Large piezoelectric performance of Sn doped BaTiO3 ceramics deviating from quadruple point

Accepted Manuscript Large piezoelectric performance of Sn doped BaTiO3 ceramics deviating from quadruple point Wenfeng Liu, Jiageng Wang, Xiaoqin Ke, Shengtao Li PII:

S0925-8388(17)31190-8

DOI:

10.1016/j.jallcom.2017.04.013

Reference:

JALCOM 41414

To appear in:

Journal of Alloys and Compounds

Received Date: 7 December 2016 Revised Date:

30 March 2017

Accepted Date: 1 April 2017

Please cite this article as: W. Liu, J. Wang, X. Ke, S. Li, Large piezoelectric performance of Sn doped BaTiO3 ceramics deviating from quadruple point, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.04.013. 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 proof before it is published in its final 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.

Large piezoelectric performance of MANUSCRIPT Sn doped BaTiO3 ceramics deviating ACCEPTED from quadruple point

Liu Wenfeng*, Wang Jiageng, Ke xiaoqin and Li Shengtao

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State Key Laboratory of Electrical Insulation and Power Equipment,School of Electrical Engineering, Xi’an Jiaotong University, Xi'an, Shaanxi, 710049, China *E-mail: [email protected]

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Abstract

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The piezoelectric performance, both weak-signal piezoelectric coefficient d33 or large-field electrostrain, were systemically measured from room temperature until their Curie temperature. Large piezoelectric coefficient d33 of 920 pC/N was found on the boundary between the tetragonal (T) and the orthorhombic (O) phase (around 50

) rather than the quadruple point with lowest free energy barrier. Further simulation work

based on the Landau free energy function also supported the experimental results, owing to the joint effect

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of the anisotropy energy and the order parameter polarization. Besides, the electrostrain of 0.15% with small hysteresis was also found on the O-T boundary. Systemically study pointed out that polarization rotation would have a significant contribution to the overall piezoelectric response.

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1. Introduction

Piezoelectrics are certain materials which can swell and shrink when applied with electric field and vice

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versa. Such material, being as the bridge between the mechanical and electrical energy, naturally leads to wide applications for sensors, actuators and transducers ranging from advanced technology to our daily life [1-2]. Pb-based perovskites has been the icon and workhorse since 1950’s and now facing a global restriction due to their toxicity. Thus there is an urgent need to develop a non-Pb substitute. There are three types of ferroeelectrics which have been intensively studied as the most promising candidates [3-5]. Firstly the BaTiO3 based ceramics are manifested with large piezoelectric performances with d33 ranging from 400 pC/N to 600 pC/N at optimal morphotropic phase boundary (MPB) compositions, however, with the critical problem of low Curie temperature around 100 moderate Curie point (>250

[6-9]. Secondly (K0.5Na0.5)NbO3 based ceramics showed

) and considerable piezoelectric performance with d33 of 200~490 pC/N

[10-13]. Besides, the (Bi0.5Na0.5)TiO3 based ceramics exhibit comparable electrostrain as lead-based ceramics of 0.15~0.2% but with considerable large coercive field (~8 kV/mm) [14-17] .

Both in Pb-based and Pb-free piezoelectric system, the common solution to promote piezoelectric

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performance is to place materials at their phase transition boundaries, either a paraelectric to ferroelectric phase boundary or multi ferroelectric phases coexisting boundary (including the most famous morphotropic phase boundary, i.e. MPB). Since the instability of the polarization at phase boundaries allows a significant polarization variation under an external stress or electric field. Despite the intense interest on the phase coexisting strategy, the key to understand to the consequent high piezoelectricity is still in quarrel. For

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instance, we initially proposed the coexistence of tetragonal (T) and rhombohedral (R) symmetry at MPB, evidenced by the synchrotron XRD results from Ehmke [18] and TEM results from Gao [19]. While soon later, the discovery of an intermediate orthorhombic (O) phase was found based on synchrotron XRD results

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from Keeble [20] and temperature spectrum of dielectric permittivity from Damjanovic [21].

Hereinto, most studies above focused on the energy barrier from the thermodynamic view, i.e.

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considering the flatness of the free energy landscape at phase coexisting region. However, merely this consideration can’t explain the experimental results that the small signal piezoelectric activity is found along the orthorhombic to tetragonal phase boundary rather than the multi-phases coexisting region reported in BZT-BCT system [6]. In the present study, researchers proposed that the overall piezoelectric performance not only benefits from the low barrier but also the large anisotropy among different energetically equivalent

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states, further we confirm this from both experiment and simulation. The Sn doped BaTiO3 system was designed with a quadruple point, at which four phases (cubic, tetragonal, orthorhombic and rhombohedral) merged together [22-23]. From this system, large piezoelectric performance with d33 over 920 pC/N was found on the boundary between the tetragonal (T) and the orthorhombic (O) phase rather than the quadruple

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point with lowest free energy barrier. Further simulation work based on the Landau free energy function also supported the experimental results, owing to the joint effect of the anisotropy energy and the order parameter

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polarization. Besides, the electrostrain of 0.15% with small hysteresis was also found on the O-T boundary. Systematical study pointed out that polarization rotation would have a significant contribution to the overall piezoelectric response. 2. Experimental

The Sn doped BaTiO3 ceramics were fabricated by the conventional solid-state reaction method from analytical reagent grade powders of BaCO3 (99.9%), TiO2 (99.9%) and SnO2 (99.9%) (Alfa Aesar Co. Ltd., USA). The initial raw powders were mixed and ball milled using zirconia balls for 6h in polyethylene jars with ethanol as media. After drying, the mixture was calcined at 1200

for 2h. The calcined powders were

crushed and ball milled again for 6h. After dried, the powders were mixed with 5 wt.% PVA solution and uniaxially pressed into disks with diameter of 10 mm and around 1 mm in thickness. The relative density of

the obtained ceramics was over 95%.

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The XRD analysis was performed by the X-ray diffractometer with CuKα (λ=0.15405nm) radiation (XRD, Rigaku Corporation, Japan). The microstructures of the sintered pellets were studied on a natural surface by scanning electron microscope (SEM, VE-9800S, KEYENCE, Japan). The grains size was calculated by the Nano measurer software. For further electrical measurements, the samples were coated with silver paste on both sides. The temperature dependent dielectric spectroscopy were measured by a LCR

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meter (3532-50 LCR, HIOKI) in the program controlled chamber. The piezoelectric coefficient d33 was obtained from a quasi-static d33 meter (ZJ-3B, Institute of Acoustics, Chinese Academy of Sciences, China). The ferroelectric hysteresis loops and the electrostrain curves were examined at room temperature using a

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Radiant precision workstation (TReK model 609A, USA) and MTI-2000 photonic sensor under the fixed frequency of 10Hz.

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3. Results and discussions

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Fig.1.(a) Phase diagram of the Sn-BaTiO3, (b) XRD profiles for different compositions at room Temperature, (c) SEM pictures of the obtained samples.

Fig.1 (a) shows the phase diagram of the Sn-BaTiO3 system obtained by the temperature dependent dielectric measurements [22]. The Curie temperature decreased with increasing the Sn dopant; meanwhile the transition temperatures of the T-O and the O-R raised. As a consequence, the four phases merged together. The quasi-quadruple point appeared at the composition of 11 mol% Sn doped BaTiO3 (abbreviated as 11Sn-BT hereafter) with the Tc around 42

; at the same time, two phase boundaries (T-O and O-R) could

be obtained. Fig.1 (b) shows the XRD profiles for different compositions at room temperature. All the samples exhibit pure perovskite structure at the room temperature. Hereinto 2Sn-BT with 1:2 splitting at (200) clearly indicates a tetragonal phase with c/a of 1.0084. When increasing the Sn dopant, the tetragonality decreased with merging of the splitting peaks and the anisotropy of the lattice decreased. Fig.1

(c) shows the SEM pictures of the obtained samples. With increasing the Sn dopant, the average grain size

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decreased. The 2Sn-BT exhibited a large average grain size of about 10 µm, which decreased monotonously to 1.6 µm as the concentration of Sn dopant increasing to 11%. Such results are due to the higher densification sintering temperature of BaSnO3 (over 1700

) comparing to the BaTiO3 ceramics [24].

Previous study has pointed out that the small grain sizes might favor the high performance [25]. Based on the Landau free energy function, the energy barrier distribution in the phase diagram of

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BaSnxTi1-xO3 can be approximated as shown in Fig.2. A sixth order Landau free energy was employed to model BaSnxTi1-xO3 system. Here the form of the free energy can be expressed as:

G = α 1 ( P1 2 + P22 + P32 ) + α 11 ( P1 2 + P22 + P32 ) 2 + α 111 ( P1 2 + P22 + P32 ) 3 + α 12 ( P1 2 P22 + P22 P32 + P1 2 P32 )

(1)

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+ α 112 ( P1 4 ( P22 + P32 ) + P24 ( P1 2 + P32 ) + P34 ( P1 2 + P22 )) + α 113 P1 2 P22 P32

where G is the Landau free energy, α1 , α11 , α111 , α12 , α112 , α113 are the Landau coefficient which depend on the

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temperature T and composition c, P=(P1,P2,P3) is the polarization vector. The temperature and composition dependency of these Landau coefficients are as follows: α1 =α10 (T -TC )

α12 = α120 (T − Ttr ) + α1200 (c − cqu )

0 α112 = α112 (c − cqu )

and

α11 = α110 | c − cqu | 0 α113 = α113 (c − cqu )

0 α111 =α111

,

where

0 0 0 are constants, Tqu , cqu are the temperature and composition at the quadruple α 10 ,α 110 , α 111 , α 120 ,α 1200 , α112 , α 113

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point, Trt is the transition temperature from tetragonal to rhombohedral phase, TC = TC00 +bc in which TC00 is the Curie temperature at c=0 and b is a constant. The constants in these equations are (in SI units unless

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0 otherwise specified): α10 = 4.124 ×105 , α110 = -4 ×108 , α120 = 1.0 ×106 , α1200 = -5.833×108 , α111 = 1.294 ×109 ,

Fig.2.contour map of energy barrier distribution

0 0 9 Tqu = 40o C , TC00 = 75 o C , b = -291.7 o C . The energy α112 = 5.833 ×108 , α 113 , cqu = 0.12 ,MANUSCRIPT = -2.3332 × 10 ACCEPTED

barrier between two domain variants of the stable phase in the phase diagram, i.e., the barrier between two R variants in the R phase region, the barrier between two O variants in the O phase region and the barrier between two T variants in the T phase region, can be obtained by calculating the minimum energy pathway as detailed in the reference [26-27]. As shown in Fig.2, the energy barrier is low at T-O and O-R phase boundary while the minimum appeared at the T-O phase boundary as denoted by the dashed line. This

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predicts that the best piezoelectric property would appear at the T-O phase boundary.

Fig.3. (a) Contour map of d33 in phase transition diagram, (b) Comparison of d33 Among xSn-BT with x = 2, 6, 8, 9, 10, 11.

Fig.3 shows the contour map of piezoelectric coefficient d33 in the phase diagram. Here, the d33 was

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obtained from the quasi-static method, which reflected the responds of the piezoelectric ceramics under the weak applied force of 0.25 N with the frequency of 50 Hz. Large d33 were obtained near the tetragonal to

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orthorhombic phase boundary from various ceramics with different compositions. More interestingly, the d33 data verified the above speculation and showed the maximum at the T-O boundary rather that the quadruple point, as shown in the Fig.3. Hereinto, the peak value of the 9Sn-BT at 50

was extraordinary high with

920 pC/N, which is even superior to that of best lead zirconate titanate ceramics [2]. Besides, the d33 of the 6Sn-BT was over 400 pC/N over the room temperature range from 20

to 60

with the peak value of

650 pC/N. The mismatch between the best piezoelectric property and the multi-phase coexisting point was also reported in the BZT-BCT ceramics. Matias [28-29] revealed the similar results that the piezoelectric d33 values along the orthorhombic to tetragonal phase boundary which was 30% higher than that in the phase convergence region. These results demonstrated that joint effect of the anisotropy energy and the order parameter polarization should be taken into consideration. On one hand, the multi-phase coexisting point

would inspire the extensive piezoelectric response by lowering the energy barrier for polarization rotation

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and extension. While on the other hand, the reduced retention of the spontaneous polarization would decrease the difference between each energy-allowed state and consequently restrain the obtained piezoelectric properties. To design the high piezoelectric materials by searching, or engineering a phase coexistence state, these two contradictory factors should be taken into account. Besides piezoelectric properties under either mechanical or electrical weak-field, the performance under large electric field was

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also measured.

Fig.4 shows the hysteresis loops of Sn-BT ceramics at room temperature under the frequency of 10 Hz. With increasing Sn content, there was a decrease in maximum and remnant polarization, as well as the

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coercive field. It can ascribe to the non-ferroelectric nature of the BaSnO3; with adding more Sn into BaTiO3, the ferroelectricity or correlation between ferroelectric dipoles decreased, resulting in reduced polarization.

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Besides, adequate Sn concentration would make BaTiO3 transfer to relaxor [22-24], thus resulted in the slim

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hysteresis loops with reduced polarization and coercive field, as indicated in Tab.1.

Fig.4.Hysteresis loops of xSn-BT ceramic for x = 2, 6, 8, 9, 10, 11.

Sn content

Pr (µC/cm2)

Pmax (µC/cm2)

Ec (kV/cm)

2Sn-BT

13.39

20.92

1.05

6Sn-BT

13.02

20.21

0.83

8Sn-BT

5.01

15.62

0.65

9Sn-BT

3.84

15.36

0.64

10Sn-BT

1.83

14.05

0.55

11Sn-BT

1.66

12.61

0.49

Tab.1. Remanent polarization (Pr), maximum polarization (Pmax) and conceive field (Ec) of xSn-BT ceramic for x = 2, 6, 8, 9, 10, 11.

Fig.5 shows the electrostrain curves under the electric field of 30 kV/cm from the Sn-doped BT

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ceramics at room temperature. All the strain curves exhibited typical butterfly shape. Different from the hysteresis loops which reflect the switchable entire domain population, the electrostrain indicates the switched non-180 degree domain population [30]. When approaching the quadruple point the strain curves got more and more slim with reduced hysteresis due to the decreased energy barrier. Besides, the

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electrostrain showed the maximum of 0.15% of the 9Sn-BT with small hysteresis, which was comparable to

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Fig.5.Electrostrain curves of xSn-BT ceramic for x = 2, 6, 8, 9, 10, 11.

that of the most widely used hard PZT ceramics. As shown in Fig.5, all the electrostrain curves could be

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roughly regarded as two different stages. The initial stage identified by the non-linear quick increase of the electrostrain was considered mainly arising from the extensive domain switching process. While the second stage generally came from the polarization extension and showed the linear increase to the external field. To characterize these two distinct physical process, two parameters d′33 defined as the maximum slope of the electrostrain and d″33 defined as the slope of the linear electrostrain part were introduced as shown in Fig.5. Electrostrain curves of each composition were measured under the electric field of 3 kV/mm under the frequency of 10 Hz within the temperature range from 20

to 80

. Fig.6 illustrates the change of d′33 and

d″33 with temperature from all the Sn-BT ceramics. For each individual composition, when approaching the T-O boundary, the d″33 exhibited maximum value. While when approaching their Tc, there was a sharp drop of d′33. The coexistence of ferroelectric phases was responsible to the high converse piezoelectric response,

the anisotropic flattening of the energy profile providing a low energy pathway for polarization rotation.

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, lying at the T-O boundary in the phase

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Moreover the 9Sn-BT exhibited an extremely high d′33 value at 40

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Fig.6.Change of dʹ33 and dʹʹ33 with temperature of xSn-BT ceramic for x = 2, 6, 8, 9, 10, 11.

diagram. Such result is consistent with the calculation result and small-signal piezoelectric property and

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demonstrated a strong domain switching behavior. It can be understood from both small energy barrier at the multi-phase boundary and considerable difference between each energetically equivalent state. While the d″33 didn’t change too much for each composition within the temperature range from 20

to 80

. Merely,

there was a slight increase when approaching to Tc due to the lower tetragonality. It is evident that the overall piezoelectric response would have a significant contribution, not only from the motion of domain walls but also to inter-ferroelectric boundaries. 4. Conclusion The piezoelectric performance of the Sn doped BaTiO3 ceramics were systemically studied either by simulation or experimental approach. Both results indicated that the strongest piezoelectric response appeared on the boundary between T and O phase rather than the quadruple point with lowest free energy

barrier. Experimentally, large piezoelectric coefficient d33 of 920 pC/N at 50

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and the electrostrain of 0.15%

with small hysteresis were found from the 9 mol% Sn doped BaTiO3 ceramics at room temperature. These results indicated that polarization rotation would have a significant contribution to the overall piezoelectric response. To design piezoelectric ceramic with high piezoelectric performance, both the thermodynamical low free energy barrier and the anisotropy between the changeable polarization states should be taken into account.

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Acknowledgments

The authors acknowledge the support of National Natural Science Foundation of China (51422704) and State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University

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(EIPE14113).

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Large piezoelectric coefficient d33 of 920 pC/N was obtained from the 9 mol% Sn doped BaTiO3 ceramics. Large electrostrain of 0.15% was also found in the 9 mol% Sn doped BaTiO3 ceramics.

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The joint effect of the anisotropy energy and the order parameter polarization benefited the large piezoelectric performance.

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Non-180° polarization rotation would have a significant contribution to the overall piezoelectric response.