Enhanced Electrical Property and Thermal Stability in Lead-Free BNT–BT–BF Ceramics

Abstract

The synergistic combination of outstanding electrical properties and exceptional thermal stability holds significant implications for advancing piezoelectric ceramic applications. In this work, lead-free ((1−x)(0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃)-xBiFeO₃ (x = 0.08, 0.10, 0.12)) ceramics were synthesized using a conventional solid-state method, with systematic investigation of phase evolution, microstructural characteristics, and their coupled effects on electromechanical performance and thermal stability. Rietveld refinement analysis revealed a rhombohedral–tetragonal (R–T) phase coexistence, where the tetragonal phase fraction maximized at x = 0.10. This structural optimization enabled the simultaneous enhancement of piezoelectricity and thermal resilience. The x = 0.10 composition achieved recorded values of d₃₃ = 132 pC/N, g₃₃ = 26.11 × 10⁻³ Vm/N, and a depolarization temperature T_d = 105 °C. These findings establish BiFeO₃ doping as a dual-functional strategy for developing high-performance lead-free ceramics.

1. Introduction

Since the discovery of lead zirconate titanate (PbZr_xTi_1−xO₃, abbreviated as PZT), it has been extensively investigated and employed in a diverse range of technological applications, encompassing sensors, transducers, and actuators [1,2,3,4]. However, the presence of lead (Pb) in PZT and other Pb-containing material compositions is crucial due to its inherent toxicity. This issue has recently prompted heightened environmental concerns, leading to the implementation of new regulations in numerous countries aimed at restricting the utilization of materials containing lead (Pb) [5,6,7]. The challenges pertaining to the further advancement of lead-free piezoelectric materials for electronic devices were comprehensively reviewed [8]. In recent decades, extensive research efforts have been devoted to investigating various perovskite-type oxides, including (Bi_0.5Na_0.5)TiO₃ (BNT), BaTiO₃ (BT), and their solid solutions. A solid solution of 0.94(Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃ (BNT–6BT), composed of BNT and BT with a Zr/Ti ratio of approximately 53/47, exhibits a morphotropic phase boundary (MBP) where both rhombohedral and tetragonal phases coexist, resulting in significantly enhanced electrical properties [9,10,11]. Due to these advancements, BNT–6BT has garnered significant attention due to its environmentally friendly lead-free composition and its exceptional dielectric, piezoelectric, and ferroelectric properties as a potential substitute for PZT.

As extensively documented, the integration of diverse materials represented a valuable strategy for generating novel multi-component systems that harness exceptional properties. For example, the incorporation of KNN into the BNT–BT system resulted in reductions in remnant polarization, coercive field, and maximum strain percentage. Additionally, the high energy storage density highlighted the potential for applications in actuators and energy storage systems [12]. The high-energy storage performance (Wrec = 1.9 J/cm³, eta = 86%) of BNT–BT–0.4BZT ceramics suggest their potential as promising candidates for pulsed-power applications [13]. The addition of Bi_0.5Ag_0.5TiO₃ in BNT–BT ceramics can cause the grain size of the ceramics to become more homogenous and possess optimum electrical properties (Ec is similar to 32.0 kV/cm, d₃₃ is similar to 172 pC/N, and kp is similar to 32.6%) [14]. BNT–BT–BiT piezoelectric ceramics exhibiting a significant strain response hold great potential as lead-free materials for actuator applications [15]. The orientation of grains presents a promising avenue for enhancing the electrocaloric and energy harvesting characteristics of lead-free BaTiO₃-based ferroelectric ceramics [16]. Although extensive studies on the BNT–BT binary system have established fundamental relationships between composition, phase structure, and electromechanical properties [17,18,19,20,21], recent advances highlight that ternary systems incorporating BiT can synergistically enhance field-induced strain through stabilized ferroelectric domains. These materials are widely regarded as highly promising candidates for replacing PZT due to their exceptional piezoelectric performance and significant remnant polarization located at or near the morphotropic phase boundary (MPB).

According to the literature, BiFeO₃ (BF) has garnered significant attention due to its commendable ferroelectric properties and elevated curie temperature [22,23,24]. Therefore, the solid solution has the potential to exhibit intriguing multi-ferroic characteristics. Considerable attention has been devoted to investigating the incorporation of BiFeO₃ into BaTiO₃ and PbTiO₃ solutions in order to achieve reliable electrical properties [25,26,27]. The addition of BiFeO₃ to NBT-based compositions enhances their piezoelectric properties and facilitates the poling process, surpassing those observed in pure NBT ceramics [28]. BiFeO₃ exhibits anti-ferromagnetic behavior up to the Neel temperature (~370 °C), whereas NBT does not display any magnetic properties. This fundamental contrast motivates the strategic incorporation of BF into NBT-based systems to engineer multifunctional materials coupling piezoelectric and antiferromagnetic responses, potentially enabling magnetoelectric effects inaccessible in pure NBT–BT ceramics. The BiFeO₃ composition was selected as the preferred dopant for the BNT–BT-based system in this study, and it represents a continuation of our ongoing efforts to develop an innovative alternative to lead-based piezoelectric ceramics, aiming for their replacement with Pb-free counterparts. The piezoelectric properties of BNT–BT ceramics exhibit a decline beyond a certain temperature due to their low depolarization temperature. Herein, we will discuss the influence of varying BF fractions on the phase, structure, electrical properties, and thermal depolarization of BNT–BT ceramics.

2. Materials and Methods

The ceramics of (1−x)(0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃)-xBiFeO₃ (x = 0.08, 0.10, 0.12), abbreviated as BNT–BT–xBF (x = 0.08, 0.10, 0.12), were synthesized using the conventional solid-state method. The raw materials of Bi₂O₃ (99.90%), BaCO₃ (99%), TiO₂ (99%), Na₂CO₃ (99.99%), and Fe₂O₃ (99%) were precisely weighed according to stoichiometry, followed by thorough mixing in anhydrous ethanol and subsequent ball milling for a duration of 12 h. The mixed materials were dried and calcined at 680 °C for 1 h. Next, the mixture was subjected to an additional 24 h ball milling process and was subsequently dried again. After adding 5% polyvinyl alcohol (PVA), the powders were compacted into discs with a diameter of 13 mm under a pressure of 9 MPa, followed by sintering at 1050 °C (the heating rate was 5 °C/min) for 2 h in air. Finally, the ceramic surface was polished with SiC paper, covered with silver paste, and polarized in a silicone oil bath for 15 min.

The phase structure was analyzed using X-Ray Diffraction (D/max–rB 12kW X–ray diffractometer, Rigaku Denki, Tokyo, Japan). The surface morphology was characterized using a scanning electron microscope (SEM, SU5000, Hitachi High–Technologies, Tokyo, Japan). The temperature dependence of the dielectric constant and loss were measured using a LCR test instrument (Agilent, E4980A, Santa Clara, CA, USA) from room temperature to 450 °C at 1 kHz, 10 kHz, and 100 kHz. The ferroelectric polarization–electrical (P–E) hysteresis loops were measured with the assistance of a ferroelectric hysteresis measurement tester (premier II, Radiant Tech., Albuquerque, NM, USA). The piezoelectricity d₃₃ was tested using specialized d₃₃ measuring equipment (ZJ-3A, Xuchang Zhongji Electric Co., Ltd., Zhengzhou, China). The k_p, Q_m, and g₃₃ values were calculated using the following formulas [29]:

$${\displaystyle \frac{1}{k_p^2}}=0.395\times {\displaystyle \frac{f_r}{f_a-f_r}}+0.574$$

(1)

$$Q_m={\displaystyle \frac{1}{4\pi (f_a-f_r)R_1(C_0+C_1)}}$$

(2)

$$g_{33}={\displaystyle \frac{d_{33}}{{\epsilon }_r{\epsilon }_0}}$$

(3)

where f_r is the resonance frequency, f_a is the antiresonance frequency, R₁ is the minimum impedance at resonance, and C₀ and C₁ are static and dynamic capacities. In line with IEEE standards, these electromechanical coupling factors were characterized using the resonance–antiresonance method with an impedance analyzer (HP4294A, Agilent Technologies, Santa Clara, CA, USA). The in-situ d₃₃ of the ceramic wafers was measured using a high temperature in-situ d₃₃ m (wide temperature range d₃₃ instrument, TZFD-900, Harbin Julang Technology Co. Ltd., Harbin, China) over the temperature range from room temperature to 230 °C.

3. Results and Discussion

3.1. Phase and Structural Analysis

The X-ray diffraction patterns of BNT–BT–xBF ceramics are presented in Figure 1. The ceramics exhibited a pristine perovskite phase structure devoid of any impurity phases. The coexistence of a rhombohedral–tetragonal multiferroic phase boundary (MPB) was observed in the 0.94(Na_0.5Bi_0.5)TiO₃-0.06BaTiO₃ ceramics [30]. According to the XRD pattern, the (111) diffraction peak at 40° exhibited a splitting phenomenon into (003) and (021) peaks, whereas the (200) peak at 46.5° also showed splitting behavior into (002) and (200) peaks. These observations were consistent with the characteristic XRD pattern of MPB composition where the rhombohedral (R) and tetragonal (T) phases coexist [31,32]. In addition, the XRD peak shifted to a lower diffraction angle, indicating that the crystal structure of the material had changed. This may be due to the fact that the ionic radius of Fe³⁺ (0.64 A) is bigger than that of the B-site ions of Ti^{4 +} (0.61 A).

To further elucidate the crystalline structural changes, the XRD data were analyzed using the two-phase model (R3c+P4mm) and refined using a Rietveld refinement approach. The coexistence of R and T phases in all samples is demonstrated in Figure 1b–d. The coexistence of R+T phases with a high T phase at x = 0.10 in the MPB lead to significantly enhanced piezoelectric performance. At the same time, the increase in T-phase content caused the ceramics to exhibit characteristics of “hard ceramics”, and the coercive field of the hysteresis loops exhibited a significant increase, as seen in Figure 1.

3.2. Microstructure and Grain Size

The SEM images and grain size distribution of BNT–BT–xBF ceramics are presented in Figure 2. As anticipated, the microstructure of BNT–BT–xBF exhibited dense granular grain packing characteristics derived from the solid-state sintering mechanism. The introduction of BF had a significant influence on the microstructure of the ceramics. The optimal doping concentration promoted the growth of ceramic grains. However, the grain size decreased with further increases in BiFO₃ content due to the presence of Fe³⁺ near the grain boundaries, which impeded grain growth [33]. The increase in grain size led to a reduction in the activation energy required for domain wall motion, thereby enhancing the electrical properties of the ceramics [34]. The results indicate that grain size has a significant influence on the piezoelectric properties, with larger grain sizes contributing to an excellent piezoelectric response.

3.3. Dielectric and Thermal Properties

The dielectric constant (ε_r) and dielectric loss (tanδ) of BNT–BT–xBF ceramics were measured from room temperature to 450 °C after polarization at frequencies of 1KHz, 10 kHz, and 100 kHz, as shown in Figure 3a–c. The first dielectric anomaly peak corresponded to the polarization temperature T_d, which determined the uppermost temperature achievable in practical ceramic applications. When x = 0.10, it has the highest depolarization temperature, which is of great significance in practical applications, as seen in Figure 3c. Simultaneously, the space charge carriers failed to keep pace with the applied electric field variations as the frequency increased, thereby diminishing their contribution to the dielectric constant and resulting in dielectric relaxation. In addition, ε_r is related to the grain size, and the relative dielectric constant exhibits a positive correlation with particle size augmentation [35]. In this study, when the average grain size reached a maximum at x = 0.10, the ε_r reached a maximum.

The ε_r–T curves of BNT–BT–xBF ceramics showed a broad dielectric peak near T_m, indicating the characteristic diffuse phase transition. The formula for the dispersion coefficient is as follows:

$${\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$\epsilon $}\right.}-{\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{${\epsilon }_m$}\right.}={\displaystyle \raisebox{1ex}{${(T-T_m)}^{\gamma }$}\!\left/ \!\raisebox{-1ex}{$C$}\right.}$$

(4)

The diffusion coefficient γ is a value ranging from 1 to 2. When the diffusion coefficient of the ceramic approaches 1, it indicates that the ceramic belongs to the category of traditional ferroelectric ceramics. Conversely, when the diffusion coefficient of the ceramic approaches 2, the ceramic is classified as a relaxation ferroelectric ceramic.

3.4. Ferroelectric and Piezoelectric Performance

The P–E and J–E loops, representing the polarization and leakage current with the electric field of the BNT–BT–xBF ceramic, are depicted in Figure 4. The J–E curve exhibited two distinct peaks, corresponding to the coercive field induced by polarization rotation. The BNT–BT–0.10BF ceramics exhibited a maximum value of 34.4 kV/cm for E_c, indicating that the phenomenon of leakage current was severe, and the process of domain reversal posed significant challenges. Consequently, a higher magnitude of polarization electric field was required to induce domain reversal in the same direction, resulting in an elliptical hysteresis loop and a decline in ferroelectric performance.

The phase angles depicted in Figure 5 played a pivotal role as essential indicators for precisely quantifying the polarization of piezoelectric ceramics. These phase angles served as a critical metric to assess the degree of alignment of the domain within the structure in response to the applied electric field. The alignment of the domain along the direction of the electric field exhibited a direct correlation with the magnitude of the phase angle, whereby the larger phase angle denoted more pronounced alignment of the domain. This alignment, in turn, led to a stronger piezoelectric response, which is the ability of the material to convert mechanical stress into electrical energy and vice versa [36,37]. The phase angle achieved its peak value. This particular composition, identified as BNT–BT–0.10BF ceramics, demonstrated a propensity for exhibiting a significantly enhanced piezoelectric response. Furthermore, this composition was characterized by the presence of a more distinct and pronounced abnormal peak within the dielectric temperature spectrum, as clearly illustrated in Figure 3b. This abnormal peak indicated a unique behavior in the response to the change of temperature, which could be attributed to the specific structural arrangement and alignment of the domain.

Table 1. Comparison of electric properties between this work and related BNT–BT-based piezoelectric ceramics.

Composition	d33 (pC/N)	Qm	εr (1kHz)	g33 (10−3 Vm/N)	kp	Td (°C)	Ref.
BNT–BT–0.08BF	124	111	542	25.84	0.18	101	This work
BNT–BT–0.10BF	132	124	571	26.11	0.20	105	This work
BNT–BT–0.12BF	111	55	558	22.47	0.20	97	This work
0.82BNT–0.15BT–0.03BNMN	110	500	520	-	-	-	[38]
BNT–BT–0.025BNiT	160	-	1000	-	-	39	[39]
0.884BNT–0.036BT–0.08BKT	116	-	2300	-	0.27	121	[40]
BNT–BT–0.04BST	277	-	1081	15.79	-	130	[41]
BNT–BT/PC	42	-	280	17.99			[42]

presents the dielectric, piezoelectric, and electromechanical coupling properties of BNT–BT–xBF ceramics at room temperature, as well as those of related BNT–BT-based piezoelectric ceramics. The values of d₃₃, ε_r, and Q_m fall within the range of 111–132 pC/N, 542–571, and 55–124, respectively. It is noteworthy that a peak in d₃₃, ε_r, and Q_m was observed at x = 0.10. The aforementioned description reveals that the incorporation of BiFeO₃ hampers certain piezoelectric properties of BNT–BT ceramics [38,39,40]. The piezoelectric, dielectric, and electromechanical coupling properties of BNT–BT–0.12BF ceramics were observed to decrease due to an increased presence of oxygen vacancies, larger distortion in the crystal cells, and the difficulty of polarization with domain pinning. The g₃₃ represents the voltage generated per unit of applied stress, and a high g₃₃ value renders the composite highly responsive for sensor applications. The addition of BiFeO₃ with a composition of x = 0.10 reached the optimum value of 26.11 × 10⁻³ Vm/N. The comparison of the g₃₃ value in this study with other previous reports [41,42] indicated a higher value.

As can be seen from Table 1, the piezoelectric constant exhibited a significant increase for BNT–BT–0.10BF ceramics. The incorporation of Bi³+ and Fe³+ ions promotes rhombohedral- and tetragonal-phase coexistence, creating a polymorphic phase boundary that facilitates polarization rotation and domain wall motion. In order to examine thermal stability, the ceramic samples were tested for d₃₃ at variable temperatures. The result presented in Figure 6 demonstrated a non-linear relationship between the piezoelectric constant and temperature, characterized by an initial increase followed by a subsequent decrease. The initial rise originates from thermally activated domain wall mobility in the ergodic relaxor phase, whereas the subsequent degradation corresponds to the onset of long-range ferroelectric order disruption. The variation rule of d₃₃ is identical to that of T_d. The introduction of BiFeO₃ resulted in a higher T_d, thereby enhancing the piezoelectricity and thermal stability of the BNT–BT solid solution. This dual mechanism—optimizing phase coexistence for low-temperature piezoelectric enhancement and reinforced covalent bonding for high-temperature stability—establishes BiFeO₃-modified ceramics as promising candidates for wide-temperature-range piezoelectric applications.

4. Conclusions

In summary, this study demonstrated that the incorporation of BiFeO₃ into the BNT–BT system can significantly enhance piezoelectric properties and thermal stability. The optimal composition of BNT–BT–0.10BF ceramics exhibited a d₃₃ of 132 pC/N, a g₃₃ of 26.11 × 10⁻³ Vm/N, and a T_d of 105 °C. The improvement can be attributed to the reduction of oxygen vacancies and the mitigation of crystal cell distortion, which facilitate polarization and domain movement. The higher T_d suggested that the BNT–BT–0.10BF ceramics can maintain their piezoelectric performance over a wider temperature range, making them suitable for applications in harsh environments. Future work should focus on further optimizing the composition to achieve even higher piezoelectric performance and better thermal stability, as well as exploring the potential of these materials in practical devices.