Significantly enhanced thermally conductive epoxy composite composed of caterpillar-like structured expanded graphite/ boron nitride nanotubes

Abstract

The trend toward miniaturization, integration and multifunctionality of modern electronics has led to a rapid increase in power density, which makes heat dissipation a critical issue. Despite the great potential of graphite-related nanocomposites in dissipating excess heat to ensure high efficiency and long lifetime of electronic devices, the practical application of these composites is limited by the ultra-low vertical thermal conductivity due to the interfacial thermal resistance between graphite layers. Here, a caterpillar-like hybrid filler was fabricated by the in situ intercalation of boron nitride nanotubes (BNNTs) between expanded graphite (EG) layers based on chemical vapor deposition technology. Owing to the optimized interfacial thermal resistance by forming covalent C-N bonding at the interface of EG and BNNT, the through-plane thermal conductivity of epoxy-based nanocomposites can be up to 5.18 Wm⁻¹ K⁻¹. In addition, the composite possessed electromagnetic interference shielding performance of 33.34 dB while maintaining electrical insulation due to the hierarchical structure. This work provided a new strategy for fabricating polymer-based composites with excellent through-plane thermal conductivity in thermal management applications.

Keywords

Boron nitride nanotubes chemical vapor deposition interfacial thermal resistance thermal management materials

Introduction

With the integration and high power densification of modern equipment, significant heat buildup can lead to thermal failure or even explosion of electrical equipment in integrated electronics, military weapons and energy storage systems.^1,2 Therefore, thermally conductive but electrically insulating materials are becoming increasingly urgent to ensure the reliability and longevity of electronic products.³ Electromagnetic interference (EMI) shielding of RF radiation is likewise a serious problem to be faced by electronic devices. The commonly used fillers are metals (e.g., silver nanoparticles and copper nanowires^4,5). However, metal fillers are expensive and heavy in weight. Carbon materials can possess a thermal conductivity comparable to that of metallic fillers, lightweight and good electrical conductivity.^6,7 Therefore, they are widely used as thermally conductive electromagnetic shielding fillers. Carbon nanomaterials, such as graphite nanoplatelets (GNPs), carbon nanotubes (CNT) and graphene, are obtained by chemical vapor deposition or chemically exfoliating, typically with ultra-high intrinsic thermal conductivity ( λ > 1500 Wm⁻¹K⁻¹ at room temperature).^8,9 Among carbon fillers, expanded graphite (EG) is considered to be the most cost-effective filler for improving thermal conductivity of polymer composites due to its low price and high thermal conductivity (400 Wm⁻¹K⁻¹). The use of graphite flakes with large lateral dimensions to reduce the lateral space density of high thermal resistance junctions can radically improve the in-plane TC.¹⁰ However, their practical application is usually limited by the ultra-low through-plane thermal conductivity, which is due to the interfacial thermal resistance that hinders phonon transport between the contacting graphite layers. Some efforts have been made to insert nanostructures (e.g., cellulose nanocrystals, silver nanoparticles, and nanoprofibrillated cellulose) between graphene layers to improve through-plane thermal conductivity. Silver nanoparticles were inserted in reduced graphene oxide (Ag/rGO) by Guo et al.⁴ The corresponding Ag/rGO/PI nanocomposite has a maximum λ of 2.12 Wm⁻¹K⁻¹. Zou et al.¹¹ prepared graphene-(graphitized polydopamine)-(carbon nanotubes) (G-gPDA-CNT) all-carbon ternary composite films. It has a maximum through-plane thermal conductivity of 2.65 Wm⁻¹K⁻¹. However, the weak interaction between graphene and nanostructures limits the effectiveness of this approach to enhance the through-plane thermal conductivity. Reducing the interfacial thermal resistance to produce satisfactorily high through-plane thermal conductivity remains challenging. Recently, Li et al. constructed 3D hybridized carbon films with graphene nanowrinkles and microhinge structures. The design provided graphitized GO/PI films in the through-plane (150 ± 7 Wm⁻¹K⁻¹) and in-plane (1428 ± 64 Wm⁻¹K⁻¹) directions with ultra-flexibility and ultra-high thermal conductivity.¹² However, the high electrical conductivity of graphene severely hinders its application in electronic packaging where insulation of thermal management materials is an essential requirement.

BNNT is an analog of CNT. But it has electrical insulating properties. It has received a lot of attention due to its high thermal conductivity and high thermal stability.^13–15 Therefore bridging the graphite layer with BN not only reduces the interfacial thermal resistance and increases the thermal conductivity of the whole plane. And it can also reduce the electrical conductivity. However, in situ growth of BN between expanded graphite layers has rarely been reported. Wei et al.¹⁶grew hexagonal boron nitride/graphene in-plane heterostructures by direct epitaxy with chemical vapor deposition on Ni(111) surfaces. Li et al. prepared exfoliated graphene-boron nitride nanotube (EG-BNNT) layered structures by constructing graphene-supported BN nanotubes¹⁷. The growth of high-quality boron nitride nanotubes is also crucial. The main methods are arc discharge, laser heating, ball mill annealing, carbon nanotube substitution and plasma methods.^18–20 These methods suffer from poor quality and impurities of the obtained products. About the development history of carbon nanotubes, the CVD method is the most likely method to achieve the batch and controlled preparation of BNNTs because of its simple equipment and easy control.^21,22

In this work, EG and BNNT heterostructures (EBNT) were prepared by in situ covalent growth of BNNTs between EG layers using high-temperature CVD. C-N covalent bonds were formed at the interface between EG and BNNT to construct vertical heat transfer paths between graphite layers. The C-N bonded interface supports phonon transfer to overcome the interfacial thermal resistance between graphite layers compared to weak van der Waals (vdW) interactions. Based on this structure, the prepared EBNT was cold-pressed into a 25.4 mm diameter and 0.5 mm thickness sheet at a pressure of 5 MPa, filled with epoxy resin under vacuum, and cured at high temperature. The orientation of the graphite flakes and the transverse vdW bonding at the micron scale were achieved by compression-induced construction of large aligned graphite flakes. The BNNTs between the layers acted as “bridges” to complete the vertical thermal conductivity pathway. In addition, the EBNT/EP composites were electrically insulating without compromising the electromagnetic shielding properties. This approach will provide a new idea to improve the thermal conductivity between graphite layers. Thermal management materials with electrical insulating properties particularly show strong potential applications in various fields.

Experimental Section

Materials

Expandable Graphite(75μm), boron oxide (B₂O₃) and N, N-Dimethylformamide (DMF) were supplied by Shanghai Aladdin Biochemical Technology Co., Ltd. Magnesium diboride (MgB₂) was provided by Shanghai Macklin Biochemical Co., Ltd. The epoxy resin is a POSS-modified diglycidyl ether of bisphenol A (DGEBA, epoxy equivalent weight of 180–200g/eq.), supplied by Shanghai Yueyi Chemical Co., Ltd. Ecotion-tech provided the epoxy-POSS (POSS-glycidyl ether oxypropyl with Mw = 1337.88 g/mol). The curing agent(Methyltetrahydrophthalic anhydride, MeTHPA) was provided by Guangzhou Gongyi Huagong Co., Ltd.

Preparation of catalyst-loaded expanded graphite

Expanded graphite was prepared by heating expandable graphite at 900°C for 1 min in the resistance furnace (SX2-4-13NP) with a 50-fold expansion factor. 20mg of expanded graphite and 50mg of catalyst magnesium boride were dipped into 20mL of DMF solution. Then, it was stirred and impregnated for 30 min under vacuum at −0.8bar. The catalyst-treated EG/MgB₂ was obtained after drying at 80°C for 12 h.

Preparation of EBNT

EBNT was prepared by intercalation BNNTs into EG layer within a CVD system as shown in Figure 1 below, according to reactions 1–3²³ and the X-ray Powder Diffraction (XRD) patterns of the products obtained after the reaction can be found in Supplement Figure S1. The catalyst-treated EG in the quartz boat was placed in the middle of a horizontal quartz tube, which was placed in a tube furnace (TF1700-60). The boron source (B₂O₃) was placed 5 cm upstream and heated to 1300°C at a rate of 5°C min⁻¹ under argon gas. EBNT growth was carried out using ammonia at a flow rate of 100 sccm for 30 min. Then the samples were cooled to room temperature under argon.

{MgB}_{2} {+ NH}_{3} \to {Mg}_{3} N_{2}

{Mg}_{3} N_{2} {+ B}_{2} O_{3} \to MgO + BN

{MgO + B}_{2} O_{3} \to {Mg}_{3} B_{2} O_{6}

Preparation of EBNT/EP

The EBNT was compacted in a stainless steel mold to obtain the EBNT framework. Further, the curing agent was added to DGEBA in the proportion of 90% epoxy value. The obtained mixture was removed to a thermostat water bath and stirred with a stirring paddle at 60°C for 1 h. The porous EBNT block was placed in the epoxy resin and vacuum (−0.1 bar) impregnated for 2 h. Finally, it has cured at 90°C for 3 h, 110°C for 2 h, and post-cured at 130°C for another 4 h. For comparison, the EG/EP composites were prepared by the same method. The loading of fillers was the same as 50 wt%. The TGA curves were presented in Supplement Figure S2.

Characterization

The morphologies of the fillers and composites were characterized via Field emission scanning electron microscope (SEM, SU8020) with 10 kV accelerating voltage and transmission electron microscope (TEM, Tecnai G2 F20). Before analysis, cryogenic fracture surfaces were covered with a thin gold layer.

Raman spectra of samples were measured using Micro Confocal Raman Spectroscopy (Renishaw inVia Reflex) with an excitation wavelength of 532 nm.

Philips X^′ Pert Pro MPD X-ray diffractometer (40 kV, 40 mA) with Cu-Ka radiation (λ = 0.154 nm) was used to take the X-ray diffraction (XRD) measurements.

The TGA curves were obtained using the thermal gravimetric analysis (Q5000 IR) heated from 20 to 1000°C with a heating rate of 10 °C/min under air conditions.

Elemental scanning was achieved by X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermo-VG Scientific).

A four-point probes electrical resistivity measurement system (RTS-9) was used to measure the samples with a volume of electrical conductivity beyond 1×10⁻⁶ S/cm. Samples with lower conductivity (≤1×10⁻⁶ S/cm) were tested on an insulation resistance tester (model LK2679A). The electrical conductivity value was calculated by the equation Supplement equation S(1).

The EMI shielding performance was investigated through vector network analyzer (Anritsu MS46322B) equipped with two waveguide-to-coaxial adaptors connected face to face. EMI SE was obtained from the scattering parameters (S11 and S21) which were measured in the frequency range of 8.2–12.4 GHz (X band) at room temperature. The values of EMI SE_T, SE_R, and SE_A were calculated according to Supplement equation S(2)−S(7).

The in-plane and through-plane thermal conductivity were obtained by Netzsch laser flash apparatus (LFA467) at room temperature according to ASTM E 1461-01. The measurement for each sample was repeated three times.

A thermal imager was used to study the thermal dissipation performance of the composite. The sample was heated using a hot plate and then cooled in air. The thermographic images of the sample were captured using an infrared camera. Temperature changes on the sample surface during the operation time were analyzed based on the thermographic image.

Results and discussion

Microstructure Characterization of EBNT

Compared with the expandable graphite (Figure 2(a)), the expanded graphite exhibited a worm-like appearance, and the surface was formed with many interconnected thin graphite flakes (Figure 2(b)). This was because after the heating treatment, the expandable graphite absorbed enough heat so that the interlayer intercalation can react with the graphite carbon. The gas generated by the reaction instantly generates a large pressure to push the graphite to expand along the Z-axis direction, which made the tight lamellae structure peel off from each other. It can be seen from Figure 2(c) and Figure 2(d) that a lot of BNNTs growed uniformly between the EG layers. While EBNT retained the complete caterpillar structure. TEM images further indicate that the BNNTs were in a tubular structure, tightly anchored to the monolayer graphite, which indicated a strong interaction. HRTEM image further confirms that the interlayer spacing is about 3.4 Å, which is consistent with the interfacial spacing of the h-BN lattice stripe (Figure 2(f)).²⁴ Figure 2(g)(h) shows the cross-sectional SEM images of the EBNT/EP composites, which exhibited interconnected networks. It was noteworthy that the epoxy matrix almost completely penetrated the internal voids of EBNT without any gaps. This may reduce the resistance to heat transfer in the EBNT/EP interface. The cross-section can be clearly seen to have lamellar and linear structures. The BNNTs showed an extracted shape, indicating that BNNTs possess certain mechanical properties. It was foreseen that the EBNT/EP composites were likely to exhibit excellent thermal and EMI performance due to the robust 3D-EBNT architecture with high quality.

The growth process of nanotubes can be seen in Figure 3 below. Firstly, the catalyst MgB₂ melted at high temperature. Then the boron source delivered by gas was dissolved in the catalyst liquid (Figure 3(b)). When the solubility reached saturation, the boron nitride started to precipitate (Figure 3(c)). With continuous lattice insertion (Supplement Figure S3(a)), lush BNNTs were gradually formed between the EG layers (Figure 4(d)).It was consistent with the typical solid-liquid-gas growth mechanism²⁵.

The crystal structures and chemical bonding in EBNT, EG and BNNS were determined as shown in Figure 4. Comparing the XRD patterns (Figure 4(a)), a significant peak of EBNT can be seen at 26.6º, matching the crystallographic plane of (002), which confirms the graphitic carbon phase (JCPDS no. 26–1080). Diffraction peaks at 41.5º, 54.8º, and 75.7º can be attributed to the (100), (004) and (110) crystallographic planes of BN (JCPDS no. 85–1068). These signals presented in EBNT indicates the formation of BNNT. The Raman spectra of the original EG and EBNT were recorded separately, as shown in Figure 4(b). The Raman spectrum of EG shows D-peak (1348 cm⁻¹) and G-peak (1581 cm⁻¹). The D peak involves a double resonance Raman process of defect scattering. The G peak is attributed to the in-plane vibration of the sp² carbon atom. The intensity ratio (I_D/I_G) of the D peak to the G peak is indicative of the degree of graphitization.²⁶ The I_D/I_G of EG is 0.32, indicating a high degree of graphitization. The characteristic peaks of EBNT can be detected at 1356 cm⁻¹ (D-peak) and 1572 cm⁻¹ (G-peak). These positions are slightly shifted compared to EG. In addition, the I_D/I_G of EBNT is significantly increased compared to EG, reflecting the presence of structural defects in the graphite. The shift of EBNT characteristic peak positions and the increase of I_D/I_G are attributed to the growth of BNNTs on the EG surface. The chemical composition of EBNT was explored by XPS as shown in Figure 4(c)-(e). The deconvolution of the N 1s and B 1s peaks showed two peaks at 398.2º (N-B) and 190.4º (B-N), as shown in Figure 4(c), (d). It indicates that BNNT is implanted on EG²⁷. Moreover, the N 1s and C 1s peaks can be deconvoluted into two components: two peaks at 397º (N-C) and 285.4º (C-N), as shown in Figure 4(c)(e). The covalent C-N bonding at the interface of EG and BNNT is demonstrated.

Electrical Conductive and EMI Shielding Performance of EBNT/EP

Considering that the electrical insulating properties of the filler may affect the ability of the nanocomposite to be used in electronic devices, the electrical conductivity of the EBNT/EP composite was measured, as shown in Figure 5(a). The conductivity of EG/EP is 3.84 × 10²S/cm. This is due to the continuous EG architecture that can serve as a high flux network for efficient electron transport. In contrast, the conductivity of the EBNT/EP composite is 1.51×10⁻¹³S/cm, which is clearly shown to be highly electrically insulating.^28,29 This indicates that despite the electrical conductivity of EG, the presence of insulating BNNTs prevents the formation of electron transport paths. The excellent electrical insulation properties of the composites contribute to the proper functioning of the composites as electronic packaging materials.³⁰ The dielectric properties are also key parameters for various applications of composite materials. Supplement Figure S5 shows the EG/EP, EBNT/EP dielectric properties at 8–12 GHz. The introduction of EBNT increases the dielectric constant of EBNT/EP composites compared to pure EP. And the dielectric loss is kept at a relatively low level with an average value of 0.05. Figure 5(b) summarizes the EMI SE_T curves of EP, EG/EP, and EBNT/EP composites at X-band.

The total SE (SE_T) can be defined as the ratio (logarithmic ratio) between the power of the transmitted electromagnetic wave and the power of the incident wave.

S E_{T} (d B) = 10 l o g_{10} \frac{P_{T}}{P_{I}} = 20 l o g_{10} \frac{E_{T}}{E_{I}} = 20 l o g_{10} \frac{H_{T}}{H_{I}}

(1)

where

P_{I}

(

E_{I}

H_{I}

) and

P_{T}

(

E_{T}

H_{T}

) are the power (electric or magnetic field strength) of the incident and transmitted electromagnetic waves (EMW), respectively.

SE_T can be expressed as SE due to reflection (SE_R), SE due to absorption (SE_A), and SE due to multiple reflections (SE_M)

{SE}_{T} = {SE}_{R} + {SE}_{A} + {SE}_{M}

(2)

Due to absorption and reflection, the attenuation of EMW by SE_M is not as high as the ones. Its value is usually negligible.

Using EM theory, SE_R and SE_A can be written as

{SE}_{R} (dB) \approx 10 log (\frac{σ_{ac}}{16 ω ε_{0} μ_{r}})

(3)

{SE}_{A} (dB) = 20 (\frac{t}{δ}) {log}_{e} = 8.68 (\frac{t}{δ}) = 8.68 t \sqrt{\frac{μ_{r} ω σ_{ac}}{2}}

(4)

δ = 8.68 (\frac{t}{{SE}_{A}})

(5)

Where

σ_{ac}

is the electrical conductivity, which depends on the dielectric behavior of the material

(σ_{ac} = ω ε_{0} ε^{''})

ω

is the angular frequency

(ω = 2 π f)

ε_{0}

is the free-space permittivity, and

μ_{r}

is the relative magnetic permeability of the sample.

Equations (3) and (4) show that both SE_R and SE_A depend directly on the value of $σ_{ac}$ . At constant $ω$ and $μ_{r}$ , SE_R and SE_A are proportional to $σ_{ac}$ and $\sqrt{σ_{ac}}$ , respectively. The equations also show that, for a given material, SE_R decreases and SE_A increases with increasing frequency.

Electromagnetic interference (EMI) shielding of RF radiation is likewise a serious problem to be faced by electronic devices. The commonly used fillers are metals (e.g., silver nanoparticles and copper nanowires^4,5). However, metal fillers are expensive and heavy in weight. Carbon materials can possess a thermal conductivity comparable to that of metallic fillers, lightweight and good electrical conductivity.^6,7 Therefore, they are widely used as thermally conductive electromagnetic shielding fillers.

This will have a promising application in thermal management materials for anti-electromagnetic interference.

The total shielding effectiveness SE_T of the pure epoxy ≈ 3 dB. With the conductive filler incorporated into the epoxy matrix, a perfect 3D carbon-based filler network is formed and EG/EP exhibits 33.07 dB EMI shielding capability. This means that the shielding efficiency η = 99.95% and only 0.05% of the incident radiation are transmitted.^31,32 Interestingly, the 3D carbon-based network with BNNTs grown between the layers still retains the original EMI shielding capability (33.34 dB). Even if the graphite packing does not form a continuous electrical conductive path, the incident EM waves can be coupled to the electrons in the individual graphite packing.³³ Figure 5(c) shows the average values of SE_T, SE_A and SE_R of the composites at 8–12 GHz to reveal the EMI shielding mechanism. For EG/EP, EBNT/EP composites, the SE_A values are higher than the SE_R values, indicating that the electromagnetic waves (EMW) absorption mechanism plays a major role in the EMI shielding process. When the incident EMW encounters the composite surface, most of the EMW is transmitted through the EG/EP, EBNT/EP composites, except for a small portion of the reflected EMW due to impedance mismatch. The incident EMW is then captured and further attenuated by absorption and multiple reflections in the continuous 3D EG architecture, and dissipated as heat. This will have a promising application in thermal management materials for anti-electromagnetic interference.

Thermal Properties of EBNT/EP

The thermogravimetric analysis (TGA) of EBNT shows a steady improvement compared to the thermal stability of EG, with the initial thermal decomposition temperature (decomposition temperature at 5% loss by weight) increasing from 574°C to 775°C (Figure 6). After 574°C, expanded graphite began to oxidize and decompose in large quantities to form carbon oxides. The thermal stability of BN was relatively good. The presence of BNNTs made the content of expanded graphite in EBNTs decrease, which led to the increase in thermal stability of EBNT. And the three-dimensional network structure of EBNT provided a tortuous path for the diffusion of gas molecules. The network significantly reduced the gas permeation rate. BNNTs act as a physical barrier in the matrix, delaying the escape of degradation products. It eventually drove the thermal decomposition temperature toward higher temperatures³⁴. The TGA curves on EG/EP and EBNT/EP composites are shown in Supplement Figure S2.

The in-plane and through-plane thermal diffusivities $(α)$ of EG/EP, EBNT/EP and EP were measured by the laser flash method as shown in Figure 7(a), (b). The through-plane thermal diffusivity of EBNT/EP (3.271 mm²s⁻¹) is 60% higher than that of EG/EP (2.067 mm²s⁻¹). The corresponding thermal conductivity (κ) can be calculated by the following equation³⁵: κ = α × ρ × C_p, where ρ is the sample density and C_p is the specific heat capacity. The specific values are in Table S2. The through-plane thermal conductivity of EBNT/EP (5.18 Wm⁻¹K⁻¹) is nearly two times higher than that of EG/EP (2.77 Wm⁻¹K⁻¹), which is 25 times higher than that of pure EP. The more efficient heat transfer capability of EBNT along the vertical direction is due to the inherent high thermal conductivity of BNNTs and the covalent C-N bonding grown at the EG/BNNTs interface.^36,37 In contrast, the in-plane thermal diffusivity of EBNT/EP (17.46 mm²s⁻¹) is 20% lower than EG/EP (21.82 mm²s⁻¹), because BNNTs growed between the EG layers. The in-plane thermal conductivity of EBNT/EP (28.11 Wm⁻¹K⁻¹) is 4% lower than EG/EP (29.23 Wm⁻¹K⁻¹), which was 140 times higher than that of pure EP. This reduction is attributed to the formation of graphite/BNNT heterojunction. It acts as a scattering center on the graphite surface and hinders the phonon transport along the graphene substrate.³⁸ To realistically evaluate the thermal conductivity, EP, EG/EP and EBNT/EP were placed on a heated plate at room temperature. The infrared thermal images and the corresponding surface temperature versus time plots were shown in Figures 7(c) and (d). It can be seen that EBNT/EP shows the highest temperature after 60 seconds of heating. To verify the usability of the EBNT/EP composite in the LED lamp, the composite was used underneath the LED lamp. A 3W LED lamp was selected and a conductive circuit with a switch was constructed as shown in Figure 7(f). This conductive path was used to verify the heat dissipation of the LED lamp. The data in Figure 7(e) were detected by the infrared camera in Figure 7(f). The real-time temperature of the LEDs from the start of an operation to 5 minutes of operation of the LEDs was observed. The temperature of the pure EP was constant at 32°C, while the temperature of the EBNT/EP composite was constant at 26°C. It was shown that the EBNT/EP composite can conduct the heat generated by the LED lamp in time and thus can avoid overheating of the LED. Therefore, the composite can play a role in the heat dissipation of LED electronics. The results showed that EBNT/EP has the best thermal management capability, which was attributed to the construction of effective heat transfer paths. Li et al.¹⁷ performed non-equilibrium molecular dynamics (NEMD) simulations and verified that the interfacial thermal resistance between graphene and BNNT connected by covalent bonds is much smaller than that of vdW interactions when the system reaches a steady state. A low coefficient of thermal expansion is another important indicator of a high-performance thermal management material. We measured the coefficient of thermal expansion of EG/EP and EBNT/EP, as shown in Supplement Figure S6. Table 1 shows the progress of epoxy-based nanocomposites in terms of thermal conductivity in recent years. Composites with direct blending of fillers have high interfacial thermal resistance and cannot achieve high loadings.³⁹ Horizontally-oriented fillers result in low through-plane thermal conductivity.^40–42 The well-conducted thermal pathway is established by the three-dimensional structure of the filler. However, the interfacial thermal resistance between fillers assembled by vdW force is still relatively large.^43,44 Our work establishes the three-dimensional structure. The lattice between EG and BNNTs is relatively matched and covalent bonds are formed. Therefore, the interfacial thermal resistance between the fillers is relatively small. In comparison with the work in recent years, our work has achieved the highest through-plane thermal conductivity while ensuring high in-plane thermal conductivity.

Table 1.

Comparison of thermal conductivity of epoxy-based nanocomposites with different filler loading

Fillers	Thermal conductivity (W/(m·K))	Loading	Year
MWCNTs³⁹	0.4	$\approx$ 6wt%	2022
GNP⁴⁰	3.25	40wt%	2020
BNNS/CNF-AlN/B	8.53(in-plane)	30wt%	2019
N⁴¹	0.87(through-plane)
BN⁴²	2.52	30wt%	2020
BN⁴³	6.07(in-plane) $\approx$ 0.6(through-plane)	$\approx$ 24.15wt%	2020
rGO/BN⁴⁴	5.05(through-plane)	13.16vol%	2018
EBNT	28.11(in-plane)5.18(through-plane)	50wt%	This work

Conclusion

In conclusion, BNNTs have been successfully intercalated between expanded graphite layers using high-temperature chemical vapor deposition to prepare caterpillar-like EBNT hybrid fillers. After compression, the transverse lap of large-sized expanded graphite flakes achieved the bonding of transverse vdW forces. While, the BNNTs, acting as a “bridge”, refined the thermal conductivity pathway in the vertical direction. The covalent bonds formed between the graphite flake and BNNTs significantly reduced the interfacial thermal resistance. As a result, the corresponding EBNT/EP nanocomposites obtained an outstanding in-plane thermal conductivity of 28.11 Wm⁻¹K⁻¹ and a high through-plane thermal conductivity of 5.18 Wm⁻¹K⁻¹, which are 140 and 25 times higher than that of pure EP, respectively. In addition, the nanocomposites exhibited both good electrical insulation (1.51×10⁻¹³S/cm) and electromagnetic shielding properties (33.34 dB) due to this well-designed architecture. This study provides new ideas and methods for the design of next-generation polymer-based electronic packaging materials.

Supplemental Material

Supplemental Material - Significantly enhanced thermally conductive epoxy composite composed of caterpillar-like structured expanded graphite/ boron nitride nanotubes

Supplemental Material for Significantly enhanced thermally conductive epoxy composite composed of caterpillar-like structured expanded graphite/ boron nitride nanotubes by Menghan Zhu, Chao Xiao, Qiqi Qu, Yunsheng Da, Yanyan Liu, Xingyou Tian and Hua Wang in High Performance Polymers

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Menghan Zhu

Qiqi Qu

Yunsheng Da

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