Functionalization of CNFs via wet chemical oxidation method in order to improve CNFs adhesion to matrix of carbon composites
Introduction
Carbon fibers (CFs) and nanofibers (CNFs) are mainly applied to reinforce polymers, much like glass fibers have been used for decades in fiber glass reinforced polyesters, with the principal difference that composites with CFs are so-called ‘advanced composites’ or ‘high performance’ composites. The progress achieved with CNFs, as compared with glass reinforcement fibers, is based on the superior stiffness of CNFs, combined with high strength and low density [1]. Carbon nanofiber reinforced carbon (CNFC) composites are a generic class of advanced composites. CNFC composites have an excellent combination of physical and mechanical properties including high thermal conductivity, low density, a low thermal linear expansion coefficient, high resistance to thermal shock, high-energy absorption capability, high specific strength, high stiffness and toughness, and most importantly, retention of mechanical properties at high temperatures up to 2800 °C (in vacuum or inert atmospheres). Due to this excellent combination of the properties, CNFC composites can be considered as the basic construction material for high-temperature applications, aerospace and aircraft industry brake systems [[2], [3], [4], [5], [6]]. Mechanical properties of CNFC composite can vary over a wide range depending on processes type, relative amounts of composite components and nanofibers structural and mechanical properties [7,8]. Carbon nanofibers consist of polyaromatic (graphite) and disordered carbons (turbostratic) structures that are shown in Fig. 1 [1]. Polyaromatic structure consists of tightly bonded, hexagonally arranged carbon layers that are held together by weak Van der Waals force (about 57 kJ/mol) [9]. The atoms within the layer plane have strong covalent bonds (about 524 kJ/mol) [10]. Sp2 hybridization of the electron orbitals in the carbon layers, makes the theoretical tensile modulus and the ultimate tensile strength of graphite extremely high, approximately 1060 GPa [11] and 106 GPa [12,13], respectively. Breaking some carbon bonds in the layer planes of polyaromatic structure lead to formation of turbostrastic structure by weak carbon bonds that decrease the structure order and mechanical properties of CNFs [1,14,15]. Polyaromatic structure volume fraction and crystalline parameters including in-plane graphitic crystallite size (La), crystalline stacking thickness (Lc) and interlayer d-spacing (d002) are the more important corresponding factors of CNFs mechanical properties [16].
Physical and mechanical properties of these composites are also highly governed by adhesion and bonding intensity between CNFs and matrix [[17], [18], [19], [20]]. Therefore, an important practical issue is the ability to provide good adhesion and bonding between the CNFs and matrix [21]. One of the best approaches to improving the CNFs adhesion to the matrix is wet chemical oxidation method. The approaches to improving the bonding can be divided into two groups; surface functionalization and wetting in order to improve chemical bonding and increasing the surface roughness and defect density in order to improve mechanical bonding [[18], [19], [20]]. This approach employs concentrated solutions of strong oxidants: HNO3, H2SO4 + HNO3, KMnO4, NaClO3, Na2Cr2O7, NaIO4, and others [[22], [23], [24]]. Treating carbon nanofibers with strong acids causes a substantial increase in the amount of defects and functional groups such as carbonyl (C=O) or carboxylic acid (COOH) groups on fibers surface [25]. Defects can be described as disorder agent that converts a certain number of C atoms hybridization from sp2 to sp3. Therefore, the relative amount of polyaromatic structure and the in-plane graphitic crystallite size (La) of CNFC composite are decreasing because of the significant increase of the defect density and the destruction degree of CNFs surface leads to deterioration of mechanical properties of CNFC composite. These effects are directly related to oxidation intensity of the acid solution [26,27]. Creation of functional groups on CNFs surface can increase CNFs adhesion and bonding to composites matrix. On the other hand, defects and destruction of CNFs surface can lead to decrease of CNFs strength and subsequently composites strength. Thus, it is more important to employ a proper oxidant acid solution in order to optimize functionalizing process. For this purpose, H2SO4/HNO3 and HNO3 acid solutions were mainly offered by previous researches [[28], [29], [30]]. CNFs modifying via these acid solutions confirmed the generation of oxygen-containing species such as carboxylic and hydroxyl species. HNO3 oxidizes the surface and sulfuric acid H2SO4 roughens the surface. Surface roughening of CNFs leads to breaking carbon carbon bonds and creation of surface destructions, which allows HNO3 to create functional groups on the surfaces of the nanofibers. On the other hand, oxidation intensity of HNO3/H2SO4 acid solution is too high, which leads to excessively destruction of CNFs surface. High destruction and roughness is what allows the functional groups to bond; however, the rougher the surface, the weaker the nanofibers [31,32]. In the present study we suggest adding HCL to H2SO4/HNO3 acid solution in order to decrease of oxidation intensity to approach functionalized CNFs with a good combination of structural and morphological characteristics, which can resulted in CNFs strong adhesion and bonding to the matrix of CNFC composite. With this work some oxygen-containing functional groups are created on the surface of CNFs without high destruction of CNFs surface. Low amount of defects and destruction on the surface of CNFs prevents excessive reduction of fibers strength as the reinforcement agent. On the other hand, functional groups created on the surface of CNFs increase the fibers adhesion and bonding to composites matrix. HCl concentration in the acid solution should be optimized in order to creation of high amount of functional groups on the CNFs surface without high surface destruction. For this, CNFs was functionalized via H2SO4/HNO3/HCl acid solutions by concentration ratio of 3:2:0, 3:2:1 and 3:2:2 (v/v). Crystalline structure and surface studies of each acid treated CNFs were characterized using the various analysis techniques including field emission scanning electron microscopy (FE-SEM), X-ray diffraction, XPS, Raman and FTIR spectroscopies. The most efficient acid solution was selected by measuring tensile strength of the acid treated CNFs reinforced carbon composites.
Section snippets
Materials
All of the required materials, including PAN (Polyacrylonitrile), DMF (Dimethyleformamide), HNO3 and H2SO4 were purchased from Sigma-Aldrich Co. Specifications of the materials have been summarized in Table 1.
Surface modifying of carbon nanofibers
In this study PAN-based electrospun CNFs was synthesized via electrospinning of 6% (w/w) PAN/DMF solution followed by stabilizing in air atmosphere (at 300 °C for 1 h) and carbonization up to 1200 °C (by heating rate of 10 °C·min−1) in a tubular furnace with cylindrical chamber of quartz,
Results and discussion
Fig. 3 shows FE-SEM images of un-treated CNFs, CNFs treated via 3:2:0, 3:2:1 and 3:2:2 (v/v) mixtures of H2SO4/HNO3/HCl acid solutions. It is evident from Fig. 3-a that the surface of un-treated CNFs are quite smooth. As seen in Fig. 3(b, c and d), surface of acid treated CNFs were partly damages and rough. Surface destruction degree decreased by increasing the HCl concentration in H2SO4/HNO3/HCl acid solution. The highest surface damage, destruction and deep cracks were observed on the surface
Conclusion
The oxidation of carbon fibers by 3:2:0, 3:2:1 and 3:2:2 (v/v) mixtures of H2SO4/HNO3/HCl acid solutions has been studied using FE-SEM, XRD, XPS, FTIR and Raman spectroscopies. The results indicate that destruction intensity and the relative amounts of functional groups created on CNFs surface have been decreased by increase of HCL concentration in the acid solution. The acid modified CNFs were applied as the reinforcement agent in the carbon‑carbon composites. The highest average value of
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors wish to thank Tabriz University for its support with this study. The results presented in this article is a part of the Ph. D. thesis by Mahdi Valinejad Qanati.
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