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High Performance Structural Flexible Strain Sensor Based on Graphene Coated Glass Fabric/Silicone Composite Yafei Fu, Yuanqing Li, Yafeng Liu, Pei Huang, Ning Hu, and Shao-Yun Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09424 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018

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ACS Applied Materials & Interfaces

High Performance Structural Flexible Strain Sensor Based on Graphene Coated Glass Fabric/Silicone Composite Ya-Fei Fu,† Yuan-Qing Li,*,† Ya-Feng Liu,† Pei Huang,† Ning Hu,†,‡ and Shao-Yun Fu,*,† †

College of Aerospace Engineering, Chongqing University, Chongqing 400044, People’s Republic of China.

‡Key

Disciplines Lab of Novel Micro-Nano Devices and System and International R &D Center of Micro-Nano

Systems and New Materials Technology, Chongqing University, Chongqing 400044, People’s Republic of China

ABSTRACT: Recently, various piezo-resistive composites with good flexibility have been developed as sensing materials for flexible strain sensors (FSSs). External forces will be applied to strain sensors when they are used in some circumstances such as wrist bending, etc. However, conventional flexible composites may fail upon subjected to external forces since they have low strengths and are unable to protect the inner vulnerable structure of flexible sensors. In this work, the reduced graphene oxide coated glass fabric (RGO@GF)/silicone composite is fabricated and used to make high performance structural flexible strain sensor. The composite is not only flexible and sensitive to strain, but also exhibits high tensile strength needed to maintain the structural integrity of the flexible strain sensor. Silicone resin and GF are employed to provide flexibility and high strength, respectively. By coating RGO on the surface of GF, the non-conductive GF becomes conductive, which renders the piezo-resistive behavior and strain sensing ability to the RGO@GF/silicone composite. The as-prepared structural flexible sensor not only possess a good strain sensitivity with a gauge factor of around 113 which is much higher than that of typical strain sensors based on metals, but can also maintain its structural integrity until the applied external force is over 800 N while the conventional flexible strain sensor fails upon subjected to an external force of only 5 N. Moreover, the as-prepared structural FSS is applied to monitor wrist movement and breathing to demonstrate its applicability. In overall, the RGO@GF/silicone composite exhibits great potentials as sensing material for structural FSSs for wrist movement, etc. KEYWORDS: composite, piezo-resistivity, strain sensor, flexibility, structural component

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INTRODUCTION

Fiber-reinforced polymers (FRP) have been widely applied in various engineering industries due to their light weight, superior mechanical properties, chemical and environmental resistance.1-4 Driven by the need for improving the material utilization degree and system energy efficiency, the interests in multifunctional composites and structures that simultaneously perform two or more structural and non-structural functions rapidly increase. For example, the structural FRPs designed for electrochemical energy storage,5-9 which can carry structural loads while simultaneously store electrochemical energy, could offer significant weight/volume savings in systems that currently rely on traditional, independent energy storage devices and load-carrying structural materials. Structural FRPs designed for structural health-monitoring (SHM) have also been reported, which can not only function to carry loads as traditional FRPs but can also be capable to sense the strain levels of the composite materials and structures. For example, Gao et al. studied the efficiency of damage sensing in fiber composites with uniformly and non-uniformly dispersed carbon nanotubes, respectively.1 Hao et al. reported the preparation of coated glass fibers with carbon nanotube and graphene by using an electrophoretic deposition process.2 By coating conductive nano-fillers, such as graphene and carbon nanotube (CNT) on the surface of glass fibers, glass fiber reinforced polymers (GFRPs) showed piezo-resistive characteristics. The GFRPs exhibited not only excellent mechanical properties but also strain sensing capability desired for SHM. Though these structural GFRPs have demonstrated strain sensing capability, but most of them are made based on relatively stiff matrices like epoxy resins,10-13 thus they lack the flexibility needed for flexible wearable devices.14-16 With the rapid development of robotics, wearable devices and intelligent systems, flexible strain sensors (FSSs) have drawn tremendous attention.16-28 Compared with traditional sensors based on


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stiff metals or semiconductors,29 FSSs generally possess large deformation ability, which enables them conformal integration with nonplanar surface, such as human skin and robotic arm, etc. Up to date, various FSSs based on capacitors,30,31 piezo-electric effect,32,33 tribo-electric effect,34,35 and piezo-resistive materials36-43 or structures,44-46 have been reported. Among them, piezoresistive FSSs have been intensively investigated due to their simple structures and facile fabrication processes, as well as low energy consumption in operation. Currently, most of the researches related to FSSs are focused on improving their sensitivity. Such as, by coating silver nanowires to patterned polydimethylsiloxane, highly sensitive FSS with a gauge factor of 150,000 was demonstrated by Liao et al.47 Inspired by the geometry of spider’s slit organ, an FSS with an ultrahigh gauge factor of 2,000 was reported by Kang et al.48 In practice, except high sensitivity, good mechanical properties such as high tensile strength are also important for FFSs, which provides them the capability to bear external forces to maintain the inner structural integrity of flexible strain sensors when subjected to external forces when used in wrist bending, etc. Such as, FFSs can not only feel the stimulus of outside world, but also have good structural stability under large external loading. It is known that most FSSs are developed based on flexible polymers, like silicone resin and rubber with low tensile strengths, 14-16 which limits their application as structural materials to carry large forces. As mentioned above, although high strength FRPs with piezo-resistivity have been reported, but they lack the flexibility desired for FSSs. Thus, it is highly desirable to develop structural FSSs with not only good flexibility and strain sensitivity but also high strength to avoid their failure upon subjected to external forces. Graphene, a two-dimensional, single-atom-thick structure of sp2-bonded carbon atoms, has attracted tremendous research interest due to its extraordinary mechanical, electrical, and thermal properties.49 Piezo-resistive behaviors have been achieved by coating graphene on the surfaces of



non-conductive glass fibers and nylon fibers, which makes them ideal to fabricate strain sensors.50 It is well known that graphene oxide (GO) can be easily obtained from natural graphite, and chemical reduction of GO is the most promising approach to large-scale production of graphene.51 In this work, conductive glass fiber (GF) fabric was firstly prepared by dip-coating graphene oxide (GO) on the surfaces of GF, and followed with HI reducing process. Then, the reduced GO coated GF (RGO@GF)/silicone composite was fabricated with a simple in-situ polymerization method. Take the advantage of GF with high mechanical performance as reinforcement filler, silicone resin with excellent flexibility as matrix, the RGO@GF/silicone composite fabricated exhibits simultaneously high tensile strength and good flexibility. Meanwhile, the RGO@GF/silicone composite shows excellent piezo-resistive behaviour with a gauge factor of 113, which also exhibits stable response to cyclic loading within a broad frequency range. Finally, its application as the structural FSS to simultaneously monitor high load and sense subtle signal caused by human body movement and breathing was demonstrated. Consideration of its high sensitivity, good flexibility combined with high tensile strength, the RGO@GF/silicone composite is highly attractive as structural FSSs for wrist movement, etc.

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EXPERIMENTAL SECTION Materials. Glass fiber fabric (EWR1000) was purchased from Suzhou Zhengtian Composite

Material Co., Ltd. Silicone resin (Ecoflex Supersoft 00-30) was supplied by Smooth-On, Inc. Graphite powder (CP, 99.95%, d < 45 μm) was obtained from Hefei Pure Technology Co., Ltd. Conductive silver paint was purchased from Guangzhou Kai Xiang Electronic Products Co., Ltd. Aluminum foil was supplied by Hong Shi Da economic and trade Co., Ltd.


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Preparation of FSS based on RGO@GF/silicone composite. GO was synthesized from graphite powder by the modified Hummer’s method,52 as described in the Supporting Information. GF fabric was cut into stripes with an area of 6×2 cm2. The GF strips obtained were soaked in 8 mg/ml GO solution for 2 days and dried at 80°C for 4 hours. Then the strips of GO coated GF were reduced with HI (57%) for 1 hour. To remove excess HI, the reduced GO coated GF (RGO@GF) strips were repeatedly washed with deionized water until the PH is close to 7, and then were dried at 80°C for 30 mins. Silicone resin was prepared by homogenously mixing the base and curing agent of Ecoflex Supersoft 00-30 in a mass ratio of 1:1. Then, RGO@GF strip was placed on a 1 mm thick precured silicone resin sheet, and silicone resin was poured on the surface of RGO@GF. After vacuum degassing, silicone resin was cured at 80°C for 4 hours. Finally, flexible RGO@GF/silicone composite with a sandwich structure and thickness around 2 mm was obtained by removing the extra part of pure silicone resin. To fabricate the structural FSS based on RGO@GF, aluminum foils as electrodes were firstly soldered with RGO@GF strips by silver paste. Then, similar to the process of RGO@GF/silicone composite fabrication, the FSS was obtained by in-situ curing the RGO@GF with electrodes and silicone resin. Characterizations. All optical pictures presented were taken by a digital camera (Canon 70D). The morphologies of GO were observed by an atomic force microscope (AFM, MFP-3D-BIO). The morphologies of GF and RGO@GF were observed by a scanning electron microscope (SEM, Phenom, XL). The resistances of RGO@GF and RGO@GF/silicone composites were measured with a two-probe method using a digital multimeter (VICTOR 86E). The specimens used for resistance measurement were silver-pasted to minimize the contact resistance between the specimens and the electrodes. The resistance responses of RGO@GF/silicone composite and FSS



sensor under loading were recorded with a high resolution digital multimeter (KEYSIGHT 34465A). A universal testing machine (UTM, Shimadzu, EZ-LX) was employed to apply tensile loading and three point bending loading. The specimens used for resistance measurement of tensile and three point bending experiment are 6×2 cm2 in dimensions. For both the tensile and threepoint bending tests, the distance between the clamps is 40 mm. Finite element analysis (FEA) of stress distribution of RGO@GF/silicone composite was conducted utilizing ABAQUS (6.14-4).

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RESULTS AND DISCUSSION

The fabrication process of the FSS based on RGO@GF/silicone composite is presented in Figure 1a. It is known that GF is non-conductive and hence is incapable to sense strain directly. To enable GF to sense strain, it was firstly coated with GO and followed by a HI reducing process to improve its conductivity. The GO employed for GF coating was prepared with a modified Hummer method,52 which is well dispersed in water with a light brown color. As shown in Figure 1b, the AFM image of GO exhibits a typical irregular two-dimension sheet structure with a size of several microns. The thickness of GO sheets measured is around 1.4 nm, indicating it is a single layer. After coating with GO, the transparent GF strips are turned to become light brown, indicating the successful coating of GO. In addition, as shown in Figure 1c and d, the surface of original GF is very smooth, while many wrinkles are seen on the surface of RGO@GF, which also confirms the existence of RGO coating layer. Because GO is almost an insulator, the electrical conductivity of GO@GF measured is only 1.1×10-5 S/m. To further improve the electrical conductivity of GO@GF, which was reduced with HI. After reduction, as indicated in Figure 1a, the light brown color of GO@GF is changed to sliver black, indicating the formation of RGO@GF. Importantly, the electrical conductivity of the


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RGO@GF is significantly improved by the reduction of GO. The effects of GO concentration on the RGO coating content and the electrical conductivity of RGO@GF are investigated as shown in Figure S1. It is obvious that the RGO coating content and the electrical conductivity of RGO@GF increase with the increase of GO concentration, and a highest RGO content of 2.4 wt% and conductivity of 21.7 S/m are achieved with the GO concentration of 8.0 mg/ml. The RCRstrain relationship is investigated in Figure S2 for various GO concentrations. The 8.0 mg/ml case corresponds to the highest RCR values. Thus, the RGO@GF with the 8.0 mg/ml RGO concentration is employed for further investigation.

Figure 1. (a) Schematics of the fabrication of RGO@GF/silicone composite, (b) AFM image of the GO and the thickness curve of GO sheet. SEM images for (c) GF and (d) RGO@GF.



Figure 2. (a) Typical stress-train curves of pure silicone and RGO@GF/silicone composite, the upper-right inset is the enlarged stress-train curve of pure silicone, the left insets indicate the strong RGO@GF/silicone composite, and the down-right insets are the FEA modelled result of RGO@GF/silicone composite with 0.1% and 0.8% tensile strain. (b) Flexural stress-strain curve of RGO@GF/silicone composite, the left insets show the flexibility of RGO@GF/silicone composite, and the right insets are the setup for three point bending test.

The stress distribution of RGO@GF/silicone composite under stretching is analyzed by finite element method. With the increase of tensile strain from 0.1% to 0.8%, as shown in the right insets of Figure 2a, the color of GF in finite element analysis (FEA) model is changed from light green to a mixture of yellow-red, while the color of silicone matrix in FEA model is steady with a blue color, which reveals that instead of silicone matrix, the GF in the composite withstands almost all of the tensile loading. Meanwhile, based on our knowledge, the tensile strength and Young’s modulus of RGO@GF/silicone composite is much higher than that of the typical FSSs reported so far. Moreover, the RGO@GF/silicone composite is highly flexible, as shown in the left insets of Figure 2b, which can be easily bent and twisted. The stress-strain curve of RGO@GF/silicone composite under the three point bending is shown in Figure 2b. Compared with tensile, the stress 8 Environment ACS Paragon Plus

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of RGO@GF/silicone composite under bending is much lower and the flexural modulus calculated is just 2.8 MPa, which is around 1000 times lower that of epoxy,13 further confirming that the RGO@GF/silicone composite is highly flexible. Take the advantage of good electrical conductivity of RGO@GF, the RGO@GF/silicone composite also exhibits a moderate electrical conductivity of 5.4 S/m, which makes it ideal for strain sensing. The strain sensing performance of the structural FSS of RGO@GF/silicone composite is investigated by monitoring its resistance under external loading. The response of the structural FSS to tensile strain is shown in Figure 3a. It is clear that the relative change of resistance (RCR, ΔR/R0) monotonically rises with the increase of tensile strain up to 1.2%, indicating a typical piezo-resistive behavior. The RCR response of the RGO@GF/silicone composite with tensile strain larger than 1.4% turns to become un-stable with large fluctuation. It is worth pointing out that the tensile stress of RGO@GF/silicone composite with 1.4% strain is as large as 14.32 MPa, which outperforms most of the flexible piezo-resistive materials. In addition, two linear stages with different slopes are observed from the RCR-strain curves, and the RCR increasing rate at stage I (strain: < 0.2%) is obviously faster than that at stage II (strain: 0.2-1.4%). The slope of the RCR-strain curve is defined as the strain sensitivity, also known as gauge factor. The gauge factor calculated corresponding to stage I and II is 113 and 29.5, respectively, which is much higher than that of conventional strain sensors based on metals and semiconductors with a gauge factor of around 2 and 43 respectively (see Table S1).29 This indicates that the present FSS it is highly sensitive to tensile strain. Compared with strain gauge based on metal foil or semiconductors, the RGO@GF/silicone composite exhibits better flexibility and compatibility with human skin, which makes it ideal for applications requiring good flexibility. Moreover, since the glass fiber fabric is weaved along vertical and horizontal directions, the RGO is non-uniformly distributed on the



surface of glass fiber fabric, which causes stress concentration in RGO@GF under loading. At the beginning of tensile, the stress concentration is released with the formation of cracks within RGO coating layers, resulting in the rapid decrease of the conductivity of RGO@GF. After release of the stress concentration, the amount of newly formed cracks under large tensile strains is reduced, which causes the decrease of gauge factor at large strains.

Figure 3. (a) RCR response of the structural FSS based on the RGO@GF/silicone composite to tensile strain, and the inset is the test setup. (b) RCR response of the structural FSS to three point bending, the insets are the FEA models at 0.1%, 0.8% and 2.5% bending strains, respectively.

The response of the structural RGO@GF/silicone composite FSS to bending is also investigated by monitoring its RCR under three point bending. As shown in Figure 3b, the RCR of the structural FSS under bending also increases monotonically with increasing the flexural strain. When the flexural strain is less than 0.2%, the RCR increases rapidly with a gauge factor around 13. As the flexural strain increases further, the RCR increasing rate becomes slower down with an average gauge factor around 1.5 and 0.3 corresponding to the strain ranges of 0.2-1.4% and 1.4-3.7%, respectively. The results show that the strain sensitivity of the RGO@GF/silicone composite 10 Environment ACS Paragon Plus

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sensor under bending is lower than that under tensile stretching. To reveal the mechanism, the stress distribution of the RGO@GF/silicone composite sensor subjected to 0.1%, 0.8% and 2.5% bending strains is simulated by FEA, respectively. As indicated in the insets of Figure 3b, under bending, the stress in RGO@GF/silicone composite is non-uniformly distributed with an obvious concentration in the middle section. Meanwhile, the stress level of RGO@GF/silicone composite under bending is lower than that under tensile, which results in the fact that the strain sensitivity of RGO@GF/silicone composite sensor to bending is lower than that to tensile. Furthermore, the effect of RGO coating content on the gauge factor of the silicone composite is investigated. Table S2 shows that the gauge factor of the RGO@GF/silicone composite is improved more than 15 times as the RGO coating content increases from 0.21 to 2.4 wt%, indicating that the RGO@GF with the highest conductivity also exhibits the highest strain sensitivity.

Figure 4. RCR response of the RGO@GF/silicone composite sensor to cyclic bending at (a) different maximum strains and (b) different loading frequencies with a peak strain of 1%.

To fully reveal the piezo-resistive behavior of the structural RGO@GF/silicone composite sensor, its RCR response to cyclic bending is studied. As shown in the Figure 4a, the RCR response



curves in each cycle are identical, indicating a steady response to the cyclic loading applied. In addition, as the peak strain increases from 1% to 3%, the RCR response remains stable with the magnitude improved from 0.7% to 2.2%. It is known that the working frequency is a key parameter for strain sensors. Thus, the effect of loading frequency is also investigated on the responsive behavior of the RGO@GF/silicone composite sensor. As shown in Figure 4b, the RGO@GF/silicone composite exhibits similar response within the loading frequency range of 0.1 to 1 Hz. Importantly, the magnitude of the RCR peak is almost same, indicating its independence of loading frequency, which is significant to its practical application.

Figure 5. RCR response of the structural FSS based on RGO@GF/silicone composite to human body motion: wrist bending (a) and breathing (b). (c) The RCR response of the structural FSS based on RGO@GF/silicone composite to increasing load.

To verify the feasibility of RGO@GF/silicone composite as the sensing material of flexible strain sensor, by attaching the RGO@GF FSS on the wrist of an adult volunteer, the wrist bending of the volunteer is traced. As shown in Figure 5a, the RCR signal monitored agrees well with the frequency and magnitude of the bending gesture. In addition, breathing is closely related with our health, and breathing monitoring is valuable to the evaluation of sports performance and the early


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warning of illness. By attaching the RGO@GF/silicone composite FSS on the inner surface of a waist belt, breathing detection is demonstrated. As shown in Figure 5b, the breathing of an adult volunteer within 100 seconds is recorded, and 34 times of breathing in total are identified, which is consistent with the real breathing status. On the other hand, the maximum effective tensile and flexural strain (ca. 1.5-4.0%) of the present silicone composite is lower than that of previously reported flexible strain sensor based on RGO without structural function28 and also lower than the maximum strain (10-25%) of human skin.53 It is worth pointing out that though the RGO@GF/silicone composite sensor in this work achieves high tensile strength at the cost of certain flexibility and this will limit the applicability of the present sensor to areas which require higher flexibility, the FSS based on RGO@GF/silicone is still suitable to monitor low strain human activities such as wrist bending and breathing, etc. due to its good flexibility, high strength and also high stability upon external forces to be shown below. Moreover, to demonstrate the advantage of the RGO@GF/silicone FSS as structural sensor to bear large loading, a regular FSS based on carbon black (CB)/silicone composite (FSS-CC) is fabricated (details are available in the Supporting Information). As shown in Figure S3, the RCR signal of FSS-CB becomes unstable with a maximum load of 5 N, indicating its multifunction. While the RCR response of FSS-RGO@GF remains stable with a maximum load of 600 N (as shown in Figure 5c), indicating the FSS-RGO@GF can act as a structural flexible strain sensor to stand large loads and protect the inner vulnerable structures of the flexible strain sensors. In addition, compared with those flexible strain sensors using RGO for strain sensing as shown in Table S1, although the RGO@GF/silicone FSS exhibits only a moderate sensitivity, but its mechanical properties such as Young’s modulus etc. are the highest, which also indicates the merit of RGO@GF/silicone composite as suitable sensing material for structural flexible strain sensors.



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CONCLUSIONS

In summary, RGO@GF/silicone composite has been fabricated and used to make the structural flexible strain sensor. The composite is not only flexible and sensitive to strain, but also exhibits high tensile strength needed for maintaining the structural integrity of the as-prepared flexible strain sensor. By coating RGO on the surface of GF, the non-conductive GF is turned to be conductive, which renders piezo-resistive behavior and strain sensing ability to the RGO@GF/silicone composite. As a result, the RGO@GF/silicone composite exhibits simultaneously a high tensile strength and a good flexibility. Importantly, the structural flexible sensor made from the RGO@GF/silicone composite shows a high strain sensitivity with a gauge factor of 113, which also exhibits stable response to cyclic loading within a broad frequency range. The structural FSS is applied to monitor the wrist movement and breathing as well as external loading bearing ability. The structural flexible sensor shows a high load-bearing ability until 800 N while the conventional flexible sensor fails at 5 N. Due to its good flexibility, high strain sensitivity, high tensile strength and high stability upon external loading, the RGO@GF/silicone composite is highly attractive as sensing material for structural flexible strain sensors. 

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: ***. Preparation of GO sheets and FSS based on CB/silicone composites, effects of GO solution concentration on the RGO coating content and the electrical conductivity, relative change of resistance response to bending of the RGO@GF/silicone composites with different GO


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concentrations, RCR response of the FSS based on RGO@GF/silicone composite and carbon black/silicone composite to step-by-step increased load, performance comparison of strain sensors.

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AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The authors are grateful to the financial support of National Natural Science Foundation of China (Grant Nos. 11672049, 51373187, 51573200), Chongqing Municipal Fundamental, Frontier Research Program (Grant No. cstc2016jcyjA0325), and Key Program for International Science and Technology Cooperation Projects of Ministry of Science and Technology of China (No. 2016YFE0125900).

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Table of contents entry

Reduced graphene oxide coated glass fabric (RGO@GF)/silicone composite is fabricated and used to make high performance structural flexible strain sensor (FSS). The RGO@GF/silicone composite is not only flexible and sensitive to strain, but also exhibits high tensile strength needed to maintain the structural integrity of the as-prepared FSS, which exhibits great potentials as sensing material for structural FSSs.


High-Performance Structural Flexible Strain ... - ACS Publications

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