Effect of LiF addition on phase structure and ...

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CERAMICS INTERNATIONAL

Ceramics International 41 (2015) 4035–4041 www.elsevier.com/locate/ceramint

Effect of LiF addition on phase structure and piezoelectric properties of (Ba,Ca)(Ti,Sn)O3 ceramics sintered at low temperature Peng-Fei Zhou, Bo-Ping Zhangn, Lei Zhao, Li-Feng Zhu School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Received 3 October 2014; received in revised form 17 November 2014; accepted 18 November 2014 Available online 26 November 2014

Abstract In this study, a series of (Ba0.95Ca0.05)(Ti0.90Sn0.10)O3–xLiF (BCTS–xLiF) (0 rx r 6 mol%) lead-free piezoceramics were fabricated by a conventional solid-state reaction method. The incorporation of LiF could significantly improve the sinterability of BCTS ceramics by reducing the sintering temperature from 1480 1C to 1250 1C, where a relative density over 90% was achieved with grown grains of 5–20 μm. X-ray diffraction and Raman spectroscopy experiments revealed that BCTS–xLiF ceramics are composed of coexisting R–O–T phases at 0 rx r 1 and R–T phases at 2 rx r 6 at room temperature. Optimal piezoelectric properties of d33 ¼ 510 pC/N, kp ¼40.9%, εr ¼ 5370, tan δ¼ 0.019 and dS/ dE¼ 961 pm/V were obtained at x ¼5, which is attributed to the coexistence of R and T phases with an optimum ratio at room temperature and dense structure with large grains. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: C. Piezoelectric properties; BCTS; LiF addition; Low-temperature sintering

1. Introduction Pb(Zr,Ti)O3 (PZT) based piezoelectric ceramics have been widely used as electronic components due to their excellent piezoelectric properties [1]. However, it is respected to replace PZT ceramics with lead-free candidates owing to the growing concern with environmental pollution and human health problem. BaTiO3 (BT) ceramic was first discovered as a piezoelectric ceramic but now mostly used as a dielectric material rather than a piezoelectric material because of its inferior piezoelectric properties to PZT. Recently, surprising high d33 values have been obtained in BaTiO3 by microwave sintering, two-step sintering and template grain growth, which have high cost or complex processes for practical mass production [2–4]. Persistent efforts have been made to fabricate high performance BaTiO3 based ceramics by economical production routes in a series of modified systems such as (Ba,Ca)(Ti,Sn)O3 (BCTS), BaTiO3–11BaSnO3 and (Ba,Ca) n

Corresponding author. Tel./fax: þ 86 10 62334195. E-mail address: [email protected] (B.-P. Zhang).

http://dx.doi.org/10.1016/j.ceramint.2014.11.094 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

(Ti,Zr)O3 [5–14]. (Ba0.98Ca0.02)(Ti0.96Sn0.04)O3 ceramic sintered at 1500 1C has a high d33 of 510 pC/N [8], higher d33 of 568 pC/N and 670 pC/N were achieved in (Ba0.95Ca0.05) (Ti0.92Sn0.08)O3 and (Ba0.95Ca0.05)(Ti0.89Sn0.11)O3 [10,11], respectively. At present, the highest d33 reaches 697 pC/N in BaTiO3–11BaSnO3 ceramic at 40 1C [12]. Outstanding piezoelectric properties of these BCTS ceramics are owing to coexisting two or more phases, which can lead to the instability of the polarization state that makes the polarization direction easily rotated by external stress or electric field [5–14]. Despite of their high piezoelectric properties, dense BaTiO3-based ceramics must be sintered at high temperature of 1400– 1500 1C, which impedes practical applications. It is well known that LiF and LiF-containing additives are very effective in lowering sintering temperature (TS) and improving density for BaTiO3 ceramic [15,16]. High relative density over 98% was obtained for BaTiO3 ceramic sintered at 900 1C by adding 2 wt% LiF–SrCO3 [15]. Dense BaTiO3 ceramic with better properties (d33 ¼ 270 pC/N, kp ¼ 45%) was also obtained at 1100 1C by adding 4 mol% LiF [16]. No one has studied the LiF-additive effect on BCTS ceramics and the detailed

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sintering mechanism of LiF doped BCTS ceramics is still unclear so far. Herein, LiF as a sintering aid was introduced to (Ba0.95Ca0.05)(Ti0.90Sn0.10)O3 ceramics to lower TS and modify the piezoelectric properties, by laying an emphasis on the effect of LiF content on sintering mechanisms, phase structure and electrical properties of BCTS ceramics.

2. Experimental procedure (Ba0.95Ca0.05)(Ti0.90Sn0.10)O3–xLiF (BCTSLix) (x¼ 0, 1, 2, 3, 4, 5 and 6 mol%) ceramics were prepared by conventional solidstate reaction method using BaCO3, CaCO3, SnO2, TiO2 and LiF powders (499%, ShanTou Xilong Chemical Factory GuangDong, China) as raw materials. The weighed powders were mixed by using a planetary ball mill with anhydrous ethanol with 300 rpm for 4 h in a nylon jar with zirconia balls and then dried at 80 1C for 8 h in oven. After calcinating dried powders at 1250 1C for 4 h, LiF in the range from 0% to 6 mol% was mixed with calcinated BCTS powders and then ball-milled again. Remilled powders were dried at 80 1C for 8 h in oven and pressed into disks of 10 mm in diameter and 1.5 mm in thickness under 80 MPa using 2 wt% polyvinyl alcohol (PVA) as binder. After burning out PVA at 650 1C for 1 h in air, samples were cooled to room temperature and reheated with a rate of 5 1C/min to 1250 1C for soaking 4 h in air, and recooled to room temperature. Silver electrodes were pasted on the top and bottom surfaces of the sintered samples and fired at 600 1C for 30 min. The polarization was performed under an electric field of 3–4 kV/mm in silicone oil bath for 30 min. The phase structure was examined by X-ray diffraction (XRD: D/max-RB, Rigaku Inc., Japan) with a Cu Kα radiation (λ¼ 1.5416 Å) filtered through a Ni foil. Density of samples was determined by the Archimedes method. The microstructure of the sintered samples was observed by scanning electron microscope (FE-SEM: SUPRATM55, Carl Zeiss, Nakano, Japan). The shrinkage curves from 50 1C to 1250 1C for BCTS and BCTSLi4 ceramics were measured by a horizontal pushrod dilatometer (Netzsch DIL402PC, Selb, Germany) with a heating rate of 5 1C/min. Dielectric constant and dielectric loss were measured using a LCR meter with a rate of 3 1C/min from  20 to 100 1C (TH2828S; Changzhou Tonghui Electronics Co., Ltd. Changzhou, China) at 1 kHz. The piezoelectric property was measured using a quasi-static piezoelectric coefficient d33 testing meter (ZJ-3A, Institute of Acoustics, Chinese Academy of Sciences, Beijing, China). The planar electromechanical coupling coefficient kp was determined by resonance–antiresonance method using an Agilent 4294A (Hewlett-Packard, Palo Alto, CA) precision impedance analyzer. The electric field-induced strains were measured using an attachment onto the TF ANALYZER 1000 ferroelectric measuring system (aixACCT Systems GmbH, Denne-wartstrasse, Aachen, Germany). Ferroelectric hysteresis loops were measured using a ferroelectric tester (RT6000HVA; Radiant Technologies, Inc., Albuquerque, NM). Raman spectra were measured by the LabRAM HR800 (Horiba JobinYvon, Paris, France).

3. Results and discussion Fig. 1 shows SEM images (a–f) of thermally etched surface, dynamic sintering curves (g) and density (h) for BCTSLix ceramics as a function of x. The pristine BCTS ceramic sintered at 1250 1C has a porous microstructure with small grains of 0.5–2 μm (Fig. 1a) and low measured density and relative density of 4.66 g/cm3 and 78% (Fig. 1h), confirming that dense BCTS ceramic is difficult to be obtained below 1250 1C since the densification TS has so far been reported to be 1450–1540 1C [5–11]. After adding LiF, a developed dense microstructure with enlarged grains of 5–20 μm (Fig. 1b–f) and high relative density over 90% (Fig. 1h) was obtained, indicating an enhanced sinterability with a reduced TS of 200 1C at least in BCTS ceramics. The sintering mechanisms of BCTS and BCTSLi4 ceramics were studied by dynamic sintering curves as shown in Fig. 1g. BCTS starts to shrink at 1050 1C compared to that at 950 1C for BaTiO3 [17], confirming higher TS of BCTS than BaTiO3. Adding LiF produces a dramatically decreased shrink temperature to 700 1C for BCTSLi4 because of the formation of eutectic liquid phase LiTiO2 [18]. Beside, the shrinkage rate increases above 950 1C, which is attributed to generated oxygen vacancies (V O ) in the matrix phase as expressed Ti4 þ =Sn4 þ

0

by the defect equation: Li þ -Li″Ti=Sn þ 32 V O . The substitution mechanisms of Li þ for high valence Ti4 þ /Sn4 þ will be discussed later in XRD patterns. The dense microstructure with enlarged grains is attributable to the emergence of V O and the intermediately formed eutectic liquid phase LiTiO2 which would enter finally the lattice. Because of the evaporate of F  is more intense than Li þ [19], and Li þ can lead to the formation of eutectic liquid phase LiTiO2 and the generation of oxygen vacancy, Li þ is more essential for reducing sintering temperature. XRD patterns in Fig. 2 exhibit a single perovskite structure for all BCTSLix samples without any trace of secondary phase in the detectable limit of XRD, suggesting that Ca2 þ , Sn4 þ and Li þ all diffused into BaTiO3 lattice to form a solid solution in the studied compositions. Compared to the case for pristine BCTS ceramic, the diffraction peaks for LiF-added samples shift gradually to low angles with increasing x as shown in Fig. 2b, indicating an enlarged lattice, which may be attributed to the substitution of Li þ (0.76 Å) for Sn4 þ (0.71 Å) or Ti4 þ (0.68 Å) at B-sites [19,20] and the smaller binding force with O2  of Li þ than that of Ti4 þ or Sn4 þ . All BCTSLix ceramics have a broad peak around 451, which is similar to BCTS–CuOx system [21], indicating the absence of a single rhombohedral (R), pseudo-cubic (PC) or cubic (C) phase since all have a symmetric singlet peak around this angle. Two peaks with intensity ratio of greater than 1 between low angle and high one appear around 83.31 in Fig. 2c, implying that a single tetragonal (T) or orthorhombic (O) phase is impossible to be presented solely, since T is singlet and O is doublet with the same intensity around 83.31, respectively [21]. So we speculate that all BCTSLix ceramics should consist of two or more symmetries at room temperature because of the

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Fig. 1. SEM images (a–f) of thermally etched surface, dynamic sintering curves (g) and density (h) for BCTSLix ceramics (a) x¼ 0; (b) x¼ 1; (c) x¼ 3; (d) x¼ 4; (e) x¼ 5; (f) x ¼6.

misfit between XRD patterns and the individual characteristics of PC, C, R, O and T symmetries. The phase structure of BCTSLix ceramics was further defined by curves of dielectric permittivity versus temperature (εr–T) and room-temperature Raman spectra. The main features of εr–T curves shown in Fig. 3a are: (i) only one phase transition point for pristine BCTS ceramic; (ii) two phase

transition points for samples at 1 r xr 6. The results reveal that TC (TC–T), TT–O and TO–R in the pristine BCTS ceramic merge to one point [21]. When LiF is added, the phase transition point separates into TC and Tx as shown in the inset of Fig. 3b. Apart from the constant TC at the vicinity of about 50 1C, Tx decreases rapidly from 48 1C to 22 1C as x increases from 1 to 2, and shows a slight falling as further increasing x.

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Fig. 2. X-ray diffraction patterns for BCTSLix ceramics (a) and enlarged XRD pattern at 44–461 (b) and 83–841 (c).

Fig. 4. Raman spectrum of BCTSLix ceramics (a) and enlarged spectrum at 100–350 cm  1 (b) measured at room temperature.

Fig. 3. Temperature dependence of dielectric constant (a), dielectric loss (b) and phase transition temperature (inset in (b)) at 1 kHz for BCTSLix ceramics.

Nevertheless, it is still difficult to exactly determine the type of phase transition(s) at Tx just by XRD and εr–T curves. Fig. 4 shows Raman spectra for BCTSLix ceramics at room temperature. Main modes at 160, 198, 250, 302, 480, 515 and 720 cm  1 are observed at 0 r xr 1 in Fig. 4a; all are the feature of R phase especially in low-wave number region at about 160 and 198 cm  1 [22,23]. Most modes are also

observed in O phase (including modes at 250, 302, 480, 515 and 720 cm  1) and T phase (including modes at 250, 302, 515 and 720 cm  1) [22–25], whose difference is the existence of mode at 480 cm  1 or not. So it is difficult to either exclude the existence of T or T and O phase(s). Raman spectra of BCTSLix ceramics with 0r x r 1 are similar to the case of Ba (Ti0.90Sn0.10)O3 [23] at  100 1C which has coexisting R, O and T phases. We speculate that at room temperature, samples at 0r x r 1 may also have coexisting R, O and T phases; as xZ 2, O phase is absent since mode at 480 cm  1 disappears or merges with the neighboring 515 cm  1 mode; samples at 2r x r 6 may have coexisting R and T phases. Besides, as x increases from 2 to 5, the relative intensity between modes at both 160 and 198 cm  1 and 190 and 250 cm  1 gets weak as shown in Fig. 4b, indicating that R phase gradually changes to T phase in their coexisting scope. These continuous decline trends about the relative intensity with x is interrupted as x ¼ 6. So, Tx in the inset of Fig. 3b may correspond to TR/O–T at x ¼ 1 in which R–T and O–T phase transitions may be coexisting,

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while Tx at 2 r xr 6 is defined as TR–T which corresponds to R–T phase transition. Fig. 5 shows the piezoelectric constant d33 and planar mode eletromechanical coupling coefficient kp at room temperature for BCTSLix ceramics as a function of x. d33 and kp of pristine BCTS ceramic are 56 pC/N and 11.55 %, respectively. With increasing x, both d33 and kp first increase and then decrease, reaching peak values of 510 pC/N and 40.94% at x¼ 5. In BaTiO3-based ceramics, two-phase or multiphase coexistence generates fewer grain boundaries constituting energy barriers and provides a favorable condition for easier motion of domain [12]. On the other hand, grain size over 1 μm can create large grain boundary and enlarge domain width, which would lead to the domain wall vibrate without less inhibition and enhance the dipole polarizability, all improve piezoelectric response [26]. While the substitution of acceptor doping Li þ for high valence ions Sn4 þ /Ti4 þ leads to the creation of oxygen vacancies because of the ionic charge compensation, which will pin the movement of the ferroelectric domain walls and result in decreased d33 and kp [20]. In this study, adding LiF can enhance density with grown grain and lead to coexisting R–O–T or R–T for BCTSLix (1rxr6) ceramics at room temperature. Meanwhile, concomitant more oxygen vacancy is harmful for d33 and kp as more Li þ entered B-sites via increasing x. Improved d33 and kp shown in Fig. 5 imply that the positive effect of adding LiF including high density, large grains and coexisting two-phase or multiphase is

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superior to that of negative effect such as oxygen vacancies in the present study. The dominant factors for improving piezoelectric properties should be case by case. Compared to the case for pristine BCTS ceramic, increased d33 and kp in BCTSLi1 ceramic are mainly attributed to the enhanced sintering density and grown grains because of their similar phase structure. Higher d33 and kp at 2rxr6 than that of x¼ 1 may be due to enlarged grains and more closer TR–T to room temperature than TR/O–T, since the relative density of BCTSLix (1rxr6) ceramics is close to each

Fig. 5. Variations of d33 and kp of BCTSLix ceramics as a function of x at room temperature.

Table 1 Electrical properties of reported (Ba,Ca)(Ti,Sn)O3 piezoelectric materials. Ceramic system

kp Sinter. d33 Temp. (pC/N) (%) (1C)

Reference

(Ba0.90Ca0.10)(Ti0.94Sn0.06)O3 (Ba0.97Ca0.03)(Ti0.94Sn0.06)O3 (Ba0.98Ca0.02)(Ti0.95Sn0.05)O3 (Ba0.98Ca0.02)(Ti0.96Sn0.04)O3 0.7Ba(Sn0.12Ti0.88)O3–0.3(Ba0.7Ca0.3)TiO3 (Ba0.95Ca0.05)(Ti0.92Sn0.08)O3 (Ba0.95Ca0.05)(Ti0.89Sn0.11)O3 (Ba0.95Ca0.05)(Ti0.90Sn0.10)O3–5 mol% LiF

1450 1500 1450 1500 1450 1480 1480 1250

[5] [6] [7] [8] [9] [10] [11] This work

405 440 464 510 530 568 670 510

43.2 45.0 43.1 48.0 55.2 47.7 43.0 40.9

Fig. 6. Ferroelectric hysteresis loops (a), Electric field-induced strain (b, c) and converse piezoelectric coefficient dS/dE (inset in (b, c)) for BCTSLix ceramics.

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other. Considering similar phase structure, density and average grain size, the disparity of d33 at 2rxr6 is ascribed to the different specific ratios of R to T phase which had be confirmed by Raman spectra in Fig. 4b. Optimal piezoelectric properties of d33 ¼ 510 pC/N and kp ¼ 40.9% were obtained at x¼ 5, which is mainly owing to the coexistence of R and T phases with an optimum component ratio at room temperature. The piezoelectric properties of recently reported BCTS piezoelectric materials are also compared in Table 1, in which all samples were fabricated by the conventional sintering method. Although BCTS ceramics all show high d33 over 400 pC/N, Ts of those ceramics is as high as 1450–1540 1C. In this work, even Ts of BCTSLi5 sample was reduced to 1250 1C, a high d33 of 510 pC/N is still attainable, demonstrating that BCTSLi5 ceramic is a promising candidate as a leadfree piezoelectric ceramic. Fig. 6a shows hysteresis loops (P–E) of BCTSLix ceramics, which all are a typical ferroelectric loop. The inset in Fig. 6a illustrates the variation of the remnant polarization (Pr) and coercive field (Ec). Pr and Ec for pristine BCTS ceramic are 3.2 μC/cm2 and 2.3 kV/cm, respectively. With increasing x, Pr first increases and then decreases, while Ec shows an inverse trend. At x¼ 5, Pr reaches peak value of 6.1 μC/cm2 and Ec gets the lowest value of 1.3 kV/cm. The easy reversals of polarization under dc bias after doping would result in improved piezoelectric responses [20], which is coincidental with a significantly increased d33 at x¼ 5. Fig. 6b and c shows electric field-induced strain and converse piezoelectric coefficient dS/dE for BCTSLix ceramics measured at room temperature until 500 and 1000 V/mm, respectively. The electrostrain increases and then decreases with x, reaching the highest value 0.48 ‰ at x ¼ 5 along with a maximum dS/dE value of 961 pm/V as measured at 500 V/mm shown in the inset of Fig. 6b, being superior to PZT whose electrostrain and dS/dE are 0.45‰ and 360–900 pm/V, respectively [14]. The superior property of BCTSLi5 may be owing to appropriate ratio of R to T at room temperature that facilitates the domain motion at an electric field [10]. Both electrostrain and dS/dE show a dependence on electric field. 4. Conclusion (Ba0.95Ca0.05)(Ti0.90Sn0.10)O3–xLiF (BCTSLix) lead-free ceramics were fabricated by a conventional sintering method at 1250 1C. The sinterability of BCTS ceramics was improved by adding LiF with high relative density over 90% and grown grains. Increasing the amount of LiF additive enhances largely the piezoelectric properties caused by grown grains and coexisting R–T phase. BCTSLi5 ceramic demonstrates excellent piezoelectric properties of d33 ¼ 510 pC/N, kp ¼ 40.9%, εr ¼ 5370, tanδ¼ 0.0194 and dS/dE ¼ 961 pm/V, which shows a promising future as a lead-free ceramic. Acknowledgments This work was supported by Specialized Research Fund for the Doctoral Program of Higher Education (Grant no.

20130006110006) and National Natural Science Foundation of China (Grant no. 51472026 and 51332002).

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