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Superior piezoelectric composite films: taking advantage of carbon nanomaterials
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 045501 (http://iopscience.iop.org/0957-4484/25/4/045501) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 25 (2014) 045501 (10pp)
doi:10.1088/0957-4484/25/4/045501
Superior piezoelectric composite films: taking advantage of carbon nanomaterials Nasser Saber1 , Sherif Araby1 , Qingshi Meng1 , Hung-Yao Hsu1 , Cheng Yan3 , Sara Azari2 , Sang-Heon Lee1 , Yanan Xu3 , Jun Ma1 and Sirong Yu4 1
School of Engineering, University of South Australia, Mawson Lakes, SA 5095, Australia School of Natural and Built Environment, University of South Australia, Mawson Lakes, SA 5095, Australia 3 School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, Brisbane, QLD 4001, Australia 4 College of Mechanical and Electronic Engineering, China University of Petroleum, Qingdao 266580, People’s Republic of China 2
E-mail: [email protected] Received 1 October 2013, revised 19 November 2013 Accepted for publication 20 November 2013 Published 7 January 2014 Abstract
Piezoelectric composites comprising an active phase of ferroelectric ceramic and a polymer matrix have recently found numerous sensory applications. However, it remains a major challenge to further improve their electromechanical response for advanced applications such as precision control and monitoring systems. We here investigated the incorporation of graphene platelets (GnPs) and multi-walled carbon nanotubes (MWNTs), each with various weight fractions, into PZT (lead zirconate titanate)/epoxy composites to produce three-phase nanocomposites. The nanocomposite films show markedly improved piezoelectric coefficients and electromechanical responses (50%) besides an enhancement of ∼200% in stiffness. The carbon nanomaterials strengthened the impact of electric field on the PZT particles by appropriately raising the electrical conductivity of the epoxy. GnPs have been proved to be far more promising in improving the poling behavior and dynamic response than MWNTs. The superior dynamic sensitivity of GnP-reinforced composite may be caused by the GnPs’ high load transfer efficiency arising from their two-dimensional geometry and good compatibility with the matrix. The reduced acoustic impedance mismatch resulting from the improved thermal conductance may also contribute to the higher sensitivity of GnP-reinforced composite. This research pointed out the potential of employing GnPs to develop highly sensitive piezoelectric composites for sensing applications. Keywords: piezoelectric composites, sensors, graphene platelets, multi-walled carbon nanotubes, poling S Online supplementary data available from stacks.iop.org/Nano/25/045501/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
characteristics of CNTs still encourage their use is sensing applications versus their emerging competitor—graphene [3, 4]. With the groundbreaking introduction of two-dimensional graphene, a tremendous amount of attention has been attracted toward its research and development for a broad range of devices, from electronics to sensors to reinforced nanocomposites [5]. Composed of honeycomb structure in a planar sheet of one carbon atom thickness [6],
The superior properties of carbon nanotubes (CNTs) have ignited intensive interest among a great many scientists for a wide variety of applications over the past two decades. CNTs comprise single-walled (SWNTs) and multi-walled carbon nanotubes (MWNTs), both of which consist of rolled graphite layers of sp2 hybridized carbon atoms [1]. Despite their remarkably high manufacturing cost [2], the lucrative 0957-4484/14/045501+10$33.00
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graphene possesses a Young’s modulus of ∼1 TPa and an ultimate strength of 130 GPa. Moreover, it features unique electrical and thermal conductivities, up to 6000 S cm−1 and 5000 W m−1 K−1 , respectively. Besides its low production cost, which is advantageous over CNTs, chirality is not a factor in graphene’s electrical conductivity [6–9]. Recently, graphene nanocomposites with various polymer matrices such as polyethylene oxide [10], polystyrene [11, 12], polyamide 12 [13] and epoxy [14–16] have been investigated. These studies suggest that the incorporation of graphene into polymers effectively improves their mechanical and functional properties. Our recently in-house developed graphene platelets (GnPs) [17, 18] offer superior properties in terms of (i) low manufacturing cost of 10–20 $ kg−1 compared to 30 $ kg−1 for graphene oxide (GnO) excluding the reduction cost [6], (ii) higher structural integrity based on a Raman ID /IG ratio of ∼0.07 [17, 18] versus ∼1.0 for reduced GnO [19] and (iii) utmost resemblance of stiffness, strength and electrical and thermal conductivities to graphene [20]. The GnPs are produced by the combination of thermally expanding a commercial graphite intercalation compound (GIC) with ultrasonication of the expanded product. Each graphene platelet consists of 1–5 graphene layers. Piezoelectric ceramic–polymer composites have been developed and applied in arrays of sensing and actuation systems since the 1970s [21–23]. These composites provide target sensing performance with the desired flexibility, advantageous over conventional one-phase rigid ceramic sensing blocks, in particular for in situ damage identification in intelligent structures [24, 25]. These sensors, permanently covering the specified areas of a host structure surface, can be used in ultrasonic and acoustic emission monitoring techniques [26–29]. The working principle is based on the fact that every structural degradation mechanism emits a specific pattern of waveform under the quasi-static and dynamic loading regimes of a structure’s operating conditions. The sensing composites are able to capture these incoming elastic wavefields, owing to the piezoelectric effect of the ferroelectric ceramic particles embedded in the matrix. The majority of previous studies have focused on two-phase piezoelectric composites, which consist of an inorganic pigment—predominantly lead zirconate titanate (PZT)—and a polymer matrix [21, 30–35]. In comparison, few studies have reported on the incorporation of a third phase into the composite. Sa-Gong et al [36] added a third conductive phase of carbon, germanium or silicon between the piezoelectric particles to facilitate the poling of piezocomposite films. Ray and Batra [25] fabricated a piezoelectric composite comprising armchair SWNTs embedded in a piezoceramic matrix. They demonstrated that the piezoelectric coefficients and the effective elastic moduli of composites containing low fractions of single-walled carbon nanotubes were significantly higher than PZT/epoxy composites. Recently, Park et al [37] used CNTs in their BaTiO3 -based nanocomposites to serve as a dispersant, stress reinforcing agent and conducting functional material. They showed that CNT-reinforced composites provide higher
output voltages as a result of the BaTiO3 nanoparticles’ better dispersion by forming a complex mixture with CNT networks. CNTs have also been compounded with polymers to make functional composites [38, 39]. Nevertheless, to the best of our knowledge, no research has incorporated GnPs into piezocomposites, and concurrently compared these nanoadditives with MWNTs regarding their effects on the structure and electromechanical and piezoelectric properties of these composites. In this study we developed three-phase, PZT-based epoxy composites including (i) utilization of cost-effective, highstructural integrity GnPs as well as MWNTs as a third phase nanofiller, (ii) study on the morphology and microstructure of the composites, (iii) investigation of the structure–property relationships of the composites in terms of micromechanical properties, poling behavior and dynamic response, and (iv) comparison between GnPs and MWNTs—as a third phase—on the durability, electromechanical and piezoelectric properties of these nanocomposites. Our investigation shows that the hybrid composite film containing carbon nanomaterial demonstrates obviously enhanced mechanical strength, poling behavior and sensitivity to excitation, and that GnPs offer more advantages over MWNTs, as evidenced by the greater improvement in electromechanical properties and poling behavior. 2. Experimental procedure 2.1. Materials
Lead zirconate titanate (PZT-5H) ceramic powder of 0.94 ± 0.21 µm in diameter (particle size distribution provided in the supplementary information, available at stacks. iop.org/Nano/25/045501/mmedia) was kindly provided by Sunnytec (Suzhou) Electronics Co., Ltd, Taiwan. Asbury Carbons (Asbury, NJ, USA) provided free samples of a commercial graphite intercalation compound (GIC, Asbury 3494). MWNTs were kindly supplied by Showa Denko. Bisphenol A/F-based epoxy resin with an epoxide equivalent weight of 189–200 g eq−1 and hardener were obtained from Huntsman, Australia. The solvent tetrahydrofuran (THF) was purchased from Sigma-Aldrich. 2.2. Material preparation and fabrication of composite films
This research started with the preparation of a number of purpose-designed piezoelectric composite films. After curing and applying silver electrodes, these films were poled and characterized for performance evaluation. Figure 1 summarizes the experimental procedures used in this study. Graphene platelets (GnPs). 1 g of GIC was transferred into a crucible preheated in a common furnace at 700 ◦ C and treated for 1 min. The expanded product was left to cool down. Safety procedures, such as placing the furnace in a fume cupboard to prevent nanoparticle-inhalation hazard and wearing safety glasses, respirator and heat resistant gloves, were required. A desired amount of the expanded product was immersed 2
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nanoparticles must be added prior to the hardener mixing stage to prevent premature crosslinking. Likewise, as-received MWNTs were incorporated to produce another three-phase nanocomposite. Film coating and curing were similarly undertaken. All films made herein were of sufficient flexibility and robustness for practical applications. 2.3. Microstructure and mechanical characterization
X-ray diffraction (XRD) spectra of the GnPs and composites were collected in a reflection mode using a Mini-Materials Analyzer (MMA) with a diffractometer equipped with curved graphite monochromators with Cu Kα radiation (wavelength ˚ The spectra were collected from 2.5◦ to of λ = 1.541 A). ◦ 35 at a scanning rate of 1◦ min−1 under a tube voltage of 35 kV with 1 kW at room temperature. Scanning electron microscopy (SEM) was employed to investigate the microstructure of the films which were coated with a thin layer of platinum and observed using a Philips XL30 FegSEM operating at an accelerating voltage of 10 kV. Nanoindentation experiments were conducted to determine the hardness and Young’s moduli of the composite films (10 mm × 10 mm × 0.65 mm). Experiments were performed using a Hysitron TI-950 TriboIndenter with a Berkovich indenter (three-sided pyramidal diamond tip) whose nominal tip radius was 100–200 nm. Probe calibration was performed on a reference sample of fused quartz (reduced modulus Er = 69.6 GPa ± 5%, hardness = 9.25 GPa ±10%). Both load and displacement were continuously monitored by a three-plate capacitive transducer. Tests were carried out under a constant loading rate up to the maximum force and then unloaded to zero under the same rate. In order to ensure experimental reproducibility, four experiments were conducted for each sample, with ten data points collected in each experiment.
Figure 1. Summary of experimental procedures.
in THF (1 wt%) in a metallic container and treated in an ultrasonication bath (200 W at 42 kHz) for 1 h below 30 ◦ C. Upon sonication, the expanded product in suspension was able to split into platelets of 2–4 nm in thickness, as evidenced in the previous works [18, 40]. This suspension was used in the following fabrication. Two-phase composites. A prescribed quantity of PZT particles (70 wt%) was suspended in THF using an ultrasonic bath for 10 min. A calculated amount of epoxy resin was dissolved in the same solvent and heated to 70 ◦ C using an oil bath and a round-bottom flask equipped with a condenser to obtain a clear solution. The PZT suspension was then added to the solution and stirred using a magnet bar at 70 ◦ C for 30 min, followed by a sonication process of 30 min below 30 ◦ C [40]. A recommended ratio of hardener (weight ratio of epoxy to hardener of 100:130) was added to the mixture and dissolved by stirring and sonication. A thin layer of the mixture was subsequently cast on a PVC plate using an adjustable casting knife and cured at room temperature for three days. The final film thickness was about 650 µm. Three-phase nanocomposites with either GnPs or MWNTs. The above described GnP suspension was added to the two-phase composite solution. It should be noted that
2.4. Sensor activation and piezoelectric measurements
Upon curing a composite film, a thin layer of silver paint (∼20 µm) was carefully brushed onto both sides of the film to serve as electrodes connecting the film with the data acquisition system. A large electric field was vertically applied using a TREK 610E power supply to pole the films at room temperature. In order to study the poling behavior, we followed the procedure described by Hanner et al [32]. To be specific, the poling voltage was applied with an increment of 200 V, allowing the film to remain at each voltage for 5 min. Then the piezoelectric coefficient measurement was carried out, followed by further increments until dielectric breakdown of the film occurred. A digital oscilloscope (Agilent Technologies) was employed to monitor the transient current during the film poling. The piezoelectric coefficients (d33 ) of the poled films were measured using a wide-range d33 meter (APC International, Ltd). 2.5. Dynamic characterization
In order to examine and compare the sensing performance of our composite films in detecting Rayleigh waves (a special 3
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Figure 2. Experimental installation of the pitch–catch setup.
type of surface acoustic wave travelling on solids), an active sensing setup was established as shown in figure 2. This pitch–catch experimental arrangement [41] consisted of (i) a wideband acoustic emission (AE) transducer (MISTRAS Group Inc.) as an actuator that was surface-mounted on an aluminum panel and (ii) a 0.650 mm thick, 20 mm square film with an active electrode area of 10 mm × 10 mm that was affixed 200 mm away from the transmitter using special vacuum grease. To produce the Rayleigh waves, the AE transducer was actuated by a 150 Vpp sinusoidal signal generated by a Rigol DG1022 waveform generator. The composite film output was amplified by a 2/4/6 preamplifier (PAC) with a fixed gain of 40 dB and the amplified signals were recorded using a two-channel PCI-2 AE board (MISTRAS Group Inc.) with a sampling rate of 2 MHz. Initially, the frequency spectrum of two-phase composite film was obtained through a frequency sweep test, by varying the actuation signal from 5 kHz to 1 MHz and subsequently plotting the spectral response calculated by a fast Fourier transform (FFT). Secondly, to investigate the time-domain responses of all composite films, the AE transducer was excited at a resonant frequency determined from the aforementioned frequency sweep test, and then the generated Rayleigh waves were captured by each nanocomposite film.
Figure 3. XRD patterns of the two-phase PZT/epoxy composite and
its graphene platelet (GnP) nanocomposites.
pattern at 2θ = 26.7◦ observed for GnPs is assigned to the 0.34 nm basal spacing between graphene layers. Upon sonication, the expanded product exfoliated and separated into mono- and multi-layer graphene, explaining the existence of diffraction at 2θ = 26.7◦ . The two-phase composite shows a minor diffraction at 2θ = 21.5◦ , attributed to the scattering of cured epoxy molecules. It also shows a large diffraction at 2θ = 31.5◦ corresponding to the perovskite phase of PZT [43]. At low GnP fractions—0.1 and 0.25 wt% –the nanocomposites show similar diffraction patterns to the two-phase composite with absence of the GnP diffraction pattern, implying full exfoliation and dispersion of GnPs. Meanwhile, with increase in the GnP fraction to 0.5 wt%, the pattern indicates a tiny diffraction at 2θ = 26.7◦ , referring to the possibility of graphene layers stacking in the matrix. Figure 4 displays SEM micrographs of the fractured surface of the two-phase PZT/epoxy composite. In figure 4(a), the light-color dispersion phase of 0.94±0.21 µm in diameter refers to clusters or aggregates of PZT particles. The higher magnification image of a typical matrix zone in figure 4(b) reveals the presence of micro-voids formed during the film fabrication process. SEM images of the three-phase MWNT/PZT/epoxy nanocomposite are presented in figure 5, where the fractions of MWNTs and PZT are 0.1 wt% and 70 wt%, respectively. It is noteworthy that 70 wt% was found to be the optimum fraction of PZT particles during the solution mixing and film fabrication process; lower values result in poor piezoelectric properties while in higher fractions, handling of the solution is
3. Results and discussion 3.1. Morphology
The thickness of the graphene platelets (GnPs) dispersed in tetrahydrofuran (THF) was measured as 3.57 ± 0.50 nm, while this value reduced to 2.51 ± 0.39 nm when the THF was replaced by N-methyl-2-pyrrolidone (NMP) [17, 18]. This thickness value depends on how well the interfacial interaction energy of the solvent–graphene matches that of graphene–graphene [42]. Upon being compounded with polymers, GnPs may exfoliate causing separation from each other; however, the layered structure in every platelet should be retained throughout the entire compounding process. Figure 3 contains the x-ray diffraction patterns of the two-phase PZT/epoxy composite and the three-phase GnP/PZT/epoxy nanocomposites. The sharp diffraction 4
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Figure 4. SEM micrographs of the 70 wt% two-phase PZT/epoxy composite.
Figure 5. SEM micrographs of the MWNT/PZT/epoxy nanocomposite.
difficult and the wet paint cannot be applied consistently using a casting knife. MWNTs can be clearly distinguished as the ridges in figure 5(a). Comparison of the matrix in figures 5(a) and (b) with that in figure 4 confirms the presence of the third phase MWNTs. These magnified images also show a decrease in void size upon incorporation of nanotube content. Figure 6 presents representative SEM micrographs of the fractured surface of the three-phase GnP/PZT/epoxy nanocomposite, where the fractions of GnPs and PZT are 0.1 wt% and 70 wt%, respectively. No obvious aggregates or clusters of GnPs can be seen in figure 6(a), but a number of ridges are visible. When a typical ridge is magnified in figure 6(b), a light-colored, folded sheet is found coexisting with a number of smaller sheets, and these should be either GnPs or their clusters. Figure 6(c) shows the detail of a typical folded sheet, where graphene layers are clearly visible; it must be a large GnP cluster comprising a few GnPs that are most likely connected with each other by epoxy resin. The
matrix deformation shown as red arrows indicates a strong interface between the cluster and the matrix, even though no interface modification was made in this work. Under loading, a fraction of load is transferred from the matrix to the GnPs and their clusters through the interface; the GnPs thus share the load and restrain the movement of matrix molecules in their vicinity. This means that GnPs are a class of suitable filler for reinforcement of piezocomposite, as supported by a previous study [17]. 3.2. Mechanical properties
Figure 7 illustrates typical load–displacement curves of indentations made at peak loads of 100 µN and 200 µN applied for the two-phase PZT/epoxy composite and the two three-phase nanocomposites, respectively. Due to the flexible nature of our samples, the applied load could not be increased further for the two-phase composite, and a relatively 5
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Figure 6. SEM micrographs of the GnP/PZT/epoxy nanocomposite.
Table 1. Summary of the mechanical properties of the PZT/epoxy composite and its nanocomposites.
MWNT/PZT/epoxy
GnP/PZT/epoxy
Nanoparticle fraction (wt%)
Young’s modulus (MPa)
Hardness (MPa)
Young’s modulus (MPa)
Hardness (MPa)
0 0.1 0.25 0.5
3.30 ± 0.46 6.00 ± 0.26 7.15 + 0.32 9.02 ± 0.48
0.235 ± 0.0962 0.774 ± 0.0813 0.869 ± 0.0791 1.11 ± 0.0847
3.30 ± 0.46 7.31 ± 0.39 8.44 ± 0.47 9.13 ± 0.54
0.235 ± 0.0962 0.794 ± 0.0932 0.875 ± 0.0742 0.950 ± 0.0686
high creep strain is noted at the peak load for all three samples. Therefore, holding of the peak load is essential to allow for dissipation of creep displacement. With regard to both of the three-phase nanocomposites, lower indentation depths and higher unloading slopes are observed owing to the nanoadditives—MWNTs and GnPs—of high stiffness. As discussed before, these additives share a fraction of the load and restrain the movement of the matrix molecules. Table 1 contains the Young’s moduli and hardnesses of the composite and nanocomposite films obtained from nanoindentation measurements. Calculation of the listed Young’s moduli was performed using the reduced elastic modulus (Er ) from nanoindentation according to the Oliver–Pharr method [44]. The given reduced elastic modulus bears the implication that both the sample and the indenter undergo elastic deformation. Er is then related to the Young’s modulus of the sample by 1 − νi2 1 − ν2 1 = + Er E Ei
same parameters for the indenter, which are taken as 1141 GPa and 0.07, respectively [46]. It is noted that the nanocomposites exhibit significantly enhanced Young’s modulus and hardness values up to 176.6% and 372.3%, respectively, with increase in the nanoparticle content. In addition, the GnP-reinforced nanocomposite yields a higher modulus and hardness than the equally loaded MWNT nanocomposite. The superior mechanical properties of the GnP-reinforced nanocomposite can be correlated to the higher specific area of GnPs as well as the stronger GnP–matrix adhesion due to the GnPs’ wrinkled surfaces. The 2D (planar) geometry of the GnPs also contributes to this enhanced mechanical performance [47]. 3.3. Piezoelectric properties and poling behavior
As a member of the ceramics family, lead zirconate titanate (PZT) is classed as a polycrystalline ferroelectric material—a special category of piezoelectrics which generally exhibit large piezoelectric responses. Due to the intricate microstructural and electric boundary conditions of individual grains, the grains in polycrystalline materials are usually divided into several domains. Since the uninhibited polarization in a material is mostly random leading to neutral macroscopic
(1)
where E is the sample’s Young’s modulus and ν represents the Poisson’s ratio of the sample which is assumed to be 0.3 for PZT-based epoxy composites [45]; Ei and νi refer to the 6
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(a)
(b) Figure 8. Plots of piezoelectric activity versus poling field of
piezocomposites reinforced with various fractions of nanoparticles.
a high concentration of the applied electric field on the PZT phase is always desirable since it results in faster and more efficient poling; however, this trend cannot take place in conventional piezocomposites where nanoadditives are absent. Our composite film consists primarily of PZT particles featuring a high electrical conductivity of the order of ∼10−13 −1 cm−1 and an epoxy matrix possessing a low conductivity (∼10−16 −1 cm−1 ). At the microscale level, this three-order magnitude difference in the electrical conductivity leads to the accumulation of space-charges at the interface between the PZT and the epoxy. This is where electrons move from a higher conductive phase to a lower one under an applied voltage [27]. The existence of such an interfacial negative potential can displace the electric field, leading to a higher field concentration in the epoxy phase. As a result, a higher external voltage is required to fully pole the film. This additional voltage would consequently increase the injected space-charge electrons, which eventually causes dielectric breakdown of the film. In this condition, a transition from Ohmic to space-charge-limited (SCL) conduction [49, 50] occurs, and thus the sensing function can no longer be performed. One way to mitigate the premature breakdown during the electrical poling step is to add a third conductive phase into the composite [36, 51], which would raise the electrical conductivity of the matrix reducing the difference in conductivity between the PZT particles and the epoxy. This in turn contributes to suppression of the accumulated space charge at the interface of the PZT and the epoxy, and thereby the electric field more efficiently impacts on the PZT particles [52–55]. This accounts for the superior poling behavior of the nanocomposite films. In this work, two types of carbon nanomaterial, i.e. multiwalled carbon nanotubes (MWNTs) and graphene platelets (GnPs), each in various weight fractions, were investigated for their effects on the poling behavior and electromechanical response of piezocomposite films. Their fractions are limited to 0.5 wt% because higher fractions would cause electrical conductivity of the film [18, 56]. The films containing 0.25 wt% and 0.5 wt% of MWNTs showed leakage currents
(c)
Figure 7. Typical load–displacement curves of indentations made
on films of (a) the two-phase PZT/epoxy composite, (b) the MWNT/PZT/epoxy nanocomposite (0.1 wt% MWNTs) and (c) the GnP/PZT/epoxy nanocomposite (0.1 wt% GnPs).
polarization, these ceramics need to be forced to the polar state through the application of a large external electric field [48]. This process of reorienting the electric dipoles along the direction of the field, called poling, is therefore a vital step to activate the piezoelectric properties of the produced composites. Poling remains a major challenge for piezoelectric composites due to the complexity of the electric field distribution in the composite film. Figure 8 depicts the piezoelectric activity of our samples when subjected to step poling. As a general trend, the three-phase nanocomposite films yield much higher piezoelectric coefficients than the two-phase composite. Moreover, dielectric breakdown in the composite and MWNT-containing nanocomposite occurs far earlier than in the GnP-containing nanocomposite. This trend in poling behavior can be explained in light of the field distribution across the films. It is known that 7
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which increase the electrical conductivity of the epoxy to close to that of the PZT particles. It is worth mentioning that the unmodified graphene–epoxy system has a percolation threshold range of 1–2 vol% (∼2–4 wt%) (figure 9), which guarantees a safe margin ensuring the non-conductivity of the films in the current work. 3.4. Sensitivity analysis
Figure 10 shows the frequency spectrum of the two-phase composite film in response to a sweeping sinusoidal excitation. We obtained a dominant resonant frequency at 30 kHz from the spectrum, and then used it to actuate different films for evaluation of their dynamic sensitivities. Figure 11 illustrates the time-domain responses of the two-phase composite, the 0.1 wt% MWNT/PZT/epoxy composite and the 0.1 wt% GnP/PZT/epoxy composite. The peak amplitude in the initial portion of the voltage response is below 1.0 mV for the two-phase and MWNT/PZT/epoxy films, whereas it partially exceeds 1.5 mV in the GnP/PZT/epoxy film. It is well known that the initial portion of a voltage–time history provides more reliable results on the incident signals, as the signals in the later time ranges might have been interfered with by the reflected signal pulses. Load transfer from the host structure to the PZT particles is paramount for our films to benefit from the high Young’s modulus and strength of carbon nanomaterials. Although the load transfer efficiency can generally be promoted by the addition of carbon nanotubes into the matrix, this is not the case in this study. The reason can be linked to the mono-directional load transfer of MWNTs due to their one-dimensional tubular structure. In comparison, GnPs are two-dimensional with a lateral size of ∼100 nm, and may either disperse in the matrix or wrap around the PZT particles of 0.94 ± 0.21 µm in diameter. The defects of GnPs, such as oxygen-containing groups, make them hydrophilic, while the other regions of GnPs are hydrophobic. Thus, GnPs may act as a surfactant and possibly wrap around the particles.
Figure 9. Electrical resistivity of epoxy and its graphene
nanocomposites (data obtained from [15]).
during high field poling owing to their porous microstructure, preventing their application in sensing functions, and thus these two fractions are excluded in the following discussion. Even though MWNTs could improve the piezoelectric coefficient of a composite film, the film did not endure high electric fields, breaking down at ∼19 kV cm−1 . This is explained in light of the one-dimensional morphology of MWNTs, which causes electrical shorting of the film at relatively low poling fields. On the other hand, the GnPreinforced films in figure 8 exhibit far higher piezoelectric activities, and withstand higher external voltages at the time of poling. This remarkable feature can be explained in terms of both microstructure and electrical conductivity aspects. As evidenced earlier by XRD and SEM results, well exfoliated, homogeneously dispersed GnPs provide a platform for smooth distribution of applied electric field. In addition, GnPs have a low ID /IG ratio of 0.07 in their Raman spectrum [17], and very good electrical conductivity. GnP/PZT/epoxy composites endure larger breakdown fields than the two-phase composite film, which can be described as being a result of the addition of conductive GnPs
Figure 10. The frequency spectrum of the two-phase composite excited by a sine signal.
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4. Conclusion
We have demonstrated the key role of multi-walled carbon nanotubes (MWNTs) and graphene platelets (GnPs) in promoting the piezoelectric and mechanical properties of PZT/epoxy composites. Upon nanomechanical investigation, these carbon nanomaterials were found to markedly improve the composites’ mechanical properties. The nanocomposites yielded higher piezoelectric coefficients and electric field endurance, and sharper signal outputs than their parent composite. GnPs made a far greater contribution to increases in piezoelectric properties and electromechanical response than MWNTs. High load transfer efficiency, reduced acoustic mismatch and efficient distribution of the external electric field were proposed as the dominant mechanisms for these improvements. Based on these findings, our study suggests the potential application of economical, high-structural integrity GnPs in superior piezoelectric composites for transducer technology. References [1] Iijima S 1991 Helical microtubules of graphitic carbon Nature 354 56–8 [2] Treacy M M J, Ebbesen T W and Gibson J M 1996 Exceptionally high Young’s modulus observed for individual carbon nanotubes Nature 381 678–80 [3] Han B, Yu X and Kwon E 2009 A self-sensing carbon nanotube/cement composite for traffic monitoring Nanotechnology 20 445501 [4] Zhao H et al 2010 Carbon nanotube yarn strain sensors Nanotechnology 21 305502 [5] Kuilla T et al 2010 Recent advances in graphene based polymer composites Prog. Polym. Sci. 35 1350–75 [6] Kim H, Abdala A A and Macosko C W 2010 Graphene/polymer nanocomposites Macromolecules 43 6515–30 [7] Steurer P et al 2009 Functionalized graphenes and thermoplastic nanocomposites based upon expanded graphite oxide Macromol. Rapid Commun. 30 316–27 [8] Kauffman D R and Star A 2010 Graphene versus carbon nanotubes for chemical sensor and fuel cell applications Analyst 135 2790–7 [9] Javadi A et al 2012 Chemically modified graphene/P (VDF-TrFE-CFE) electroactive polymer nanocomposites with superior electromechanical performance J. Mater. Chem. 22 830–4 [10] Mahmoud W E 2011 Morphology and physical properties of poly(ethylene oxide) loaded graphene nanocomposites prepared by two different techniques Eur. Polym. J. 47 1534–40 [11] Lu Y et al 2011 Polystyrene/graphene composite electrode fabricated by in situ polymerization for capillary electrophoretic determination of bioactive constituents in Herba Houttuyniae Electrophoresis 32 1906–12 [12] Stankovich S et al 2006 Graphene-based composite materials Nature 442 282–6 [13] Yan D et al 2012 Improved electrical conductivity of polyamide 12/graphene nanocomposites with maleated polyethylene-octene rubber prepared by melt compounding ACS Appl. Mater. Interfaces 4 4740–5 [14] Araby S et al 2013 A novel approach to electrically and thermally conductive elastomers using graphene Polymer 54 3663–70
Figure 11. Time-domain responses of different nanocomposites:
(a) two-phase, (b) 0.1 wt% MWNTs and (c) 0.1 wt% GnPs.
More importantly, since GnPs improved the modulus of the epoxy in section 3.2 and they enhanced the epoxy toughness remarkably in a previous study [17], there must be a high level of interaction between the GnPs and the matrix, resulting in efficient load transfer from the matrix to the PZT particles. In other words, the contribution of the GnPs can be described as a ‘relay’ function to efficiently transmit the incoming elastic waves from the underlying structure to the target PZT particles. Another critical factor affecting the sensitivity of piezoelectric composite sensors is acoustic impedance mismatch. Acoustic impedance, defined as the product of the material density and the sound velocity of the medium, predominantly determines the percentage of the incident elastic waves transmitted from the host structure to the piezoelectric composite. It is well known that two-dimensional graphene platelets can markedly improve the thermal conductance of polymers; this in turn leads to enhanced phonon transfer from the substrate to the nanocomposite film which accounts for the improved dynamic response of the GnP-reinforced film in this work. 9
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[36] Sa-Gong G et al 1986 Poling flexible piezoelectric composites Ferroelectr. Lett. Sect. 5 131–42 [37] Park K-I et al 2012 Flexible nanocomposite generator made of BaTiO3 nanoparticles and graphitic carbons Adv. Mater. 24 2999–3004 [38] Li F et al 2013 Relations between carbon nanotubes’ length and their composites’ mechanical and functional performance Polymer 54 2158–65 [39] Li S et al 2008 Effect of acid and TETA modification on mechanical properties of MWCNTs/epoxy composites J. Mater. Sci. 43 2653–8 [40] Zaman I et al 2011 Epoxy/graphene platelets nanocomposites with two levels of interface strength Polymer 52 1603–11 [41] Raghavan A and Cesnik C E S 2005 Damage Prognosis: For Aerospace, Civil and Mechanical Systems (Hoboken, NJ: Wiley) pp 235–58 (Lamb-Wave Based Structural Health Monitoring) [42] Hernandez Y et al 2008 High-yield production of graphene by liquid-phase exfoliation of graphite Nature Nanotechnol. 3 563–8 [43] Chen X and Shi Y 2013 PZT nano active fiber composites-based acoustic emission sensor Selected Topics in Micro/Nano-Robotics for Biomedical Applications (Berlin: Springer) pp 9–22 [44] Oliver W C and Pharr G M 1992 Improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments J. Mater. Res. 7 1564–83 [45] Gururaja T et al 1985 Piezoelectric composite materials for ultrasonic transducer applications. Part I: resonant modes of vibration of PZT rod-polymer composites IEEE Trans. Sonics Ultrason 32 481–98 [46] Mitchell B S 2004 An Introduction to Materials Engineering and Science for Chemical and Materials Engineers (New York: Wiley) pp 380–536 [47] Rafiee M A et al 2009 Enhanced mechanical properties of nanocomposites at low graphene content ACS Nano 3 3884–90 [48] Damjanovic D 2005 Hysteresis in piezoelectric and ferroelectric materials Sci. Hysteresis 3 337–465 [49] Rose A 1955 Space-charge-limited currents in solids Phys. Rev. 97 1538–44 [50] Lampert M A and Mark P 1970 Current Injection in Solids (New York: Academic) pp 112–38 [51] Sagong G, Safari A and Jang S-J 1991 Easily poled 0–3 piezoelectric composites for transducer applications US Patent No. 5043622 [52] Ruschau G R et al 1989 0–3 ceramic/polymer composite chemical sensors Sensors Actuators 20 269–75 [53] Yoshikawa S et al 1990 Piezoresistivity in polymer–ceramic composites J. Am. Ceram. Soc. 73 263–7 [54] McLachlan D S, Blaszkiewicz M and Newnham R E 1990 Electrical resistivity of composites J. Am. Ceram. Soc. 73 2187–203 [55] Ruschau G R, Yoshikawa S and Newnham R E 1992 Resistivities of conductive composites J. Appl. Phys. 72 953–9 [56] Ramaratnam A and Jalili N 2006 Reinforcement of piezoelectric polymers with carbon nanotubes: pathway to next-generation sensors J. Intell. Mater. Syst. Struct. 17 199–208
[15] Ma J et al 2013 Covalently bonded interfaces for polymer/graphene composites J. Mater. Chem. A 1 4255–64 [16] Araby S et al 2013 Melt compounding with graphene to develop functional, high-performance elastomers Nanotechnology 24 165601 [17] Zaman I et al 2012 From carbon nanotubes and silicate layers to graphene platelets for polymer nanocomposites Nanoscale 4 4578 [18] Zaman I et al 2012 A facile approach to chemically modified graphene and its polymer nanocomposites Adv. Funct. Mater. 22 2735–43 [19] Zhao B et al 2012 Supercapacitor performances of thermally reduced graphene oxide J. Power Sources 198 423–7 [20] Young R J et al 2012 The mechanics of graphene nanocomposites: a review Compos. Sci. Technol. 72 1459–76 [21] Newnham R E, Skinner D P and Cross L E 1978 Connectivity and piezoelectric-pyroelectric composites Mater. Res. Bull. 13 525–36 [22] Skinner D P, Newnham R E and Cross L E 1978 Flexible composite transducers Mater. Res. Bull. 13 599–607 [23] Furukawa T, Ishida K and Fukada E 1979 Piezoelectric properties in the composite systems of polymers and PZT ceramics J. Appl. Phys. 50 4904–12 [24] Meyers F N et al 2013 Active sensing and damage detection using piezoelectric zinc oxide-based nanocomposites Nanotechnology 24 185501 [25] Ray M C and Batra R C 2007 A single-walled carbon nanotube reinforced 1–3 piezoelectric composite for active control of smart structures Smart Mater. Struct. 16 1936–47 [26] Li X and Zhang Y 2008 Analytical study of piezoelectric paint sensor for acoustic emission-based fracture monitoring Fatigue Fract. Eng. Mater. Struct. 31 684–94 [27] Egusa S and Iwasawa N 1995 Thickness dependence of the poling and current–voltage characteristics of paint films made up of lead zirconate titanate ceramic powder and epoxy resin J. Appl. Phys. 78 6060–70 [28] Pickwell A J, Dorey R A and Mba D 2011 Thick-film acoustic emission sensors for use in structurally integrated condition—monitoring applications IEEE Trans. Ultrason. Ferroelectr. Freq. Control 58 1994–2000 [29] Wenger M P et al 1999 Characterization and evaluation of piezoelectric composite bimorphs for in situ acoustic emission sensors Polym. Eng. Sci. 39 508–18 [30] Newnham R E 1986 Composite electroceramics Ferroelectrics 68 1–32 [31] Safari A 1994 Development of piezoelectric composites for transducers J. Phys. III 4 1129–49 [32] Hanner K A et al 1989 Thin-film 0–3 polymer piezoelectric ceramic composites—piezoelectric paints Ferroelectrics 100 255–60 [33] Egusa S and Iwasawa N 1993 Piezoelectric paints: preparation and application as built-in vibration sensors of structural materials J. Mater. Sci. 28 1667–72 [34] van den Ende D A et al 2010 Improving the d33 and g33 properties of 0–3 piezoelectric composites by dielectrophoresis J. Appl. Phys. 107 024107 [35] Sakamoto W K, Souza E d and Das-Gupta D K 2001 Electroactive properties of flexible piezoelectric composites Mater. Res. 4 201–4
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Superior piezoelectric composite films: taking advantage of carbon nanomaterials
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 045501 (http://iopscience.iop.org/0957-4484/25/4/045501) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 25 (2014) 045501 (10pp)
doi:10.1088/0957-4484/25/4/045501
Superior piezoelectric composite films: taking advantage of carbon nanomaterials Nasser Saber1 , Sherif Araby1 , Qingshi Meng1 , Hung-Yao Hsu1 , Cheng Yan3 , Sara Azari2 , Sang-Heon Lee1 , Yanan Xu3 , Jun Ma1 and Sirong Yu4 1
School of Engineering, University of South Australia, Mawson Lakes, SA 5095, Australia School of Natural and Built Environment, University of South Australia, Mawson Lakes, SA 5095, Australia 3 School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, Brisbane, QLD 4001, Australia 4 College of Mechanical and Electronic Engineering, China University of Petroleum, Qingdao 266580, People’s Republic of China 2
E-mail: [email protected] Received 1 October 2013, revised 19 November 2013 Accepted for publication 20 November 2013 Published 7 January 2014 Abstract
Piezoelectric composites comprising an active phase of ferroelectric ceramic and a polymer matrix have recently found numerous sensory applications. However, it remains a major challenge to further improve their electromechanical response for advanced applications such as precision control and monitoring systems. We here investigated the incorporation of graphene platelets (GnPs) and multi-walled carbon nanotubes (MWNTs), each with various weight fractions, into PZT (lead zirconate titanate)/epoxy composites to produce three-phase nanocomposites. The nanocomposite films show markedly improved piezoelectric coefficients and electromechanical responses (50%) besides an enhancement of ∼200% in stiffness. The carbon nanomaterials strengthened the impact of electric field on the PZT particles by appropriately raising the electrical conductivity of the epoxy. GnPs have been proved to be far more promising in improving the poling behavior and dynamic response than MWNTs. The superior dynamic sensitivity of GnP-reinforced composite may be caused by the GnPs’ high load transfer efficiency arising from their two-dimensional geometry and good compatibility with the matrix. The reduced acoustic impedance mismatch resulting from the improved thermal conductance may also contribute to the higher sensitivity of GnP-reinforced composite. This research pointed out the potential of employing GnPs to develop highly sensitive piezoelectric composites for sensing applications. Keywords: piezoelectric composites, sensors, graphene platelets, multi-walled carbon nanotubes, poling S Online supplementary data available from stacks.iop.org/Nano/25/045501/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
characteristics of CNTs still encourage their use is sensing applications versus their emerging competitor—graphene [3, 4]. With the groundbreaking introduction of two-dimensional graphene, a tremendous amount of attention has been attracted toward its research and development for a broad range of devices, from electronics to sensors to reinforced nanocomposites [5]. Composed of honeycomb structure in a planar sheet of one carbon atom thickness [6],
The superior properties of carbon nanotubes (CNTs) have ignited intensive interest among a great many scientists for a wide variety of applications over the past two decades. CNTs comprise single-walled (SWNTs) and multi-walled carbon nanotubes (MWNTs), both of which consist of rolled graphite layers of sp2 hybridized carbon atoms [1]. Despite their remarkably high manufacturing cost [2], the lucrative 0957-4484/14/045501+10$33.00
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graphene possesses a Young’s modulus of ∼1 TPa and an ultimate strength of 130 GPa. Moreover, it features unique electrical and thermal conductivities, up to 6000 S cm−1 and 5000 W m−1 K−1 , respectively. Besides its low production cost, which is advantageous over CNTs, chirality is not a factor in graphene’s electrical conductivity [6–9]. Recently, graphene nanocomposites with various polymer matrices such as polyethylene oxide [10], polystyrene [11, 12], polyamide 12 [13] and epoxy [14–16] have been investigated. These studies suggest that the incorporation of graphene into polymers effectively improves their mechanical and functional properties. Our recently in-house developed graphene platelets (GnPs) [17, 18] offer superior properties in terms of (i) low manufacturing cost of 10–20 $ kg−1 compared to 30 $ kg−1 for graphene oxide (GnO) excluding the reduction cost [6], (ii) higher structural integrity based on a Raman ID /IG ratio of ∼0.07 [17, 18] versus ∼1.0 for reduced GnO [19] and (iii) utmost resemblance of stiffness, strength and electrical and thermal conductivities to graphene [20]. The GnPs are produced by the combination of thermally expanding a commercial graphite intercalation compound (GIC) with ultrasonication of the expanded product. Each graphene platelet consists of 1–5 graphene layers. Piezoelectric ceramic–polymer composites have been developed and applied in arrays of sensing and actuation systems since the 1970s [21–23]. These composites provide target sensing performance with the desired flexibility, advantageous over conventional one-phase rigid ceramic sensing blocks, in particular for in situ damage identification in intelligent structures [24, 25]. These sensors, permanently covering the specified areas of a host structure surface, can be used in ultrasonic and acoustic emission monitoring techniques [26–29]. The working principle is based on the fact that every structural degradation mechanism emits a specific pattern of waveform under the quasi-static and dynamic loading regimes of a structure’s operating conditions. The sensing composites are able to capture these incoming elastic wavefields, owing to the piezoelectric effect of the ferroelectric ceramic particles embedded in the matrix. The majority of previous studies have focused on two-phase piezoelectric composites, which consist of an inorganic pigment—predominantly lead zirconate titanate (PZT)—and a polymer matrix [21, 30–35]. In comparison, few studies have reported on the incorporation of a third phase into the composite. Sa-Gong et al [36] added a third conductive phase of carbon, germanium or silicon between the piezoelectric particles to facilitate the poling of piezocomposite films. Ray and Batra [25] fabricated a piezoelectric composite comprising armchair SWNTs embedded in a piezoceramic matrix. They demonstrated that the piezoelectric coefficients and the effective elastic moduli of composites containing low fractions of single-walled carbon nanotubes were significantly higher than PZT/epoxy composites. Recently, Park et al [37] used CNTs in their BaTiO3 -based nanocomposites to serve as a dispersant, stress reinforcing agent and conducting functional material. They showed that CNT-reinforced composites provide higher
output voltages as a result of the BaTiO3 nanoparticles’ better dispersion by forming a complex mixture with CNT networks. CNTs have also been compounded with polymers to make functional composites [38, 39]. Nevertheless, to the best of our knowledge, no research has incorporated GnPs into piezocomposites, and concurrently compared these nanoadditives with MWNTs regarding their effects on the structure and electromechanical and piezoelectric properties of these composites. In this study we developed three-phase, PZT-based epoxy composites including (i) utilization of cost-effective, highstructural integrity GnPs as well as MWNTs as a third phase nanofiller, (ii) study on the morphology and microstructure of the composites, (iii) investigation of the structure–property relationships of the composites in terms of micromechanical properties, poling behavior and dynamic response, and (iv) comparison between GnPs and MWNTs—as a third phase—on the durability, electromechanical and piezoelectric properties of these nanocomposites. Our investigation shows that the hybrid composite film containing carbon nanomaterial demonstrates obviously enhanced mechanical strength, poling behavior and sensitivity to excitation, and that GnPs offer more advantages over MWNTs, as evidenced by the greater improvement in electromechanical properties and poling behavior. 2. Experimental procedure 2.1. Materials
Lead zirconate titanate (PZT-5H) ceramic powder of 0.94 ± 0.21 µm in diameter (particle size distribution provided in the supplementary information, available at stacks. iop.org/Nano/25/045501/mmedia) was kindly provided by Sunnytec (Suzhou) Electronics Co., Ltd, Taiwan. Asbury Carbons (Asbury, NJ, USA) provided free samples of a commercial graphite intercalation compound (GIC, Asbury 3494). MWNTs were kindly supplied by Showa Denko. Bisphenol A/F-based epoxy resin with an epoxide equivalent weight of 189–200 g eq−1 and hardener were obtained from Huntsman, Australia. The solvent tetrahydrofuran (THF) was purchased from Sigma-Aldrich. 2.2. Material preparation and fabrication of composite films
This research started with the preparation of a number of purpose-designed piezoelectric composite films. After curing and applying silver electrodes, these films were poled and characterized for performance evaluation. Figure 1 summarizes the experimental procedures used in this study. Graphene platelets (GnPs). 1 g of GIC was transferred into a crucible preheated in a common furnace at 700 ◦ C and treated for 1 min. The expanded product was left to cool down. Safety procedures, such as placing the furnace in a fume cupboard to prevent nanoparticle-inhalation hazard and wearing safety glasses, respirator and heat resistant gloves, were required. A desired amount of the expanded product was immersed 2
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nanoparticles must be added prior to the hardener mixing stage to prevent premature crosslinking. Likewise, as-received MWNTs were incorporated to produce another three-phase nanocomposite. Film coating and curing were similarly undertaken. All films made herein were of sufficient flexibility and robustness for practical applications. 2.3. Microstructure and mechanical characterization
X-ray diffraction (XRD) spectra of the GnPs and composites were collected in a reflection mode using a Mini-Materials Analyzer (MMA) with a diffractometer equipped with curved graphite monochromators with Cu Kα radiation (wavelength ˚ The spectra were collected from 2.5◦ to of λ = 1.541 A). ◦ 35 at a scanning rate of 1◦ min−1 under a tube voltage of 35 kV with 1 kW at room temperature. Scanning electron microscopy (SEM) was employed to investigate the microstructure of the films which were coated with a thin layer of platinum and observed using a Philips XL30 FegSEM operating at an accelerating voltage of 10 kV. Nanoindentation experiments were conducted to determine the hardness and Young’s moduli of the composite films (10 mm × 10 mm × 0.65 mm). Experiments were performed using a Hysitron TI-950 TriboIndenter with a Berkovich indenter (three-sided pyramidal diamond tip) whose nominal tip radius was 100–200 nm. Probe calibration was performed on a reference sample of fused quartz (reduced modulus Er = 69.6 GPa ± 5%, hardness = 9.25 GPa ±10%). Both load and displacement were continuously monitored by a three-plate capacitive transducer. Tests were carried out under a constant loading rate up to the maximum force and then unloaded to zero under the same rate. In order to ensure experimental reproducibility, four experiments were conducted for each sample, with ten data points collected in each experiment.
Figure 1. Summary of experimental procedures.
in THF (1 wt%) in a metallic container and treated in an ultrasonication bath (200 W at 42 kHz) for 1 h below 30 ◦ C. Upon sonication, the expanded product in suspension was able to split into platelets of 2–4 nm in thickness, as evidenced in the previous works [18, 40]. This suspension was used in the following fabrication. Two-phase composites. A prescribed quantity of PZT particles (70 wt%) was suspended in THF using an ultrasonic bath for 10 min. A calculated amount of epoxy resin was dissolved in the same solvent and heated to 70 ◦ C using an oil bath and a round-bottom flask equipped with a condenser to obtain a clear solution. The PZT suspension was then added to the solution and stirred using a magnet bar at 70 ◦ C for 30 min, followed by a sonication process of 30 min below 30 ◦ C [40]. A recommended ratio of hardener (weight ratio of epoxy to hardener of 100:130) was added to the mixture and dissolved by stirring and sonication. A thin layer of the mixture was subsequently cast on a PVC plate using an adjustable casting knife and cured at room temperature for three days. The final film thickness was about 650 µm. Three-phase nanocomposites with either GnPs or MWNTs. The above described GnP suspension was added to the two-phase composite solution. It should be noted that
2.4. Sensor activation and piezoelectric measurements
Upon curing a composite film, a thin layer of silver paint (∼20 µm) was carefully brushed onto both sides of the film to serve as electrodes connecting the film with the data acquisition system. A large electric field was vertically applied using a TREK 610E power supply to pole the films at room temperature. In order to study the poling behavior, we followed the procedure described by Hanner et al [32]. To be specific, the poling voltage was applied with an increment of 200 V, allowing the film to remain at each voltage for 5 min. Then the piezoelectric coefficient measurement was carried out, followed by further increments until dielectric breakdown of the film occurred. A digital oscilloscope (Agilent Technologies) was employed to monitor the transient current during the film poling. The piezoelectric coefficients (d33 ) of the poled films were measured using a wide-range d33 meter (APC International, Ltd). 2.5. Dynamic characterization
In order to examine and compare the sensing performance of our composite films in detecting Rayleigh waves (a special 3
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Figure 2. Experimental installation of the pitch–catch setup.
type of surface acoustic wave travelling on solids), an active sensing setup was established as shown in figure 2. This pitch–catch experimental arrangement [41] consisted of (i) a wideband acoustic emission (AE) transducer (MISTRAS Group Inc.) as an actuator that was surface-mounted on an aluminum panel and (ii) a 0.650 mm thick, 20 mm square film with an active electrode area of 10 mm × 10 mm that was affixed 200 mm away from the transmitter using special vacuum grease. To produce the Rayleigh waves, the AE transducer was actuated by a 150 Vpp sinusoidal signal generated by a Rigol DG1022 waveform generator. The composite film output was amplified by a 2/4/6 preamplifier (PAC) with a fixed gain of 40 dB and the amplified signals were recorded using a two-channel PCI-2 AE board (MISTRAS Group Inc.) with a sampling rate of 2 MHz. Initially, the frequency spectrum of two-phase composite film was obtained through a frequency sweep test, by varying the actuation signal from 5 kHz to 1 MHz and subsequently plotting the spectral response calculated by a fast Fourier transform (FFT). Secondly, to investigate the time-domain responses of all composite films, the AE transducer was excited at a resonant frequency determined from the aforementioned frequency sweep test, and then the generated Rayleigh waves were captured by each nanocomposite film.
Figure 3. XRD patterns of the two-phase PZT/epoxy composite and
its graphene platelet (GnP) nanocomposites.
pattern at 2θ = 26.7◦ observed for GnPs is assigned to the 0.34 nm basal spacing between graphene layers. Upon sonication, the expanded product exfoliated and separated into mono- and multi-layer graphene, explaining the existence of diffraction at 2θ = 26.7◦ . The two-phase composite shows a minor diffraction at 2θ = 21.5◦ , attributed to the scattering of cured epoxy molecules. It also shows a large diffraction at 2θ = 31.5◦ corresponding to the perovskite phase of PZT [43]. At low GnP fractions—0.1 and 0.25 wt% –the nanocomposites show similar diffraction patterns to the two-phase composite with absence of the GnP diffraction pattern, implying full exfoliation and dispersion of GnPs. Meanwhile, with increase in the GnP fraction to 0.5 wt%, the pattern indicates a tiny diffraction at 2θ = 26.7◦ , referring to the possibility of graphene layers stacking in the matrix. Figure 4 displays SEM micrographs of the fractured surface of the two-phase PZT/epoxy composite. In figure 4(a), the light-color dispersion phase of 0.94±0.21 µm in diameter refers to clusters or aggregates of PZT particles. The higher magnification image of a typical matrix zone in figure 4(b) reveals the presence of micro-voids formed during the film fabrication process. SEM images of the three-phase MWNT/PZT/epoxy nanocomposite are presented in figure 5, where the fractions of MWNTs and PZT are 0.1 wt% and 70 wt%, respectively. It is noteworthy that 70 wt% was found to be the optimum fraction of PZT particles during the solution mixing and film fabrication process; lower values result in poor piezoelectric properties while in higher fractions, handling of the solution is
3. Results and discussion 3.1. Morphology
The thickness of the graphene platelets (GnPs) dispersed in tetrahydrofuran (THF) was measured as 3.57 ± 0.50 nm, while this value reduced to 2.51 ± 0.39 nm when the THF was replaced by N-methyl-2-pyrrolidone (NMP) [17, 18]. This thickness value depends on how well the interfacial interaction energy of the solvent–graphene matches that of graphene–graphene [42]. Upon being compounded with polymers, GnPs may exfoliate causing separation from each other; however, the layered structure in every platelet should be retained throughout the entire compounding process. Figure 3 contains the x-ray diffraction patterns of the two-phase PZT/epoxy composite and the three-phase GnP/PZT/epoxy nanocomposites. The sharp diffraction 4
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Figure 4. SEM micrographs of the 70 wt% two-phase PZT/epoxy composite.
Figure 5. SEM micrographs of the MWNT/PZT/epoxy nanocomposite.
difficult and the wet paint cannot be applied consistently using a casting knife. MWNTs can be clearly distinguished as the ridges in figure 5(a). Comparison of the matrix in figures 5(a) and (b) with that in figure 4 confirms the presence of the third phase MWNTs. These magnified images also show a decrease in void size upon incorporation of nanotube content. Figure 6 presents representative SEM micrographs of the fractured surface of the three-phase GnP/PZT/epoxy nanocomposite, where the fractions of GnPs and PZT are 0.1 wt% and 70 wt%, respectively. No obvious aggregates or clusters of GnPs can be seen in figure 6(a), but a number of ridges are visible. When a typical ridge is magnified in figure 6(b), a light-colored, folded sheet is found coexisting with a number of smaller sheets, and these should be either GnPs or their clusters. Figure 6(c) shows the detail of a typical folded sheet, where graphene layers are clearly visible; it must be a large GnP cluster comprising a few GnPs that are most likely connected with each other by epoxy resin. The
matrix deformation shown as red arrows indicates a strong interface between the cluster and the matrix, even though no interface modification was made in this work. Under loading, a fraction of load is transferred from the matrix to the GnPs and their clusters through the interface; the GnPs thus share the load and restrain the movement of matrix molecules in their vicinity. This means that GnPs are a class of suitable filler for reinforcement of piezocomposite, as supported by a previous study [17]. 3.2. Mechanical properties
Figure 7 illustrates typical load–displacement curves of indentations made at peak loads of 100 µN and 200 µN applied for the two-phase PZT/epoxy composite and the two three-phase nanocomposites, respectively. Due to the flexible nature of our samples, the applied load could not be increased further for the two-phase composite, and a relatively 5
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Figure 6. SEM micrographs of the GnP/PZT/epoxy nanocomposite.
Table 1. Summary of the mechanical properties of the PZT/epoxy composite and its nanocomposites.
MWNT/PZT/epoxy
GnP/PZT/epoxy
Nanoparticle fraction (wt%)
Young’s modulus (MPa)
Hardness (MPa)
Young’s modulus (MPa)
Hardness (MPa)
0 0.1 0.25 0.5
3.30 ± 0.46 6.00 ± 0.26 7.15 + 0.32 9.02 ± 0.48
0.235 ± 0.0962 0.774 ± 0.0813 0.869 ± 0.0791 1.11 ± 0.0847
3.30 ± 0.46 7.31 ± 0.39 8.44 ± 0.47 9.13 ± 0.54
0.235 ± 0.0962 0.794 ± 0.0932 0.875 ± 0.0742 0.950 ± 0.0686
high creep strain is noted at the peak load for all three samples. Therefore, holding of the peak load is essential to allow for dissipation of creep displacement. With regard to both of the three-phase nanocomposites, lower indentation depths and higher unloading slopes are observed owing to the nanoadditives—MWNTs and GnPs—of high stiffness. As discussed before, these additives share a fraction of the load and restrain the movement of the matrix molecules. Table 1 contains the Young’s moduli and hardnesses of the composite and nanocomposite films obtained from nanoindentation measurements. Calculation of the listed Young’s moduli was performed using the reduced elastic modulus (Er ) from nanoindentation according to the Oliver–Pharr method [44]. The given reduced elastic modulus bears the implication that both the sample and the indenter undergo elastic deformation. Er is then related to the Young’s modulus of the sample by 1 − νi2 1 − ν2 1 = + Er E Ei
same parameters for the indenter, which are taken as 1141 GPa and 0.07, respectively [46]. It is noted that the nanocomposites exhibit significantly enhanced Young’s modulus and hardness values up to 176.6% and 372.3%, respectively, with increase in the nanoparticle content. In addition, the GnP-reinforced nanocomposite yields a higher modulus and hardness than the equally loaded MWNT nanocomposite. The superior mechanical properties of the GnP-reinforced nanocomposite can be correlated to the higher specific area of GnPs as well as the stronger GnP–matrix adhesion due to the GnPs’ wrinkled surfaces. The 2D (planar) geometry of the GnPs also contributes to this enhanced mechanical performance [47]. 3.3. Piezoelectric properties and poling behavior
As a member of the ceramics family, lead zirconate titanate (PZT) is classed as a polycrystalline ferroelectric material—a special category of piezoelectrics which generally exhibit large piezoelectric responses. Due to the intricate microstructural and electric boundary conditions of individual grains, the grains in polycrystalline materials are usually divided into several domains. Since the uninhibited polarization in a material is mostly random leading to neutral macroscopic
(1)
where E is the sample’s Young’s modulus and ν represents the Poisson’s ratio of the sample which is assumed to be 0.3 for PZT-based epoxy composites [45]; Ei and νi refer to the 6
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(a)
(b) Figure 8. Plots of piezoelectric activity versus poling field of
piezocomposites reinforced with various fractions of nanoparticles.
a high concentration of the applied electric field on the PZT phase is always desirable since it results in faster and more efficient poling; however, this trend cannot take place in conventional piezocomposites where nanoadditives are absent. Our composite film consists primarily of PZT particles featuring a high electrical conductivity of the order of ∼10−13 −1 cm−1 and an epoxy matrix possessing a low conductivity (∼10−16 −1 cm−1 ). At the microscale level, this three-order magnitude difference in the electrical conductivity leads to the accumulation of space-charges at the interface between the PZT and the epoxy. This is where electrons move from a higher conductive phase to a lower one under an applied voltage [27]. The existence of such an interfacial negative potential can displace the electric field, leading to a higher field concentration in the epoxy phase. As a result, a higher external voltage is required to fully pole the film. This additional voltage would consequently increase the injected space-charge electrons, which eventually causes dielectric breakdown of the film. In this condition, a transition from Ohmic to space-charge-limited (SCL) conduction [49, 50] occurs, and thus the sensing function can no longer be performed. One way to mitigate the premature breakdown during the electrical poling step is to add a third conductive phase into the composite [36, 51], which would raise the electrical conductivity of the matrix reducing the difference in conductivity between the PZT particles and the epoxy. This in turn contributes to suppression of the accumulated space charge at the interface of the PZT and the epoxy, and thereby the electric field more efficiently impacts on the PZT particles [52–55]. This accounts for the superior poling behavior of the nanocomposite films. In this work, two types of carbon nanomaterial, i.e. multiwalled carbon nanotubes (MWNTs) and graphene platelets (GnPs), each in various weight fractions, were investigated for their effects on the poling behavior and electromechanical response of piezocomposite films. Their fractions are limited to 0.5 wt% because higher fractions would cause electrical conductivity of the film [18, 56]. The films containing 0.25 wt% and 0.5 wt% of MWNTs showed leakage currents
(c)
Figure 7. Typical load–displacement curves of indentations made
on films of (a) the two-phase PZT/epoxy composite, (b) the MWNT/PZT/epoxy nanocomposite (0.1 wt% MWNTs) and (c) the GnP/PZT/epoxy nanocomposite (0.1 wt% GnPs).
polarization, these ceramics need to be forced to the polar state through the application of a large external electric field [48]. This process of reorienting the electric dipoles along the direction of the field, called poling, is therefore a vital step to activate the piezoelectric properties of the produced composites. Poling remains a major challenge for piezoelectric composites due to the complexity of the electric field distribution in the composite film. Figure 8 depicts the piezoelectric activity of our samples when subjected to step poling. As a general trend, the three-phase nanocomposite films yield much higher piezoelectric coefficients than the two-phase composite. Moreover, dielectric breakdown in the composite and MWNT-containing nanocomposite occurs far earlier than in the GnP-containing nanocomposite. This trend in poling behavior can be explained in light of the field distribution across the films. It is known that 7
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which increase the electrical conductivity of the epoxy to close to that of the PZT particles. It is worth mentioning that the unmodified graphene–epoxy system has a percolation threshold range of 1–2 vol% (∼2–4 wt%) (figure 9), which guarantees a safe margin ensuring the non-conductivity of the films in the current work. 3.4. Sensitivity analysis
Figure 10 shows the frequency spectrum of the two-phase composite film in response to a sweeping sinusoidal excitation. We obtained a dominant resonant frequency at 30 kHz from the spectrum, and then used it to actuate different films for evaluation of their dynamic sensitivities. Figure 11 illustrates the time-domain responses of the two-phase composite, the 0.1 wt% MWNT/PZT/epoxy composite and the 0.1 wt% GnP/PZT/epoxy composite. The peak amplitude in the initial portion of the voltage response is below 1.0 mV for the two-phase and MWNT/PZT/epoxy films, whereas it partially exceeds 1.5 mV in the GnP/PZT/epoxy film. It is well known that the initial portion of a voltage–time history provides more reliable results on the incident signals, as the signals in the later time ranges might have been interfered with by the reflected signal pulses. Load transfer from the host structure to the PZT particles is paramount for our films to benefit from the high Young’s modulus and strength of carbon nanomaterials. Although the load transfer efficiency can generally be promoted by the addition of carbon nanotubes into the matrix, this is not the case in this study. The reason can be linked to the mono-directional load transfer of MWNTs due to their one-dimensional tubular structure. In comparison, GnPs are two-dimensional with a lateral size of ∼100 nm, and may either disperse in the matrix or wrap around the PZT particles of 0.94 ± 0.21 µm in diameter. The defects of GnPs, such as oxygen-containing groups, make them hydrophilic, while the other regions of GnPs are hydrophobic. Thus, GnPs may act as a surfactant and possibly wrap around the particles.
Figure 9. Electrical resistivity of epoxy and its graphene
nanocomposites (data obtained from [15]).
during high field poling owing to their porous microstructure, preventing their application in sensing functions, and thus these two fractions are excluded in the following discussion. Even though MWNTs could improve the piezoelectric coefficient of a composite film, the film did not endure high electric fields, breaking down at ∼19 kV cm−1 . This is explained in light of the one-dimensional morphology of MWNTs, which causes electrical shorting of the film at relatively low poling fields. On the other hand, the GnPreinforced films in figure 8 exhibit far higher piezoelectric activities, and withstand higher external voltages at the time of poling. This remarkable feature can be explained in terms of both microstructure and electrical conductivity aspects. As evidenced earlier by XRD and SEM results, well exfoliated, homogeneously dispersed GnPs provide a platform for smooth distribution of applied electric field. In addition, GnPs have a low ID /IG ratio of 0.07 in their Raman spectrum [17], and very good electrical conductivity. GnP/PZT/epoxy composites endure larger breakdown fields than the two-phase composite film, which can be described as being a result of the addition of conductive GnPs
Figure 10. The frequency spectrum of the two-phase composite excited by a sine signal.
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4. Conclusion
We have demonstrated the key role of multi-walled carbon nanotubes (MWNTs) and graphene platelets (GnPs) in promoting the piezoelectric and mechanical properties of PZT/epoxy composites. Upon nanomechanical investigation, these carbon nanomaterials were found to markedly improve the composites’ mechanical properties. The nanocomposites yielded higher piezoelectric coefficients and electric field endurance, and sharper signal outputs than their parent composite. GnPs made a far greater contribution to increases in piezoelectric properties and electromechanical response than MWNTs. High load transfer efficiency, reduced acoustic mismatch and efficient distribution of the external electric field were proposed as the dominant mechanisms for these improvements. Based on these findings, our study suggests the potential application of economical, high-structural integrity GnPs in superior piezoelectric composites for transducer technology. References [1] Iijima S 1991 Helical microtubules of graphitic carbon Nature 354 56–8 [2] Treacy M M J, Ebbesen T W and Gibson J M 1996 Exceptionally high Young’s modulus observed for individual carbon nanotubes Nature 381 678–80 [3] Han B, Yu X and Kwon E 2009 A self-sensing carbon nanotube/cement composite for traffic monitoring Nanotechnology 20 445501 [4] Zhao H et al 2010 Carbon nanotube yarn strain sensors Nanotechnology 21 305502 [5] Kuilla T et al 2010 Recent advances in graphene based polymer composites Prog. Polym. Sci. 35 1350–75 [6] Kim H, Abdala A A and Macosko C W 2010 Graphene/polymer nanocomposites Macromolecules 43 6515–30 [7] Steurer P et al 2009 Functionalized graphenes and thermoplastic nanocomposites based upon expanded graphite oxide Macromol. Rapid Commun. 30 316–27 [8] Kauffman D R and Star A 2010 Graphene versus carbon nanotubes for chemical sensor and fuel cell applications Analyst 135 2790–7 [9] Javadi A et al 2012 Chemically modified graphene/P (VDF-TrFE-CFE) electroactive polymer nanocomposites with superior electromechanical performance J. Mater. Chem. 22 830–4 [10] Mahmoud W E 2011 Morphology and physical properties of poly(ethylene oxide) loaded graphene nanocomposites prepared by two different techniques Eur. Polym. J. 47 1534–40 [11] Lu Y et al 2011 Polystyrene/graphene composite electrode fabricated by in situ polymerization for capillary electrophoretic determination of bioactive constituents in Herba Houttuyniae Electrophoresis 32 1906–12 [12] Stankovich S et al 2006 Graphene-based composite materials Nature 442 282–6 [13] Yan D et al 2012 Improved electrical conductivity of polyamide 12/graphene nanocomposites with maleated polyethylene-octene rubber prepared by melt compounding ACS Appl. Mater. Interfaces 4 4740–5 [14] Araby S et al 2013 A novel approach to electrically and thermally conductive elastomers using graphene Polymer 54 3663–70
Figure 11. Time-domain responses of different nanocomposites:
(a) two-phase, (b) 0.1 wt% MWNTs and (c) 0.1 wt% GnPs.
More importantly, since GnPs improved the modulus of the epoxy in section 3.2 and they enhanced the epoxy toughness remarkably in a previous study [17], there must be a high level of interaction between the GnPs and the matrix, resulting in efficient load transfer from the matrix to the PZT particles. In other words, the contribution of the GnPs can be described as a ‘relay’ function to efficiently transmit the incoming elastic waves from the underlying structure to the target PZT particles. Another critical factor affecting the sensitivity of piezoelectric composite sensors is acoustic impedance mismatch. Acoustic impedance, defined as the product of the material density and the sound velocity of the medium, predominantly determines the percentage of the incident elastic waves transmitted from the host structure to the piezoelectric composite. It is well known that two-dimensional graphene platelets can markedly improve the thermal conductance of polymers; this in turn leads to enhanced phonon transfer from the substrate to the nanocomposite film which accounts for the improved dynamic response of the GnP-reinforced film in this work. 9
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