Sensitive response of GNP/epoxy coatings as strain sensors: Analysis of tensile-compressive and reversible cyclic behavior

1 Sensitive response of GNP/epoxy coatings as strain sensors: analysis of tensile-compressive and reversible cyclic behavior. Xoan F. Sánchez-Romate1, R. Moriche*1,2, A. Jiménez-Suárez1, M. Sánchez1, S. G. Prolongo1, A. Ureña1 1Materials Science and Engineering Area, University Rey Juan Carlos, C/Tulipán s/n, Móstoles, 28933, Madrid, Spain 2Dpto. de Física de la Materia Condensada, ICMS, CSIC-Universidad de Sevilla, Apdo. 1065, 41080 Sevilla, Spain *Corresponding author: rm[email protected] Abstract The electromechanical performance and reversibility of sensitive GNP/epoxy strain sensors were experimentally and theoretically analyzed. Under tensile loads, the strain sensors showed lower sensitivity and more linearity than bulk sensors, behavior attributed to a slight preferential orientation of the GNPs along the in-plane direction. The Gauge Factor (GF) obtained was 9.1 ± 0.9 and 11 ± 1 for strain values up to 0.005 mm/mm and above 0.015 mm/mm, respectively. In contrast, the electromechanical response when subjected to compressive strain is more complex and three different regions are distinguished: (I) diminution, (II) stabilization and (III) increase of the normalized electrical resistance. Here, GF under compressive loads was negative at low strain values (region I), being -13 ± 2, and positive at high strain (region III), with a value of 8 ± 1. Theoretical analysis revealed that at low strain, there is prevalence of in-plane tunneling mechanisms whereas at higher strain, the out of plane mechanisms dominate, explaining the apparently anomalous behavior at compressive loads. Additionally, strain sensors showed high reversibility with cyclic load in the electromechanical response, but under compressive forces, the loading-unloading electrical resistance curve was asymmetric due to the opening and closing of microcavities and defects in the vicinities of the GNPs. Keywords: Polymer-matrix composites (PMCs), Graphene nanoplatelets, Electrical properties, Analytical modelling, Sensors, SHM 2 1. Introduction Carbon nanoparticles show exceptional properties as it has been observed in many studies, leading to Young’s Modulus of 250 GPa to 1 TPa as well as superior thermal and electrical conductivities, ~ 3000 W/mK and ~ 106 S/m, respectively [1-4]. In fact, they enhance electrical conductivity of typical non-conductive polymers making them electrically conductive by the creation of percolating networks inside the material, leading to electrical conductivities up to 1000-1500 S/cm [5-9]. For these reasons, their use has increased during the last years, covering a wide range of applications such as lightning strike protection, electromagnetic shielding or structural health monitoring (SHM) [10-12]. Specifically, their use for SHM purposes is particularly interesting as these percolating networks change when subjected to applied strain, promoting a variation of the electrical resistance that can be monitored [13-15]. More specifically, they have demonstrated excellent capabilities for real-time curing behavior monitoring purposes [16]. To date, there are many research on sensing capabilities of graphene nanoplatelets (GNPs), proving their exceptional potential for SHM applications. In this regard, many studies have demonstrated their high electrical sensitivity when subjected to mechanical strain. In fact, the gauge factor (GF), defined as the change in normalized resistance divided by the applied strain, is much higher than carbon nanotube (CNT) based nanocomposites [17-19]. This is due to the combined effect of their piezoresistive behavior as well as the tunneling mechanisms between adjacent carbon nanoparticles, which are more prevalent in case of GNPs due to their 2D structure [20,21]. Therefore, their use as strain gauges is attracting the interest of many researchers since their addition induces a drastic improvement on mechanical performance and yields to high gauge factors (from 2 at low strain levels up to around 150 for flexible sensors) [9,15,22]. This work, thus, aims to explore the strain sensing capabilities of GNP based composites in their application as strain surface sensors. Here, dispersion technique plays a crucial role in the behavior of the strain sensors. In this context, ultrasonication process seems to be a powerful dispersion technique to achieve a good homogenization of GNPs inside the non-conductive media 3 [9,22]. However, three roll milling process has proven to be a complementary technique as it induces some stretching effects on nanoplatelets that improves the dispersion state of the nanofillers [23]. For this reason, a combined ultrasonication-three roll milling process will be used to processing the GNP/epoxy mixture. Moreover, a further understanding of electromechanical properties by analytical modelling is carried out. In this regard, there are many studies trying to model the electrical properties of GNP based nanocomposites [24-27]. However, most of these studies do not provide a proper overview of electromechanical properties as do not take some parameters such as dispersion state or carbon nanoparticle interactions into account. Furthermore, the electrical behavior under compressive loads is not yet well understood as some studies have reported an increase of electrical resistance at compressive strain that, at first sight, is not easy to explain [28,29]. For these reasons, tensile and compressive tests will be carried out in the developed GNP strain sensors. The idea is to further explore the main mechanisms governing the electromechanical behavior of the proposed materials. In this regard, compressive behavior will be deeply analyzed by an analytical modelling which takes Poisson effect into account. This model is based in a previously one developed for bulk nanocomposites and proved at tensile load conditions [21]. Here, the interactions between nanoparticles have a crucial effect in electrical properties, in a similar way than for CNT based nanocomposites [30] and the effect of dispersion and GNP distribution inside the matrix will be deeply explored to better understanding the previously commented anomalous behavior at compressive load. Furthermore, the reversibility of the proposed strain sensors will be also investigated by an analysis at different maximum load levels and at tensile and compressive stresses. It would highlight the potential and applicability of the GNP nanocomposite based strain sensors. 4 2. Materials and methods 2.1. Materials The epoxy matrix of the surface sensors was bisphenol A diglycidyl ether (DGEBA) monomer (Araldite LY556, Huntsman) cured with an aromatic amine (Araldite XB3473, Huntsman). Graphene nanoplatelets were grade M25 from XGScience, with an average lateral size of 25 µm and a thickness lower than 6 nm. They were used as nanoreinforcement to achieve electrically conductive nanocomposites. The GNPs content used was 3 wt%, based on a previous published work [3], which is near the percolation threshold to maximize the electrical sensitivity to strain. 2.2. Manufacturing of GNP/epoxy surface sensors Dispersion of the GNPs into the epoxy matrix was carried out following a sonication and calandering two-step method. Initially, GNPs were incorporated into the DGEBA monomer and dispersed by probe sonication (Hielscher UP400S) with an amplitude of 50 % and a cycle of 0.5 s for 45 minutes. After sonication was completed, three cycles of three roll milling, using a mini-calander (Exakt 80E, Exakt GmbH), were applied with a gap distance between rolls of 5 µm and an increasing speed of 250, 300 and 350 rpm. A deeper analysis of the morphology of the nanocomposites and GNPs structure changes induced by the dispersion process have been published in a previous work proving the combined exfoliating and stretching effect of both dispersion procedures [31]. Once dispersion was done, the mixture was degassed under vacuum for 15 minutes and hardener was added. This degasification step is carried out before the hardener addition due to the low gel time of the mixture. When the mixture was homogeneous, surface sensors were prepared by using a polytetrafluoroethylene (PTFE) mask on the surface of a glass reinforced composite substrate to produce a sensor with dimensions 50 x 5 mm. Dimensions of the substrates were 100 x 12 x 3 mm. Curing cycle was set at 140 °C for 8 hours and then, the polymeric mask was removed. 5 2.3. Electromechanical and microstructural characterization The electromechanical performance of the GNP/epoxy strain sensors was analyzed under different load conditions. Mechanical tests were carried out in a Zwick/Roell Z100 kN machine following the correspondent ASTM standard for tensile (ASTM D 3039/D3039M – 00), compression (ASTM D3410) and flexural (ASTM D790 – 02) tests of polymer matrix reinforced composites. Test rate at both tensile and compression tests were fixed at 5 mm/min and 1.23 mm/min for flexural ones. Additionally, the maximum strain set in tests was below the value of the strain at break and yield strength, based on a previous published work [32]. Tensile elastic modulus, strength and elongation at break were 2.54 ± 0.07 GPa, 55.7 ± 3.7 MPa and 0.030 ± 0.004, respectively. Flexural elastic modulus, strength and elongation at break were 2.5 ± 0.4 GPa, 83.6 ± 7.9 MPa and 0.028 ± 0.004, respectively. The electrical response of the strain sensors was simultaneously recorded by using a source-meter Agilent 34410A. In order to measure the electrical resistance, two copper electrodes were attached to the surface of the strain sensors with silver paint to minimize the electrical contact resistance. To avoid detachment during the tests, the electrodes were fixed with hot melt adhesive. Electrical sensitivity was determined as the change of the normalized resistance, ∆𝑅/𝑅𝑂 divided by the applied strain, ε at different strain levels. Thickness of strain sensors has been determined by using an optical profilometer ZETA Z-20 and the microstructure of transversal section has been analyzed by Scanning Electron Microscopy (SEM) with a S-3400N apparatus from Hitachi. Samples were coated by sputtering with a thin layer of gold for a proper observation. 6 3. Results and discussion 3.1. Electromechanical characterization of GNP/epoxy strain sensors The in-plane dimensions of the GNP/epoxy strain sensors, as it has been previously indicated, were 50 × 5 mm. Figure 1 shows the optical profilometry of the strain sensors, being the thickness ~ 300 µm. Furthermore, Figure 2 shows a representative SEM image of transversal section of strain sensors. Here, a good homogeneity of GNP distribution is observed due to the combined effect of ultrasonication, which induces partial exfoliation of GNP layers and three roll milling, which induces as stretching effect on thinner GNP particles due to the shear forces involved during dispersion [33]. 0200 400 600 800 1000 0 200 400 600 800 Substrate Height (m) Distance (m) 300 m Sensor Figure 1. Analysis of thickness of surface sensors by optical profilometry: (a) 3D profile and (b) cross-sectional profile. (a) (b) µm µm 7 Figure 2. SEM image of the cross-section of GNP/epoxy surface sensors showing the distribution of GNPs within the cured nanocomposite. 3.1.1 Experimental analysis of sensitivity at tensile/compression Figures 3a and 3b show the electromechanical response of the GNP/epoxy strain sensors under uniaxial tensile loads up to two different levels of maximum load, 2500 and 4000 N, in order to avoid failure of sensors. Two different maximum loads are used with the aim of analyzing possible changes in tendencies related to the strain level reached in the material, as opening of microcavities or internal defects, which can condition the electromechanical behavior and recovery of sensors. It can be seen that the normalized electrical resistance experiences an exponential growth as the one observed in volumetric sensors, behavior that has been previously published [34], which corresponds to a higher contribution of the tunneling conductivity along the GNPs paths. When load ceases, the strain sensors fully recover the initial state, demonstrating the reversibility of the created electrically conductive network. The sensitivity obtained was 9.1 ± 0.9 and 11 ± 1 for strain values up to 0.005 mm/mm and above 0.015 mm/mm, respectively. These values are lower to that obtained in the equivalent volumetric sensor, reinforced with the same GNPs content. The observed reduction is ~30 % for low strain values and ~60 % in the case of higher strain. This fact also evidences the higher linearity of the electrical response, as there is no significant difference between sensitivity values at low and high strain levels, which is indicative of a higher contribution of the electrical contact-based mechanisms. The increase in the contribution of this mechanism could be attributed to a slight preferential orientation of the GNPs 8 along the in-plane direction (Figure 2) or the volume analyzed, which is smaller than the one analyzed in volumetric samples [24]. This slight preferential orientation contributes to the creation of a more efficient 2D - electrical network, which results in higher concentration of electrical contacts of overlying GNPs and, consequently, in a higher linearity of the electrical response while the sensor is strained. The electrical response of the GNP/epoxy strain sensors under compressive load was also studied. Representative results up to two different levels of maximum loads, 2000 and 6000 N, are shown in Figures 3c and 3d, respectively. Three different regions can be differentiated, similar to the ones obtained in volumetric sensors and the electrical response previously published by Han et al. [35] for similar systems. Initially, there is a diminution of the normalized electrical resistance (I), which appears at the two levels of maximum load, associated to the approximation of the adjacent GNPs, as well as an increase of contact area related to overlaying GNPs; followed by stabilization (II) and a consecutive increase (III), associated to separation of the GNPs and the partial interruption of electrically conductive paths, only observed in Figure 3d. As a consequence, calculated sensitivity under compressive loads was negative at low strain values (region I), being -13 ± 2; and positive at high strain (region III), with a value of 8 ± 1. The strain associated to the change in tendency is located at 0.018 mm/mm. Comparing these values to the ones of the volumetric sensors, the sensitivity is higher in both strain ranges [34]. Although the reasons are not clear, differences can be attributed to the volume contributing to the electrical measurement, which is lower in these strain sensors as mentioned above. However, although similar results under compressive load has been previously reported, as commented, it is necessary a deeper understanding of the electromechanical behavior of the proposed sensors subjected to this type of load. 9 0,000 0,005 0,010 0,015 0,020 0,025 0 10 20 30 40 50 60 70 Stress (MPa) Strain (mm/mm) 0,00 0,02 0,04 0,06 0,08 0,10 0,12 DR/Ro (per unit) 0,00 0,01 0,02 0,03 0,04 0 20 40 60 80 100 120 Stress (MPa) Strain (mm/mm) 0,00 0,05 0,10 0,15 0,20 0,25 0,30 DR/Ro (per unit) 0,00 0,01 0,02 0,03 0 5 10 15 20 25 Stress (MPa) Strain (mm/mm) -0,05 0,00 0,05 0,10 DR/Ro (per unit) 0,00 0,02 0,04 -20 0 20 40 60 80 100 120 140 160 180 Stress (MPa) Strain (mm/mm) -0,15 -0,10 -0,05 0,00 0,05 0,10 0,15 DR/Ro (per unit) Figure 3. Electromechanical response of GNP/epoxy surface sensors under uniaxial load: (a, b) tensile up to (a) 2500 and (b) 4000 N, and (c, d) compression up to (c) 2000 and (d) 6000 N. 3.1.2 Theoretical analysis of electromechanical behavior The analysis of the electromechanical properties, based on a theoretical model previously published [21], is carried out to better understanding the electrical properties of developed surface strain sensors. This theoretical model is based on two types of tunneling mechanisms: out-of-plane (or type I) and in-plane (or type II). The electrical resistance variation is estimated by following the well-known exponential formula [36,37] as a function of tunneling distance: (b) (a) (d) (c) 16 At strain values lower than 0.018 mm/mm, the reduction of sensitivity during unloading, related to the loading values, increases with the maximum applied load, being a diminution of 18, 21 and 27 % for 4000, 5000 and 6000 N, respectively. Additionally, there is also a decrease in the amplitude of the electrical signal, showing decays of 9, 17 and 29 %, respectively. Although the electrical performance varies depending on the maximum strain, i.e. sensitivity and amplitude of the electrical signal, these values remains the same for the different cycles at a same maximum load. This fact is important to be pointed out as it demonstrates reproducibility of the results. It can be affirmed that developed GNP/epoxy strain sensors present, in a certain way, reversibility if the whole cycle is treated like a module as it is repetitive with cycles. Table 1. Electrical parameters during compressive cyclic loading and unloading. Max. Load Cycle Sensitivity ɛ < 0.018 mm/mm Signal amplitude ΔR/Ro Sensitivity ɛ > 0.018 mm/mm Loading Unloading Loading Unloading Loading Unloading 4000 1 -19 ± 2 16 ± 1 0.132 ± 0.004 0.127 ± 0.005 - - 5000 1 -18 ± 2 14 ± 2 0.154 ± 0.001 0.128 ± 0.002 5.6 ± 0.6 -1.6 ± 0.3 6000 1 -17 ± 2 13 ± 2 0.164 ± 0.008 0.116 ± 0.005 7 ± 2 -5.4 ± 0.9 The reduction of the amplitude of the electrical signal with cycles, and the absence of changes after successive cycles with a same maximum strain, can be explained by the opening of micro-cavities in the vicinities of the GNPs, which has been previously corroborated in a published work [29]. The authors previously cited, Han et al. [35], also supported this statement in their published work. They propose that compressive forces induces the reduction of internal defects, such as holes, fissures, etc. This phenomenon leads to a reversible electrical response below a stress threshold, which authors suggest to be located at a 30% of the materials strength and is in accordance with the obtained behavior. Other authors, as Zhai et al. [41], have also published asymmetrical electrical responses in similar systems. Based on these discussions, at 17 first stages, due to the compressive stress, internal defects are closed as well as GNPs approximate each other, contributing to a diminution of the electrical. Once the stress is high enough, micro-cavities can be more prevalent, leading to an increase of the normalized electrical resistance of the strain sensors. These defects cannot be recovered during unloading but they can be closed during the initial stage of the next cycle. This fact justifies the increase of the amplitude and the negative slope of the base line. In this regard, Figure 9 shows a simplified model of mechanisms that occurs during deformation under tensile and compressive loads. Figure 9. Simplified model of modifications induced in the electrical network created by the GNPs under tensile and compressive loads showing more significant mechanisms. 4. Conclusions An experimental to theoretical study of strain monitoring mechanisms of GNP/epoxy strain sensors is carried out, It is observed that the sensitivity at tensile loading is lower than the one of volumetric sensors from other studies. More specifically, measured GF was 9.1 ± 0.9 and 11 ± 1 for strain values up to 0.005 mm/mm and above 0.015 mm/mm, respectively This fact is associated to a higher linearity based on a prevalence of contact conducting mechanisms. Additionally, 18 electromechanical behavior when subjected to compressive strain is quite complex. An initial decrease is observed as expected due to a reduction in the tunneling distance between adjacent nanoparticles. Then, an increase of electrical resistance is noticed with calculated GF being negative at low strain values (region I), of -13 ± 2, and positive at high strain (region III), with a value of 8 ± 1. This is explained by the Poisson effect, leading to an increase of tunneling distance in transversal direction. By theoretical analysis, based on two types of contacts between GNPs: in-plane and out of plane, it can be elucidated that at lower strain level, there is prevalence of in-plane tunneling mechanisms whereas at higher strain levels the out of plane mechanisms dominate. Tensile-compressive cyclic tests demonstrate the reversibility of the proposed strain sensors with similar baseline and peak resistance values. The slight differences during compressive cyclic tests can be attributed to the closing of internal defects during the cycle, leading to a different electrical response at load-unloading cycles. 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