Structural and electrical properties of SrTiO3-modified

Structural and electrical properties of SrTiO3-modified Bi0.5(Na,K)0.5TiO3 leadfree ceramics Volkan Kalem

Journal of Materials Science: Materials in Electronics ISSN 0957-4522 J Mater Sci: Mater Electron DOI 10.1007/s10854-016-4879-5

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Author's personal copy J Mater Sci: Mater Electron DOI 10.1007/s10854-016-4879-5

Structural and electrical properties of SrTiO3-modified Bi0.5(Na,K)0.5TiO3 lead-free ceramics Volkan Kalem1

Received: 17 February 2016 / Accepted: 21 April 2016 Ó Springer Science+Business Media New York 2016

Abstract Polycrystalline (1 - x)[0.8Bi0.5Na0.5TiO3– 0.2Bi0.5K0.5TiO3]–(x)[SrTiO3] (x = 0 to 0.20) lead-free piezoelectric ceramics were synthesized via a solid-state reaction route. Microstructural and compositional analyses have been carried out using X-ray diffraction (XRD) and scanning electron microscope (SEM). XRD patterns indicated that undoped composition (x = 0) had a tetragonal phase dominant structure, however SrTiO3 (ST) incorporations resulted in a phase shift to rhombohedral symmetry. SEM studies showed that increasing ST doping led to a decrease in grain size. Effect of ST doping on the piezoelectric and dielectric properties were also investigated. The results revealed that the dielectric constant increased gradually with the increase of ST. The ceramic having 12 mol% ST possessed the optimum properties (d33 = 205 pC/N, kp = 0.36, KT = 1520, tand = 6.1 %, Qm = 87, Tm = 285 °C) enabling this composition a suitable lead-free candidate for piezoelectric actuator applications.

1 Introduction Lead-based piezoelectrics such as lead zirconate titanate (Pb[Zr,Ti]O3, PZT), lead manganese niobate-lead titanate (Pb[Mg,Nb]O3–PbTiO3, PMN–PT), lead zinc niobate-lead titanate (Pb[Zn,Nb]O3–PbTiO3, PZN–PT) are widely used in electronic devices thanks to their enhanced piezoelectric properties around their morphotropic phase boundaries & Volkan Kalem [email protected] 1

Department of Metallurgical and Materials Engineering, Selcuk University, 42130 Konya, Turkey

(MPB). Due to toxicity and volatility of lead, there has been an increasing demand for the development of leadfree materials from the viewpoint of environmental protection. Lead-free piezoelectrics of perovskite-, tungsten bronze-, and bismuth layer-structures have been extensively studied in the last two decades. Among them, perovskite type ferroelectrics such as bismuth sodium titanate ([Bi0.5Na0.5]TiO3, BNT)-, potassium sodium niobate ([K0.5Na0.5]NbO3, KNN)-, barium titanate (BaTiO3, BT)-, and sodium tantalite (NaTaO3, NT)-based materials exhibit high piezoelectric properties; especially a large piezoelectric constant (d) [1–7]. BNT-based ferroelectrics are considered to be the most promising lead-free materials since its solid solutions with PT, BT, or bismuth potassium titanate ([Bi0.5K0.5]TiO3, BKT) have enhanced electrical properties due to an MPB between the end compounds, similar to the lead-based systems. Especially (1 - x)BNT–xBKT (BNKT) based compositions have come into prominence owing to their extra high strain values which are comparable to PZT. The origins of this characteristic are believed to be a reversible phase transition from a nonpolar phase to a ferroelectric phase and a ‘‘point-defect-mediated’’ reversible domain switching under applied electric fields [7, 8]. At room temperature, BNT and BKT have rhombohedral and tetragonal structures, respectively. It was reported that their solid solutions have an MPB near x = 0.16–0.20 [9–12]. The study of Sasaki et al. [9] as well as some recent studies [10–18] showed that relatively good electrical properties could be obtained in BNKT system in the vicinity of the MPB compositions: kp % 0.26–0.35, kt % 0.42, k33 % 0.54–0.56, KT % 630–1700, Qm % 110–195, d33 % 105–195 pC/N, and tand % 0.03–0.07. The enhancement in electrical properties for such mixtures was attributed to co-existence of rhombohedral (R) and

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Author's personal copy J Mater Sci: Mater Electron

tetragonal (T) phases. Numerous researchers have tried to improve the piezoelectric properties of BNKT based ceramics by forming solid solutions and by doping. Role of solid solution of A0 B0 O3 type dopants such as BiAlO3, LiTaO3, CaZrO3, BaTiO3 and etc., were studied to improve piezoelectric, dielectric and ferroelectric properties [8]. Few reports have been published on the effect of Sr2? as 0 0 A B O3 type dopant (SrTiO3, ST, or SrZrO3, SZ) on dielectric and piezoelectric properties, and phase structure of BNKT ceramics near MPB. Hussain et al. [19] studied the field induced strain, polarization and dielectric response of (1 - y)[Bi0.5(Na0.80K0.20)0.5TiO3]–ySrZrO3 ceramics. They reported a maximum piezoelectric constant, d33 of 190 pC/N for y = 0.02 and a normalized strain, Smax/Emax (d33*) of 617 pm/V for y = 0.03. The reason for the increased piezoelectric strain constants was explained on the basis of partial substitution of Sr2? at A-site and Zr4? at B-site. This partial substitution was believed to be causing a local distortion in BNKT unit cell, which in turn led to nonpolar, and ferroelectric phase transition under applied electric fields. Effect of ST in a BNKT composition was first investigated by Yoo et al. [20] in 2004. The chemical formula of the studied composition was Bi0.5(Na0.84K0.16)0.5TiO3. This composition was also modified using 0.3 wt% Nb2O5, but the utilization of Nb5? was not disclosed. Although the selected (1 - x)BNT–xBKT composition is close to the rhombohedral side (x \ 0.18) of the MPB, it was concluded that the crystal structure showed a transition from tetragonal symmetry to a tetragonal–rhombohedral coexistence region with increasing Sr doping. The highest piezoelectric constant (d33) of 205 pC/N was achieved at 6 mol % Sr doping. In a recent study, Bai et al. [21] prepared \001[ textured 0.83BNT–0.17BKT ceramics using plate-like ST templates, up to 27 mol%, fabricated by templated grain growth method. This composition also lies close to the rhombohedral side of the MPB. They claimed that the piezoelectric constant (d33) decreases with increasing ST content up to 15 mol%, and diminishes with further ST doping. Although the BNKT compositions with rhombohedral symmetry provided higher electromechanical coupling coefficients, compositions close to tetragonal side (x [ 0.20) of the MPB led to ceramics with higher dielectric constant and piezoelectric strain constant [9]. The only study, to the best of my knowledge, on the effect of ST doping in a tetragonal 0.80BNT–0.20BKT composition, was conducted by Wang et al. [22]. They examined the strain behavior and ferroelectric properties in terms of the ST doping. But the amount of ST was confined to 3 and 5 mol%. Best strain behavior (Smax/Emax) of 600 pm/V was observed for 5 mol% ST added composition. Data on piezoelectric properties such as piezoelectric constant

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(d33 - pC/N), electromechanical coupling factor (k) or mechanical quality factor (Qm) was not provided. In view of the scarcity of information on electromechanical properties of ST-modified BNKT ceramics, the focus of the present study was to examine in detail the effect of ST doping in a wide range (up to 20 mol%) on the dielectric and piezoelectric properties of 0.80BNT–0.20BKT ceramics and the changes in the associated microstructures.

2 Materials and methods 2.1 Ceramic preparation High purity oxide [Bi2O3 (99.999 %) and TiO2 (99.99 %)] and carbonate [Na2CO3 (99.5 %), K2CO3 (99.0 %), and SrCO3 (99.9 %)] constituents from Sigma-Aldrich were used to prepare the desired compositions of BNKT–ST ceramics by the mixed-oxide method. The composition range of 0 B x B 0.20 was examined, where the composition is specified by a molecular formula of (1 - x)[0.8Bi0.5Na0.5TiO3–0.2Bi0.5K0.5TiO3]–(x)[SrTiO3]. Each powder batch was blended thoroughly via ballmilling for 24 h. The dried mixture was compacted as a slug in a hardened steel die and then calcined at 850 °C for 4 h. The product was ball milled again for another 24 h and dried. The XRD patterns of the calcined powders revealed the formation of pure perovskite phase during that stage of the thermal treatment. As an example, Fig. 1 shows the powder XRD pattern at room temperature for the calcined

Fig. 1 XRD pattern of the 0.80BNKT–0.20ST sample after calcination


sample with x = 0.20. All millings were done with stabilized zirconia balls in a medium of ethanol. Green ceramic discs, each measuring 13 mm in diameter and 1 mm in thickness, were prepared from the STmodified powders plasticized by 2 wt% PVA addition. The discs were compacted by uniaxial pressing in a hardened tool steel die under a load of 300 MPa. After binder burnout in air at 600 °C, the discs were sintered by soaking at 1150 °C for 3 h. The sintering was conducted in a closed crucible assembly, which contained BNKT bedding to inhibit the evaporation of Bi, Na, and K and to maintain the chemical composition. 2.2 Characterization The microstructural studies were conducted mainly on polished sections under a SM Zeiss LS-10 field emission scanning electron microscope (FE-SEM). SEM images of the sintered pellets were obtained from the fracture surfaces of the ceramic samples. Preceding the SEM work, the sample surfaces were sputtered with gold. The average grain size was calculated on the SEM micrographs by the usual linear intercept method [23]. The extent of porosity in the ceramics was also ascertained from examination of their surfaces under SEM. XRD analyses were performed for monitoring the powder synthesis process, and quantification of the tetragonal and rhombohedral phase fractions present in sintered ceramics. A Bruker D8 Advance model diffractometer was used with Cu-Ka radiation; the diffraction data were collected over the 2h range from 20° to 80° with a step size of 0.03°. For electromechanical measurements, the flat surfaces of the sintered discs were lapped and then metalized with a silver paste. Painted silver electrodes were fired at 650 °C for 30 min. For the poling process, the electroded discs were exposed to a DC electric field of 4 kV/mm for 30 min in a silicon oil bath at 100 °C. The piezoelectric strain coefficient (d33) of each disc was measured 24 h after poling, by a Berlincourt d33-meter. kp and Qm were determined by the resonance/anti-resonance method according to the IRE standards using a HP4194A impedance analyzer. The reliability of the data was checked by conducting measurements on duplicate samples; the experimental uncertainty was confined to ±2 % of the reported values. Free dielectric constant (KT) and loss tangent (tand) of the poled ceramics were calculated from the capacitance and dissipation factor values measured at 1 kHz and from the sample dimensions. The relative permittivity maximum temperature, Tm, of poled discs was determined by establishing the variation of dielectric permittivity with temperature in the range of 25–400 °C. These measurements

were done at 1 kHz frequency by placing the samples in a small muffle furnace in which the data on permittivity were taken upon heating the ceramics at a rate of 2 °C/min.

3 Results and discussion 3.1 Physical and structural properties The crystal structure of the BNKT–ST samples characterized by XRD and Ka2-eliminated patterns are depicted in Fig. 2. They indicated that a pure perovskite structure existed in all specimens. This confirms the total solubility of Sr in the BNT–BKT lattice without forming any impurity phase. As reported earlier, the 0.80BNT–0.20BKT composition is expected to lie on the tetragonal side of the MPB [9]. Ka2 peaks were eliminated in order to clearly understand the phase transformations between tetragonal, rhombohedral and any pseudo-cubic phases. Since XRD patterns do not include Ka2 peaks, the peak splitting near 46° for the undoped composition was believed to belong to the tetragonal symmetry. The experimental diffraction lines of any MPB composition consisted of three separated peaks between 2h values of 45° and 47°. Two of the peaks, with the intensity of the one being approximately half of the other, corresponded to the line splitting of the tetragonal phase and the third corresponded to the central peak of the rhombohedral phase. The {200} peak splits into (002)– (200) and confirms tetragonal phase. In order to resolve for

Fig. 2 Room temperature XRD patterns (1 - x)BNKT–xST ceramic samples

of

the

sintered

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the tetragonal and rhombohedral fractions, Peakfit v4.0 software was used to deconvolute the peaks assuming Lorentzian line shape. The relative volume fractions of rhombohedral and tetragonal phases, %R and %T, were estimated by integrated intensities of XRD peaks. The results obtained by processing the integrated intensities are shown in Fig. 3. The composition without the ST dopant was mainly tetragonal. ST doping into BNKT caused an increase in rhombohedral fraction. On further increasing the dopant content beyond *10 mol% the perovskite structure changed from tetragonal to rhombohedral phase dominant structure as (002) and (200) peaks merged into a single (200) peak. It was concluded from these results that the possible substitution of Sr2? at the A-site (Bi, Na, and K) of the formulation of these ceramics shifted the MPB to the BKT-rich region. The difference between the ionic radii of Sr2? ion (118 pm) and the A-site ions (Bi3?: 103 pm, Na?: 102 pm, K?: 138 pm) results in the alteration of both the lattice constant and lattice energy, which in turn leads to a phase transformation for the stabilization of the structure [8, 24]. As a result, the incorporation of Sr into the perovskite structure stabilized the rhombohedral phase against the tetragonal one. Figure 4 shows the SEM micrographs of fracture surfaces of SrTiO3-modified BNKT ceramics. All samples have mainly quadrate grains and the sizes were affected by the amount of SrTiO3. The doping refined the microstructure; average grain size of undoped BNKT ceramics was about 3.2 lm and decreased to *1.4 lm with 20 mol% SrTiO3 incorporation. The decrease of grain size with increasing ST is probably due to the greater difficulty of diffusion, which is a result of the lattice distortion caused by Sr2? substitution at the A-site. The ceramic with 12 mol% had the most uniform and compact structure. Excessive ST doping led to a bimodal grain size distribution; some sub-micron grains with accompanying larger

Fig. 3 Proportions of phases T and R in the (1 - x)BNKT–xST ceramics as a function of ST content

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grains of 2–3 lm average size (Fig. 4d). The non-homogeneity in grain sizes is associated with two effects, (1) the pinning effect of grain boundary due to the segregation of smaller sized particles and (2) possible replacement of larger A-site ions (K?) by smaller Sr2? ions mostly in the grain boundary area [25]. 3.2 Dielectric and piezoelectric properties Effect of ST doping on the dielectric and piezoelectric properties of BNKT ceramics was examined. Figure 5 shows the change in the electromechanical properties with the increase in dopant concentration. ST doping was very effective on electromechanical properties of ceramics. The free dielectric constant, KT shows a monotonous increase with increasing ST content, indicating a steady enhancement in polarization, previously explained as being due to the elimination of the effect of compression of the 180° domains in the tetragonal phase [26]. On the other hand, planar coupling coefficient (kp) and piezoelectric charge constant (d33) increased with an increase in ST content up to a certain point and then decreased. A maximum kp and d33 of 0.36 and 205 pC/N were obtained for x = 0.12, respectively. ST doping into BNKT caused a transition from tetragonal phase dominant region to a tetragonal– rhombohedral coexistence region. The compositions near that region around 8–12 mol% ST exhibited higher piezoelectric values since the activation energy for domain motion is low near the two coexisting phases. Apart from the phase coexistence, type of dopant is also effective on the domain wall motion. Donor ions can be formed owing to the substitution of Sr2? for ions of lower valence (Na?, K?) at the A-site. Donors, also known as softeners, lead to some A-site vacancies, which in turn provide a relaxation of the strain caused by domain reorientation. As a result of easier domain wall motion, enhanced electrical properties were obtained in BNKT ceramics by ST doping. In addition to the phase coexistence, the domain wall motion is also affected by the grain size. Since grain boundaries act as additional pinning points, the domain wall motion inhibited as the grain size decreased. Additions of ST caused a reduction in the grain size, implying that the domain size was also reduced. Under these conditions, the domains got clamped gradually by increasing Sr substitution due to the pinning action of grain boundaries and the dominance of space charge effects. Hence, reductions in the electromechanical properties were observed at higher levels of ST doping. Figure 6 presents the variation of mechanical quality factor (Qm) and dielectric loss tangent (tand) as a function of ST doping. They changed in almost the opposite sense with increasing amount of ST. Dielectric loss was higher for 8 and 12 mol% ST doped compositions in which the

Author's personal copy J Mater Sci: Mater Electron Fig. 4 SEM micrographs of the representative compositions: a BNKT, b 0.96BNKT–0.04ST, c 0.88BNKT–0.12ST, d 0.80BNKT–0.20ST

Fig. 5 Change in dielectric constant (KT), planar electromechanical coupling coefficient (kp), and piezoelectric charge constant (d33) of BNKT–ST ceramics as a function of ST content

tetragonal and rhombohedral phases coexisted. On the other hand, as expected, mechanical quality factor was lower at this level of doping. Furthermore, ST doping beyond 12 mol% caused Qm to increase in the rhombohedral phase region. This effect can also be correlated to the decreasing grain size of ceramics with ST addition. In piezoelectric ceramics, an increase in d33 or kp accompanies with a decrease in Qm which is mainly due to the domain structure. Increase in wall mobility results in an observed reduction in mechanical quality factor and an increase in piezoelectric coefficients and coupling factors. As a result, the plot of Qm in Fig. 6 changed in the opposite direction as compared with those of d33 and kp. The change

in Qm was consistent with the results reported by Yoo et al. [20] for 0.84BNT–0.16BKT ceramics. It should be noted that the Qm values of the BNKT ceramics studied in the present work were lower than the one reported in the mentioned study due to difference in BNT/BKT ratio. Figure 7 shows the temperature dependence of dielectric constant as a function of ST doping in BNKT ceramics. Two dielectric anomalies were observed; the relative permittivity maximum temperature (Tm) around 280–290 °C and the depolarization temperature (Td) around 50–100 °C. The depolarization temperature indicates the ferroelectric– antiferroelectric phase transition. Both Tm and Td shifted toward lower temperatures with an increase in ST content.

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Author's personal copy J Mater Sci: Mater Electron Table 1 Physical and electrical properties of 0.88BNKT–0.12ST ceramic g/cm3

Density (q) T

Fig. 6 Change in mechanical quality factor (Qm) and loss tangent of BNKT–ST ceramics as a function of ST content

5.60

Dielectric constant (K )

–

1520

Dielectric loss tangent (tand)

%

6.1

Piezoelectric charge constant (d33)

pC/N

205

Electromechanical coupling coefficient (kp)

–

0.36

Mechanical quality factor (Qm)

–

87

Relative permittivity maximum temperature (Tm)

°C

285

The ceramic composition designated as 0.88BNKT– 0.12ST displayed the highest d33 and kp values coupled with a large KT. Therefore, despite the higher KT values attained with 0.16ST and 0.20ST, the one having 0.12ST was chosen as the optimized composition. The data on physical and electrical properties of the mentioned composition is given in Table 1. It should be noted that the effective electrical properties per unit weight of lead-free piezoelectrics are almost two times higher than their actual values since the density of lead-free piezoelectric materials are approximately half of the density of their lead-based counterparts.

4 Conclusions

Fig. 7 Temperature dependence of dielectric constant of unpoled ceramics, measured at 1 Hz between 25 and 400 °C

The relative permittivity maximum temperature of the undoped BNKT ceramic was reported as 280–300 °C [10, 12, 18]. The maximum dielectric constant of *6000 was obtained for the sample with 12 mol% ST. The dielectric curves exhibit broad transition peaks with increasing ST doping around Td and Tm, indicating the characteristics of diffuse phase transition, which implies a relaxor-type phase transition. Relaxation-like response of ST doped ceramics was attributed to the compositional fluctuation and/or inhomogeneous crystal structure at the nanometer scale. The reason for these structural changes were explained on the basis of unit cell distortion, dipolar moment variation, and strain generation in the lattice due to Sr2? incorporation at the A-site [19, 25, 27]. Tm of BNKT ceramic was 307 °C and decreased to a value of 277 °C at the maximum doping level of 20 mol% ST (inset in Fig. 7). Similar behavior in the relative permittivity maximum temperature change with ST doping has been observed in BNT-based lead-free piezoelectrics [21, 24].

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Effect of SrTiO3 doping on phase structure and electromechanical properties of (1 - x)[0.80BNT–0.20BKT]– (x)ST lead-free ceramics have been studied. Ceramic samples with very high density and pure perovskite structure were synthesized. SrTiO3 doped samples exhibited good piezoelectric and dielectric properties in the vicinity of the phase coexistence region. The phase structure shifted from a tetragonal dominant coexistence region to single rhombohedral symmetry with increasing ST content. In addition, ST doping also caused a gradual increase in the dielectric constant from *1000 to 1800. The examination of changes in dielectric and piezoelectric properties of doped BNKT samples leads to the determination of optimum composition as 0.88BNKT–0.12ST. Optimum piezoelectric and dielectric properties of KT = 1520, d33 = 205 pC/N and kp = 0.36 have been obtained for the ceramic with x = 0.12, together with a high Tm (285 °C). These results suggest that such a material system is a promising lead-free candidate for piezoelectric actuator applications. Acknowledgments This work was partly supported by the Scientific Research Foundation (BAP) of Selcuk University (Project No: 13601227).


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