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Nanofibrillated Cellulose/MgO@rGO composite films with highly anisotropic thermal conductivity and electrical insulation ⁎
Meng Ma1, Lin Xu1, Lele Qiao, Si Chen, Yanqin Shi, Huiwen He, Xu Wang College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, PR China
H I GH L IG H T S
hybrid thermal conductive filler • The (MgO@rGO) was prepared by in-situ reaction.
NFC composite films with highly ani• sotropic thermal conductivity were
G R A P H I C A L A B S T R A C T
Nanofibrillated cellulose (NFC)-based composite films were prepared via a facile vacuum-assisted filtration and mechanical compression. High thermal conductivity anisotropy was achieved by the layered structure. MgO nano-particles were introduced into NFC composites to maintain electrical insulation of the films which exhibited superior electrical resistivity above 1011 Ω·m.
prepared.
reduced the interface thermal • MgO resistance of NFC composite film. NFC composite films maintain • The electrical insulation. composite films facilitate the heat • NFC dissipation of electronic devices.
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanofibrillated cellulose MgO@rGO hybrid filler Heat dissipation Anisotropic thermal conductivity Electrical insulation Structure-performance relationship
Graphene, an ideal two-dimensional material with high thermal conductivity, is widely used in the field of thermal management. However, its excellent electrical conductivity becomes an inevitable obstacle for applying in the electronic fields where electrical insulation materials are demanded. Herein, the reduced graphene oxide (rGO) decorated by magnesium oxide (MgO) particles was used as the hybrid thermal conducting filler to prepare the high thermal conductivity and electrical insulation nanofibrillated cellulose (NFC)-based composite films via a facile vacuum-assisted filtration and mechanical compression. In this way, MgO nano-particles not only reduced the interface thermal resistance between rGO and NFC but also cut the electric conductive pathways of rGO to enhance the thermal conductivity and maintain electrical insulation of the films. Simultaneously, mechanical compression caused the compacted layered structure formed by vacuum filtration along the in-plane direction. Thus, the obtained composite film exhibited high thermal conductivity and anisotropy. The in-plane and cross-plane thermal conductivity reached 7.45 W/(m·K) and 0.32 W/(m·K), respectively, when the filler content increased to 20 wt%, along with a high thermal conductivity anisotropy of 23 and superior electrical resistivity above 1011 Ω·m. Moreover, in the simulation test of the infrared camera, it has been demonstrated that the composite film dissipated heat quickly by the surface temperature variations of light-emitting-diode (LED) chips fixed on the composite film substrate with time. Therefore, the NFC-based composite films have great application prospects in the heat dissipation of electronic devices.
1. Introduction Nowadays, with the development of technology, the modern
electronic devices tend to be lighted in weight, miniaturized in size and integrated in electronic component, resulting in accumulation of heat [1–3]. The heat dissipation of the electronic devices has become an
⁎
Corresponding author. E-mail address: [email protected] (X. Wang). 1 Meng Ma and Lin Xu contributed equally to this work. https://doi.org/10.1016/j.cej.2019.123714 Received 9 August 2019; Received in revised form 2 December 2019; Accepted 4 December 2019 1385-8947/ © 2019 Published by Elsevier B.V.
Please cite this article as: Meng Ma, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123714
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maintain electrical insulation of the films. Besides, the pretreatment of the filler made the thermal conductivity of final NFC/MgO@rGO composite films higher than that of NFC/MgO/rGO composite films. Furthermore, the combination of one-dimensional (1D) NFC, two-dimensional (2D) rGO, and three-dimensional (3D) MgO formed hierarchical composite films with the layered structure by vacuum-assisted filtration. Thus, the obtained NFC composite films exhibited highly anisotropic thermal conductivity and electrical insulation. At a loading of 20 wt%, the composite film showed a dramatically improved inplane thermal conductivity of 7.45 W/(m·K) compared to that of pure NFC film (1.02 W/(m·K)), accompanying with high surface electrical resistivity (1.96 × 1011 Ω) and volume electrical resistivity (3.01 × 1011 Ω·m), which would be the best choice for heat dissipation between adjacent electronic components.
important factor to evaluate their comprehensive performances because the heat accumulation will reduce the accuracy and service life of the devices, even be a fire hazard [4,5]. Therefore, it is of great significance to achieve the efficient heat transfer of the current electronics. Polymers are widely used for electronic packaging [6] due to their light mass, easy processability, excellent mechanical property, and insulation property [7–9]. Besides, nanofibrillated cellulose (NFC) [10–13] as the bio-based polymer material has attracted wide attention owing to its excellent biocompatibility, biodegradability, and insulation property. However, the poor heat-conductivity is the fatal weakness of the NFC used for electronic packaging. Thus, adding thermal conducting fillers is the easiest way to improve the heat dissipating capability of NFCbased composites [14–17]. There are many researches on thermal conductive fillers used in polymer materials [18–20]. In order to maintain the electrical insulating properties of polymer materials, researchers prefer to choose the fillers with poor electrical conductivity, such as aluminum oxide (Al2O3), zinc oxide (ZnO), magnesium oxide (MgO), aluminum nitride (AlN), boron nitride (BN) or other metal oxides, nitrides, and so on. Gao et al. [18]. reported that the geometry of the filler had a non-negligible effect on the improvement of the thermal conductivity. The results show that the thermal conductivity of the composite reached 2.05 W/ (m·K) higher than that of smaller spherical Al2O3 particles when 60 vol % spherical Al2O3 particles with diameter of 75 μm were added. Additionally, laminal fillers are easier to form the thermal network in the matrix compared with spheroidal fillers. Lee et al. [19]. prepared heat conductive epoxy resin loaded with AlN powder, which showed the thermal conductivity of 3.39 W/(m·K) with the AlN content of 57 vol%. Moreover, laminal fillers exhibit to be ideal thermal conductive fillers due to their unique two-dimensional structures which improve the inplane thermal conduction properties of the film materials. Zhang et al. [20]. used AlN nanosheets and cellulose nanofibrils to fabricate a thermally conductive film with the in-plane thermal conductivity of 4.20 W/(m·K) for 25 wt% of AlN. However, there is a limitation to further enhance the thermal conductivity of composite since the heat transfer ability of these insulating fillers is not so high. Consequently, the filler exhibiting a high thermal conductivity and electrical conductivity is gradually to be a research focus [21]. Song et al [15]. utilized nanodiamond (ND) to increase the thermal conductivity of NFC film. At a low ND content of 0.5 wt%, the film displayed a high in-plane thermal conductivity of 9.820 W/(m·K), which is far better than that of the NFC/AlN film containing 25 wt% AlN nanosheets as mentioned above. Besides, graphene [22], carbon nanotube (CNT) [23], and silver nanowires (AgNWs) [24] are the most typical materials with excellent thermal conductivity and electrical conductivity. Unfortunately, the addition of these fillers will inevitably lead to a significant improvement in the electrical conductivity of the composites, which will limit their application in electronic fields where electrical insulation is required. Thus, the problem how to keep the electrical insulation of the composites after conductive fillers added is indispensable. To solve this problem, in this paper, we used electrical insulating filler to decorate the surface of electrically conductive filler to prepare the hybrid fillers with good heat conduction and electrical insulation. Firstly, the graphene oxide (GO) decorated by magnesium hydroxide (Mg(OH)2) particles (Mg(OH)2@GO) was synthesized by chemical co-precipitation through strong interaction of hydroxyl groups between GO and Mg(OH)2. Subsequently, the thermal conducting compound filler of reduced graphene oxide (rGO) decorated by magnesium oxide (MgO) particles (MgO@rGO) was prepared by thermal reduction of Mg(OH)2@GO at a high temperature, avoiding the use of hydrazine hydrate, sodium borohydride or other toxic reducing agents. After calcination, there were strong interaction between MgO and rGO so that the interfacial thermal resistance in the composite films would decrease and the thermal conductivity would improve. In the dispersion liquid, the MgO nano-particles not only promoted the dispersion of rGO in NFC but also cut off the electric conductive pathways of rGO to
2. Experimental section 2.1. Materials Nanofibrillated celluloses were supplied by Zhongshan Nanofibrils New Material Co., Ltd. Raw graphite powder with an average size of 30 μm and a purity above 95% was purchased from Qindao Guangli Co., Ltd. Magnesium chloride hexahydrate (MgCl2·6H2O) was obtained from Taicang Meida Reagent Co., Ltd. Sodium nitrate (NaNO3, 99.0%, Guanghua Technology Co., Ltd.), sulfuric acid (H2SO4, 95.0%-98.0%, Xilong Scientific Co., Ltd.), potassium permanganate (KMnO4, 99.5%, Yonghua Chemical Technology Co., Ltd.), hydrogen peroxide (H2O2, 30.0%, Yonghua Chemical Technology Co., Ltd.), sodium hydroxide (NaOH, 96.0%, Xilong Scientific Co., Ltd.) and all other chemical regents were of analytical grade and were used without further purification. 2.2. Preparation of GO GO was prepared using the modified Hummers method [25]. First, 5 g of graphite powder and 3.75 g of sodium nitrate were added to a three-necked flask and pre-blended in an ice bath environment. Then, 200 mL pre-refrigerated concentrated H2SO4 was slowly added to the mixture, and a homogeneous solution was formed with mechanical stirring. After a few minutes, 40 g of KMnO4 was gradually added to the mixture with a 10 g/20 min feeding speed and the system was always in an ice bath when KMnO4 was added. The homogeneous solution was continued stirring for 24 h at room temperature. Next, 500 mL of dilute 5 wt% H2SO4 was delivered at a 3.3 mL/min flow rate with mechanical stirring. After 24 h, 26.97 mL H2O2 was added to the mixture and stirred overnight until no bubbles appear. At the same time, the mixture solution had turned golden. The reaction products were obtained after acid washing, water washing and freeze-drying. 2.3. Preparation of MgO@rGO MgO@rGO was preparation as follows. 0.1 g of GO was added into a deionized water solution under magnetic stirring and ultrasonic processing to obtain a homogeneous solution. Subsequently, 2 g of MgCl2·6H2O was poured to the GO solution and the resulting mixture was further stirred about 4 h at 80 °C. 100 mL of a NaOH solution (0.2 mol/L) was gradually added to the mixture at a 3.3 mL/min flow rate with the magnetic stirring continued for 12 h. The obtained mixture Mg(OH)2@GO was collected by centrifugation and freeze-drying. Finally, MgO@rGO was calcined 8 h at 600 °C under N2 protection. The preparation schematic for MgO@rGO was illustrated in Fig. 1. 2.4. Preparation of NFC/MgO@rGO films A certain amount of NFC, MgO@rGO and deionized water were mixed with a continuously ultrasonic treatment of 90 min at room 2
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Fig. 1. Schematic of the fabrication of NFC/MgO@rGO films.
thickness. The results presented were an average of at least five individual tests per specimen.
temperature to obtain the homogeneous NFC/MgO@rGO dispersion with the fixed concentration (NFC and MgO@rGO) of 5 mg/g. The NFC/MgO@rGO films were prepared by the vacuum-assisted filtration on PP filter membrane with the diameter of 50 mm and the pore size of 0.45 μm, respectively. Subsequently, the films were compressed at 50 kPa and dried at room temperature for 2 days. The preparation schematic for NFC/MgO@rGO films was illustrated in Fig. 1. The NFC/ MgO@rGO films with 5, 10, 15, and 20 wt% MgO@rGO were obtained by adjusting the weight ratio of NFC and MgO@rGO. Moreover, film samples for pure NFC and NFC/rGO were prepared with the same method mentioned above. For comparison, MgO@rGO was replaced by MgO/rGO to prepare NFC/MgO/rGO films following the same steps.
3. Results and discussion In this study, GO was prepared from the flake graphite through the modified Hummers method. In order to prove its successful synthesis, FTIR, XRD, and AFM were conducted. In the FTIR spectra of Fig. S1(a), the appearance of oxygen-containing functional groups (-OH, -COOH, -C-O-C-) confirmed the successful oxidation of graphite. Fig. S1(b) shows the XRD patterns of graphite and GO. The diffraction peak of graphite at 2θ = 26.5° disappeared after chemical oxidation and shifted to a lower position of 8.8°, corresponding to (0 0 2) plane of GO. Fig. S1(c) reveals the thin sheet topography of as-synthesized GO with a thickness of 1 nm, consisting with the characteristic of GO single layer sheet [26]. The reduction degree of GO will affect the thermal conductivity of the NFC composite films by influencing the inherent thermal conductivity of rGO. In order to reveal the surface state of MgO@rGO, the XPS was utilized with GO, rGO, and MgO@rGO for comparison, which was shown in Fig. 2(a–d). Compared with GO, Fig. 2(a) showed a significant increase in the C/O atomic ratio of rGO, indicating the successful reduction of GO. The C/O atomic ratio of MgO@rGO (0.87) is far below that of rGO (10.63), which is attributed to the existence of oxygen element in MgO. Two new peaks at 1304.6 eV and 305.6 eV corresponding to Mg1s and MgKLL appear respectively in the XPS spectrum of MgO@rGO, indicating that the MgO is successfully loaded on the rGO surface. To further prove the successful reduction of GO and the loading of MgO in MgO@rGO, XRD was used and the patterns of GO, rGO, MgO@ rGO were shown in Fig. 2(e). For GO, the diffraction peak at 2θ = 8.8° indicated the (0 0 2) plane. After thermal reduction, the XRD pattern of rGO shifted to a higher and broad peak which widely spread in range of 2θ = 16.4°~26.5°, implying the rGO was successfully prepared. Besides, it is a wide dispersion peak different from the crystalline graphite. And MgO@rGO exhibited the characteristic peaks at 2θ = 36.5°, 42.8°, 62.1°, 74.3°, and 78.3°, corresponding to (1 1 1), (2 0 0), (2 2 0), (3 3 1), and (2 2 2) lattice planes, which belonged to simple cubic system MgO materials (JCPDS no. 75-0447) [27]. To obtain the MgO mass fraction decorated on the surface of rGO, TGA was measured. As shown in Fig. 2(f), the weight loss of GO started below 100 °C, which is ascribed to the evaporation of absorbed water. Between 200 °C and 300 °C, the weight loss of GO is due to the thermal decomposition of the labile oxygen-containing groups. The weightlessness characteristic of GO in air atmosphere is approximately the same as that in nitrogen atmosphere before 300 °C as shown in Fig. S2. However, the images appear significantly different when the
2.5. Characterization The prepared GO was characterized by X-ray diffraction (XRD, Ultima IV, Rigaku) with Cu Ka radiation (λ = 0.154 nm), Fourier transform infrared (FTIR, Thermo fisher Nicolet 6700, USA) and atomic force microscope (AFM, Bruker Dimension, USA). The rGO and MgO@ rGO were also characterized by XRD. The morphologies of rGO and MgO@rGO were observed with field emission-scanning electron microscopy (FE-SEM, FEI Co., Netherlands) at 15 kV accelerating voltage and transmission electron microscopy (TEM, JEOLJEM-1010) at 80 kV. The oxygen content on the GO, rGO, and MgO@rGO were obtained by X-ray photoelectron spectroscopy (XPS, EscaLab 250X, Thermo Fisher, USA). The weight ratio of rGO and MgO was determined through thermogravimetric analysis (TGA, TA Q5000IR, USA) under the air atmosphere with a heating rate of 10 °C/min and the measurements were taken use of 5–10 mg samples,which were dried at 45 °C overnight under vacuum before testing. The thermal conductivities of the NFC/ MgO@rGO films were calculated by the formula TC = αCpρ. The thermal diffusivity (α) was conducted on a Netzsch LFA 467 Nanoflash at 25 °C. And the results presented were an average of at least three individual tests per film. The specific heat capacity (Cp) was characterized by DSC (TA Q2000, USA), in which the films were heated at the rate of 5 °C/min under the nitrogen atmosphere. The density (ρ) was calculated from the weight and volume of the sample. The surface temperatures of light-emitting-diode (LED) chips fixed on the pure NFC film and NFC composite film were recorded by an infrared thermograph (C3000, Hangzhou Meisheng Infrared Optoelectronic Technology Co., Ltd.). The surface electrical resistivity (ρs) and volume electrical resistivity (ρv) were conducted at room temperature using the volumetric surface resistance tester (EST-121, Beijing Guance Precision Electric Instrument Equipment Co., Ltd.). The stress–strain curves were tested on the Instron 5966 tester (USA) with a load cell of 500 N at room temperature with a speed of 10 mm/min. All the specimens were cut from the films and possessed uniform size of 30 × 4 mm with varying 3
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Fig. 2. XPS wide scan spectra of (a) GO, rGO, and MgO@rGO. XPS C1s spectra of (b) GO, (c) rGO, and (d) MgO@rGO. XRD patterns of (e) GO, rGO, and MgO@rGO. TGA curves in air atmosphere of (f) GO, rGO, and MgO@rGO.
film and 80NFC/20MgO@rGO film were measured by SEM, as shown in Fig. 4(c–d). The pure NFC film (Fig. 4(c)) presents a densely layered structure so that the heat prefers to deliver along the in-plane direction leading to the anisotropy of thermal conductivity. Meanwhile, the high mechanical property of pure NFC film is closely related to the tight alignment of celluloses. As shown in Fig. 4(d), the NFC-based composite film with 20 wt% MgO@rGO fillers still maintains this typical layered structure and the tight stacking of 1D NFC, 2D rGO, and 3D MgO components has a positive effect on the improvement of mechanical property of the film. In theory, the MgO@rGO fillers will arranged along the in-plane direction, but the MgO@rGO fillers are too small to be observed. Therefore, the elemental mappings of fracture surface of 80NFC/20MgO@rGO film exhibit uniform dispersion, as shown in Fig. 4(e–f). For the surface of 80NFC/20MgO@rGO film, there is no doubt that the fillers are well dispersed in NFC as shown in Fig. S5. The thermal conductivities of NFC film and NFC/MgO@rGO composite films filled with different filler content were measured by flash method at 25 °C, and the results were shown in Fig. 5. The NFC is a poor conductor [28,29] of heat with the λx (in-plane direction) of 1.02 W/ (m·K) and the λz (cross-plane direction) of 0.02 W/(m·K). The thermal conductivity of NFC/MgO@rGO composite films increased significantly when the thermal conductive filler was added, as shown in Fig. 5(a). The λx and λz increased greatly up to 7.45 W/(m·K) and 0.32 W/(m·K), respectively, when the MgO@rGO addition amount was 20 wt%. There appear to be different from the λx to λz at the same thermal conductivity filler content, which is attributed to the film-forming process. During the vacuum-assisted filtration, the NFC and MgO@rGO were arranged layer by layer under the gravity action so that the MgO@rGO was inclined to lap each other along the plane, accompanied by the formation of the thermally conductive network which was depicted in Fig. 5(c). However, the interface thermal resistance between the NFC and MgO@rGO was the main cause of the low efficiency of the heat dissipation, leading to the poor thermal conductivity cross the plane of the film. The thermal conductivity anisotropy of the NFC/MgO@rGO films is shown in Fig. S6. When the filler content ranges from 0 to 20 wt %, the NFC composite films always maintain gratifying anisotropy of thermal conductivity which is expected to avoid the influence of
temperature continued to increase. The curve of GO becomes stable in the nitrogen atmosphere, indicating that the thermal reduction was basically completed. In contrast, the GO in the air atmosphere was completely burnt without residue. After thermal reduction, the rGO is very stable at low temperature with nearly no mass loss and exhibits only one main weight loss step in air atmosphere above 450 °C, which is due to the decomposition of the carbon skeleton. As for MgO@rGO, the law of mass loss by heating is in accordance with rGO. There is just the carbon skeleton of the rGO decomposition while the MgO persists. According to the thermogravimetric residues of the above samples, it can be concluded that the quantity of MgO decorated on the rGO was about 87.0 wt%. Moreover, the SEM and TEM were employed to analyze the surface morphology of rGO and MgO@rGO, as shown in Fig. 3. There is a distinct difference that rGO (Fig. 3(a–b)) exhibits a relatively smooth surface without any impurities while the thin sheet surface of MgO@ rGO (Fig. 3(d–e)) is decorated with some MgO nano-particles. In addition, the TEM image of rGO (Fig. 3(c)) shows a typically wrinkled structure and the size of rGO sheet is about 2–3 μm. Comparatively, the size of MgO@rGO (Fig. 3(f)) measured by TEM is mainly consistent with that of rGO, as shown in Fig. 3(e). As shown in Fig. 3(f), the MgO nano-particles of MgO@rGO tend to concentrate on the surface of rGO rather than existing alone after ultrasonic dispersion, which indicate that there is strong interaction between MgO and rGO. As observed from Fig. S3, the size of MgO nano-particles is in the range from 10 to 70 nm and the average diameter of the MgO is approximately 30 nm. To further investigate the thickness of the hybrid thermally conductive fillers, the atomic force microscope (AFM) was used and the results were shown in Fig. S4. The thickness of the hybrid filler sheet is approximately 2–7 nm due to the surface loading of MgO nanoparticles. The NFC-based composite films with a series of MgO@rGO content were prepared by the vacuum-assisted filtration. The optical images of the pure NFC film and 80NFC/20MgO@rGO film are displayed in Fig. 4(a-b). The pure NFC film has high optical transparency, while the light transmittance decreases rapidly with the addition of the MgO@ rGO. In order to study the relationship between internal structure of the film and its property, the fractured surface morphology of pure NFC 4
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Fig. 3. SEM images of (a-b) rGO and (d-e) MgO@rGO. TEM images of (c) rGO and (f) MgO@rGO.
applications in electrical apparatus [30–32]. Fig. 7 presents the surface electrical resistivity (ρs) and volume electrical resistivity (ρv) of the NFC and NFC-based composites. Pure NFC film has excellent electrical insulation performance, with the ρs and ρv to be 3.17 × 1011 Ω and 1.22 × 1013 Ω·m, respectively. As for NFC/rGO films, no matter surface electrical resistivity or volume electrical resistivity, the results show that there is a decline of at least 7 orders of magnitude even if the rGO content is only 5 wt%, which is ascribed to the excellent electrical conductivity of rGO. As a contrast, NFC/MgO@rGO films exhibit the excellent electrical resistivity, even when the filler content reaches up to 20 wt%, the NFC/MgO/rGO films also maintain excellent electrical insulation property due to a large amount of MgO which acts as a barrier to prevent the forming of rGO conductive network. Based on the above results, we proposed the heat conduction mechanism of the NFC-based composite films. The addition of rGO can form a heat conduction way to increase the thermal conductivity of films. The MgO plays an important role in cutting electric conductive pathways off so as to maintain the electrical insulation property of the composite films. To further illustrate the heat transfer process of the film, the schematic diagram is depicted in Fig. 8. For NFC/rGO films, the heat transfer path and conductive path can be formed by the overlap of closely arranged rGO. Since the percolation threshold of conductive path is far lower than that of the heat path, the improvement of thermal conductivity will inevitably accompany with the rapid ascension of electrical conductivity. In order to keep the electrical insulation of the NFC composite film, MgO filler which has good thermal conductivity and excellent electrical resistance property is added. However, the NFC/MgO/rGO film prepared by directly mixing displays a big interface thermal resistance between NFC and MgO (Rth-1), rGO and MgO (Rth-2), NFC and rGO (Rth-3). As we known, phonon transfer [33] is the main way of thermal transport in the insulating materials. But at the interfaces, phonon scattering [34] would hinder the heat transfer. Additionally, the improvement of thermal conductivity by reducing phonon scattering is a desirable way. Thus, the in-situ growth of MgO decorated on the surface of rGO will reduce the interface thermal resistance and phonon scattering to obtain the better thermal conductivity. For the comparison of the in-plane thermal conductivities and electric insulation properties of our NFC/MgO@rGO composite film with those previously reported thermally conductive films, the results
neighboring electronic components because the heat tends to dissipate along the in-plane direction. In order to demonstrate the decorated of MgO on the surface of rGO is necessary, NFC/rGO films and NFC/MgO/ rGO films were prepared by directly mixing of NFC, rGO, and MgO as comparison samples. The thermal conductivity of these films was shown in Figs. 5(b) and S7. For 80NFC/rGO film, the thermal conductivity is slightly higher than that of 80NFC/20MgO@rGO film due to the difference in inherent thermal conductivity between rGO and MgO. Besides, the 80NFC/20MgO/rGO film exhibits slightly improvement in thermal conductivity due to the non-negligible increasement of interface thermal resistance, compared to 80NFC/20rGO film and 80NFC/20MgO@rGO film. As shown in Fig. S7, the thermal conductivity of NFC/MgO/rGO films also exhibits anisotropy, but lower than that of NFC/MgO@rGO films at the same filler content. Thus, the pre-treatment of thermal conductive filler is indispensable. In order to visually assess the heat dissipation ability of NFC-based composite film, the pure NFC film and 80NFC/20MgO@rGO film were used as the substrate to fix LED chips. The surface temperatures of LED chips were monitored by an infrared thermal imager. The optical and thermal images of the LED chips are shown in Fig. 6 and the specific temperature values are listed in Table 1. The temperature point 1 (PO1) is the maximum temperature of the LED chip fixed on pure NFC film substrate while the temperature point 2 (PO2) is the maximum temperature of the LED chip fixed on 80NFC/20MgO@rGO film substrate. Before electrifying, PO1 and PO2 are almost identical at around 38 °C. A significant rise in temperature occurs and continuously increases with time when the LED chips start to work. Besides, POI is always higher than PO2 at the same time and the difference value between them gradually increases over time. For example, the PO1 reached up to 53.8 °C and PO2 reached up to 50.1 °C while the temperature difference value was calculated to be 3.7 °C when the LED chips worked for 1 min. The temperature difference value was raised to 8.4 °C when the LED chips worked for 3 min. This is due to the higher thermal conductivity of 80NFC/20MgO@rGO film than that of pure NFC film as mentioned above. The LED chip fixed on 80NFC/20MgO@rGO film substrate can keep the surface temperature at low lever because of the composite film’s ability to dissipate heat timely. Therefore, NFC/MgO@rGO composite films show a great potential in the application for heat dissipation of electronic devices. The electric insulation property is primary requested for 5
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Fig. 4. Optical image of the prepared pure NFC film (a) and 80NFC/20MgO@rGO film (b). SEM images of fracture surfaces of the pure NFC film (c) and 80NFC/ 20MgO@rGO film (d). C elemental mapping (e) and Mg elemental mapping (f) of fracture surface of 80NFC/20MgO@rGO film.
properties as shown in Fig. S9. To improve the mechanical properties of the NFC composite films, it is a good choice to enhance the interfacial adhesion between matrix and filler by adding a binder such as dopamine (DA) and tannic acid (TA) in subsequent research. At least, the mechanical properties of NFC-based composite films are in an acceptable range due to the conservation of compacted layered structure. Meanwhile, the anisotropic thermal conductivity of composite films is also derived from this layered structure.
are summarized in Table 2. Obviously, the insulating nitride is preferred to add into polymer for increasing the thermal conductivity. In fact, the results are unsatisfactory with high filling of thermal conducting fillers. For example, Zhang et al. [20] used AlN as fillers to improve the thermal conductivity of CNF film and the CNF/AlN film reached 4.2 W/(m·K) with 25 wt% fillers adding. Song et al. utilized GP [37] and RGO [36] as the fillers to accelerate the heat transfer of NFC film but with excellent electrical conductivity. In this work, NFC/ MgO@rGO film exhibited an expected anisotropic thermal conductivity and electrical insulation. In practical application, mechanical properties [39–42] are one of the most important characteristics of the materials. The typical stress − strain curves of NFC/MgO@rGO composite films are shown in Fig. S8. The results indicate that there is a slightly deterioration of the composite films compared to the pure NFC film. The pure NFC film is one of the degradable polymers with high strength, existing a tensile strength of 99.6 MPa and an elongation at break of 4.7%. After adding MgO@rGO, the tensile strength of the composite films shows a downward trend due to the weak interface adhesion between NFC matrix and MgO@rGO fillers. The SEM images of fracture surfaces of pure NFC film and NFC composite films exhibit some voids leading to poor mechanical
4. Conclusions In summary, NFC-based nanocomposite films filled with different content of MgO@rGO were successfully prepared by the vacuum-assisted filtration. Decorating the MgO onto the surface of rGO can not only facilitate the homogeneous dispersion of rGO fillers in the NFC matrix but also prevent the formation of electric conductive pathways without affecting the thermal conductive pathways. When the content of MgO@rGO filler increased to 20 wt%, the in-plane thermal conductivity of the composite film raised to 7.45 W/(m·K), which is 7.3 times that of pure NFC. Meanwhile, the cross-plane thermal conductivity of the composite film was only 0.32 W/(m·K), far lower than 6
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Fig. 5. Thermal conductivity of the NFC/MgO@rGO films (a), NFC composite films (b), and schematic diagram of thermal conductivity of the NFC composite films (c).
Fig. 6. Optical and thermal images of the LED chips fixed on the pure NFC film and 80NFC/20MgO@rGO film substrate.
composite films exhibit superior surface electrical resistivity above 1011 Ω and superior volume electrical resistivity above 1011 Ω·m. Therefore, the prepared composite films with excellent thermal conductivities and electrical insulating properties can be widely used in the field of electronic devices.
Table 1 The temperatures of the LED chips fixed on the pure NFC film and 80NFC/ 20MgO@rGO film substrate at 12 V. Time (min)
Temperature Point 1 (oC) (pure NFC film substrate)
Temperature Point 2 (oC) (80NFC/ 20MgO@rGO film substrate)
0.0 0.5 1.0 2.0 3.0
38.1 53.8 58.0 60.3 62.4
38.3 50.1 52.8 53.9 54.0
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
that of in-plane value, which would reduce the impact on adjacent electronic components due to the anisotropy of the thermal conductivity when using in thermal management. Moreover, NFC-based
Acknowledgments This work was supported by the National Natural Science 7
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Fig. 7. The surface electrical resistivity (a) and volume electrical resistivity (b) of the NFC composite films.
Fig. 8. Schematic diagram of thermal conductivities and electrical resistivities of NFC composite films. Table 2 Comparison of in-plane TC and electric insulation of this work with other polymer-based thermally conductive films. composites
filler content (wt%)
in-plane TC (W·m−1·K−1)
PVDF/m-BN CNF/AlN RGO/CNF NFC/GP NFC/FCNT Al2O3–AgNP/epoxy-H NFC/MgO@rGO
30 25 50 6 35 50 20
7.29 4.2 7.3 9.0 14.1 6.71 7.45
η (λx/λz)
Insulator (yes or no)
test method
yearreferences
11 56
Y Y N
LFA467 LFA467 LFA 447 LFA 447 LFA 467 LFA 467 LFA 467
2018 [35] 2019 [20] 2017 [36] 2017 [37] 2018 [17] 2017 [38] this work
21 23
Y Y Y
References
Foundation of China (grant no. 21504078), the Natural Science Foundation of Zhejiang Province (grant no. LY20E030008), the Research Project of Canal Cup of Zhejiang University of Technology, and the China Scholarship Council.
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Nanofibrillated Cellulose/MgO@rGO composite films with highly anisotropic thermal conductivity and electrical insulation ⁎
Meng Ma1, Lin Xu1, Lele Qiao, Si Chen, Yanqin Shi, Huiwen He, Xu Wang College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, PR China
H I GH L IG H T S
hybrid thermal conductive filler • The (MgO@rGO) was prepared by in-situ reaction.
NFC composite films with highly ani• sotropic thermal conductivity were
G R A P H I C A L A B S T R A C T
Nanofibrillated cellulose (NFC)-based composite films were prepared via a facile vacuum-assisted filtration and mechanical compression. High thermal conductivity anisotropy was achieved by the layered structure. MgO nano-particles were introduced into NFC composites to maintain electrical insulation of the films which exhibited superior electrical resistivity above 1011 Ω·m.
prepared.
reduced the interface thermal • MgO resistance of NFC composite film. NFC composite films maintain • The electrical insulation. composite films facilitate the heat • NFC dissipation of electronic devices.
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanofibrillated cellulose MgO@rGO hybrid filler Heat dissipation Anisotropic thermal conductivity Electrical insulation Structure-performance relationship
Graphene, an ideal two-dimensional material with high thermal conductivity, is widely used in the field of thermal management. However, its excellent electrical conductivity becomes an inevitable obstacle for applying in the electronic fields where electrical insulation materials are demanded. Herein, the reduced graphene oxide (rGO) decorated by magnesium oxide (MgO) particles was used as the hybrid thermal conducting filler to prepare the high thermal conductivity and electrical insulation nanofibrillated cellulose (NFC)-based composite films via a facile vacuum-assisted filtration and mechanical compression. In this way, MgO nano-particles not only reduced the interface thermal resistance between rGO and NFC but also cut the electric conductive pathways of rGO to enhance the thermal conductivity and maintain electrical insulation of the films. Simultaneously, mechanical compression caused the compacted layered structure formed by vacuum filtration along the in-plane direction. Thus, the obtained composite film exhibited high thermal conductivity and anisotropy. The in-plane and cross-plane thermal conductivity reached 7.45 W/(m·K) and 0.32 W/(m·K), respectively, when the filler content increased to 20 wt%, along with a high thermal conductivity anisotropy of 23 and superior electrical resistivity above 1011 Ω·m. Moreover, in the simulation test of the infrared camera, it has been demonstrated that the composite film dissipated heat quickly by the surface temperature variations of light-emitting-diode (LED) chips fixed on the composite film substrate with time. Therefore, the NFC-based composite films have great application prospects in the heat dissipation of electronic devices.
1. Introduction Nowadays, with the development of technology, the modern
electronic devices tend to be lighted in weight, miniaturized in size and integrated in electronic component, resulting in accumulation of heat [1–3]. The heat dissipation of the electronic devices has become an
⁎
Corresponding author. E-mail address: [email protected] (X. Wang). 1 Meng Ma and Lin Xu contributed equally to this work. https://doi.org/10.1016/j.cej.2019.123714 Received 9 August 2019; Received in revised form 2 December 2019; Accepted 4 December 2019 1385-8947/ © 2019 Published by Elsevier B.V.
Please cite this article as: Meng Ma, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123714
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maintain electrical insulation of the films. Besides, the pretreatment of the filler made the thermal conductivity of final NFC/MgO@rGO composite films higher than that of NFC/MgO/rGO composite films. Furthermore, the combination of one-dimensional (1D) NFC, two-dimensional (2D) rGO, and three-dimensional (3D) MgO formed hierarchical composite films with the layered structure by vacuum-assisted filtration. Thus, the obtained NFC composite films exhibited highly anisotropic thermal conductivity and electrical insulation. At a loading of 20 wt%, the composite film showed a dramatically improved inplane thermal conductivity of 7.45 W/(m·K) compared to that of pure NFC film (1.02 W/(m·K)), accompanying with high surface electrical resistivity (1.96 × 1011 Ω) and volume electrical resistivity (3.01 × 1011 Ω·m), which would be the best choice for heat dissipation between adjacent electronic components.
important factor to evaluate their comprehensive performances because the heat accumulation will reduce the accuracy and service life of the devices, even be a fire hazard [4,5]. Therefore, it is of great significance to achieve the efficient heat transfer of the current electronics. Polymers are widely used for electronic packaging [6] due to their light mass, easy processability, excellent mechanical property, and insulation property [7–9]. Besides, nanofibrillated cellulose (NFC) [10–13] as the bio-based polymer material has attracted wide attention owing to its excellent biocompatibility, biodegradability, and insulation property. However, the poor heat-conductivity is the fatal weakness of the NFC used for electronic packaging. Thus, adding thermal conducting fillers is the easiest way to improve the heat dissipating capability of NFCbased composites [14–17]. There are many researches on thermal conductive fillers used in polymer materials [18–20]. In order to maintain the electrical insulating properties of polymer materials, researchers prefer to choose the fillers with poor electrical conductivity, such as aluminum oxide (Al2O3), zinc oxide (ZnO), magnesium oxide (MgO), aluminum nitride (AlN), boron nitride (BN) or other metal oxides, nitrides, and so on. Gao et al. [18]. reported that the geometry of the filler had a non-negligible effect on the improvement of the thermal conductivity. The results show that the thermal conductivity of the composite reached 2.05 W/ (m·K) higher than that of smaller spherical Al2O3 particles when 60 vol % spherical Al2O3 particles with diameter of 75 μm were added. Additionally, laminal fillers are easier to form the thermal network in the matrix compared with spheroidal fillers. Lee et al. [19]. prepared heat conductive epoxy resin loaded with AlN powder, which showed the thermal conductivity of 3.39 W/(m·K) with the AlN content of 57 vol%. Moreover, laminal fillers exhibit to be ideal thermal conductive fillers due to their unique two-dimensional structures which improve the inplane thermal conduction properties of the film materials. Zhang et al. [20]. used AlN nanosheets and cellulose nanofibrils to fabricate a thermally conductive film with the in-plane thermal conductivity of 4.20 W/(m·K) for 25 wt% of AlN. However, there is a limitation to further enhance the thermal conductivity of composite since the heat transfer ability of these insulating fillers is not so high. Consequently, the filler exhibiting a high thermal conductivity and electrical conductivity is gradually to be a research focus [21]. Song et al [15]. utilized nanodiamond (ND) to increase the thermal conductivity of NFC film. At a low ND content of 0.5 wt%, the film displayed a high in-plane thermal conductivity of 9.820 W/(m·K), which is far better than that of the NFC/AlN film containing 25 wt% AlN nanosheets as mentioned above. Besides, graphene [22], carbon nanotube (CNT) [23], and silver nanowires (AgNWs) [24] are the most typical materials with excellent thermal conductivity and electrical conductivity. Unfortunately, the addition of these fillers will inevitably lead to a significant improvement in the electrical conductivity of the composites, which will limit their application in electronic fields where electrical insulation is required. Thus, the problem how to keep the electrical insulation of the composites after conductive fillers added is indispensable. To solve this problem, in this paper, we used electrical insulating filler to decorate the surface of electrically conductive filler to prepare the hybrid fillers with good heat conduction and electrical insulation. Firstly, the graphene oxide (GO) decorated by magnesium hydroxide (Mg(OH)2) particles (Mg(OH)2@GO) was synthesized by chemical co-precipitation through strong interaction of hydroxyl groups between GO and Mg(OH)2. Subsequently, the thermal conducting compound filler of reduced graphene oxide (rGO) decorated by magnesium oxide (MgO) particles (MgO@rGO) was prepared by thermal reduction of Mg(OH)2@GO at a high temperature, avoiding the use of hydrazine hydrate, sodium borohydride or other toxic reducing agents. After calcination, there were strong interaction between MgO and rGO so that the interfacial thermal resistance in the composite films would decrease and the thermal conductivity would improve. In the dispersion liquid, the MgO nano-particles not only promoted the dispersion of rGO in NFC but also cut off the electric conductive pathways of rGO to
2. Experimental section 2.1. Materials Nanofibrillated celluloses were supplied by Zhongshan Nanofibrils New Material Co., Ltd. Raw graphite powder with an average size of 30 μm and a purity above 95% was purchased from Qindao Guangli Co., Ltd. Magnesium chloride hexahydrate (MgCl2·6H2O) was obtained from Taicang Meida Reagent Co., Ltd. Sodium nitrate (NaNO3, 99.0%, Guanghua Technology Co., Ltd.), sulfuric acid (H2SO4, 95.0%-98.0%, Xilong Scientific Co., Ltd.), potassium permanganate (KMnO4, 99.5%, Yonghua Chemical Technology Co., Ltd.), hydrogen peroxide (H2O2, 30.0%, Yonghua Chemical Technology Co., Ltd.), sodium hydroxide (NaOH, 96.0%, Xilong Scientific Co., Ltd.) and all other chemical regents were of analytical grade and were used without further purification. 2.2. Preparation of GO GO was prepared using the modified Hummers method [25]. First, 5 g of graphite powder and 3.75 g of sodium nitrate were added to a three-necked flask and pre-blended in an ice bath environment. Then, 200 mL pre-refrigerated concentrated H2SO4 was slowly added to the mixture, and a homogeneous solution was formed with mechanical stirring. After a few minutes, 40 g of KMnO4 was gradually added to the mixture with a 10 g/20 min feeding speed and the system was always in an ice bath when KMnO4 was added. The homogeneous solution was continued stirring for 24 h at room temperature. Next, 500 mL of dilute 5 wt% H2SO4 was delivered at a 3.3 mL/min flow rate with mechanical stirring. After 24 h, 26.97 mL H2O2 was added to the mixture and stirred overnight until no bubbles appear. At the same time, the mixture solution had turned golden. The reaction products were obtained after acid washing, water washing and freeze-drying. 2.3. Preparation of MgO@rGO MgO@rGO was preparation as follows. 0.1 g of GO was added into a deionized water solution under magnetic stirring and ultrasonic processing to obtain a homogeneous solution. Subsequently, 2 g of MgCl2·6H2O was poured to the GO solution and the resulting mixture was further stirred about 4 h at 80 °C. 100 mL of a NaOH solution (0.2 mol/L) was gradually added to the mixture at a 3.3 mL/min flow rate with the magnetic stirring continued for 12 h. The obtained mixture Mg(OH)2@GO was collected by centrifugation and freeze-drying. Finally, MgO@rGO was calcined 8 h at 600 °C under N2 protection. The preparation schematic for MgO@rGO was illustrated in Fig. 1. 2.4. Preparation of NFC/MgO@rGO films A certain amount of NFC, MgO@rGO and deionized water were mixed with a continuously ultrasonic treatment of 90 min at room 2
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Fig. 1. Schematic of the fabrication of NFC/MgO@rGO films.
thickness. The results presented were an average of at least five individual tests per specimen.
temperature to obtain the homogeneous NFC/MgO@rGO dispersion with the fixed concentration (NFC and MgO@rGO) of 5 mg/g. The NFC/MgO@rGO films were prepared by the vacuum-assisted filtration on PP filter membrane with the diameter of 50 mm and the pore size of 0.45 μm, respectively. Subsequently, the films were compressed at 50 kPa and dried at room temperature for 2 days. The preparation schematic for NFC/MgO@rGO films was illustrated in Fig. 1. The NFC/ MgO@rGO films with 5, 10, 15, and 20 wt% MgO@rGO were obtained by adjusting the weight ratio of NFC and MgO@rGO. Moreover, film samples for pure NFC and NFC/rGO were prepared with the same method mentioned above. For comparison, MgO@rGO was replaced by MgO/rGO to prepare NFC/MgO/rGO films following the same steps.
3. Results and discussion In this study, GO was prepared from the flake graphite through the modified Hummers method. In order to prove its successful synthesis, FTIR, XRD, and AFM were conducted. In the FTIR spectra of Fig. S1(a), the appearance of oxygen-containing functional groups (-OH, -COOH, -C-O-C-) confirmed the successful oxidation of graphite. Fig. S1(b) shows the XRD patterns of graphite and GO. The diffraction peak of graphite at 2θ = 26.5° disappeared after chemical oxidation and shifted to a lower position of 8.8°, corresponding to (0 0 2) plane of GO. Fig. S1(c) reveals the thin sheet topography of as-synthesized GO with a thickness of 1 nm, consisting with the characteristic of GO single layer sheet [26]. The reduction degree of GO will affect the thermal conductivity of the NFC composite films by influencing the inherent thermal conductivity of rGO. In order to reveal the surface state of MgO@rGO, the XPS was utilized with GO, rGO, and MgO@rGO for comparison, which was shown in Fig. 2(a–d). Compared with GO, Fig. 2(a) showed a significant increase in the C/O atomic ratio of rGO, indicating the successful reduction of GO. The C/O atomic ratio of MgO@rGO (0.87) is far below that of rGO (10.63), which is attributed to the existence of oxygen element in MgO. Two new peaks at 1304.6 eV and 305.6 eV corresponding to Mg1s and MgKLL appear respectively in the XPS spectrum of MgO@rGO, indicating that the MgO is successfully loaded on the rGO surface. To further prove the successful reduction of GO and the loading of MgO in MgO@rGO, XRD was used and the patterns of GO, rGO, MgO@ rGO were shown in Fig. 2(e). For GO, the diffraction peak at 2θ = 8.8° indicated the (0 0 2) plane. After thermal reduction, the XRD pattern of rGO shifted to a higher and broad peak which widely spread in range of 2θ = 16.4°~26.5°, implying the rGO was successfully prepared. Besides, it is a wide dispersion peak different from the crystalline graphite. And MgO@rGO exhibited the characteristic peaks at 2θ = 36.5°, 42.8°, 62.1°, 74.3°, and 78.3°, corresponding to (1 1 1), (2 0 0), (2 2 0), (3 3 1), and (2 2 2) lattice planes, which belonged to simple cubic system MgO materials (JCPDS no. 75-0447) [27]. To obtain the MgO mass fraction decorated on the surface of rGO, TGA was measured. As shown in Fig. 2(f), the weight loss of GO started below 100 °C, which is ascribed to the evaporation of absorbed water. Between 200 °C and 300 °C, the weight loss of GO is due to the thermal decomposition of the labile oxygen-containing groups. The weightlessness characteristic of GO in air atmosphere is approximately the same as that in nitrogen atmosphere before 300 °C as shown in Fig. S2. However, the images appear significantly different when the
2.5. Characterization The prepared GO was characterized by X-ray diffraction (XRD, Ultima IV, Rigaku) with Cu Ka radiation (λ = 0.154 nm), Fourier transform infrared (FTIR, Thermo fisher Nicolet 6700, USA) and atomic force microscope (AFM, Bruker Dimension, USA). The rGO and MgO@ rGO were also characterized by XRD. The morphologies of rGO and MgO@rGO were observed with field emission-scanning electron microscopy (FE-SEM, FEI Co., Netherlands) at 15 kV accelerating voltage and transmission electron microscopy (TEM, JEOLJEM-1010) at 80 kV. The oxygen content on the GO, rGO, and MgO@rGO were obtained by X-ray photoelectron spectroscopy (XPS, EscaLab 250X, Thermo Fisher, USA). The weight ratio of rGO and MgO was determined through thermogravimetric analysis (TGA, TA Q5000IR, USA) under the air atmosphere with a heating rate of 10 °C/min and the measurements were taken use of 5–10 mg samples,which were dried at 45 °C overnight under vacuum before testing. The thermal conductivities of the NFC/ MgO@rGO films were calculated by the formula TC = αCpρ. The thermal diffusivity (α) was conducted on a Netzsch LFA 467 Nanoflash at 25 °C. And the results presented were an average of at least three individual tests per film. The specific heat capacity (Cp) was characterized by DSC (TA Q2000, USA), in which the films were heated at the rate of 5 °C/min under the nitrogen atmosphere. The density (ρ) was calculated from the weight and volume of the sample. The surface temperatures of light-emitting-diode (LED) chips fixed on the pure NFC film and NFC composite film were recorded by an infrared thermograph (C3000, Hangzhou Meisheng Infrared Optoelectronic Technology Co., Ltd.). The surface electrical resistivity (ρs) and volume electrical resistivity (ρv) were conducted at room temperature using the volumetric surface resistance tester (EST-121, Beijing Guance Precision Electric Instrument Equipment Co., Ltd.). The stress–strain curves were tested on the Instron 5966 tester (USA) with a load cell of 500 N at room temperature with a speed of 10 mm/min. All the specimens were cut from the films and possessed uniform size of 30 × 4 mm with varying 3
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Fig. 2. XPS wide scan spectra of (a) GO, rGO, and MgO@rGO. XPS C1s spectra of (b) GO, (c) rGO, and (d) MgO@rGO. XRD patterns of (e) GO, rGO, and MgO@rGO. TGA curves in air atmosphere of (f) GO, rGO, and MgO@rGO.
film and 80NFC/20MgO@rGO film were measured by SEM, as shown in Fig. 4(c–d). The pure NFC film (Fig. 4(c)) presents a densely layered structure so that the heat prefers to deliver along the in-plane direction leading to the anisotropy of thermal conductivity. Meanwhile, the high mechanical property of pure NFC film is closely related to the tight alignment of celluloses. As shown in Fig. 4(d), the NFC-based composite film with 20 wt% MgO@rGO fillers still maintains this typical layered structure and the tight stacking of 1D NFC, 2D rGO, and 3D MgO components has a positive effect on the improvement of mechanical property of the film. In theory, the MgO@rGO fillers will arranged along the in-plane direction, but the MgO@rGO fillers are too small to be observed. Therefore, the elemental mappings of fracture surface of 80NFC/20MgO@rGO film exhibit uniform dispersion, as shown in Fig. 4(e–f). For the surface of 80NFC/20MgO@rGO film, there is no doubt that the fillers are well dispersed in NFC as shown in Fig. S5. The thermal conductivities of NFC film and NFC/MgO@rGO composite films filled with different filler content were measured by flash method at 25 °C, and the results were shown in Fig. 5. The NFC is a poor conductor [28,29] of heat with the λx (in-plane direction) of 1.02 W/ (m·K) and the λz (cross-plane direction) of 0.02 W/(m·K). The thermal conductivity of NFC/MgO@rGO composite films increased significantly when the thermal conductive filler was added, as shown in Fig. 5(a). The λx and λz increased greatly up to 7.45 W/(m·K) and 0.32 W/(m·K), respectively, when the MgO@rGO addition amount was 20 wt%. There appear to be different from the λx to λz at the same thermal conductivity filler content, which is attributed to the film-forming process. During the vacuum-assisted filtration, the NFC and MgO@rGO were arranged layer by layer under the gravity action so that the MgO@rGO was inclined to lap each other along the plane, accompanied by the formation of the thermally conductive network which was depicted in Fig. 5(c). However, the interface thermal resistance between the NFC and MgO@rGO was the main cause of the low efficiency of the heat dissipation, leading to the poor thermal conductivity cross the plane of the film. The thermal conductivity anisotropy of the NFC/MgO@rGO films is shown in Fig. S6. When the filler content ranges from 0 to 20 wt %, the NFC composite films always maintain gratifying anisotropy of thermal conductivity which is expected to avoid the influence of
temperature continued to increase. The curve of GO becomes stable in the nitrogen atmosphere, indicating that the thermal reduction was basically completed. In contrast, the GO in the air atmosphere was completely burnt without residue. After thermal reduction, the rGO is very stable at low temperature with nearly no mass loss and exhibits only one main weight loss step in air atmosphere above 450 °C, which is due to the decomposition of the carbon skeleton. As for MgO@rGO, the law of mass loss by heating is in accordance with rGO. There is just the carbon skeleton of the rGO decomposition while the MgO persists. According to the thermogravimetric residues of the above samples, it can be concluded that the quantity of MgO decorated on the rGO was about 87.0 wt%. Moreover, the SEM and TEM were employed to analyze the surface morphology of rGO and MgO@rGO, as shown in Fig. 3. There is a distinct difference that rGO (Fig. 3(a–b)) exhibits a relatively smooth surface without any impurities while the thin sheet surface of MgO@ rGO (Fig. 3(d–e)) is decorated with some MgO nano-particles. In addition, the TEM image of rGO (Fig. 3(c)) shows a typically wrinkled structure and the size of rGO sheet is about 2–3 μm. Comparatively, the size of MgO@rGO (Fig. 3(f)) measured by TEM is mainly consistent with that of rGO, as shown in Fig. 3(e). As shown in Fig. 3(f), the MgO nano-particles of MgO@rGO tend to concentrate on the surface of rGO rather than existing alone after ultrasonic dispersion, which indicate that there is strong interaction between MgO and rGO. As observed from Fig. S3, the size of MgO nano-particles is in the range from 10 to 70 nm and the average diameter of the MgO is approximately 30 nm. To further investigate the thickness of the hybrid thermally conductive fillers, the atomic force microscope (AFM) was used and the results were shown in Fig. S4. The thickness of the hybrid filler sheet is approximately 2–7 nm due to the surface loading of MgO nanoparticles. The NFC-based composite films with a series of MgO@rGO content were prepared by the vacuum-assisted filtration. The optical images of the pure NFC film and 80NFC/20MgO@rGO film are displayed in Fig. 4(a-b). The pure NFC film has high optical transparency, while the light transmittance decreases rapidly with the addition of the MgO@ rGO. In order to study the relationship between internal structure of the film and its property, the fractured surface morphology of pure NFC 4
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Fig. 3. SEM images of (a-b) rGO and (d-e) MgO@rGO. TEM images of (c) rGO and (f) MgO@rGO.
applications in electrical apparatus [30–32]. Fig. 7 presents the surface electrical resistivity (ρs) and volume electrical resistivity (ρv) of the NFC and NFC-based composites. Pure NFC film has excellent electrical insulation performance, with the ρs and ρv to be 3.17 × 1011 Ω and 1.22 × 1013 Ω·m, respectively. As for NFC/rGO films, no matter surface electrical resistivity or volume electrical resistivity, the results show that there is a decline of at least 7 orders of magnitude even if the rGO content is only 5 wt%, which is ascribed to the excellent electrical conductivity of rGO. As a contrast, NFC/MgO@rGO films exhibit the excellent electrical resistivity, even when the filler content reaches up to 20 wt%, the NFC/MgO/rGO films also maintain excellent electrical insulation property due to a large amount of MgO which acts as a barrier to prevent the forming of rGO conductive network. Based on the above results, we proposed the heat conduction mechanism of the NFC-based composite films. The addition of rGO can form a heat conduction way to increase the thermal conductivity of films. The MgO plays an important role in cutting electric conductive pathways off so as to maintain the electrical insulation property of the composite films. To further illustrate the heat transfer process of the film, the schematic diagram is depicted in Fig. 8. For NFC/rGO films, the heat transfer path and conductive path can be formed by the overlap of closely arranged rGO. Since the percolation threshold of conductive path is far lower than that of the heat path, the improvement of thermal conductivity will inevitably accompany with the rapid ascension of electrical conductivity. In order to keep the electrical insulation of the NFC composite film, MgO filler which has good thermal conductivity and excellent electrical resistance property is added. However, the NFC/MgO/rGO film prepared by directly mixing displays a big interface thermal resistance between NFC and MgO (Rth-1), rGO and MgO (Rth-2), NFC and rGO (Rth-3). As we known, phonon transfer [33] is the main way of thermal transport in the insulating materials. But at the interfaces, phonon scattering [34] would hinder the heat transfer. Additionally, the improvement of thermal conductivity by reducing phonon scattering is a desirable way. Thus, the in-situ growth of MgO decorated on the surface of rGO will reduce the interface thermal resistance and phonon scattering to obtain the better thermal conductivity. For the comparison of the in-plane thermal conductivities and electric insulation properties of our NFC/MgO@rGO composite film with those previously reported thermally conductive films, the results
neighboring electronic components because the heat tends to dissipate along the in-plane direction. In order to demonstrate the decorated of MgO on the surface of rGO is necessary, NFC/rGO films and NFC/MgO/ rGO films were prepared by directly mixing of NFC, rGO, and MgO as comparison samples. The thermal conductivity of these films was shown in Figs. 5(b) and S7. For 80NFC/rGO film, the thermal conductivity is slightly higher than that of 80NFC/20MgO@rGO film due to the difference in inherent thermal conductivity between rGO and MgO. Besides, the 80NFC/20MgO/rGO film exhibits slightly improvement in thermal conductivity due to the non-negligible increasement of interface thermal resistance, compared to 80NFC/20rGO film and 80NFC/20MgO@rGO film. As shown in Fig. S7, the thermal conductivity of NFC/MgO/rGO films also exhibits anisotropy, but lower than that of NFC/MgO@rGO films at the same filler content. Thus, the pre-treatment of thermal conductive filler is indispensable. In order to visually assess the heat dissipation ability of NFC-based composite film, the pure NFC film and 80NFC/20MgO@rGO film were used as the substrate to fix LED chips. The surface temperatures of LED chips were monitored by an infrared thermal imager. The optical and thermal images of the LED chips are shown in Fig. 6 and the specific temperature values are listed in Table 1. The temperature point 1 (PO1) is the maximum temperature of the LED chip fixed on pure NFC film substrate while the temperature point 2 (PO2) is the maximum temperature of the LED chip fixed on 80NFC/20MgO@rGO film substrate. Before electrifying, PO1 and PO2 are almost identical at around 38 °C. A significant rise in temperature occurs and continuously increases with time when the LED chips start to work. Besides, POI is always higher than PO2 at the same time and the difference value between them gradually increases over time. For example, the PO1 reached up to 53.8 °C and PO2 reached up to 50.1 °C while the temperature difference value was calculated to be 3.7 °C when the LED chips worked for 1 min. The temperature difference value was raised to 8.4 °C when the LED chips worked for 3 min. This is due to the higher thermal conductivity of 80NFC/20MgO@rGO film than that of pure NFC film as mentioned above. The LED chip fixed on 80NFC/20MgO@rGO film substrate can keep the surface temperature at low lever because of the composite film’s ability to dissipate heat timely. Therefore, NFC/MgO@rGO composite films show a great potential in the application for heat dissipation of electronic devices. The electric insulation property is primary requested for 5
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Fig. 4. Optical image of the prepared pure NFC film (a) and 80NFC/20MgO@rGO film (b). SEM images of fracture surfaces of the pure NFC film (c) and 80NFC/ 20MgO@rGO film (d). C elemental mapping (e) and Mg elemental mapping (f) of fracture surface of 80NFC/20MgO@rGO film.
properties as shown in Fig. S9. To improve the mechanical properties of the NFC composite films, it is a good choice to enhance the interfacial adhesion between matrix and filler by adding a binder such as dopamine (DA) and tannic acid (TA) in subsequent research. At least, the mechanical properties of NFC-based composite films are in an acceptable range due to the conservation of compacted layered structure. Meanwhile, the anisotropic thermal conductivity of composite films is also derived from this layered structure.
are summarized in Table 2. Obviously, the insulating nitride is preferred to add into polymer for increasing the thermal conductivity. In fact, the results are unsatisfactory with high filling of thermal conducting fillers. For example, Zhang et al. [20] used AlN as fillers to improve the thermal conductivity of CNF film and the CNF/AlN film reached 4.2 W/(m·K) with 25 wt% fillers adding. Song et al. utilized GP [37] and RGO [36] as the fillers to accelerate the heat transfer of NFC film but with excellent electrical conductivity. In this work, NFC/ MgO@rGO film exhibited an expected anisotropic thermal conductivity and electrical insulation. In practical application, mechanical properties [39–42] are one of the most important characteristics of the materials. The typical stress − strain curves of NFC/MgO@rGO composite films are shown in Fig. S8. The results indicate that there is a slightly deterioration of the composite films compared to the pure NFC film. The pure NFC film is one of the degradable polymers with high strength, existing a tensile strength of 99.6 MPa and an elongation at break of 4.7%. After adding MgO@rGO, the tensile strength of the composite films shows a downward trend due to the weak interface adhesion between NFC matrix and MgO@rGO fillers. The SEM images of fracture surfaces of pure NFC film and NFC composite films exhibit some voids leading to poor mechanical
4. Conclusions In summary, NFC-based nanocomposite films filled with different content of MgO@rGO were successfully prepared by the vacuum-assisted filtration. Decorating the MgO onto the surface of rGO can not only facilitate the homogeneous dispersion of rGO fillers in the NFC matrix but also prevent the formation of electric conductive pathways without affecting the thermal conductive pathways. When the content of MgO@rGO filler increased to 20 wt%, the in-plane thermal conductivity of the composite film raised to 7.45 W/(m·K), which is 7.3 times that of pure NFC. Meanwhile, the cross-plane thermal conductivity of the composite film was only 0.32 W/(m·K), far lower than 6
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Fig. 5. Thermal conductivity of the NFC/MgO@rGO films (a), NFC composite films (b), and schematic diagram of thermal conductivity of the NFC composite films (c).
Fig. 6. Optical and thermal images of the LED chips fixed on the pure NFC film and 80NFC/20MgO@rGO film substrate.
composite films exhibit superior surface electrical resistivity above 1011 Ω and superior volume electrical resistivity above 1011 Ω·m. Therefore, the prepared composite films with excellent thermal conductivities and electrical insulating properties can be widely used in the field of electronic devices.
Table 1 The temperatures of the LED chips fixed on the pure NFC film and 80NFC/ 20MgO@rGO film substrate at 12 V. Time (min)
Temperature Point 1 (oC) (pure NFC film substrate)
Temperature Point 2 (oC) (80NFC/ 20MgO@rGO film substrate)
0.0 0.5 1.0 2.0 3.0
38.1 53.8 58.0 60.3 62.4
38.3 50.1 52.8 53.9 54.0
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
that of in-plane value, which would reduce the impact on adjacent electronic components due to the anisotropy of the thermal conductivity when using in thermal management. Moreover, NFC-based
Acknowledgments This work was supported by the National Natural Science 7
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Fig. 7. The surface electrical resistivity (a) and volume electrical resistivity (b) of the NFC composite films.
Fig. 8. Schematic diagram of thermal conductivities and electrical resistivities of NFC composite films. Table 2 Comparison of in-plane TC and electric insulation of this work with other polymer-based thermally conductive films. composites
filler content (wt%)
in-plane TC (W·m−1·K−1)
PVDF/m-BN CNF/AlN RGO/CNF NFC/GP NFC/FCNT Al2O3–AgNP/epoxy-H NFC/MgO@rGO
30 25 50 6 35 50 20
7.29 4.2 7.3 9.0 14.1 6.71 7.45
η (λx/λz)
Insulator (yes or no)
test method
yearreferences
11 56
Y Y N
LFA467 LFA467 LFA 447 LFA 447 LFA 467 LFA 467 LFA 467
2018 [35] 2019 [20] 2017 [36] 2017 [37] 2018 [17] 2017 [38] this work
21 23
Y Y Y
References
Foundation of China (grant no. 21504078), the Natural Science Foundation of Zhejiang Province (grant no. LY20E030008), the Research Project of Canal Cup of Zhejiang University of Technology, and the China Scholarship Council.
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