Materials Science and Engineering B 130 (2006) 94–100

Effect of MnO addition on structure and electrical properties of(Na0.5Bi0.5)0.94Ba0.06TiO3 ceramics prepared by citrate method

Qing Xu a,∗, Xin-Liang Chen a, Wen Chen a, Min Chen a, Shu-Long Xu a,Bok-Hee Kim b, Joong-Hee Lee b

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

Received 15 October 2005; received in revised form 4 January 2006; accepted 19 February 2006

Abstract

(Na0.5Bi0.5)0.94Ba0.06TiO3 ceramics added with 0–0.8 wt.% MnO were prepared by a citrate method, and the influence of the MnO addition onthe structure and electrical properties was investigated. The results indicate that the addition of small amounts of MnO did not cause a remarkablechange in crystal structure, but resulted in an evident evolution in microstructure. The dielectric constant (εr) and piezoelectric constant (d33)s0at©

K

1

itSo(CebcliTt(dt

0d

ignificantly decrease with increasing MnO content, while the electromechanical coupling factor (kp) presents a slight variation in the range of.25–0.28. The dissipation factor (tan δ) and mechanical quality factor (Qm) attain a minimum value of 1.5% and a maximum value of 304 whendding 0.4 and 0.5 wt.% MnO, respectively. This research demonstrates that doping effect and microstructural evolution contribute cooperativelyo the electrical properties of the ceramics.

2006 Elsevier B.V. All rights reserved.

eywords: Doping effect; Ferroelectrics; Manganese; Microstructure; Perovskite; Piezoelectricity

. Introduction

In these years, there is a growing interest of investigat-ng lead-free piezoelectric materials as a possible alternativeo widely used lead zirconate titanate (PZT)-based ceramics.odium bismuth titanate (Na0.5Bi0.5)TiO3 (NBT), is a per-vskite ferroelectric with a relatively large remanent polarizationPr = 38 �C/cm2) at room temperature and a relatively highurie temperature (Tc = 320 ◦C) [1]. Due to its strong ferro-lectricity at room temperature, NBT has been considered toe a promising candidate material for lead-free piezoelectriceramics. However, pure NBT suffers from a poling prob-em because of its high coercive field (Ec = 73 kV/cm), mak-ng it difficult in obtaining the desired piezoelectric properties.o solve this poling problem, various NBT-based solid solu-

ions have been developed [2–5]. Among these solid solutionsNa0.5Bi0.5)1−xBaxTiO3 (NBT-BT) system has attracted a greateal of attention owing to the existence of a rhombohedral-etragonal morphotropic phase boundary (MPB) near x = 0.06.

Compared with pure NBT, the NBT-BT compositions near theMPB provide substantially improved poling and piezoelectricproperties [2,6]. Recently, various oxides have been employedas additives for the NBT-BT compositions near the MPB, caus-ing different effects on the electrical properties depending onthe nature of the oxide additives [6–10].

Adding Mn is an often-adopted strategy to tailor the electri-cal properties of PZT-based piezoelectric ceramics [11]. MnOhas been used as an additive for several NBT-based ceramics,including NBT [12], (Na0.5Bi0.5)0.87(Sr0.5Ca0.5)0.13TiO3 [3],(Na0.5Bi0.5)0.94Ba0.06TiO3 [9] and (Na0.5Bi0.5)0.92Ba0.08TiO3[10]. The material constants of (Na0.5Bi0.5)0.94Ba0.06TiO3ceramics added with various amounts of MnCO3 have been eval-uated by Kaewkamnerd et al. with respect to the application ofthe material in Lamb wave devices [9]. It has been demonstratedthat (Na0.5Bi0.5)0.94Ba0.06TiO3 added with 0.5 wt.% MnCO3 ischaracterized by improved dielectric property, high mechanicalstrength and satisfactory electromechanical coupling factor, andcan be used as a new substrate material for Lamb wave devices.Despite of these investigations, the role of Mn on the struc-ture and electrical properties of NBT-based ceramics remains

∗ Corresponding author. Tel.: +86 27 87863277; fax: +86 27 87864580.E-mail address: xuqing@mail.whut.edu.cn (Q. Xu).

somewhat ambiguous. Therefore, further investigation on thissubject is necessary. On the other hand, NBT-based ceramics

921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2006.02.051

Q. Xu et al. / Materials Science and Engineering B 130 (2006) 94–100 95

are usually produced by the conventional solid-state method.Recently, research efforts have been devoted to the preparationof the material by various wet chemical methods, such as cit-rate method [13], hydrothermal process [14] and stearic acidgel route [15]. In continuation of our earlier research on theelectrical properties of NBT-BT ceramics prepared by a citratemethod [16], (Na0.5Bi0.5)0.94Ba0.06TiO3 ceramics added withvarious amounts of MnO were prepared by the citrate method inthis work, and the influence of the MnO addition on the structureand electrical properties was examined.

2. Experimental

The powders with the nominal composition of(Na0.5Bi0.5)0.94Ba0.06TiO3 + x wt.% MnO (x = 0–0.8) weresynthesized by a citrate method. Reagent grade NaNO3,Bi(NO3)3·5H2O, Ba(NO3)2, tetrabutyl titanate, Mn(NO3)2aqueous solution (50 wt.%) and citric acid were used as startingmaterials. Tetrabutyl titanate was first dissolved into a citricacid solution and various nitrates were then added, followed bya stirring to yield a transparent aqueous solution. The mole ratioof citric acid to the total metal cation content was 1.25. Theprecursor solution was heated to form a sol and subsequently a

Fig. 1. XRD patterns of the powders with various MnO contents.

gel. The gel was calcined at 600 ◦C for 1 h in air. The detail ofthe synthesis process has been described elsewhere [16]. Thecalcined powders were pressed into discs of 19 mm in diameterand 1 mm in thickness, and then sintered at 1150 ◦C for 2 h inair.

Fig. 2. SEM micrographs of the powders wi

th (a) x = 0, (b) x = 0.4, and (c) x = 0.6.

96 Q. Xu et al. / Materials Science and Engineering B 130 (2006) 94–100

The crystal structure of calcined powders and ceramic spec-imens was examined by a Rigaku D/MAX-RB X-ray diffrac-tometer using Cu K� radiation. The morphology of calcinedpowders and microstructure of ceramic specimens were inves-tigated by a Jeol JSM-5610LV scanning electron microscope(SEM). For ceramic specimens, thermally etched surfaces wereused for SEM observation. The ceramic specimens were pol-ished to ensure surface flatness and painted with silver pasteon both surfaces as electrodes. The dielectric properties weremeasured using a HP4294 impedance analyzer at 1 kHz. Thespecimens for measuring piezoelectric properties were poled ina silicon oil bath at 80 ◦C under 3.0 kV/mm for 15 min. Thepiezoelectric constant (d33) was measured using a quasistaticd33 meter based on the Berlincourt method at 110 Hz. Theelectromechanical coupling factor (kp), mechanical quality fac-tor (Qm) and frequency constant (Np) were measured by theresonance–antiresonance method using the HP4294 impedanceanalyzer. The polarization-field (P–E) hysteresis loop was mea-sured at room temperature 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 of the pow-ders with various MnO contents. A perovskite structure was

certified for the powders together with a trace amount of inter-mediate phase (Bi4Ti3O12). Fig. 2 shows the SEM micrographsof the powders with various MnO contents. It can be seen that thepowders have similar morphology, consisting of homogeneousand fine particles of 100–200 nm.

A pure perovskite structure was identified for the resultingceramic specimens. It was noticed that the color of the specimenwith x = 0 is yellowish, whereas that of the specimens added withMnO became black and got darker with increasing MnO content.Considering the very small addition amount of MnO, the colorchange with MnO addition infers the incorporation of Mn intothe lattice [12].

Fig. 3 shows the XRD patterns in the 2θ ranges of 38–42◦and 45–48◦, respectively, for the ceramic specimens with variousMnO contents. A (0 0 3)/(0 2 1) peak splitting, corresponding toa rhombohedral symmetry, and a (0 0 2)/(2 0 0) peak splitting,corresponding to a tetragonal symmetry, were detected for thespecimen with x = 0. It characterizes a coexistence of rhombohe-dral and tetragonal phases, which is consistent with the natureof the specimen with a composition near the MPB [2,6]. Thespecimens added with MnO maintain the coexistence of the twophases. It indicates that the addition of small amounts of MnOdid not give rise to an obvious change in crystal structure. Fig. 4shows the SEM micrographs of the ceramic specimens withvarious MnO contents. There is a monotonic increase in grain

Fig. 3. XRD patterns of the ceramic specimens with various Mn

O contents in the 2θ ranges of (a) 38–42◦ and (b) 45–48◦.

Q. Xu et al. / Materials Science and Engineering B 130 (2006) 94–100 97

Fig. 4. SEM micrographs of the ceramic specimens with (a) x = 0, (b) x = 0.2, (c) x = 0.5, and (d) x = 0.8.

size with enhancing MnO content. This implies that within theamount range of MnO addition in the present work, Mn mainlydissolved into the perovskite structure, because the accumula-tion of Mn at the grain boundaries would inhibit grain growth[17]. Moreover, it can be observed that distinct pores appeared inthe triangle regions formed by the grains in the specimens withrelatively high MnO contents (Fig. 4(c) and (d)). These resultsreveal that the MnO addition caused a significant evolution inmicrostructure.

Fig. 5 shows typical P–E hysteresis loops of the ceramicspecimens with various MnO contents. A well-saturated P–Ehysteresis loop was obtained for the specimen with x = 0 underan electrical field of 100 kV/cm, showing a relatively largeremanent polarization (Pr) of 37.1 �C/cm2 and a relatively lowcoercive field (Ec) of 42.7 kV/cm. For the specimens addedwith MnO, the values of applicable electrical field decline to50–70 kV/cm due to an increase in leakage current. Applyinga higher electrical field resulted in an electrical breakdown ofthe specimens or an obvious distortion of measured loops. Asa result, the P–E hysteresis loops of the specimens added withMnO are not fully saturated, making it unavailable in determin-ing the real values of remanent polarization and coercive field forthese specimens. Moreover, for the specimens added with MnO,

several scattered points deviating form the normal locus of theloops (Fig. 5(b) and (c)) and obvious asymmetry of the measuredloop (Fig. 5(c)) can be distinguished. It suggests that even underrelatively low electrical filed, the contribution of leakage currentis still evident for the MnO added specimens. Hence, it can bededuced that the MnO addition led to a reduction in resistivity.

Fig. 6 shows the dielectric and piezoelectric properties of theceramic specimens as a function of MnO content. The dielectricconstant (εr) and piezoelectric constant (d33) display a similarchange, rapidly decreasing with MnO content over the rangeof x = 0–0.4 and then slightly varying with further increasedMnO content. The variation trend of dissipation factor (tan δ)with MnO content is inverse to that of mechanical quality fac-tor (Qm), which attain a minimum value of 1.5% at x = 0.4and a maximum value of 304 at x = 0.5, respectively. A rathersimilar result has been previously reported for MnCO3 added(Na0.5Bi0.5)0.94Ba0.06TiO3 ceramics prepared by the conven-tional solid state method, with the peak value of the mechanicalquality factor (Qm ∼ 370) occurring when 0.5 wt.% MnCO3 wasadded [9]. Comparatively, no considerable influence of the MnOaddition on the electromechanical coupling factor (kp) and fre-quency constant (Np) were found, with the electromechanicalcoupling factor and frequency constant slightly changing in the

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Fig. 5. P–E hysteresis loops of the ceramic specimens with (a) x = 0, (b) x = 0.4, and (c) x = 0.6.

ranges of 0.25–0.28 and 2960–3100 Hz m, respectively. In Ref.[9], it has been revealed that the addition of moderate amount ofMn (0.5 wt.% MnCO3) into (Na0.5Bi0.5)0.94Ba0.06TiO3 resultedin a reduced dissipation factor, an obviously enhanced mechan-ical quality factor and a slightly increased frequency constantwhile maintaining a high electromechanical coupling factor. Theresults of the present work are generally consistent with the earlyresearch.

It has been reported that Mn can be present in perovskitematerials with multiple valence states [11]. Some recent resultssuggested that Mn coexists mainly in Mn2+ and Mn3+ statesin the perovskite structure piezoelectric ceramics such as PZT-based system [17,18]. In this research, Mn was introduced into(Na0.5Bi0.5)0.94Ba0.06TiO3 composition in the form of Mn2+.Considering the versatility of Mn in valence state as a transi-tional metal element, the oxidization of partial Mn2+ to Mn3+

during the sintering in air can be expected. Mn2+ and Mn3+ have

cationic radii of 0.80 A and 0.66 A, respectively, close to that ofTi4+ (0.68 A). Thus, Mn2+ and Mn3+ can enter into the octahedralsite of the perovskite structure to substitute for Ti4+ because ofradius matching. Accompanying this occurrence, oxygen vacan-cies were created to maintain electrical neutrality. Similar to thecase of PZT-based piezoelectric ceramics, the incorporation ofMn into the perovskite structure as an acceptor can generate ahard effect on the electrical properties. On the other hand, theformation of oxygen vacancies is beneficial for the mass trans-port during sintering. This is presumably responsible for thepromoted grain growth with the MnO addition.

The variation of the electrical properties with the addition ofMnO can be tentatively interpreted with respect to the dopingeffect and microstructural evolution. When the addition amountof MnO is relatively low (x ≤ 0.4), the hard doping effect on theelectrical properties appears to be predominant. In the low MnOcontent region (x ≤ 0.4), the decrease of resistivity, dielectric

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Fig. 6. Dielectric and piezoelectric properties of the ceramic specimens as a function of MnO content.

constant, dissipation factor and piezoelectric constant togetherwith the enhancement of mechanical quality factor correspondwell to the feature of a hard doping effect on the electrical prop-erties. In the case of relatively high MnO content (x > 0.4), thegrain growth became remarkable. The increase of grain sizefavors improving piezoelectric properties, which is known asgrain size effect [8,19]. The grain size effect compensates thedecrease of piezoelectric constant due to the hard doping effect.This is assumedly responsible for the slight fall of piezoelectricconstant when x > 0.4. The significant rise of dissipation factorand degradation of mechanical quality factor in the high MnOcontent region may be attributed to the appearance of distinctpores in the microstructure [18]. Therefore, it is likely that thedoping and microstructural effects contribute to the electricalproperties of the ceramics in a cooperative way.

4. Conclusions

(Na0.5Bi0.5)0.94Ba0.06TiO3 powders added with variousamounts of MnO (x = 0–0.8 wt.%) have been synthesized by acitrate method. The powders show a fine and uniform morphol-ogy, consisting of particles of 100–200 nm. The structure andelectrical properties of the resulting ceramic specimens havebeen investigated. It was found that the addition of small amountsof MnO did not cause an obvious change in crystal structure, but

led to a significant evolution in microstructure. A considerableinfluence of the MnO addition on the ferroelectric, dielectric andpiezoelectric properties was detected for the ceramics. It wasconsidered that the variation of the electrical properties with theMnO addition may be attributed to a joint contribution of dopingeffect and microstructural evolution.

Acknowledgements

This work was financially supported by the Natural ScienceFoundation of China (grant no. 50272044). It is grateful tothe Natural Science Foundation of China (grant no. 50410529)and Korea Science and Engineering Foundation (grant no. F01-2004-000-10084-0) for jointly supporting the research. One ofthe authors (Q. Xu) expresses sincere acknowledgement to Pro-fessor Kohji Toda of the Department of Electrical and ElectronicEngineering, National Defense Academy of Japan, for his kindhelp in providing an important reference.

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