Electric field induced strain, switching and energy storage behaviour of lead free Barium Zirconium Titanate ceramic

Author’s Accepted Manuscript Electric field induced strain, switching and energy storage behaviour of lead free Barium Zirconium Titanate ceramic T. Badapanda, S. Chaterjee, Anupam Mishra, Rajeev Ranjan, S. Anwar www.elsevier.com/locate/physb

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S0921-4526(17)30402-7 http://dx.doi.org/10.1016/j.physb.2017.07.013 PHYSB310080

To appear in: Physica B: Physics of Condensed Matter Received date: 10 May 2017 Revised date: 5 July 2017 Accepted date: 6 July 2017 Cite this article as: T. Badapanda, S. Chaterjee, Anupam Mishra, Rajeev Ranjan and S. Anwar, Electric field induced strain, switching and energy storage behaviour of lead free Barium Zirconium Titanate ceramic, Physica B: Physics of Condensed Matter, http://dx.doi.org/10.1016/j.physb.2017.07.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Electric field induced strain, switching and energy storage behaviour of lead free Barium Zirconium Titanate ceramic T.Badapanda1*, S. Chaterjee2*, Anupam Mishra3, Rajeev Ranjan3, S.Anwar4 1

Department of Physics, C.V. Raman College of Engineering, Bhubaneswar, Odisha 752054, India Department of Mechanical Engineering, C.V. Raman College of Engineering, Bhubaneswar, Odisha 752054, India 3 Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India 4 Colloids & Materials Chemistry, Institute of Minerals and Materials Technology, Bhubaneswar, Odisha, India751013 2

[email protected] [email protected] *

Corresponding author’s. Tel: +91-9437306100 Fax: +91-674 2113593 Abstract There is a huge demand of lead-free high performance ceramics with large strain, low hysteresis loss and high-energy storage ability at room temperature. In this context, we investigated the large electric field induced strain, switching behaviour and energy storage properties of BaZr0.05Ti0.95O3 ceramic (BZT) prepared by high energy ball milling technique, reportedly exhibiting a triple point transition near the room temperature. The X-ray diffraction of the BZT ceramic confirms orthorhombic symmetry with space group Amm2 at room temperature. The room temperature dielectric study reveals that there is a negligible variation of dielectric constant and dielectric loss with frequency. The polarization behaviour at various applied electric fields was studied and the energy storage densities were obtained from the integral area of P-E loops. Electric field induced strain behaviour has been studied with due emphasis on the electrostrictive response at room temperature. The ferroelectric and electromechanical properties derived from the P-E and S-E loops suggest that the present ceramic encompass the properties of actuation and energy storage simultaneously. Keywords: Piezoelectric ceramic; High energy ball milling; Electric field induced strain; Electrostriction; Energy storage efficiency.

1. Introduction: Piezoelectric ceramics are widely used in applications, such as actuators, energy storage devices, sensors, resonators and ultrasonic transducers. Lead based perovskite oxides have

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been the most prominent candidate of piezoelectric technology due to their excellent piezoelectric properties, ease of processing, and low cost. These are the complex perovskite oxide materials containing two cation species with different charges randomly distributed on the B site of the ABO3 structure [1]. The B site cation order strongly influences the dielectric and ferroelectric properties in this complex perovskite and hence these materials are superior in the application point of view. The best piezoelectric and ferroelectric properties are found in lead based compositions exhibiting morphotropic phase boundary (MPB) starting from a triple point of a paraelectric cubic phase (C), ferroelectric rhombohedral (R), and tetragonal (T) phase [2,3]. This triple point is shown to be a tricritical point, and this leads to an anisotropically flattened energy profile so that the polarization can be easily rotated by external stress or electric field between <111>R and <001>T states [4,5]. Though, environmental concerns with lead based materials have stimulated intensive search for leadfree piezoelectric ceramics worldwide, reports on single phase lead-free material depicting a triple point phase transition is rare in the open literature. In recent years, BaZrxTi1-xO3 materials have been exploited for its high dielectric, ferroelectric and piezoelectric properties. The dielectric and ferroelectric properties of BZT are largely dependent on the amount of Zr substitution [6-8]. BZT shows promising piezoelectric properties in the range (0.03 ≤ x ≤ 0.08) [9-12]. We have recently reported [13] the dielectric and ferroelectric properties of BaZr0.05TiO3 ceramic prepared by high energy ball milling technique. We observed a triple point transition in the temperature dependent dielectric study showing all the transition near the room temperature. Rehrig et al. [14] studied the piezoelectric properties of BZT single crystal (x=0.045 and 0.05) and for pseudocubic (001) orientation, reporting remnant polarization of 13 μC/cm2 at 15 kV/cm at room temperature. They also showed a high unipolar strain value of 0.48% at a maximum field of 60 kV/cm. Recently, Kalyani et al. [15] reported that the system exhibits coexistence

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of ferroelectric phases over an extended composition range 0.02 20 kV/cm), residing outside the Rayleigh region, the piezoelectric properties vary nonlinearly with the electric field due to electrostriction [20], thus generating additional strain. Electrostriction produces an expansion in the direction of the field regardless of its polarity and this expansion relaxes back to zero when the field is removed. Thus, large strain generated through combined piezoelectricity and electrostriction effects can be favourably harvested for suitable extension of the operating range of actuator. At the same time, evaluation of the polarization behaviour is necessary to predict the energy storage utilization under large electric field. It is well known that the properties of the material depends on the shape and size. Hence, the synthesis route has a great influence on the physical behaviour of materials. Mostly, samples are prepared by solid-state reaction and wet chemical route. Recently, Mechanochemical synthesis or high energy mechanical milling is considered to be a suitable method for sample preparation. Intensive milling increases the area of contact

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between the reactant particles due to the reduction in particle size and allows fresh surfaces to come into contact. Therefore, solid- state reactions that normally require high temperatures occur at relatively lower temperature. The mechanical technique is superior to both the conventional solid-state reaction and the wet chemistry based processing routes due to its cost effectiveness. Furthermore, the mechano-chemically derived ceramic powders are nanosized and homogeneous, hence, demonstrate much better sinterability than those synthesised by the conventional solid-state reaction and wet chemical processes [21].

In the present work, we report the effect of large electric field on the polarization and strain behaviour of BaZr0.05Ti0.95O3 ceramic prepared by high energy ball milling. The extent of electrostrictive behaviour exhibited by the composition will be studied through evaluation of electrostrictive coefficient. In this paper, we further report the effect of electric field on the energy storage properties of the BZT ceramic.

2. Experimental Techniques: BaZr0.05Ti0.95O3 (BZT) ceramic was prepared by high energy ball milling (Fritsch planetary) technique with ball-to-powder ratio (BPR) at 20:1 by weight. Stoichiometric amount of BaCO3 (99.9%), ZrO2 (99.8%) and TiO2 (99.95%) were mixed using tungsten carbide balls (10 mm diameter) with toluene as the milling media operating for 5 hrs at 300 RPM. Milled powders were heat treated at 11000C for 4 hrs by using a programmable furnace. For structural analysis the calcinied powder was characterized by X-Ray Diffraction, using a Philips diffractometer Model PW-1830 with Cu Kα (λ = 1.5418 Å) radiation at a scanning rate of 20 min-1. Discs were pressed uniaxially at 200 MPa with 2 wt% polyvinyl alcohol added as a binder. Then these discs were sintered at 12000C for 4 hrs and silver contacts were deposited on opposite disc faces. The average particle size was measured by transmission

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electron microscope (TEM), model CM200 (Philips, USA) operated at 200 kV and the morphological study was carried out by JSM T330 scanning electron microscope (SEM) (JEOL, USA) operated at 25 kV. The room temperature dielectric measurement was carried out by N4L PSM impedance analyzer in the frequency range of 100Hz to 1MHz. The polarization and electric field driven strain measurements were carried out by using Radiant Precision Premier II Ferroelectric loop tester along with a MTI strain measurement set up. The electric filed induced measurements were carried out using triangular waveform fields of amplitude varying from 20 kV/cm to 50 kV/cm at a frequency of 1Hz.

3. Results and Discussion: 3.1 Structural and Microstructural Study: The XRD pattern of the powder calcinied at 11000C for 4hrs is shown in Fig 1 (a), which indicates a pure perovskite phase. The calcination temperature of the material prepared by high energy ball milling is quite low than that of the sample prepared by solid state reaction route [22]. The nearly overlapping doublet peaks of nearly equal intensity in the pseudocubic {200}pc profile (shown in the inset) confirms the orthorhombic structure with a space group of Amm2. Diffraction peaks related to the deleterious phase were not detected which indicates a monophasic ceramic. In an earlier report [13], the authors have reported the detail structural analysis carried out by Rietveld refinement, super cell structure, and Raman spectroscopy. TEM image in Fig. 1(b) reveals that BZT is formed by the agglomeration of several particles with irregular morphologies and different particle sizes of around 100nm (±5). Fig. 1(c) shows selected-area electron diffraction (SAED) patterns which confirm that the BZT powder are well crystallized and is of perovskite-type. The surface morphology

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has been analysed by scanning electron microscope (SEM) image of the BZT ceramic and is shown in Fig. 1 (d). The SEM image shows well dense grain with irregular shape and the average grain size is found to be 7μm (±1). Again, ferroelectric domains with lamellar and water-mark characters are clearly observed from the image. 3.2 Room temperature dielectric study: Fig. 2(a) shows the frequency dependence of dielectric constant at room temperature. Fig. 2(a) confirms that the dielectric constant value is comparatively higher at a lower frequency and decreases with increase in frequency. However, in the high frequency region, dielectric constant becomes almost frequency independent. In a dielectric material, polarization occurs due to contributions from electronic, ionic, dipolar and space charge regions [23]. At lower frequencies, all types of polarization contribute to polarization; as the frequency increases, the contribution from different polarizations decreases, and at a higher frequency, only dipolar and electronic polarization mainly contribute to polarization. Moreover, as the frequency increases, dielectric constant slightly decreases because dipoles begin to lag behind the field. Thus, the overall variation of dielectric constant with frequency can be attributed to interplay of contributions from electronic, ionic, dipolar and space charge regions, and, electric field. The dielectric constant obtained from the present study (using high energy ball milling) is higher than that of the sample prepared by sol gel route and solid state reaction route [24, 21]. Fig. 2 (b) shows the variation of dielectric loss of BZT ceramic with frequency. It is observed that the dielectric loss is very less compared to the samples prepared by sol gel technique [24]. High dielectric constant and low dielectric makes the material more suitable for various applications.

3.3 Polarization vs. Electric field: Figure 3 shows the room temperature P-E hysteresis loops for various unipolar (Fig. 3a) and bipolar (Fig. 3b) electric fields. Well saturated hysteresis loop is observed for all the applied 6

electric fields. The maximum polarization (Pmax), saturation polarization (Ps) and remnant polarization (Pr) for unipolar field are shown in Table 1. It is observed that the maximum polarization, saturation polarization (Ps) and remnant polarization increase with increase in electric field. With an increase in applied voltage, domain reorientation occurs along the applied field direction thus resulting in typical well saturated electric field induced ferroelectric polarization hysteresis loops. The ferroelectricity in BZT is due to off-centering of Ti4+/Zr4+ ions in TiO6/ZrO6 octahedra. The remnant polarization of the sample is lower than that of the other BZT compositions [25,26]. Further, the coercive field (Ec) obtained from the bipolar field cycle (Fig. 3b) is shown in Table 1. Low coercive field (Ec) of the samples suggests that the BZT sample is ferroelectrically soft and very low electric field is sufficient to cause the switching of domains. It is a well-acclaimed fact that ferroelectric properties of materials are affected by their crystal phase, composition, microstructure, lattice defects and imperfections like oxygen vacancies [27]. Oxygen vacancies may disturb the domain wall motion by screening of the polarization charge. Also, the random field in the surrounding of dipolar defects can considerably reduce the activation barrier needed for the nucleation of ferroelectric domains, resulting in a lower value of Ec [28,29]. The remnant polarization and the coercive field of the BZT sample prepared by high energy ball milling is quite low in comparison to the sample prepared by solid state reaction route [30]. However, the (Pmax-Pr) value is quite high. The observed high value of (Pmax-Pr) and low value of Ec over the entire range of applied electric field can be favourably exploited for actuator and energy storage applications.

On application of electric field, switching of domains is believed to occur by the formation of new domain nuclei. The rate of change in polarization (jE) in the direction of electric field, E can be defined as [31]

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̂

(1)

where ρ is the number of nucleation sites per unit volume, dPE is the associated change in polarization along the applied electric field direction, Γ is the rate of formation of nuclei, Ps, spontaneous polarization and, δ the angle between Ps and ̂ before and after the switching. According to Eq. (1), at higher electric field, the increase in rate of formation of nuclei leads to increase in polarization current as shown in Fig. 4. The peak value of current observed in both forward and reverse cycles indicate the occurrence of a cyclic and uniform remnant polarization in both directions, a typical characteristic of ferroelectric materials.

The energy density (Ed) of a capacitor is calculated from the P–E hysteresis loop (Fig 3a), and is the integral area of the P–E loop (charge- lower branch of P–E curve or dischargeupper branch of P–E curve) and y-axis, given by Ed = ∫EdP, where E is applied electric field and P is polarization [32,33]. The stored energy can be estimated using polarization response under an applied electric field from polarization P–E loops, excluding the hysteresis losses. The ratio of discharge curve energy density (Ed)d to that of the charge curve energy density (Ed)c can be used to evaluate the energy storage efficiency (η = (Ed)d/(Ed)c) [34]. The calculated discharge and charge curve energy densities and energy storage efficiency for different fields are listed in Table 2. The obtained energy storage efficiency for the current compositions are higher than that of other lead free compositions [35,36].

3.4 Strain vs. Electric field: The strain–electric field (S–E) hysteresis loops of the BZT ceramic at room temperature for various uniploar and bipolar electric fields are shown in Fig. 5(a, b). Symmetric nature of butterfly strain–electric field loops (Fig.5b) suggests the piezoelectric nature of the BZT 8

ceramic. The difference between zero field strain and the lowest strain which is known as the negative strain (Sneg) is only visible in the bipolar cycle. The maximum value of Sneg obtained at 50 kV/cm is found to be as low as ~ 0.007%. Thus, there is an absence of polarization back switching in S-E curves.

The normalized strain

which corresponds to the inverse

piezoelectric effect, is defined as

, pm/V

(2)

where Smax is the maximum unipolar strain and Emax is the maximum applied electric field. The

values for the materials in this study are listed in Table 1. It is observed that the

value is higher at lower field. The large electromechanical strains in this composition is further evidence that the crystal lattice is elastically ‘‘soft’’ which is consistent with the low coercive field [37]. For the present composition, large recoverable strains with a maximum of 0.124% obtained under the applied electric fields might be useful for actuator applications. For the present study, in which, sample is prepared by high energy ball milling, the strain is very high with respect to the sample prepared by solid state reaction route [38]. The degree of hysteresis (ΔS/Smax) was also calculated from the unipolar strain deviation (ΔS) during the rise and fall with the field at half of the maximum field divided by the maximum strain Smax, and is presented in Table 1. It is observed that the degree of hysteresis decreases with electric field and reaches a minimum at 50 kV/cm. Significant lower value of the degree of hysteresis in the present ceramic might be useful for potential application as positioning actuators [39]. Such hysteresis free nature in a material reduces the additional circuitry that would otherwise be required in the form of a feedback circuit, to nullify hysteresis in materials and decreases heating up of peripheral electronics [40].

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It is reported that the electric field-induced strain consists of intrinsic contributions (electrostriction or piezoelectric effects) and extrinsic contributions produced by domain wall movement [41]. The intrinsic effect is due to the deformation of the crystal lattices by the electric field, which is aligned with the true piezoelectric response (linear strain response) or the electrostrictive response (the strain has quadratic function). While the extrinsic effect is aligned with the nonlinear response with hysteretic S-E loops produced by domain wall movement. In order to evaluate the effect of electrostriction for the present composition, S-P2 curves are plotted as shown in Figure 6. For electrostrictive ceramics, the strain curve follows a linear relationship with the square of the polarization (P2) which is expressed as (3)

where the parameter Q33 is electrostrictive coefficient and is presented in Table 1. The obtained Q33 values are comparable with the reported lead free electrostrictive materials [4243]. Thus, for large electric fields outside the Rayleigh region, electrostrictive behaviour of the present composition is a major contributor to the observed large strain. 4. Conclusions: The present work focuses on the electric field induced polarization and strain behaviour of single phase lead-free BaZr0.05Ti0.95O3 ceramic depicting a triple point phase transition at room temperature. The structural and microstructural study of the high energy ball milled samples confirmed the occurrence of well crystallized monophasic ceramic with well characterized ferroelectric domains. The variation of dielectric permittivity and dielectric loss with frequency shows a negligible variation with respect to frequency. Large recoverable strains with significant contributions of electrostrictive behaviour at room temperature are observed in usable range of electric field. High values of (Pmax-Pr) with low degrees of hysteresis result in appreciable room temperature energy storage efficiency. The nature of

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P-E and S-E loops and the derived parameters suggest possible application of the present class of lead-free ceramic in actuators and energy storage devices concurrently.

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Fig.1: (a) X-ray diffraction (b) TEM image (c) SAED patterns (d) SEM image of BaZr0.05Ti0.95O3 ceramic Fig. 2: Frequency dependent (a) dielectric permittivity (b) dielectric loss of BaZr0.05Ti0.95O3 ceramic at room temperature Fig. 3: Room temperature P-E loop of BaZr0.05Ti0.95O3 ceramic at various (a) unipolar (b) bipolar fields Fig. 4: Switching current of the bipolar measurements of BaZr0.05Ti0.95O3 ceramic at roomtemperature with various applied fields Fig. 5: Room temperature S-E loop of BaZr0.05Ti0.95O3 ceramic at various (a) unipolar (b) bipolar fields Fig. 6: Plot of S-P2 of BaZr0.05Ti0.95O3 ceramic

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(a) Intensity (a.u)

(a)

44.5

20

30

40

50

45.0

60

45.5

70

80

2 (degree) 

(b)

15

(c)

(d)

Fig 1 (a-d)

16

2000

dielectric permittivity r

(a) 1800 1600 1400 1200 1000 100

1000

10000

100000

1000000

Frequency (Hz)

0.10

(b) 0.08

tan 

0.06 0.04 0.02 0.00 100

1000

10000

100000

Frequency (Hz)

Fig. 2

17

1000000

(a)

20 kV/cm 30 kV/cm 40 kV/cm 50 kV/cm

20

2

Polarization (C/cm )

24

16 12 8 4 0

0

5

10 15 20 25 30 35 40 45 50 55

2

Polaroization (C/cm )

Electric field (kV/cm)

25 20 15 10 5 0 -5 -10 -15 -20 -25

20 kV/cm 30 kV/cm 40 kV/cm 50 kV/cm

(b)

-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60

Electric Field (kV/cm)

Fig.3

18

polarization current (A)

0.6 0.4

20 kV/cm 30 kV/cm 40 kV/cm 50 kV/cm

0.2 0.0 -0.2 -0.4 -0.6 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60

Electric Field (kV/cm)

Fig. 4

20 kV/cm 30 kV/cm 40 kV/cm 50 kV/cm

0.12

strain (%)

0.10 0.08

(a)

0.06 0.04 0.02 0.00

0

5

10 15 20 25 30 35 40 45 50 55

Electric Field (kV/cm)

19

0.14 0.12

20 kV/cm 30 kV/cm 40 kV/cm 50 kV/cm

strain (%)

0.10 0.08 0.06

(b)

0.04 0.02 0.00

-0.02 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60

Electric Field (kV/cm) Fig. 5

0.14 20 kV/cm 30 kV/cm 40 kV/cm 50 kV/cm

0.12

strain (%)

0.10 0.08 0.06 0.04 0.02 0.00

0

50 100 150 200 250 300 350 400 450 2



2

4

P ( C /cm ) Fig. 6

Table 1 Parameters obtained from room temeperature ferroelectric and electric field induced strain study of BaZr0.05Ti0.95O3 ceramic 20

Maximum Electric filed (Emax)( kV/cm)

20

30

40

50

Pr (μC/cm2)

1.15

1.92

2.53

2.92

Ps (μC/cm2)

10.36

12.31

14.26

14.75

Pmax (μC/cm2)

15.03

17.52

18.94

19.85

Ec (kV/cm)

2.44

2.85

3.30

3.63

Smax (%)

0.063

0.087

0.107

0.124

Q33 (m4/C2)

2.76x10-2

2.81 x10-2

2.92 x10-2

3.12 x10-2

d33*(pm/V)

315

290

267

248

ΔS/Smax (%)

22

13

7

4

Table 2 Electric field-dependent discharge , charge energy densities and energy storage efficiency of BaZr0.05Ti0.95O3 ceramic sample at room temperature Maximum Electric filed (Emax)(kV/cm)

Discharge energy density (Ed)d (J/cm3)

50

0.218

0.302

72

40

0.162

0.248

65

30

0.109

0.192

56

20

0.058

0.130

44

Charge energy density (Ed)c (J/cm3)

21

Energy storage efficiency (%) (η=(Ed)d /(Ed)c)