Статьи на перевод PVDF_P(VDF-TrFE) / 2012-Dodds,J-Thesis (Development of Piezoelectric Zinc Oxide Nanoparticle-Poly(Vinylidene Fluoride))

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Figure 6: Hysteresis Output, Increasing Voltage

Representative D-E hysteresis loops for one film with increasing amounts of voltages are shown here. It can be seen that increasing the maximum voltage leads to an increase in the remnant polarization.

Based on the previous results, it was decided to consistently apply voltages up to 50 MV-m-1 to maximize film piezoelectricity while minimizing the number of film breakdowns. PVDF-TrFE/ZnO thin films of different ZnO concentrations were subjected to high voltage AC signals for characterizing each of their electric polarization-to-electric field hysteretic responses. Representative D-E hysteresis loops corresponding to each ZnO concentration are plotted and overlaid in Figure 7. First, it can be seen that the hysteresis loops transitions from an approximately linear line for 0 wt.% ZnO-based thin films to increasingly larger hysteresis loops with increasing ZnO concentrations. This linearity indicates that no domain shifting was occurring at that poling voltage for the 0

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wt.% ZnO film; its piezoelectricity was not being improved by poling. More hysteresis in films with higher ZnO concentrations indicates higher piezoelectricity.

Secondly, remnant polarization, an important indicator of bulk film piezoelectricity, was determined by calculating the intersection of the D-E hysteresis loop with the electric displacement axis. In other words, remnant polarization is the electric displacement when there is no electric field applied across the material. Theoretically, remnant polarization indicates the distance between the center of mass of the positive and the negative charges in the material or an equivalent dipole moment at

Figure 7: Hysteresis Output, Changing ZnO Content

Representative D-E hysteresis loops for films of different ZnO concentrations have been obtained after ferroelectric characterization and are overlaid as shown here. The maximum applied electric field is 50 MV-m-1. The remnant polarization increases with greater ZnO weight content.

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zero electric field. The larger this charge separation, the more charge is generated across the material from the same amount of force [26]. It was shown that there is a linear relationship between remnant polarization and a material’s piezoelectricity [76].

The remnant polarization for representative films, as calculated from Figure 7, increased from approximately 0 for 0 wt.% ZnO up to 0.0098 C-m-2 for 20 wt.% ZnO films. Films with 5, 10, and 15 wt.% ZnO showed average remnant polarizations of 0.0008, 0.0036, and 0.0078 C-m-2, respectively. It is clear that remnant polarization, and similarly bulk film piezoelectricity, increased in tandem with greater concentrations of ZnO nanoparticles embedded in the PVDF-TrFE polymer matrix. Because these thin films were subjected to the same electrical excitation (i.e., same voltage, frequency, and ambient conditions), the change in remnant polarization was due to the increasing ZnO content. These results suggested that in addition to aligning the PVDF-TrFE electrical domains in the polymer matrix (which was the same for all specimens), bulk film piezoelectricity increased with increasing ZnO concentrations because more piezoelectric ZnO nanoparticles participated and were also being aligned to the applied electric field.

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Table 1: Remnant Polarization, Changing ZnO Content

Further investigating this same phenomenon revealed similar trends for films cycled at higher voltages (Table 1). It should be noted that the maximum breakdown voltage of the film decreased with increasing ZnO content, meaning that there is at least one negative tradeoff to increasing the percentage of ZnO in the material. This suggests that films with lower ZnO concentrations can be poled at higher magnitudes than those with higher ZnO concentrations; however, this phenomenon is not investigated beyond this initial inquiry for this thesis.

3.4 High Electric Field Poling

The films’ piezoelectric properties were enhanced by high electric field poling, following the experimental setup similar to that described in Section 3.3.1. After spin coating of the PVDF-TrFE/ZnO thin films, their electrical domains were randomly oriented and bulk film piezoelectricity was low. However, the steady application of a high voltage direct current (DC) field would align the majority of the electrical domains in the same direction, thereby enhancing piezoelectricity; this is in contrast with ferroelectric testing discussed in Section 3.3, which used an AC field for characterization and not for

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a permanent and consistent material change [76]. For this experiment, PVDF-TrFE/ZnO thin films were immersed in silicone oil and heated to 75 °C [77, 78]. Then, the Ultravolt amplifier was employed for amplifying the DC voltage signal generated by an Agilent E3642A DC power supply, up to 50 MV-m-1. Each film was poled by applying the high electric field across its thickness, changed appropriately as per the film’s thickness in order to maintain an electrical field of 50 MV-m-1. The magnitude of the DC electric field was chosen based on other work involving poling of piezoelectric polymers [78], and it was also found that high concentrations (15 and 20 wt.% ZnO) of PVDF-TrFE/ZnO thin films broke down when given higher applied electric fields [71].

During poling, the oscilloscope and a Sawyer-Tower circuit were used together to monitor the voltage across each specimen. The film temperature, maintained at 75 °C during poling, was subsequently cooled to room temperature before removal of the applied electric field to ensure that the aligned domains were locked in their poled positions [55]. Lastly, film capacitance was measured before and after poling to ensure that the measurements remained the same, that the films remained intact, and that the films had not electrically broken down. After this initial film characterization and enhancement by poling, films needed to be further characterized specifically for sensing and actuation for SHM applications.

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Chapter 4: Sensing

4.1 Introduction

The objective of this chapter is to characterize the sensing performance of poled PVDF-TrFE/ZnO thin films. Commercial poled PVDF-TrFE films were employed as the control and served as the baseline for comparison. The films’ sensing performance was validated using a variety of experiments, including hammer impact testing, free vibration testing, and load frame testing. Hammer impact testing involved using a hammer fixture to consistently excite the film; this allowed for easy comparison between different ZnO concentrations in films. Free vibration testing involved adhering the free-standing film to a cantilever and exciting that cantilever with an initial displacement; this was performed to validate the film as a dynamic strain sensor. Load frame testing employed a load frame to sinusoidally displace the thin film under test; this was done to validate piezoelectricity in the 1-3 direction of the film. All tests measured the voltage output of the film.

4.2 Hammer Impact Sensing Testing

Hammer impact testing, using a simple lab-fabricated device, provided a quick and consistent assessment of the PVDF-TrFE/ZnO films’ piezoelectric sensing response. The device used a hammer to strike a test substrate, which then generated a guide wave that radiated outwards from the point of impact. Both ZnO-based and commercial PVDF-TrFE thin films affixed onto the substrate were electrically interrogated for measuring the generated guided waves. This hammer impact device

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was also able to provide a range of impact forces and a range of impact distances in order to determine how sensor response changed with changing impact parameters.

4.2.1 Hammer Impact Sensing Setup

In order to understand this thin film’s response to guided wave excitations, a setup needed to be constructed which could apply a consistent amount of excitation to the film. The hammer impact apparatus is shown in Figure 8. The hammer was released at certain angles (or heights) relative to the surface of a test substrate. The impact apparatus was designed such that the same hammer impact strikes could be reproduced over numerous tests, simply by controlling the hammer release angle, release height, and impact location.

Validation of PVDF-TrFE/ZnO thin film piezoelectricity and comparison of sensing performances for films with different concentrations of nanoparticles were conducted using hammer impact tests. Spin coating and poling (at 50 MV-m-1) based on the procedures mentioned in Sections 3.2 and 3.4 were employed for preparing a PVDFTrFE/ZnO thin film to be mounted onto the surface of one end of a glass microscope slide substrate. A commercial poled PVDF-TrFE thin film, controlled to be the same size as the test specimen, was mounted onto the surface of the opposite end of the substrate using a cyanoacrylate adhesive. Finally, the entire slide, including the attached films, was mounted in the customized hammer impact test apparatus and secured in the specimen holder as shown in Figure 8.

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Figure 8: Hammer Validation Setup

This schematic and picture of the hammer impact testing apparatus demonstrate how this device provided a consistent force to the same impact location and recorded this information using the electrical probes.

The hammer was dropped such that impact occurred halfway between the two films. It should be mentioned that the impact location did not have any piezoelectric transducers instrumented. In this configuration, vibrations induced in the center of the slide allowed guided waves to propagate to either end of the test structure. The guided waves arrived at the piezoelectric transducers with the same phase and magnitudes. After the hammer was dropped at a 6º angle above the horizontal, the voltage time history responses of both films were simultaneously measured using an Agilent oscilloscope with electrodes across the thickness of the films. To avoid extra measurements associated with hammer bouncing, the oscilloscope trigger was conditioned to measure only the initial voltage response of the film. Since the films are piezoelectric, no input power was supplied and only the films’ voltage responses were recorded. The magnitude of voltage generated was directly correlated to bulk film

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piezoelectricity, where higher generated voltages suggested greater thin film piezoelectricity.

In addition, the sensitivity of the film with regard to different impact energies was assessed. Different energies were generated using the impact device to release the hammer from various angles depending on the location of the pin. Impact energy was calculated using potential energy of the hammer directly above the point of impact (PE=mgh). To prevent cracking of the glass slide substrate, the maximum impact angle was 12º; to provide a detectable impact, the minimum impact angle was 2.5º.

The distance between hammer impact location and the sensing film was also investigated for its effect on the maximum voltage response. The specimen holder was moved such that the hammer impacted the substrate at a location ranging from 10 to 25 mm away from the edge of the specimen electrode (in increments of 2.5 mm), at 2.5º relative to the horizontal.

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4.2.2 Hammer Impact Sensing Results

Figure 9, a representative time history response for hammer impact testing, demonstrates that increasing ZnO content of a film generally provided increased voltage sensitivity to elastic waves in the substrate. This is consistent with the increase in remnant polarization as was demonstrated with the hysteretic characterization of these nanocomposites (Section 3.3.2). A higher initial voltage peak response indicates that the film was more sensitive to surface waves and hence exhibited greater piezoelectricity.

From Figure 9, it is noted that the decay of the voltage response is proportional with respect to different ZnO concentrations. Figure 10 plots the average and standard

Figure 9: Hammer Impact Time Results

The voltage time-history responses of representative thin films with different ZnO concentrations subjected to hammer impact testing are overlaid. The results suggest that increasing ZnO concentration increases the film’s sensitivity to hammer impact.