- Email: [email protected]
Structural and electrical properties of La3+-doped Na0.5Bi4.5Ti4O15-Bi4Ti3O12 inter-growth high temperature piezoceramics
Ceramics International 43 (2017) 6446–6452
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Structural and electrical properties of La3+-doped Na0.5Bi4.5Ti4O15Bi4Ti3O12 inter-growth high temperature piezoceramics
MARK
⁎
Yalin Jiang, Xiangping Jiang , Chao Chen, Yunjing Chen, Xingan Jiang, Na Tu, Xiang Xia, Yuhan Luo, Sheng Zhu Jiangxi Key Laboratory of Advanced Ceramic Materials, Department of Material Science and Engineering, Jing de zhen Ceramic Institute, 333001 Jing de zhen, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Ceramics Structural Piezoelectric materials Na0.5Bi4.5Ti4O15-Bi4Ti3O12
New lead-free inter-growth piezoelectric ceramics, Na0.5Bi8.5-xLaxTi7O27 (NBT-BIT-xLa, 0.00≤x≤1.00), were prepared by the conventional solid-state method. Structural and electrical properties of NBT-BIT-xLa were studied. All the NBT-BIT-xLa samples exhibited a single inter-growth structured phase. XRD and Raman spectroscopy revealed a reduced orthorhombicity, which strongly supports the variation of dielectric and ferroelectric properties. Plate-like grains were found to decrease with the increasing x contents. Impedance spectra analysis indicated that oxygen vacancy defects dominated the contributions to the electrical conductivity. The increased activation energies for dc conductivity evidenced the reduction of oxygen vacancy concentration after La substitution, inducing the enhancement in piezoelectric constant (d33) and remanent polarization (2Pr). The studies of thermal depoling indicated that the optimal d33 of NBT-BIT-0.50La ceramics still remained 22 pC/N at 500 °C, implying that this ceramics could be potentially applied into high temperature devices.
1. Introduction Bismuth layered structure ferroelectrics (BLSFs), also known as Aurivillius family of oxides, have been considered as a candidate for the high-temperature ferro/piezo-electric devices [1–3]. The crystal structure of BLSFs is composed of pseud-perovskite layers (Am-1BmO3m 2and bismuth oxide (Bi2O2)2+ layers alternating arrangement +12) along the c-axis. Their formulas are usually expressed as (Bi2O2)2+(Am21BmO3m+1) [4,5], wherein A cations are in 12-fold coordination, and B cations are in 6-fold coordination. The inter-growth could be composed of two known BLSFs formula along the c-axis in a combination manner [6]. Na0.5Bi4.5Ti4O15-Bi4Ti3O12 (NBT-BIT) is a typical inter-growth BLSFs, which is a regular inter-growth of Bi4Ti3O12 (m=3) and Na0.5Bi4.5Ti4O15 (m=4). NBT-BIT possesses superior ferroelectric property and high Curie temperature (about 650 °C), but a low d33 (≤10 pC/N) and relatively high dielectric loss at temperature above 500 °C [7]. Due to the requirements for the high temperature ferro/ piezo-electric application, higher d33 and low dielectric losses are significant considerations for NBT-BIT [8]. Moreover, oxygen vacancy usually causes the increase in electrical conductivity and the reduction in ferro/piezo-electric properties [9,10]. Thus, suppressing oxygen
⁎
vacancy concentration is also an urgent problem in BLSFs. Currently, A/B-site or mixture modifications with rare-earth ions has been reported to be an effective method to enhance electric properties and suppress oxygen vacancy of BLSFs. For instance, Fei et al. reported that La3+-doping inter-growth structured CaBi4Ti4O15-Bi4Ti3O12 at A-site exhibited a lager d33 of 23.8 pC/N and a lower dielectric loss [11]. With La3+ substitution for Bi3+, the SrBi1.7La0.3Nb2O9 showed the improved remanent polarization and electrical field-induced strain [12]. Long et al. reported the substitution of La3+ for Bi3+ in Na0.5Bi2.5Nb2O9 at Asite could increase 2Pr and d33 owing to the reduced oxygen vacancy concentrations [13]. However, there are few reports on their electrical properties for La3+ doped NBT-BIT systems. Hence, in this study, Na0.5Bi8.5-xLaxTi7O27 (NBT-BIT-xLa) were prepared by the conventional solid-state method, and the influence of La3+-doped on crystal structure, dielectric, ferroelectric and piezoelectric properties were systematically investigated. 2. Experimental Na0.5Bi8.5-xLaxTi7O27 (NBT-BIT-xLa) ceramics (with x=0.00, 0.25, 0.50, 0.75 and 1.00) were prepared by the conventional solid-state method. High purity Na2CO3 (99.8%), Bi2O3 (99.999%), TiO2
Corresponding author. E-mail address: [email protected] (X. Jiang).
http://dx.doi.org/10.1016/j.ceramint.2017.02.059 Received 26 October 2016; Received in revised form 6 February 2017; Accepted 14 February 2017 Available online 16 February 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 43 (2017) 6446–6452
Y. Jiang et al.
(99.99%), and La2O3 (99.95%) were used as raw materials. The chemical compositions were weighted accurately, mixed with alcohol ball milled in polyethylene bottles with ZrO2 balls for 24 h, and calcined at 800 °C for 6 h in air. Then, the calcinated NBT-BIT-xLa powders were ground and granulated with 5 wt% polyvinyl alcohol (PVA) binder. The granulated mixture under 18 MPa pressed into pellets of approximately 12 mm in diameter and about 1.2 mm in thickness. Finally, the pellets were sintered at 1050 °C for 4 h in the airtight crucibles. For electrical and dielectric property measurement, silver electrodes are coated on each surfaces of the pellets and heated at 800 °C for 20 min. The crystal structure for samples was obtained by X-ray diffraction (XRD) analysis (D8 Advanced, Bruker, Germany). Raman spectra were measured with an instrument (LabRAM HR800, Horiba Jobin Yvon, Lyon, France) under a 532 nm argon ion laser. The micro-structure characterization was conducted by a scanning electron microscope (FESEM, JSM-5610LV; JEOL, Japan). The dielectric spectra measurement was conducted with Agilent 4294 A impedance analyzer in a frequency ranging from 100 Hz~ 1 MHz. Impedance data (Z) was measured at the sweeping frequency ranging from 100 Hz to 1 MHz and at the temperature ranging from 410 °C to 590 °C. Impedance data are fitted by Zview software according to a designed equivalent circuit. The ferroelectric hysteresis loops were measured at 10 Hz frequency through automated loop tracer (TF analyzer 2000). The sample was poled in silicone oil at 180 °C under a dc electric field of 10– 14 kV cm−1 for 30 min. The piezoelectric constant d33 was determined by a piezo-d33 meter (ZJ-3A, China) at room temperature.
Fig. 2. Rietveld refinements for (a) NBT-BIT and (b) NBN-BIT-0.50La at room temperature.
performed with the orthorhombic I2cm space group [17], by using GSAS software. Fig. 2 shows refinement result of NBT-BIT and NBTBIT-0.50La, which well fits the observed data. The refinement index, Rwp, Rp and chi2 are 9.80%, 7.43% and 8.281 for NBT-BIT and 9.84%, 7.42% and 7.760 for NBT-BIT-0.50La, respectively. The refined lattice parameter and orthorhombicity for all compositions are listed in Table 1. In our past studies, the refined lattice parameter (c) is in agreement with the calculative result of HRTEM, and this image suggests that the inter-growth structure consists of Bi4Ti3O12 with m=3 and Na0.5Bi4.5Ti4O15 with m=4 alternating arrangement [18]. According to A. Chakrabarti et. [19], the orthorhombicity is calculated by the formula of 2(a-b)/(a+b), which can be evaluated by the structural distortion of pseudo-perovskite blocks. From Table 1, it is observed that the orthorhombicity gradually reduces with La3+ substitution. Thus, the decreased orthorhombicity and split diffraction peaks (201)/(021) indicate that structural distortion is weakened with the increasing La3+-doping content in NBT-BIT-xLa system. Fig. 3(a) shows the Raman patterns of NBT-BIT-xLa ceramics from 50 to 1000 cm−1 under 532 nm laser source at the room temperature. To get the accurate peak positions for various modes, the Raman spectra fitted by the Lorentzian as their individual modes are shown in Fig. 3. In this study, the curve is decomposed into ten Lorentzian peak labeled as v1 to v10, respectively. Usually, Raman bands below 200 cm−1 are associated with the heavier mass ions. For BLSFs, v1 mode corresponds to the vibrations of Bi3+ cations in the (Bi2O2)2+ layer, while v2 and v3 modes originate from perovskite A-site ions [20– 22]. Raman bands above 200 cm−1 are related to the vibrations of TiO6 octahedra. The most prominent mode v5 is associated with the torsional bending vibration of TiO6 octahedra, which can be expressed as F2g symmetry. v4 and v6 modes locate around mode v5 as a weak shoulder, and correspond to F2u and F1u symmetry, respectively [23– 25]. These modes are related to the distortion of TiO6 octahedra and
3. Results and discussion The XRD patterns of NBT-BIT-xLa (0.00≤x≤1.00) ceramics measured at the 2θ angle of (a) 10°–60° and (b) 29°–34° are shown in Fig. 1. All diffraction patterns for samples can be indexed into the standard card PDF#32-1044 [Na0.5Bi8.5Ti7O27] and the strongest diffraction peak is (118) direction, which suggests that pure intergrowth NBT-BIT-xLa ceramics are obtained and there are no impurities found [14]. Moreover, it can be seen from Fig. 1(b) that the diffraction peaks of NBT-BIT-xLa ceramics show there is no obvious shift with the increasing La3+-doping concentration, which is attributed to the almost similar ionic radius of La3+(1.160 Å, 12CN) compared to Bi3+(1.170 Å, 12CN) [15]. In addition, the split diffraction peaks (201)/ (021) can be an indicator of orthorhombic symmetry [16]. Interestingly, the split diffraction peaks (201)/(021) becomes relatively weaker and nearly disappears with the increasing La3+-doping substitution, which suggests the structural symmetry transition from orthorhombic to tetragonal. To further investigate the structural distortion of NBT-BIT-xLa ceramics, XRD Rietveld refinements were
Table 1 Lattice parameters of NBT-BIT-xLa ceramics.
Fig. 1. (a) XRD patterns of NBT-BIT-xLa ceramics from 10° to 60° and (b) XRD patterns for the (118), (201) and (021) peaks from 29° to 34°.
6447
compound
a (Å)
b (Å)
c (Å)
V (Å3)
orthorhombicity
NBT-BIT NBT-BIT-0.25La NBT-BIT-0.50La NBT-BIT-0.75La NBT-BIT-1.00La
5.44623 5.44520 5.44430 5.42224 5.42640
5.41432 5.41777 5.42177 5.42782 5.43025
73.7326 73.7551 73.6990 73.6443 73.6470
2174.20 2175.83 2175.42 2167.42 2170.14
0.00588 0.00505 0.00415 0.00103 0.00071
Ceramics International 43 (2017) 6446–6452
Y. Jiang et al.
Fig. 3. Raman patterns of NBT-BIT-xLa ceramics and fitted by Lorentzian respectively.
doping. In addition, v7 and v8 modes gradually merge into one mode and v9 mode disappears simultaneously. These changes are rationally attributed to a decrease in TiO6 octahedra distortion [23–25], which well supports the XRD results and concludes that the structural distortion is decreased. To further understand the structural phase transition of the NBTBIT-xLa ceramics, Fig. 5 presents the temperature dependence of Raman spectra (23–800 °C) for the pure NBT-BIT. It can be observed that a remarkable softening phenomenon occurs in almost all the Raman modes with the increasing temperature. v4, v5 and v6 modes gradually overlap together, and v8 and v9 mode gradually disappear, which is extremely similar to the observations of La3+-doped NBT-BIT (see Fig. 3). According to Long et al. [27,28], temperature increasing can weaken the structural symmetry and distortion. Thus, this well confirms that the structural distortion is reduced after La3+ substitution in BIT-NBT system. Fig. 6 shows the surface SEM images of the NBT-BIT-xLa ceramics
they are silent in the ideal TiO6 octahedra. v7 and v8 modes derive from the opposing excursion of the external apical oxygen atoms of the octahedral and correspond to the Eg symmetry [26]. Furthermore, v10 mode is related to the stretching from the oxygen octahedron. Fig. 4 shows the Raman shift with various La3+-doping contents for NBT-BIT-xLa ceramics. At low frequency, v2 and v3 modes demonstrate a blue shift and v1 remains nearly unchanged with the increasing x from 0.00 to 0.75. It reveals that La3+ ions are substituting for the A sites of pseudo-perovskite layer for x≤0.75. However, with further La3+ doping, v1 mode begins to shift towards a low-frequency side and v3 mode continues blue shift. This indicates that La3+ ions begin to partly incorporate into the (Bi2O2)2+ layers for x≥0.75, which is similar to the reported phenomenon of La3+/Nd3+-doped in SrBi4Ti4O15-Bi4Ti3O12 [20,21]. Moreover, the softening of v2 mode can be observed from Fig. 3, implying a gradually reduced orthorhombicity in the NBT-BITxLa ceramics [25]. At high frequency, v4 mode was observed to shift towards high frequency and v6 mode weakens with the increasing La3+-
Fig. 4. Compositional dependence of Raman mode for NBT-BIT-xLa ceramics.
6448
Ceramics International 43 (2017) 6446–6452
Y. Jiang et al.
Fig. 7. Temperature dependence of the dielectric permittivity εr and dielectric loss tanδ of NBT-BIT-xLa ceramics at 100 kHz frequency.
Fig. 5. Various temperature (23–800 °C) Raman patterns of pure NBT-BIT ceramics.
found to obviously decrease with more stability as the increasing La3+ content, which is significance of high temperature ferro/piezo-electric application. The high-temperature tanδ in BLSFs is related to oxygen vacancy (VO) and electric conductivity [34]. Generally, VO is formed during sintered owing to volatilization of the Bi3+ ions, as shown in the follow Eq. (1).
with x=0.00–1.00. It can be observed that the grains show typical plate-like structure, which is caused by the higher anisotropic growth rate in the a-b plane, compared with the c-axis with the lower surface energies [29,30]. Oxygen vacancies could promote the transport for the oxygen ions in the crystal lattices and increase the speed of diffusion in the grain boundaries and grains [31]. In this paper, with the increasing La3+ content, the grain sizes are observed to gradually decrease probably due to the suppression of oxygen vacancies, which is discussed later. Fig. 7 shows the temperature dependence of the dielectric permittivity εr and dielectric loss tanδ at 100 kHz for NBT-BIT-xLa. It can be found that all the samples possess two dielectric peaks for the higher temperature (Cure temperature, Tc) and the lower temperature (Tm), which are corresponding to the phase transition of the BIT part and the ferroelectric to paraelectric phase transition, respectively [32]. As x increases from 0.00 to 1.00, Tc decreases significantly from 656 °C to 549 °C indicating the reduction of Tc after doping the La3+ ions. The decreasing of Tc is cause by the fact that the non-polarity La3+ ions substituted for the polarity Bi3+ ions with 6 s2 lone pair electrons in the A-site leading to the decreasing of the structural distortion [33], which is confirmed by the XRD and Raman results. Therefore, the decreasing of structural distortion makes contribution to decreased Tc for NBTBIT-xLa. Moreover, the high-temperature dielectric loss (tanδ) can be
•• 2B iBi + 3OO → 2V′′′ Bi + 3VO + B i2O3 ↑ .
(1)
The nonvolatile La3+ substitution for Bi3+ decreases the formation of VO. Thus, the suppressed VO contributes to the decreasing hightemperature tanδ, as like the trend has been reported by many investigators [34–36]. To further elucidate the change of VO concentrations in La3+-doped ceramics, complex impedance spectroscopy studies were invoked. The impedance characteristics of ceramics can be segregated into three different contributions of electrical components corresponding to grain, grain boundary and electrode interface [37,38]. However, in our studies a simulation of complex impedance was conducted with the designed (CR)(CQR) equivalent circuit for NBT-BIT ceramics at 490 °C (see Fig. 8a), wherein C is a capacitance, R is a resistant and Q is a constant phase element (CPE) which is an element to express the electrical behavior of space charge carries [38]. The smaller fitting semicircle in the high frequency is associated with the grain effect and
Fig. 6. SEM images of the surface of the NBT-BIT-xLa ceramics: (a) x=0.00 (b) x=0.25 (c) x=0.50 (d) x=0.75 (f) x=1.00.
6449
Ceramics International 43 (2017) 6446–6452
Y. Jiang et al.
Fig. 8. Complex impedance plots corresponding to NBT-BIT ceramics at 490 °C (a), complex impedance plots of NBT-BIT-xLa ceramics at different temperature: (b) NBT-BIT, (c) NBTBIT-0.25La, (d) NBT-BIT-0.50La, (e) NBT-BIT-0.75La and (f) NBT-BIT-1.00La.
linear and calculated to be in the range of 1.04–1.39 eV, which corresponds to the required energy to create and move VO defects [34,40]. The obtained results indicate that VO provides the dominating contributions to the electrical conductivity. Moreover, in Fig. 8(f) a deviation in the Arrhenius plot is observed, which corresponds to Curie transition. The EA (below Tc) gradually increases with the increasing La3+-doping contents, which can be explained by the fact that the stable La3+ substituting for the Bi3+ results in the reduction in VO concentrations. Fig. 9 shows the ferroelectric hysteresis (P-E) loops of NBT-BIT-xLa ceramics measured at 10 Hz and room temperature. However, the P-E hysteresis loop has not reached saturation under the condition of applying 100 kV/cm voltage, which is attributed to hard alignment of the ferroelectric domain of NBT-BIT-xLa to saturation polarization at room temperature [41]. From Fig. 9, it can be seen that the remanent polarization (2Pr) increases as x≤0.50, then decreases with the increasing La3+ substitution. The largest 2Pr value of NBT-BIT0.50La ceramics (about 10.3 μC/cm2) is three times more than that
the bigger one in the low frequency is assigned to the grain boundary effect. Fitting results of complex impedance indicate that the grain boundary effect dominates the impedance behaviors at the entire measuring temperature range. Fig. 8 shows the complex impedance plots of NBT-BIT-xLa ceramics at various temperatures, and all the curves can be well fitted into the (CR)(CQR) equivalent circuits. With the increasing temperature the observed curve gradually bends toward to the real axis (Z′), indicating the resistance gradual decreased [39]. Moreover, the resistance obviously increases with the introducing La3+, which is probably attributed to the decreased VO-related carries concentrations. The activation energy (EA) for dc conductivity can be expressed by the Arrhenius relations:
σ = σ0 exp( − EA / kT )
(2)
wherein σ0 is the pre-exponential factor, k is the Boltzmann constant, the EA is the activation energy and the T is absolute temperature [34,35,40]. As shown in insets, the activation energy EA is fitted by ion 6450
Ceramics International 43 (2017) 6446–6452
Y. Jiang et al.
annealing. The d33 for the optimal NBT-BIT-0.50 compositions remains unchanged until 500 °C, indicating that this NBT-BIT-0.50 La ceramics can be applied for relatively high temperature devices. 4. Conclusion Structural and electrical properties of NBT-BIT-xLa ceramics were studied. XRD and Raman spectroscopy revealed a reduced orthorhombicity, which well supports variations of dielectric and ferroelectric properties. In addition, it was found that La3+ partly incorporated into the (Bi2O2)2+ layers for x above 0.75. The Tc obviously decreased from 656 to 549 °C. The La3+ substitution for Bi3+ resulted in a reduction in oxygen vacancy and a decrease in grain size. The suppressed oxygen vacancies contributed to the increasing of the electrical resistivity, reduced the dc conduction and enhanced 2Pr. The value of d33 and 2Pr simultaneous increased at first and then decreased, and the optimal electrical properties was obtained in the ceramics with x=0.50. Thermal depoling studies indicated that the optimal d33 of NBT-BIT-0.50La ceramics still remained 22 pC/N at 500 °C, implying that this ceramics could be potentially applied into high temperature devices.
Fig. 9. Ferroelectric hysteresis loops of NBT-BIT-xLa ceramics at room temperature.
Acknowledgments This work is financially supported by the National Natural Science Foundation of China (51562014, 51262009, 51602135), Natural Science Foundation of Jiangxi, China (20133ACB20002, 20142BAB216009), the Foundation Provincial Department of Education (GJJ150931 and GJJ150911) and innovation training program of Jingdezhen Ceramic Institute (212050-008), partially sponsored by Foundation of Training Academic and Technical Leaders for Main Majors of Jiangxi (Grant No. 2010DD00800). References [1] H.X. Yan, H.T. Zhang, R. Ubic, M.J. Reece, J. Liu, Z. Shen, Z. Zhang, A lead-free high Curie point ferroelectric ceramic, CaBi2Nb2O9, Adv. Mater. 17 (2005) 1261–1265. [2] H. Yan, H. Zhang, M.J. Reece, X. Dong, Thermal depoling of high Curie point Aurivillius phase ferroelectric ceramics, Appl. Phys. Lett. 87 (2005) 082911. [3] X.P. Jiang, Q. Yang, S.L. Zhou, C. Chen, Y. Chen, N. Tu, Z.D. Yu, Microstructure and properties of high-temperature materials (1-x)Na0.5Bi2.5Nb2O9-xLiNbO3, J. Am. Ceram. Soc. 94 (2011) 1109–1113. [4] D. Peng, X. Wang, C. Xu, X. Yao, J. Lin, T. Sun, Bright upconversion emission, increased Tc, enhanced ferroelectric and piezoelectric properties in Er-doped CaBi4Ti4O15 multifunctional ferroelectric oxides, J. Am. Ceram. Soc. 96 (2013) 184–190. [5] X. Wang, X. Jiang, H. Jiang, J. Jiang, Effects of B-site Co2O3 doping on microstructure and electrical properties of Na0.25K0.25Bi2.5Nb2O9 ceramics, J. Alloy. Compd. 646 (2015) 528–531. [6] G. Parida, J. Bera, Electrical properties of niobium doped Bi4Ti3O12-SrBi4Ti4O15, intergrowth ferroelectrics, Ceram. Int. 40 (2014) 3139–3144. [7] A. Yokoi, J. Sugishita, Ferroelectric properties of mixed bismuth layer-structured Na0.5Bi8.5Ti7O27, ceramic and SrxNa0.5−x/2Bi8.5-x/2Ti7O27, solid solutions, J. Alloy. Compd. 452 (2008) 467–472. [8] S. Zhang, F. Yu, Piezoelectric materials for high temperature sensor, J. Am. Ceram. Soc. 94 (2011) 3153. [9] B.H. Park, S.J. Hyun, S.D. Bu, T.W. Noh, J. Lee, H.D. Kim, T.H. Kim, W. Jo, Differences in nature of defects between SrBi2Ta2O9 and Bi4Ti3O12, Appl. Phys. Lett. 74 (1999) 1907. [10] S.D. Bu, B.H. Park, B.S. Kang, S.H. Kang, T.W. Noh, W. Jo, Influence of the laser fluence on the electrical properties of pulsed-laser-deposited SrBi2Ta2O9 thin films, Appl. Phys. Lett. 75 (1999) 1155. [11] L. Fei, Z. Zhou, S. Hui, X. Dong, Y. Li, Structure and electric properties of lanthanum-doped CaBi4Ti4O15-Bi4Ti3O12 intergrowth ferroelectric, Mater. Lett. 156 (2015) 165–168. [12] P.Y. Fang, H.Q. Fan, J. Li, F.J. Liang, Lanthanum induced larger polarization and dielectric relaxation in Aurivillius phase SrBi2−xLaxNb2O9 ferroelectric ceramics, J. Appl. Phys. 107 (2010) 064104. [13] C.B. Long, H.Q. Fan, M.M. Li, High temperature Aurivillius piezoelectrics: the effect of (Li, Ln) modification on the structure and properties of (Li,Ln)0.06(Na,Bi)0.44Bi2Nb2O9 (Ln˭Ce, Nd, La and Y), Dalton Trans. 42 (2013) 3561. [14] C.M. Wang, J.F. Wang, High performance Aurivillius phase sodium-potassium bismuth itanate lead-free piezoelectric ceramics with lithium and cerium modification, Appl. Phys. Lett. 89 (2006) 202905.
Fig. 10. Temperature dependence of piezoelectric constant (d33) of NBT-BIT-xLa ceramics.
of the pure one (about 3.0 μC/cm2). According to Anita et al. [42], VO defects act as space charge and pin the domain wall, which easily deteriorates 2Pr. As mentioned in Fig. 8, the stable La3+ substitution for the Bi3+ results in the reduction in VO concentrations, which contributes to the increase in 2Pr. However, it is noted from Figs. 3 and 5 that La3+ substituting for Bi3+ also decreases the lattice distortion, which can reduce Pr to some extent [27]. Thus, 2Pr values largely depend on the competition effect between VO and structural distortion. In the low La3+-doped concentration, VO dominants the increase 2Pr. With the further increasing La3+-doping concentration, the lattice distortion is significantly decreased, which dominants the deterioration of 2Pr. So, 2Pr slightly decreases when x is at 0.50. Especially, as mentioned by Fig. 4, above x≥0.75 La3+ ions partly enter into the (Bi2O2)2+ layer causing the dramatic relaxation of lattice distortion, which well supports the decrease of 2Pr as x≥0.75. Similar phenomenon has been proved in the SrBi4Ti4O15-Bi4Ti3O12 [20,21]. In addition, the value of piezoelectric constant (d33) for NBT-BIT-xLa ceramics were measured at room temperature. NBT-BIT-0.50La sample achieved the optimal d33 (23 pC/N), which is twice more than that of NBT-BIT (10 pC/N). Interestingly, the values of d33 and 2Pr vary similarly with different La3+-doping content (see inset of Fig. 9), which can be well expressed by the phenomenological theory d33=2ε0Q11Pr [43]. Moreover, the increased resistivity in La3+-doping ceramics can be poled under larger applied electric fields [44]. Thus, the significant increase of d33 should be attributed to the enhanced 2Pr and resistivity. Fig. 10 shows the thermal depoling behavior of d33 of NBT-BIT-xLa ceramics. NBT-BIT-xLa ceramics possess good stability after thermal 6451
Ceramics International 43 (2017) 6446–6452
Y. Jiang et al. [15] T. Zheng, J.G. Wu, Enhanced piezoelectric activity in high-temperature Bi1-xySmxLayFeO3 lead-free ceramics, J. Mater. Chem. C 3 (2015) 3684. [16] S. Kumara, S. Kundua, D.A. Ochoab, J.E. Garciab, K.B.R. Varma, Raman scattering, microstructural and dielectric studies on Ba1-xCaxBi4Ti4O15 ceramics, Mater. Chem. Phys. 136 (2012) 680. [17] G. Parida, J. Bera, Effect of La-substitution on the structure, dielectric and ferroelectric properties of Nb modified SrBi8Ti7O27 ceramics, Mater. Res. Bull. 68 (2015) 155–159. [18] Y.L. Jiang, X.P. Jiang, C. Chen, Y.J. Chen, X.J. Jiang, N. Tu, Photoluminescence and electrical properties of Er3+-doped Na0.5Bi4.5Ti4O15-Bi4Ti3O12 inter-growth ferroelectric ceramics, Front. Mater. Sci. 11 (2017) 51–58. [19] A. Chakrabarti, J. Bera, Structure and relaxor behavior of BaBi4Ti4-xZrxO15 ceramics, Curr. Appl. Phys. 10 (2010) 574–579. [20] J. Zhu, X.B. Chen, J.H. He, J.C. Shen, Raman scattering investigations on lanthanum-doped Bi4Ti3O12-SrBi4Ti4O15 intergrowth ferroelectrics, J. Solid State Chem. 178 (2005) 2832–2837. [21] W. Wang, S.P. Gu, X.Y. Mao, X.B. Chen, Effect of Nd modification on electrical properties of mixed-layer Aurivillius phase Bi4Ti3O12-SrBi4Ti4O15, J. Appl. Phys. 102 (2007) 024102. [22] C.B. Long, Q. Chang, Y. Wu, W.F. He, Y.H. Li, H.Q. Fan, New layer-structured ferroelectric polycrystalline materials, Na0.5NdxBi4.5−xTi4O15: crystal structures, electrical properties and conduction behaviors, J. Mater. Chem. C 3 (2015) 8852. [23] X.A. Jiang, X.P. Jiang, C. Chen, Y.J. Chen, B.C. Zhang, Photoluminescence, structural, and electrical properties of Erbium-doped Na0.5Bi4.5Ti4O15 ferroelectric ceramics, J. Am. Ceram. Soc. 99 (2016) 1332–1339. [24] X.P. Jiang, X.A. Jiang, C. Chen, N. Tu, Y.J. Chen, B.C. Zhang, Effect of potassium sodium niobate (KNN) substitution on the structural and electrical properties of Na0.5Bi4.5Ti4O15 ceramics, J. Phys. D. Appl. Phys. 49 (2016) 125101. [25] J.J. Liu, G.T. Zou, Y.R. Jin, Raman scattering study of Na0.5Bi4.5Ti4O15 and its solid solutions, J. Phys. Chem. Solids 57 (1996) 1653–1658. [26] R. Bokolia, O.P. Thakur, V.K. Raic, S.K. Sharma, K. Sreenivas, Dielectric, ferroelectric and photoluminescence properties of Er3+ doped Bi4Ti3O12 ferroelectric ceramics, Ceram. Int. 41 (2015) 6055–6066. [27] C.B. Long, H.Q. Fan, P.R. Ren, Structure, phase transition behaviors and electrical properties of Nd substituted Aurivillius polycrystallines Na0.5NdxBi2.5−xNb2O9 (x=0.1, 0.2, 0.3, and 0.5), Inorg. Chem. 52 (2013) 5045–5054. [28] C.B. Long, H.Q. Fan, M.M. Li, Q. Li, Crystal structure and enhanced piezoelectric properties of scandium and bismuth-excess modified Aurivillius oxides, Na0.5xBi2.5+xNb2−xScxO9, Eur. J. Inorg. Chem. 32 (2013) 5622–5630. [29] Z.R. Yao, R.Q. Chu, Z.J. Xu, J.G. Hao, G.R. Li, Enhanced electrical properties of (Li,Ce) co-doped Sr(Na0.5Bi0.5)Bi4Ti5O18 high temperature piezoceramics, RSC Adv. 6 (2016) 33387. [30] P. Xiao, Q.J. Zheng, M.J. Tian, Y.Q. Guo, X.C. Wu, C.G. Xu, D.M. Lin, Enhanced
[31]
[32] [33]
[34]
[35] [36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
6452
ferroelectricity/piezoelectricity, bright blue/yellow emission and excellent thermal stability in Ca1−x(LiDy)x/2Bi4Ti4O15 lead-free multifunctional ceramics, RSC Adv. 6 (2016) 16387–16394. H.C. Du, L.J. Tang, S. Kaskel, Preparation, microstructure, and ferroelectric properties of Bi3.25La0.75Ti3−xMxO12 (M=Mo, W, Nb, V) ceramics, J. Phys. Chem. C 113 (2009) 1329–1339. W. Wang, D. Shan, J.B. Sun, X.Y. Mao, Aliovalent B-site modification on three- and four-layer Aurivillius intergrowth, J. Appl. Phys. 103 (2008) 044102. Z.G. Yi, Y.X. Li, J.T. Zeng, Q.B. Yang, D. Wang, Y.Q. Lu, Q.R. Yin, Lanthanum distribution and dielectric properties of intergrowth Bi5-xLaxTiNbWO15 ferroelectrics, Appl. Phys. Lett. 87 (2005) 202901. S. Kumar, K.B.R. Varma, Influence of lanthanum doping on the dielectric, ferroelectric and relaxor behaviour of barium bismuth titanate ceramics, J. Phys. D: Appl. Phys. 42 (2009) 075405. G. Parida, J. Beran, Electrical properties of niobium doped Bi4Ti3O12-SrBi4Ti4O15 intergrowth ferroelectrics, Ceram. Int. 40 (2014) 3139–3144. C.B. Long, H.Q. Fan, Effect of lanthanum substitution at A site on structure and enhanced properties of new Aurivillius oxide K0.25Na0.25La0.5Bi2Nb2O9, Dalton Trans. 41 (2012) 11046. F. Rehman, J.B. Li, Y.K. Dou, J.S. Zhang, Y.J. Zhao, M. Rizwan, S. Khalid, H.B. Jin, Dielectric relaxations and electrical properties of Aurivillius Bi3.5La0.5Ti2Fe0.5Nb0.5O12 ceramics, J. Alloy. Compd. 654 (2016) 315–320. F. Rehman, H.B. Jin, J.B. Li, Effect of reduction/oxidation annealing on dielectric relaxation and electrical properties of Aurivillius Na0.5Gd0.5Bi4Ti4O15 ceramics, RSC Adv. 6 (2016) 35102–35109. A. Moure, L. Pardo, Microstructure and texture dependence of the dielectric anomalies and dc conductivity of Bi3TiNbO9 ferroelectric ceramics, J. Appl. Phys. 97 (2005) 084103. Q. Xu, M.T. Lanagan, W. Luo, L. Zhang, J. Xie, H. Hao, M.H. Cao, Z.H. Yao, H.X. Liu, Electrical properties and relaxation behavior of Bi0.5Na0.5TiO3-BaTiO3 ceramics modified with NaNbO3, J. Eur. Ceram. Soc. 36 (2016) 2469–2477. D. Peng, H. Zou, C. Xu, X. Wang, X. Yao, Er doped BaBi4Ti4O15 multifunctional ferroelectrics: up-conversion photoluminescence, dielectric and ferroelectric properties, J. Alloy. Compd. 552 (2013) 463–468. A. Khokhar, P.K. Goyal, O.P. Thakur, A.K. Shukla, K. Sreenivas, Influence of lanthanum distribution on dielectric and ferroelectric properties of BaBi4xLaxTi4O15 ceramics, Mater. Chem. Phys. 152 (2015) 13–25. M.L. Chi, X.W. Xiao, K.W. Kwok, Photoluminescence, ferroelectric, dielectric and piezoelectric properties of Er-doped BNT-BT multifunctional ceramics, Appl. Surf. Sci. 336 (2015) 314–320. Z.Y. Zhou, Y.C. Li, S.P. Hui, X.L. Dong, Effect of tungsten doping in bismuthlayered Na0.5Bi2.5Nb2O9 high temperature piezoceramics, Appl. Phys. Lett. 104 (2014) 012904.
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Structural and electrical properties of La3+-doped Na0.5Bi4.5Ti4O15Bi4Ti3O12 inter-growth high temperature piezoceramics
MARK
⁎
Yalin Jiang, Xiangping Jiang , Chao Chen, Yunjing Chen, Xingan Jiang, Na Tu, Xiang Xia, Yuhan Luo, Sheng Zhu Jiangxi Key Laboratory of Advanced Ceramic Materials, Department of Material Science and Engineering, Jing de zhen Ceramic Institute, 333001 Jing de zhen, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Ceramics Structural Piezoelectric materials Na0.5Bi4.5Ti4O15-Bi4Ti3O12
New lead-free inter-growth piezoelectric ceramics, Na0.5Bi8.5-xLaxTi7O27 (NBT-BIT-xLa, 0.00≤x≤1.00), were prepared by the conventional solid-state method. Structural and electrical properties of NBT-BIT-xLa were studied. All the NBT-BIT-xLa samples exhibited a single inter-growth structured phase. XRD and Raman spectroscopy revealed a reduced orthorhombicity, which strongly supports the variation of dielectric and ferroelectric properties. Plate-like grains were found to decrease with the increasing x contents. Impedance spectra analysis indicated that oxygen vacancy defects dominated the contributions to the electrical conductivity. The increased activation energies for dc conductivity evidenced the reduction of oxygen vacancy concentration after La substitution, inducing the enhancement in piezoelectric constant (d33) and remanent polarization (2Pr). The studies of thermal depoling indicated that the optimal d33 of NBT-BIT-0.50La ceramics still remained 22 pC/N at 500 °C, implying that this ceramics could be potentially applied into high temperature devices.
1. Introduction Bismuth layered structure ferroelectrics (BLSFs), also known as Aurivillius family of oxides, have been considered as a candidate for the high-temperature ferro/piezo-electric devices [1–3]. The crystal structure of BLSFs is composed of pseud-perovskite layers (Am-1BmO3m 2and bismuth oxide (Bi2O2)2+ layers alternating arrangement +12) along the c-axis. Their formulas are usually expressed as (Bi2O2)2+(Am21BmO3m+1) [4,5], wherein A cations are in 12-fold coordination, and B cations are in 6-fold coordination. The inter-growth could be composed of two known BLSFs formula along the c-axis in a combination manner [6]. Na0.5Bi4.5Ti4O15-Bi4Ti3O12 (NBT-BIT) is a typical inter-growth BLSFs, which is a regular inter-growth of Bi4Ti3O12 (m=3) and Na0.5Bi4.5Ti4O15 (m=4). NBT-BIT possesses superior ferroelectric property and high Curie temperature (about 650 °C), but a low d33 (≤10 pC/N) and relatively high dielectric loss at temperature above 500 °C [7]. Due to the requirements for the high temperature ferro/ piezo-electric application, higher d33 and low dielectric losses are significant considerations for NBT-BIT [8]. Moreover, oxygen vacancy usually causes the increase in electrical conductivity and the reduction in ferro/piezo-electric properties [9,10]. Thus, suppressing oxygen
⁎
vacancy concentration is also an urgent problem in BLSFs. Currently, A/B-site or mixture modifications with rare-earth ions has been reported to be an effective method to enhance electric properties and suppress oxygen vacancy of BLSFs. For instance, Fei et al. reported that La3+-doping inter-growth structured CaBi4Ti4O15-Bi4Ti3O12 at A-site exhibited a lager d33 of 23.8 pC/N and a lower dielectric loss [11]. With La3+ substitution for Bi3+, the SrBi1.7La0.3Nb2O9 showed the improved remanent polarization and electrical field-induced strain [12]. Long et al. reported the substitution of La3+ for Bi3+ in Na0.5Bi2.5Nb2O9 at Asite could increase 2Pr and d33 owing to the reduced oxygen vacancy concentrations [13]. However, there are few reports on their electrical properties for La3+ doped NBT-BIT systems. Hence, in this study, Na0.5Bi8.5-xLaxTi7O27 (NBT-BIT-xLa) were prepared by the conventional solid-state method, and the influence of La3+-doped on crystal structure, dielectric, ferroelectric and piezoelectric properties were systematically investigated. 2. Experimental Na0.5Bi8.5-xLaxTi7O27 (NBT-BIT-xLa) ceramics (with x=0.00, 0.25, 0.50, 0.75 and 1.00) were prepared by the conventional solid-state method. High purity Na2CO3 (99.8%), Bi2O3 (99.999%), TiO2
Corresponding author. E-mail address: [email protected] (X. Jiang).
http://dx.doi.org/10.1016/j.ceramint.2017.02.059 Received 26 October 2016; Received in revised form 6 February 2017; Accepted 14 February 2017 Available online 16 February 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 43 (2017) 6446–6452
Y. Jiang et al.
(99.99%), and La2O3 (99.95%) were used as raw materials. The chemical compositions were weighted accurately, mixed with alcohol ball milled in polyethylene bottles with ZrO2 balls for 24 h, and calcined at 800 °C for 6 h in air. Then, the calcinated NBT-BIT-xLa powders were ground and granulated with 5 wt% polyvinyl alcohol (PVA) binder. The granulated mixture under 18 MPa pressed into pellets of approximately 12 mm in diameter and about 1.2 mm in thickness. Finally, the pellets were sintered at 1050 °C for 4 h in the airtight crucibles. For electrical and dielectric property measurement, silver electrodes are coated on each surfaces of the pellets and heated at 800 °C for 20 min. The crystal structure for samples was obtained by X-ray diffraction (XRD) analysis (D8 Advanced, Bruker, Germany). Raman spectra were measured with an instrument (LabRAM HR800, Horiba Jobin Yvon, Lyon, France) under a 532 nm argon ion laser. The micro-structure characterization was conducted by a scanning electron microscope (FESEM, JSM-5610LV; JEOL, Japan). The dielectric spectra measurement was conducted with Agilent 4294 A impedance analyzer in a frequency ranging from 100 Hz~ 1 MHz. Impedance data (Z) was measured at the sweeping frequency ranging from 100 Hz to 1 MHz and at the temperature ranging from 410 °C to 590 °C. Impedance data are fitted by Zview software according to a designed equivalent circuit. The ferroelectric hysteresis loops were measured at 10 Hz frequency through automated loop tracer (TF analyzer 2000). The sample was poled in silicone oil at 180 °C under a dc electric field of 10– 14 kV cm−1 for 30 min. The piezoelectric constant d33 was determined by a piezo-d33 meter (ZJ-3A, China) at room temperature.
Fig. 2. Rietveld refinements for (a) NBT-BIT and (b) NBN-BIT-0.50La at room temperature.
performed with the orthorhombic I2cm space group [17], by using GSAS software. Fig. 2 shows refinement result of NBT-BIT and NBTBIT-0.50La, which well fits the observed data. The refinement index, Rwp, Rp and chi2 are 9.80%, 7.43% and 8.281 for NBT-BIT and 9.84%, 7.42% and 7.760 for NBT-BIT-0.50La, respectively. The refined lattice parameter and orthorhombicity for all compositions are listed in Table 1. In our past studies, the refined lattice parameter (c) is in agreement with the calculative result of HRTEM, and this image suggests that the inter-growth structure consists of Bi4Ti3O12 with m=3 and Na0.5Bi4.5Ti4O15 with m=4 alternating arrangement [18]. According to A. Chakrabarti et. [19], the orthorhombicity is calculated by the formula of 2(a-b)/(a+b), which can be evaluated by the structural distortion of pseudo-perovskite blocks. From Table 1, it is observed that the orthorhombicity gradually reduces with La3+ substitution. Thus, the decreased orthorhombicity and split diffraction peaks (201)/(021) indicate that structural distortion is weakened with the increasing La3+-doping content in NBT-BIT-xLa system. Fig. 3(a) shows the Raman patterns of NBT-BIT-xLa ceramics from 50 to 1000 cm−1 under 532 nm laser source at the room temperature. To get the accurate peak positions for various modes, the Raman spectra fitted by the Lorentzian as their individual modes are shown in Fig. 3. In this study, the curve is decomposed into ten Lorentzian peak labeled as v1 to v10, respectively. Usually, Raman bands below 200 cm−1 are associated with the heavier mass ions. For BLSFs, v1 mode corresponds to the vibrations of Bi3+ cations in the (Bi2O2)2+ layer, while v2 and v3 modes originate from perovskite A-site ions [20– 22]. Raman bands above 200 cm−1 are related to the vibrations of TiO6 octahedra. The most prominent mode v5 is associated with the torsional bending vibration of TiO6 octahedra, which can be expressed as F2g symmetry. v4 and v6 modes locate around mode v5 as a weak shoulder, and correspond to F2u and F1u symmetry, respectively [23– 25]. These modes are related to the distortion of TiO6 octahedra and
3. Results and discussion The XRD patterns of NBT-BIT-xLa (0.00≤x≤1.00) ceramics measured at the 2θ angle of (a) 10°–60° and (b) 29°–34° are shown in Fig. 1. All diffraction patterns for samples can be indexed into the standard card PDF#32-1044 [Na0.5Bi8.5Ti7O27] and the strongest diffraction peak is (118) direction, which suggests that pure intergrowth NBT-BIT-xLa ceramics are obtained and there are no impurities found [14]. Moreover, it can be seen from Fig. 1(b) that the diffraction peaks of NBT-BIT-xLa ceramics show there is no obvious shift with the increasing La3+-doping concentration, which is attributed to the almost similar ionic radius of La3+(1.160 Å, 12CN) compared to Bi3+(1.170 Å, 12CN) [15]. In addition, the split diffraction peaks (201)/ (021) can be an indicator of orthorhombic symmetry [16]. Interestingly, the split diffraction peaks (201)/(021) becomes relatively weaker and nearly disappears with the increasing La3+-doping substitution, which suggests the structural symmetry transition from orthorhombic to tetragonal. To further investigate the structural distortion of NBT-BIT-xLa ceramics, XRD Rietveld refinements were
Table 1 Lattice parameters of NBT-BIT-xLa ceramics.
Fig. 1. (a) XRD patterns of NBT-BIT-xLa ceramics from 10° to 60° and (b) XRD patterns for the (118), (201) and (021) peaks from 29° to 34°.
6447
compound
a (Å)
b (Å)
c (Å)
V (Å3)
orthorhombicity
NBT-BIT NBT-BIT-0.25La NBT-BIT-0.50La NBT-BIT-0.75La NBT-BIT-1.00La
5.44623 5.44520 5.44430 5.42224 5.42640
5.41432 5.41777 5.42177 5.42782 5.43025
73.7326 73.7551 73.6990 73.6443 73.6470
2174.20 2175.83 2175.42 2167.42 2170.14
0.00588 0.00505 0.00415 0.00103 0.00071
Ceramics International 43 (2017) 6446–6452
Y. Jiang et al.
Fig. 3. Raman patterns of NBT-BIT-xLa ceramics and fitted by Lorentzian respectively.
doping. In addition, v7 and v8 modes gradually merge into one mode and v9 mode disappears simultaneously. These changes are rationally attributed to a decrease in TiO6 octahedra distortion [23–25], which well supports the XRD results and concludes that the structural distortion is decreased. To further understand the structural phase transition of the NBTBIT-xLa ceramics, Fig. 5 presents the temperature dependence of Raman spectra (23–800 °C) for the pure NBT-BIT. It can be observed that a remarkable softening phenomenon occurs in almost all the Raman modes with the increasing temperature. v4, v5 and v6 modes gradually overlap together, and v8 and v9 mode gradually disappear, which is extremely similar to the observations of La3+-doped NBT-BIT (see Fig. 3). According to Long et al. [27,28], temperature increasing can weaken the structural symmetry and distortion. Thus, this well confirms that the structural distortion is reduced after La3+ substitution in BIT-NBT system. Fig. 6 shows the surface SEM images of the NBT-BIT-xLa ceramics
they are silent in the ideal TiO6 octahedra. v7 and v8 modes derive from the opposing excursion of the external apical oxygen atoms of the octahedral and correspond to the Eg symmetry [26]. Furthermore, v10 mode is related to the stretching from the oxygen octahedron. Fig. 4 shows the Raman shift with various La3+-doping contents for NBT-BIT-xLa ceramics. At low frequency, v2 and v3 modes demonstrate a blue shift and v1 remains nearly unchanged with the increasing x from 0.00 to 0.75. It reveals that La3+ ions are substituting for the A sites of pseudo-perovskite layer for x≤0.75. However, with further La3+ doping, v1 mode begins to shift towards a low-frequency side and v3 mode continues blue shift. This indicates that La3+ ions begin to partly incorporate into the (Bi2O2)2+ layers for x≥0.75, which is similar to the reported phenomenon of La3+/Nd3+-doped in SrBi4Ti4O15-Bi4Ti3O12 [20,21]. Moreover, the softening of v2 mode can be observed from Fig. 3, implying a gradually reduced orthorhombicity in the NBT-BITxLa ceramics [25]. At high frequency, v4 mode was observed to shift towards high frequency and v6 mode weakens with the increasing La3+-
Fig. 4. Compositional dependence of Raman mode for NBT-BIT-xLa ceramics.
6448
Ceramics International 43 (2017) 6446–6452
Y. Jiang et al.
Fig. 7. Temperature dependence of the dielectric permittivity εr and dielectric loss tanδ of NBT-BIT-xLa ceramics at 100 kHz frequency.
Fig. 5. Various temperature (23–800 °C) Raman patterns of pure NBT-BIT ceramics.
found to obviously decrease with more stability as the increasing La3+ content, which is significance of high temperature ferro/piezo-electric application. The high-temperature tanδ in BLSFs is related to oxygen vacancy (VO) and electric conductivity [34]. Generally, VO is formed during sintered owing to volatilization of the Bi3+ ions, as shown in the follow Eq. (1).
with x=0.00–1.00. It can be observed that the grains show typical plate-like structure, which is caused by the higher anisotropic growth rate in the a-b plane, compared with the c-axis with the lower surface energies [29,30]. Oxygen vacancies could promote the transport for the oxygen ions in the crystal lattices and increase the speed of diffusion in the grain boundaries and grains [31]. In this paper, with the increasing La3+ content, the grain sizes are observed to gradually decrease probably due to the suppression of oxygen vacancies, which is discussed later. Fig. 7 shows the temperature dependence of the dielectric permittivity εr and dielectric loss tanδ at 100 kHz for NBT-BIT-xLa. It can be found that all the samples possess two dielectric peaks for the higher temperature (Cure temperature, Tc) and the lower temperature (Tm), which are corresponding to the phase transition of the BIT part and the ferroelectric to paraelectric phase transition, respectively [32]. As x increases from 0.00 to 1.00, Tc decreases significantly from 656 °C to 549 °C indicating the reduction of Tc after doping the La3+ ions. The decreasing of Tc is cause by the fact that the non-polarity La3+ ions substituted for the polarity Bi3+ ions with 6 s2 lone pair electrons in the A-site leading to the decreasing of the structural distortion [33], which is confirmed by the XRD and Raman results. Therefore, the decreasing of structural distortion makes contribution to decreased Tc for NBTBIT-xLa. Moreover, the high-temperature dielectric loss (tanδ) can be
•• 2B iBi + 3OO → 2V′′′ Bi + 3VO + B i2O3 ↑ .
(1)
The nonvolatile La3+ substitution for Bi3+ decreases the formation of VO. Thus, the suppressed VO contributes to the decreasing hightemperature tanδ, as like the trend has been reported by many investigators [34–36]. To further elucidate the change of VO concentrations in La3+-doped ceramics, complex impedance spectroscopy studies were invoked. The impedance characteristics of ceramics can be segregated into three different contributions of electrical components corresponding to grain, grain boundary and electrode interface [37,38]. However, in our studies a simulation of complex impedance was conducted with the designed (CR)(CQR) equivalent circuit for NBT-BIT ceramics at 490 °C (see Fig. 8a), wherein C is a capacitance, R is a resistant and Q is a constant phase element (CPE) which is an element to express the electrical behavior of space charge carries [38]. The smaller fitting semicircle in the high frequency is associated with the grain effect and
Fig. 6. SEM images of the surface of the NBT-BIT-xLa ceramics: (a) x=0.00 (b) x=0.25 (c) x=0.50 (d) x=0.75 (f) x=1.00.
6449
Ceramics International 43 (2017) 6446–6452
Y. Jiang et al.
Fig. 8. Complex impedance plots corresponding to NBT-BIT ceramics at 490 °C (a), complex impedance plots of NBT-BIT-xLa ceramics at different temperature: (b) NBT-BIT, (c) NBTBIT-0.25La, (d) NBT-BIT-0.50La, (e) NBT-BIT-0.75La and (f) NBT-BIT-1.00La.
linear and calculated to be in the range of 1.04–1.39 eV, which corresponds to the required energy to create and move VO defects [34,40]. The obtained results indicate that VO provides the dominating contributions to the electrical conductivity. Moreover, in Fig. 8(f) a deviation in the Arrhenius plot is observed, which corresponds to Curie transition. The EA (below Tc) gradually increases with the increasing La3+-doping contents, which can be explained by the fact that the stable La3+ substituting for the Bi3+ results in the reduction in VO concentrations. Fig. 9 shows the ferroelectric hysteresis (P-E) loops of NBT-BIT-xLa ceramics measured at 10 Hz and room temperature. However, the P-E hysteresis loop has not reached saturation under the condition of applying 100 kV/cm voltage, which is attributed to hard alignment of the ferroelectric domain of NBT-BIT-xLa to saturation polarization at room temperature [41]. From Fig. 9, it can be seen that the remanent polarization (2Pr) increases as x≤0.50, then decreases with the increasing La3+ substitution. The largest 2Pr value of NBT-BIT0.50La ceramics (about 10.3 μC/cm2) is three times more than that
the bigger one in the low frequency is assigned to the grain boundary effect. Fitting results of complex impedance indicate that the grain boundary effect dominates the impedance behaviors at the entire measuring temperature range. Fig. 8 shows the complex impedance plots of NBT-BIT-xLa ceramics at various temperatures, and all the curves can be well fitted into the (CR)(CQR) equivalent circuits. With the increasing temperature the observed curve gradually bends toward to the real axis (Z′), indicating the resistance gradual decreased [39]. Moreover, the resistance obviously increases with the introducing La3+, which is probably attributed to the decreased VO-related carries concentrations. The activation energy (EA) for dc conductivity can be expressed by the Arrhenius relations:
σ = σ0 exp( − EA / kT )
(2)
wherein σ0 is the pre-exponential factor, k is the Boltzmann constant, the EA is the activation energy and the T is absolute temperature [34,35,40]. As shown in insets, the activation energy EA is fitted by ion 6450
Ceramics International 43 (2017) 6446–6452
Y. Jiang et al.
annealing. The d33 for the optimal NBT-BIT-0.50 compositions remains unchanged until 500 °C, indicating that this NBT-BIT-0.50 La ceramics can be applied for relatively high temperature devices. 4. Conclusion Structural and electrical properties of NBT-BIT-xLa ceramics were studied. XRD and Raman spectroscopy revealed a reduced orthorhombicity, which well supports variations of dielectric and ferroelectric properties. In addition, it was found that La3+ partly incorporated into the (Bi2O2)2+ layers for x above 0.75. The Tc obviously decreased from 656 to 549 °C. The La3+ substitution for Bi3+ resulted in a reduction in oxygen vacancy and a decrease in grain size. The suppressed oxygen vacancies contributed to the increasing of the electrical resistivity, reduced the dc conduction and enhanced 2Pr. The value of d33 and 2Pr simultaneous increased at first and then decreased, and the optimal electrical properties was obtained in the ceramics with x=0.50. Thermal depoling studies indicated that the optimal d33 of NBT-BIT-0.50La ceramics still remained 22 pC/N at 500 °C, implying that this ceramics could be potentially applied into high temperature devices.
Fig. 9. Ferroelectric hysteresis loops of NBT-BIT-xLa ceramics at room temperature.
Acknowledgments This work is financially supported by the National Natural Science Foundation of China (51562014, 51262009, 51602135), Natural Science Foundation of Jiangxi, China (20133ACB20002, 20142BAB216009), the Foundation Provincial Department of Education (GJJ150931 and GJJ150911) and innovation training program of Jingdezhen Ceramic Institute (212050-008), partially sponsored by Foundation of Training Academic and Technical Leaders for Main Majors of Jiangxi (Grant No. 2010DD00800). References [1] H.X. Yan, H.T. Zhang, R. Ubic, M.J. Reece, J. Liu, Z. Shen, Z. Zhang, A lead-free high Curie point ferroelectric ceramic, CaBi2Nb2O9, Adv. Mater. 17 (2005) 1261–1265. [2] H. Yan, H. Zhang, M.J. Reece, X. Dong, Thermal depoling of high Curie point Aurivillius phase ferroelectric ceramics, Appl. Phys. Lett. 87 (2005) 082911. [3] X.P. Jiang, Q. Yang, S.L. Zhou, C. Chen, Y. Chen, N. Tu, Z.D. Yu, Microstructure and properties of high-temperature materials (1-x)Na0.5Bi2.5Nb2O9-xLiNbO3, J. Am. Ceram. Soc. 94 (2011) 1109–1113. [4] D. Peng, X. Wang, C. Xu, X. Yao, J. Lin, T. Sun, Bright upconversion emission, increased Tc, enhanced ferroelectric and piezoelectric properties in Er-doped CaBi4Ti4O15 multifunctional ferroelectric oxides, J. Am. Ceram. Soc. 96 (2013) 184–190. [5] X. Wang, X. Jiang, H. Jiang, J. Jiang, Effects of B-site Co2O3 doping on microstructure and electrical properties of Na0.25K0.25Bi2.5Nb2O9 ceramics, J. Alloy. Compd. 646 (2015) 528–531. [6] G. Parida, J. Bera, Electrical properties of niobium doped Bi4Ti3O12-SrBi4Ti4O15, intergrowth ferroelectrics, Ceram. Int. 40 (2014) 3139–3144. [7] A. Yokoi, J. Sugishita, Ferroelectric properties of mixed bismuth layer-structured Na0.5Bi8.5Ti7O27, ceramic and SrxNa0.5−x/2Bi8.5-x/2Ti7O27, solid solutions, J. Alloy. Compd. 452 (2008) 467–472. [8] S. Zhang, F. Yu, Piezoelectric materials for high temperature sensor, J. Am. Ceram. Soc. 94 (2011) 3153. [9] B.H. Park, S.J. Hyun, S.D. Bu, T.W. Noh, J. Lee, H.D. Kim, T.H. Kim, W. Jo, Differences in nature of defects between SrBi2Ta2O9 and Bi4Ti3O12, Appl. Phys. Lett. 74 (1999) 1907. [10] S.D. Bu, B.H. Park, B.S. Kang, S.H. Kang, T.W. Noh, W. Jo, Influence of the laser fluence on the electrical properties of pulsed-laser-deposited SrBi2Ta2O9 thin films, Appl. Phys. Lett. 75 (1999) 1155. [11] L. Fei, Z. Zhou, S. Hui, X. Dong, Y. Li, Structure and electric properties of lanthanum-doped CaBi4Ti4O15-Bi4Ti3O12 intergrowth ferroelectric, Mater. Lett. 156 (2015) 165–168. [12] P.Y. Fang, H.Q. Fan, J. Li, F.J. Liang, Lanthanum induced larger polarization and dielectric relaxation in Aurivillius phase SrBi2−xLaxNb2O9 ferroelectric ceramics, J. Appl. Phys. 107 (2010) 064104. [13] C.B. Long, H.Q. Fan, M.M. Li, High temperature Aurivillius piezoelectrics: the effect of (Li, Ln) modification on the structure and properties of (Li,Ln)0.06(Na,Bi)0.44Bi2Nb2O9 (Ln˭Ce, Nd, La and Y), Dalton Trans. 42 (2013) 3561. [14] C.M. Wang, J.F. Wang, High performance Aurivillius phase sodium-potassium bismuth itanate lead-free piezoelectric ceramics with lithium and cerium modification, Appl. Phys. Lett. 89 (2006) 202905.
Fig. 10. Temperature dependence of piezoelectric constant (d33) of NBT-BIT-xLa ceramics.
of the pure one (about 3.0 μC/cm2). According to Anita et al. [42], VO defects act as space charge and pin the domain wall, which easily deteriorates 2Pr. As mentioned in Fig. 8, the stable La3+ substitution for the Bi3+ results in the reduction in VO concentrations, which contributes to the increase in 2Pr. However, it is noted from Figs. 3 and 5 that La3+ substituting for Bi3+ also decreases the lattice distortion, which can reduce Pr to some extent [27]. Thus, 2Pr values largely depend on the competition effect between VO and structural distortion. In the low La3+-doped concentration, VO dominants the increase 2Pr. With the further increasing La3+-doping concentration, the lattice distortion is significantly decreased, which dominants the deterioration of 2Pr. So, 2Pr slightly decreases when x is at 0.50. Especially, as mentioned by Fig. 4, above x≥0.75 La3+ ions partly enter into the (Bi2O2)2+ layer causing the dramatic relaxation of lattice distortion, which well supports the decrease of 2Pr as x≥0.75. Similar phenomenon has been proved in the SrBi4Ti4O15-Bi4Ti3O12 [20,21]. In addition, the value of piezoelectric constant (d33) for NBT-BIT-xLa ceramics were measured at room temperature. NBT-BIT-0.50La sample achieved the optimal d33 (23 pC/N), which is twice more than that of NBT-BIT (10 pC/N). Interestingly, the values of d33 and 2Pr vary similarly with different La3+-doping content (see inset of Fig. 9), which can be well expressed by the phenomenological theory d33=2ε0Q11Pr [43]. Moreover, the increased resistivity in La3+-doping ceramics can be poled under larger applied electric fields [44]. Thus, the significant increase of d33 should be attributed to the enhanced 2Pr and resistivity. Fig. 10 shows the thermal depoling behavior of d33 of NBT-BIT-xLa ceramics. NBT-BIT-xLa ceramics possess good stability after thermal 6451
Ceramics International 43 (2017) 6446–6452
Y. Jiang et al. [15] T. Zheng, J.G. Wu, Enhanced piezoelectric activity in high-temperature Bi1-xySmxLayFeO3 lead-free ceramics, J. Mater. Chem. C 3 (2015) 3684. [16] S. Kumara, S. Kundua, D.A. Ochoab, J.E. Garciab, K.B.R. Varma, Raman scattering, microstructural and dielectric studies on Ba1-xCaxBi4Ti4O15 ceramics, Mater. Chem. Phys. 136 (2012) 680. [17] G. Parida, J. Bera, Effect of La-substitution on the structure, dielectric and ferroelectric properties of Nb modified SrBi8Ti7O27 ceramics, Mater. Res. Bull. 68 (2015) 155–159. [18] Y.L. Jiang, X.P. Jiang, C. Chen, Y.J. Chen, X.J. Jiang, N. Tu, Photoluminescence and electrical properties of Er3+-doped Na0.5Bi4.5Ti4O15-Bi4Ti3O12 inter-growth ferroelectric ceramics, Front. Mater. Sci. 11 (2017) 51–58. [19] A. Chakrabarti, J. Bera, Structure and relaxor behavior of BaBi4Ti4-xZrxO15 ceramics, Curr. Appl. Phys. 10 (2010) 574–579. [20] J. Zhu, X.B. Chen, J.H. He, J.C. Shen, Raman scattering investigations on lanthanum-doped Bi4Ti3O12-SrBi4Ti4O15 intergrowth ferroelectrics, J. Solid State Chem. 178 (2005) 2832–2837. [21] W. Wang, S.P. Gu, X.Y. Mao, X.B. Chen, Effect of Nd modification on electrical properties of mixed-layer Aurivillius phase Bi4Ti3O12-SrBi4Ti4O15, J. Appl. Phys. 102 (2007) 024102. [22] C.B. Long, Q. Chang, Y. Wu, W.F. He, Y.H. Li, H.Q. Fan, New layer-structured ferroelectric polycrystalline materials, Na0.5NdxBi4.5−xTi4O15: crystal structures, electrical properties and conduction behaviors, J. Mater. Chem. C 3 (2015) 8852. [23] X.A. Jiang, X.P. Jiang, C. Chen, Y.J. Chen, B.C. Zhang, Photoluminescence, structural, and electrical properties of Erbium-doped Na0.5Bi4.5Ti4O15 ferroelectric ceramics, J. Am. Ceram. Soc. 99 (2016) 1332–1339. [24] X.P. Jiang, X.A. Jiang, C. Chen, N. Tu, Y.J. Chen, B.C. Zhang, Effect of potassium sodium niobate (KNN) substitution on the structural and electrical properties of Na0.5Bi4.5Ti4O15 ceramics, J. Phys. D. Appl. Phys. 49 (2016) 125101. [25] J.J. Liu, G.T. Zou, Y.R. Jin, Raman scattering study of Na0.5Bi4.5Ti4O15 and its solid solutions, J. Phys. Chem. Solids 57 (1996) 1653–1658. [26] R. Bokolia, O.P. Thakur, V.K. Raic, S.K. Sharma, K. Sreenivas, Dielectric, ferroelectric and photoluminescence properties of Er3+ doped Bi4Ti3O12 ferroelectric ceramics, Ceram. Int. 41 (2015) 6055–6066. [27] C.B. Long, H.Q. Fan, P.R. Ren, Structure, phase transition behaviors and electrical properties of Nd substituted Aurivillius polycrystallines Na0.5NdxBi2.5−xNb2O9 (x=0.1, 0.2, 0.3, and 0.5), Inorg. Chem. 52 (2013) 5045–5054. [28] C.B. Long, H.Q. Fan, M.M. Li, Q. Li, Crystal structure and enhanced piezoelectric properties of scandium and bismuth-excess modified Aurivillius oxides, Na0.5xBi2.5+xNb2−xScxO9, Eur. J. Inorg. Chem. 32 (2013) 5622–5630. [29] Z.R. Yao, R.Q. Chu, Z.J. Xu, J.G. Hao, G.R. Li, Enhanced electrical properties of (Li,Ce) co-doped Sr(Na0.5Bi0.5)Bi4Ti5O18 high temperature piezoceramics, RSC Adv. 6 (2016) 33387. [30] P. Xiao, Q.J. Zheng, M.J. Tian, Y.Q. Guo, X.C. Wu, C.G. Xu, D.M. Lin, Enhanced
[31]
[32] [33]
[34]
[35] [36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
6452
ferroelectricity/piezoelectricity, bright blue/yellow emission and excellent thermal stability in Ca1−x(LiDy)x/2Bi4Ti4O15 lead-free multifunctional ceramics, RSC Adv. 6 (2016) 16387–16394. H.C. Du, L.J. Tang, S. Kaskel, Preparation, microstructure, and ferroelectric properties of Bi3.25La0.75Ti3−xMxO12 (M=Mo, W, Nb, V) ceramics, J. Phys. Chem. C 113 (2009) 1329–1339. W. Wang, D. Shan, J.B. Sun, X.Y. Mao, Aliovalent B-site modification on three- and four-layer Aurivillius intergrowth, J. Appl. Phys. 103 (2008) 044102. Z.G. Yi, Y.X. Li, J.T. Zeng, Q.B. Yang, D. Wang, Y.Q. Lu, Q.R. Yin, Lanthanum distribution and dielectric properties of intergrowth Bi5-xLaxTiNbWO15 ferroelectrics, Appl. Phys. Lett. 87 (2005) 202901. S. Kumar, K.B.R. Varma, Influence of lanthanum doping on the dielectric, ferroelectric and relaxor behaviour of barium bismuth titanate ceramics, J. Phys. D: Appl. Phys. 42 (2009) 075405. G. Parida, J. Beran, Electrical properties of niobium doped Bi4Ti3O12-SrBi4Ti4O15 intergrowth ferroelectrics, Ceram. Int. 40 (2014) 3139–3144. C.B. Long, H.Q. Fan, Effect of lanthanum substitution at A site on structure and enhanced properties of new Aurivillius oxide K0.25Na0.25La0.5Bi2Nb2O9, Dalton Trans. 41 (2012) 11046. F. Rehman, J.B. Li, Y.K. Dou, J.S. Zhang, Y.J. Zhao, M. Rizwan, S. Khalid, H.B. Jin, Dielectric relaxations and electrical properties of Aurivillius Bi3.5La0.5Ti2Fe0.5Nb0.5O12 ceramics, J. Alloy. Compd. 654 (2016) 315–320. F. Rehman, H.B. Jin, J.B. Li, Effect of reduction/oxidation annealing on dielectric relaxation and electrical properties of Aurivillius Na0.5Gd0.5Bi4Ti4O15 ceramics, RSC Adv. 6 (2016) 35102–35109. A. Moure, L. Pardo, Microstructure and texture dependence of the dielectric anomalies and dc conductivity of Bi3TiNbO9 ferroelectric ceramics, J. Appl. Phys. 97 (2005) 084103. Q. Xu, M.T. Lanagan, W. Luo, L. Zhang, J. Xie, H. Hao, M.H. Cao, Z.H. Yao, H.X. Liu, Electrical properties and relaxation behavior of Bi0.5Na0.5TiO3-BaTiO3 ceramics modified with NaNbO3, J. Eur. Ceram. Soc. 36 (2016) 2469–2477. D. Peng, H. Zou, C. Xu, X. Wang, X. Yao, Er doped BaBi4Ti4O15 multifunctional ferroelectrics: up-conversion photoluminescence, dielectric and ferroelectric properties, J. Alloy. Compd. 552 (2013) 463–468. A. Khokhar, P.K. Goyal, O.P. Thakur, A.K. Shukla, K. Sreenivas, Influence of lanthanum distribution on dielectric and ferroelectric properties of BaBi4xLaxTi4O15 ceramics, Mater. Chem. Phys. 152 (2015) 13–25. M.L. Chi, X.W. Xiao, K.W. Kwok, Photoluminescence, ferroelectric, dielectric and piezoelectric properties of Er-doped BNT-BT multifunctional ceramics, Appl. Surf. Sci. 336 (2015) 314–320. Z.Y. Zhou, Y.C. Li, S.P. Hui, X.L. Dong, Effect of tungsten doping in bismuthlayered Na0.5Bi2.5Nb2O9 high temperature piezoceramics, Appl. Phys. Lett. 104 (2014) 012904.