Effect of composition on electrical properties of lead-free Bi0.5(Na0.80K0.20)0.5TiO3-(Ba0.98Nd0.02)TiO3 piezoelectric ceramicsPharatree Jaita, Anucha Watcharapasorn, and Sukanda Jiansirisomboon

Citation: Journal of Applied Physics 114, 027005 (2013); doi: 10.1063/1.4811813 View online: http://dx.doi.org/10.1063/1.4811813 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/114/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Phase transitional behavior and electric field-induced large strain in alkali niobate-modifiedBi0.5(Na0.80K0.20)0.5TiO3 lead-free piezoceramics J. Appl. Phys. 115, 034101 (2014); 10.1063/1.4862187 Phase transitions, relaxor behavior, and large strain response in LiNbO3-modified Bi0.5(Na0.80K0.20)0.5TiO3lead-free piezoceramics J. Appl. Phys. 114, 044103 (2013); 10.1063/1.4816047 Bipolar piezoelectric fatigue of Bi(Zn0.5Ti0.5)O3-(Bi0.5K0.5)TiO3-(Bi0.5Na0.5)TiO3 Pb-free ceramics Appl. Phys. Lett. 101, 042905 (2012); 10.1063/1.4738770 Effect of lattice occupation behavior of Li+ cations on microstructure and electrical properties of(Bi1/2Na1/2)TiO3-based lead-free piezoceramics J. Appl. Phys. 109, 054102 (2011); 10.1063/1.3555598 Phase structure, dielectric properties, and relaxor behavior of ( K 0.5 Na 0.5 ) NbO 3 – ( Ba 0.5 Sr 0.5 ) TiO 3lead-free solid solution for high temperature applications J. Appl. Phys. 105, 124104 (2009); 10.1063/1.3153128

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Effect of composition on electrical properties of lead-freeBi0.5(Na0.80K0.20)0.5TiO3-(Ba0.98Nd0.02)TiO3 piezoelectric ceramics

Pharatree Jaita,1 Anucha Watcharapasorn,1,2 and Sukanda Jiansirisomboon1,2,a)

1Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200,Thailand2Materials Science Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand

(Received 29 October 2012; accepted 17 January 2013; published online 10 July 2013)

Lead-free piezoelectric ceramics with the composition of (1�x)Bi0.5(Na0.80K0.20)0.5TiO3-x(Ba0.98Nd0.02)TiO3 or (1�x) BNKT-xBNdT (with x¼ 0–0.20 mol fraction) have been synthesized

by a conventional mixed-oxide method. The compositional dependence of phase structure and

electrical properties of the ceramics were systemically studied. The optimum sintering temperature

of all BNKT-BNdT ceramics was found to be 1125 �C. X-ray diffraction pattern suggested that

BNdT effectively diffused into BNKT lattice during sintering to form a solid solution with a

perovskite structure. Scanning electron micrographs showed a slight reduction of grain size when

BNdT was added. It was found that BNKT-0.10BNdT ceramic exhibited optimum electrical

properties (er¼ 1716, tand¼ 0.0701, Tc¼ 327 �C, and d33¼ 211 pC/N), suggesting that this

composition has a potential to be one of a promising lead-free piezoelectric candidate for dielectric

and piezoelectric applications. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4811813]

I. INTRODUCTION

Lead-based piezoelectric ceramics such as lead zircon-

ate titanate or Pb(Zr, Ti)O3 have dominated the commercial

market for more than half a century since its discovery in

1950s.1 They are widely used in electronic industry for vari-

ous devices such as piezoelectric transformer, ultrasonic

equipment, actuator, sensor, and multilayered capacitor.2

However, the waste of products containing lead oxide (PbO)

causes a serious ecological problem and it is strongly harm-

ful for the human health.3 Recently, according to the stern

restriction of environmental pollution such as waste electri-

cal and electronic equipment (WEEE) and restriction of haz-

ardous substance (RoSH), the development of lead-free

piezoelectric ceramics capable of replacing lead-based pie-

zoelectric ceramics is strongly required.4

Among the potential materials that could substitute for

lead-based piezoelectric ceramics are the solid solutions

of (1�x)Bi0.5Na0.5TiO3-xBi0.5K0.5TiO3 (BNT-BKT). The

Bi0.5(Na1�xKx)0.5TiO3 or BNKT system is widely investi-

gated as it is also lead free ferroelectric material.2,5 It has

attracted considerable attention, because of the existence of

rhombohedral-tetragonal morphotropic phase boundary

(MPB) near x¼ 0.16–0.20.5–7 The dielectric, piezoelectric,

and electromechanical properties are sharply enhanced at

this boundary. The maximum dielectric constant (er¼ 1030)

and piezoelectric coefficient (d33¼ 151 pC/N, d31¼ 46.9 pC/N)

were obtained at the composition of x¼ 0.20.7 In addition,

BNKT has been investigated in terms of various dopants and

the formation of solid solutions with other compounds.

Previous studies have demonstrated that BNKT-based

compositions modified with BaTiO3,8–11 SrTiO3,3,12

Bi(Zn0.5Ti0.5)O3,13 (K0.5Na0.5)NbO3,14 BiAlO3,15 and

Ba(Zr0.04Ti0.96)O316 have better piezoelectric properties and

easier treatment in poling process when being compared

with that of pure BNKT ceramic.

It is well known that barium neodymium titanate or

BNdT as well as modified barium titanate (BaTiO3) has a tet-

ragonal phase at room temperature. The addition of Nd3þ as

a dopant can also modify crystal structure and improve

dielectric properties of BaTiO3 ceramic. For BNdT composi-

tion, Ba2þ is displaced by Nd3þ at A-site (aliovalent A-site

doping), which can lead to A-site cation disorder and defect

structure. Nd doped BaTiO3 has been paid significant atten-

tion by many researchers. In 2008, Yao et al.17 investigated

phase and dielectric behavior of Ba1�xNdxTiO3 ceramics.

They found that the addition of Nd3þ as a dopant can modify

the crystal structure of BaTiO3. X-ray diffraction (XRD)

indicated that Ba1�xNdxTiO3 ceramic had a tetragonal phase

at x< 0.05, while it had a cubic phase at x� 0.05. The Curie

temperature (Tc) was shifted to lower value when Nd3þwas

added. At the composition of x¼ 0.02, the room temperature

dielectric constant was maximum (�8000) in tetragonal

phase region with the Tc � 76 �C.

There is no report on the properties of Bi0.5

(Na0.80K0.20)0.5TiO3�x(Ba0.98Nd0.02)TiO3 binary system

up to date. Accordingly, attempts to combine the advantages

of both BNKT and BNdT compounds were then carried

out. In this study, the binary system of (1�x)Bi0.5

(Na0.80K0.20)0.5TiO3-x(Ba0.98Nd0.02)TiO3 was prepared by a

conventional mixed-oxide technique and conducted research

on phase, physical properties, and microstructure of the

ceramics. The effect of BNdT concentration on electrical

properties of BNKT ceramic was also investigated and dis-

cussed in detail.

II. EXPERIMENTAL

The Bi0.5(Na0.40K0.10)TiO3 and (Ba0.98Nd0.02)TiO3 pow-

ders were prepared by a conventional mixed-oxide method.a)Electronic mail: sukanda.jian@cmu.ac.th

0021-8979/2013/114(2)/027005/6/$30.00 VC 2013 AIP Publishing LLC114, 027005-1

JOURNAL OF APPLIED PHYSICS 114, 027005 (2013)

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The starting materials used in this study were Bi2O3 (98%,

Fluka), Na2CO3 (99.5%, Carlo Erba), TiO2 (99%, Riedel-de

Ha€en), K2CO3 (99%, Riedel-de Ha€en), BaCO3 (98.5%,

Fluka), and Nd2O3 (99.9%, Sigma-Aldrich). All carbonate

powders were first dried at 120 �C for 24 h in order to remove

any moisture. The raw materials of BNKT and BNdT were

stoichiometrically weighted and mixed by ball milling for

24 h in ethanol. The slurry was dried using oven-drying

method. The dried BNKT and BNdT powders were sepa-

rately calcined in closed Al2O3 crucible at 900 �C for 2 h and

1100 �C for 2 h, respectively. Both calcined powders were

then weighed, mixed, and dried to produce the mixed pow-

ders of (1�x)Bi0.5(Na0.80K0.20)0.5TiO3�x(Ba0.98Nd0.02)TiO3

or (1�x)BNKT-xBNdT, when x¼ 0, 0.05, 0.10, 0.15, and

0.20 mol fraction. After drying and sieving, the powders

were granulated by adding a few drops of 3 wt. % polyvinyl

alcohol (PVA) as a binder and were then pressed into disks

with 10 mm in diameter and about 1.3 mm in thickness. The

green disks were preheated in air at 500 �C for 1 h to remove

organic binder and then sintered at 1050–1175 �C for 2 h

with a heating/cooling rate of 5 �C/min.

Crystal structures of mixed powders and sintered

ceramics were examined in 2h range of 10�–80� using an

X-ray diffractometer (XRD-Phillip, X-pert) with CuKa radi-

ation (step 0.02�). Bulk densities of all ceramics were deter-

mined using Archimedes’ method. The theoretical densities

of all samples were calculated based on the theoretical

densities of BNKT (5.84 g/cm3)18 and BT (6.01 g/cm3).19

Surface morphologies of the ceramics were observed using

scanning electron microscope (SEM, JEOL JSM-6335 F).

Grain size of each sample was determined by a mean linear

intercept method from SEM micrographs. The average grain

size was estimated by counting the number of grains inter-

cepted by one or more straight lines.

In order to measure electrical properties, the samples

were carefully polished to 1 mm thickness and painted with

the silver paste on both sides before being fired at 600 �C for

15 min to form electrodes. Dielectric properties were deter-

mined ranging from 25 �C to 500 �C with a frequency of

10 kHz using 4284A LCR-meter connected to a high temper-

ature furnace. A standard Sawyer-Tower circuit was used to

measure ferroelectric hysteresis loop. AC electric field at

55 kV/cm was applied to the samples and polarization-

electric field (P-E) loops were recorded by a digital oscillo-

scope. Prior to the measurement of the piezoelectric proper-

ties, the samples were poled at 60 �C in silicone oil by

applying DC electric field of 5 kV/mm for 15 min.

Piezoelectric coefficient (d33) was recorded from 1-day aged

samples using d33-meter (KCF technologies, S5865) at a

frequency of 50 Hz.

III. RESULTS AND DISCUSSION

A relationship between densities of BNKT-BNdT

ceramics at various sintering temperatures between 1050 and

1175 �C is shown in Fig. 1. At higher sintering temperature

of 1175 �C, some samples containing 15–20 mol. % of BNdT

started to melt. These sintered samples were, therefore,

excluded from further investigations. As can be seen in these

figures, all BNKT-BNdT ceramics achieved their maximum

density at 1125 �C and the values were rather similar (rang-

ing from 5.77 to 5.81 g/cm3), corresponding to at least 98%

of their theoretical values as listed in Table I. It should be

noted that the relative density values of BNKT-BNdT

ceramics in this work were close to the BNT-BKT-BT ter-

nary system previously reported by Zhang et al.11 who

obtained the relative density value around 97.5%–98.2%.

Thus, the samples sintered at 1125 �C were selected for fur-

ther characterizations.

Figure 2(a) shows X-ray diffraction patterns of

(1�x)BNKT-xBNdT mixed powders (with x¼ 0, 0.05, 0.10,

0.15, and 0.20 mol fraction) scanned with a wide range of

2h¼ 10�– 80�. In these patterns, no second phases were

detected for all compositions. When 5 mol. % of BNdT was

added into BNKT powder, the (110) main peak correspond-

ing to 2h � 32� was slightly split of BNdT peak. The peaks

FIG. 1. Plots of density as a function of sintering temperature of BNKT-

BNdT ceramics with different BNdT-added contents.

TABLE I. Physical and electrical properties of (1�x)BNKT-xBNdT ceramics sintered at 1125 �C.

x Relative density (%) Grain size (lm) Td (�C) Tm (�C) era tan da Pr (lC/cm2) Ec (kV/cm) Rsq d33 (pC/N)

0 99.48 0.60 6 0.09 135 320 1419 0.0479 30.48 31.49 1.12 178

0.05 99.46 0.40 6 0.05 136 320 1602 0.0620 16.30 11.87 0.57 174

0.10 98.73 0.41 6 0.05 148 327 1716 0.0701 20.15 22.95 0.93 211

0.15 98.47 0.52 6 0.09 163 326 1284 0.0593 21.45 30.10 0.99 123

0.20 98.17 0.58 6 0.09 168 310 918 0.0430 21.05 32.71 0.98 119

aDielectric data obtained at room temperature and at a frequency of 1 kHz.

027005-2 Jaita, Watcharapasorn, and Jiansirisomboon J. Appl. Phys. 114, 027005 (2013)

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belonged to BNdT were dominant with increasing the

amount of BNdT up to 20 mol. %.

X-ray diffraction patterns at room temperature (�25 �C)

of optimally sintered BNKT-BNdT ceramics are shown in

Fig. 2(b). As can be seen from these patterns, all ceramics

possessed pure perovskite phase and no secondary phase

could be certified, indicating that BNdT has completely

diffused into BNKT lattice to form a homogenous solid solu-

tion. The solubility limit of BNdT in BNKT lattice is

believed to be more than 20 mol. % because no trace of sec-

ondary phases was detected in XRD patterns. With increas-

ing BNdT content, the diffraction peaks shifted to lower

angles. Because of the differences in ionic radius of Bi3þ

(1.17 A), Naþ (1.18 A), Kþ (1.33 A), Ba2þ (1.42 A), and Nd3þ

(1.109 A) when Ba2þ and Nd3þ filled in A-site, lattice distor-

tion occurred which induced the lattice constant to change

and then the diffraction peaks were shifted.20 Similar obser-

vation of the diffraction peaks shifted to lower angles in the

90BNT-5BKT-5BT ternary system was also previously

reported by Wang et al.8 The results of more detailed XRD

analysis performed in narrow range of 2h¼ 44�–48� are

shown in Fig. 3(a). It is widely known that BNT is rhombo-

hedral, whereas BKT is tetragonal at room temperature. In

agreements with previous works, a MPB in the solid solu-

tions of BNT-BKT existed near x¼ 0.16–0.20.6,7 Based on

the XRD pattern in Fig. 3(a), the structure of BNKT ceramic

presented features of mixed rhombohedral-tetragonal sym-

metry but showed a domination of rhombohedral over tetrag-

onal structure. At x¼ 0 .05, the peak around 46.5� was

featured with slightly splitting of (002) and (200) peaks. The

splitting of (002)/(200) peaks at 2h � 46.5� clearly domi-

nated at the composition of x¼ 0.10. The (002) peak was

found to broaden and increase in intensity, while its intensity

was decreased in (200) peak. This indicated that the crystal-

line structure at this composition was nearly the same

amount of coexisting rhombohedral-tetragonal phases of

BNKT-BNdT system. However, it is interesting to note that

the addition of BNdT content greater than 10 mol. % led to

wider separation of the (002) and (200) peaks which indi-

cated the tetragonal-rich phase. This behavior became

clearer after the analysis of calculated lattice parameters

(a and c) indicated an increase in tetragonality (c/a) values

as shown in Fig. 3(b).

SEM micrographs of the polished and thermally etched

surfaces obtained from (1�x)BNKT�xBNdT ceramics (with

x¼ 0, 0.05, 0.10, 0.15, and 0.20 mol fraction) sintered at

1125 �C are shown in Fig. 4. From SEM images, all samples

were dense with a well-developed microstructure and a gran-

ular morphology. The addition of BNdT had an influence on

the average grain size (see grain size values in Table I). As

FIG. 2. X-ray diffraction patterns of

BNKT-BNdT with 2h¼ 10�–80�:(a) mixed powders and (b) ceramics

sintered at 1125 �C.

FIG. 3. X-ray diffraction patterns of

(a) BNKT-BNdT ceramics sintered at

1125 �C with 2h¼ 44�–48� and (b) lat-

tice parameters as function of compo-

sition for BNKT-BNdT system.

027005-3 Jaita, Watcharapasorn, and Jiansirisomboon J. Appl. Phys. 114, 027005 (2013)

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shown in Fig. 4, pure BNKT ceramic was well crystallized

with a diameter of 0.6 lm. The additional small amount of

BNdT (x¼ 0.05–0.10) into BNKT slightly inhibited grain

growth as can be seen from a slight drop of average grain

size from 0.6 lm for pure BNKT to �0.40 lm for BNdT-

added samples. The mixture of small and large grains started

to be present at this composition. Upon further increasing

BNdT up x¼ 0 .15–0.20, the grain size increased and some-

what the coarse grain is formed. The mixture of small and

large grains was still be observed.

Room temperature dielectric constant (er) and dielectric

loss (tand) plotted as a function of BNdT content at fre-

quency of 1 kHz are shown in Fig. 5(a). The dielectric values

are also listed in Table I. The er of pure BNKT ceramic was

found to be 1419 with tan d of 0.0479. The addition of BNdT

into BNKT trended to enhance er up to 1716 at the composi-

tion of x¼ 0.10 and then started to decrease with further

increasing BNdT. The er value at the composition of

x¼ 0.10 in this work was found to be higher than the er value

(�1648) of the 85.25BNT-10.995BKT-3.755BT ternary sys-

tem previously reported by Dai et al.10 Variation of tan dvalue with increasing BNdT content had similar trend to that

of er. At the composition near x¼ 0.10, the crystalline struc-

ture was considered to contain nearly the same amount of

coexisting rhombohedral and tetragonal phases. Both of

these phases produced a large number of polarization direc-

tions and enhanced crystallographic orientations under elec-

tric field. This promoted the movement and polarization of

ferroelectric active ions which led to an increase in er.16

The dielectric constant versus temperature curves of

BNKT-BNdT ceramics at the frequency of 10 kHz are

shown in Fig. 5(b). Dielectric constant at high temperature

(em) of pure BNKT ceramic was found to be 5114 with tan dof 0.0187. For BNdT-added sample, the em of 5595 was

observed in BNKT-0.05BNdT sample and then em value

slightly dropped with further increasing BNdT content. The

tan d of all ceramics had similar trend to that of em and varied

between 0.0187 and 0.0261. It has been reported that

BNT-based ceramics exhibited two dielectric anomalies

FIG. 4. SEM micrographs of (1�x)BNKT-xBNdT ceramics sintered at 1125 �C where (a) x¼ 0, (b) x¼ 0.05, (c) x¼ 0.10, (d) x¼ 0.15, and (e) x¼ 0.20.

FIG. 5. Plots of (a) room temperature dielectric constant and dielectric loss

as a function of BNdT content and (b) temperature dependence on dielectric

constant and dielectric loss of BNKT-BNdT ceramics sintered at 1125 �C.

027005-4 Jaita, Watcharapasorn, and Jiansirisomboon J. Appl. Phys. 114, 027005 (2013)

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corresponding to the maximum dielectric constant tempera-

ture (Tm: the temperature at which dielectric constant reaches

the maximum, Tm could be considered as the Curie point: Tc)

and depolarization temperature (Td: the temperature at which

the phase transition from rhombohedral to tetragonal).21 As

can be seen from Fig. 5(b), the em versus temperature curves

were similar for all compositions. Two transition tempera-

tures of Td and Tm were observed from room temperature

(�25 �C) to high temperature (�500 �C). The dielectric

curves of BNKT ceramic exhibited a broad of Tm peak.

Further increasing BNdT, the Tm peak gradually broadened,

suggesting that BNdT concentration induced a diffuse phase

transition in BNKT-BNdT system. The Td and Tm of pure

BNKT ceramic in this study were found to be 135 �C and

320 �C, respectively, which were in agreement with the pre-

viously reported work by Sasaki et al.7 For BNKT-BNdT

samples group, Td significantly increased with increasing

BNdT content. However, Tm reached a maximum value of

327 �C at x¼ 0.10. The Tm value at this composition was

higher than that observed earlier in the 85.2BNT-12BKT-

2.8BT ternary system previously reported by Nagata et al.9

who obtained the Tm � 301. With further increasing BNdT

over 10 mol. %, Tm decreases (see Fig. 5(a)). In general, an

obvious improvement in dielectric properties of BNT-based

ceramics through modification was accompanied by a signifi-

cant decrease in Td. This was believed to be an effect of addi-

tives in BNT-based composition which caused defects such

as vacancies and lattice deformation, and they facilitated the

domain movement leading to higher dielectric properties but

decreasing Td.8 However, BNKT-BNdT multi-component

system not only gave good dielectric performance but also

had a high Td.

The polarization-electric field (P-E) hysteresis loops of

(1�x)BNKT-xBNdT ceramics (with x¼ 0, 0.05, 0.10, 0.15,

and 0.20 mol fraction) sintered at 1125 �C are shown in Fig. 6.

As evident in the hysteresis curves, all samples exhibited

well-saturated P-E loops at room temperature. Pure BNKT

ceramic displayed typical well-saturated P-E hysteresis loops

with the maximum coercive field (Ec � 31.49 kV/cm), rema-

nent polarization (Pr � 30.48 lC/cm2), and loop squareness

(Rsq � 1.12), which are all in good agreement with previ-

ously reported work by Yoshii et al.22 However, BNdT

exerted significant influences on shape and polarization val-

ues of the loops. The addition of BNdT into BNKT ceramic,

however, slightly degraded of ferroelectric behavior, as can

be seen from a reduction trend in Pr, Ec, and Rsq values. The

addition of 10–20 mol. % BNdT samples showed similar of

Pr (�20.15–21.45 lC/cm2) and Rsq (0.93–0.99). It is interest-

ing to note that the Pr value of BNKT-BNdT samples in this

work was close to the 75BNT-20BKT-5BT ternary system

previously reported by Wang et al.8 who obtained Pr � 20

lC/cm2. On the other hand, Ec tended to increase with

increasing amount of BNdT. Since BNdT by itself was

known to have lower value of Pr (�3.68 lC/cm2) and Ec

(�2.65 kV/cm) compared to that pure BNKT, this seemed to

be the reason for a reduction of both Pr and Rsq observed in

BNdT-added samples.

Piezoelectric coefficient (d33) values with the variation

of BNdT content are listed in Table I. The values were found

to be in range of 119–211 pC/N. For pure BNKT, d33 showed

a value of 178 pC/N which was close to the value observed

earlier by Hiruma et al.23 With increasing BNdT, d33 value

increased to the maximum value of 211 pC/N at x¼ 0.10.

This value was higher than that observed earlier in the

FIG. 6. Plots of polarization as a function of electric field of (1-x)BNKT-xBNdT ceramics sintered at 1125 �C (a) x¼ 0, (b) x¼ 0.05, (c) x¼ 0.10, (d) x¼ 0.15,

and (e) x¼ 0.20.

027005-5 Jaita, Watcharapasorn, and Jiansirisomboon J. Appl. Phys. 114, 027005 (2013)

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85.2BNT-12BKT-2.8BT ternary system (d33 � 191 pC/N)

by Nagata et al.9 As the crystal structure of BNKT-

0.10BNdT ceramic was nearly a coexistence of rhombohe-

dral and tetragonal phases, thus a flexibility increase in the

domain wall could effectively occur. Thus, it can be said that

the optimal piezoelectric properties would occur at this com-

position. Moreover, as we have referred, considering the

ionic radius, Ba2þ and Nd3þ could enter into A-site not

B-site. During sintering, Bi3þ in BNKT might leave the

ceramics and form some vacancies in the lattice because

Bi3þ was easily volatile at high temperature. It was thus pos-

sible for Ba2þ and Nd3þ to fill in Bi3þ vacancies. As Ba2þ

and Nd3þ have a radius of 1.42 A and 1.11 A, respectively,

which are different from 1.17 A of Bi3þ when Ba2þ and

Nd3þ filled into A-site, it caused the slack of BNKT.20 The

lattice deformation could enhance the motion of domains

which led to the improvement of piezoelectric properties.

Additionally, Ba2þ and Nd3þ could also occupy A-site of

Naþ (1.18 A) and Kþ (1.33 A). In this case, Ba2þ and Nd3þ

acted as a donor and led to some vacancies of A-site in the

lattice, which facilitated the movement of the domain and

thus significantly improved the piezoelectric perform-

ance.21,24 The d33 value was then dropped to the minimum

value of 119 pC/N at x ¼ 0.20. This was supported by phase

analysis using X-ray diffraction in Fig. 3(a) which indicated

a deviation of the composition from the mixed

rhombohedral-tetragonal phases of BNKT-BNdT system to

the tetragonal-rich phase of BNdT. The change in crystal

structure to be more tetragonal-rich phase may also contrib-

ute to the reduction in piezoelectric performances of BNKT-

BNdT ceramics. This result was similar to the reduction of

d33 observed in previous work on BNKT-BZT system.16

IV. CONCLUSION

Lead-free piezoelectric ceramics of (1-x)Bi0.5

(Na0.80K0.20)0.5TiO3-x(Ba0.98Nd0.02)TiO3 or (1-x)BNKT-

xBNdT system with x¼ 0–0.20 mol fraction were success-

fully synthesized by a conventional mixed-oxide method.

The effects of BNdT concentration on phase, microstructure,

and electrical properties were studied in detail. The optimum

sintering temperature of all ceramics was found to be

1125 �C at which all samples had the relative densities of at

least 98% of their theoretical values. A stable solid solution

was formed between BNKT and BNdT; a pure perovskite

phase was demonstrated in these ceramics. The small addi-

tion of BNdT (x¼ 0.05–0.10) into BNKT ceramic caused a

reduction of grain size. The addition of BNdT was also

found to improve both dielectric and piezoelectric properties

of BNKT ceramic. A large dielectric properties (er¼ 1716,

Tc¼ 327 �C) and piezoelectric coefficient (d33¼ 211 pC/N)

were obtained at the optimal composition of x¼ 0.10. As a

result, BNKT-0.10BNdT ceramic is a promising candidate

material among lead-free piezoelectric ceramics.

ACKNOWLEDGMENTS

This work was financially supported by the Thailand

Research Fund (TRF) and the National Research University

Project under Thailand’s Office of the Higher Education

Commission (OHEC). The Faculty of Science and the

Graduate School, Chiang Mai University is also acknowl-

edged. P. Jaita would like to acknowledge financial support

from the TRF through the Royal Golden Jubilee Ph.D.

Program.

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