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

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composites have also been characterized and investigated for their poling properties [55]. Thostenson and Chou [56] have found that carbon nanotubes increase the strength of nanocomposites, which may prove useful when constructing piezoelectric nanocomposites for SHM. Finally, AlN/diamond composites have been investigated for their fast guided-wave propagation characteristics [57].

ZnO is a piezoelectric material that has been used for structural health monitoring applications for the past few decades. Initially, ZnO has been fabricated into thin films and used as an actuator. It has been characterized for sensing and actuation [58] and been used as a bulk acoustic wave generator [59, 60], as a dynamic strain sensor [61], and for detecting damage in composite structures [62]. More recently, ZnO has become a more relevant material in a number of applications, including SHM, due to its possible complex nano-architectures.

Advancements in the nanotechnology domain have motivated the development of piezoelectric nanocomposites, particularly those featuring ZnO-based nanomaterials (e.g., nanowires, nanobelts, nanosprings, sheets, and nanotubes) [63-65]. ZnO possess a wurtzite crystal structure, which is pyroelectric in nature. Amad et al. [66] have characterized the optical, electrical, and mechanical properties of ZnO nanoparticles, while Huang et al. [67] have characterized the mechanical properties of nanowires [66]. The piezoelectricity of ZnO-based nanocomposites has been demonstrated and it has been used as a dynamic strain sensor and energy harvester [68, 69]. A ZnO-based composite sensor, based on inserting ZnO nanoparticles into commercial paper, has been shown through basic characteristic studies (e.g. lab testing employing tensile loading on a steel beam) to exhibit excellent mechanical flexibility [70].

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One interesting composite that has been recently researched is ZnO incorporated within PVDF [71]. Both of these materials are piezoelectric and can enhance each other’s properties. The nanocomposite’s piezoelectric performance has been characterized using impedance analysis, ultraviolet-visible analysis, and X-ray diffraction [72, 73]. Fabrication has extended beyond solvent casting and faster techniques such as electro-spinning ZnO onto PVDF mats or dipping ZnO mats into PVDF solutions have also been achieved [74]. However, prior to their use for SHM, a more thorough characterization of key properties needs to take place, and a more detailed analysis of their sensing and actuation performance in laboratory settings is needed.

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Chapter 3: Material Fabrication and Ferroelectric Testing

3.1 Introduction

The objective of this chapter is to present an experimental approach for fabricating piezoelectric nanocomposites using PVDF-TrFE and ZnO nanoparticles. Throughout this thesis, these thin films will be referred to as PVDF-TrFE/ZnO films, transducers, sensors, and/or actuators, depending on the context. First, nanocomposite fabrication using spin coating is discussed in detail. Next, ferroelectric characterization of the newly fabricated material is discussed. It has been determined that increasing quantities of ZnO within a PVDF film gives increasing piezoelectricity up to a point. Finally, the method of permanently enhancing the piezoelectric properties of the film through the application of a high electric field (poling) is discussed.

3.2 Nanocomposite Fabrication

PVDF-TrFE/ZnO nanocomposites were fabricated by spin coating of dispersed nanoparticle solutions. The following raw materials used for film fabrication were identified. ZnO nanoparticles (spherical, 20 nm diameter) were obtained from Nano Amor and PVDF-TrFE (65:35) powder was obtained from Measurement Specialties. Commercial poled PVDF-TrFE thin films (28 m thick) were purchased from Measurement Specialties and were used for comparison purposes. The solvent used in this study, methyl ethyl ketone (MEK), along with disposable laboratory supplies, glass microscope slide substrates, and other materials, were from Fisher Scientific. Electrode components such as conductive copper tape and colloidal silver paste were purchased

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from Ted Pella. Finally, discrete circuit elements (such as resistors and capacitors) and electronic components used for assembling the Sawyer-Tower and poling circuits were from Digi-Key.

An overview of the film fabrication procedure using spin coating has been illustrated schematically in Figure 3. First, the PVDF-TrFE copolymer was completely dissolved in the polar solvent MEK using a magnetic stirrer and by heating the solution to 50 ºC [73]. Secondly, ZnO nanoparticles were added to the PVDF-TrFE solution at appropriate proportions so as to obtain 0 to 20 wt.% ZnO solutions. A total of five unique sets of solutions were prepared by varying ZnO concentrations in 5 wt.% increments. It should be mentioned that films constructed with a ZnO concentration of greater than 20 wt.% were too brittle for practical use. Each PVDF-TrFE/ZnO solution was prepared in 20 mL batches and stored in scintillation vials, which was then subjected to bath ultrasonication (135 W, 42 kHz) to disperse the nanoparticles. Sonication was performed for 180 min or until adequate dispersion was visually observed. Without ultrasonication, the ZnO nanoparticles agglomerated and were unsuitable for film fabrication. This agglomeration meant that the unique properties and high piezoelectricity offered by ZnO nanoparticles could not be fully realized.

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Figure 3: Fabrication Flowchart

To fabricate PVDF-TrFE/ZnO thin films, the solution was bath ultrasonicated (a). Next, the solutions were spin coated to form thin films on electrode-coated substrates, annealing after each layer (b). Finally, e-beam deposition was used to deposit the top electrodes (c).

The next step in the fabrication process was spin coating thin films using the dispersed ZnO-based solutions. Prior to spin coating, an electron-beam (e-beam) evaporator was used to deposit a thin (~150 nm) aluminum (Al) electrode onto glass microscope slides or flexible polymer sheets (0.01 mm thick). The Al-coated substrate was then mounted in a Laurell spin processor where spin coating was performed to deposit layers of PVDF-TrFE solution. Spinning was performed first at 400 rpm for 5 s then directly followed by spinning at 3,000 rpm for 30 s. This two-step procedure was adopted with the first step spreading the solution so that it covered the whole slide and with the next step spinning the solution to the desired thickness of 3 to 4 m per layer (i.e., when 150 mg-ml-1 of the PVDF-TrFE copolymer in MEK was employed; a higher concentration produced a thicker film). Upon spin coating, the slide was annealed at 140 ºC for 2 min before additional layers were spin coated for obtaining thicker films. Once

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the desired film thickness was achieved, the film underwent a final annealing step at 140

C for 2 h; this final annealing step was important for safeguarding against film breakdown during high-voltage poling. Finally, e-beam deposition was employed (in conjunction with a plastic mask) for depositing three rectangular Al electrodes (each approximately 1 1 cm2) onto the film surface. Extraneous film between each electrode was removed by selective dissolving using MEK or by mechanical etching using a razor blade. Thus, this fabrication procedure yielded three PVDF-TrFE/ZnO thin film specimens for each spin-coated substrate.

The morphology of films with different ZnO concentrations was examined using scanning electron microscopy (SEM), and the results are presented in Figure 4. It was seen that films with higher ZnO concentrations have noticeably more ZnO agglomerates. It was observed that even the film with 0 wt.% ZnO has some clumps, presumably of PVDF. This indicated that the film construction process did not produce a perfectly uniform film, which undoubtedly gave rise to experimental error and variability between sample sets.

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The SEM images of films with (a) 0, (b) 5, (c) 10, and (d) 15 wt.% ZnO nanoparticles show the distribution of ZnO agglomerates within the PVDF-TrFE matrix.

Some of these films were lifted off of their substrates in order to make freestanding films. First, the edges of the films were removed by mechanical etching. The films and glass substrates were then immersed in a 1 vol.% hydrofluoric acid (HF) solution. The acid slowly undercut and etched away the glass substrate beneath the PVDF-TrFE/ZnO film, thereby allowing the film to peel off the substrate. The film floated

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to the surface of the HF solution and was carefully removed with tweezers. The etched film was successively dropped in deionized (DI) water baths for rinsing out remnant acid on the film surface. This process completed the fabrication of freestanding PVDFTrFE/ZnO nanocomposites used in this research.

3.3 Ferroelectric Testing

It has been mentioned in Chapter 2 that ferroelectric materials are pyroelectric materials which can have their polar domains shifted and even reversed under the application of a strong-enough electric field. The extent of this change with applied electric field has been measured and relates directly to piezoelectricity. The objective of this section is to characterize the ferroelectric properties of thin films fabricated with different concentrations of ZnO nanoparticles. Briefly, these ferroelectric characterization tests were performed by applying a high voltage AC signal to the thin films in conjunction with a Sawyer-Tower circuit and film response was measured using a mixed-signal digital oscilloscope.

3.3.1 Ferroelectric Setup

The piezoelectric potential of PVDF-TrFE/ZnO thin films was characterized by measuring their electric displacement or polarization (D) in response to a high AC electric field following the procedure previously adopted by Dodds et al. [71]. A 10 Hz AC waveform was applied across the thickness of the film using an Agilent 33210A arbitrary waveform generator and an Ultravolt 5HVA24 high voltage module (amplifier). Equation

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3.1 provided a way to calculate the maximum applied electric field.

E is the applied electric field across the film, V is the applied voltage, and t is the film thickness. E varied between around 40 MV-m-1 and above 100 MV-m-1 for this characterization experiment. A Dektak IIA profilometer was used to measure the thickness of each film and the voltage required to achieve the proper applied electric field was calculated for each specimen. The applied electric field and the corresponding film polarization were measured using a Sawyer-Tower circuit and an Agilent MSO8104A mixed-signal oscilloscope (Figure 5). The resulting D-E hysteresis responses of the PVDF-TrFE/ZnO films were used for determining their remnant polarizations, which are indicative of the piezoelectric potential of the bulk nanocomposite. It should also be mentioned that the films could be reused after film breakdown (i.e., when the electric field across the material become so high that it electrically shorted) by re-annealing and re-poling the film [75].

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Figure 5: Ferroelectric Setup

Ferroelectric characterization was achieved by applying an amplified AC voltage across a

film specimen. A picture of the experimental setup is shown.

3.3.2 Ferroelectric Results

A Sawyer-Tower circuit was able to effectively measure the D-E hysteresis loops for the various PVDF-TrFE/ZnO thin films tested. First, the maximum voltage amplitude was increased over time for each specimen and D-E hysteresis loops corresponding to different maximum voltages were recorded. Figure 6 shows an example of the D-E hysteretic responses of a 10 wt.% PVDF-TrFE/ZnO film subjected to different magnitudes of excitation. This figure clearly shows that higher maximum electric fields increased its hysteresis response. This increased hysteresis indicated that more domains within the material were changing their orientations, and as a result, the material was becoming more piezoelectric. A balance must be achieved between reorienting the maximum number of domains and being able to electrically excite every film consistently without having a significant number of breakdowns.