A

Tectpc©

K

1

mtnqpotobfatapmdtwf

YT

0h

Available online at www.sciencedirect.com

ScienceDirect

Journal of the European Ceramic Society 34 (2014) 2849–2855

A new poling method for piezoelectric ceramics with thick film

Yung Ting ∗, Hariyanto Gunawan, Jain-Zhi Zhong, Chun-Wei ChiuDepartment of Mechanical Engineering, Chung Yuan Christian University, Chung Li 32023, Taiwan

Available online 24 January 2014

bstract

he poling process plays an important role in the interaction with the crystal structure in a number of ways to obtain ferroelectricity and piezo-lectricity. However, many factors influence the effectiveness of poling process. In the conventional poling process, electric field is applied to theeramics via the metal electrode. The drawback of conventional poling method is the occasional occurrence of fracture and crack due to subjectiono high electric voltage. In this study, a new poling method by using ITO (Indium Tin Oxide) glass for piezoelectric ceramics with thick film is

roposed. The constraint of applying high electric field has been overcome and is verified with the use of energy level concept. As compared toonventional method, using ITO for poling will preserve better performance via various experimental testing.

2014 Elsevier Ltd. All rights reserved.

eywords: Piezoelectric ceramics; Thick film; Poling; ITO

ccmdiaaSamiiwdaoetp

. Introduction

The piezoelectric ceramic material is popularly used inany applications due to its intrinsic electro-elastic interaction

hrough piezoelectricity or ferroelectricity effects. The combi-ation of piezoelectric material and thin film technology attractsuite a few of research works. In this decade, thick film ofiezoelectric material is used to make miscellaneous actuatorsr sensors such as motor, vibrator, surface acoustic wave sensor,orque sensor, and pyroelectric detector.1–3 Other advantagesf ferroelectric ceramics are chemically and mechanically sta-le at room temperature, and relatively easy to prepare in theorm of polycrystalline samples.4 Piezoelectric materials gener-te an electrical potential in response to an applied stress. Notehat, all ferroelectrics are piezoelectric but not all piezoelectricre ferroelectric. Also some ferroelectrics show a very smalliezoelectric effect. As known, crystalline of the piezoelectricaterial influences the piezoelectricity and ferroelectricity. A

ipole is generated by a shift of charge inside the crystal lat-

ice or a distortion of the crystal. Therefore, poling can interactith crystal in numerous ways, and determines the quality and

unction of piezoelectricity as well as ferroelectricity. In the

∗ Corresponding author at: Department of Mechanical Engineering, Chunguan Christian University, 200 Chung Pei Road, Chung Li 32023, Taiwan.el.: +886 3 265 4319; fax: +886 3 2654399.

E-mail address: yung@cycu.edu.tw (Y. Ting).

sccatmoZa

955-2219/$ – see front matter © 2014 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.jeurceramsoc.2014.01.010

onventional poling process, an electric field is applied to theeramic via the metal electrode that generates rotating of dipoleoment in the direction of the applied electric field. However,

efects such as fracture, crack, dislocation are the problems existn conventional poling under high applied electric field.5,6 Quite

few of research works have been investigated on the causend effect between electric field and fracture. To name a few,uo et al. connected the fracture concept of energy release ratend stress intensity factor to piezoelectric ceramics by imple-enting another factor of electrical energy release rate and field

ntensity.7,8 Park et al. investigated similar research by apply-ng electric field parallel and perpendicular to the crack.9 Itas concluded that the release rate of mechanical strain energyefined by the crack closure integral is a suitable parameter for

fracture criterion of piezoelectric ceramics. Gao et al. devel-ped a strip electrical saturation model to explain the fractureffect resultant from applied electric field.10 Ru used strip satura-ion model to study the conducting crack with limited electricalolarization.11 Yang et al. inspected the shielding effect by thewitch wake of ferroelectric domain to ascertain the cause ofrack.12 Suo explored different conducting path and found basicracking mechanism while the switching zone was localizedround the end of a conducting electrode.13 Electric field at theip of conducting layer is non-homogenous and much higher in

agnitude than the nominal applied electric field. Lynch et al.bserved the dielectric breakdown happened inside the crack.14

hang et al. found local partial electric discharge would maken impermeable crack and conduct electricity as well as change

2850 Y. Ting et al. / Journal of the European Ce

Nomenclature

EA acceptor levelEC conduction bandECN conduction band ITOECP conduction band PZTED donor levelEF Fermi levelEFM Fermi level metalEFN Fermi level ITOEFP Fermi level PZTEG energy gapEGN energy gap ITOEGP energy gap PZTEV valence bandEVN valence band ITOEVP valence band PZTID inside diameter of PZT sampleOD outside diameter of PZT sampleVbiN built-in potential barrier ITOVbiP built-in potential barrier PZTWM work function metalWN work function ITOWP work function PZTXN electron affinity ITOXP electron affinity PZT�EC conduction level difference�EF Fermi level difference�EV valence level difference

tspatlepoauutrgIIfgfiecoc

ditIWtah

2

tmmetpddtbcmltCeofi

wEvcUdolbtosIpdsedbFsaand such a semiconductor is called n-type impurity semicon-

φB potential barrier (Schottky barrier)

he failure behavior of piezoelectric materials.15 All the abovetudies imply that crack occurs due to high electric field in therocess of poling. Crack appears will lead to electric breakdownnd affect polarization. Although an attempt is to limit the elec-ric field in poling in order to avoid from cracking, however,ow piezoelectric constant is obtained. Hence, applying highlectric field to preserve good piezoelectricity without crackinghenomenon is demanding. In this article, a new poling methodf using Indium Tin Oxide (ITO) glass is proposed, which willchieve the target without the disadvantaged problems whilesing conventional poling method. ITO coated glass is widelysed for a number of applications due to its electrical conduc-ivity and optical transparency.16,17 ITO provides small surfaceoughness with size of crystallites of 10–15 nm, which are homo-eneous and relatively flat. With small electrical resistance ofTO, limited current is allowed to flow through the ceramics.TO is an ideal candidate of optimum charge carrier transportor having high energy band.18 In our previous work,19,20 ITOlass was used for poling the polymer material. Higher electriceld can be applied during polarization so that more dipole is ori-nted. As a result, higher piezoelectric constant is obtained. The

onventional poling method utilizes metal electrode depositednto surface of piezoelectric ceramics that would easily producerack/microcrack, and cause electrical breakdown as well as

ddF

ramic Society 34 (2014) 2849–2855

isturb polarization. Different from using metal electrode, ITOs deposited onto a soda lime float glass and used as a conductoro transmit electric field. Sandwich structure constructed withTO glass – Piezoceramics – ITO glass is employed for poling.

ith high energy band, ITO is able to obtain high peak veloci-ies. Therefore, high electron transfer interface between the ITOnd piezoelectric ceramics is formed, that allows for applyingigher electric field.

. Energy band concept

Poling condition is required to obtain optimal piezoelec-ric characteristics; however, it significantly varies for differentaterials. Generally, the purpose of poling process is to induceaximum degree of domain alignment by implementing low-

st electric field at temperature as close as possible to roomemperature. Electrode is usually coated onto the surface ofiezoelectric ceramic to transmit electric field so as to rotateomain orientation. Quality of piezoelectric constant is stronglyependent material aspects and the interface between the elec-rodes of piezoelectric material. Phenomena of conductivity cane described by the concentration and the mobility of chargearrier. If the atoms are far away from each other, the interactionay be disregarded, and the electrons will have the same energy

evels. Conversely, if the atoms close each other and form crystal,he electron levels are split into mutual interaction of electrons.onsider applying electric field to crystal for polarization forxample, can generate negative charge with respect to positivene. This circumstance means the valence band is completelylled energy of valence electron as depicted in Fig. 1.21

Fig. 1 shows the band model of various types of materialshere the shaded areas represent filling of energy band, andG is energy gap.21 The higher energy electrons go beyond thealence band as an effect of applying electric field. So they areapable of moving through the crystal and making conduction.nlike the condition shown in Fig. 1(a), good electrical con-uctivity of many metals is achieved results from overlappingf the conduction and valence bands since there is a sufficientlyarge number of electrons can move shown in Fig. 1(b). Theand model of ceramics without electron capability of movinghrough the crystal is an insulator shown in Fig. 1(c). Band modelf semiconductor has small energy gap expressed by EG < 2 eV,o the electrons can move while encountered with electric field.f a semiconductor with electron holes in the valence band islaced in the electric field, electrons drifting in between the con-uction band and valence band may happen. Therefore, jump ofuch an electron to electron hole is possible. Energy level oflectron in the energy gap is illustrated in Fig. 2. Fermi level isefined as the highest occupied molecular orbital in the valenceand at 0 K, in which many states are available to accept electron.or semiconductor, the Fermi level is located in the band gapince the valence and conduction band is separate. The impuritytoms give up electrons to the conduction band is called donors,

uctor since the charge carrier is of negatively charged electronsrifting in the conduction band. In donor-type of energy gap, theermi level EF is close to the lower edge of conduction band EC,

Y. Ting et al. / Journal of the European Ceramic Society 34 (2014) 2849–2855 2851

etals,

aFhtaiiItg

hnEetto

oeeeeatiacwwrrI

Fo

damTttdcePPiFtcjthe electric current flows only in one direction, and rectificationoccurs. Reduction of the barrier of the electron is instrumentalto inject electron successfully.25 To achieve the same purpose,

Fig. 1. Band model: (a) and (b) m

nd the donor levels ED is greater than EF but lower than EC.ig. 2(b) shows another type of impurity semiconductor, whichas high electron and forms negative ions easily. Therefore, elec-rons from the valence band are trapped by impurity atoms. Thetoms trap electrons is called acceptor and the semiconductors called p-type impurity semiconductor since the charge carriers of positively charged electron drifting in the valence band.22

n acceptor-type of energy gap, the Fermi level EF is close tohe upper edge of valence band EV, and the acceptor level EA isreater than EV but lower than EF.

Considering the charge carrier in terms of band model willave large conduction and valence bands; in other words, a largeumber of energy moves electrons or electron holes in a crystal.lectric field applied to the crystal will change the motions oflectrons against the field with a given velocity. The higher elec-ric field is applied, the higher mobility of electron is achievedo increase ions migration. As a result, more polarization isbtained.

In Fig. 3(a), unlike semiconductor, note that the Fermi levelf metal electrode separates the energy levels of occupied andmpty electrons, and the work function of a metal implies itslectron affinity as well as its ionization energy. When the metallectrode attached to PZT, both of the Fermi levels must bequivalent, i.e., EFM = EFP, presented in Fig. 3(b). As explainedbove, the electrons will be trapped in p-type semiconductor, sohat a high electron affinity will occur and easily form negativeons; therefore, the surface acceptors become negatively chargednd the downward depletion of surface band is happened as aonsequence. The change of band edge of semiconductor (PZT)ith respect to new equilibrium value is seen in Fig. 3(b). Theork function of metal and PZT is expressed by WM and WP

espectively. Their difference is the built-in potential VbiP thatepresents the barrier need to overcome and obtain current flow.n PZT, a downward bending of the energy band will not only

ig. 2. Impurity levels of the (a) donor-type, (b) acceptor-type associated withccurrence of impurity atoms in solid solution in crystals.

Fb

(c) insulator, (d) semiconductor.

ecrease the concentration of hole to result in lower charge butlso increase the ionization of acceptor to result in local for-ation of depletion as well as large amount of space charge.he barrier of hole is smaller than that of electron in the con-

act between the metal electrode and PZT, which is favor ofhe injection of hole. Ideally, if the work function of semicon-uctor (PZT) WP is greater than that of metal WM, an ohmicontact happens to allow the electric current to freely flow inither direction across interface. However, the work function ofZT, in fact, is smaller than that of metal. For example, usingt as metal, the work function of metal is WPt = 5.65 eV which

s greater than the work function of PZT WP = 4.53 eV.23,24 Inig. 3(b), while an electric field is applied to the metal elec-

rode, electron injection on the conduction band occurs withurrent flow, however, electrons from the metal are difficult toump over a large potential barrier (Schottky barrier) φB. Hence,

ig. 3. Band diagram of interface between the metal electrode and PZT (a)efore contact, (b) after contact.

2852 Y. Ting et al. / Journal of the European Ceramic Society 34 (2014) 2849–2855

F(

etfdcclmrtPvrpbh

oTtssiaia

Ft

bNtbVrBEalachtteIebtddfacsdbca

3

3

al1tt

ig. 4. Band diagram of interface between the PZT and ITO (a) before contact,b) after contact.

lectrons can overcome the barrier while applying higher elec-ric field. Nonetheless, applying high electric field has the sameunction as the breakdown voltage has become; therefore, break-own effect is more probable to ensue.26 As the voltage andurrent increase, a net charge in the interface forms double-arrier injection and will initiate breakdown effect. In fact, aarge amount of conducting electrons exist in the contact of the

etal and PZT. Thus, breakdown occurs when the ionizationate exceeds the recombination rate.27 Besides that, for the p-ype semiconductor like PZT, conduction of electron or hole inZT is not possible because the charge carrier cannot be pro-ided in the contact between the metal electrode and PZT due toelatively large difference between EF and EV. As a result, theositive space charge required to induce bending of the energyand cannot be generated, and the electron hole flow is hardlyappened.

With the band model presented in Fig. 1(d), energy gapf the semiconductors, PZT (Pb(Zr,Ti)O3) and ITO (Indiumin Oxide), is 3.4 eV and 4.2 eV respectively.23,28 The con-

act between them is determined by the alignment of the bandtructures across the interface and the possible existence ofpace-charge layers as well as the action of electronegativ-ty. Since semiconductor exhibits different electronegativities,

charge transferred across the interface will occur, and annterface dipole will then be formed as expected. PZT as ancceptor possesses an abundance of p-type impurities indexed

a–r

ig. 5. Sandwich structure of ITO glass – Piezoceramics – ITO glass at roomemperature.

y P, whereas ITO as a donor prefers n-type impurity indexed by. Following Fig. 3, the ideal band diagram of interface between

he PZT and ITO is shown in Fig. 4. Depletion with upwardending is happened in ITO, therefore, the built-in potentialbiN is equal to the difference between WP and WN that rep-

esents the barrier need to overcome and obtain current flow.oth PZT and ITO have their conductive potential ECP andCN, valence potential EVP and EVN, and electron affinity XP

nd XN. When the difference between them in the conductionevel and the valence level, i.e., �EC = ECN − ECP = XN − XP

nd �EV = EVP − EVN = (EGN + XN) − (EGP + XP), are not zero,harge transfer happens and there is a charge flow from theigher energy level (ITO) to the lower one (PZT). In Fig. 4,he valence band of PZT with energy level of EVP is greaterhan the valence band of ITO with energy level of EVN, so thelectron holes near valence band of PZT will penetrate intoTO. Similarly, electrons in the conduction band of PZT withnergy level of ECP will infiltrate into ITO. As known, the bondsetween two semiconductors are covalent bond, which are par-ially ionic, so that the interface dipole is expected to obtain. Theifference between the electronegativities of PZT and ITO mayetermine the dipole contribution influential to the charge trans-erred across the interface. This interface dipole contributionffects the domain orientation of PZT, so higher piezoelectriconstant can be expected as a consequence. Since constructiontructure between the PZT and ITO is heterostructure semicon-uctor, the electrons and holes transfer across the interface cane happened in two directions, and their distribution and flowan thus be controlled. As a result high electric field can bepplied to this structure.

. Experiment

.1. Poling

New poling technique using ITO glass is proposed in thisrticle. ITO glass is used in our approach that consists of an ITOayer of 150 nm thickness coated onto a soda lime float glass of.1 mm to obtain electrical resistance of 12 �/sq.29 Piezoelec-ric ceramic PZT 5A with ring type (OD = 25 mm, ID = 4 mm,hickness = 0.5 mm) is used in the experiment. Electric field is

pplied to the sandwich structure constructed with ITO glass

Piezoceramics – ITO glass. Two polarization conditions, atoom temperature shown in Fig. 5 and in silicone oil medium

Y. Ting et al. / Journal of the European Ceramic Society 34 (2014) 2849–2855 2853

Fa

aTt

3

tEw4srm

4

4

eimtob

tit

4

te(otpaprti

ipr

ig. 6. Sandwich structure of ITO glass – Piezoceramics – ITO glass at temper-ture of 120 ◦C.

t temperature of 120 ◦C shown in Fig. 6, are considered to use.he electric field in the range of 800 V to 2.8 kV is applied in

he process of poling for 15 min.

.2. Characterization

Fourier Transformed Infrared Spectroscopy (FTIR) is usedo investigate the PZT material of poling and no-poling. Perkin-lmer FTIR Spectrum One is set in the range of 450–1600 cm−1

ave number with average of 32 scans and a resolution of cm−1. X-ray diffraction (XRD) and scanning electron micro-cope (SEM) is also used to observe the perovskite phase of PZTespectively. The piezoelectric coefficient d33 of the ceramics iseasured by d33 meter.

. Result and discussion

.1. FTIR

Fig. 7 shows the phase transition of ferroelectric for differ-nt poling conditions measured by FTIR. The intensity value isncreased after poling. Applying higher electric field is instru-

ental to increase intensity. Apparently, using ITO is better

han the conventional method. Moreover, poling with siliconeil obtains higher intensity than that without silicone oil mainlyecause the temperature is raised above 120 ◦C much higher

Fig. 7. FTIR spectra of ceramic at different poling.

wpc

4

tgabtibtwPF

Fig. 8. Phase of sample PZT measured by XRD.

han poling at room temperature of 27 ◦C without silicone oil. Its known that temperature is influential to atom mobility. Orien-ation of the dipole thus becomes easier in higher temperature.

.2. X-ray diffraction (XRD)

XRD characterization of sample PZT is shown in Fig. 8. Notehat, the sample exhibits dominant crystal orientation of ferro-lectric perovskite phase (1 1 0) and minor rhombohedral phase1 1 1) and tetragonal phase (2 0 0). Phenomenon of co-existencef various phases is observed, which is similar to previous inves-igation works.30–32 As known, composition near morphotropichase boundary (MPB), which is between the rhombohedralnd tetragonal regions, indicates an explicit enhancement ofiezoelectric properties.33 Therefore, electrical property in theegion of polymorphic phase transition, i.e., from rhombohedralo tetragonal phase, will obtain optimal properties due to thencreased dipolar states.

While no poling on the PZT sample shown in Fig. 8, theres no peak of perovskite, rhombohedral, and tetragonal. Multieaks appear after poling, which implies occurrence of dipoleotation. In Fig. 8, higher electric field up to 2.8 kV can be appliedhile using ITO with silicone oil. Hence, the proposed ITOoling method preserves better results in comparison with theonventional poling method.

.3. Scanning electron microscope (SEM)

SEM micrograph is used to investigate the surface polariza-ion of piezoelectric material. Brightness-contrast image distin-uishes the positive from negative domains. Fig. 9 shows the topnd side view of the sample. The image of positive domains isrighter than that of the negative domains. In Fig. 9(a), orienta-ion of dipole is random before poling. After poling, the dipoles attempted to rotate in one direction. However, it is impossi-le to make 100% reorientation of dipole. Poling conditions forhe case studies in Fig. 9(b) and (c) with no (w/o) silicon oil as

ell as those in Fig. 9(d) and (e) with silicon oil are compared.ercentage of bright-dark area in Fig. 9(b) is higher than inig. 9(c) indicates the dipole direction is more uniform and will

2854 Y. Ting et al. / Journal of the European Ceramic Society 34 (2014) 2849–2855

Fig. 9. SEM micrograph of Piezoelectric ceramic of top and front view: (a) Nopoling; (b) ITO poling w/o oil, 900 V; (c) ITO poling w/o oil, 800 V; (d) ITOpoling w/oil, 800 V; (e) ITO poling w/oil, 900 V; (f) ITO poling w/oil, 2 kV.

Table 1d33 properties.

Condition d33 (pC/N)

Conventional, electric field E = 0.8 kV 283ITO poling w/o oil, E = 0.8 kV 296ITO poling w/o oil, E = 0.9 kV 328ITO poling w/oil, E = 0.8 kV 250ITO poling w/oil, E = 0.9 kV 317II

pvoib

thpt

4

dc

4

icsao

Fe

TO poling w/oil, E = 2 kV 344TO poling w/oil, E = 2.8 kV 380

reserve higher ferroelectric properties while applying higheroltage. Similar results are concluded for the cases with siliconil shown in Fig. 9(d) and (e). As expected, better performances reached for the case study of using silicon oil. In particular,etter result will achieve while 2 kV is applied in poling.

It is known that only boundary contrast can be observed onhe polar surface. Fig. 9(b)–(f) shows most of boundary domainsave been poled and expressed with bright contrast areas. Thishenomenon of polar surface is related to pyroelectric proper-ies, which is not further addressed in this article.

.4. Piezoelectric d33 properties

Piezoelectric strain coefficient of the sample is measured by33 meter. Table 1 shows the d33 properties under different polingonditions.

.5. Impedance and phase

Impedance and phase of the sample is measured bympedance analyzer. The capacitance of piezoelectric ceramican be calculated by using frequency, phase and impedance

hown in Fig. 10 for an example of poling with no oil as wells electric field 900 V. The impedance, phase and capacitancef the sample under different poling conditions are listed in

ig. 10. (a) Impedance and (b) phase of ceramic for ITO poling w/o oil andlectric field 900 V.

Y. Ting et al. / Journal of the European Ceramic Society 34 (2014) 2849–2855 2855

Table 2Impedance and phase.

Condition Impedance (�) Frequency (kHz) Capacitance (nF)

Conventional, E = 0.8 kV 28.364 74.4 10.5ITO poling w/o oil, E = 0.8 kV 37.3 72.8 9.1ITO poling w/o oil, E = 0.9 kV 28.1 74 11.2ITO poling w/oil, E = 0.8 kV 26.3 74.4 9.8ITO poling w/oil, E = 0.9 kV 29.3 72.8 13.7II

Tdh

5

aeetaHtotFt

A

ub

R

1

1

1

1

1

1

1

1

1

1

2

2

2

2

2

2

22

2

23

3

3

TO poling w/oil, E = 2 kV 23.6

TO poling w/oil, E = 2.8 kV 22.8

able 2. Large capacitance indicates PZT gains higher actuatingisplacement and energy store. Therefore, the case of applyingigher electric field is superior to the others.

. Conclusions

Via energy level concept, the reason of using ITO for polingllowed to employ with higher electric field is investigated andstablished. Different from the conventional method using metallectrode, ITO is used for poling. From experiment, it is verifiedhat using ITO can significantly increase the poling voltage rangend reduce the prevalence of cracking of piezoelectric material.igh electric field up to 2.8 kV/cm can be applied to the ceramic

hick film while using ITO. With high electric field, orientationf dipole is more effective; therefore the ferroelectric proper-ies are improved. Several different testing methods includingTIR, XRD, SEM, and impedance analyzer, are implemented

o characterize the function of using ITO poling method.

cknowledgments

This research was supported by National Science Councilnder the Grant NSC98-2221-E-033-035-MY2, and sponsoredy Soundwide Technology Corp.

eferences

1. Kushida K, Takeuchi H. Piezoelectricity of c-axis oriented PbTiO3 thinfilms. Appl Phys Lett 1987;50:1800.

2. Srewenivas K, Sayer M. Characterization of Pb(Zr,Ti)O3 thin film depositedfrom multielement metal targets. J Appl Phys 1988;64:1484.

3. Iijima K, Yomita Y, Takayama R, Veda I. Preparation of c-axis orientedPbTiO3 thin films and their crystallographic, dielectric, and pyroelectricproperties. J Appl Phys 1986;60:361.

4. Brazier M, McElfresh M, Mansour S. Unconventional hysteresis behav-ior in compositionall graded Pb(Zr,Ti)O3 thin films. Appl Phys Lett (a)1998;72:1121.

5. Parton VZ. Fracture mechanics of piezoelectric materials. Acta Astronaut1976;3:671–83.

6. Park YE. Crack extension force in a piezoelectric material. ASME J ApplMech 1990;57:647–53.

7. Suo Z, Kuo CM, Barnett DM, Willis JR. Fracture mechanics for piezoelectricceramics. J Mech Phys Solid 1992;40:739–65.

8. Suo Z. Mechanics concepts for failure in ferroelectric ceramics. Smart StructMater 1991;AD-24/AMD-123:1–6.

9. Park S, Sun CT. Fracture criteria for piezoelectric ceramics. J Am CeramSoc 1995;78(6):1475.

0. Gao H, Fulton CC. Electrical non-linearity in fracture of piezoelectric ceram-ics. ASME Appl Mech Rev 1997;50:S56–63.

3

75.2 17.574.4 28.6

1. Ru CQ. Conducting cracks in a piezoelectric ceramic of limited electricalpolarization. J Mech Phys Solids 1999;47:2125–46.

2. Yang W, Zhu T. Switch toughening of ferroelectrics subjected to electricfields. J Mech Phys Solids 1998;46:291–311.

3. Suo Z. Models for breakdown-resistant dielectric and ferroelectric ceramics.J Mech Phys Solid 1993;41:1155.

4. Lynch CS, Yang W, Collier L, Suo Z, McMeeking RM. Electric field inducedcracking in ferroelectric ceramics. Ferroelectrics 1995;166:11–30.

5. Zhang TY, Zhao M, Tong P. Fracture of piezoelectric ceramics. Adv ApplMech 2001;38:147–289.

6. Hsieh J, Liu CJ, Ju YH. Response characteristics of lead phthalocyanine gassensor: effects of film thickness and crystal morphology. Thin Solid Films1998;322:98.

7. Kim H, Howitz JS, Kushto G, Pique A, Kafafi ZH, Gilmore CM, ChriseyDB. Effect of film thickness on the properties of indium tin oxide thin films.J Appl Phys 2000;88:6021.

8. Lof R, van Veenendaal M, Koopmans B, Jonkman H, Sawatzky G.Band gap, excions, and coulomb interaction in solid C60. Phys Rev Lett1992;68(26):3924–7.

9. Ting Y, Gunawan H, Chiu CW, Chong JZ. A new approach for surface polingof polyvinylidene fluoride (PVDF). Ferroelectrics 2013;446:18–27.

0. Ting Y, Gunawan H, Sugondo A, Chiu CW. A new approach ofpolyvinylidene fluoride (PVDF) poling method for higher electric response.Ferroelectrics 2013;446:28–38.

1. Pampuch R. Ceramic material an introduction to their properties. New York:Elsevier; 1976.

2. Monch W. Semiconductor surface and interfaces. Heidelberg: Springer Ver-lag; 1993.

3. Wouters DJ, Willems GJ, Maes HE. Electrical conductivity in ferroelectricthin film. Microelectron Eng 1995;29(1–4):249–56.

4. Wang C, Cao D, Zheng F, Dong W, Fang L, Shen M. Electronic supple-mentary information photocathodic behavior of ferroelectric Pb(Zr,Ti)O3

films decorated with silver nanoparticles. Chem Commun 2013;49:3769–71.

5. Waser R. Inscience and technology of electroceramic thin film. Dordrecht,Netherlands: Kluwer Acad. Publ.; 1995.

6. Scott JF. Ferroelectric memories. Heidelberg: Springer Verlag; 2000.7. hench LL, West JK. Principle of electronic ceramic. New York, USA: John

Wiley & Sons Inc.; 1990.8. Olesik J. Field Induced emission mechanism in ITO/glass/ITO system. J

Phys 2007;79:012023.9. Visiontek Systems Ltd. http://www.visionteksystems.co.uk/ito-glass.htm0. Ding AL, Wang HF. Phase transitions and electrical properties of (Na0.5K0.5)

NbO3–Bi(Sc0.5Fe0.5)O3 lead-free piezoelectric ceramics. J Ceram ProcessRes 2010;11(1):44–6.

1. Marinkovic B, Aya T, Koser H. Characterization of ferroelectric materialproperties of multifunctional lead zironate titanate for energy harvestingsensor nodes. J Appl Phys 2011;109:0149041–149044.

2. Tuttle BA, Headley TJ, Bunker BC, Schwartz RW, Zender TJ, Hernan-dez CL, Goodnow DC, Tissot RJ, Michael J, Carim AH. Microstructuralevolution of Pb(Zr,Ti)O3 thin films prepared by hybrid metallo-organic

decomposition. J Mater Res 1992;7:1876.

3. Randall CA, Kim N, Kucera JP, Cao W, Shrout TR. Intrinsic and extrin-sic size effect in fine-grained morphotropic-phase-boundary lead zirconatetitanate ceramics. J Am Ceram Soc 1998;81(3):677–88.