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Graphene oxide grafted carbon fiber reinforced siliconborocarbonitride ceramics with enhanced thermal stability
Carbon 95 (2015) 157e165
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Graphene oxide grafted carbon fiber reinforced siliconborocarbonitride ceramics with enhanced thermal stability Wenbo Han, Guangdong Zhao*, Xinghong Zhang**, Shanbao Zhou, Peng Wang, Yumin An, Baosheng Xu National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, Centre for Composite Materials and Structures, Harbin Institute of Technology, Harbin, 150080, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 May 2015 Received in revised form 16 July 2015 Accepted 11 August 2015 Available online 13 August 2015
Hierarchical structure consisting of graphene oxide (GO) grafted onto carbon fiber (CF) has been synthesized to improve the interfacial properties between the CFs and polymer matrix. The modified CFs are coated with siliconborocarbonitride (SiBCN) preceramic polymer by in situ polymerization to enhance their antioxidant and ablation properties. X-ray photoelectron spectroscopy were used to monitor the composition of the composites. Scanning electron microscopy images revealed that GO was successfully grafted onto CF (CF-g-GO) and coated with SiBCN preceramic polymer (CF-g-GO/SiBCN). Loading of SiBCN preceramic polymer on the surface of CF-g-GO increased remarkably compared to that on the surface of untreated CFs. Pyrolysis of CF-g-GO/SiBCN preceramic polymer at 1400 C in inert atmosphere led to the formation of SiBCN ceramics with the approximate elemental composition of Si3N4/SiC/BN. TG results show that the thermal stability of the CF with ceramic layer has improved noticeably at high temperature. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Graphene oxide Carbon fiber Chemical grafting SiBCN ceramic Thermal stability
1. Introduction Carbon-fiber reinforced polymer (CFRP) composites have attracted continuous attention over the past half century because they offer excellent mechanical properties, high strength-to weight-ratio, high rigidity, and chemical resistance. They exhibit potential applications in aerospace, automotive and civil engineering, and numerous industrial fields [1e3]. Besides, CFRP also possesses good in-plane tensile properties attributed to its equivalent weight in comparison to the traditional metallic materials. However, the performance of CFRP is often limited due to the existence of a weak interface between the fiber and surrounding polymer matrix [4]. This is ascribed to the smooth and inert surface of CF. Therefore, the interface between the CFs and matrix remains an important area of study and it plays a major role in determining the macroscopic shear, transverse, and out of plane properties of a composite [5].
* Corresponding author. Tel.: þ86 451 86403016; fax: þ86 451 86403016. ** Corresponding author. E-mail addresses: [email protected] (G. Zhao), [email protected] (X. Zhang). http://dx.doi.org/10.1016/j.carbon.2015.08.028 0008-6223/© 2015 Elsevier Ltd. All rights reserved.
Numerous techniques have been developed to improve the interfacial properties by modifying CFs using micrometer and nanometer scale materials such as carbon nanotube (CNT), graphene and silicon carbide nanofibers [6e9]. In general, these techniques include chemical vapor deposition (CVD) and chemical modification. CVD is one of the most frequently used approaches to modify the CFs. CVD leads to the direct growth of the CNTs onto the surface of the CF; however, the method has an important limitation that the catalyst and high temperature deposition condition destroy the surface of the CFs leading to the degradation of the mechanical properties [10]. Moreover, CFs contain a high fraction of graphitic carbon leading to its low reactivity. Alternatively, grafting CNTs onto the surface of CF through chemical reaction has also been investigated extensively. CNTs are grafted onto the surface of CFs by chemical reactions using polyhedral oligomeric silsesquioxanes, poly(amido amine), and hexamethylenediamine as coupling agents [6e9]. The chemical modification results in obvious improvement in the interfacial properties, in particular, wettability and surface energy, of the CFs. The method of chemical grafting can significantly improve the surface roughness of the CFs and is beneficial for the CFs to integrate with the polymer matrix [11e15]. Recently, graphene, which has been extensively applied in diverse areas including field effect transistors, organic solar cells,
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chemical/bio sensors, and transparent electrodes in photovoltaic devices, has attracted significant attention [16e18]. Research on graphene contributes remarkably to the advancements in material fields [19e25]. Several fascinating properties of graphene were discovered through the investigation of pristine graphene including extremely high charge mobility with 2.3% absorption of visible light, thermal conductivity, the highest strength, and the highest theoretical specific surface area. However, two critical issues are encountered which include fabrication of cost effective graphene nanostructures produced at large scale and its high efficiency because graphene sheets are difficult to be incorporated and distributed homogeneously into various matrices [26]. The solution to the above mentioned problem has been provided in terms of an attractive approach to produce low cost graphene oxide/reduced graphene oxide (GO/RGO) sheets in large quantities [27,28]. The method includes the incorporation of several groups such as hydroxyl, carboxyl, epoxy, and other similar groups, on the surface of GO. Incorporation of groups lead to the efficient combination of graphene with the matrix. Besides, the active groups provide numerous options to graft GO onto the CF by different multifunctional coupling agents and different chemical reactions. Moreover, synthesis of polymer shows significant importance for both CFRP and its potential applications because they exhibit excellent chemical, thermal, and mechanical properties. This research mainly focuses on the preceramic polymers [29]. In the past decades, extensive research efforts have been devoted to the preceramic polymers which act as precursors for the fabrication of mainly silicon (Si)-based advanced ceramics [30]. Decomposition, crystallization, phase separation, and creep of the precursor polymers result in the formation of ceramics exhibiting excellent stability at high temperature (up to 2000 C) [31]. Thus, the transformation of precursor polymers to the ceramics leads to the formation of polymer-derived ceramics (PDCs). Moreover, preceramic polymers owing to their multiple compositions have been used as polymer matrix to fabricate CF reinforced polymer composites. Numerous important engineering fields suitable for potential application of PDCs include high-temperature-resistant materials (energy materials, automotive, aerospace), hard materials, chemical engineering (catalyst support, food- and biotechnology), and functional materials in electrical engineering as well as in micro/nanoelectronics. Synthesis of preceramic polymers involve low temperature polymerization and cross-linking reactions, followed by pyrolysis at high temperatures (1000e1600 C) in an inert atmosphere [32e35]. The structure and composition of monomers play an important role in controlling the composition and final structure of ceramics. Lee et al. synthesized siliconborocarbonitride (SiBCN) preceramic polymers using three monomers, trichlorosilane (HSiCl3), boron trichloride (BCl3), and hexamethydisilazane (HMDZ), at a molar ratio of about 1:1:4. The product soobtained exhibited an amorphous structure with cross-linked bonds during the elimination of (CH3)3SiCl. Pyrolysis of SiBCN preceramic polymers at 1600 C converted them to ceramics via phase transformation; thus, providing hydrothermal stability under autoclave test [36]. Jansen et al. developed borazine derivative [B{CH(CH3) (SiCl3)}NH]3 (TSEB)using 1-(trichlorosilyl)-1-(dichloroboryl)ethane (Cl3SiCH(CH3)BCl2) with HMDZ under ambient conditions. The specially designed molecule served as a single source precursor for the synthesis of SiBCN preceramic polymers. Pursuing this philosophy, polymerization of TSEB with methylamine as the cross-linking reagent led to the fabrication of highly homogeneous preceramic polymer. The resulting amorphous SiBCN of the approximate elemental composition of Si3B3N5C4 exhibited an outstanding thermal durability at 2000 C under inert conditions and it was also stable in pure oxygen up to at least 1300 C [37]. Bernard et al. fabricated SiBCN preceramic polymers
using two monomers, namely, dichloromethylvinylsilane (CH2] CHSiCH3Cl2) and borane dimethylsulfide (BH3$S(CH3)2), subsequently, it was processed into polymer green fibers by a melt spinning process. Shaping processing at 200 C followed by pyrolysis at 1400 C in a nitrogen atmosphere resulted in the formation of the amorphous and stable Si3.0B1.0C5.0N2.4 ceramic fibers with tensile strength of 1.3 GPa and Young's modulus of 170 GPa [38]. Various monomers used for synthesis of SiBCN preceramic polymers have a common characteristic, i.e., they contain active chlorine, which undergoes substitution reaction with the hydroxyl groups present on the surface of polymer matrix. In general, prior to grafting of GO onto CFs, they are treated with concentrated nitric acid by liquid phase oxidation method, which incorporates numerous hydroxyl and carboxyl groups on the surface of CF; moreover, GO itself contains a large number of hydroxyl groups. Thus, the monomers with active chlorine can directly react with hydroxyl groups present on CF and GO. Therefore, the chlorinated compound not only acts as a monomer to synthesize the SiBCN preceramic polymers, but also chemically combines CF, GO, and SiBCN preceramic polymers as a whole by acting as an adhesive. In this study, we proposed a novel method to synthesize the SiBCN preceramic polymer on the modified CF surface by in situ polymerization. In particular, CFs grafted with GO were employed to fabricate CF-g-GO/SiBCN preceramic polymer hierarchical structure. Pyrolysis of CF-g-GO/SiBCN preceramic polymer at 1400 C in inert atmosphere led to the formation of Si3N4/SiC/BN ceramics. The composite material exhibited good stability at high temperature. 2. Experimental section 2.1. Materials All solvents, monomers, and other chemicals were purchased from Aldrich unless otherwise stated. CF (Jiangsu tianniao Company, China) had an average diameter of 7 mm. In this study, following chemicals were employed: (3-aminopropyl)triethoxysilane (99%), (3-glycidyloxypro-pyl)trimethoxysilane (98%), boron trichloride solution (BCl3, 1.0 M in methylene chloride), trichlorosilane (HSiCl3, 99%), and hexamethyldisilazane (HDMZ, 99.9%). GO was synthesized according to the literature methods [28]. Other chemicals were of analytical grade and used as received. 2.2. Preparation of GO amine GO was fabricated according to the modified Hummers' method. In a typical synthesis, GO was taken in a round bottomed flask and dispersed in ethanol (100 mL) for 1 h. Subsequently, (3aminopropyl)triethoxysilane in deionized water was added to the stirring reaction mixture. The reaction mixture was heated to 60 C for 24 h. Later, the contents were cooled to room temperature and the solvent was removed by centrifugation. The so-obtained solid product was dried under vacuum until constant weight was obtained; thus, leading to the formation of GO amine. 2.3. Grafting of GO onto CF (CF-g-GO) CF was added to acetone with magnetic stirring at 70 C for 48 h, filtered, and thoroughly washed with acetone. CF was dried under vacuum, and dried CF and concentrated nitric acid were taken in a round bottomed flask. The mixture was stirred at 80 C for 3 h. The solid products were dried under vacuum until constant weight was obtained. GO amine (0.5 g) was dispersed by ultrasonication for 2 h in dimethylformamide (DMF, 100 mL) taken in a round bottomed flask. Subsequently, acidified CF was added and the reaction
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mixture was stirred at 110 C for 24 h. The product was filtered and thoroughly washed with ethanol. The solid sample was collected and dried under vacuum until constant weight was obtained. 2.4. Preparation of SiBCN preceramic polymer coating on CF-g-GO In a typical reaction, GO grafted CF, BCl3, HSiCl3, and HDMZ were successively added to a round bottomed flask, and the mixture was degassed with nitrogen for 30 min. The reaction mixture was magnetically stirred at 70 C for 3 h and then heated to 200 C for 3 h. The reaction product was distilled under reduced pressure, then the solid sample was collected, and dried under vacuum until constant weight was acquired which resulted in the formation of the desired product CF-g-GO/SiBCN preceramic polymer. 2.5. Preparation of CF-g-GO/SiBCN ceramic composites Pyrolysis of CF-g-GO/SiBCN ceramic composites led to the formation of SiBCN ceramics. CF-g-GO/SiBCN preceramic polymer was first heated in nitrogen atmosphere from 25 to 900 C at a heating rate of 10 C min1. The temperature was held constant at 900 C for 30 min, and then the polymer was heated to 1400 C, and held for 1 h in a silicate tube furnace. The contents were cooled down to room temperature which resulted in the formation of the composites. 2.6. Characterization The morphology of the fabricated materials was investigated by scanning electron microscopy (SEM, Sirion 200 FEI Netherlands). The elemental compositions and chemical binding states were analyzed by X-ray photoelectron spectroscopy (XPS, Escalab 250, USA), and Fourier transform infrared spectroscopy (FTIR, PerkineElmer 2000, USA) using KBr disks. The thermal stability of the materials was characterized by thermogravimetric analyses (TGA, TA Instruments TGA 2050, USA) at a rate of 10 C min1 under nitrogen. The mechanical properties of fiber samples were recorded with Nanomechanical testing system (UTM, Agilent T150, USA). 3. Results and discussion We reported a novel method to synthesize the SiBCN ceramic on the modified CFs surface by in situ polymerization (Fig. 1). The CFs were grafted with GO using (3-aminopropyl)triethoxysilane and (3-glycidyloxypropyl) trimethoxysilane as the coupling agents. The CF-g-GO hierarchical structure contains numerous hydroxyl groups capable of reacting with chlorine atoms present in the monomer units and improving the interfacial properties of the CF by increasing the surface area and mechanical interlocking. First, GO was fabricated via a modified Hummers' method. (3Aminopropyl)triethoxysilane was hydrolyzed under alkaline condition to generate silanol, which reacted with hydroxyl group present on the GO to form GO amine. The structure of GO amine is shown in Fig. 1. IR analysis was used to monitor the changes occurring in the chemical bonds during the reaction and the target structure was identified. Fig. 2 clearly exhibits the formation of various covalent bonds with (3-aminopropyl)triethoxysilane due to graphene modification. A strong stretching vibration absorption peak corresponding to hydroxyl and amino groups appears at 3320e 3670 cm1. Another sharp stretching vibration absorption peak is observed at 1084 cm1, which is attributed to CeN bond and SieO bond. The SieO bond was derived from the dehydration reaction between CeOH and SieOH. CeH bond stretching absorptions at 2895 and 2934 cm1 correspond to CH2 connected to the amino
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group. The results demonstrate that the amino group successfully modified the surface of GO. The elemental compositions of the surfaces of GO and GO amine were investigated by XPS, and the elemental contents are listed in Table S1. The chemical compositions of GO mainly consists of C, N, and O and the chemical bonds include CeC, COOH, CeOH, and C] O. The values listed in Table S2 indicate that modification by amino groups results in the appearance of nitrogen atoms. Fig. 3a exhibits that C1s scans of GO show a sharp peak at around 284.8 eV and two broad shoulder peaks at 286.3e290.1 eV assigned to sp2-hybridized carbon and single- or double-bonded carbon atoms including CeOH, C]O, and OeC]O. Different contents of hydroxyl and carboxyl groups were due to different preparation conditions of GO. The values listed in Table S2 and Fig. 4 exhibit changes in the different chemical bond contents and fitting curves by LorentzianeGaussian method. When amino-modified GO reacts with silanol it is observed, that the content of CeOH bond partly decreases and results in the generation of CeOeSi bond. Fig. 4 displays that C1s scans of GO amine also show an intense peak at around 284.8 eV and two broad shoulder peaks at 286.0e291.0 eV, and a new chemical bond CeN appears during modification process, which is attributed to the amino coupling agent. Thus, the bonds on the surface of the GO amine include CeC, COOH (OeC]O), CeOH (CeO), carbonyl group (C]O), and CeNH2 (CeN). In the region of silicon XPS response, two visible peaks at 153.3 and 101.6 eV are detected. The peak at 101.6 eV is attributed to the silicon hydride bond structural units which usually exist in juncture between GO and the coupling agent. The existence of peak at 401.3 eV confirms the presence of Ncontaining group (CeNH2) on the surface of the GO amine. These peaks indicate that the coupling agent containing amino group has successfully and effectively reacted with GO. Amino group can react with epoxy group by ring-opening method under certain conditions such as high temperature; thus, this method which combines two different materials by chemical bond is exercisable. In this study, GO grafted onto CF via ring-opening reaction was investigated, and the synthetic route is depicted in Fig. 1. (3-Aminopropyl)triethoxysilane and (3-glycidyloxypropyl)-trimethoxysilane were used as coupling agents with amino and epoxy groups. On completion of the grafting reaction, the residual epoxy groups and the newly generated hydroxyl groups on the surface of CF-g-GO could significantly improve the interfacial properties between the hierarchical structure and the polymer matrix. The hydroxyl groups react with the monomers of SiBCN preceramic polymer, forming a strong chemical bond at the composites interface. First, CFs were oxidized in concentrated nitric acid at 80 C for 3 h. Oxidation leads to the formation of large number of hydroxyl, carboxyl, and epoxy groups on the surface of CF. (3glycidyloxypropyl)trimethoxysilane is hydrolyzed to generate silanol, which gets connected to the CF by dehydration reaction; thus, leading to the formation of epoxy groups modified CF. Second, amino-functionalized GO is used as reinforced material to prepare CF-g-GO hierarchical structure. The ring-opening reaction was conducted in DMF at 110 C for 24 h. Untreated CF consisted of high orientation graphite sheets; therefore, the surface was smooth and had chemical inertia. The surface of CF treated with acetone and acid exhibited the appearance of a few narrow and shallow grooves leading to a partial decrease in the mechanical properties of the CF; however, its surface reactivity was significantly improved. Therefore, to increase the mechanical properties of the CF, it was grafted with GO. Although roughness of the CF-g-GO obviously increased, the mechanical properties such as tensile strength and toughness were enhanced because GO was distributed uniformly on the fiber
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Fig. 1. Schematic illustration of the preparation process for CF-g-GO hierarchical structure with SiBCN ceramic layer.
Fig. 2. FTIR spectra of GO and GO amine.
surface. Besides, GO could fix the polymer matrix on the surface of CF by mechanical interlocking action. The elemental compositions of the surface of the CFs in different stages were investigated by XPS, and the scanning curves are exhibited in Fig. 5. The chemical elemental compositions of the acidified CF (CFeCOOH) and CF with epoxy groups (CFeepoxy) are basically composed of C, O, and a small amount of Si and N in CFeepoxy. The chemical bonds include CeC, CeOH, C]O, and COOH. Fig. 6a and b clearly display that C1s scans of CFeCOOH and CFeepoxy similarly show a strong peak at around 284.8 eV and two broad shoulder peaks at 286.0e290.3 eV assigned to sp2hybridized carbon and single- or double-bonded carbon atoms including CeO, C]O, and OeC]O. The contents of CeO bond on
the surface of CFeepoxy increase rapidly compared to that on the surface of the acidified CF which can be attributed to the fact that the acidified CF reacts with the coupling agent containing epoxy group (Table S3). Fig. 6c shows that modification of CF with GO amine leads to the appearance of nitrogen on the surface of CF-gGO hierarchical structure and shows the existence of five chemical bonds, namely, CeC, CeO, C]O, CeN, and OeC]O. The CeN bonds, in particular, are derived from two parts, from ring-opening reaction between the amino group and epoxy group, and another is due to GO. Therefore, XPS in Fig. 6c demonstrate that the GO amino group is successfully grafted onto the surface of the CFs. The mechanical property of CFs was studied by single fiber tensile testing. The tensile strength results for the untreated CFs, acidified CFs, and GO grafted CFs are presented in Fig. 7, respectively. Compared to the untreated CF, the tensile strength of the acid treated CF decrease from 2.58 to 2.25 GPa. This is ascribed to the fact that acid oxidation leads to the destruction of the orderly arrangement of the graphite sheets present on the surface of the CF which results in the emergence of the defects. However, modification by grafting leads to an increase in the tensile strength to 2.64 GPa and Young's modulus approaches 264 GPa. In this study, toughness was investigated to evaluate the mechanical performance of the CF in different stages. Fig. 8 exhibits the toughness of different CFs revealing that the toughness of acid treated CF decreases from 14.7 to 12.0 MJ/m3 compared to untreated CF. Further, the toughness of CF modified by GO obviously increases to 16.6 MJ/m3. Thus, the addition of GO is beneficial to improve the toughness of the carbon. The above mentioned results indicated that the CF-gGO hierarchical structure did not have any negative effects on the tensile strength of the fiber. Mechanical interlocking can enhance the interfacial properties through repairing graphite sheet defects and increasing surface roughness. The chemical bonds also play an important role in improving interfacial properties during the processes involving the modification of CF. Significantly, the modification of the surface of CF mainly involves two targets, first is the
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Fig. 3. XPS curves a) GO and GO amine, b) Si 2s and SieO of GO amine.
Fig. 4. XPS fitting curves of C1s of a) GO, b) GO amine.
improvement of the interfacial properties as discussed above and second, is to provide hydroxyl groups capable of reacting with chlorine atoms in the monomers. The combination of hydroxyl groups and chlorine atoms conglomerates CF, GO, and polymer matrix as a whole through chemical bonding. During the coating process, all the reactions occurred on the surface of CF-g-GO as solid support. Two monomers, BCl3 and HSiCl3, possessing chlorine atoms as functional groups, participated in the polymerization reaction. The chlorine atoms combined with hydroxyl groups on the CF-g-GO hierarchical structure (Fig. 9). In general, BCl3 and HSiCl3 react with HDMZ via condensation polymerization. At the first stage, the reaction was conducted at 70 C for 3 h, and the main products were composed of low molecular weight oligomers. Subsequently, the temperature was increased to 200 C and the reaction was conducted for another 3 h; thus, the target SiBCN preceramic polymer was generated by intermolecular cross-linking reaction. Two factors might affect the polymer loading efficiency on the hierarchical structure surface. First was grafting ratio, when GO was grafted onto CF, increase in the grafting ratio and graft density resulted in an increase in the polymer loading efficiency due to
Fig. 5. XPS spectrum of CFs in different stages a) CFeCOOH, b) CFeepoxy, and c) CF-gGO.
Fig. 6. XPS spectrum and fitting curves of C1s a) CFeCOOH, b) CFeepoxy, and c) CF-gGO.
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Fig. 7. A typical tensile curve with different CFs a) un-treated CF, b) CFeCOOH, and c) CF-g-GO.
Fig. 8. Single fiber Young's modulus and toughness of different CFs a) untreated CF, b) CFeCOOH, and c) CF-g-GO.
smooth surface resulting from enhanced difficulty for polymer adsorption. Second, was the synergistic effect originating due to the presence of hydroxyl groups on the surface of CF-g-GO, the content of which was usually decided by controlling the temperature and time of the oxidation of CF without affecting the mechanical properties of CF. The aforementioned two factors had a relevance to some extent, because increase in the hydroxyl content on the CF significantly improved the interfacial reactivity to enhance the GO grafting ratio. IR analysis was performed to identify the bonds formation of the CFs in different processes with the objective of identifying the expected bonds after modification. The FTIR spectrum of the acidified CF shown in Fig. 10a indicates the formation of various covalent bonds. The absorption peak corresponding to the stretching vibration of the OH unit is observed at 3451 and 3140 cm1. The bending vibration absorption peak attributed to CeH group appears at 1391 cm1. Fig. 10b shows the appearance of two new peaks at 2933 and 2857 cm1 which are due to CH2 on the epoxy coupling agent. This indicates that (3glycidyloxypropyl)trimethoxysilane has reacted with the acidified CF. Fig. 10c displays the sharp peaks at 3610 and 1744 cm1, which are assigned to the formation of NeH bond belonging to the amino coupling agent, demonstrating that the GO amine is grafted onto CF by ring-opening reaction. The FTIR curve of the CF-g-GO coating
with SiBCN preceramic polymer is shown in Fig. 10d. Asymmetric stretching bands of the NeH and SieH units are observed at 3389, 1164, and 2144 cm1, respectively. A strong and broad vibration band overlapping with BeN and CeH group appears at 1465 and 1366 cm1. The peaks at 2951 and 2894 cm1 appear separately, which are due to the formation of CeH bond and SieC bond. The CeH and SieC bond stretching absorptions overlap at 1253 cm1. A sharp peak at 835 cm1 is due to the SieN group attached to boron atom. Finally, the SieNeSi bond derived from the intermolecular reaction appears at 928 cm1. Moreover, signals of chemical bonds containing oxygen are not detected, which indicates complete coating of the CF by SiBCN preceramic polymer. CF-g-GO/SiBCN preceramic polymers were converted to the CFg-GO/SiBCN ceramic composites by pyrolysis at 1400 C for 1 h (Fig. 9). The polymer-to-ceramic transformation consisted of two steps, first, was cross-linking of the polymers at low temperature (T < 400 C) leading to infusible organiceinorganic networks; and second was ceramization via pyrolysis at temperatures up to 1400 C leading to the fabrication of SiBCN ceramic. Although mainly amorphous ceramics are obtained upon pyrolysis, subsequent annealing at high temperatures (1400 C < T < 2000 C) converts them to crystalline materials. A key factor to increase the ceramic yield is the prevention of the loss of low molecular weight components of the precursor as well as their fragmentation processes during ceramization. Furthermore, the surroundings of CF-gGO were cladded with ceramic coating, which also effectively enhanced the CF tolerance under extreme high temperature environment. Surface morphologies of the CFs in different stages are shown in Fig. 11. GO sheets along the fiber can be observed on the surface of the CFs (Fig. 11a). SiBCN preceramic polymers are uniformly coated onto the fiber surface (Fig. 11b) after the in situ polymerization. These sheets act as the wedges on the fiber surface, which can increase the surface roughness and prevent the exfoliation of the polymers. The SiBCN ceramic coating synthesized via pyrolysis at 1400 C can be seen in Fig. 11c. There has a noteworthiness problem that the CFs may not be shielded by the ceramic layer effectively. To this problem, it can be solved by regulating the react ratio between the monomers and CF-g-GO to form enough thickness SiBCN preceramic layer on the CFs, which can ensure the graphene oxide could be shielded by the polymer layer. Fig 11b shows that the polymer coating and its thickness can reach ca. 1.1 mm, it could coat the graphene oxide effectively and the SiBCN preceramic would have a significant contraction on the surface of CF-g-GO in the process of high temperature crosslinking. It was also beneficial to fix the graphene oxide on the surface of CFs tightly. Moreover, the GO offers a reaction medium that the formation of chemical bonds can reinforce the interfacial binding force among the CF, GO, and polymer matrix. The micrometer scale ceramic particles derived from the SiBCN preceramic polymers coating onto CF-g-GO by the process of crystallization can prevent the contact of oxygen present in the air and effectively encourage the antioxidant properties of the composite materials. The thermal gravimetric analysis technique in a flowing air atmosphere was utilized to investigate the oxidation behavior of graphene oxide, SiBCN preceramic, SiBCN ceramic and CF-g-GO/ SiBCN ceramic, simultaneously the untreated CFs were used as reference substance. The TG curves of the CFs are shown in Fig. 12a. The untreated CFs are stable during their initial heating from 28 to 300 C, and the weight remains ca. 98%. Further, the TGA shows that the fibers exhibit no significant drop off in mass until over 579 C, which is ascribed to the oxidation of carbon resulting in the formation of carbon monoxide (CO) and carbon dioxide (CO2) during the heating process. Approximately at 799 C, the CF is completely oxidized to gases. The GO weight has a dramatic decline from 28 to
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Fig. 9. Preparation of CF-g-GO/SiBCN preceramic polymer and CF-g-GO/SiBCN ceramic composites.
Fig. 10. FTIR spectra of a) CFeCOOH, b) CFeepoxy, c) CF-g-GO, and d) CF-g-GO/SiBCN polymer.
Fig. 12. TG curves of (a) untreated CF, (b) GO, (c) SiBCN preceramic, (d) SiBCN ceramic and (e) CF-g-GO/SiBCN ceramic at a heating rate of 10 C min1 in air from 25 to 1200 C.
Fig. 11. SEM images of different CFs a) CF-g-GO, b) CF-g-GO/SiBCN preceramic, and c) CF-g-GO/SiBCN ceramic coating.
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610 C until constant weight (Fig. 12b), it is owing to a great quantity of defects on the surface of GO due to concentrated nitric acid oxidation. Fig. 12c and d show the SiBCN preceramic and SiBCN ceramic retain 75% and 90% residual mass until 1200 C under air atmosphere, respectively. That indicates the SiBCN ceramic has better thermal stability than the SiBCN precercamic under air atmosphere. And the pure SiBCN ceramic has a weight loss of 11.5% from 25 to 900 C, it is attributed to the boron nitride in SiBCN ceramic is partly oxidized to boron oxide and boron oxide melting point is 445 C. So the weight loss is due to the boron oxide evaporation from 720 to 870 C. When the CF-g-GO/SiBCN ceramic is oxidized in air at 1200 C, the residual mass reaches to 85%, that demonstrates the CFs coated with SiBCN ceramics layer are stable toward oxidation compared to the untreated CFs. But CF-g-GO/ SiBCN ceramic decreases rapidly until about 678e866 C, the sudden acceleration in the weight loss of the CF-g-GO/SiBCN ceramic can be mainly ascribed to the boron nitride oxidation in SiBCN ceramic layer as mentioned above. Moreover, because of the SiBCN preceramic pyrolysis on the surface of carbon fiber under nitrogen atmosphere, it emerges a mass of gas that leads to many mesoporous appearance in the SiBCN ceramic layer. The distribution of the pore diameter on the surface of CF-g-GO/SiBCN ceramic was depicted in Fig S1. By the high temperature oxidation in air atmosphere, the SiBCN ceramics were partly converted to silica and boron oxide, which had a glassy structure in high temperature and were able to flow and seal the mesoporous and surface defects to protect the carbon fiber from the oxidization in aerobic environment effectively. So the SiBCN ceramic coating could effectively prevent the CFs from the oxygen in the air. 4. Conclusions In this study, GO grafted CF reinforced SiBCN preceramic polymer composite was fabricated by a facile in situ polymerization. The hierarchical structure formed by this method enhanced the interfacial adhesion between the fibers and polymer matrix by providing strong mechanical interlocking property and chemical bonds, which were responsible for the effective combination of CF, GO, and polymer matrix. Specifically, the grafting ratio and graft density of GO on the fiber surface could be altered by employing different reaction conditions. Moreover, the mechanical properties of the CF modified by graphene were evidently enhanced, which effectively solved the disadvantage carbon fiber after acidification performance reduction. Pyrolysis of CF-g-GO/SiBCN preceramic polymer at 1400 C in inert atmosphere led to the formation of SiBCN ceramics with the approximate elemental composition of Si3N4/SiC/BN. CF-g-GO coated with SiBCN ceramic composite exhibited an excellent antioxidant capability under high temperature environment. The results indicated that chemical grafting technique is a potential and operable method for preparing the CF reinforced polymer composite materials with enhanced thermal stability. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carbon.2015.08.028. References [1] T.M. Keller, Oxidative protection of carbon fibers with poly(carboraneesiloxaneeacetylene), Carbon 40 (2011) 225e229. [2] K. Pingkarawat, T. Bhat, D.A. Craze, C.H. Wang, R.J. Varleyb, A.P. Mouritz, Healing of carbon fibreeepoxy composites using thermoplastic additives, Polym. Chem. 4 (2013) 5007e5015.
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Carbon journal homepage: www.elsevier.com/locate/carbon
Graphene oxide grafted carbon fiber reinforced siliconborocarbonitride ceramics with enhanced thermal stability Wenbo Han, Guangdong Zhao*, Xinghong Zhang**, Shanbao Zhou, Peng Wang, Yumin An, Baosheng Xu National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, Centre for Composite Materials and Structures, Harbin Institute of Technology, Harbin, 150080, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 May 2015 Received in revised form 16 July 2015 Accepted 11 August 2015 Available online 13 August 2015
Hierarchical structure consisting of graphene oxide (GO) grafted onto carbon fiber (CF) has been synthesized to improve the interfacial properties between the CFs and polymer matrix. The modified CFs are coated with siliconborocarbonitride (SiBCN) preceramic polymer by in situ polymerization to enhance their antioxidant and ablation properties. X-ray photoelectron spectroscopy were used to monitor the composition of the composites. Scanning electron microscopy images revealed that GO was successfully grafted onto CF (CF-g-GO) and coated with SiBCN preceramic polymer (CF-g-GO/SiBCN). Loading of SiBCN preceramic polymer on the surface of CF-g-GO increased remarkably compared to that on the surface of untreated CFs. Pyrolysis of CF-g-GO/SiBCN preceramic polymer at 1400 C in inert atmosphere led to the formation of SiBCN ceramics with the approximate elemental composition of Si3N4/SiC/BN. TG results show that the thermal stability of the CF with ceramic layer has improved noticeably at high temperature. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Graphene oxide Carbon fiber Chemical grafting SiBCN ceramic Thermal stability
1. Introduction Carbon-fiber reinforced polymer (CFRP) composites have attracted continuous attention over the past half century because they offer excellent mechanical properties, high strength-to weight-ratio, high rigidity, and chemical resistance. They exhibit potential applications in aerospace, automotive and civil engineering, and numerous industrial fields [1e3]. Besides, CFRP also possesses good in-plane tensile properties attributed to its equivalent weight in comparison to the traditional metallic materials. However, the performance of CFRP is often limited due to the existence of a weak interface between the fiber and surrounding polymer matrix [4]. This is ascribed to the smooth and inert surface of CF. Therefore, the interface between the CFs and matrix remains an important area of study and it plays a major role in determining the macroscopic shear, transverse, and out of plane properties of a composite [5].
* Corresponding author. Tel.: þ86 451 86403016; fax: þ86 451 86403016. ** Corresponding author. E-mail addresses: [email protected] (G. Zhao), [email protected] (X. Zhang). http://dx.doi.org/10.1016/j.carbon.2015.08.028 0008-6223/© 2015 Elsevier Ltd. All rights reserved.
Numerous techniques have been developed to improve the interfacial properties by modifying CFs using micrometer and nanometer scale materials such as carbon nanotube (CNT), graphene and silicon carbide nanofibers [6e9]. In general, these techniques include chemical vapor deposition (CVD) and chemical modification. CVD is one of the most frequently used approaches to modify the CFs. CVD leads to the direct growth of the CNTs onto the surface of the CF; however, the method has an important limitation that the catalyst and high temperature deposition condition destroy the surface of the CFs leading to the degradation of the mechanical properties [10]. Moreover, CFs contain a high fraction of graphitic carbon leading to its low reactivity. Alternatively, grafting CNTs onto the surface of CF through chemical reaction has also been investigated extensively. CNTs are grafted onto the surface of CFs by chemical reactions using polyhedral oligomeric silsesquioxanes, poly(amido amine), and hexamethylenediamine as coupling agents [6e9]. The chemical modification results in obvious improvement in the interfacial properties, in particular, wettability and surface energy, of the CFs. The method of chemical grafting can significantly improve the surface roughness of the CFs and is beneficial for the CFs to integrate with the polymer matrix [11e15]. Recently, graphene, which has been extensively applied in diverse areas including field effect transistors, organic solar cells,
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chemical/bio sensors, and transparent electrodes in photovoltaic devices, has attracted significant attention [16e18]. Research on graphene contributes remarkably to the advancements in material fields [19e25]. Several fascinating properties of graphene were discovered through the investigation of pristine graphene including extremely high charge mobility with 2.3% absorption of visible light, thermal conductivity, the highest strength, and the highest theoretical specific surface area. However, two critical issues are encountered which include fabrication of cost effective graphene nanostructures produced at large scale and its high efficiency because graphene sheets are difficult to be incorporated and distributed homogeneously into various matrices [26]. The solution to the above mentioned problem has been provided in terms of an attractive approach to produce low cost graphene oxide/reduced graphene oxide (GO/RGO) sheets in large quantities [27,28]. The method includes the incorporation of several groups such as hydroxyl, carboxyl, epoxy, and other similar groups, on the surface of GO. Incorporation of groups lead to the efficient combination of graphene with the matrix. Besides, the active groups provide numerous options to graft GO onto the CF by different multifunctional coupling agents and different chemical reactions. Moreover, synthesis of polymer shows significant importance for both CFRP and its potential applications because they exhibit excellent chemical, thermal, and mechanical properties. This research mainly focuses on the preceramic polymers [29]. In the past decades, extensive research efforts have been devoted to the preceramic polymers which act as precursors for the fabrication of mainly silicon (Si)-based advanced ceramics [30]. Decomposition, crystallization, phase separation, and creep of the precursor polymers result in the formation of ceramics exhibiting excellent stability at high temperature (up to 2000 C) [31]. Thus, the transformation of precursor polymers to the ceramics leads to the formation of polymer-derived ceramics (PDCs). Moreover, preceramic polymers owing to their multiple compositions have been used as polymer matrix to fabricate CF reinforced polymer composites. Numerous important engineering fields suitable for potential application of PDCs include high-temperature-resistant materials (energy materials, automotive, aerospace), hard materials, chemical engineering (catalyst support, food- and biotechnology), and functional materials in electrical engineering as well as in micro/nanoelectronics. Synthesis of preceramic polymers involve low temperature polymerization and cross-linking reactions, followed by pyrolysis at high temperatures (1000e1600 C) in an inert atmosphere [32e35]. The structure and composition of monomers play an important role in controlling the composition and final structure of ceramics. Lee et al. synthesized siliconborocarbonitride (SiBCN) preceramic polymers using three monomers, trichlorosilane (HSiCl3), boron trichloride (BCl3), and hexamethydisilazane (HMDZ), at a molar ratio of about 1:1:4. The product soobtained exhibited an amorphous structure with cross-linked bonds during the elimination of (CH3)3SiCl. Pyrolysis of SiBCN preceramic polymers at 1600 C converted them to ceramics via phase transformation; thus, providing hydrothermal stability under autoclave test [36]. Jansen et al. developed borazine derivative [B{CH(CH3) (SiCl3)}NH]3 (TSEB)using 1-(trichlorosilyl)-1-(dichloroboryl)ethane (Cl3SiCH(CH3)BCl2) with HMDZ under ambient conditions. The specially designed molecule served as a single source precursor for the synthesis of SiBCN preceramic polymers. Pursuing this philosophy, polymerization of TSEB with methylamine as the cross-linking reagent led to the fabrication of highly homogeneous preceramic polymer. The resulting amorphous SiBCN of the approximate elemental composition of Si3B3N5C4 exhibited an outstanding thermal durability at 2000 C under inert conditions and it was also stable in pure oxygen up to at least 1300 C [37]. Bernard et al. fabricated SiBCN preceramic polymers
using two monomers, namely, dichloromethylvinylsilane (CH2] CHSiCH3Cl2) and borane dimethylsulfide (BH3$S(CH3)2), subsequently, it was processed into polymer green fibers by a melt spinning process. Shaping processing at 200 C followed by pyrolysis at 1400 C in a nitrogen atmosphere resulted in the formation of the amorphous and stable Si3.0B1.0C5.0N2.4 ceramic fibers with tensile strength of 1.3 GPa and Young's modulus of 170 GPa [38]. Various monomers used for synthesis of SiBCN preceramic polymers have a common characteristic, i.e., they contain active chlorine, which undergoes substitution reaction with the hydroxyl groups present on the surface of polymer matrix. In general, prior to grafting of GO onto CFs, they are treated with concentrated nitric acid by liquid phase oxidation method, which incorporates numerous hydroxyl and carboxyl groups on the surface of CF; moreover, GO itself contains a large number of hydroxyl groups. Thus, the monomers with active chlorine can directly react with hydroxyl groups present on CF and GO. Therefore, the chlorinated compound not only acts as a monomer to synthesize the SiBCN preceramic polymers, but also chemically combines CF, GO, and SiBCN preceramic polymers as a whole by acting as an adhesive. In this study, we proposed a novel method to synthesize the SiBCN preceramic polymer on the modified CF surface by in situ polymerization. In particular, CFs grafted with GO were employed to fabricate CF-g-GO/SiBCN preceramic polymer hierarchical structure. Pyrolysis of CF-g-GO/SiBCN preceramic polymer at 1400 C in inert atmosphere led to the formation of Si3N4/SiC/BN ceramics. The composite material exhibited good stability at high temperature. 2. Experimental section 2.1. Materials All solvents, monomers, and other chemicals were purchased from Aldrich unless otherwise stated. CF (Jiangsu tianniao Company, China) had an average diameter of 7 mm. In this study, following chemicals were employed: (3-aminopropyl)triethoxysilane (99%), (3-glycidyloxypro-pyl)trimethoxysilane (98%), boron trichloride solution (BCl3, 1.0 M in methylene chloride), trichlorosilane (HSiCl3, 99%), and hexamethyldisilazane (HDMZ, 99.9%). GO was synthesized according to the literature methods [28]. Other chemicals were of analytical grade and used as received. 2.2. Preparation of GO amine GO was fabricated according to the modified Hummers' method. In a typical synthesis, GO was taken in a round bottomed flask and dispersed in ethanol (100 mL) for 1 h. Subsequently, (3aminopropyl)triethoxysilane in deionized water was added to the stirring reaction mixture. The reaction mixture was heated to 60 C for 24 h. Later, the contents were cooled to room temperature and the solvent was removed by centrifugation. The so-obtained solid product was dried under vacuum until constant weight was obtained; thus, leading to the formation of GO amine. 2.3. Grafting of GO onto CF (CF-g-GO) CF was added to acetone with magnetic stirring at 70 C for 48 h, filtered, and thoroughly washed with acetone. CF was dried under vacuum, and dried CF and concentrated nitric acid were taken in a round bottomed flask. The mixture was stirred at 80 C for 3 h. The solid products were dried under vacuum until constant weight was obtained. GO amine (0.5 g) was dispersed by ultrasonication for 2 h in dimethylformamide (DMF, 100 mL) taken in a round bottomed flask. Subsequently, acidified CF was added and the reaction
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mixture was stirred at 110 C for 24 h. The product was filtered and thoroughly washed with ethanol. The solid sample was collected and dried under vacuum until constant weight was obtained. 2.4. Preparation of SiBCN preceramic polymer coating on CF-g-GO In a typical reaction, GO grafted CF, BCl3, HSiCl3, and HDMZ were successively added to a round bottomed flask, and the mixture was degassed with nitrogen for 30 min. The reaction mixture was magnetically stirred at 70 C for 3 h and then heated to 200 C for 3 h. The reaction product was distilled under reduced pressure, then the solid sample was collected, and dried under vacuum until constant weight was acquired which resulted in the formation of the desired product CF-g-GO/SiBCN preceramic polymer. 2.5. Preparation of CF-g-GO/SiBCN ceramic composites Pyrolysis of CF-g-GO/SiBCN ceramic composites led to the formation of SiBCN ceramics. CF-g-GO/SiBCN preceramic polymer was first heated in nitrogen atmosphere from 25 to 900 C at a heating rate of 10 C min1. The temperature was held constant at 900 C for 30 min, and then the polymer was heated to 1400 C, and held for 1 h in a silicate tube furnace. The contents were cooled down to room temperature which resulted in the formation of the composites. 2.6. Characterization The morphology of the fabricated materials was investigated by scanning electron microscopy (SEM, Sirion 200 FEI Netherlands). The elemental compositions and chemical binding states were analyzed by X-ray photoelectron spectroscopy (XPS, Escalab 250, USA), and Fourier transform infrared spectroscopy (FTIR, PerkineElmer 2000, USA) using KBr disks. The thermal stability of the materials was characterized by thermogravimetric analyses (TGA, TA Instruments TGA 2050, USA) at a rate of 10 C min1 under nitrogen. The mechanical properties of fiber samples were recorded with Nanomechanical testing system (UTM, Agilent T150, USA). 3. Results and discussion We reported a novel method to synthesize the SiBCN ceramic on the modified CFs surface by in situ polymerization (Fig. 1). The CFs were grafted with GO using (3-aminopropyl)triethoxysilane and (3-glycidyloxypropyl) trimethoxysilane as the coupling agents. The CF-g-GO hierarchical structure contains numerous hydroxyl groups capable of reacting with chlorine atoms present in the monomer units and improving the interfacial properties of the CF by increasing the surface area and mechanical interlocking. First, GO was fabricated via a modified Hummers' method. (3Aminopropyl)triethoxysilane was hydrolyzed under alkaline condition to generate silanol, which reacted with hydroxyl group present on the GO to form GO amine. The structure of GO amine is shown in Fig. 1. IR analysis was used to monitor the changes occurring in the chemical bonds during the reaction and the target structure was identified. Fig. 2 clearly exhibits the formation of various covalent bonds with (3-aminopropyl)triethoxysilane due to graphene modification. A strong stretching vibration absorption peak corresponding to hydroxyl and amino groups appears at 3320e 3670 cm1. Another sharp stretching vibration absorption peak is observed at 1084 cm1, which is attributed to CeN bond and SieO bond. The SieO bond was derived from the dehydration reaction between CeOH and SieOH. CeH bond stretching absorptions at 2895 and 2934 cm1 correspond to CH2 connected to the amino
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group. The results demonstrate that the amino group successfully modified the surface of GO. The elemental compositions of the surfaces of GO and GO amine were investigated by XPS, and the elemental contents are listed in Table S1. The chemical compositions of GO mainly consists of C, N, and O and the chemical bonds include CeC, COOH, CeOH, and C] O. The values listed in Table S2 indicate that modification by amino groups results in the appearance of nitrogen atoms. Fig. 3a exhibits that C1s scans of GO show a sharp peak at around 284.8 eV and two broad shoulder peaks at 286.3e290.1 eV assigned to sp2-hybridized carbon and single- or double-bonded carbon atoms including CeOH, C]O, and OeC]O. Different contents of hydroxyl and carboxyl groups were due to different preparation conditions of GO. The values listed in Table S2 and Fig. 4 exhibit changes in the different chemical bond contents and fitting curves by LorentzianeGaussian method. When amino-modified GO reacts with silanol it is observed, that the content of CeOH bond partly decreases and results in the generation of CeOeSi bond. Fig. 4 displays that C1s scans of GO amine also show an intense peak at around 284.8 eV and two broad shoulder peaks at 286.0e291.0 eV, and a new chemical bond CeN appears during modification process, which is attributed to the amino coupling agent. Thus, the bonds on the surface of the GO amine include CeC, COOH (OeC]O), CeOH (CeO), carbonyl group (C]O), and CeNH2 (CeN). In the region of silicon XPS response, two visible peaks at 153.3 and 101.6 eV are detected. The peak at 101.6 eV is attributed to the silicon hydride bond structural units which usually exist in juncture between GO and the coupling agent. The existence of peak at 401.3 eV confirms the presence of Ncontaining group (CeNH2) on the surface of the GO amine. These peaks indicate that the coupling agent containing amino group has successfully and effectively reacted with GO. Amino group can react with epoxy group by ring-opening method under certain conditions such as high temperature; thus, this method which combines two different materials by chemical bond is exercisable. In this study, GO grafted onto CF via ring-opening reaction was investigated, and the synthetic route is depicted in Fig. 1. (3-Aminopropyl)triethoxysilane and (3-glycidyloxypropyl)-trimethoxysilane were used as coupling agents with amino and epoxy groups. On completion of the grafting reaction, the residual epoxy groups and the newly generated hydroxyl groups on the surface of CF-g-GO could significantly improve the interfacial properties between the hierarchical structure and the polymer matrix. The hydroxyl groups react with the monomers of SiBCN preceramic polymer, forming a strong chemical bond at the composites interface. First, CFs were oxidized in concentrated nitric acid at 80 C for 3 h. Oxidation leads to the formation of large number of hydroxyl, carboxyl, and epoxy groups on the surface of CF. (3glycidyloxypropyl)trimethoxysilane is hydrolyzed to generate silanol, which gets connected to the CF by dehydration reaction; thus, leading to the formation of epoxy groups modified CF. Second, amino-functionalized GO is used as reinforced material to prepare CF-g-GO hierarchical structure. The ring-opening reaction was conducted in DMF at 110 C for 24 h. Untreated CF consisted of high orientation graphite sheets; therefore, the surface was smooth and had chemical inertia. The surface of CF treated with acetone and acid exhibited the appearance of a few narrow and shallow grooves leading to a partial decrease in the mechanical properties of the CF; however, its surface reactivity was significantly improved. Therefore, to increase the mechanical properties of the CF, it was grafted with GO. Although roughness of the CF-g-GO obviously increased, the mechanical properties such as tensile strength and toughness were enhanced because GO was distributed uniformly on the fiber
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Fig. 1. Schematic illustration of the preparation process for CF-g-GO hierarchical structure with SiBCN ceramic layer.
Fig. 2. FTIR spectra of GO and GO amine.
surface. Besides, GO could fix the polymer matrix on the surface of CF by mechanical interlocking action. The elemental compositions of the surface of the CFs in different stages were investigated by XPS, and the scanning curves are exhibited in Fig. 5. The chemical elemental compositions of the acidified CF (CFeCOOH) and CF with epoxy groups (CFeepoxy) are basically composed of C, O, and a small amount of Si and N in CFeepoxy. The chemical bonds include CeC, CeOH, C]O, and COOH. Fig. 6a and b clearly display that C1s scans of CFeCOOH and CFeepoxy similarly show a strong peak at around 284.8 eV and two broad shoulder peaks at 286.0e290.3 eV assigned to sp2hybridized carbon and single- or double-bonded carbon atoms including CeO, C]O, and OeC]O. The contents of CeO bond on
the surface of CFeepoxy increase rapidly compared to that on the surface of the acidified CF which can be attributed to the fact that the acidified CF reacts with the coupling agent containing epoxy group (Table S3). Fig. 6c shows that modification of CF with GO amine leads to the appearance of nitrogen on the surface of CF-gGO hierarchical structure and shows the existence of five chemical bonds, namely, CeC, CeO, C]O, CeN, and OeC]O. The CeN bonds, in particular, are derived from two parts, from ring-opening reaction between the amino group and epoxy group, and another is due to GO. Therefore, XPS in Fig. 6c demonstrate that the GO amino group is successfully grafted onto the surface of the CFs. The mechanical property of CFs was studied by single fiber tensile testing. The tensile strength results for the untreated CFs, acidified CFs, and GO grafted CFs are presented in Fig. 7, respectively. Compared to the untreated CF, the tensile strength of the acid treated CF decrease from 2.58 to 2.25 GPa. This is ascribed to the fact that acid oxidation leads to the destruction of the orderly arrangement of the graphite sheets present on the surface of the CF which results in the emergence of the defects. However, modification by grafting leads to an increase in the tensile strength to 2.64 GPa and Young's modulus approaches 264 GPa. In this study, toughness was investigated to evaluate the mechanical performance of the CF in different stages. Fig. 8 exhibits the toughness of different CFs revealing that the toughness of acid treated CF decreases from 14.7 to 12.0 MJ/m3 compared to untreated CF. Further, the toughness of CF modified by GO obviously increases to 16.6 MJ/m3. Thus, the addition of GO is beneficial to improve the toughness of the carbon. The above mentioned results indicated that the CF-gGO hierarchical structure did not have any negative effects on the tensile strength of the fiber. Mechanical interlocking can enhance the interfacial properties through repairing graphite sheet defects and increasing surface roughness. The chemical bonds also play an important role in improving interfacial properties during the processes involving the modification of CF. Significantly, the modification of the surface of CF mainly involves two targets, first is the
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Fig. 3. XPS curves a) GO and GO amine, b) Si 2s and SieO of GO amine.
Fig. 4. XPS fitting curves of C1s of a) GO, b) GO amine.
improvement of the interfacial properties as discussed above and second, is to provide hydroxyl groups capable of reacting with chlorine atoms in the monomers. The combination of hydroxyl groups and chlorine atoms conglomerates CF, GO, and polymer matrix as a whole through chemical bonding. During the coating process, all the reactions occurred on the surface of CF-g-GO as solid support. Two monomers, BCl3 and HSiCl3, possessing chlorine atoms as functional groups, participated in the polymerization reaction. The chlorine atoms combined with hydroxyl groups on the CF-g-GO hierarchical structure (Fig. 9). In general, BCl3 and HSiCl3 react with HDMZ via condensation polymerization. At the first stage, the reaction was conducted at 70 C for 3 h, and the main products were composed of low molecular weight oligomers. Subsequently, the temperature was increased to 200 C and the reaction was conducted for another 3 h; thus, the target SiBCN preceramic polymer was generated by intermolecular cross-linking reaction. Two factors might affect the polymer loading efficiency on the hierarchical structure surface. First was grafting ratio, when GO was grafted onto CF, increase in the grafting ratio and graft density resulted in an increase in the polymer loading efficiency due to
Fig. 5. XPS spectrum of CFs in different stages a) CFeCOOH, b) CFeepoxy, and c) CF-gGO.
Fig. 6. XPS spectrum and fitting curves of C1s a) CFeCOOH, b) CFeepoxy, and c) CF-gGO.
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Fig. 7. A typical tensile curve with different CFs a) un-treated CF, b) CFeCOOH, and c) CF-g-GO.
Fig. 8. Single fiber Young's modulus and toughness of different CFs a) untreated CF, b) CFeCOOH, and c) CF-g-GO.
smooth surface resulting from enhanced difficulty for polymer adsorption. Second, was the synergistic effect originating due to the presence of hydroxyl groups on the surface of CF-g-GO, the content of which was usually decided by controlling the temperature and time of the oxidation of CF without affecting the mechanical properties of CF. The aforementioned two factors had a relevance to some extent, because increase in the hydroxyl content on the CF significantly improved the interfacial reactivity to enhance the GO grafting ratio. IR analysis was performed to identify the bonds formation of the CFs in different processes with the objective of identifying the expected bonds after modification. The FTIR spectrum of the acidified CF shown in Fig. 10a indicates the formation of various covalent bonds. The absorption peak corresponding to the stretching vibration of the OH unit is observed at 3451 and 3140 cm1. The bending vibration absorption peak attributed to CeH group appears at 1391 cm1. Fig. 10b shows the appearance of two new peaks at 2933 and 2857 cm1 which are due to CH2 on the epoxy coupling agent. This indicates that (3glycidyloxypropyl)trimethoxysilane has reacted with the acidified CF. Fig. 10c displays the sharp peaks at 3610 and 1744 cm1, which are assigned to the formation of NeH bond belonging to the amino coupling agent, demonstrating that the GO amine is grafted onto CF by ring-opening reaction. The FTIR curve of the CF-g-GO coating
with SiBCN preceramic polymer is shown in Fig. 10d. Asymmetric stretching bands of the NeH and SieH units are observed at 3389, 1164, and 2144 cm1, respectively. A strong and broad vibration band overlapping with BeN and CeH group appears at 1465 and 1366 cm1. The peaks at 2951 and 2894 cm1 appear separately, which are due to the formation of CeH bond and SieC bond. The CeH and SieC bond stretching absorptions overlap at 1253 cm1. A sharp peak at 835 cm1 is due to the SieN group attached to boron atom. Finally, the SieNeSi bond derived from the intermolecular reaction appears at 928 cm1. Moreover, signals of chemical bonds containing oxygen are not detected, which indicates complete coating of the CF by SiBCN preceramic polymer. CF-g-GO/SiBCN preceramic polymers were converted to the CFg-GO/SiBCN ceramic composites by pyrolysis at 1400 C for 1 h (Fig. 9). The polymer-to-ceramic transformation consisted of two steps, first, was cross-linking of the polymers at low temperature (T < 400 C) leading to infusible organiceinorganic networks; and second was ceramization via pyrolysis at temperatures up to 1400 C leading to the fabrication of SiBCN ceramic. Although mainly amorphous ceramics are obtained upon pyrolysis, subsequent annealing at high temperatures (1400 C < T < 2000 C) converts them to crystalline materials. A key factor to increase the ceramic yield is the prevention of the loss of low molecular weight components of the precursor as well as their fragmentation processes during ceramization. Furthermore, the surroundings of CF-gGO were cladded with ceramic coating, which also effectively enhanced the CF tolerance under extreme high temperature environment. Surface morphologies of the CFs in different stages are shown in Fig. 11. GO sheets along the fiber can be observed on the surface of the CFs (Fig. 11a). SiBCN preceramic polymers are uniformly coated onto the fiber surface (Fig. 11b) after the in situ polymerization. These sheets act as the wedges on the fiber surface, which can increase the surface roughness and prevent the exfoliation of the polymers. The SiBCN ceramic coating synthesized via pyrolysis at 1400 C can be seen in Fig. 11c. There has a noteworthiness problem that the CFs may not be shielded by the ceramic layer effectively. To this problem, it can be solved by regulating the react ratio between the monomers and CF-g-GO to form enough thickness SiBCN preceramic layer on the CFs, which can ensure the graphene oxide could be shielded by the polymer layer. Fig 11b shows that the polymer coating and its thickness can reach ca. 1.1 mm, it could coat the graphene oxide effectively and the SiBCN preceramic would have a significant contraction on the surface of CF-g-GO in the process of high temperature crosslinking. It was also beneficial to fix the graphene oxide on the surface of CFs tightly. Moreover, the GO offers a reaction medium that the formation of chemical bonds can reinforce the interfacial binding force among the CF, GO, and polymer matrix. The micrometer scale ceramic particles derived from the SiBCN preceramic polymers coating onto CF-g-GO by the process of crystallization can prevent the contact of oxygen present in the air and effectively encourage the antioxidant properties of the composite materials. The thermal gravimetric analysis technique in a flowing air atmosphere was utilized to investigate the oxidation behavior of graphene oxide, SiBCN preceramic, SiBCN ceramic and CF-g-GO/ SiBCN ceramic, simultaneously the untreated CFs were used as reference substance. The TG curves of the CFs are shown in Fig. 12a. The untreated CFs are stable during their initial heating from 28 to 300 C, and the weight remains ca. 98%. Further, the TGA shows that the fibers exhibit no significant drop off in mass until over 579 C, which is ascribed to the oxidation of carbon resulting in the formation of carbon monoxide (CO) and carbon dioxide (CO2) during the heating process. Approximately at 799 C, the CF is completely oxidized to gases. The GO weight has a dramatic decline from 28 to
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Fig. 9. Preparation of CF-g-GO/SiBCN preceramic polymer and CF-g-GO/SiBCN ceramic composites.
Fig. 10. FTIR spectra of a) CFeCOOH, b) CFeepoxy, c) CF-g-GO, and d) CF-g-GO/SiBCN polymer.
Fig. 12. TG curves of (a) untreated CF, (b) GO, (c) SiBCN preceramic, (d) SiBCN ceramic and (e) CF-g-GO/SiBCN ceramic at a heating rate of 10 C min1 in air from 25 to 1200 C.
Fig. 11. SEM images of different CFs a) CF-g-GO, b) CF-g-GO/SiBCN preceramic, and c) CF-g-GO/SiBCN ceramic coating.
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610 C until constant weight (Fig. 12b), it is owing to a great quantity of defects on the surface of GO due to concentrated nitric acid oxidation. Fig. 12c and d show the SiBCN preceramic and SiBCN ceramic retain 75% and 90% residual mass until 1200 C under air atmosphere, respectively. That indicates the SiBCN ceramic has better thermal stability than the SiBCN precercamic under air atmosphere. And the pure SiBCN ceramic has a weight loss of 11.5% from 25 to 900 C, it is attributed to the boron nitride in SiBCN ceramic is partly oxidized to boron oxide and boron oxide melting point is 445 C. So the weight loss is due to the boron oxide evaporation from 720 to 870 C. When the CF-g-GO/SiBCN ceramic is oxidized in air at 1200 C, the residual mass reaches to 85%, that demonstrates the CFs coated with SiBCN ceramics layer are stable toward oxidation compared to the untreated CFs. But CF-g-GO/ SiBCN ceramic decreases rapidly until about 678e866 C, the sudden acceleration in the weight loss of the CF-g-GO/SiBCN ceramic can be mainly ascribed to the boron nitride oxidation in SiBCN ceramic layer as mentioned above. Moreover, because of the SiBCN preceramic pyrolysis on the surface of carbon fiber under nitrogen atmosphere, it emerges a mass of gas that leads to many mesoporous appearance in the SiBCN ceramic layer. The distribution of the pore diameter on the surface of CF-g-GO/SiBCN ceramic was depicted in Fig S1. By the high temperature oxidation in air atmosphere, the SiBCN ceramics were partly converted to silica and boron oxide, which had a glassy structure in high temperature and were able to flow and seal the mesoporous and surface defects to protect the carbon fiber from the oxidization in aerobic environment effectively. So the SiBCN ceramic coating could effectively prevent the CFs from the oxygen in the air. 4. Conclusions In this study, GO grafted CF reinforced SiBCN preceramic polymer composite was fabricated by a facile in situ polymerization. The hierarchical structure formed by this method enhanced the interfacial adhesion between the fibers and polymer matrix by providing strong mechanical interlocking property and chemical bonds, which were responsible for the effective combination of CF, GO, and polymer matrix. Specifically, the grafting ratio and graft density of GO on the fiber surface could be altered by employing different reaction conditions. Moreover, the mechanical properties of the CF modified by graphene were evidently enhanced, which effectively solved the disadvantage carbon fiber after acidification performance reduction. Pyrolysis of CF-g-GO/SiBCN preceramic polymer at 1400 C in inert atmosphere led to the formation of SiBCN ceramics with the approximate elemental composition of Si3N4/SiC/BN. CF-g-GO coated with SiBCN ceramic composite exhibited an excellent antioxidant capability under high temperature environment. The results indicated that chemical grafting technique is a potential and operable method for preparing the CF reinforced polymer composite materials with enhanced thermal stability. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carbon.2015.08.028. References [1] T.M. Keller, Oxidative protection of carbon fibers with poly(carboraneesiloxaneeacetylene), Carbon 40 (2011) 225e229. [2] K. Pingkarawat, T. Bhat, D.A. Craze, C.H. Wang, R.J. Varleyb, A.P. Mouritz, Healing of carbon fibreeepoxy composites using thermoplastic additives, Polym. Chem. 4 (2013) 5007e5015.
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