3D Vertically Aligned BNNS Network with Long-Range Continuous

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3D Vertically Aligned BNNS Network with Long-Range Continuous Channels for Achieving a Highly Thermally Conductive Composite Xiongwei Wang, and Peiyi Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09398 • Publication Date (Web): 30 Jul 2019 Downloaded from pubs.acs.org on July 31, 2019

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ACS Applied Materials & Interfaces

3D Vertically Aligned BNNS Network with Long-Range Continuous Channels for Achieving a Highly Thermally Conductive Composite Xiongwei Wang1,2, Peiyi Wu1,2*

1State

Key Laboratory for Modification of Chemical Fibers and Polymer Materials,

College of Chemistry, Chemical Engineering and Biotechnology, Center for Advanced Low-Dimension Materials, Donghua University, Shanghai 201620, China 2State

Key Laboratory of Molecular Engineering of Polymers, Department of

Macromolecular Science, Fudan University, Shanghai 200433, P. R. China

E-mail：[email protected]

Keywords: nanofibrillated cellulose; boron nitride; segregated network; thermal conductivity; epoxy composite

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Abstract: Construction of a three-dimensional (3D) vertically aligned filler network in polymer matrix has been believed an effective method to attain a large through-plane thermal conductivity enhancement at relatively low filler loading. However, it is still a challenge to construct a vertically aligned filler network composed of many long-range continuous pore channels in polymer matrix for the high-flux heat-conduction. To address this problem, herein, nanofibrillated cellulose (NFCs) assisted unidirectional freeze-drying of boron nitride nanosheets (BNNSs) slurry was used to prepare a novel epoxy composite containing 3D vertically aligned BNNS network with long-range continuous pore channels. The vertically aligned and nacre-mimetic channels make the composite possess a high through-plane thermal conductivity of 1.56 W m-1 K-1 at an extremely low BNNSs loading of 4.4 vol%, and a significant thermal conductivity enhancement efficiency of 167.3 per 1 vol% filler. Therefore ， we think this work is expected to give a significant insight in the preparation of polymer composite with high heat-conduction efficiency to address the heat dissipation of modern electronics.


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1. Introduction Recently, boron nitride nanosheets (BNNSs) have attracted an extensive interest in the preparation of high-performance polymer composites with desirable electrical insulation and high thermal conductivity (TC) because of their high theoretical thermal conductivity (600 W m-1 K-1) and band gap (~5.5 eV)1-3. In order to achieve a significant TC enhancement, building a continuous segregated 3D-BNNS network in polymer matrix is very necessary4-7. While for the composites prepared by the conventional blending methods, due to the inevitable agglomeration of BNNSs, a large amount of filler (above 30 vol%) is always needed to form a continuous BNNS network in the polymer matrix, thereby leading to the considerable polymer/filler interfacial thermal resistance and low TC enhancement efficiency8-11. Although surface modification can observably improve the dispersion of BNNSs as well as reduce the thermal resistance cross the polymer/filler interface, the composite’s TC enhancement efficiency is still not desirable enough12-14. With the purpose to significantly improve the TC enhancement efficiency, introducing an interconnected 3D-BNNS segregated network in polymer matrix is considered to be one of the most effective strategies15-19. Because the 3D-BNNS segregated network can make full use of the heat-conduction capability of BNNSs and act as the macroscopic “expressway” for the rapid transport of phonons throughout the composite by running on them, eventually resulting in a dramatically reduced polymer/filler interfacial thermal resistance16-18, 20-22. For instance, Tian et al. prepared a gelatin-mediated BN porous architecture via a direct foaming method and then 3 ACS Paragon Plus Environment


impregnated them with epoxy resin18. The obtained epoxy composite integrated with 3D-BNNS segregated network possesses a high through-plane TC of 3.48 W m-1 K-1 at a relatively low BNNS loading of 24.4 wt%. However, the isotropic feature of this BN segregated network means a zigzag heat conduction path. Given the TC anisotropy of BNNSs, regulating their orientation along the through-plane direction to form 3D vertically-aligned and interconnected network in polymer matrix is expected to make the heat transfer path straighter, and then promote the heat conduction throughout the composite. To date, many efforts have been devoted to realizing the alignment of BNNSs in the through-plane direction of segregated network: such as the electrospinning combined with rational arrangement proposed by Chen et al.23, hydrothermal treatment of large-sized graphene oxide (GO) at alkali addition conditions reported by An et al.24, and unidirectional freeze-drying method reported by Zeng et al.25-27. Among them, the unidirectional freeze-drying of a uniform aqueous BNNSs slurry containing various macromolecular binders (PVA, PVP, cellulose, GO, etc.) is the most efficient approach because of its more facile and eco-friendly processing conditions, stronger applicability to various materials and larger potentials for scale-up production22, 25, 28-30. Although the unidirectional freeze-drying has been extensively used to prepare the vertically aligned 3D-BNNS networks, the pore channels of them all show a poor long-range continuity and a large number of horizontal branches bridged in the vertical direction25,

28, 30.

The presence of these structural defects is expected to

weaken the heat conduction. Recently, Pan et al. prepared a honeycomb-like 4 ACS Paragon Plus Environment

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framework with long-range continuous and vertically aligned pore channels via the unidirectional freeze-drying of nanofibrillated cellulose (NFCs) aqueous slurry31. Wicklein et al. further found that even introducing some inorganic building blocks (GO nanosheets and sepiolite nanorods) into the NFCs slurry, a regular and long-range continuous network with vertically aligned structure can still be formed after unidirectional freeze-drying32. Therefore, it inspires us that the unidirectional freeze-drying of NFCs assisted high-concentration BNNSs aqueous slurry is likely to prepare a vertically aligned 3D-BNNS network with long-range continuous pore channels. After infiltration of epoxy resin, the unique structure can make the phonons transmit almost linearly along the long-range continuous walls of vertically aligned 3D-BNNS network, and then significantly improve the composite’s TC enhancement efficiency. In this work, we prepared a vertically aligned anisotropic 3D-BNNS network (3D-Aniso-BNNS) with long-range continuous and nacre-like channels via NFCs assisted unidirectional freeze-drying and then impregnated epoxy resin into this framework to obtain the final epoxy composite (Epoxy/3D-Aniso-BNNS). The amphiphilic NFCs can not only promote the uniform dispersion of hydrophobic BNNSs in water, but also act as the role of “mortar” to direct the assembly of the “brick” of BNNSs to form a vertically aligned and long-range continuous 3D-BNNSs network during the unidirectional freeze. Benefit from this unique structure, the prepared epoxy composite achieves a high through-plane TC of 1.56 W m-1 K-1 at a low BNNSs loading of 4.4 vol%, and the corresponding TC enhancement relative to 5 ACS Paragon Plus Environment


epoxy resin is 736 %. We hope this work could provide a significant insight to subtly regulate the channel structure of 3D segregated filler network and then to prepare the highly thermally conductive polymer composites.

2. Experiential Section 2.1 Materials Hexagonal boron nitride (h-BN) was purchased from Alfa-Aesar. TEMPO, sodium hypochlorite aqueous solution (6-14 wt%) and o-dichloroaniline methane (MOCA) were purchased from Aladdin Chemical Reagent. Isopropanol (IPA), sodium bromide (NaBr) and sodium hydroxide (NaOH) were provided by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Microcrystalline cellulose (MCC) was provided by Houcheng fine chemical Co. LTD. (Shanghai, China). Bisphenol-F epoxy resin was purchased from Taiwan Nanya plastics. 2.2 Preparation of few-layered BNNSs 5 g of h-BN powders were uniformly dispersed in 500 ml IPA and the mixture was then continuously sonicated for 24 h to exfoliate the bulk h-BN into few-layered BNNSs in a sonication bath with an output power of 100 W (KQ5200DB). After that, the suspension was centrifuged at 2000 rpm for 12 min to remove the non-exfoliated BNNSs. The supernatant was used to collect the exfoliated BNNSs by removing the solvent. 2.3 Preparation of 3D-Aniso-BNNS network NFCs were prepared via a TEMPO-mediated oxidation method reported by our previous literature33. A certain amount of BNNS powders were first uniformly 6 ACS Paragon Plus Environment

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dispersed in NFC aqueous solution (5 mg/ml, PH=7) by bath sonication for 10 min and tip sonication for 5 min. Herein, it should be noted that the mass ratio of BNNSs to NFCs was invariably fixed at 7:3. After concentration by rotary evaporation, the concentrated slurry was poured into a cylindrical Teflon mold, and then placed on the top of a copper cylinder that partially immersed in liquid nitrogen bath for unidirectional freezing. After that, the frozen suspensions further freeze-dried to obtain the vertically aligned 3D-Aniso-BNNS scaffolds with different densities. For comparison, the isotropic 3D-BNNS (3D-Iso-BNNS) scaffolds with different densities were also prepared by the same steps except that the freezing of the concentrated suspension was carried out in a -18 oC refrigerator. 2.4 Preparation of Epoxy/3D-Aniso-BNNS composite The 3D-Aniso-BNNS scaffold was further infiltrated with low viscosity epoxy resin under vacuum to prepare the Epoxy/3D-Aniso-BNNS composite. The experimental details are as follows: 10 g curing agent (MOCA) was first uniformly dissolved in 30 g bisphenol-F epoxy resin at 100 oC. After that, the porous scaffold was impregnated with the epoxy resin in a vacuum oven at 120 oC for 15-30 min to ensure the complete filling. Then the system was further cured at 150 oC for 2 h and 180

oC

for 2.5 h at the normal pressure, eventually obtaining the final

Epoxy/3D-Aniso-BNNS composite. Additionally, the procedure to prepare the Epoxy/3D-Iso-BNNS composite was the same as above. 2.5 Characterization The morphology of the pristine h-BN and the exfoliated BNNSs was 7 ACS Paragon Plus Environment


characterized by Transmission electron microscopy (TEM) (JEOL JEM2011 F, Japan) at 200 kV. Atomic force microscopy (AFM) (Bruker Multimode 8, Germany) was used to estimate the thickness of BNNSs by the tapping mode. The cross-sectional and lateral-sectional morphology of 3D-BNNS scaffolds and the corresponding epoxy composites were observed by field emission scanning electron microscopy (FESEM) (Zeiss Ultra 55, Germany). XRD patterns of the BNNSs before and after exfoliation were recorded on X’pert PRO PANalytical (Netherland) with Ni-filtered Cu Kα radiation (40 kV, 40 mA). Raman spectra of the BNNSs before and after exfoliation were collected on a micro-Raman spectrometer (HORIBA XploRA, France). The through-plane thermal diffusivity (α, m2 s-1) of the epoxy composites were measured on a laser-flash diffusivity instrument LFA 447 (NETZSCH, Germany). Before the measurement, the circular samples (Ø12.5mm×1.5 mm) with smooth surface were first spray-coated by graphite ink. The specific heat (c, J g-1 K-1) was measured using a differential scanning calorimeter (DSC, TA Q2000, American). The density (ρ, g cm-3) of the samples was calculated by the equation: ρ=m/ν and the eventual thermal conductivity (λ, W m-1 K-1) was calculated by the equation: λ=α × c × ρ.


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3. Results and discussion

Figure 1 ． TEM image (a) and high-resolution TEM image (b) of the exfoliated BNNSs, inserts show its edge and SAED pattern. AFM image (c) and the corresponding height profile (d) of the exfoliated BNNSs; Statistical analysis of BNNSs in length (e) and thickness (f) according to the SEM image and AFM image. The ultrathin BNNSs are more favorable to construct a free-standing and high-strength 3D porous monolith, thus the pristine h-BN was first exfoliated into single- or few-layered BNNSs by the consecutive bath sonication in IPA. Figure S1 is the morphology of original h-BN and exfoliated BNNSs. One can see that h-BN possesses a typical layered structure composed of many closely packed lamellas (Figure S1a). After sonication, both of the thickness and lateral size of them show a remarkable decrease, indicating the successful exfoliation of h-BN (Figure S1b). From the TEM image, one can more clearly see the ultrathin feature of the exfoliated BNNSs because of their large electron transparency (Figure 1a). The high-resolution TEM image further reveals the flawless crystalline structure in BNNSs, which is consistent with the well-defined hexagonal diffraction lattice recorded by the selected area electron diffraction (SAED) pattern (Figure 1b). The edge-counting method 9 ACS Paragon Plus Environment


reveals the exfoliated BNNSs consist of 6-8 layers. As a proof, AFM characterization was further used to accurately evaluate the thickness of BNNSs. As shown in Figure 1c and d, the thickness of the exfoliated BNNSs can be calculated to be 2-2.5 nm from the corresponding height profile, suggesting the few-layered feature of the exfoliated BNNSs. Given the significant influence of the BNNS’s thickness and lateral size on the heat conduction performance of final composite, the statistical analysis of the thickness and lateral size of BNNSs was further performed by selecting above 60 samples in AFM and SEM images. As shown in Figure 1e and f, majority of the exfoliated BNNSs have a lateral size in 0.5-2.5 μm and thickness in 1.5-3.0 nm, suggesting the typical lamellar structure of BNNSs with high aspect ratio. To further ascertain the structural evolution of h-BN before and after exfoliation, the XRD pattern and Raman spectra of the original h-BN and exfoliated BNNSs were both collected. XRD pattern of h-BN displays five characteristic diffraction peaks at 2θ=26.7o, 41.6o, 43.8o, 50.1o and 55.2o, which are assigned to the (002), (100), (101), (102) and (004) planes, respectively (Figure S2a)34. After exfoliation, the (002) diffraction peak broadens, and the relative intensity ratio of (004) to (100) diffraction peaks shows a large increase, which imply the successful exfoliation of h-BN into multi-layered BNNSs34-35. In addition, the Raman spectra of h-BN and BNNSs reveal an apparent shift of the characteristic peak from 1369.8 cm-1 to 1372.2 cm-1 after exfoliation, which is related to the reduced layer number of h-BN (Figure S2b)36.


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Figure 2．(a) TEM image of NFCs; (b) optical images of BNNSs and NFCs assisted BNNSs aqueous dispersions; (c) optical images of the prepared 3D-Aniso-BNNS network; (d) top-view image of 3D-NFC network with the density of ~17.3 mg/cm3; top-view images of 3D-Aniso-BNNS network with different density: (e) ~17.3 mg/cm3, (f) ~34.7 mg/cm3, (g) 69.3 mg/cm3 and (h) 138.6 mg/cm3. Amphiphilic NFCs were prepared via a TEMPO mediated oxidation of microcrystalline cellulose in an alkaline environment. TEM image of the synthesized NFCs shows a wire-like morphology with micro-sized length and nano-sized diameter (Figure 2a). Moreover, NFCs show a strong surface electronegativity in water with a ζ-potential of -45 mV, which is attributed to the oxidation of the hydroxymethyl into carboxylate during the TEMPO-mediated oxidation process33. The presence of these carboxylates endows NFCs with amphiphilic nature, and meanwhile enables them have an excellent dispersion in water by electrostatic repulsion37-38. Therefore, NFCs were selected as the surfactant to promote the dispersion of hydrophobic BNNSs in aqueous phase. As shown in Figure 2b, the bare BNNSs are difficult to disperse in water even after intense sonication. The introduction of NFCs (the mass ratio of NFCs 11 ACS Paragon Plus Environment


to BNNSs was fixed at 3:7) can significantly improve the dispersion of BNNSs in water, and the obtained suspension shows a good colloidal stability. Further reducing the NFCs or increasing the BNNSs content would cause a small number of BNNSs separate from the aqueous suspension. That is to say, BNNSs have reached the limit to attain a uniform and stable dispersion at the BNNSs/NFCs ratio of 7:3. In order to clarify the interaction between NFCs and BNNSs, AFM and FTIR characterizations were employed. After removing the free NFCs from NFC/BNNS suspension by multiple centrifugations, AFM image of the remaining BNNSs reveals that there still have many NFCs anchored on the surface of BNNSs (Figure S3). This phenomenon directly confirms the existence of strong interaction between NFCs and BNNSs. To figure out whether the existence of hydrogen bond between them or not, the NFCs and NFC/BNNS suspensions were dropped onto the silicon wafer for FTIR characterization, respectively. However, the result shows that the introduction of BNNSs don’t create a prominent shift of –OH stretching vibration bands of NFCs, indicating the negligible hydrogen bonding interaction between BNNSs and NFCs (Figure S4). In a related work, Zeng et al. considered the strong hydrophobic to hydrophobic interaction between NFCs and boron nitride nanotubes (BNNTs) render the homogeneous dispersion of BNNTs in water37. Therefore, we think the strong affinity of NFCs and BNNSs in this work might also be derived from the hydrophobic to hydrophobic interaction between them. The as-prepared uniform aqueous dispersion of NFCs and BNNSs was further concentrated to obtain mixed slurry, and then placed at the top of a copper rod which 12 ACS Paragon Plus Environment

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has about two-thirds part immersed in liquid nitrogen for unidirectional freezing. As the schematic model shown in Figure 3, due to the large temperature gradient in the vertical direction of the slurry, the ice crystals would first nucleate at the bottom and then vertically grow along the temperature gradient to form the array of vertically aligned ice columns39-40. At the same time, NFCs and BNNSs are expelled into the gap of adjacent ice columns, and then assemble to form orderly oriented structure along the vertical direction driven by the vertical shear forces20,

25, 28.

After

freeze-drying, a vertically aligned 3D-BNNS network with long-range continuous pore channels is obtained. By using the Teflon mold of different geometries, 3D-BNNS framework with various dimensions and shapes can be prepared (Figure 2c). Since the crucial role of the microstructure of 3D-BNNS on the high-efficiency heat conduction for composites, the morphology of the 3D-Aniso-BNNS network with different densities in the perpendicular and horizontal directions to ice growth are systematically studied by SEM. As shown in Figure 2d, after unidirectional freeze-drying, the top-view image of 3D-NFCs network exhibits a well-defined and vertically-aligned open-framework structure. However, when 70 % NFCs are replaced by BNNSs, a prominent change in pore structure has been occurred for the prepared 3D-BNNS network (Figure 2e). Although the vertically-aligned pore structure along the direction of ice growth has been basically maintained, the pores perpendicular to the direction of ice growth have a disorder assembly with many side branches. That’s because during the oriented freezing process, the structure of the 3D-BNNS architecture is dependent on many factors including the growth rate of ice columns, 13 ACS Paragon Plus Environment


the concentration of building blocks, and the moving speed of building blocks20, 28. Therefore, the introduction of BNNSs might hinder the migration rate of building blocks during the vertical growth of ice crystals, resulting in the relatively loose and disordered pore structure. As the density increases to ~34.7 mg/cm3, an orderly vertically aligned lamellar network is formed (Figure 2f). With the further increasing density of 3D-BNNS network (~69.3 and ~138.6 mg/cm3), the well-organized array of walls is well maintained, but meanwhile the space between adjacent layered walls decreases (Figure 2g-h).

Figure 3．Schematic illustration of the NFCs assisted unidirectional freeze-drying of BNNSs suspension and the preparation of Epoxy/3D-Aniso-BNNS composite. In order to further demonstrate the microstructure of pore channels in 3D-Aniso-BNNS network, the side-view images of them with different densities were 14 ACS Paragon Plus Environment

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also studied. As shown in Figure 4a, the 3D-NFCs network exhibits continuous oriented pore channels with smooth walls. While the addition of BNNSs would increase the roughness of pore walls, the well-organized structure is still preserved (Figure 4b-e). On the other hand, at the low density of ~17.3 mg/cm3, the poor continuity of channels with a large number of holes is observed. Once the density increases to ~34.7 mg/cm3, the holes embedded on the pore walls have completely disappeared but replaced by the long-range continuous and vertically aligned pore channels. Further increasing architecture density is prone to causing the broaden width of pore channels and the shrunken space between adjacent walls. Moreover, the high-magnified image further reveals that BNNSs and NFCs tightly interconnect with each other to ensure the structural integrality and robustness for the 3D-BNNS network, and the BNNSs show a highly oriented alignment along the channels (Figure 4f and Figure S5).

Figure 4. (a) Side-view morphology of 3D-NFC network with the density of ~17.3 mg/cm3; side-view morphology of 3D-Aniso-BNNS network with different density: (b) ~17.3 mg/cm3, (c) ~34.7 mg/cm3, (d) 69.3 mg/cm3 and (e) 138.6 mg/cm3; (f) the magnified image of pore channel.



Figure 5. (a) Photographs of Epoxy/3D-Aniso-BNNS composite before and after epoxy infiltration; top-view (b and c) and side-view (d and e) morphology of Epoxy/3D-Aniso-BNNS composite The as-prepared 3D-Aniso-BNNS network was further infiltrated by epoxy resin to obtain the corresponding epoxy composite integrated with 3D segregated BNNS network (Epoxy/3D-Aniso-BNNS). Figure 5a displays the optical image of the cured epoxy composite. It can be seen from the naked eye that the backfilling of epoxy resin does not cause a perceptible damage to the 3D-Aniso-BNNS network because of its robust interconnected structure. The mechanical performance of the pure epoxy resin and the epoxy composite with 4.4 vol% BNNSs shows that the incorporation of 3D-Aniso-BNNS network would cause an obvious decease both in tensile strength and break elongation of the pure epoxy resin, but still able to meet the demands of 16 ACS Paragon Plus Environment

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using as a thermal interface martial (Figure S6). To further clarify the microstructure of the composite, the top-view and side-view fractured morphology were also explored. As shown in Figure 5b and d, the pore channels of the 3D-Aniso-BNNS network have been fully filled with epoxy resin, but still retain the vertically aligned interconnected structure. From the high-magnification SEM image, one can further see a good interfacial adhesion between the epoxy resin and 3D-BNNS skeleton, which might be ascribed to the favorable compatibility between NFCs and epoxy resin (Figure 5c and e). Importantly, the vertically aligned pore channels and the high orientation of BNNSs along the channels are in favor of reducing the path tortuosity and interfacial thermal resistance of phonon transfer in the through-plane direction of the composites23, 28. Therefore, the 3D vertically-aligned segregated BNNS network in epoxy resin can be used as the effective expressways for high-flux and fast heat conduction, promoting the significant enhancement of through-plane TC. For

comparison,

the

morphology

of

isotropic

3D-BNNS

network

(3D-Iso-BNNS) prepared by general freeze-drying and their corresponding epoxy composites have also been investigated. As shown in Figure 6a and b, the non-directional freeze-drying gives rise to an isotropic 3D-BNNS network with randomly distributed porous structure. The increased density of 3D-BNNS network can enhance the density of pores and the thickness of walls. After impregnation of epoxy resin, the cross-sectional SEM images of the composites exhibit the full infiltration of epoxy resin into the interconnected porous architecture, resulting in the formation of 3D isotropic segregated BNNS network inside the epoxy resin (Figure 6c 17 ACS Paragon Plus Environment


and d).

Figure 6. Cross-sectional morphology of 3D-Iso-BNNS network with the density of ~74.9 mg/cm3 (a) and ~146.2 mg/cm3 (b); the corresponding cross-sectional morphology of Epoxy/3D-Iso-BNNS composites (c and d) To further confirm the orientation of BNNSs in Epoxy/3D-Aniso-BNNS and Epoxy/3D-Iso-BNNS composites, XRD characterization was performed. Many previous studies have found that the intensity ratio of (002) to (100) diffraction peak of BNNSs (I002/I100) can be used to qualitatively reflect the orientation level of BNNSs in the polymer matrix30, 41-43. Generally, the larger of the I002/I100 value, the higher of the orientation degree of BNNSs42-43. Figure 7a shows the XRD diffraction patterns of Epoxy/3D-Aniso-BNNS and Epoxy/3D-Iso-BNNS composites in the vertical direction. It can be seen that I002/I100 ratio of Epoxy/3D-Iso-BNNS composite is only 4, indicating the random distribution of BNNSs in this composite. Moreover, it is unusual that the I002/I100 ratio of Epoxy/3D-Aniso-BNNS composites is less than 1. For further verification, the XRD diffraction pattern of Epoxy/3D-Aniso-BNNS 18 ACS Paragon Plus Environment

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composite in the lateral direction has also been investigated. As shown in Figure 7b, the I002/I100 ratio calculated from the lateral direction of Epoxy/3D-Aniso-BNNS composite is as high as 413. These results adequately confirm the highly vertical alignment of BNNSs in the Epoxy/3D-Aniso-BNNS composite.

Figure 7. (a) XRD patterns of Epoxy/3D-Iso-BNNS and Epoxy/3D-Aniso-BNNS composites in the vertical direction; (b) XRD pattern of Epoxy/3D-Aniso-BNNS composite in the lateral direction Through-plane TCs of Epoxy/3D-Aniso-BNNS and Epoxy/3D-Iso-BNNS composites with different BNNSs loading were tested by laser flash method at room temperature. Figure 8a shows that the through-plane TC of epoxy resin is only 0.187 W m-1 K-1. With the increasing addition of BNNSs, the TC values of these two kinds of epoxy composites both show a significant enhancement, which are mainly benefitted from the increasing number of heat transfer pathway in the high-density segregated 3D-BNNS network. While the TC value of Epoxy/3D-Aniso-BNNS composites is always significantly higher than that of Epoxy/3D-Iso-BNNS composites at the same filler loading, and this trend becomes more prominent at the higher filler content. For instance, the Epoxy/3D-Aniso-BNNS composite achieves a high through-plane TC of 1.56 W m-1 K-1 at a low BNNSs content of 4.4 vol%, corresponding to 736 % TC enhancement compared with pure epoxy resin (Figure 19 ACS Paragon Plus Environment


8b). But the through-plane TC of Epoxy/3D-Iso-BNNS composite with 4.64 vol% can

Figure 8. Through-plane TC (a) and TC enhancement (b) of Epoxy/3D-Iso-BNNS and Epoxy/3D-Aniso-BNNS composites with different BNNSs loading; (c) Schematic illustration of the heat-conduction mechanism in Epoxy/3D-Aniso-BNNS and Epoxy/3D-Iso-BNNS composites only reach to 1.12 W m-1 K-1, which is equivalent to 501 % TC enhancement of epoxy resin. The large TC difference between these two kinds of composites is mainly attributed to the straighter and wider heat transfer pathway in Epoxy/3D-Aniso-BNNS composite for the high -flux phonon conduction along the through-plane direction compared to the zigzag heat transfer pathway in Epoxy/3D-Iso-BNNS composite (Figure 8c)20,

25.

Additionally, the Epoxy/BNNS composites containing randomly

dispersive BNNSs have also been prepared for comparison. The result shows that the through-plane TC of Epoxy/BNNS composite with 4.56 vol% BNNS is only 0.324 W m-1 K-1, suggesting the important role of constructing a segregated interconnected 20 ACS Paragon Plus Environment

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BNNS network on attaining the significant TC enhancement at a lowest possible filler loading for the composite. In order to further exhibit the superiority of NFCs-assisted vertical alignment of BNNSs on the TC enhancement of composite, the through-plane TC enhancement per 1 vol% filler loading is calculated to represent the TC enhancement efficiency (η) of the filler. The equation of η is as follows:

=

TC-TC0 100% 100 TC0

where TC0 and TC is the thermal conductivity of epoxy resin and epoxy composite with a certain BNNS loading, respectively, and ν represents the volume fraction of BNNS in the composite. According to this equation, the through-plane TC and the corresponding η values of some typical thermally conductive and electrically insulating composites are listed in Table 1. One can see that the η value of Epoxy/3D-Aniso-BNNS composite is larger than those of most related composites, indicating that the 3D vertically aligned BNNS network with long-range continuous channels has a comparable or superior heat conduction performance to many other related polymer/BN composites. To visually demonstrate the heat-conduction capability of these epoxy composites, the epoxy resin, Epoxy/3D-Iso-BNNS and Epoxy/3D-Aniso-BNNS composites were simultaneously placed on a same hot stage at 100 oC for heating, during which a portable infrared camera was utilized to record the instantaneous surface temperature distribution of the specimens after different heating time. As shown in Figure 9, the surface temperature of Epoxy/3D-Aniso-BNNS composite shows a slightly faster increase than that of Epoxy/3D-Iso-BNNS, but much faster 21 ACS Paragon Plus Environment


Page 22 of 28

than that of epoxy resin, which is consistent with the through-plane TCs of these three samples. Table 1 Comparison of through-plane TC and η of this work with the previously reported related polymer composites. Sample

Filler loading (vol%)

TC (W

TC Enhancement

m-1 K-1)

(%)

η

YearRef.

Epoxy/3D-BNNS

9.6

3.13

1574

161

20175

PDMS/PVA/BNNS

15.6

1.94

977

62.7

201723

Epoxy/3D-SiCNW

2.17

1.67

827.8

406.6

201820

Epoxy/3D-BNNS-rGO

13.16

5.05

2705

205.5

201828

Epoxy/3D-BNNS

9.29

2.4

~1100

118.3

201525

PDMS/3D-BNNS

13.9

1.7

1033

74.3

201629

Epoxy/Ver-h-BN

40

9.0

~4400

~110

201727

Epoxy/3D-h-BN

34

4.4

~2100

61.7

201730

PMMA/BNNS

~69

10.22

4545

65.8

201611

PDMS/NFC/C3N4

9.16

1.94

978

107

201826

Epoxy/3D porous BN

~15

3.48

~1640

109.3

201818

Epoxy/3D-Aniso-BNNS

4.4

1.56

736

167.3

Current

Figure 9. Infrared thermal images of pure epoxy resin, Epoxy/3D-Iso-BNNS composite with 4.64 vol% BNNS and Epoxy/3D-Aniso-BNNS composite with 4.4 vol% BNNS at different heating time. 22 ACS Paragon Plus Environment

Page 23 of 28 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


4. Conclusions In summary, we have prepared a novel epoxy composite containing 3D vertically aligned segregated BNNS network with long-range continuous pore channels via a NFCs assisted unidirectional freeze-drying of BNNS slurry and the subsequent epoxy infiltration. The vertically aligned and nacre-mimetic channels assembled by the highly oriented BNNS nanoplatelets can effectively reduce the interface thermal resistance and provide high-flux heat conduction in the through-plane direction of the composite.

Benefiting

from

the

unique

structure,

the

prepared

Epoxy/3D-Aniso-BNNS composite exhibits a high through-plane TC of 1.56 W m-1 K-1 at an extremely low filler of 4.4 vol%, and its TC enhancement efficiency relative to the filler amount is also superior to most of the previously-reported counterparts. ASSOCIATED CONTENT Supporting Information Available. SEM images, XRD patterns and Raman spectra of original h-BN and exfoliated BNNSs; AFM images and FTIR spectra of the NFCs and NFCs-functionalized BNNSs AUTHOR INFORMATION Corresponding Author *Authors for Correspondence: [email protected] ORCID 23 ACS Paragon Plus Environment


Peiyi Wu: 0000-0001-7235-210X Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We gratefully acknowledge the ﬁnancial support from the Ministry of Science & Technology of China (No. 2016YFA0203302). Reference: (1) Guerra, V.; Wan, C.; McNally, T. Thermal Conductivity of 2D Nano-Structured Boron Nitride (BN) and Its Composites with Polymers. Prog. Mater. Sci. 2019, 100, 170-186. (2) Fang, H.; Bai, S.-L.; Wong, C. P. "White Graphene" - Hexagonal Boron Nitride Based Polymeric Composites and Their Application in Thermal Management. Compos. Commun. 2016, 2, 19-24. (3) Kuang, Z.; Chen, Y.; Lu, Y.; Liu, L.; Hu, S.; Wen, S.; Mao, Y.; Zhang, L. Fabrication of Highly Oriented Hexagonal Boron Nitride Nanosheet/Elastomer Nanocomposites with High Thermal Conductivity. Small 2015, 11, 1655-1659. (4) Wu, K.; Lei, C.; Huang, R.; Yang, W.; Chai, S.; Geng, C.; Chen, F.; Feng, Q. Design and Preparation of a Unique Segregated Double Network with Excellent Thermal Conductive Property. ACS Appl. Mater. Interfaces 2017, 9, 7637-7647. (5) 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. 2017, 27, 1604754. (6) Li, Z.; Kong, J.; Ju, D.; Cao, Z.; Han, L.; Dong, L. Thermal Conductivity Enhancement of Poly(3-hydroxylbutyrate) Composites by Constructing Segregated Structure with The Aid of Poly(ethylene oxide). Compos. Sci. Technol. 2017, 149, 185-191. (7) Zhu, Z.; Li, C.; Songfeng, E.; Xie, L.; Geng, R.; Lin, C.-T.; Li, L.; Yao, Y. Enhanced Thermal Conductivity of Polyurethane Composites via Engineering Small/Large Sizes Interconnected Boron Nitride Nanosheets. Compos. Sci. Technol. 2019, 170, 93-100. (8) Yang, N.; Xu, C.; Hou, J.; Yao, Y.; Zhang, Q.; Grami, M. E.; He, L.; Wang, N.; Qu, X. Preparation and Properties of Thermally Conductive Polyimide/Boron Nitride Composites. RSC Adv. 2016, 6, 18279-18287. (9) Zhang, R.-C.; Sun, D.; Lu, A.; Askari, S.; Macias-Montero, M.; Joseph, P.; Dixon, D.; Ostrikov, K.; Maguire, P.; Mariotti, D. Microplasma Processed Ultrathin Boron Nitride Nanosheets for Polymer Nanocomposites with Enhanced Thermal Transport Performance. ACS Appl. Mater. Interfaces 2016, 8, 13567-13572. 24 ACS Paragon Plus Environment

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3D Vertically Aligned BNNS Network with Long-Range Continuous

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