Current Applied Physics 10 (2010) 1174–1181

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Current Applied Physics

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Phase transitions and large electric field-induced strain in BiAlO3-modifiedBi0.5(Na, K)0.5TiO3 lead-free piezoelectric ceramics

Aman Ullah a, Chang Won Ahn b, Ali Hussain c, Sun Young Lee a, Hai Joon Lee a, Ill Won Kim a,*

a Department of Physics, University of Ulsan, Ulsan 680-749, Republic of Koreab Convergence Components R&D Division, KETI, Seongnam-si 463-816, Republic of Koreac School of Materials Science and Engineering, University of Ulsan, Ulsan 680-749, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 14 December 2009Received in revised form 1 February 2010Accepted 4 February 2010Available online 12 February 2010

Keywords:PerovskitePhase transitionStrainNormalized strain

1567-1739/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.cap.2010.02.006

* Corresponding author. Tel.: +82 52 259 2323; faxE-mail address: kimiw@mail.ulsan.ac.kr (I.W. Kim

Lead-free piezoelectric (1 � x)(Bi0.5(Na0.78K0.22)0.5TiO3)–xBiAlO3 (abbreviated BNKT22–BA, x = 0.00–0.100)ceramics were synthesized using a conventional sintering technique. The incorporation of BA into theBNKT22 lattice was investigated by X-ray diffraction (XRD), and the dielectric and ferroelectric character-izations and electric field-induced strain behavior. We found that the structural and electrical properties ofBNKT22 ceramics are significantly influenced by the presence of BA content. X-ray diffraction revealed apure perovskite phase for x 6 0.050. A phase transformation from tetragonal to pseudocubic was observedat x = 0.050. As BA content increased, the maximum dielectric constant continuously decreased, and thedepolarization temperature (Td) shifted towards lower temperatures. The polarization and strain hyster-esis loops indicate that the addition of BA significantly disrupts the ferroelectric order. The destabilizationof the ferroelectric order is accompanied by an enhancement of bipolar and unipolar strains. In particular,a very large electric field-induced strain (S = 0.35%) and a normalized strain (d�33 = Smax/Emax = 592 pm/V)were observed at x = 0.030, near the tetragonal–pseudocubic phase boundary. These results suggested thatthe BNKT22–BA system is a promising candidate for high performance, lead-free electromechanicalapplications.

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1. Introduction

Lead-based piezoelectric ceramics with perovskite structure,such as Pb(Zr, Ti)O3 (PZT), exhibit high electric field-induced strainresponse and excellent electromechanical properties, and as a re-sult have come to dominate the piezoelectric actuator market[1,2]. Because of the environmental issues associated with lead,however, many researchers have focused heavily on lead-freeceramics in an effort to replace commercial Pb(Zr, Ti)O3 for use indevice fabrication [3,4].

Lead-free piezoelectric ceramics exhibiting superior electrome-chanical responses have been formulated near the morphotropicphase boundary (MPB). Among the lead-free piezoelectric ceramicsso far developed, Bi0.5Na0.5TiO3 (BNT) and Bi0.5K0.5TiO3 (BKT) sys-tems have received a great deal of attention due to their excellentferroelectric and piezoelectric properties and their near rhombohe-dral–tetragonal (MPB) compositions [5]. In a BNT–BKT binary sys-tem the tetragonal side Bi0.5(Na0.78K0.22)TiO3 (BNKT22) of the MPBcomposition possess a high strain of 0.23% and a dynamic piezo-electric coefficient (d�33) of 291 pm/V at an applied electrical field

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of 80 kV/cm, and therefore can be considered for application inelectromechanical devices [6,7].

BiAlO3 (BA) has recently received considerable attention due toits excellent ferroelectric properties [8]. Theoretical calculationspredict that BA has a very large spontaneous polarization of about76 lC/cm2 and a Curie temperature of about 800 K [8]. Moreover,theoretical calculations predict its crystal structure to have perov-skite-like rhombohedral symmetry at room temperature [8]. Zyl-berberg et al. have synthesized BA and have confirmed that it isindeed ferroelectric and has a Curie temperature Tc > 520 �C [9].The dielectric, ferroelectric, and piezoelectric properties of BA arecomparable to those of BiFeO3 and SrBi2Ta2O9, making it a promis-ing new high-Tc, lead-free ferroelectric material for memory appli-cations [9]. However, its poor thermal stability and the extremeconditions used to synthesize this material limit its usability intechnological applications [9]. Therefore, it is favorable to stabilizeBA by incorporating it into other perovskite materials to form solidsolutions. Watanabe et al. [10] have fabricated (1 � x)(Bi0.5Na0.5)-TiO3–xBiAlO3 ferroelectric ceramics and evaluated their electricalproperties. Recently, Yu and Ye synthesized a (1 � x)(Bi0.5Na0.5)-TiO3–xBiAlO3 (BNT–BA) ceramic system and reported that it enjoysexcellent ferroelectric and piezoelectric properties compared topure BNT ceramics [11]. Despite the system having many interest-

A. Ullah et al. / Current Applied Physics 10 (2010) 1174–1181 1175

ing properties, however, no reports to date exist regardingBNKT22–BA ceramic systems.

In the present study, solid solution ceramics between two lead-free candidates, bismuth sodium potassium titanate (BNKT22) andbismuth aluminates (BA), (1 � x)(Bi0.5(Na0.78K0.22)0.5TiO3)–xBiAlO3

(BNKT22–BA), were investigated: their phase transition, micro-structure, dielectric, and ferroelectric properties were evaluated.In addition, we emphasize on the electric field-induced strain per-formance of a BNKT22–BA ceramic system. We expect that thisstudy will be helpful for understanding the properties of theBNKT22–BA ceramic system and in the promotion of its practicalapplications in electromechanical devices.

Fig. 1. X-ray diffraction (XRD) patterns of (1 � x)BNKT22–xBA ceramics (x = 0.00–0.100) in the 2h ranges of (a) 20–80� and (b) 35–50�. (�) Denotes the secondaryphase.

2. Experimental procedures

(1 � x)BNKT22–xBA (0 6 x 6 0.100) ceramics were synthesizedusing a conventional sintering technique from raw starting materi-als, namely Bi2O3, TiO2, Al2O3 (99.9%, High Purity Chemicals),Na2CO3 (99.9%, Cerac Specialty Inorganics), and K2CO3 (P99%, Sig-ma–Aldrich). Before weighing, these powders were dried in anoven at 100 �C for 12 h. For each composition, the starting materi-als were weighed according to the stoichiometric formula and ballmilled for 24 h in ethanol with zirconia balls. The dried slurrieswere calcined at 800 �C for 2 h and then ball milled again for24 h. The powders were pulverized, mixed with an aqueous poly-vinyl alcohol (PVA) solution and pressed into green disks with adiameter of 13 mm under a pressure of 70 MPa.

Sintering was carried out at 1150 �C for 2 h in covered aluminacrucibles. To prevent the vaporization of Bi, Na, and K, the diskswere embedded in a powder of the same composition. The relativedensity of each sample was determined by the Archimedes meth-od. The crystal structures of the ceramics were characterized by X-ray diffractometer (XRD, X’pert PRO MRD, Philips, KBSI, Busan Cen-ter). The surface morphology was observed using scanning electronmicroscopy (JSM-5610LV). Electrical measurements were carriedout on sintered ground disks. The circular surfaces of the diskswere covered with a thin layer of silver paste and fired at 700 �Cfor 30 min. The temperature dependence of the dielectric proper-ties were measured using an impedance analyzer (HP4192A) overa temperature range of 30–550 �C. The ferroelectric hysteresisloops were measured in silicon oil with the aid of a Sawyer–Towercircuit to apply an electric field with a triangular waveform. Elec-tric field-induced strains were measured in silicon oil with a linearvariable differential transducer (LVDT) system.

Fig. 2. Lattice constants a, c, and tetragonality (c/a) as functions of x in (1 � x)-BNKT22–xBA ceramics.

3. Results and discussion

Fig. 1a shows the X-ray diffraction patterns of the (1 � x)-BNKT22–xBA ceramics (x = 0.00–0.100) as 2h varies over the range20–80� The BNKT22–BA ceramics exhibit a pure perovskite struc-ture at x 6 0.050, suggesting that BA has diffused into the BNKT22lattice to form a homogeneous solid solution. A secondary phaseappeared at x P 0.060 around 2h = 24–31� and is marked with astar in the XRD pattern. This secondary phase was identified asBi2Al4O9 (PDF No. 74-1097). The concentration of secondary phaseincreased with increasing BA content. The appearance of the sec-ondary phase indicated that the solubility limit of BA in BNKT22is at x � 0.060. This is possibly due to the instability of the BAperovskite structure, which decomposes at high temperatures[9]. Belik et al. have synthesized BA ceramics and observed similarimpurity phases with peaks at 2h = 26–31.2� [12]. Fig. 1b illustratedetailed XRD analysis of the BNKT22–BA ceramics in the 2h rangeof 35–50�. In agreement with previously reported studies [5,6],pure BNKT22 ceramic (i.e. x = 0) has a tetragonal symmetry, as evi-denced by the splitting of (0 0 2)/(2 0 0) peaks at a 2h of around 46�

and a single (1 1 1) peak at around 2h of 40�. However, the tetragonaldistortion gradually decreased with increasing BA content, and(0 0 2)/(2 0 0) split peaks combined into a single (2 0 0) peak atx = 0.050, demonstrating that the crystal structure of BNKT22–BAceramics transformed from tetragonal to pseudocubic symmetry.

This structural transformation can be explained using the toler-ance factor and bond angles. In general, the tolerance factor is de-fined for ABO3 perovskites by the formula, t ¼ ðrA þ rOÞ=

ffiffiffi

2pðrB þ rOÞ

where rA, rB, and rO are the radii of A, B, and O ions, respectively[13]. In BNKT22–BA ceramics, the tolerance factor of BA is largerthan that of BNKT22. Therefore, the incorporation of BA into BNKTslightly increased the tolerance factor of the BNKT22–BA ceramicsystem. Kentaro et al. elucidated the role of A-site and B-site ionsin interlanthanide ABO3 perovskite structures and found that withincreasing B-site ionic radius, the B–O–B bond angle decreases, andso the distortion of the crystal structure from the ideal cubic perov-skite structure increases with the ionic radius of the B-site ion [14].In BNKT22–BA ceramics, the ionic radius of Al3+ (r = 53.5 pm) issmaller than that of Ti4+ (r = 60.5 pm), occupying the B-site in theBNKT22 lattice. Therefore, the B–O–B bond angle is expected to in-crease by the addition of BA in view of the above explanation [14].Thus, the distortion of the crystal structure decreased due to the io-nic radius of the B-site Al3+ ion, resulting in the crystal structuretransformation of BNKT22–BA lattice from a tetragonal to a pseu-docubic phase.

Fig. 3. SEM micrographs of the (1 � x)BNKT22–xBA ceramics: (a) x = 0.00, (b)x = 0.015, (c) x = 0.030, (d) x = 0.040, (e) x = 0.075, and (f) x = 0.100.

Fig. 4. Dielectric constant and loss of (1 � x)BNKT22–xBA ceramics as a function of tex = 0.040, (f) x = 0.045, (g) x = 0.050, and (h) x = 0.060.

1176 A. Ullah et al. / Current Applied Physics 10 (2010) 1174–1181

Fig. 2 shows the lattice constants a and c and the tetragonality(c/a) as functions of BA content in BNKT22–BA ceramics. It can beseen that the values of c and c/a decreased with increasing BA con-tent. However, the lattice constant ‘a’ remained roughly constant.The decreasing of tetragonality clearly indicates that the additionof BA decreased the lattice anisotropy of BNKT22–BA ceramic sys-tem, and it also changed the crystal symmetry from tetragonal topseudocubic.

Fig. 3 shows SEM micrographs of the BNKT22–BA ceramics withx = 0.00, 0.015, 0.030, 0.040, 0.075, and 0.100. Addition of BA hadlittle influence on the average grain size of BNKT22–BA ceramics,although the average grain size decreased slightly with increasingBA content. Conversely, addition of BA resulted in an obviouschange in grain morphology. Specifically, the grain profile shiftedtoward a neat, clear cubic, with some rectangular shapes up tox = 0.040. Further increasing BA concentrations up to x = 0.100changed the grains completely into a rectangular shape that incor-porated a small amount of pores.

Fig. 4 shows the temperature dependence of the dielectric con-stant and dielectric loss of BNKT22–BA ceramics withx = 0.00, 0.025, 0.030, 0.035, 0.040, 0.045, 0.050, and 0.060 at fre-quency of 1, 10, 50, and 100 kHz. Similar to the pure BNT and otherBNT-based ceramics [15–17], two dielectric anomalies were ob-served, at all measured frequencies, and correspond to the maxi-mum dielectric constant temperature (Tm) and depolarizationtemperature (Td), respectively. All of the ceramics exhibit a broadtransition peak suggesting that the transition is a diffused phasetransition [18].

mperature and frequency: (a) x = 0.00, (b) x = 0.025, (c) x = 0.030, (d) x = 0.035, (e)

Table 1Electromechanical properties of BNKT22–BA piezoceramics.

x values Density (g/cm3) Pm (lC/cm2) Pr (lC/cm2) Ec (kV/cm) er (100 kHz) Tan d (100 kHz) Strain (%) d�33 (pm/V)

0.00 5.88 42 29 26 6521 0.053 0.17 2960.015 5.92 43 26 16 5972 0.075 0.21 3500.025 5.98 38 19 12 5049 0.074 0.33 5330.030 5.98 35 8 11 4953 0.076 0.35 5920.035 6.01 35 8 11 4944 0.076 0.30 5080.040 5.99 35 8 10.7 4620 0.077 0.20 3400.045 5.89 31 7.8 10.5 4262 0.080 0.17 2870.050 5.83 30 7.8 10.6 3938 0.078 0.16 2690.060 5.80 28 7.5 10.5 3593 0.079 0.12 2030.075 5.76 20 5 9 3050 0.081 0.09 152

A. Ullah et al. / Current Applied Physics 10 (2010) 1174–1181 1177

It is widely known that dielectric properties are related to phasestructure and domain alignment. The relative dielectric constant er

and dielectric loss tan d at a frequency of 100 kHz are summarizedin Table 1. The BNKT22 ceramics without the addition of BA hadtetragonal symmetry and exhibited a high dielectric constant of6521 at 100 kHz. As BA content increased, the tetragonal distortiongradually decreased, resulting in a continuous decrease in the max-imum dielectric constant at Tm. Moreover, the Td peak was shiftedtoward lower temperatures with increasing BA content. Tan d var-ied between 0.05 and 0.08. This continuous decrease of the maxi-mum dielectric constant and the shift of the Td peak towardlower temperatures ascribed to the decreased polarization in theceramics as shown in Fig. 5. Because BA incorporation reducedthe tetragonality of the crystal structure of the BNKT22–BA cera-mic system as shown in Fig. 2. In addition, the low dielectric con-stant and high dielectric loss at higher BA concentrations can alsobe attributed to the pseudocubic symmetry and to the formation ofthe secondary phase.

Fig. 5 shows P–E hysteresis loops of BNKT22–BA ceramics mea-sured at room temperature. The BNKT22–BA ceramics with x = 0.00exhibit typical ferroelectric hysteresis loop having large remnantpolarization and maximum polarization of 29 lC/cm2 and 42 lC/cm2, respectively, and the coercive field of 26 kV/cm. As the hyster-

Fig. 5. Room temperature P–E hysteresis l

esis curves show, BA exerts significant influence on the loop’sshape and polarization values. The remnant polarization (Pr), max-imum polarization (Pm), and coercive field (Ec) as functions of BAcontent are summarized in Fig. 6, and are listed in Table 1. The pro-files of P–E hysteresis loops are consistent with the XRD analysis.As BA content increases, the tetragonal distortion gradually de-creases, resulting in a slightly pinched-type hysteresis loop, a de-crease in Pr and Ec, and a small decrease in Pm values. However,at higher BA content, when the sample has pseudocubic symmetry,both Pr and Ec were drastically lower and the hysteresis curvesslimmed, nearly exhibiting almost the behavior of linear dielectricmaterials. Still, the role of pseudocubic symmetry and the forma-tion of secondary phases at higher BA concentrations could notbe ignored, due to their strong influence on the ferroelectric prop-erties. The significant decrease in Pr and Ec and small decrease in Pm

together with the slightly pinched-type character of P–E hysteresisloops demonstrated that the long-range ferroelectric order whichwas dominant in pure BNKT22 was disrupted. However, the pres-ence of traces of ferroelectric order at higher BA content at zeroelectric field is also evident, since remnant polarization (Pr =4 lC/cm2 at x = 0.075) is not negligible. The destabilization of fer-roelectric order, along with an obvious change in loops shape, indi-cates that BA induced a phase transition in the BNKT22 lattice.

oops of (1 � x)BNKT22–xBA ceramics.

Fig. 6. Maximum polarization (Pm), remnant polarization (Pr), and coercive field (Ec)as functions of x in (1 � x)BNKT22–xBA ceramics.

1178 A. Ullah et al. / Current Applied Physics 10 (2010) 1174–1181

This kind of phase transition in BNT-based ceramics washypothesized to be a phase transition from ferroelectric to antifer-roelectric [7,19,20]. However, many researchers disagree with thishypothesis [18,21–24,25].

Temperature-dependent P–E hysteresis loops were performedto monitor the type of phase transition occurring during the addi-tion of BA into BNKT22 ceramics. Fig. 7 shows the P–E loops of theBNKT22–BA ceramics with x = 0.00, 0.015, 0.030, and 0.035 at var-ious temperatures at an electric field of 50 kV/cm. The BNKT22–BAceramics with x = 0.00 exhibits a nearly square (Pr = 23lC/cm2,Ec = 33 kV/cm) P–E hysteresis loop, indicating the ferroelectric nat-ure of BNKT22 ceramics. As the temperature increases, the loop be-comes narrower, decreasing the Pr and Ec values. At temperaturesnear the depolarization temperature (Td) � 130 �C, the loop be-comes slightly pinched but maintains the typical ferroelectric fea-ture due the observed Pr and Ec values (10 lC/cm2 and 7 kV/cm,respectively). However, with further increases in temperature

Fig. 7. The P–E hysteresis loops of (1 � x)BNKT22–xBA ceramics at various

above Td, the loop becomes narrower, leads to drastic decreasesin both Pr and Ec values. In addition, the observed pinched-typecharacter also disappears, and the P–E hysteresis loops are similarto those of a linear dielectric material. As shown in Fig. 7b–d, theBNKT22–BA ceramics with x = 0.015, 0.030, and 0.035 exhibit sim-ilar temperature dependences of ferroelectric properties, i.e., aslight pinched-type near Td, and the behavior of a nearly lineardielectric material above Td. However, the temperature for thepinched-type P–E hysteresis loop decreased with increasing BAconcentration. This indicates that BA lowers the depolarizationtemperature (Td), which agrees with the dielectric measurements.It is also observed that the temperature corresponding to thepinched-type hysteresis loop is slightly lower than Td. The differ-ence between these temperatures may be due to the diffuse tran-sition that takes place over a wide temperature range.

Recently, many researchers have investigated BNT-basedceramics to explore the behavior of pinched-type P–E hysteresisloops. The in situ transmission electron microscopy (TEM) of aBNT–BKT–BLT–BT system showed no crystallographic evidence ofantiferroelectric domains near Td [21]. Fan et al. [26] reported theexistence of a non-polar phase instead of an antiferroelectric oneabove Td. Recently, Jo et al. [27] investigated KNN-modified BNT–BT lead-free piezoelectric ceramics and proposed a non-polarphase instead of an antiferroelectric phase. On the basis of the re-ported work of BNT-based ceramics [18,21–27] and the tempera-ture dependence of dielectric and ferroelectric properties ofBNKT22–BA ceramics, it is therefore suggested that the pinch-typecharacter in P–E hysteresis loop resulted from the electromechan-ical interaction between the polar and non-polar phases whichcoexist and have the average free energy close to each other andtransform each other [18,23,26–28].

Fig. 8 shows the bipolar electric field-induced strain curves ofBNKT22–BA ceramics measured at room temperature. PureBNKT22 ceramics without the addition of BA exhibit a butterfly-

temperatures: (a) x = 0.00, (b) x = 0.015, (c) x = 0.030, and (d) x = 0.035.

Fig. 8. Bipolar S–E loops of (1 � x)BNKT22–xBA ceramics.

Fig. 9. Negative strain as a function of x in (1 � x)BNKT22–xBA ceramics.

A. Ullah et al. / Current Applied Physics 10 (2010) 1174–1181 1179

shaped curve typical of ferroelectric material with a maximumstrain of 0.15%. As BA content increases, the curve changes shapeand the maximum strain increases. On the other hand, the ‘‘nega-tive strain,” which denotes the difference between zero field strainand the lowest strain, decreased with increasing BA content [20].

The data also characterize the changes in bipolar S–E shapechanges with BA concentration. The ‘‘negative strain” is summa-rized as a function of BA content in Fig. 9. At lower BA content(x = 0.015), the negative strain increased slightly, whereas a sharpdecrease is observed at x = 0.030. Beyond this composition, thenegative strain gradually vanished. On the other hand, the totalstrain increased significantly with increasing BA content, reacheda maximum value of 0.35% at x = 0.030, decreased slowly up tox = 0.040, and thereafter decreased drastically. We suggest the fol-lowing explanation for the observed bipolar strain behavior inBNKT22–BA ceramics. At lower BA content (x 6 0.015), the tetrag-onal ferroelectric phase is dominant, and contributed to the strain,in the intermediate stage 0.025 6 x 6 0.035; here, BNKT22–BAceramics has coexisting ferroelectric (polar) and non-polar phases(evidenced by the slightly pinched-type hysteresis loop and polar-ization values), and both phases contribute to the strain, resultingin large strain. Beyond this narrow region, the non-polar phasedominates and the contribution from the ferroelectric domainswitching gradually disappears, resulting in the observed strain.

Fig. 10 shows the unipolar field-induced strain curves ofBNKT22–BA ceramics measured at room temperature. As in bipolarstrain, unipolar strain increased significantly with increasing BAcontent up to x = 0.030 and then gradually decreased up tox = 0.040. Beyond that, the strain value drops drastically. Thefield-induced strain S (%) and normalized strain d�33 of BNKT22–BA ceramics as functions of BA content are depicted in Fig. 11.The detailed maximum strain S (%) and d�33 values of BNKT22–BAceramics are shown in Table 1. A large strain (S = 0.35%) and nor-malized strain (d�33 = Smax/Emax = 592 pm/V) were obtained forx = 0.030 at an applied electric field of 60 kV/cm. A brief compari-son of BNKT22–BA with some other reported lead-free BNT-basedceramics data is drawn in Table 2 [6,7,10,19,29]. The Smax/Emax va-

lue of BNKT22–BA ceramic is higher than those of lead-free BNT-based ceramics systems, which suggests a promising lead-freealternative material for electromechanical applications.

BA has excellent ferroelectricity characteristics [9], and pureBNKT22 exhibited typical ferroelectric order with large Pr and Ec

of 29 lC/cm2 and 26 kV/cm, respectively. However, the additionof BA into a BNKT22 lattice disrupts the long-range ferroelectric or-der of pure BNKT22 ceramics, leading to a significant decrease in Pr

and Ec values. The significant decrease in polarization values, alongwith a noticeable change in loop shape, with increasing BA contentindicates that BA induced a phase transition in the BNKT22 latticefrom a ferroelectric (polar) to a non-polar phase, and passesthrough an intermediate stage that exhibits both ferroelectricand non-polar characteristics. The free energy of the ferroelectricphase seems so comparable to that of the non-polar phase at zerofields that it can be easily induced by an external electric field andsaturated at 60 kV/cm as shown in Fig. 5 [27]. Furthermore, thenon-polar phase dominates at higher BA concentrations (i.e.x = 0.075), apparently delays the transformation from non-polarto ferroelectric phase, as evidenced by the significant decrease inthe maximum polarization from 42 to 18 lC/cm2.

Fig. 10. Unipolar S–E loops of (1 � x)BNKT22–xBA ceramics.

Fig. 11. Electric field-induced strain (%) and normalized strain d�33 as functions of xin (1 � x)BNKT22–xBA ceramics.

Table 2Comparison of normalized strain d�33 = (Smax/Emax) of BNT-based materials.

BNT-based materials Normalized strain d�33 = Smax/Emax

(pm/V)References

BNKT22–BA 592 Currentwork

BNT–BKT 291 [6]BNT–BA 122 [10]BNT–BT 240 [19]BNT–KN 498 [7]BNT–BT–KNN2 560 [19]BNT–NN 259 [7]BNT–ST 488 [29]

1180 A. Ullah et al. / Current Applied Physics 10 (2010) 1174–1181

The results of this study suggest that a large unipolar strain, lo-cated only in a narrow region (i.e., around x = 0.030) in which bothferroelectric and non-polar phases coexist in the BNKT22–BA cera-mic system, exhibits competitive free energy. In addition, beyondthis narrow region, either the ferroelectric or the non-polar phasedominates. Neither of these phases in a single form can deliver astrain as large as that measured from the composition (x = 0.025–

0.035) close to the boundary between tetragonal and pseudocubicphases. Therefore on the basis of structure, P–E hysteresis loops,and S–E loops, this large strain at x = 0.030 can be attributed tothe coexistence of ferroelectric and non-polar phases.

4. Conclusions

Lead-free (1 � x)Bi0.5(Na0.78K0.22)0.5TiO3–xBiAlO3 (BNKT22–BA,x = 0.00–0.100) piezoelectric ceramics were successfully synthe-sized using a conventional sintering technique. A single perovskitephase was formed for x 6 0.050. XRD revealed a phase transitionfrom tetragonal to pseudocubic phase at x = 0.050. The P–E loopsand S–E loops demonstrated that the BNKT22–BA ceramics gradu-ally changed from ferroelectric to a non-polar phase passingthrough an intermediate stage which has both ferroelectric andnon-polar characteristics. The dielectric constant at Tm graduallydecreased with increasing BA content, which can be ascribed tothe decreased polarization and tetragonal distortion. Lastly, thecomposition (x = 0.030) close to the boundary between ferroelec-tric and non-polar phases exhibited a very large strain of 0.35%and a normalized strain d�33 of 592 pm/V at an electric field of60 kV/cm, ascribed to the coexistence of ferroelectric and non-po-lar phase induced by the chemical modification.

Acknowledgements

This work was financially supported by the Fundamental R&DProgram for Core Technology of Materials funded by the Ministryof Knowledge Economy, Republic of Korea. The authors alsoacknowledge the Ministry of Education, Science Technology(MEST) and Korea Industrial Technology Foundation (KOTEF)through the Human Resource Training Project for Regional Innova-tion for their financial support.

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