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

39

Figure 10: Relative Maximum Voltage Response by ZnO wt.%

The results of hammer impact testing are summarized and the average relative maximum voltage is plotted as a function of ZnO concentrations.

error of the mean of the relative maximum voltage response for films of different ZnO concentrations. The relative maximum voltage (VR,Max) was calculated by computing the ZnO-based film’s maximum voltage (VMax-ZnO) divided by the maximum voltage generated by the corresponding commercial PVDF-TrFE specimen mounted on the same slide (VMax-PVDF), as shown in in Equation 4.1:

Equations 4.1 used the commercial PVDF’s maximum generated voltage as a reference so that any experimental errors such as slight variations in initial impact energies between trials or between films are adequately noted.

Thus, VR,Max should theoretically be equal to 1 for tests conducted on the 0 wt.% PVDF-TrFE/ZnO films, assuming they were poled in the same direction and in the same manner as the commercial PVDF-TrFE specimens. In this case, it can be seen that the

40

average VR,Max for 0 wt.% PVDF-TrFE/ZnO films is ~1.3 (Figure 10), thereby suggesting that the films fabricated for this study are more sensitive than their commercial counterparts. While it is unlikely that the PVDF-TrFE/ZnO films are poled to possess higher piezoelectricity than the commercial specimens, it is more likely that the commercial films have been poled in a slightly different manner. Nevertheless, the data corresponding to the 0% case serves as the baseline for comparison.

Next, it can also be observed that the average relative maximum voltage increases in tandem with increasing ZnO concentrations. This result suggests that increasing ZnO content enhances the piezoelectricity of PVDF-TrFE thin films, and greater ZnO content provides greater enhancement, up to a point. It is likely that the enhancement will plateau and asymptotically approach some maximum limit (approaching the piezoelectricity of pure ZnO films). Thus, based on the results shown in the previous two figures, it can be concluded that given the same amount of mechanical excitation, poled PVDF-TrFE films with more embedded ZnO nanoparticles will be able to produce a higher voltage output, thereby making this film even more viable for sensing and SHM applications.

41

Figure 11: Maximum Voltage Compared with Impact Energy

The peak voltages (normalized by film thicknesses) generated by different energy impacts are plotted. In addition, representative results are shown for each thin film sample set. Increasing impact energy increases the measured peak voltage response.

Film sensitivity to hammer impact energy has also been investigated. As expected, higher impact energies correspond linearly with higher peak voltage responses, as can be seen in Figure 11. Since piezoelectricity is a linear phenomenon (as discussed in Section 2.3), this linear correlation with energy increase is appropriate and expected. As before, increasing ZnO concentrations in the film give increasing peak voltage responses.

42

Figure 12: Maximum Voltage Compared with Impact Distance

These results demonstrate that the voltage response of the thin film decreases with increasing distance between the film and location of impact. The result for a 20 wt.% PVDF-TrFE/ZnO thin film is shown.

Piezoelectric sensitivity to impact distance from the PVDF-TrFE film has also been explored. Figure 12 demonstrates an example of the observed phenomenon. For all films, but specifically in this case for a specimen with 20 wt.% ZnO nanoparticles, an inverse relationship is observed between increasing distances (between the impact location and the film electrode) versus the measured voltage response (Figure 12). This result suggests that there is a limit on the maximum distance from which an impact can be detected. Future testing will be conducted to determine the maximum distance at which these films can sense impact on other substrates.

43

4.3 Free Vibration Sensing Testing

Free vibration sensing testing is another method to test the sensitivity of the PVDF-TrFE/ZnO film. Both the free vibration testing and the later described load frame testing are analogous to testing performed on carbon nanotube/PVDF composite films by Ramaratnam and Jalili [79]. Specifically, the free vibration testing was performed similarly to testing on PZT-based piezoelectric paint by Zhang [80]. This test helps to determine if the film would be an effective dynamic strain sensor.

4.3.1 Free Vibration Sensing Setup

Free vibration testing was performed with films deposited onto a cellulose acetate substrate (rather than a glass substrate) to give greater film flexibility. Films were poled as before (Section 3.4) in order to enhance their piezoelectricity. The film specimens were then affixed onto polyvinyl chloride (PVC) thin plate specimens (30 7

0.3 cm3) using a cyanoacrylate adhesive. A metal-foil strain gage (Tokyo Sokki Kenkyujo) was also attached to the PVC cantilevered beam adjacent to the PVDFTrFE/ZnO films. As shown in Figure 13, each PVC beam with the attached thin films and strain gage was clamped to a lab bench at one end and an initial excitation was provided to initiate free vibration (i.e., via an initial displacement). Voltage was measured across three PVDF-TrFE/ZnO specimens simultaneously using an oscilloscope, with its fourth channel employed for measuring strain gage output. A Wheatstone bridge and amplification circuit was interfaced with the strain gage to enhance the SNR of strain gage outputs. Since all four channels were recorded simultaneously, the raw data from

44

Figure 13: Free Vibration Setup

The experimental setup for the free-vibration test is shown. The image on the left shows the use of an Agilent oscilloscope for data acquisition and the image on the right shows the simultaneous querying of three PVDF-TrFE/ZnO thin films (along with the strain gage adjacent to the film specimens).

the piezoelectric nanocomposites and from the strain gage were time synchronized and were compared side-by-side.

4.3.2 Free Vibration Sensing Results

Free vibration validation tests have been performed according to the procedure outlined in Section 4.3.1. Voltage generated across the films during free vibration has been directly compared with measured strains. Dynamic strain, as calculated in Equation 4.2, has been considered rather than strain because a piezoelectric material supplies a voltage due to a change in instantaneous strain, rather than due to a constant level of applied strain (see Section 2.3).

45

Figure 14 provides two examples of thin film free vibration generated voltage responses overlaid with the dynamic strain time history (which was calculated from strain gage measurements). The top plot of the figure shows the response for a typical 0 wt.% PVDF-TrFE/ZnO film, whereas the bottom plot shows the response for a representative 10 wt.% PVDF-TrFE/ZnO film. It should be mentioned that the raw data have been down-sampled by 50 in order to produce the plots shown. From both graphs in Figure 14, it can be seen that the dynamic strain and the voltage generated have the same

Figure 14: Free Vibration: Dynamic Strain Time Response

Free vibration testing has been conducted, and two representative thin film voltage time history responses are plotted and overlaid with the dynamic strain time history. The top plot shows the response for a representative 0 wt.% PVDF-TrFE/ZnO thin film, whereas the bottom plot shows the response from a 10 wt.% ZnO thin film.

46

phase and frequency and change in tandem with one another, thereby confirming that these nanocomposites can be used for dynamic strain sensing. Films with more ZnO nanoparticles are more sensitive, which is as expected due to their higher piezoelectricity.

Figure 15 shows a more direct relationship between thin film piezoelectric response and dynamic strain. Here, only the first few cycles of vibration are plotted for clarity. As expected, the relationship between voltage and applied dynamic strain is linear without offset. Similar to Figure 14, the top graph of Figure 15 presents the results from the same 0 wt.% ZnO specimen shown previously, and the bottom graph shows the results corresponding to the same 10 wt.% ZnO specimen. Linear least-squares best-fit lines have also been computed and overlaid on top of the data. The correlation coefficients have been calculated to be 0.959 and 0.966 for the 0 wt.% and 20 wt.% PVDF-TrFE/ZnO thin films, respectively, thereby demonstrating a strong correlation between dynamic strain and voltage generated. It should be mentioned that other specimens tested also exhibited similar types of responses, and only two representative results have been shown.

In addition to the observed positive correlation, the slopes of these least-squares best-fit lines are equivalent to the dynamic strain sensitivity (SD) of these films and can be calculated using Equation 4.3 as follows.

47

Figure 15: Free Vibration: Voltage Response vs. Dynamic Strain

The results from Figure 14 are processed and expressed differently here. Thin film voltage response is plotted as a function of dynamic strain, and it is clear from these results that there is a linear relationship between voltage and dynamic strain, thereby indicating the potential for these films for use as dynamic strain sensors.

Here, V is the change in voltage over a time step t and is the change in

strain over the same time step t. As expected, the 0 wt.% ZnO specimen has a sensitivity of 1.928, lower than the dynamic strain sensitivity for the 10 wt.% ZnO specimen, which is 3.24 in this case. These results and trends are consistent with the

48

Figure 16: TestResources Load Frame

A picture of the load frame used for thin film testing is shown above.

ferroelectric characterization and hammer sensitivity test results as discussed previously.

4.4 Load Frame Sensing Testing

A load frame has also been used to excite the PVDF-TrFE/ZnO film to test its sensing abilities and linearity. The load cell head will excite the film sinusoidally in order to validate 1-3 piezoelectricity in PVDF-TrFE/ZnO films.

4.4.1 Load Frame Sensing Setup

Load frame testing began with the PVDF-TrFE/ZnO film clamped at both ends in a TestResources 150R load frame (see Figure 16). As in free vibration testing, the film has been deposited on a cellulose acetate flexible substrate. Copper tape has been attached to both ends to enable electrical connectivity to the oscilloscope, with one channel employed to measure the voltage generated across the thickness of the specimen. The TestResources was commanded to load the film specimen following a sinusoidal strain pattern after employing a small initial position offset, such that the film was in tension for the entire loading cycle. It was expected that this excitation would