Highly thermally conductive carbon nanotubes pillared exfoliated graphite/polyimide composites

In this work, carbon nanotubes pillared grew on exfoliated graphite by the microwave-assisted method is utilized as thermally conductive fillers (CPEG) in polyimide (PI) to fabricate CPEG/PI thermally conductive composites with the combining ways of “in-situ polymerization, electrospinning, lay-up, and hot-pressing”. The prepared CPEG/PI composites realized the maximum thermal conductivity (λ, 1.92 W m−1 K−1) at low CPEG amount (10 wt%), much higher than that of pure PI (0.28 W m−1 K−1). The λ of CPEG/PI composites show almost no change after 1000 cycles of heating and cooling at the temperature of 25−100 °C. The finite element analysis simulates the nano-/microscale heat transfer in CPEG/PI composites to reveal the internal reason of the λ enhancement. The improved thermal conductivity model and empirical equation could better reflect the actual λ change trend of CPEG/PI composites. The actual application test shows the CPEG/PI composites could significantly reduce the operating temperature of the CPU in mobile phone.


INTRODUCTION
The miniaturization, lightness, and intelligence of consumer electronics, the large-scale construction of communication base station brought about by 5G commercialization, and the rapid development of batteries in new energy vehicle (NEV) and other fields have all put forward higher requirements for highly thermally conductive materials [1][2][3] . Polymer-based composites are used ubiquitously in the above areas ranging from substrate, frame to interlayer medium, and sealing material, due to their lightweight, easy processing, and excellent chemical stability 4,5 . However, the intrinsic low thermal conductivity (λ) of polymers cannot meet the increasing heat conduction and dissipation demand in the fields of high-end electronic, 5G, and NEV, etc. 6,7 .
At present, there are two main ways to improve the λ of polymer materials. One is synthesizing new-type polymers with high intrinsic λ (such as liquid crystal epoxy and polyimide [8][9][10] , triphenylene-based discotic liquid crystal polymers 11 , conjugated poly(3-hexylthiophene) 12 , etc.), the other is adding highly thermally conductive fillers (such as graphene 13 , carbon nanotubes (CNT) 14 , boron nitride nanosheets (BNNS) 15 , etc.) into the polymer matrix. The latter is simple, efficient, and suitable for industrial production 16,17 . However, while ensuring the required thermal conductivity, the amount of thermally conductive fillers should be reduced as much as possible to maintain the inherent advantages of polymer-based composites such as lightweight, easy processing, and excellent mechanical properties 5 .
One-dimensional (1D) CNT and two-dimensional (2D) graphite derivatives are favored by researchers in the field of thermally conductive fillers due to their ultra-high λ [18][19][20] . By now, a number of researchers directly adding CNT and graphite derivatives into the polymer to improve the λ of the polymer-based composites 21,22 . Wang et al. 23 blended graphene nanoplatelets (GNP), CNT and polyvinylidene fluoride (PVDF) to prepare CNT/GNP/PVDF thermally conductive composites. When the amount of CNT/GNP was 22 wt% (CNT:GNP = 1:10, wt:wt), the λ of the CNT/GNP/PVDF thermally conductive composites reached 1.92 W m −1 K −1 , while the λ of GNP/PVDF and CNT/PVDF thermally conductive composites prepared with the same amount of fillers were 1.64 and 0.56 W m −1 K −1 , respectively. Kim et al. 24 prepared multiwalled CNT/GNP/polycarbonate (MWCNT/GNP/PC) thermally conductive composites by the blending method, and confirmed that the MWCNT/GNP hybrid fillers had better thermal conductivity improvement effect than MWCNT or GNP single fillers. When the amount of fillers was 20 wt%, the λ of MWCNT/GNP/PC, GNP/ PC, and MWCNT/PC thermally conductive composites were 1.39, 1.13, and 0.74 W m −1 K −1 , respectively, mainly attributed to the synergistic effect of the line-surface structure of CNT/graphite derivative hybrid fillers 25 . The 1D CNT can be used as the basic unit to bridge adjacent 2D graphite derivative sheets, constructing more effective thermal conduction pathways 26 . The introduction of 2D graphite derivatives can effectively promote the uniform dispersion of CNT in the polymer matrix, benefiting the formation of thermal conduction pathways with relatively low fillers amount 27 . However, it is also found that the simple blending of CNT and graphite derivatives has limited enhancement in the λ of polymer-based composites. On the one hand, the bridging effect is limited, and it is easily destroyed during the blending process. On the other hand, the agglomeration problem of graphite derivatives and CNT cannot be solved effectively 28 .
In order to solve the above problems better, researchers have tried to connect CNT and graphite derivatives by chemical or physical interaction. Li et al. 29 built a kind of thermally conductive fillers with hierarchical structure by electrostatic self-assembly between the positively charged (3-aminopropyl)trimethoxysilane (APTMS) modified CNT (A-CNT) and negatively charged graphene oxide (GO), then fabricated GO-A-CNT/PVDF thermally conductive composites by solution compounding method. When the amount of GO-A-CNT was 10 wt% and the mass ratio of A-CNT to GO was 4:1, the λ of GO-A-CNT/PVDF composites reached 1.56 W m −1 K −1 , the λ enhancement of 610% was achieved compared with that of pure PVDF. However, the λ of 10 wt% CNT/PVDF composites was only 0.43 W m −1 K −1 . Zhao et al. 30 prepared a kind of hybrid fillers by utilizing the reaction between dodecylamine-modified graphene nanoplatelets (DA-GNP) and γ-aminopropyl-triethoxysilane treated MWCNT (f-MWCNT), and then fabricated DA-GNS/f-MWCNT/cyanate ester resin (CE) thermally conductive composites. The results showed that chemical modification significantly improved the dispersion performance of f-MWCNT/DA-GNP, and led to an effective decrease in the interfacial thermal resistance. When the amount of DA-GNS/f-MWCNT was 5 wt%, the λ of (DA-GNS/f-MWCNT)/CE composites reached 0.86 W m −1 K −1 , 3.2 times higher than the pure CE matrix (0.26 W m −1 K −1 ). The λ of f-MWCNT/CE and DA-GNS/CE thermally conductive composites were only 0.65 and 0.40 W m −1 K −1 , respectively. In our previous research works, Gu et al. 31 grafted aminated MWCNT to the surface of GO and reduced it to obtain f-MWCNT-g-rGO thermally conductive fillers, and then fabricated f-MWCNT-g-rGO/polyimide (PI) thermally conductive composites via "in-situ polymerization, electrospinning, lay-up and hot-pressing" technique. When the amount of f-MWCNT-g-rGO was 10 wt% and the mass ratio of f-MWCNT to rGO was 1:2, the λ of f-MWCNT-g-rGO/PI thermally conductive composites reached 1.60 W m −1 K −1 , much higher than the λ (1.21 W m −1 K −1 ) of the rGO/PI composites at the same amount of rGO.
It can be seen that the method of modifying and chemical bonding CNT and graphite derivatives can effectively solve the agglomeration problems of CNT and graphite derivatives, which is to the benefit of improving the λ of polymer-based composites compared with directly mixing thermally conductive fillers and polymer matrix. However, most of these methods require modification of CNT or graphite derivatives which will reduce their intrinsic λ to a certain extent 31 . In addition, the CNT mostly stack on the graphite derivative sheets in a disordered manner (the CNT axis is parallel to the graphite derivative plane) in the CNT/graphite derivative hybrid fillers prepared by directly blending or chemical bonding 32 . However, phonons in CNT and graphite derivatives easily transfer in the axial or in-plane direction, and scatter severely in the radial or through-plane direction due to the sp 2 hybrid structure of carbon atoms 33 . Therefore, the structure that CNT randomly stack on the graphite derivative sheets makes the CNT/graphite derivative hybrid fillers have low through-plane λ (λ ⊥ ).
An effective method to solve the above problems is making CNT pillared grow on the surface of the graphite derivative to construct three-dimensional (3D) structured thermally conductive fillers. The structure of CNT pillared standing on the surface of the graphite derivative can effectively reduce the thermal resistance at the junction between CNT and graphite derivative, avoiding heat scattering caused by the overlap among the CNT, giving full play to the ultra-high axis λ of CNT and in-plane λ of graphite derivative 34 . All the above advantages will realize a marked increase in the λ of polymer-based thermally conductive composites under the condition of low amount of thermally conductive fillers. Liu et al. 34 used the chemical vapor deposition (CVD) method, realizing CNT vertically grew on the surface of graphene. The calculation results showed the interfacial thermal resistance between CNT and graphene decreased from 7.5 × 10 −7 m 2 K −1 W −1 under natural contact conditions to 9 × 10 −10 m 2 K −1 W −1 . Feng et al. 35 also utilized the CVD method to make CNT grew on the surface of expanded graphite, and the λ ⊥ (38 W m −1 K −1 ) was 170% higher than that of pure expanded graphite. However, most of the 3D-structured CNT/graphite derivatives that have been reported so far were prepared based on the CVD method, which has disadvantages such as low efficiency, high cost, and high demands of reaction conditions 36,37 .
The microwave-assisted method does not require strict experimental conditions, and can greatly shorten the reaction time (only tens of seconds) to prepare 3D-structured CNT/graphite derivatives fillers 38 . At present, a number of carbon nanomaterials have been synthesized by the microwave-assisted technique 38,39 .
However, the CNT in the 3D-structured CNT/graphite derivatives fillers prepared by the microwave-assisted method mostly distributes in the form of random coils, the dispersibility and λ ⊥ are not ideal. It is still a challenging and valuable work to prepare 3D-structured CNT/graphite derivative thermally conductive fillers with CNT vertically grown on the graphite derivative sheets by the microwave-assisted method. In this paper, the 3D-structured thermally conductive fillers with CNT pillared grown on the surface of exfoliated graphite (CPEG) were prepared simply and efficiently by the microwave-assisted method. The effects of microwave reaction time, gas atmosphere, and raw material ratio on the morphological characteristics, orientation degree, crystal structure, etc. of CPEG were studied in detail. Scanning electron microscope (SEM), transmission electron microscope (TEM), Raman spectroscopy (Raman), and X-ray diffraction (XRD) were used to characterize the morphologies and structures of CPEG thermally conductive fillers, and the microwave-assisted method was optimized according to the orientation of CNT in CPEG. The optimized CPEG was used to fabricate CPEG/PI thermally conductive composites by the ways of "in-situ polymerization, electrospinning, lay-up, and hot-pressing". The effect of the amount of CPEG on the λ, glass transition temperature (T g ), heat resistance temperature (T HRI ), and other properties of CPEG/PI thermally composites were studied in detail. Finite element analysis was used to simulate the nano-/microscale heat transfer in CPEG/PI thermally conductive composites, analyzing the essential reasons that CPEG significantly improves the λ of CPEG/PI thermally conductive composites from the microscopic perspective. Based on the effective medium theory and the first law of thermodynamics, a more widely applicable thermal conductivity model and equation was established. On this basis, the λ stability of the CPEG/PI thermally conductive composites was verified by repeatedly testing the λ in the heating-cooling process, and the actual application test of the CPU in mobile phone was used to analyze the heat conduction and dissipation capability of CPEG/PI thermally conductive composites.

RESULTS AND DISCUSSION Preparation of CPEG and CPEG/PI thermally conductive composites
The preparation schematic diagram of CPEG and CPEG/PI thermally conductive composites are shown in Fig. 1. EG is prepared by liquid-phase exfoliation of pre-expanded graphite, and its lateral size (~5.5 μm) and thickness (~0.2 μm) are smaller than that of graphite (lateral size of~8.5 μm, thickness of 0.7 μm). Besides, the surface of EG is smooth and there are no impurities ( Supplementary Fig. 2), indicating that the method used in this paper can effectively exfoliate and purify graphite. After microwave treatment, the appearance of EG is more fluffy and the color is darkened (Supplementary Fig. 1a). Both PAA and CPEG/ PAA fibers (Supplementary Fig. 1b, c) prepared by electrospinning have a high degree of orientation. The apparent color of CPEG/PI composites deepens with the increase of the amount of CPEG ( Supplementary Fig. 1d).

Characterization of CPEG
In order to efficiently and stably prepare 3D-structured thermally conductive fillers with CNT pillared grown on the surface of EG (CPEG), this work optimized the gas atmosphere, reaction time, and the mass ratio of FC to EG during the microwave reaction process. In the air atmosphere, almost no CNT grows on the surface of EG ( Supplementary Fig. 3). Considering the hydrogen (H 2 ) can regulate the growth of CNT, but it is flammable and dangerous, this work uses hydrogen/argon (H 2 /Ar) mixture gas (volume fraction of H 2 is 5%). In general, CNT can be grown under the given reaction time and the mass ratio of FC and EG ( Supplementary Fig. 4). The length of CNT increases with the increase of FC proportion and reaction time. When the mass ratio of FC to EG is greater than 0.25 and the reaction time is 90 s, the CNT distributes on the surface of EG like random coils. When the mass ratio of FC to EG is 0.5 and the reaction time is 60 s, CNT all pillared stands on the surface of EG and has obvious alignment structures. The length and diameter of CNT are approximately 600 and 50 nm, respectively. Therefore, the optimal growth conditions for CPEG determined in this paper are H 2 /Ar mixture gas, the reaction time of 60 s, and the mass ratio of FC to EG of 0.5. The CPEG mentioned later is prepared under this condition. Figure 2 shows the structure and morphology characterization of CPEG. As shown in the SEM images of Fig. 2a, CNT uniformly pillared stands on the surface of EG, and there is a white particle at its top. The roots of CNT embed on the surface of EG, and both sides of the EG sheet have CNT growth, and some CNT contains particles in the middle part (Fig. 2b). Figure 2c clearly shows the CNT has a multi-walled structure, and the CNT completely coats  the particle. High magnification TEM (Fig. 2d) shows that the wall spacing of CNT is 0.34 nm, which corresponds to the (002) lattice plane of the graphene carbon phase 40 . The EDS elemental mapping images of CPEG (Fig. 2e, f) shows the homogeneous distribution of C element throughout the nanotube and the particle contains the iron element. There are three main characteristic peaks in the Raman spectrum of CPEG (Fig. 2g). The D peak (~1350 cm −1 ) is a disordered vibration peak, indicating defects in the carbon atom lattice. The G peak (~1580 cm −1 ) is caused by the in-plane vibration of sp 2 carbon atoms, reflecting the degree the close-packed hexagonal graphite crystal structure of carbon atoms. The G′ peak (~2700 cm −1 ) is two-phonon resonance second-order Raman peak, which is used to characterize the stacking of carbon atoms in the sample 41 . Generally, the intensity ratio of D peak to G peak (I D /I G ) is used to indicate the degree of disorder. The value I D /I G (0.62) of CPEG is significantly higher than that of EG (0.12), mainly attributed to that the CNT on the surface of EG, disturbs the in-plane vibration of the EG carbon atoms. At the same time, defects are inevitably introduced into CNT during its growth process 42 , resulting in increased defects in the overall structure of CPEG. XRD is used to further characterize the crystal structure of CPEG (Fig. 2h). Both EG and CPEG have obvious diffraction peaks at 2θ = 26.38°, corresponding to the (002) lattice plane of graphite. It shows that the crystal structure of graphite has no change during the pre-expansion, ultrasonication exfoliating, and microwave treatment process. The remaining diffraction peaks in the XRD spectrum of CPEG are consistent with the standard diffraction peaks of Fe 3 C, indicating that the particles in CPEG contain Fe 3 C. Therefore, the growth mechanism of CNT in CPEG can be inferred: EG heats up rapidly in the microwave field, and the H 2 promotes the decomposition of FC to produce metallic iron and hydrocarbons. After that, the carbon source dissolves into the iron particles to form Fe 3 C. After saturation of the carbon in the iron particles, solid carbon precipitates outward and grows into CNT 43 . Furthermore, XPS (Fig. 2i) is carried out to determine the element composition and the chemical state of CPEG. The signals of C, O, and Fe are evidenced at~284,~533, and~708 eV. Among the signals, the narrow XPS spectrum of Fe 2p exhibits two peaks at 707.5 and 720.2 eV (inset in Fig. 2i) corresponding to the Fe 2p 3/2 and Fe 2p 1/2 spin-orbit peaks of Fe, confirming the formation of Fe in CPEG 44 . Besides, EG has a high carbon/oxygen atom concentration ratio (~32.2), indicating that the preexpansion and ultrasonication exfoliating process did not cause excessive oxidation of graphite.
Morphologies of CPEG/PI fibers and composites SEM images of CPEG/PI fibers are shown in Fig. 3. The pure PI fibers have a smooth surface and uniform diameter distribution (Figs. 3a, 2.2−2.5 μm). With the increase of the amount of CPEG, the diameter of CPEG/PI fibers decrease firstly and then increases (Fig. 3b-e). The main reason is that the electrical conductivity and viscosity of the CPEG/PAA solution increase with the amount of CPEG increases. (Due to the CPEG/PI fibers are obtained from CPEG/PAA fibers, so the paper only discusses the diameter variation of CPEG/PAA fibers.) The enhancement in the electrical conductivity increases the surface charge density of the CPEG/PAA jet and the electrostatic force received in the electric field, causing more jet splits and reducing the diameter of the prepared CPEG/ PAA fibers. However, the increase in surface tension makes it difficult to split the CPEG/PAA jet, so that the diameter of the CPEG/PAA fibers increases. When the amount of CPEG is 1 wt%, the electrical conductivity of the CPEG/PAA solution increases greatly, reducing the diameter of the CPEG/PAA fibers. With the continuous increase of the amount of CPEG, the viscous stress of the CPEG/PAA solution is dominant. The effect of electrical conductivity weakens, and the diameter of the CPEG/PAA fibers increases. In addition, with the increase of CPEG amount, the number of beads in the CPEG/PI fibers gradually increases. The reason is that CPEG has high specific surface area, and the CNT on its surface has large length/diameter ratio, which makes CPEG easy to agglomerate. As the amount of CPEG increases, its volume fraction increases rapidly, which is not conducive to the uniform dispersion of CPEG in the CPEG/PAA solution, so that beads are formed in the CPEG/PAA fibers 45 . However, the overall dispersion of CPEG in the CPEG/PI fibers is relatively uniform (Fig. 3f), which is attributed to the shear effect of the CPEG/PAA solution passing through the needle during the electrospinning process and the high-speed stretching effect of the electric field on the CPEG/PAA jet. In addition, pure PI fibers and CPEG/PI fibers are oriented in the same direction, mainly ascribed to the self-made collection device of a cylinder with a copper wire frame. On the one hand, the cylinder rotates at high speed, and when its surface linear velocity is consistent with the settling velocity of fibers, a better orientation effect can be achieved. On the other hand, every two parallel wires on the cylinder can be regarded as parallel electrodes to induce fibers orientation 46 . The cross-section morphologies ( Supplementary Fig. 5) of CPEG/PI composites have great changes with the amount of CPEG increases. The pure PI has a relatively flat fracture, and the partial domain is relatively rough, which means the PI is brittle and has a certain degree of toughness. With the addition of CPEG, the roughness and the number of folds in the CPEG/PI composites increase significantly, and there are lots of dimples fracture in the SEM images. It shows that the addition of CPEG helps to improve the toughness of CPEG/PI composites. However, when the amount of CPEG reaches 10 wt%, the number of dimples fracture is significantly reduced, and lamellar shape appears on the cross-section, indicating that the composites have a brittle fracture.
Thermal conductivity of CPEG/PI composites Figure 4a shows the λ of CEG/PI and CPEG/PI thermally conductive composites. It can be seen that the λ of the two kinds of composites enhance with the increase in the amount of thermally conductive fillers (CEG or CPEG), and the λ of the CPEG/PI thermally conductive composites has a faster growth rate. Under the same amount of thermally conductive fillers, the λ of CPEG/PI thermally conductive composites is higher than that of CEG/PI thermally conductive composites. When the amount of CPEG is 10 wt%, the λ of CPEG/PI thermally conductive composites reaches 1.92 W m −1 K −1 , which is about seven times that of pure PI (0.28 W m −1 K −1 ), 1.7 times than that of 10 wt% CEG/PI thermally conductive composites (1.12 W m −1 K −1 ). The possible reasons are summarized as follows: (1) CPEG itself has high λ, and the 3D structure of the CNT pillared grown on EG gives it excellent inplane and through-plane λ (λ ∥ and λ ⊥ ), which is conducive to rapid heat transfer along the 3D direction 47,48 . (2) Electrospinning facilitates the orientation of PI molecular chains, reducing phonon scattering, and increasing the intrinsic λ of the PI matrix. In addition, CPEG distributed on the PI fibers, the orientation and overlap of CPEG/PI fibers are beneficial to increasing the contact among CPEG, promoting the formation of CPEG−CPEG thermal conduction pathways, and increasing the λ of CPEG/PI composites effectively 49,50 . The λ of previous reports on CNT and graphite derivative filled polymer-based thermally conductive composites are summarized in Supplementary Table 1. It can be seen that the CPEG prepared in this paper has distinct strength in improving the λ of polymer-based thermally conductive composites.
In order to deeply analyze the essential reasons that the CPEG/ PI thermally conductive composites have better thermal conductivity than CEG/PI thermally conductive composites, this paper utilizes COMSOL Multiphysics software to simulate the nano-/ microscale heat transfer in composites (Supplementary Note 3), the results are shown in Fig. 5. For CEG/PI composites, the temperature at 0.1 s is slightly higher than the temperature at 0.01 s, and the temperature distribution is nonuniform. The CPEG/ PI thermally conductive composites have no temperature difference in 0.01 and 0.1 s and show the uniform distribution of temperature. In addition, at the same time point, the overall temperature of the CPEG/PI thermally conductive composites is higher than that of the CEG/PI thermally conductive composites. The main reason is that the CNT in the CEG is distributed randomly on the surface of the EG, and only some CNT interact with the EG through van der Waals forces, which results in high contact thermal resistance and does not conducive to thermal conduction (Fig. 5a). The overlap among CNT makes the heat transfer along the radial direction of CNT, which will cause thermal diffusion. Moreover, the low radial λ of CNT and the high thermal resistance in the contact point of CNT generate lots of heat loss, all of which result in non-uniform temperature distribution on the CNT (Fig.  5b, c). In addition, when the heat is transferred from the CNT to EG, the thermal diffusion is extremely nonuniform due to the partial contact between CNT and EG ( Supplementary Fig. 6a), which makes a non-uniform temperature distribution in CEG (Fig.  5c). As for CPEG/PI thermally conductive composites, CNT and EG have high λ in axial and in-plane directions, respectively. The 3D structure of CNT pillared grown on the EG sheet is beneficial to the thermal conduction to the surface of EG along the CNT axis (Fig.  5d), meanwhile, rapid heat diffusion in the EG sheet induces the temperature of CPEG to rise quickly. In addition, there is no interference among the oriented CNT, the CNT and EG are tightly bonded (Supplementary Fig. 6b) so that the heat loss in the CPEG is low. The high distribution density of CNT ensures that the heat can be conducted in time. All the advantages make sure uniform temperature distribution in CPEG (Fig. 5e, f).
In addition, in view of the narrow application range of existing thermal conductivity models and empirical equations in the calculation of the λ of polymer-based thermally conductive composites. A more widely applicable thermal conductivity model and empirical equation for calculating the effective thermal conductivity of polymer-based composites is established based on the effective medium theory (EMT) 51 and the law of conservation of energy (Supplementary Note 4).
The theoretical λ of CPEG/PI thermally conductive composites are calculated by Eq. 1 and classical empirical equations are shown in Fig. 4b. It can be seen that the thermal conductivity model and empirical equation established in this paper have a better fitting degree of theoretical calculation/experimental results than the classical empirical equations, and can better reflect the actual λ change of CPEG/PI thermally conductive composites. Figure 4c shows the λ of the CPEG/PI thermally conductive composites changes with testing temperature. It can be found that the variation trend of λ with temperature is basically similar for CPEG/PI thermally conductive composites with different amount of CPEG. When the temperature increases from 25 to 100°C, the λ of the CPEG/PI thermally conductive composites is basically unchanged. In addition, the λ stability of the composites after repeated heating and cooling cycles is very important to ensure the performance of the product. In this paper, the CPEG/PI thermally conductive composites are heated from 25 to 100°C, and then cooled to 25°C, and the λ is tested at 25°C after every 100 cycles. The test results are shown in Fig. 4d. It can be clearly seen that the λ of the CPEG/PI composites still maintains excellent stability after 1000 cycles in the temperature range of 25−100°C, indicating its excellent environmental temperature tolerance. In addition, this paper tested the glass transition temperature (T g ) and heat resistance temperature (T HRI ) of CPEG/PI thermally conductive composites by DSC and TGA ( Supplementary Fig. 7).
The characteristic values are shown in Supplementary Table 2. It can be seen that the 5% weight loss temperature of CPEG/PI thermally conductive composites is higher than 500°C, and its T g and T HRI both increase with the increase of the CPEG amount. The T g and T HRI of the CPEG/PI thermally conductive composites with 10 wt% CPEG are 214.7 and 293.0°C, respectively, showing excellent heat resistance property. On the one hand, the PI backbone contains a large number of aromatic and imide rings, giving it high T g and thermal decomposition temperature 52,53 . On the other hand, CPEG has high specific surface area, which restricts the movement of the PI molecular chains segments, increasing the T g of the CPEG/PI composites 54 . Besides, the heat resistance of CPEG itself is higher than that of the PI matrix. The introduction of CPEG can improve the heat resistance of CPEG/PI composites. In addition, the formation of perfect 3D thermal conduction networks by CPEG will also facilitate the heat transfer and reduce the negative impact of the degradation of PI matrix due to local overheating caused by heat concentration.
In order to show the thermal conduction of the CPEG/PI thermally conductive composites more intuitively, this work places the CPEG/PI thermally conductive composites on a hot/cold plate with constant temperature and records the surface temperature by infrared thermography. The results are shown in Fig. 6a, b. It can be seen from Fig. 6a that the color of the infrared images of the CPEG/PI thermally conductive composites gradually turns red with the heating time extension, indicating its surface temperature gradually increases. At the same time, the CPEG/PI thermally conductive composites with more CPEG amount have higher surface temperature, indicating that it has better λ. After the samples are heated up for 60 s, the samples were quickly transferred to a 25°C stainless steel plate, and the surface temperature variation during the cooling process was recorded with the same infrared thermography. The relationship between the sample temperature and time is shown in Fig. 6b. It can be seen that the CPEG/PI thermally conductive composites with more CPEG amount not only have a faster heating rate, but also a higher surface temperature. Similarly, in the cooling process, the CPEG/PI thermally conductive composites containing more CPEG amount have a faster cooling rate and a lower surface temperature, indicating that the CPEG/PI thermally conductive composites prepared in this paper have excellent heat conduction and dissipation performance.
In order to further evaluate the heat conduction and dissipation performance of the CPEG/PI thermally conductive composites in practical applications. This work integrates the 10 wt% CPEG/PI thermally conductive composites with the CPU in mobile phone (Fig. 6c, d), and records the CPU temperature by CPU-Z software during the video playing period. After the phone is sufficiently cooled, removing the CPEG/PI thermally conductive composites to disclose the CPU (Fig. 6e, f) and playing the same video continuously for 1 h. The relationship of the CPU temperature with time is shown in Fig. 6g. It can be seen that when the CPU temperature is stabilized, the temperature of the CPU integrated with CPEG/PI thermally conductive composites (~47.5°C) is significantly lower than that of the integrated without CPEG/PI thermally conductive composites (~52.3°C). It shows that the CPEG/PI thermally conductive composites have excellent heat conduction and dissipation capability as well as practical application value.

Preparation of CPEG
Two grams of graphite, 80 mL of H 2 SO 4 , and 10 g of (NH 4 ) 2 S 2 O 8 were added into a flask, stirring evenly at 35°C until no bubble emission. Then, 200 mL of H 2 O was added into the flask drop by drop to ensure the temperature of the mixture under 60°C. The mixture was filtrated and washed several times until the filtrate was neutral, and then the filter cake was collected and dispersed in 100 mL of NMP and ultrasonicated (200 KHz) for 1 h. Finally, the exfoliated graphite (EG) was obtained after filtrating the NMP mixture followed by washing with deionized water and drying. After that, a certain amount of FC and EG were mixed uniformly and sealed in the quartz tube, put tube into the microwave oven (900 W, 2450 MHz), and CPEG with different morphologies were obtained by changing the gas atmosphere in the quartz tube and the microwave heating time.
Fabrication of the CPEG/PI thermally conductive composites APB was dissolved in a mixed solvent of DMAc/THF (3/2, wt/wt), and a certain amount of CPEG was added and dispersed by ultrasonication. Under ice bath condition, a certain amount of ODPA (ODPA/APB = 1.01/1, mol/mol) was added into the above solution and stirred for 4 h to prepare the CPEG/polyamide acid (CPEG/PAA) solution. The CPEG/PAA fibers mats were obtained after electrospinning the CPEG/PAA solution with the technology as follows: distance between the needle and collection device is 30 cm, voltage of 20 kV, injection speed of 0.1 mm/min, and rotational speed of 700 rpm. The CPEG/PAA fibers mats were placed into the 80°C oven for 12 h for drying and then thermal imidization (120°C/1 h + 200°C/ 1 h + 250°C/1 h, heating rate of 1°C/min) to obtain CEPG/PI fibers mats. Finally, the CPEG/PI fibers mats were cut into 20*20 mm shape, then lay-up in a mold and hot-pressing at 320°C/10 MPa for 40 min to obtain the CPEG/PI thermally conductive composites (20*20*2 mm). As a comparison, EG and purchased CNT were mixed directly in a certain ratio (CNT/EG = 3/ 1, wt/wt. This ratio is determined by the ratio of CNT to EG in CPEG) to obtain CNT/EG (CEG) thermally conductive fillers, and then CEG/PI thermally conductive composites were fabricated according to the above method.

DATA AVAILABILITY
The data that support the findings of this study are available from the corresponding author upon reasonable request. Fig. 6 Heat dissipation of CPEG/PI composites. Infrared images of CPEG/PI thermally conductive composites on the hot plate (a) and its surface temperature (b), optical photo (c) and infrared thermal images (d) of CPU integrated with CPEG/PI thermally conductive composites, optical photo (e) and infrared thermal images (f) of bare CPU, temperature of CPU during the working process (g).