Temperature independence of piezoelectric properties for high-performance BiFeO3–BaTiO3 lead-free piezoelectric ceramics up to 300 °C

The temperature-dependence behaviors of ferroelectric, piezoelectric, kp and electrical-field-induced strain were carefully evaluated for high-performance BiFeO3–0.3BaTiO3 (BF–0.3BT) ceramics. There results indicate, combined with Rayleigh analysis and temperature-dependence XRD and PFM, that the increase of strain and large signal with increasing the temperature from room temperature to 180 °C is related to the joint effect of intrinsic contribution (lattice expansion) and extrinsic contribution (domain switching). With further increasing the temperature to 300 °C, the large signal d33 and electrical-field-induced strain mildly decrease because of the increase of conductivity for BF–0.3BT ceramics. However, different from strain and large signal the small signal d33(E0) and kp exhibit excellent temperature stability behavior as the temperature increases from room temperature to 300 °C.

It is well known that industrial implementation not only demands good piezoelectric performance at room temperature but also hopes an excellent stability for piezoelectric properties in response to temperature changes. Despite the d 33 of BF-BT system has achieved major breakthroughs 20 and even it can meet the industrial application, how about the temperature stability of piezoelectric properties for the BF-BT ceramics is, and whether it is similar with the PZT system? Very few articles 31 point out that the ferroelectric property for BF-BT ceramics was enhanced with increasing the temperature from room temperature to 150 C. Nevertheless, there have been few investigations of the temperature dependence of piezoelectric properties for BF-BT ceramics. In addition, the reasons which cause the improvement of ferroelectric and k p properties are still vague for BF-BT ceramics so far.
In this work, 0.7BiFeO 3 -0.3BaTiO 3 (abbreviated as BF-0.3BT) ceramics were prepared to reveal the temperature-dependence behavior of its ferroelectric, piezoelectric, k p and electrical-eld-induced strain properties in the wide temperature range between room temperature and 300 C. Because of the joint effect of intrinsic contribution (lattice expansion tested by temperature-dependence XRD) and extrinsic contribution (domain switching proved by temperature-dependence PFM), the remanent polarization (P r ), strain and large signal d *

33
increase with increasing the temperature from room temperature to 180 C. This result is also veried by Rayleigh analysis models, which indicates that the extrinsic contribution and intrinsic contribution (d int Â 10 À12 ) increase from 18.70% and 161.84 m V À1 at 24 C to 34.45% and 207.06 m V À1 at 100 C. With further increasing the temperature from 180 C to 300 C, the large signal d 33 and electrical-eld-induced strain mildly decrease. However, different the strain and large signal d * 33 ; the small signal d 33 (E 0 ) and k p for BF-0.3BT lead-free piezoelectric ceramics exhibit an excellent temperature-insensitive behavior at measured temperature range between the room temperature and 300 C because it has high T C $475 C and stability phase structure below T C , which is even superior to PZT system. This result offers an evidence for industrial application of BF-BT ceramics to replace Pb-based piezoelectric ceramics in hightemperature piezoelectric devices.

Experimental procedure
Nano-BaTiO 3 (about 100 nm, 99%), Fe 2 O 3 (99%) and Bi 2 O 3 (99%) powders were used as raw materials, which were weighed on the basis of a composition of 0.7BiFeO 3 -0.3BaTiO 3 (abbreviated as BF-0.3BT), and mixed in nylon tank. Aer dried and calcined at 800 C for 12 h, the resultant powders were milled again and pressed into disks of 10 mm in diameter and 1.5 mm in thickness under 80 MPa using 3 wt% polyvinyl alcohol (PVA) as the binder, followed by burning the binder at 650 C by 5 C min À1 and for 2 h. Aer removing the PVA, the samples were sintered at 1000 C with holding for 6 h, and then furnace cooled to room temperature. The sintered specimens were coated with silver for the electrical measurement. The coated samples were poled under a DC eld of 4 kV mm À1 at 130 C for 30 min in a silicone oil bath.
The crystallographic structures were investigated by using Xray diffraction (XRD: D/max-RB, Rigaku Inc., Japan) with a Cu Ka radiation (l ¼ 0.15406 nm) ltered through a Ni foil. Bulk densities of samples were measured using the Archimedes method. The temperature dependence of dielectric properties was examined using a programmable furnace with an LCR analyzer (TH2828S) at 1 kHz in temperature range of 20 C to 650 C. Microstructure of the sintered samples was observed by eld emission scanning electron microscopy (FESEM, SUPRATM 55, Japan). Piezoelectric properties were measured using a quasi-static piezoelectric coefficient testing meter (ZJ-3A, Institute of Acoustics, Chinese Academy of Sciences, Beijing, China). Field-dependent and temperature-dependent parameters including d 33 (E) hysteresis loops, ferroelectric hysteresis (P-E) loops and unipolar strain S(E) were measured with ferroelectric measuring equipment (aixACCT TF Analyzer 2000, Germany). The temperature-dependent planar electromechanical coupling coefficient k p was determined by resonance-antiresonance method using an Agilent 4294A precision impedance analyzer (Hewlett-Packard, Palo Alto, CA). The temperature-dependent of leakage currents (J) were measured by precision 10 kV HVI-SC precision materials analyzer (Radiant technologies INC, USA). In addition, the PFM experiments were carried out by a commercial atomic force microscope (MFP-3D, Asylum Research, USA)

Results and discussion
Temperature-dependent P-E loops of BF-0.3BT ceramics under a eld of 4.0 kV mm À1 at the frequency of 1 Hz are shown in Fig. 1(a-e). The samples exhibit a relative saturation P-E hysteresis loops measured at room temperature as shown in Fig. 1(a) and only a single polarization current peak located in E 1 corresponding to the normal domain switching is observed during electric loading. However, the E 1 is larger than coercive eld (E C ). This indicates that the domain switching is suppressed by the defect dipole under a low electric eld. With increasing the temperature to 100 C, a typical ferroelectric polarization hysteresis loop is detected, along with E 1 ¼ E C , indicating that increase of temperature is benet for reducing the pinning of defect dipole and promoting the domain switching. As the temperature reaches to 180 C, the E 1 is lower than E C as shown in Fig. 1(e), suggesting that the conductivity of samples increases, caused by the increase of migration of defect ions. The detailed variations of P r , P max and E C with different temperature are shown in Fig. 1(f). As the temperature increases, the E C monotonously decreases. Instead, the P r and P max from 13.57 mC cm À2 and 17.03 mC cm À2 increase to 54.23 mC cm À2 and 46.44 mC cm À2 with increasing the temperature from 25 C to 180 C, respectively. In addition, it also can nd that the P r is larger than P max when the temperature is over 160 C. This may be because the fact that the leakage abruptly increases as the temperature is over 160 C as shown in Fig. S1. † Temperature-dependent electrical-eld-induced strain (S-E loop) under a eld of 5.5 kV mm À1 is shown in Fig. 2(a). The strain of samples measured at room temperature is 0.15%, which is comparable to that of so PZT ceramics (typical strain values 0.15% at 2 kV mm À1 ) 32 and 0.95(Na 0.49 K 0.49 Li 0.02 ) (Nb 0.8 Ta 0.2 )O 3 -0.05CaZrO 3 (CZ5) ceramics (0.16% at 6 kV mm À1 ). 13 With increasing the temperature, the strain increases. This phenomenon is consistent with the PZT system, such as PZT-5H 32 and PZT4, 10 indicating that the BF-0.3BT ceramics exhibit slight temperature-dependent behaviors. As the temperature increases  from 30 C to 180 C, the strain increases from 0.15% to 0.237%. Different from strain, the small signal d 33 (E) exhibits few changes with increasing the temperature as shown in Fig. 2(b) and (c). Fig. 2(b) shows the eld-dependent piezoelectric coefficient d 33 (E) loops, which were measured by applying a triangular signal of 4.0 kV mm À1 at the frequency of 1 Hz with an superimposed AC voltage of 25 V at 250 Hz, and were used to characterize the quasi-static d 33 . The equivalence between the piezoelectric constant value achieved by the d 33 (E) measurement and that acquired from quasi-static d 33 meter has also been conrmed by Fialka et al. 33 and Yao. 34 Generally, the positive value at zero eld in d 33 (E) loops is considered as piezoelectric constant d 33 . The small signal d 33 (E 0 ) of BF-0.3BT ceramics is about 195 pm V À1 at room temperature that is comparable to the piezoelectric coefficient d 33 (205 pC N À1 ) measured by a quasi-static piezoelectric constant apparatus. Therefore, the temperature-dependence electric-eld-dependent piezoelectric coefficient d 33 (E) loops were used to represent the temperature-dependence d 33 . From the Fig. 2(b), it can nd that the small signal d 33 (E 0 ) has little change as the temperature increases from 30 C to 160 C. The detailed variations for small signal d 33 (E 0 ) and larger signal d *

33
with different temperature are shown in Fig. 2(c). With increasing the temperature from 30 C to 180 C, the d * 33 increases from 260 pm V À1 to 404 pm V À1 . It is well known that the piezoelectric coefficient is comprised of both an intrinsic ðd * int Þ and extrinsic ðd * ext Þ components as shown in eqn (1). The intrinsic piezoelectric contribution is related to the lattice variation. However, the extrinsic contribution is associated with domain switching and phase boundary motions. Here the increases of d * 33 with increasing temperature may also be ascribed to crystal lattice variation and domain switching. Different from large signal d * 33 ; small signal d 33 (E 0 ) exhibits an evident temperature-stability behavior. This indicates that BF-0.3BT ceramics have a good promising in high temperature sensors areas. Fig. 2(d) shows the temperature dependence of dielectric permittivity 3 r and dielectric loss factor tan d for BF-0.3BT ceramics measured at 1 kHz between 20 C and 650 C. Only one 3 r peak is detected, corresponding to Curie temperature T C ¼ 475 C, which is superior to other lead-free piezoelectric systems, such as: KNN, BNT-BT, BT, and most Pb-based systems, further indicating that BF-BT ceramics have a good promising in high temperature sensors and actuators. In addition, the fracture surface SEM of samples is shown in inset of Fig. 2(d), in which few pores, caused by Bi 2 O 3 's volatilization at sintering process, are detected. The relative density of the BF-0.3BT ceramics is about 93%.
In order to clarify the effect of intrinsic and extrinsic contribution on the P r , d 33 (E 0 ) and d * 33 ; the temperature dependence of XRD for BF-0.3BT ceramics was measured and shown in Fig. 3(a) and S2. † The crystal structure is R and T twophase coexistence at room temperature as shown in Fig. S3 † and does not change until the temperature over 450 C as shown in Fig. 3(a) and S2. † Moreover, with increasing the temperature, the diffraction peaks shi to the low angle, indicating that the lattice expands as shown in Fig. 3(a). From XRD results, it is still difficult to exclude the effect of extrinsic contribution on the piezoelectric properties. The Rayleigh model, which was originally used for analyzing the nonlinear response of ferromagnetic materials 35,36 and also successfully applied to ferroelectric materials to describe the linear dependence of piezoelectric constant on the electric eld amplitude recently, is one of the simplest approaches to obtaining details on nonlinear phenomena due to the domain motion. Thus, the intrinsic and extrinsic contribution on strain and piezoelectric coefficient can be evaluated by using the Rayleigh model analysis as shown in Fig. S4. † The BF-0.3BT ceramics exhibit linear Rayleigh behavior over the range of 0.5 # E 0 # 1.8 kV mm À1 measured at room temperature as shown in Fig. 3(b) and 0.5 # E 0 # 1.0 kV mm À1 measured at 100 C as shown in Fig. 3(c), respectively. With increasing the electric eld, the extrinsic contribution linearly increases, resulting in the increase of d 33 . Comparing with the samples measured at room temperature, with increasing the temperature both of intrinsic and extrinsic contribution increase as shown in Table 1. The intrinsic contribution value is 161.84 pm/V at room temperature, and increases to 207.06 pm V À1 at 100 C. This may be related to the lattice expansion. The ratio of extrinsic contribution using E 0 ¼ 0.8 kV mm À1 increases from 18.7% at 24 C to 34.45% at 100 C as shown in Table 1, indicating that the increase of temperature is propitious to domain switching. The apparatus PFM facilitates the observation of the components of spontaneous polarization vectors in domains, Fig. 4 PFM scanning results of out-of-plane amplitude and phase for the sample measured at 25 C (a1) and (a2), and 100 C (b1) and (b2). which consequently may shed light on the variation of the domain. In situ PFM phase and piezoresponse of BF-0.3BT ceramics were investigated at different temperatures 25 C and 100 C, as depicted in Fig. 4. Both micro and nano domain are observed in the samples at 25 C. As the temperature increases to 100 C, obvious domain switching (at 1 and 2 areas) was detected as shown in Fig. 4(b2), further verifying that the increase of temperature can release the pinning of ionic defect and facilitate the domain switching.
In order to further measure the temperature stability of electrical-eld-induced strain, d * 33 ; d 33 (E 0 ) and k p for BF-0.3BT ceramics, the temperature-dependent strain curves, d 33 (E 0 ) and k p from room temperature to 300 C were tested as shown in Fig. 5. The strain under a eld of 0.5 kV mm À1 slightly increases with increasing the temperature from 24 C to 200 C, and then it mildly decreases when the temperature increases from 200 C to 300 C as shown in Fig. 5(a). The slight increase of strain is consistent with previous results as shown in Fig. 2, which is due to the joint effect of intrinsic contribution (lattice expansion) and extrinsic contribution (domain switching). As the temperature further increases, the decrease of strain may be attributed to the increases of conductivity, which is adverse to the polarization of ceramics and dipole alignment. 30 The temperature-dependent strain behavior (in terms of temperaturedependent d * 33 normalized to its room temperature value d * 33 RT ) of several representative piezoceramics is also provided in Fig. 5(b) for comparison. It is noted that the strain variation of BF-0.3BT ceramics is superior to all these piezoelectric ceramics (including PZT-5H, 32 PZT4, 10 KNN-3T-0.5Mn, 38 KNLN-BZ-BNT 39 and BNT-BT-KNN 37 ). BF-0.3BT ceramics can endure temperature as high as 300 C and the large signal d * 33 almost has a little change from room temperature up to 300 C. In contrast, the range of applications of the PZT-5H, PZT4, KNN-3T-0.5Mn, KNLN-BZ-BNT and BNT-BT-KNN systems is restricted below 200 C due to their low Curie temperature T C . The temperature-dependence piezoelectric properties d 33 (E 0 ) are shown in Fig. 5(c). The small signal d 33 (E 0 ) also exhibit excellent temperature-insensitive behavior. This phenomenon is different from that of CZ5, in which d 33 drops fast with increasing temperature in the temperature range between room temperature and 175 C, due to the existence of polycrystalline phase boundary (PPT) around room temperature. 13 The excellent temperature stability behavior of d 33 (E 0 ) for BF-0.3BT system indicates that BF-0.3BT ceramic have a good prospects for replacing the PZT systems in the high-temperature devices. The temperature-dependence planar electromechanical coupling factor (k p ), which was determined by resonanceantiresonance method at different temperatures as shown in Fig. S5, † provides the further evidence for the temperatureinsensitive behavior of BF-0.3BT ceramics as shown in Fig. 5(d). The variation of k p is less than 10% within the temperature range from room temperature up to 300 C, which is better than textured KNN-3T ceramics. 40 In addition, the BF-0.3BT ceramics also show excellent high-temperature anti-aging behavior, as shown in Fig. S6. † Aer experiencing a heat treatment at 300 C or 400 C for 500 h, the variation of the piezoelectric coefficient (d 33 ) is less than 5%.

Conclusions
The temperature-dependent behaviors of ferroelectric, piezoelectric, k p and electrical-eld-induced strain were investigated to uncover the temperature stability of high-performance BF-0.3BT piezoceramics. Because of the joint effect of intrinsic (lattice expansion) and extrinsic contribution (domain switching), both electrical-eld-induced strain and large signal d * 33 for BF-0.3BT ceramics increased with increasing the temperature from room temperature to 180 C. As the temperature furthering increases, the large signal d 33 and electrical-eldinduced strain mildly decreased, which was due to the increase of conductivity for BF-0.3BT ceramics. No matter what, the strain and d * 33 values at 300 C are still higher than that at room temperature. In addition, the d 33 (E 0 ) and k p exhibit excellent temperature-insensitive behavior in the wide temperature range between room temperature and 300 C, which revealed the promising potential of using BF-0.3BT ceramics in high-temperature piezoelectric devices.

Conflicts of interest
There are no conicts to declare.