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A New Approach for Surface Poling ofPolyvinylidene Fluoride (PVDF)Yung Ting a , Hariyanto Gunawan a , Chun-Wei Chiu a & Jain-ZhiZhong aa Department of Mechanical Engineering , Chung Yuan ChristianUniversity , No. 200, Chung Pei Rd., Chung , Li , 32023 , TaiwanPublished online: 23 Sep 2013.

To cite this article: Yung Ting , Hariyanto Gunawan , Chun-Wei Chiu & Jain-Zhi Zhong (2013) A NewApproach for Surface Poling of Polyvinylidene Fluoride (PVDF), Ferroelectrics, 446:1, 18-27, DOI:10.1080/00150193.2013.820978

To link to this article: http://dx.doi.org/10.1080/00150193.2013.820978

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Ferroelectrics, 446:18–27, 2013Copyright © Taylor & Francis Group, LLCISSN: 0015-0193 print / 1563-5112 onlineDOI: 10.1080/00150193.2013.820978

A New Approach for Surface Polingof Polyvinylidene Fluoride (PVDF)

YUNG TING,∗ HARIYANTO GUNAWAN, CHUN-WEI CHIU,AND JAIN-ZHI ZHONG

Department of Mechanical Engineering, Chung Yuan Christian University,No. 200, Chung Pei Rd., Chung, Li 32023, Taiwan

PVDF has been popularly used for miscellaneous sensor and energy harvesting pur-poses. The advantages of flexible and light characteristics in particular enhance moreinterest. A new approach of using ITO glass for surface poling is investigated in thisstudy. Instead of using metal electrode onto the PVDF surface, ITO is coated onto glassfor conducting electrode. It can provide an easy way of electrode pattern design. Also,higher electric field can be applied in the process of poling so that higher response isachieved. Properties of PVDF films are characterized by using d31 meter to measurethe piezoelectric constant and using Fourier Transformed Infrared Spectroscopy (FTIR)and Differential Scanning Calorimeter (DSC) to measure β-crystallization. From theexperiment results, it indicates that PVDF film makes use of the ITO glass poling methodwill gain higher output voltage and current as compared to traditional poling technique.Comb type electrode pattern, for example, is efficient to generate higher output responsefor planar polarization and is able to control the desired output voltage and current.Using a whole piece of ITO glass for poling with pattern design can avoid from thedefect of non-uniform polarization at the corners that usually happens in using metalelectrode. Compared to the PVDF embedded in a cantilever and a pressing structure,the latter one will gain more output response.

Keyword PVDF; ITO; surface poling

I. Introduction

In recent decades, polymer material attracts very high attention for researchers to investigatevarious applications of sensor or renewable energy. Polyvinylidene fluoride (PVDF) filmsis one of electroactive material that can generate electricity by using the piezoelectric directeffect. Especially, the flexibility and lightness as well as and good mechanical strength [1,2] would be an ideal candidate. The β-phase in the PVDF films is the most useful phasewith respect to piezoelectric characteristic. Through the mechanical treatment of stretchingand polarization can provoke and change the unpoled α-phase into β-phase. It is known thatenergy generated from PVDF films is quite small. In the previous investigation [3], stackand zig-zag types of PVDF were proposed to increase output power generated by PVDFfilm. It was found that stress effect along the planar direction obtains larger output than thethickness direction. Hence, surface poling is important. Also, stretching, electrode pattern,and poling are influential to the performance of PVDF. Stretching PVDF film to transform

Received November 13, 2012; in final form February 26, 2013.∗Corresponding Author. E-mail: yung@cycu.edu.tw

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New Approach for Surface Poling of PVDF 19

α-phase into β-phase has been studied in [4–6]. It shows that stretching of PVDF filmwith ratio greater than 3 would obtain more β-crystallinity of PVDF film and satisfy forthe later poling. Besides stretching, poling process is significant for increasing the contentof β-phase. Quite a few of poling methods have been investigated [7–10]. Traditionalpoling technique makes use of metal plate to apply electric field in the environment ofgradient increasing temperature. However, high electric field is limited due to occurrenceof flashover or arcing. Corona is another polarization method induced by inhomogeneousstrain coupled with distribution of space charge in the film [8]. A monoenergetic electronbeam was used to irradiate the surface and form space-charge on PVDF film for polingpurpose. With partial penetration electrons, non-uniform polarization in the sample wouldlikely happen [11]. Photo-thermal and photo-induced poling are other methods that generatedipole orientation periodically. A drawback is the thermal stability problem would disturbthe dipole orientation and cause non-uniform dipole orientation across the film thickness[12, 13]. Although the above several methods allow high electric field, small area of polingis the crucial disadvantage. In this article, ITO glass was used as conductor to transmit highelectric field to reorient dipole of PVDF film at room temperature.

Considering the direction of polarization, poling acting perpendicular to film surfaceas depicted in Figure 1 is called the thickness polarization and parallel to film surface asdepicted in Figure 2 is called the planar polarization. Thickness polarization is widely usedfor transducer or sensor. However, capacitance is small due to small thickness of film [14].Also, smaller film thickness would decrease polarization effect and influence crystallinity[15, 16]. Conversely, planar polarization usually associated with pattern design will gainhigher capacitance by increasing electrode length. As illustrated in Figure 2, the interdigitalelectrode (IDE) pattern is widely used to build sensors such as strain gage, surface acousticwave, structural health monitoring, or for energy harvesting applications. Surface polingis used for planar polarization to obtain high sensitivity of IDE [17]. However, in IDEpattern, length, width and number of finger are determinant factors. The polarization is

Figure 1. Thickness polarization.

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Figure 2. Planar polarization.

inhomogeneous in magnitude and direction in the area of electrode finger especially sincedipole is not reorient during poling process.

Generally, IDE pattern is deposited onto piezoelectric material. However, depositionof electrode onto polymer substrate is difficult due to limitation in high temperature pro-cessing of polymer film. Therefore, poor conductivity of electrode on the flexible substratewill cause voltage drop during polarization [18]. Instead, ITO layer is deposited on theglass, which is used as substrate. Thus, good conductivity between ITO layer and glass isobtained.

In addition to the material design, structure of PVDF film is also influential to theperformance. Two fundamental types, the cantilever and pressing structures are used forcomparison.

II. Design of Electrode

In this article, the popular comb type electrode pattern is designed as shown in Figure 2.ITO glass poling technique is proposed in order to enhance piezoelectric charge constantand obtain higher output voltage. In traditional poling method, metal electrode is coatedonto the surface of PVDF film. Limited applied electric field is allowable for flashover andarcing will occur when high electric field is applied. Moreover, the difference of thermalexpansion between the electrode and polymer, and the gas as well as moisture absorptionof polymer induces degradation of conductivity and adhesion will cause voltage dropduring polarization. Different from the traditional poling method, ITO is deposited onto asoda lime float glass, which is used as conductor to transmit electric field. The sandwichstructure constructed with ITO glass—PVDF film—bare glass is carried out poling processat room temperature as shown in Figure 3. In particular, pattern design is usually seen forbetter output electricity and sensitivity. For instance, using the comb type pattern of metalelectrode for poling, non-uniform and inhomogeneous poling effect will happen at thecorners [19]. Using ITO glass poling method allows higher electric field applied in polingis able to solve this problem.

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New Approach for Surface Poling of PVDF 21

Figure 3. Poling with electrode pattern on the sandwich structure.

III. Experiment

A. Poling

PVDF film with thickness of 120 μm is stretched with ratio R = 6 at constant temperature80◦C. ITO layer of thickness 150 nm with comb pattern using magnetron sputtering methodis coated onto a soda lime float glass of 1.1 mm. Equal width w and gap g of electrodepattern is depicted in Figure 4.

Electric field of 700 kv/cm is applied to the sandwich structure illustrated in Figure 3for 15 minutes. Same size of PVDF sample employed by traditional method is selected asan example for comparison with the one made by ITO glass poling method.

B. Characterization

The content of β-phase of PVDF sample is investigated by Differential scanning calorimeter(DSC) and Fourier Transformed Infrared Spectroscopy (FTIR). DSC model of Perkin-ElmerDSC 7 is operated at heating rate of 10◦C/min. Temperature is set in the range of 50∼200◦Cfor the melting heat enthalpy. Perkin-Elmer FTIR Spectrum One is set in with wavenumberin the range of 450–1600 cm−1 with average of 32 scans and resolution of 4 cm−1.

Figure 4. Comb pattern of ITO glass.

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Degree of crystallinity is calculated using differential scanning calorimeter (DSC).Degree of crystallinity can be calculated by [20].

X = �Hf

�H ∗f

(1)

where X is degree of crystallinity, �Hf is the fusion enthalpy of the sample PVDF and�H ∗

f is fusion enthalpy with 100% crystallinity, which is 102.5 J/g given in [21].Fraction of β-phase crystals F(β) in each sample is calculated by [6, 22]:

F (β) = Aβ

(kβ/kα)Aα + Aβ

, (2)

where Aα and Aβ is determined by absorption bands for α- and β-phases, respectively, andcan be estimated by

Aα,β = C.Kα,β .Xα,β .t, (3)

where C is monomer concentration, α and β subscripts refer to the crystalline phases. K isthe absorption coefficient at the respective wave number, and X is degree of crystallinitydefined in eq.(1), t is thickness of PVDF film [22].

C. Electric Response

Piezoelectric charge constant d31 of PVDF is characterized by d31 meter (KCF PM-3500).PVDF sample is bonded by two pieces of polyethylene HDPE plastic. The generally usedcantilever and pressing structures as presented in Figures 5 and 6 are chosen for testing.Corresponding to different point force applied to PVDF sample, output voltage and currentresponse is measured using Keithley Electrometer model of 617.

Figure 5. PVDF embedded in a pressing structure.

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New Approach for Surface Poling of PVDF 23

Figure 6. PVDF embedded in a cantilever structure.

IV. Result and Discussion

A. Material Properties

DSC curve of PVDF sample is shown in Figure 7. As seen, the crystallization endothermicpeak is higher while using ITO poling method. It results from higher electric field can beapplied that will create more dipole orientation. Enthalpy of PVDF sample is estimatedabout 43.25 J/g and 46.87 J/g for traditional and ITO poling methods respectively bymeasuring the effective area of each peak in Figure 7. With the measured fusion enthalpy,degree of crystallinity of PVDF sample is calculated as 0.42 and 0.45 for traditional andITO poling methods respectively by using eq. (1).

The β-phase crystallinity was also confirmed by FTIR spectra as shown in Figure 8. It isseen that the absorption peaks occur at 782 cm−1 for α-phase and 862 cm−1 for β-phase. Theabsorption coefficient Kα and Kβ for the α-phase and β-phase are 6.1×104 and 7.7×104 cm2/

Figure 7. DSC curve of traditional and ITO poling.

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Figure 8. FTIR spectra of traditional and ITO poling.

mole respectively, and the monomer concentration is 0.0305 mole/cm3 [6]. As seen inFigure 8, the intensity of β is higher for ITO than traditional poling method. By using eq.(2)–(3), the β-phase fraction F(β) after stretching process is calculated about 78.6% and84.9% for the conditions of traditional and ITO poling methods respectively.

In traditional poling method, smaller piezoelectric charge coefficient d31 and loweroutput response is obtained. When higher electric field is applied in traditional polingmethod, flashover and arcing is arisen so as to disturb and prevent polarization process[23]. Besides that, deposition of metal electrode onto thin-film type of polymer in high-temperature processing and difference of thermal expansion as well as gas and moistureabsorption would lead to poor conductivity and adhesion; thus, it will cause voltage drop

Figure 9. d31 vs applied voltage (Color figure available online).

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New Approach for Surface Poling of PVDF 25

Figure 10. Traditional vs ITO poling for a cantilever structure (Color figure available online).

during polarization. In ITO poling method, ITO layer is used as an electrode and depositedon soda lime float glass, which preserves good conductivity. Voltage drop during polingcan thus be minimized efficiently. As seen in Figure 9, higher electric field up to 700 kv/cmcan be applied while using ITO. Piezoelectric charge coefficient d31 of PVDF sampleis measured as 28 pC/N and 16 pC/N for ITO poling and traditional poling methodsrespectively. As compared to traditional poling method, piezoelectric charge coefficientd31 and fraction of β-phase are greater mainly due to the more effective dipole orientationwhen applying high electric field.

B. Electricity Generation

Figures 10 and 11 show the voltage and current response of PVDF film constructed as acantilever and a pressing structure by using the ITO and traditional poling methods.

Figure 11. Traditional vs ITO poling for a pressing structure (Color figure available online).

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As seen that, voltage and current response using ITO poling method is much greaterthan the traditional poling method. In comparison the cantilever structure with the pressingstructure, the latter one gains higher response. The outcome results from inhomogeneousstress and stress concentration in one direction in the cantilever structure. However, it isnoted that greater stress effect occurs not in the thickness direction but along the planardirection in the pressing structure. Hence, d31 surface poling is significant and demanding.

V. Conclusions

Developing of PVDF by using the electrode pattern and ITO poling techniques is studiedin this article. The popular comb type electrode pattern is selected as an example for planarpolarization. IR spectra using FTIR is carried out to see the absorption band of sample.The degree of crystallinity is measured about 0.42 and 0.45 using DSC for traditional andITO poling method respectively. The β-fraction is calculated about 78.6% and 84.9% fortraditional and ITO poling method respectively. Piezoelectric charge coefficient d31 is 16and 28 pC/N for traditional and ITO poling methods respectively. With these performanceindices, the conclusion can be drawn that the proposed ITO poling technique is superiorto the traditional method. Moreover, ITO glass itself has small resistance of 12 ohm/sq,can reduce current flow, therefore it can eliminate the occurrence of flashover or arcing.Furthermore, smooth surface of ITO glass is instrumental to complete close contact withPVDF film, which is influential to poling process. For specific pattern design such as combtype, uniform poling effect is achieved without the defect at the corners while using metalelectrode in the traditional method. With d31 surface poling, using pressing structure willobtain larger output response than the cantilever structure.

Acknowledgments

This research was supported by National Science Council under the grant NSC98-2221-E-033-035-MY2, and the project of the specific research fields in the CYCU as grantCYCU-98-CR-ME, and sponsored by Soundwide Technology Corp.

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