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IEEE-NMDC 2012 Conference Paper Proceedings of the 2012 IEEE Nanotechnology Material and Devices Conference October 16-19, 2012, Hawaii, USA Investigation of PVDF-TrFE Nanofibers for Energy Harvesting Sumon Dey, Mohsen Purahmad*, Suman Sinha Ray Alexander L. Yarin, Mitra Dutta, Fellow, IEEE *[email protected] Abstract- We have investigated the copolymer polyvinylidene energy harvesting, the generated pulse width is a critical fluoride, (PVDF-trifluoroethylene) for energy harvesting. parameter which should be considered during interface circuit Polyvinylidene fluoride (PVDF) nanofibers were electrospun on indium tin oxide (ITO) coated plastic. The electrical response of design and it can strongly affect the operation of interface nanofibers at different frequencies was investigated. The circuit and the net energy gain. experimental results demonstrate that the duty cycle of electrical The aim of this paper is to describe the effect of frequency response pulses is increased as the frequency of vibration is on the generated pulse widths from randomly oriented PVDF- increased. By using the fast Fourier transform (FFT) of the TrFE nanofibers. Experimental results demonstrate the change response pulses, the maximum power extracted has been calculated. of pulse widths on changing the frequency of the applied mechanical pressure. In addition, we also calculated maximum attainable power from PVDF-TrFE nanofibers. NTRODUCTION I. I A promising method of energy harvesting is the use II. PIEZOELECTRICITY EFFECT piezoelectric materials to capitalize on the ambient vibrations. Since the demand for high-performance wireless sensors is Piezoelectricity is the result of the rearrangement of increasing continuously, energy harvesting has been the focus electronic charges in materials with no inversion symmetry of much research for this application. Vibration driven energy such as PZT, PVDF and ZnO. A detailed study on the harvesters have been given attention as replacement for micro- piezoelectricity effect in nanoscale size has been reported in [7, batteries in various wireless sensor networks, for underground 8]. The constitutive relations describing the piezoelectric or remote areas, and for health monitoring systems [1,2]. behavior in materials can be expressed as the following [3, 9]. Development of energy harvesters will provide a significant advantage to such wireless devices and sensors in distributed T =−e.E + cE : S (1) locations or in hard to reach areas and without the need for D = ε S .E + e : S (2) frequent battery replacements. Nanofiber-based piezoelectric energy generators could be scalable power sources applicable where T is the stress, c is the elastic stiffness constant, e is in various electrical devices and systems by scavenging the polarization constant, E is the electric field, D is the electric mechanical energy from the environment [3,4]. Piezoelectric displacement, S is the strain and, and Ɛ is the electric materials such as polyvinylidene fluoride (PVDF) and lead permittivity. The copolymer of PVDF with trifluoroethylene is zirconate titanate (PZT) have been more extensively studied frequently used as a piezoelectric material. The piezoelectric because of their high piezoelectric coefficients. Energy properties of PVDF-TrFE arise because of the remnant harvesters and generators made of PVDF or PZT can be polarization obtained by orienting the crystalline phase in fabricated by means of electrospinning processes such as strong poling field. Due to get piezoelectric behaviors, material conventional, modified or near-field electrospinning (NFES) should be below its curie temperature. [4]. Polymer based piezoelectric materials gives us some The copolymer exhibits paraelectric behaviors above its advantages over the other piezoelectric materials. curie temperature. Piezoelectric behaviors arise because of the Polyvinylidene fluoride (PVDF-TrFE) is inexpensive, easy to ferroelectric properties of materials, where the copolymer process, light weight and more flexible compared to the other generates an electric field based on applied stress or change in piezoelectric materials [5]. The piezoelectric energy harvesting shape. Since piezoelectric properties of this copolymer depend devices have two main parts; the piezoelectric material and on the degree of crystallinity and thin PVDF-TrFE copolymers interface circuit design [6]. In order to make improvements in show higher phase separation under stronger electric fields, energy storage, an energy harvesting circuit or interface circuit PVDF-TrFE is a very interesting material for energy harvesting is inserted between the piezoelectric device and the actual load. [10]. Hence, the efficiency of the interfacing circuit also plays a major role during the conversion of mechanical energy to electrical energy. In the case of interface circuits used in ,((( 21 overnight until a clear solution was achieved. For electrospinning, the flow rate was kept equal to 0.25 ml/h under the application of electric field ~1-2 kV/cm. To produce the PVDF nanofibers, the ITO coated plastic was used as a conductive substrate which is also suitable for vibration purposes. An optical image of the as-electrospun nanofibers is shown in figure 2. The contacts were formed by using silver paste and the data was captured using the HP Infinium oscilloscope. Fig. 1. Schematic of the electrospinning setup and poling process. Fig. 2. The optical image of the as-electrospun nanofibers. III. EXPERIMENTAL The basic experimental arrangement and the possible dipole orientation during the electrospinning process are shown in figure 1. The body of the polymer liquid droplet becomes charged due to the application of the high electric field on the liquid droplet and the electrostatic repulsion counteracts the surface tension. As a result the droplet is stretched. Due to molecular cohesion of the liquid, the stream does not break up. If molecule cohesion is not tight enough, then droplets are electro-sprayed and a jet of charged liquid is formed. Due to the charge migration to the surface of the fiber the jet is elongated by a whipping process set up by the electrostatic repulsion initiated at the small bends in the fiber. This process is carried on until it is deposited on the grounded collector, which leads to the formation of uniform fibers with nanometer scale diameters. For the set up for the electrospinning of the polymer solution is described elsewhere Fig. 3. The electrical response of PVDF nanofibers at different vibration [11]. A copolymer of PVDF-TrFE in the ratio of 70:30 was frequencies, (a) at frequency=2.67Hz (b) at frequency=3.73Hz and (c) at used. A solution 1 g of PVDF-TrFE was mixed with 3 g of N- frequency=6.54Hz. dimethylformamide (DMF) and 2 g of acetone was used and was heated on a hotplate at 750 C with vigorous stirring 22 lower than theoretical value [14] and the measured internal (a) resistance is around 13 MΩ. B. Effect of vibration frequency on generated pulse width The electrical response of the PVDF-TrFE nanofibers is shown in figure 3. As can be seen the typical amplitude of pulses are around 1 V with a duty cycle of about few ms. To investigate the effect of vibrational frequency on the duty cycle of generated electrical pulses, the electrical response of PVDF- TrFE nanofibers at three different frequencies were measured. The electrical response presented in Figure 3 demonstrates that by increasing the frequency, the duty cycle of the pulses are enhanced and this enhancement is not a function of the amplitude of electrical response. The duty cycle is increased from 5ms at the frequency 2.7 Hz to 34 ms at frequency 6.5 (b) Hz. C. Estimation of maximum output power To calculate the maximum output power which can be extracted from the electrical response of nanofibers, the fast Fourier transform (FFT) technique was used. By considering a single time period of the electrical response illustrated in figure 4 (a), the FFT data shown at Figure 4 (b) were obtained. As seen in Figure 4(b) the main frequency which has the maximum amplitude is located at frequency 0.6 Hz. The corresponding sine waves of odd harmonics are also presented at figure 4 (c). Considering the main harmonic located at frequency 0.6 and ignoring other harmonics and by considering one external resistance load equal to the internal resistance of (c) energy harvester, the power were calculated which is 0.6 nW. V. CONCLUSION In summary, the growth of electrospun nanofibers of PVDF- TrFE has been performed and the electrical response of has been investigated. It is shown that the duty cycle of response pulses is increasing as the frequency of vibration is increased. Using the FFT technique the maximum output power also is calculate which is around 0.6 nW. Partial support of this effort is provided by ELORET via a DARPA SBIR Phase II award through AMREC via Contract No. W31P4Q-10-C-0093. Fig. 4. (a) one time period of electrical response of PVDF nanofibers. (b) Fast REFERENCES Fourier Transform (FFT) for one cycle output voltage and (c) corresponding sin waves of odd frequency. [1] Z. L. Wang, “Energy Harvesting for Self-Powered Nanosystems,” J. Nano Research, vol. 1, pp. 1- 8, 2008. [2] S. Saadon, O. Sidek, “Ambient Vibration-based MEMS Piezoelectric Energy Harvester for Green Energy Source,” IEEE Conference IV. RESULTS AND DISCUSSION proceeding ICMSAO, 2011. A. Internal impedance [3] M. Alexe, S. Senz, M. A. Schubert, D. Hesse, and U. Gosele, “Energy Harvesting Using Nanowires?,” J. Adv. Mater., vol. 20, pp.4021–4026, The dielectric constants for the PVDF-TrFE copolymer can 2008. be varied from 9.0 to 13.1 [12, 13]. Using the parallel plate [4] J. Chang, M. Dommer, C. Chang, L. Lin, “Piezoelectric nanofibers for capacitor model and dielectric constant 10, the theoretical energy scavenging applications,” J. Science Direct Nano Energy, vol. 1, pp. 356–371, 2012. capacitance value should be 1150 [14]. In our case, measured [5] D. Vatansever, R.L. Hadimani, T. Shah and E. Siores “Investigation of capacitance value was approximately 110 pF which is pretty Energy Harvesting from Renewable Sources with PVDF and PZT” J.
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