Enhancement in electro-strain behavior by La3+ substitution in lead free BaZr0.05Ti0.95O3 ceramics

Materials Letters 97 (2013) 40–43

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Enhancement in electro-strain behavior by La3 þ substitution in lead free BaZr0.05Ti0.95O3 ceramics Sandeep Mahajan a,c,n, Divya Haridas b, K. Sreenivas c, O.P. Thakur d, Chandra Prakash e a

Centre for Materials for Electronics Technology (C-MET), IDA Phase-III, Cherlapally, HCl (PO), Hyderabad 500051, India Keshav Mahavidyalaya, H-4-5 Zone, Pitampura, Delhi 110034, India c Department of Physics and Astrophysics, University of Delhi, Delhi 110054, India d Electroceramics Group, Solid State Physics Laboratory, Lucknow Road, Delhi 110054, India e Directorate of Extramural Research & Intellectual Property Rights (ER&IPR), DRDO Bhawan, New Delhi 110011, India b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 November 2012 Accepted 12 January 2013 Available online 29 January 2013

Recently, lead-free piezoelectric ceramics have widely attracted interest among the research community, as they are considered to be potential substitutes for lead-based ceramics which causes environmental pollution due to the presence of hazardous lead oxide in them. In the present study we have synthesized La doped Ba1  x/2Lax/3(ZryTi1  y)O3 (x ¼ 0–0.06 in steps of 0.02; y¼ 0, 0.05) ceramics by the conventional solid-state reaction method. Results reveal that electric field induced strain increases from  0.08% to  0.18% (at 40 kV/cm applied field) for x ¼ 0.02 and y¼ 0.05 of lanthanum and zirconium substitution respectively compared to undoped BaTiO3. It was also observed that, sample with 2 mol% lanthanum could sustain higher electric field upto 120 kV/cm during strain measurement (without electrical breakdown) and results showed highest strain value of  0.3% in this system. & 2013 Elsevier B.V. All rights reserved.

Keywords: Ceramics BZT XRD Piezoelectric materials Strain

1. Introduction Historically, BaTiO3 was the first polycrystalline piezoelectric material, but was soon replaced by PbZrTiO3 (PZT) ceramics because the latter has much better piezoelectric properties and is widely used in actuator, sensor, and transducer applications [1–3]. However, PZT is now facing a big challenge due to the environmental hazard by its toxic lead. In the last decade, great effort has been made to search for high performance lead-free materials and compounds such as K0.5Na0.5NbO3-based, Bi0.5Na0.5 Ti03-based, Bi4Ti3O12-based, SrBi2Ta2O9-based, and BaTiO3-based materials exhibiting promising piezoelectric properties and these are extensively studied [4–7]. In particular, compositionally modified solid solution of BaTiO3 and BaZrO3 [BaZrxTi1 xO3 (BZT)], identified in the 1950s—although initially research was focused on temperature dependence of dielectric properties for capacitor applications, has been exploited for its promising piezoelectric properties in the range (0.03r xr0.08) [8–12]. Compositional behavior of BZT is also being studied across the complete phase diagram (0.00rxr1.00) and reported the dependence on composition, properties extending

n Corresponding author at: Centre for Materials for Electronics Technology (C-MET), IDA Phase-III, Cherlapally, HCl (PO), Hyderabad 500051, India. Tel.: þ91 40 27267006; fax: þ 91 40 27261658. E-mail addresses: [email protected] (S. Mahajan), [email protected] (O.P. Thakur).

0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.01.057

from simple dielectric to relaxor ferroelectric was reported[13]. Rherig et al. studied piezoelectric properties of BZT single crystal (x¼0.045 and 0.05) and for pseudocubic (001) orientation, reporting remanent polarization of 13 mC/cm2 at 15 kV/cm. They also showed a high field d33 of 340–355 pC/N and unipolar strain value of 0.48% for the same composition which has orthorhombic structure at room temperature [14]. These properties stimulate BZT for further study as a promising substitute for lead-based piezoelectrics. Compared with single crystal growth, polycrystalline ceramic materials are easy to prepare and a wide range of compositions can be made. Compositional alterations to BZT have been studied either with ‘‘donar-doping’’ with an ionic compensation mechanism that induces cationic vacancies/defect and alters the properties or with ‘‘acceptor-doping’’ to further improve material performance [15–16]. Recent studies on rare earth (La, Pr, Nd, Sm, Eu, Dy, Ho, etc.) and other elements (Al, Mn, Bi, Ca, etc.) doping in BZT ceramics have shown the effects on structural, dielectric, relaxor and electrical properties, which find potential applications in various tunable microwave devices [17–24]. Maura et al. reported strain and vacancy cluster behavior of vanadium and tungsten-doped BaZr0.01Ti0.9O3 ceramics [16]. Our earlier reported work on BZT has been focused on study of its structural, dielectric, relaxor, ferroelectric, piezoelectric and impedance spectroscopy properties, etc. by doping of different elements (e.g. Bi, Nd, and Ca) and preparation of compound by different methods [22–25]. The aim of the present work is to

S. Mahajan et al. / Materials Letters 97 (2013) 40–43

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improve piezoelectric properties of BaZr0.05T0.95O3 ceramic via stabilizing the low temperature orthorhombic phase of BaTiO3, since it has potentially higher piezoelectric response and further doping of La at A-site. As far as we are aware, systematic crystal structure, dielectric and piezoelectric properties of La doped BaZr0.05T0.95O3 ceramics have not been reported earlier, although many studies of individual aspects have been made toward finding ferroelectric tunable ceramic capacitor applications.

out using an Agilent 4294A Impedance analyzer interfaced with a PC. Strain was measured using a SS50 strain measurement system (Sensor Tech., Canada). The experimental details including sample preparation for the abovementioned measurements are discussed in detail earlier [22–25].

2. Experimental procedure

Structure and microstructure analysis: Fig. 1 illustrates the room temperature XRD pattern of BLZT ceramic with different amounts of La and Zr doping with pure BaTiO3 (BT). All ceramic samples are found to be single phase with tetragonal structure for pure BT, orthorhombic structure for x ¼0–0.02 and rhombohedral structure for x¼0.04–0.06 which are confirmed by powder ‘‘X’’ fitting software. It can be seen that with substitution of Zr in pure BT the peaks are shifted toward lower angle side and with further increasing in the value of x the peak again shifted to higher angle side. The shifting of peaks in lower angle side is attributed to the ˚ as compared to Ti4 þ (0.60 A) ˚ larger ionic radius of Zr4 þ (0.72 A) which resulted in higher d-values and thereby peaks are shifted toward lower angle side [9] (Fig. 1, inset). With further substitution level of La3 þ , the diffraction peaks are shifted toward higher angle side indicating contraction in the unit cell (or, decrease in

x = 0.04

c

x = 0.02

b

BaZr0.05Ti0.95O

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20000 15000 10000 5000

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(201)

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d c b a 44.0 44.05 44.0 44.5 46.0

Rhombohedral

Dielectric Constant (ε')

e

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500 kHz

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BT

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The stoichiometric ratio of starting chemicals BaCO3 (99.9%), TiO2 (99.9 þ%), ZrO2 (99.9%) and La2O3 (99.9%) (Aldrich chemical) were weighed for the composition Ba1  x/2Lax/3(ZryTi1  y)O3 hereafter referred as BLZT (x ¼0–0.06 and in steps of 0.02 and y¼0 and 0.05). After milling and drying, calcination was done in a high purity alumina crucible at 1200 1C for 6 h in a conventional furnace. The calcined powder was again ball milled for about 24 h and then dried and sieved. The sieved powders were pressed in the form of rod at a pressure of 200 MPa using a Cold Isostatic Press (M/S Autoclave Engineers, USA). These rods were sintered at 1400 1C for 6 h with 3 1C/min heating and cooling rate. X-ray diffraction (XRD) analysis of the sample was carried out using a Philips Diffractometer (model PW3020, Netherlands) with CuKa ˚ radiation. Microstructural information was obtained (1.5418 A) by SEM (Leo 1430, Japan). Dielectric measurements were carried

3. Results and discussion

30

40

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60

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Temperature (°C)

2θ(degree) Fig. 1. XRD patterns of (a) pure BT, (b) BaZr0.05T0.95O3, (c) x ¼0.02, (d) 0.04 and (e) 0.06 doped La2O3.

X = 0.00 Y = 0.05

10μm

Fig. 3. Temperature dependence of dielectric constant (e0 ) for BaZr0.05T0.95O3 and inset (a) for pure BT and (b) for x¼ 0.02 doped La2O3 at various frequencies.

X = 0.02 Y = 0.05

Fig. 2. Scanning electron micrographs for (a) x ¼0 and y¼ 0.05 and (b) x ¼0.02 and y¼ 0.05.

2μm

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S. Mahajan et al. / Materials Letters 97 (2013) 40–43

Dielectric and piezoelectric properties: Dielectric constant of all the samples was measured as a function of temperature in the frequency range of 100 Hz–500 kHz. Fig. 3 represents the variation of dielectric constant with temperature at five different frequencies for x¼0.02 and y¼0.05. Insets (a) and (b) represent the dielectric constant for undoped BaTiO3 (x¼0 and y¼0) and BZT5 (x¼0 and y¼0.05) samples respectively. It has been observed that with increase in substitution level of lanthanum, Curie temperature (Tc) decreases whereas other two transition temperatures (Trhom/ortho and Tortho/tetra) shift to higher temperature and merge at x¼ 0.06. Closer observation of the peaks at transition temperature reveals that no frequency dispersion was observed for all the substitution range. Chou et al. observed typical relaxor behavior in La2O3 doped BZT and reported that La3 þ ion substitutes for A-site Ba2 þ while marinating the perovskite structure of solid solution and Curie temperature shifted to lower temperature with substitution, which well supported our results [29]. Fig. 4(a)–(f) represents the strain electric field (S–E loop) for La3þ substituted BZT ceramic system. It is well known in lead based

lattice parameter). This can be understood as the ionic radius of ˚ which is too large to replace Ti/Zr (0.60 A/0.72 ˚ ˚ La3 þ (1.36 A) A) ˚ site as it has larger ionic radius, whereas it replaces Ba2 þ (1.61 A) which results in lower d-values and hence peaks are shifted toward higher angle side. Microstructural information is obtained by polished and thermally etched sample by SEM. Typical SEM micrograph is shown in Fig. 2(a) for x¼ 0 and y¼0.05 and 2(b) for x¼ 0.02 and y¼0.05. It can be inferred from the figure that porosity decreases with increase in La3 þ substitution. Grain size was found to decrease from 16 mm (x ¼0 and y¼0.05) to 1 mm with increase in lanthanum substitution (x ¼0.06). The incorporation of trivalent rare-earth cations such as lanthanum in the perovskite lattice of BaTiO3 modifies the microstructural properties of doped BaTiO3. The larger ionic size rare-earth ions (La3 þ ) predominantly dissolve in A-sites, and act as donors and it slows down the overall diffusion rate during the sintering process. The dissolution of lanthanum dopant into perovskite lattice may decrease the oxygen vacancies and contribute to decrease of the overall cationic transport, thus diminishing the diffusion rate, thereby reducing the grain size [26–28].

0.20

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0.20 0.3 0.15 0.2 0.10 0.1

0.05 x = 0.06 y = 0.05

x = 0.02 y = 0.05

x = 0.06

At high field x = 0.02

0.0

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Electric Field (kV/cm) Fig. 4. Strain (S) vs electric field (E) for (a) pure BT, (b) BaZr0.05T0.95O3, (c) x ¼0.02, (d) 0.04 and (e) 0.06 doped La2O3, (f) x¼ 0.02, at higher applied field.

S. Mahajan et al. / Materials Letters 97 (2013) 40–43

systems that substitution of La3 þ makes the materials more soft resulting in an increase in squareness and also decrease in coercive field for PZT system [3]. In the present study, we observed a drastic increase in electric field induced strain with substitution compared to pure BaTiO3 (x¼y¼0.00). It can be seen (Fig. 4(c)) for x¼0.02 sample, the strain level reaches  0.18% at 40 kV/cm. It can be seen that as we further increase the lanthanum substitution in BZT lattice, the value of strain decreases with meager hysteresis; the reason may be grain size refinement effect. These parameters are comparable to those of soft lead zirconate titanate (PZT) materials (S¼  0.2% at 40 kV/cm). Fig. 4(f) represents the strain behavior at higher electric field (120 kV/cm) and it reaches a maximum value of 0.3%. In this situation, three ferroelectric phases are pinched near the room temperature and it was expected that near the pinched point all the polar vectors in three phases [i.e. T(6)þO(12)þR(8)¼26] contribute to strain level due to small difference in the free energy of various phases and as a result higher value of strain [10,30–32]. A careful inspection of the S–E loop reveals that there are two apparent linear regions at low fields (Er30 kV/cm) and high fields (EZ40 kV/cm) and one transition in between for the samples with x¼0.02 which corresponds to possible steps of the reorientation of domains induced by external electric fields. At Er30 kV/cm, some hysteresis is present which is associated with the domain reorientation in the samples. At electric field higher than 40 kV/cm, the strain is hysteresis free, indicating a stable single domain/poling state induced by the high electric fields as reported by Zhi et al. [12]. The degree of hysteresis is also calculated from the strain deviation during the rise and fall with the field Dx at half of the maximum field (20 kV/cm) divided by the maximum strain xmax at 40 kV/cm. A significant reduction in hysteresis was observed with increasing substitution of La2O3 in BZT5. This characteristic indicates its usefulness as a potential application for positioning actuators.

Acknowledgments One of the authors SM, wants to acknowledge the Director, Solid State Physics Lab, DRDO, Delhi for providing experimental facilities for this work and awarding scholarship.

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4. Conclusions Phase pure Ba1  x/2Lax/3(ZryTi1  y)O3 ceramic was prepared by the conventional solid-state reaction method. Room temperature XRD study shows the occurrence of different crystal structures with substitution of lanthanum in BZT. The rare earth substitution ions are effective for the improvement of electro-strain from 0.08% to 0.18%. Degree of hysteresis also decreases from 25.4% to 2% with substitution.

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