Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010, pp. 1102∼1105
Phase Transition, Microstructures and Electromechanical Properties of BiAlO3 -modified Bi0.5 (Na,K)0.5 TiO3 Lead-free Piezoelectric Ceramics Aman Ullah, Sun Young Lee, Hai Joon Lee and Ill Won Kim∗ Department of Physics, University of Ulsan, Ulsan 680-749, South Korea
Chang Won Ahn and Hak-In Hwang Convergence Components R&D Division, Korea Electronics Technology Institute, Seongnam 463-816, South Korea
Ali Hussain and Jae Shin Lee School of Material Science and Engineering, University of Ulsan, Ulsan 680-749, South Korea (Received 2 February 2010, in final form 6 July 2010) Lead-free piezoelectric (1 - x)(Bi0.5 (Na0.75 K0.25 )0.5 TiO3 )-xBiAlO3 (BNKT25-BA, 0 ≤ x ≤ 0.10) ceramics were synthesized using a conventional solid state reaction method. The incorporation of BA into the BNKT25 lattice was investigated by using X-ray diffraction (XRD), ferroelectric characterizations and electric-field-induced strain behavior. X-ray diffraction revealed a pure perovskite phase for x ≤ 0.050. A phase transformation from tetragonal to pseudocubic was observed at x = 0.050. The polarization hysteresis loops of the BNTK25-BA ceramics indicate that the addition of BA significantly disrupts the ferroelectric order of BNKT25 ceramics. The destabilization of the ferroelectric order is accompanied by an enhancement of the unipolar strain. In particular, a large electric-field-induced strain (S = 0.29%) and a normalized strain (d∗33 = Smax /Emax = 484 pm/V) were observed at x = 0.025, near the tetragonal-pseudocubic phase boundary. These results suggest that the BNKT25-BA system is a promising candidate for lead-free electromechanical applications. PACS numbers: 77.65.Ly, 77.65.Bn, 77.65.-j Keywords: Ferroelectrics, Phase transition, Polarization, Electric-field-induced strain DOI: 10.3938/jkps.57.1102
I. INTRODUCTION
Lead-based piezoelectric ceramics such as lead zirconate titanate (PZT) are widely used in various electromechanical devices due to their excellent piezoelectric properties [1,2]. However, environmental issues have raised the need for nonhazardous materials with properties comparable to that of Pb-based materials. Thus, considerable effort has been devoted towards the development of lead-free piezoelectric ceramics [3,4]. Among the developed lead-free piezoelectric ceramics, a bismuth-sodium-titanate (Bi0.5 Na0.5 TiO3 (BNT)) and bismuth-potassium-titanate (Bi0.5 K0.5 TiO3 (BKT)) solid solution has attracted considerable attention on account of the existence of a rhombohedraltetragonal morphotropic phase boundary (MPB) in the range of 16-20 mol% BKT in the BNT-BKT system [5]. In the BNT-BKT binary system, the tetragonal side Bi0.5 (Na0.75 K0.25 )0.5 TiO3 (BNKT25) of the MPB com∗ E-mail:
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position is one of the superior candidate materials for lead-free actuator applications because of its high depolarization temperature (Td ) and excellent electromechanical properties [6]. BiAlO3 (BA) has recently received considerable attention due to its excellent ferroelectric properties [7]. Zylberberg et al. synthesized BA and confirmed that it was indeed, ferroelectric and had a Curie temperature Tc > 520 ◦ C [8]. Recently, Yu and Ye synthesized the (Bi0.5 Na0.5 )TiO3 -BiAlO3 (BNT-BA) ceramic system and reported excellent ferroelectric and piezoelectric properties comparable to those of pure BNT ceramics [9]. Despite the system having many interesting properties, however, no reports to date exist regarding (1 - x)(Bi0.5 (Na0.75 K0.25 )0.5 TiO3 )-xBiAlO3 ceramic systems. In this study, BiAlO3 -modified Bi0.5 (Na0.75 K0.25 )0.5 TiO3 , (1 - x)BNKT25-xBA, ceramics were investigated from the standpoint of phase transition, ferroelectric characterization and electric-fieldinduced strain behavior.
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Phase Transition, Microstructures and Electromechanical Properties · · · – Aman Ullah et al.
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Fig. 1. (Color online) X-ray diffraction (XRD) patterns of (1 - x)BNKT25-xBA ceramics in the 2θ range of 20 - 60◦ . (*) denotes the secondary phase.
Fig. 2. SEM micrographs of (1 - x)BNKT25-xBA ceramics sintered at 1150 ◦ C for 2 h (a) x = 0, (b) x = 0.025, (c) x = 0.035, and (d) x = 0.075.
II. EXPERIMENTAL PROCEDURE
secondary phase was identified as Bi2 Al4 O9 (PDF No. 74-1097). This is possibly due to the instability of the BA perovskite structure, which decomposes at high temperatures [8]. Belik et al. synthesized BA ceramics and observed similar impurity phases with peaks at 2θ = 26◦ - 31.2◦ [10]. Additionally, in agreement with the previously reported studies [6], the pure BNKT25 ceramic has tetragonal symmetry, as evidenced by the splitting of the (002)/(200) peaks at a 2θ of around 46◦ and a single (111) peak at around a 2θ of 40◦ . However, with increasing BA content, the tetragonal distortion gradually decreased, and the (002)/(200) split peaks of the tetragonal phase merged into a single (200) peak at x = 0.050, indicating that the crystal structure of the BNKT25-BA ceramics had transformed from a tetragonal to a pseudocubic symmetry. This structural transformation can be explained using the tolerance factor and the bond angles [11,12]. In the BNKT25-BA system, the tolerance factor of BA is larger than that of pure BNKT25. Therefore, the addition of BA into BNKT25 slightly increased the tolerance factor of the BNKT25-BA ceramic system. Moreover, the ionic radius of Al3+ (r = 53.5 pm) is smaller than that of Ti4+ (r = 60.5 pm). Therefore, the B-O-B bond angle is expected to be increased by the addition of BA [12]. Thus, the distortion of the crystal structure was decreased due to the ionic radius of the B-site Al3+ ion, resulting in a crystal structure transformation of BNKT25-BA lattice from a tetragonal to a pseudocubic phase. Figure 2 shows SEM micrographs of the BNKT25-BA ceramics with x = 0, 0.025, 0.035, and 0.075. Addition of BA had little influence on the average grain size of the BNKT25-BA ceramics although the average grain size decreased slightly with increasing BA content. Conversely, addition of BA into BNKT25 resulted in an obvious change in grain morphology. Specifically, the grains profile shifted towards neat and clear cubic, concurrently
(1 - x)BNKT25-xBA (0 ≤ x ≤ 0.10) ceramics were prepared by using a solid state reaction method from raw starting materials, namely, Bi2 O3 , TiO2 , Al2 O3 (99.9%, High Purity Chemicals), Na2 CO3 (99.9%, Cerac Specialty Inorganics), and K2 CO3 (≥99%, Sigma-Aldrich). All starting materials were weighed in stoichiometric amounts and ball-milled for 24 h in ethanol. The dried slurries were calcined at 800 ◦ C for 2 h and ball-milled again for 24 h. The powders were then pulverized, mixed with an aqueous polyvinyl alcohol (PVA) solution, and pressed into green disks under a pressure of 100 MPa. Sintering was carried out at 1150 ◦ C for 2 h. The crystal structures of the ceramics were characterized using an X-ray diffractometer (XRD, PRO MRD, Philips), and the microstructures were observed by using scanning electron microscopy (SEM, JSM-5610LV). Ferroelectric hysteresis loops were measured in silicon oil with the aid of a Sawyer-Tower circuit. Electric-fieldinduced strains were measured in silicon oil at 0.1 Hz by using a linear variable differential transducer (LVDT) system. The samples were poled in silicon oil by applying an electric field of 3 - 4 kV/mm for 30 min, and the piezoelectric constant d33 was measured using the Quasi-static method with a d33 meter.
III. RESULTS AND DISCUSSION Figure 1 shows the X-ray diffraction patterns of the (1 - x)BNKT25-xBA (0 ≤ x ≤ 0.100) ceramics in the 2θ range of 20 - 60◦ . The BNKT25-BA ceramics exhibit a pure perovskite structure at x ≤ 0.050. A secondary phase appeared at x = 0.075 around 2θ = 24◦ - 31◦ and is marked with a star in the XRD pattern. This
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Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010
Fig. 3. (Color online) P -E hysteresis loops of (1 x)BNKT25-xBA (x = 0, 0.015, 0.025, 0.035, and 0.075) ceramics.
with rectangular, shapes for BA contents up to x = 0.035. Upon further increases in the BA concentration, the grain profiles changed completely into rectangular shapes that incorporated a small number of pores. Figure 3 shows the room-temperature polarization hysteresis loops of the BNKT25-BA ceramics. The polarization hysteresis loop of pure BNKT25 exhibited typical ferroelectric behavior, having a large remnant polarization and a maximum polarization of 22 µC/cm2 and 39 µC/cm2 , respectively, and a coercive field of 20 kV/cm. The profiles of the P − E hysteresis loops are consistent with the XRD analysis. As the BA content was increased, the tetragonal distortion gradually decreases, resulting in a slightly pinched-type hysteresis loop, a significant decrease in Pr and Ec , and a small decrease in Pm values. However, at a higher BA content of x = 0.075, when the sample had pseudocubic symmetry, both Pr and Ec were drastically lower, and the hysteresis curves slimmed, nearly exhibiting almost the behavior of linear dielectric materials. The significant decreases in Pr and Ec and the small decrease in Pm , together with the slightly pinched-type character of the P − E hysteresis loops demonstrated that the long-range ferroelectric order that was dominant in pure BNKT25 was disrupted. However, the presence of traces of ferroelectric order at higher BA contents at zero electric field was also evident because the remnant polarization (Pr = 4 µC/cm2 at x = 0.075) was not negligible. The destabilization of the ferroelectric order, along with the noticeable change in the loop shape, with increasing BA content indicates that BA induced a phase transition in the BNKT25 lattice from a ferroelectric (polar) to a non-polar phase with an intermediate stage (x = 0.015-0.035) that exhibits both ferroelectric and non-polar characteristics [13,14]. The free energy of the ferroelectric phase at x = 0.025 seems to be so competitive with that of the non-polar phase at zero fields that it can be easily induced by an external electric field and is saturated at 60 kV/cm [14]. Furthermore, the non-polar phase dominates at higher BA
Fig. 4. Piezoelectric constant d33 as a function of x in (1-x)BNKT25-xBA ceramics.
Fig. 5. (Color online) Unipolar S-E loops of (1-x) BNKT25-xBA (x = 0, 0.015, 0.025, 0.035, and 0.075) ceramics.
concentrations and apparently delays the transformation from the non-polar to the ferroelectric phase, which is evident from the significant decrease in the maximum polarization from 39 to 18 µC/cm2 at x = 0.075. Figure 4 shows the piezoelectric constant d33 of the BNKT25-BA ceramics as a function of BA content. The d33 value decreased significantly with increasing BA content. This decreasing trend in d33 is not surprising because according to the thermodynamic theory of ferroelectrics [15], d33 can be expressed as d33 = 2Q11 Pr T33 , where Q11 represents the electrostrictive coefficient, which is constant for perovskite materials, and Pr and T33 represent the remnant polarization and dielectric constant of the material. Since d33 is proportional to the remnant polarization (Pr ), at x = 0.025, the Pr value is drastically decreased from 22 µC/cm2 for pure BNKT25 to 10 µC/cm2 , which results in a significant reduction of d33 from 162 pC/N for pure BNKT25 to 32 pC/N for x = 0.025. At higher BA content (x = 0.075), the P r is so small (Pr = 4 µC/cm2 ) that the piezoelectric constant d33 is very close to zero. Figure 5 shows the unipolar field-induced strain curves
Phase Transition, Microstructures and Electromechanical Properties · · · – Aman Ullah et al.
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for x ≤ 0.050. XRD revealed a phase transition from the tetragonal to the pseudocubic phase at x = 0.050. The P − E hysteresis loops demonstrated that the BNKT25BA ceramics gradually changed from a ferroelectric to a non-polar phase, passing through an intermediate stage that had both ferroelectric and non-polar characteristics. Finally, the composition (x = 0.025) close to the boundary between the ferroelectric and the non-polar phases exhibited a very large strain of 0.29% and a normalized strain d∗33 of 484 pm/V at an electric field of 60 kV/cm, which are ascribed to the coexistence of ferroelectric (polar) and non-polar phases induced by the chemical modification. Fig. 6. Strain (%) and normalized strain (d∗33 ) as functions of x in (1 - x)BNKT25-xBA ceramics.
ACKNOWLEDGMENTS of the BNKT25-BA ceramics measured at room temperature. The strain increased significantly with increasing BA content up to x = 0.025, but decreased thereafter. A large strain of 0.29% was obtained for the BNKT25BA ceramics at x = 0.025. The field-induced strain S (%) and the normalized strain d∗33 of the BNKT25-BA ceramics as functions of BA content are depicted in Fig. 6. A large d∗33 of 484 pm/V was obtained for x = 0.025 at an applied electric field of 60 kV/cm. BA is indeed ferroelectric [8], and pure BNKT25 exhibits a typical ferroelectric order; however, the addition of BA appears to disrupt the long-range ferroelectric order of BNKT25 ceramics, leading to the development of a non-polar phase that passes through an intermediate stage, where the observed remnant polarization in the P - E hysteresis loops may indicate the presence of mixed ferroelectric and non-polar phases. The non-polarity becomes more pronounced with increasing concentrations of BA (evidenced by the significant decrease in maximum polarization). Therefore, the results of this study suggest that a large unipolar strain, located only in a narrow region (i.e., around x = 0.025) in which both ferroelectric and non-polar phases coexist in the BNKT25-BA ceramic system, exhibits a competitive free energy. Beyond this narrow region, either the ferroelectric or the non-polar phase dominates. Both of these phases in a single form would be unable to deliver a strain as large as that measured from the composition (x = 0.015 - 0.035). Therefore, on the basis of structure and P − E hysteresis loops, this large strain at x = 0.025 can be attributed to the coexistence of ferroelectric and non-polar phases.
IV. CONCLUSIONS Lead-free (1 - x)Bi0.5 (Na0.75 K0.25 )0.5 TiO3 -xBiAlO3 (BNKT25-BA, x = 0 - 0.100) piezoelectric ceramics were successfully synthesized using a conventional solid state reaction method. A single perovskite phase was formed
This work was financially supported by the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. The authors also acknowledge the Ministry of Education, Science Technology (MEST) and the Korea Industrial Technology Foundation (KOTEF) through the Human Resource Training Project for Regional Innovation.
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