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Microstructure and Piezoelectric Properties of ZnO-added (Na0.5K0.5)NbO3 Ceramics

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2004 Jpn. J. Appl. Phys. 43 L1072

(http://iopscience.iop.org/1347-4065/43/8B/L1072)

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Microstructure and Piezoelectric Properties of ZnO-added (Na0:5K0:5)NbO3 Ceramics

Seung-Ho PARK, Cheol-Woo AHN, Sahn NAHM� and Jae-Sung SONG1

Department of Materials Science and Engineering, Korea University, 1-5 Ka, Anam-Dong, Sungbuk-Ku, Seoul 136-701, Korea1Electric and Magnetic Devices Group, Korea Electro-Technology Research Institute, 28-1 Sungju-Dong, Chang-won 641-120, Korea

(Received May 10, 2004; accepted June 18, 2004; published July 23, 2004)

The orthorhombic structure of (Na0:5K0:5)NbO3 (NKN) ceramics was maintained when ZnO was added. NKN ceramics have aporous microstructure and dissolve when they are exposed to water. However, as ZnO was added, a dense microstructure wasdeveloped and deliquescence was not observed. The mechanical quality factor (Qm) and coercive field (Ec) increased with theaddition of ZnO, indicating that ZnO acted as a hardener in the NKN ceramics. The dielectric constant ("T3="o), piezoelectricconstant (d33) and electromechanical coupling factor (kp) increased when a small amount of ZnO was added, which might bedue to the increase in density. The good dielectric and piezoelectric properties of "T3="o ¼ 500, d33 ¼ 121 and kp ¼ 0:4 wereobtained for the NKN ceramics with 1.0mol% ZnO. [DOI: 10.1143/JJAP.43.L1072]

KEYWORDS: (Na0:5K0:5)NbO3, lead-free ceramics, deliquescence, piezoelectric properties, ZnO

Lead zirconate titanite (PZT) ceramics have been widelyused as piezoelectric materials because of their excellentpiezoelectric properties.1) However, PZT ceramics causeserious environmental problems because of PbO. Therefore,investigations on lead-free materials, which can replace PZTceramics, have increased. The KNbO3 ceramic is one of thecandidate lead-free materials because the single crystalshows excellent piezoelectric properties and high Curietemperature.2) The solid solution of ferroelectric KNbO3 andantiferroelectric NaNbO3 also exhibit good piezoelectricproperties.3–5) Particularly, the K0:5Na0:5O3 ceramics formthe morphotropic phase boundary and show excellentpiezoelectric properties of d33 ¼ 80 pC/N, kr ¼ 0:34{0:39,Qm ¼ 130, and "T3="o ¼ 290.4,5) However, the hot pressmethod is usually used to obtain a dense ceramic because itis very difficult to obtain a dense NKN ceramic by theconventional sintering process.3–6) Moreover, when NKNceramics are exposed to water, they show deliquescence. Toavoid these problems, (1� x)NKN-xPbTiO3, (1� x)NKN-xSrTiO3 and (1� x)KNbO3-xLaFeO3 ceramics were inves-tigated but their piezoelectric properties were found to beunsatisfactory.7–9) The PbO-added NKN ceramics werereported to have good piezoelectric properties withoutdeliquescence, but prolonged sintering was necessary toobtain good piezoelectric properties.10)

In this work, ZnO was added to the (Na0:5K0:5)NbO3

ceramics and the microstructure and piezoelectric propertiesof the specimens were investigated. The addition of ZnOwas expected to improve the piezoelectric properties ofNKN ceramics by enhancing their sinterability.

Na0:5K0:5NbO3 þ xZnO with 0 � x � 9:0mol% were pre-pared from oxides of >99% purity by conventional solid-state synthesis. The oxide compounds of K2CO3 (Wako PureChemical Industries Ltd., Osaka, Japan), Na2CO3 2 (YakuriPure Chemicals, Kyoto, Japan) and Nb2O5 (High PurityChemicals, Chiyoda Sakado, Japan) were mixed for 24 h in anylon jar with zirconia balls, then dried, and calcined at950�C for 16 h. After remilling with ZnO (Aldrich ChemicalCompany Inc., Milwaukee, WI), the powder was dried andpressed into discs under a pressure of 100 kgf/cm2 andsintered at 1050�C for 2 hours. The crystal structure andmicrostructure of the specimens were examined by X-ray

diffraction (XRD: Rigaku D/max-RC) and scanning electronmicroscopy (SEM: Hitachi S-4300, Japan). The densities ofthe sintered specimens were measured by a water-immersiontechnique. A silver electrode was printed on the lappedsurfaces, and the specimens were poled in silicone oil at120–150�C by applying a dc field of 2.5–3 kV/mm for60min. The electromechanical coupling factor and piezo-electric and dielectric properties were determined using apiezo d33 meter (Channel Product DT-3300) and animpedance analyzer (Model HP 4194) on the basis of IEEEstandards. The dielectric constant as a function of temper-ature was obtained using an LCR meter (HP model 4284) inan automated temperature-controlled furnace, which wasinterfaced with a computer for data acquisition.

Figure 1 shows the X-ray diffraction pattern of(Na0:5K0:5)NbO3 þ xZnO ceramics with 0:0 � x � 9:0mol% sintered at 1050�C for 2 h. The (Na0:5K0:5)NbO3

ceramic has an orthorhombic structure. When ZnO wasadded, the peak position and the peak shape did not change.Therefore, the ZnO-added NKN ceramics are considered tohave an orthorhombic structure. However, when ZnOexceeded 1.0mol%, the peaks for ZnO indicated by the full

10 20 30 40 50 60 70 80

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Fig. 1. X-ray diffraction pattern of (Na0:5K0:5)NbO3 þ xZnO ceramics

with 0:0 � x � 9:0mol% sintered at 1050�C for 2 h.�Corresponding author. E-mail address: snahm@korea.ac.kr

Japanese Journal of Applied Physics

Vol. 43, No. 8B, 2004, pp. L 1072–L 1074

#2004 The Japan Society of Applied Physics

L 1072

circles were observed, indicating that the ZnO second phaseformed when a large amount of ZnO was added. All of theZnO-added NKN ceramics showed no deliquescence whenthey were exposed to water for a long time. Therefore, theaddition of ZnO inhibited the formation of the unstablesecond phase, resulting in the stable NKN phase. The ZnO-added NKN sintered at 1000�C exhibited a similar X-raydiffraction pattern but specimens were not sintered below1000�C.

The microstructure of the ZnO-added NKN was inves-tigated using SEM, as shown in Figs. 2(a)–2(c). Figure 2(a)shows the SEM image of the fracture surface of the NKNceramics. A porous microstructure with many pores wasdeveloped and the average grain size was approximately1.5 mm. When 1.0mol% ZnO was added, a dense micro-structure without pores was formed and some of the grainsstarted to grow as shown in Fig. 2(b). The grain growth wascompleted when the ZnO content exceeded 5mol% and theaverage grain size of the NKN was about 2.5 mm. The liquidphase was found in the ZnO-added NKN ceramics. Thus, theincrease in grain size and the development of a densemicrostructure could be explained by the presence of theliquid phase. Since the melting temperature of ZnO is high(about 1500�C), the liquid phase is not a ZnO phase.Moreover, the melting temperatures of the (1� x)ZnO-xNb2O5 solid solutions are high (above 1285�C). Thus, the(1� x)ZnO-xNb2O5 solid solution cannot be a liquid phaseeither. On the other hand, it is generally accepted that someof the K2O evaporates during sintering. Therefore, the liquidphase might be a mixture of Na2O and ZnO or Na2O, Nb2O5

and ZnO.Figures 3(a) and 3(b) show the variations in dielectric

constant and dielectric loss with temperature for the NKNþxZnO ceramics with 0:0 � x � 9:0mol% sintered at 1050�Cfor 2 h. NKN ceramics have two phase transition temper-atures at 200�C and 420�C, corresponding to the transitiontemperatures of cubic to tetragonal and tetragonal to

orthorhombic, respectively. When a small amount of ZnOwas added, both transition temperatures were slightlydecreased. The decrease in transition temperature impliesthat ZnO entered the matrix of the NKN ceramics. However,since the transition temperature did not further decrease withincreased addition of ZnO, the amount of ZnO, whichentered the matrix of the NKN ceramics, might be limited.The loss of the specimens with x � 0:1 is very small but itincreased with the addition of ZnO.

Figure 4 shows the polarization vs electric field curves ofthe specimens sintered at 1050�C for 2 h. The remnantpolarization (Pr) of the NKN ceramic is about 8 mC/cm2,which increased with the addition of ZnO, exhibiting amaximum value of 16 mC/cm2 as 1.0mol% ZnO was added.The increase in Pr might be related to the increase in bulkdensity. In addition, when the ZnO content exceeded1.0mol%, the Pr decreased and the coercive field increased.Therefore, it is considered that ZnO, which entered thematrix of the NKN ceramics, might behave as a hardener.

Figure 5 shows the variations in the bulk density, Qm,"T3="o, d33 and kp values of the ZnO-added NKN ceramicssintered at 1050�C for 2 h. The bulk density increased withthe addition of a small amount of ZnO but it decreased when

( c )

( a ) ( b )

10 mµ

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Fig. 2. SEM image of fracture surface of (Na0:5K0:5)NbO3 þ xZnO

ceramics with (a) x ¼ 0:0mol%, (b) x ¼ 1:0mol% and x ¼ 9:0mol%

sintered at 1050�C for 2 h.

( a )

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Fig. 3. Variations in (a) dielectric constant and (b) dielectric loss with

temperature for NKNþ xZnO ceramics sintered at 1050�C for 2 h.

Jpn. J. Appl. Phys., Vol. 43, No. 8B (2004) S.-H. PARK et al. L 1073

the ZnO content exceeded 1.0mol%. The formation of adense microstructure through the liquid-phase sintering wasattributed to the increase in bulk density. However, when alarge amount of ZnO was added, grain growth occurred,which might be responsible for the decrease in bulk density.The Qm increased with the addition of ZnO. This result alsoimplies that ZnO acted as a hardener in the NKN ceramics.The "T3="o increased with the addition of ZnO and showed amaximum value when x is 1.0mol%. The d33 and kp valuesexhibited similar variations with the addition of ZnO. Theincrease in "T3="o, d33 and kp might be explained by theincrease in bulk density. However, since ZnO behaves as ahardener in NKN ceramics, the "T3="o, d33 and kp areexpected to decrease with increased addition of ZnO. This iswell illustrated in Fig. 5. The NKN ceramic with 1.0mol%ZnO shows good dielectric and piezoelectric values of "T3="o ¼ 500, d33 ¼ 123 and kp ¼ 0:4 without deliquescence.

Bulk density increased with the addition of a smallamount of ZnO but it decreased when a large amount of ZnOwas added. The liquid phase was found in the ZnO addedNKN, which might be responsible for the increase in bulkdensity through the improvement of sinterability. Phasetransition temperatures decreased when ZnO was added. Thecoercive field and Qm of the specimens increased with theaddition of ZnO. Therefore, ZnO is considered to act as ahardener in NKN ceramics. The "T3="o, d33 and kp increasedwith the addition of a small amount of ZnO. The improve-

ment of the "T3="o, d33 and kp was explained by the increasein density. The specimen with 1.0mol% ZnO sintered at1050�C for 2 h exhibited good dielectric and piezoelectricproperties of "T3="o ¼ 500, d33 ¼ 123 and kp ¼ 0:4. More-over, all of the ZnO-added NKN ceramics showed nodeliquescence when they were exposed to water for a longtime. Therefore, the addition of ZnO was very effective inboth improving the piezoelectric properties and preventingthe deliquescence of NKN ceramics.

This research was supported by a grant from the Centerfor Advanced Materials Processing of the 21 CenturyFrontier R&D Program, funded by the Ministry of Scienceand Technology of the Republic of Korea.

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7113.

-4 -2 0 2 4

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Fig. 4. Polarization vs electric field curves of (Na0:5K0:5)NbO3 þ xZnO

ceramics with 0:0 � x � 9:0mol% sintered at 1050�C for 2 h.

0 10

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Fig. 5. Variations in bulk density, Qm, "T3="o, d33 and kp values of ZnO-

added NKN ceramics sintered at 1050�C for 2 h.

L 1074 Jpn. J. Appl. Phys., Vol. 43, No. 8B (2004) S.-H. PARK et al.