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Superior mechanical properties of sulfonated graphene reinforced carbon-graphite composites
Carbon 148 (2019) 378e386
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Carbon journal homepage: www.elsevier.com/locate/carbon
Superior mechanical properties of sulfonated graphene reinforced carbon-graphite composites Chuanjun Tu a, *, Lirui Hong a, Tenghui Song a, Xuanke Li a, Qinbao Dou b, Yichao Ding c, Tingting Liao c, Sisi Zhang a, Guoqiang Gao d, Ziqiu Wang a, Yonghua Jiang e a
College of Materials Science and Engineering, Hunan Province Key Laboratory for Advanced Carbon Materials and Applied Technology, Hunan University, Changsha, 410082, China Shandong Hengyu Carbon Co Ltd, Zibo, 256401, China c School of Materials Engineering, Chengdu Technological University, Chengdu, 611730, China d School of Electrical Engineering, Southwest Jiaotong University, Chengdu, 611730, China e Suzhou Gaotong New Material Technology Co., Ltd, Suzhou, 215000, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 November 2018 Received in revised form 19 March 2019 Accepted 1 April 2019 Available online 4 April 2019
The unavoidable defects in the structure of carbon-graphite composites, such as microcracks, have severe effects on their behaviors and performances in application. The introduction of graphene in carbongraphite materials is considered as an effective strategy to regulate their structures and properties. Here, sulfonated graphene with abundant sulfonic acid groups was introduced into ultrafine petroleum coke powders, which could effectively prevent and reduce the generation of microcracks during preparation process of carbon-graphite composite. As a consequent, the prepared composites reinforced by doping 1 wt% of sulfonated graphene demonstrate obviously enhanced mechanical properties with flexural and compressive strength of 57.5 ± 3.8 MPa and 121.0 ± 2.3 MPa, respectively, which is 2.5 times higher than that of pure carbon-graphite counterparts. It indicates that the addition of sulfonated graphene as carbon reinforcement in carbon-graphite composites effectively improve interaction between tar pitch and coke powders, which increases the carbonized degree in roasting process, thus substantially polish up the structure integrity and the performance of carbon-graphite composites. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Carbon-graphite composites are important strategic materials which have been given a high priority in the application of aerospace and nuclear energy. Actually, due to their low density, high strength, wonderful corrosion resistance, remarkable thermal shock resistance, good electrical conductivity, and excellent selflubrication, carbon-graphite composites also have been widely commercialized as aircraft brakes, engine shafts sealers and high temperature gas flushing resistance [1e5]. However, there are many random cracks and voids in carbon-graphite composites generated due to the phase separation between different components during heat-treatment process. Those defects have a baneful influence on the mechanical strength and toughness of carbon materials, thus, consecutive carburizing and densification are
* Corresponding author. E-mail addresses: [email protected], [email protected] (C. Tu). https://doi.org/10.1016/j.carbon.2019.04.001 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
frequently required in order to recover the strength degradation [6e8]. Various reinforcements, especially ceramic materials, have been exploited in recent years, which contribute to microstructure optimization, surface defects modification and multi-phase compatibility in carbon-graphite composites [9e12]. For instance, SiC nanopowder might function as fillers which decrease the porosity and the highest pore fraction, endowing the hybrid materials with improved mechanical strength, thermal conductivity, and oxidation resistance [9]. Liquid silicon infiltration into carbon matrix could fill the pore via in-situ generation of SiC network, producing a higher density [10]. In addition, boron compounds promote the graphitization of coke, offering a graphite/B4C composites, which display an excellent mechanical property [11]. The crack generated between SiC phase and carbon matrix could be refilled by boron-based sealant [12]. Lastly, carbon-graphite composites doped with either ZrB2 or ZrC demonstrate an obviously lower mass and linear ablation rates than that of pure carbon counterparts due to the formation of ZrO2 protective layer on the surface of the materials [13,14]. However, ceramic composites
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might suffer a poor self-lubrication property and study on other reinforcements is highly demanded. In recent decades, carbon reinforcements have also been widely exploited in order to optimize the properties of carbon-graphite composites. Commercial carbons generally demonstrated an enhanced oxidation resistance while carbon fiber was introduced into the carbon matrix [15]. Mesocarbon microbeads mixed with acid oxidized carbon fiber offer an increased bending strength of 79 MPa due to the formation of strong bonding between carbon fiber and carbon matrix [16]. Furthermore, carbon nanotubes were also used as additives for materials reinforcing [17e19]. For example, multi-walled carbon nanotube could improve the mechanical performance of mesophase pitch matrix with maximal increment of 147% and 400% for bending strength and stiffness, respectively [17]. A 8 times increase for Young modulus and 30e40% decrease for weight could be achieved, according to molecular dynamic calculation, for carbon nanotube reinforced graphite materials than that of pure graphite [19]. Carbon materials prepared from semicoke with 1 wt% of carbon nanotube exhibited flexural strength of 110 MPa, accompanying with increased electrical and thermal conductivity [20]. However, the dispersibility and compatibility of carbon fiber and carbon nanotube in most graphite materials remains a challenge for the preparation of carbon-graphite composites. Graphene, since first emergence in 2004, has drawn much attention due to its immaculate 2D crystal structure which accompanied with both excellent electronic and remarkable mechanics properties [21,22]. Various researches where graphene was employed as reinforcement to enhance the physical properties of polymers matrices have been frequently reported [23e26]. Actually, graphene made a great contribution to increase the mechanical strength of polymers, such as polypropylene [24], epoxy resin [25] and poly(vinyl alcohol) [26]. In addition, other composites such as metals [27], ceramics [28], and cements [29] also demonstrated an improved mechanical performance due to the increasing of structural integrity and refilling of pores and cracks by graphene. Nevertheless, graphene reinforced carbon-graphite composite is rare and need further investigation. Herein, a sulfonated graphene with rich sulfonic acid groups is reported to prepare carbon-graphite composites via a facile preparation process. The obtained sulfonated graphene reinforced carbon-graphite composite demonstrates an obviously better mechanical strength and tribological performance than pure carbon materials counterparts, which is attribute to the excellent affinity of sulfonated graphene to other components and the promotion of graphite crystalline growth during roasting process. 2. Experimental section 2.1. Synthesis of sulfonated graphene modified coke (SGC) SGC was prepared according to the following procedures. Generally, commercial available sulfonated graphene (SG, 1 wt% of the total mass of tar pitch and coke powder) with basic properties listed in Table 1 was dispersed in water under sonication for 2 h. Then, waterborne epoxy resin (10 wt% of sulfonated graphene) was
Table 1 Basic properties of the commercial obtained SG. Main parameters
Index
Diameter (um) Solubility in water Carbon content (wt) Specific surface area (g/m2)
75e100 >20% 97.5% 1917
379
added into SG suspension to offer a homogeneous mixture solution. Afterwards, the mixture was directly introduced into ultrafine coke powder (the size range of coke powder is 13e23 mm) via kneading process at 100 C for 5 h. After in-situ curing, the generated crude materials were further dried in oven and crushed into powder to give the final SGC-1.0. Other SGCs, such as SGC-0.5 and SGC-1.5, were also prepared according to the similar procedure with various SG contents. 2.2. Preparation of carbon-graphite composites (CGC) The prepared SGC-1.0 was mixed evenly with high-pure tar pitch (softening point at 85e90 C) via kneading process with temperature of 190 C for 1 h. Then the obtained crude mixture was processed sequentially by hot rolling, milling, stamping (molding pressure 1e1.2 tons/cm2, 1e2 h, material temperature 90e100 C) and roasting (calcination time 350e400 h, final temperature 1050e1200 C) to offer the final carbon-graphite composites CGC1.0. Other CGCs were also synthesized using a similar procedure with different starting materials. For comparison, CGC-0 was prepared using pure coke powder as starting carbon aggregation phase. The preparation flow chart of the carbon-graphite composites was shown in Fig. 1 and percentage of three components were listed in Table 2. 2.3. Characterization The morphology of SG, cross section (obtained by sawing) and fractured surface (obtained by striking with a pendulum bob) were investigated by environmental scanning electron microscope (ESEM, Quanta FEI 200, Netherlands). The pore area and fraction of cross section were counted by Image J under the same threshold after converting to binary images and setting parameters. The elements mapping of CGC was conducted on energy dispersive X-ray spectroscopy (EDS) at a magnification of 5000 and the detection area of samples were 0.0108 mm2. The surface of SG was also recorded by transmission electron microscope (TEM, JEM-3010, Japan) with an acceleration voltage of 200 kV. And the thickness of SG was analyzed using atomic force microscopy (AFM, Bioscope System, Germany) in the peakforce tapping mode with resonance frequency of ~300 kHZ. Fourier transform infrared spectroscopy (FTIR, IR Affinity-1, Japan) was employed in the spectral range of 650e3800 cm1 at a resolution of 4 cm1 to clarify the functional groups of SG. The polarizing optical microscope (POM, BX53MRF-S, Japan) was conducted to observe the overall lumpy structure of CGC. The graphite crystalline structure of CGC was investigated using X-ray diffractometer (XRD, Y-500, Denmark) with Cu Ka radiation (l ¼ 1.5406 Å) in the 2q range from 10 to 50 with the scanning rate of 0.02 s1. The effect of SG on tar pitch was analyzed via the softening point and adhesion test of tar pitch with different content of SG. The softening point was measured according to the Ring and Ball method by using two stainless balls (diameter of 9.5 mm and 3.50 ± 0.05 g in weight) placed on tar pitch. And the average heating temperature at which dropped the stainless balls on tar pitch by 25 mm was eventually determined for softening point. The measurement range of total immersion thermometer in the test is 30e180 C and index value of 0.5 C. Adhesion test of SG modified tar pitch was carried out by following procedures. Petroleum coke powders were contained in cylinder (Ф20 mm 150 mm) with a detachable bottom and covered modified tar pitch on the surface of coke powders. The samples were heated to 150 C and insulated with 2 h to ensure completely permeation of tar pitch. Differences of adhesion amount of samples were contrasted with ratios (Ka),
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Fig. 1. Flow chart for the preparation of CGCs.
Table 2 Percentage of three components for the preparation of CGCs. Entry
Name
Coke powder (wt%)
Tar pitch (wt%)
SG (wt%, percent of the total mass of tar pitch and coke powder)
1 2 3 4
CGC-0 CGC-0.5 CGC-1.0 CGC-1.5
68e72 68e72 68e72 68e72
32e28 32e28 32e28 32e28
0 0.5 1.0 1.5
which were total weight of coke powders to the weight of not absorbed. Each experiment repeated for three times and average value was obtained. The simulated carbonization process and thermal stability of CGCs were all investigated through thermogravimetric analysis (TGA) which were carried out in thermal analyzer (STA 449C, Germany) with temperature increasing from room temperature to 800 C with a rate of 10 C min1. Thermal conductivity and thermal diffusivity were investigated with laser thermal conductivity meter (Netzsch LFA467, America) at 25 C. Each test was repeated for three times. Compressive and flexural strength were performed on electronic universal material testing machine (INSTRON-3382) at room temperature. Cube (20.0 mm 20.0 mm 20.0 mm) specimens and rectangular specimens (50.0 mm 20.0 mm 20.0 mm) with span of 30 mm were respectively loading at speed of 60 Mpa s 1 and 1 mm min1 for compressive and flexural tests. Micro mechanical property was recorded by Nanoindentation tester (Hysitron Ttriboindenter) under the loading force of 750 mN. Friction coefficient was detected via MM-200 wear test machine by dry grinding under 240.1 N loading force at a speed of 0.42 m s1 for 2 h. Each test was repeated for three times. 3. Results and discussion SEM (Fig. 2a) and TEM images (Fig. 2b) reveal the wrinkled and thin layer structure of SG. The thickness of SG is estimated to be ~4 nm from the AFM analysis. The presence of functional groups on the surface of SG are classified in FTIR spectroscopy (Fig. 2e). The broad peak at 3405 cm1 is attributed to the stretching vibrations of
O-H [30]. C¼O stretching vibrations and skeletal vibrations of graphene planes are reflected on peaks at 1724 and 1620 cm1 [30,31]. The peak located at 1187 cm1 is assigned to O-S symmetric stretching and S¼O asymmetric stretching vibrations. Moreover, the peak at 1029 cm1 is identified for S-phenyl stretching vibration, corroborating the existence of sulfonic acid groups on the graphene planes [32,33]. Note that the peak intensity of C¼O, S¼O and S-phenyl stretching vibration is strong, demonstrating high contents of carboxyl and sulfonic group. The chemical structure of SG was also illustrated in Fig. 2f to provide more structural information. SEM images of CGCs (Fig. 3a) show that there are many macropores relate to gas channels that result from the decomposition of organic components in roasting process. In contrast, much denser surfaces are observed for CGCs with the addition of SG, indicating that SG could effectively suppress the expansion of pores. This is further supported by results derived from void area statistics (insert in Fig. 3aed). The average void size of CGC-1.0 is estimated to be 28.0 mm2, which is much smaller than that of CGC-0 (Table 3). Interestingly, the average size and total area of voids increase when more SG were introduced, as demonstrated in CGC-1.5. This is attributed to the aggregation feature of excessive SG between different carbon phases that destroys the layer structure of SG, which reduces the promotion effect on powder sintering [34e36]. The results here also suggest that optimized SG amount is highly desirable. Fractured surface morphology of CGCs were also provided to correlate the SG amount with the state of defects in the composites. As shown in Fig. 3e (more SEM images of fractured surfaces were shown in Fig. S1), a trapezoid channel with width decreasing from
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Fig. 2. (a) SEM images of SG, (b) TEM images of SG, (c) AFM image and (d) corresponding thickness curve of SG, (e) FTIR spectrum and (f) molecular analog diagram of SG. (A colour version of this figure can be viewed online.)
Fig. 3. SEM images of (aed) the cross section and (eeh) fractured surface of CGC-0, CGC-0.5, CGC-1.0 and CGC-1.5. Insets are binary images from Image J software. (A colour version of this figure can be viewed online.)
Table 3 Void area statistics on cross section of CGCs. SG content (wt%)
Total area (mm2)
Average size (mm2)
Area fraction (%)
0 0.5 1.0 1.5
1299340.9 1059667.3 828636.2 1092301.1
61.3 39.5 28.0 43.4
27.3 22.2 17.4 22.9
4.9 mm to 1.4 mm is found in CGC-0. However, the microcrack in CGC-0.5 range from 0.8 mm to 3.3 mm, which is narrower with respect to CGC-0. There is nearly no microcrack in CGC-1.0 (Fig. 3g). The size evolution of microcrack follows in the order of CGC0 > CGC-0.5 > CGC-1.0, highlighting the important role played by SG in keeping the structural integrity of the composites. Further
increase in SG amounts create microcrack again, as shown in CGC1.5, which agrees well with the aforementioned result. This also indicates that 1.0 wt% is the optimized weight percentage of SG in our work. Typical fractured surface analysis further support this claim. As shown in Fig. 4, CGC-1.0 exhibits a much denser surface, in addition to the higher contents of sulfur (1.28%) and oxygen (26.29%) that left over by SG. Moreover, the uniform distribution of sulfur also displays the great dispersion of SG, which results in the final improvement of structural integrity. The flaw such as microcrack in composites is generally regarded as the stress concentrative point, which significantly affects the mechanical properties of materials. There are actually three main ways accounting for defect generation: i), the impurity in both carbon aggregation and tar pitch. While most of them could be removed during the calcination process, the residual heteroatoms,
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Fig. 4. (a, d) Typical SEM images and (d, e) corresponding sulfur distribution maps of CGC-0 and CGC-1.0. Contents of various elements of (left of Fig. 4c) CGC-0 and (right of Fig. 4c) CGC-1.0. (A colour version of this figure can be viewed online.)
such as nitrogen and oxygen could markedly disturb the creation of new carbon skeleton; ii) the pyrolysis and collapse of tar pitch. It is easily for tar pitch to fill into the gaps in coke powders at the high temperature. However, it tends to pyrolysis and collapse during the calcination, which produce new cracks; iii) the mismatch in the thermal expansion coefficient between carbon aggregation phase and coal tar pitch. To capture the generation and propagation of microcracks upon carbonization process, carbon-graphite composites are roasted at 450 C and 600 C. These temperatures are high enough for the rearrange of tar pitch molecular structure and the formation of stable semi-coke structure. The related composites were labeled as CGC-0-450, CGC-0-600, CGC-1.0-450 and CGC-1.0-600, respectively. As shown in Fig. 5a, there are obviously larger cracks in CGC0-450 compared to CGC-1.0-450, indicating that SG could act on the process of thermal decomposition of tar pitch (300e450 C) and reduce the propagation of microcracks. When the temperature increases up to 600 C, the size of cracks that appear in CGC-0-600 (Fig. 5c) are still large after the growth stage of aromatic plane (450e600 C). In contrast, the slender cracks occurs in CGC-1.0-450 is limited that only smaller microcracks appeared in CGC-1.0-600. Meanwhile, due to the formation of stable carbon skeleton, the large-sized cracks in CGC-0 could not be shrinked in the structure perfection stage of high-temperature roasting (after 600 C), which
is also confirmed by SEM images (Fig. 3). Therefore, SG could effectively limit the generation and propagation of microcracks in roasting process. In CGC composites, SG acts as anchor to bridge different components together, which improves the affinity and adhesion of tar pitch to coke powder. This is supported by the increase in softening point and adhesion ratio (Ka) of tar pitch. As shown in Table 4, the softening point is determined to be 116.4 C for CGC-1.0, which is higher than that of pure tar pitch (105.3 C). This is attributed to the two dimensional feature of SG, which enhances the degree of crosslinking and entanglement of tar pitch molecules, thus decreasing the movements of chain segments. In addition, the abundant sulfonic acid groups also provide better affinity between tar pitch and ultrafine coke powder in the initial stage of carbonization. The higher Ka value (1.67) of tar pitch with 1 wt% SG further corroborates the improvements in adhesion ability of tar pitch to coke powder. Meanwhile, the good affinity is critically important for mass transfer and fusion of coke particles in roasting process. As shown in Fig. 6a, TGA measurements for tar pitch, SG, CGC-0 and CGC-1.0 before sintering were conducted to clarify the function mechanism of SG in roasting process. The first weight loss stage before 150 C in SG is attributed to the absorbed and bound water. The second weight loss between 300 C and 600 C is due to the decomposition of sulfonic group [37e39]. The tar pitch starts to lose from 210 C and stabilized at 570 C with final residues of 40%, which is coincided with previous report [40]. Both CGC-0 and CGC-1.0 demonstrate similar decomposition behavior and final residues (87%). However, the initial carbonization temperature of CGC-1.0 is 500 C, which is much lower than that of CGC-0. It is concluded that SG in the mixture could facilitate the extension of six-membered ring structure of tar pitch molecular, which greatly reduce initial carbonization time and temperature. Thus, more time will be provided in CCG-1.0 for improvement of carbon skeleton at high temperature and the structural integrity will be enhanced, which is also confirmed by SEM images of cross-section and fractured surface.
Table 4 Softening points and adhesion ratios (Ka) of tar pitch. SG content (wt%) Fig. 5. (a) SEM images of CGC-0-450, (b) CGC-1.0-450, (c) CGC-0-600 and (d) CGC-1.0600 in the fracture section. Scale bar: 5 mm. The representative microcracks were marked with circles. (A colour version of this figure can be viewed online.)
Tar pitch softening point ( C) Ka
0
1.0
105.3 1.51
116.4 1.67
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383
Fig. 6. (a) The TGA curves of tar pitch (black), SG (red), CGC-0 before sintering (blue) and CGC-1.0 before sintering (green) in N2 atmosphere. (b) TGA curves of CGC-0 and CGC-1.0 in air. (c) XRD profiles of CGC-0 and CGC-1.0. POM images (d) of CGC-0 and (e) CGC-1.0. (A colour version of this figure can be viewed online.)
The degree of perfection in structure is also reflect in thermal stability of composites that structural defects tend to cause worse high-temperature oxidation resistance. Therefore, the TGA measurements of the obtained CGC-0 and CGC-1.0 were also investigate, as shown in Fig. 6b. CGC-0 exhibits ~5% weight loss below 200 C due to the removal of absorbed water and thermal decomposition begins at 530 C with carbon residues around 15% after 800 C. In contrast, an obviously higher thermal stability could be detected in CGC-1.0. While underwent a slight decrease (~1%) at 100 C due to remove of water, CGC-1.0 could stabilize until 580 C where thermal decomposition happens. The amount of carbon residues is around 55% at 800 C, which is 3.7 times than that of CGC-0. This is attribute to the improvement of oxidation resistance that results from the decreased defects inside CGC-1.0 after a longer time of structural perfection. SG also act as seed crystals to promote the growth of graphite crystallites, and thus improving the structural integrity of CGC-1.0. As depicted in Fig. 6c, differing from the broad peak feature of CGC-0, a new and sharp peak at 2q ¼ 26.4 , corresponding to a narrower d-spacing of 3.372 Å, is identified for CGC-1.0, in addition to the broad peaks at around 25.5 (002),
43.0 (100) and 44.3 (100) [41]. This suggests the high graphitization degree of CGC-1.0. More evidence can be found from block structures in POM images. Apparently, CGC-1.0 consists of coke blocks in larger size compare to CGC-0, demonstrating a better fusion and growth of coke powder after roasting. The strong interactions among SG, coke powder and tar pitch account for this by promoting the densification and growth of coke particles at high temperature. All these, contribute to the dense and intact denser morphology of fractured surface and cross section, improved structural integrity and enhanced mechanical properties of CGCs. The representative compressive stress-strain and flexural stressstrain curves of CGCs are depicted in Fig. 7 and the average strengths are also inserted into diagrams. Compared with CGC-0, both flexural strength and compressive strength are dramatically enhanced with the introduction of SG. This is due to the improvement of structural integrity and decrease of cracks, consisting with aforementioned morphological observations. Furthermore, the CGC-1.0 delivers the highest compressive and flexural strength, which is 3.5 folds of that of composites materials without SG. The contribution of SG is reduced in CGC-1.5, which is attribute to the
Fig. 7. (a) Representative compressive stress-strain and (b) flexural stress-strain curves of CGCs. Insets are average compressive and flexural strength of CGCs over three tests. (c) The friction coefficient of CGCs in horizontal and vertical directions. Error bar: standard deviation. (A colour version of this figure can be viewed online.)
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Table 5 Thermal conductivity and diffusivity of CGC-0 and CGC-1.0. Sample name Thermalconductivity (W m1 K1) Thermal diffusivity (m2 s1)
In-plane Through-plane In-plane Through-plane
CGC-0
CGC-1.0
3.143 ± 0.088 3.173 ± 0.057 2.243 ± 0.028 2.093 ± 0.018
2.474 ± 0.081 2.742 ± 0.016 1.814 ± 0.019 1.987 ± 0.069
Fig. 8. (a) Nanoindentation load-displacement curves of CGCs under the maximum load of 750 mN, (b) Young's Modulus (up part) and Hardness-displacement (down part) analysis of CGC-0 (black) and CGC-1.0 (red) versus maximum displacement. Insets are impressions of CGC-0 and CGC-1.0 after loading. (A colour version of this figure can be viewed online.)
appearance of more microcraks inside the composite due to the aggregation or overlap of excessive SG. In addition, escaping gas results from pyrolysis process of overmuch sulfonic groups may also cause cracks or pores, and thus degrade the mechanical of composites. The extremely high compressive strength (121 ± 3.8 MPa) and flexural strength (57.5 ± 2.3 MPa) of CGC-1.0 is comparable to that of most traditional carbon-graphite composites. Friction coefficients were also recorded via wear test for all the CGCs as shown in Fig. 7c. Due to the structure anisotropy, friction coefficient shows a slight difference between horizontal and vertical section during the measurement. As expected, the addition of SG endowed the composites with a lower friction coefficient, which is attribute to a higher graphitization degree of CGC-1.0 than CGC0 that provide more fragments of graphite crystalline to form lubricating friction film for a lower friction coefficient [42]. The friction coefficient for CGC-1.0 is approximately half of that of CGC0 while demonstrates a slight increase compared to CGC-0.5, which may cause by many adhesion phenomenon in tests. However, as listed in Table 5, both thermal conductivity and thermal diffusivity of CGC-1.0 is a bit lower than that of CGC-0, indicating heat transmission performances have been slightly decrease with the addition of graphene, which may result from the residual sulfonic acid groups in graphene layers between two carbon phases even after roasting. Nanoindentation has been established as a powerful method to characterize the mechanical behaviors of materials on a submicron scale [43e47]. As shown in Fig. 8a, a typical load-displacement curve is obtained for both CGC-0 and CGC-1.0 under the maximum indentation load of 750 mN. Clearly, CGC-0 deformed much easier than that of CGC-1.0 with maximum depth of 6700 nm and 4500 nm, respectively. There are pop-ins observed in loading process, indicating probably the present of cracks and fractures in CGC-0 [45,46]. However, no steps (pop-ins and pop-outs) and discontinuities are detected in loading and unloading curve of CGC1.0. In addition, a higher percent of elastic recovery is found for CGC-1.0 (35%) than that of CGC-0 (22%) with a calculation method as previous report [46,48]. From the impressions remain on the surface, the morphology shown a brittle fracture after loading. And the size on CGC-1.0 is only 10.3 mm which is narrower than that of CGC-0 (15.4 mm), which imply that the surface of CGC-0 is easier to
be broken under the same load. Furthermore, hardness and Young's modulus of CGCs were also recorded as a function of maximum displacement as shown in Fig. 8b. It is noted that CGC-1.0 appears higher hardness and modulus, indicative of the improvement of micromechanical performance. Simultaneously, the indentation size effect also appears in composites during the loading process, especially the obvious decrease of hardness and modulus in CGC-1.0 which probably attribute to the increased quantity of graphite crystalline and attendant strain gradient plasticity caused by geometrically necessary dislocation [49,50]. The average hardness and Young's modulus is 1.29 GPa, 18.1 GPa for CGC-0 and 2.15 GPa, 21.5 GPa for CGC-1.0, respectively.
4. Conclusion Sulfonated graphene has been introduced successfully into the mixture of tar pitch and ultrafine coke powders, producing a sulfonated graphene reinforced carbon-graphite composite. The addition of graphene could effectively improve the integrity of the carbon skeleton and reduce the generation and extension of cracks inside of carbon composites. The sulfonated graphene used here not only increases the affinity of tar pitch to coke powder that facilitates the fusion of coke powders upon carbonization process, but also act as crystal seeds to induce the growth of graphite crystallites in roasting process, thus reducing the propagation of harmful penetrating cracks. All these, contribute to the improved mechanical performances for carbon-graphite composites in terms of high mechanical strength and low friction coefficient, and offer new opportunities for their widely applications in high temperature carbon-graphite sealing material or jet nozzle.
Acknowledgements The authors thank financial support from National Natural Science Foundation of China (Nos. 51772081, 51837009, 51577158), Natural Science Foundation of Hunan Province (Nos. 2016JJ2024, 2016JJ2025) and National Youth Science Fund project (No. 51607147).
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Carbon journal homepage: www.elsevier.com/locate/carbon
Superior mechanical properties of sulfonated graphene reinforced carbon-graphite composites Chuanjun Tu a, *, Lirui Hong a, Tenghui Song a, Xuanke Li a, Qinbao Dou b, Yichao Ding c, Tingting Liao c, Sisi Zhang a, Guoqiang Gao d, Ziqiu Wang a, Yonghua Jiang e a
College of Materials Science and Engineering, Hunan Province Key Laboratory for Advanced Carbon Materials and Applied Technology, Hunan University, Changsha, 410082, China Shandong Hengyu Carbon Co Ltd, Zibo, 256401, China c School of Materials Engineering, Chengdu Technological University, Chengdu, 611730, China d School of Electrical Engineering, Southwest Jiaotong University, Chengdu, 611730, China e Suzhou Gaotong New Material Technology Co., Ltd, Suzhou, 215000, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 November 2018 Received in revised form 19 March 2019 Accepted 1 April 2019 Available online 4 April 2019
The unavoidable defects in the structure of carbon-graphite composites, such as microcracks, have severe effects on their behaviors and performances in application. The introduction of graphene in carbongraphite materials is considered as an effective strategy to regulate their structures and properties. Here, sulfonated graphene with abundant sulfonic acid groups was introduced into ultrafine petroleum coke powders, which could effectively prevent and reduce the generation of microcracks during preparation process of carbon-graphite composite. As a consequent, the prepared composites reinforced by doping 1 wt% of sulfonated graphene demonstrate obviously enhanced mechanical properties with flexural and compressive strength of 57.5 ± 3.8 MPa and 121.0 ± 2.3 MPa, respectively, which is 2.5 times higher than that of pure carbon-graphite counterparts. It indicates that the addition of sulfonated graphene as carbon reinforcement in carbon-graphite composites effectively improve interaction between tar pitch and coke powders, which increases the carbonized degree in roasting process, thus substantially polish up the structure integrity and the performance of carbon-graphite composites. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Carbon-graphite composites are important strategic materials which have been given a high priority in the application of aerospace and nuclear energy. Actually, due to their low density, high strength, wonderful corrosion resistance, remarkable thermal shock resistance, good electrical conductivity, and excellent selflubrication, carbon-graphite composites also have been widely commercialized as aircraft brakes, engine shafts sealers and high temperature gas flushing resistance [1e5]. However, there are many random cracks and voids in carbon-graphite composites generated due to the phase separation between different components during heat-treatment process. Those defects have a baneful influence on the mechanical strength and toughness of carbon materials, thus, consecutive carburizing and densification are
* Corresponding author. E-mail addresses: [email protected], [email protected] (C. Tu). https://doi.org/10.1016/j.carbon.2019.04.001 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
frequently required in order to recover the strength degradation [6e8]. Various reinforcements, especially ceramic materials, have been exploited in recent years, which contribute to microstructure optimization, surface defects modification and multi-phase compatibility in carbon-graphite composites [9e12]. For instance, SiC nanopowder might function as fillers which decrease the porosity and the highest pore fraction, endowing the hybrid materials with improved mechanical strength, thermal conductivity, and oxidation resistance [9]. Liquid silicon infiltration into carbon matrix could fill the pore via in-situ generation of SiC network, producing a higher density [10]. In addition, boron compounds promote the graphitization of coke, offering a graphite/B4C composites, which display an excellent mechanical property [11]. The crack generated between SiC phase and carbon matrix could be refilled by boron-based sealant [12]. Lastly, carbon-graphite composites doped with either ZrB2 or ZrC demonstrate an obviously lower mass and linear ablation rates than that of pure carbon counterparts due to the formation of ZrO2 protective layer on the surface of the materials [13,14]. However, ceramic composites
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might suffer a poor self-lubrication property and study on other reinforcements is highly demanded. In recent decades, carbon reinforcements have also been widely exploited in order to optimize the properties of carbon-graphite composites. Commercial carbons generally demonstrated an enhanced oxidation resistance while carbon fiber was introduced into the carbon matrix [15]. Mesocarbon microbeads mixed with acid oxidized carbon fiber offer an increased bending strength of 79 MPa due to the formation of strong bonding between carbon fiber and carbon matrix [16]. Furthermore, carbon nanotubes were also used as additives for materials reinforcing [17e19]. For example, multi-walled carbon nanotube could improve the mechanical performance of mesophase pitch matrix with maximal increment of 147% and 400% for bending strength and stiffness, respectively [17]. A 8 times increase for Young modulus and 30e40% decrease for weight could be achieved, according to molecular dynamic calculation, for carbon nanotube reinforced graphite materials than that of pure graphite [19]. Carbon materials prepared from semicoke with 1 wt% of carbon nanotube exhibited flexural strength of 110 MPa, accompanying with increased electrical and thermal conductivity [20]. However, the dispersibility and compatibility of carbon fiber and carbon nanotube in most graphite materials remains a challenge for the preparation of carbon-graphite composites. Graphene, since first emergence in 2004, has drawn much attention due to its immaculate 2D crystal structure which accompanied with both excellent electronic and remarkable mechanics properties [21,22]. Various researches where graphene was employed as reinforcement to enhance the physical properties of polymers matrices have been frequently reported [23e26]. Actually, graphene made a great contribution to increase the mechanical strength of polymers, such as polypropylene [24], epoxy resin [25] and poly(vinyl alcohol) [26]. In addition, other composites such as metals [27], ceramics [28], and cements [29] also demonstrated an improved mechanical performance due to the increasing of structural integrity and refilling of pores and cracks by graphene. Nevertheless, graphene reinforced carbon-graphite composite is rare and need further investigation. Herein, a sulfonated graphene with rich sulfonic acid groups is reported to prepare carbon-graphite composites via a facile preparation process. The obtained sulfonated graphene reinforced carbon-graphite composite demonstrates an obviously better mechanical strength and tribological performance than pure carbon materials counterparts, which is attribute to the excellent affinity of sulfonated graphene to other components and the promotion of graphite crystalline growth during roasting process. 2. Experimental section 2.1. Synthesis of sulfonated graphene modified coke (SGC) SGC was prepared according to the following procedures. Generally, commercial available sulfonated graphene (SG, 1 wt% of the total mass of tar pitch and coke powder) with basic properties listed in Table 1 was dispersed in water under sonication for 2 h. Then, waterborne epoxy resin (10 wt% of sulfonated graphene) was
Table 1 Basic properties of the commercial obtained SG. Main parameters
Index
Diameter (um) Solubility in water Carbon content (wt) Specific surface area (g/m2)
75e100 >20% 97.5% 1917
379
added into SG suspension to offer a homogeneous mixture solution. Afterwards, the mixture was directly introduced into ultrafine coke powder (the size range of coke powder is 13e23 mm) via kneading process at 100 C for 5 h. After in-situ curing, the generated crude materials were further dried in oven and crushed into powder to give the final SGC-1.0. Other SGCs, such as SGC-0.5 and SGC-1.5, were also prepared according to the similar procedure with various SG contents. 2.2. Preparation of carbon-graphite composites (CGC) The prepared SGC-1.0 was mixed evenly with high-pure tar pitch (softening point at 85e90 C) via kneading process with temperature of 190 C for 1 h. Then the obtained crude mixture was processed sequentially by hot rolling, milling, stamping (molding pressure 1e1.2 tons/cm2, 1e2 h, material temperature 90e100 C) and roasting (calcination time 350e400 h, final temperature 1050e1200 C) to offer the final carbon-graphite composites CGC1.0. Other CGCs were also synthesized using a similar procedure with different starting materials. For comparison, CGC-0 was prepared using pure coke powder as starting carbon aggregation phase. The preparation flow chart of the carbon-graphite composites was shown in Fig. 1 and percentage of three components were listed in Table 2. 2.3. Characterization The morphology of SG, cross section (obtained by sawing) and fractured surface (obtained by striking with a pendulum bob) were investigated by environmental scanning electron microscope (ESEM, Quanta FEI 200, Netherlands). The pore area and fraction of cross section were counted by Image J under the same threshold after converting to binary images and setting parameters. The elements mapping of CGC was conducted on energy dispersive X-ray spectroscopy (EDS) at a magnification of 5000 and the detection area of samples were 0.0108 mm2. The surface of SG was also recorded by transmission electron microscope (TEM, JEM-3010, Japan) with an acceleration voltage of 200 kV. And the thickness of SG was analyzed using atomic force microscopy (AFM, Bioscope System, Germany) in the peakforce tapping mode with resonance frequency of ~300 kHZ. Fourier transform infrared spectroscopy (FTIR, IR Affinity-1, Japan) was employed in the spectral range of 650e3800 cm1 at a resolution of 4 cm1 to clarify the functional groups of SG. The polarizing optical microscope (POM, BX53MRF-S, Japan) was conducted to observe the overall lumpy structure of CGC. The graphite crystalline structure of CGC was investigated using X-ray diffractometer (XRD, Y-500, Denmark) with Cu Ka radiation (l ¼ 1.5406 Å) in the 2q range from 10 to 50 with the scanning rate of 0.02 s1. The effect of SG on tar pitch was analyzed via the softening point and adhesion test of tar pitch with different content of SG. The softening point was measured according to the Ring and Ball method by using two stainless balls (diameter of 9.5 mm and 3.50 ± 0.05 g in weight) placed on tar pitch. And the average heating temperature at which dropped the stainless balls on tar pitch by 25 mm was eventually determined for softening point. The measurement range of total immersion thermometer in the test is 30e180 C and index value of 0.5 C. Adhesion test of SG modified tar pitch was carried out by following procedures. Petroleum coke powders were contained in cylinder (Ф20 mm 150 mm) with a detachable bottom and covered modified tar pitch on the surface of coke powders. The samples were heated to 150 C and insulated with 2 h to ensure completely permeation of tar pitch. Differences of adhesion amount of samples were contrasted with ratios (Ka),
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Fig. 1. Flow chart for the preparation of CGCs.
Table 2 Percentage of three components for the preparation of CGCs. Entry
Name
Coke powder (wt%)
Tar pitch (wt%)
SG (wt%, percent of the total mass of tar pitch and coke powder)
1 2 3 4
CGC-0 CGC-0.5 CGC-1.0 CGC-1.5
68e72 68e72 68e72 68e72
32e28 32e28 32e28 32e28
0 0.5 1.0 1.5
which were total weight of coke powders to the weight of not absorbed. Each experiment repeated for three times and average value was obtained. The simulated carbonization process and thermal stability of CGCs were all investigated through thermogravimetric analysis (TGA) which were carried out in thermal analyzer (STA 449C, Germany) with temperature increasing from room temperature to 800 C with a rate of 10 C min1. Thermal conductivity and thermal diffusivity were investigated with laser thermal conductivity meter (Netzsch LFA467, America) at 25 C. Each test was repeated for three times. Compressive and flexural strength were performed on electronic universal material testing machine (INSTRON-3382) at room temperature. Cube (20.0 mm 20.0 mm 20.0 mm) specimens and rectangular specimens (50.0 mm 20.0 mm 20.0 mm) with span of 30 mm were respectively loading at speed of 60 Mpa s 1 and 1 mm min1 for compressive and flexural tests. Micro mechanical property was recorded by Nanoindentation tester (Hysitron Ttriboindenter) under the loading force of 750 mN. Friction coefficient was detected via MM-200 wear test machine by dry grinding under 240.1 N loading force at a speed of 0.42 m s1 for 2 h. Each test was repeated for three times. 3. Results and discussion SEM (Fig. 2a) and TEM images (Fig. 2b) reveal the wrinkled and thin layer structure of SG. The thickness of SG is estimated to be ~4 nm from the AFM analysis. The presence of functional groups on the surface of SG are classified in FTIR spectroscopy (Fig. 2e). The broad peak at 3405 cm1 is attributed to the stretching vibrations of
O-H [30]. C¼O stretching vibrations and skeletal vibrations of graphene planes are reflected on peaks at 1724 and 1620 cm1 [30,31]. The peak located at 1187 cm1 is assigned to O-S symmetric stretching and S¼O asymmetric stretching vibrations. Moreover, the peak at 1029 cm1 is identified for S-phenyl stretching vibration, corroborating the existence of sulfonic acid groups on the graphene planes [32,33]. Note that the peak intensity of C¼O, S¼O and S-phenyl stretching vibration is strong, demonstrating high contents of carboxyl and sulfonic group. The chemical structure of SG was also illustrated in Fig. 2f to provide more structural information. SEM images of CGCs (Fig. 3a) show that there are many macropores relate to gas channels that result from the decomposition of organic components in roasting process. In contrast, much denser surfaces are observed for CGCs with the addition of SG, indicating that SG could effectively suppress the expansion of pores. This is further supported by results derived from void area statistics (insert in Fig. 3aed). The average void size of CGC-1.0 is estimated to be 28.0 mm2, which is much smaller than that of CGC-0 (Table 3). Interestingly, the average size and total area of voids increase when more SG were introduced, as demonstrated in CGC-1.5. This is attributed to the aggregation feature of excessive SG between different carbon phases that destroys the layer structure of SG, which reduces the promotion effect on powder sintering [34e36]. The results here also suggest that optimized SG amount is highly desirable. Fractured surface morphology of CGCs were also provided to correlate the SG amount with the state of defects in the composites. As shown in Fig. 3e (more SEM images of fractured surfaces were shown in Fig. S1), a trapezoid channel with width decreasing from
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381
Fig. 2. (a) SEM images of SG, (b) TEM images of SG, (c) AFM image and (d) corresponding thickness curve of SG, (e) FTIR spectrum and (f) molecular analog diagram of SG. (A colour version of this figure can be viewed online.)
Fig. 3. SEM images of (aed) the cross section and (eeh) fractured surface of CGC-0, CGC-0.5, CGC-1.0 and CGC-1.5. Insets are binary images from Image J software. (A colour version of this figure can be viewed online.)
Table 3 Void area statistics on cross section of CGCs. SG content (wt%)
Total area (mm2)
Average size (mm2)
Area fraction (%)
0 0.5 1.0 1.5
1299340.9 1059667.3 828636.2 1092301.1
61.3 39.5 28.0 43.4
27.3 22.2 17.4 22.9
4.9 mm to 1.4 mm is found in CGC-0. However, the microcrack in CGC-0.5 range from 0.8 mm to 3.3 mm, which is narrower with respect to CGC-0. There is nearly no microcrack in CGC-1.0 (Fig. 3g). The size evolution of microcrack follows in the order of CGC0 > CGC-0.5 > CGC-1.0, highlighting the important role played by SG in keeping the structural integrity of the composites. Further
increase in SG amounts create microcrack again, as shown in CGC1.5, which agrees well with the aforementioned result. This also indicates that 1.0 wt% is the optimized weight percentage of SG in our work. Typical fractured surface analysis further support this claim. As shown in Fig. 4, CGC-1.0 exhibits a much denser surface, in addition to the higher contents of sulfur (1.28%) and oxygen (26.29%) that left over by SG. Moreover, the uniform distribution of sulfur also displays the great dispersion of SG, which results in the final improvement of structural integrity. The flaw such as microcrack in composites is generally regarded as the stress concentrative point, which significantly affects the mechanical properties of materials. There are actually three main ways accounting for defect generation: i), the impurity in both carbon aggregation and tar pitch. While most of them could be removed during the calcination process, the residual heteroatoms,
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Fig. 4. (a, d) Typical SEM images and (d, e) corresponding sulfur distribution maps of CGC-0 and CGC-1.0. Contents of various elements of (left of Fig. 4c) CGC-0 and (right of Fig. 4c) CGC-1.0. (A colour version of this figure can be viewed online.)
such as nitrogen and oxygen could markedly disturb the creation of new carbon skeleton; ii) the pyrolysis and collapse of tar pitch. It is easily for tar pitch to fill into the gaps in coke powders at the high temperature. However, it tends to pyrolysis and collapse during the calcination, which produce new cracks; iii) the mismatch in the thermal expansion coefficient between carbon aggregation phase and coal tar pitch. To capture the generation and propagation of microcracks upon carbonization process, carbon-graphite composites are roasted at 450 C and 600 C. These temperatures are high enough for the rearrange of tar pitch molecular structure and the formation of stable semi-coke structure. The related composites were labeled as CGC-0-450, CGC-0-600, CGC-1.0-450 and CGC-1.0-600, respectively. As shown in Fig. 5a, there are obviously larger cracks in CGC0-450 compared to CGC-1.0-450, indicating that SG could act on the process of thermal decomposition of tar pitch (300e450 C) and reduce the propagation of microcracks. When the temperature increases up to 600 C, the size of cracks that appear in CGC-0-600 (Fig. 5c) are still large after the growth stage of aromatic plane (450e600 C). In contrast, the slender cracks occurs in CGC-1.0-450 is limited that only smaller microcracks appeared in CGC-1.0-600. Meanwhile, due to the formation of stable carbon skeleton, the large-sized cracks in CGC-0 could not be shrinked in the structure perfection stage of high-temperature roasting (after 600 C), which
is also confirmed by SEM images (Fig. 3). Therefore, SG could effectively limit the generation and propagation of microcracks in roasting process. In CGC composites, SG acts as anchor to bridge different components together, which improves the affinity and adhesion of tar pitch to coke powder. This is supported by the increase in softening point and adhesion ratio (Ka) of tar pitch. As shown in Table 4, the softening point is determined to be 116.4 C for CGC-1.0, which is higher than that of pure tar pitch (105.3 C). This is attributed to the two dimensional feature of SG, which enhances the degree of crosslinking and entanglement of tar pitch molecules, thus decreasing the movements of chain segments. In addition, the abundant sulfonic acid groups also provide better affinity between tar pitch and ultrafine coke powder in the initial stage of carbonization. The higher Ka value (1.67) of tar pitch with 1 wt% SG further corroborates the improvements in adhesion ability of tar pitch to coke powder. Meanwhile, the good affinity is critically important for mass transfer and fusion of coke particles in roasting process. As shown in Fig. 6a, TGA measurements for tar pitch, SG, CGC-0 and CGC-1.0 before sintering were conducted to clarify the function mechanism of SG in roasting process. The first weight loss stage before 150 C in SG is attributed to the absorbed and bound water. The second weight loss between 300 C and 600 C is due to the decomposition of sulfonic group [37e39]. The tar pitch starts to lose from 210 C and stabilized at 570 C with final residues of 40%, which is coincided with previous report [40]. Both CGC-0 and CGC-1.0 demonstrate similar decomposition behavior and final residues (87%). However, the initial carbonization temperature of CGC-1.0 is 500 C, which is much lower than that of CGC-0. It is concluded that SG in the mixture could facilitate the extension of six-membered ring structure of tar pitch molecular, which greatly reduce initial carbonization time and temperature. Thus, more time will be provided in CCG-1.0 for improvement of carbon skeleton at high temperature and the structural integrity will be enhanced, which is also confirmed by SEM images of cross-section and fractured surface.
Table 4 Softening points and adhesion ratios (Ka) of tar pitch. SG content (wt%) Fig. 5. (a) SEM images of CGC-0-450, (b) CGC-1.0-450, (c) CGC-0-600 and (d) CGC-1.0600 in the fracture section. Scale bar: 5 mm. The representative microcracks were marked with circles. (A colour version of this figure can be viewed online.)
Tar pitch softening point ( C) Ka
0
1.0
105.3 1.51
116.4 1.67
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Fig. 6. (a) The TGA curves of tar pitch (black), SG (red), CGC-0 before sintering (blue) and CGC-1.0 before sintering (green) in N2 atmosphere. (b) TGA curves of CGC-0 and CGC-1.0 in air. (c) XRD profiles of CGC-0 and CGC-1.0. POM images (d) of CGC-0 and (e) CGC-1.0. (A colour version of this figure can be viewed online.)
The degree of perfection in structure is also reflect in thermal stability of composites that structural defects tend to cause worse high-temperature oxidation resistance. Therefore, the TGA measurements of the obtained CGC-0 and CGC-1.0 were also investigate, as shown in Fig. 6b. CGC-0 exhibits ~5% weight loss below 200 C due to the removal of absorbed water and thermal decomposition begins at 530 C with carbon residues around 15% after 800 C. In contrast, an obviously higher thermal stability could be detected in CGC-1.0. While underwent a slight decrease (~1%) at 100 C due to remove of water, CGC-1.0 could stabilize until 580 C where thermal decomposition happens. The amount of carbon residues is around 55% at 800 C, which is 3.7 times than that of CGC-0. This is attribute to the improvement of oxidation resistance that results from the decreased defects inside CGC-1.0 after a longer time of structural perfection. SG also act as seed crystals to promote the growth of graphite crystallites, and thus improving the structural integrity of CGC-1.0. As depicted in Fig. 6c, differing from the broad peak feature of CGC-0, a new and sharp peak at 2q ¼ 26.4 , corresponding to a narrower d-spacing of 3.372 Å, is identified for CGC-1.0, in addition to the broad peaks at around 25.5 (002),
43.0 (100) and 44.3 (100) [41]. This suggests the high graphitization degree of CGC-1.0. More evidence can be found from block structures in POM images. Apparently, CGC-1.0 consists of coke blocks in larger size compare to CGC-0, demonstrating a better fusion and growth of coke powder after roasting. The strong interactions among SG, coke powder and tar pitch account for this by promoting the densification and growth of coke particles at high temperature. All these, contribute to the dense and intact denser morphology of fractured surface and cross section, improved structural integrity and enhanced mechanical properties of CGCs. The representative compressive stress-strain and flexural stressstrain curves of CGCs are depicted in Fig. 7 and the average strengths are also inserted into diagrams. Compared with CGC-0, both flexural strength and compressive strength are dramatically enhanced with the introduction of SG. This is due to the improvement of structural integrity and decrease of cracks, consisting with aforementioned morphological observations. Furthermore, the CGC-1.0 delivers the highest compressive and flexural strength, which is 3.5 folds of that of composites materials without SG. The contribution of SG is reduced in CGC-1.5, which is attribute to the
Fig. 7. (a) Representative compressive stress-strain and (b) flexural stress-strain curves of CGCs. Insets are average compressive and flexural strength of CGCs over three tests. (c) The friction coefficient of CGCs in horizontal and vertical directions. Error bar: standard deviation. (A colour version of this figure can be viewed online.)
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Table 5 Thermal conductivity and diffusivity of CGC-0 and CGC-1.0. Sample name Thermalconductivity (W m1 K1) Thermal diffusivity (m2 s1)
In-plane Through-plane In-plane Through-plane
CGC-0
CGC-1.0
3.143 ± 0.088 3.173 ± 0.057 2.243 ± 0.028 2.093 ± 0.018
2.474 ± 0.081 2.742 ± 0.016 1.814 ± 0.019 1.987 ± 0.069
Fig. 8. (a) Nanoindentation load-displacement curves of CGCs under the maximum load of 750 mN, (b) Young's Modulus (up part) and Hardness-displacement (down part) analysis of CGC-0 (black) and CGC-1.0 (red) versus maximum displacement. Insets are impressions of CGC-0 and CGC-1.0 after loading. (A colour version of this figure can be viewed online.)
appearance of more microcraks inside the composite due to the aggregation or overlap of excessive SG. In addition, escaping gas results from pyrolysis process of overmuch sulfonic groups may also cause cracks or pores, and thus degrade the mechanical of composites. The extremely high compressive strength (121 ± 3.8 MPa) and flexural strength (57.5 ± 2.3 MPa) of CGC-1.0 is comparable to that of most traditional carbon-graphite composites. Friction coefficients were also recorded via wear test for all the CGCs as shown in Fig. 7c. Due to the structure anisotropy, friction coefficient shows a slight difference between horizontal and vertical section during the measurement. As expected, the addition of SG endowed the composites with a lower friction coefficient, which is attribute to a higher graphitization degree of CGC-1.0 than CGC0 that provide more fragments of graphite crystalline to form lubricating friction film for a lower friction coefficient [42]. The friction coefficient for CGC-1.0 is approximately half of that of CGC0 while demonstrates a slight increase compared to CGC-0.5, which may cause by many adhesion phenomenon in tests. However, as listed in Table 5, both thermal conductivity and thermal diffusivity of CGC-1.0 is a bit lower than that of CGC-0, indicating heat transmission performances have been slightly decrease with the addition of graphene, which may result from the residual sulfonic acid groups in graphene layers between two carbon phases even after roasting. Nanoindentation has been established as a powerful method to characterize the mechanical behaviors of materials on a submicron scale [43e47]. As shown in Fig. 8a, a typical load-displacement curve is obtained for both CGC-0 and CGC-1.0 under the maximum indentation load of 750 mN. Clearly, CGC-0 deformed much easier than that of CGC-1.0 with maximum depth of 6700 nm and 4500 nm, respectively. There are pop-ins observed in loading process, indicating probably the present of cracks and fractures in CGC-0 [45,46]. However, no steps (pop-ins and pop-outs) and discontinuities are detected in loading and unloading curve of CGC1.0. In addition, a higher percent of elastic recovery is found for CGC-1.0 (35%) than that of CGC-0 (22%) with a calculation method as previous report [46,48]. From the impressions remain on the surface, the morphology shown a brittle fracture after loading. And the size on CGC-1.0 is only 10.3 mm which is narrower than that of CGC-0 (15.4 mm), which imply that the surface of CGC-0 is easier to
be broken under the same load. Furthermore, hardness and Young's modulus of CGCs were also recorded as a function of maximum displacement as shown in Fig. 8b. It is noted that CGC-1.0 appears higher hardness and modulus, indicative of the improvement of micromechanical performance. Simultaneously, the indentation size effect also appears in composites during the loading process, especially the obvious decrease of hardness and modulus in CGC-1.0 which probably attribute to the increased quantity of graphite crystalline and attendant strain gradient plasticity caused by geometrically necessary dislocation [49,50]. The average hardness and Young's modulus is 1.29 GPa, 18.1 GPa for CGC-0 and 2.15 GPa, 21.5 GPa for CGC-1.0, respectively.
4. Conclusion Sulfonated graphene has been introduced successfully into the mixture of tar pitch and ultrafine coke powders, producing a sulfonated graphene reinforced carbon-graphite composite. The addition of graphene could effectively improve the integrity of the carbon skeleton and reduce the generation and extension of cracks inside of carbon composites. The sulfonated graphene used here not only increases the affinity of tar pitch to coke powder that facilitates the fusion of coke powders upon carbonization process, but also act as crystal seeds to induce the growth of graphite crystallites in roasting process, thus reducing the propagation of harmful penetrating cracks. All these, contribute to the improved mechanical performances for carbon-graphite composites in terms of high mechanical strength and low friction coefficient, and offer new opportunities for their widely applications in high temperature carbon-graphite sealing material or jet nozzle.
Acknowledgements The authors thank financial support from National Natural Science Foundation of China (Nos. 51772081, 51837009, 51577158), Natural Science Foundation of Hunan Province (Nos. 2016JJ2024, 2016JJ2025) and National Youth Science Fund project (No. 51607147).
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