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Acta Materialia 56 (2008) 642–650

Effect of CoO additive on structure and electrical propertiesof (Na0.5Bi0.5)0.93Ba0.07TiO3 ceramics prepared by the citrate method

Qing Xu a,*, Min Chen a,b, Wen Chen a, Han-Xing Liu a, Bok-Hee Kim b, Byung-Kuk Ahn b

a School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, Chinab Faculty of Advanced Materials Engineering, Chonbuk National University, Chonju 561756, Republic of Korea

Received 29 March 2007; received in revised form 4 September 2007; accepted 11 October 2007Available online 26 November 2007

Abstract

(Na0.5Bi0.5)0.93Ba0.07TiO3 ceramics with added 0–0.8 wt.% CoO were prepared by a citrate method and the influence of the CoO addi-tive on the structure and electrical properties of the ceramics was investigated. All the specimens maintained a rhombohedral–tetragonalphase coexistence in crystal structure and the addition of CoO caused a remarkably promoted grain growth. Adding CoO led to a dis-appearance of the response in the dielectric constant (er) to the ferroelectric–antiferroelectric transition and increased the diffuseness ofthe dielectric constant peak around 230 �C. Polarization–electric field hysteresis loops at varied temperatures revealed that adding CoOserved to increase the depolarization temperature (Td). The addition of CoO tailored the dielectric, piezoelectric and ferroelectric prop-erties at room temperature basically following a hard doped effect. The specimen with 0.8 wt.% CoO added showed a low dissipationfactor (tand) of 0.8% and a high mechanical quality factor (Qm) of 297 while retaining a piezoelectric constant (d33) of 137 pC N�1.� 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Perovskites; Oxides; Electroceramics; Piezoelectricity; Ferroelectricity

1. Introduction

There has been continuous research interest in recentdecades regarding lead-free piezoelectric materials aspotential alternatives to the widely used lead zirconate tita-nate (PZT) based piezoelectric ceramics. Sodium bismuthtitanate, (Na0.5Bi0.5)TiO3 (NBT), is a perovskite-type(ABO3) ferroelectric with a relatively large remanent polar-ization (Pr = 38 lC cm�2) at room temperature and a rela-tively high Curie temperature (Tc = 320 �C) [1]. Due to itsstrong ferroelectricity at room temperature, NBT has beenconsidered as a promising candidate material for lead-freepiezoelectric ceramics. However, pure NBT suffers from apoling problem because of its high coercive field(Ec = 73 kV cm�1), making it difficult to obtain the desiredpiezoelectric properties. To solve this poling problem, vari-eties of NBT-based solid solutions have been developed

1359-6454/$30.00 � 2007 Acta Materialia Inc. Published by Elsevier Ltd. All

doi:10.1016/j.actamat.2007.10.014

* Corresponding author. Tel.: +86 27 878 63277; fax: +86 27 878 64580.E-mail address: xuqing@whut.edu.cn (Q. Xu).

[2–5]. Among them, the (Na0.5Bi0.5)1�xBaxTiO3 (NBT–BT) system has attracted considerable attention on accountof the existence of a rhombohedral–tetragonal morpho-tropic phase boundary (MPB) near x = 0.06. Comparedwith pure NBT, the NBT–BT compositions near theMPB provide obviously decreased coercive fields and sub-stantially improved piezoelectric properties [2].

Nevertheless, the electrical properties of these NBT–BTcompositions are still far from satisfactory in terms of prac-tical application. So improving the electrical properties ofthe NBT–BT system remains as an issue to be resolved.In general, there are currently two approaches to thisend. One approach is microstructural modification. Recentresearch has reported a high Berlincourt piezoelectric con-stant, d33, of 200 pC N�1 in highly textured (volume frac-tion �90%) (Na0.5Bi0.5)0.945Ba0.055TiO3 ceramics withÆ00 1æ orientation fabricated by a templated grain growth(TGG) technique using tabular SrTiO3 as the template[6]. Another approach to improve the electrical propertiesis compositional modification. Extensive research efforts

rights reserved.

Fig. 1. XRD patterns of calcined powders with various amounts of addedCoO.

Q. Xu et al. / Acta Materialia 56 (2008) 642–650 643

have been devoted to searching for new compositions withimproved electrical properties based on the NBT–BT bin-ary system. It has been demonstrated that a ternary systemdesign of NBT–BT–(K0.5Bi0.5)TiO3 (KBT) is favorable toimproving piezoelectric and ferroelectric properties [7,8].On the other hand, introducing oxide additives into theNBT–BT system provides a versatile route to composi-tional adjustment. Various oxides have been employed asadditives for NBT–BT compositions near the MPB, withdiffering effects on the electrical properties, depending onthe nature of the oxide additives [9–14].

Similar to the cases of PZT-based piezoelectric ceramics[15,16], transition metal oxides are often used as additivesfor modifying the electrical properties of NBT-basedceramics. The effects of MnO on the structure and electricalproperties have been evaluated with regard to the amountof the additive for the NBT–BT ceramics with variousMPB compositions, such as (Na0.5Bi0.5)0.94Ba0.06TiO3

[9,14] and (Na0.5Bi0.5)0.92Ba0.08TiO3 [13]. It has been foundthat (Na0.5Bi0.5)0.94Ba0.06TiO3 ceramics with 0.5 wt.%MnCO3 added can be used as a substrate material forLamb wave devices due to improved dielectric properties,high mechanical strength and a satisfactory electromechan-ical coupling factor [9]. By comparison, only a few previouspapers had reported the effects of cobalt oxide addition onthe structure and electrical properties of NBT–BT ceramicswith MPB compositions [10,12]. Moreover, the addition ofcobalt oxide was limited to an invariant amount in theseresearches. Especially, there exist somewhat discrepantviews concerning the role of added cobalt oxide in thestructure and electrical properties. Thus systematicresearch is warranted on this subject, from the viewpointof both tailoring electrical properties and elucidating themechanism of electrical property change.

In this work, (Na0.5Bi0.5)0.93Ba0.07TiO3 was selected as arepresentative of MPB compositions in the NBT–BT sys-tem. In continuation of our earlier research on the synthe-sis of the NBT–BT system by the citrate method [17],(Na0.5Bi0.5)0.93Ba0.07TiO3 ceramics with varied amountsof CoO added were prepared by the citrate method andthe influence of adding CoO on the structure and electricalproperties was examined.

2. Experimental

Powders with a nominal composition of (Na0.5Bi0.5)0.93-Ba0.07TiO3 + x wt.% CoO (x = 0–0.8) were synthesized bythe citrate method. Reagent grade NaNO3, Bi(NO3)3 Æ5H2O, Ba(NO3)2, tetrabutyl titanate, Co(NO3)2Æ5H2O andcitric acid were used as starting materials. Tetrabutyl tita-nate was first dissolved in a citric acid solution and variousnitrates were then added, followed by stirring to yield atransparent aqueous solution. The mole ratio of citric acidto the total metal cation content was 1.25. The precursorsolution was heated to form a sol and subsequently a gel.The resulting gel was pulverized and calcined at 600 �Cfor 1 h in air. The detail of the synthesis process has been

described elsewhere [17]. The calcined powders were uniax-ially pressed under a pressure of 300 MPa into discs of19 mm in diameter and 1 mm in thickness and then sinteredat 1150 �C for 2 h in air.

The phase purities of calcined powders and crystal struc-ture of ceramic specimens were examined with a PhilipsX’pert PBO X-ray diffractometer using Cu Ka radiation.The morphology of calcined powders and the microstruc-ture of ceramic specimens were investigated with a JEOLJSM-5610LV scanning electron microscope (SEM). Forceramic specimens, polished and thermally etched surfaceswere used for SEM observation. The ceramic specimenswere polished to ensure surface flatness and painted withsilver paste on both surfaces as electrodes. The dielectricproperties, including the constant (er) and the dissipationfactor (tand), were measured using a HP4294 impedanceanalyzer at 1 kHz at room temperature. The temperaturedependence of the dielectric constant was measured usinga programmable furnace and a TH2818 automatic compo-nent analyzer (0.02–300 kHz) at 10 kHz upon heating at aheating rate of 1 �C min�1. The specimens for measuringpiezoelectric properties were poled in a silicon oil bath at60 �C under 3.0 kV mm�1 for 15 min. The piezoelectricconstant (d33) was measured using a quasistatic d33 meterbased on the Berlincourt method at 110 Hz. The electrome-chanical coupling factor (kp) and the mechanical qualityfactor (Qm) were measured by the resonance–antiresonancemethod using the HP4294 impedance analyzer. The polar-ization–electric field (P–E) hysteresis loop was measured atvaried temperatures by a Radiant precision workstationbased on a standard Sawyer–Tower circuit at 50 Hz.

3. Results and discussion

Fig. 1 shows the X-ray diffraction (XRD) patterns ofcalcined powders with various amounts of CoO added. Aperovskite phase was certified for the powders and no sec-ondary phase could be found within the sensitivity ofXRD. SEM observation indicated that the powders had

Fig. 3. XRD patterns of the ceramic specimens with various CoOamounts in the 2h ranges of (a) 38–42� and (b) 45–48�.

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a rather similar morphology, consisting of fine and uniformparticles around 100 nm, as shown in Fig. 2. Additionally,it was observed that the color of the powder with x = 0 wasyellowish, whereas powders with other compositions werereddish brown. Considering the very small amounts ofCoO added, this color change implies the incorporationof Co into the lattice.

A pure perovskite structure was identified for the cera-mic specimens. Fig. 3 shows the XRD patterns in the 2hranges of 38–42� and 45–48�, respectively, for the ceramicspecimens with various amounts of added CoO. Obvioussplitting of XRD peaks was detected for the specimens withx = 0. They can be assigned to a (003)/(0 21) peak splittingand a (002)/(2 00) peak splitting according to a rhombohe-dral symmetry and a tetragonal symmetry, respectively[10]. This characterizes a coexistence of rhombohedraland tetragonal phases, which is consistent with the natureof the specimen with an MPB composition [2,10]. The spec-imens with CoO added maintained the coexistence of thetwo phases. Compared with the specimen with x = 0, thespecimens with CoO added showed, to different extents,shifts in the position of XRD peaks towards lower diffrac-tion angle directions. This is an evidence for the dissolutionof Co into the structure. Moreover, one can see that thespecimens displayed a progressive peak shift towards lowerdiffraction angle directions with increasing amounts ofCoO until x = 0.4 above which the shift was reduced. Thisphenomenon can be qualitatively explained with respect totwo converse effects on the crystal structure caused by theCo incorporation. In the present work, cobalt was intro-duced into a (Na0.5Bi0.5)0.93Ba0.07TiO3 composition in theform of Co2+. According to Shannon’s effective ionic radiiwith a coordination number of six, Co2+ has a radius of0.65 A, which is close to that of Ti4+ (0.61 A) [18]. There-fore, Co2+ can enter into the sixfold coordinated B site ofthe perovskite structure to substitute for Ti4+ because ofradius matching. The structure of ABO3-type perovskitescan be viewed as a network of [BO6] oxygen octahedra.

Fig. 2. SEM micrograph of the powder with x = 0.2.

The substitution of the relatively larger Co2+ for the rela-tively smaller Ti4+ led to an enlargement of the unit cells.This radius effect is presumably responsible for the steadyshift of the XRD peak positions to lower diffraction angledirections with increasing CoO amount when x 6 0.4. Onthe other hand, an inverse effect of the Co substitutionon crystal structure should also be taken into consider-ation. Due to a lower valence state compared with Ti4+,the incorporation of Co2+ into the octahedral site of thestructure produced excess negative charges. To maintainan overall electrical neutrality, oxygen vacancies were cre-ated for compensation purposes. The generation of oxygenvacancies resulted in a distortion and contraction of theunit cells from the viewpoint of crystal chemistry. This oxy-gen vacancy effect is considered to be the main reason forthe reduction of the XRD peak shift in the specimens withx = 0.6 and x = 0.8. That is, the peak shift in the specimenswith CoO added relative to the specimen with x = 0 relieson the co-contribution of these two distinct effects. FromFig. 3 one can suggest that the radius effect seems to bethe main contributing factor at relatively low CoOamounts, while the oxygen vacancy effect appears to bedominant at relatively high CoO amounts.

Fig. 4 shows SEM micrographs of the ceramic speci-mens with various amounts of added CoO. Dense micro-structures can be seen for the specimens, displaying anevidently promoted grain growth with the addition ofCoO. As is generally recognized, the presence of oxygenvacancies in oxide systems is beneficial to mass transportduring sintering. This is assumed to be responsible for

Fig. 4. SEM micrographs of the ceramic specimens with (a) x = 0, (b) x = 0.3, (c) x = 0.4 and (d) x = 0.6.

Q. Xu et al. / Acta Materialia 56 (2008) 642–650 645

the promoted grain growth with the addition of CoO.Moreover, the monotonic increase of grain size with CoOamount also suggests that the added CoO mainly dissolvedinto the perovskite structure, otherwise, as is well-known,the segregation of the oxide additive at grain boundarieswould inhibit grain growth [19].

Fig. 5 shows the temperature dependence of the dielec-tric constant (er) of the ceramic specimens with variousamounts of added CoO. For the specimen with x = 0(Fig. 5a), one can see an obvious dielectric dispersion withincreasing temperature. There are two dielectric anoma-lies—a weak hump and a broad dielectric constantpeak—within the measuring temperature range. The resultis rather analogous to those obtained in NBT–BT ceramicswith MPB compositions [2,11,12]. The two anomalies areattributable to a ferroelectric–antiferroelectric transitionand a subsequent transition to a paraelectric state, respec-tively [2,11]. The dashed lines in Fig. 5a indicate two char-acteristic temperatures. Td, corresponding to theferroelectric–antiferroelectric transition, is termed the‘‘depolarization temperature’’, because the specimen isbasically depolarized and loses piezoelectric activity overthe temperature. The specimen with x = 0 showed a depo-

larization temperature of �100 �C, which is very close tothat of (Na0.5Bi0.5)0.94Ba0.06TiO3 ceramics achieved froma thermally stimulated depolarization current (TSDC)measurement [10]. The Tmax refers to the temperaturewhere the peak value of the dielectric constant occurs.The diffuse phase transition behavior in the specimen withx = 0 is in agreement with the nature of the NBT–BT sys-tem as a relaxor ferroelectric. The diffuse phase transitionin complex perovskite-type relaxor ferroelectrics has beeninterpreted from different viewpoints [20,21]. In the presentwork, it is considered that the diffuse phase transition in thespecimen with x = 0 should be closely related to the coex-istence of complex cations (Na+, Bi3+ and Ba2+), whichpossess similar radii but different charges and electronicconfigurations, at an equivalent crystallographic site.

It was found that the addition of CoO resulted in a dis-appearance of the low-temperature hump in the dielectricconstant curve, as shown in Fig. 5b and c. A similar phe-nomenon was observed in (Na0.5Bi0.5)0.94Ba0.06TiO3 ceram-ics with a small amount of Co2O3 added [12]. It is ofinterest that double P–E hysteresis loops, indicative of anantiferroelectric state, were observed for both the speci-mens with or without CoO addition at temperatures below

Fig. 5. Temperature dependence of the dielectric constant, er, of theceramic specimens with various CoO amounts.

Fig. 6. (1/er � 1/er max) vs. (T � Tmax)2/2er max plot of the specimen withx = 0.4.

Fig. 7. Diffuseness factor, d, of the ceramic specimens as a function ofCoO amount.

646 Q. Xu et al. / Acta Materialia 56 (2008) 642–650

their Tmax values. This implies that the specimens withadded CoO might also undergo a ferroelectric–antiferro-electric transition with increasing temperature, despite thelack of a visible response in the dielectric constant. Inspec-tion of the P–E hysteresis loops at various temperatureswill be presented in the following part. The Tmax valuesof the specimens with different CoO amounts slightly var-ied around 230 �C. For each specimen, it was found thatthe dielectric constants at temperatures above Tmax couldbe described by the following quadratic formula:

1

er

� 1

er max

¼ ðT � T maxÞ2

2er maxd2

ð1Þ

where er max represents the maximum dielectric constant atTmax, T is a temperature above the Tmax and d is the dif-fuseness factor, which quantifies the diffuse characteristicof the phase transition [22]. Fig. 6 shows the typical plotof (1/er � 1/er max) vs. (T � Tmax)2/2er max. Similar linearplots were also obtained for other specimens. The valuesof the diffuseness factor were derived from (1/er �1/er max) vs. (T � Tmax)2/2er max plots of the specimens bylinear fitting. Fig. 7 shows the diffuseness factor of the cera-mic specimens as a function of the amount of added CoO.One can see that the diffuseness factor tends to rise withincreasing CoO amount. The dissolution of Co2+ into the

structure of (Na0.5Bi0.5)0.93Ba0.07TiO3 can be given as thecause for this phenomenon, which increased local composi-tional inhomogeneity and as a result led to a more diffusivedielectric response.

At room temperature, saturated P–E hysteresis loopswere observed over the whole composition range investi-gated. The values of remanent polarization (Pr) and coer-cive field (Ec) were determined from the measured loops.Fig. 8 shows the remanent polarization and coercive fieldof the ceramic specimens at room temperature as a func-tion of the CoO amount. It can be seen that the coercivefield tends to be larger with increasing CoO amount, whilethe remanent polarization increases with CoO amountthrough a maximum value near x = 0.3 and then declines.

The P–E hysteresis loops of the specimens were alsoexamined at elevated temperatures. Fig. 9 shows the P–Ehysteresis loops of the specimen with x = 0 at various tem-peratures. Compared with the P–E hysteresis loop mea-sured at 30 �C, the loop measured at 80 �C becamenarrow but kept a typical ferroelectric feature. At a furtherelevated temperature of 120 �C, the specimen presented a

Fig. 8. Remanent polarization Pr and coercive field Ec of the ceramicspecimens at room temperature as a function of CoO amount.

Q. Xu et al. / Acta Materialia 56 (2008) 642–650 647

double hysteresis loop, reflecting the appearance of an anti-ferroelectric state. This result is generally in accordancewith the depolarization temperature of the specimen beingnear 100 �C, as indicated by the dashed line in Fig. 5a. Itwas noticed that the double hysteresis loop still reserveda small degree of remanent polarization. This can beascribed to the coexistence of ferroelectric and antiferro-electric states in the specimen at that temperature [23].The specimens with relatively low amounts of added CoO(x 6 0.3) presented an identical evolution of P–E hysteresisloops with measuring temperature to that of the specimenwith x = 0. Fig. 10 shows the P–E hysteresis loops of thespecimen with x = 0.6 at various temperatures. One cansee that the loops became narrower with higher measuringtemperatures, with a double hysteresis loop being observedat 150 �C. Similar results were also acquired in the speci-mens with x = 0.4 and x = 0.8. Thus, it is rational to sug-gest that the addition of CoO serves to increase thedepolarization temperature of the (Na0.5Bi0.5)0.93Ba0.07-

TiO3 composition. It implies an increase in the temperaturestability of ferroelectric domains [10]. The depolarizationtemperature of NBT-based piezoelectric ceramics has beenregarded as an important factor in practical applications,

Fig. 9. P–E hysteresis loops of the ceramic spe

with the increase of depolarization temperature enablinga wider applicable temperature range [8,24].

The influence of the CoO addition on the phase transi-tion behavior can be explained in terms of the incorpora-tion of Co into the lattice and the generation of oxygenvacancies. Successive ferroelectric–antiferroelectric–para-electric transitions with increasing temperature had previ-ously been observed in varieties of lead-based complexperovskite compounds and NBT-based compositions[23,25]. The intermediate antiferroelectric state has beendefined as an incommensurate structure, which may notstrictly be classified as a normal antiferroelectric stateand can be explained in light of a subtle modulation ofspontaneous polarization [23,26,27]. The weak hump inthe dielectric constant curve, as shown in Fig. 5a, can beregarded as a response to such incommensurate modula-tion. Based upon the transmission electron microscopy(TEM) observation, it has been revealed that there is adirect interaction between ferroelectric domains and theincommensurate modulation, with ferroelectric domainwalls playing a role as barriers [26]. As is well known, oxy-gen vacancies in perovskite-type ferroelectrics have aclamping effect on the motion of domain walls [28]. Then,it is plausible that the clamping effect associated with theappearance of oxygen vacancies caused by adding CoOcould dynamically suppress the degree of modulating spon-taneous polarization. This is believed to be the main reasonfor the disappearance of the weak hump in the dielectricconstant curves of the specimens with added CoO, asshown in Fig. 5b and c.

Furthermore, it is reasonable to deduce that the clamp-ing effect would become pronounced with increasing con-centration of oxygen vacancies. Thus, compared with thespecimens without Co addition or with relatively lowCoO amounts, those with relatively high CoO amountsnecessitate a higher temperature to overcome the clampingeffect on ferroelectric domain walls. In other words, it iscomparatively difficult to thermally drive the ferroelec-tric–antiferroelectric transition for these specimens. As aresult, the specimens with relatively high CoO amountsshow an increase in depolarization temperature.

cimen with x = 0 at various temperatures.

Fig. 10. P–E hysteresis loops of the ceramic specimen with x = 0.6 at various temperatures.

Fig. 11. Piezoelectric and dielectric properties of the ceramic specimens asa function of CoO amount.

648 Q. Xu et al. / Acta Materialia 56 (2008) 642–650

The change of coercive field with CoO amount in Fig. 8can also be assigned to the clamping effect caused by oxy-gen vacancies. The clamping effect restrains the reversal ofspontaneous polarization of ferroelectric domains under anapplied electrical field. This is assumed to be responsiblefor the increase of coercive field with the addition ofCoO. The maximum value of remanent polarizationaround x = 0.3 in Fig. 8 can be roughly understood in rela-tion to the co-contribution of the radius effect and the oxy-gen vacancy effect on crystal structure, as indicated byXRD analysis (Fig. 3). As is well established, the spontane-ous polarization in a perovskite-type ferroelectric mainlyoriginates from the displacement of B site cations withinthe [BO6] octahedra. At relatively low CoO amounts, theradius effect appears to be predominant, which resultedin an increase in the dimension of the unit cells. This ben-efits the displacement of Ti4+, and thus the improvement ofspontaneous polarization. In the case of relatively highCoO amounts, the oxygen vacancy effect turned out to beprevailing, leading to a decrease in dimension of the unitcells. This is unfavorable to the movement of Ti4+ and actsas a detrimental factor to spontaneous polarization. There-fore, the occurrence of the remanent polarization maxi-mum near x = 0.3 can be regarded as a result of thecompetition between these two distinct effects on structure.

Fig. 11 shows the piezoelectric and dielectric properties ofthe ceramic specimens as a function of CoO amount. Thespecimen with x = 0 gives a relatively large piezoelectricconstant (d33) of 176 pC N�1, which is rather similar toour previous result of 180 pC N�1 obtained in

(Na0.5Bi0.5)0.94Ba0.06TiO3 ceramics prepared by the citratemethod [17]. These values are much larger than thoseobtained in NBT–BT ceramics with various MPB composi-

Q. Xu et al. / Acta Materialia 56 (2008) 642–650 649

tions, which were prepared by the conventional solid-statemethod [2,10,29]. This demonstrates the advantage of thecitrate method in producing NBT–BT ceramics, attributableto the fine and uniform morphology together with highchemical homogeneity of the powders derived from the cit-rate method [17]. Meanwhile, the specimen with x = 0 exhib-ited a relatively large dissipation factor (tand) of 3.8% and arelatively small mechanical quality factor (Qm) of 91. Withthe increase of CoO amount, the piezoelectric constant,dielectric constant and dissipation factor displayed a mono-tonic decline, while the mechanical quality factor providedan inverse trend. The electromechanical coupling factor(kp) seems to be comparatively insensitive to the additionof CoO, with the parameter fluctuating within the range of20.4–23.2%. In comparison with the specimen with x = 0,adding 0.8 wt.% CoO yielded an obviously lowered dissipa-tion factor of 0.8% and an apparently increased mechanicalquality factor of 297, accompanied by a reduction of the pie-zoelectric constant from 176 to 137 pC N�1. The consider-ably decreased dissipation factor and substantiallyincreased mechanical quality factor allow a higher conver-sion efficiency between electrical and mechanical power.

The monotonic decrease of the piezoelectric constantwith increasing CoO amount can be attributed to theclamping effect associated with oxygen vacancies. Theclamping effect hindered sufficient reorientation of ferro-electric domains during electrical poling. This is consideredto be responsible for the decrease of the piezoelectric con-stant with the addition of CoO. The inner attrition resultingfrom the motion of ferroelectric domains has been regardedas an important cause of energy dissipation in NBT-basedceramics [13,30]. The clamping effect caused by oxygenvacancies can restrain the motion of ferroelectric domains,and thus reduce the inner attrition. It is suggested to be thereason for the reduction of the dissipation factor and theincrease of the mechanical quality factor with increasingCoO amount. Furthermore, it can be noticed in Fig. 11 thatthere was a relatively rapid decline of the dissipation factorfrom x = 0.3 to x = 0.4, compared with a slow and nearlylinear decrease of the parameter with CoO amount whenx 6 0.3. Correspondingly, the mechanical quality factoralso shows a facilitated increase after x = 0.4. These resultsindicate an increased influence of oxygen vacancies on theelectrical properties at relatively high CoO amounts.

In general, the variation of dielectric and piezoelectricproperties with increasing CoO amount reveals a typical fea-ture of the hard doping effect on electrical properties, coin-ciding well with the substitution of Co2+ for Ti4+ as anacceptor. A similar result was achieved in (Na0.5Bi0.5)0.92-

Ba0.08TiO3 ceramics with a small amount of Co2O3 added[10]. However, previous research had also reported aninconsistent result. It has been shown that adding a smallamount of Co2O3 (�1.0 at.%) to (Na0.5Bi0.5)0.94Ba0.06TiO3

resulted in an increase in the piezoelectric constant, d33,from 117 to 139 pC N�1 [12]. This abnormal change in pie-zoelectric properties was explained with a grain size effect,namely, a sound grain growth with the addition of Co2O3

enables a consummate development of ferroelectricdomains and thus improves the piezoelectric properties. Inthe present work, in spite of a remarkably promoted graingrowth with the CoO addition, the contribution of such agrain size effect to the electrical properties seems to havebeen insignificant, with the addition of CoO tailoring theelectrical properties predominately according to a hard dop-ing effect.

4. Conclusions

Fine and uniform (Na0.5Bi0.5)0.93Ba0.07TiO3 powderswith 0–0.8 wt.% CoO added have been produced by thecitrate method. The structure and electrical properties ofthe resulting ceramic specimens have been investigated.All the specimens maintained a coexistence of rhombohe-dral and tetragonal phases, while the addition of CoOapparently promoted grain growth. This research demon-strates a considerable influence of CoO additive on thedielectric, piezoelectric and ferroelectric properties. Theaddition of CoO led to a disappearance of the responseto ferroelectric–antiferroelectric transition in the dielectricconstant (er) vs. temperature curve and increased the dif-fuseness of the dielectric constant peak around 230 �C.Inspection of the P–E hysteresis loops at various tempera-tures suggests that adding CoO serves to increase the depo-larization temperature (Td). At room temperature, theaddition of CoO resulted in a decrease of the dielectricconstant, dissipation factor (tand) and piezoelectric con-stant (d33), together with an increase of the mechanicalquality factor (Qm) and coercive field (Ec). The variationof the electrical properties has been interpreted withrespect to the incorporation of Co into the perovskitestructure. Compared with (Na0.5Bi0.5)0.93Ba0.07TiO3, thespecimen with 0.8 wt.% CoO added gave an obviouslyreduced dissipation factor (0.8%), a remarkably increasedmechanical quality factor (297) and a lowered piezoelectricconstant (137 pC N�1).

Acknowledgements

This work was financially supported by the Program forNew Century Excellent Talents in University (Grant No.NCET-04-0724). We are grateful to the Natural ScienceFoundation of China (Grant No. 50272044 and50410529) and the Korea Science and Engineering Foun-dation (Grant No. F01-2004-000-10084-0) for jointly sup-porting the research.

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