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Sensors and Actuators A 200 (2013) 107– 113

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators A: Physical

jo ur nal homepage: www.elsev ier .com/ locate /sna

i-based lead-free ceramic multilayer actuators usinggPd–(Na0.51K0.47Li0.02)(Nb0.8Ta0.2)O3 composite inner electrodes

an-Quyet Nguyena, Jin-Kyu Kanga, Hyoung-Su Hana, Hyun-Young Leea, Soon-Jong Jeongb,hang-Won Ahnc, Ill-Won Kimc, Jae-Shin Leea,∗

School of Materials Science and Engineering, University of Ulsan, Ulsan, Republic of KoreaKorea Electrotechnology Research Institute, Changwon 642-120, Republic of KoreaDepartment of Physics, University of Ulsan, Republic of Korea

r t i c l e i n f o

rticle history:eceived 30 May 2012eceived in revised form 9 September 2012ccepted 31 October 2012vailable online 6 November 2012

a b s t r a c t

We investigated the effect of inner electrode composition on the microstructure and electromechanicalproperties of Bi0.5(Na0.82K0.18)0.5TiO3 (BNKT) multilayer ceramic actuators. Using tape-casting and screenprinting techniques, multilayered laminates were prepared by stacking green sheets of Nb-modifiedBNKT ceramic layers with AgPd (70/30 in weight ratio) thick films as the inner electrode. After co-firing attemperatures of 1100–1140 ◦C for 4 h in air, the multilayered structure revealed significant camber as well

eywords:ultilayer actuator

ead-free ceramicsnternal electrodehermal shrinkage

as cracks due to the mismatch in thermal shrinkage between the ceramic and the AgPd layer. However,the addition of 10 wt% (K0.47Na0.51Li0.02)(Nb0.8Ta0.2)O3 (KNLNT) powder into the inner electrode markedlysuppressed the generation of co-firing-induced defects. More surprisingly, the normalized electric-fieldinduced strain (Smax/Emax) of BNKT multilayer actuators was enhanced from 110 pm/V to 350 pm/V byadding ceramic KNLNT into the AgPd inner electrode.

lectric field-induced strain

. Introduction

Piezoelectric multilayer actuators (MLAs) have played impor-ant roles in high precision mechatronic systems [1,2]. MLAs areenerally fabricated by co-firing a multilayered structure, in whichiezoelectrically active layers and electrically conducting layers arelternatively laminated. Such heterogeneous structure inevitablynduces thermal mismatch between compositionally different lay-rs during high temperature co-firing steps, which often lead toicrocracks at their interfaces, macroscopic delaminations, and

nternally remnant stresses [3–6] that are generally responsible forechanical failures during operation. In particular, the stress con-

entration at the tip of the inner electrode has been reported ashe origin of crack propagation under periodic actuation [7–9]. Thetress concentration was reported to be effectively relieved by aoderate design of the inner electrode configuration such as the

nsertion of floating gate electrodes between ceramic layers [10].The intrinsic thermal mismatch between ceramic and electrode

ayers was, however, found to be significantly reduced by apply-

ng ceramic–metal composites to the inner electrode layer [11–14].ccording to our previous work [13,14] on Pb-based multilayereramic actuators, the addition of ceramic powder to the inner

∗ Corresponding author. Tel.: +82 52 259 2245; fax: +82 52 259 1688.E-mail address: jslee@ulsan.ac.kr (J.-S. Lee).

924-4247/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.sna.2012.10.036

© 2012 Elsevier B.V. All rights reserved.

electrode layer remarkably increased throughput during fabrica-tion by reducing the co-firing failure rate [13]. The metal–ceramiccomposite electrode further decreased the co-firing-inducedcamber of multilayered laminates, and enhanced the mean time-to-failure (MTTF) of MLAs under a cyclic loading [14].

Recently, lead-free piezoceramics attract much attentionbecause of the environmental concerns on Pb-containing mate-rials including lead zirconate titanate (PZT) that includes morethan 60 wt% Pb [15–17]. Among various lead-free piezoelectricmaterials, (1 − x)BNT–xBKT (BNKT) solid solutions have been ofgreat interests due to their excellent ferroelectric and piezoelec-tric properties near the rhombohedral–tetragonal morphotropicphase boundary (MPB) within a range of 0.16 ≤ x ≤ 0.20 [18,19].More recently, we found that a giant strain of Smax/Emax = 641 pm/Vin BNKT ceramics can be obtained when 3 mol% Ti4+ ions were sub-stituted with Nb5+ ions [20]. Although hundreds of studies havereported the physical properties of BNT-based lead-free ceramics,only a few are available on BNT-based MLAs [21–23].

In this work, we investigated the co-firing behavior andstrain properties of BNT-based lead-free multilayer ceramic actu-ators. In contrast to the findings of previous works [15–17] thatthe ceramic–metal composite inner electrodes utilized the mix-

ture of Ag or AgPd with the same ceramic composition withthe piezoelectrically active layer, the main contribution of thisstudy is the addition of a compositionally different ferroelec-tric ceramic into the inner electrode of BNKT MLAs. We selected

108 V.-Q. Nguyen et al. / Sensors and Actuators A 200 (2013) 107– 113

metry of a multilayer ceramic actuator.

(ttKo((mfrtbed

2

lds(w2cmpcs

d(lwd

TS

Fig. 1. (a) Photograph and (b) geo

K0.47Na0.51Li0.02)(Nb0.8Ta0.2)O3 (KNLNT) as a ceramic additiveo AgPd instead of electromechanically active BNKT composi-ion on the basis of the following two reasons. One is thatNLNT was reported to be thermally stable with AgPd through-ut high temperature co-firing cycles [24,25]. The other is thatK0.5Na0.5)NbO3 (KNN) enhances the electric-field-induced strainEFIS) of BNKT ceramics [26,27]. In this study, we compared the

icrostructure and EFIS properties of BNKT MLAs using two dif-erent electrodes: AgPd and a composite of AgPd–KNNLT. As aesult, this work enlightens the design of MLAs by demonstratinghat the electromechanical properties of a MLA can be enhancedy a heterogeneous combination of compositions between thelectromechanically active ceramic layer and the electrically con-ucting metal–ceramic composite layer.

. Experiments

The ceramic composition of the electromechanically activeayer was Bi0.5(Na0.82K0.18)0.5(Ti0.97Nb0.03)O3 (BNKTNb). The pow-er was synthesized by a solid state reaction route using powdertate reagents including Bi2O3 (99.9%), Na2CO3 (99.9%), K2CO3>99%), TiO2 (99.9%), and Nb2O5 (99.9%). First, the powders wereeighed according to the chemical formula and then ball-milled for

4 h in ethanol solution with zirconia balls. The slurry was dried andalcined at 850 ◦C for 2 h and then ball-milled again using the sameilling media for 48 h. AgPd–ceramic composite pastes were pre-

ared by adding 10 wt% (K0.47Na0.51Li0.02)(Nb0.8Ta0.2)O3 (KNLNT)eramic powders to a commercial AgPd (70/30 in weight ratio ofolid content) paste (SJA-73-731, Sung Jee Tech, Korea).

Fig. 1 shows a typical MLA specimen prepared in this work. Theimension of a co-fired specimen was about 7 mm (L) × 5.5 mm

W) × 1.6 mm (H), in which 14 BNKTNb and 13 inner electrodeayers were alternatively stacked. BNKTNb ceramic green sheets

ere prepared by tape-casting a slurry containing BNKTNb pow-er, methyl ethyl ketone (99% purity, MEK, Samchun Chemical Co.,

able 1hrinkages, remanent polarization (Pr) and coercive field (Ec) of MLAs as a function of Ts w

Ts (◦C) Shrinkage (%)

Lateral Thickness Volume

AgPd AgPd + KNLNT AgPd AgPd + KNLNT AgPd

1100 15.40 16.10 19.91 15.12 42.76

1110 16.51 16.34 19.90 17.65 44.22

1120 17.07 17.02 19.00 18.48 44.31

1130 16.97 17.16 19.82 18.86 44.78

1140 16.23 16.01 19.00 16.20 43.38

Fig. 2. Schematic of co-firing-induced deflection of a specimen.

Pyeongtaek, Korea) as a solvent, polyvinyl butyral (99%, Aldrich Co.,Milwaukee, WI, USA) as a binder, dibutyl phthalate (99%, KantoChemical Co., Tokyo, Japan) as a plasticizer, and KD-1 (UniqemaCo., Everberg, Belgium) as a dispersant. The slurry was tape-castedusing a doctor blade to a thickness of about 200 �m and then driedat room temperature. AgPd or AgPd–ceramic composite paste was

screen-printed on the ceramic sheet as the inner electrode. Theceramic sheets with the thick film electrode were stacked and thenpressed at 50 ◦C under 50 MPa pressure for 2 min to enhance theadhesion strength between layers. Before sintering, the binder was

ith two different internal electrodes.

Pr (�C/cm2) Ec (kV/cm)

AgPd AgPd + KNLNT AgPd AgPd + KNLNT

AgPd + KNLNT

40.28 11.5 4.5 22.3 15.542.50 13.4 5.4 21.7 13.343.85 15.5 6.6 21.7 13.544.34 11.7 3.8 21.7 17.441.00 – – – –

V.-Q. Nguyen et al. / Sensors and Actuators A 200 (2013) 107– 113 109

110 0 11 10 1120 1130 114 00

1

2

3

4

5

BNKT Nb/AgPd-KNN LT

Sintering temperature (ºC)

Cu

rva

ture

, 1

/R (

1/m

)

BNKT Nb/AgPd

Fft

bwb

sbstRwJs7t

◦

Fi

ig. 3. Co-firing-induced camber of BNKTNb/electrode multilayer specimens as aunction of co-firing temperature for AgPd and AgPd–KNLNT composite inner elec-rodes.

urnt out in air by heating the samples at 500 ◦C for 10 h. Co-firingas carried out in a covered alumina crucible at temperatures

etween 1100 ◦C and 1140 ◦C with a soaking time of 4 h in air.After co-firing, thermally induced camber of a multilayered

pecimen was determined on the basis of the method proposedy Jean et al. [28]. As depicted in Fig. 2, the curvature (1/R) of apecimen was calculated by measuring the deflection h. The crys-al structure was analyzed using an X-ray diffractometer (XRD,AD III, Rigaku, Japan) and the surface morphology was observedith a field-emission scanning electron microscope (FE-SEM, JEOL,

SM-65OFF, Japan). Electrical measurements were carried out aftercreen-printing Ag paste on both sides of a MLA and firing at00 ◦C for 30 min. Data on field-dependent polarization (P–E) hys-eresis loops for unpoled specimens were acquired at 200 mHz

ig. 5. Cross-sectional micrograph of polished BNKTNb multilayer actuator surfaces usndicated temperatures for 4 h in air.

Fig. 4. X-ray diffraction patterns of AgPd–BNKTNb composite co-fired at 1130 Cfor 4 h in air. The result was compared with those of an as-dried AgPd pellet and anas-sintered BNKTNb ceramic specimen.

using a modified Sawyer–Tower circuit with a 15 �F capacitor. Thedependence of mechanical strain S on an external electric field Ewas obtained in silicon oil bath using a linear variable differentialtransducer.

3. Results and discussion

We measured the lateral, thickness and volume shrinkage ofMLAs as a function of co-firing temperature (Ts) for two different

internal electrodes and summarized the results in Table 1. Withincreasing Ts, both lateral and volume shrinkage increased and thenreached maximum values at Ts of 1130 ◦C. The fact that greater lev-els of shrinkage were observed in the direction of thickness than in

ing AgPd and AgPd–KNLNT inner electrodes. The specimens were co-fired at the

110 V.-Q. Nguyen et al. / Sensors and Actuators A 200 (2013) 107– 113

F s in Be

tewpA

tmfcfiitwAtntstit

ig. 6. Magnified micrographs showing BNKTNb/electrode/BNKTNb interface regionlectrode. The specimens were co-fired at the indicated temperatures for 4 h in air.

he lateral direction might be attributed to larger shrinkage in thelectrode layer than in the ceramic layer. According to our previousork on Nb-doped BNKT ceramics [20], the optimal sintering tem-erature was found to be 1150–1175 ◦C while the melting point ofgPd (70/30 in weight ratio) was reported to be about 1150 ◦C [29].

It has been reported that the co-firing of metal/ceramic mul-ilayers can lead to the development of camber [1,2] due to

ismatches in thermal shrinkage between different layers. There-ore, we examined the effect of inner electrode composition on theo-firing induced camber of BNKTNb MLAs. Fig. 3 compares the co-ring induced curvature between BNKTNb MLAs with two different

nner electrodes. With increasing co-firing temperature, the curva-ure increased and reached maximum values at 1110 ◦C for MLAsith AgPd–KNLNT internal electrodes and at 1120 ◦C for MLAs withgPd internal electrode. Further increase in the co-firing tempera-

ure decreased the curvature to minimum at 1130 ◦C. It should beoted that AgPd internal electrodes resulted in much higher curva-ure of MLAs than AgPd–KNNLT internal electrodes, which is very

imilar to previous results on KNLNT MLAs [25], where it was foundhat KNLNT–AgPd inner electrode significantly reduced the firingnduced camber of KNLNT MLAs. The present result also indicateshat the thermal shrinkage mismatch between internal electrode

NKTNb multilayer ceramic actuators using either AgPd or AgPd–KNLNT as the inner

and ceramic layers can be largely reduced by adding KNLNT ceramicinto the AgPd internal electrode.

For the successful application of a conducting material to theinner electrode of multilayer ceramic devices, the conductor shouldhave good thermal and chemical stability throughout the high tem-perature co-firing processes. It was reported that there is littledetectable reaction between KNLNT and AgPd multilayers duringco-firing at temperatures over 1100 ◦C [24,25]. Recently, it was alsoreported that KNN is completely soluble to BNKT at concentrationsin the range of 0–10 mol% [27]. To check the thermal and chem-ical stability of AgPd (70/30), the crystal structure of a co-firedAgPd–BNKTNb composite was analyzed using XRD and the resultis given in Fig. 4. The diffraction patterns of pure AgPd and BNKTNbwere found to be face-centered cubic and tetragonal, respectively.In addition, both reflections were also observed in the co-fired mix-ture of a AgPd–BNKTNb composite without any secondary phaseas seen in Fig. 4, indicating that there was little significant reactionbetween AgPd and BNKTNb during the high temperature co-firing

step. This result implies that AgPd can be used as a thermally stableinner electrode in BNKTNb MLAs.

To examine the thermal stability between BNKTNb and AgPdlayers, cross-sectional microstructures of MLA specimens were

V.-Q. Nguyen et al. / Sensors and Actuators A 200 (2013) 107– 113 111

F (P–E)

omwttsHMAattsa(tifii1f

hs

Ft

ig. 7. Effect of inner electrode composition on the polarization versus electric field

bserved using a FE-SEM. Fig. 5 compares the cross-sectionalicrostructures of BNKTNb/AgPd and BNKTNb. AgPd–KNLNT MLAsere co-fired at different temperatures for 4 h in air. Dark areas in

he photo correspond to BNKTNb layers and bright lines betweenhem indicate are electrode layers. Samples co-fired at 1110 ◦Chowed dense BNKTNb layers with good thickness uniformity.owever, vertical cracks (region A) were observed in BNKTNb/AgPdLAs while such defects were not found in specimens with

gPd–KNLNT composite electrodes. This result indicates that theddition of KNLNT into AgPd inner electrode significantly con-ribute to the suppression of co-firing induced cracks by reducinghe mismatch in the thermal shrinkage between two layers. Ashown in Fig. 5(c) and (d), further elevating the co-firing temper-ture up to 1140 ◦C, which was close to the melting point of AgPd70/30) of about 1150 ◦C [29], resulted in the failure of inner elec-rode. In the case of BNKTNb/AgPd MLAs co-fired at 1140 ◦C, as seenn Fig. 5(c), the inner electrode layers became thinner than those co-red at 1110 ◦C in Fig. 5(a). In particular, MLAs using AgPd–KNLNT

nner electrodes showed a local disconnection after co-firing at140 ◦C like the region B in Fig. 5(d), which may lead to the electrical

ailure of MLAs.

To more clearly observe the BNKTNb/electrode interface,igher magnification micrographs were taken from the samepecimens. Fig. 6 displays magnified micrographs showing

ig. 8. Effect of inner electrode composition on the unipolar electric field-induced strain (ures.

hysteresis loop of BNKTNb multilayer actuators co-fired at different temperatures.

BNKTNb/electrode/BNKTNb interface regions in the specimen witheither AgPd or AgPd–KNLNT as the inner electrode. The grain sizeof BNKTNb increases with increasing co-firing temperature whileporosity in the BNKTNb layer decreases. Looking into the innerelectrode, we can find that there are differences in the thicknessand connectivity of the AgPd layer between BNKTNb/AgPd andBNKTNb/AgPd–KNLNT MLAs. In the case of BNKTNb/AgPd MLAsas shown in Fig. 6(a), (c), and (e), the AgPd layer reveals goodthickness uniformity with a thickness of about 3 �m regardlessof co-firing temperature. As shown in Fig. 6(b) and (d), however,BNKTNb/AgPd–KNLNT MLAs showed thinner AgPd layers withpoorer thickness uniformity, finally resulting in disconnection afterco-firing at 1140 ◦C as can be seen in Fig. 6(f).

Fig. 7 shows the polarization versus electric field (P–E) hys-teresis loops for specimens co-fired at different temperatures. Theremnant polarizations (Pr) and coercive electric fields (Ec) of theBNKTNb MLAs with AgPd and AgPd–KNLNT internal electrodes areshown in Table 1. As shown in Fig. 7, when the sintering tempera-ture was increased up to 1120 ◦C, the Pr of the BNKTNb MLAs withAgPd and AgPd–KNLNT internal electrodes decrease to 15.5 and

2

6.6 �C/cm , respectively. Whereas the Ec of the BNKTNb MLAs withAgPd and AgPd + KNNLT internal electrodes increase to 21.7 and13.5 kV/cm, respectively. Upon increasing the sintering tempera-ture over 1120 ◦C, the Pr of the BNKTNb MLAs with AgPd internal

S–E) hysteresis loop of BNKTNb multilayer actuators co-fired at different tempera-

112 V.-Q. Nguyen et al. / Sensors and Act

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ig. 9. Co-firing temperature dependence of the normalized strain Smax/Emax of BNK-Nb multilayer actuators using either AgPd or AgPd–KNLNT as the inner electrode.

lectrodes decreased continuously while that of BNKTNb MLAsith AgPd internal electrodes increased. The specimens sintered at

140 ◦C showed little response, probably due to the disconnectionn the inner electrodes as confirmed in SEM micrographs in Fig. 5.

An important finding in the P–E measurement is that KNLNTdded to the inner electrode strongly affected the ferroelectricroperties of BNKTNb layers. According to a recent work on KNN-odified BNKT ceramics by Hussain et al. [27], the addition of KNN

nto BNKT formed a single perovskite structure at concentrations inhe range of 0–10 mol% KNN and furthermore degraded the ferro-lectricity of BNKT. In addition, a study of BNT–BKT–KNN ternaryystem by Anton et al. [26] also demonstrated the disruption oferroelectricity by KNN modification. Considering these reports onNN-modified BNKT, we believe that KNN added into AgPd layeras diffused into the electromechanically active BNKTNb layer andodified its ferroelectricity.For electromechanical actuators, the electric field-induced

train property of a MLA in a unipolar mode is more importanthan that in a bipolar mode. Fig. 8 displays the unipolar EFIS prop-rties of BNKTNb MLAs as a function of co-firing temperature forwo different electrode compositions. From the unipolar S–E loopshown in Fig. 8, it is seen that the S–E loop is strongly dependent onoth Ts and the inner electrode composition. The strain peaked at120 ◦C for Ag/Pd internal electrodes (Smax = 0.068%), and at 1130 ◦Cor AgPd–KNNLT internal electrodes (Smax = 0.207%), respectively.

One of the most important parameters for electromechanicalaterials is the normalized strain Smax/Emax that is often called

s high field d33 value based on the unit of pm/V. The normal-zed strain of our specimens was determined from the S–E loopn Fig. 8 and re-plotted in Fig. 9 as a function of Ts. In the case ofNKTNb/AgPd MLAs, the Smax/Emax was increased to 110 pm/V withlevating Ts up to 1120 ◦C and then decreased with further elevat-ng Ts, resulting in failures at Ts = 1140 ◦C in the inner electrode aseen in the cross-sectional micrographs in Fig. 5. However, the addi-ion of KNLNT into AgPd layers greatly improved the Smax/Emax ashown in Fig. 9. The Smax/Emax of BNKTNb/AgPd–KNLNT MLA spec-mens sintered at 1130 ◦C reached a maximum value of 350 pm/V,

hich is much higher than those of AgPd inner electrode MLAs. Thisesult is consistent with recent reports on KNN-modified BNT–BKT26,27], which reported the enhancement in the electric field-nduced strain with the disruption of ferroelectricity in BNKT byNN modification.

. Conclusions

We investigated the effect of inner electrode composition onhe microstructure and electromechanical properties of lead-free

[

uators A 200 (2013) 107– 113

BNT-based ceramic multilayer actuators. By adding KNLNT to theinner electrode layer of BNKTNb MLAs, we successfully resolved theco-firing-induced camber and eliminated cracks due to the ther-mal mismatch between ceramic and metal layers. The normalizedelectric-field induced strain (Smax/Emax) of BNKTNb multilayer actu-ators was enhanced from 110 pm/V to 350 pm/V by adding ceramicKNLNT into the AgPd inner electrode. Our approach significantlycontributes to the co-firing of multilayer ceramic devices by pro-viding more versatility in the choice of ferroelectric additives to theinner electrode of multilayer ceramic devices.

Acknowledgements

This work was financially supported by the National ResearchFoundation (NRF), Republic of Korea, under contract no. 2010001-4113 and was partly supported by the Ministry of Knowledge andEconomy, Republic of Korea, under the Fundamental R&D Programfor Core Technology of Materials.

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