The relationship between phase structure and piezoelectric characteristics is demonstrated by producing a sequence of (1 − x)BiFeO3xBaTiO3 ceramics. A morphotropic phase boundary between the R and PC phases in BF–xBT ceramics is observed within the 0.285 ≤ x ≤ 0.315 range. High piezoelectric properties with a piezoelectric constant (d33) of 174 pC/N and a planar electromechanical coupling coefficient of 0.33 are achieved in BF–xBT ceramics at 0.285 ≤ x ≤ 0.315 due to the coexistence of the R and PC phases. In addition, a slight variation (±15%) in Δd33/d33 over the temperature range of 25–400 °C is noted in BF–xBT ceramics at 0.285 ≤ x ≤ 0.315, indicating that BF–xBT ceramics hold promising potential for use in high-temperature applications.

Piezoelectric materials are recognized for their unique capability to convert mechanical energy into electrical energy and vice versa, presenting substantial potential across various applications, including actuators, sensors, transducers, and others.1–3 Historically, lead-based piezoelectric ceramics, such as Pb(Zr,Ti)O3 (PZT), have been predominant in practical uses due to their superior piezoelectric properties and excellent temperature stability.3–5 However, in consideration of human health and environmental impacts, recent years have seen a shift toward lead-free materials, such as (K,Na)NbO3, (Bi,Na)TiO3, BiFeO3, BaTiO3, and their derivatives, which have become prominent in piezoceramics.6–11 However, these lead-free piezoelectric ceramics often exhibit a low Curie temperature (TC), constraining their use at high temperatures.

Bismuth ferrite (BiFeO3, BF) ceramic, possessing a rhombohedral perovskite structure and exhibiting a high TC of 830 °C along with a substantial polarization value (PS = 90–100 µC/cm2), is potent for high-temperature applications.12–15 However, challenges such as the reduction in Fe valence state and the volatilization of Bi3+ during sintering generate oxygen vacancies (Vö), resulting in poor electrical insulation, increased leakage current density, enhanced dielectric loss (tan δ = 15%), and inadequately saturated PE loops (Pr = 2.5 µC/cm2; EC = 40 kV/cm).16 In addition, impurity phases, such as Bi2Fe4O9, Bi25FeO39, and Bi25FeO40, which appear at similar temperatures, compromise the phase stability of pure BF ceramic, complicating the synthesis of a single-phase BF ceramic.17,18 In order to address these limitations, researchers have extensively implemented strategies such as partial substitution at A- and B-sites (A: La, Y, and Nd; B: Al, Ga, Zr1/2Mn1/2, Co, Mg1/2Ti1/2, Zn1/2Ti1/2, and Ni1/2Ti1/2) and forming BF solid solutions with other ABO3 perovskites (e.g., BaTiO3, SrTiO3, and Bi0.5K0.5TiO3).13–15,19–23,25,26 The BiFeO3–BaTiO3 system (BF–xBT) has gained increasing attention for its high TC (above 580 °C) and relatively high piezoelectricity.23,24,27–29 In 2015, Lee et al.28 reported that BF–0.33BT ceramics with 3 mol. % Ga-doping exhibited both a high TC and a significant d33 value, positioning the BF–xBT system as a promising alternative to lead-based piezoceramics. More recently, in 2023, Song et al.31 have described the 0.7BF–0.3BT ceramic, which was poled at 120 °C and 50 kV/cm, achieving an impressive d33 of 205 pC/N, thereby enhancing the piezoelectric performance of the system. Although substantial progress has been made in advancing the piezoelectric properties of BF–xBT ceramics, continued focus on improving piezoelectricity and temperature stability is essential.

This study employs the traditional solid-state method to prepare a series of (1 − x)BiFeO3xBaTiO3 ceramics. The volatilization of Bi2O3 at high temperatures is counteracted by adding extra Bi content. A systematic investigation is conducted using XRD, PE, IE, and d33T to assess the impact of BT on phase structure, piezoelectric properties, and thermal stability of BF–xBT. XRD patterns revealed that the BF–xBT ceramics with 0.285 ≤ x ≤ 0.315 are positioned in the morphotropic phase boundary (MPB) between the rhombohedral (R) and pseudocubic (PC) phases. Effective management of the MPB allows for precise adjustment of the phase ratios by regulating the BT content as domain switching progresses. Hence, ceramics with x values of 0.285–0.315 exhibit relatively high piezoelectric properties with d33 = 174 pC/N and kp = 0.33. These ceramics also demonstrate excellent temperature stability with Δd33/d33 < ±15% over the 25–400 °C temperature range, highlighting the potential of BF–xBT ceramics in high-temperature applications.

Following the stoichiometric ratios of (1 − x)BiFeO3xBaTiO3 (abbreviated as BF–xBT; 0.270 ≤ x ≤ 0.330), the team weighed raw Bi2O3 (99.9%), BaCO3 (99.9%), Fe2O3 (99.9%), and TiO2 (99.9%) powders and ball-milled them in anhydrous ethanol for 12 h. To compensate for the loss of Bi2O3 during the sintering process, an additional 10 mol. % Bi2O3 was incorporated into the stoichiometric composition. After calcining for 2 h at 800 °C, the dry mixture was ball-milled again for 12 h. Following the drying process, the powders were compressed into circular disks, 10 mm in diameter and 1.5 mm thick, using a binder consisting of 2% polyvinyl alcohol (PVA) under a pressure of 300 MPa. The disks underwent a 2 h debonding process at 600 °C, followed by sintering at 980 °C for 3 h with a heating rate of 5 °C/min. The sample surfaces were meticulously polished using 2000 mesh silicon carbide papers, then coated with silver pastes, and subjected to a solidification process at 600 °C for 30 min to consolidate the silver pastes and alleviate the stress induced during polishing. After coating, the samples were subjected to a 5 kV/mm electric field at a temperature of 90 °C for 30 min in the presence of silicone oil.

An x-ray diffractometer (XRD, D8 Discover, Bruker Axs Gmbh, Germany) was utilized to determine the phase structure. Field emission scanning electron microscopy (SEM, SU8020, Hitachi High-Tech, Japan) was used to observe the microstructure. The temperature dependence of permittivity was measured at 1 kHz during heating (2 °C/min) using a precision LCR meter (TH2827C, Tonghui, China) in a temperature-regulated chamber. The piezoelectric constant (d33) was measured using a quasi-static d33 testing meter (ZJ-3AN, Institute of Acoustics Academic, China). The samples were annealed post-poling at 25–500 °C for 20 min to characterize the temperature stability of BF–xBT ceramics. Ferroelectric hysteresis (PE) loops and current–field (IE) loops were measured using a high voltage amplifier (aixACCT, TF Analyzer 2000, Germany) at 1 Hz. The dielectric properties and electromechanical coupling factor kp were measured using an impedance analyzer (4294 A, Agilent, USA).

Figure 1(a) depicts the XRD patterns of BF–xBT ceramics measured at room temperature within 0.27 ≤ x ≤ 0.33. The enlarged sections in Figs. 1(b)1(d) emphasize additional details. Each sample displays a typical perovskite structure without any impurity phase, indicating a stable solid solution formed. The vertical lines indicate the standard diffraction peaks for BiFeO3 (BF) with R symmetry (PDF No. 71-2494) and BaTiO3 (BT) with PC phase (PDF No. 75-0461). The bimodal peaks observed at ∼32°, 39°, and 57° in the BF–0.270BT ceramic align with the characteristic peaks of the R phase as indicated by the standard card, showing that these samples consist of a single R phase. As the BT content increases, the intensity of the (104)R and (006)R peaks gradually decreases. When the BT content exceeds 0.315, only a single peak near 32°, 39°, and 57° is present, indicating that the samples’ crystalline structure at x = 0.330 is of single PC symmetry. These results demonstrate that with the increase in BT, the BF–xBT ceramics undergo a phase transition from R to PC phases, facilitating the formation of MPB of the R–PC phase for 0.285 ≤ x ≤ 0.315.

FIG. 1.

XRD patterns of the BF–xBT ceramics in selected 2θ ranges of (a) 2θ = 20–70°, (b) 2θ = 31–32.5°, (c) 2θ = 38–40°, and (d) 2θ = 55–58°.

FIG. 1.

XRD patterns of the BF–xBT ceramics in selected 2θ ranges of (a) 2θ = 20–70°, (b) 2θ = 31–32.5°, (c) 2θ = 38–40°, and (d) 2θ = 55–58°.

Close modal

Figure 2 shows the SEM images of BF–xBT ceramics. The images reveal well-defined grain boundaries and a high-density level across all samples of BF–xBT ceramics. Bi2O3 melts at temperatures above 830 °C; therefore, during the sintering process, a liquid phase emerges, promoting elemental diffusion in the solid solutions and enhancing their density. The average grain sizes, calculated using MATLAB, range from 4.41 to 4.66 µm, indicating that variations in BT content do not significantly influence grain sizes.

FIG. 2.

SEM images of BF–xBT ceramics: (a) x = 0.270; (b) x = 0.285; (c) x = 0.300; (d) x = 0.315; and (e) x = 0.330.

FIG. 2.

SEM images of BF–xBT ceramics: (a) x = 0.270; (b) x = 0.285; (c) x = 0.300; (d) x = 0.315; and (e) x = 0.330.

Close modal

Figure 3 shows the temperature-related changes in the dielectric constants (εr) and dielectric loss factor (tan δ) measured at 1 kHz for BF–xBT ceramics. A single dielectric peak is observed above 450 °C, which corresponds to the Curie temperature (TC). Initially, TC exhibits an upward trend, followed by a decline, peaking at 545 °C when x = 0.315, as shown in Fig. 3(a). Before reaching 400 °C, the tan δ remains below 3 and gradually increases to 14 at 500 °C as conductivity increases. These results demonstrate that BF–xBT ceramics offer a reliable prospect for high-temperature applications.

FIG. 3.

Temperature dependences of dielectric constants (a) and dielectric loss (b) of BF–xBT ceramics.

FIG. 3.

Temperature dependences of dielectric constants (a) and dielectric loss (b) of BF–xBT ceramics.

Close modal

The domain structure significantly influences the piezoelectric and ferroelectric properties of ceramics. The polarization–electric field (P–E) loops and the I–E loops of piezoelectric ceramics arise from domain switching and domain wall movement.30 The PE and IE loops for the BF–xBT ceramics are recorded at room temperature, as depicted in Fig. 4, under an electric field strength of 60 kV/cm. Due to domain switching during electric loading, typical ferroelectric PE loops and a single polarization current peak near the coercive field (EC) are observed in each sample. The maximum domain number is noted at x = 0.285, as Fig. 4(b) shows, with current peaks rising and then falling. Figure 4(c) displays the changes in the remnant polarization (2Pr) and 2EC. As the BT content increases, 2Pr stabilizes near 52.3 µC/cm2. The 2EC exhibits a varied trend, increasing from 43.0 µC/cm at x = 0.270 to 66.7 µC/cm at x = 0.330, possibly due to increased defects that enhance domain-wall pinning in the BF–xBT ceramics. In addition, the asymmetry observed in the PE loops, as shown in Fig. 4(a), can also be attributed to these defects.

FIG. 4.

Ferroelectric hysteresis (PE) loops (a), current–electrical field (IE) loops (b), and variation of polarization (2Pr) and electric field (2EC) (c) of BF–xBT ceramics.

FIG. 4.

Ferroelectric hysteresis (PE) loops (a), current–electrical field (IE) loops (b), and variation of polarization (2Pr) and electric field (2EC) (c) of BF–xBT ceramics.

Close modal

Figure 5 illustrates the piezoelectric coefficient (d33), planar mode electromechanical coupling coefficient (kp), and mechanical quality factor (Qm) for BF–xBT ceramics. As the BT content increases, d33 and kp rise from 156 pC/N and 0.29 to 168–174 pC/N and 0.33 at x = 0.285–0.315, respectively, and then decrease to 150 pC/N and 0.27 as the BT content increases to 0.330. Qm remains at 33 throughout. High piezoelectric responses of d33 = 168–174 pC/N and kp = 0.33 are achieved at x = 0.285–0.315. This enhancement can be ascribed to the phase structure in R–PC coexistence, which facilitates polarization rotation and extension among various ferroelectric phases by minimizing the energy barrier involved, thereby significantly enhancing these processes.

FIG. 5.

Piezoelectric coefficient (d33), planar mode electromechanical coupling coefficient (kp), and mechanical quality factor (Qm) of BF–xBT ceramics.

FIG. 5.

Piezoelectric coefficient (d33), planar mode electromechanical coupling coefficient (kp), and mechanical quality factor (Qm) of BF–xBT ceramics.

Close modal

Figure 6 displays the impact of annealing on the piezoelectric properties of BF–xBT ceramics. The d33 for BF–0.285BT, BF–0.300BT, and BF–0.315BT ceramics annealed at 25–400 °C remains above 170 pC/N and then gradually decreases as the annealing temperature approaches TC. Figure 6(b) presents the calculated Δd33/d33 for BF–xBT ceramics to assess the temperature stability. A variation of ±15% is observed for BF–xBT ceramics with x = 0.285–0.315 over the temperature range of 25–400 °C. Consequently, a large piezoelectricity (d33 ∼174 pC/N) and an outstanding temperature stability (Δd33/d33 < ±15%) are simultaneously attained in the BF–xBT ceramics.

FIG. 6.

Temperature-dependent piezoelectric coefficients (d33T) (a) and temperature stability Δd33/d33 [(d33(T) − d33(RT))/d33(RT)*100%] (b) of BF–xBT ceramics.

FIG. 6.

Temperature-dependent piezoelectric coefficients (d33T) (a) and temperature stability Δd33/d33 [(d33(T) − d33(RT))/d33(RT)*100%] (b) of BF–xBT ceramics.

Close modal

This study systematically investigates the impact of BT content on the electrical properties and phase structure of BF–xBT ceramics using XRD, PE, IE, and d33T. These results indicated that domain switching can be enhanced by regulating the appropriate BT content due to the effective control of the MPB. Hence, the material exhibited excellent piezoelectric properties, with a d33 of 174 pC/N and a kp of 0.33. In addition, the ceramics demonstrated outstanding temperature stability, with a Δd33/d33 of less than ±15% over the temperature range from 25 to 400 °C at BT contents ranging from 0.285 to 0.315. This research into BT content will stimulate further investigations into the development of BF–BT-based ceramics for high-temperature applications.

This work was supported by the Guangxi Innovation-Driven Development Project (Grant No. AA18118030).

The author has no conflicts to disclose.

H.R.W. conducted the simulation and experiments, collected the data, and wrote the manuscript draft. X.H.Z. supervised the whole work. All authors contributed to the preparation of the final manuscript.

Hao-Ran Wu: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Writing – original draft (equal). Xiu-Hai Zhang: Funding acquisition (equal); Supervision (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article.

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