Статьи на перевод PVDF_P(VDF-TrFE) / Piezoelectric Characterization of PVDF-TrFE Thin Films

Abstract— The objective of this letter is the fabrication and piezoelectric characterization of poly(vinylidene fluoride)- trifluoroethylene (PVDF-TrFE) thin films enhanced with zinc oxide (ZnO) nanoparticles. The incorporation of piezoelectric ZnO nanoparticles will enhance bulk film piezoelectricity while preserving the mechanical flexibility of PVDF-TrFE. The nanocomposites are fabricated by spin coating. Piezoelectric properties are measured by applying electric fields while measuring their response using a Sawyer-Tower circuit.

Index Terms— Ferroelectric hysteresis, piezoelectric polymer, PVDF copolymer, zinc oxide nanoparticle.

I. INTRODUCTION

PIEZOELECTRIC materials have been widely adopted for various applications (e.g., biomedical engineering, damage detection, automotive engines, and electronics, among others) due to their versatility as sensors, actuators, or energy harvesters. Lead zirconate titanate (PZT) is a piezo-ceramic with a high piezoelectric coefficient and has been used for active sensing of cracks and for delamination detection in composite materials, to name a few [1]. Alternatively, poly(vinylidene fluoride) (PVDF) and PVDF-trifluoroethylene (TrFE) (i.e., its copolymer) are piezoelectric polymers with lower piezoelectricity but can conform to complex structural surfaces due to their polymeric nature. PVDF has been used

for dynamic strain sensing and energy harvesting [2, 3]. Ideally, piezoelectric materials should possess high piezo-

electricity like PZTs, while remaining conformable and flexible like piezo-polymers. In particular, zinc oxide (ZnO)- based nanomaterials have been regarded as a next-generation piezoelectric material due to their inherent piezoelectricity [4]. A potential method for simultaneously achieving improved piezoelectric and mechanical performance is by embedding piezoelectric ZnO nanomaterials in composite architectures. In fact, ZnO has already been embedded in PVDF [5] and PSS/PVA [6] polymer matrices for enhancing piezoelectricity.

Manuscript received September 15, 2011; revised December 14, 2011; accepted December 19, 2011. Date of publication December 28, 2011; date of current version April 25, 2012. This work was supported in part by the University of California Institute for Mexico and the U.S. and El Consejo Nacional de Ciencia y Tecnología Program and the College of Engineering, University of California, Davis. The associate editor coordinating the review of this letter and approving it for publication was Dr. Patrick Ruther.

J. S. Dodds and K. J. Loh are with the Civil and Environmental Engineering Department, University of California, Davis, CA 95616-5294 USA (e-mail: jsdodds@ucdavis.edu; kjloh@ucdavis.edu).

F. N. Meyers is with the Mechanical and Aerospace Engineering Department, University of California, Davis, CA 95616-5294 USA (e-mail: fnmeyers@ucdavis.edu).

Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2011.2182043

The goal of this study is to enhance and characterize the piezoelectricity of PVDF-TrFE thin films embedded with various concentrations of ZnO nanoparticles (NP). To date, such composites have undergone some dielectric characterization but have not yet had their piezoelectric properties fully characterized [6]. This letter starts with an overview of the fabrication method and characterization procedure. The characterization results, including polarization hysteresis loops, are then presented and discussed.

II.EXPERIMENTAL DETAILS

A.Thin Film Fabrication

Nanocomposite fabrication begins by dissolving 150 mgmL−1 of PVDF-TrFE in methyl ethyl ketone. ZnO NPs are added to the solution to achieve ZnO weight fractions ranging from 0% to 20% in 5% increments. The solutions are bath sonicated (135 W, 42 kHz) for up to 6 h to ensure ZnO suspension. Care is taken to ensure that ZnO NPs are adequately dispersed so that the resulting thin films also possess distinct ZnO concentrations. Suspension also prevents agglomerations that can serve as defects in the film.

The solutions are then used as is for spin coating thin films onto glass microscope slides with a thin 150 nm layer of aluminum (i.e., by e-beam deposition). A Laurell WS-650Mz- 23NPP spin processor is employed for spin coating. Here, ZnO/PVDF-TrFE solutions are pipetted onto the aluminum surface, followed by spreading at 450 rpm for 15 s and then spinning at 3,000 rpm for 30 s. Thermal annealing at 150 °C is performed for enhancing film quality. The aforementioned spread, spin, and anneal procedure is repeated for depositing multiple layers until film thicknesses range from 6 to 11 μm. Finally, e-beam is employed for depositing the top aluminum electrode. Three specimens are fabricated simultaneously, and12 specimens are made for each ZnO weight fraction set.

B. Piezoelectric Characterization

Piezoelectric characterization is performed by applying a 10 Hz high electric field across the thickness of the films. Use of a 10 Hz waveform is guided by Furukawa et al. [7], where the ferroelectric domain switching time of PVDF at 200 MV-m−1 is only 4 μs; 10 Hz thus provides sufficient time for domain switching in the ZnO/PVDF-TrFE films. An Agilent 33210A arbitrary waveform generator is used for producing the output sinusoidal signal and then amplified to the desired magnitude using an Ultravolt 5HVA24 high voltage module. The applied electric field (E = Va /d , where Va is applied voltage, and d is film thickness) and film response are measured using an Agilent MSO8104A mixed-signal

Electric field [MV-m−1]

Fig. 1. Hysteresis loops of PVDF-TrFE thin films with different weight fractions of ZnO nanoparticles measured at 60 MV-m−1.

TABLE I

REMNANT POLARIZATION ( DR ) RELATED TO ZINC OXIDE

WEIGHT FRACTION

oscilloscope connected to a Sawyer-Tower (ST) circuit [8]. It should be noted that testing is conducted at room temperature, and the samples are immersed in silicone oil so as to prevent electrical arcing.

Thin film electric displacement or polarization (D) is calculated using Eq. (1):

where ε is dielectric constant, V is voltage across the sample, C is the reference capacitor’s capacitance in the ST circuit, and A is the sample’s surface area. Because ε is so small, the first term in Eq. (1) is assumed to be negligible. The results can be expressed and plotted to give D − E hysteresis loops indicating piezoelectric response. In this study, fatigue bias (if observed) is numerically removed during data processing.

III. RESULTS AND DISCUSSIONS

The results from the previous piezoelectric characterization tests are summarized in Table I. Table I shows three columns of results, where columns 1 and 2 show the average remnant polarization and the corresponding standard error of the mean, measured at an applied electric field of 60 and 75 MV-m−1, respectively. It should be mentioned that remnant polarization, or DR , is obtained by calculating the y-intercept of each

sample’s D − E hysteretic response [9]. Higher remnant or permanent polarization suggests greater alignment of ZnO NPs and improved piezoelectricity. A representative set of D − E hysteresis loops are also plotted and shown in Fig. 1.

First, Fig. 1 demonstrates the notable effect that ZnO concentration has on remnant polarization. The results show that increasing remnant polarization is attained with increasing ZnO weight fractions. These results hold true even when the applied electric field is increased to 75 MV-m−1 (Table I), thereby suggesting that piezoelectric ZnO nanoparticles do enhance bulk film piezoelectricity when embedded within PVDF-TrFE piezo-polymer matrices. However, from Table I, it can be seen that the remnant polarization appears to become saturated (i.e., approximately 21 or 22 mC-m−2) at 75 MV-m−1. The last column of Table I shows film breakdown voltage versus ZnO weight fractions. A clear trend is observed where maximum remnant polarization at lower applied electric fields are observed as ZnO weight fraction increases (i.e., breakdown voltage decreases with increasing ZnO).

It has been reported that PVDF-TrFE films have a remnant polarization of 50 mC-m−2 at 120 MV-m−1[9]. The average remnant polarization at breakdown of pure PVDF-TrFE used in this study is 34 mC-m−2 at 113 MV-m−1. This result shows that additional gains in DR can be achieved, but the current set of films break down at lower applied electric fields and thus achieve slightly lower remnant polarization.

IV. CONCLUSION

This study presents the fabrication and piezoelectric testing of ZnO/PVDF-TrFE thin films. ZnO weight fractions have been varied from 0% to 20% in 5% increments. The results show that, for a given level of applied high electric field, higher remnant polarization can be obtained with increasing ZnO content. However, greater gains in bulk film piezoelectricity can be expected by fabricating higher quality films (i.e., with fewer number of defects). Future work will also entail directly measuring film piezoelectric coefficients.

REFERENCES

[1]X. P. Qing, H.-L. Chan, S. J. Beard, and A. Kumar, “An active diagnostic system for structural health monitoring of rocket engines,” J. Intell. Mater. Syst. Struct., vol. 17, no. 7, pp. 619–628, Jul. 2006.

[2]H. Gao, M. J. Guers, J. L. Rose, G. Zhao, and C. Kwan, “Ultrasonic guided wave annular array transducers for structural health monitoring,” in Proc. AIP Conf., vol. 820. 2006, pp. 1680–1686.

[3]S. P. Beeby, M. J. Tudor, and N. M. White, “Energy harvesting vibration sources for microsystems applications,” Meas. Sci. Technol., vol. 17, no. 12, pp. R175–R195, 2006.

[4]Z. L. Wang, “Zinc oxide nanostructures: Growth, properties and applications,” J. Phys.: Condens. Mat., vol. 16, no. 25, pp. R829–R858, 2004.

[5]P. I. Devi and K. Ramachandran, “Dielectric studies on hybridised PVDF–ZnO nanocomposites,” J. Experim. Nanosci., vol. 6, no. 3, pp. 281–293, 2011.

[6]K. J. Loh and D. Chang, “Zinc oxide nanoparticle-polymeric thin films for dynamic strain sensing,” J. Mater. Sci., vol. 46, no. 1, pp. 228–237, 2011.

[7]T. Furukawa and G. E. Johnson, “Measurements of ferroelectric switching characteristics in polyvinylidene fluoride,” Appl. Phys. Lett., vol. 38, no. 12, pp. 1027–1029, 1981.

[8]T. T. Wang, The Applications of Ferroelectric Polymers, T. T. Wang, Ed. New York: Chapman & Hall, 1987, p. 72.

[9]T. Furukawa, M. Date, and E. Fukada, “Hysteresis phenomena in polyvinylidene fluoride under high electric field,” J. Appl. Phys., vol. 51, no. 2, pp. 1135–1141, 1980.