High Curie temperature BiFeO 3 -BaTiO 3 lead-free piezoelectric ceramics: Ga 3+ doping and enhanced insulation properties

BiFeO 3 -BaTiO 3 (BF-BT) is a promising high temperature lead-free piezoceramics due to their excellent piezoelectric properties with high Curie temperature ( T C >500 °C). While the high leakage current density severely restricted its application. In this work, the leakage mechanism relative to the dielectric properties and piezoelectric properties were systematically studied with a special emphasis on gallium (Ga 3+ ) adding effect in 0.7BiFe (1- x ) Ga x O 3 -0.3BaTiO 3 (BFGa x -BT, 0≤ x ≤0.10) ceramics. A high resistivity ( ρ ) of 2.73×10 12 Ω·cm -1 and low leakage current ( J ) of 7.78×10 -9 A·cm -2 were achieved at x =0.06, which attributes to the low oxygen vacancies ( O V  ). The J - E curves reveal different type of conduction process in BFGa x -BT ceramics, including Ohmic conduction, space-charge-limited conduction (SCLC) mechanism and interface-limited Schottky emission. The BFGa0.06-BT ceramic exhibits excellent piezoelectric performance: d 33 =174 pC·N -1 , T C =497 °C, k p =29 %.


Introduction
Lead-based Pb(Ti, Zr)O3 (PZT) ceramics are primary commercial piezoelectric ceramics and widely used in sensors, brakes and transducers [1]. However, the application of lead-based piezoelectric ceramics is restricted by the toxicity of lead, which promotes the development of lead-free piezoelectric ceramics, such However, the narrow synthesis temperature range and Bi volatilization during solid-state reaction produce the formation of Bi25FeO40 and Bi2Fe4O9 phases and are obstacles for preparing pure (1-x)BF-xBT phase [16][17][18]. Besides, part of Fe 3+ will transform into Fe 2+ during calcining and sintering process. These factors all lead to the generation of a large amount of oxygen vacancies ( O V  ), resulting in a high leakage current (J) and a large dielectric loss, which is harmful for polarization and obtaining saturated P-E hysteresis loop [19][20]. Lots of attempts on reducing J and enhancing direct current resistivity (ρ) have been performed by optimizing preparation process and doping modification. Lee et al. inhibited the formation of secondary Bi2Fe4O9 and/or Bi25FeO39 phases by water quenching method, whereby a low leakage current density of 7.5×10 -10 A·cm -2 and an excellent piezoelectric property of d33=240 pC·N -1 achieved in 0.67BF-0.33BT ceramics [21]. Using nano-BaTiO3/Bi2O3/Fe2O3 as raw materials instead of BaCO3/TiO2/Bi2O3/Fe2O3 to prepare BF-BT ceramics, Cheng et al. realized an increased d33=210 pC·N -1 from 164 pC·N -1 in 0.7BF-0.3BT along with an order of magnitude improved insulation resistance [22].
Besides, the use of stability elements, like La, Ga, Bi(Zn1/2Ti1/2)O3 and Nd etc., to replace A and/or B site of BF is also effective to enhance the electrical performance of (1-x)BF-xBT ceramics [23][24][25][26]. It is reported that the water quenched BF-33BT-3BiGaO3 ceramic exhibited excellent piezoelectric properties d33=402 pC·N -1 with high TC=454 °C, which attributes to the low J=1×10 -10 A·cm -2 under the electric field of 50 kV·cm -1 [21]. However, the water quenching process is difficult to commercialize for its poor repeatability.
It is necessary to prepare excellent performance Ga-doped BF-BT using traditional methods. Zhou et al.

Experimental section
The conventional solid-state reaction method was used to prepare the 0.7BiFe(1-x)GaxO3-0.3BaTiO3 (0≤x≤0.10) (BFGax-BT) ceramics using Fe2O3, Bi2O3, (>99 %, both from Shantou Xilong Chemical Factory Guangdong, China), nano-particle BT (>99.9 %, from Aladdin, C-phase) and Ga2O3(>99.99 %, from Aladdin). These raw materials were weighed according to the chemical formula, and then mixed thoroughly in ethanol for 24 h by using a planetary ball mill. After calcining the mixtures at 800 o C for 6 h, the powders were re-milled, and then pressed into pellets of 10 mm in diameter and 1 mm in thickness under 80 MPa with 2 wt% polyvinyl alcohol (PVA). After excluding binder at 650 o C for 2 h, the pressed pellets were sintered at 980 o C for 3 h with the heating rate of 5 o C·min -1 . The sintered specimens were pasted with silver on both surface and fired at 600 o C for 20 min, then poled under a DC field of 3.5 kV at 100 o C for 10 min in a silicone oil bath.
The X-ray diffraction (XRD) with Cu Kα radiation (D/max-RB, Rigaku Inc., Japan) was used to examine crystalline structure of the sintered samples. Rietveld refinement was performed by using Materials studio software. The microstructure of the sintered samples was observed using field emission scanning electron microscopy (FESEM, SUPRATM 55, Japan). Bulk density was measured by using the Archimedes' method. The powders chemical state of the prepared ceramics was characterized by X-ray photoelectron spectroscopy (XPS: Thermo Fisher ESCALAB 250Xi) using Al Kα radiation (hν = 1486.6 eV) as an X-ray source. Dielectric properties were measured using a programmable furnace with a LCR analyzer (TH2828S) at 100 kHz in temperature range of 25 o C to 600 o C. The piezoelectric properties were measured using a quasi-static piezoelectric coefficient testing meter (ZJ-3A, Institute of Acoustics, Chinese Academy of science, Beijing, China). The kp and the mechanical quality factor (Qm) were determined by resonance-antiresonance method using an Agilent 4294A precision impedance analyzer (Hewlett-Packard, Palo Alto, CA). The J and ρ was measured by AT683 insulation test instrument (Applent instruments Ltd, China). A ferroelectric measuring equipment (aixACCT TF Analyzer 1000, Germany) was used to obtain the ferroelectric hysteresis (P-E) loops and field-induced strain (S-E) loops.

Results and discussion
Fig. 1a-b shows the ρ-E and J-E curves of poled BFGax-BT ceramics measured at room temperature (RT). The ρ of all samples rapidly decreases with E increasing at E≤8 kV·cm -1 . When E>8 kV·cm -1 , the ρ of 0≤x≤0.02 still decreases, while that of 0.04≤x≤0.1 hold steady. The ρ of x=0.06 is around 2.73×10 12 Ω·cm -1 under E=12 kV·cm -1 , which is larger than that of BF-BT (0.55×10 12 Ω·cm -1 ), indicating an effectively enhanced the insulation of BF-BT ceramics via Ga-doping. The J of BFGax-BT ceramics shows a different trend, which increases with E increasing. According to the previous reported transport behavior of BiFeO3 and other similar ferroelectric perovskite oxides [20,[28][29][30][31][32], the relationship between the leakage current (J) and the applied electric field (E) for Ohmic conduction, space-charge-limited conduction (SCLC) mechanism and interface-limited Schottky emission are expressed as in Eq. (1-3) as follow: Ohmic Where e is the electron charge, μ is the free carrier mobility, Ne is the density of the thermally stimulated electrons.
Where ɛr is the static dielectric constant, ɛ0 is the permittivity of free space, μ is the free carrier mobility, d is the thickness of thin film, and θ is the ratio of the total density of free electrons to the trapped electrons.
Where A is constant, T is absolute temperature, φb is the voltage barrier, kB is the Boltzmann constant, e is the electron charge, ɛ0 is the permittivity of free space and K is the optical dielectric. It is clearly according to Eqs.
(1-2) that the ln(J) is proportional to ln(E) for Ohmic conduction and SCLC mechanism, whose slope (S) of ln(J)-ln(E) is 1 or 2, respectively. While the ln(J) is proportional to E 1/2 in Eq.
The curves of ln(J)-ln(E) for BFGax-BT ceramics in Fig. 2a [20,[28][29][30][31][32]. As x increases to 0.04, the ln(J)-ln(E) plot in Fig. 2c shows a linear behavior with S=1.12, suggesting that the leakage mechanisms transform to a pure Ohmic conduction. When 0.06≤x≤0.1 at E≤8 kV·cm -1 , the ln(J)-ln(E) plots in Fig. 2d-f show a nonlinear behavior, thus single Ohmic conduction and SCLC mechanism can be ruled out. In order to confirm the leakage mechanism in this region, the ln(J)-E 1/2 curves for 0.06≤x≤0.1 are plotted in Fig. 2g. It is seen that the curves show obvious linear characteristic, suggesting the Schottky emission mechanism dominates according to Eq. 3. The same conduction mechanism was reported in Bi0.9La0.1FeO3 at low applied electric field [31]. When E>8 kV·cm -1 , the S of 0.06≤x≤0.1 in Fig. 2d-f is 0.92, 0.85 and 1.04, respectively, which correspond to Ohmic conduction.
The slopes for ln(J)-ln(E) and ln(J)-E 1/2 are plotted in Fig. 2h, which exhibit the leakage mechanism transformation.
In order to analysis the valence of the constituent elements, high resolution XPS spectral analyses were performed for BFGax-BT ceramics. The relative percent of Fe 3+ and OⅠ for BFGax-BT ceramics as a function of x are shown in Fig. 3a, along with the ρ at 12 kV·cm -1 . The detailed information of Bi 4f, Ba 3d, Ti 2p Fe 2p and O 1s can be seen in Fig. S1. It was found in Fig S1a that Fig. S2a-e. After peak fitting, the relative percentages of Fe 3+ (Fe 3+ (%)) and Fe 2+ (Fe 2+ (%)) were calculated from peak areas using equation increasing as shown in Fig. 3a. The maximum of OⅠ and ρ are achieved at x=0.06, which is 63 % and 2.73×10 12 Ω·cm -1 . It is reasonable that Ga-doping will reduce O1 V  derived from the valence of Fe, leading to the different leakage mechanism, which is benefited for improving ρ.  Table S1, it is obvious the variation with x of both TC and R phase fraction in Fig. 3d shows a good correction, which suggests the more R phase, the higher TC. Fig. 3 (a) x-dependence of the relative percentage of Fe 3+ and OⅠ for BFGax-BT ceramics, along with the ρ at 12 kV·cm -1 ; Temperature dependence of dielectric loss (tanδ) (b) and dielectric constant (εr) (c) measured at 100Hz for BFGax-BT ceramics; (d) The x-dependence of Curie temperature (TC) and R phase fraction.
Fig. 4a-c shows P-E loops at RT, bipolar S-E curves and unipolar S-E curves measured at 50 kV·cm -1 and 1 Hz for BFGax-BT ceramics, and the maximum polarization (Pm), remanent polarization (Pr), coercive field (EC), bias electric field (Ei), positive strain (Spos), negative strain (Sneg), unipolar strain (Suni) and ΔP 2 (the square of polarization difference, Pm 2 -Pr 2 ) as a function of x are summarized in Fig. 4d-f. All BFGax-BT ceramics exhibit saturated P-E loops and butterfly S-E curves. As x increasing, the Ei in Fig. 4d increases firstly and then decreases. The maximum Ei is shown at x=0.02, indicating the existence of lots of defect-dipole, which results in the largest Pm=28.7 μC·cm -2 and Pr =23.4 μC·cm -2 because of the free point defects will be oriented under electric field . As x≥0.8, the structure is far from the appreciate phase fraction, which causes the decreased Pr and Pm. Fig. 4e shows the x-dependence of Spos and Sneg for BFGax-BT ceramics. Spos is a sum of intrinsic piezoelectric lattice strain and extrinsic reversible domain switching strain, while Sneg involves the non-180° domain reorientation [38][39]. The increased Spos=0.14 % and Sneg=-0.06 % at x=0.08 caused by the increased PC phase results in the raise of distortion, which is consistent with the change of phase fraction. However, the structure of BFGax-BT ceramics turns to single phase fraction at x=0.10, which lead to the decrease of Spos and Sneg. It is reported that the relationship between field-induced strain (S) and the polarization (P) usually written as S=QΔP 2 , where Q represents the electrostrictive coefficient [40][41][42]. The x-dependence of Suni and ΔP 2 are contrasted in Fig. 4f, exhibiting exactly the same traces that the maximum value ΔP 2 =347 μC 2 ·cm -4 and Suni=0.14 % are achieved at x=0.08. Thus, the unipolar strain behavior in this work is attributed to the constant values of ΔP 2 .   indicating the BFGax-BT system has a promising prospect in high temperature piezoelectric devices.

Conflicts of interest
There are no conflicts of interest to declare.

Acknowledgments
This work was supported by National Natural Science Foundation of China (grant No. 52072028 and 52032007).

Electronic Supplementary Material
Supplementary material about XPS patterns, XRD patterns，Rietveld refined results of XRD patterns and SEM images for BFGax-BT ceramics is available in the online version of this article at https:// doi.org/10.1007/s40145-....