Investigation of a new lead-free Bi0.5(Na0.40K0.10)TiO3-(Ba0.7Sr0.3)TiO3 piezoelectric ceramic
© Jaita et al; licensee Springer. 2012
Received: 6 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
Lead-free piezoelectric compositions of the (1-x)Bi0.5(Na0.40K0.10)TiO3-x(Ba0.7Sr0.3)TiO3 system (when x = 0, 0.05, 0.10, 0.15, and 0.20) were fabricated using a solid-state mixed oxide method and sintered between 1,050°C and 1,175°C for 2 h. The effect of (Ba0.7Sr0.3)TiO3 [BST] content on phase, microstructure, and electrical properties was investigated. The optimum sintering temperature was 1,125°C at which all compositions had densities of at least 98% of their theoretical values. X-ray diffraction patterns that showed tetragonality were increased with the increasing BST. Scanning electron micrographs showed a slight reduction of grain size when BST was added. The addition of BST was also found to improve the dielectric and piezoelectric properties of the BNKT ceramic. A large room-temperature dielectric constant, ε r (1,609), and piezoelectric coefficient, d33 (214 pC/N), were obtained at an optimal composition of x = 0.10.
Keywordsceramics X-ray diffraction dielectric properties microstructure piezoelectricity
Although Pb(Zr, Ti)O3 has played a dominant role in piezoelectric materials, waste of products containing Pb causes a crucial environmental problem. Thus, it is urgent to search for lead-free piezoelectric ceramics with excellent properties comparable to those found in lead-based ceramics.
Because it has a large remanent polarization [P r ] of approximately 38 μC/cm2 and a high Curie temperature [T c ] of approximately 320°C, (Bi0.5Na0.5)TiO3 [BNT] is a candidate material for a lead-free piezoelectric ceramic. However, poling difficulties due to its high coercive field [E c ] of approximately 73 kV/cm and high conductivity often require some modifications. It has been reported that BNT-based compositions modified with BaTiO3 , (Ba, Sr)TiO3 , and Ba(Zr, Ti)O3  showed improved piezoelectric properties. Another modification based on the work of Sasaki et al.  showed that Bi0.5(Na1-xK x )0.5TiO3 ceramic had a morphotropic phase boundary [MPB] between rhombohedral and tetragonal phases near x = 0.16 to 0.20, at which a relatively high d33 of 151 pC/N was obtained.
Aside from BNT, lead-free barium strontium titanate, (Ba1-xSr x )TiO3, as well as doped BaTiO3 are currently important dielectric materials for capacitor applications . The main purpose of adding Sr2+ into BaTiO3 is to shift the T c (approximately 130°C) towards room temperature, offering a high dielectric constant and a low dielectric loss, tanδ . At x = 0.3 composition, a relatively high permittivity was achieved. Recently, Lee et al.  have studied the (1-x)(Bi0.5Na0.5)TiO3-x(Ba0.7Sr0.3)TiO3 system. The addition of (Ba0.7Sr0.3)TiO3 into (Bi0.5Na0.5)TiO3 generated a phase transition from rhombohedral to tetragonal. The improvement of both dielectric and piezoelectric performances was found at an MPB of x = 0.08.
In order to develop a new material system with both high piezoelectric and dielectric performances, (1-x)Bi0.5(Na0.40K0.10)TiO3-x(Ba0.7Sr0.3)TiO3 [(1-x)BNKT-x BST] (x = 0 to 0.20) ceramics were prepared. The effect of the BST concentration on phase, microstructure, and electrical properties of the ceramics was investigated and discussed.
Conventional mixed-oxide technique was used to prepare Bi0.5(Na0.40K0.10)TiO3 and (Ba0.7Sr0.3)TiO3 powders. The starting materials were Bi2O3, Na2CO3, TiO2, K2CO3, BaCO3, and SrCO3. A stoichiometric amount of BNKT and BST powders was weighed, ball-milled for 24 h, and dried using the oven-drying method. BNKT and BST powders were separately calcined for 2 h at 900°C for BNKT and 1,100°C for BST. The calcined powders were then weighed, mixed, and oven-dried to produce the mixed powders of (1-x)BNKT-x BST (when x = 0, 0.05, 0.10, 0.15, and 0.20). After drying and sieving, a few drops of 3 wt.% PVA binders were added before being uniaxially pressed into pellets of 10 mm in diameter. These pellets were covered with their own powders and subsequently sintered at 1,050°C to 1,175°C for 2 h with a heating/cooling rate of 5°C/min.
Phase evolution was examined using an X-ray diffraction [XRD] diffractometer (X'Pert, PANalytical B.V., Almelo, The Netherlands). Bulk densities were determined using Archimedes' method. The theoretical densities of all samples were calculated based on the theoretical densities of BNKT (5.84 g/cm3)  and BST (5.75 g/cm3) . Surfaces of the ceramics were observed using a scanning electron microscope [SEM] (JSM-6335F, JEOL Ltd., Akishima, Tokyo, Japan). Grain size was determined by mean linear intercept method.
For electrical measurements, two parallel surfaces were polished and painted with silver paste for electrical contacts. Dielectric properties were determined at 25°C to 500°C with a frequency of 10 kHz using a 4284A-LCR meter (Agilent Technologies, Santa Clara, CA, USA) connected to a high-temperature furnace. A standard Sawyer-Tower circuit was used to measure the hysteresis loop. The samples were poled at 60°C in a stirred silicone oil bath by applying a DC electric field of 5 kV/mm for 15 min, and piezoelectric measurements were then carried out using a d33-meter (S5865, KCF Technologies, Inc., State College, PA, USA).
Results and discussion
Physical and electrical properties of (1-x)BNKT-x BST ceramics sintered at 1,125°C
Grain size (μm)
T c (°C)
ε r a
P r (μC/cm2)
E c (kV/cm)
5.81 ± 0.01
0.60 ± 0.09
5.80 ± 0.02
0.40 ± 0.04
5.77 ± 0.01
0.39 ± 0.04
5.76 ± 0.01
0.46 ± 0.07
5.72 ± 0.01
0.47 ± 0.06
Piezoelectric coefficients of (1-x)BNKT-x BST ceramics are listed in Table 1. The d33 of pure BNKT ceramic was 178 pC/N, which was close to the value of 165 pC/N observed earlier by Hiruma et al. . The highest d33 of 214 pC/N was observed for the BNKT-0.10BST ceramic. As the crystal structure of BNKT-0.10BST was nearly a coexistence of rhombohedral and tetragonal phases, a flexibility increase in the domain wall could effectively occur. Moreover, E c of this composition was lower than that of pure BNKT, whereas P r was maintained. Thus, it is obvious that the optimal piezoelectric properties would occur in this composition. The d33 decreased with the further increasing BST content of over 10 mol%. This was supported by phase analysis using XRD which indicated a deviation of the composition from the mixed rhombohedral and tetragonal phases of BNKT-BST system to mainly the tetragonal BST phase. In addition, the change in crystal structure to being more tetragonal may also contribute to the reduction in the piezoelectric performance of BNKT-BST ceramics similar to the reduction in d33 observed in the previous work on BNKT-BZT system .
New (1-x)BNKT-x BST ceramics were successfully fabricated. The optimum sintering temperature of all ceramics was 1,125°C. XRD indicated that the addition of BST into BNKT caused a change in crystal structure and increase in lattice parameters. The addition of BST also inhibited grain growth. The incorporation of 10 mol% BST was found to be an optimum condition that could enhance ε r and d33 to the maximum values of 1,609 and 214 pC/N, respectively. In addition, it also possessed a relatively low E c , while T c and P r were quite comparable to that of pure BNKT. Therefore, BNKT-0.10BST ceramic is a promising candidate as a new lead-free piezoelectric ceramic which can be further used in actuator applications.
This work is 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 acknowledged. PJ would like to acknowledge financial support from the TRF through the Royal Golden Jubilee Ph.D. Program.
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