Статьи на перевод PVDF_P(VDF-TrFE) / Enhancing the piezoelectric performance of PVDF-TrFE thin films

Enhancing the piezoelectric performance of PVDF-TrFE thin films using zinc oxide nanoparticles

John S. Doddsa, Frederick N. Meyersb, Kenneth J. Loh*,a

a Civil & Environmental Engineering, University of California, Davis, CA 95616-5294; b Mechanical & Aerospace Engineering, University of California, Davis, CA 95616

ABSTRACT

Structural health monitoring (SHM) is crucial for detecting sudden and progressive damage and for preventing catastrophic structural failure. Piezoelectric materials have been widely adopted for their use as sensors and as actuators. Piezoceramics (such as lead zirconate titanate) offer high piezoelectricity but are mechanically brittle. Poly(vinylidene fluoride) (PVDF) piezopolymers are conformable to complex structural surfaces but exhibit lower piezoelectricity. So as to achieve a combination of these desirable properties, piezoelectric zinc oxide (ZnO) nanomaterials are proposed for embedment in flexible polymer matrices during fabrication to yield high-performance piezoelectric nanocomposites. The main objective of this research is to characterize the piezoelectricity of nanocomposites formed by embedding ZnO nanoparticles in a PVDF-trifluoroethylene (TrFE) matrix. Film fabrication is performed by dispersing ZnO into a PVDFTrFE solution and then by spin coating the solution onto a rigid substrate. A high electric field is applied to each of the films for poling, and the films’ remnant polarization is quantified by measuring their ferroelectric response using a Sawyer-Tower circuit. Graphs of electric field compared to electric displacement can be obtained for determining the films’ piezoelectricity. Finally, validation of their sensing performance is achieved by hammer impact testing.

Keywords: ferroelectric hysteresis, nanocomposite, piezoelectric, PVDF, sensing, zinc oxide nanoparticle

1. INTRODUCTION

Structural health monitoring (SHM) systems provide important information relevant to structural performance and damage formation/propagation and are needed for facilitating repair efforts to prevent catastrophic structural failure. However, current structural health management approaches rely on visual inspection, tethered transducers, and bulky expensive instrumentation, all of which are time-, labor-, and cost-intensive. Some examples of structural systems that can benefit from a more efficient and effective SHM approach are pipelines, aircrafts, and bridges, to name a few. For instance, U.S. gas companies spend $300 million every year for detecting and repairing gas pipeline leaks [1]. More specifically, these decaying pipelines primarily rely on visual inspection, which is often incapable of identifying small cracks that can ultimately cause pipeline rupture [2, 3]. The Pacific Gas and Electric (PG&E) Company pipeline rupture of 2011 (in San Bruno, CA) could have been prevented had a thorough pipeline monitoring system been installed [4]. Similarly, visual inspection is also the predominant approach for monitoring the integrity of aircrafts [5]. For example, the inability to detect fatigue cracks has led to a Southwest Airlines aircraft’s fuselage to tear open in-flight (2011) [6]. Another area where more effective SHM systems can be beneficial is in civil infrastructure monitoring. Currently, significant costs are allocated for visually inspecting bridges on a biannual basis [7], and yet, the nation’s infrastructure remains in a highly deteriorated state [8, 9]. Having a proper SHM system installed in these structures can reduce costs associated with monitoring while simultaneously freeing resources that can be redirected for providing greater attention to criticality deficient structures. The dire need for effective SHM systems has led to the development and implementation of novel sensor networks in bridges such as the Golden Gate Bridge and the Hale Boggs Bridge [10, 11].

Among the diverse research activities dedicated to SHM and sensor development, piezoelectric transducers have attracted significant attention due to their ability to be used simultaneously as sensors and actuators. The unique feature of piezoelectric materials is their ability to develop an electric potential in response to time-varying applied strains and also conversely become strained when excited by an applied voltage. These effects occur because of an imbalance in the dipoles within their crystal structure. As electric domains in the material are strained, positive and negative charges tend in different directions, thereby producing charge separations, which will generate a difference in charge across the material. The direct piezoelectric effect (i.e., charge from strain) is useful for sensing, whereas the converse piezoelectric

Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2012, edited by Masayoshi Tomizuka, Chung-Bang Yun, Jerome P. Lynch, Proc. of SPIE Vol. 8345, 834515 · © 2012 SPIE · CCC code: 0277-786X/12/$18 · doi: 10.1117/12.915072

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effect (i.e., strain from charge) can be used for actuation. In typical SHM applications, piezoceramic lead zirconate titanate (PZT) and piezopolymer poly(vinylidene fluoride) (PVDF) (or their copolymers) have been used.

These piezoelectric sensors and actuators have demonstrated the capability of detecting cracks and delamination in structural components using techniques such as acoustic emissions, active sensing, and dynamic strain sensing [12-14]. Acoustic emission monitoring methods are able to detect very small cracks in structures by passively sensing acoustic waves generated due to crack opening and propagation [12]. In this case, piezoelectric elements used for sensing do not rely on an external power source. A piezoelectric device can also act as an actuator by actively propagating Lamb waves in thin or plate-like structural components. Simple pitch-catch and pulse-echo strategies using piezoelectric sensors/actuators have been demonstrated for detecting damage in metallic and fiber-reinforced polymer composite panels [15, 16]. A piezoelectric monitoring system placed on an aircraft surface has been shown to be able to detect cracks [17]. These piezoelectric transducers have also been used for other applications such as for guided-wave pipeline monitoring [18].

While PVDF-(trifluoroethylene) (-TrFE) copolymers have been used for sensing and actuation [19], their piezoelectricity (as measured by their piezoelectric coefficients) are significantly lower than that of PZTs. However, they do possess high flexibility and small form factors that make them ideal candidates for instrumentation on complex structural surfaces or for embedment within composite materials. Its lower piezoelectricity has motivated and spurred the development of PVDF and PVDF-TrFE nanocomposites that feature improved bulk film piezoelectricity while preserving their inherent favorable mechanical attributes. Examples include PZT particles embedded in PVDF that exhibit sensitivity to temperature and pressure changes [20], PVDF with embedded carbon nanotubes for strain sensing [21], and PVDF with Al2O3 for increasing acoustic wave velocity [22].

A specific form of piezoelectric nanocomposite that has gained considerable attention is one based on embedding piezoelectric zinc oxide (ZnO) nanomaterials within PVDF and PVDF-TrFE polymer matrices [23]. ZnO nanostructures such as nanobelts, nanowires, and nanosprings have been explored by Wang [24]. ZnO has been embedded in a paper matrix for increased flexibility and used as a strain sensor under static and dynamic loading [25]. ZnO has also been embedded in a poly(sodium 4-styrenesulfonate) matrix and has been validated for dynamic strain sensing [26]. So as to increase bulk film piezoelectricity, ZnO has been incorporated with PVDF, and dielectric characterization studies have been performed [27]. ZnO and PVDF have also been combined and constructed radially, resulting in an increased dielectric constant [28]. However, the piezoelectric performance of ZnO/PVDF-based nanocomposites has not been comprehensively characterized, and it remains unclear as to how much piezoelectric enhancement can be attained by embedding ZnO nanomaterials within a PVDF or PVDF-TrFE matrix. A better understanding of their material properties and sensing/actuation performance will facilitate their adoption and use for SHM applications.

Thus, the objective of this study is to characterize the piezoelectric performance of nanocomposites formed by dispersing ZnO nanoparticles in a PVDF-TrFE thin film matrix and to demonstrate their potential for sensing and actuation. The outline of this paper is as follows. First, the nanocomposite fabrication procedure using spin coating of dispersed ZnO nanoparticles in PVDF-TrFE solutions is discussed. Second, ferroelectric characterization and determination of the films’ remnant polarization are conducted. Then, the as-fabricated thin films are subjected to high-electric field poling to align their electric domains, thereby achieving enhanced piezoelectric performance. Once the films have been poled, the films are mounted in a hammer impact testing apparatus. The hammer impacts the substrate, and the nanocomposite prototype transducers are employed for measuring surface acoustic waves due to impact (i.e., for sensing validation).

2.SPECIMEN FABRICATION AND PREPARATION

2.1Film Fabrication and Preparation

Fabrication of ZnO/PVDF-TrFE thin films was based on spin coating dispersed nanomaterial solutions onto a rigid substrate and is also schematically illustrated in Figure 1. The first step in the fabrication process was to prepare the ZnO nanoparticle solution. The process began by dissolving 150 mg-mL-1 of PVDF-TrFE powder (65:35 mole) (Measurement Specialties) in methyl ethyl ketone (MEK) at 50 ºC. This concentration was selected to ensure proper solution viscosity for spin coating as will be discussed later. Then, an appropriate amount of ZnO nanoparticles (20 nm diameter, Nano Amor) was mixed in the PVDF-TrFE solution to obtain a 10 wt.% mixture. It should be mentioned that ZnO nanoparticles tended to agglomerate in solution, which suggested that the initial mixture was not suitable for spin coating. Therefore, to eliminate these agglomerations, the solution was subjected to bath ultrasonication (135 W, 42 kHz)

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Figure 1. PVDF-TrFE and ZnO in MEK (a) were mixed and (b) bath sonicated, (c) followed by spin coating the dispersed 10 wt.% ZnO/PVDF-TrFE solutions. After spin coating, the films were annealed, and (e) top aluminum electrodes were deposited using an e- beam evaporator.

for at least 180 min to obtain dispersed 10 wt.% ZnO nanoparticles in PVDF-TrFE solutions. These solutions were used as-is for spin coating the ZnO/PVDF-TrFE thin films.

Spin coating was selected as the method used for assembling ZnO/PVDF-TrFE nanocomposites due to its flexibility, ease of use, and short fabrication times. Prior to spin coating, glass microscope slide substrates (75 x 25 mm2) were prepared by using an electron beam (e-beam) evaporator to deposit a thin (~150 nm) layer of aluminum. The aluminum layer formed the bottom electrode that was electrically connected to the thin films. The substrate was then mounted in a Laurell WS-650Mz-23NPP spin processor. Then, small quantities of the dispersed ZnO/PVDF-TrFE solutions were pipetted onto the aluminum-coated substrate. The spin processor was commanded to first spread the solution at 400 rpm for 5 s and then to spin the solution at 3,000 rpm for 30 s. Immediately after spinning, the coated slide was removed and placed into a heated furnace for annealing at 150 ºC for 2 min. The aforementioned procedure yielded films approximately 4 μm thick. Multiple layers were spin coated consecutively following the same procedure, with annealing taking place between each coat. Once the desired thickness was achieved, a final annealing step (again at 150 ºC) for 2 h was performed for evaporating any excess solvent or moisture that might have been present in the films.

The final step in specimen preparation was to deposit a top electrode to provide specimen’s functionality in the thickness direction. Here, a plastic mask with three rectangular-shaped openings was made; each rectangle measured approximately 1.2 x 1 cm2 (i.e., the longer direction was oriented perpendicularly to the length of the slide). The mask was placed over the ZnO/PVDF-TrFE thin film and the glass substrate, and e-beam was employed again to deposit ~150 nm thick layer of aluminum directly on top of the film. This procedure allowed the simultaneous deposition of three top electrodes onto the thin film specimen. Then, a combination of mechanical (using a razor blade) and chemical (using MEK solvent) etching was used for removing film between the top electrodes while also exposing the bottom (or ground) electrodes. Thus, this procedure yielded three ZnO/PVDF-TrFE specimens per substrate or fabrication cycle. The benefits of using this technique were achieving greater consistency while increasing the number of samples fabricated and tested.

In this study, control specimens were also prepared using commercially poled PVDF-TrFE thin films from Measurement Specialties. Since these specimens were not metalized, e-beam evaporation and a similar masking technique were used for depositing aluminum topand bottom-electrodes. The area of the electrode was maintained at 1.2 x 1 cm2. Then, the control specimens were mounted onto structural surfaces (e.g., glass substrates) using a cyanoacrylate-based strain gage adhesive. It should also be mentioned that this mounting procedure was different than the ones used for ZnO/PVDFTrFE thin films. As mentioned earlier, ZnO/PVDF-TrFE thin films were directly deposited onto slide substrates and were tested as is. However, since the commercial PVDF-TrFE films required additional processing for depositing electrodes, they were epoxy-mounted onto the glass substrates. The difference in mounting procedure could have caused noise and losses in the control specimen measurements. A total of eight prototype thin film slides (i.e., 24 unique specimens) were fabricated and tested in this study.

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Figure 2. In part (a), the sample is kept at 70ºC in silicone oil while 50 MV-m-1 is applied. Part (b) shows the equipment set-up where a DC source is amplified to the correct voltage with an oscilloscope for monitoring.

2.2High-Voltage Poling

Poling is a process to create or enhance piezoelectric properties in materials. PVDF-TrFE and ZnO are inherently piezoelectric, but when these nanocomposites were formed using techniques such as spin coating (Section 2.1), their electric domains become distributed randomly throughout the material. On the other hand, when a high voltage is applied through the film thickness and above its Curie temperature, most electrical domains will realign in the direction of the electric field (Figure 2a) [29]. Electric domain alignment greatly enhances bulk film piezoelectricity.

In this study, the ZnO/PVDF-TrFE specimens were poled by applying a 50 MV-m-1 high electric field. The voltage source was provided by an Agilent E3642A DC power supply and then amplified by an Ultravolt 5HVA24 high voltage module as shown in Figure 2b. The magnitude of voltage required for poling at the desired electric field was determined by knowing the thickness of the film, which was obtained by measuring them beforehand using a Dektak IIA Profileometer. The magnitude of the high electric field was selected because the films rarely broke down at this voltage, and the applied field was high enough such that it promoted alignment of the electric domains. During poling, each specimen was immersed in silicone oil (to prevent electrical arcing and to provide a stable surrounding temperature) and maintained at 70 ºC using a hot plate. The applied electric field was slowly increased to up to 50 MV-m-1 (i.e., approximately 500 V) in order to prevent breakdown. Poling occurred for 2 min at 70 ºC, and then the temperature was reduced to room temperature while keeping the same high voltage in order to lock in the electric domains [30].

3.PIEZOELECTRIC CHARACTERIZATION TEST PROCEDURE

3.1Ferroelectric Testing

Ferroelectric testing allows one to characterize a piezoelectric material’s remnant polarization, which is directly correlated with bulk film piezoelectric sensitivity. Remnant polarization in piezoelectric materials is simply the material’s electric polarization corresponding to zero ambient electric field. Ferroelectric testing, when conducted using low frequency alternating current (AC) signals, causes electric domains to switch orientations at that same frequency. When more electric domains are able to change their orientations, remnant polarization increases.

Ferroelectric testing was conducted by using the aforementioned high voltage amplifier in conjunction with an Agilent 33250A arbitrary waveform function generator connected to a ZnO/PVDF-TrFE film. A similar testing procedure was also reported in a previous study by Dodds et al. [31]. The input and ground probes were connected to the film’s topand bottom-electrodes, and the function generator (and amplifier) was commanded to supply a high voltage AC signal at a frequency of 10 Hz. To allow dipoles to reorient slowly, the voltage was increased from 0 MV-m-1 to 70 MV-m-1 over a period of time, waiting for stabilization of the signal to occur after every increase of 5 MV-m-1. It should be mentioned that film breakdown occurred for certain specimens, and great care was required to ensure that the film do not become electrically shorted during poling. The output response of the film was recorded using a Sawyer-Tower circuit [32]. An Agilent MSO8104A mixed-signal oscilloscope recorded the film’s response. The electric displacement (D) was calculated using the voltage response across the parallel reference capacitor as shown in Equation 1:

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Figure 3. (a) The hammer impact testing apparatus is shown and has been used for validation of the ZnO/PVDF-TrFE thin film’s sensing performance. (b) A picture of the actual experimental test setup is also shown.

The first term in Equation 1 can be neglected, because the dielectric constant (ε) is relatively small. C is the capacitance of the reference capacitor, V is the voltage measured across that capacitor (obtained from the oscilloscope), and A is the surface area of the poling electrode.

Using the known applied electric field (E) and the calculated electric displacement or polarization (Equation 1), the results can be represented as D-E hysteresis curves as will be discussed later in Section 4. Using the D-E hysteresis loops, remnant polarization is determined by identifying the polarization when the electric field across the film is zero. Higher remnant polarization indicates higher piezoelectricity, where it indicates that more electric domains have reoriented themselves in the preferred direction (i.e., parallel with the poling direction) [33].

3.2Sensing Validation using Hammer Impact Tests

A customized hammer striking system was fabricated as shown in Figure 3, and the apparatus was used to evaluate the sensing performance of ZnO/PVDF-TrFE thin films. The specimens used were PVDF-based films deposited or mounted onto glass microscope slide substrates. On one end of the slide was the as-deposited and poled ZnO/PVDF-TrFE nanocomposite. A commercially poled PVDF-TrFE thin film was affixed onto the opposite end of the glass slide following the procedure mentioned in Section 2.1. The slide (and films) was mounted in the hammer impact apparatus’s aluminum mount, and plastic clamps held the substrate in place throughout testing; plastic clamps were used because they are electrically insulating. The substrate is then adjusted and positioned on the mount such that the hammer struck a location on the substrate that was equidistant from both the commercial and the nanocomposite piezoelectric sensors. The average distance to both of the electrode edges was approximately 2 cm.

Measurement of the commercial and nanocomposite sensor responses was conducted by recording sample voltage time histories using an Agilent MSO8104A mixed-signal oscilloscope. Again, the oscilloscope measured the voltage across the thickness of both sensors and can record both responses simultaneously using two separate measurement channels. The oscilloscope was set and commanded such that measurement began immediately following the first impact (i.e., based on the first spike in voltage) and stopped immediately thereafter so that no bounces of the hammer were recorded. Standard hammer drops were from 6º above the horizontal. Pre-machining through-holes and quick-release pins in the hammer impact apparatus ensured reproducibility of hammer drop angles and heights. Thus, during testing, the hammer was held by the release pin at a pre-determined angle, the pin was released, and the hammer dropped.

In addition to the standard 6º hammer impact drop, drops ranging from 2.5º up to 12º (i.e., limited by the minimum force necessary to ensure a voltage response and the amount of force that would break the glass slide) were also conducted. The drop angles were varied so as to vary the amount of impact energy delivered to the substrate. The amount of impact energy was calculated based on the potential energy (PE) of the hammer (using PE=mgh). The voltage time history and maximum voltage output obtained from the first impact was recorded similar to the aforementioned procedure.

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Figure 4: The D-E hysteresis loops for a representative ZnO/PVDF-TrFE thin film subjected to different applied electric fields are shown.

4.RESULTS AND DISCUSSIONS

4.1Ferroelectric Characterization Results

As mentioned in Section 3.1, loops of the electric displacement (D) versus the applied high electric field (E) have been measured using a modified Sawyer-Tower Circuit. Each film specimen has been excited by high voltage AC signals at a frequency of 10 Hz. Figure 4 plots the D-E hysteresis loops of a representative ZnO/PVDF-TrFE thin film subjected to different magnitudes of applied electric field. From Figure 4, it can be seen that the D-E hysteresis loops increase in size and area with increasing applied electric fields. More importantly (and as mentioned briefly in Section 3.1), the film’s remnant polarization can be extracted by identifying the electric displacement or polarization corresponding to zero applied electric field (i.e., the y-intercept). First, from the results shown in Figure 4, only a linear D-E response is seen when the specimen is excited by lower electric fields (e.g., 38 MV-m-1). In contrast, at applied electric fields greater than ~40 MV-m-1, hysteresis and remnant polarization start to appear. This result indicates that above a certain minimum voltage, electrical domains start shifting and the dipoles start reorienting themselves.

As mentioned before, increasing remnant polarization is correlated to increasing piezoelectric sensitivity. All of the specimens fabricated have been subjected to ferroelectric characterization tests, and the remnant polarization for maximum applied electric fields of 38, 47, 56, and 63 MV-m-1 are 0.16, 0.72, 4.4, and 10 mC-m-2, respectively. Others have reported remnant polarization of approximately 10 mC-m-2 for pure PVDF at 80 MV-m-1, and this is comparable to the results obtained for the ZnO/PVDF-TrFE thin films tested in this study [34]. However, it should be mentioned that pure PVDF thin films required an applied electric field of 80 MV-m-1 for achieving a remnant polarization of 10 mC-m-2, whereas the same can be achieved by applying 63 MV-m-1 to the ZnO-based thin films. This suggests that greater remnant polarization (or essentially bulk film piezoelectricity) could be achieved if the films are subjected to higher applied electric fields. By increasing the magnitude of the applied electric field, more electric domains will be able to reorient themselves in the preferred direction (thickness direction). It would also make sense that the remnant polarization gains would start to level off at higher voltages, because fewer dipoles will be available for reorientation and alignment. Unfortunately, this possibility was not explored in this study, because many films broke down at these higher applied electric fields. Future studies will focus on improving film quality and for characterizing their maximum remnant polarization.

4.2Sensing Validation Results from Hammer Impact Tests

Recall from Section 3.2 that the sensing performance of ZnO/PVDF-TrFE thin films has been characterized by hammer impact testing. Here, a hammer has been released and dropped at pre-defined heights (or angles) and has been used to strike a substrate. The generated surface acoustic waves have been monitored and measured using the prototype

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impact, and in addition, exhibited greater piezoelectricity and higher maximum normalized voltage response. Moreover, different impact energies were also explored, and the film also demonstrated linear sensitivity to impact energy. Future work will focus on using these nanocomposites for actuation and damage detection.

6. ACKNOWLEDGEMENTS

The authors gratefully acknowledge the support provided by the UC-MEXUS-CONACYT program, as well as additional support from the College of Engineering, University of California, Davis. The authors also thank the Northern California Nanotechnology Center for assistance with film thickness measurement and e-beam deposition.

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