# Dopamine-Mediated Graphene Bridging Hexagonal Boron Nitride for Large-Scale Composite Films with Enhanced Thermal Conductivity and Electrical Insulation

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## Abstract

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_{PDA}) and hexagonal boron nitride (BN

_{PDA}), as well as bacterial cellulose (BC). The introduction of the PDA layer greatly improves the interface compatibility between hybrid fillers and BC matrix, and the presence of GNS

_{PDA}-bridging significantly increases the probability of effective contact with BN

_{PDA}fillers, which is beneficial to form a denser and complete “BN-GNS-BN” heat conduction pathway and tight filler–matrix network, as supported by the Foygel model fitting and numerical simulation. The resulting BC/BN

_{PDA}/GNS

_{PDA}film shows the thermal conductivity and tensile strength of 34.9 W·m

^{−1}·K

^{−1}and 30.9 MPa, which separately increases to 161% and 155% relative to the BC/BN

_{PDA}film. It was found that the low electrically conductive and high thermal conductive properties can be well balanced by tuning the mass ratio of GNS

_{PDA}at 5 wt%, and the electrical conductivity caused by GNS

_{PDA}can be effectively blocked by the BN

_{PDA}filler network, giving the low electrical conductivity of 1.8 × 10

^{−10}S·cm

^{−1}. Meanwhile, the BC/BN

_{PDA}/GNS

_{PDA}composite films effectively transfer the heat and diminish the hot-spot temperature in cooling LED chip module application. Thus, the present study may pave the way to promoting the industrialization of scalable thermal management devices.

## 1. Introduction

^{−1}) and high thermal conductivity (180 W·m

^{−1}·K

^{−1}) [11], which is an ideal filler for the fabrication of the highly thermal conductive and electrical insulation composite materials. For example, BN has been used for embedding into the polymer matrix (comprising natural rubber [12], epoxy resin [13], and cellulose nanofibers [14]) to fabricate functional composite films [15]. However, the existence of interfacial thermal resistance (R

_{I}) significantly affects the heat transfer efficiency along BN filler pathways, for example, the interfaces between BN fillers or between filler and matrix. Thus, many studies have pointed out that the R

_{I}can be reduced by enhancing the interface compatibility through an interface modification strategy [16,17]. In fact, the improvement extent of thermal conductivity also has certain limitations only when reducing the interfacial thermal resistance. This is because the thermal contact resistance (R

_{c}) caused by the poor contact between fillers is another key parameter that determines the final thermal conductivity of composites. It is reported that the contact probability can be effectively increased by introducing another additional filler instead of using only one filler component [18,19]. For instance, the zero-dimensional silver nanoparticles and one-dimensional silicon carbide nanowires were selected to connect BN fillers [20,21]. Compared with the “point-to-point” contact type of zero/one-dimensional materials, two-dimensional materials provide the “face-to-face” contact sites between the thermal conductive fillers [22], which seem to be a very effective strategy for improving effective contact and building a bridge of fillers.

^{−1}·K

^{−1}) and tensile strength (130 GPa) [23]. Nevertheless, this kind of ultra-high electrical conductivity (10

^{6}S·m

^{−1}) makes it hard to directly apply it to the thermal management devices of electronics and the power industry which require good electrical insulation [24]. So, GNS can be used as a candidate for other, supplementary materials of the main BN filler in order to exploit hybrid composites with excellent thermal conductivity and dielectric properties. However, it is difficult to uniformly disperse two heterogeneous fillers due to the poor interface compatibility, resulting in numerous agglomeration and serious phase separation problems. Moreover, even the most extensive methods of preparing composites, such as hot pressing, vacuum filtration, and freeze-drying, are still far from the industrialized production of scalable and highly arranged composites [25,26,27]. Therefore, it is highly desirable for the fabrication of large-scale and regularly oriented hybrid composite materials with excellent insulation and thermal conductivity through a convenient and facile approach.

_{PDA}/GNS

_{PDA}composite films were prepared through a layer-by-layer (LBL) scraping method. The results demonstrate that the flat and highly directional microstructure of BC/BN

_{PDA}/G

_{PDA}composite films were successfully built, and the high anisotropic thermal conductivity, enhanced tensile strength, and excellent electrical insulation performances can be effectively balanced by regulating the critical value of the mass ratio of GNS

_{PDA}fillers. It was found that the main BN

_{PDA}fillers can be bridged by flexible GNS

_{PDA}filler to build a continuous and highly oriented thermally conductive pathway under the optimal mass ratio of GNS

_{PDA}of 5 wt%; meanwhile, the electrical conductivity arising from GNS

_{PDA}can also be effectively blocked by the electrical insulation of h-BN filler. Furthermore, the BC/BN

_{PDA}/GNS

_{PDA}films show high efficiency in transferring the hot-spot heat from the LED chip, demonstrating great potential applications as thermal management electronic devices.

## 2. Experimental Section

#### 2.1. Materials

#### 2.2. Preparation of Composite Films

_{PDA}and GNS

_{PDA}, respectively. Then, the BN

_{PDA}and GNS

_{PDA}fillers were obtained after drying for 12 h at 60 °C, and the desired content ratio of BN

_{PDA}and GNS

_{PDA}fillers were added into BC solution (2 mg/mL) and stirred for 3 min. Subsequently, the mixture was cast onto nylon film and heated at 100 °C for 5 min. Finally, the large-scale films were obtained using the layer-by-layer (LBL) scraping method. The composite films composed of BC and hybrid fillers were represented as BC/BN

_{PDA}/GNS

_{PDA}-X, and X is the mass content of GNS

_{PDA}in the hybrid fillers (X = 0, 1, 3, 5, 7, and 10).

#### 2.3. Characterization

_{PDA}, GNS, and GNS

_{PDA}fillers was performed on a Raman spectroscope (inVia Reflex, Renishaw, London, UK) with a 532 nm laser excitation wavelength. The thermal degradations of BN, BN

_{PDA}, GNS, and GNS

_{PDA}fillers were tested by thermogravimetry (TGA, INNUO TGA-1000, Yingnuo, Shanghai, China) under a nitrogen atmosphere (10 °C·min

^{−1}, 50–800 °C). X-ray photoelectron spectra (XPS) recorded the chemical composition of BN, BN

_{PDA}, GNS, and GNS

_{PDA}fillers on an ESCA instrument (Physical Electronics, MA, USA). Isothermal titration calorimetry (ITC) experiments were performed with a Microcal VP-ITC apparatus at 298.15 K. An X-ray diffractometer (XRD, Rigaku SmartLab 9 kW, Japan) analyzed the orientation of hybrid fillers with a diffraction angle in the range of 2–90°. The mechanical properties of the composite films were tested using a tensile testing machine (AI-7000-LAU10, Gotech Testing Machines Co., Ltd., Dongguan, China). Dielectric properties and the AC conductivity were acquired by the Tong Hui TH26077 Precision LCR Meter in the frequency range of 1000 Hz to 10 MHz. Thermal images were recorded by an infrared camera (Ti400, Fluke, Everett, WA, USA). The numerical simulation software (ANSYS, 16.0) was employed to simulate the heat transfer characteristics of composite films. The anisotropic thermal conductivities of composite films were calculated by the equation: k = α × ρ × C, where α, ρ, and C, respectively, correspond to the thermal diffusivity, density, and specific heat capacity of composite films, which is measured by the laser flashing method (LFA 467, NanoFlash, Netzch, Germany). C was determined through the following equation: C = C

_{BN}× ω + C

_{BC}× (1 − ω), where ω is the filler content and C

_{BN}and C

_{BC}were separately measured by a differential scanning calorimeter (NETZSCH DSC214, Germany).

## 3. Results and Discussion

#### 3.1. Characterizations of PDA Modified BN, GNS, and Composite Films

^{−1}(Figure S2) [28]. Figure 1a displays Raman spectra of the original BN, and GNS fillers and modified BN

_{PDA}and GNS

_{PDA}fillers. The characteristic peak of BN filler appears at 1364 cm

^{−1}[10]. After the PDA modification, a new absorption peak appears near 1557 cm

^{−1}, which is attributed to the hydroxyl groups arising from PDA layer on the surface of the BN filler [22]. The Raman shifts at 1263 (weak D-band) and 1575 cm

^{−1}(strong G-band) are the characteristic peaks of GNS [24]. After the introduction of PDA, the D-band peak shifts to 1266 cm

^{−1}while the G-band peak decreases to 1564 cm

^{−1}, which is attributed to the deformation of the catechol groups of the PDA [6]. Figure 1b shows the TGA curves of BN and GNS before and after PDA modification, and BN has no thermal weight loss in the temperature range of 50 to 800 °C, whereas the BN

_{PDA}shows thermal degradation between 300 and 650 °C and weight loss reaches 1.89 wt% at 800 °C [5]. GNS and GNS

_{PDA}both exhibit obvious thermal degradation, but GNS

_{PDA}has a greater weight loss of 3.68 wt% due to the PDA-coated layer. Figure 1c presents the XPS spectra of PDA modified fillers. Compared with the untreated BN filler, the peak intensities of the O and C elements increase but the B and N elements are diminished [5]. For GNS

_{PDA}, the N element appears relative to the untreated GNS. In addition, ITC was employed to investigate the thermodynamic behavior of the interactions between BN (or GNS) and PDA (Figure 1d) [5,29]. When the PDA solution was titrated into the BN (or GNS) solution, the observed enthalpy changes (ΔH

_{obs}) are negative, indicating that the combination of PDA and BN (or GNS) is accompanied by exothermic process. The ΔH

_{obs}value gradually decreases to zero plateau with the addition of PDA solution, which manifests the saturation of the noncovalent binding between PDA and BN or (GNS). The binding constants (K

_{d}) of PDA with BN or GNS can be derived by fitting the ITC curves, and the resulting K

_{d}values give 1.14 × 10

^{3}and 3.53 × 10

^{3}M

^{−1}, respectively, which demonstrates that PDA and BN (or GNS) are effectively combined, and these two fillers have a comparable binding ability with PDA. In brief, this evidence proves that BN and GNS fillers are successfully modified by PDA.

_{PDA}(the mass ratio of BN fillers is 70 wt%), BC/BN

_{PDA}/GNS

_{PDA}-5 (the total mass ratio of hybrid fillers is 70 wt% and the mass ratio of GNS

_{PDA}is 5 wt%), and BC/BN

_{PDA}/GNS

_{PDA}-10 (the total mass ratio of hybrid fillers is 70 wt% and the mass ratio of GNS

_{PDA}is 10 wt%) were observed from SEM. Figure 2a presents the cross-sectional morphology of the BC/BN

_{PDA}film, which shows the partially oriented stacking of BN

_{PDA}fillers (as revealed by the red arrows) [5]. After the introduction of 5 wt% GNS

_{PDA}, the cross-section of BC/BN

_{PDA}/GNS

_{PDA}-5 film exhibits more regular arrangement and forms longer networks of hybrid fillers relative to BC/BN

_{PDA}film, as shown in Figure 2b. This may be because GNS

_{PDA}plays a key role in face-to-face contact with the BN

_{PDA}filler, making the formation a continuous network structure inside the BC/BN

_{PDA}/GNS

_{PDA}-5 film. Simultaneously, the energy dispersive spectrometer (EDS) mapping of the BC/BN

_{PDA}/GNS

_{PDA}-5 film (Figure 2d) clearly shows the lamellar distributions of B, N, and C elements, which confirms the well-stacked layered structure of the hybrid fillers. Moreover, when the mass ratio of GNS

_{PDA}increases to 10 wt%, the uniform and close networks of BC/BN

_{PDA}/GNS

_{PDA}-10 film appear in Figure 2c. Additionally, this result indicates that the probability of effective contact between hybrid fillers significantly increases, which is beneficial for forming a denser and complete “BN-GNS-BN” pathway.

_{PDA}and BC/BN

_{PDA}/GNS

_{PDA}-5 were performed. As shown in Figure 2e, two characteristic peaks arising from BC nanofiber appear at 15° and 23°. Moreover, the characteristic peaks at 27° and 42° arise from the (002) and (100) planes of the BN fillers, and the intensities of these two peaks are expressed as I

_{002}and I

_{100}. It is reported that the increased values of I

_{002}/I

_{100}means the improved in-plane orientation of BN fillers for the composite film [20]. Take the example of the BC/BN

_{PDA}/GNS

_{PDA}-5 film: the I

_{002}/I

_{100}value of BC/BN

_{PDA}/GNS

_{PDA}-5 film gives 132, which is obviously higher than that of BC/BN

_{PDA}films (89) [5]. Thus, by the aid of the flexible GNS

_{PDA}, the orientation of BN fillers along the in-plane direction is greatly enhanced. This result is consistent with the observed morphology features, which shows that the BC/BN

_{PDA}/GNS

_{PDA}film exhibits increased in-plane interconnected structures relative to BC/BN

_{PDA}films. In general, the distribution of GNS

_{PDA}in the hybrid fillers is closely related to the formation of thermal contact channels in composite films, which directly determines the heat transfer efficiency. As illustrated in Figure 2f–h, the BC/BN

_{PDA}film shows randomly oriented BN networks. After the introduction of GNS

_{PDA}filler, the BN

_{PDA}fillers can be connected by GNS

_{PDA}fillers to form the more continuous and longer “BN-GNS-BN” pathways (Figure 2g,h), which are beneficial for filling the interfacial gap between BN and BC and improving the insufficient contact between the brittle BN fillers. Therefore, by introducing a small amount of GNS

_{PDA}filler, the interface compatibility between hybrid fillers and BC matrix is improved, and more importantly, the possibility of effective contact between hybrid fillers is also enhanced, which is expected to diminish the heat transfer resistance in the interfacial thermal resistance (R

_{I}) and thermal contact resistance (R

_{c}).

#### 3.2. Thermal Conductivity of Composite Films

_{PDA}films obtained by vacuum filtration are 21.6 W·m

^{−1}·K

^{−1}[5], and these values are close to the results of BC/BN

_{PDA}films prepared using the LBL scraping method (21.7 W·m

^{−1}·K

^{−1}). Figure 3a shows that the BC/BN

_{PDA}/GNS

_{PDA}-5 composite films always exhibit higher in-plane thermal conductivities than BC/BN

_{PDA}films at varied filler contents. In particular, the in-plane thermal conductivity of the BC/BN

_{PDA}/GNS

_{PDA}-5 composite film reaches 34.9 W·m

^{−1}·K

^{−1}at the hybrid filler mass ratio of 70 wt%, which increases to 161% relative to the BC/BN

_{PDA}film (21.7 W·m

^{−1}·K

^{−1}) [5]. By contrast, Figure 3b displays the through-plane thermal conductivities of the BC/BN

_{PDA}/GNS

_{PDA}-5 composite films, which are of lower values than those of BC/BN

_{PDA}films at different filler contents. This may be because the introduction of a small amount of GNS

_{PDA}(5 wt%) plays an important role in bridging the main BN filler (95 wt%), which greatly increases the contact of BN fillers along the in-plane direction, as observed from the continuous and well-stacked BN structure in Figure 2b, which is conducive to enhancing the anisotropic heat transfer efficiency of BC/BN

_{PDA}/GNS

_{PDA}-5 composite films. The effect of GNS

_{PDA}contents on the in-plane thermal conductivities of the BC/BN

_{PDA}/GNS

_{PDA}films (the total content of hybrid fillers is fixed at 70 wt%) is also considered. Since the high content of GNS will lead to poor electrical insulation performance [6], the influence of GNS

_{PDA}contents in the range of 0~10 wt% on the in-plane thermal conductivity is investigated in Figure 3c. It can be clearly observed that the in-plane thermal conductivity of the composite film has a significant increase when the mass ratio of GNS

_{PDA}reaches 5 wt%, but the improvement extent of in-plane thermal conductivities is limited by further increasing the mass ratio of GNS

_{PDA}. Therefore, the synergistic effect between GNS

_{PDA}and BN

_{PDA}may exist in improving heat transfer efficiency at the critical GNS

_{PDA}content of 5 wt%.

_{PDA}(5 wt%) on the thermal conductivity of the composite films, and thus, the effective medium theory (EMT) [25] and Foygel model [10] were used to investigate their influence on interfacial thermal resistance (R

_{I}) and thermal contact resistance (R

_{c}) of inner composite films. According to the in-plane thermal conductivity of the composite films, the EMT model is employed to calculate the R

_{I}values between filler and BC matrix before and after the introduction of GNS

_{PDA}, and the corresponding equations are shown below:

_{p}(3.05 W·m

^{−1}·K

^{−1}, Table S1), k

_{f}(180 W·m

^{−1}·K

^{−1}), and k are the in-plane thermal conductivities of BC matrix, BN fillers, and composite films, respectively; f is the volume fraction of filler; d (0.5 μm) and L (7.0 μm) are the thickness and lateral size of BN (Figure S1); and R

_{I}is obtained by using the EMT equations (Figure S2). The resulting R

_{I}of BC and BN

_{PDA}/GNS

_{PDA}-5 gives 1.83 × 10

^{−8}m

^{2}·K·W

^{−1}, which is evidently lower than the BC and BN

_{PDA}(3.06 × 10

^{−8}m

^{2}·K·W

^{−1}). These results demonstrate that a small amount of GNS

_{PDA}(5 wt%) eliminates interfacial gaps, thereby reducing the interfacial thermal resistance between the filler and the BC matrix.

_{PDA}filler on the R

_{c}of composite films, which can be described as follows (calculated details in S2 in the Supplementary Materials):

_{0}is a pre-exponential factor ratio determined by the interconnected BN networks; f

_{c}is the measured and critical volume fraction of the hybrid fillers in the films; and τ is the conductivity exponent determined by the aspect ratio of the BN. The f

_{c}, k

_{0}, and τ values are 15 vol%, 44, and 0.62 for BC/BN

_{PDA}/GNS

_{PDA}-5 and 10 vol%, 28, and 0.56 for BC/BN

_{PDA}, respectively, and R

_{c}is obtained by combing the above Foygel model equations. As a result, the R

_{c}between partially connected “BN-GNS-BN” interfaces is calculated to be 0.9 × 10

^{5}K·W

^{−1}, which is approximately half of that of “BN-BN” interfaces (2.1 × 10

^{5}K·W

^{−1}), which demonstrates more contact probability of filler and efficient phonon transport in BC/BN

_{PDA}/GNS

_{PDA}-5 films (Figure 3d).

_{PDA}filler is simulated using the finite element, and finite element models, boundary conditions, and mesh distributions were detailed described in S3 in the Supplementary Materials. Figure 3e displays the calculated heat flux distributions and thermal conductivities of BC/BN

_{PDA}and BC/BN

_{PDA}/GNS

_{PDA}-5 composite films. For BC/BN

_{PDA}film, the BN fillers exhibit obvious randomly oriented distributions, and the heat flux vector is mainly distributed along the BN pathway. Meanwhile, the noncontact BN fillers contain numerous BN–BN interfaces, and large thermal scattering remains at the edges of h-BN fillers. By contrast, for BC/BN

_{PDA}/GNS

_{PDA}-5 film, the higher heat flux vector is mainly distributed along the “BN-GNS-BN” pathway due to the adjacent BN

_{PDA}fillers are bridged by flexible GNS

_{PDA}. Additionally, the relative longer and denser thermally conductive pathways along hybrid fillers are formed, which effectively minimizes the phonon interface scattering. The numerical thermal conductivities of BC/BN

_{PDA}and BC/BN

_{PDA}/GNS

_{PDA}-5 composite films are separately 21.0 and 34.7 W·m

^{−1}·K

^{−1}, which are approximately consistent with the experimental values (21.7 and 34.9 W·m

^{−1}·K

^{−1}) at the fixed filler content of 70 wt% (Figure 3a). Furthermore, Figure 3f further compares the thermal conductivities of BC/BN

_{PDA}/GNS

_{PDA}-5 composite films in this work with BN-based fillers composites from the literature at the fixed filler content [5,19,30,31,32,33,34,35,36,37], which suggests that the BC/BN

_{PDA}/GNS

_{PDA}-5 films prepared using present strategy show satisfactory in-plane thermal conductivity.

#### 3.3. Mechanical and Dielectric Properties of Composite Films

_{PDA}and the BC/BN

_{PDA}/GNS

_{PDA}-5 composite films at varied filler contents. It can be seen that the pure BC film shows good mechanical strength (66.1 MPa), which is similar to the results found in the literature [5,38]. Evidently, the BC/BN

_{PDA}/GNS

_{PDA}-5 films exhibit higher tensile strength and elongation at break relative to BC/BN

_{PDA}films. For example, the tensile strength of the BC/BN

_{PDA}/GNS

_{PDA}-5 film is 30.9 MPa at 70 wt% filler content, which increases to 156% relative to BC/BN

_{PDA}film (19.8 MPa). At the same time, the elongation at break of BC/BN

_{PDA}/GNS

_{PDA}-5 film reaches 4.1%, which increases to 205% compared with BC/BN

_{PDA}film (2.0%). This can be analyzed through the microstructures in SEM images. On the basis of the “nacre-like” lamellar structures of BC/BN

_{PDA}films, there are few interfacial gaps between BN

_{PDA}and BC nanofibers (Figure 2a), which destroy the formation of continuous and complete the BC nanofiber networks. With the introduction of PDA-modified GNS filler, the GNS

_{PDA}has a better match with BC matrix in intrinsic flexibility and can be completely covered by the BC nanofibers and BN

_{PDA}fillers (Figure 2b), thus forming a more continuous network structure within the film. Therefore, the GNS

_{PDA}filler can play a flexible bridge role in connecting isolated and brittle BN fillers, thus effectively transferring the stress and delaying the deformation of the composite film. In addition, we also performed the stress−strain experiments on the BC/BN

_{PDA}/GNS

_{PDA}-5 composite films at varied filler contents, as presented in Figure S8a. The ultimate tensile strength and elongation at break at different filler contents are basically consistent with the experimental results in Figure 4a,b. For example, at the fixed filler content of 70 wt%, the ultimate tensile strength and elongation at break are 29.7 MPa and 4.0%, respectively, which are basically consistent with results from Figure 4a,b (30.9 MPa and 4.1%). Figure S8b shows that the BC/BN

_{PDA}/GNS

_{PDA}-5 composite films at the hybrid filler content of 70 wt% can easily lift a 500 g weight, indicating the robust mechanical property [39,40]. Figure 5c presents the tensile recovery result of the BC/BN

_{PDA}/GNS

_{PDA}-5 film at the fixed filler content of 70 wt%, and it was found that when the tensile strain of the film gradually increases from 1.5 to 4%, the stress of the film at 1.5% strain is 3.0, 4.3, 6.5, 7.4, and 8.5 MPa, respectively, exhibiting an obvious increasing trend. This is because when the film is stretched, the ultrahigh aspect-ratios of the BN

_{PDA}, GNS

_{PDA}, and BC matrix will be further distributed along the in-plane orientation, forming a tighter network structure, thus strengthening the mechanical properties.

^{5}cm

^{2}·V

^{−1}·s

^{−1}) [24], which leads to poor electrical insulation properties. It was found, surprisingly, that the electrical insulation properties can be regulated by controlling the mass ratio of GNS

_{PDA}. Figure 4d,e depicts the dielectric constant (ε) and dielectric loss (tanδ) of composite films at varied filler content. It can be seen that the ε and tanδ values of BC/BN

_{PDA}/GNS

_{PDA}-5 films are clearly reduced with the increase in hybrid fillers contents, which has a similar trend seen in the literature [5,37]. For example, the ε and tanδ values of the BC/BN

_{PDA}/GNS

_{PDA}-5 composite film with 70 wt% hybrid fillers contents separately reach the extremely low values of 6.3 and 0.03 at the frequency of 1000 Hz, which are lower than that of the film with 10 wt% hybrid fillers (8.1 and 0.05). This is because the BN is the main filler, which exhibits an extremely low values of ε and tanδ (~4, ~5 × 10

^{−4}) [42]; that being said, some studies have reported that the addition of GNS causes the enhancement of ε due to the interfacial polarization effect and the formation of the micro-capacitance [43]. However, for the hybrid filler of the prepared BC/BN

_{PDA}/GNS

_{PDA}-5 film, the content of GNS filler (5 wt%) is much lower than that of the BN filler (95 wt%), and the small amount of GNS is isolated by numerous BN fillers, so the GNS filler fails to form micro-capacitances inner composites. However, when the mass ratio of GNS

_{PDA}increases to 10 wt%, the ε and tanδ values for the BC/BN

_{PDA}/GNS

_{PDA}-10 film increase to 7.8 and 0.05, which are apparently increased relative to the BC/BN

_{PDA}/GNS

_{PDA}-5 film. This result shows that when the amount of GNS fillers exceeds the critical value, GNS fillers might connect to each other, resulting in the interfacial polarization effect [44]. Therefore, when the content of GNP is about 5 wt%, it is beneficial to ensure that the composite film exhibits excellent dielectric properties. Moreover, as presented in Figure 4f, at the fixed GNS

_{PDA}of 5 wt%, the AC conductivity of BC/BN

_{PDA}/GNS

_{PDA}-5 film can be dramatically decreased with the increase in the hybrid fillers contents, further showing an obvious enhancement of electrical insulation properties [37]. However, when the mass ratio of GNS

_{PDA}further increases to 10 wt %, the AC conductivity of the BC/BN

_{PDA}/GNS

_{PDA}-10 film further increases to 6.5 × 10

^{−10}S·cm

^{−1}, which is a significant enhancement compared to the BC/BN

_{PDA}/GNS

_{PDA}-5 film (1.8 × 10

^{−10}S·cm

^{−1}). This may be because when the mass ratio of GNS

_{PDA}is close to a critical value (5 wt%), the electrical conductivity caused by GNS

_{PDA}could be largely cut off by the excellent electrical insulation of h-BN filler networks. As a result, the low dielectric constant, dielectric loss, and electrical conductivity, as well as high thermal conductive properties, can be effectively balanced by simply tuning the mass ratio of the supplementary GNS

_{PDA}filler.

#### 3.4. Heat Dissipation Application of the BC/BN_{PDA}/GNS_{PDA}-5 Composite Films

_{PDA}/GNS

_{PDA}-5 composite films have potential applications in the thermal management of cooling high-power electronic devices. Here, the cooling efficiency of three composite films, including pure the BC, BC/BN

_{PDA}, and BC/BN

_{PDA}/GNS

_{PDA}-5 films, is compared through heat dissipation experiments. As displayed in Figure 5a, the LED chip (10 W) was placed in the middle of films, and the aluminum plate was directly mounted on the films to ensure effective heat diffusion. The IR camera was used to record the hot-spot temperature changes and the temperature distributions images of LED chips. As shown in Figure 5b,c, the LED chip achieves a lower temperature of 74.4 °C within 200 s using the BC/BN

_{PDA}/GNS

_{PDA}-5 films, which is, respectively, 22 °C and 13.9 °C lower than when using the BC (96.4 °C) and BC/BN

_{PDA}films (88.3 °C). It is worth noting that the operation temperature of LED chips closely relates to their working lifetimes; for example, it is reported that every 10 °C drop in chip temperature can extend its service life by 50% [5]. Therefore, the BC/BN

_{PDA}/GNS

_{PDA}-5 film can effectively dissipate the heat and quickly reduce the hot spot temperature, which reveals great potential for use in electronic packaging and electrical thermal management applications.

## 4. Conclusions

_{PDA}-bridging significantly increased the probability of effective contact with BN

_{PDA}fillers. The reduction in R

_{I}and R

_{c}was conducive to the formation of the well stacked and highly oriented microstructure of the BC/BN

_{PDA}/GNS

_{PDA}composite film, which was beneficial when forming a denser and complete “BN-GNS-BN” heat conduction pathway and tight filler–matrix network. For example, the BC/BN

_{PDA}/GNS

_{PDA}composite film shows a thermal conductivity and tensile strength of 34.9 W·m

^{−1}·K

^{−1}and 30.9 MPa, which separately increased to 161% and 155% relative to the BC/BN

_{PDA}film. Although the introduction of the high electric conductivity GNS

_{PDA}may affect the electrical insulation of composites, it was found that the low electrically conductive and high thermal conductive properties can be well balanced by regulating the critical value of the mass ratio of GNS

_{PDA}fillers. At the optimal GNS

_{PDA}mass ratio of 5 wt%, the electrical conductivity caused by GNS

_{PDA}could be effectively blocked by the BN

_{PDA}filler network, thereby resulting in a low electrical conductivity of 1.8 × 10

^{−10}S·cm

^{−1}. Moreover, the BC/BN

_{PDA}/GNS

_{PDA}composite films effectively transfer heat and diminish the hot spot temperature in LED chip cooling applications. The present concise strategy simultaneously meets the high anisotropic thermal conductivity and low electrical conductivity demands, and it may pave the way to promoting the potential of the industrialization of scalable thermal management devices.

## Supplementary Materials

_{I}values were calculated by fitting the in-plane thermal conductivities of the BC/BN

_{PDA}and BC/BN

_{PDA}/GNS

_{PDA}-5 composite film based on the EMT model. Figure S5. Foygel model fitting curves of BC/BN

_{PDA}and BC/BN

_{PDA}/GNS

_{PDA}-5 composite films, respectively. Figure S6. Linear fitting results based on Foygel model for (a) BC/BN

_{PDA}and (b) BC/BN

_{PDA}/GNS

_{PDA}-5 composite films. Figure S7. (a,b) The finite element models of the composite films: BC/BN

_{PDA}and BC/BN

_{PDA}/GNS

_{PDA}-5. (c,d) The mesh distributions of the composite films: BC/BN

_{PDA}and BC/BN

_{PDA}/GNS

_{PDA}-5. Figure S8. (a) Stress−strain curves of BC/BN

_{PDA}/GNS

_{PDA}-5 composite films at varied filler content. (b) The optical image of 70 wt % BC/BN

_{PDA}/GNS

_{PDA}-5 film lifting a 500 g weight. Table S1. The in-plane (α

_{‖}) and through-plane (α

_{⊥}) thermal diffusivity, density (ρ), and specific heat capacity (C) of composite films at different filler contents. References [45,46] are cited in the Supplementary Materials.

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

## References

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**Scheme 1.**Schematic illustration of design strategy. Construction of the large-scale BC/BN

_{PDA}/GNS

_{PDA}composite films from BC nanofibers, PDA-modified BN, and GNS through the LBL scraping method. The highly oriented composite films exhibit enhanced thermal and insulating performance, as well as excellent mechanical robustness.

**Figure 1.**(

**a**) Raman spectra, (

**b**) TGA analysis, and (

**c**) XPS spectra of BN, BN

_{PDA}, GNS, and GNS

_{PDA}. (

**d**) ITC analysis of the enthalpy changes (ΔH

_{obs}) relative to the PDA/BN (or GNS) molar ratio.

**Figure 2.**(

**a**–

**c**) SEM images of cross section of three composite films: BC/BN

_{PDA}, BC/BN

_{PDA}/GNS

_{PDA}-5, and BC/BN

_{PDA}/GNS

_{PDA}-10. (

**d**) Elemental mapping images of B, N, and C in the BC/BN

_{PDA}/GNS

_{PDA}-5 composite film. (

**e**) XRD patterns of BC/BN

_{PDA}and BC/BN

_{PDA}/GNS

_{PDA}-5 composite films. (

**f**–

**h**) Schematic diagrams of the distributions of hybrid fillers.

**Figure 3.**(

**a**) In-plane and (

**b**) through-plane thermal conductivities of two composite films: BC/BN

_{PDA}and BC/BN

_{PDA}/GNS

_{PDA}-5 at different filler contents. (

**c**) In-plane thermal conductivities of the 70 wt% BC/BN

_{PDA}/GNS

_{PDA}composite films at different mass ratios of GNS

_{PDA}. (

**d**) Schematic diagram of the effect of contact thermal resistance on phonon transport. (

**e**) Numerical simulation of heat transfer behaviors between BC/BN

_{PDA}and BC/BN

_{PDA}/GNS

_{PDA}-5 films in the preferred direction. (

**f**) Comparisons of in-plane thermal conductivity of this work with the reported BN-based composites from the literature.

**Figure 4.**(

**a**) Tensile strength and (

**b**) elongation at break of the BC/BN

_{PDA}and BC/BN

_{PDA}/GNS

_{PDA}-5 films with different hybrid fillers contents. (

**c**) The tensile recovery curves of the 70 wt% BC/BN

_{PDA}/GNS

_{PDA}-5 films. (

**d**) Dielectric constant (ε), (

**e**) dielectric loss (tanδ), and (

**f**) AC conductivity of the BC/BN

_{PDA}/GNS

_{PDA}composite films at varied hybrid fillers contents.

**Figure 5.**(

**a**) Experimental configuration for testing the heat transfer performance of BC, BC/BN

_{PDA}, and BC/BN

_{PDA}/GNS

_{PDA}-5 films applied to LED chip heat dissipation. (

**b**) Temperature evolution of LED chip as a function of running time and (

**c**) the corresponding IR images of temperature distributions of BC, BC/BN

_{PDA}, and BC/BN

_{PDA}/GNS

_{PDA}-5 films.

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## Share and Cite

**MDPI and ACS Style**

Li, S.; Shen, Y.; Jia, X.; Xu, M.; Zong, R.; Liu, G.; Liu, B.; Huai, X. Dopamine-Mediated Graphene Bridging Hexagonal Boron Nitride for Large-Scale Composite Films with Enhanced Thermal Conductivity and Electrical Insulation. *Nanomaterials* **2023**, *13*, 1210.
https://doi.org/10.3390/nano13071210

**AMA Style**

Li S, Shen Y, Jia X, Xu M, Zong R, Liu G, Liu B, Huai X. Dopamine-Mediated Graphene Bridging Hexagonal Boron Nitride for Large-Scale Composite Films with Enhanced Thermal Conductivity and Electrical Insulation. *Nanomaterials*. 2023; 13(7):1210.
https://doi.org/10.3390/nano13071210

**Chicago/Turabian Style**

Li, Shikun, Yutan Shen, Xiao Jia, Min Xu, Ruoyu Zong, Guohua Liu, Bin Liu, and Xiulan Huai. 2023. "Dopamine-Mediated Graphene Bridging Hexagonal Boron Nitride for Large-Scale Composite Films with Enhanced Thermal Conductivity and Electrical Insulation" *Nanomaterials* 13, no. 7: 1210.
https://doi.org/10.3390/nano13071210