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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: [email protected]
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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April 2015
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
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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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April 2015
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
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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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