Massively parallel aligned microfibers-based

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Jul 17, 2013 - situ, oriented poled near-field electrospinning. Yiin-Kuen Fuh ... 2.8 mW cmА3.8. Polyvinylidene fluoride (PVDF) is another alternative material.

Massively parallel aligned microfibers-based harvester deposited via in situ, oriented poled near-field electrospinning Yiin-Kuen Fuh, Shao-Yu Chen, and Jia-Cheng Ye Citation: Appl. Phys. Lett. 103, 033114 (2013); doi: 10.1063/1.4813909 View online: http://dx.doi.org/10.1063/1.4813909 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v103/i3 Published by the AIP Publishing LLC.

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APPLIED PHYSICS LETTERS 103, 033114 (2013)

Massively parallel aligned microfibers-based harvester deposited via in situ, oriented poled near-field electrospinning Yiin-Kuen Fuh,1 Shao-Yu Chen,1,a) and Jia-Cheng Ye2 1

Department of Mechanical Engineering, National Central University, No. 300, Jhongda Rd., Jhongli City, Taoyuan County 32001, Taiwan 2 Energy Engineering, National Central University, No. 300, Jhongda Rd., Jhongli City, Taoyuan County 32001, Taiwan

(Received 25 March 2013; accepted 28 June 2013; published online 17 July 2013) In this study, we demonstrate a direct-write, in situ poled polyvinylidene fluoride power generator via near-field electrospinning and fully encapsulated on a flexible substrate. An unique polarity alignment and a total of 500 microfibers continuously deposited in parallel and serial configurations are capable of producing a peak output voltage of 1.7 V and the current of 300 nA, which is two to three orders of magnitude increase in both voltage/current outputs when compared with near-field electrospinning setup of a single nanofiber and the similar amount of microfibers with postpoling C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4813909] treatment. V Harvesting tiny amounts of mechanical energy and packaging highly integrated, compact power density energy sources are the key driving forces for the recent development of nanogenerators (NGs). The original significant contribution dates back to 2006, at which time piezoelectric zinc oxide (ZnO) nanowires (NWs) arrays converted nanoscale mechanical energy into electrical energy.1 A fully packaged, vertically aligned ZnO NW array that utilized a zigzag metal electrode with a small gap was also demonstrated to drive ultrasonic waves with a 3–5 fold increase in direct-current output of 20–30 mV, and 1.2 nA were generated from a total of 1000 NWs.2 Radially grown ZnO around textile fibers were presented as a simple, low-cost approach, and only 1–3 mV, 5 pA were generated by brushing the NWs bundles with respect to each other.3 A flexible power generator based on cyclic stretching–releasing of a piezoelectric fine wire was reported to generate an electrical output of 20–50 mV, 400–750 pA, respectively.4 In order to make NGs a reality, the integration of large numbers of NW energy harvesters into a single power source is indispensable; this process demands a low-cost, scalable alignment technique for NWs. A lateral integration of 700 rows of ZnO NWs reportedly produced a peak voltage of 1.26 V and current of 20 nA at a low strain of 0.19%.5 Furthermore, a scalable sweeping-printing method was proposed to fabricate flexible high output NG.6 By simply dispersing the conical shape of the as-grown ZnO NWs onto a flat polymer film to form a rational “composite” structure and an output voltage up to 2 V, current 300 nA was produced under a compressive strain of 0.11% at a straining rate of 3.67% s1.7 Another piezoelectric material suitable for NWs configuration is PbZrxTi1xO3 (PZT). A single array of PZT NWs produced a peak output voltage of 0.7 V, current density of 4 lA cm2 and an average power density of 2.8 mW cm3.8 Polyvinylidene fluoride (PVDF) is another alternative material for piezoelectricity and nanofibers (NFs) with diameters ranging from 70 to 400 nm produced by electrospinning.9 Considering the in vivo applications, a hybrid device consisting of a a)

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piezoelectric PVDF NFs NG and a flexible enzymatic biofuel cell for harvesting the biochemical (glucose/O2) energy is proposed.10 Concerning the controllability and patterning capabilities, a near-field electrospinning (NFES) process has been proposed to deposit solid NFs in a direct, continuous, and controllable manner.11 Controlled complex patterns such as circular shapes and grid arrays on large and flat areas can be obtained.12 A direct-write PVDF NF with high energy conversion efficiency was reported to generate electrical outputs of 5–30 mV and 0.5–3 nA for a single NF.13 In order to amplify the harvested energy, PVDF electrospun NFs arranged in a series and/or in parallel with patterned combshape gold electrodes.14 The extremely lower power output for the parallel 500 NFs arrays may attribute to the less effectiveness of post poling, partial reverse generation of neighboring electrodes, or encapsulation loss. A recent summary report highlighting the latest NGs developments can be found in Ref. 15. In this paper, we demonstrate a direct-write and in situ poled NG via NFES process to deposit polymeric PVDF NFs arrays fully encapsulated on a flexible substrate. The performance in this paper is the landmark progress when compared with NFES setup of a single nanofiber and the similar amount of NFs with post poling treatment.13,14 Figure 1(a) illustrates the schematic diagram and working principle of the electrode design in both a serial and parallel configuration. The direct-write NFES11,12 PVDF fibers with in situ electrical poling and mechanical stretching were used to construct NG onto a flexible substrate polyvinyl chloride (PVC), with both ends tightly bonded by silver paste to the substrate and interconnects. With the direct-write NFES, suspended multiple fibers across 11 copper contact electrodes (made of 55 lm-thick copper foils) were deposited on an insulation PVC substrate. This process was realized by using an x–y stage (Parker, Inc.) to control the deposition speed and direction of the substrate, achieving a desirable pattern at specific regions of interest. The same writing direction of all microfibers (MFs) must be ensured such that the crystallographic orientations of the horizontal MFs were aligned along the sweeping direction in situ. Figure1(b) is the schematic diagram with the working principle of the electrospun power

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FIG. 1. (a) Schematic diagram of the in situ electrical poling direct-write and near-field electrospun PVDF fibers with mechanical stretching to construct power generator onto a flexible substrate of PVC, with silver paste used to tightly bond both ends of the substrate. Electrical voltage and current superposition in the serial/parallel configuration can be easily achieved. (b) Working principle of the voltage and current scaling-up superposition for the electrospun MFs are integrated as one large array energy harvesting device. As mechanical deformation was induced on the substrate, tensile strain and a corresponding piezoelectric potential in the MFs can be created, where the “6” signs indicate the polarity of the local piezoelectric potential created in situ inside the MFs.

generator (PG). PVDF piezoelectric MFs were direct-write and in situ poled on a flexible substrate and fixed to electrodes at both ends. The aligned dipoles are necessary since electrical potentials can only be generated under axial strain by bending the bottom plastic substrate. The physical stability of the entire structure was maintained by encapsulating a thin layer of insulating polydimethylsiloxane (PDMS) inside. Bending the plastic substrate, an axial stress was induced and a piezoelectric potential was generated according to the piezoelectricity of PVDF. The diameter and surface morphology of both the PVDF MFs were analyzed using scanning electron

Appl. Phys. Lett. 103, 033114 (2013)

microscopy (SEM, JSM-6700f, JEOL Co., Japan) at 10 kV accelerating voltage. The devices of a current preamplifier (AutoLab) and a voltage preamplifier (Agilent, DSO1020) were used to measure current and voltage output, respectively. A DC linear motor was used to provide strain for the PG measurement. Figure 2(a) shows optical images of a fabricated device with electrospun MFs on top of the plastic substrate before the encapsulation of PDMS is applied. When an axial stress is applied by bending the plastic substrate, there are about 500 active working contacts to collect charges generated from these PVDF MFs. The as-spun PVDF fibers have diameters ranging from 900 nm to 2.5 lm with the separation distance (200–600 lm) between two metallic electrodes as shown in the SEM photo on the right-side of Figure 2(b). The AFM images of PVDF nanofibres fabricated via NFES are shown in Figure 2(d), indicating the diameter is c.a. 2.2 lm. Figure 3 investigates the performance of output voltage and current that can be greatly enhanced by superpositioning a number of PGs in a series and in parallel configurations. Then a cyclic stretching-releasing deformation at a strain of 0.5% and 15 Hz by a linear motor was used to periodically deform the electrospun PG. For example, the PG opencircuit output voltage was connected in a series, leading to an output voltage of 1.5–1.7 V when the substrate was stretched and released repeatedly (Fig. 3(a)); short-circuit currents were measured and the peak current can exceed 200 nA (Fig. 3(b)). The corresponding insets are the enlarged view of the boxed area for one cycle of mechanical deformation and the detailed shapes of peaks. The polarity test is crucial for validating that the measured results were coming from the piezoelectric responses instead of artifacts.4,13 Experimentally, if the polarity of the contacts has been changed while the shape of the response remains the same, then the signal is coming from the noise or other forms instead of piezoelectric responses.

FIG. 2. (a) A total of about 50 parallel MFs and 11 electrode pairs have been fabricated on top of a flexible substrate.(b) Optical images of a fabricated device with parallel aligned MFs. The working gap between two electrodes is in the range of 200-600 lm. (c) Enlarged SEM photomicrograph showing a NF with diameter of 2 lm. Scale bar, 10 lm. (d) Atomic Force Microscope (AFM) images of PVDF MFs fabricated via NFES.

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FIG. 3. The output voltage and current versus cyclic stretching-releasing deformation at a strain of 0.5% and 15 Hz. (a) The output voltage of the PG can be enhanced by integrating aligned MFs in a series. The device produces an output voltage of 1.51.7 V. (b) When connected in parallel, device produces an output current of 200300 nA. Insets in the right panels of (a) and (b) are enlarged views of the output voltage and current for one cycle of the mechanical deformation.

The forward and reverse connections in the voltage measurements are depicted in Figure 4(a). The magnified pattern of one single bending and release process is illustrated in the bottom plots. The peak voltage in the forward direction was measured to be 1.7 V, while the reverse connection was 1.2 V. Similar to the voltage measurement, the current measurement result is plotted in Figure 4(b). The polarity check for the piezoelectric response was confirmed since the shape of the response was flipped. The peak currents in the forward and reverse connections were 300 nA and 250 nA, respectively.16

Figure 5 shows the responses of the PG under various cycling frequencies ranging from 3 to 15 Hz for the same applied external strain of 0.5%. In the experimental setup, the cycling frequency can be directly related to the strain rate since the linear motor controls the stretch-release cycle for a given applied strain. Measurement results show that both the output voltages and currents increase proportionally to the increase of the cycling frequency during both the stretch and release cycles. We measured the average output of peak voltages for cycling frequencies of 3, 5, 10, 15 Hz to be 0.2, 0.35, 0.8, and 1.7 V, and the average output to

FIG. 4. Polarity measurements were validated via (a) forward and reverse connections of voltage generated by the device. When the measurement polarity was switched, the response shape also changed. Peak voltage was measured to be about 1.7 V in the forward connection and 1.2 V in the reverse connection. (b) The current generated by the device and the response shape, which was flipped when the measurement polarity was switched. The peak current was measured to be as high as 300 nA in the forward connection and 250 nA in the reverse connection.

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FIG. 5. Magnitude of the output (a) voltage and (b) current of a PVDF PG subject to different stretch-release cycling frequencies of 3–15 Hz and a strain of 0.5%. The typical diameters of the electrospun MFs are in the range of 0.9–2.5 lm with lengths of 200–600 lm. When gradually increasing the applied frequency from 3 to 5, 10, and 15 Hz, the output voltage also increases almost proportionally.

be 60, 90, 200, and 300 nA, respectively, for a period of 5 s. Theoretically, the results are consistent with fundamental piezoelectric theory i ¼ d33EA_e ,13 where i is the generated current, d33 is the piezoelectric charge constant, E is the Young’s modulus, A is the cross-sectional area, and e_ is the applied strain rate. Intuitively adopting the above formula, the increase of a pressing force acting on the MFs will directly result in larger deformation, as well as the linear scale up of output voltages and currents. In summary, we have designed and implemented a simple electrode configuration for easy scaling-up of both voltage and current via the electrical superposition principle. Electrical superposition is realized by connecting MFs collectively and effectively in parallel and serial patterns to achieve a high current and high voltage output, respectively. Utilizing the NFES direct-write technique to align piezoelectric PVDF MFs on flexible substrates, as well as the unique properties of in situ mechanical stretching and electrical poling, can guarantee the scaling-up of the piezoelectric potentials. The PGs that comprised 500 rows of well-aligned PVDF MFs are capable of producing a peak output voltage of 1.7 V and the current reached up to 300 nA. This landmark achievement is two to three orders of magnitude increase in both voltage/current outputs when compared with

a single nanofiber and NFES setup with post poling treatment due to opposite poling direction for a similar amount of NFs.14 1

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