Materials Transactions, Vol 56, No (2015) pp 1378 to 1382 Special Issue on Nanostructured Functional Materials and Their Applications © 2015 The Japan Institute of Metals and Materials Microstructural and Ferroelectric Properties of Bi0.5(Na,K)0.5TiO3-Based Modified by Bi0.5Li0.5TiO3 Lead-Free Piezoelectric Ceramics D D Dung1,+, D V Thiet1,2, N V Quyet3, L H Bac4 and S Cho2 Department of General Physics, School of Engineering Physics, Ha Noi University of Science and Technology, Dai Co Viet Road, Ha Noi, Viet Nam Department of Physic, University of Ulsan, Ulsan 680-749, Republic of Korea Hanautech Co., Ltd., 832, Tamnip-dong, Yuseong-gu, Daejeon, Republic of Korea Department of Optics and Optoelectronics, School of Engineering Physics, Ha Noi University of Science and Technology, Dai Co Viet Road, Ha Noi, Viet Nam Lead-free ceramics of composition (0.97 ¹ x)Bi0.5Na0.4K0.1TiO3-0.03BiAlO3-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 mol% BLTO-added which results from the phase transition from rhombohedral to tetragonal structure The maximum spontaneous polarization increased from 26.5 µC/cm2 to 30.8 µC/cm2 for 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 Introduction Pb(Ti1¹xZrx)O3 (PZT)-based piezoceramics currently dominate the electronic industry however the search for an appropriate lead-free replacement due to effects on environment 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/Emax when isovalent ions Hf 4+ 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/Emax values.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 electric field 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.612) In fact, the tolerance factors just only evaluate the perovskite and nonperovskite 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 + Corresponding author, E-mail: dung.dangduc@hust.edu.vn co-dopants at both A- and B-site with similarly concentration because of diffuse element during sintering process Among them, BiAlO3 seems 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 BiAlO3 result in limit its usability in technological application.17) Thank to well solidsolution 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 mol.% BiAlO3 solid solution in Bi0.5(Na0.8K0.2)0.5TiO3 which resulted from transition from the coexistence of rhombohedral and tetragonal phase into pseudocubic 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 composition.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 BiAlO3 solid 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(Bi0.5La0.5)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/Emax values 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 Microstructural and Ferroelectric Properties of Bi0.5(Na,K)0.5TiO3-Based Modified by Bi0.5Li0.5TiO3 Lead-Free Piezoelectric Ceramics 1379 Fig (a) X-ray diffraction pattern of BNKTBA-xBLTO ceramics as function of BLTO doping level x, and (b) the magnified XRD pattern in the 2ª ranges of 38°42° and 44°48° the 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 0.97Bi0.5Na0.4K0.1TiO30.03BiAlO3 (BNKTBA) ceramics The electric-field-induced strain (Smax/Emax) values increased from 410 pm/V to 688 pm/V for 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 mol% BLTO solid solution in BNKTBA, and then it decreased as BLTO further added Experimental Procedure The (0.97 ¹ x)Bi0.5Na0.4K0.1TiO3-0.03BiAlO3-xBi0.5Li0.5TiO3 (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 a field 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 Results and Discussions Figure 1(a) shows the XRD patterns of the BNKTBAxBLTO 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 mol%, indicating a transition from the morphotropic phase region, in which there coexist rhombohedral 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 obtained via BLTO-modified BNKTBA ceramics Figure 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 BiAlO3 modified-BNKT by Ullah et al.1820) 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 shows the polarization versus electric field hysteresis loops of all samples at room temperature The nonzero remnant polarization (Pr) and coercive field (EC) 1380 D D Dung, D V Thiet, N V Quyet, L H Bac and S Cho Fig 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 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 results indicated that all ceramic specimens have typical P-E hysteresis loops of ferroelectric materials at room temperature Interestingly, the maximum spontaneous (Pm) values increased from 26.5 µC/cm2 to 30.8 µC/cm2 for 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 EC values 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)0.5Ti0.95Sn0.05O3,23) 0.85Bi0.5Na0.5TiO3-0.10Bi0.5K0.5TiO30.05BaTiO3,26) or NaNbO3.27) In addition, it is clear from Fig that the specimens with BLTO content added over 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 Bi0.5Na0.5TiO3 and 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 pinchedtype 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 Microstructural and Ferroelectric Properties of Bi0.5(Na,K)0.5TiO3-Based Modified by Bi0.5Li0.5TiO3 Lead-Free Piezoelectric Ceramics 1381 Fig (a) Bipolar strain hysteresis loops and (b) the positive and negative strain values of BNKTBA-xBLTO ceramics as function of BLTO doping x level (c) Unipolar strain hysteresis loops and (d) the normalized strain and Smax/Emax values as a function of BLTO content BNKTBA exhibited a butterfly-shaped curve with a maximum strain (Smax) of 0.229% and negative strain (Sneg) of 0.047% Noted that the negative strain denoted the difference between zero field strain and the lowest strain.20) The BLTOadded BNKTBA displayed increased Smax values up to 0.337% at 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 = mol% and then decreased beyond this critical composition The field-induced strain Smax (%) and normalized strain (Smax/Emax) of BNKTBA ceramics as a function of BLTO content are depicted in Fig 4(d) The Smax and Smax/Emax values of BNKTAB were 0.205% and 410 pm/V, respectively A large strain of 0.344% at an applied electric field of kV/mm corresponding to Smax/Emax of 688 pm/V obtained in BLTO-added BNKTBA with x = mol% However, the Smax values 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.1822) 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 mol% BLTO which results from phase transition from rhombohedral to tetragonal structure 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hysteresis loops and (b) the positive and negative strain values of BNKTBA-xBLTO ceramics as function of BLTO doping x level (c) Unipolar strain hysteresis loops and (d) the normalized strain and Smax/Emax