The electric-field-induced strain values were increased from 410 pm/V to 688 pm/V for 6 mol% BLTO-added which results from the phase transition from rhombohedral to tetragonal structure..
Trang 1Lead-free ceramics of composition (0.97 ¹ x)Bi 0.5 Na0.4K0.1TiO3-0.03BiAlO3-xBi0.5Li0.5TiO3(BNKTBA-xBLTO) were synthesized using
solid state technique The strong enhancements in ferroelectric and electric- field-induced strain were obtained The electric-field-induced strain
values were increased from 410 pm/V to 688 pm/V for 6 mol% BLTO-added which results from the phase transition from rhombohedral to
tetragonal structure The maximum spontaneous polarization increased from 26.5 µC /cm 2 to 30.8 µC /cm 2 for 4 mol % BLTO solid solution in
BNKTBA and then decreased as BLTO was further added We expect that this work could be helpful for further understanding the original
enhancement in electrical field-induced strain in lead-free BNKT-based ceramics due to comparison between A- and B-site co-modifications.
[doi:10.2320 /matertrans.MA201553]
(Received January 30, 2015; Accepted July 1, 2015; Published August 25, 2015)
Keywords: ferroelectric, bismuth sodium potassium titanate, bismuth lithium titanate, bismuth aluminum oxide, lead-free
1 Introduction
Pb(Ti1¹xZrx)O3(PZT)-based piezoceramics currently
dom-inate the electronic industry however the search for an
appropriate lead-free replacement due to effects on
environ-ment and human health.1)Among various lead-free systems,
modified-Bi0.5(Na,K)0.5TiO3 (BNKT) ceramics seem to be a
candidate for real application in piezoelectric devices due to
giant electric field-induced strain.24) Recently, our review
work on the current development BNKT-based indicated that
the dynamic coefficient (Smax/Emax) could be compared with
soft PZT-based materials.5) In addition, it was reported that
dynamic coefficient in BNKT ceramics can be enhanced
when the most of B-site and/or A-site were modified Hussain
et al obtained the enhancement of Smax/Emaxwhen isovalent
ions Hf4+ and Zr4+ replaced Ti4+ at B-site.6,7) Do et al
reported that trivalent Y3+ and alionvalent Ta5+ modified
Ti4+were increased the Smax/Emaxvalues.8,9)Similarly, Dinh
et al reported the enhancement Smax/Emax due to substitute
Bi3+ by La3+ at A-site.10) Recently, Nguyen et al archived
strong enhancement of the electricfield induced strain due to
the co-substitution in both A-site (Li+ substituted Na+) and
B-site (Ta5+and Sn4+substituted Ti4+).11,12)The explanation
for the enhancement in Smax/Emax originates from phase
transition from polar to non-polar due to expansion tolerance
factor and/or promotion of oxygen vacancies.6 12)In fact, the
tolerance factors just only evaluate the perovskite and
non-perovskite structure and it could not show the relationship
between tolerance factors with structure symmetry.1315)
Therefore, the origin of phase transition was still unclear
In addition, the solid solutions of secondary ferroelectric
perovskite materials with lead-free BNKT-based were also
found to be enhanced Smax/Emax values In fact, the solid
solution of AABAO3 perovskite materials could considered as
co-dopants at both A- and B-site with similarly concentration because of diffuse element during sintering process Among them, BiAlO3seems to be a good candidate for solid-solution with BNKT.1620) The theoretical calculations predict that BiAlO3 has perovskite-like rhombohedral symmetry with a large spontaneous polarization of about 76 µC/cm2 and a Curie temperature (TC) around 800 K.16)Experimental results confined that BiAlO3 exhibited ferroelectric with TC over
793 K However, the poor thermal stability and extreme conditions requirement to synthesize BiAlO3result in limit its usability in technological application.17)Thank to well solid-solution with lead-free Bi0.5(Na,K)0.5TiO3-based ceramics, Ullah et al reported the highest value of Smax/Emax of 391 pm/V for 5 mol.% BiAlO3 solid solution in Bi0.5(Na0.8
-K0.2)0.5TiO3 which resulted from transition from the coex-istence of rhombohedral and tetragonal phase into pseudo-cubic phase.18)Interestingly, Ullah et al pointed out that the
Smax/Emax values increased up to 533 pm/V in 0.975[Bi0.5 -(Na0.78K0.22)0.5TiO3]0.025BiAlO3 which displayed on the tetragonal side of the mophotropic phase boundary compo-sition.19) Moreover, the normalized strain Smax/Emax value
of 592 pm/V at near the tetragonal-pseudocubic phase boundary was obtained in 0.970[Bi0.5(Na0.78K0.22)0.5TiO3 ]-0.030BiAlO3 solid solution.20) However, Fu et al reported that only distorted structures were obtained and there were not exhibited phase transition due to BiAlO3solid solution in
Bi0.5(Na0.82K0.18)0.5TiO3.21) In fact, co-modification at A-site and B-site further enhanced the Smax/Emax up to 579 pm/V obtained in lead-free 0.99Bi0.5(Na0.78K0.22)0.5TiO30.01(Bi0.5
-La0.5)AlO3 composition, resulting from distorted tetragonal structure.22) These results were important to point out that: i) the mechanism in enhancement of Smax/Emax values was unclear and ii) the modification of A-site was more sensitive
to Smax/Emaxvalues than that of B-site Recently, we reported that the A-site modified by Li+ in lead-free BNKT-based ceramics caused phase transition and resulted in enhancement
+Corresponding author, E-mail: dung.dangduc@hust.edu.vn
Trang 2the Smax/Emax values.11,12,23) In addition, we reported that
the distorted tetragonal and rhombohedral structure due to
Li+ ions modified Bi0.5(Na0.78K0.22)0.5Ti0.97Zr0.03O3 lead-free
piezoceramics which were possible to increase the Smax/Emax
from 600 pm/V to 643 pm/V due to Li+-added.24)
In this work, we reported the effect of Bi0.5Li0.5TiO3
(BLTO) solid solution in lead-free 0.97Bi0.5Na0.4K0.1TiO3
-0.03BiAlO3(BNKTBA) ceramics The electric-field-induced
strain (Smax/Emax) values increased from 410 pm/V to 688
pm/V for 6 mol% BLTO-added which results from phase
transition from rhombohedral to tetragonal structure The
maximum spontaneous (Pm) value increased from 26.5
µC/cm2 to 30.8 µC/cm2 for 4 mol% BLTO solid solution
in BNKTBA, and then it decreased as BLTO further added
2 Experimental Procedure
The (0.97¹ x)Bi0.5Na0.4K0.1TiO3-0.03BiAlO3-xBi0.5Li0.5
-TiO3 (x= 0.00, 0.02, 0.04, 0.06, 0.08, and 0.10) ceramics
were prepared by a conventional solid state reaction route
The raw materials were powders composed of Bi2O3, K2CO3,
TiO2, Li2CO3 (99.9%, Kojundo Chemical), Al2O3 (99.9%
High Purity Chemicals) and Na2CO3 (99.9%, Ceramic
Specialty Inorganics) The full details of samples fabrication
could be found elsewhere.23,24)The surface morphology was
observed with afield emission scanning electron microscope
(FE-SEM) The crystalline structures of the samples were
characterized by X-ray diffraction (XRD) in range 2ª from
10° to 70°, step 0.02° The polarization-electric fields (P-E)
and electric field-induced strain hysteresis loops were
measured in silicon oil using a modified SawyerTower
circuit and linear variable differential transducer system,
respectively
3 Results and Discussions
Figure 1(a) shows the XRD patterns of the
BNKTBA-xBLTO ceramics in the 2ª range of 10°70° From Fig 1(a),
it indicated that all samples exhibited single typical ABO3
perovskite diffraction peaks and no impurity phases can be
detected The absence of impurity phases indicated that the
Al3+and Li+ions successfully diffused into the BNKT lattice
to from a homogenous solid solution with the perovskite structure or the second phase cannot be detected because of the small amount BNKTBA specimen shows the coexistence between the rhombohedral and tetragonal symmetry as evidence of splitting diffraction patterns around 40° and 46°, respectively, as shown in Fig 1(b) However, the peaks reflection at 40° merged into a single peaks when BLTO content ricked than 6 mol%, indicating a transition from the morphotropic phase region, in which there coexist rhombo-hedral and tetragonal phase, to a tetragonal phase region In addition, the diffraction peaks shifted to high angle as the increasing BLTO content This can cause by the different size
of Li+ and Na+ when it diffused into the BNKT lattice to from a solid solution The ionic size of Li+ (0.98¡) were smaller than that of Na+ (1.18¡) The effect of Li+ ions dopant on phase transition was well reported in the lead-free ferroelectric materials Fu et al reported that the A-site Li+ driven orthorhombic-tetragonal ferroelectric phase transition
in lead-free (Na,K)(Nb,Sb)O3-LiTaO3.25) Nguyen et al obtained the polar phase transition to non-polar phase due
to Li-modified at A-site of BNKT-Ta.11) However, the Li+ ions promotion the tetragonal phase developed from pseudocubic phase in BNKT-Sn.12,23) In other word, the phase transition combination with distorted structure ob-tained via BLTO-modified BNKTBA ceramics
Figure 2 shows the FE-SEM micrographs of the surface for the BNKTBA-xBLTO ceramics with x= 0.00, 0.02, 0.04, 0.06, 0.08 and 0.10 A dense microstructure with some distinct holes is observed for the BNKTBA ceramics, as seen
in Fig 2(a) In addition, the grains grew into neat rectangular and cubic shapes which were in agreement with result of BiAlO3modified-BNKT by Ullah et al.18 20)The addition of BLTO lead to an obviously changed in shape and size of the grain, as seen in Fig 2(b)(d) The rectangular and cubic shapes changed to layer-to-layer-like structure and grain sizes increased as increasing the BLTO content This may be attributed to the low melting temperature of Li compounds that appears to promote the formation of liquid phase during sintering
Figure 3 shows the polarization versus electric field hysteresis loops of all samples at room temperature The nonzero remnant polarization (P) and coercive field (E )
in the 2 ª ranges of 38°42° and 44°48°.
Trang 3results indicated that all ceramic specimens have typical P-E
hysteresis loops of ferroelectric materials at room
temper-ature Interestingly, the maximum spontaneous (Pm) values
increased from 26.5 µC/cm2 to 30.8 µC/cm2 for 4 mol%
BLTO solid solution in BNKTBA, indicating that the Li+
ions strongly enhanced the ferroelectric properties of
BNKTBA ceramics However, the Pm values decreased to
22.3 µC/cm2 when BLTO amount further added up to
10 mol% In addition, the ECvalues decreased from 1.44 to
0.94 kV/mm The enhancement the ferroelectric properties
of Li-modified lead-free ceramics materials was also reported
in several solid solutions such as Bi0.5(Na0.82K0.18)0.5
-Ti0.95Sn0.05O3,23) 0.85Bi0.5Na0.5TiO3-0.10Bi0.5K0.5TiO3
-0.05BaTiO3,26) or NaNbO3.27) In addition, it is clear from
Fig 3 that the specimens with BLTO content added over
6 mol% ceramics which exhibited a slightly pinched-type P-E
loop The origin of the pinched-type character in the P-E loops of BNKT-modified ceramics can be considered as i) polar nanoregion in a non-polar matrix, ii) defect dipoles and iii) antiferroelectric.7,28,29)It is noted that the antiferroelectric phase has not been confirmed to exist in Bi0.5Na0.5TiO3and
Bi0.5Na0.5TiO3-based ceramics.7) The XRD study indicated that the polar phases were existed and there were no phase transition from polar to non-polar In fact, the Li+ ions can substituted at Ti4+ions site and give rise to the generation of oxygen vacancies to form the defect dipoles.23,24,30,31) Therefore, we suggested that the observations of pinched-type loops at high BLTO level dopants dominated from defect dipoles
Bipolar electric-field-induced strain of the ceramics is shown in Fig 4(a) All the ceramics showed butterfly-shaped curves that are distinct features of ferroelectric materials The
Fig 2 FE-SEM images of BNKTBA-xBLTO ceramics as function of BLTO doping level x 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 3 Room temperature P-E hysteresis loops of BNKTBA-xBLTO ceramics as function of BLTO doping level 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.
Trang 4BNKTBA exhibited a butterfly-shaped curve with a
max-imum strain (Smax) of 0.229% and negative strain (Sneg) of
0.047% Noted that the negative strain denoted the difference
between zerofield strain and the lowest strain.20)The
BLTO-added BNKTBA displayed increased Smax values up to
0.337% at 6 mol% then decreased to 0.273% by further
added BLTO content up to 10 mol% In addition, the Sneg
values gradually decreased to 0.004% which corresponded to
10 mol% BLTO-added BNKTBA Furthermore, the strain
enhancements by BLTO-added were observed in unipolar
S-E loops as shown in Fig 4(c) The unipolar strain
significantly increased with increasing BLTO concentration
until x= 6 mol% and then decreased beyond this critical
composition The field-induced strain Smax(%) and
normal-ized strain (Smax/Emax) of BNKTBA ceramics as a function
of BLTO content are depicted in Fig 4(d) The Smax and
Smax/Emaxvalues of BNKTAB were 0.205% and 410 pm/V,
respectively A large strain of 0.344% at an applied electric
field of 5 kV/mm corresponding to Smax/Emaxof 688 pm/V
However, the Smaxvalues decreased as increasing the BLTO
content and it went down to 0.295% with corresponding
to Smax/Emax values of 590 pm/V at BLTO-added up to
10 mol%, but the results were still comparable and/or higher
than that only BiAlO3 (³592 pm/V) and Bi0.5La0.5AlO3
(³579 pm/V) modified BNKT ceramics.18 22)
4 Conclusion
The lead-free ceramics of BNKTBA modified with BLTO
have been synthesized using solid state synthesis techniques
The highest Smax/Emax values of 688 pm/V obtained for
6 mol% BLTO which results from phase transition from
rhombohedral to tetragonal structure The P values
increas-ed from 26.5 µC/cm2 to 30.8 µC/cm2 for 4 mol% BLTO solid solution in BNKT-BA then decreased as BLTO was further added We expect that this work could be helpful for further understanding the original enhancement in electrical field-induced strain in lead-free BNKT-based due to compar-ison between A- and B-site co-modifications
Acknowledgments
The authors would like to thank Prof Ill Won Kim for providing the measurement facilities This research is funded
by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02-2012.62 Sunglae Cho acknowledges the support from Energy Efficiency & Resources program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Korean Ministry of Knowledge Economy (20132020000110)
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