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AlN hybrid films for thermal management of flexible energy storage devices
Carbohydrate Polymers 213 (2019) 228–235
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Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol
Highly thermal conductivity of CNF/AlN hybrid films for thermal management of flexible energy storage devices Kun Zhanga,b,1, Peng Taoa,b,1, Yuehua Zhanga,b, Xiaoping Liaoa, Shuangxi Niea,b, a b
T
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School of Light Industry and Food Engineering, Guangxi University, Nanning 530004, China Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, Nanning 530004, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermal conductive papers Aluminum nitride Cellulose nanofibrils Thermal management Flexible energy storage devices
As energy storage devices are becoming more highly integrated, it is inevitable that heat accumulation will occur under high power working conditions. Finding efficient thermal management materials for cooling down electronic components is an urgent problem for energy storage devices. In this work, a thermally conductive film with tailorable macroproperties is fabricated by using a simple vacuum filtration method, using cellulose nanofibrils as the polymer substrate and assembly with aluminum nitride nanosheets. The interaction between the cellulose nanofibrils and aluminum nitride nanosheets is studied, and the electronic components made using the composite exhibit excellent thermal conductivity, thermal stability and mechanical flexibility. A high thermal conductivity is achieved along the film surface (up to 4.20 W/mK for 25 wt.% of aluminum nitride). This green material can effectively promote potential applications as lateral heat spreaders in flexible energy storage devices and the thermal conductivity may facilitate the applications in thermal management.
1. Introduction Electronic devices are developing in the direction of increasing miniaturization, high integration and multifunctionality (Wang, Wang, Fu, Wang, & Yu, 2018). Inevitably, some energy storage components accumulate heat under the high-power conditions (Xing et al., 2019). This heat degrades the performance of some components, thereby affecting the overall performance of electronic devices (Wu, Lei et al., 2017). The search for efficient thermal management materials for cooling electronic equipment has become an urgent problem for modern energy storage devices (Chen, Huang, Zhu, & Jiang, 2017; Moore & Shi, 2014). Polymer materials are considered to be highly efficient thermal management materials for solving the problem of heat accumulation (Chen et al., 2016; Dong et al., 2019). Generally, the thermal conductivity of polymeric materials is only 0.1–0.5 W/mK and is improved by adding inorganic fillers with high thermal conductivity to the polymer materials (Zeng et al., 2017). In recent years, many researchers have studied the thermal conductivity of polymer composites, such as the materials formed by the addition of copper (Wang, Cheng, Wang, Sun, & Gao, 2014), gold (Balachander et al., 2013), silver (Shen & Feng, 2018; Xu, Munari, Dalton, Mathewson, & Razeeb, 2009) and other metal nanowires, carbon nanotubes (Han & Fina, 2011) and graphene (Song et al., 2013) to polymer composites. Although metal
nanowires, carbon nanotubes, and graphene improve the thermal conductivity of polymer composites to a certain extent, they inevitably also increase the electrical conductivity of the composites, limiting their application in some electrical insulation devices. The electrical insulation properties of thermal management materials play an important role in electrical insulation devices. Inorganic nitrides have become a hot research topic in the field of thermal management materials in recent years due to their high thermal conductivity and good electrical insulation. Yao et al. prepared a composite film of boron nitride nanosheets and graphene oxide with a thermal conductivity of 29.8 W/mK (Yao et al., 2016). Hong et al. added aluminum nitride and boron nitride to the epoxy resin matrix. They found that the thermal conductivity of the composites reaches the maximum value of 8 W/mK for the 1:1 ratio of aluminum nitride and boron nitride (Hong et al., 2012). Fu et al. first introduced the hydroxyl group to the boron nitride powder under molten alkali conditions. The hydroxylated boron nitride film shows a good thermal conductivity in the direction of the surface, reaching 51.1 W/mK. The thermal conductivity of the dihydroxyl group was increased by 14% after a low temperature annealing treatment (Fu et al., 2017). However, pure inorganic nitride film materials are highly brittle and have poor mechanical properties, greatly limiting their use, particularly in flexible electronic devices (Wu, Fang et al., 2017). By combining these materials with polymers,
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Corresponding author at: School of Light Industry and Food Engineering, Guangxi University, Nanning 530004, China. E-mail address: [email protected] (S. Nie). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.carbpol.2019.02.087 Received 29 December 2018; Received in revised form 20 February 2019; Accepted 25 February 2019 Available online 01 March 2019 0144-8617/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. (a) Suspension with an AlN additions of 0%, 5%, 10%, 15%, 20%, 25%, and 50%; (b) TEM image of pure CNF; (c) TEM image of pure AlN; (d) Transmittance of the CNF-AlN suspension with different amounts of added AlN.
Fig. 2. (a) Schematic of the fabrication process of the CNF-AlN nanocomposites; (b) Optical images of the CNF-AlN nanocomposites with diff ;erent AlN content.
thermal conductivity (Wu, Fang et al., 2017). Zhu et al. interspersed a layer of boron nitride nanosheets with two-dimensional structure in CNF. This special combination structure enables the composite material to reach a high thermal conductivity of 145.7 W/mK for the boron nitride nanosheet content of 50% (Zhu et al., 2014). These studies have showed that CNF can be a substrate for inorganic nitrides enabling the replacement of traditional high-molecular-weight polymers by a new generation of green electronic substrates. However, an increase in the amount of the added inorganic nitride leads to a degradation of the mechanical properties of the composite, and the uneven dispersion of the nitride also lowers the thermal conductivity. Therefore, to improve the mechanical and thermal conductivities of the composites, it is highly important to design the nitride and CNF assembly modes and improve the dispersion uniformity of the nitride. In this work, a simple vacuum filtration method was used to assemble CNF as a polymer substrate with aluminum nitride (AlN) nanosheets to prepare a high thermal conductivity composite. The interaction between CNF and aluminum nitride nanosheets was studied, and
their mechanical properties can be greatly improved. Cellulose is abundant in nature and is a renewable, environmentally friendly polymer (Fan et al., 2017; Lin, Wu, Zhang, Liu, & Nie, 2018; Nie et al., 2014; Pei et al., 2016; Yao, Nie, Yuan, Wang, & Qin, 2015; Zhang, Nie, Qin, & Wang, 2019). Cellulose nanofibrils (CNF) have good biodegradability, excellent mechanical strength and mechanical flexibility (Li, Wang, Wang, Qin, & Wu, 2019; Nie, Zhang, Zhang et al., 2018; Yao et al., 2017). CNF and inorganic nitride composite materials combine the advantages of cellulose and inorganic nitrides, expanding their range of application in electronic devices. Zeng et al. synthesized a one-dimensional structure of boron nitride nanotubes with CNF to prepare a new polymeric material that which effectively reduced the thermal resistance of the interface. Due to the inherent high thermal conductivity of boron nitride, for the boron nitride content of 25%, the composite exhibits a high thermal conductivity of 21.39 W/mK (Zeng et al., 2017). In the study of Wu et al., the boron nitride was functionalized with urea and then was combined with CNF, obtaining a foldable composite membrane with high mechanical strength and high 229
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Fig. 3. SEM images (a) Typical cross-section morphology of CNF; (b) CNF-25%AlN Typical cross-section morphology of the composite films; (c–h) Composite film surfaces with the AlN addition of 0%, 5%, 10%, 15%, 20%, and 25%.
2.2. Sample preparation
the effect of the aluminum nitride dosage on the properties of the composites was examined. Our investigations focused on the thermal conductivity, thermal stability and mechanical properties of the composite. The electronic components produced by this composite material show excellent performance. The developed composite is a green and renewable matrix material that can effectively promote the development of green flexible energy storage devices.
Bagasse pulp bleaching further purified cellulose with high purity can be obtained (Dai et al., 2016; Nie, Yao, Wang, & Qin, 2016; Zhang, Nie, Qin, Zhang, & Wang, 2018). Bleached bagasse dry pulp (20 g) was placed in a 1 L Erlenmeyer flask, and then, 5% potassium hydroxide solution (640 mL) was added. The bleached bagasse pulp was heated to 90 ℃ in a water bath and stirred at this temperature for 2 h. After the treatment, the pulp was placed in a 300-mesh polyethylene bag and washed with deionized water until the filtrate was neutral, and the hemicellulose was removed (Sehaqui, Liu, Zhou, & Berglund, 2010). The obtained sample was placed in a 1 L Erlenmeyer flask, and deionized water (640 mL) was added to adjust the pulp consistency to 3%. Then, 30% sodium chlorite (6 g) was added, and glacial acetic acid was added to adjust the pH of the pulp to 4–5. The temperature of the sample was adjusted to 70–80 ℃ in a water bath, and the mixture was stirred at this temperature for 1 h. This step was repeated three times. Finally, the sample was washed with deionized water until the filtrate became neutral, and then, the residual lignin was removed (Okahisa,
2. Material and methods 2.1. Materials and chemicals Bleached bagasse pulp from Guangxi Guitang Group Co., Ltd. (Guangxi, China), AlN nanopowders (average diameter less than 100 nm) and sodium chlorite were purchased from Sigma-Aldrich (Germany). Sealed bags, potassium hydroxide, sodium and glacial acetic acid were purchased from Guangxi Nanning Boyu Co., Ltd. (Guangxi, China). Deionized water was used for all of the experiment, and all of the chemical compounds were of analytical grade. 230
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Fig. 4. (a) Infrared spectra of CNF, AlN and CNF-AlN composites; (b) Schematic of interaction between the CNF and AlN; (c) XRD and (d) Thermogravimetric analysis results for the CNF-AlN nanocomposites; (e) A photograph of the AlN film shows its flexibility; (f) Tensile strength vs AlN content in the composite films.
ultrasonically mixed with nano-aluminum nitride for 24 h. The ultrasonic power was set to 50 W in order to avoid damage to the crystal structure of aluminum nitride. After the end of the ultrasonication, the mixed solution was centrifuged at 500 rpm for 5 min, and the supernatant was vacuum-filtered through a PTFE membrane with a diameter of 50 mm and a pore size of 0.22 μm. Finally, the CNF-AlN composite was obtained by vacuum drying at 50 ℃ for 24 h.
Table 1 Thermal conductivity properties of the CNF composite films. Sample
a⊥ (mm2/s)
a∥ (mm2/s)
ρ (g/cm3)
C (KJ/K)
TC⊥ (W/mK)
TC∥ (W/mK)
CNF CNF-5%AlN CNF-10%AlN CNF-15%AlN CNF-20%AlN CNF-25%AlN
0.125 0.210 0.273 0.307 0.342 0.333
0.798 2.831 2.550 2.726 2.838 3.576
1.15 1.19 1.24 1.25 1.21 1.24
1.35 0.92 1.02 0.98 1.06 0.95
0.19 0.23 0.35 0.38 0.44 0.39
1.24 3.10 3.20 3.30 3.60 4.20
2.3. Analysis and testing procedures The light transmittance of the CNF-AlN composite was measured using an ultraviolet-visible spectrophotometer (SPECORD 50 PLUS, Jena analysis, Germany). The CNF-AlN composite suspension with a mass fraction of 1 wt.% was tested at room temperature and the test wavelength was varied from 400 nm to 1100 nm. The CNF-AlN composites were analyzed by ATR-FTIR (TENSOR II, Brook Technology, Germany). The sample was examined at the wavelengths in the 400–4000 cm−1 range. The sample was thoroughly crushed and dried in an oven at 105 ℃ for 6 h. The sample was compressed with potassium bromide and then examined by ATR-FTIR infrared spectroscopy (Zhang, Zhang, Yan, Zhang, & Nie, 2018). The mass fraction of the CNF or AlN was controlled to be
Yoshida, Miyaguchi, & Yano, 2009). Distilled water was added to the chemically purified pulp until the pulp consistency was 2%, and then the solution was allowed to dissolve for 30 min. Next, a Superfine grinding mill (MKZA10-15J, Japan) was used for grinding, and the gap between the two grinding discs was controlled to be minus 4 grids below 0 points (Ruan, Ai, & Lu, 2014). The number of grinding passes of all samples was 10. After the grinding, the sample was homogenized with a high-pressure microfluidizer (M-110EH-30, USA) to obtain the CNF (Nie, Zhang, Lin et al., 2018). The prepared CNF were diluted to 1% and stored in a refrigerator for use. The prepared CNF were formulated into a suspension (240 mL) with a mass fraction of 0.1% and 231
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0.005–0.01 wt.%, and 1˜2 drops of CNF or AlN suspension were added to cover the 200-mesh copper mesh carbon support film. Interference from any contaminants during the period was avoided, and the samples were naturally dried for 12 h at room temperature. Two drops of a dye (uranyl acetate) were dropped on the dried copper grid, requiring the droplets to cover the copper grid. Then, the sample was placed in the dark for 10–30 min. After the dyeing was completed, the excess dye was removed by filter paper, and the sample was naturally dried at room temperature. The copper mesh was observed with a transmission electron microscope (Hitachi HT7700, Japan) at a magnification of 15,000; the operating voltage of the electron microscope was 100 kV, and the current was 10 μA. The surface topography and cross-sectional morphology of the composite were examined using a scanning electron microscope (PHENOM F16502, Netherlands) with a test voltage of 15 kV. The sample was cryocracked using liquid nitrogen for cross-sectional observation. The sample was sprayed with gold under vacuum and then observed. The crystallinity of the CNF-AlN film was analyzed using a high-resolution X-ray diffractometer (MINFLEX600, Japan). The dried CNF-AlN film was cut into 2 × 2 cm samples with scissors and was attached to a glass sample plate. We ensured that the film was flat on the sample plate, and the sample holder position was controlled to remain constant. The scanning angle 2θ was varied in steps of 0.02°, and the scan was performed in the 2θ range from 5° to 60° in 2.5 s. The freeze-dried samples were analyzed in thermogravimetric tests using a simultaneous thermal analyzer (STA 449F5, NETZSCH, German). All of the tests were carried out under a nitrogen atmosphere, and the samples were filled into an alumina crucible. A sample approximately 10 mg was weighed, and the temperature was raised from 25 ℃ to 800 ℃ at a heating rate of 10 ℃/min for thermogravimetric analysis; this procedure was repeated three times for each sample. The mechanical properties of the CNF-AlN composite films were evaluated using a vertical tension machine and Bluehill software (AMETEK LS1, USA). The operating conditions were as follows: the
Fig. 5. (a) In-plane and cross-plane TC of the CNF-AlN composite films; (b) Schematic diagram of the heat dissipation along the surface of CNF-AlN composite films.
Fig. 6. (a) Optical image of CNF and CNF-25%AlN film as the LED chip substrates; (b) The surface temperature at central area close to the chip and (c) 10 mm close to the chip. 232
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could increase the thickness of the composite film, and a uniform layer of AlN nanosheets is interspersed in the CNF. The uniform arrangement of the AlN nanosheets is beneficial for improving the thermal conductivity of the composite. Fig. 3c–h show the SEM images of the surfaces of the CNF-AlN composites with different AlN contents. It is observed that the AlN distribution is highly uniform and that AlN is interlaced with CNF in a network structure. As shown in Fig. 3d–f, when the AlN content is less smaller than 15%, the AlN nanosheets rarely interweave into the network structure. With the increase in the AlN addition, the stacking of AlN nanosheets appears, affecting the mechanical properties of the CNF-AlN composite paper. To reveal the interactions between the CNF and AlN, infrared spectra were used to analyze the CNF, AlN and CNF-AlN nanocomposites. As shown in Fig. 4a, a strong wide infrared absorption band centered on 731 cm−1 is observed for the pure AlN, which is due to the Al-N stretching vibration absorption peak. Pure AlN also has several additional infrared absorption peaks at 1384, 1634, and 3348 cm−1. The absorption peak at 1384 cm−1 is produced by the infrared absorption of (AlN)2 (Lv et al., 2008), The absorption peaks at 1634 and 3348 cm−1 are caused by the H2O adsorbed on the surface of the AlN and KBr crystals. The FTIR spectrum of the pure CNF has an eOH stretching vibration absorption peak at 3348 cm−1. The absorption peak near 2910 cm−1 is mainly due to the stretching vibrations of eCH, eCH2 or eCH3 in the CNF. The absorption peak near 1634 cm−1 is mainly due to the stretching vibration absorption peak derived from the C]O ester group in the CNF. The peak at 1065 cm−1 is due to a stretching vibration of the CeOeC asymmetric ring in the pyran ring (Nie et al., 2015; Swain, Dash, Behera, Kisku, & Behera, 2013). The characteristic absorption peaks of the CNF-AlN nanocomposites are similar to those of pure CNF, while the strong Al-N absorption peak of pure AlN does not appear in the FTIR spectra of the CNF-AlN nanocomposites. This finding does not necessarily imply that the bond between the Al and N atoms is broken in the CNF-AlN nanocomposite; this is because the preparation process only involved treatments by ultrasonication and vacuum filtration, and these two treatments are not sufficiently harsh to break the Al-N bond. The most important reason for the disappearance of the absorption peak of Al-N bond in the CNFAlN nanocomposites is that when the CNF and AlN are mixed to form a homogeneous solution, the hydroxyl-rich CNF easily adsorb to the surface of AlN, and hydrogen bonds are formed between the H and N atoms. The presence of the bond weakens the absorption due to the vibration of the Al-N bond (Fig. 4b). In addition, the presence of hydrogen bonds also has a certain effect on the thermal and mechanical properties of the CNF-AlN nanocomposites. The crystallinity of the pure CNF, pure AlN and CNF-AlN nanocomposites with various dosages of AlN were analyzed by XRD. As shown in Fig. 4c, the main diffraction peaks of pure CNF are found at 2θ = 16.3° and 22.5° (French & Santiago Cintrón, 2013; Peng et al., 2013), and the main diffraction peak of pure AlN is found at 2θ = 36° (Zhou et al., 2014). With the increased AlN addition, the diffraction peak position of the CNF-AlN composites changes slowly from that of the pure CNF, and the diffraction peaks at 2θ = 16.3° and 22.5° decrease continuously. When the amount of the added AlN reaches 10%, a new diffraction peak appears at 2θ = 20.5°, and its intensity increases with increasing amount of the added AlN. When the amount of added AlN reaches 15%, a new diffraction peak appears at 2θ = 40°, and its intensity increases with increasing AlN content. This result indicates that the interaction between the CNF and AlN shifts the position of the diffraction peak. To study the effect of the amount of the added AlN on the thermal stability of the CNF-AlN nanocomposites, thermogravimetric analysis (TGA) was carried out on composites with different AlN content. The TGA samples were heated from 25 ℃ to 800 ℃ under a nitrogen atmosphere. As shown in Fig. 4d, pure AlN has almost no mass loss in this temperature range, indicating that AlN has high thermal stability and oxidation resistance. The initial mass loss of the CNF-AlN
stretching speed was set to 1 mm/min, the load cell was 80 N, and the distance was 20 mm. The samples were mounted between the fixtures and were measured in an ISO constant temperature and humidity laboratory conditions (23 ℃ and 50% relative humidity). The in-plane thermal diffusivity of the CNF-AlN composite (diameter of 25.4 mm) was measured using the laser flash method (NETZSCH LFA467, Germany). The thermal conductivity was calculated according to the following formula at room temperature of 25 ℃： TC = α * ρ* Ccomposites Ccomposites=CAlN*Ф+CCNF*(1−Ф) where TC is thermal conductivity, α is thermal diffusivity, ρ is density, C is specific heat capacity, and Ф is the volume fraction; the final result for thermal conductivity was obtained as the average of at least three measurements (Wu, Fang et al., 2017). 3. Results and discussion The repulsive force produced by the carboxyl groups between the CNF fibers gives rise to CNF suspension with good dispersion and high transparency and disperses the inorganic filler evenly and stably (Nie, Zhang, Lin et al., 2018; Vincent, Heintz, Silvain, & Chandra, 2011). The stability of the CNF-AlN suspension is essential for the formation of a uniform composite film. As Fig. 1a, the suspension of the pure CNF and obtained by ultrasonic blending of AlN and CNF are stabilized for more than one week. CNF has abundant surface hydroxyl groups, leading to easy adsorption on the AlN surface in the CNF-AlN suspensions. As observed from the TEM images of pure AlN and CNFs presented in Fig. 1b and c, respectively, the average diameter of the CNF is approximately 50 nm, and one CNF is several μm long. The average diameter of AlN is 70 nm, and its average length is approximately 200 nm. As observed from Fig. 1d, the pure CNF suspension with no AlN addition has light transmittance of 69% at the wavelength of 550 nm, whereas light transmittance of 55% is obtained for the suspension with 5 wt.% added AlN. When the amount of the added AlN added is increased further, the UV transmittance of the CNF-AlN suspension gradually decreases due to the increasing amount of added AlN. When the amount of AlN is 20 wt.%, the transmittance at 550 nm is only 6%. When the AlN content is 50 wt.%, the transmittance is less than 1%, and the whiteness of the composite material reaches 85% ISO. A paper with customized optical properties can be used for many purposes. For example, 5 wt.% AlN composite paper can be used as a transparent substrate for flexible electronic display products. The composite paper with 50 wt.% AlN can be used as an opaque substrate for flexible solar panels. After the CNF-AlN suspension was uniformly dispersed by ultrasonication, the CNF-AlN composite films were prepared by vacuum filtration (Fig. 2a). A series of CNF-AlN composite films can be prepared by controlling the amount of added AlN to vary from 0 to 50 wt.%. As shown in Fig. 2b, the pure CNF paper has good light transmittance, and the light transmittance decreases with the increase in the amount of the added AlN. When the AlN addition exceeds 5 wt.%, the CNF-AlN composite paper becomes opaque. In fact, the transparency of the CNFAlN composite does not affect its potential for use as a substrate for cooling electronic devices. When the AlN addition exceeds 50 wt.%, it is difficult to form a CNF-AlN composite paper. Fig. 3 shows the SEM images of a CNF-AlN nanocomposite. The cross-sectional structure of a composite material has a certain effect on the thermal conductivity and mechanical properties of the material itself. Fig. 3a presents an SEM image of a cross-section of a pure CNF film (thickness: 32.82 μm) showing a densely packed layered structure. Pure CNF films can achieve high mechanical strength, which is closely related to the dense layered structure (Zeng et al., 2017). Fig. 3b shows an SEM image of the cross-section of the CNF-25%AlN composite film (thickness: 44.39 μm). It is clearly observed that the addition of AlN 233
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nanocomposites is found at approximately 100 ℃ and is mainly due to the elimination of the moisture on the CNF surface. The pyrolysis process of the composite material mainly occurs at approximately 350 ℃, at which point the CNF are essentially completely decomposed. With increasing AlN content, the mass loss of the nanocomposites decreases, showing that the addition of AlN increases the thermal stability of the CNF-AlN nanocomposites. This effect is because the thermal conductivity (170 W/mK) of AlN is much higher than that of CNF (0.03 W/ mK), so AlN in the composite can better absorb heat and limits the movement of heat to the CNF polymer chain, resulting in the CNF polymer chains beginning to degrade at higher temperatures (Yang, Hsiang, Huang, Huang, & Han, 2017). In addition, the interaction of AlN with CNF also increases the thermal resistance of the nanocomposite and reduces the heat loss. Flexibility and strength are important indicators for the evaluation of the mechanical properties of thermally conductive materials. Fig. 4e demonstrates the good flexibility of the CNF-AlN composite obtained in this study. It is observed from Fig. 4e that the tensile strength of the nanocomposite is 116.1 MPa when the amount of added AlN is 5%. The tensile strength of the composite decreases with the increase in the AlN amount. It should be noted that AlN is highly brittle and is not easily interwoven into a film. In the CNF-AlN composite, CNF acts as an adhesive to tightly bind the AlN nanosheets. The mixing of CNF and AlN is conducive to better alignment of the AlN nanosheets, enabling the full interconnection of AlN with itself and between AlN and the CNF. At the same time, a large number of amino groups (eNH2) and hydroxyl groups (eOH) are present at the edge of AlN, which easily form hydrogen bonds with the hydroxyl groups (eOH) and carboxyl groups (eCOOH) found on the CNF surface(Zeng et al., 2017), leading to the formation of a high-mechanical-strength nanocompositevia hydrogen bonding interactions. In addition, the CNF-AlN composites form a compact, dense and regular layered three-dimensional structure under vacuum suction filtration under high-vacuum conditions. This ordered and uniform structure formed by the strong interfacial interaction between the CNF and AlN is similar to the layered structure of pearls. This special structure is another important factor that contributes to the formation of nanocomposites with high mechanical strengths. The in-plane and cross-plane thermal conductivities of the composites with different AlN additions were analyzed to study the thermal conductivity of the CNF-AlN nanocomposites. Table 1 shows the parameters of the thermal conductivity and the test conditions for the composite materials at room temperature of 25 ℃. The thermal conductivity is calculated as shown in Fig. 5. It is observed that both the inand cross-plane thermal conductivities of the CNF-AlN nanocomposites show an upward trend. In particular, the increase in the in-plane thermal conductivity is more pronounced (Fig. 5a). An examination of the data presented in Table 1 shows that the thermal conductivity of the pure CNF film in the plane is 1.24 W/mK, which is almost five times higher that of the conventional high-molecular-weight polymer. Uetani et al. studied the thermal conductivity of the CNF in different crystal forms and found that a higher crystallinity of the CNF leads to a higher thermal conductivity (Uetani, Okada, & Oyama, 2015). This reason is because the in-plane direction is favorable for phonon transport for a highly crystalline CNF (Wu, Fang et al., 2017). When the AlN addition amount is 25%, the thermal conductivity of the composite film reaches 4.20 W/mK; this result is related to the inherent high thermal conductivity of the pure AlN. The 1D CNF decrease the contact between the AlN nanosheets. After the ultrasonic vacuum filtration process of the CNF-AlN suspension, the AlN nanosheets are arranged neatly and evenly, providing a greater likelihood of contacting and overlapping each other. Thus, an efficient and continuous AlN network structure can be formed that greatly reduces the in-plane thermal resistance (Fig. 5b). However, the thermal conductivity of the CNF-AlN composite is not significantly increased in the cross-plane direction. The thermal conductivity of the pure CNF film is 0.19 W/mK and is only 0.39 W/mK with an AlN content of 25%. The cross-plane thermal conductivity is
one order of magnitude lower than the in-plane thermal conductivity. This result may be due to the higher anisotropy of the nanocomposite thermal conductivity so that the in-plane phonon transport is easier than the cross-plane phonon transport (Zeng et al., 2017). In order to further prove the potential performance of CNF-AlN nanocomposite films as a thermal management materials in flexible energy storage devices, the crystal light-emitting diode (LED) chips were attached to CNF film and CNF-25%AlN composite film (Fig. 6a). The LED was served as the main heat source, and the surface temperature variations of CNF and CNF-25%AlN film at central area close to the chip and 10 mm close to the chip with the LED working for 300 s were measured by using an infrared thermomete. Fig. 6b and c showed the surface temperature variations of the composite film at central area close to the LED chip and 10 mm close to the LED chip, respectively. It can be seen that the heating rate of CNF film was higher than that of CNF-25% AlN film at both central area close to the LED chip and 10 mm close to the LED chip. The final temperature of CNF film is also higher than that of CNF-25%AlN film. The CNF film showed a higher surface temperature of 47.8℃ than that of CNF-25%AlN film (40.1℃) at the central area close to the chip. Similarly, the CNF film showed a higher surface temperature of 31.6℃ than that of CNF-25%AlN film (29.2℃) at 10 mm close to the chip. These results indicated that the CNF-AlN composite film exhibits excellent heat dissipation performance, and it can effectively transfer the heat out, so that the composite can achieve a good goal of cooling. On the basis of the above results, the CNF-AlN composite film with highly thermal conductivity can effectively promote potential applications as lateral heat spreaders in flexible energy storage devices and may facilitate the applications in thermal management. 4. Conclusions Under ultrasonication, AlN was uniformly and stably dispersed in the CNF suspension by strong interactions. Then, the compact, dense, light and regular-layered three-dimensional structure of the CNF-AlN nanocomposite material was obtained by vacuum filtration. The composite exhibits good thermal conductivity, thermal stability and high mechanical strength. The in-plane thermal conductivity reached 4.20 W/mK when the amount of the added AlN was 25 wt.%. The addition of AlN increases the thermal impedance of the nanocomposite, reducing its mass loss in high-temperature environments and improving its thermal stability. Based on these excellent properties, CNF-AlN nanocomposites are expected to be suitable for operation in extreme environments and are excellent substrates for flexible electronic devices. However, the incorporation of AlN nanosheets led composite films inevitably bereaved of transparency. Therefore, this highly thermal conductivity flexible NFC/AlN composite films can serve as a lateral heat spreader in the thermal management applications where film transparency were not strictly required. Acknowledgements This project was supported by the National Natural Science Foundation of China (31760192) and the Guangxi Natural Science Foundation of China (2018GXNSFDA281050). References Balachander, N., Seshadri, I., Mehta, R. J., Schadler, L. S., Borca-Tasciuc, T., Keblinski, P., ... Ramanath, G. (2013). Nanowire-filled polymer composites with ultrahigh thermal conductivity. Applied Physics Letters, 102(9), 093117. Chen, H., Ginzburg, V. V., Yang, J., Yang, Y., Liu, W., Huang, Y., ... Chen, B. (2016). Thermal conductivity of polymer-based composites: Fundamentals and applications. Progress in Polymer Science, 59, 41–85. Chen, J., Huang, X., Zhu, Y., & Jiang, P. (2017). Cellulose nanofiber supported 3D interconnected BN nanosheets for epoxy nanocomposites with ultrahigh thermal management capability. Advanced Functional Materials, 27(5), 1604754. Dai, Y., Song, X., Gao, C., He, S., Nie, S., & Qin, C. (2016). Xylanase-aided chlorine
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Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol
Highly thermal conductivity of CNF/AlN hybrid films for thermal management of flexible energy storage devices Kun Zhanga,b,1, Peng Taoa,b,1, Yuehua Zhanga,b, Xiaoping Liaoa, Shuangxi Niea,b, a b
T
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School of Light Industry and Food Engineering, Guangxi University, Nanning 530004, China Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, Nanning 530004, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermal conductive papers Aluminum nitride Cellulose nanofibrils Thermal management Flexible energy storage devices
As energy storage devices are becoming more highly integrated, it is inevitable that heat accumulation will occur under high power working conditions. Finding efficient thermal management materials for cooling down electronic components is an urgent problem for energy storage devices. In this work, a thermally conductive film with tailorable macroproperties is fabricated by using a simple vacuum filtration method, using cellulose nanofibrils as the polymer substrate and assembly with aluminum nitride nanosheets. The interaction between the cellulose nanofibrils and aluminum nitride nanosheets is studied, and the electronic components made using the composite exhibit excellent thermal conductivity, thermal stability and mechanical flexibility. A high thermal conductivity is achieved along the film surface (up to 4.20 W/mK for 25 wt.% of aluminum nitride). This green material can effectively promote potential applications as lateral heat spreaders in flexible energy storage devices and the thermal conductivity may facilitate the applications in thermal management.
1. Introduction Electronic devices are developing in the direction of increasing miniaturization, high integration and multifunctionality (Wang, Wang, Fu, Wang, & Yu, 2018). Inevitably, some energy storage components accumulate heat under the high-power conditions (Xing et al., 2019). This heat degrades the performance of some components, thereby affecting the overall performance of electronic devices (Wu, Lei et al., 2017). The search for efficient thermal management materials for cooling electronic equipment has become an urgent problem for modern energy storage devices (Chen, Huang, Zhu, & Jiang, 2017; Moore & Shi, 2014). Polymer materials are considered to be highly efficient thermal management materials for solving the problem of heat accumulation (Chen et al., 2016; Dong et al., 2019). Generally, the thermal conductivity of polymeric materials is only 0.1–0.5 W/mK and is improved by adding inorganic fillers with high thermal conductivity to the polymer materials (Zeng et al., 2017). In recent years, many researchers have studied the thermal conductivity of polymer composites, such as the materials formed by the addition of copper (Wang, Cheng, Wang, Sun, & Gao, 2014), gold (Balachander et al., 2013), silver (Shen & Feng, 2018; Xu, Munari, Dalton, Mathewson, & Razeeb, 2009) and other metal nanowires, carbon nanotubes (Han & Fina, 2011) and graphene (Song et al., 2013) to polymer composites. Although metal
nanowires, carbon nanotubes, and graphene improve the thermal conductivity of polymer composites to a certain extent, they inevitably also increase the electrical conductivity of the composites, limiting their application in some electrical insulation devices. The electrical insulation properties of thermal management materials play an important role in electrical insulation devices. Inorganic nitrides have become a hot research topic in the field of thermal management materials in recent years due to their high thermal conductivity and good electrical insulation. Yao et al. prepared a composite film of boron nitride nanosheets and graphene oxide with a thermal conductivity of 29.8 W/mK (Yao et al., 2016). Hong et al. added aluminum nitride and boron nitride to the epoxy resin matrix. They found that the thermal conductivity of the composites reaches the maximum value of 8 W/mK for the 1:1 ratio of aluminum nitride and boron nitride (Hong et al., 2012). Fu et al. first introduced the hydroxyl group to the boron nitride powder under molten alkali conditions. The hydroxylated boron nitride film shows a good thermal conductivity in the direction of the surface, reaching 51.1 W/mK. The thermal conductivity of the dihydroxyl group was increased by 14% after a low temperature annealing treatment (Fu et al., 2017). However, pure inorganic nitride film materials are highly brittle and have poor mechanical properties, greatly limiting their use, particularly in flexible electronic devices (Wu, Fang et al., 2017). By combining these materials with polymers,
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Corresponding author at: School of Light Industry and Food Engineering, Guangxi University, Nanning 530004, China. E-mail address: [email protected] (S. Nie). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.carbpol.2019.02.087 Received 29 December 2018; Received in revised form 20 February 2019; Accepted 25 February 2019 Available online 01 March 2019 0144-8617/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. (a) Suspension with an AlN additions of 0%, 5%, 10%, 15%, 20%, 25%, and 50%; (b) TEM image of pure CNF; (c) TEM image of pure AlN; (d) Transmittance of the CNF-AlN suspension with different amounts of added AlN.
Fig. 2. (a) Schematic of the fabrication process of the CNF-AlN nanocomposites; (b) Optical images of the CNF-AlN nanocomposites with diff ;erent AlN content.
thermal conductivity (Wu, Fang et al., 2017). Zhu et al. interspersed a layer of boron nitride nanosheets with two-dimensional structure in CNF. This special combination structure enables the composite material to reach a high thermal conductivity of 145.7 W/mK for the boron nitride nanosheet content of 50% (Zhu et al., 2014). These studies have showed that CNF can be a substrate for inorganic nitrides enabling the replacement of traditional high-molecular-weight polymers by a new generation of green electronic substrates. However, an increase in the amount of the added inorganic nitride leads to a degradation of the mechanical properties of the composite, and the uneven dispersion of the nitride also lowers the thermal conductivity. Therefore, to improve the mechanical and thermal conductivities of the composites, it is highly important to design the nitride and CNF assembly modes and improve the dispersion uniformity of the nitride. In this work, a simple vacuum filtration method was used to assemble CNF as a polymer substrate with aluminum nitride (AlN) nanosheets to prepare a high thermal conductivity composite. The interaction between CNF and aluminum nitride nanosheets was studied, and
their mechanical properties can be greatly improved. Cellulose is abundant in nature and is a renewable, environmentally friendly polymer (Fan et al., 2017; Lin, Wu, Zhang, Liu, & Nie, 2018; Nie et al., 2014; Pei et al., 2016; Yao, Nie, Yuan, Wang, & Qin, 2015; Zhang, Nie, Qin, & Wang, 2019). Cellulose nanofibrils (CNF) have good biodegradability, excellent mechanical strength and mechanical flexibility (Li, Wang, Wang, Qin, & Wu, 2019; Nie, Zhang, Zhang et al., 2018; Yao et al., 2017). CNF and inorganic nitride composite materials combine the advantages of cellulose and inorganic nitrides, expanding their range of application in electronic devices. Zeng et al. synthesized a one-dimensional structure of boron nitride nanotubes with CNF to prepare a new polymeric material that which effectively reduced the thermal resistance of the interface. Due to the inherent high thermal conductivity of boron nitride, for the boron nitride content of 25%, the composite exhibits a high thermal conductivity of 21.39 W/mK (Zeng et al., 2017). In the study of Wu et al., the boron nitride was functionalized with urea and then was combined with CNF, obtaining a foldable composite membrane with high mechanical strength and high 229
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Fig. 3. SEM images (a) Typical cross-section morphology of CNF; (b) CNF-25%AlN Typical cross-section morphology of the composite films; (c–h) Composite film surfaces with the AlN addition of 0%, 5%, 10%, 15%, 20%, and 25%.
2.2. Sample preparation
the effect of the aluminum nitride dosage on the properties of the composites was examined. Our investigations focused on the thermal conductivity, thermal stability and mechanical properties of the composite. The electronic components produced by this composite material show excellent performance. The developed composite is a green and renewable matrix material that can effectively promote the development of green flexible energy storage devices.
Bagasse pulp bleaching further purified cellulose with high purity can be obtained (Dai et al., 2016; Nie, Yao, Wang, & Qin, 2016; Zhang, Nie, Qin, Zhang, & Wang, 2018). Bleached bagasse dry pulp (20 g) was placed in a 1 L Erlenmeyer flask, and then, 5% potassium hydroxide solution (640 mL) was added. The bleached bagasse pulp was heated to 90 ℃ in a water bath and stirred at this temperature for 2 h. After the treatment, the pulp was placed in a 300-mesh polyethylene bag and washed with deionized water until the filtrate was neutral, and the hemicellulose was removed (Sehaqui, Liu, Zhou, & Berglund, 2010). The obtained sample was placed in a 1 L Erlenmeyer flask, and deionized water (640 mL) was added to adjust the pulp consistency to 3%. Then, 30% sodium chlorite (6 g) was added, and glacial acetic acid was added to adjust the pH of the pulp to 4–5. The temperature of the sample was adjusted to 70–80 ℃ in a water bath, and the mixture was stirred at this temperature for 1 h. This step was repeated three times. Finally, the sample was washed with deionized water until the filtrate became neutral, and then, the residual lignin was removed (Okahisa,
2. Material and methods 2.1. Materials and chemicals Bleached bagasse pulp from Guangxi Guitang Group Co., Ltd. (Guangxi, China), AlN nanopowders (average diameter less than 100 nm) and sodium chlorite were purchased from Sigma-Aldrich (Germany). Sealed bags, potassium hydroxide, sodium and glacial acetic acid were purchased from Guangxi Nanning Boyu Co., Ltd. (Guangxi, China). Deionized water was used for all of the experiment, and all of the chemical compounds were of analytical grade. 230
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Fig. 4. (a) Infrared spectra of CNF, AlN and CNF-AlN composites; (b) Schematic of interaction between the CNF and AlN; (c) XRD and (d) Thermogravimetric analysis results for the CNF-AlN nanocomposites; (e) A photograph of the AlN film shows its flexibility; (f) Tensile strength vs AlN content in the composite films.
ultrasonically mixed with nano-aluminum nitride for 24 h. The ultrasonic power was set to 50 W in order to avoid damage to the crystal structure of aluminum nitride. After the end of the ultrasonication, the mixed solution was centrifuged at 500 rpm for 5 min, and the supernatant was vacuum-filtered through a PTFE membrane with a diameter of 50 mm and a pore size of 0.22 μm. Finally, the CNF-AlN composite was obtained by vacuum drying at 50 ℃ for 24 h.
Table 1 Thermal conductivity properties of the CNF composite films. Sample
a⊥ (mm2/s)
a∥ (mm2/s)
ρ (g/cm3)
C (KJ/K)
TC⊥ (W/mK)
TC∥ (W/mK)
CNF CNF-5%AlN CNF-10%AlN CNF-15%AlN CNF-20%AlN CNF-25%AlN
0.125 0.210 0.273 0.307 0.342 0.333
0.798 2.831 2.550 2.726 2.838 3.576
1.15 1.19 1.24 1.25 1.21 1.24
1.35 0.92 1.02 0.98 1.06 0.95
0.19 0.23 0.35 0.38 0.44 0.39
1.24 3.10 3.20 3.30 3.60 4.20
2.3. Analysis and testing procedures The light transmittance of the CNF-AlN composite was measured using an ultraviolet-visible spectrophotometer (SPECORD 50 PLUS, Jena analysis, Germany). The CNF-AlN composite suspension with a mass fraction of 1 wt.% was tested at room temperature and the test wavelength was varied from 400 nm to 1100 nm. The CNF-AlN composites were analyzed by ATR-FTIR (TENSOR II, Brook Technology, Germany). The sample was examined at the wavelengths in the 400–4000 cm−1 range. The sample was thoroughly crushed and dried in an oven at 105 ℃ for 6 h. The sample was compressed with potassium bromide and then examined by ATR-FTIR infrared spectroscopy (Zhang, Zhang, Yan, Zhang, & Nie, 2018). The mass fraction of the CNF or AlN was controlled to be
Yoshida, Miyaguchi, & Yano, 2009). Distilled water was added to the chemically purified pulp until the pulp consistency was 2%, and then the solution was allowed to dissolve for 30 min. Next, a Superfine grinding mill (MKZA10-15J, Japan) was used for grinding, and the gap between the two grinding discs was controlled to be minus 4 grids below 0 points (Ruan, Ai, & Lu, 2014). The number of grinding passes of all samples was 10. After the grinding, the sample was homogenized with a high-pressure microfluidizer (M-110EH-30, USA) to obtain the CNF (Nie, Zhang, Lin et al., 2018). The prepared CNF were diluted to 1% and stored in a refrigerator for use. The prepared CNF were formulated into a suspension (240 mL) with a mass fraction of 0.1% and 231
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0.005–0.01 wt.%, and 1˜2 drops of CNF or AlN suspension were added to cover the 200-mesh copper mesh carbon support film. Interference from any contaminants during the period was avoided, and the samples were naturally dried for 12 h at room temperature. Two drops of a dye (uranyl acetate) were dropped on the dried copper grid, requiring the droplets to cover the copper grid. Then, the sample was placed in the dark for 10–30 min. After the dyeing was completed, the excess dye was removed by filter paper, and the sample was naturally dried at room temperature. The copper mesh was observed with a transmission electron microscope (Hitachi HT7700, Japan) at a magnification of 15,000; the operating voltage of the electron microscope was 100 kV, and the current was 10 μA. The surface topography and cross-sectional morphology of the composite were examined using a scanning electron microscope (PHENOM F16502, Netherlands) with a test voltage of 15 kV. The sample was cryocracked using liquid nitrogen for cross-sectional observation. The sample was sprayed with gold under vacuum and then observed. The crystallinity of the CNF-AlN film was analyzed using a high-resolution X-ray diffractometer (MINFLEX600, Japan). The dried CNF-AlN film was cut into 2 × 2 cm samples with scissors and was attached to a glass sample plate. We ensured that the film was flat on the sample plate, and the sample holder position was controlled to remain constant. The scanning angle 2θ was varied in steps of 0.02°, and the scan was performed in the 2θ range from 5° to 60° in 2.5 s. The freeze-dried samples were analyzed in thermogravimetric tests using a simultaneous thermal analyzer (STA 449F5, NETZSCH, German). All of the tests were carried out under a nitrogen atmosphere, and the samples were filled into an alumina crucible. A sample approximately 10 mg was weighed, and the temperature was raised from 25 ℃ to 800 ℃ at a heating rate of 10 ℃/min for thermogravimetric analysis; this procedure was repeated three times for each sample. The mechanical properties of the CNF-AlN composite films were evaluated using a vertical tension machine and Bluehill software (AMETEK LS1, USA). The operating conditions were as follows: the
Fig. 5. (a) In-plane and cross-plane TC of the CNF-AlN composite films; (b) Schematic diagram of the heat dissipation along the surface of CNF-AlN composite films.
Fig. 6. (a) Optical image of CNF and CNF-25%AlN film as the LED chip substrates; (b) The surface temperature at central area close to the chip and (c) 10 mm close to the chip. 232
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could increase the thickness of the composite film, and a uniform layer of AlN nanosheets is interspersed in the CNF. The uniform arrangement of the AlN nanosheets is beneficial for improving the thermal conductivity of the composite. Fig. 3c–h show the SEM images of the surfaces of the CNF-AlN composites with different AlN contents. It is observed that the AlN distribution is highly uniform and that AlN is interlaced with CNF in a network structure. As shown in Fig. 3d–f, when the AlN content is less smaller than 15%, the AlN nanosheets rarely interweave into the network structure. With the increase in the AlN addition, the stacking of AlN nanosheets appears, affecting the mechanical properties of the CNF-AlN composite paper. To reveal the interactions between the CNF and AlN, infrared spectra were used to analyze the CNF, AlN and CNF-AlN nanocomposites. As shown in Fig. 4a, a strong wide infrared absorption band centered on 731 cm−1 is observed for the pure AlN, which is due to the Al-N stretching vibration absorption peak. Pure AlN also has several additional infrared absorption peaks at 1384, 1634, and 3348 cm−1. The absorption peak at 1384 cm−1 is produced by the infrared absorption of (AlN)2 (Lv et al., 2008), The absorption peaks at 1634 and 3348 cm−1 are caused by the H2O adsorbed on the surface of the AlN and KBr crystals. The FTIR spectrum of the pure CNF has an eOH stretching vibration absorption peak at 3348 cm−1. The absorption peak near 2910 cm−1 is mainly due to the stretching vibrations of eCH, eCH2 or eCH3 in the CNF. The absorption peak near 1634 cm−1 is mainly due to the stretching vibration absorption peak derived from the C]O ester group in the CNF. The peak at 1065 cm−1 is due to a stretching vibration of the CeOeC asymmetric ring in the pyran ring (Nie et al., 2015; Swain, Dash, Behera, Kisku, & Behera, 2013). The characteristic absorption peaks of the CNF-AlN nanocomposites are similar to those of pure CNF, while the strong Al-N absorption peak of pure AlN does not appear in the FTIR spectra of the CNF-AlN nanocomposites. This finding does not necessarily imply that the bond between the Al and N atoms is broken in the CNF-AlN nanocomposite; this is because the preparation process only involved treatments by ultrasonication and vacuum filtration, and these two treatments are not sufficiently harsh to break the Al-N bond. The most important reason for the disappearance of the absorption peak of Al-N bond in the CNFAlN nanocomposites is that when the CNF and AlN are mixed to form a homogeneous solution, the hydroxyl-rich CNF easily adsorb to the surface of AlN, and hydrogen bonds are formed between the H and N atoms. The presence of the bond weakens the absorption due to the vibration of the Al-N bond (Fig. 4b). In addition, the presence of hydrogen bonds also has a certain effect on the thermal and mechanical properties of the CNF-AlN nanocomposites. The crystallinity of the pure CNF, pure AlN and CNF-AlN nanocomposites with various dosages of AlN were analyzed by XRD. As shown in Fig. 4c, the main diffraction peaks of pure CNF are found at 2θ = 16.3° and 22.5° (French & Santiago Cintrón, 2013; Peng et al., 2013), and the main diffraction peak of pure AlN is found at 2θ = 36° (Zhou et al., 2014). With the increased AlN addition, the diffraction peak position of the CNF-AlN composites changes slowly from that of the pure CNF, and the diffraction peaks at 2θ = 16.3° and 22.5° decrease continuously. When the amount of the added AlN reaches 10%, a new diffraction peak appears at 2θ = 20.5°, and its intensity increases with increasing amount of the added AlN. When the amount of added AlN reaches 15%, a new diffraction peak appears at 2θ = 40°, and its intensity increases with increasing AlN content. This result indicates that the interaction between the CNF and AlN shifts the position of the diffraction peak. To study the effect of the amount of the added AlN on the thermal stability of the CNF-AlN nanocomposites, thermogravimetric analysis (TGA) was carried out on composites with different AlN content. The TGA samples were heated from 25 ℃ to 800 ℃ under a nitrogen atmosphere. As shown in Fig. 4d, pure AlN has almost no mass loss in this temperature range, indicating that AlN has high thermal stability and oxidation resistance. The initial mass loss of the CNF-AlN
stretching speed was set to 1 mm/min, the load cell was 80 N, and the distance was 20 mm. The samples were mounted between the fixtures and were measured in an ISO constant temperature and humidity laboratory conditions (23 ℃ and 50% relative humidity). The in-plane thermal diffusivity of the CNF-AlN composite (diameter of 25.4 mm) was measured using the laser flash method (NETZSCH LFA467, Germany). The thermal conductivity was calculated according to the following formula at room temperature of 25 ℃： TC = α * ρ* Ccomposites Ccomposites=CAlN*Ф+CCNF*(1−Ф) where TC is thermal conductivity, α is thermal diffusivity, ρ is density, C is specific heat capacity, and Ф is the volume fraction; the final result for thermal conductivity was obtained as the average of at least three measurements (Wu, Fang et al., 2017). 3. Results and discussion The repulsive force produced by the carboxyl groups between the CNF fibers gives rise to CNF suspension with good dispersion and high transparency and disperses the inorganic filler evenly and stably (Nie, Zhang, Lin et al., 2018; Vincent, Heintz, Silvain, & Chandra, 2011). The stability of the CNF-AlN suspension is essential for the formation of a uniform composite film. As Fig. 1a, the suspension of the pure CNF and obtained by ultrasonic blending of AlN and CNF are stabilized for more than one week. CNF has abundant surface hydroxyl groups, leading to easy adsorption on the AlN surface in the CNF-AlN suspensions. As observed from the TEM images of pure AlN and CNFs presented in Fig. 1b and c, respectively, the average diameter of the CNF is approximately 50 nm, and one CNF is several μm long. The average diameter of AlN is 70 nm, and its average length is approximately 200 nm. As observed from Fig. 1d, the pure CNF suspension with no AlN addition has light transmittance of 69% at the wavelength of 550 nm, whereas light transmittance of 55% is obtained for the suspension with 5 wt.% added AlN. When the amount of the added AlN added is increased further, the UV transmittance of the CNF-AlN suspension gradually decreases due to the increasing amount of added AlN. When the amount of AlN is 20 wt.%, the transmittance at 550 nm is only 6%. When the AlN content is 50 wt.%, the transmittance is less than 1%, and the whiteness of the composite material reaches 85% ISO. A paper with customized optical properties can be used for many purposes. For example, 5 wt.% AlN composite paper can be used as a transparent substrate for flexible electronic display products. The composite paper with 50 wt.% AlN can be used as an opaque substrate for flexible solar panels. After the CNF-AlN suspension was uniformly dispersed by ultrasonication, the CNF-AlN composite films were prepared by vacuum filtration (Fig. 2a). A series of CNF-AlN composite films can be prepared by controlling the amount of added AlN to vary from 0 to 50 wt.%. As shown in Fig. 2b, the pure CNF paper has good light transmittance, and the light transmittance decreases with the increase in the amount of the added AlN. When the AlN addition exceeds 5 wt.%, the CNF-AlN composite paper becomes opaque. In fact, the transparency of the CNFAlN composite does not affect its potential for use as a substrate for cooling electronic devices. When the AlN addition exceeds 50 wt.%, it is difficult to form a CNF-AlN composite paper. Fig. 3 shows the SEM images of a CNF-AlN nanocomposite. The cross-sectional structure of a composite material has a certain effect on the thermal conductivity and mechanical properties of the material itself. Fig. 3a presents an SEM image of a cross-section of a pure CNF film (thickness: 32.82 μm) showing a densely packed layered structure. Pure CNF films can achieve high mechanical strength, which is closely related to the dense layered structure (Zeng et al., 2017). Fig. 3b shows an SEM image of the cross-section of the CNF-25%AlN composite film (thickness: 44.39 μm). It is clearly observed that the addition of AlN 233
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nanocomposites is found at approximately 100 ℃ and is mainly due to the elimination of the moisture on the CNF surface. The pyrolysis process of the composite material mainly occurs at approximately 350 ℃, at which point the CNF are essentially completely decomposed. With increasing AlN content, the mass loss of the nanocomposites decreases, showing that the addition of AlN increases the thermal stability of the CNF-AlN nanocomposites. This effect is because the thermal conductivity (170 W/mK) of AlN is much higher than that of CNF (0.03 W/ mK), so AlN in the composite can better absorb heat and limits the movement of heat to the CNF polymer chain, resulting in the CNF polymer chains beginning to degrade at higher temperatures (Yang, Hsiang, Huang, Huang, & Han, 2017). In addition, the interaction of AlN with CNF also increases the thermal resistance of the nanocomposite and reduces the heat loss. Flexibility and strength are important indicators for the evaluation of the mechanical properties of thermally conductive materials. Fig. 4e demonstrates the good flexibility of the CNF-AlN composite obtained in this study. It is observed from Fig. 4e that the tensile strength of the nanocomposite is 116.1 MPa when the amount of added AlN is 5%. The tensile strength of the composite decreases with the increase in the AlN amount. It should be noted that AlN is highly brittle and is not easily interwoven into a film. In the CNF-AlN composite, CNF acts as an adhesive to tightly bind the AlN nanosheets. The mixing of CNF and AlN is conducive to better alignment of the AlN nanosheets, enabling the full interconnection of AlN with itself and between AlN and the CNF. At the same time, a large number of amino groups (eNH2) and hydroxyl groups (eOH) are present at the edge of AlN, which easily form hydrogen bonds with the hydroxyl groups (eOH) and carboxyl groups (eCOOH) found on the CNF surface(Zeng et al., 2017), leading to the formation of a high-mechanical-strength nanocompositevia hydrogen bonding interactions. In addition, the CNF-AlN composites form a compact, dense and regular layered three-dimensional structure under vacuum suction filtration under high-vacuum conditions. This ordered and uniform structure formed by the strong interfacial interaction between the CNF and AlN is similar to the layered structure of pearls. This special structure is another important factor that contributes to the formation of nanocomposites with high mechanical strengths. The in-plane and cross-plane thermal conductivities of the composites with different AlN additions were analyzed to study the thermal conductivity of the CNF-AlN nanocomposites. Table 1 shows the parameters of the thermal conductivity and the test conditions for the composite materials at room temperature of 25 ℃. The thermal conductivity is calculated as shown in Fig. 5. It is observed that both the inand cross-plane thermal conductivities of the CNF-AlN nanocomposites show an upward trend. In particular, the increase in the in-plane thermal conductivity is more pronounced (Fig. 5a). An examination of the data presented in Table 1 shows that the thermal conductivity of the pure CNF film in the plane is 1.24 W/mK, which is almost five times higher that of the conventional high-molecular-weight polymer. Uetani et al. studied the thermal conductivity of the CNF in different crystal forms and found that a higher crystallinity of the CNF leads to a higher thermal conductivity (Uetani, Okada, & Oyama, 2015). This reason is because the in-plane direction is favorable for phonon transport for a highly crystalline CNF (Wu, Fang et al., 2017). When the AlN addition amount is 25%, the thermal conductivity of the composite film reaches 4.20 W/mK; this result is related to the inherent high thermal conductivity of the pure AlN. The 1D CNF decrease the contact between the AlN nanosheets. After the ultrasonic vacuum filtration process of the CNF-AlN suspension, the AlN nanosheets are arranged neatly and evenly, providing a greater likelihood of contacting and overlapping each other. Thus, an efficient and continuous AlN network structure can be formed that greatly reduces the in-plane thermal resistance (Fig. 5b). However, the thermal conductivity of the CNF-AlN composite is not significantly increased in the cross-plane direction. The thermal conductivity of the pure CNF film is 0.19 W/mK and is only 0.39 W/mK with an AlN content of 25%. The cross-plane thermal conductivity is
one order of magnitude lower than the in-plane thermal conductivity. This result may be due to the higher anisotropy of the nanocomposite thermal conductivity so that the in-plane phonon transport is easier than the cross-plane phonon transport (Zeng et al., 2017). In order to further prove the potential performance of CNF-AlN nanocomposite films as a thermal management materials in flexible energy storage devices, the crystal light-emitting diode (LED) chips were attached to CNF film and CNF-25%AlN composite film (Fig. 6a). The LED was served as the main heat source, and the surface temperature variations of CNF and CNF-25%AlN film at central area close to the chip and 10 mm close to the chip with the LED working for 300 s were measured by using an infrared thermomete. Fig. 6b and c showed the surface temperature variations of the composite film at central area close to the LED chip and 10 mm close to the LED chip, respectively. It can be seen that the heating rate of CNF film was higher than that of CNF-25% AlN film at both central area close to the LED chip and 10 mm close to the LED chip. The final temperature of CNF film is also higher than that of CNF-25%AlN film. The CNF film showed a higher surface temperature of 47.8℃ than that of CNF-25%AlN film (40.1℃) at the central area close to the chip. Similarly, the CNF film showed a higher surface temperature of 31.6℃ than that of CNF-25%AlN film (29.2℃) at 10 mm close to the chip. These results indicated that the CNF-AlN composite film exhibits excellent heat dissipation performance, and it can effectively transfer the heat out, so that the composite can achieve a good goal of cooling. On the basis of the above results, the CNF-AlN composite film with highly thermal conductivity can effectively promote potential applications as lateral heat spreaders in flexible energy storage devices and may facilitate the applications in thermal management. 4. Conclusions Under ultrasonication, AlN was uniformly and stably dispersed in the CNF suspension by strong interactions. Then, the compact, dense, light and regular-layered three-dimensional structure of the CNF-AlN nanocomposite material was obtained by vacuum filtration. The composite exhibits good thermal conductivity, thermal stability and high mechanical strength. The in-plane thermal conductivity reached 4.20 W/mK when the amount of the added AlN was 25 wt.%. The addition of AlN increases the thermal impedance of the nanocomposite, reducing its mass loss in high-temperature environments and improving its thermal stability. Based on these excellent properties, CNF-AlN nanocomposites are expected to be suitable for operation in extreme environments and are excellent substrates for flexible electronic devices. However, the incorporation of AlN nanosheets led composite films inevitably bereaved of transparency. Therefore, this highly thermal conductivity flexible NFC/AlN composite films can serve as a lateral heat spreader in the thermal management applications where film transparency were not strictly required. Acknowledgements This project was supported by the National Natural Science Foundation of China (31760192) and the Guangxi Natural Science Foundation of China (2018GXNSFDA281050). References Balachander, N., Seshadri, I., Mehta, R. J., Schadler, L. S., Borca-Tasciuc, T., Keblinski, P., ... Ramanath, G. (2013). Nanowire-filled polymer composites with ultrahigh thermal conductivity. Applied Physics Letters, 102(9), 093117. Chen, H., Ginzburg, V. V., Yang, J., Yang, Y., Liu, W., Huang, Y., ... Chen, B. (2016). Thermal conductivity of polymer-based composites: Fundamentals and applications. Progress in Polymer Science, 59, 41–85. Chen, J., Huang, X., Zhu, Y., & Jiang, P. (2017). Cellulose nanofiber supported 3D interconnected BN nanosheets for epoxy nanocomposites with ultrahigh thermal management capability. Advanced Functional Materials, 27(5), 1604754. Dai, Y., Song, X., Gao, C., He, S., Nie, S., & Qin, C. (2016). Xylanase-aided chlorine
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