Highly Thermally conductive fluorinated graphene films with superior

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Applications of Polymer, Composite, and Coating Materials

Highly Thermally conductive fluorinated graphene films with superior electrical insulation and mechanical flexibility Xiongwei Wang, and Peiyi Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07377 • Publication Date (Web): 28 May 2019 Downloaded from http://pubs.acs.org on May 29, 2019

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ACS Applied Materials & Interfaces

Highly Thermally conductive fluorinated graphene films with superior electrical insulation and mechanical flexibility Xiongwei Wang1,2, Peiyi Wu1,2*

1State

of

Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College

Chemistry,

Chemical

Engineering

and

Biotechnology,

Center

for

Advanced

Low-Dimension Materials, Donghua University, Shanghai 201620, China 2State

Key

Laboratory

of

Molecular

Engineering

of

Polymers,

Department

of

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

E-mail：[email protected]

Keywords: fluorinated graphene; thermal conductivity; electrical insulation; composite film; mechanical flexibility

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Abstract: Graphene-based heat spreading films have captured high attention in academic study and commercial application because of their extremely high thermal conductivity and desired flexibility. However, the electrical conductivity limits their utilizations in many electronic fields. Herein, to address this problem, fluorinated graphene (F-graphene) that exfoliated from commercial fluorinated graphite was first used to prepare the flexible free-standing composite film via vacuum filtration of uniform PVA-assisted F-graphene suspension. The well-organized alignment of F-graphene lamellas makes the composite film show an ultrahigh in-plane thermal conductivity of 61.3 W m-1 K-1 at 93 wt% F-graphene. Despite at such high filler loading, the fabricated F-graphene film still possesses a superior electrical insulation property. Therefore, these results suggest that F-graphene, as the novel thermally conductive filler, demonstrates fascinating characters in preparation of thermally conductive yet electrically insulating nanocomposite.


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1. Introduction In the past decade, the rapid development of portable devices (e. g. cell phone, tablet PCs and other smart hardware) toward multi-functionalization, lighter and thinner or even bendability put forward higher requirements for their inside electronic components in miniaturization, integration and high-power densification on.1-3 Consequently, the escalation of heat flux per square area would give rise to severe heat dissipation issues that compromise the lifetime and reliability of electronic devices.4-6 Using anisotropic thermally conductive films with high in-plane thermal conductivity (TC), good electrical insulation, flexible and robust structure is an accessible way to address the excessive heat.7-12 Thereby they are expected to have large potential applications in next-generation portable and flexible wearable electronic devices.7-8, 13 Graphene, as the most promising two-dimensional (2D) nanomaterial, possesses extremely high TC of ~5300 W m-1 K-1, excellent mechanical flexibility and high aspect ratio.14-15 These superior comprehensive properties make it a promising candidate to replace conventional thermally conductive fillers in the preparation of thermal management materials.14-18 Among them, the graphene-based heat spreading films that prepared by graphitization of graphene oxide film have captured more attention due to their ultrahigh macroscopic TC above 1000 W m-1 K-1 and outstanding flexibility.14, 19-20 For example, Liu et al. prepared an ultrathin free-standing graphene film via evaporation of GO suspension on aluminum substrate together with high-temperature graphitization and the in-plane TC can reach up to 3200 W m-1 K-1.19 Unfortunately, despite the admirable heat-conducting performance of these graphene-based films, the excellent electrical conductivity limits their 3 ACS Paragon Plus Environment


use in highly-integrated electronics. Therefore, in view of the requirement of electrical insulation, boron nitride nanosheet (BNNS), a well-known analogue of graphene with wide band gap (~5.9 eV) and high TC, is regarded as the most promising alternative of graphene.21-23 So far, many kinds of thermally conductive BNNS films have been prepared by introducing little organic macromolecules as interlayered binder to ensure the structure integrality.7, 9, 24-26 However, the in-plane TC values (recorded by laser-flash method) of these BNNS composite films are all below 60 W m-1 K-1 due to the relatively low intrinsic TC and chemical inertness of BNNSs. Fluorination is an efficient method to tune the bandgap of graphene, because the invasion of fluorine would cause structural transformation of C-C bonds from sp2 to sp3 configuration.27-30 Reflecting on the intrinsic properties, the electrical conductivity of graphene is inclined to show a rapid drop from electrical conductor to insulator as the increasing coverage of fluorine.30-32 Nevertheless, the variation of TC of fluorinated graphene versus fluorination degree is vastly different from its electrical conductivity. The simulated theoretical TC of fluorinated graphene by Huang et al. showed a U-shaped change with the increasing fluorination and eventually achieved ~35 % of the original graphene at 100 % coverage of fluorine.33 That is to say, the completely fluorinated graphene can maintain a high theoretical TC of above 1800 W m-1 K-1 together with a favorable electrical insulation and 2D flexibility, which are comparable or superior to BNNSs. This finding provides a firm theoretical basis for the practical use of F-graphene. Therefore, the above-mentioned discussions inspires us that using the commercial highly fluorinated graphene as building blocks to construct thermally conductive film are promising to acquire high thermal 4 ACS Paragon Plus Environment

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conductivity as well as superior electrical insulation. On the other hand, F-graphene as thermally conductive filler has never been reported in the preparation of thermal dissipation materials, to the best of our knowledge. In a previous work, we synthesized cellulose nanofiber / fluorinated carbon nanotubes (CNF/F-CNTs) composite film and found that this film achieved a peak TC of 14.1 W m-1 K-1 at 35 wt% F-CNTs.34 The decreased TC is likely due to the weak linking effect of 1D CNF on adjacent FCNTs at high filler loading. Given the superior filler-filler contact of 2D fillers to 1D fillers, in this work, we used 2D F-graphene nanosheets to prepare a flexible free-standing composite film with well-organized “brick-and-mortar” structure by a vacuum-assisted filtration method (Figure 1). The pristine F-graphite was first exfoliated into few-layered F-graphene by bath sonication in isopropanol. A little amount of PVA was further introduced to facilitate the dispersion of exfoliated F-graphene in water and meanwhile act as the binder to enhance the linking betwen adjacent F-graphene nanosheets. Benefit from this unique structure, the fabricated F-graphene composite film exhibits an ultrahigh in-plane thermal conductivity of 61.3 W m-1 K-1 at 93 wt% F-graphene as well as good electrical insulation. Therefore, the favorable comprehensive performance would endow this composite film with large potential applications in efficient thermal management of modern portable and flexible electronic devices.



2. Experiential Section 2.1 Materials Fluorinated graphite was purchased from Yuancheng tech. Co. Ltd. (Hubei, China). Isopropanol (IPA) was provided by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Poly(vinyl alcohol) (Mw ~145000) was purchased from Aladdin Chemical Reagent. Deionized water was used for all experiments. 2.2 Preparation of few-layered F-graphene nanosheets 4 g of F-graphite powder was first dispersed in 500 ml of isopropanol. This mixture was placed into a sonication bath with an output power of 100 W for 24 h to peel off the F-graphite into few-layered F-graphene. After that, the non-exfoliated F-graphite was removed by centrifugation at 3000 rpm for 10 min. 2.3 Preparation of F-graphene composite film PVA was first dissolved in water at 80 oC to prepare 6 wt% PVA aqueous solution. The concentrated F-graphene/IPA suspension was further transferred into aqueous suspension through dialysis in deionized water. Then the obtained F-graphene aqueous suspension was diluted and certain amount of PVA solution was added to facilitate the uniform dispersion of fluorographene in water by sonication for 60 min. The prepared F-graphene/PVA aqueous suspension was vacuum-filtered using a mixed cellulose ester membrane (50 mm in diameter and 0.22 μm in pore size) to obtain a free-standing F-graphene composite film (15-20 μm in thickness) after removing the basal membrane 2.4 Characterization The morphology of exfoliated F-graphene was performed in TEM (JEOL JEM2011 F, 6 ACS Paragon Plus Environment

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Japan) with a operation voltage of 200 kV and the fractured cross-sectional morphology of the F-graphene composite films were observed by field emission SEM (Zeiss Ultra 55, Germany). The thickness of exfoliated F-graphene was measured by Atomic force microscopy (AFM) (Bruker Multimode 8, Germany) in a tapping mode. XRD patterns and Raman spectra of the pristine and exfoliated F-graphite were recorded on X’pert PRO PANalytical (Netherland) with Ni-filtered Cu Kα radiation (40 kV, 40 mA) and micro-Raman spectrometer (HORIBA XploRA, France), respectively. The elemental composition and bonding mode were detected by X-ray photoelectron spectroscopy (XPS) RBD on an upgraded PHI-5000C ESCA system (Kratos Elmer) with Mg Kα radiation ( hv = 1253.6 eV). Thermogravimetric analysis (TGA) was carried out on a Metter Toledo TGA 1 (Switzerland) from 100 to 800 oC with a heating rate of 20 oC/min in N2 atmosphere. The tensile stress-strain curves of the F-graphene composite films were measured on a universal electronic tensile machine (UTM4000, SUNS, China). FT-IR spectrum was collected on a Nicolet Nexus 470 spectrometer. The volume resistivity of the composite films was measured on an insulation resistance tester of ZC-90G (Yuanzhong electronic, Shanghai). The in-plane thermal diffusivity (α, m2 s-1) of F-graphene composite film was detected by a laser-flash diffusivity instrument LFA 467 (NETZSCH, Germany). The specific heat (c, J g-1 K-1) of F-graphene composite films was measured on differential scanning calorimeter (DSC) (TA Q2000, American). The density (ρ, g cm-3) of the specimens was calculated by the equation of ρ=m/ν. The eventual thermal conductivity (λ, W m-1 K-1) of F-graphene specimens was calculated by the following equation: λ=α × c × ρ. The surface temperature distribution of the customized composite substrate was detected by an infrared thermal probe (FLIR ORE@PRO) controlled by IPhone. 7 ACS Paragon Plus Environment


3．Results and Discussion

Figure 1. Scheme of the exfoliation of F-graphite and the preparation process F-graphene composite films To fully fulfil their intrinsic property, exfoliation of F-graphite into few-layered F-graphene nanosheets is necessary. As shown in Figure 1, sonication-assisted liquid phase exfoliation was used to counteract the van der waals interaction between graphite fluoride layers and then peel off few-layered F-graphene nanosheets in isopropanol (IPA). IPA is one kind of good intercalated molecules to promote the separation of stacked F-graphite layers.27, 35

After centrifugation, the morphology of the exfoliated F-graphene nanosheets was observed

by SEM and TEM. As shown in Figure S1a, the pristine F-graphite powders show typical laminated structure stacked by a plenty of F-graphene sheets. After liquid exfoliation, the thickness of the nanosheets is significantly reduced due to the consecutive separation of F-graphene from bulk F-graphite (Figure S1b). Moreover, the strong cavitation during 8 ACS Paragon Plus Environment

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sonication would inevitably break the exfoliated nanosheets, leading to the decreased lateral size. TEM image further reveals that the obtained F-graphene nanoplatelets show a transparent and wrinkled lamellar structure, indicating the ultrathin feature of them (Figure 2a). Due to the matching surface energy, the collected F-graphene show extremely uniform dispersion in IPA with a clear Tyndall effect and excellent dispersion stability for at least three days (Figure S2). It is well known that the invasion of fluorine would deeply change the well-organized hexagonal structure of graphene through the structural transformation of C-C bonds from sp2 to sp3.29-30 Therefore, high-resolution TEM characterization is also employed to observe the exfoliated F-graphene (Figure 2b). One can see that the hexagonal atomic structure of graphene is almost completely destroyed, leaving continuous unordered fluorinated region. The irregular atomic arrangement with rich sp3 C-C mode endows the F-graphene with favorable electrical insulating property.31 To more directly reflect the exfoliation effect, the thickness of the obtained F-graphene nanosheets was evaluated by AFM analysis. Figure 2c-d reveals the thin lamellar topography of the exfoliated F-graphene with a thickness of ~2.3 nm, suggesting that IPA-assisted sonication can reliably peel off the F-graphite into few-layered F-graphene nanosheets. In order to have a more comprehensive understanding on the exfoliation effect of F-graphene nanosheets, statistical analysis of the thickness and lateral size by counting over 60 pieces of F-graphene was adopted to estimate their size distribution. As shown in Figure 2e-f, most of the exfoliated F-graphene nanoplatelets have a thickness at 2-5 nm and lateral size at 0.5-2.5 μm.



Figure 2. (a) TEM and (b) high-resolution TEM images of the exfoliated F-graphene nanosheets; Inset is the suspension of F-graphene in isopropanol; AFM images (c) and the corresponding height profile (d) of F-graphene; Statistical distribution of the thickness (e) and lateral size (f) of the F-graphene nanoplatelets. In a bid to gain the information of the structural change during exfoliation, XRD patterns and Raman spectra of pristine F-graphite and F-graphene were both collected. As shown in Figure 3a, three characteristic peaks of (001), (002) and (100) reflections can be observed in F-graphite.36-37 The broad (001) reflection at 14.6o, corresponding to a d-spacing of ca. 0.62 nm, is associated with the stacking of fluorinated layer along the stacking direction. Compared with graphite (ca. 0.33 nm), the expanded interplanar distance in F-graphite is mainly attributed to the intercalation of fluorine atoms between adjacent lamellas and the deterioration of aromatic conjugated π-π stacking caused by incorporated fluorines.38 The (002) reflection that related with the stacking of graphite structure is very weak, indicating the high fluorination level. Another broad diffraction peak of (100) at 41.2o is commonly assigned to the in-plane length of C-C network in the reticular system.36 After exfoliation, the intensity of (100) reflection shows a tremendous reduction, suggesting the appreciable break of F-graphene during exfoliation. Moreover, Raman spectroscopy was further employed to perceive the structure information of F-graphite and F-graphene (Figure 3b). Similar to the 10 ACS Paragon Plus Environment

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common carbon-based materials, two characteristic peaks of D band and G band are observed. ID/IG ratio, which is commonly used to reflect the atomic-scale conjugation, is estimated to be 1.36 for F-graphite and 1.17 for F-graphene, respectively, indicating the partial restoration of sp2 structure during exfoliation process. Due to the extremely high electronegativity of fluorine, C-F bonds in fluorinated graphene materials show the unique chemical reactivity, and they are susceptible to reductive defluorination under light irradiation, nucleophilic attack, or vigorous sonication36,

39-42.

Consequently, when the adjacent fluorine atoms in

stoichiometric carbon-fluorine scaffold split simultaneously, the rebuilding of C=C bonds would be occurred40, 43.

Figure 3. XRD patterns (a) and Raman spectra (b) of the exfoliated F-graphene and pristine F-graphite For the deep understanding, XPS spectroscopy was further used to monitor the changes of elemental composition and bonding mode from pristine F-graphite to exfoliated F-graphene. As shown in Figure 4a, the F-graphite shows two prominent signals of C 1s and F 1s. While after sonication process, another puny signal of O starts to appear, which might be ascribed to the interposition of oxygen at the edges of the broken F-graphene nanosheets during sonication process.44 The presence of oxygen-containing functional group in F-graphene is further confirmed by the prominent weight loss of F-graphene at 200-300 oC 11 ACS Paragon Plus Environment


compared to F-graphite in TGA curve (Figure 4b). Furthermore, according to the integral area and sensitivity factor of the corresponding characteristic peaks, the fraction of F and the F/C atomic ratio of F-graphite can be calculated to be 58.65 at% and 1.42, respectively, compared with 53.63 at% and 1.2 for F-graphene. The declined F content of F-graphene after exfoliation is a direct evidence to confirm the departure of fluorine from F-graphene during sonication process, which is consistent with the results in Raman characterization. Figure 4c and d demonstrate the high-resolution C 1s XPS spectra of pristine F-graphite and F-graphene with fitted peaks. It can be seen that the C 1s spectrum of F-graphite consists of five subcomponents, 291.4, 289.8, 287.6, 286.3 and 284.6 eV, which corresponds to CF2, C-F, C-CF, C-C and C=C, respectively.34, 36-37 Among them, C-F is the most reigning component. Interestingly, after exfoliation, the relative proportion of CF2 shows a significant loses accompanied with a prominent enhancement of the components of C-CF, C-O and C=C. The decreased CF2 and the increased C-CF and C=C indicates the occurrence of fluorine departure during exfoliation to restore the sp2 structure, which is in agreement with the reduced ID/IG ratio of F-graphene in Raman observation. Such a phenomenon could be explained that the increasing stretching vibration energy of C-F bond delivered by the consecutive sonication would eventually give rise to the removal of F.32, 36 The CF2 groups, which are usually located at the defects or edges, has larger free vibration than the C-F modes located at the flawless carbon network. Thus the CF2 mode is more easily to decompose under sonic power.36


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Figure 4. (a) XPS survey spectra and (b) TGA curves of exfoliated F-graphene and pristine F-graphite; High-resolution C1s XPS spectrum of pristine F-graphite (c) and exfoliated F-graphene (d).

Figure 5. Surface morphology (a-d) and cross-section fractured morphology (e-m) of F-graphene composite films with F-graphene loading of 93 wt% (a, e and i), 88.6 wt% (b, f and j), 81.8 wt% (e, g and k) and 73.3 wt% (d, h and m), respectively.



The exfoliated F-graphene nanosheets with high fluorination degree were used to prepare the free-standing F-graphene film via a vacuum-assisted filtration technique. In this case, it is well known that a homogeneous, dilute suspension of building blocks plays a critical role to obtain a well-organized film. For this purpose, a little amount of water-soluble PVA was added to improve the dispersion of F-graphene in water. As shown in Figure S3, the PVA-assisted F-graphene aqueous dispersion has a desirable dispersion uniformity and stability with a typical Tyndall effect. As the filtration proceeds, the high aspect-ratio F-graphene nanosheets are prone to lie horizontally in sequence to form highly anisotropic film.7, 45 PVA, as the binder, can further enhance the interaction between adjacent F-graphene nanosheets in composite film. The content of F-graphene in the final composite films was roughly estimated by TGA (Figure S4). Figure 5 shows the surface and cross-section morphology of F-graphene composite film with different compositions. The surface morphology shows the compact surface structure embedded with many horizontally aligned platelets along in-plane direction (Figure 5a-d). As the increasing addition of PVA, the excess PVA covers up the abrupt F-graphene platelets, leading to the slight decrease of surface roughness. The fractured cross-section of F-graphene film further reveals that the F-graphene nanosheets are indeed well stacked layer-by-layer to form highly-oriented structure along the in-plane direction (Figure 5e-h). Moreover, the closely stacking of building blocks indicates the strong interaction between F-graphene and PVA chains, which contributes to reducing the interfacial thermal resistance of F-graphene nanosheets (Figure 5i-m).46 However, the orderly alignment of F-graphene gradually decreases with the increasing addition of PVA. This observation could be explained that excess PVA would fully fill the gap between adjacent 14 ACS Paragon Plus Environment

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nanosheets and then weaken the regulating effect of vacuum pressure on the orderly alignment of F-graphene flakes.13

Figure 6. (a) Typical tensile stress-strain curves of F-graphene composite films with different compositions; (b) optical images to display the good mechanical flexibility of F-graphene composite films; (c) FT-IR spectra of PVA and F-graphene composite films; (d) Volume electrical resistance of F-graphene composite films with different F-graphene loading. To explore the mechanical properties of the F-graphene films with different PVA content, the tensile stress-strain curves of them were investigated. As shown in Figure 6a, the F-graphene composite film with 93 wt% F-graphene possesses a relatively low tensile strength of 27.3 MPa, while the mechanical property of pure F-graphene film is too poor to form a free-standing film, suggesting the gluing effect of PVA chains on F-graphene plates. The increasing PVA content in composite films simultaneously improves their tensile strength and breaking elongation. When the PVA concentration reaches to 26.7 wt%, the F-graphene film can achieve a high tensile strength of 84.4 MPa at a braking elongation of 5.7%. The photograph displayed in Figure 6b is to further visually demonstrate the outstanding 15 ACS Paragon Plus Environment


mechanical flexibility of F-graphene composite film. One can see that the composite film with 88.6 wt% F-graphene can accommodate with repeating bending deformation of human finger and the film with 73 wt% F-graphene can even tolerate more complicated folding into a paper plane without any damage. In view of the large brittleness of pure F-graphene film, the significantly enhanced mechanical performance of F-graphene composite film is mainly associate with the strong interaction between F-graphene and PVA chains.37 FT-IR spectroscopy was utilized to clarify the interaction between them. Compared with pure PVA, the OH vibration peak of the F-graphene composite film shows a positive shift from 3272 to 3306 cm-1, confirming the strong hydrogen bonding interaction between F-graphene and PVA chains (C-F...H-O) (Figure 6c).47 Moreover, it has been widely reported that the introduction of fluorine would cause a rapid electrical conductivity loss of the graphene due to the serious deterioration of aromatic conjugation structure. On the other hand, for thermal conductive materials used in flexible electronic devices, the desired electrical insulation property has an equal importance to their flexibility. Therefore, the volume electrical resistivity of F-graphene composite films was further investigated (Figure 6d). It can be seen that all of the F-graphene composite films have an admirable electrical insulation property (>1011 Ω cm), although the increasing addition of F-graphene results in a slight decrease of volume electrical resistivity.


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Figure 7. (a) In-plane and through-plane TC of F-graphene composite films versus F-graphene loading; (b) Schematic illustration of the heat-conduction mechanism in F-graphene film with low and high PVA content; (c) Experimental density, theoretical density and void fraction of F-graphene composite films with various compositions. As a flexible thermally conductive film, the in-plane TC is a very important parameter to determine its heat dissipation capability. Therefore ， the in-plane TC of F-graphene composite film with different PVA content was investigated. As shown in Figure 7a, the in-plane TC of the composite film with 93 wt% F-graphene can reach up to an ultrahigh value of 61.3 W m-1 K-1. As the increasing PVA, the F-graphene film shows a steady decrement in TC but still maintains a high level of 21.6 W m-1 K-1 at 73.3 wt% F-graphene. As the schematic models shown in Figure 7b, this phenomenon can be explained as follows: at the low PVA concentration, the PVA chains uniformly distribute between F-graphene galleries to remove the interlayered gap, and meanwhile act as the bridges to tightly connect the adjacent F-graphene lamellas via the strong H-bond interaction between them, leading to the relatively 17 ACS Paragon Plus Environment


gentle phonon scattering at the interfaces of adjacent F-graphene nanosheets and thus the high TC improvement.24-25,

46

Nevertheless, at the high concentration of PVA, the excess PVA

without directly interacting with F-graphene would enlarge the distance of adjacent F-graphene nanosheets and destroy the highly oriented alignment of F-graphene, and meanwhile as a thermally insulating layer are expected to cause a serious interlayered phonon scattering, thereby resulting in the decreased heat conduction capability of F-graphene films.13 Moreover, the presence of voids in composites is believed to largely restrain the heat conduction because of their extremely low thermal conductivity. Therefore, the void fraction in F-graphene composite films with different compositions is estimated from the relationship between the theoretical and experimental densities by the following equations: Void fraction 

theoretical 

theoretical  exp erimental theoretical 1 wF-graphene wPVA +

 F-graphene

 PVA

where wF-graphene and wPVA represent the mass fraction of F-graphen and PVA in composite films, respectively, and ρF-graphene and ρPVA are the densities of F-graphene (~3.14 g/cm3) and PVA (1.30 g/cm3), respectively.48 According to the equation, the calculated void fraction of the composite films is shown in Figure 7c. It can be seen that although the void content inside the composite films gradually increases with the increased F-graphene loading, they are all at low level (60 wt%). Clearly ， most of BN-based composite films deliver an in-plane TC below 60 W m-1 K-1(LFA method). However, it is also found that the in-plane TC of BN-based film reported by Lei et al can reach up to 120 W m-1 K-1 or even 212 W m-1 K-1.9, 25 We notice that the measurement of TC in Lei’s works is performed by a steady-state method, which is not a commonly-used method in TC testing of highly anisotropic materials.7 Therefore, we think that the extremely large difference of TC value between the previously-reported BNNS films might be attributed to the different test method. In this case, the in-plane TC of our F-graphene film is superior to most of previous BN-based composite films (LFA method). Table 1. Comparison of F-graphene film with other electrically insulating composite films containing high filler concentration (>60 wt%) on the in-plane thermal conductivity Filler content

Thermal conductivity

(wt%)

(W m-1 K-1)

BNNS/PVA

94

BNNS/PVA

Testing method

YearReference

6.9

Laser-flash

201513

90

120

Steady-state

201725

BNNS/PDDA

90

212.8

Steady-state

20179

f-BNNS/CNF

70

30.25

Laser-flash

20177

BNNS/GO

95

29.8

Laser-flash

201610

BNNS-OH

100

58.3

Laser-flash

201751

CMC/SiC@Ag

>65

34

Laser-flash

201752

SiC@Ag/BNNS@Ag/PVA

95

21.7

Laser-flash

201653

PDA@BNNS/CNC

94

40

Laser-flash

201854

BNNS/TPU

95

50.3

Laser-flash

201855

Sample



F-graphene/PVA

93

61.3

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Laser-flash

This work

On the other hand, given the large structural anisotropy of F-graphene film, the through-plane TC of them with different compositions had also been investigated. As shown in Figure 7a, the through-plane TC of F-graphene films all show a low level ranging from 0.3 to 0.5 W m-1 K-1, suggesting the extremely high TC anisotropy of them. The “U” shaped changes of through-plane TC with the PVA content might be attributed to the decreased in-plane alignment of F-graphene nanosheets at high PVA concentration. In a word, the above-mentioned outstanding comprehensive performances of F-graphene composite films make them have large potential applications in thermal management of modern portable or flexible electronics devices, in which the consecutive heat generated by the highly-integrated or -powered components (e. g. CPU, LED backlight, RAM and battery) should be dissipated in time to ensure the working stability of the devices (Figure 7b). Therefore, to evaluate the heat-conduction and electrical insulation stability of F-graphene film in the possible application scenario, the in-plane TC and volume resistivity of F-graphene-93 composite film after different bending cycles were further measured. As shown in Figure S5, both of the in-plane TC and volume resistivity of F-graphene-93 composite film show a slight fluctuation during the 2000 bending cycles, suggesting their strong tolerance to the repeating bending behavior.


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Figure 8. Optical photographs of LED chips integrated with commercial PI (a), F-graphene films with 73.3 wt% F-graphene (b) and F-graphene films with 93 wt% F-graphene (c); the infrared thermal images of the commercial PI (d), F-graphene films with 73.3 wt% F-graphene (e) and F-graphene films with 93 wt% F-graphene (f) after working for 10 min. To further visually display the heat dissipation capability of F-graphene composite films in practical applications, they were utilized as the substrates to connect with a commercial LED chip (3 W) by silver paste. Polyimide film that widely used as flexible substrates to support various electronic components was also selected for comparison. When the LED chips lit up and work for 10 min, the surface temperature distribution of these substrates was recorded by an iphone-controlled infrared thermal camera. As shown in Figure 8, the maximum and average temperatures of F-graphene composite film detected from the selected box region are both lower than those of the commercial PI film, especially for the F-graphene film with 93 wt% F-graphene. This result indicates the efficient heat dissipation capability of F-graphene composite film with high filler loading.

3. Conclusions In summary, highly thermally conductive and electrically insulating F-graphene composite film containing slight amount of PVA was fabricated through a vacuum filtration 21 ACS Paragon Plus Environment


technique. The introduction of PVA is purposed to facilitate the uniform dispersion of F-graphene in water and enhance the linking between adjacent F-graphene nanosheets. Due to the well-organized alignment of F-graphene flakes along the in-plane direction, the prepared composite films exhibit an ultrahigh in-plane TC of 61.3 W m-1 K-1 as well as desired mechanical flexibility. Moreover, the composite films with such high F-graphene still maintain a good electrical insulation property. Therefore, this F-graphene film is expected to show potential applications in modern portable and flexible electronic devices for heat dissipation. ASSOCIATED CONTENT Supporting Information Available. Experimental details, SEM images of pristine F-graphite and exfoliated F-graphene, and TGA curves of F-graphene composite films with different compositions AUTHOR INFORMATION Corresponding Author *Authors for Correspondence: [email protected] ORCID Peiyi Wu: 0000-0001-7235-210X Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We gratefully acknowledge the ﬁnancial support from the Ministry of Science & Technology 22 ACS Paragon Plus Environment

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of China (No. 2016YFA0203302).

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TOC


Highly Thermally conductive fluorinated graphene films with superior

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