Optimization of functional property of lead-free Bismuth ferrite (BiFeO₃)-based ceramics by the synergistic approach of Bismuth (Bi)-excess and thermal quenching

1. Introduction

Piezoelectric materials mutually interconvert electrical and mechanical energy, and they are widely used in electromechanical devices such as actuators, energy storage capacitor applications, nanogenerators for energy harvesting, nano-positioners, nanosensors, piezocatalysts, switching and sensing devices, and transducers [1]. However, commercially available piezoelectric materials are lead-based materials, and their widespread use raises environmental and public health issues, necessitating the fabrication of lead-free alternatives [2]. Recent research that looked at the impacts of lead exposure in 2019 found that in just that one year, lead exposure caused about 5 million adult casualties from cardiovascular disease. The study provided an estimate of the death toll, which fell within the range of 2,305,000 to 8,271,000 fatalities. The economic expenditures of lead exposure in just 2019 were estimated to be a staggering US$6.0 trillion [3]. Therefore, the design and development of lead-free piezoelectric ceramics have drawn the interest of the research community.

BiFeO₃ (BFO), a lead-free multiferroic material, exhibits piezoelectric properties, enabling the conversion of electrical energy into and mechanical energy and vice versa. This makes BFO an attractive material for various applications, including electromechanical devices, actuators, and energy storage capacitors. For example, a piezoelectric sensor used in fuel injectors of the aircraft or diesel engines is working in the harsh environment of over 200°C [4]. However, the upper limit of the operating temperature of a piezoelectric device is almost half of its T_C due to the depolarization and electrical conductivity at elevated temperatures [5]. The main advantage of BFO is the large theoretical polarizations (P_r ∼ 90-100 µC/cm²) response, high Curie temperature (T_C) ∼ 830°C and G-type anti-ferromagnetism near room temperature [6]. Hence, the BFO has a high potential for magnetoelectric sensors, transducers, and spintronic applications [7-12]. However, Fe ion spin is slightly leaning away from their absolute antiferromagnetic order by the lone pair electrons of Bi³⁺ and creates a cycloidal spiral structure with a range of 62 nm [7]. Moreover, the volatile nature of Bi₂O₃ creates Bi and oxygen vacancies ($V_{{\text{Bi}}^{3+}}^{\text{'''}}$ and $V_{{\text{O}}^{2-}}^{}$) together with ${\left({\text{Fe}}_{{\text{Fe}}^{3+}}^{2+}\right)}^{\text{'}}$ due to Fe³⁺ to Fe²⁺ transition [13]. Nam et al. [14] reported that during slow cooling, accumulation of defects creates defect dipoles such as [($V_{{\text{O}}^{2-}}^{}$)- ${\left({\text{Fe}}_{{\text{Fe}}^{3+}}^{2+}\right)}^{\text{'}}$ and ($V_{{\text{O}}^{2-}}^{}$)-$V_{{\text{Bi}}^{3+}}^{\text{'''}}$]. These defects dipoles suppress the domain walls motion and inhibit ferroelectric performance. Furthermore, the thermodynamic instability during slow cooling also gives rise to secondary phases, such as Fe-rich phase: Bi₂Fe₄O₉ and Bi-rich phase: Bi₂₅FeO₃₉, which stabilize defects and impede domain wall motion under applied field. As a result, the ferroelectric response is suppressed [12,15].

The electrical characteristics of BFO-based piezoceramics can be enriched by modifying the processing parameters such as annealing, quenching and sintering temperatures, milling, dwell times, and sintering atmosphere [16-20]. The partial substitution of Bi by rare-earth elements like Gd³⁺, Eu³⁺, La³⁺, Sm³⁺, and Nd³⁺ can suppress cycloidal spin structure and liberation of the latent polarization response [21-23]. Among the other elements, Sm³⁺ is more effective due to the smaller ionic radius (∼1.24 Å) relative to Bi³⁺ (∼1.34 Å) which can induce large lattice distortion [24]. Recently many reports have shown that the lattice distortion creates local structure heterogeneity due to difference in the ionic radii of the chemical modifier and improves the multifunctional property of the perovskite materials [5, 25,26]. Besides, the replacement of Bi by Sm in BFO reduces $V_{{\text{O}}^{2-}}^{}$ because the Sm-O (0.724) bond is stronger than the Bi-O (0.395) bond [12]. Thermal quenching suppresses secondary phases and improves the functional properties. Mostly, some sintering additives such as CuO, MnO_2, and Li₂CO₃ are added to control the Bi-loss during high temperatures [27-29]. However, the Bi-excess in the BFO-based ceramic is an effective way of enhancing their electrical properties [30,31]. Still, further systematic investigations into the processing conditions are required to improve functional properties because the functional properties of BFO-based materials are highly dependent on the processing conditions, and experimentally obtained polarization values are way inferior than theoretically predicted values.

Unlike earlier studies on BFO-based ceramics, which have often focused on a specific single challenge, this work takes a comprehensive approach to address all of the major challenges encountered during BFO ceramic synthesis, an area that is yet to be explored. Therefore, a synergistic approach of calcination temperature optimization, optimum amount of Bi-excess, thermal quenching, and chemical doping were applied for the improvement of electrical property, as shown in Figure 1.

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Figure 1.

Schematic diagram of synergistic approach for improvement of electrical property in BFO-based materials.

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2. Materials and Methods

The lead-free ferroelectric materials 0.97Bi_{0.92(1- x)}Sm_0.08FeO₃–0.03BaTiO₃ (B _xSF-BT; x = 0.99, 1.00, 1.01, 1.03, and 1.05) were formulated by solid-state method from precursors, Bi₂O₃ (99.90 %), Fe₂O₃ (≥ 99.0 %), BaCO₃ (≥ 99 %), and TiO₂ (≥ 99.9 %). The mixing of powder was done in ethanol (EtOH) solution using ball milling for 24 h. The EtOH was evaporated from the composition powder in the oven at 120 ^oC and followed by calcination at 600 ^oC, 700 ^oC, and 750 ^oC for 2 h. The 5 mol% polyvinyl alcohol was added and pressed into pellets with 10 mm diameter. All green pellets were sintered in the air for 3 h at 890 ^oC. After completing sintering the pellets were thermally quenched in water in order to avoid the formation of Fe-rich (Bi₂Fe₄O₉) and Bi‐rich (Bi₂₅FeO₃₉) secondary phases. X-ray diffractometer (XRD, JDX-3532 & Japan) was used for the structural characterization. Microstructural and chemical characterizations of as-sintered samples were performed using scanning electron microscopy (SEM, JSM5910 & Japan). The silver was pasted on both flat surfaces of the pellets to make capacitor-like samples for electrical characterization. The electric field-induced (P-E) loops were measured using a modified Sawyer-Tower circuit (Radiant, RT6000 HVS). The direct piezoelectric constant (d₃₃) was measured by using piezo-d₃₃-meter (IACAS, ZJ-6B) after DC-bias poling in a silicone oil bath applying 120 kV/cm for 30 minutes.

3. Results and Discussion

Figure 2(a) displays the XRD patterns of the powder calcined at 600°C, 700°C, and 750°C. It can be seen that at a low calcination temperature of 600°C, many secondary peaks were observed. However, at 700°C most of these secondary peaks significantly disappeared. One can see that no obvious change occurred in the XRD peaks of the samples at 700°C to 750°C calcination temperature (Figure 2b). Therefore, the optimized calcination temperature for the investigated ceramics should be 700°C. Figure 2(c) represents the XRD patterns of the calcined powder at 700°C for the B _xSF-BT system with x = 0.99, 1.00, 1.01, 1.03, and 1.05 samples. In the case of Bi-deficient composition, i.e. x = 0.99, the lower intensity of the XRD peaks revealed the low crystallinity nature. However, for other compositions x ≥ 1.00, the high-intensity peaks were noted with some minor unwanted secondary phases. These secondary peaks may be associated with the Bi-rich and Fe-rich phases, which are mostly observed in BFO-based ceramics [32]. As given in Figure 2(d), a clear splitting in the (111) expendend XRD peak near 2θ ˜ 39-40° indicate the apparent rhombohedral phase. A significant shift occurred in the XRD peak toward the lower angle for the x = 1.05 sample which may be related with the lattice volume expansion [33].

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Figure 2.

(a) Calcined powder XRD patterns at 600°C, 700°C, and 750°C for B _xSF-BT with x = 1.03. (b) Magnified view for the (111) XRD peak at 2θ ≈ 38°-41°. (c) The 700°C calcined powder XRD patterns for B _xSF-BT system (with x = 0.99, 1.00, 1.01, 1.03 and 1.05) and (d) extended view for the (111) XRD peak at 2θ ≈ 38°-41°.

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Figure 3(a) represents XRD for the sintered ceramics samples with different Bi-contents. The secondary phases detected in the calcined samples were significantly suppressed in the sintered samples. However, still some minor peaks of Fe-rich (Bi₂Fe₄O₉) and Bi-rich (Bi₂₅FeO₃) secondary phases were observed near 2θ = 33° and 2θ = 27°-28°/37°-38° for the x = 0.99 and x = 1.05 samples, respectively. These peaks were matched with Fe-rich (Bi₂Fe₄O₉) and Bi-rich (Bi₂₅FeO₄₀) phases. Previously, similar secondary peaks were also detected in the XRD of BFO-based ceramics [34, 13]. For x = 0.99 a minor peak of Fe-rich is associated with Bi₂O₃ deficiency and the Bi-rich peak is related to Bi-excess powder. All peaks coincide with the rhombohedral phase with PDF # 71-2494, indicating the formation of stable R3c symmetry BiFeO₃ ceramic [32, 13]. Additionally, as evident from Figure 3(b), a clear splitting can be observed near the 2θ = 39°-40,° also suggesting the rhombohedral phase structure. Normally, the peak splitting width (∆θ) in the XRD peaks represents the lattice distortion, and shifting the peak’s position towards the lower angle signifies the lattice volume expansion [35]. The maximum shift and a high lattice distortion can be observed for Bi = 1.01 and 1.03 samples, as shown in Figure 3(c) and (d). The high lattice distortion predicts the maximum ferroelectric and piezoelectric response [36, 37].

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Figure 3.

(a) The sintered ceramic XRD patterns for B _xSF-BT with x = 0.99, 1.00, 1.01, 1.03, and 1.05, and (b) extended XRD peak at 2θ = 38°-41°. (c) XRD peaks position of the (111) and (1-11), (d) The (111) peak splitting width (Δθ) as a function of Bi-content.

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Figure 4(a) represents the room temperature P–E loops under the applied field of 130 kV/cm for the BxSF–BT (with 0.99 ≤ x ≤ 1.05) ceramics system calcined at 700°C. The variation of saturation polarization (P_s) and remnant polarization (P_r) as a function of Bi-content has been illustrated in Figure 4(b). It can be seen that, for x ≤ 1.01 samples the slim and slanted P–E hysteresis loops were observed. Initially, for the x = 0.99 sample, the P_r ≈ 1.0 µC/cm² and P_s ≈ 2.6 µC/cm² were noted. Mostly, in BFO-based ceramics, the formation of non-perovskite unwanted secondary phases suppresses the ferroelectric response, because these secondary phases exhibit non-polar cubic structures [34]. For the optimized amount of Bi-content (x = 1.03), a highly saturated P–E loop was observed with enhanced polarization response (P_r = 15.5 µC/cm² and P_s = 17.1 µC/cm²). On the other hand, for the high content of Bi (x = 1.05), again the ferroelectric response decreases. The suppression of ferroelectric performance is mainly associated with the stabilization of the non-polar Bi-rich phase. The ferroelectric results are consistent with the XRD results mentioned in Figure 3.

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Figure 4.

(a) Ferroelectric Polarization-electric field (P-E) hysteresis loops for B _xSF-BT with x = 0.99, 1.00, 1.01, 1.03, and 1.05 samples calcined at 700°C. (b) The saturation polarization (P_s) and remnant polarization (P_r) as a function of Bi-content.

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For chemical analysis, energy-dispersive X-ray spectroscopy (EDS) is a powerful tool for confirming the elemental composition of the synthesized sample [37]. Therefore, the EDS analysis was performed in order to verify the correct stoichiometry of the solid solutions. Figure 5 shows the EDS spectrum of the optimized sample with x = 1.03 ceramic. The absence of the other foreign elements and the presence of Bi³⁺, Ba²⁺, Sm³⁺, Ti⁴⁺, and Fe⁴⁺ elements confirm the phase purity of the synthesized sample. Moreover, the ratio of A-site (i.e., Bi³⁺, Ba²⁺, Sm³⁺) and B-site (i.e., Ti⁴⁺, Fe⁴⁺) is almost near to the stoichiometric ratio of the x = 1.03 composition.

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Figure 5.

Quantitative and qualitative EDS analysis for the optimized sample with x = 1.03 of Bi-content.

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The uniform grain size distribution with densified microstructure is highly required for enhanced electrical properties [38]. Figure 6(a) depicts the density (ρ) of the developed ceramics system as a function of Bi contents. The density was measured by using the Archimedes principle. Initially, for the x = 0.99 sample, the ρ ≈ 8.33 g/cm³ was noted, which gradually grew up to ρ ≈ 8.50 g/cm³ until the x = 1.03 sample. However, for the higher order of Bi content the density again decreased. The reduction in density factor for x = 1.05 may be related to the volatilization of a higher amount of Bi₂O₃ excess powder [31]. This highly densified optimum composition predicts excellent electrical performance as previously reported in the lead-free ceramics [39, 40]. Figure 6(b) shows the SEM image and grain size distribution of the x = 1.03 as-sintered ceramic sample. The sample has well-sintered and tightly bound grains with an average grain size of approximately 3 µm. The homogenous microstructure without unwanted liquid phase, visible pores and cracks, confirms that the selected sintering temperature is optimal for the investigated ceramics.

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Figure 6.

(a) Density for B _xSF-BT system as a function of Bi-contents, and (b) SEM image for x = 1.03 sample.

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In BFO-based materials, the loss of Bi₂O₃ creates defects such as $V_{{\text{Bi}}^{3+}}^{\text{ '''}}$, $V_{\text{O}}^{••}$ and ($F{e}_{F{e}^{3+}}^{2+}$)′ according to the following Eq. (1) [41].

The defect charges can combine due to electrostatic force and create complex defect dipoles (${\text{Fe}}_{F{e}^{3+}}^{2{+}^{\text{'}}}$−$V_{\text{O}}^{••}$) or ($V_{{\text{Bi}}^{3+}}^{\text{ '''}}$−$V_{\text{O}}^{••}$). These defect dipoles generate barriers for domain reorientations during the applied electric field, resulting in the suppression of ferroelectic and piezoelectric characteristics [42]. However, thermal quenching avoids the attachment of defect charges and depinning domain wall motion [14]. Moreover, the thermal quenching also inhibited Bi/Fe-rich phases and improved the piezoelectric constant (d₃₃) [34]. The Sm-doping reduces $V_{{\text{O}}^{2-}}^{}$ due to the stronger ionic bond Sm-O as compared with Bi-O [43]. Additionally, the 3 mol% Bi-excess further reduces defect charges and suppresses the domain pinning effect, which releases the latent polarization response under the externally applied field [31]. Therefore, the investigated ceramic samples were quenched in water from their sintering temperature and subsequently poled at an electric field of 120 kV/cm for a duration of 30 minutes. The d₃₃ ≈ 18 pC/N of the stoichiometric composition significantly improved to 59 pC/N for the 3 mol% of the Bi-excess sample, as shown in Figure 7(a). As shown in Figure 7(b), the highest d₃₃ of 59 pC/N of this work for x = 1.03 sample is higher than those of previously reported rare-earth doped BiFeO₃ ceramics [44-49,13]. Hence, the Sm-doping, thermal quenching, and optimization of Bi-excess is an effective strategy for the improvement of the d₃₃ value. This work provides a path for future work for the improvement of functional property.

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Figure 7.

(a) Piezoelectric coefficient (d₃₃) of this work as a function of Bi-contents and (b) comparison with other lead-free rare-earth doped BiFeO₃ ceramics.

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4. Conclusions

In this work, the crystal structure, microstructure, and ferroelectric characteristics of BxSF–BT system (x = 0.99, 1.00, 1.01, 1.03, and 1.05) were studied systematically. All samples showed a perovskite phase with minor secondary phases in Bi-deficient and over-doped Bi-excess samples. The formation of secondary phases was suppressed by thermal quenching and also adding 3 mol% Bi-excess powder. The optimal composition, x = 1.03 Bi-content, exhibited a dense microstructure with definite grain boundaries and maximal lattice distortion, resulting in the highest polarization response P_r = 15.5 µC/cm². Furthermore, the piezoelectric charge constant d₃₃ was significantly improved from 18 pC/N of the stoichiometric composition to 59 pC/N for the 3 mol% Bi-excess sample. The obtained piezoelectric charge constant value is relattively higher than previousely reported work.