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A role of BNLT compound addition on structure and properties of PZT ceramics
Solid State Sciences 12 (2010) 1608e1614
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A role of BNLT compound addition on structure and properties of PZT ceramics P. Jaita, A. Watcharapasorn, S. Jiansirisomboon* Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 May 2010 Received in revised form 3 July 2010 Accepted 6 July 2010 Available online 14 July 2010
In this research, effects of lead-free bismuth sodium lanthanum titanate (BNLT) addition on structure and properties of lead zirconate titanate (PZT) ceramics were investigated. PZT ceramics with addition of 0.1 e3.0 wt%BNLT were fabricated by a solid-state mixed oxide method and sintering at 1050e1200 C for 2 h to obtain dense ceramics with at least 96% of theoretical density. X-ray diffraction indicated that complete solid solution occurred for all compositions. Phase identification showed both tetragonal and rhombohedral perovskite structure of PZT with no BNLT phase detected. Scanning electron micrographs of fractured PZT/BNLT ceramics showed equiaxed grain shape with both transgranular and intergranular fracture modes. Addition of BNLT was also found to reduce densification and effectively limited grain growth of PZT ceramic. Optimum Hv and KIC values were found to be 4.85 GPa and 1.56 MPa.m1/2 for PZT/ 0.5 wt%BNLT sample. Among PZT/BNLT samples, room temperature dielectric constant seemed to be improved with increasing BNLT content. The maximum piezoelectric coefficient values were observed in pure PZT ceramic and were slightly decreased in BNLT-added samples. Small reduction of remanent polarization and coercive field in hysteresis loops was observed in BNLT-added samples, indicating a slightly suppressed ferroelectric interaction in this material system. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: PZT BNLT Mechanical properties Dielectric Piezoelectric Ferroelectric
1. Introduction Lead zirconate titanate (PZT) is a solid solution of ferroelectric PbTiO3 (Tc ¼ 490 C) and antiferroelectric PbZrO3 (Tc ¼ 230 C) at different Zr/Ti ratios. PZT is one of the most widely used piezoelectric materials having perovskite structure ABO3 [1]. Extraordinary high piezoelectric activities are found at the morphotopic phase boundary (MPB) composition corresponding to Pb (Zr0.52Ti0.48)O3 [2,3]. This compound also possesses high spontaneous polarization, high Curie temperature (Tc ¼ 390 C), good thermal stability during operation, high electromechanical coupling coefficient and easy poling, etc. [4]. The effects of doping on various physical and chemical properties of this material are well known and some have already been exploited as highperformance piezoelectrics and ferroelectrics [5]. Bismuth sodium titanate (BNT) is one of the excellent lead-free piezoelectric materials which was discovered by Smolenskii et al. [6]. It has interesting electrical properties such as good dielectric constant (er) and acceptable piezoelectric coefficient (d33). Its crystal structure was found to be rhombohedral at room temperature. Pure BNT ceramic normally has Curie temperature of about 320 C, remanent polarization (Pr) of 38 mC/cm2 and coercive field
* Corresponding author. Tel.: þ66 53 941921x631; fax: þ66 53 943445. E-mail address: [email protected] (S. Jiansirisomboon). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.07.008
(Ec) of 73 kV/cm [4,7,8]. However, Bi ion is highly volatile at high temperature above 1130 C during sintering, making this material difficult to pole due to its high conductivity which eventually results in rather low piezoelectric properties [9]. To solve this problem, several solid solutions of BNT with PbTiO3 [10], BaTiO3 [11] and CaTiO3 [12] have been studied. Another way to improve the properties of BNT is by addition of a modifier using rare-earth element such as La. The addition of La to replace Bi and Na sites was found to induce lattice distortion and cause changes in microstructure. A small amount of La (1.72 at%) added in BNT also led to an improvement in dielectric and piezoelectric properties [13]. With the growing interest in developing new materials for device applications, a large number of ferroelectric oxide ceramics have been studied covering a wide range of composition. The study of PZT-based solid solutions with BNT was firstly reported by Kitagawa et al., whose results showed that mechanical strength and piezoelectric properties of BNT-added PZT composite ceramics were improved over those of pure PZT ceramic [14]. Thus, it would be interesting to combine two ideas, i.e. (1) addition of La3þ in BNT as suggested by Herabut and Safari [13] and (2) formation of a solid solution as suggested by Kitagawa et al. [14] in order to produce new ferroelectric ceramics based on PZT/BNLT system. In this research, PZT/BNLT ceramic system was fabricated by a conventional solid-state mixed oxide and sintering methods. The powder mixture of PZT/xBNLT with x ¼ 0, 0.1, 0.5, 1.0 and 3.0 wt% were in-house prepared, pressed into pellet and sintered at various
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Fig. 1. X-ray diffraction patterns of PZT/BNLT calcined powders (a) 2q ¼ 10e80 and (b) 2q ¼ 42e47.
temperatures to form high density ceramics. A role of added BNLT concentration on density, phase evolution, microstructure, mechanical, dielectric, piezoelectric and ferroelectric properties of PZT ceramics were investigated and discussed. 2. Experimental PZT (PbZr0.52Ti0.48O3) and BNLT (Bi0.487Na0.487La0.017TiO3) powders were separately prepared by a conventional mixed oxide method. The starting materials used in this study were PbO (99%, Fluka), ZrO2 (99%, Riedel-de Haën), TiO2 (99%, Riedel-de Haën), Bi2O3 (98%, Fluka), La2O3 (99%, Cerac) and Na2CO3 (99.5%, Carlo Erba). Stoichiometric amount of starting powders were weighed and ball-milled using zirconia milling media for 24 h in distilled water as dispersion medium and dried using a freeze-drying method. Dried PZT powder was calcined in a closed Al2O3 crucible at a temperature of 800 C for 2 h. For BNLT powder was also calcined at the same temperature of 800 C for 2 h. The PZT powders were weighed and mixed with 0, 0.1, 0.5, 1.0 and 3.0 wt% of BNLT powder before being ball-milled again for 24 h in distilled water and dried using the same procedure mentioned earlier. After drying and sieving, a few drops of 5 wt% PVA (polyvinyl alcohol) was added to the mixed powders as a binder before being pressed into pellet with a diameter of 10 mm using a uniaxial press. Binder removal was carried out by heating the pellets at 500 C for 1 h. These pellets were subsequently sintered at 1050, 1100, 1150 and 1200 C for 2 h dwell time with a heating/cooling rate of 5 C/min in PbO-rich atmosphere using PbZrO3 powder. Phase identification of powders and ceramics were investigated in 2q range of 10e80 by an X-ray diffractometer (XRD, Phillip Model X-pert). Bragg’s law, 2dsinq ¼ l, was used to calculate the interplanar spacing (d) of planes (200)Te(002)T of tetragonal (T) PZT phase; q was the Bragg angle and l was the wavelength of CuKa radiation (w1.542 Å). The values of d and Miller indices (hkl) of (200)Te(002)T tetragonal PZT peaks were then substituted into equation (1) for determination of the lattice parameter a and c and hence the tetragonality (c/a).
1 ¼ d2
h2 þ k2 a2
Bulk densities of the ceramics were determined using Archimedes’ method with ASTM standard C 373-88. The theoretical densities of all ceramic samples were calculated based on theoretical densities of BNLT (5.960 g/cm3) [13] and PZT (8.006 g/cm3) [15]. Observation of fractured surfaces of the ceramics was carried out using scanning electron microscopy (SEM, JEOL JSM-6335F). Grain size of each sample was measured by a mean linear intercept method from SEM micrographs. For the measurement of mechanical properties, the samples were polished to a mirror finish using 1 mm diamond paste. The well-polished ceramics were then subjected to microhardness tester (MXT-a) for Vickers hardness (Hv) determination. The indentation load used was 1000 N which was applied for 15 s holding time. Fracture toughness (KIC) was also determined from cracks’ length propagated from four corners of indentation impression following the method described by Anstis et al. [16].
þ
l2 c2
(1)
Fig. 2. Plots of relative density as a function of sintering temperature of PZT/BNLT ceramic with different of BNLT content.
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Table 1 Physical and mechanical properties of PZT/BNLT ceramics. BNLT content (wt%)
Density (g/cm3)
0 0.1 0.5 1.0 3.0
7.78 7.78 7.77 7.76 7.67
0.03 0.02 0.01 0.01 0.03
Grain size (mm)
Tetragonality; c/a Calcined powder
Ceramic
1.0291 1.0294 1.0309 1.0314 1.0309
1.0164 1.0108 1.0128 1.0128 1.0256
For electrical characterization, two parallel surfaces of the sintered ceramics were polished and painted with silver paste for electrical contacts. Room temperature dielectric constant (er) and dielectric loss (tand) were measured using LCZ-meter (HewlettePackard 4192A) at 1, 10, 100, 200, 500 and 1000 kHz frequencies. For the measurement of piezoelectric properties, all samples were poled at 120 C in a stirred silicone oil bath by applying a DC electric field 4 kV/mm for 10 min. The piezoelectric coefficient (d33) of the samples was measured using d33-meter (KCF technologies S5865) at a frequency of 50 Hz. Ferroelectric hysteresis loop of each sample was obtained using a computer controlled modified Sawyer-Tower circuit. The electric field was applied to a sample by a high voltage AC amplifier at 30 kV/cm with the input sinusoidal signal of 50 Hz from a function generator. The polarizationeelectric field (PeE) loop was then recorded by a digital oscilloscope. Remanent polarization (Pr), maximum polarization (Pmax), coercive field (Ec), maximum field (Emax) and loop squareness (Rsq) values were then determined from hysteresis loops. The squareness ratio of hysteresis loop was calculated using the ratio of Pr at zero electric field to saturated polarization (Ps) obtained at some finite field strength below dielectric breakdown, i.e. Pr/Ps. According to Haertling [3], the squareness can be used to measure not only the deviation in polarization axis but also that in electric field axis with an empirical equation: Rsq ¼ (Pr/Ps) þ (P1.1Ec/Pr). 3. Results and discussion X-ray diffraction patterns of PZT/xBNLT (x ¼ 0, 0.1, 0.5, 1.0 and 3.0 wt%) and pure BNLT calcined powders are shown in Fig. 1. Pure PZT powder had a composition near morphotopic phase boundary
3.18 1.79 1.69 1.74 1.96
0.34 0.24 0.23 0.14 0.20
Hv (GPa)
3.52 4.34 4.85 4.38 3.72
0.02 0.01 0.08 0.04 0.01
KIC (MPa.m1/2)
1.00 1.12 1.56 0.95 0.73
0.07 0.08 0.18 0.05 0.04
which consisted of tetragonal structure (JCPDS no. 33-0784) and rhombohedral structure (JCPDS no. 73-2022) while pure BNLT powder had rhombohedral structure, which was also in agreement with the previously reported by Herabut and Safari [13]. Addition of BNLT did not cause any significant change in X-ray diffraction patterns of the powders and both tetragonal and rhombohedral phases of PZT were maintained. Similar BNT phase disappeared for the samples containing 0.5 and 1.0 wt%BNT and only PZT perovskite phases were observed in PZT/BNT system [14]. Since no secondary phase was observed in these patterns, it appeared that BNLT was completely dissolved in the PZT matrix. A relationship between relative density and sintering temperature of ceramics with various added BNLT contents is shown in Fig. 2. The data clearly showed that both temperature and BNLT concentration had significant influence on density of sintered ceramics. The densities of all samples increased with increasing sintering temperature. All ceramics achieved their maximum density values at the sintering temperature of 1150 C except PZT/ 3.0 wt%BNLT ceramic whose highest density was observed when sintered at 1200 C. Although a slight decrease in density was observed at 1200 C for samples containing <3.0 wt%BNLT, the densities of all ceramics were rather similar i.e. within 7.67e7.78 g/cm3 (Table 1). The samples sintered at this temperature were therefore selected for further characterization. It should be noted that comparable the density values of PZT/BNLT ceramics in this work were slightly higher than PZT/BNT ceramics to those previously reported by Kitagawa et al. [14] at the same composition. X-ray diffraction patterns of PZT/BNLT ceramics sintered at 1200 C are shown in Fig. 3. Based on peak profile fitting within 2q
Fig. 3. X-ray diffraction patterns of PZT/BNLT ceramics (a) 2q ¼ 10e80 and (b) 2q ¼ 42e47.
P. Jaita et al. / Solid State Sciences 12 (2010) 1608e1614
range of 42e47, pure PZT had a phase which consisted of tetragonal structure (JCPDS no. 33-0784) and rhombohedral structure (JCPDS no. 73-2022). With increasing content of added BNLT, there seemed to be a merging of (020)R reflection of rhombohedral (R) phase and (200)Te(002)T peaks of tetragonal (T) phase in XRD patterns. However, comparing the peak intensity ratio of (020)R and (200)T reflections of both powders and ceramics, it was more likely that PZT/BNLT ceramics contained mainly tetragonal phase with an apparent reduction in tetragonality (i.e. c/a ratio). From the observed result, the slightly distortion of XRD patterns seemed to be attributed to the differences in ionic radii of Bi3þ [1.17 Å], Naþ1 [1.18 Å], La3þ [1.16 Å] and Pb2þ [1.29 Å] as well as the role of cation vacancies created by charge compensation mechanism. For PZT/ 3.0 wt%BNLT ceramic, the phase also contained mainly tetragonal structure but the change in tetragonality when compared to that of its calcined powder was less than other PZT/BNLT samples, the presence of rhombohedral structure could therefore be observed.
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The exact reasons of this discrepancy for PZT/3.0 wt%BNLT sample were not clearly known but compositional homogeneity in the sample could be the main factor causing this observed result. Table 1 lists the values of lattice parameters and tetragonality ratio (c/a) previously discussed. SEM micrographs of fractured PZT/BNLT ceramics sintered at 1200 C are shown in Fig. 4. Grain sizes of all ceramics were measured and the values are given in Table 1. It was obvious that BNLT addition into PZT led to change in average grain size but did not significantly change the density as shown in Fig. 5. All compositions of PZT/BNLT ceramics possessed equiaxed grains. Microstructure of pure PZT revealed large grains size of w3.2 mm with relatively wide grain size distribution. The grain size of PZT/ BNLT ceramics was found to decrease almost by half with addition of only 0.1 wt%BNLT and remained rather constant with further increasing BNLT concentration. The reduction in grain size was mainly attributed to grain growth inhibition as a result of solute
Fig. 4. SEM micrographs of PZT/xBNLT ceramics (a) x ¼ 0, (b) x ¼ 0.1, (c) x ¼ 0.5, (d) x ¼ 1.0 and (e) x ¼ 3.0 wt%.
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drag effect. Since solute diffusion near grain boundary region is usually slower than intrinsic diffusion of host atoms across the boundary plane and becomes rate-limiting for grain boundary movement. This seemed to be the main mechanism governing the observed microstructure similar to those reported in a number of previous work on solid solutions and doped systems, for examples, Nb-added BaTiO3 [17] and MgO-doped Al2O3 [17]. Mechanical properties of the ceramics in terms of Vickers hardness (Hv) and fracture toughness (KIC) were investigated. Their dependence on material compositions are shown in Fig. 6 and the corresponding values are also listed in Table 1. It could be seen that while pure PZT ceramic had Hv of 3.52 GPa, addition of small amount of BNLT caused a slight increase in Hv up to 4.85 GPa in PZT/ 0.5 wt%BNLT composition. These values seemed to higher than those observed earlier in PZT/0.5 wt%BNLT composition by Kitagawa et al. [14] who obtained Hv value of about 4.1 GPa. According to HallePetch relation [18], grain boundaries are known as stress concentration sites, which act as effective obstacles to dislocation pile-up in the adjacent grains and hence leading to harder material. However, among doped samples, grain sizes were nearly the same and therefore other factors might play a role which caused a slight drop in Hv value. From KIC study, it was found that its dependence on composition followed that of Hv. An increase in KIC in samples
containing small BNLT concentration was most likely due to the effect of reduced grain size. Based on the multi-domain grain model, ceramics with smaller grain size could possess single domain state and residual stress generated during sintering could not be released, leading to improvement in apparent KIC as was also observed in PLZT (9/65/53) ceramic system [19]. Further increasing BNLT content >0.5 wt% was found to decrease KIC of ceramics. In this study, it seemed that drop in density value of PZT/3.0 wt%BNLT and grain boundary strengthening played an important role. From SEM micrographs of the fractured surfaces (Fig. 4), PZT exhibited mainly intergranular fracture mode which indicated its relatively weak grain boundaries. Addition of 0.1 wt%BNLT caused fracture behavior to switch from intergranular to transgranular mode as seen in Fig. 4(b), suggesting an increase in grain boundary strength. A further increase in BNLT content over 1.0 wt% gradually changed the fracture mode from transgranular to that of intergranular but with apparently smaller grain size. Hence, it seemed that the maximum hardness and toughness values obtained in PZT/0.5 wt% BNLT ceramic in this study could be correlated to the combination of smaller grain size and optimum strength of grains and grain boundaries. Dielectric constant and dielectric loss, at room temperature, for PZT/BNLT ceramics sintered at 1200 C are shown in Fig. 7 and the corresponding values are also given in Table 2. The dielectric constant was found to slightly decrease with small addition of BNLT but the value increased with further increasing BNLT content. The maximum dielectric constant (er ¼ 1099) and dielectric loss (tand ¼ 0.029) at 1 kHz were observed for PZT/3.0 wt%BNLT. Since BNLT is a donor-doped material [13], this A-site substitution of PZT resulted in a creation of cation vacancies and reduction in the number of oxygen vacancies in PZT system. It has been known that oxygen vacancies are the main cause of domain wall clamping which leads to the lowering of dielectric constant [20]. Thus, it seemed that addition of high BNLT content caused a reduction of oxygen vacancies and led to enhance dielectric constant compared to that of pure PZT sample which contained more oxygen vacancies. From this study, addition of BNLT into PZT ceramic could thus significantly improve dielectric constant. The piezoelectric coefficient (d33) values of PZT/BNLT ceramics are listed in Table 2. The values were found to be in the range of 214e312 pC/N. The highest d33 value of 312 pC/N was observed in pure PZT ceramic, which was in agreement with that observed earlier by Barzegar et al. [21] who obtained d33 value of about 328 pC/N. For PZT/BNLT ceramics, the d33 values were generally
Fig. 6. Plots of Vickers hardness and fracture toughness as a function of BNLT content of PZT/BNLT ceramics.
Fig. 7. Plots of room temperature dielectric constant and dielectric loss values of PZT/ BNLT ceramics as a function of BNLT content.
Fig. 5. Plots of relative density and grain size as a function of BNLT content of PZT/BNLT ceramics.
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Table 2 Electrical properties of PZT/BNLT ceramics. BNLT content (wt%) 0 0.1 0.5 1.0 3.0
Electrical properties
er*
tand*
Pr (mC/cm2)
Ec (kV/cm)
Pr/Pmax
Ec/Emax
Rsq
d33 (1012 C/N)
877 818 889 964 1099
0.0263 0.0173 0.0141 0.0181 0.0290
10.46 8.07 7.90 7.61 7.10
12.00 8.24 8.23 7.88 9.14
0.55 0.44 0.43 0.44 0.45
0.41 0.28 0.29 0.27 0.31
0.84 0.57 0.55 0.56 0.57
312 217 251 279 214
* Dielectric data obtained at room temperature and at a frequency of 1 kHz.
lower than that of pure PZT with no obvious dependence on BNLT concentration for the range investigated. Since BNLT was more difficult to be poled than PZT, this decrease in d33 would be expected. In addition, highest piezoelectric coefficient was observed at MPB composition at which both rhombohedral and tetragonal phases were present [4], the lower d33 values in our samples seemed reasonable and were supported by phase analysis using X-ray diffraction which indicated the compositions slightly deviated from original MPB of pure PZT. Polarizationeelectric field (PeE) hysteresis loops of PZT/BNLT ceramics are shown in Fig. 8. Because of the temperature and field dependence of ferroelectric properties of the ceramics, these parameters have been normalized in form of Pr/Pmax and Ec/Emax [22]. These ratios were also listed in Table 2 along with the loop squareness (Rsq). The polarization loop of pure PZT ceramic showed maximum coercive field (Ec w 12 kV/cm) and remanent polarization (Pr w 10.5 mC/cm2). Addition of BNLT led to a decreasing trend in Ec with minimum value of 7.88 kV/cm in PZT/1.0 wt%BNLT sample. Although grain size reduction in PZT/BNLT samples could play a role in determining ferroelectric properties, it seemed that the effect of A-site cation substitution was more pronounced. In general, ceramic with smaller grain size had limited domain reorientation and the coercive field should increase. However, the observed ferroelectric hysteresis loops suggested donor-like doping with creation of cation vacancies in PZT/BNLT whose typical effect was to cause a decrease in coercive field due to easier ionic motion and hence improved polarizability and domain wall motion in this system [20]. The ferroelectric characteristic of the ceramics can be assessed with the hysteresis loop squareness which is roughly determined
from the ratio of Pr/Ps where Pr is the remanent polarization at zero electric field and Ps is the saturation polarization obtained at some finite field strength below the dielectric breakdown [22]. The loop squareness was often used to measure not only the deviation in polarization axis but also that in electric field axis with the empirical expression Rsq ¼ (Pr/Ps) þ (P1.1Ec/Pr), where P1.1Ec is the polarization at the field equal to 1.1Ec. For the ideal hysteresis loop, Rsq is equal to 2.00. As listed in Table 2, Rsq of pure PZT ceramic showed a value of 0.84, indicating typical ferroelectric properties of this material. Addition of BNLT into PZT ceramic dropped Rsq value to w0.55e0.57 regardless of BNLT content. This clearly showed that when BNLT was added to the PZT, the hysteresis curves became slimmer which was a characteristic of suppressed ferroelectric interaction. Similar reduction in Rsq was also observed in leadbased PMNePZT system [23]. Despite the fact that PZT/BNLT ceramics in this study showed promising mechanical and room temperature dielectric properties, further characterization of PZT/ BNLT on high temperature dielectric properties would be beneficial to confirm the possibility of using this material for transducer applications. 4. Conclusions In this study, PZT/xBNLT (x ¼ 0, 0.1, 0.5, 1.0 and 3.0 wt%) ceramics were successfully fabricated by a solid-state mixed oxide method. The optimum sintering temperature for this ceramic system was found to be 1200 C at which all samples had densities at least 96% of their theoretical values. X-ray diffraction analysis of the ceramics indicated that an addition of BNLT caused slight change in tetragonality. An addition of BNLT in PZT ceramics inhibited grain growth which resulted a reduction of grain size. This, thus, improved Hv of the ceramics and improved KIC in samples containing 0.5 wt%BNLT concentration. Room temperature dielectric constant was found to improve with an addition of >0.1 wt%BNLT. Addition of BNLT into PZT ceramics also affected their piezoelectric and ferroelectric properties. The maximum piezoelectric coefficient values were observed in pure PZT ceramic and were slightly decreased in BNLT-added samples. Slimmer hysteresis loops seen for modified PZT samples suggested a possible use of this material in transducer applications. From this viewpoint, a new PZT/BNLT system provided useful information for material development. It could be seen that suitable content of added BNLT into PZT ceramic could optimize mechanical, dielectric, piezoelectric and ferroelectric properties. Acknowledgements
Fig. 8. Plots of polarization as a function of electric field of PZT/BNLT ceramics with different of BNLT content.
This work is financially supported by the Thailand Research Fund and the National Research University, Chiang Mai University under the Office of the Higher Education Commission. The Faculty of Science and the Graduate School, Chiang Mai University is also acknowledged.
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A role of BNLT compound addition on structure and properties of PZT ceramics P. Jaita, A. Watcharapasorn, S. Jiansirisomboon* Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 May 2010 Received in revised form 3 July 2010 Accepted 6 July 2010 Available online 14 July 2010
In this research, effects of lead-free bismuth sodium lanthanum titanate (BNLT) addition on structure and properties of lead zirconate titanate (PZT) ceramics were investigated. PZT ceramics with addition of 0.1 e3.0 wt%BNLT were fabricated by a solid-state mixed oxide method and sintering at 1050e1200 C for 2 h to obtain dense ceramics with at least 96% of theoretical density. X-ray diffraction indicated that complete solid solution occurred for all compositions. Phase identification showed both tetragonal and rhombohedral perovskite structure of PZT with no BNLT phase detected. Scanning electron micrographs of fractured PZT/BNLT ceramics showed equiaxed grain shape with both transgranular and intergranular fracture modes. Addition of BNLT was also found to reduce densification and effectively limited grain growth of PZT ceramic. Optimum Hv and KIC values were found to be 4.85 GPa and 1.56 MPa.m1/2 for PZT/ 0.5 wt%BNLT sample. Among PZT/BNLT samples, room temperature dielectric constant seemed to be improved with increasing BNLT content. The maximum piezoelectric coefficient values were observed in pure PZT ceramic and were slightly decreased in BNLT-added samples. Small reduction of remanent polarization and coercive field in hysteresis loops was observed in BNLT-added samples, indicating a slightly suppressed ferroelectric interaction in this material system. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: PZT BNLT Mechanical properties Dielectric Piezoelectric Ferroelectric
1. Introduction Lead zirconate titanate (PZT) is a solid solution of ferroelectric PbTiO3 (Tc ¼ 490 C) and antiferroelectric PbZrO3 (Tc ¼ 230 C) at different Zr/Ti ratios. PZT is one of the most widely used piezoelectric materials having perovskite structure ABO3 [1]. Extraordinary high piezoelectric activities are found at the morphotopic phase boundary (MPB) composition corresponding to Pb (Zr0.52Ti0.48)O3 [2,3]. This compound also possesses high spontaneous polarization, high Curie temperature (Tc ¼ 390 C), good thermal stability during operation, high electromechanical coupling coefficient and easy poling, etc. [4]. The effects of doping on various physical and chemical properties of this material are well known and some have already been exploited as highperformance piezoelectrics and ferroelectrics [5]. Bismuth sodium titanate (BNT) is one of the excellent lead-free piezoelectric materials which was discovered by Smolenskii et al. [6]. It has interesting electrical properties such as good dielectric constant (er) and acceptable piezoelectric coefficient (d33). Its crystal structure was found to be rhombohedral at room temperature. Pure BNT ceramic normally has Curie temperature of about 320 C, remanent polarization (Pr) of 38 mC/cm2 and coercive field
* Corresponding author. Tel.: þ66 53 941921x631; fax: þ66 53 943445. E-mail address: [email protected] (S. Jiansirisomboon). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.07.008
(Ec) of 73 kV/cm [4,7,8]. However, Bi ion is highly volatile at high temperature above 1130 C during sintering, making this material difficult to pole due to its high conductivity which eventually results in rather low piezoelectric properties [9]. To solve this problem, several solid solutions of BNT with PbTiO3 [10], BaTiO3 [11] and CaTiO3 [12] have been studied. Another way to improve the properties of BNT is by addition of a modifier using rare-earth element such as La. The addition of La to replace Bi and Na sites was found to induce lattice distortion and cause changes in microstructure. A small amount of La (1.72 at%) added in BNT also led to an improvement in dielectric and piezoelectric properties [13]. With the growing interest in developing new materials for device applications, a large number of ferroelectric oxide ceramics have been studied covering a wide range of composition. The study of PZT-based solid solutions with BNT was firstly reported by Kitagawa et al., whose results showed that mechanical strength and piezoelectric properties of BNT-added PZT composite ceramics were improved over those of pure PZT ceramic [14]. Thus, it would be interesting to combine two ideas, i.e. (1) addition of La3þ in BNT as suggested by Herabut and Safari [13] and (2) formation of a solid solution as suggested by Kitagawa et al. [14] in order to produce new ferroelectric ceramics based on PZT/BNLT system. In this research, PZT/BNLT ceramic system was fabricated by a conventional solid-state mixed oxide and sintering methods. The powder mixture of PZT/xBNLT with x ¼ 0, 0.1, 0.5, 1.0 and 3.0 wt% were in-house prepared, pressed into pellet and sintered at various
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Fig. 1. X-ray diffraction patterns of PZT/BNLT calcined powders (a) 2q ¼ 10e80 and (b) 2q ¼ 42e47.
temperatures to form high density ceramics. A role of added BNLT concentration on density, phase evolution, microstructure, mechanical, dielectric, piezoelectric and ferroelectric properties of PZT ceramics were investigated and discussed. 2. Experimental PZT (PbZr0.52Ti0.48O3) and BNLT (Bi0.487Na0.487La0.017TiO3) powders were separately prepared by a conventional mixed oxide method. The starting materials used in this study were PbO (99%, Fluka), ZrO2 (99%, Riedel-de Haën), TiO2 (99%, Riedel-de Haën), Bi2O3 (98%, Fluka), La2O3 (99%, Cerac) and Na2CO3 (99.5%, Carlo Erba). Stoichiometric amount of starting powders were weighed and ball-milled using zirconia milling media for 24 h in distilled water as dispersion medium and dried using a freeze-drying method. Dried PZT powder was calcined in a closed Al2O3 crucible at a temperature of 800 C for 2 h. For BNLT powder was also calcined at the same temperature of 800 C for 2 h. The PZT powders were weighed and mixed with 0, 0.1, 0.5, 1.0 and 3.0 wt% of BNLT powder before being ball-milled again for 24 h in distilled water and dried using the same procedure mentioned earlier. After drying and sieving, a few drops of 5 wt% PVA (polyvinyl alcohol) was added to the mixed powders as a binder before being pressed into pellet with a diameter of 10 mm using a uniaxial press. Binder removal was carried out by heating the pellets at 500 C for 1 h. These pellets were subsequently sintered at 1050, 1100, 1150 and 1200 C for 2 h dwell time with a heating/cooling rate of 5 C/min in PbO-rich atmosphere using PbZrO3 powder. Phase identification of powders and ceramics were investigated in 2q range of 10e80 by an X-ray diffractometer (XRD, Phillip Model X-pert). Bragg’s law, 2dsinq ¼ l, was used to calculate the interplanar spacing (d) of planes (200)Te(002)T of tetragonal (T) PZT phase; q was the Bragg angle and l was the wavelength of CuKa radiation (w1.542 Å). The values of d and Miller indices (hkl) of (200)Te(002)T tetragonal PZT peaks were then substituted into equation (1) for determination of the lattice parameter a and c and hence the tetragonality (c/a).
1 ¼ d2
h2 þ k2 a2
Bulk densities of the ceramics were determined using Archimedes’ method with ASTM standard C 373-88. The theoretical densities of all ceramic samples were calculated based on theoretical densities of BNLT (5.960 g/cm3) [13] and PZT (8.006 g/cm3) [15]. Observation of fractured surfaces of the ceramics was carried out using scanning electron microscopy (SEM, JEOL JSM-6335F). Grain size of each sample was measured by a mean linear intercept method from SEM micrographs. For the measurement of mechanical properties, the samples were polished to a mirror finish using 1 mm diamond paste. The well-polished ceramics were then subjected to microhardness tester (MXT-a) for Vickers hardness (Hv) determination. The indentation load used was 1000 N which was applied for 15 s holding time. Fracture toughness (KIC) was also determined from cracks’ length propagated from four corners of indentation impression following the method described by Anstis et al. [16].
þ
l2 c2
(1)
Fig. 2. Plots of relative density as a function of sintering temperature of PZT/BNLT ceramic with different of BNLT content.
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Table 1 Physical and mechanical properties of PZT/BNLT ceramics. BNLT content (wt%)
Density (g/cm3)
0 0.1 0.5 1.0 3.0
7.78 7.78 7.77 7.76 7.67
0.03 0.02 0.01 0.01 0.03
Grain size (mm)
Tetragonality; c/a Calcined powder
Ceramic
1.0291 1.0294 1.0309 1.0314 1.0309
1.0164 1.0108 1.0128 1.0128 1.0256
For electrical characterization, two parallel surfaces of the sintered ceramics were polished and painted with silver paste for electrical contacts. Room temperature dielectric constant (er) and dielectric loss (tand) were measured using LCZ-meter (HewlettePackard 4192A) at 1, 10, 100, 200, 500 and 1000 kHz frequencies. For the measurement of piezoelectric properties, all samples were poled at 120 C in a stirred silicone oil bath by applying a DC electric field 4 kV/mm for 10 min. The piezoelectric coefficient (d33) of the samples was measured using d33-meter (KCF technologies S5865) at a frequency of 50 Hz. Ferroelectric hysteresis loop of each sample was obtained using a computer controlled modified Sawyer-Tower circuit. The electric field was applied to a sample by a high voltage AC amplifier at 30 kV/cm with the input sinusoidal signal of 50 Hz from a function generator. The polarizationeelectric field (PeE) loop was then recorded by a digital oscilloscope. Remanent polarization (Pr), maximum polarization (Pmax), coercive field (Ec), maximum field (Emax) and loop squareness (Rsq) values were then determined from hysteresis loops. The squareness ratio of hysteresis loop was calculated using the ratio of Pr at zero electric field to saturated polarization (Ps) obtained at some finite field strength below dielectric breakdown, i.e. Pr/Ps. According to Haertling [3], the squareness can be used to measure not only the deviation in polarization axis but also that in electric field axis with an empirical equation: Rsq ¼ (Pr/Ps) þ (P1.1Ec/Pr). 3. Results and discussion X-ray diffraction patterns of PZT/xBNLT (x ¼ 0, 0.1, 0.5, 1.0 and 3.0 wt%) and pure BNLT calcined powders are shown in Fig. 1. Pure PZT powder had a composition near morphotopic phase boundary
3.18 1.79 1.69 1.74 1.96
0.34 0.24 0.23 0.14 0.20
Hv (GPa)
3.52 4.34 4.85 4.38 3.72
0.02 0.01 0.08 0.04 0.01
KIC (MPa.m1/2)
1.00 1.12 1.56 0.95 0.73
0.07 0.08 0.18 0.05 0.04
which consisted of tetragonal structure (JCPDS no. 33-0784) and rhombohedral structure (JCPDS no. 73-2022) while pure BNLT powder had rhombohedral structure, which was also in agreement with the previously reported by Herabut and Safari [13]. Addition of BNLT did not cause any significant change in X-ray diffraction patterns of the powders and both tetragonal and rhombohedral phases of PZT were maintained. Similar BNT phase disappeared for the samples containing 0.5 and 1.0 wt%BNT and only PZT perovskite phases were observed in PZT/BNT system [14]. Since no secondary phase was observed in these patterns, it appeared that BNLT was completely dissolved in the PZT matrix. A relationship between relative density and sintering temperature of ceramics with various added BNLT contents is shown in Fig. 2. The data clearly showed that both temperature and BNLT concentration had significant influence on density of sintered ceramics. The densities of all samples increased with increasing sintering temperature. All ceramics achieved their maximum density values at the sintering temperature of 1150 C except PZT/ 3.0 wt%BNLT ceramic whose highest density was observed when sintered at 1200 C. Although a slight decrease in density was observed at 1200 C for samples containing <3.0 wt%BNLT, the densities of all ceramics were rather similar i.e. within 7.67e7.78 g/cm3 (Table 1). The samples sintered at this temperature were therefore selected for further characterization. It should be noted that comparable the density values of PZT/BNLT ceramics in this work were slightly higher than PZT/BNT ceramics to those previously reported by Kitagawa et al. [14] at the same composition. X-ray diffraction patterns of PZT/BNLT ceramics sintered at 1200 C are shown in Fig. 3. Based on peak profile fitting within 2q
Fig. 3. X-ray diffraction patterns of PZT/BNLT ceramics (a) 2q ¼ 10e80 and (b) 2q ¼ 42e47.
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range of 42e47, pure PZT had a phase which consisted of tetragonal structure (JCPDS no. 33-0784) and rhombohedral structure (JCPDS no. 73-2022). With increasing content of added BNLT, there seemed to be a merging of (020)R reflection of rhombohedral (R) phase and (200)Te(002)T peaks of tetragonal (T) phase in XRD patterns. However, comparing the peak intensity ratio of (020)R and (200)T reflections of both powders and ceramics, it was more likely that PZT/BNLT ceramics contained mainly tetragonal phase with an apparent reduction in tetragonality (i.e. c/a ratio). From the observed result, the slightly distortion of XRD patterns seemed to be attributed to the differences in ionic radii of Bi3þ [1.17 Å], Naþ1 [1.18 Å], La3þ [1.16 Å] and Pb2þ [1.29 Å] as well as the role of cation vacancies created by charge compensation mechanism. For PZT/ 3.0 wt%BNLT ceramic, the phase also contained mainly tetragonal structure but the change in tetragonality when compared to that of its calcined powder was less than other PZT/BNLT samples, the presence of rhombohedral structure could therefore be observed.
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The exact reasons of this discrepancy for PZT/3.0 wt%BNLT sample were not clearly known but compositional homogeneity in the sample could be the main factor causing this observed result. Table 1 lists the values of lattice parameters and tetragonality ratio (c/a) previously discussed. SEM micrographs of fractured PZT/BNLT ceramics sintered at 1200 C are shown in Fig. 4. Grain sizes of all ceramics were measured and the values are given in Table 1. It was obvious that BNLT addition into PZT led to change in average grain size but did not significantly change the density as shown in Fig. 5. All compositions of PZT/BNLT ceramics possessed equiaxed grains. Microstructure of pure PZT revealed large grains size of w3.2 mm with relatively wide grain size distribution. The grain size of PZT/ BNLT ceramics was found to decrease almost by half with addition of only 0.1 wt%BNLT and remained rather constant with further increasing BNLT concentration. The reduction in grain size was mainly attributed to grain growth inhibition as a result of solute
Fig. 4. SEM micrographs of PZT/xBNLT ceramics (a) x ¼ 0, (b) x ¼ 0.1, (c) x ¼ 0.5, (d) x ¼ 1.0 and (e) x ¼ 3.0 wt%.
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drag effect. Since solute diffusion near grain boundary region is usually slower than intrinsic diffusion of host atoms across the boundary plane and becomes rate-limiting for grain boundary movement. This seemed to be the main mechanism governing the observed microstructure similar to those reported in a number of previous work on solid solutions and doped systems, for examples, Nb-added BaTiO3 [17] and MgO-doped Al2O3 [17]. Mechanical properties of the ceramics in terms of Vickers hardness (Hv) and fracture toughness (KIC) were investigated. Their dependence on material compositions are shown in Fig. 6 and the corresponding values are also listed in Table 1. It could be seen that while pure PZT ceramic had Hv of 3.52 GPa, addition of small amount of BNLT caused a slight increase in Hv up to 4.85 GPa in PZT/ 0.5 wt%BNLT composition. These values seemed to higher than those observed earlier in PZT/0.5 wt%BNLT composition by Kitagawa et al. [14] who obtained Hv value of about 4.1 GPa. According to HallePetch relation [18], grain boundaries are known as stress concentration sites, which act as effective obstacles to dislocation pile-up in the adjacent grains and hence leading to harder material. However, among doped samples, grain sizes were nearly the same and therefore other factors might play a role which caused a slight drop in Hv value. From KIC study, it was found that its dependence on composition followed that of Hv. An increase in KIC in samples
containing small BNLT concentration was most likely due to the effect of reduced grain size. Based on the multi-domain grain model, ceramics with smaller grain size could possess single domain state and residual stress generated during sintering could not be released, leading to improvement in apparent KIC as was also observed in PLZT (9/65/53) ceramic system [19]. Further increasing BNLT content >0.5 wt% was found to decrease KIC of ceramics. In this study, it seemed that drop in density value of PZT/3.0 wt%BNLT and grain boundary strengthening played an important role. From SEM micrographs of the fractured surfaces (Fig. 4), PZT exhibited mainly intergranular fracture mode which indicated its relatively weak grain boundaries. Addition of 0.1 wt%BNLT caused fracture behavior to switch from intergranular to transgranular mode as seen in Fig. 4(b), suggesting an increase in grain boundary strength. A further increase in BNLT content over 1.0 wt% gradually changed the fracture mode from transgranular to that of intergranular but with apparently smaller grain size. Hence, it seemed that the maximum hardness and toughness values obtained in PZT/0.5 wt% BNLT ceramic in this study could be correlated to the combination of smaller grain size and optimum strength of grains and grain boundaries. Dielectric constant and dielectric loss, at room temperature, for PZT/BNLT ceramics sintered at 1200 C are shown in Fig. 7 and the corresponding values are also given in Table 2. The dielectric constant was found to slightly decrease with small addition of BNLT but the value increased with further increasing BNLT content. The maximum dielectric constant (er ¼ 1099) and dielectric loss (tand ¼ 0.029) at 1 kHz were observed for PZT/3.0 wt%BNLT. Since BNLT is a donor-doped material [13], this A-site substitution of PZT resulted in a creation of cation vacancies and reduction in the number of oxygen vacancies in PZT system. It has been known that oxygen vacancies are the main cause of domain wall clamping which leads to the lowering of dielectric constant [20]. Thus, it seemed that addition of high BNLT content caused a reduction of oxygen vacancies and led to enhance dielectric constant compared to that of pure PZT sample which contained more oxygen vacancies. From this study, addition of BNLT into PZT ceramic could thus significantly improve dielectric constant. The piezoelectric coefficient (d33) values of PZT/BNLT ceramics are listed in Table 2. The values were found to be in the range of 214e312 pC/N. The highest d33 value of 312 pC/N was observed in pure PZT ceramic, which was in agreement with that observed earlier by Barzegar et al. [21] who obtained d33 value of about 328 pC/N. For PZT/BNLT ceramics, the d33 values were generally
Fig. 6. Plots of Vickers hardness and fracture toughness as a function of BNLT content of PZT/BNLT ceramics.
Fig. 7. Plots of room temperature dielectric constant and dielectric loss values of PZT/ BNLT ceramics as a function of BNLT content.
Fig. 5. Plots of relative density and grain size as a function of BNLT content of PZT/BNLT ceramics.
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Table 2 Electrical properties of PZT/BNLT ceramics. BNLT content (wt%) 0 0.1 0.5 1.0 3.0
Electrical properties
er*
tand*
Pr (mC/cm2)
Ec (kV/cm)
Pr/Pmax
Ec/Emax
Rsq
d33 (1012 C/N)
877 818 889 964 1099
0.0263 0.0173 0.0141 0.0181 0.0290
10.46 8.07 7.90 7.61 7.10
12.00 8.24 8.23 7.88 9.14
0.55 0.44 0.43 0.44 0.45
0.41 0.28 0.29 0.27 0.31
0.84 0.57 0.55 0.56 0.57
312 217 251 279 214
* Dielectric data obtained at room temperature and at a frequency of 1 kHz.
lower than that of pure PZT with no obvious dependence on BNLT concentration for the range investigated. Since BNLT was more difficult to be poled than PZT, this decrease in d33 would be expected. In addition, highest piezoelectric coefficient was observed at MPB composition at which both rhombohedral and tetragonal phases were present [4], the lower d33 values in our samples seemed reasonable and were supported by phase analysis using X-ray diffraction which indicated the compositions slightly deviated from original MPB of pure PZT. Polarizationeelectric field (PeE) hysteresis loops of PZT/BNLT ceramics are shown in Fig. 8. Because of the temperature and field dependence of ferroelectric properties of the ceramics, these parameters have been normalized in form of Pr/Pmax and Ec/Emax [22]. These ratios were also listed in Table 2 along with the loop squareness (Rsq). The polarization loop of pure PZT ceramic showed maximum coercive field (Ec w 12 kV/cm) and remanent polarization (Pr w 10.5 mC/cm2). Addition of BNLT led to a decreasing trend in Ec with minimum value of 7.88 kV/cm in PZT/1.0 wt%BNLT sample. Although grain size reduction in PZT/BNLT samples could play a role in determining ferroelectric properties, it seemed that the effect of A-site cation substitution was more pronounced. In general, ceramic with smaller grain size had limited domain reorientation and the coercive field should increase. However, the observed ferroelectric hysteresis loops suggested donor-like doping with creation of cation vacancies in PZT/BNLT whose typical effect was to cause a decrease in coercive field due to easier ionic motion and hence improved polarizability and domain wall motion in this system [20]. The ferroelectric characteristic of the ceramics can be assessed with the hysteresis loop squareness which is roughly determined
from the ratio of Pr/Ps where Pr is the remanent polarization at zero electric field and Ps is the saturation polarization obtained at some finite field strength below the dielectric breakdown [22]. The loop squareness was often used to measure not only the deviation in polarization axis but also that in electric field axis with the empirical expression Rsq ¼ (Pr/Ps) þ (P1.1Ec/Pr), where P1.1Ec is the polarization at the field equal to 1.1Ec. For the ideal hysteresis loop, Rsq is equal to 2.00. As listed in Table 2, Rsq of pure PZT ceramic showed a value of 0.84, indicating typical ferroelectric properties of this material. Addition of BNLT into PZT ceramic dropped Rsq value to w0.55e0.57 regardless of BNLT content. This clearly showed that when BNLT was added to the PZT, the hysteresis curves became slimmer which was a characteristic of suppressed ferroelectric interaction. Similar reduction in Rsq was also observed in leadbased PMNePZT system [23]. Despite the fact that PZT/BNLT ceramics in this study showed promising mechanical and room temperature dielectric properties, further characterization of PZT/ BNLT on high temperature dielectric properties would be beneficial to confirm the possibility of using this material for transducer applications. 4. Conclusions In this study, PZT/xBNLT (x ¼ 0, 0.1, 0.5, 1.0 and 3.0 wt%) ceramics were successfully fabricated by a solid-state mixed oxide method. The optimum sintering temperature for this ceramic system was found to be 1200 C at which all samples had densities at least 96% of their theoretical values. X-ray diffraction analysis of the ceramics indicated that an addition of BNLT caused slight change in tetragonality. An addition of BNLT in PZT ceramics inhibited grain growth which resulted a reduction of grain size. This, thus, improved Hv of the ceramics and improved KIC in samples containing 0.5 wt%BNLT concentration. Room temperature dielectric constant was found to improve with an addition of >0.1 wt%BNLT. Addition of BNLT into PZT ceramics also affected their piezoelectric and ferroelectric properties. The maximum piezoelectric coefficient values were observed in pure PZT ceramic and were slightly decreased in BNLT-added samples. Slimmer hysteresis loops seen for modified PZT samples suggested a possible use of this material in transducer applications. From this viewpoint, a new PZT/BNLT system provided useful information for material development. It could be seen that suitable content of added BNLT into PZT ceramic could optimize mechanical, dielectric, piezoelectric and ferroelectric properties. Acknowledgements
Fig. 8. Plots of polarization as a function of electric field of PZT/BNLT ceramics with different of BNLT content.
This work is financially supported by the Thailand Research Fund and the National Research University, Chiang Mai University under the Office of the Higher Education Commission. The Faculty of Science and the Graduate School, Chiang Mai University is also acknowledged.
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