Journal of Materials Science & Technology  2019 , 35 (9): 1989-1995 https://doi.org/10.1016/j.jmst.2019.05.027

Orginal Article

Preparation of highly conductive graphene-coated glass fibers by sol-gel and dip-coating method

Minghe Fangab, Xuhai Xiongac, Yabin Haoab, Tengxin Zhangab, Han Wangab, Hui-Ming Chengabd*, You Zengab*

a Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
b School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China
c Liaoning Key Laboratory of Advanced Polymer Matrix Composites, Shenyang Aerospace University, Shenyang 110136, China
d Shenzhen Geim Graphene Center, Tsinghua-Berkeley Shenzhen Institute, Shenzhen 518055, China

Corresponding authors:   *Corresponding authors at: Shenyang National Laboratory for Materials Science,Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China.E-mail addresses: cheng@imr.ac.cn (H.-M. Cheng), yzeng@imr.ac.cn (Y. Zeng).*Corresponding authors at: Shenyang National Laboratory for Materials Science,Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China.E-mail addresses: cheng@imr.ac.cn (H.-M. Cheng), yzeng@imr.ac.cn (Y. Zeng).

Received: 2019-03-6

Revised:  2019-05-6

Accepted:  2019-05-7

Online:  2019-09-20

Copyright:  2019 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

In order to fabricate highly-conductive glass fibers using graphene as multi-functional coatings, we reported the preparation of graphene-coated glass fibers with high electrical conductivity through sol-gel and dip-coating technique in a simple way. Graphene oxide (GO) was partially reduced to graphene hydrosol, and then glass fibers were dipped and coated with the reduced GO (rGO). After repeated sol-gel and dip-coating treatment, the glass fibers were fully covered with rGO coatings, and consequently exhibited increased hydrophobicity and high electrical conductivity. The graphene-coated fibers exhibited good electrical conductivity of 24.9 S/cm, being higher than that of other nanocarbon-coated fibers and commercial carbon fibers, which is mainly attributed to the high intrinsic electrical conductivity of rGO and full coverage of fiber surfaces. The wettability and electrical conductivity of the coated fibers strongly depended on the dip-coating times and coating thickness, which is closely associated with coverage degree and compact structure of the graphene coatings. By virtue of high conductivity and easy operation, the graphene-coated glass fibers have great potential to be used as flexible conductive wires, highly-sensitive sensors, and multi-functional fibers in many fields.

Keywords: Graphene ; Glass fiber ; Sol-gel ; Dip-coating ; Coating ; Electrical conductivity

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Minghe Fang, Xuhai Xiong, Yabin Hao, Tengxin Zhang, Han Wang, Hui-Ming Cheng, You Zeng. Preparation of highly conductive graphene-coated glass fibers by sol-gel and dip-coating method[J]. Journal of Materials Science & Technology, 2019, 35(9): 1989-1995 https://doi.org/10.1016/j.jmst.2019.05.027

1. Introduction

Conductive fibers have attracted tremendous interest due to their great potential applications in flexible sensors, wearable electronics, and light-weight electromagnetic shielding [[1], [2], [3]]. Metal fibers possess extremely-high electrical conductivity, but they have intrinsic disadvantages of high density and poor anti-corrosion. Carbon fibers have been commercially used as electric heating wires for a long period, but there still exists unsatisfactory electrical conductivity of less than 10 S/cm. Some conductive polymeric fibers like polyaniline and polypyrrole also have limitations of high cost, low strength, and complicated fabrication process [4]. Among all the commercial fibers, glass fibers (GFs) are most widely-used by virtues of their low cost and high mechanical properties. However, significant improvement in electrical conduction of GFs is still challenging. Coating glass fibers with conductive materials has been proved to be a feasible method for obtaining high electrical conductivity. Some conductive coating materials, such as polyaniline [5], metal nanoparticles [6,7], and carbon nanotubes [8], have been used to fabricate conductive GFs. And a few coating techniques, such as spin-coating [9], electrostatic deposition or assembly [10], ion sputtering or evaporation [11], chemical plating or electroplating [12,13], have been rapidly developed in recent years. Moreover, some relevant electro-thermal phenomena of the conductive composite structure, such as conductance loss with elevated temperature [14], positive temperature coefficient effect [15], and dielectric performance [16], have also been widely investigated. It is worth pointing out that, high-quality coating and strong interfacial bonding are extremely essential for ensuring high electrical conductivity of coated fibers. However, conventional nanoparticles are rather difficult to achieve full coverage and strong interactions due to their ball-like morphology and size-mismatch with fibers, consequently resulting in discontinuous transport pathways and low electrical conductivity.

Graphene possesses unique 2D sheet-like structure, extremely high electrical conductivity, high mechanical strength, and good chemical stability [[17], [18], [19], [20]]. Comparing with other nanoparticles, graphene can be much easier to be flattened and aligned along fiber surfaces under shearing force, which can result in full coverage and layered structures [21]. On the other hand, strong π-π interactions existing between adjacent graphene are beneficial to forming interconnected electron-transport pathways for high electrical conductivity. In recent years, graphene-coated GFs have been prepared through electrostatic self-assembly [10], coupling agent [22,23], electrophoretic deposition [24,25], and dip-coating methods [[26], [27], [28]]. Among these methods, the dip-coating technique is the most effective way to obtain aligned and layered structures for high-quality coatings on GFs [29]. R. Moriche et al dispersed graphene into aqueous solution with addition of coupling agents, and then coated glass fabrics with graphene using dip-coating method. They reported that the coated fabrics exhibited drastic change in electrical resistance during deformation of composites [30,31]. Balaji et al coated glass fiber bundles with a mixture of reduced graphene oxide (rGO) and epoxy through dip-coating technique, and they found that the obtained fibers exhibited high sensitivity of electrical resistance to strain [32]. It is worth pointing out that these graphene-coated glass fibers exhibit rather low electrical conductivity of 10-2 S/m despite the full coverage and layered structure of graphene coatings. The reason for such low conductivity lies in the discontinuous transport pathways and high contact resistance due to the presence of insulating coupling agents or epoxy adhesive. Therefore, preparation of highly-conductive graphene coatings without any insulating additive is the key point to greatly decrease contact resistance for high electrical conductivity.

Fortunately, graphene nanosheets can be self-assembled into macroscopic structures like aerogels, films, and fibers through sol-gel process [33,34], where GO is partially reduced into rGO hydrosol, and the increased π-π interactions between rGO play an important role of adhesive or cross-linker in firmly connecting graphene together. This sol-gel self-assembly makes it feasible to fabricate continuous graphene coatings on GFs in a simple way. In that case, we expect that graphene-coated fibers with layered structure and high conductivity can be achieved by a combination of sol-gel and dip-coating technique. As far as we know, such a preparation method for obtaining highly-conductive graphene-coated GFs has not been reported yet.

The aim of this work is to prepare graphene-coated glass fibers with high electrical conductivity through sol-gel and dip-coating technique. We first partially reduced GO into rGO to form graphene hydrosol, and subsequently GFs were dipped into the hydrosol and coated with rGO. After repeated reduction and drying treatment, the graphene-coated GFs with layered structures were obtained. Morphology, microstructure, and electrical properties of the coated fibers were investigated in detail. We found that the surface wettability and electrical conductivity of the coated fibers strongly depend on the dip-coating times and coating thickness, which is closely associated with the coverage degree and compact structure of the graphene coatings. The obtained graphene-coated fibers exhibited a high electrical conductivity of 24.9 S/cm due to the high intrinsic electrical conductivity of rGO and full coverage of fiber surfaces.

2. Experimental

2.1. Raw materials

Graphene oxide (GO) was prepared by modified Hummers method using natural graphite as precursors. Analytical-grade reagents of sodium nitrate, concentrated sulfuric acid, potassium permanganate (KMnO4), hydrochloric acid, and hydrogen peroxide with 30% concentration, were purchased from Sinopharm Chemical Reagent Co. Ltd. China. Hydrazine hydrate with 80% concentration was purchased from Damao chemical reagent factory, China. Glass fibers with a monofilament diameter of 22.8 μm were purchased from Tianjin Qingke composite CO., LTD. The GFs were pre-treated at 180 °C for 4 h to eliminate organic sizing agents in advance.

2.2. Preparation of graphene coated glass fibers

Graphene oxide (GO) was prepared using a modified Hummers method as follows: 2 g graphite powders, 1 g NaNO3, and 92 mL H2SO4 were added into a 500 mL flask cooled in an ice bath. The mixture was constantly stirred for 1 h with gradual addition of 6 g KMnO4. The solution was diluted with distilled water, and then 20 mL H2O2 was added to terminate the reaction. The obtained suspension was centrifuged and washed with 10% HCl solution and distilled water repeatedly. As a result, GO aqueous suspension with uniform dispersion was obtained. In order to form graphene hydrosol for high-quality coating, hydrazine hydrate as a reduction agent was added into the GO suspension at 0.5% volume fraction, and the mixture was heated at 50 °C for 1 h without stirring. During the treatment, the GO was partially reduced into rGO by hydrazine hydrate, consequently forming crosslink-like graphene hydrosol through strong π-π interactions between rGO [35]. Subsequently, GFs were quickly dipped and coated with the hydrosol at a speed of approximately 0.1 m/s. Thereafter, the coated fibers were dried in an oven at 120 °C for 30 min to further reduce rGO and remove residual hydrazine hydrate and water. As a result, rGO was firmly coated on fibers through the sol-gel and dip-coating treatment. By repeating these processes, GFs can be fully covered with rGO, and consequently the electrical conductivity of GFs can be greatly improved. The dip-coating cycles varied from 1 to 20, and correspondingly the obtained fibers were marked as rGO-GF1 to rGO-GF20, respectively.

2.3. Characterizations

Morphology of fibers was observed by using a scanning electron microscope (SEM, NavoSEM430, FEI) at 10 kV. Surface properties like contact angle and wettability of fibers were measured using the Wilhemly plate method on a dynamic surface tension meter (CDCA-100 F). Raman spectra of samples were recorded on Jobin Yvon LabRam HR800, excited by 632.8 nm laser. X-ray photoelectron spectra (XPS) of samples were recorded on Escalab 250 for analyzing elemental compositions. X-ray diffractometer (XRD, D/max 2400 with Cu kα radiation) was used to analyze crystalline structure of samples. Thermogravimetric analysis (TGA) was conducted on Netzsch STA-499C to evaluate composition content of samples. For electrical measurement, the coated fibers were clamped with electrodes at a distance of 4 cm (l), and conductive silver adhesive was used to minimize contact resistance between samples and electrodes. Current-voltage (I-V) characteristics were measured for calculation of electrical resistance (R) on a portable electrochemical workstation (SP-200), and volume electrical resistivity (ρ) of the coated fibers or the graphene coatings was calculated according to the equation: ρ=R × S/l, where S is cross-section area of the coated fibers or graphene coatings.

3. Results and discussion

3.1. Graphene coating onto glass fibers

GO possesses good dispersion stability in aqueous solution due to strong electrostatic repulsion between the charged GO. Although GO can be easily attached onto GFs through electrostatic attraction or chemical bonding [10,22,23], it is rather difficult to obtain full coverage and multi-layered structures due to intrinsic repulsion between the GO and the coated fibers. Moreover, electrical conductivity of GO is too low to meet antistatic requirement. In order to obtain continuous and multi-layered graphene coatings, we partially reduced GO into rGO hydrosol for decreasing repulsive force and consequently forming strong π-π interactions between rGO. Fig. 1 shows preparation schematic of the graphene-coated fibers through sol-gel and dip-coating technique. We can see that, after hydrazine hydrate treatment, the dispersed GO was partially reduced and aggregated into rGO hydrosol due to the decreased repulsion and increased π-π interactions [35]. Such treatment is beneficial to obtaining multi-layered structures and full coverage for high-efficiency electron transport. In addition, electrical conductivity of the graphene coatings can be further improved after reduction treatment due to the fewer defects and higher structural integrity of rGO [32,36]. In our work, the electrical conductivity greatly increased from 10-5 S/cm for GO to 69.5 S/cm for rGO after reduction treatment. On the other hand, dip-coating technique is one of the most suitable methods to obtain the multi-layered structure of graphene coatings [37]. The shearing force generated in fiber-pulling process can effectively stretch and align the rGO along fiber surfaces for full coverage. Moreover, the drying treatment at elevated temperatures is beneficial to further reducing the rGO and forming the compact structure as a result of volume shrinkage for high conductivity and strong interfacial interactions. After repeated dip-coating cycles, the GFs can be fully covered with rGO coatings and consequently possess high electrical conductivity.

Fig. 1.   Preparation schematic of graphene-coated GFs through sol-gel and dip-coating technique.

3.2. Surface morphology

Fig. 2 shows surface morphology of the coated GFs. Comparing with the smooth GF surfaces shown in Fig. 2a, the coated GFs show rough and uneven surfaces due to the existence of graphene layers. We can see clearly from Fig. 2b that the graphene sheets were well spread out on GF surfaces, which is mainly attributed to the 2D planar structure of graphene, large diameter/thickness ratio, high flexibility, and parallel alignment under shearing force. Since GFs possess much larger size than graphene sheets, they still cannot be fully covered with rGO even after 3 dip-coating cycles. It is worth pointing out that the coating quality and coverage degree play an important role in influencing electron transport capability. Full coverage and highly-conductive graphene are two important factors for obtaining high electrical conductivity of coated fibers. For conventional coating methods like electrostatic adsorption and chemical bonding, GFs are rather difficult to be fully coated with GO even after repeated coating cycles due to intrinsic repulsion between the charged GO and coated fibers. Because of the unsatisfactory coating and low electrical conductivity, the GO-coated fibers were only used to investigate the mechanical reinforcement in composites or the change in electrical resistance as stress/strain sensors so far [10,24,30]. In our work, GO was partially reduced into rGO, greatly improving attraction between the rGO and coated fibers. As a result, the multi-layered structure and full coverage can be easily achieved through repeated dip-coating process. It can be seen from Fig. 2e that full coverage and interconnecting networks have been obtained after 5 dip-coating cycles. In addition, we can clearly observe the wrinkled and crumpled morphology of the rGO coatings, similar to graphene fibers [[38], [39], [40], [41]]. Such wrinkled morphology is mainly attributed to intermittent dip-coating and dehydration shrinkage (See Fig. 2i) [42]. The volume shrinkage is beneficial to compacting the rGO coatings into a dense structure and forming strong interactions between the rGO coatings and the GFs [43]. We measured diameters of the coated fibers using SEM. It can be seen from Fig. 2j that the diameters gradually increased with the dip-coating times, from 22.8 μm for the GFs to 42.5 μm for the rGO-GF20. Such proportional relation of diameters with dip-coating times indicates that the coating quality can be well controlled in a simple way. Therefore, we can see that the GFs can be fully covered with rGO to construct high-efficiency electron-transport pathways for high electrical conductivity using the sol-gel and dip-coating technique.

Fig. 2.   SEM images of (a) GF, (b) rGO-GF1, (c) rGO-GF2, (d) rGO-GF3, (e) rGO-GF5, (f) rGO-GF10, (g) rGO-GF15 and (h) rGO-GF20. (i) Formation of the wrinkled rGO coatings onto GFs, and (j) the diameters of the coated fibers as a function of dip-coating times. Scale bar: 50 μm.

3.3. Surface wettability

shows changes in surface wettability of GFs after graphene coating. Surface contact angles (θ) of fibers were measured and calculated using the Wilhelmy plate method [[44], [45], [46]], as shown in Fig. 3a. It can be seen from Fig. 3b that the contact angles of the coated fibers gradually increase with dip-coating times, from 23° for the neat GFs to 105° for the rGO-GF10, implying that the graphene coatings greatly increased the GF wettability to water. Generally speaking, GFs are the typical hydrophilic materials, while rGO processes the hydrophobicity [47]. When the hydrophilic GFs were partially covered with rGO, the corresponding contact angles gradually increased, as shown in Fig. 3b. Once GFs were fully covered with the rGO coatings after 5 dip-coating cycles, the contact angle reached up to 105°, and maintained almost the same value even increasing the dip-coating times. The dependence of contact angle on dip-coating times is consistent with the coverage degree observed from SEM images in Fig. 2. Such change in wettability before and after graphene coating can also be confirmed through optic microscopic observation (see Fig. 3c-d). It is worth pointing out that the surface wettability depends on not only the hydrophobicity or hydrophilicity, but also the surface roughness of coated fibers. The existence of graphene on fiber surface can increase surface roughness, resulting in high hydrophobicity [48]. It has been reported that the increased hydrophobicity and high roughness is beneficial to improving wettability of the coated fibers with polymeric resins and obtaining remarkable mechanical reinforcement of composites [49,50]. Therefore, the graphene coating can greatly improve the surface wettability of GFs due to the full coverage, high rGO hydrophobicity, and increased surface roughness.

Fig. 3.   (a) Schematic of contact angle measurement of GFs using the Wilhelmy method, (b) the contact angle of the coated fibers as a function of dip-coating times, and the optic microscopic images of water droplet on (c) GF and (d) rGO-GF.

3.4. Composition of rGO-coated GFs

We measured composition of the coated GFs using TGA, XPS, XRD, and Raman spectroscopy shown in Fig. 4. It can be seen from TGA curves that the coated GFs show obvious mass loss due to oxidation of rGO at elevated temperatures in air. At 700 °C, the rGO has been completely decomposed, and the corresponding mass loss can be used to determine the mass fraction of rGO coatings on GFs. We can see from the inset of Fig. 4a that the mass fraction of rGO is approximately proportional to the dip-coating times, implying that the number of layers and the mass of rGO coatings can be well controlled by simply adjusting dip-coating cycles. Surface chemical compositions were analyzed using XPS, as shown in Fig. 4b. We can see the typical characteristic peaks of O 1s, Si 2s, and Si 2p for GFs and a slight C 1s peak from residual sizing agents existed even after high-temperature treatment. After the sol-gel and dip-coating treatment, the rGO-GFs exhibit a strong C 1s peak due to the existence of rGO coatings and a slight N 1s peak from hydrazine hydrate. In addition, we can observe typical crystalline peaks of graphitic lattices at 12.5° and 26.6° for the rGO-GFs in comparison with the broad dispersion peak of amorphous SiO2 for GFs (See Fig. 4c) [51,52]. We further measured Raman spectra of the coated fibers, and they exhibited characteristic peaks of graphene at 1360 and 1580 cm-1 from amorphous sp3 and ordered sp2 structures respectively [53]. In Fig. 4d, the rGO coatings exhibits relatively higher intensity of D mode at 1360 cm-1 than GO, implying more disordered carbon structures. Such increased intensity is mainly attributed to the formation of small-size sp2 domains (disordered carbon) during GO reduction for interconnecting graphene together, which is consistent with that of rGO reported in literature [54]. Therefore, we can see that the GFs can be fully coated with rGO, and the coating thickness can be well controlled by simply adjusting dip-coating times. The high-quality rGO coating is beneficial to improving electrical conductivity of GFs.

Fig. 4.   (a) TGA curves, (b) XPS spectra, (c) XRD patterns, and (d) Raman spectra of the rGO-coated GFs.

3.5. Electrical properties

We measured the electrical properties of the rGO-coated GFs using an electrochemical workstation shown in Fig. 5. It can be seen from Fig. 5a that all the coated fibers with over 5 coating cycles exhibit good linear relationship of current with applied voltage, showing typical ohmic characteristics and high electrical conduction. For the rGO-GFs with less than 5 cycles, their electrical resistance cannot be obtained due to the incomplete coverage and extremely high electrical insulation. It reveals that the electrical resistance of the coated fibers strongly depends on dip-coating times and coverage degree. In Fig. 5b, the electrical resistance drastically decreases with coating times, from 97 kOhm for rGO-GF5 to 11 kOhm for rGO-GF20. This resistance dependence on coating times is closely associated with the number of electron transport pathways in cross-section areas. The more the dip-coating times, the larger of the graphene coating thickness, resulting in the more conductive pathways and lower electrical resistance. In order to eliminate the influence of sample sizes on electron transport, we further calculated volume electrical conductivity (ρ) of the coated fibers and the graphene coatings respectively, according to the equation: ρ=R ×l/S (see Fig. 5c-d). In our work, the rGO-GF20 exhibits high electrical conductivity of 24.9 S/cm, and its corresponding rGO coatings show the conductivity of 35.21 S/cm. It reveals that the graphene coating can greatly improve electrical conductivity of the insulating GFs, which is mainly attributed to the high intrinsic conductivity of rGO, full coverage of GFs, and high-efficiency electron-transport networks.

Fig. 5.   (a) I-V curves, (b) electrical resistance, (c) volume electrical conductivity of the coated GFs and rGO coatings, and (d) conductivity dependence of the rGO coatings on coating thickness (inset: heterogeneous structure).

It can be clearly seen from Fig. 5c-d that the rGO coatings still exhibit strong dependence of electrical conductivity on coating times and coating thickness, although the influence of sample sizes has been considered and eliminated in the calculation of electrical conductivity. The rGO coatings in the rGO-GF20 show electrical conductivity of 35.21 S/cm, higher than 19.05 S/cm for that in the rGO-GF5. The larger the coating thickness, the higher the electrical conductivity. According to theoretical analysis and numerical simulation, the intrinsic conductivity of multi-layer graphene decreases with graphitic layer number or thickness, but this size-dependence of conductivity is only effective in the nano-size range [16]. In our work, the dependence of electrical conductivity on coating thickness is closely associated with heterogeneous structures of rGO coatings generated during various dip-coating cycles and corresponding electron transport mechanism. For the rGO-GF20, they underwent 20-time dip-coating and heat-treatment cycles, and the inner rGO layers were repeatedly compacted and thermally reduced, consequently resulting in higher reduction degree, more compact structures, and lower contact resistance than that in the outer layers. The resultant heterogeneous and gradiently-reduced structure of the rGO coatings was illustrated in the inset of Fig. 5d. As a contrast, the rGO-GF5 underwent only 5-time cycles, and consequently possessed lower reduction degree and looser structure of rGO coatings than the rGO-GF20, resulting in much more interface scattering and lower electron mobility. In that case, electrons are much easier to be quickly transported through the highly reduced and compact rGO coatings in rGO-GF20 than that in rGO-GF5, consequently exhibiting higher electrical conductivity. Moreover, according to electron transport mechanism [16], the electrons in the inner layers of rGO-GF20 are easily transported through ballistic scattering with high mobility. Although the electrons at the interface are scattered by GF lattices, the obtained conductivity of rGO-GF20 is satisfactory due to the high reduction degree and compact structure of graphene coatings.

We further compared the electrical conductivity of the graphene-coated GFs with other coated GFs reported in literature. In our work, the lowest electrical conductivity of the rGO-coated GFs is 6.58 S/cm for the rGO-GF5, still much higher than 10-4 S/cm for graphene-coated glass fabrics [30], 0.2 S/cm for CNT-coated GFs [8], and 6 S/cm for polyaniline-coated GFs [5]. As mentioned above, electrical conductivity of the coated fibers strongly depends on many influencing factors, such as intrinsic electrical conductivity of coating materials, coverage degree, interconnecting conductive pathway, compact structure, and coating thickness. In our work, the rGO possesses higher electrical conductivity than CNTs and polyaniline [5,8], being beneficial to realizing high-efficiency electron transport. On the other hand, the 2D planar characteristic of graphene is helpful to achieve full coverage and well-aligned structure for construction of interconnecting pathways, consequently exhibiting low contact resistance and high electrical conductivity. Moreover, the coating thickness can be well controlled by simply adjusting dip-coating times for obtaining desired electrical conductivity. In addition, the electrical conduction and equivalent conductivity of the graphene-coated glass fibers can be theoretically simulated using the network conductivity formula for overall evaluation and comparison of conduction efficiency between various nanocomposites [55]. Notably, our rGO-coated GFs exhibit higher electrical conductivity than commercial carbon fibers with conductivity of less than 10 S/cm, showing great potential to be widely applied as multi-functional dielectric materials with negative dielectric constant (negative permittivity) [[56], [57], [58]], high mechanical strength, and good electrochemical activity [[59], [60], [61], [62]]. It is also worth pointing out that the electrical conductivity of our rGO-coated fibers is lower than that of metal-coated GFs due to high contact resistance and interface scattering between graphene sheets [6,7], but the rGO-coated fibers still possess advantages of light weight, high flexibility, and good anti-corrosion performance. By virtues of the high electrical conductivity and easy operation, the graphene-coated GFs exhibit great potential to be used as novel sensors, flexible conductive wires, and multi-functional fibers in high-performance composites.

4. Conclusion

Highly conductive graphene-coated glass fibers were prepared using sol-gel and dip-coating technique, and their microstructures, surface properties, and electrical conductivity were investigated in detail. The GFs can be fully coated with rGO, greatly improving surface wettability and electrical conductivity. The graphene-coated GFs possess satisfactory electrical conductivity of 24.9 S/cm, which is mainly attributed to the high intrinsic conductivity of rGO, compact structures, and high-efficiency electron-transport pathways. Moreover, we found that electrical conductivity of the coated fibers exhibited strong dependence on coating thickness, which is associated with the reduction degree and compact structure of rGO coatings. By virtues of the high electrical conductivity, easy operation, and controllable multi-layered structures, the graphene-coated glass fibers exhibit great potential to be used as flexible conducting wires, multi-functional fibers, and highly-sensitive sensors in many fields.

Acknowledgements

The authors acknowledge financial supports from the National Natural Science Foundation of China (No. 51802317), Department of Science and Technology of Shenyang City (No. 17-231-1-66), and Shenyang National Laboratory for Materials Science (No. 2017RP11).

The authors have declared that no competing interests exist.


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