electronics and optoelectronics [43], such composites with carbonaceous fillers are not applicable as TIMs in applications in which both thermal conductivity and electrical insulation are required. Because current TIMs are based on polymers filled with thermally conductive inclusions, these materials require high filler volume fractions of up to 70% to achieve a thermal conductivity of 1e5 W/mK for the composite [1]. Therefore, the ability to attain a thermal conductivity of 1.56 W/mK for polysiloxane/BN nanocomposites with filler contents as low as 15 vol% BN while preserving the electrically insulating nature of the composites is an outstanding achievement.

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Fig. 10. Schematic model of the generation of higher-conduction routes through LDPBNs using various applications of electric fields.

The application of a DC or AC electric field to align carbonaceous materials such as CNTs has been described as one of the best methods of obtaining electrical conductivity using a low filler content [17,44]. Furthermore, such field-induced self-assembled networks or dendrite structures of CNTs can facilitate electron transfer in polymers of lower filler content than in the case of polymers with random distributions of CNTs. However, the present research demonstrates that increasing the population of linearly assembled structures of nanofillers d in this case, the number of bridge-like linear assemblies of BNs, or LDPBNs d does not always induce a proportional enhancement in the thermal conductivity of polymer-based composites. The thermal conduction in an insulating polymer is primarily facilitated by acoustic phonons, which are very susceptible to ensembles of filler particles that affect the matrix frequencies [7], TBRs [45,46], and the low thermal dissipation of amorphous polymers [47]. Furthermore, estimates based on a phonon mean free path of approximately 1 nm in polymers [48] suggest that the creation of narrower filler-to-filler gaps by thickening the linear filler structures, which enhances the linear thermal conductivity by facilitating phonon transfer by minimizing heat dissipation to the polymer, produces a greater enhancement in thermal conductivity than that achieved by increasing the population of linear filler structures, which consequently results in wider filler-to-filler gaps, especially for polymer-mediated composites.

mK, was approximately 4 times higher than the conductivity of the composite without LDPBN structures (0.4 W/mK) and 15 times that of the polysiloxane matrix (0.3 W/mK). In addition, the typical wide band gap of BN helped maintain the electrically insulating nature of the composite even after the addition of BN and the formation of bridge-like LDPBNs in the composite film. This field-induced technique for the fabrication of self-assembled stretched bundle structures resulted in superior thermally conductive and highly electrically insulating functional materials with small filler contents (
15 vol%), and it represents a novel approach in the field of nano- and microcomposite functional materials.

4. Conclusion

References

The generation of highly thermally conductive routes through linear densely packed boron nitride nanosheets (LDPBNs) was demonstrated in the fabrication of insulating polymer nanocomposites. The LDPBN structures anchored to the composite surfaces were fabricated through the coordinated effects of polarization, dipoleedipole moments, electrophoresis, and Coulombic attraction, and the structural and thickness variations of the assembly were controlled by varying the applied electric field and the BN content. Among the three applied electric-field conditions (AC, DC, and switching DC), although the application of an AC field produced the largest total number of LDPBNs, the application of a switching DC field caused the largest number of LDPBN bundles to become anchored to the surfaces of the composite film, resulting in a thickening of the LDPBN bundles and a narrowing of the interparticle gaps. This LDPBN thickening enhanced the thermal conductivity of the composite films, and this effect gradually intensified as the aspect ratio and loading level of the BN nanosheets were increased. Consequently, the thermal conductivity of the composite that contained 15 vol% BN-GP, which was 1.56 W/

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Acknowledgement This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT &Future Planning (No. 2015R1A5A1037548) and the Fundamental R&D Program for Core Technology of Materials (10050890, Chalcogenide nanostructurebased room-temperature (25  C) H2 &H2S gas sensors with low power consumption) and the Human Resources Development program (No.20154030200680) of the Korea Institute of Energy Technology Evaluation and Planning(KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy, Republic of Korea.

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