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Direct-write PVDF nonwoven fiber fabric energy harvesters via the hollow cylindrical near-field

electrospinning process

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2014 Smart Mater. Struct. 23 025003

(http://iopscience.iop.org/0964-1726/23/2/025003)

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Smart Mater. Struct. 23 (2014) 025003 (11pp) doi:10.1088/0964-1726/23/2/025003

Direct-write PVDF nonwoven fiber fabricenergy harvesters via the hollow cylindricalnear-field electrospinning process

Z H Liu1,2,3, C T Pan1,2,6, L W Lin4, J C Huang5 and Z Y Ou1

1 Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-Sen University,Kaohsiung 80424, Taiwan2 Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University; National ScienceCouncil Core Facilities Laboratory for Nano-Science and Nano-Technology inKaohsiung–Pingtung area, Taiwan3 Electronics and Optoelectronics Research Laboratories, Industrial Technology Research Institute,Hsinchu 310, Taiwan4 Department of Mechanical Engineering and Berkeley Sensor and Actuator Center,University of California, Berkeley, CA 94720, USA5 Department of Materials and Optoelectronic Science, National Sun Yat-Sen University,Kaohsiung 80424, Taiwan

E-mail: panct@mail.nsysu.edu.tw

Received 24 September 2013, revised 5 November 2013Accepted for publication 20 November 2013Published 13 December 2013

AbstractOne-dimensional piezoelectric nanomaterials have attracted great attention in recent years for theirpossible applications in mechanical energy scavenging devices. However, it is difficult to control thestructural diameter, length, and density of these fibers fabricated by micro/nano-technologies. This workpresents a hollow cylindrical near-field electrospinning (HCNFES) process to address production andperformance issues encountered previously in either far-field electrospinning (FFES) or near-fieldelectrospinning (NFES) processes. Oriented polyvinylidene fluoride (PVDF) fibers in the form ofnonwoven fabric have been directly written on a glass tube for aligned piezoelectricity. Under a high insitu electrical poling field and strong mechanical stretching (the tangential speed on the glass tubecollector is about 1989.3 mm s−1), the HCNFES process is able to uniformly deposit large arrays ofPVDF fibers with good concentrations of piezoelectric β-phase. The nonwoven fiber fabric (NFF) istransferred onto a polyethylene terephthalate (PET) substrate and fixed at both ends using copper foilelectrodes as a flexible textile-fiber-based PVDF energy harvester. Repeated stretching and releasing ofPVDF NFF with a strain of 0.05% at 7 Hz produces a maximum peak voltage and current at 76 mV and39 nA, respectively.

Keywords: electrospinning, near-field electrospinning, PVDF, energy harvester, nonwoven fiber fabricS Online supplementary data available from stacks.iop.org/SMS/23/025003/mmedia

(Some figures may appear in colour only in the online journal)

1. Introduction

Vibration energy harvesters or energy scavengers capable ofconverting mechanical energy, such as irregular vibrations,

6 Address for correspondence: Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-Sen University, No. 70 Lien-haiRoad Kaohsiung 80424, Taiwan.

airflows, and human activities, into electricity could powerelectronic systems for various applications [1]. However, thistype of energy harvester typically faces common problemssuch as low energy conversion efficiency, wide spectraof vibration frequencies, and time-dependent amplitudes.Several research directions are important in constructinggood piezoelectric energy harvesters, such as the production

10964-1726/14/025003+11$33.00 c© 2014 IOP Publishing Ltd Printed in the UK

Smart Mater. Struct. 23 (2014) 025003 Z H Liu et al

of piezoelectric materials with high electro-mechanicalconversion efficiencies, the design of mechanical structuresresponding to wide ranges of frequencies, and the schemesin building up matching electrical circuits. In the area ofpiezoelectric materials, organic piezoelectric nanomaterialsare attractive for their potential advantages in terms oflow manufacturing cost, high resistance to fatigue, andenvironmental friendliness [2, 3]. Therefore, this work focuseson energy harvesting characteristics utilizing a modifiednear-field electrospinning process to make piezoelectricpolyvinylidene fluoride (PVDF) fibers. The hollow cylindricalnear-field electrospinning (HCNFES) process is proposedand explored to make well-aligned electrospun piezoelectricPVDF fiber arrays on a glass tube collector at roomtemperature without the necessity of a post-annealing andelectrical poling process. As such, it is believed that the newlyintroduced fabrication process could be the manufacturingfoundation for a new class of energy harvesters made oforganic nanofibers.

Previously, inorganic nanomaterials such as lead zir-conate titanate (PZT) [4–7] and zinc oxide (ZnO) [8–10]have demonstrated considerable progress toward self-poweredenergy harvesters. These piezoelectric fibers can be fabricatedusing various methods, including chemical vapor depositionand physical vapor deposition [11, 12]. On the other hand,electrospinning processes have been used to make organicnanofibers, and, with the right selection of piezoelectricmaterials, electrospun fibers could also be used as energyharvesters [13]. Yee et al have demonstrated the feasibilityof using far-field electrospinning (FFES) and a rotating diskto collect aligned electrospun PVDF fibers [14]. These fibersare emitted from the Taylor cone in the fabrication processby overcoming the surface tension and viscoelasticity ofpolymer droplet solutions [15]. Electrostatic force is used tostretch the fiber and the polymer jet can chaotically bend intocomplex paths, causing random deposition of nanofibers [16].As a result, it is difficult to produce aligned PVDF fibersusing FFES. Fennessey [17] and Kim [18] have investigateddifferent techniques to align electrospun fibers using a rotatingcollector. Wang [19] has modified the electric field for betteralignments and the application of fundamental physics andchemistry for better process controls [20]. Recently, thedirect-write electrospinning technique using NFES has beenintroduced [21–23]. Unlike the conventional FFES process,NFES only needs a small electric bias voltage to producecontinuous fibers with ultrafine diameters. Under the in situelectrical poling procedure, the solidified PVDF polymersolution can transform PVDF polymer from the non-polarα-phase into the polar β-phase [13]. However, the breakdownvoltage of air is ∼106 V m−1, which is too low for theelectric poling process, and a polymer thin film has beenproposed to replace air to apply higher electric field fora better poling process [24]. In this work, in order toprevent possible electrical shorts during the electrospinningprocess [25], direct-write NFES on a glass tube under ahigh applied voltage has been demonstrated. Furthermore,a strong mechanical stretching effect is implemented byrotating the glass tube collector with a tangential speed

of 400 mm s−1. As such, PVDF nanofibers with smallerdiameters and fewer defects can be produced as comparedwith the previously demonstrated NFES process on a flatsilicon wafer collector using an X–Y stage with motionspeed of 80 mm s−1 [26]. In a previous report [27], thepiezoelectric constants of the ultrathin films have decreasedbecause of the internal defects. In another report, piezoelectricPVDF fibers with smaller diameters have higher energyconversion efficiency [13] for several possible reasons: (1)large deformations prior to failure; (2) surface/interfaceeffects and high surface-to-volume ratios enhancing materialproperties [3]; and (3) fewer defects as compared with thePVDF thin films owing to a higher degree of crystallinity andchain orientation [28]. Compared with commercially availablePVDF thin films, PVDF fibers produced by the NFES processhave excellent properties, including scalability, flexibility, andgreater piezoelectric constant (d33 ∼ −57.6 pm V−1; g33 ∼

−400×10−3 V m N−1; PVDF thin films, d33 ∼ −15 pm V−1;g33 ∼ −308× 10−3 V m N−1) [22, 29].

This work modifies the conventional FFES and NFESprocesses to achieve good fiber alignment, piezoelectricityand manufacturability of PVDF nonwoven fiber fabrics(NFFs). By the introduction of a glass tube and rotating glasstube collector, PVDF fibers with small diameters, smoothsurface morphology, high density, and good piezoelectricityhave been accomplished using the HCNFES process. Assuch, this new process could be a suitable building block toconstruct energy harvesters as self-sufficient power sources invarious applications.

2. Working principle of HCNFES

PVDF is a semicrystalline ferroelectric polymer withfour different crystal forms, α, β, γ , and δ. The randomdistribution of dipole moments in the non-polar α-phase leadsthem to offset each other without exhibiting a piezoelectricproperty. The α-phase structure can be transformed to thepolar β-phase by mechanical deformation and electricalpoling to achieve piezoelectric characteristics. In the processof NFES, a grounded wafer was placed on the XY stageto collect electrospun fibers. The polymer jet could createspirals and pile-ups at the sharp corners of the electrospinningpaths owing to the unavoidable time delay from the XYstage controller [25]. Furthermore, because the collector is agood electrical conductor; it is prone to short circuits underhigh bias voltage, resulting in discontinuous fiber production.In the HCNFES process, a rotating glass tube collector(diameter, 20 mm; thickness, 1 mm; length, 200 mm) is usedto collect the electrospun fibers. A copper foil is placed inthe internal wall of the glass tube and an electrical brushis attached to ground the copper electrode. The glass tubecollector significantly reduces the occurrence of short circuits.By employing a DC motor to turn the tube collector, andcontrolling the uniaxial movement, this method is able torapidly and continuously collect electrospun PVDF fibers.The experimental setup details are shown in figure 1(a), whichincludes a stainless needle (internal diameter: 0.381 mm),syringe pump (Harvard PHD 22/2000), high voltage power

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Figure 1. (a) Schematic diagram of the HCNFES process showing possible directions of dipoles and the applied electric field. The inset isthe simulation results of the electrostatic potential streamlines for the case of a 0.5 mm gap between the tip of the needle and the surface ofthe glass tube. The PVDF polymer solution experiences mechanical stretching and in situ electric poling during the formation of polymerfibers due to the high electrostatic field. The crystalline structures of the non-polar α-phase and the polar β-phase are shown on the rightside. Both the electric field and mechanical stretching help to convert the non-polar α-phase to become the polar β-phase. (b) Opticalphotographs showing experimental results of the HCNFES processes. The formation of the Taylor cone is observed first, and the polymer jetis emitted toward the collector and both electrostatic and tangential forces stretch the semi-solidified PVDF fiber.

supply (HP 6515A), and XY control platform (Tanlian ElectroOptics TL-ST9090-50). A high voltage of 10–16 kV is usedand the tip-to-tube distance is about 0.5 mm. The tubecollector is controlled to rotate at velocities of 900 and1900 rpm and the corresponding tangential speed on thesurface is 942.3–1989.3 mm s−1. The XY control platformhas a motion speed of 2 mm s−1 and a traveling distanceof 50 mm. Figure 1(b) shows sequential optical photographsof the formation of the Taylor cone underneath the needleelectrode, the formation and ejection of the polymer jet, andthe deposition of orderly fibers, respectively.

3. The optimal parameters of the HCNFES process

3.1. PVDF polymer solution preparation

PVDF solutions for the HCNFES process were prepared usingthe following steps. Dimethyl sulfoxide (DMSO) was usedas the solvent for PVDF powder (Mw = 534 000). Acetoneand surfactant (ZONYL R©UR) were applied to improve theevaporation rate and to reduce the surface tension of the PVDFsolution, respectively. The ratio of DMSO:acetone was setas 1:1, and the surfactant was fixed at 0.2 g. Various PVDF

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Table 1. The compositions of the PVDF polymer solutions.

PVDF powderSolvent

(DMSO:acetone) Surfactant

Weight percentage(PVDF/solvent) (wt%) Weight (g) DMSO (g) Acetone (g) Weight (g)

16 0.8 2.5 2.5 0.218 0.9 2.5 2.5 0.220 1 2.5 2.5 0.2

Figure 2. Applied DC voltage versus diameter of HCNFES PVDFfibers (the variance of the data point is ∼2%–4%).

solution concentrations of 16, 18, and 20 (wt%) have beenprepared to characterize the viscosity and surface tensionproperties of PVDF solutions in the HCNFES process. Thethree PVDF solutions with various concentrations are listedin table 1. Typically, a solution amount of about 6–6.5 g(5 g solvent) is prepared to assure a correct weighing. First,PVDF powder was stirred in acetone uniformly using acontainer to prevent the evaporation of solvent for 30 min.Second, surfactant and DMSO were uniformly mixed for30 min. Finally, the two solutions were mixed and stirred for60 min forming a homogeneous PVDF solution, which wasplaced in a vacuum chamber for 15 min in order to removeair bubbles. The experimental results show that, when thesolution concentration is lower than 16 wt%, the HCNFESprocess becomes discontinuous and results in non-uniformfiber diameters. PVDF solution with 18 wt% concentration ischosen to achieve good electrospun fibers.

3.2. Operating parameters and fiber diameters

The typical electric poling requirement for PVDF is higherthan 107 V m−1 [5]. Various DC voltages of 10, 12, 14, and16 kV have been tested in this work. The tube rotating velocity(tangential speed on tube surface), XY control platform speed,and PVDF solution concentration are fixed at 1500 rpm(1570.5 mm s−1), 2 mm s−1, and 18 wt%, respectively.The relationship between the DC voltages and diameters ofHCNFES PVDF fibers is shown in figure 2. The error rangeof each fiber diameter under a specific DC voltage is drawnbased on eight samples measured directly from SEM. Each

Figure 3. Rotation velocity versus diameter of HCNFES PVDFfibers (the variance of the data points is ∼2–4%).

data point is averaged from at least three measurements alongthe longitudinal axis of the PVDF fiber. The average fiberdiameter is in the range from 200 nm to 1.16 µm underdifferent applied voltages. In continuous HCNFES, the effectof the Coulombic repulsion force in the polymer jet can bediminished by a short tip-to-tube distance (the tip-to-tubedistance is fixed at 0.5 mm). Compared with the conventionalelectrospinning process, the unstable stretching effect duringthe whipping process is replaced with a stable tangentialforce in the HCNFES process such that the initial polymerjet diameter dominates the final fiber diameter rather thantangential force stretching [23]. It is further noted that asufficiently high voltage results in smaller fiber diameter,which is consistent with the previous results based on theNFES process. In contrast, under a lower voltage, the appliedelectrical field cannot generate sufficient electrostatic forces todeform the polymer meniscus into a symmetrical Taylor cone,which would result in an unstable polymer droplet, making itdifficult to control the ejection of a jet, and thus ejects a thickjet.

Various rotating velocities of 900, 1100, 1300, 1500,1700, and 1900 rpm have been used to fabricate PVDFfibers and the applied DC voltage, XY control platformspeed, and PVDF solution concentration have been fixed at16 kV, 2 mm s−1, and 18 wt%, respectively. Figure 3 showsthe relationship between the diameter of HCNFES PVDFfibers and the rotation speed. Results show that the fiberdiameter is inversely proportional to the rotation speed. Theaverage fiber diameter is in the range of 100 nm–1 µm.Under low rotational speed, the deposition speed of thefibers exceeds the moving speed of the collector, resulting

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Figure 4. (a) Optical photographs showing electrospun PVDF fiber arrays on a glass tube collector. (b) The top optical photograph showswell aligned PVDF NFF on a glass tube collector and a piece of PVDF NFF is easily extracted from the glass tube collector. The bottomSEM image shows dense deposition of fibers to form the NFFs. (c) A close-up SEM image shows some tiny pores on the surface oflarge-diameter fibers. The final solvent evaporation process could have resulted in tiny holes left behind on the surface of the fibers.

in disorganized and disordered collections. On the otherhand, at very high rotational speed, electrospun fibers canbe mechanically broken apart, resulting in discontinuousdeposition of broken fibers. When the tube rotates at speedsbetween 900 to 1900 rpm, good and continuous electrospunfibers are collected with diameters of less than 1.2 µm. Thetangential force caused by the rotating tube provides strongmechanical stretching for the PVDF fibers, which could leadto better dipole alignments along the longitudinal direction ofthe fibers for enhanced β-phase concentrations.

3.3. Material and structural characterizations

The fiber arrays fabricated by HCNFES can have controllablestructural thickness based on layer-by-layer assembly asshown in figures 4(a) and (b). One can easily remove the tubecollector and extract the NFF, as illustrated in figure 4(b).Moreover, after a long period of collection time, there couldbe slight randomness. The main reason could be the presenceof residual charges on the electrospun fibers [30] (figure 4(b)).Defects such as tiny holes and voids are often observed onlarge-diameter PVDF fibers after the fibers have completely

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Figure 5. XRD diffraction characterizations of PVDF NFFs. (a) PVDF powder; (b)–(d) electrospun NFFs using (b) 16 wt%, (c) 18 wt%,and (d) 20 wt% of PVDF solutions and under various DC voltages of 10, 12, 14, and 16 kV.

dried, as shown in figure 4(c). Since the distance betweenthe electrode and collector is short, the solvent may not havecompletely evaporated when the fibers are deposited on thecollector. The final solvent evaporation process could haveresulted in tiny holes left behind on the surface of theselarge-diameter fibers, as shown. Furthermore, it is observedthat, under higher mechanical stretching speed, ultrathinPVDF fibers (less than 1µm) can be constructed, and the extramechanical stretching seems to help alleviate surface defectproblems such as holes and voids.

3.4. Material characterizations of PVDF NFFs

X-ray diffraction (XRD) is used to characterize PVDF NFFsmade by the HCNFES process with in situ electric polingand mechanical stretching. Figure 5(a) is the result from theoriginal PVDF powder before the electrospinning process,and it clearly shows strong presence of the non-polar α-phase,with diffraction peaks at 17.9◦ [100], 18.4◦ [020], 20.1◦ [110],and 26.7◦ [021]. Figures 5(b)–(d) are XRD results forelectrospun PVDF NFFs with concentrations of 16 wt%,18 wt%, and 20 wt%, respectively, and under various DCvoltages of 10, 12, 14, and 16 kV. In these plots, one

can observe the diffraction peaks around 20.6◦, representingβ-phase PVDF. Specifically, figures 5(b) and (c) show a peakat 20.6◦–20.98◦, which corresponds to [110] β-phase [31]while the α-phase peaks, [100], [110], and [021], havedisappeared. It is also observed that at the high concentrationof 20 wt% PVDF (close to its equilibrium solubility limit),homogeneous solution is difficult to achieve [23, 32], and thisreduces the intensity of the β-phase, as shown in figure 5(d).

These XRD results suggest that the β-phase PVDF NFFscan be constructed by the HCNFES process. In order to obtaingood piezoelectricity, high structural density (small fiberdiameter), and smooth surface morphology, it is found that thebest operating parameters in the prototype experiments are thecombination of 18 wt% PVDF with DC voltage of 14 kV andtube rotation velocity of 1900 rpm.

4. Energy harvesting demonstrations

4.1. Mechanical straining rate induced electrical energyoutputs

The setup of a piezoelectric PVDF NFF energy harvester isillustrated in figure 6(a). The construction of the electrospun

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Figure 6. (a) Schematic diagram of the prototype measurement system including the packaged PVDF fiber bundle on a plastic structure. Itis estimated that the fiber bundle has length of 1 mm and diameter of 200 µm and there are about 100 fibers. (b) Voltage generated from thefiber bundle is measured under an applied mechanical strain with a frequency of 5 Hz. The magnitude of the tensile strain is 0.05% within aperiod of 25 ms. The generated peak voltage is about −25 mV under the setup of forward electrical connection. (c) The electrical energyoutput results under the setup of reverse electrical connection. (d) Maximum peak current is about −7.9 nA under the setup of forwardelectrical connection.

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Figure 7. (a) The design of a flexible textile-fiber-based PVDF harvester and (b) a vibration measurement system.

PVDF NFF depends on the layer-by-layer deposition processvia controlling the movements of the rotating glass tube.In the prototype experiments, an NFF sample is assembledin the form of a bundle with length of 1 mm, diameter of200µm for about 100 fibers connected in parallel, and averageindividual fiber diameter of about 1.6 µm. The packagingprocess includes the following steps. First, the PVDF fiberbundle is placed on a PET substrate, with both ends tightlybonded to two Cu foil electrodes. Silver paste is then appliedat both ends and the entire structure is packaged inside athin flexible polymer to maintain its physical stability. The

fiber bundle is characterized under repeated external strainsby bending the assembled package repeatedly. Alternatingvoltage and current can be generated via the transverse mode(d33) piezoelectric effect. Figure 6(b) shows the response ofinduced electric potential when a periodic force is appliedmanually by hand to the PVDF fiber bundle. Under a tensilestrain at 0.05% with a short period of 25 ms and frequencyof 5 Hz, a piezoelectric field (EZ < 0) is created in thefibers to induce a high piezopotential (V+). The piezopotentialcan serve as the driving force for the flow of electrons inthe external load resistor, and the electrons are driven back

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Figure 8. The voltage and current output of a flexible textile-fiber-based PVDF harvester by exciting at 7 Hz with a tensile strain of 0.05%and fast strain rate of 18 ms. (a) Peak voltage is about −76 mV. (b) Peak current is about 49 nAp−p (Imax = −39 nA).

through the load to ground by this potential. During thestretching process, the peak voltage with a load resistor of10 M� is about −25 mV. The peak current is −7.9 nA(figure 6(d)). The piezopotential gradually vanishes due tothe screening effect of the charge carriers and the externalfree carriers are driven to balance this piezopotential. As theapplied tensile strain is released, the piezopotential decreasesand the locally accumulated free carriers at both ends of thefibers can quickly flow, which creates a circular flow of theelectrons in the external circuit. As such, a positive potentialsignal is generated in the strain releasing process. The heightsof the voltage peaks for the stretch and release processesappear to be different as the strain rates are different. When themeasuring system is connected in reverse fashion as illustratedin figure 6(c), the response signals exhibit reversed responsesas compared to those in figure 6(b). This test is importantto validate that the results are coming from the piezoelectricresponses instead of other effects. If the signal is coming fromthe noise or other forms instead of piezoelectric responses,the shape of the response should remain the same even if thepolarity of the contacts has been changed. Since the inducedpiezoelectric response has its own polarity based on the

electrical poling direction, the electrical measurements shouldhave reversed responses when the polarity of the contacts isreversed.

4.2. Power generation from flexible NFFs

Figure 7(a) shows the setup of the PVDF NFFs withnanofibers connected in parallel to collectively contribute thecurrent generations. The aligned PVDF NFF is first placed ona PET substrate with both ends tightly bonded to the two Cufoil electrodes using silver paste. To obtain a better protectionbetween NFF and electrodes, a thin flexible polymer layeris added on top to package the entire structure. Figure 7(b)shows the setup of applied external load via a rotary motorto produce low-frequency excitation. The induced electricoutputs from the harvester are measured using a voltagemeasurement meter (NI 9234, National Instruments) and acurrent measurement meter (CHI 611D electrochemical workstation, CH Instruments). Under the applied load frequencyof 7 Hz with a tensile strain of 0.05% and about 18 msfor the duration of the strain, the generated peak voltage(with a load resistor of 10 M�) is averaged as −76 mVunder the setup of forward electrical connection as shown in

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Figure 9. (a) Output voltage and (b) current of flexibletextile-fiber-based PVDF harvester under different stretch andrelease cycling frequencies of 2–7 Hz with 0.05% strain.

figure 8(a). The maximum power remains at 577.6 pW cm−2.The corresponding peak current is measured at −39 nA asshown in figure 8(b). Due to different strain rates in thestretching and releasing processes, the voltage and currentpeaks appear to be different in these processes.

Moreover, the total amount of charge Q created by onesingle fiber is small. Rapid release of these electrons caninduce a significant electric voltage/current pulse owing toVL ≈ RLQ/1t [33], where VL is measured voltage acrossthe load, 1t is the time interval for the charge releasingprocess and RL is the load resistor. With fundamentalpiezoelectric theory, I = d33EAε/1t [34], where I is thegenerated current, d33 is the piezoelectric constant, E isYoung’s modulus, A is the cross-sectional area, and ε is theapplied strain. The electrical outputs of voltage and currentare affected by strain rate. Figure 9 shows the responses ofa flexible textile-fiber-based PVDF harvester under variouscycling frequencies ranging from 2 to 7 Hz for the sameapplied external tensile strain at 0.05%. The output voltagein figure 9(a) and current in figure 9(b) increases during thestretching cycle under higher frequency as strain rate increasesunder higher frequency. Furthermore, the peak values duringthe stretch and release processes are different and it can beattributed to different strain rates in the two processes.

5. Conclusions

In summary, this study demonstrates the design and fabri-cation of flexible textile-fiber-based PVDF NFF harvestersfabricated by the HCNFES process. The technique ofHCNFES can control the fiber diameter, length, and densityto produce orderly PVDF fiber arrays and NFFs. Underhigh electrical field and strong mechanical stretching, thealignment of dipoles along the longitudinal direction canbe achieved. By adjusting the PVDF solution concentrationto 18 wt%, the DC voltage bias to 14 kV, and the tuberotation velocity to 1900 rpm, good piezoelectric PVDF fiberswith small diameter, smooth surface morphology, and goodpiezoelectric β-phase have been constructed in this work. Thesolidified PVDF fibers can be removed from the collectorand placed onto other substrates as flexible energy harvesters.When the flexible textile-fiber-based PVDF harvester isexcited, electrical outputs of voltage and current are generatedas electrons accumulated at both ends of the fibers are quicklyreleased due to the piezoelectric responses. HCNFES providesa simple yet scalable way to fabricate large bundles of uniformPVDF fibers with good controllability, which could promotethe applications of sub-micro/nano-polymer fibers in variousfields, including green energy, flexible sensing, bio-medicine,and micro/nano-systems.

Acknowledgments

The authors would like to thank the National ScienceCouncil of Taiwan for its financial support under grantNSC-100-2628-E-110-006-MY3. We also sincerely thankthe National Science Council Core Facilities Laboratory forNano-Science and Nano-Technology in National Sun Yat-senUniversity, Kaohsiung-Pingtung area, Taiwan, for support.

References

[1] Wang Z L 2011 Nanogenerators for Self-powered Devices andSystems (Atlanta: Georgia Institute of Technology)ISBN 978-1-4507-8016-2

[2] Vidhate S, Shaito A, Chung J and D’Souza N A 2009Crystallization, mechanical, and rheological behavior ofpolyvinylidene fluoride/carbon nanofiber composites Appl.Polym. Sci. 112 254–60

[3] Fang X Q, Liu J X and Gupta V 2013 Fundamentalformulations and recent achievements in piezoelectricnano-structures: a review Nanoscale 5 1716–26

[4] Chen X, Xu S Y and Yao N 2010 1.6 V nanogenerator formechanical energy harvesting using PZT nanofibers NanoLett. 10 2133–7

[5] Chang J, Dommer M, Chang C and Lin L W 2012Piezoelectric nanofibers for energy scavenging applicationsNano Energy 1 356–71

[6] Chen X, Xu S, Yao N, Xu W and Shi Y 2009 Potentialmeasurement from a single lead ziroconate titanatenanofiber using a nanomanipulator Appl. Phys. Lett.94 253113

[7] Zhang G, Xu S and Shi Y 2011 Electromechanical coupling oflead zirconate titanate nanofibres Micro Nano Lett. 6 59–61

[8] Zhu G, Yang R S, Wang S H and Wang Z L 2010 Flexiblehigh-output nanogenerator based on lateral ZnO nanowirearray Nano Lett. 10 3151–5

10

Smart Mater. Struct. 23 (2014) 025003 Z H Liu et al

[9] Wang Z L and Song J 2006 Piezoelectric nanogeneratorsbased on zinc oxide nanowire arrays Science 312 242–6

[10] Qin Y, Wang X and Wang Z L 2008 Microfibre–nanowirehybrid structure for energy scavenging Nature 451 809–13

[11] Yang R, Qin Y, Dai L and Wang Z L 2008 Power generationwith laterally packaged piezoelectric fine wires NatureNanotechnol. 4 34–9

[12] Cha S N, Seo J S, Kim S M, Kim H J, Park Y J and Kim S W2010 Sound-driven piezoelectric nanowire-basednanogenerators Adv. Mater. 22 4726–30

[13] Chang C, Tran V H, Wang J, Fuh Y K and Lin L W 2010Direct-write piezoelectric polymeric nanogenerator withhigh energy conversion efficiency Nano Lett. 10 726–31

[14] Yee W A, Kotaki M, Liu Y and Lu X 2007 Morphology,polymorphism behavior and molecular orientation ofelectrospun poly(vinylidene fluoride) fibers Polymer48 512–21

[15] Theron A, Zussman E and Yarin A L 2001 Electrostaticfield-assisted alignment of electrospun nanofibersNanotechnology 12 384–90

[16] Reneker D H, Yarin A L, Fong H and Koombhongse S 2000Bending instability of electrically charged liquid jets ofpolymer solutions in electrospinning J. Appl. Phys.87 4531–47

[17] Fennessey S F and Farris R J 2004 Fabrication of aligned andmolecularly oriented electrospun polyacrylonitrilenanofibers and the mechanical behavior of their twistedyarns Polymer 45 4217–25

[18] Kim K W, Lee K H, Khil M S, Ho Y S and Kim H Y 2004The effect of molecular weight and the linear velocity ofdrum surface on the properties of electrospun poly(ethyleneterephthalate) nonwovens Fiber Polym. 5 122–7

[19] Li D, Wang Y and Xia Y 2003 Electrospinning of polymericand ceramic nanofibers as uniaxially aligned arrays NanoLett. 3 1167–71

[20] Theron S A, Zussman E and Yarin A L 2004 Experimentalinvestigation of the governing parameters in theelectrospinning of polymer solutions Polymer 45 2017–30

[21] Liu Z H, Pan C T, Lin L W and Lai H W 2013 Piezoelectricproperties of PVDF/MWCNT nanofiber using near-fieldelectrospinning Sensors Actuators A 193 13–24

[22] Pu J, Yan X, Jiang Y, Chang C and Lin L W 2010Piezoelectric actuation of a direct write electrospun PVDF

fiber IEEE 23th Int. Conf. on Micro Electro MechanicalSystems (MEMS) (Hong Kong, Jan.) pp 1163–6

[23] Sun D, Chang C, Li S and Lin L W 2006 Near-fieldelectrospinning Nano Lett. 6 839–42

[24] Chang J and Lin L W 2011 Large array electrospun PVDFnanogenerators on a flexible substrate Transducers’11: 16thInt. Conf. Solid-State Sensors, Actuators and Microsystems(Beijing, China, June 5–9) pp 747–50

[25] Liu Z H, Lin L W, Pan C T and Ou Z Y 2012 Pre-strainedpiezoelectric PVDF nanofiber array fabricated by near-fieldelectrospining on cylindrical process for flexible energyconversion ICMDME 2012: 1st Int. Conf. Machine DesignManufacturing Engineering (Jeju Island, Korea, May11-12)

[26] Ou Z Y, Liu Z H, Pan C T, Lin L W, Chen Y J and Lai H W2012 Study on piezoelectric properties of near-fieldelectrospinning PVDF/MWCNT nano-fiber NEMS 2012:7th Annual IEEE Int. Conf. Nano/Micro Engineered andMolecular Systems (Kyoto, March) pp 173–6

[27] Dunn S 2003 Determination of cross sectional variation offerroelectric properties for thin film (ca. 500 nm) PZT(30/70) via PFM Integr. Ferroelectr. 59 1505–12

[28] Gu S-Y, Wu Q-L, Ren J and Vancso G J 2005 Mechanicalproperties of a single electrospun fiber and its structuresMacromol. Rapid Commun. 26 716–20

[29] Pu J, Yan X, Jiang Y, Chang C and Lin L 2010 Piezoelectricactuation of direct-write electrospun fibers SensorsActuators A 164 131–6

[30] Yee W A, Kotaki M, Liu Y and Lu X 2007 Morphologypolymorphism behavior and molecular orientation ofelectrospun poly(vinylidene fluoride) fibers Polymer48 512–21

[31] Arshak K, Ryan C and Campion M 2002 Pressure inducedpiezoelectric β-phase crystallization in screen printedpolymer thick films ISE11: 11th Int. Symp. Electretspp 411–4

[32] Chang C, Limkrailassiri K and Lin L W 2008 Continuousnear-field electrospinning for large area deposition oforderly nanofiber patterns Appl. Phys. Lett. 93 123111

[33] Wang Z L 2008 Towards self-powered nanosystems: fromnanogenerators to nanopiezotronics Adv. Funct. Mater.18 3553–67

[34] Sirohi J and Chopra I 2000 Fundamental understanding ofpiezoelectric strain sensors J. Intell. Mater. Syst. Struct.11 246–75

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