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Utilizing ammonium persulfate assisted expansion to fabricate flexible expanded graphite films with excellent thermal conductivity by introducing wrinkles
Carbon 153 (2019) 565e574
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Carbon journal homepage: www.elsevier.com/locate/carbon
Utilizing ammonium persulfate assisted expansion to fabricate flexible expanded graphite films with excellent thermal conductivity by introducing wrinkles Yuhang Liu 1, Bingxin Qu 1, Xunen Wu, Yuxin Tian, Kai Wu, Bowen Yu, Rongni Du, Qiang Fu**, Feng Chen* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 May 2019 Received in revised form 4 July 2019 Accepted 21 July 2019 Available online 22 July 2019
Traditional expanded graphite (EG) films exhibit excellent electric and thermal conductivities, due to weak oxidation degree induced low defect density. However, the stiff sheet structure makes EG films very brittle, which greatly limits its applications in next generation flexible electric devices. To overcome this challenge, ammonium persulfate ((NH4)2S2O8) is employed as both expansion agent and weak oxidation agent. As a result, the weak oxidation can effectively reduce the surface energy mismatching between EG and water, making it possible for EG to be exfoliated in water by high-speed shearing combined with sonication. Most particularly, the expansion step introduces plenty of wrinkles into the EG sheets, which notably improve the flexibility of EG films. Besides, using raw graphite with large lateral size and less (NH4)2S2O8 can greatly improve the final overall performances. The prepared EG films exhibit good electric and thermal conductivities of 2977 S/cm, 854 W/mK, respectively. 33.1 dB EMI shielding property is obtained with the film thickness of only 10 mm. Moreover, 800 times direct bending can be overcome without any structure break. This facial large scale and environmentally friendly method endows the films with great potential applications in flexible electric devices. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, the rapid development of micro-processors in smart phones has drawn great attention, due to the intense demands by application software. The fast calculation speed of microprocessors requires relatively high power, generating high temperature hot spots. The extortionate temperature can greatly reduce the life time of micro-processors. Besides, micro-processors are forced to reduce the calculation speed to protect the system but obviously decreasing the performances. Thus, heat dissipation devices are urgently needed to solve this problem [1,2]. The most commercially used heat dissipation device in smart phones or computers is metals, especially copper. However, heat is mainly spread by the high density of transport electrons, resulting in intrinsically defining the maximum thermal conductivity of 429 W/
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q. Fu), [email protected] (F. Chen). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.carbon.2019.07.079 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
mK for silver among metals. Besides, brittleness and high density are two great limitations for their use in flexible electric devices. By comparison, carbon materials can reach very high thermal conductivity as their heat conductivity is usually dominated by photons, the vibrations in their crystal lattice. The outstanding flexibility and low density endow carbon films with great potential applications in next generation flexible and wearable electric devices. Graphene and its derivatives, such as graphene nanosheets (GNS); reduced graphene oxide (rGO), the most widely studied 2D sheet materials, consist of a plane layer of conjugated carbon double bonds arranged in a honeycomb lattice [3e5]. The string bonding, low atomic mass, high crystal structure and regularity entrust graphene with outstanding electric transportation speed of 1500 cm2/V, thermal conductivity of 5300 W/mK [6,7]. Besides, the thin sheet thickness endows graphene films with remarkable flexibility. These excellent overall properties make graphene an alternative promising building block for next generation flexible heat dissipation devices. However, the most promising and scalable method to prepare graphene is from rGO route. The performances
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of carbon materials are often limited by the density of defects, holes, interfaces, boundaries, impurities [8]. GO after harsh oxidation consists plenty of defects. In order to restore these defects and recover the high electric, thermal conductivities, many different attempts have been tried. Typically, the chemically converted rGO often exhibits very low electric conductivity, such as 72 S/cm and 298 S/cm, due to the extremely incomplete reducing [9,10]. To overcome this problem, high temperature thermally reducing was proposed. For instance, Tian el al, thermally reduced GNS/GE composite films at 1600 C for 2 h. With the adding of 20% GE, thermal diffusivity of 150 mm2/s was obtained [11]. Hou el al, reported a GE/GE film thermally reduced at 1060 C for 2 h with electric conductivity of 850 S/cm and thermal conductivity of 220 W/mK [12]. Shen et al., reported GE films with extremely high thermal conductivity of 1100 W/mK and outstanding EMI shielding performance of 20 dB at the thickness of 8.4 mm by thermally reducing at 2000 C for 2 h [13]. However, two important issues should be stressed. Despite the fact that excellent thermal and electric conductivities are obtained, special equipment, severe conditions, lots of time and energy are required, making it extremely difficult for them to be scaled up for commercial needs. Most importantly, the high temperature graphitizing leads to the brittleness. No 180 direct bending is reported by these works. Thus, light weight films with super electric, thermal conductivities together with good mechanical flexibility are urgently demanded throng a facile method. Expanded graphite (EG) is the most widely used carbon material in industrial applications, due to its easy and cheap preparation process [14,15]. Besides, without any severe oxidation, EG has very low defect density and high C/O ratio, leading to good thermal and electric conductivities. However, due to the decomposition of intercalation agent concentrated sulfuric acid (98% H2SO4), the expansion step of EG releases lots of toxic gases, like sulfur dioxide, heavily polluting the environment and corroding the production equipment [16,17]. The intercalation step and expansion step are separated. Thus, extra drying step and thermal dilation, reqiring high temperature, are needed. Most importatly, the thick and stiff strcture of EG sheets results in very low mechanical properties and brittleness of EG films, greatly restricting its applications in flexible electric devices [18e20]. Although, many efforts have been made to endow EG films with some flexibility. The sacrifice of electric and thermal conductivities is evident [21e23]. How to regulate and control the intrinsic structure of EG sheets to endow EG films with good flexibilty still remains a great chanllenge. In this work, we designed to introduce some stretchable and foldable wrinkles into EG sheets to give EG films good flexibility. Ammonium persulfate ((NH4)2S2O8) is the right choice. Unlike traditional methods to prepare EG, the decomposition of (NH4)2S2O8 as gas expansion agent is directly progressed in 98% H2SO4, requiring no extra energy and releasing no toxic gas. The weak oxidation introduced by (NH4)2S2O8 can effectively reduce the surface energy mismatching between EG and water, making it possible for EG to be exfoliated in water by high-speed shearing combined with sonication. Besides, compared with traditional methods to prepare EG, the simultaneous intercalation step and expansion step can ensure full expansion. Most particularly, plenty of wrinkles are introduced through the gas expansion process. As a result, the EG films intrinsically reveal outstanding flexibility. This method to prepare flexible EG films is facial, cheap, environmentally friendly and can be simply scaled up for industrial applications. In this work, (NH4)2S2O8 is applied as both gas expansion agent and weak oxidation agent. After the gas expansion process, the sheet-like graphite was successfully converted to worm-like EG. Besides, the surface energy mismatching between EG and water is effectively reduced due to the weak oxidation introduced by
(NH4)2S2O8. The improved specific area as well as weak oxidation degree makes it possible for EG to be exfoliated in water by highspeed shearing and sonication. With choosing the best preparation condition, the obtained EG films exhibit outstanding electric, thermal conductivities of 2977 S/cm, 854 W/mK, respectively. Besides, the pure EG films can suffer from at least 800 times 180 direct bending without any structure deformation. We believe the remarkable comprehensive properties of the EG films can attract great attention in next generation foldable electric devices in smart phones. 2. Experimental 2.1. Chemicals Graphite flakes of different sizes and Graphite intercalation compound (GIC) were kindly supplied by Qingdao Jin Ri Lai Graphite Co. Ltd (32 mesh ¼ 500 mm; 100 mesh ¼ 150 mm; 400 mesh ¼ 38 mm). Concentrated sulfuric acid (H2SO4, AR, 95%e98%), Ammonium persulfate ((NH4)2S2O8, AR, >98%) were purchased from Kermal Chemical Reagent Plant, China. All the reagents were straightly used as obtained. 2.2. (NH4)2S2O8 treated graphite flakes into expanded graphite (EG) Graphite flakes were first sieved by sifters (32 mush, using 30e35 mesh sifters, recorded as L; 100 mush, suing 80e120 mesh sifters, recorded as M; 400 mesh, using 325e500 mesh sifters, recorded as S). 2 g graphite and 80 ml 98% H2SO4 were mixed and heated to 35 C under constant swirling. 10 g (NH4)2S2O8 was slowly added into the mixture. The gas expansion step must be conducted in an open reactor due to the release of gases (The safety evaluation and amplified EG preparation can be viewed in SP and Fig. S1a, S2a-f). The reaction was held at 35 C for 1 h until no obvious bubble observed. 80 ml deionized water was added by drops to avoid the temperature exceeding 60 C. The mixture was filtrated on a poly tetra fluoroethylene (PTFE) membrane (10 cm in diameter and 0.25 mm in pore size). Plenty of deionized water was poured onto the surface of EG under filtrating. The EG was washed for several times until the filtrate was neutral. The schematic diagram of gas expansion process is shown in Fig. 1a and b. 1-n represents the weight ratio between graphite and (NH4)2S2O8. O-L expresses large sized graphite after (NH4)2S2O8 expansion and oxidation. 2.3. The preparation process of flexible EG films The obtained EG was re-dispersed in 200 ml deionized water. The suspension was high-speed sheared at 8 kr/min for 30 min (IKA T25 digital ultra turrax) under 300 W ultrasonication to exfoliate the EG. To keep the uniformity of the suspension, constant mechanical agitation was applied. 10 ml suspension was further filtrated on a PTFE membrane (50 mm in diameter, 0.22 mm in pore size) for each EG film. The films were dried in a blast oven set at 60 C for 6 h 800 C thermally annealing for 5 min in muffle furnace was applied to reduce the EG films. A simple 20 Mpa pressure for 5 min was used to compact the EG films. After the pressing process, the film thickness decreased from 213 mm to 46 mm, indicating the density increases 4.6 times. The improved compact stacking of EG sheets can endow the EG films with better thermal and electric transportation pathways. Thus, the compressing process is extremely important for high quality EG film preparation. The schematic diagram of flexible EG film preparation can be viewed in Fig. 1c rO-L represents the large sized graphite after (NH4)2S2O8 oxidation and then thermally reducing at 800 C for 5 min.
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Fig. 1. Schematic diagram: (a) The preparation process of EG; (b) (NH4)2S2O8 assisted gas expansion; (c) The preparation process of flexible EG films. (A colour version of this figure can be viewed online.)
2.4. Materials characterization The gas components were recorded on a gas chromatography (GC, Fuli GC9790 Plus, China) for full spectrum scanning. The gases generated from the reaction were carefully led into an air trap. Scanning electron microscopy (SEM) images were taken on a SEM (JEOL JSM-5900LV) at an accelerating voltage of 5 kV. Powder samples were simply sprayed onto the conducting resin without spraying gold. The hydrophilic property of the films was conducted on water contact angle instrument (DSA 100 M). Raman spectra data were obtained on an Andor SR-500i Raman microscope with a 532 nm laser source. X-ray photoelectron spectroscopy (XPS) data were performed with an Escalab 250Xi photoelectron spectrometer with Al Ka (1486.6 eV) with a pass energy of 30 eV. Besides, the Xray source of 150 W was used for high resolution scan (C, O elements were characterized). The electric conductivity was measured on a four-point probe equipment (Four-point probe Technology Ltd, China). X-ray diffraction (XRD) data were collected on an X-ray diffractometer (Philips X'Pert pro MPD, Holland), using monochromatic Cu Ka1 radiation (l ¼ 1.5406 Å) at 40 KV. Thermogravimetric Analysis (TGA) was tested on a TG 209F1 Iris (Netzsch, Germany) with the heating rate of 10 C/min from 100 C to 600 C (All the samples were heated at 100 C for 10 min to remove the free water). The thermal diffusivity was measured by laser flash method on Netzsch LFA 467 NanoFlash instrument. All samples were uniformly processed into round shape with a diameter of 2.54 cm. The thermal conductivity (K) is calculated using the equation: K ¼ r Cp a. Here, r is the density and directly measured by mass and volume, and Cp is the specific heat capacity tested from the differential scanning calorimetry (DSC 1/700 Mettler-Toledo). The sample was measured with the temperature rising rate of 10 C/min from 20 C to 80 C, applying sapphire as reference sample. The mechanical properties were performed on a dynamic mechanical analyzer (DMA Q800 analyzer TA instruments, U.S.A). The tensile tests were conducted in a controlled force mode with a preload force of 0.1 N and a force rate of 0.5 N/min. All samples were cut with 20 mm in length and 3 mm in width and napped using film tension clamps with a clamp compliance of 1.5 N. Electromagnetic Interference (EMI) shielding properties at different frequencies (X-band: 8.2e12.4 GHz frequency) were conducted on
Agilent N5247A vector network analyzer (The detailed characterization method of EMI shielding can be viewed in SP and Fig. S3a). All the samples were carefully cut with a diameter of 13 mm. The whole EMI shielding (SEtotal) is counted by adding absorption (SEA) and reflection (SER) of microwaves. Moreover, the power coefficients of reflection (R), absorption (A) and transmission (T) can be counted from the measured parameters and the relationship is as follows: R þ A þ T ¼ 1 (1, 2, 3). The A, R and T of the samples can be calculated from the below equations respectively: SEtotal ¼ 10logT
(1)
SER ¼ 10log(1-R)
(2)
SEA ¼ SEtotal-SEReSEM
(3)
The SEM indicates the microwave multiple internal reflection and can be ignored when SEtotal 15 dB. 3. Results and discussion 3.1. The mechanism of improved flexibility for (NH4)2S2O8 expanded EG films Fig. 2a depicts the (NH4)2S2O8 gas expansion process. After the adding of gas expansion agent (NH4)2S2O8, many bubbles quickly generated in a few seconds (Video S1). After 1 h, almost no bubble produced, indicating the end of expansion step. Thus, we applied 1 h as the appropriate reaction time. Gas chromatography (GC) was applied to detect the gas components (Fig. S4a). The retention time peak at 3.588 belongs to oxygen while the peak at 4.421 is attributed to nitrogen. No other gas can be observed at the full spectrum scanning. Two possible decomposition equations are proposed (Fig. S4b). Table S1 shows a comparison table between (NH4)2S2O8 and other expansion agents. Compared with other gas expansion agents, the gas expansion agent (NH4)2S2O8 releases no toxic gas and requires no high temperature and microwave for expansion. Moreover, because (NH4)2S2O8 in 98% H2SO4 can intercalate and expand graphite at the same time in only 1 h, no other drying process and extra acid are required. Thus, applying (NH4)2S2O8 to
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Fig. 2. Digital photos: (a) Graphite in 98% H2SO4; (NH4)2S2O8 assisted gas expansion; After (NH4)2S2O8 gas expansion; EG in diluted H2SO4 (98% H2SO4: Water ¼ 1 : 2 by volume); (b) Graphite in 98% H2SO4; After (NH4)2S2O8 gas expansion; EG in diluted H2SO4 (98% H2SO4: Water ¼ 1 : 2 by volume); EG in neutral water; Exfoliated EG in neutral water (Up: as obtained; Down: after 1 h); (c) Graphite; EG; EG after exfoliation (All the weight is 0.1 g); (d) Graphite films; r1-5 films. (A colour version of this figure can be viewed online.)
prepare expanded graphite (EG) is very environmentally friendly and efficient. The formation of 98% H2SO4 intercalated graphite was detected by the appearance of deep-green color [24]. After adding water to dilute 98% H2SO4, the diluted H2SO4 quits from the interlayer of EG, transforming the color from deep-green to the original color for graphite (Silvery white). Due to the relatively high density of graphite compared with 98% H2SO4, raw graphite settled to the bottle (Fig. 2b). After (NH4)2S2O8 gas expansion, the volume of EG rapidly increases. Besides, EG sheets floated onto the surface of the suspension, implying the formation of porous structure. The porous structure, containing many gases into the interlayer, can support EG to float. After the exfoliation process of high-speed shearing combined with sonication, EG settled to the bottom again, indicating the removal of gas holes. This phenomenon certifies that the exfoliation process is effective. The water contact angle was conducted to study the hydrophilic property of EG (Figs. S5aec). Compared with the raw graphite (Water contact angle: 87.8 ), EG after gas expansion shows a lower angel of 63.7, reflecting some hydrophilic functional groups are introduced. The improved hydrophilic property is beneficial to the exfoliation of EG in water, due to the reducing of surface energy mismatching between EG and water. After the reducing process, the water contact angle is recovered to 84.7 with the removal of functional groups. Thus, the improved specific area and introduced weak oxidation greatly decrease the difficulty of exfoliation in water (Fig. 3b and Fig. 5c1). Compared with other exfoliation methods which often require organic solvents or surfactants to reduce the surface energy mismatching between graphite and water, we directly applied high-speed shearing to form fast vortices, enabling possibility to exfoliate EG in water [25,26]. Thus, the exfoliation method makes no pollution to the environment. The stacking density is directly calculated from the freeze-drying samples (Fig. 2c). The stacking density improved from 1.55 ml/g to 26.5 ml/g, due to the introduction of porous structure while decreased from 26.5 ml/g to 17.5 ml/g after exfoliation, indicating the removal of holes. Most importantly, the raw graphite is hard to be processed into a
complete film and the obtained films are extremely brittle (Fig. 2d). Obvious cracks are formed after a slight bend. By comparison, after the (NH4)2S2O8 expansion and exfoliation process, the EG films show excellent flexibility. The direct 180 bending leads no break of the films and only a weak crease is found. Moreover, (NH4)2S2O8 expanded EG without exfoliation was also filtrated (Figs. S6a and b). The result indicates the porous structure goes against film preparation. The films are brittle. Thus, both (NH4)2S2O8 expansion and exfoliation process are important in producing flexible EG films. The reason will be further discussed in the following section. Supplementary video related to this article can be found at https://doi.org/10.1016/j.carbon.2019.07.079. In order to explain the improved flexibility and film forming property, SEM was applied to investigate the microstructure of products from each reaction step. The raw graphite has very regular and rigid sheet structure (Fig. 3a). Besides, the surface is very smooth without any wrinkles. After, (NH4)2S2O8 gas expansion, graphite expanded many times along its Z axis to form worm-like EG (Fig. 3b). Most importantly, many wrinkles are observed in the EG sheets. One possible wrinkle forming mechanism is proposed. Microscope was applied to monitor the gas expansion process. After the adding of (NH4)2S2O8, many bubbles generate from the interlayer of graphite sheets (Fig. S7b). These gases, escaping from the interlayer of graphite, produce strong forces on the expanded graphite (EG) sheets. However, the weak oxidation by (NH4)2S2O8 is not enough to cause the exfoliation and destruction of EG sheets. Thus, the strong force, caused by gas escaping, directly acts on the EG sheets, forming plenty of wrinkles. Many wrinkles in the EG sheets can be evidently observed by the microscope. Besides, the volume of EG increases remarkably compared with raw graphite (Figs. S7a and c). These results are in accordance with the SEM images. The worm-like EG is unable to form compact stacking, making the EG films very fragile (Figs. S6a and b). After the exfoliation process, the worm-like EG is effectively exfoliated into sheet-like (Fig. 3c). Unlike the worm-like EG, the exfoliated sheetlike EG can notably orientate along with the water flow by
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Fig. 3. SEM images: (a) Graphite; (b) EG; (c) EG after exfoliation; (d) Graphite films; (e) EG films; (f) Schematic diagram of wrinkles in stretching and folding. (A colour version of this figure can be viewed online.)
vacuum filtration and form compact stacking. Besides, one interesting phenomenon should be stressed is that many wrinkles still remain on the exfoliated EG sheets. The micrometer structure on the EG surface contains puffy wrinkles, produced by the gas expansion step (Fig. 3c). After the compressing process, the puffy wrinkles are compacted. Thus, the macroscopic wrinkles change to nanoscopic wrinkles (Fig. 3e). Besides, the nanoscopic wrinkles contribute greatly to the flexibility of EG films. The schematic diagram of compacted wrinkles is shown in Fig. 3f. Fig. 3d and e shows the surface morphology of graphite films and exfoliated EG films. Raw graphite films are very smooth while EG films are uneven and contain many wrinkles on their surface. These wrinkles contribute greatly to the flexibility of EG films. Without any wrinkles, raw graphite films are easily broken by the stretching and folding stress. In contrast, with plenty of wrinkles, the wrinkles parallel to the stressing direction can be extended to avoid direct fracture in stretching model while the wrinkles perpendicular to the stressing direction can be partially spread. Thus, EG films markedly display super folding performances. Fig. 3f illustrates the stretchable and foldable wrinkles under tensile stress. All the EG sheets used in the following parts are EG after exfoliation. To further demonstrate the importance of wrinkles, commercial EG was applied to prepare commercial EG films for comparison with (NH4)2S2O8 expanded EG films. The commercial EG films also show silver white colour in sight (Fig. S8a). But the films are extremely brittle. Even a single direct folding can lead to the breakdown. The SEM images indicate the commercial EG sheets are very stiff and almost no puffy wrinkle is observed (Figs. S9a and b). Also, the surface of commercial EG films is very smooth, just like raw graphite films. Thus, without any wrinkles, the commercial EG films show bad flexibility.
3.2. The size effect of raw graphite on the electric conductivity for EG films Firstly, the size effect of raw graphite on the electric conductivity is carefully investigated. Three different sized graphite is mechanically sieved, namely 32 mush; 100 mush; 400 mush (Fig. 4a1-a3). The weight ratio of expansion agent to the raw graphite and
expansion time are fixed at 1:5 and 1 h, respectively. The defect restoration of each sample with conjugated structure is carefully detected by X-ray diffraction (XRD), Raman spectrum and X-ray photoelectron spectroscopy (XPS). The XRD data for all samples reveal an evident diffraction peak at 26.6 (Corresponding to the interlayer spacing of 3.35 Å), indicating the (002) lattice plane of graphite, typically (Figs. S10a and b) [27]. Large sized graphite shows stronger graphite characteristic peak, reflecting more complete crystallization structure. Another evidence is that large sized graphite possesses thick thickness (Figs. S11a and b). After (NH4)2S2O8 gas expansion, the graphite characteristic peak is sharply reduced, implying the positional disorders in the EG sheets and the reducing of sheet thickness, caused by expansion, exfoliation and the following turbostratic restacking of the constituent graphene nanosheets. Two peaks are observed in the Raman spectrum (Fig. 4b1-b3). The D-band (1330-1340 cm1) is attributed to the sp3 hybrid defect carbon of six membered rings while the Gband (1580-1600 cm1) belongs to the E2g phonons at the Brillouin zero center, representing the sp2 hybrid conjugated double bands [28]. All the data have been normalized by the G band intensity. The intensity ratio of D-band and G-band (ID/IG) indicates the average density of defects. XPS spectrum is applied to monitor the C/O ratio (Fig. 4c1-c3). The XPS full spectrum data are normalized by the C1s intensity. With the increased lateral size of graphite, the defect density ratio decreased from 0.215 to 0.0287 while the C/O ratio improved from 63.4 to 554. Often, low defect density and high C/O ratio can endow carbon materials with better electric conductivity [29]. Thus, we can infer large sized graphite is better raw material for preparing high quality EG with better electric conductivity. After (NH4)2S2O8 gas expansion, some oxygen functional groups are introduced to EG. For large sized graphite, the C/O ratio decreases from 554 to 10.7 while the defect density increases from 0.0287 to 0.336 due to the (NH4)2S2O8 oxidation, damaging the conjugated double bonds. Besides, compared with large sized EG, small sized EG contains more oxygen functional groups and defects. The reason can be attributed to the less complete crystallization s0tructure, making graphite with small lateral size easier to be oxidized. The reducing temperature is initially confirmed by the TGA curves
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Fig. 4. SEM images: (a1) Large sized graphite; (a2) Medium sized graphite; (a3) Small sized graphite; Raman spectrum: (b1) Different sized graphite; (b2) Different sized EG after oxidation; (b3) Different sized EG after reducing; XPS spectrum: (b1) Different sized graphite; (b2) Different sized EG after oxidation; (b3) Different sized EG after reducing; (d) Electric conductivity; (e) Schematic diagram of electric pathways for different sized EG. (A colour version of this figure can be viewed online.)
(Fig. S12a). The typical TGA curve of large sized EG displays two decomposing peaks. The first peak at 216 C is assigned to the decomposition of oxygen functional groups like hydroxyl, carbonyl and carboxyl groups. The second peak at 321 C belongs to the organic sulfur decomposition peak [30]. In order to ensure the full elimination of these oxygen functional groups, we applied 800 C thermally annealing for 5 min in muffle furnace to reduce the EG. After the reducing process, almost no weight loss from the TGA curve is observed, indicating EG is notably reduced at the chosen condition (Fig. S12b). After reducing, large sized EG exhibits the highest C/O ratio of 126 and lowest defect density (ID/IG: 0.140). The two characteristic parameters are extremely much better than
most chemically converted carbon materials (Table S2). Moreover, the characteristic peak at 26.6 increases after thermally reducing, indicating the restoration of the graphite crystallization structure (Fig. S8b). The electric conductivity was measured by the four-point probe. The large sized EG films show outstanding electric conductivity, namely 2977 S/cm, which is 2.1 times higher than medium sized EG films and 5.83 times higher than small sized EG (Fig. 4d). The reason can be attributed to the following factors: (1) Large sized EG shows the highest C/O ratio and lowest defect density (ID/ IG: 0.14), endowing EG sheets with well in-plane electron transportation. (2) The stacking of large sized EG can greatly decrease the overlapping sites and increase the overlapping area, indeed,
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reducing the steric hindrance of hopping electrons. The schematic diagram of electric pathways for different sized EG is shown in Fig. 4e. Peak fitting is applied to C1s spectrum to detect the functional group changing (Figs. S13aec). The peak location of CeC/C¼C is calibrated to be 284.6 eV. The C1s spectrum of oxidized carbon samples often possess four kinds of carbon bonds: CeC/C¼C (at 284.6 eV), CeOeC and CeO (at 286.6 eV), C¼O (287.8 eV), and OeC¼O (at 289 eV), respectively [31]. Detailed functional group content is shown in Table S3. Raw large sized graphite only contains 7.42% epoxy and hydroxyl groups. After oxidation, the mole content of epoxy and hydroxyl groups increases to 17.4% and 4.23% carbonyl group is newly introduced to EG, endowing EG with light hydrophilicity and reducing the surface energy mismatching between EG and water. Thus, the EG can be exfoliated via turbulent flow in water. After thermally reducing, carbonyl groups are completely removed and epoxy, hydroxyl groups decreased to 9.32%. Only 1.97% improvement of oxygen functional groups is observed, compared with raw graphite. Thus, the reducing condition is very efficient and the oxidation introduced by (NH4)2S2O8 is very easy to be removed with the comparison of chemically converted graphene from traditional Hummers' method (Table S2). The EG films show excellent electric conductivity up to 2977 S/cm. In the experimental step, ammonium ions are introduced along with (NH4)2S2O8. XPS spectrum is employed to investigate the N-containing functional groups in the prepared large sized EG after exfoliation and thermally annealing (Figs. S14a and b). The total amount of N atom is around 0.12 at%. Moreover, the N 1s spectrum exhibits the CeN bonds, indicating some N-containing functional group is grafted. But, the total amount is less than 0.5%. Such microscale N-containing functional groups can be ignored. 3.3. The influence of oxidation dosage on the electric conductivity and the overall properties for the best EG film sample In this section, all the samples were prepared from large sized graphite to obtain the best performances. The influence of oxidation dosage on the performances is carefully discussed. We applied four different mass ratios and found that EG can not form complete films just like raw graphite when the ratio is 1:2 (Graphite: (NH4)2S2O8 by weight) (Figs. S15a and b). It can be inferred that insufficient mass of (NH4)2S2O8 can not entirely expand graphite, making it difficult for EG to form complete films. The XRD data indicate the graphite crystallization structure is gradually destroyed with the increasing of oxidation content (Fig. 5a1, a2). After reducing, the graphite crystallization is restored and the EG with the least oxidation mass shows the best crystallization structure. Besides, with the improvement of oxidation mass, the defect density improves remarkably from 0.336 to 0.903 (ID/IG ratio) (Fig. 5b1, b2). After thermally reducing, the defects partially recover from 0.336 to 0.140 for 1e5 to r1-5. Other samples also show the same tendency, but the reduced EG with the least oxidation mass exhibits the mimimum defect density with the lowest ID/IG ratio. XPS spectrum displays the C/O ratio gradually decreases with the improvement of oxidation mass (Fig. 5c1, c2). Besides, more weight loss is observed with the oxidation mass improving in the TGA curves (Fig. S16a). The results are in concordance with the XPS data. After reducing, the same tendency is observed. r1-5 possesses the lowest oxygen content and defect density. Four-point method was used to measure the electric conductivity. The electric conductivity of oxidized EG decreased from 1897 S/cm to 951.3 S/cm with the increasing of oxidation mass (Fig. 5d). The reason can be attributed to the increased oxygen content and defect density, which greatly hinder the transportation of electrons. After thermally reducing, the oxygen functional groups are removed and the defects are repaired. As a result, the
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electric conductivity for each sample dramatically improves. EG with the least oxidation mass exhibits the best performances. The electric conductivity increased by 1.57 times from 1897 S/cm to 2977 S/cm. Thus, we can conclude that due to the introduction of less oxygen content and defects, using less (NH4)2S2O8 is beneficial to electric conductivity. However, too little oxidation mass may make it difficult for EG sheets to form a complete film due to insufficient expansion. 1e5 (Graphite: (NH4)2S2O8 by weight) ratio can lead to the delicate balance between film forming property and electric conductivity. 1e5 ratio is chosen as the best sample to discuss its thermal and mechanical properties. The in-plane thermal diffusivity (TD) of each sample was measured by the laser method (Fig. S17a). The TD value of 1e5 EG films is tested to be 347 mm2/s while after reducing, the TD notably improves to be 606 mm2/s due to the restoration of conjugated p-p structure (Fig. 5e). Typically, the in-plane thermal conductivity can be calculated from the following equation: K ¼ TD Cp r. The sample density is directly counted by the mass and volume to be 1.83 g/cm3 and 1.88 g/cm3 for 1e5 and r1-5, respectively. The density slightly improved after reducing, indicating denser stack of EG sheets. The specific heat (Cp) was measured by the DSC to be 0.75 J/kgoC at 25 C for EG (Fig. S17b). The thermal conductivity of r1-5 is calculated to be 854 W/mK. The most commonly used heat dissipation device in computers is copper. Such high thermal conductivity for our flexible EG films is 2.12 times higher than commercially used copper (401 W/mK). However, the density of EG films (1.88 g/cm3) is far below copper (8.96 g/cm3). Thus, the EG films are very suitable for light weight, small electric devices in our daily life. The mechanical properties for EG films after reducing are much higher than EG films before reducing (Tensile strength: from 11.3 Mpa to 9.12 Mpa; Elongation at break: from 0.763% to 0.588%) (Fig. 5f). The improved mechanical properties can be attributed to the following reasons: the restored defects and conjugated structure can improve the intrinsic performance of EG sheets. And the recovered p-p bonds can increase the interaction force between EG sheets. In order to demonstrate the high heat transfer speed of the EG films, infrared images were used to visually manifest the excellent heat dissipation ability (Fig. 5g and h). Before the smart phone working, the temperature distribution is almost the same. After the gaming program working for long, the range of temperature can be obviously detected. The temperature is 4.4 C lower with our EG films. The heat dissipation ability is remarkably improved with the assistance of EG films. Small CPUs in smart phones are easily affected by the near electric devices. Thus, flexible films with high thermal conductivity are not the only requirement. Efficient EMI shielding properties must also be taken into consideration. EG films prepared from 1:5 (Graphite: (NH4)2S2O8 by weight) after reducing was chosen as the sample with the best electric and thermal conductivities to study its EMI shielding properties. As shown in Fig. 6a, all the EMI shielding data exhibit weak frequency dependent properties. Thus, EMI shielding value at 10.3 GHz can be chosen to represent the average EMI shielding properties. Typically, the total EMI shielding (SEtotal) properties can be calculated from the sum of microwave absorption (SEA), microwave reflection (SER) and microwave multiple reflection (SEM can be ignored when the SEtotal is more than 15 dB). With the decreasing of film thickness, the EMI shielding properties show progressively decreasing tendency, due to the reducing of highly conductive EG sheets blocking the entering microwaves. The lowest total EMI shielding properties are obtained to be 33.1 dB with the film thickness of only 10 mm. However, the EMI shielding value is still higher than the industrial EMI shielding requirement (20 dB). In order to understand the EMI shielding mechanism, A, R, T EMI shielding coefficients are carefully calculated from the measured parameters (Fig. 6b). The T coefficient is
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Fig. 5. EG obtained from different oxidation ratios: (a1) XRD data; (b1) Raman spectrum; (c1) XPS spectrum; EG after reducing: (a2) XRD data; (b2) Raman spectrum; (c2) XPS spectrum; (d) Electric conductivity; (e) Thermal conductivity; (f) Mechanical properties; (g, h) Infrared images and corresponding temperature distribution curves (Left: rest; Right: work). (A colour version of this figure can be viewed online.)
Fig. 6. (a) EMI shielding properties in the frequency range of 8.2e12.4 GHz with different film thickness (Thickness error: ± 2 mm); (b) SEA, SER and SEtotal at the frequency range of 10.3 GHz and the corresponding EMI coefficients of R, A and T; (c) Direct folding circles; (d) Paper crane. (A colour version of this figure can be viewed online.)
0.005% which is far smaller than the R coefficient (87.3%) and A coefficient (12.6%), indicating the EG films are very effective in shielding the microwave. Besides, the reflection of microwave is the major mechanism. A possible mechanism of the EG films opposing microwave is proposed. The impedance mismatches on the EG surface reflect the majority of the microwaves [32,33]. The rest of microwaves enter into the films. The microwaves are continually absorbed by the EG sheets due to their good electric conductivity, leading to the fading of microwave energy. Finally, weak multiple internal reflection happens on the nearby EG sheets, owing to the tight stacking of EG sheet layer. The schematic diagram is shown in Fig. S18a. Direct 180 bending circles were applied to investigate the flexibility of the EG films (Fig. 6c). The normalized resistance almost keeps consistent even after 800 times direct bending, implying no structure crack is formed. Thus, the EG films are highly flexible and can be folded into a silvery paper crane (Fig. 6d). In recent years, carbon films with high electric, thermal conductivities and EMI shielding properties are often produced from
chemical converting or thermally reducing at extremely high temperature of graphene oxide (GO) or graphene nanosheets (GNS). The production process for them is often complex and requires toxic oxidation agents and intercalation agents, like KMnO4, CrCl3. Besides, the conditions of high temperature reducing require harsh equipment and cost lots of time and energy. This is indeed not in favor of commercial production. Thus, a facial method to prepare flexible carbon films with excellent thermal and EMI shielding properties is urgently needed. Using (NH4)2S2O8 to treat graphite is the right choice. Table 1 shows a comparison of our EG films with the previously reported net carbon films on their electric, thermal conductivities. Typically, the overall performances are better than most samples listed here except the film after 2850 C thermally reducing. But it is worth noticing that (NH4)2S2O8 is a green expansion agent due to the release of no toxic gas and containing no heavy metal. Besides, the weak oxidation makes it possible for EG to be exfoliated in water via turbulence flow, avoiding the use of polluting organic solvents or surfactants. We
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Table 1 A comparison of our EG film with the previously reported net carbon films on their electric, thermal conductivities. Methods
d(S/cm) K (W/mK) Tensile strength (MPa) Ref. no
Ball mill with oxalic acid (12 h) and dispersed in NMP, annealing at 600 C (2 h) Sonication in nitric acid and sulfuric acid (48e72 h) Sonication in NMP (50 h) Sonication in SC/water (10 h) Sonication in nitric acid and sulfuric acid (72 h), annealing at 1060 C (2 h) Sonication in SC/water (400 h), annealing at 500 C (2 h) Sonication in acetone/DMF (20 min), annealing at 250 C (14 h), mechanical compression (5 MPa) Ball milling in NMP (6 h), annealing at 2850 C (2 h), mechanical compression (30 MPa) Sonication in isopropanol (2 h) and PEI (0.5 h), annealing at 340 C (2 h), mechanical compression (100 psi, 1 h) Sonication in PEI/water (1 h), annealing at 340 C (2 h), mechanical compression (100 psi, 1 h) Chemical vapor deposition (NH4)2S2O8 expansion (1 h), thermally reducing (800 C, 5 min), mechanical compression (20Mpa, 5 min)
277 300 180 e 850 175 1443 2231 e 880 1136 2977
e e e 110 220 e e 1529 178 200 e 854
e e 12e18 e e 15e33 e e e e 22 11.3
[34] [35] [36] [37] [12] [38] [39] [40] [41] [42] [43] This work
s: Electrical conductivity; K: in-plane thermal conductivity; TS: tensile strength; NMP: N-methyl-pyrrolidone; SC: sodium cholate; PEI: polyethyleneimine.
believe the mechanical flexibility together with its outstanding electric, thermal and EMI shielding properties can endow the films with great potential use in next generation flexible electric devices. 4. Conclusion In conclusion, we have introduced plenty of wrinkles to endow expanded graphite (EG) films with super flexibility via ammonium persulfate ((NH4)2S2O8) gas expansion. The use of high quality and large sized graphite can greatly improve the overall performances of EG films. Besides, chosen appropriate amount of (NH4)2S2O8 to expand graphite (1 : 5; graphite: (NH4)2S2O8 by weight) can endow EG sheets with good film forming property and outstanding electric and thermal properties. Moreover, the slight oxidation introduced by (NH4)2S2O8 can effectively reduce the surface energy mismatching between EG and water. Thus, the as obtained worm-like EG can be effectively exfoliated into sheet-like in water. As a result, after a simple thermally reducing process and compression, the EG films exhibit high electric conductivity of 2977 S/cm and thermal conductivity up to 854 W/mK, respectively. 33.1 dB EMI shielding property is obtained with the film thickness of only 10 mm. The EG films can endure at least 800 times of repeated folding without any structure crack. Such facial, scalable and multifunctional EG films together with its easy producing process, outstanding electric and thermal conductivities open up great potential uses in next generation of high frequency flexible electronic devices, like wearable smart phones. Notes The authors declare no competing financial interest. Acknowledgments We would like to express our sincere thanks to the National Natural Science Foundation of China for financial support (Grant No. 51721091 and 51573102). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.07.079. References [1] Y. Zhang, Y.-J. Heo, Y.-R. Son, I. In, K.-H. An, B.-J. Kim, S.-J. Park, Recent advanced thermal interfacial materials: a review of conducting mechanisms and parameters of carbon materials, Carbon 142 (2019) 445e460. [2] W. Feng, M. Qin, Y. Feng, Toward highly thermally conductive all-carbon
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Carbon journal homepage: www.elsevier.com/locate/carbon
Utilizing ammonium persulfate assisted expansion to fabricate flexible expanded graphite films with excellent thermal conductivity by introducing wrinkles Yuhang Liu 1, Bingxin Qu 1, Xunen Wu, Yuxin Tian, Kai Wu, Bowen Yu, Rongni Du, Qiang Fu**, Feng Chen* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 May 2019 Received in revised form 4 July 2019 Accepted 21 July 2019 Available online 22 July 2019
Traditional expanded graphite (EG) films exhibit excellent electric and thermal conductivities, due to weak oxidation degree induced low defect density. However, the stiff sheet structure makes EG films very brittle, which greatly limits its applications in next generation flexible electric devices. To overcome this challenge, ammonium persulfate ((NH4)2S2O8) is employed as both expansion agent and weak oxidation agent. As a result, the weak oxidation can effectively reduce the surface energy mismatching between EG and water, making it possible for EG to be exfoliated in water by high-speed shearing combined with sonication. Most particularly, the expansion step introduces plenty of wrinkles into the EG sheets, which notably improve the flexibility of EG films. Besides, using raw graphite with large lateral size and less (NH4)2S2O8 can greatly improve the final overall performances. The prepared EG films exhibit good electric and thermal conductivities of 2977 S/cm, 854 W/mK, respectively. 33.1 dB EMI shielding property is obtained with the film thickness of only 10 mm. Moreover, 800 times direct bending can be overcome without any structure break. This facial large scale and environmentally friendly method endows the films with great potential applications in flexible electric devices. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, the rapid development of micro-processors in smart phones has drawn great attention, due to the intense demands by application software. The fast calculation speed of microprocessors requires relatively high power, generating high temperature hot spots. The extortionate temperature can greatly reduce the life time of micro-processors. Besides, micro-processors are forced to reduce the calculation speed to protect the system but obviously decreasing the performances. Thus, heat dissipation devices are urgently needed to solve this problem [1,2]. The most commercially used heat dissipation device in smart phones or computers is metals, especially copper. However, heat is mainly spread by the high density of transport electrons, resulting in intrinsically defining the maximum thermal conductivity of 429 W/
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q. Fu), [email protected] (F. Chen). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.carbon.2019.07.079 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
mK for silver among metals. Besides, brittleness and high density are two great limitations for their use in flexible electric devices. By comparison, carbon materials can reach very high thermal conductivity as their heat conductivity is usually dominated by photons, the vibrations in their crystal lattice. The outstanding flexibility and low density endow carbon films with great potential applications in next generation flexible and wearable electric devices. Graphene and its derivatives, such as graphene nanosheets (GNS); reduced graphene oxide (rGO), the most widely studied 2D sheet materials, consist of a plane layer of conjugated carbon double bonds arranged in a honeycomb lattice [3e5]. The string bonding, low atomic mass, high crystal structure and regularity entrust graphene with outstanding electric transportation speed of 1500 cm2/V, thermal conductivity of 5300 W/mK [6,7]. Besides, the thin sheet thickness endows graphene films with remarkable flexibility. These excellent overall properties make graphene an alternative promising building block for next generation flexible heat dissipation devices. However, the most promising and scalable method to prepare graphene is from rGO route. The performances
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of carbon materials are often limited by the density of defects, holes, interfaces, boundaries, impurities [8]. GO after harsh oxidation consists plenty of defects. In order to restore these defects and recover the high electric, thermal conductivities, many different attempts have been tried. Typically, the chemically converted rGO often exhibits very low electric conductivity, such as 72 S/cm and 298 S/cm, due to the extremely incomplete reducing [9,10]. To overcome this problem, high temperature thermally reducing was proposed. For instance, Tian el al, thermally reduced GNS/GE composite films at 1600 C for 2 h. With the adding of 20% GE, thermal diffusivity of 150 mm2/s was obtained [11]. Hou el al, reported a GE/GE film thermally reduced at 1060 C for 2 h with electric conductivity of 850 S/cm and thermal conductivity of 220 W/mK [12]. Shen et al., reported GE films with extremely high thermal conductivity of 1100 W/mK and outstanding EMI shielding performance of 20 dB at the thickness of 8.4 mm by thermally reducing at 2000 C for 2 h [13]. However, two important issues should be stressed. Despite the fact that excellent thermal and electric conductivities are obtained, special equipment, severe conditions, lots of time and energy are required, making it extremely difficult for them to be scaled up for commercial needs. Most importantly, the high temperature graphitizing leads to the brittleness. No 180 direct bending is reported by these works. Thus, light weight films with super electric, thermal conductivities together with good mechanical flexibility are urgently demanded throng a facile method. Expanded graphite (EG) is the most widely used carbon material in industrial applications, due to its easy and cheap preparation process [14,15]. Besides, without any severe oxidation, EG has very low defect density and high C/O ratio, leading to good thermal and electric conductivities. However, due to the decomposition of intercalation agent concentrated sulfuric acid (98% H2SO4), the expansion step of EG releases lots of toxic gases, like sulfur dioxide, heavily polluting the environment and corroding the production equipment [16,17]. The intercalation step and expansion step are separated. Thus, extra drying step and thermal dilation, reqiring high temperature, are needed. Most importatly, the thick and stiff strcture of EG sheets results in very low mechanical properties and brittleness of EG films, greatly restricting its applications in flexible electric devices [18e20]. Although, many efforts have been made to endow EG films with some flexibility. The sacrifice of electric and thermal conductivities is evident [21e23]. How to regulate and control the intrinsic structure of EG sheets to endow EG films with good flexibilty still remains a great chanllenge. In this work, we designed to introduce some stretchable and foldable wrinkles into EG sheets to give EG films good flexibility. Ammonium persulfate ((NH4)2S2O8) is the right choice. Unlike traditional methods to prepare EG, the decomposition of (NH4)2S2O8 as gas expansion agent is directly progressed in 98% H2SO4, requiring no extra energy and releasing no toxic gas. The weak oxidation introduced by (NH4)2S2O8 can effectively reduce the surface energy mismatching between EG and water, making it possible for EG to be exfoliated in water by high-speed shearing combined with sonication. Besides, compared with traditional methods to prepare EG, the simultaneous intercalation step and expansion step can ensure full expansion. Most particularly, plenty of wrinkles are introduced through the gas expansion process. As a result, the EG films intrinsically reveal outstanding flexibility. This method to prepare flexible EG films is facial, cheap, environmentally friendly and can be simply scaled up for industrial applications. In this work, (NH4)2S2O8 is applied as both gas expansion agent and weak oxidation agent. After the gas expansion process, the sheet-like graphite was successfully converted to worm-like EG. Besides, the surface energy mismatching between EG and water is effectively reduced due to the weak oxidation introduced by
(NH4)2S2O8. The improved specific area as well as weak oxidation degree makes it possible for EG to be exfoliated in water by highspeed shearing and sonication. With choosing the best preparation condition, the obtained EG films exhibit outstanding electric, thermal conductivities of 2977 S/cm, 854 W/mK, respectively. Besides, the pure EG films can suffer from at least 800 times 180 direct bending without any structure deformation. We believe the remarkable comprehensive properties of the EG films can attract great attention in next generation foldable electric devices in smart phones. 2. Experimental 2.1. Chemicals Graphite flakes of different sizes and Graphite intercalation compound (GIC) were kindly supplied by Qingdao Jin Ri Lai Graphite Co. Ltd (32 mesh ¼ 500 mm; 100 mesh ¼ 150 mm; 400 mesh ¼ 38 mm). Concentrated sulfuric acid (H2SO4, AR, 95%e98%), Ammonium persulfate ((NH4)2S2O8, AR, >98%) were purchased from Kermal Chemical Reagent Plant, China. All the reagents were straightly used as obtained. 2.2. (NH4)2S2O8 treated graphite flakes into expanded graphite (EG) Graphite flakes were first sieved by sifters (32 mush, using 30e35 mesh sifters, recorded as L; 100 mush, suing 80e120 mesh sifters, recorded as M; 400 mesh, using 325e500 mesh sifters, recorded as S). 2 g graphite and 80 ml 98% H2SO4 were mixed and heated to 35 C under constant swirling. 10 g (NH4)2S2O8 was slowly added into the mixture. The gas expansion step must be conducted in an open reactor due to the release of gases (The safety evaluation and amplified EG preparation can be viewed in SP and Fig. S1a, S2a-f). The reaction was held at 35 C for 1 h until no obvious bubble observed. 80 ml deionized water was added by drops to avoid the temperature exceeding 60 C. The mixture was filtrated on a poly tetra fluoroethylene (PTFE) membrane (10 cm in diameter and 0.25 mm in pore size). Plenty of deionized water was poured onto the surface of EG under filtrating. The EG was washed for several times until the filtrate was neutral. The schematic diagram of gas expansion process is shown in Fig. 1a and b. 1-n represents the weight ratio between graphite and (NH4)2S2O8. O-L expresses large sized graphite after (NH4)2S2O8 expansion and oxidation. 2.3. The preparation process of flexible EG films The obtained EG was re-dispersed in 200 ml deionized water. The suspension was high-speed sheared at 8 kr/min for 30 min (IKA T25 digital ultra turrax) under 300 W ultrasonication to exfoliate the EG. To keep the uniformity of the suspension, constant mechanical agitation was applied. 10 ml suspension was further filtrated on a PTFE membrane (50 mm in diameter, 0.22 mm in pore size) for each EG film. The films were dried in a blast oven set at 60 C for 6 h 800 C thermally annealing for 5 min in muffle furnace was applied to reduce the EG films. A simple 20 Mpa pressure for 5 min was used to compact the EG films. After the pressing process, the film thickness decreased from 213 mm to 46 mm, indicating the density increases 4.6 times. The improved compact stacking of EG sheets can endow the EG films with better thermal and electric transportation pathways. Thus, the compressing process is extremely important for high quality EG film preparation. The schematic diagram of flexible EG film preparation can be viewed in Fig. 1c rO-L represents the large sized graphite after (NH4)2S2O8 oxidation and then thermally reducing at 800 C for 5 min.
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Fig. 1. Schematic diagram: (a) The preparation process of EG; (b) (NH4)2S2O8 assisted gas expansion; (c) The preparation process of flexible EG films. (A colour version of this figure can be viewed online.)
2.4. Materials characterization The gas components were recorded on a gas chromatography (GC, Fuli GC9790 Plus, China) for full spectrum scanning. The gases generated from the reaction were carefully led into an air trap. Scanning electron microscopy (SEM) images were taken on a SEM (JEOL JSM-5900LV) at an accelerating voltage of 5 kV. Powder samples were simply sprayed onto the conducting resin without spraying gold. The hydrophilic property of the films was conducted on water contact angle instrument (DSA 100 M). Raman spectra data were obtained on an Andor SR-500i Raman microscope with a 532 nm laser source. X-ray photoelectron spectroscopy (XPS) data were performed with an Escalab 250Xi photoelectron spectrometer with Al Ka (1486.6 eV) with a pass energy of 30 eV. Besides, the Xray source of 150 W was used for high resolution scan (C, O elements were characterized). The electric conductivity was measured on a four-point probe equipment (Four-point probe Technology Ltd, China). X-ray diffraction (XRD) data were collected on an X-ray diffractometer (Philips X'Pert pro MPD, Holland), using monochromatic Cu Ka1 radiation (l ¼ 1.5406 Å) at 40 KV. Thermogravimetric Analysis (TGA) was tested on a TG 209F1 Iris (Netzsch, Germany) with the heating rate of 10 C/min from 100 C to 600 C (All the samples were heated at 100 C for 10 min to remove the free water). The thermal diffusivity was measured by laser flash method on Netzsch LFA 467 NanoFlash instrument. All samples were uniformly processed into round shape with a diameter of 2.54 cm. The thermal conductivity (K) is calculated using the equation: K ¼ r Cp a. Here, r is the density and directly measured by mass and volume, and Cp is the specific heat capacity tested from the differential scanning calorimetry (DSC 1/700 Mettler-Toledo). The sample was measured with the temperature rising rate of 10 C/min from 20 C to 80 C, applying sapphire as reference sample. The mechanical properties were performed on a dynamic mechanical analyzer (DMA Q800 analyzer TA instruments, U.S.A). The tensile tests were conducted in a controlled force mode with a preload force of 0.1 N and a force rate of 0.5 N/min. All samples were cut with 20 mm in length and 3 mm in width and napped using film tension clamps with a clamp compliance of 1.5 N. Electromagnetic Interference (EMI) shielding properties at different frequencies (X-band: 8.2e12.4 GHz frequency) were conducted on
Agilent N5247A vector network analyzer (The detailed characterization method of EMI shielding can be viewed in SP and Fig. S3a). All the samples were carefully cut with a diameter of 13 mm. The whole EMI shielding (SEtotal) is counted by adding absorption (SEA) and reflection (SER) of microwaves. Moreover, the power coefficients of reflection (R), absorption (A) and transmission (T) can be counted from the measured parameters and the relationship is as follows: R þ A þ T ¼ 1 (1, 2, 3). The A, R and T of the samples can be calculated from the below equations respectively: SEtotal ¼ 10logT
(1)
SER ¼ 10log(1-R)
(2)
SEA ¼ SEtotal-SEReSEM
(3)
The SEM indicates the microwave multiple internal reflection and can be ignored when SEtotal 15 dB. 3. Results and discussion 3.1. The mechanism of improved flexibility for (NH4)2S2O8 expanded EG films Fig. 2a depicts the (NH4)2S2O8 gas expansion process. After the adding of gas expansion agent (NH4)2S2O8, many bubbles quickly generated in a few seconds (Video S1). After 1 h, almost no bubble produced, indicating the end of expansion step. Thus, we applied 1 h as the appropriate reaction time. Gas chromatography (GC) was applied to detect the gas components (Fig. S4a). The retention time peak at 3.588 belongs to oxygen while the peak at 4.421 is attributed to nitrogen. No other gas can be observed at the full spectrum scanning. Two possible decomposition equations are proposed (Fig. S4b). Table S1 shows a comparison table between (NH4)2S2O8 and other expansion agents. Compared with other gas expansion agents, the gas expansion agent (NH4)2S2O8 releases no toxic gas and requires no high temperature and microwave for expansion. Moreover, because (NH4)2S2O8 in 98% H2SO4 can intercalate and expand graphite at the same time in only 1 h, no other drying process and extra acid are required. Thus, applying (NH4)2S2O8 to
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Fig. 2. Digital photos: (a) Graphite in 98% H2SO4; (NH4)2S2O8 assisted gas expansion; After (NH4)2S2O8 gas expansion; EG in diluted H2SO4 (98% H2SO4: Water ¼ 1 : 2 by volume); (b) Graphite in 98% H2SO4; After (NH4)2S2O8 gas expansion; EG in diluted H2SO4 (98% H2SO4: Water ¼ 1 : 2 by volume); EG in neutral water; Exfoliated EG in neutral water (Up: as obtained; Down: after 1 h); (c) Graphite; EG; EG after exfoliation (All the weight is 0.1 g); (d) Graphite films; r1-5 films. (A colour version of this figure can be viewed online.)
prepare expanded graphite (EG) is very environmentally friendly and efficient. The formation of 98% H2SO4 intercalated graphite was detected by the appearance of deep-green color [24]. After adding water to dilute 98% H2SO4, the diluted H2SO4 quits from the interlayer of EG, transforming the color from deep-green to the original color for graphite (Silvery white). Due to the relatively high density of graphite compared with 98% H2SO4, raw graphite settled to the bottle (Fig. 2b). After (NH4)2S2O8 gas expansion, the volume of EG rapidly increases. Besides, EG sheets floated onto the surface of the suspension, implying the formation of porous structure. The porous structure, containing many gases into the interlayer, can support EG to float. After the exfoliation process of high-speed shearing combined with sonication, EG settled to the bottom again, indicating the removal of gas holes. This phenomenon certifies that the exfoliation process is effective. The water contact angle was conducted to study the hydrophilic property of EG (Figs. S5aec). Compared with the raw graphite (Water contact angle: 87.8 ), EG after gas expansion shows a lower angel of 63.7, reflecting some hydrophilic functional groups are introduced. The improved hydrophilic property is beneficial to the exfoliation of EG in water, due to the reducing of surface energy mismatching between EG and water. After the reducing process, the water contact angle is recovered to 84.7 with the removal of functional groups. Thus, the improved specific area and introduced weak oxidation greatly decrease the difficulty of exfoliation in water (Fig. 3b and Fig. 5c1). Compared with other exfoliation methods which often require organic solvents or surfactants to reduce the surface energy mismatching between graphite and water, we directly applied high-speed shearing to form fast vortices, enabling possibility to exfoliate EG in water [25,26]. Thus, the exfoliation method makes no pollution to the environment. The stacking density is directly calculated from the freeze-drying samples (Fig. 2c). The stacking density improved from 1.55 ml/g to 26.5 ml/g, due to the introduction of porous structure while decreased from 26.5 ml/g to 17.5 ml/g after exfoliation, indicating the removal of holes. Most importantly, the raw graphite is hard to be processed into a
complete film and the obtained films are extremely brittle (Fig. 2d). Obvious cracks are formed after a slight bend. By comparison, after the (NH4)2S2O8 expansion and exfoliation process, the EG films show excellent flexibility. The direct 180 bending leads no break of the films and only a weak crease is found. Moreover, (NH4)2S2O8 expanded EG without exfoliation was also filtrated (Figs. S6a and b). The result indicates the porous structure goes against film preparation. The films are brittle. Thus, both (NH4)2S2O8 expansion and exfoliation process are important in producing flexible EG films. The reason will be further discussed in the following section. Supplementary video related to this article can be found at https://doi.org/10.1016/j.carbon.2019.07.079. In order to explain the improved flexibility and film forming property, SEM was applied to investigate the microstructure of products from each reaction step. The raw graphite has very regular and rigid sheet structure (Fig. 3a). Besides, the surface is very smooth without any wrinkles. After, (NH4)2S2O8 gas expansion, graphite expanded many times along its Z axis to form worm-like EG (Fig. 3b). Most importantly, many wrinkles are observed in the EG sheets. One possible wrinkle forming mechanism is proposed. Microscope was applied to monitor the gas expansion process. After the adding of (NH4)2S2O8, many bubbles generate from the interlayer of graphite sheets (Fig. S7b). These gases, escaping from the interlayer of graphite, produce strong forces on the expanded graphite (EG) sheets. However, the weak oxidation by (NH4)2S2O8 is not enough to cause the exfoliation and destruction of EG sheets. Thus, the strong force, caused by gas escaping, directly acts on the EG sheets, forming plenty of wrinkles. Many wrinkles in the EG sheets can be evidently observed by the microscope. Besides, the volume of EG increases remarkably compared with raw graphite (Figs. S7a and c). These results are in accordance with the SEM images. The worm-like EG is unable to form compact stacking, making the EG films very fragile (Figs. S6a and b). After the exfoliation process, the worm-like EG is effectively exfoliated into sheet-like (Fig. 3c). Unlike the worm-like EG, the exfoliated sheetlike EG can notably orientate along with the water flow by
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Fig. 3. SEM images: (a) Graphite; (b) EG; (c) EG after exfoliation; (d) Graphite films; (e) EG films; (f) Schematic diagram of wrinkles in stretching and folding. (A colour version of this figure can be viewed online.)
vacuum filtration and form compact stacking. Besides, one interesting phenomenon should be stressed is that many wrinkles still remain on the exfoliated EG sheets. The micrometer structure on the EG surface contains puffy wrinkles, produced by the gas expansion step (Fig. 3c). After the compressing process, the puffy wrinkles are compacted. Thus, the macroscopic wrinkles change to nanoscopic wrinkles (Fig. 3e). Besides, the nanoscopic wrinkles contribute greatly to the flexibility of EG films. The schematic diagram of compacted wrinkles is shown in Fig. 3f. Fig. 3d and e shows the surface morphology of graphite films and exfoliated EG films. Raw graphite films are very smooth while EG films are uneven and contain many wrinkles on their surface. These wrinkles contribute greatly to the flexibility of EG films. Without any wrinkles, raw graphite films are easily broken by the stretching and folding stress. In contrast, with plenty of wrinkles, the wrinkles parallel to the stressing direction can be extended to avoid direct fracture in stretching model while the wrinkles perpendicular to the stressing direction can be partially spread. Thus, EG films markedly display super folding performances. Fig. 3f illustrates the stretchable and foldable wrinkles under tensile stress. All the EG sheets used in the following parts are EG after exfoliation. To further demonstrate the importance of wrinkles, commercial EG was applied to prepare commercial EG films for comparison with (NH4)2S2O8 expanded EG films. The commercial EG films also show silver white colour in sight (Fig. S8a). But the films are extremely brittle. Even a single direct folding can lead to the breakdown. The SEM images indicate the commercial EG sheets are very stiff and almost no puffy wrinkle is observed (Figs. S9a and b). Also, the surface of commercial EG films is very smooth, just like raw graphite films. Thus, without any wrinkles, the commercial EG films show bad flexibility.
3.2. The size effect of raw graphite on the electric conductivity for EG films Firstly, the size effect of raw graphite on the electric conductivity is carefully investigated. Three different sized graphite is mechanically sieved, namely 32 mush; 100 mush; 400 mush (Fig. 4a1-a3). The weight ratio of expansion agent to the raw graphite and
expansion time are fixed at 1:5 and 1 h, respectively. The defect restoration of each sample with conjugated structure is carefully detected by X-ray diffraction (XRD), Raman spectrum and X-ray photoelectron spectroscopy (XPS). The XRD data for all samples reveal an evident diffraction peak at 26.6 (Corresponding to the interlayer spacing of 3.35 Å), indicating the (002) lattice plane of graphite, typically (Figs. S10a and b) [27]. Large sized graphite shows stronger graphite characteristic peak, reflecting more complete crystallization structure. Another evidence is that large sized graphite possesses thick thickness (Figs. S11a and b). After (NH4)2S2O8 gas expansion, the graphite characteristic peak is sharply reduced, implying the positional disorders in the EG sheets and the reducing of sheet thickness, caused by expansion, exfoliation and the following turbostratic restacking of the constituent graphene nanosheets. Two peaks are observed in the Raman spectrum (Fig. 4b1-b3). The D-band (1330-1340 cm1) is attributed to the sp3 hybrid defect carbon of six membered rings while the Gband (1580-1600 cm1) belongs to the E2g phonons at the Brillouin zero center, representing the sp2 hybrid conjugated double bands [28]. All the data have been normalized by the G band intensity. The intensity ratio of D-band and G-band (ID/IG) indicates the average density of defects. XPS spectrum is applied to monitor the C/O ratio (Fig. 4c1-c3). The XPS full spectrum data are normalized by the C1s intensity. With the increased lateral size of graphite, the defect density ratio decreased from 0.215 to 0.0287 while the C/O ratio improved from 63.4 to 554. Often, low defect density and high C/O ratio can endow carbon materials with better electric conductivity [29]. Thus, we can infer large sized graphite is better raw material for preparing high quality EG with better electric conductivity. After (NH4)2S2O8 gas expansion, some oxygen functional groups are introduced to EG. For large sized graphite, the C/O ratio decreases from 554 to 10.7 while the defect density increases from 0.0287 to 0.336 due to the (NH4)2S2O8 oxidation, damaging the conjugated double bonds. Besides, compared with large sized EG, small sized EG contains more oxygen functional groups and defects. The reason can be attributed to the less complete crystallization s0tructure, making graphite with small lateral size easier to be oxidized. The reducing temperature is initially confirmed by the TGA curves
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Fig. 4. SEM images: (a1) Large sized graphite; (a2) Medium sized graphite; (a3) Small sized graphite; Raman spectrum: (b1) Different sized graphite; (b2) Different sized EG after oxidation; (b3) Different sized EG after reducing; XPS spectrum: (b1) Different sized graphite; (b2) Different sized EG after oxidation; (b3) Different sized EG after reducing; (d) Electric conductivity; (e) Schematic diagram of electric pathways for different sized EG. (A colour version of this figure can be viewed online.)
(Fig. S12a). The typical TGA curve of large sized EG displays two decomposing peaks. The first peak at 216 C is assigned to the decomposition of oxygen functional groups like hydroxyl, carbonyl and carboxyl groups. The second peak at 321 C belongs to the organic sulfur decomposition peak [30]. In order to ensure the full elimination of these oxygen functional groups, we applied 800 C thermally annealing for 5 min in muffle furnace to reduce the EG. After the reducing process, almost no weight loss from the TGA curve is observed, indicating EG is notably reduced at the chosen condition (Fig. S12b). After reducing, large sized EG exhibits the highest C/O ratio of 126 and lowest defect density (ID/IG: 0.140). The two characteristic parameters are extremely much better than
most chemically converted carbon materials (Table S2). Moreover, the characteristic peak at 26.6 increases after thermally reducing, indicating the restoration of the graphite crystallization structure (Fig. S8b). The electric conductivity was measured by the four-point probe. The large sized EG films show outstanding electric conductivity, namely 2977 S/cm, which is 2.1 times higher than medium sized EG films and 5.83 times higher than small sized EG (Fig. 4d). The reason can be attributed to the following factors: (1) Large sized EG shows the highest C/O ratio and lowest defect density (ID/ IG: 0.14), endowing EG sheets with well in-plane electron transportation. (2) The stacking of large sized EG can greatly decrease the overlapping sites and increase the overlapping area, indeed,
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reducing the steric hindrance of hopping electrons. The schematic diagram of electric pathways for different sized EG is shown in Fig. 4e. Peak fitting is applied to C1s spectrum to detect the functional group changing (Figs. S13aec). The peak location of CeC/C¼C is calibrated to be 284.6 eV. The C1s spectrum of oxidized carbon samples often possess four kinds of carbon bonds: CeC/C¼C (at 284.6 eV), CeOeC and CeO (at 286.6 eV), C¼O (287.8 eV), and OeC¼O (at 289 eV), respectively [31]. Detailed functional group content is shown in Table S3. Raw large sized graphite only contains 7.42% epoxy and hydroxyl groups. After oxidation, the mole content of epoxy and hydroxyl groups increases to 17.4% and 4.23% carbonyl group is newly introduced to EG, endowing EG with light hydrophilicity and reducing the surface energy mismatching between EG and water. Thus, the EG can be exfoliated via turbulent flow in water. After thermally reducing, carbonyl groups are completely removed and epoxy, hydroxyl groups decreased to 9.32%. Only 1.97% improvement of oxygen functional groups is observed, compared with raw graphite. Thus, the reducing condition is very efficient and the oxidation introduced by (NH4)2S2O8 is very easy to be removed with the comparison of chemically converted graphene from traditional Hummers' method (Table S2). The EG films show excellent electric conductivity up to 2977 S/cm. In the experimental step, ammonium ions are introduced along with (NH4)2S2O8. XPS spectrum is employed to investigate the N-containing functional groups in the prepared large sized EG after exfoliation and thermally annealing (Figs. S14a and b). The total amount of N atom is around 0.12 at%. Moreover, the N 1s spectrum exhibits the CeN bonds, indicating some N-containing functional group is grafted. But, the total amount is less than 0.5%. Such microscale N-containing functional groups can be ignored. 3.3. The influence of oxidation dosage on the electric conductivity and the overall properties for the best EG film sample In this section, all the samples were prepared from large sized graphite to obtain the best performances. The influence of oxidation dosage on the performances is carefully discussed. We applied four different mass ratios and found that EG can not form complete films just like raw graphite when the ratio is 1:2 (Graphite: (NH4)2S2O8 by weight) (Figs. S15a and b). It can be inferred that insufficient mass of (NH4)2S2O8 can not entirely expand graphite, making it difficult for EG to form complete films. The XRD data indicate the graphite crystallization structure is gradually destroyed with the increasing of oxidation content (Fig. 5a1, a2). After reducing, the graphite crystallization is restored and the EG with the least oxidation mass shows the best crystallization structure. Besides, with the improvement of oxidation mass, the defect density improves remarkably from 0.336 to 0.903 (ID/IG ratio) (Fig. 5b1, b2). After thermally reducing, the defects partially recover from 0.336 to 0.140 for 1e5 to r1-5. Other samples also show the same tendency, but the reduced EG with the least oxidation mass exhibits the mimimum defect density with the lowest ID/IG ratio. XPS spectrum displays the C/O ratio gradually decreases with the improvement of oxidation mass (Fig. 5c1, c2). Besides, more weight loss is observed with the oxidation mass improving in the TGA curves (Fig. S16a). The results are in concordance with the XPS data. After reducing, the same tendency is observed. r1-5 possesses the lowest oxygen content and defect density. Four-point method was used to measure the electric conductivity. The electric conductivity of oxidized EG decreased from 1897 S/cm to 951.3 S/cm with the increasing of oxidation mass (Fig. 5d). The reason can be attributed to the increased oxygen content and defect density, which greatly hinder the transportation of electrons. After thermally reducing, the oxygen functional groups are removed and the defects are repaired. As a result, the
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electric conductivity for each sample dramatically improves. EG with the least oxidation mass exhibits the best performances. The electric conductivity increased by 1.57 times from 1897 S/cm to 2977 S/cm. Thus, we can conclude that due to the introduction of less oxygen content and defects, using less (NH4)2S2O8 is beneficial to electric conductivity. However, too little oxidation mass may make it difficult for EG sheets to form a complete film due to insufficient expansion. 1e5 (Graphite: (NH4)2S2O8 by weight) ratio can lead to the delicate balance between film forming property and electric conductivity. 1e5 ratio is chosen as the best sample to discuss its thermal and mechanical properties. The in-plane thermal diffusivity (TD) of each sample was measured by the laser method (Fig. S17a). The TD value of 1e5 EG films is tested to be 347 mm2/s while after reducing, the TD notably improves to be 606 mm2/s due to the restoration of conjugated p-p structure (Fig. 5e). Typically, the in-plane thermal conductivity can be calculated from the following equation: K ¼ TD Cp r. The sample density is directly counted by the mass and volume to be 1.83 g/cm3 and 1.88 g/cm3 for 1e5 and r1-5, respectively. The density slightly improved after reducing, indicating denser stack of EG sheets. The specific heat (Cp) was measured by the DSC to be 0.75 J/kgoC at 25 C for EG (Fig. S17b). The thermal conductivity of r1-5 is calculated to be 854 W/mK. The most commonly used heat dissipation device in computers is copper. Such high thermal conductivity for our flexible EG films is 2.12 times higher than commercially used copper (401 W/mK). However, the density of EG films (1.88 g/cm3) is far below copper (8.96 g/cm3). Thus, the EG films are very suitable for light weight, small electric devices in our daily life. The mechanical properties for EG films after reducing are much higher than EG films before reducing (Tensile strength: from 11.3 Mpa to 9.12 Mpa; Elongation at break: from 0.763% to 0.588%) (Fig. 5f). The improved mechanical properties can be attributed to the following reasons: the restored defects and conjugated structure can improve the intrinsic performance of EG sheets. And the recovered p-p bonds can increase the interaction force between EG sheets. In order to demonstrate the high heat transfer speed of the EG films, infrared images were used to visually manifest the excellent heat dissipation ability (Fig. 5g and h). Before the smart phone working, the temperature distribution is almost the same. After the gaming program working for long, the range of temperature can be obviously detected. The temperature is 4.4 C lower with our EG films. The heat dissipation ability is remarkably improved with the assistance of EG films. Small CPUs in smart phones are easily affected by the near electric devices. Thus, flexible films with high thermal conductivity are not the only requirement. Efficient EMI shielding properties must also be taken into consideration. EG films prepared from 1:5 (Graphite: (NH4)2S2O8 by weight) after reducing was chosen as the sample with the best electric and thermal conductivities to study its EMI shielding properties. As shown in Fig. 6a, all the EMI shielding data exhibit weak frequency dependent properties. Thus, EMI shielding value at 10.3 GHz can be chosen to represent the average EMI shielding properties. Typically, the total EMI shielding (SEtotal) properties can be calculated from the sum of microwave absorption (SEA), microwave reflection (SER) and microwave multiple reflection (SEM can be ignored when the SEtotal is more than 15 dB). With the decreasing of film thickness, the EMI shielding properties show progressively decreasing tendency, due to the reducing of highly conductive EG sheets blocking the entering microwaves. The lowest total EMI shielding properties are obtained to be 33.1 dB with the film thickness of only 10 mm. However, the EMI shielding value is still higher than the industrial EMI shielding requirement (20 dB). In order to understand the EMI shielding mechanism, A, R, T EMI shielding coefficients are carefully calculated from the measured parameters (Fig. 6b). The T coefficient is
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Fig. 5. EG obtained from different oxidation ratios: (a1) XRD data; (b1) Raman spectrum; (c1) XPS spectrum; EG after reducing: (a2) XRD data; (b2) Raman spectrum; (c2) XPS spectrum; (d) Electric conductivity; (e) Thermal conductivity; (f) Mechanical properties; (g, h) Infrared images and corresponding temperature distribution curves (Left: rest; Right: work). (A colour version of this figure can be viewed online.)
Fig. 6. (a) EMI shielding properties in the frequency range of 8.2e12.4 GHz with different film thickness (Thickness error: ± 2 mm); (b) SEA, SER and SEtotal at the frequency range of 10.3 GHz and the corresponding EMI coefficients of R, A and T; (c) Direct folding circles; (d) Paper crane. (A colour version of this figure can be viewed online.)
0.005% which is far smaller than the R coefficient (87.3%) and A coefficient (12.6%), indicating the EG films are very effective in shielding the microwave. Besides, the reflection of microwave is the major mechanism. A possible mechanism of the EG films opposing microwave is proposed. The impedance mismatches on the EG surface reflect the majority of the microwaves [32,33]. The rest of microwaves enter into the films. The microwaves are continually absorbed by the EG sheets due to their good electric conductivity, leading to the fading of microwave energy. Finally, weak multiple internal reflection happens on the nearby EG sheets, owing to the tight stacking of EG sheet layer. The schematic diagram is shown in Fig. S18a. Direct 180 bending circles were applied to investigate the flexibility of the EG films (Fig. 6c). The normalized resistance almost keeps consistent even after 800 times direct bending, implying no structure crack is formed. Thus, the EG films are highly flexible and can be folded into a silvery paper crane (Fig. 6d). In recent years, carbon films with high electric, thermal conductivities and EMI shielding properties are often produced from
chemical converting or thermally reducing at extremely high temperature of graphene oxide (GO) or graphene nanosheets (GNS). The production process for them is often complex and requires toxic oxidation agents and intercalation agents, like KMnO4, CrCl3. Besides, the conditions of high temperature reducing require harsh equipment and cost lots of time and energy. This is indeed not in favor of commercial production. Thus, a facial method to prepare flexible carbon films with excellent thermal and EMI shielding properties is urgently needed. Using (NH4)2S2O8 to treat graphite is the right choice. Table 1 shows a comparison of our EG films with the previously reported net carbon films on their electric, thermal conductivities. Typically, the overall performances are better than most samples listed here except the film after 2850 C thermally reducing. But it is worth noticing that (NH4)2S2O8 is a green expansion agent due to the release of no toxic gas and containing no heavy metal. Besides, the weak oxidation makes it possible for EG to be exfoliated in water via turbulence flow, avoiding the use of polluting organic solvents or surfactants. We
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Table 1 A comparison of our EG film with the previously reported net carbon films on their electric, thermal conductivities. Methods
d(S/cm) K (W/mK) Tensile strength (MPa) Ref. no
Ball mill with oxalic acid (12 h) and dispersed in NMP, annealing at 600 C (2 h) Sonication in nitric acid and sulfuric acid (48e72 h) Sonication in NMP (50 h) Sonication in SC/water (10 h) Sonication in nitric acid and sulfuric acid (72 h), annealing at 1060 C (2 h) Sonication in SC/water (400 h), annealing at 500 C (2 h) Sonication in acetone/DMF (20 min), annealing at 250 C (14 h), mechanical compression (5 MPa) Ball milling in NMP (6 h), annealing at 2850 C (2 h), mechanical compression (30 MPa) Sonication in isopropanol (2 h) and PEI (0.5 h), annealing at 340 C (2 h), mechanical compression (100 psi, 1 h) Sonication in PEI/water (1 h), annealing at 340 C (2 h), mechanical compression (100 psi, 1 h) Chemical vapor deposition (NH4)2S2O8 expansion (1 h), thermally reducing (800 C, 5 min), mechanical compression (20Mpa, 5 min)
277 300 180 e 850 175 1443 2231 e 880 1136 2977
e e e 110 220 e e 1529 178 200 e 854
e e 12e18 e e 15e33 e e e e 22 11.3
[34] [35] [36] [37] [12] [38] [39] [40] [41] [42] [43] This work
s: Electrical conductivity; K: in-plane thermal conductivity; TS: tensile strength; NMP: N-methyl-pyrrolidone; SC: sodium cholate; PEI: polyethyleneimine.
believe the mechanical flexibility together with its outstanding electric, thermal and EMI shielding properties can endow the films with great potential use in next generation flexible electric devices. 4. Conclusion In conclusion, we have introduced plenty of wrinkles to endow expanded graphite (EG) films with super flexibility via ammonium persulfate ((NH4)2S2O8) gas expansion. The use of high quality and large sized graphite can greatly improve the overall performances of EG films. Besides, chosen appropriate amount of (NH4)2S2O8 to expand graphite (1 : 5; graphite: (NH4)2S2O8 by weight) can endow EG sheets with good film forming property and outstanding electric and thermal properties. Moreover, the slight oxidation introduced by (NH4)2S2O8 can effectively reduce the surface energy mismatching between EG and water. Thus, the as obtained worm-like EG can be effectively exfoliated into sheet-like in water. As a result, after a simple thermally reducing process and compression, the EG films exhibit high electric conductivity of 2977 S/cm and thermal conductivity up to 854 W/mK, respectively. 33.1 dB EMI shielding property is obtained with the film thickness of only 10 mm. The EG films can endure at least 800 times of repeated folding without any structure crack. Such facial, scalable and multifunctional EG films together with its easy producing process, outstanding electric and thermal conductivities open up great potential uses in next generation of high frequency flexible electronic devices, like wearable smart phones. Notes The authors declare no competing financial interest. Acknowledgments We would like to express our sincere thanks to the National Natural Science Foundation of China for financial support (Grant No. 51721091 and 51573102). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.07.079. References [1] Y. Zhang, Y.-J. Heo, Y.-R. Son, I. In, K.-H. An, B.-J. Kim, S.-J. Park, Recent advanced thermal interfacial materials: a review of conducting mechanisms and parameters of carbon materials, Carbon 142 (2019) 445e460. [2] W. Feng, M. Qin, Y. Feng, Toward highly thermally conductive all-carbon
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