Large strain with ultra-low hysteresis and enhanced energy storage performance of Mn-doped 0.65Bi0.5Na0.5TiO3-0.35SrTiO3 lead-free ceramics

In this paper, the electric-field-induced strain behavior and energy storage performance of MnO-doped 0.65Bi0.5Na0.5TiO3-0.35SrTiO3 (NBT-ST-xMn) lead-free ceramics has been investigated. After the introduction of MnO into NBT-ST ceramics, pinched and double P-E hysteresis loops with high Pmax and negligible Pr can be observed due to the introduction of defect dipoles. As a result, a relatively high strain of 0.22% with ultra-low hysteresis of 14% was achieved under a moderate electric field of 60 kV/cm at x=1.0 mol.%. Excellent energy storage performance of 1.14 and 1.17 J cm with a high η of 83 and 80% are achieved at x=0.5 and 1.0 mol.%, respectively. Meanwhile, high electrostriction coeffcient of 0.022 mC with pure electrostrictive characteristics was obtained at x=0.5 mol.%. The results illustrate that the proper selection of base composition and effective chemical modifier can made the NBT-ST an outstanding candidate for actuators and energy storage devices.


Introduction
Piezoelectric ceramics are widely applied in industrial devices such as actuators, sensors and energy storage capacitors because of their excellent electromechanical properties [1][2][3][4][5][6][7][8]. Currently, the lead-based systems, such as Pb(Zr,Ti)O3 (PZT) still dominated the global piezoelectric ceramics market due to their outstanding piezoelectric properties and thermal stability [1,6]. However, lead-based materials contain much lead oxide, which is harm to the environment and human beings. Therefore, it is very necessary to seek lead-free ceramics with excellent performance to replace lead-containing materials.
In recent years, some efforts have been devoted to reduce the hysteresis of NBT-based materials [23][24][25][26][27][28][29][30][31][32][33][34]. Ullah et al. [23] tailored the hysteresis behavior as low as 40% in Nb-doped NBT-KBT-Ba0.7Sr0.3TiO3 (BNT-BKT-BST) ceramic. Li et al. [25] found the introduction of A-site vacancies (VA) and oxygen vacancies (VO) into NBT-KBT-Sr0.8Bi0.1□0.1Ti0.8Zr0.2O2.95 system could be beneficial for the reduction of strain hysteresis. The design of ceramic/ceramic composite consisting of an ergodic relaxor (matrix) and a nonergodic or ferroelectric phase (seed) was also identified as 3 typical feature that could reduce the strain hysteresis [28][29][30]. In our previous work, both of the strain value and the hysteresis of NBT-BT-ST and NBT-ST systems could be improved simultaneously by the introducing of defect dipoles [26,34]. High strain of 0.24% under 80 kV/cm with ultra-low hysteresis (about 10%) was obtained in Mn-doped NBT-BT-ST system while large strain of 0.32% with small hysteresis of 28% was realized at 60 kV/cm in Mn-doped 0.7NBT-0.3ST system. However, the applied electric field in Mn-doped NBT-BT-ST system was too high and the strain hysteresis in Mn-doped 0.7NBT-0.3ST system was still not small enough. It is well-known that 3 Results and discussion X-ray diffraction patterns of the NBT-ST-xMn ceramics are displayed in Fig. 1.
As shown in Fig. 1(a), all samples exhibited a single phase perovskite structure without apparent secondary phases, which proved that the MnO had completely incorporated into the structure of the 0.65NBT-0.35ST ceramics. The characterization of (200) peaks of the four samples was investigated in detail to obtain the phase structure evolution, as shown in Fig. 1  NBT-based ceramics has also been observed in our previous research [26,34].
The dielectric constant (εr) and dielectric loss tangent (tanδ) as a function of temperature for NBT-ST-xMn ceramics measured at 1, 10 and 100 kHz are displayed in Fig.3 (a)-(d). As εr-T curves shown, one distinctive dielectric anomaly with a strong frequency-dependent dispersion at a lower temperature and a relatively weak frequency-dependent dispersion at a higher temperature are observed for all the 5 samples. One distinctive dielectric anomaly observed in εr-T curves proved that the phase structure is the T phase at room temperature, which is in accordance with the XRD analysis. The temperature at which the εr reaches its maximum value is assigned to the curie temperature Tm, which corresponds to the phase transition from the relaxation phase to the paraelectric phase [35]. With x increasing, the Tm (1kHz) increases obviously while the maximum value of εr enhances firstly and then have some decrease, as shown in Fig.3 (e). The enhanced Tm is beneficial for the temperature stability of strains. Moreover, all the samples exhibit broadness peaks at Tm, suggesting a relaxor behavior in these ceramics.
For relaxor ferroelectrics, the diffuseness in the phase transition can be described by the following equation [36] 1/εr-1/εmax=C - (T-Tmax) (1) where C is the Curie-like coefficient and  is the degree of relaxation ranging between  In order to clearly compare the actuation properties of NBT-based materials, data from previous literatures that devoted to reduce the hysteresis are listed in Table 1.
Obviously, the hysteresis of NBT-based ceramics is still more than 20% by doping  Large strain with ultra-low hysteresis is beneficial to obtain purely electrostrictive effect. Fig. 6 presents the S-P 2 plots derived from the corresponding polarization and strain hysteresis loops of NBT-ST-xMn ceramics. The electrostrictive effect can be calculated by the formula: S=Q33P 2 , where S, Q33, and P are the strain, electrostrictive coefficient, and polarization, respectively [26]. The hysteresis has made the S-P 2 curve deviate slightly from a quadratic relationship at x=0.0% and x=1.5%. For 0.5 % and 1.0 % Mn-doped ceramics, a pretty linear dependence of strain on polarization square can be noted, classifying that "purely" electrostrictive effects are achieved. The calculated Q33 value for 0.5 % Mn-doped NBT-ST ceramic is 0.022 m 4 /C 2 , which is no smaller than that of the representative electrostrictive materials in the literature [24,26,31,34].
Except for the high strain with ultra-low hysteresis, Mn doping also contribute to enhance the energy storage density of 0.65NBT-0.35ST ceramics. Generally, energy-storage density of nonlinear dielectric ceramics can be calculated by the unipolar P-E loop with the following equations: [42] 8 where Wrec, Wloss and η denote the recoverable energy storage density, energy loss density and energy storage efficiency, respectively. It can be seen from the above equations that large Pmax, small Pr and high BDS is very important to achieve high energy storage density. In order to investigate the electric-field-strength dependence distribution [26,43], as shown in Fig. 9(a). The average BDS is determined by fitting lines and the results are displayed in the inset of Fig. 9(a)

Conclusions
In conclusion, NBT-ST-xMn lead-free ceramics were designed and prepared by solid state synthesis method. It has been found that the addition of MnO also can induce defect dipoles into 0.65NBT-0.35ST ceramic, which result in double P-E hysteresis loops with high Pmax and small Pr. Accordingly, a relatively high strain of 0.22% with ultra-low hysteresis of 14% was achieved under a moderate electric field of 60 kV/cm at x=1.0 mol.% due to the introduction of defect dipoles, the enhancement of relaxor behavior and the increased grain size. Excellent energy storage performance of 1.14 and 1.17 J cm -3 with a high η of 83 and 80% were also 10 achieved at x=0.5 and 1.0 mol.% due to the enhanced BDS and Pmax-Pr value.
Meanwhile, high electrostriction coeffcient of 0.022 m 4 C 2 with pure electrostrictive characteristics was obtained at x=0.5 mol.%. The findings demonstrate that MnO-doped NBT-based ceramics are promising to be applicable for actuators and energy storage devices.