Investigation of BiFeO₃-BaTiO₃ lead-free piezoelectric ceramics with nonstoichiometric bismuth

INTRODUCTION

Piezoelectric ceramics are extensively employed in electronic devices, such as sensors, actuators, filters, ultrasonic devices, etc., which are realized through the mutual conversion between mechanical and electrical energies^[1-3]. Over decades, Pb(Zr_xTi_1-x)O₃-based piezoelectric ceramics have been used dominantly in commercial devices. However, the toxicity of lead (Pb) can cause irreversible damage to human health and the environment, which promotes research hotspots on lead-free materials^[4-6]. Among the lead-free materials, (K,Na)NbO₃ (KNN)-based and Bi_1/2Na_1/2TiO₃ (BNT)-based piezoelectric ceramics are considered to be promising candidates for lead-based piezoelectric ceramics^[7-10]. Despite the high piezoelectric coefficient ($$\ d_{33}$$ > 500 pC/N), KNN ceramics suffer from certain drawbacks, such as the K and Na volatilization, unstable phase structures near room temperature, and low Curie temperatures (T_C) ~200 °C^[1,11-13]. BNT-based ceramics used to exhibit high T_C, but the volatilization of Bi and Na elements leads to unstable chemical compositions and a high coercive field; consequently, superior polarization is difficult to achieve^[14-17].

In recent years, BiFeO₃-xBaTiO₃ (BF-xBT) piezoelectric ceramics have emerged as competitive candidates in lead-free materials^[2,18-20]. As a kind of multiferroic material, BF has a rhombohedral phase perovskite structure (ABO₃), which has attracted significant attention because of its high T_C (830 °C) and excellent spontaneous polarization (P_s = 90-100 μC/cm²)^[21,22]. Recent studies of BF single crystal^[23-25], polycrystalline thin film^[26-29], and epitaxial thin film^[30-34] have also been conducted, which has given researchers additional ideas for exploring and application. However, the synthesis of pure BF is usually accompanied by the generation of impurities, where excess Fe₂O₃ exceeding 5 mol % leads to the formation of pyrochlore Bi₂Fe₄O₉ and gamma-Fe₂O₃^[35]. Additionally, the volatilization of Bi and the valence reduction of Fe³⁺ to Fe²⁺ at high temperatures cause high leakage currents, resulting in challenges for practical applications^[36-38]. BT is a traditional ferroelectric material exhibiting a tetragonal phase structure at room temperature and possesses a low T_C of ~120 °C^[39]. It has been found that the sintering of BF-xBT solid solution can effectively suppress the generation of secondary or impurity phases and reduce the leakage current. Specifically, a morphotropic phase boundary (MPB) can be constructed when x approaches 0.30 ~ 0.35, which contributes to enhanced dielectric, piezoelectric, and ferroelectric properties while maintaining a high T_C^[40-42].

However, one of the notable disadvantages in BF-xBT-based piezoelectric ceramics is the Bi₂O₃ volatilization during the sintering process, resulting in poor electrical resistivity and piezoelectric performance. The volatilization of Bi₂O₃ can be described by the following defect Equation (1)^[43]

The volatilization of Bi₂O₃ leads to the generation of Bi vacancies ($$\ V_{\mathrm{Bi}}^{3-}$$) and O vacancies ($$\ V_{\mathrm{O}}^{2+}$$) within the ceramics. Various strategies have been attempted to solve the problem. One of the most common strategies is using nonstoichiometric (excess) Bi to compensate for the loss of Bi during high-temperature sintering, as listed in Table 1.

In this study, a series of 0.70B_1+xF-0.30BT (x = -0.01, 0, 0.01, 0.02, 0.03, 0.04) ceramics were fabricated using the conventional solid-state reaction method. The influence of Bi₂O₃ compensation on the phase structure, microstructure, dielectric, ferroelectric, and piezoelectric properties of ceramics are systematically investigated.

MATERIALS AND METHODS

0.70Bi_1+xFeO₃-0.30BaTiO₃ (B_1+xF-BT, x = -0.01, 0.00, 0.01, 0.02, 0.03, 0.04) piezoelectric ceramics were fabricated using a solid-state reaction process. Bi₂O₃ (99%, Sinopharm, China), Fe₂O₃ (99.9%, Aladdin, China), BaCO₃ (99%, Sinopharm, China), and TiO₂ (98%, Sinopharm, China) powders were employed as raw materials. All powders were weighed according to stoichiometric ratios and ball-milled for 24 h using zirconia balls in ethanol. The mixed slurry was dried and calcined at 750 °C in a sealed alumina crucible for 2 h, followed by a secondary ball-milling process for 12 h. The calcined powder was mixed with a 10 wt% polyvinyl alcohol (PVA) binder, and the green pellets with a diameter of 10 mm were formed under a pressure of 127 MPa. The green pellets were kept at 600 °C for 2 h to burn out PVA and then sintered into ceramics at 1,010 °C for 3 h to obtain ceramics. Silver paste was sintered on parallel ceramic surfaces to form electrodes for electrical measurements.

The crystal structure and morphology were probed by the X-ray powder diffraction (XRD, D8 Advance X, Bruker, Germany) with Cu-K_α radiation and the scanning electron microscope (SEM, Apreo 2, Thermo Scientific, United States) equipped with an energy-dispersive spectroscopy (EDS) detector, respectively. The dielectric and impedance properties were measured via a precision LCR meter (E4980A, Agilent Technologies, United States) connected to a high-temperature dielectric test system (DMS-1000, Balab Technology, China). The ferroelectric hysteresis (PE) loops and fieldinduced strain (SE) curves were collected by a ferroelectric tester station (PK-10E, PolyK Technologies, United States). The piezoelectric coefficient ($$\ d_{33}$$) is recorded by a quasi-static $$\ d_{33}$$ meter (SA1303A, PolyK Technology, United States).

RESULTS AND DISCUSSION

The XRD results reveal that all B_1+xF-BT compositions exhibit a perovskite structure with a phase mixture consisting of cubic (Pm$$\ \overline{3}$$m) and rhombohedral (R3c) phases, as shown in Figure 1A. The impurity of Bi₂₅FeO₄₀, a common occurrence in the BF-BT system^[37], is appeared when x ≥ 0.03. The enlarged (111) and (200) peaks show insignificant peak shifts with varying Bi₂O₃ content. The refined results for each component are displayed in Figure 1B and Table 2. The cubic phase gradually decreases as the Bi₂O₃ content increases and tends to stabilize at x ≥ 0.01, according to this Table. This demonstrates that the phase structure changes from the Bi₂O₃-deficient to the Bi₂O₃-excess tends to change from the cubic phase to the rhombohedral phase, while the excess Bi₂O₃ phase structure has little effect, which is also reflected in the lattice parameter.

Figure 1. (A) XRD patterns of the B_1+xF-BT ceramics, (B) the Rietveld refinement results for x = 0.01.

Figure 2 shows the surface morphology of the B_1+xF-BT ceramics in different compositions. The ceramic surface exhibits a compact morphology with clear grain boundaries and seldom pores. The calculated relative density is higher than 95% for all compositions [Supplementary Figure 1], which is consistent with the observations from SEM results. Statistical analysis of the grain size distributions reveals that the grains tend to increase with enriching the Bi₂O₃ content. The ceramic grain size increases from 4.01 μm of x = -0.01 to 9.62 μm of x = 0.04. It is evident that the excess Bi₂O₃ not only compensates for volatilization but also acts as a sintering aid promoting grain growth, which is consistent with the literature^[46,47].

Figure 2. SEM images and grain size distribution in B_1+xF-BT ceramics with x = (A) -0.01, (B) 0.00, (C) 0.01, (D) 0.02, (E) 0.03, and (F) 0.04.

Figure 3 shows the backscattered electron (BSE) images of the polished surface of B_1+xF-BT ceramics with the corresponding elemental mapping results by EDS. A BSE image is used to see the dark and bright contrast, showing the light and heavy element distributions to evidence the core-shell structure in grains, and EDS helps further identify the exact elements in the core and shell regions. The images reveal a non-uniform distribution of elements inside the ceramic, leading to a distinct core-shell microstructure, which is caused by immiscibility of the dominantly ionically bonded BT and covalently bonded BF phases and the microscopic segregation of elements that forms during the slow cooling process of sintering^[3,53-56]. Murakami et al. synthesized the 0.05BiScO₃-(0.95-x)BaTiO₃-xBiFeO₃ ceramics without a discernible core-shell microstructure^[53]; it is proven that the BF-BT lattice can be replaced by dopants in the narrow range of the ionic radius (R_Sc3+: 0.745; R_Ti4+:0.605; R_Fe3+:0.645Å)/electronegativity (E_Sc3+:1.3; E_Ti4+:1.5; E_Fe3+:1.8) difference so as to prevent phase separation during slow cooling processes. Notably, there is a noticeable contrast between light and dark regions in Figure 3. The core, enriched with Bi and Fe, appears brighter, while the shell, enriched with Ba and Ti, appears relatively darker. In addition, the BSE images of x = -0.01 and x = 0.03 and the distribution of EDS elements are shown in Supplementary Figure 2 and Supplementary Figure 3.

Figure 3. BSE images of B_1+xF-BT ceramics: x = (A) -0.01, (B) 0.00, (C) 0.01, (D) 0.02, (E) 0.03, (F) 0.04, and EDS elemental mapping results (G) Bi, (H) Fe, (I) Ba, (J) Ti, and (K) O, (L) the scanned points in a core and a shell region and scanned line, EDS data of points and line scan on the core-shell in B_1.01F-BT ceramic.

Figure 4A presents the temperature-dependent behavior of relative permittivity (ε_r) and dielectric loss (tanδ) at 10 kHz. It reveals that as the Bi₂O₃ content increases, the dielectric peak becomes narrower, and the maximum relative dielectric constant (ε_r) gradually increases, indicating a reduction of diffuse behavior. It is evident from the spectra that the dielectric peaks of the ceramics exhibit asymmetry, which is related to the presence of a core-shell structure within the ceramic grains. As plotted in Figure 4A, the tanδ of ceramics exhibits an abrupt increase around the T_m (temperature exhibiting the maximum ε_r), suggesting that there is a transition from the diffuse ferroelectric to the paraelectric phase [Figure 4B]. Figure 4C illustrates the relaxation factor (γ) calculated at 10 kHz, which demonstrates that the value of γ decreases as the x content increases, ranging from γ = 1.79 at x = -0.01 to γ = 1.27 at x = 0.04. Figure 4D displays the variation of ΔT_m between 1 kHz and 1 MHz for each component in the temperature spectrum. It is evident that ΔT_m tends to decrease with increasing x content, which reveals that the ferroelectricity of the ceramic becomes more prominent, aligning with the decreasing γ depicted in Figure 4C.

Figure 4. Temperature-dependent ε_r and tanδ of (A) -0.01 ≤ x ≤ 0.04 measured at 10 kHz, (B) the T_m at 10 kHz, (C) the relaxation coefficient γ at 10 kHz, (D) the ΔT_m from 1 kHz to 1 MHz for the B_1+xF-BT ceramics.

The complex impedance (Z^*) plots of B_1+xF-BT ceramics at 400 °C are shown in Figure 5A, where Z' and Z'' represent the real part and imaginary part of Z^*, respectively^[57]. At 400 °C, the total impedance initially increases from 36.5 kΩ·cm at x = -0.01 to 44.4 kΩ·cm at x = 0.01 and then decreases with incorporating more Bi₂O₃ content, which indicates that the composition of x = 0.01 is the most electrically resistive. The Z" and M"/ε₀ plots of x = 0.01 at 300 °C are plotted in Figure 5B, illustrating the electrical heterogeneity associated with various conductive components. Three peaks are found in the plots corresponding to the three conductive components. Z" exhibits a single peak related to the grain boundary response (component 1), while M"/ε₀ shows a strong peak in the low-frequency region and a weak peak in the high-frequency region, which is ascribed to the electrical heterogeneity from the core-shell structure. In this study, the strong peak represents the shell response (component 2), whereas the weaker peak is considered as the core response (component 3). The resistance (R) and capacitance (C) of all conductive components at 325 °C were calculated based on the peaks of Z" and M"/ε₀ [Table 3]. The R values of components 1 and 2 reach the maximum in the composition of x = 0.01. However, the R value of component 3 did not change significantly with the increase of Bi₂O₃ content. Additionally, the resistance of components 1 and 2 is two orders of magnitude higher than that of component 3, which matches the frequency of the peaks of the three components in Figure 5B. It is worth noting that component 3 exhibits a capacitance that is an order of magnitude higher than components 1 and 2, indicating the formation of an electrically conducting core and a nonconductive shell. Figure 5C shows the Arrhenius plots of the grain shell, core, and boundary, and the calculated activation energy calculated by fitting is shown in Figure 5D. The activation energy of the shell (1.06-1.14 eV) is generally lower than that of the core (1.15-1.28 eV) and grain boundary (1.09-1.16 eV).

Figure 5. (A)Temperature-dependent Z^* plots for B_1+xF-BT ceramics at 400 °C, (B) Combined Z" and M"/ε₀ spectroscopic plots at 300 °C for x = 0.01, (C) Arrhenius plots and (D) the activation energy for the B_1+xF-BT ceramics.

Figure 6A illustrates the PE loops of the B_1+xF-BT ceramics at 60 kV/cm under a frequency of 1 Hz, and the corresponding P_r and E_C are plotted in Figure 6B. The PE loops show typical ferroelectric features without observations of leakage characteristics at high field amplitudes. However, when x = -0.01, 0.00, the loops demonstrate the phenomenon of leakage conduction, resulting in relatively high values of P_r and E_C, which is mainly attributed to the formation of Bi and O vacancies caused by the Bi₂O₃ volatilization^[43]. When x ≥ 0.01, the leakage conductivity of BF-BT is significantly reduced, leading to stable values of P_r and E_C. The electric SE loops of ceramics measured at 60 kV/cm are shown in Figure 6C. It can be seen that the strain value increases as x increases from -0.01 to 0.01, reaching a maximum value of 0.146% at x = 0.01, and then the strain value decreases. The strains are calculated by averaging the positive strains obtained at ±60 kV/cm and used for deriving the $$\ d_{33}^{*}$$ values, as plotted in Figure 6D. Notably, the highest $$\ d_{33}^{*}$$ = 243 pm/V and $$\ d_{33}$$ = 217 pC/N are achieved at x = 0.01, evidencing that the suitable amount of compensation of Bi₂O₃ is effective in improving the piezoelectric properties of the BF-BT ceramics. In addition, the inset image of Figure 6D depicts the temperature dependence of $$\ d_{33}$$ for x = 0.01. It demonstrates that the $$\ d_{33}$$ value has a great temperature sensitivity and decreases above 300 °C as a result of the gradual depolarization at high temperatures.

Figure 6. Ferroelectric properties of (A) PE loops at 60 kV/cm and associated (B) P_r and E_C of the B_1+xF-BT ceramics. (C) SE loops at 60 kV/cm and (D) d₃₃ and d^*₃₃ as a function of x concentration for the B_1+xF-BT ceramics; the inset image shows the temperature dependence of d₃₃ for x = 0.01.

CONCLUSIONS

In this work, a series of 0.70B_1+xF-0.30BT-based lead-free piezoelectric ceramics were systematically studied. The addition of Bi₂O₃ to 0.70BF-BT ceramics plays a crucial role in compensating for the volatilization of Bi elements during high-temperature sintering, leading to enhanced dielectric and piezoelectric performance. The phase structure of the ceramics is barely influenced while varying the Bi₂O₃ content, where all compositions exhibit a typical perovskite structure with a rhombohedral-cubic phase mixture. The microscopy results indicate an increasing trend of grain size as more Bi₂O₃ content is incorporated into the composition. The BSE images and element mappings reveal core-shell microstructures in the ceramics, which are attributed to the segregation of elements during the sintering process. Moreover, the higher Bi₂O₃ content leads to narrower dielectric peaks, higher maximum ε_r, and diminished relaxation factor, indicating a deteriorated relaxor behavior. The ferroelectric properties of the ceramics, as demonstrated by PE loops, show that the excess of Bi₂O₃ helps improve the leakage conductivity while stabilizing the P_r and E_C. The piezoelectric properties of the ceramics are optimized at x = 0.01 with $$\ d_{33}$$ ~217 pC/N and $$\ d_{33}^{*}$$ ~243 pm/V, but it has a great temperature sensitivity. The nonstoichiometric optimization of the Bi₂O₃ content in BF-BT was investigated in this work. The findings can be utilized as a foundation for further research into the BF-BT system.