Journal of the Korean Physical Society, Vol. 66, No. 8, April 2015, pp. 13171322 Brief Reports

Enhancement of the Electrical-eld-induced Strain in Lead-freeBi0.5(Na,K)0.5TiO3-based Piezoelectric Ceramics: Role of the Phase Transition

Nguyen Van Quyet, Luong Huu Bac and Dang Duc Dung

School of Engineering Physics, Ha Noi University of Science and Technology, Ha Noi, Viet Nam andSchool of Materials Science and Engineering, University of Ulsan, Ulsan 680-749, Korea

(Received 12 February 2015, in nal form 13 March 2015)

In this work, a strong enhancement of the electric-eld-induced strain in Bi0.5(Na,K)0.5TiO3-based ceramics was observed via lithium(Li) addition. The Li-added Bi0.5(Na,K)0.5TiO3-basedceramics exhibited a strain of 0.40% under an electric eld of 6 kV/mm, which was almost twicethe value without the Li dopant (0.21%). We obtained the highest Smax/Emax value of 668 pm/Vfor 4-mol% Li addition, which was due to the phase transition from pseudocubic to rhombohedralsymmetry and/or to the distorted tetragonal structure. We suggest that controlling the phasetransition in ferroelectric materials is a way to enhance the electric-eld-induced giant strain andthat the phase transition from the non-polar phase to the polar phase results in a giant electric-eld-induced strain, which overcomes the result due to the phase transition from the polar phase to thenon-polar phase and/or the distorted structure. We expect our work to open new ways to enhancethe electric-led-induced giant strain to a value that is comparable to the value for Pb(Zr,Ti)O3(PZT)-based ceramics.

PACS numbers: 77.65.-j, 77.22.-d, 81.05.Je, 85.50.-nKeywords: Lead-free, BNKT ceramic, Phase transition, LithiumDOI: 10.3938/jkps.66.1317

I. INTRODUCTION

Recently, research focused on lead-free piezoceramicshas increased rapidly to replace lead zirconate titanate(PZT) in the electronics industry because PZT contain-ing more than 60-wt% Pb pollutes the environment andis harmful to human health. Among various lead-freesystems, our review work on the current developmentBi0.5(Na,K)0.5TiO3-based ceramics indicated that thedynamic piezoelectric coecient (d33) could be comparedwith that of soft PZT-based materials [1]. Pure lead-free piezoelectric Bi0.5(Na,K)0.5TiO3 (BNKT) ceramicsdisplay a low electric-eld-induced strain with dynamiccoecient values around 250 pm/V [2]. However, thelarge electric-eld-induced strains in BNKT ceramics canbe strongly enhanced when Ti4+ ions at B-sites are re-placed with either isovalent ions such as Hf4+ [3], Zr4+ [4]or Sn4+ [5], aliovalent ions including Nb5+ [6] and Ta5+[7], or trivalent ions such as Y3+ [8]. In addition, A-site modication such as Li+ or Ag+ at Na-sites [9,10],rare-earth ions (Sm3+, La3+ or Nd3+) at Bi-sites [1113], or both Li+ and La3+ at Na-site and Bi-sites [14],respectively, have been found to enhance the electric-eld-induced strain.

E-mail: dung.dangduc@hust.edu.vn

Moreover, a solid solution of various BNKT ceram-ics was reported to enhance the electric-eld-inducedstrain. Lead-free BNKT ceramics are easily fabricated,but exhibit unexpected properties such as a low Curietemperature, a high coecient eld, a low electric-eld-induced strain, etc. while secondary ferroelectricmaterials show good properties but exhibit unstablephases and/or require fabrication under extreme condi-tions [15]. Therefore, the purpose of those works wasto improve the properties of BNKT ceramics by usinga combination with perovskite ABO3 as a solid solu-tion. When solid solutions of perovskite ABO3 com-pounds such as K0.5Na0.5NbO3 [16], Bi(Zn0.5Ti0.5)O3[17], Sr(K1/4Nb3/4)O3 [18], LiTaO3 [19], LiNbO3 [20],BiAlO3 [21], BaTiO3 [22], Bi0.5La0.5AlO3 [23], SrZrO3[24], CaZrO3 [25], etc., were added to BNKT ceram-ics, the piezoelectric properties were strongly improved.Recently, in a BNKT-modied sample, a single dopantelement such as Zr4+, Nb5+, La3+ or Ta5+ ions, witha small amount of a perovskite solid-solution, such asBa0.7Sr0.3TiO3, LiSbO3 etc., was codoped at B-sites inBNKT ceramics [2629]. The maximum values of the dy-namic piezoelectric coecients (Smax/Emax) found in themodied BNKT ceramics are summarized in Fig. 1. Theresults indicate that the electric-eld-induced strain isstrongly enhanced by modications at A- and/or B-sitesin the BNKT ceramics and that the values of that strain

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Fig. 1. (Color online) The d33 values for various dopantsand/or solid-solutions with various amounts of the perovskiteABO3 in lead-free piezoelectric Bi0.5(Na,K)0.5TiO3 ceramics.

were comparable to those of soft-PZT ceramics. How-ever, the origin of the enhanced electric-eld-inducedstrain is still under debate.

Based on our research, enhanced electric-eld-inducedgiant strain in BNKT-based ceramics most likely origi-nated from i) a distorted tetragonal/rhombohedral struc-ture, ii) a phase transition from a tetragonal and/or arhombohedral phase to a pseudocubic phase, iii) a re-version phase transition from a pseudocubic phase to atetragonal and/or a rhombohedral phase caused by dop-ing and/or using a solid solution with other ABO3 per-ovskite phases, as shown in Fig. 2. However, severalquestions were raised that need to be further understood.First, the explanation of the phase transition based onthe distortion of the structure factor (tolerance factor)caused by dopants and/or a solid solution with a ABO3preovskite phase seems to be inexact because the struc-ture factor is only used to estimate the perovskite or non-perovkite structure and the tolerance factor could notbe used to determine the crystal structure, tetragonal,rhombohedral, orthorhombic or pseudocubic symmetry.Second, the phase transitions which have been reportedin literature from a polar (tetragonal and/or rhombohe-dral) to a pseudocubic phase occur quickly when severalpercent of dopants and/or solid solutions (2 3 mol%)are added, which indicates that the polar phases at themorphtropic phase boundary are very unstable. Third,if the phase transition from a polar to a non-polar phaseoccurs for a dopant, then the phase was recovered whenthe tolerance factor returned to its initial value. Under-standing the reason for the phase transition in modiedBNKT ceramics will help us to control the desire phasesand obtain good electrical-eld-induced strain to satisfythe requirement of a green material for the environmentand human health.

Recently, our work has focused on the eect of Lidopants in lead-free BNKT-based ceramics because Li+

Fig. 2. (Color online) Schematic for a possible way toobserve the enhancement of the electric-eld-induced strainin lead-free piezoelectric Bi0.5(Na,K)0.5TiO3-based ceramicswith a modied structure.

addition was found to suppress both the formation ofa second phase and Ti3+/4+ valence transitions [30,31].The rhombohedral and the tetragonal structural sym-metries were found to transition to pseudocubic sym-metry in Li-added BNKT modied with Ta [32]. Bothdistorted rhombohedral and tetragonal structures wereobtained in Li-added BNKT modied with Zr4+ [33].Interestingly, the pseudocubic phase transitioned to thetetragonal phase in Li-added BNKT modied with Sn4+[34,35]. Recently, we obtained both a distorted tetrago-nal and a distorted rhombohedral structure in of BNKTmodied with CaZrO3, which resulted in a strong en-hancement of the electric-eld-induced strain [36]. Inaddition, we obtained a thermally-induced phase transi-tion in (Li,Ta)-codoped BNKT ceramics, where the pseu-docubic phase dominated at low sintering temperatureswhile the tetragonal and the rhombohedral phases weremore stable at high sintering temperatures [37].

In this work, the eect of Li addition in lead-free0.97Bi0.5(Na,K)0.5TiO3-0.03CaZrO3 ceramics was inves-tigated at low sintering temperatures. The pseudocu-bic and the tetragonal phases were obtained withoutLi dopants, and coexisting tetragonal and rhombohedralphases were obtained after the introduction of Li+ ions.The Smax/Emax increased up to 668 pm/V with 4-mol%Li dopant, which was almost two times higher than thatof materials without a Li dopant (333 pm/V).

II. EXPERIMENTS

The Li-modied 0.97Bi0.5(Na0.80K0.20)0.5TiO3-0.03CaZrO3 (BNKT-CZ) ceramics were prepared byusing a conventional solid-state reaction route. The rawmaterials were powders of Bi2O3, K2CO3, TiO2, Li2CO3,CaCO3 (99.9%, Kojundo Chemical), Na2CO3 (99.9%,Ceramic Specialty Inorganics), and ZrO2 (99%, CERACSpecialty Inorganics). The compositions investigatedin this work were 0.97Bi0.5(Na0.80xLixK0.20)0.5TiO3-0.03CaZrO3 (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.10). Toprevent the vaporization of Bi, Na, and K, we embeddedthe disks in powders of identical compositions. The

Enhancement of the Electrical-eld-induced Strain Nguyen Van Quyet et al. -1319-

Fig. 3. FE-SEM images of the Li-modied BNKT-CZ ceramic specimens with (a) x = 0.00, (b) x = 0.02, (c) x = 0.04, (d)x = 0.06, (e) x = 0.08 and (f) x = 0.10.

Fig. 4. (Color online) (a) X-ray diraction patterns of Li-doped BNKT-CZ ceramics as a function of the Li-doping level x,and (b) the magnied XRD patterns in the 2 ranges of 37.0 42.0 and 45.0 49.0.

green compacts were well sintered in a covered aluminacrucible at 1100 C for 2 h in air.

The surface morphology was observed with a eld-emission scanning electron microscope (FE-SEM). Thecrystalline structures of the samples were character-ized by using X-ray diraction (XRD). The mechanicalstrains due to an external electric eld were measuredusing a linear variable dierential transformer.

III. RESULTS AND DISCUSSION

Figure 3 shows the FE-SEM micrographs of the frac-tured surfaces of Li-modied BNKT-CZ ceramics for dif-ferent amounts of Li addition. A dense microstructurewith some distinct pores was observed for the BNKT-CZceramics, as seen in Fig. 3(a). A compact microstructure

was also seen in the Li-modied BNKT-CZ ceramics. Inaddition, homogeneous structures without pores were ob-tained as the amount of Li substitution was increased, asseen in Figs. 3(b)(f).

Figure 4(a) shows the XRD patterns of the Li-modiedBNKT-CZ ceramics. All samples showed a single-phaseperovskite structure without any traces of secondaryphases, indicating that the Li+ ions had been successfullydiused into the lattice. Magnications of the XRD pat-terns in the ranges from 38.0 to 42.0 and from 44.0to 48.0 are shown in Fig. 4(b). The single (111)PCpeaks at a 2 of around 39.9 for the BNKT-CZ ce-ramic samples are evidence for the pseudocubic symme-try. These results are in agreement with those recentlyreported by Hong et al. for the CaZrO3-modied struc-ture of lead-free BNKT ceramics, which resulted fromrandom lattice diusion of Ca2+ and Zr4+ ions as a solidsolution at both A- and B-sites to promote a phase tran-

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Fig. 5. (Color online) (a) Unipolar strain hysteresis loops of Li-doped BNKT-CZ ceramic specimens, and (b) the d33 valuesas a function of the Li content.

sition from the tetragonal to the pseudocubic phase inthe BNKT ceramics [25]. In addition, the low sinteringtemperature possibly favored pseudocubic phase stabil-ity [37]. Interestingly, the (111)PC peaks tended to splitto (003)R/(210)R due to Li+ replacing Na+ at A-sites,indicating that a phase transition from a pseudocubic toa rhombohedral phase had occurred. In addition, thepositions of the XRD peaks were found to shift to higherangles when Li was added in amounts up to 8 mol%, in-dicating that the addition of Li increased the local com-pressive strain. These result can be understood based onthe dierent radii of Li+ (0.092 nm in 8-fold coordina-tion) and Na+ (0.136 nm in 12-fold coordination) whenLi+ ions ll at Na sites [38]. Interestingly, the positionsof the peaks shifted back to lower angles when the addedamount of Li+ exceeded 8 mol%. These results suggestthat Li+ ions can also ll octahedral sites, resulting inlarger lattice constants, because the ionic radius of Ti4+(0.0605 nm in 6-fold coordination) is comparable to theionic radius of Li+ (0.076 nm in 6-fold coordination) [38].The eect of multisite Li+ substitution is well known forboth lead-based and lead-free piezoelectric materials [33,35,39,40]. We recently determined the eect of multisiteLi+ substitution in BNKT-modied-with-Sn or BNKT-modied-with-Zr piezoelectric materials [33,35]. In otherwords, the rhombohedral phase transitioned from a pseu-docubic structure in Li-added lead-free BNKT-modied-with-CaZrO3 ceramics, which resulted from Li+ replac-ing Na+ at the A-sites.

The strain enhancements caused by Li substitutionwere observed in the unipolar S-E loops, as shown inFig. 5(a). The result is clear evidence for an enhance-ment of the electric-eld-induced strain in Li-added lead-free BNKT-CZ ceramics. The strain values were stronglyenhanced by a hundred percent (from 0.21% to 0.40% at6 kV/mm) after the introduction of Li+ at 4 mol% intothe BNKT-CZ ceramics, but it decreased to 0.16% as theLi+ concentration was increased to 10 mol%. The high-eld actuator piezoelectric coecients were calculatedfrom the ratio of the maximum strain (Smax) to the max-

imum applied electrical eld (Emax). The strains andthe normalized strains of BNKT-CZ ceramics as func-tions of the Li content are depicted in Fig. 5(b). TheLi-undoped lead-free BNKT-CZ samples exhibited anSmax/Emax value of 335 pm/V, which was smaller thanthe Smax/Emax value of 617 pm/V reported by Hong etal. for CaZrO3-modied BNKT ceramics [25]. Our ob-servation of smaller Smax/Emax values in BNKT-CZ ce-ramics could be understood as being due to the low sin-tering temperature, which resulted in dierent coexistingstable phases [25,37]. The highest Smax/Emax values was668 pm/V for BNKT-CZ ceramics with a Li addition of4 mol%, which was almost two times the value withoutLi addition, and was larger than the values reported byHong et al. [25].

Ferroelectric crystals are characterized by their asym-metric or polar structures, e.g., tetragonal, rhombohe-dral, orthorhombic, etc. In an external electric eld, ionsundergo asymmetric displacements, which results in asmall change in the crystal dimension proportional to theapplied electric eld [41,42]. However, the eect is gener-ally very small, thus limiting its usefulness. The mecha-nism for observing a giant electric-eld-induced strain isstill a subject of debate in both lead-based and lead-freeferroelectric materials.

Uchino and Pan et al. proposed that the giantelectric-eld-induced strain in PZT-based materials orig-inated from a phase transition from an antiferroelec-tric to a ferroelectric phase under an electric eld [42,43]. Zhang et al. proposed that the high strainin the lead-free Bi0.5Na0.5TiO3-BaTiO3-K0.5Na0.5NbO3system came from a signicant volume change during theeld-induced antiferroelectric-ferroelectric phase transi-tion and from the domain contribution by the inducedferroelectric phase [44, 45]. Jo et al. suggested thatthe large strain response in (K0.5Na0.5)NbO3-modied(Bi0.5Na0.5)TiO3-BaTiO3 lead-free piezoceramics wasdue to the presence of a non-polar phase that brought thesystem back to its unpoled state once the applied elec-tric eld had been removed, which led to a large unpolar

Enhancement of the Electrical-eld-induced Strain Nguyen Van Quyet et al. -1321-

strain [46]. In addition, Lee et al. reported that the giantelectric-eld-induced strains were attributed to a transi-tion from a non-polar to a ferroelectric phases in BNKT-BiAlO3 small grains with ferroelectric Bi0.5Na0.5TiO3large grains when an external electric eld was applied[47]. Ullah et al. suggested that the origin of the largeelectric-eld-induced strain was an inherently large elec-trostrictive strain combined with an additional strainhaving been introduced during the electric-eld-inducedphase transition [48]. Recently, Lee et al., via observa-tion of a giant electric-eld-induced strain in Sn-dopedBNKT ceramics, suggested a model on the basis of thecoexistence of polar nano-regions and a nonpolar matrix,which could reversibly transform into a polar ferroelec-tric phase under cyclic elds [49]. Ren reported that theobservation of a giant electric-eld-induced strain wasstrongly related to a reversible domain-switching mech-anism in which the switching of non-180 domains by therestoring force was provided by the general symmetry-conforming properties of point defects [50]. Recently, thehigh electric-eld-induced strain was mostly observed inthe boundary ferroelectric-paraelectric phase transition[38,16,19,21,2329]. In addition, a distorted structureat the morphortropic phase boundary due to the dopantand/or the solid solution with an ABO3 perovskite couldexplain the enhanced d33 values [33,36]. Interestingly, werecently obtained a tetragonal phase grown from a pseu-docubic phase that also displayed an enhanced electric-eld-induced strain [34,35,37]. Remarkably, a solid solu-tion of CaZrO3 with BNKT ceramics resulted in a phasetransition from a tetragonal and rhombohedral phase toa pseudocubic phase [25]. However, our work indicatedthat the possible phases were reversed from a pseudocu-bic to a rhombohedral structure via doping, resulting ina strong enhancement of the electric-eld-induced strain,which overcame the value observed for the phase transi-tion from a tetragonal and rhombohedral structure to apseudocubic structure. Therefore, we suggest that con-trolling the transition from the non-polar phase to thepolar phase may be the key to enhancing the electrical-eld-induced giant strain in lead-free materials to valuesthat are expected to be comparable to those for lead-based materials.

IV. CONCLUSION

The rhombohedral phase growing from the pseu-docubic phase due to Li substitution at the A-sitesof Bi0.5(Na,K)0.5TiO3-based ceramics enhanced theelectric-eld-induced giant strain. The highest electrical-strain was 668 pm/V for 4-mol% Li dopant. Our obser-vation indicated that the rhombohedral structural phaseformed in the pseudocubic phase displayed values of d33higher than that of the pseudocubic phase formed in theferroelectric phase. These results suggest a new way toenhance the electric-led-induced giant strain to values

comparable to those for PZT-based ceramics.

ACKNOWLEDGMENTS

This work was nancially supported by the Ministry ofEducation and Training, Vietnam, under project numberB 2013.01.55.

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