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Current Applied Physics 11 (2011) S143eS148

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Current Applied Physics

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Ferroelectric properties of rare-earth oxides doped(K0.4Na0.6)0.95Li0.05(Nb0.95Sb0.05)O3 lead-free piezoceramics

Zhuo Wang, Yanqin Zhuo, Dingquan Xiao*, Wenjuan Wu, Cheng Zhang, Xiaolei Huang, Jianguo ZhuDepartment of Materials Science, Sichuan University, Chengdu 610064, China

a r t i c l e i n f o

Article history:Received 28 June 2010Received in revised form26 October 2010Accepted 28 December 2010Available online 4 January 2011

Keywords:Lead-free ferroelectric ceramicKNNLSPr2O3

Yb2O3

Ferroelectric properties

* Corresponding author. Tel.: þ86 28 85412415; faxE-mail address: nic0402@scu.edu.cn (D. Xiao).

1567-1739/$ e see front matter � 2011 Elsevier B.V.doi:10.1016/j.cap.2010.12.033

a b s t r a c t

The rare-earth oxides (Pr2O3 and Yb2O3)-doped (K0.4Na0.6)0.95Li0.05(Nb0.95Sb0.05)O3 [KNNLS] ceramicswere prepared using conventional solid sintering method for improving the ferroelectric properties ofKNN-based ceramics. The effects of rare-earth oxides on the crystal structure, microstructure, dielectricand ferroelectric properties of the ceramics were systematically studied. The Curie temperature (Tc)decreases slightly with the increase of Pr2O3 and Yb2O3. The results show that Pr2O3 and Yb2O3 cangreatly enhance the ferroelectric properties of KNNLS. The KNNLS-0.1 mol% Pr2O3 ceramic and KNNLS-0.1 mol% Yb2O3 ceramic show excellent ferroelectric properties: Pr ¼ 42.8 mC/cm2, Ec ¼ 7.2 kV/cm andPr ¼ 37.3 mC/cm2, Ec ¼ 12.8 kV/cm, respectively.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Lead-based piezoelectric ceramics, Pb(Zr1 � xTix)O3 (PZT), havebeen widely used for actuators, transducers, sensors, and otherelectromechanical devices due to their excellent properties [1].However, the lead element in these piezoelectric ceramics is above60%, causing great harm to environment and human health becauseof its remarkable toxicity [2]. Therefore, it is essential to developlead-free piezoelectric ceramics with outstanding properties toreplace the lead-based piezoelectric ceramics.

Nowadays, (K,Na)NbO3 (KNN)-based ceramics are consideredto be one of the most promising lead-free piezoelectric alternativesfor PZT owing to their high piezoelectricity, good ferroelectricproperties, and compatibility with human tissue [3e5]. But theevaporation of Kþ and/or Naþ at high temperature makes it difficultto obtain high-density KNN-based ceramics by conventional sin-tering method. To improve the bulk density and piezoelectricproperties of KNN-based ceramics, different perovskite componentsare added into KNN to form a new solid solution, such asKNNeLiNbO3 [6] and KNNeLiSbO3 [7]. The enhanced piezoelectricproperties of themodifiedKNN-based ceramics shouldbe attributedto the polymorphic phase transition (PPT) near room temperature.

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As well known, piezoelectric ceramics, such as Pb(Zr1 � xTix)O3(PZT) ceramics, are ferroelectric ceramics as well. PZT ceramics andfilms have also been widely used for ferroelectric devices due totheir excellent properties. For the same reasons as mentionedabove, it is essential to develop lead-free piezoelectric ceramicswith outstanding ferroelectric properties to replace the lead-basedferroelectric ceramics. However, it was found that KNN-basedceramics have low remanent polarization [8,9], and the ferroelec-tric properties of these ceramics need to be improved.

As reported, doping rare-earth oxides, such as Pr2O3 and Yb2O3,can optimize the electrical performance of the PZT or barium tita-nate ceramics [10,11]. However, there were few reports on KNNceramic families doped with Pr2O3 and Yb2O3. In the authors’ groupthe piezoelectric properties of (K0.4Na0.6)0.95Li0.05 (Nb0.95 Sb0.05)O3(abbreviated KNNLS) were investigated, showing that the ceramicspossess good piezoelectric properties [12,13]. In this work, Pr2O3and Yb2O3 modified (K0.4Na0.6)0.95Li0.05 (Nb0.95 Sb0.05)O3 ceramicswere prepared, and the effects of doping Pr2O3 or Yb2O3 into KNNLSon the crystal structure, microstructure and ferroelectric propertiesof the ceramics were mainly investigated.

2. Experimental

(K0.4Na0.6)0.95Li0.05 (Nb0.95 Sb0.05)O3 � x mol% Pr2O3 [KNNLS-x%Pr2O3] (x ¼ 0, 0.05, 0.08, 0.1, and 0.15, respectively) ceramics and(K0.4Na0.6)0.95Li0.05 (Nb0.95 Sb0.05)O3� ymol%Yb2O3 [KNNLS-y%Yb2O3]

Fig. 1. X-ray diffraction patterns of (a) Pr2O3-doped and (b) Yb2O3-doped KNNLS ceramics at room temperature.

Z. Wang et al. / Current Applied Physics 11 (2011) S143eS148S144

(y ¼ 0, 0.05, 0.1, 0.5, and 1.5, respectively) were prepared by theconventional fabrication technique. Na2CO3 (99.8%), Li2CO3 (99.99%),K2CO3 (99%), Nb2O5 (99.5%), Sb2O3 (99.99%), Pr2O3 (99.9%) and Yb2O3(99.99%) were used as starting raw materials. The weighed rawmaterials according to the chemical formulaweremixed in a nylon jarwith agate ball for 24 h using absolute alcohol as the medium. Aftercalcinations at 850 �C for 6 h, the dried powders were subsequentlypressed intodisksof 1.0 cm indiameter and0.8mmin thicknessunder10 MPa using PVA as a binder. After burning off PVA, the pellets weresintered at 1080e1120 �C for 2 h or 3 h in air. Silver pastewas sinteredonboth sidesof the samplesat700 �C for10min to formtheelectrodes

Fig. 2. SEM images of KNNLS-x mol% Pr2O3 ceramics: (a) x ¼

for the electrical measurements. The samples were then left forferroelectric measurements.

The crystal structures of the specimens were examined by theX-ray diffraction (XRD) using Cu Ka radiation (l ¼ 1.54178�A) in theq � 2q scan mode (DX1000, Dandong, China). The instrument wasperformed in the setting conditions of 40 kV and 25 mA with thescanning speed of 3.6�/min. The surface morphologies of the sin-tered specimens were observed by a scanning electron microscope(SEM) (JSM-5900, Japan). The temperature dependence of thedielectric constantof the ceramicswasexaminedusingan LCRmeter(HP 4980, Agilent, USA). The remanent polarization Pr and coercive

0, (b) x ¼ 0.05, (c) x ¼ 0.1, and (d) x ¼ 1.5, respectively.

Fig. 3. SEM images of KNNLS-y mol% Yb2O3 ceramics: (a) y ¼ 0.05, (b) y ¼ 0.1, (c) y ¼ 0.5, and (d) y ¼ 1, respectively.

Fig. 4. Temperature dependence of 3r and tan d for (a) KNNLS-x mol% Pr2O3 at 10 kHz and (b) KNNLS-y mol% Yb2O3 ceramics at 100 kHz, and (c) KNLNS-0.05 mol% Pr2O3 and (d)KNNLS-0.1 mol% Yb2O3 ceramics in the frequency ranging from 0.1 Hz to 1000 kHz, respectively.

Z. Wang et al. / Current Applied Physics 11 (2011) S143eS148 S145

Z. Wang et al. / Current Applied Physics 11 (2011) S143eS148S146

field Ec were determined from PeE hysteresis loops measured bya Radiant Precision Workstation (USA).

Fig. 5. The dispersity of (a) KNNLS-x mol% Pr2O3 ceramics and (b) KNNLS-y mol%Yb2O3 ceramics.

3. Results and discussion

Fig.1(a) shows the XRDpatterns of KNNLS-x% Pr2O3 ceramics. Allthe Pr2O3-doped KNNLS ceramics show the single-phase perovskitestructure with orthorhombicetetragonal coexistence phase. Thisphenomenon suggests that the doping of Pr2O3 cannot change thesystem’s phase structure. Fig. 1(b) shows the XRD patterns ofKNNLS-y% Yb2O3 ceramics. The Yb2O3-doped KNNLS ceramics alsoshow the perovskite structure with orthorhombicetetragonalcoexistence phase. In addition, when the Yb2O3 content is less than0.5mol%, no secondaryphase couldbeobserved.However,when theYb2O3 content is larger than 0.5 mol%, a trace amount of thesecondary phase can be detected, which will greatly influence theproperties of the KNNLS-y% Yb2O3 ceramics, as discussed below.

Fig. 2 shows the SEM images of KNNLS-x% Pr2O3 ceramics with(a) x ¼ 0, (b) x ¼ 0.05, (c) x ¼ 0.1, and (d) x ¼ 1.5, respectively. Fig. 3shows the SEM images of KNNLS-y% Yb2O3 ceramics with (a)y¼ 0.05, (b) y¼ 0.1, (c) y¼ 0.5, and (d) y¼ 1, respectively. From Figs.2 and 3 one can see that as the addition amount of Pr2O3 or Yb2O3increases, the grain size of the ceramics decreases gradually, for theA-sites vacancies caused by doping Pr2O3 or Yb2O3 limit the growthof the crystalline grains. It is observed from the results that thePr2O3 or Yb2O3 addition is an effective way to inhibit the graingrowth.

Fig. 4 shows the temperature dependence of 3r and tan d for (a)KNNLS-xmol% Pr2O3 ceramics (x¼ 0, 0.03, 0.05, 0.08, 0.01, and 0.15,respectively) at 10 kHz, (b) KNNLS-y mol% Yb2O3 ceramics (y ¼ 0,0.05, and 0.1, respectively) at 100 kHz, (c) KNLNS-0.05 mol% Pr2O3ceramics in the frequency ranging from 0.1 Hz to 1000 kHz, and (d)KNNLS-0.1 mol% Yb2O3 ceramics in the frequency ranging from0.1 Hz to 1000 kHz. From Fig. 4(a) and (b) one can see that thedielectric constant vs temperature curves are not apparentlyinfluenced by the addition of Pr2O3 and Yb2O3. Compared with thepure KNNLS samples, the addition of rare-earth oxides (Pr2O3 andYb2O3) reduces the Curie temperatures (Tc). In addition, theorthorhombic and tetragonal transition (Toet) of the KNNLS-xmol%Pr2O3 and KNNLS-y mol% Yb2O3 ceramics is near or below roomtemperature. It confirms that the crystalline structure of theKNNLS-x mol% Pr2O3 and KNNLS-y mol% Yb2O3 ceramics is thecoexistence of the orthorhombic and tetragonal phases at roomtemperature, which is in agreement with the XRD results asobserved in Fig. 1. The tan d of KNNLS-x mol% Pr2O3 ceramicsremains almost unchanged (w0.05) with increasing temperatureup to 400 �C, but rises abruptly with further increasing. While forKNNLS-y mol% Yb2O3 ceramics, the tan d has little reliance on thetemperature, approximately 0.05. It is observed, from Fig. 4(c) and(d), that the maximum of 3r decreases as the measurementfrequency increases, which suggests that these ceramics showsome evidence of diffuse phase transition.

In order to further characterize the dielectric dispersion anddiffuseness of the ceramics, the modified CurieeWeiss law wasemployed:

1=3r � 1=3m ¼ C�1ðT � TmÞg (1)

where 3m is the maximum value of 3r at the phase transitiontemperature Tm, and g is the degree of diffuseness and C is theCurie-like constant. It is noted that g can have a value ranging from1 for a normal ferroelectric to 2 for an ideal relaxor ferroelectric.Fig. 5 shows the lg(1/3r � 1/3max) vs lg(T � Tm) measured at 10 kHzfor the (a) KNNLS-x mol% Pr2O3 and (b) KNNLS-y mol% Yb2O3ceramics. All the samples exhibit a linear relationship. By least-

squares fitting the experimental data to the modified CurieeWeisslaw, g was determined [14]. As shown in Fig. 5(a), the g slightlydecreases and then increases with the increase of the additionamount of Pr2O3, giving the minimum value of g ¼ 1.292 forKNNLS-0.1 mol% Pr2O3 ceramics. All the g is far below 2, revealingthat the normal ferroelectric characteristics appear predominatelyin KNNLS-x mol% Pr2O3 ceramics at 10 kHz. While the g of KNNLS-y mol% Yb2O3 ceramics is about 1.5 as shown in Fig. 5(b), whichsuggests that the ceramics tend to behave as relaxor ferroelectrics.The relaxor behavior should be attributed to the cationic disorderinduced by both A-sites and B-sites substitutions.

Fig. 6(a) shows the PeE loops at room temperature for theKNNLS-xmol% Pr2O3 ceramics (x¼ 0, 0.03, 0.05, 0.08, 0.01, and 0.15,respectively) under the electricfield of 4 kV/mmat 20Hz and (b) thePeE loops for the KNNLS-ymol%Yb2O3 ceramics (y¼ 0, 0.05, and 0.1,respectively) under the electric field of 6 kV/mm at 20 Hz. All theKNNLS-x mol% Pr2O3 ceramics and KNNLS-y mol% Yb2O3 ceramicsexhibitwell saturated PeE loops. The compositional dependences ofPr and Ec for KNLNS-x% Pr2O3 (x ¼ 0e0.15) ceramics and KNLNS-y%Yb2O3 (y ¼ 0e0.1) ceramics at room temperature are shown inFig. 7(a) and (b), respectively. It is found from Fig. 7(a) that forKNNLS-x mol% Pr2O3 ceramics, Pr increases sharply from 22.6 mC/cm2 to 42.8 mC/cm2 as x increases from 0 to 0.1, and then decreasesslightly with increasing x. However Ec firstly increases, then rapidlydrops to a minimum (Ec ¼ 7.23 kV/cm) at x ¼ 0.1 with increasing x,

Fig. 6. PeE loops of (a) KNLNS-x% Pr2O3 (x ¼ 0e0.15) and (b) KNLNS-y% Yb2O3 (x ¼ 0e0.1) ceramics at room temperature.

Z. Wang et al. / Current Applied Physics 11 (2011) S143eS148 S147

and then increases again with the further increase of x. Fig. 7(b)shows, for KNNLS-y mol% Yb2O3 ceramics, Pr monotonicallyincreaseswith increasing ywhile Ec decreases, giving themaximumofPr (37.3mC/cm2) and theminimumof Ec (12.8kV/cm) aty¼0.1. Theincrease in Pr suggests that the addition of Pr2O3 and Yb2O3 wouldenhance the ferroelectricity of the ceramics. Especially, with 0.1mol% Pr2O3 or 0.1 mol% Yb2O3 addition, the ceramics exhibit well satu-rated PeE loops with a large Pr and a small Ec. Moreover, it is indi-cated that Pr2O3 is more effective than Yb2O3 in improving the

Fig. 7. Compositional dependence of Pr and Ec of (a) KNLNS-x% Pr2O3 (x ¼ 0e0.15) and(b) KNLNS-y% Yb2O3 (y ¼ 0e0.1) ceramics at room temperature.

ferroelectric properties of KNNLS ceramics. These results are prob-ably attributed to the lattice distortion brought by modifying Pr2O3or Yb2O3, making the domain reverse more easily [1].

4. Conclusions

KNNLS-x mol% Pr2O3 and KNNLS-y mol% Yb2O3 ceramics wereprepared by conventional sintering method. All the ceramics showorthorhombicetetragonal coexistence phase at room tempera-ture. The additives of the rare-earth oxides (Pr2O3 and Yb2O3) willinhibit the growth of the crystalline grain of the ceramics. Withthe increasing of Pr2O3 and Yb2O3, Tc decreases slightly. Comparedwith the pure KNNLS ceramics (Pr¼ 22.6 mC/cm2, Ec¼ 13.4 kV/cm),the ferroelectric properties of KNNLS-0.1 mol% Pr2O3 ceramics(Pr ¼ 42.8 mC/cm2, Ec ¼ 7.2 kV/cm) and KNNLS-0.1 mol% Yb2O3

ceramics (Pr ¼ 37.3 mC/cm2, Ec ¼ 12.8 kV/cm) are greatly improved.It is indicated that Pr2O3 is more effective than Yb2O3 in improvingthe ferroelectric properties of KNNLS ceramics. These lead-freeferroelectric ceramics may be suitable for the applications inFRAM or other ferroelectric devices due to their very good ferro-electric properties.

Acknowledgment

This work was supported by National Science Foundation ofChina (NSFC Nos. 50772068 and 50972095) and Foundation ofDoctor Training Program in University and College in China (Nos.20030610035, and 20080610020).

Appendix. Supplementary data

Supplementary data related to this article can be found online atdoi:10.1016/j.cap.2010.12.033.

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