Effects BNT compound incorporated on structure and electrical properties of PZT ceramic

Current Applied Physics 11 (2011) S77eS81

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Effects BNT compound incorporated on structure and electrical properties of PZT ceramic P. Jaita a, A. Watcharapasorn a, b, S. Jiansirisomboon a, b, * a b

Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand Materials Science Research Center, 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 26 June 2010 Received in revised form 18 November 2010 Accepted 7 March 2011 Available online 16 March 2011

Ferroelectric ceramics with formula Pb(Zr0.52Ti0.48)O3/x(Bi0.5Na0.5)TiO3 (when x ¼ 0, 0.1, 0.5, 1.0 and 3.0 wt%) were prepared by a solid-state mixed-oxide method and sintered at the temperature between 1050 and 1200  C for 2 h to obtain dense ceramics. It was found that the optimum sintering temperature was 1200  C at which all the samples had relative density at least 96% of their theoretical values. Phase analysis using X-ray diffraction showed tetragonal and rhombohedral perovskite structure of PZT with no BNT peak detected, indicating that completed solid solutions occurred for all compositions. Scanning electron micrographs of fractured PZT/BNT ceramics showed equiaxed grain shape with mixed-mode of transgranular and intergranular fractures. Addition of BNT significantly decreased grain size of the PZT ceramic. Measurement of room temperature dielectric constant indicated a gradual increase with increasing BNT content. Results of ferroelectric characterization showed a slight decrease of remanent polarization and coercive field for BNT-added samples, suggesting ceramics which could be easily poled. Good piezoelectric coefficient (d33) could be maintained and comparable to that of pure PZT ceramic for the sample with 1.0 wt% BNT addition. Ó 2011 Elsevier B.V. All rights reserved.

Keywords: PZT/BNT Microstructure Electrical properties Ferroelectric materials

1. Introduction Lead zirconate titanate (PZT) ferroelectric materials with perovskite structure has been widely used for various applications such as sensors, filters, oscillators, actuators and nonvolatile ferroelectric random access memories (NvFRAM) because of their excellent ferroelectric and piezoelectric properties [1,2]. PZT is a solid solution of ferroelectric PbTiO3 (Tc ¼ 490  C) and antiferroelectric PbZrO3 (Tc ¼ 230  C) in different Zr/Ti ratios [3e5]. It has good piezoelectric properties, particularly at composition where Zr:Ti w 52:48 close to a morphotropic phase boundary (MPB) between tetragonal and rhombohedral phases [6]. This compound also possesses high spontaneous polarization, good thermal stability during operation, high electromechanical coupling coefficient and ease of poling [7]. Therefore, PZT is the most popular ferroelectric material used in electronic devices and transducer applications [6e8]. Furthermore, it has a high Curie temperature (Tc) of about 390  C which allows devices to be operated at relatively high temperature [9,10].

* Corresponding author. Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, 239 Huay Kaew Rd., Suthep, Chiang Mai 50200, Thailand. Tel.: þ66 53 941921x631; fax: þ66 53 943445. E-mail address: [email protected] (S. Jiansirisomboon). 1567-1739/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2011.03.012

Bismuth sodium titanate (BNT) discovered by Smolensky et al. in 1960 [11] is one of several attractive lead-free perovskite structure materials with high mechanical bending strength [11]. BNT also has interesting electrical properties such as good dielectric constant (3r w 406) and acceptable piezoelectric coefficient (d33 w 94 pC/N) [9]. BNT possesses strong ferroelectric properties at a relatively high Curie temperature (Tc ¼ 320  C) with large remanent polarization (Pr ¼ 38 mC/cm2) and coercive field (Ec ¼ 73 kV/cm) at room temperature [9,12e14]. Furthermore, it also allows free control of sintering atmosphere and no lead pollution during fabrication process [15]. With their complementary features, the solid solutions between PZT and BNT (PZT/BNT system) are expected to exhibit better properties than those of single phase PZT and BNT. Furthermore, the properties can also be tailored over a wide range by changing the compositions to meet the strict requirements for specific applications. The study of PZT-based solid solutions with BNT was firstly reported by Kitagawa et al. [16], whose results showed that mechanical strength and piezoelectric properties of BNT-added PZT ceramics were improved over those of pure PZT. Since dielectric and ferroelectric properties of PZT/BNT ceramics have not been studied, the effects of BNT compound on structure and electrical properties such as dielectric, ferroelectric and piezoelectric of PZT ceramic were therefore investigated in this work.

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2. Experimental Pb(Zr0.52Ti0.48)O3 and (Bi0.5Na0.5)TiO3 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) 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 and BNT powders were separately calcined in a closed alumina crucible at the same temperature of 800  C for 2 h. PZT/xBNT (with x ¼ 0, 0.1, 0.5, 1.0 and 3.0 wt%) powders were then formulated from the PZT and BNT component by employing the similar mixed-oxide procedure and dried using a freeze-drying method. 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 pressed with 1.5 ton weight. Binder removal was carried out by heating the pellets to 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 a covered alumina crucible filled with PbZrO3 powder in order to avoid compositional deviation by PbO volatilization during sintering. Phase identification of powders and ceramics were investigated in 2q range of 10 to 80 using an X-ray diffractometer (XRD, Phillip X-pert). The full width at half maximum (FWHM) values using a certain hkl peak for all ceramic samples were also obtained [17]. Bulk densities of sintered ceramics were determined by Archimedes’ method. The theoretical densities of all samples were calculated based on the theoretical densities of BNT (5.990 g/cm3) [15] and PZT (8.006 g/cm3) [18]. Fractured surfaces of all ceramics were observed using scanning electron microscopy (SEM, JEOL JSM6335F). Grain size of each sample was measured by a mean linear intercept method from the SEM micrographs. For electrical measurements, two parallel surfaces of sintered ceramics were polished and painted with silver paste for electrical contacts. Dielectric properties were measured at temperature in a range of 25  Ce500  C with a measured frequency of 100 kHz using 4284A LCR-meter connected to a high temperature furnace. Ferroelectric hysteresis loop of each sample was obtained using a computer controlled modified SawyereTower circuit. The electric field was applied to a sample by a high voltage AC amplifier at 30 kV. The polarization-electric field (P-E) 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 the 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 [10], 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 ðPr =Ps Þ þ ðP1:1 Ec =Pr Þ. For 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 (S5865) at a frequency of 50 Hz.

Fig. 1. X-ray diffraction patterns of calcined PZT/BNT powders.

no. 73-2022). The range of 2q ¼ 42e47, which corresponded to (020)R reflection of rhombohedral (R) structure and (002)Te(200)T peaks of tetragonal (T) structure, suggested that there was no change in X-ray diffraction patterns of the powders when BNT was added into PZT and no detectable impurity phases. Both tetragonal and rhombohedral phases were maintained. For sintering study of green pellets at various temperatures, it was found that the optimum sintering temperature was 1200  C at which all samples had densities ranging from 7.62 to 7.78 g/cm3, corresponding to at least 96% of theoretical values. The values were similar to those reported by Kitagawa et al. [16] for the same composition. It should also be noted that addition of BNT caused a slight decrease in sample density. X-ray diffraction patterns in the range of 2q ¼ 10e60 of PZT/ xBNT ceramics sintered at 1200  C are shown in Fig. 2(a). The range of 2q ¼ 42e47 in X-ray patterns shown in Fig. 2(b) suggested a slight change in lattice parameters and tetragonality (c/a) ratio compared to the powder. Based on graphical analysis, pure PZT sample and those containing BNT up to 1.0 wt% consisted of both rhombohedral and tetragonal phases. With increasing the content of BNT, no BNT or other second-phase peaks were observed and the tetragonal unit

3. Results and discussion X-ray diffraction patterns of PZT/xBNT powders with x ¼ 0, 0.1, 0.5, 1.0 and 3.0 wt% in range of 2q ¼ 10e80 are shown in Fig. 1. It can be seen that pure PZT powder had a composition near the morphotopic phase boundary (MPB) which consisted of tetragonal structure (JCPDS no. 33-0784) and rhombohedral structure (JCPDS

Fig. 2. X-ray diffraction patterns of PZT/BNT ceramics sintered at 1200 2q ¼ 10e60 and (b) 2q ¼ 42e47.

C

(a)

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cell became slightly distorted. The contribution of rhombohedral phase decreased as deduced from (002)T/(020)R/(200)T reflections, resulting in all diffraction peaks gradually shifted toward tetragonal structure, thus the sample containing 3.0 wt% BNT showed mainly tetragonal structure. Based on these observations it seemed to indicate that BNT completely dissolved in PZT matrix. The full width at half maximum (FWHM) value was calculated from (101)-type peak in X-ray diffraction pattern and these values are also included in Table 1. Pure PZT has the FWHM value of about 0.41. It should be noted that BNT addition did not have a significant effect on FWHM values. These values are quite similar for all ceramic compositions. Hence, there are virtually no differences in the degree of crystallinity in our samples. The SEM micrographs of fractured surfaces of PZT/BNT ceramics sintered at 1200  C are shown in Fig. 3. Pure PZT ceramic revealed larger grains with relatively wide grain size distribution compared to BNT-added samples. All compositions of PZT/BNT ceramics possessed normal equiaxed grains structure with mixed intergranular and transgranular fracture mode. The grain sizes of PZT/ xBNT ceramics sintered at 1200  C were analyzed from fractured samples and their values are listed in Table 1. For pure PZT, the average grain size was w3.18 mm. Addition of 0.1 wt% BNT into PZT rapidly dropped the grain size by half and remained rather constant with further increasing BNT content. The inhibition of grain growth kinetics was due to BNT solute-drag effect. Since solute diffusion near the grain boundary region is usually slower than the intrinsic diffusion of host atoms across the boundary plane, this became a rate-limiting factor and therefore effectively slowed the grain boundary movement. This seemed to be the main mechanism governing the observed microstructure which was similar to the previous work on Nb-added BaTiO3 and MgO-doped Al2O3 [19]. Dielectric constant and dielectric loss measured at 100 kHz of PZT/xBNT ceramics sintered at 1200  C plotted as a function of temperature are shown in Fig. 4(a) and (b), respectively. The related dielectric properties at room temperature are also listed in Table 1. The Curie temperature (Tc) of pure PZT ceramic in this study was 388  C, which was in agreement with the value of 390  C previously reported by Haertling [10]. Among BNT-added samples, small reduction of Tc values was observed with increasing BNT content which was to be expected because the BNT ceramic has a lower value of Tc (w320  C) compared with that of PZT. Similar compositional dependence of Tc was observed in PZT-BT system [20]. At Curie temperature, a maximum dielectric constant (3r ¼ 16500) and dielectric loss (tand ¼ 0.1800) were observed for pure PZT ceramic. This result was partly attributed to the grain size effect. Large grains normally contain multiple domains whose wall motion results in an increase in the dielectric constant at the Curie point. For ceramics with smaller grain size, smaller number of domains or, in same case, even a single domain can form inside each grain. The movement of these domain walls was restricted by the grain boundaries, thus leading to lower dielectric constant at the Curie point when compared to the large grain [8,21]. Thus, addition of 0.5 wt% BNT decreased high temperature dielectric constant to a minimum value of 9690 but the value increased with further increasing BNT content greater than 0.5 wt%. At room temperature, dielectric

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constant and dielectric loss values are also given in Table 1. Dielectric constant of pure PZT ceramic was found to be 877 with dielectric loss value of 0.026. The addition of 0.1 wt% BNT slightly dropped dielectric constant value by w100 but it then increased with further increasing in BNT content. However, dielectric loss value also increased. The maximum dielectric constant (3r ¼ 1027) and dielectric loss (tand ¼ 0.0474) were observed in PZT/3.0 wt% BNT. Since BNT is virtually an isovalent additive in which (Bi0.5Na0.5) has an effective charge of þ2, which is the same as Pb. It seemed therefore that the increase in room temperature dielectric constant was mainly caused by the lowering of Tc. The piezoelectric coefficient (d33) values of PZT/xBNT ceramics sintered at 1200  C are listed in Table 1. The values were found to be in the range of 209 to 312 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. [22] who obtained d33 value of about 328 pC/N. For PZT/BNT ceramics group, since BNT was more difficult to be poled than PZT, this decrease in d33 value would be expected. Among BNT-doped samples, d33 value tended to increase with BNT content from 0.1 wt% up to 1.0 wt% and then decreased for 3.0 wt% BNT concentration. Hence, the composition of 1.0 wt% BNT could be near the optimum composition. The reason for this high value was expected to be due to charge compensation and defects involved in Bi3þ/Na1þ ionic substitution in Pb2þ sites. The exact defect chemistry of this system should be further characterized but, at the moment, it could only be said that at 1.0 wt% BNT, the sample had better polarizability than other samples. This was partly due to the observation in the ferroelectric measurement that the coercive field of this sample was lowest. It can be seen that d33 values of PZT/BNT samples were generally lower than that of pure PZT with no obvious dependence on BNT concentration for the range investigated. In addition the highest piezoelectric coefficient of PZT was normally observed at MPB composition at which both rhombohedral and tetragonal phases were present [9]. The lower d33 values in our PZT/BNT samples when compared to pure PZT seemed reasonable. This was also supported by phase analysis using X-ray diffraction as indicated the compositions slightly deviated from original MPB of pure PZT. Polarization-electric field (P-E) hysteresis loops of PZT/xBNT ceramics sintered at 1200  C are illustrated in Fig. 5 and loop squareness (Rsq) values are also listed in Table 1. The hysteresis loop of pure PZT ceramic showed the maximum coercive field (Ec w 12 kV/cm) and remanent polarization (Pr w 10.5 mC/cm2). These results seemed to agree with those observed earlier by Yimnirun et al. [23] who reported Ec and Pr values of 10 kV/cm and 12.5 mC/cm2, respectively. The addition of BNT led to a decreasing trend in Ec and Pr with the minimum values of 7.63 kV/cm and 4.35 mC/cm2 in PZT/1.0 wt% BNT. Although grain size reduction in PZT/BNT samples could play a role in determining ferroelectric properties, it seemed in this study that the effect of dopant substitution was much more pronounced. Since BNT by itself was known to require much larger field to produce saturated polarization when compared to PZT, this seemed to be the reason for slimmer loops observed in PZT/xBNT ceramics. In addition, the change of crystal structure to be more tetragonal may also

Table 1 Physical and electrical properties of PZT/BNT ceramics sintered at 1200  C. BNT content (wt%)

Density (g/cm3)

0 0.1 0.5 1.0 3.0

7.78 7.77 7.75 7.72 7.62

a

    

0.03 0.02 0.01 0.01 0.06

FWHM of (101) ( )

Grain size (mm)

0.41 0.43 0.43 0.42 0.38

3.18 1.78 1.77 1.81 2.06

    

0.34 0.16 0.19 0.18 0.21

Dielectric data obtained at room temperature and at a frequency of 1 kHz.

Tc ( C)

3ra

tanda

Pr (mC/cm2)

Ec (kV/cm)

Rsq

d33 (pC/N)

388 383 380 380 377

877 770 884 916 1027

0.0263 0.0255 0.0429 0.0468 0.0474

10.46 9.66 9.63 4.35 6.60

12.00 8.81 8.73 7.63 10.34

0.84 0.65 0.65 0.44 0.60

312 212 230 301 209

    

9.42 5.38 7.36 6.47 9.43

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Fig. 3. SEM micrographs of PZT/xBNT ceramics sintered at 1200  C (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%.

contribute to this reduction in remanent polarization. It was shown by Berlincourt et al. [24] that remanent polarization of PZT decreased with increasing Ti/Zr ratio within tetragonal region. Similar trend was also observed in PZT doped with small amount of BaTiO3 [20]. As listed in Table 1, the loop squareness (Rsq) value was found to be a maximum of 0.84 for pure PZT ceramic which indicated typical ferroelectric properties of this material. The addition

of BNT 0.1e0.5 wt% into PZT ceramic dropped Rsq value to w0.65 and decreased to a minimum value of 0.44 for PZT/1.0 wt% BNT. From this investigation, the improvement of electrical properties of PZT/BNT ceramics could be achieved. It clearly showed that BNTadded samples exhibited better dielectric properties than single phase PZT or BNT while piezoelectric properties were comparable to those of monolithic PZT ceramic and better than pure BNT. On

Fig. 4. Plots of temperature dependence on (a) dielectric constant and (b) dielectric loss of PZT/BNT ceramics sintered at 1200  C, the measurement was done at a frequency of 100 kHz.

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Thailand’s Office of the Higher Education Commission (OHEC). The Faculty of Science and the Graduate School, Chiang Mai University is also acknowledged.

References

Fig. 5. Plots of polarization as a function of electric field of PZT/BNT ceramics sintered at 1200  C with different BNT content.

the other hand, BNT-added samples exhibited a characteristic of suppressed ferroelectric interaction which Yimnirun et al. [23] suggested that this type of slim hysteresis curve may be suitable for transducer applications. 4. Conclusions In this study, PZT/xBNT (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 relative densities at least 96% of their theoretical values. X-ray diffraction analysis of the ceramics indicated that the addition of BNT into PZT ceramics caused a slight change in lattice parameters and phases. BNT was also found to inhibit the grain growth which directly resulted in a reduction of grain size for all sintering temperatures. The dielectric constant at room temperature was found to be substantially improved with an addition >0.1 wt% BNT. The addition BNT into PZT ceramic also affected their ferroelectric and piezoelectric properties. The maximum piezoelectric coefficient values were observed in pure PZT ceramic and were slightly decreased in BNT-added samples. From this view point, it could be seen that the suitable content of added BNT could optimize dielectric, ferroelectric and piezoelectric properties of PZT ceramic. Acknowledgments This work is financially supported by the Thailand Research Fund (TRF) and the National Research University Project under

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