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

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5.4 Actuator Fabrication and Characteristics

Piezopolymer actuators were constructed via spin coating in a manner similar to that of Section 3.2. Its electrode patterning and film thickness were optimized for maximizing actuation power.

5.4.1 Piezopolymer Actuator Design

Finger-based actuator construction has been determined to be the best method to induce actuation on a structural surface using the PVDF-TrFE/ZnO thin film. A transducer operating as simply a plane electrode through the thickness of the film is not be able to take advantage of the specific mode selectivity involved in a fingered electrode. In fact, research conducted by Jin et al. [103] has found that an IDT is more sensitive (i.e., it can give a greater SNR for pitch-catch testing in a specific frequency range) than a plane electrode, when fabricated to be of similar size and using the same structural material. It has been found that for some monitoring scenarios, planar piezoceramic transducers have been better able to detect cracks for SHM, but fingered transducers (i.e., IDTs in this case) have been more effective at specific frequencies [46]. Also, fingered electrode actuators have been able to excite surface waves in a specific direction (perpendicular to the fingers), so this is useful for pitch-catch measurements made at specific locations on a structure.

Recall that fingered electrodes are constructed either as comb or IDT electrodes. For this thesis, comb actuator construction has been chosen over IDT construction due to its relative simplicity. IDT construction requires complete separation of one finger from another with detailed etching, which adds complexity and uncertainty to the process

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without provided significant advantages in actuation power and sensitivity. Additionally, IDTs also require adjacent comb electrodes to be excited 180° out-of-phase with one another, which adds electrical circuitry to the pitch-catch setup [84]. Actuator geometrical adjustments can be made in order to optimize single modes for actuation, thereby making it easier to measure certain kinds of damage [104]. Also, increasing the number of sine wave burst peaks used to excite the actuator can increase the SNR, but having too many bursts can lead to problems with resolving closely spaced damages in the structure [97]

In this research, a comb actuator design was employed to form PVDF-TrFE/ZnO actuators. Prior to reaching the final actuator design, a parametric study was conducted to assess the optimal finger spacing. The design of the finger widths followed the principle that finger widths should be half of the center-to-center finger spacing. This finger width rule-of-thumb was determined experimentally by Rose et al. [102]. In addition, it was found that fingers with spacing greater than 1 mm could not achieve the desired actuation characteristics. On the other hand, electrodes with a spacing of less than 0.225 mm were difficult to construct using the photolithographic fabrication processes employed herein (as will be described in Section 5.4.2). Therefore, it was determined that PVDF-TrFE/ZnO films could use finger spacing ranging from 0.225 to 0.9 mm. The number of fingers allowed with these dimensions and spacing was approximately between 150 and 40, respectively. In order for the transducer to excite a specific wavelength, an infinite number of fingers would have been ideal, but practical sizing and fabrication considerations limited the actual number of fingers that can be constructed [87].

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Figure 21: Actuator Construction

The construction of a PVDF-TrFE/ZnO fingered actuator is detailed in the above schematic.

5.4.2 Piezopolymer Actuator Fabrication

Fabrication of the PVDF-TrFE/ZnO actuator was based on depositing comb electrodes and the piezoelectric nanocomposite onto 125 m-thick Kapton films (see Figure 21). Kapton film, obtained from DuPont, is a thin flexible polymer that can withstand a wide range of temperatures without changing its properties. It has been used as the substrate for flexible printed circuit boards, thus making it an appropriate choice for this application. First, the Kapton film was cut to an appropriate size (~3 3 in2 or 75

75 mm2) and cleaned thoroughly with isopropanol. Second, it was temporarily adhered to a wafer using a thin layer of DI water. Bubbles underneath the film were removed by smoothing the film surface with a Teflon straight-edge.

Next, photolithography, in concert with copper sputtering, was performed to create a variety of comb patterns on the Kapton substrate. A layer of LOL2000 polymer

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was spin coated onto the Kapton substrate, annealed, and then followed by spinning another layer of S1813 photoresist. The film was then masked and exposed to UV light, using a mask pattern with a variety of comb configurations with different finger spacing. The photoresist was chemically developed in CD-26, which undercut into the LOL2000 layer and created a surface for the final etchant to etch into. The development process involved placing the film in a CD-26 bath and gently agitating it until red streamers stopped coming off of the film. To finish the development process, the film was then removed and dipped into a clean bath of CD-26 for 10 s, before being transferred to a DI water bath.

Next, sputtering was employed to deposit a 250 nm-thick layer of copper on top of the patterned Kapton film. Upon deposition, unwanted copper was lifted off using acetone, which etched into the S1813 photoresist. The remaining exposed LOL2000 was lifted off with cyclopentanone. The final product was a substrate of Kapton with six comb electrodes deposited on it. The substrate was used as is for spin coating PVDFTrFE thin films and for actuator fabrication. The photolithography film fabrication process can be seen schematically in Figure 22.

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Figure 22: Photolithography

The photolithography process to construct a PVDF-TrFE/ZnO film is shown in this schematic. For a more general overview of the fabrication process, please refer to Figure 21.

After depositing the copper comb electrode onto the Kapton substrate, a 5 wt.% ZnO in PVDF-TrFE solution was spin coated on top. It should be mentioned that, prior to spin coating, the edges of the electrodes were masked such that the bottom electrodes could be exposed for electrical connectivity even after spin coating. This ZnO concentration was chosen to strike a balance between higher bulk film piezoelectricity and mechanical robustness (i.e., to avoid attaining a film that is too brittle). Unlike sensor fabrication, a higher concentration of PVDF-TrFE solution (i.e., 30 wt.% PVDF-TrFE in MEK) was used to create a thicker nanocomposite. After spin coating, this yielded PVDF-TrFE/ZnO films with an average thickness of ~10 μm. The film was then annealed at 100 ºC for 60 s. Finally, the edges of the film were again masked to prevent shorting,

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Figure 23: Final Actuator Product

This schematic illustrates the PVDF-TrFE/ZnO actuator thin film. The plan view displays the

plane top electrode as cut-away.

and a plane copper electrode was sputtered on top of the film (i.e., in order to enable film poling across its thickness).

The final procedure entailed making electrical connections to the top and bottom electrodes. After removing the aforementioned masks, copper tape was connected to both the top and bottom electrodes. Silver paste was painted over both connections to minimize contact impedance. Electrical tape was affixed onto each connection to ensure their mechanical robustness and to prevent electrodes from being damaged during testing. The completed film is illustrated schematically in Figure 23.

High-field electrical poling was conducted in the same manner as described in Section 3.4 and was conducted across the thickness of the film. Keeping in mind the piezoelectric notation from before, the direction perpendicular to the Kapton substrate was the 3 (poling) direction.

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5.5 Pitch-Catch Active Sensing Experimental Setup

A pitch-catch active sensing methodology, where guided waves traveled one way from the actuator to the sensor, was chosen for validating PVDF-TrFE/ZnO thin film actuation capabilities due to its relative simplicity. For this testing, an aluminum pipe (4 mm thick, 100 mm outer diameter, and 584 mm long) was employed as the structural substrate to simulate a real-life SHM application (i.e., of buried metallic pipelines). Pipes are well suited for piezopolymer transducers, as these transducers are flexible enough to conform to the pipe geometry and curvature. Using this setup, the actuator “pinched” the structure to which it was attached and the resulting waves traveled through the pipe, using it as a waveguide. The guided wave traveled through the pipe and was received by a piezoelectric sensor on the other end of the pipe (from 0.25 to 0.4 m away). For the sensor, this series of tests used a commercial piezoelectric Macro Fiber Composite (MFC) sensor from Smart Material. The entire setup can be seen in Figure 24.

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Figure 24: Pitch-Catch Physical Setup

The physical setup of the pitch-catch test is displayed here, with the MFC sensor on the left and the PVDF-TrFE/ZnO actuator on the right.

The PVDF-TrFE/ZnO actuators were driven and excited by a 30 kHz five-cycle sine wave tone burst. A frequency of 30 kHz was parametrically determined to be the optimal frequency of excitation for this specific setup. In order for a piezoelectric thin film to send powerful-enough waves through a structure for SHM, a Lab Systems A-303 amplifier was used to amplify an electric signal up to 100 V at 30 kHz at 40 W. The initial input signal, a five-peaked tone burst, was provided by an Agilent 33210A arbitrary waveform generator. A Hamming window was employed to modulate the original signal (a 30 kHz sine wave) to center on a specific frequency. This burst was amplified with the Lab Systems amplifier to 200 V peak-to-peak (see Figure 25). The function generator was commanded to excite the actuator every 100 ms. This time lag allowed the sensor to measure the response and each sensor response did not interfere with previous signals. Separate responses were averaged to improve measurement SNR, as is described later in more detail.

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Figure 25: Voltage Impulse on the PVDF-TrFE/ZnO Actuator

The excitation voltage shown here was used to excite the PVDF-TrFE/ZnO film as an actuator. This particular pulse is six periods long and has a center frequency of 30 kHz after filtering with a Hamming window.

In order to obtain high-resolution sensor measurements from the MFC transducers, certain adjustments to the pitch-catch setup were required. First, the oscilloscope was grounded to the structure under test. Without this grounding, a relatively large amount of EMI noise was measured. This precaution was also necessary to prevent overloading the oscilloscope. Second, the oscilloscope was commanded to measure 64 consecutive measurements and then average the raw data to improve the sensor’s SNR. The oscilloscope was also set to sample at high-resolution mode. In addition, due to a low level of ground noise present in the electrical system (~60 Hz), a high pass filter function was also performed to eliminate this low frequency noise. Lastly, a low-pass filter set to 30 MHz was also employed.

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Figure 26: Non-Permanent Damage

A hose clamp was used to induce non-permanent damage. This clamp disrupted acoustic waves in the aluminum pipe, changing the MFC sensor signal and thus demonstrating that damage occurred.

Using this pitch-catch active sensing setup, various tests were conducted to collect data corresponding to the undamaged and damaged pipe cases. Non-permanent damage was induced on the structure using a hose clamp. For the purposes of this thesis, the pipe without the clamp on it is referred to as “pristine” pipe, and the pipe with the hose clamp employed is referred to as “damaged” pipe. The clamp, shown in Figure 26, was placed halfway between the sensor and the actuator and tightened with a screwdriver to induce stresses in the pipe. Guided waves traveled differently through this stressed section than through the pristine pipe, causing a difference in the signal output across the MFC measured by the oscilloscope. Guided waves exhibited greater attenuation when the pipe was stressed. The result was smaller-amplitude guided waves and lower sensor voltages recorded (for experimental studies confirming this phenomenon, refer to Section 5.6.2).