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Enhanced piezoelectricity in a robust and harmonious multilayer assembly of electrospun nanofiber mats and microbead-based electrodes Young Won Kim, Han Bit Lee, Si-Mo Yeon, Jean Ho Park, Hye Jin Lee, Jonghun Yoon, and Suk Hee Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18259 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 22, 2018

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ACS Applied Materials & Interfaces

Enhanced piezoelectricity in a robust and harmonious multilayer assembly of electrospun nanofiber mats and microbead-based electrodes

Young Won Kim1,2, Han Bit Lee1,2, Si Mo Yeon1, Jeanho Park1, Hye Jin Lee1, and Jonghun Yoon2, Suk Hee Park1*

1

Micro/Nano Scale Manufacturing R&D Group, Korea Institute of Industrial

Technology, Hanggaul-ro 143, Sangnok-gu, 426-910, Ansan-si, Gyeonggi-do, Republic of Korea 2

Department of Mechanical Engineering, Hanyang University, Ansan-si, Gyeonggi-do, 15588, Republic of Korea

*

To whom correspondence should be addressed.

Tel: +82-31-8040-6829; Fax: +82-31-8040-6820; E-mail: [email protected]

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Here, we present a simple yet highly efficient method to enhance the output performance of a piezoelectric device containing electrospun nanofiber mats. Multiple nanofiber mats were assembled together to harness larger piezoelectric sources in the as-spun fibers, thereby providing enhanced voltage and current outputs compared to those of a single-mat device. In addition to the multilayer assembly, microbead-based electrodes were integrated with the nanofiber mats to deliver a complexed compression and tension force excitation to the piezoelectric layers. A vacuum-packing process was performed to attain a tight and wellorganized assembly of the device components even though the total thickness was several millimeters. The integrated piezoelectric device exhibited a maximum voltage and current of 10.4 V and 2.3 µA, respectively. Furthermore, the robust integrity of the device components could provide high-precision sensitivity to perceive small pressures down to approximately 100 Pa while retaining a linear input-output relationship.

Keywords: piezoelectric nanofiber, electrospinning, microbead, vacuum packing, energy generator


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1. INTRODUCTION A number of piezoelectric materials have been demonstrated to be useful in energy harvesting devices and high-precision sensors because of their unique ability to achieve mechanical-to-electrical energy conversion.1,2 These materials can be classified into two groups, namely, polymer and ceramic. These devices have been developed into functional devices by exploiting advantages or overcoming weaknesses. Polymeric systems typically exhibit inferior piezoelectric performance compared to inorganic ceramic materials because of the lower piezoelectric stress constant and coupling coefficient.3 However, piezoelectric polymers can still be used in a wide range of applications in sensors and energy harvesters owing to their inherent superiority in terms of flexibility and easier processing characteristics. Recently, a geometric scale-down approach for piezoelectric components, which yields components down to microns and nanometers, has attracted considerable attention because of these components’ multiplied and diversified exertion of stress and strain, which leads to an increased piezoelectric potential.4,5 Specifically, nanostructure-based systems are appealing as quasi one- or two-dimensional forms, such as nanowires,6–8 nanorods,9,10 nanofibers,11–16 and nanosheets.17 Among these various forms, nanofibers can be easily implemented in a productive process known as electrospinning, where liquid-phase materials are ejected and stretched to form a long and continuous fiber under a high electric field. The representative semi-crystalline polymers, such as poly(vinylidene fluoride) (PVDF) and its copolymer of poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE), have shown considerable potential in various applications of nanofiber-based piezoelectric devices.2,18–20 In this approach, many electrospun nanofibers with random orientations, which form naturally because of the high electric field, have a preferential direction of polarization that is perpendicular to the planar direction of the fiber mat.21–25 In these cases, the perpendicular piezoelectricity could be achieved without additional electrical poling process owing to the



dipole orientation inherently synchronized with the direction of electric field. Mandal et al. postulated the normal direction of dipoles to the fiber mat and demonstrated the potential for amplified charge generation by stacking the piezoelectric layers in the same direction.22 By assuming that the superposition of nanofiber layers will increase the total dipole moments and enhance the piezoelectric performance, we developed a robust packing process that could embrace multiple stacks of electrospun nanofiber mats. For closed contact between nanofiber mats and other components, the mats and components were enveloped in flexible plastic films that were thermally packed using a vacuum. In addition to the quantitative aspect of the piezoelectric sources, the output performance could be further improved by introducing microbead-based topographic electrodes into the packed device. As the piezoelectric layers were integrated conformably according to the microscale topography of the electrode, the external input force exerted onto the device was distributed as a complex set of tension and compression. These complexed forces finally activated the dipoles of the piezoelectric layers in varied directions, which further amplified the electrical power generation. The enhanced piezoelectric device composed of various components was demonstrated to be both a high-performance power generator and high-precision sensor.

2. MATERIALS AND METHODS 2.1. Electrospinning of piezoelectric nanofiber mats Figure 1a illustrates the overall configuration of the developed device, which includes multiple piezoelectric nanofiber mats, microbead-based electrodes and other main components used for packing. For electrospinning, a polymer solution was prepared by dissolving PVDF-TrFE granules (70/30 Mol%, Piezotech, France) in a mixture (4:6 by


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volume) of acetone and dimethylformamide at a concentration of 14% (w/v). The solution was infused at a flow rate of 60 µL/min through a 25-gauge (0.25 mm inner diameter) metal needle. The electrospinning process was performed under a high electric field of 12 kV with a distance of 20 cm from the needle tip to the collecting surface. The bottom right SEM image in Figure 1a shows the as-spun fibrous mat. The constituent fibers had defect-free and continuous forms with a nanometer-scale diameter of 435 ± 84 nm.

2.2. Close-packed piezo-device integrated with microbeads With regard to the microtopography-based electrode, the topographic features were implemented using a microbead array composed of numerous conductive soldering beads (750 µm diameter, DS Hi-Metal, Korea), as shown in the top right photographs in Figure 1a. The microbead array was transferred onto a supporting sheet that was blade-coated with silver conductive paste (8330S-21G, MG Chemicals, Canada), which created the microbeadbased electrode. For the core of the as-prepared nanofiber mats, a pair of electrodes was assembled in a sandwich form and packed within thin nylon-polyethylene composite films to envelop the entire assembly, as shown in the schematic diagram in Figure 1a. To achieve close interactions between the integrated components, a vacuum was applied inside the packing films while thermally sealing the edges of the device assembly (Figure 1b). The vacuum-assisted packing process not only attained a robustly assembled entity but also achieved a closely packed system that led to the effective delivery of input forces to the piezoelectric core layer.

2.3. Characterization of integrated piezoelectric device The output performances of integrated devices were evaluated by analyzing the



values of voltage and current, which were measured from oscilloscope (DPO2024B, Tektronix, USA) and electrometer (Keithley 6514, Keithley Instruments Inc., USA), respectively. The integrated device was hit by a flat dish which was sufficiently large to cover the surface area of device. The piezoelectric device included a square nanofiber mat with a side length of 25 mm. As the device was fixed on a scale, we could measure the impact force while hitting. For evaluation of sensitivity, the device was loaded on a customized measurement system, which was mainly composed of a shaker and a function generator. The shaker was controlled by the function generator and stimulated the sensor surface (78.5 mm2 in area) with a precisely controlled force or displacement. The applied force and displacement were measured by a built-in load cell (UMMA 5kgf, Dacell, Korea) and a laser displacement sensor (LK-H027 K, Keyence, Japan), respectively. To practically confirm the high sensitivity, dropping tests were carried out with a small leaf, a grain of rice, and a water droplet. They were dropped at a distance of 10 mm from the surface of device.

3. RESULTS AND DISCUSSION 3.1. Piezoelectric performance of electrospun mat Prior to demonstrating the integrated device, we first analyzed the characteristics of electrospun nanofiber mats in terms of piezoelectricity. An electrospun mat with a greater thickness, which implies more constituent fibers and increased dipole moments, would be expected to generate more power. Figure 2a shows that the thickness of the nanofiber mat increases with increases in the electrospinning time. The rate of this increase in thickness became slower at longer electrospinning times, particularly after 30 min. The piling-up nanofibers, as the electrospinning process proceeded, covered the collector surface and


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diminished the electric field of the needle tip-to-collector, which decelerated the productivity of the as-spun fibers. Therefore, it would be more efficient to stack the multiple as-spun mats spun for a shorter time than to use a single thicker mat produced using a longer time. In that sense, we determined the optimal spinning time to be 30 min, which provided an approximately 85 µm-thick mat. Similar to typical piezoelectric polymers, all samples with various spinning times had specific peaks of the β-phase crystalline structures at 508, 840, 1285 and 1431 cm-1 in the Fourier transform infrared (FTIR) spectra, as shown in Figure 2b. In the piezoelectric material, the degree of crystallinity can be evaluated by relative β-phase content which is calculated as =

/ +

Where F(β) is the β-phase content; Aα and Aβ are the absorbance at 766 and 841 cm-1; Kα and Kβ the are the coefficients of absorption at respective wavenumber of which values are 6.1 × 104 and 7.7 × 104 cm2/mol, respectively.26–28 The values of crystallinity were measured 74.7 ± 1.1, 77.4 ± 2.1 and 77.5 ± 1.5 % for the as-spun samples of 5, 30 and 120 min respectively. There was no significant difference between the samples of varied time conditions. These comparable crystallinities of the samples could be also revealed in XRD patterns with a characteristic peak around 19.4°, which has been implied the reflection of βcrystal in PVDF-TrFE (Supporting Information, Figure S1).21,29 To test the piezoelectricity experimentally, the three time-varied samples were integrated into the vacuum-packed devices discussed above. In this experiment, we excluded the microbead array such that the effect of the thickness of the fiber mat could be solely determined. Figure 2c and d show a comparison of piezoelectric output performances in terms of the voltage and current, respectively. Although the spinning time increased from 30 min to 120 min, there was no distinguishable difference in the output voltage and current



between the trials. With these experimental results, the rationale for selecting an intermediate spinning time, such as 30 min, could also be assured considering the piezoelectricity performance as well as the aforementioned productivity.

3.2. Output performance of integrated device To evaluate the resulting piezoelectricity generated from our proposed construction, which was characterized by close packing, multiple stacking, and microtopography-based integration, we prepared several types of piezoelectric devices that varied in terms of the included components and the packing methods. We first confirmed the advantage of close packing by comparing non-packed (N1 and N5) and vacuum-packed (P1 and P5) specimens, where the number indicates the number of integrated nanofiber mats. For the non-packed specimens, the nanofiber mats and electrodes were simply stacked and edge-fixed with an adhesive. Because the components intimately contacted the least possible void space in the vacuum-packed device, the piezoelectric charges could be efficiently transferred across the electrodes. Therefore, the output voltage and current increased by an average of 1.9 and 3.6 times compared with the values of N1 and P1 in Figure 3a and b, respectively. All average values were calculated by picking five positive peaks in the outputs. Similar enhancements were observed in the five-layer stacked specimens (N5 and P5). The output voltage and current of P5 reached up to 0.94 V and 0.29 µA, respectively, compared to 0.55 V and 0.098 µA in N5. Next, beyond the simple piezolayer-to-electrode architecture, we integrated additional microbead-based electrode components inside the piezoelectric device. We expected that the inserted microbeads would generate an auxiliary transmission of mechanical force in contact with the piezoelectric layer. Figure 3c, d and e show the cross-


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section of each packed device depending on the inclusion of microbeads. Compared to the simply packed device without microbeads (Figure 3c), the microbead-integrated devices (Figure 3d and e) exhibited deformations of the core piezoelectric layers, which occurred during the vacuum-packing process with microbeads. When the microbead array was integrated on the single side of the nanofiber mat, the packing pressure forced the microbeads to generate an indentation on the mat, as shown in Figure 3d. Whereas this integration resulted in a slight deformation, the double-side integration, as shown in Figure 3e, caused a large deformation on the nanofiber mat that was naturally in line with the morphology of the microbead array. Owing to the regular arrangement of the microbeads, the core nanofiber mat was embedded in a periodically corrugated shape and integrated with few voids in the packed device. As expected, the microbead-embedded devices (P-SB1, P-SB5, P-DB1 and P-DB5) exhibited higher voltage and current values than the flat-type devices without microbeads, such as N1, N5, P1 and P5 (Figure 3a and b). In the device packed with a single nanofiber mat and a single-side bead array (P-SB1), the average output voltage and current were 0.7 V and 0.32 µA, respectively, representing increases of 1.6 and 1.5 fold compared to the values of the device without beads (P1). Additionally, in the device packed with a five-layer nanofiber mat and a single-side bead array (P-SB5), the output voltage and current values increased to 1.1 V and 0.55 µA, respectively, which are 1.2- and 1.9-fold higher than the values of P5. We could obtain further amplification of the output voltage and current for the device packed with a double-sided bead array (P-DB1 and P-DB5). High voltage and current values of 5.1 V and 1.3 µA, respectively, were achieved for the device integrated with a fivelayer nanofiber mat and a double-side bead array (P-DB5). Compared to the increasing values from P1 to P-DB1, both the voltage and current values increased by an unpredictably large extent from P5 to P-DB5, which increases of 8.2 and 3.1 fold, respectively. Typical



piezoelectric systems have shown the outputs increasing proportionally to the applied pressures.24,30 Likewise, in our device (P-DB5), both outputs of voltage and current were increasingly amplified as the applied pressure increased (Figure 4a and b). When a large pressure as much as 288 kPa was imposed onto the device, the outputs of voltage and current were generated up to 10.4 V and 2.3 µA, respectively.

3.3. Finite element analysis of integrated device We used finite element models to simulate the packed assemblies of the nanofiber mat and microbead array to clarify the combinatory effect of the core components in the device. Following the actual integrated devices, each model included components with realistic dimensions, such as an 80 µm thickness for the five-layer nanofiber mat and a 760 µm diameter for the microbeads. The material properties of the core nanofiber mat were assumed to be the previously known values of PVDF-based materials, including a Young’s modulus of 10.5 MPa, a density of 1.78 g/cm3 of and a Poisson’s ratio of 0.39.31,32 Three models were established depending on the presence of the microbead array, as shown in Figure 5a-c. We analyzed the simulated results of the stress distribution when they were postulated to be exerted by a sufficiently large pressure of 50 kPa on the top. As expected, in the simply packed device (Figure 5a, P5), all elements were under even compression, and the compression was directly transmitted from the external pressure. Furthermore, the microbead-included models displayed a combinative distribution of compression and tension, as shown in Figure 5b and c. Notably, the model with the double-sided bead array (Figure 5c, P-DB5) exhibited a more harmonious distribution of compression-to-tension compared to the model with a single-sided bead array (Figure 5b, P-SB5), which exhibited a compressionbiased distribution. The highest tensile stress of 130 kPa was generated at the convex region


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between the beads in model P-DB5 (Figure 5c). In contrast, model P-SB5 exhibited a maximum tensile stress of only 26 kPa. The exceptionally high outputs of specimen P-DB5 were likely caused by the high levels of tensile stress together with the wide range of the stress distribution. Piezoelectric devices using electrospun nanofibers are typically driven only by compressive force exerted in the vertical direction of the nanofiber mat plane.21–25 These devices have been formulated with their preferred electrical polarization direction to be perpendicular to the planar direction of fiber mat, which stimulates the vertical dipoles though a compressive force (in other words, working in d33 mode). However, previous studies have revealed that the alignment of dipoles is determined by the stretching force of the electric field during electrospinning process, which was arranged to be the longitudinal direction of the as-spun fibers.16,33,34 Such piezoelectric devices preferentially work in d31 mode, which is generally activated under tension along the nanofiber axis. In this sense, when the compressive and tensile factors were both valid for piezoelectricity of electrospun fibers, the ultrahigh outputs of the P-DB5 device could be explained by the synergistic excitation of the compression and tension that were harmoniously distributed in the piezoelectric mat, as shown in Figure 5c.

3.4. Comparison to other piezoelectric devices Table I shows the output performances of several representative piezoelectric devices using electrospun nanofibers made of polymeric materials.13,16,21,22,29,34–40 With monolithic PVDF material, the peak voltage of devices rarely exceeded 1 V. Yu et al. demonstrated a high output voltage value of up to 6 V from their nanogenerator using electrospun composite PVDF nanofibers and carbon nanotubes.39 Dhakras et al. achieved an enhanced output voltage value of 14V by means of multilayer composite structure of PVDF-TrFE fiber mat



and BaTiO3 nanoparticle paste.40 These comparative results of hybrid materials indicate that our device (P-DB5), which generated a comparable voltage of greater than 10 V without using any filler or composite, can be further enhanced when an appropriate composite material is matched. With regard to the peak current of the nanofiber-based energy generators, previous studies except for the functional composites could rarely exceed several hundreds of nA, as shown in Table I. Compared to previous attempts, our device (P-DB5) was far superior, with an average peak current of 2.3 µA. The overwhelmingly high current was likely caused by the strong contact between the piezoelectric mat and microbead electrode, as well as the aforementioned tension-compression complexed excitation. We could closely pack the device components because of the vacuum-assisted packing process, which naturally deformed the piezoelectric layer according to the topography of the microbead array (Figure 3e). The deformation of the piezolayer might increase the total contact area with the electrode. Therefore, the proximity and large contact area would further enhance both the output current and voltage. Furthermore, such robust and close integration also allowed for endurance against repetitive compression for energy generation. It can be seen that the stable outputs were generated during exertion of 1000-cycled compressive force. Additionally, even after two months, the output performance was not changed (Figure 6). The vacuum-assisted packing method confirmed its advantages in terms of not only enhanced piezoelectricity but also protective feature of integrated device. This is expected to provide universal availability in a variety of harsh environment where piezoelectric devices are frequently exposed.

3.5. Sensing performance of integrated device Piezoelectric devices have been applied to not only energy generators but also highsensitivity mechanical sensors.22,33 To confirm their applicability to highly sensitive devices,


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we set and tested our piezoelectric devices using a custom-made measurement system, which was equipped with a function generator, an electrodynamic shaker containing a load cell, and a data acquisition module. Figure 7a shows the output waveforms of our device, which was pushed by the shaker driven by the function generator set at 3 Hz with varied amplitudes. Under three different compressive inputs of less than 1 kPa, the corresponded outputs displayed clearly discernible periodic waveforms even at the input of approximately 100 Pa. Figure 7b shows the positive peaks of the waveforms generated from the four different piezoelectric devices depending on the presence of microbeads and the number of included nanofiber mats (P1, P5, P-DB1 and P-DB5). As expected, the P-DB5 device exhibited the greatest output-per-input slope and the greatest peak values for the overall inputs among all devices. In addition, in terms of input-output linearity, the P-DB1 and P-DB5 devices had minimal deviations from their linear relationship between the input and output, whereas the simply packed devices (P1 and P5) exhibited larger deviations. Figure 7c verified the physical sensing performance of our device. When dropping several light objects onto the device, such as a small leaf (30 mg), a grain of rice (25 mg), and a water droplet (25 mg), this device generated discernible peak signals that corresponded to the objects.

4. CONCLUSION In summary, we report a new concept for a nanogenerator that contains multiple electrospun PVDF-TrFE nanofiber mats and microbead-based electrodes. The integration with multiple, stacked piezoelectric layers enhanced the output voltage and current owing to the quantitative increase in nanofibers as charge-generating sources. In conjunction with the larger capacity of the device, the microbead array in the electrode could further improve the piezoelectric outputs. The microscale periodic curvedness of the bead array generated a



naturally corrugated deformation of the core nanofiber layer according to the microtopography, which enabled the compressive and tensile factors to contribute to the piezoelectric outputs. Using this method, we could obtain a peak voltage of 10.4 V and a peak current of 2.3 µA, which were 43.6- and 39.2-fold greater outputs, respectively, compared to the device simply-integrated (without vacuum packing) with a single mat and no bead array. In addition, we demonstrated the feasibility of our device as a high-sensitivity pressure sensor, retaining linear precision down to approximately 100 Pa. Collectively, these achievements were possible owing to the robust vacuum-assisted packing method, which could flexibly accommodate the various inner components even if they were several millimeters thick. We expect that this versatile packing method can provide many further applications in piezoelectric systems by integrating a variety of functional materials into the device.

ASSOCIATED CONTENT Supporting Information Figure S1: XRD patterns of PVDF-TrFE nanofiber mats electrospun for different spinning time.

ACKNOWLEDGMENT This work was supported by the R&D Convergence Program of the MSIP (Ministry of Science, ICT and Future Planning) and NST (National Research Council of Science & Technology) of the Republic of Korea (Grant CAP-13-1-KITECH) as well as a KITECH (Korea Institute of Industrial Technology) internal project.


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Liu, Z.; Zhang, S.; Jin, Y. M.; Ouyang, H.; Zou, Y.; Wang, X. X.; Xie, L. X.; Li, Z.

Flexible Piezoelectric Nanogenerator in Wearable Self-Powered Active Sensor for Respiration and Healthcare Monitoring. Semicond. Sci. Technol. 2017, 32 (6), 064004. (36) Fang, J.; Wang, X.; Lin, T. Electrical Power Generator from Randomly Oriented Electrospun Poly (Vinylidene Fluoride) Nanofibre Membranes. J. Mater. Chem. 2011, 21 (30), 11088–11091. (37) Wang, X.; Yang, B.; Liu, J.; Zhu, Y.; Yang, C.; He, Q. A Flexible TriboelectricPiezoelectric

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PDMS/MWCNT for Wearable Devices. Sci. Rep. 2016, 6, 36409.


Nanofibers

and

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Figure 1. (a) Illustration of the overall components of the piezoelectric device with microscopic photographs of the microbead array and an SEM image of the electrospun nanofiber mat. (b) Photographs before and after the vacuum-assisted thermal packing process.


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Figure 2. (a) Relationship between the thickness of the nanofiber mat and the electrospinning time. Each thickness is the mean and standard deviation. (b) FTIR spectra of the nanofiber mats electrospun for different times (5, 20, and 120 min). Measurements of the (c) open-circuit voltage and (d) short-circuit current for the three different as-spun nanofiber devices.



Figure 3. Output performances of different types of packed devices in terms of (a) voltage and (b) current. The inset symbols indicate the packing types. Cross-sectional images of the vacuum-packed devices (c) without beads (P5), (d) with a single-side bead array (P-SB5), and (e) with a double-side bead array (P-DB5). The three devices all included a five-layer nanofiber mat. The images in (c), (d) and (e) are at the same magnification.


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Figure 4. Relationship between the output performances of (a) voltage and (b) current varied by the applied pressure using the device with double-side bead array (P-DB5).



Figure 5. Simulated stress distributions of the models of vacuum-packed devices (a) without beads (P5), (b) with a single-side bead array (P-SB5), and c) with a double-side bead array (PDB5).


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Figure 6. Result of durability test performed by exertion of cycled compressive force (around 120 kPa) on the vacuum-packed piezoelectric device (P-DB5). Measurement results after two months (right dashed box).



Figure 7. (a) Output waveforms generated from the P-DB5 device for three different periodic compressions. (b) Relationship between the input pressure and output signals using the devices packed with the varied components. (c) Output signals of the drop test results using a small leaf, a grain of rice, and a water droplet.


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TABLE I. Output performances of various piezoelectric nanogenerators using PVDF-based electrospun nanofiber polymers

Reference

16

34

13

35

36

22

29

37

This paper

38

39

21

40

Voltage (V)

0.03

0.778

1

1.5

6.4

0.8

1.2

2.5

10.4

5.02

6

7.9

14

Current (um)

0.003

N/A

N/A

0.4

N/A

N/A

0.082

N/A

2.3

N/A

N/A

4.5

4.5

Material

PVDF

P(VDF-TrFE)


Composite with inorganic particles


Graphical Table of Contents / Abstract Graphic


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Enhanced Piezoelectricity in a Robust and ... - ACS Publications

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