High-temperature property of flexible polyvinylidene fluoride/(Na0.5K0.5)NbO3 vibration energy harvester

Norihisa MATSUBARA, Teruaki FUCHIGAMI* and Ken-ichi KAKIMOTO*,**,³

Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya Institute of Technology,Gokiso-cho, Showa-ku, Nagoya 466–8555, Japan*Department of Life Science and Applied Chemistry, Graduate School of Engineering, Nagoya Institute of Technology,Gokiso-cho, Showa-ku, Nagoya 466–8555, Japan

**Frontier Research Institute for Materials Science, Nagoya Institute of Technology,Gokiso-cho, Showa-ku, Nagoya 466–8555, Japan

Flexible polyvinylidene fluoride/(Na0.5K0.5)NbO3 (PVDF/NKN) composite harvester was first characterized at high temperature.The origin of high flexibility is the unique structure assembled by alternately stacking four-sheets and three-fabrics (4-3harvester). The 4-3 harvester was fabricated by uniformly dispersing piezoelectric NKN particles within a PVDF-sheet and aPVDF-fabric layer. Electric power-output of the 4-3 harvester was measured by using a custom-designed apparatus in thetemperature range from 25 to 150°C and was compared with that of PVDF/BaTiO3 (BT) harvester. The PVDF/NKN 4-3harvesters and PVDF/BT 4-3 harvesters showed significantly different temperature dependences in the temperature range from25 to 150°C. Electric power-output obtained at 25°C by applying an oscillation at 75Hz in the thickness direction for the PVDF/NKN 4-3 harvesters and PVDF/BT 4-3 harvesters were 5.7 and 3.5 nW/cm2 per an amplitude, respectively. In particular, attemperature above 130°C, the PVDF/NKN 4-3 harvester showed a stable output performance from 3.3 to 3.8 nW/cm2, whereasthe PVDF/BT 4-3 harvester demonstrated no electric power-output. Thus, the harvester composed of high TC piezoelectricparticles such as NKN is suitable as a vibration energy harvester operated under high-temperature environment.©2017 The Ceramic Society of Japan. All rights reserved.

Key-words : Piezoelectric vibration energy harvester, Polymer/ceramic composite, Lead-free piezoelectric material, Sodiumpotassium niobate, Electrospinning, High-temperature measurement

[Received December 22, 2016; Accepted February 7, 2017]

1. Introduction

Energy harvesting technologies such as solar photovoltaic gen-eration, wind power generation, and vibration-powered generatorhave been paid much attention as an eco-friendly energy tech-nology because they can convert an energy discarded from thesurrounding environment into electrical energy.1)­5) Among thesetechnologies, piezoelectric vibration energy harvesters (PVEHs)has been expected to realize the stand-alone power system for thesensor device. Using PVEHs as an electric power supply systemin the sensor devices, it is possible to obtain relatively high electricpower-output which is enough to operate the sensor devices, thus,it will be possible to reduce a number of cables for driving thedevices and to miniaturize the industrial sensor devices.6)­10)

Numerous studies have been performed so far to improve thegenerating efficiency of PVEHs.11)­15) For instance, designingmore efficient piezoelectric device structure such as cantilevershape, thin-film, and fiber structures has been studied.16)­20) Thecombination of flexibility and power generation is important toconvert vibration into electric energy for PVEHs. Therefore, acombination of piezoelectric ceramics and polymers in the bestway to obtain both advantages of two materials such as goodpower generation and a mechanical flexibility. Our groups haveproposed that the novel multilayer structure composed of piezo-

electric (Na,K)NbO3 (NKN) particles and fibrous polymer.21)

It showed good mechanical flexibility and a maximum power-output of 145 nW/cm2 per an amplitude in the longitudinal direc-tion, and the power-output depended on a direction of the appliedvibration due to a change of strain propagation from polymermatrix to piezoelectric ceramics. The multilayer structure wascomposed of four-polyvinyl alcohol (PVA)-sheets and three-polyvinylidene fluoride (PVDF)-fabrics. NKN particles wereuniformly dispersed within the PVA sheets and the PVDF fabrics.The polymer sheets and fabrics with uniformly-dispersed NKNparticles were laminated alternately (4-3 harvester). In our previ-ous reports, electric power-outputs of the proposed 4-3 harvesterwas investigated only at room temperature.21) Sensor devices arealso in great demand at high-temperature environment less than150°C such as a muffler-part and an engine-room of automobile.Seon-Bae Kim et al.22) have investigated the effect of temperatureon the electric power-output for lead zirconate titanate (PZT)-bulkand PZT-MEMS harvester. PZT-MEMS harvester showed lowerthermal degradation rate than PZT-bulk harvester in the temper-ature range from 25 to 100°C. Furthermore, Jingen Wu et al.23)

have reported that electric power-output property of BiScO3­

PbTiO3 (BSPT) bimorph-PVEHs which have a high curie tem-perature (TC ³450°C). A voltage of the BSPT bimorph-PVEHsinitially increased in the temperature range from 25 to 150°C,however, the voltage decreased dramatically above 150°C. Thisphenomenon was related to the change of piezoelectric constante31 with increasing temperature and thermal depolarization effectof the BSPT. Generally, bulk ceramic bimorph-PVEHs showed a

³ Corresponding author: K. Kakimoto; E-mail: kakimoto.kenichi@nitech.ac.jp

‡ Preface for this article: DOI http://doi.org/10.2109/jcersj2.125.P6-1

Journal of the Ceramic Society of Japan 125 [6] 449-453 2017 Full paper

©2017 The Ceramic Society of Japan

DOI http://doi.org/10.2109/jcersj2.16322

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high generation performance, however, bulk ceramics are suscep-tible to fatigue crack when subjected to high frequency cyclicloading or large displacement. Development of flexible PVEHscomposed of polymer/piezoelectric ceramic composite is impor-tant for a practical application of the PVEHs because the PVEHsshould be placed at curved surface such as the muffler-part. Highflexibility of polymer matrix is useful to attach the PVEHs toanyplace. In spite of the fact that many kinds of the PVEHs ofpolymer/piezoelectric ceramic composite have been studied,11),14)

there is no information about temperature dependency of electricpower-output properties as far as we searched.In this study, a 4-3 harvester composed of PVDF sheets and

fabrics with uniformly-dispersed NKN particles were fabricated.PVDF is suitable for the high-temperature environment becauseit is one of the super engineering plastics, and it has good heatresistance properties (the melting point: 150­170°C).24),25) Wemeasured the electric power-output of the PVEHs by the custom-designed apparatus. Furthermore, the d33 values of the heat-treated harvester were measured to determine the depolarizationtemperature (Td) of NKN particles, and the relationship betweenelectric power-output and the Td was also investigated.

2. Experimental methods

NKN powder was synthesized through a solid-state reactionmethod. Na2CO3 (99.9%), K2CO3 (99.9%), and Nb2O5 (99.9%)were weighted according to the formula (Na0.5K0.5)NbO3 andmixed by a wet-ball-milling for 16 h. The mixed powder wassintered by two-step thermal treatments at 910°C for 10 h and at1098°C for 2 h. The sintered powder was grinded for 30min toobtain a homogeneous powder. Polyvinylidene fluoride (PVDF,Takiron Co., Ltd., Japan) was dissolved into N,N-dimethyl-formamide (DMF, Kishida chemical Co., Ltd., Japan) at 110°C toprepare a PVDF solution. The initial concentration of this solutionwas 1.0 g PVDF per 30mL DMF. The NKN powder was addedwith the ratio of 50 vol% into the PVDF solution and that slurrywas stirred for 1 h, and the PVDF/NKN sheets in the thickness of40¯m were obtained by an extrusion technique on an aluminaplate at 110°C from slurry. PVDF/NKN composite fabrics wereprepared as follows. The NKN powder was added with the ratioof 50 vol% put into a PVDF solution (ca. 0.12 g/mL) and thencarbon particles (JEOL Ltd., Japan) were mixed with the ratio of1% into the PVDF/NKN slurry to rise electrical conductivity. It isknown that a uniform fiber is formed via electrospinning by risingelectrical conductivity of the slurry. The mixed PVDF/NKNslurry solution was placed in a 5mL plastic syringe (Terumo Co.,Ltd., Japan) with a needle inner diameter range of 0.80mm. Theflow speed was 1.5ml/h and the distance from the tip of a syringeto a grounded collector was 10 cm. PVDF/NKN compositefabrics were obtained by the electrospinning 18 « 1 kV voltage.Both the fabricated PVDF/NKN fabrics and the PVDF/NKNsheets were cut into 28 © 13mm. Four-PVDF/NKN sheets andthree-PVDF/NKN fabrics were laminated alternately and pressedby a hot pressing at 40MPa and 95°C. The 4-3 lamination waspolarized by corona poling method in the thickness directionunder ³20 kV for 30min. An Ag electrode was formed on thefacing surfaces of the poled samples and a Cu-tape was attachedon that Ag electrode. The prepared vibration energy harvesters(4-3 harvester) were sealed with PTFE-tape film (Chukoh Co.,Ltd., Japan) to adhere the surface electrode. Furthermore, by useof BaTiO3 powder (BT; Kojundo chemical laboratory Co., Ltd.,Japan), PVDF/BT vibration energy harvesters were also fabri-cated by a same fabrication method of 4-3 harvester of PVDF/NKN.

Microstructure and morphology of PVDF/particle fabric wereinvestigated by scanning electron microscopy (SEM; TechnexMighty-8, Japan). The fibrous diameter distribution of the fabricwas determined from over 150 fibers in the SEM images at 5000©magnification using an Image J software. As heat-resistance testof piezoelectric properties, 4-3 harvesters were placed on a hot-plate and heated for 20min at the certain temperature in thetemperature range from 25 to 250°C. After cooling of the 4-3harvesters to room temperature, the piezoelectric d33 values weremeasured by a quasi-static d33 meter (Academia Sinica ZJ-6B,China) at 110Hz. To measure a temperature dependency of theelectric power-output, a custom-designed apparatus was con-structed as schematically shown in Fig. 1. The harvester was fixedat one side, and an external repeated vibration was induced to thethickness direction in the stroke of 2.6¯m at temperature from25 to 150°C and frequency at 75Hz. In order to stabilize thetemperature, the direction of applied vibration was limited to thethickness direction. The harvester was connected with the externalresistor, and impedance matching was optimized to obtain maxi-mum electric power-output (Fig. 2). The output voltages weremonitored by an oscilloscope (Keysight Technologies, USA) andthe electric power-output was calculated. Strain gauges werebonded on the samples, and the tensile mechanical properties ofPVDF/NKN laminated composites were measured in the tem-perature range from 25 to 100°C by a compression stress tester(Imoto Co., Ltd., IMC-90FB, Japan). Young’s modulus was calcu-lated from the value of their strain in the strain range from 0.05 to0.15% at each temperature.

3. Results and discussion

3.1 Structural characterizationThere was no difference in appearance of the PVDF/NKN and

PVDF/BT vibration energy harvesters, and the thickness of twoharvesters was both approximately 300¯m. Figure 3 showedSEM images and histograms in diameter of the PVDF/NKN andPVDF/BT fibers. The average particle sizes of NKN particlesand BT particles were 1.9 « 0.6 and 0.8 « 0.3¯m in diameter,respectively. The average particle sizes were almost same as sizesof starting powders. The average fiber diameter of PVDF/NKNand PVDF/BT fiber were 139 « 54 and 144 « 53 nm, respec-tively. There was no significant change of the diameter in PVDF/

Fig. 1. (a) Schematic illustration of the measurement apparatus forin-situ temperature dependence of electric power-output, (b) side eleva-tion of the measurement apparatus, (c) temperature schedule.

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NKN and PVDF/BT fiber. In addition, both NKN and BT parti-cles were well dispersed in the three-dimensional PVDF fiber andentangled in the PVDF polymer chain. Thus, there was no signifi-cant difference between their morphologies. It is interesting thatthe type of raw material and particle sizes doesn’t affect neitherfiber diameter distribution and fiber morphology.

3.2 Electric power-output characterizationFigure 4 showed piezoelectric d33 values after the heat-

treatment in temperature range from 25 to 250°C. Although a

melting point of PVDF is around 160°C, interestingly, PVDFcomposed ceramics did not melt above Tm and kept the shapeuntil 250°C. The d33 values for the 4-3 harvesters of PVDF/NKNand PVDF/BT were 12 « 1.7 and 7.9 « 1.4 pC/N after heat-treatment at 25°C, respectively. The d33 value of a 4-3 harvesterfabricated from only pure PVDF was also measured, and it was³1.0 pC/N. Therefore, we think the piezoelectric performance isclosely related to the property of dispersed piezoelectric ceramicsand a flexibility of the polymer matrix. The d33 values for the 4-3harvester of PVDF/BT linearly decreased with increasing heat-treatment temperature, and the d33 values were 0 pC/N at tem-perature above 145°C. The 4-3 harvester of PVDF/NKN showeda stable piezoelectric performance from 25 to 115°C. Althoughthe d33 values for the 4-3 harvester of PVDF/NKN were gradu-ally decreased from 145°C, the 4-3 harvester still showed 6.8pC/N even after heat-treatment at 250°C. Matsudo et al.26) havereported the temperature dependence of the electromechanicalcoupling factor kp for NKN ceramics. The kp value at room tem-perature was 0.37 and the values were stable until TO-T (³210°C).The kp was gradually decreased above TO-T. Thus, the degradationof the d33 values for the 4-3 harvester of PVDF/NKN above210°C was caused by the depolarization of NKN particles. V.Sencadas et al.25) reported that the melting point (Tm) of poledand non-poled ¢-PVDF were 156.3 and 160.1°C, respectively.The Tm of PVDF is decided according to the crystalline phase andpoled condition, and flexible fabric layer having a large specificsurface area is sensitive to heat. Thus, the melting of PVDFcaused the structure change of flexible fabric layer, and the d33values for the 4-3 harvester of PVDF/NKN could be decreasedfrom 145 to 250°C.The temperature dependencies of electric power-output charac-

teristics were measured in the thickness direction by the custom-designed equipment at 75Hz in the temperature range from 25 to150°C (Fig. 5). From the SEM images, there were no differencein the fiber morphology of PVDF/NKN and PVDF/BT. On theother hand, their temperature dependencies of the electric power-output were significantly different. The 4-3 harvester of PVDF/BT generated 195mVand 3.5 nW/cm2 per an amplitude at 25°C.The electric power-output of the 4-3 harvester of PVDF/BT wasgradually decreased in the temperature range from 25 to 120°C,and no power output signal was observed above 130°C. Thisresults showed a similar decreasing tendency of piezoelectric d33values for the 4-3 harvester of PVDF/BTwith increasing temper-ature. It is seen that the electric power-output and depolarizationfor temperature were related closely. In contrast, the 4-3 harvester

Fig. 2. (a) Scheme of the experiment setup, (b) the electric power-output dependence for 4-3 harvester on load.

Fig. 3. SEM images of (a) PVDF/NKN 50vol% fiber and (b) PVDF/BT 50 vol% fiber, histograms of (c) PVDF/NKN 50 vol% fiber and(d) PVDF/BT 50 vol% fiber The average particle sizes are 1.9 « 0.6¯mfor NKN particle and 0.8 « 0.3¯m for BT particle, respectively.

Fig. 4. Piezoelectric constant d33 values at room temperature for 4-3harvester of PVDF/NKN (circle) and 4-3 harvester of PVDF/BT(triangle) in the heat-treatment temperature range from 25 to 250°C.

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of PVDF/NKN generated 250mVand 5.7 nW/cm2 per an ampli-tude at 25°C, and both output voltage and electric power-outputwere almost constant from 25 to 70°C. Although the electricpower-output was gradually decreased from 80°C, the 4-3 har-vester of PVDF/NKN still showed generated performance at150°C, and its power-ouwtput was 3.8 nW/cm2 per an amplitude.These differences should be related to the depolarization temper-ature (Td) of the dispersed piezoelectric particles into PVDF. It iswell known that the Curie temperature (TC) for BT is ³130°C,and the depolarization occurs before TC.27) It is assumed that theelectric power-output of the 4-3 harvester of PVDF/BT graduallydecreased by the depolarization of BT particles with increasingtemperature, and above TC of BT, polarization of the BT particleswas not enough to generate the electric power-output. On theother hand, NKN has higher TC (³400°C) than that of BT. Ourgroups have previously revealed that the d33 value of NKNceramics showed an increase tendency up to 150°C by InverseMethod and strain measurement method.28),29) Therefore, theelectric power-output of the 4-3 harvester of PVDF/NKN wasgenerated even at 150°C due to the high depolarization temper-ature of NKN.Figure 6 showed a comparison between the ratio of the

output voltage and piezoelectric d33 values for 4-3 harvester ofPVDF/NKN. The ratio of the output voltage and d33 values forthe 4-3 harvester of PVDF/NKN showed a different tendency

with increasing temperature. The change ratio of d33 values wasapproximately 10% from 25 to 145°C, however, the change ratioof output voltage was 18% from 25 to 150°C. The differenceappeared especially in the temperature range from 80 to 150°C.Temperature dependency of the strain and the stress for a 4-3

lamination of PVDF/NKN measured in the temperature rangefrom 25 to 100°C were shown in Fig. 7(b). The max strain was0.42% at 25°C, and it decreased with increasing temperature.Finally, the max strain decreased to 0.22%. On the other hand,max stress was constant up to 40°C, and it gradually decreasedfrom around 10.1 to 2.89MPa. Figure 7(c) showed the young’smodulus of the 4-3 lamination of PVDF/NKN estimated fromthe result of their strain-stress curves in the strain range from 0.05to 0.15%. Young’s modulus was linearly decreased from 3.35to 2.76GPa with increasing temperature from 25 to 100°C. Thisresult indicated that flexibility of the 4-3 lamination of PVDF/NKN was maintained in the temperature range from 25 to 100°C,and the 4-3 lamination of PVDF/NKN became more flexiblewith increasing temperature. It has been reported that an amor-phous relaxation of PVDF existed around 70°C.30),31) This relaxa-tion induces some mobility within the chains that may initiate theloosing of molecular orientation.25) Thus, the decrease tendency

Fig. 5. Temperature dependency of electric power-output properties for4-3 harvester of PVDF/NKN (circle) and 4-3 harvester of PVDF/BT(triangle) in the temperature range from 25 to 150°C. The solid line isoutput voltage and the dashed line is electric power-output.

Fig. 6. A comparison between the ratio of output voltage and piezo-electric constant d33 for 4-3 harvester of PVDF/NKN. The solid line is anoutput voltage and the dashed line is a piezoelectric constant d33.

Fig. 7. (a) Schematic illustration of tensile test. Temperature depend-ency of (b) the strain and the stress, (c) the Young’s modulus estimatedfrom the stress­strain curves for 4-3 lamination of PVDF/NKN in thetemperature range from 25 to 100°C.

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of Young’s modulus of the 4-3 lamination were related to theproperty of polymer material.In terms of the temperature dependency of the electric power-

output, the 4-3 harvester of PVDF/NKN showed the decreasetendency from 80°C. We expected that the electric power-outputfor the 4-3 harvester of PVDF/NKN showed stable values up to150°C, because the crystal structure of NKN forms an ortho-rhombic perovskite at wide temperature from room temperatureto approximately 200°C.32) Accordingly, the decrease of electricpower-output for the 4-3 harvester of PVDF/NKN could berelated the hardness variation of the polymer/ceramic multilayerstructure, especially the hardness variation of the polymer mate-rial with temperature due to the molecular motion of PVDF.Therefore, a polymer layer with high temperature resistance isnecessary to utilize the characteristic of NKN and to use theharvester for high temperature environment.

4. Conclusions

In this study, the flexible energy harvester with 4-3 multilayerstructure was fabricated by laminating the super engineeringpolymer sheets and fabrics with uniformly dispersed BT or NKNparticles, and the temperature dependency of electric power-output was investigated in the temperature range from 25 to150°C. The electric power-output of the PVDF/BT 4-3 harvesterwas linearly decreased with temperature, and above 130°C, thePVDF/BT 4-3 harvester didn’t generate the power-output. On thecontrary, even at 150°C, the PVDF/NKN 4-3 harvester generated80% of output voltage at 25°C. The decrease could be related tothe thermal property of polymer matrix rather than the property ofNKN particles which have high TC. Therefore, using a polymermatrix which has both flexibility and high temperature resistanceabove 150°C, the harvester including NKN particles is applicableto high-temperature application.

Acknowledgment This research was supported by a Grant-in-Aid for Challenging Exploratory Research (26630313) from theJapan Society for the Promotion of Science.

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