Environmentally friendly BiFeO₃-based piezoceramics: Processing, optimization, and electromechanical properties for high-temperature applications

1. Introduction

Nowadays, the world is facing serious issues due to ecological degradation caused by industrial effluents, which results in depleting water, soil, and marine life, raising severe concerns about climate change. The echo-friendly piezomaterials are considered viable agents for real applications since they may generate clean and green energy through mechanical forces. These materials can harness mechanical energy from various sources, such as vibrations, wind, and water flow, and convert it into electrical energy. This can minimize reliance on fossil fuels while also lowering greenhouse gas emissions. Piezoelectric materials are non-toxic and biocompatible, making them an appealing choice for applications where environmental sustainability is critical. Piezomaterial converts electrical energy into mechanical strain and mechanical energy into an electrical signal. High-temperature piezomaterials are used in fuel injectors and many other sensors for engine monitoring (Guo et al., 2024). The magnitude of static (d₃₃) and dynamic (d₃₃*) piezoelectric constants, along with their operating temperature range, provides information about the device performance. The lead-based piezomaterials demonstrated high piezoelectric characteristics near the morphotrophic phase boundary (MPB) (Saito et al., 2004, Li et al., 2022). However, increasing apprehensions about the toxicity of lead-based piezomaterials restrict their use in real applications (Maqbool et al., 2018). In fuel injectors utilized in diesel engines, the lower limit is ∼200°C (Habib et al., 2023). Generally, the piezoelectric devices are operated at half of T_C due to the thermal depolarization at high temperatures (Lee et al., 2018). Lead-free BiFeO₃-BaTiO₃ (BF-BT) piezoelectric materials have gained considerable attention due to their high T_C > 400°C for their possible uses at elevated temperature applications (Tan et al., 2022). Unfortunately, Bi₂O₃ volatilization at high sintering temperatures induces an electrical leakage current that causes thermal instability in piezoelectric performance and also creates difficulties in electrical poling (Lv et al., 2024). Furthermore, the non-ferroelectric impurity phase formation (Fe-rich or Bi-rich) through slow cooling suppresses piezoelectric performance (Habib et al., 2022). The Bi-deficiency can be compensated by adding some extra amount of B₂O₃ powder during chemical composition preparation, and secondary phases can be suppressed by thermal quenching (Ahmed et al., 2021; Yi et al., 2021). Previous investigation shows that the rare-earth element (i.e., Nd, Sm, and La) donor-doped lead-based ceramics exhibited high piezoelectric characteristics (Li et al., 2018; Narayan et al., 2018; Li et al., 2022; Liu et al., 2022). Similarly, the lead-free donor-doping in BF-BT ceramics suppresses leakage current and improves piezoelectric performance (Habib et al., 2020; Habib et al., 2023; Habib et al., 2023; Habib et al., 2023).

Several strategies have been undertaken to synthesize the new lead-free piezomaterials, such as (Bi_1/2Na_1/2)TiO₃, (Ba,Ca)TiO₃-Ba(Ti,Zr)O₃, and (K,Na)NbO₃ with good piezoelectric performances (Hao et al., 2014; Wu et al., 2016; Zheng et al., 2018). Saito et al. achieved superb piezoelectric characteristics, i.e., d₃₃^* ≈750 pm/V and d₃₃ of 416 pC/N with T_C of 253°C in the textured KNN-based piezomaterials, compared to PZT polycrystalline ceramics (Saito et al., 2004). However, good-quality textured piezomaterials synthesis is costly, and several variables are mandatory to be controlled in preparation.

Recently, lead-free BiFeO₃-BaTiO₃, (BF−BT) showed high T_C ≥ 400°C and relatively good characteristics near MPB composition. However, BF-based ceramics have two fundamental issues, such as the generation of secondary phases, i.e., (Bi₂Fe₄O₉ and Bi₂₅FeO₃₉) through cooling in the furnace and electrical leakage current owing to Bi₂O₃ volatilization at high sintering temperature that causes Fe³⁺ to Fe²⁺ transition and triggers oxygen vacancy formation for charge neutralization (Rojac et al., 2014; Ahmed et al., 2021). These unwanted phases have cubic structures that deteriorate the piezoelectric functioning, and defect charges inhibit domain switching under an electric field. The variable temperature range can be suppressed by applying thermal quenching to avoid secondary phases, while defect charges can be compensated either by adding Bi-excess powder or donor doping (Ahmed et al., 2021; Habib et al., 2022; Habib et al., 2022). Many authors optimized the piezoelectric characteristics of pure BF-BT ceramics (Cheng et al., 2018; Lee et al., 2018; Ahmed et al., 2020; Park et al., 2022). However, pure BF-BT piezoceramics displayed micro-sized domains with a deep free energy profile, so their ferroelectric domain switching is relatively difficult, and their properties are not matched with commercial PZT ceramics. The composition design tactic is a valuable parameter to achieve excellent piezoelectric characteristics. Thus, in the current work, a new lead-free donor-doped BF-BT composition was developed as per the formula 0.7Bi_1.03FeO₃-0.3Ba_0.99La_0.01TiO₃ near the MPB, and its properties were optimized based on sintering temperature. The substitution of Ba²⁺ (1.61 Å) with La³⁺ (1.36 Å) donor doping provokes local structure heterogeneity owing to valence difference and ionic radius mismatch (Chen et al., 2022; Yan et al., 2022; Habib et al., 2023). This local structure inhomogeneity induces extra interaction energy, that a flattened free-energy profile for domain switching and results in relatively higher piezoelectric characteristics (Li et al., 2018).

Previously different strategies have been applied in order to improve piezoelectric property of the lead-free BF-BT ceramics including, Bi-excess (Ahmed et al., 2021; Yi et al., 2021; Wang et al., 2022; Yang et al., 2022), chemical doping (Habib et al., 2022; Habib et al., 2022; Tan et al., 2022; Zhang et al., 2022), synthesis conditions (Lee et al., 2018; Zhu et al., 2018; Zhang et al., 2019; Ahmed et al., 2020; Yang et al., 2022), thermal quenching (Qin et al., 2018; Lee et al., 2019; Akram et al., 2021; Wang et al., 2022), MPB composition design (Lee et al., 2015; Habib et al., 2020; Xun et al., 2021; Habib et al., 2023), and AC-bias poling (Kim et al., 2019; Kim et al., 2022). In this work, a synergistic approach of 3 mol% Bi-excess, 1 mol% donor doping, sintering optimization, water quenching, and AC-poling is applied to achieve thermally stable optimal piezoelectric performance. The detailed description of the experimental method and synergistic approach for the improvement of piezoelectric performance has been depicted by a schematic diagram in Fig. 1.

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Fig. 1.

A flow for the solid state reaction process and a schematic diagram of the TF analyzer system for ferroelectric and piezoelectric property measurement.

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2. Materials and Methods

In this study, the solid-state reaction approach was employed to synthesize lead-free donor-doped BF-BT polycrystalline piezoelectric ceramics according to the chemical formula Bi_1.03FeO₃-Ba_0.99La_0.01TiO₃. The starting materials, Bi₂O₃ (99.9%), La₂O₃ (99.99%), Fe₂O₃ (≥99%), BaCO₃ (≥99%), and TiO₂ (≥99.9%) were acquired from Sigma Aldrich. All these precursor powders were precisely measured and blended to guarantee accurate stoichiometric ratios. During chemical composition weighing, the 3 mol% Bi-excess powder was added for the purpose of compensating for Bi₂O₃ evaporation during high-temperature sintering. The 1 mol% La³⁺-donor doping further suppresses defect charges and enhances piezoelectric performance. Varying temperatures (940°C ≤ T_S ≤ 980°C) were applied to sinter the specimens for 3 h in air before being water quenched. The phase structure was evaluated using an X-ray diffractometer (XRD, Rigaku, MiniFlex II with Cu-K). The micrographs of the specimens were acquired using a scanning electron microscope (SEM, JP/JSM5200, Jeol). Following the Pt ion sputtering technique, a homogeneous coating of silver paste is applied to both surfaces of a ceramic material for electrical evaluation. The room temperature strain-electric field (S-E) loops were detected at 1 Hz using a TF analyzer (Aix ACCT, TF analyzer 2000, Germany), and temperature-dependent S-E loops measurements were performed from room temperature up to 140°C, with 20°C increments under the same field and frequency. The converse piezoelectric values were calculated by d₃₃^* = S/E, where S and E show induced piezoelectric strain and applied electric field, respectively. Dielectric properties were examined by utilizing an impedance analyzer (Agilent, HsP4192A) at 10 kHz in the temperature range of 25-700°C, with a heating rate of 1°/min. Utilizing a d₃₃-meter (IACAS, ZJ-6B with 0.25 N and 110 Hz), the d₃₃ values were determined.

3. Results and Discussion

Fig. 2(a) represents the XRD for specimens sintered at various temperatures, T_S of 940, 950, 960, 970, and 980°C. During XRD measurements, a thin line of Si is pasted on the surface of the sample as a reference peak to correct the tilting and height error. All of the detected diffraction peaks were indexed according to the pseudocubic phase, and dotted lines represent the conventional Si reference peaks. For detecting any possible splitting in the XRD, the enlarged view of the (111) and (200) peaks has been shown in Fig. 2(b). Normally, the splitting or asymmetry in the (111) peaks indicates the R phase, while splitting in the (100) or (200) peak shows the T phase structure (Zhou et al., 2020). At lower temperatures of 940 and 950°C, no clear splitting can be detected in both (111) and (200) peaks. As the temperature increased to 960°C and 970°C, an apparent splitting was identified in the (111) peak near 39° and broadness in the (200) diffraction peak near 45-46°, indicating the R and T mixed phases. With further increasing sintering temperature, the (111) peak gets sharper, resulting transformation to the cubic-like phase.

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Fig. 2.

(a) XRD patterns for the lead-free La-donor doped BF-30BT ceramics sintered at various temperatures (940°C ≤ TS ≤ 980°C) and (b) magnified view for the (100) and (111) peaks.

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Fig. 3 displays the surface morphology, statistical distribution of average grain size for samples sintered at 940, 950, 960, 970, and 980°C. Fig. 3(a) illustrates that at a relatively low sintering temperature (T_s = 940°C), the sample had small grains with narrow grain size distributions. Moreover, some holes were detected in this sample, which suggests that this low sintering temperature is insufficient for full grain growth and appropriate densification. As the sintering temperature increased to 970°C and 980°C, pore-free and fully densified microstructure with distinct grain boundaries can be detected, where the grain sizes are distributed over a broad range, as illustrated in Figs. 3(b-e). The average grain sizes were noted as 1.9 μm, 3.1 μm, 4.3 μm, 5.3 μm, and 5.5 μm for the sample at T_s 940°C, 950°C, 960°C, 970°C, and 980°C. The average grain size distribution and percentage density increased from 86.4% to 92.6% as T_s increased from 940°C to 980°C, as given in Fig. 3(f). However, for the T_s of 980°C, some abnormal grain growth and poor densification may be due to the refractory nature of Bi₂O₃ at high temperature.

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Fig. 3.

(a-e) depicts the surface morphology with grain size distribution of the samples sintered at 940°C, 950°C, 960°C, 970°C, and 980°C. (f) Average grain size as a function of sintering temperature.

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Fig. 4 illustrates the dielectric constant (ε_r) and tangent loss (tanδ) as a function of temperature with heating rates of 1°/min at 10 kHz. Dielectric characteristics strongly depend on the average grain size (Lee et al., 2018). At lower T_s of 940°C and 950°C, the diffused phase transition can be observed as shown in Fig. 4(a). These results suggest that the sintering temperature is not sufficient for appropriate densification and complete grain growth. A prominent transition peak with a higher T_C = 530°C was seen in the dielectric constant, which abruptly increased when the T_s rose to 960°C or above. This higher dielectric response could be related to a densified microstructure and appropriate grain size. For the T_s = 980°C, ε_r value again slightly reduced, which may be owing to Bi₂O₃ evaporation at high heat-treatment temperature. Fig. 4(b) shows the temperature-dependent tanδ in the temperature range 25-400°C. Near the room temperature, the tanδ values of 0.09 are nearly identical for all samples. Interestingly, the dielectric loss dramatically decreases to tanδ ≈ 0.02 for the T_s ≥ 960°C in 150-200°C temperature range, which is almost five times smaller than that of T_s ≤ 950°C as shown in Fig. 4(b). However, beyond 300°C, the tanδ values increase significantly, indicating a surge in thermally induced electrical conductivity.

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Fig. 4.

(a, b) Temperature-dependent variation of dielectric constant and dielectric loss of the sample at various sintering temperatures TS i.e., (940°C ≤ TS ≤ 980°C) at 1 kHz.

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Room temperature field-induced polarization (P-E) loops have been shown in Fig. 5(a). As the T_s increased from 940°C to 970°C, a significant improvement occurred in the electrical polarization as shown in Fig. 5(b). The samples at lower T_s, such as 940°C and 950°C, exhibited smaller remanent polarization (P_r) values of 14.59 μC/cm² and 17.58 μC/cm², respectively. As the T_s increased to 970°C, the highest P_r value of 30.2 μC/cm² was observed. The squared-shaped and highly saturated P−E loop with enhanced ferroelectric response suggests a high piezoelectric response (Kim et al., 2017; Nam et al., 2018; Lee et al., 2019). The value of P_r fell somewhat as the T_S increased more. The fall in P_r value is due to the huge number of pores caused by Bi-volatilization at high sintering temperatures.

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Fig. 5.

(a,b) Ferroelectric P-E hysteresis loop of BF-30BT ceramics sintered at various sintering temperatures (940°C ≤ TS ≤ 980°C).

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The bipolar/unipolar strain (S−E) curves were measured under the applied field of 50 kV/cm, as shown in Figs. 6(a,b). As the Ts increased to 960°C or above well well-saturated S-E loops were observed with a noticeable negative strain (-S_neg) value. For an ideal piezoelectric material, the coercive field (E_c) and field at the negative strain (E_n) of the S−E loop are almost equal (Habib et al., 2023). It is generally accepted that the magnitude of -S_neg value is directly proportional to the non-reversible switching of the domain under a bipolar applied electric field. A high -S_neg of 0.17% and lower ΔE of 4.7 kV/cm values for the sample with T_s of 970°C revealed better domain switching, like an ideal piezoceramic Fig. 6(c). For this sample, the highest S_max strain is achieved in both bipolar and unipolar S-E loops, as given in Fig. 6(d). Recently, it has been proven that d₃₃ significantly improved after AC-poling (Kim et al., 2019; Kim et al., 2022). Therefore, all samples were AC poled by 1000 cycles and then DC poled at 50 kV/cm for 30 min. A high d₃₃ = 297 pC/N was achieved at the T_s of 970°C under the conventional DC-bias poling, which is further improved to 344 pC/N after AC poling, as evident from Fig. 6(e). Mostly, it is hard to achieve both high d₃₃ and d₃₃^* in one sample. Importantly, in this work, the simultaneously high 450 pm/V and d₃₃ of 344 pC/N are realized in a single composition at T_s of 970°C, as shown in Fig. 6(f). This improved d₃₃ and d₃₃^* together with a high T_C of 530°C in a single composition are promising findings in the eco-friendly BF-BT ceramics.

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Fig. 6.

(a, b) Bipolar and unipolar S-E curve at different Ts of 940°C, 950°C, 960°C, 970°C, and 980°C. (c) The -Sneg, and ΔE (d) unipolar and bipolar Smax (e) d33 after DC and AC+DC poling, and (f) d33* of the unipolar and bipolar loops as a function of sintering temperature.

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For real application in a device, not only enhanced d₃₃ and d₃₃^* are required, but also the thermal stability is highly desired. Therefore, the temperature-dependent S-E loops are measured at 25, 40, 60, 80, 100, 120, and 140°C, as given in Figs. 7(a,b). Generally, based on microstructure morphology and physical properties, piezoelectric materials are divided into two categories (Habib et al., 2022). The type-I piezoelectrics are composed of short-range order, where abundant polar nanoregions (PNRs) are embedded in a non-polar matrix (Tao et al., 2019). However, the type-II piezoelectrics consist of hybrid nano-domains with a polar matrix (Tao et al., 2019; Habib et al., 2022). Normally, the piezoelectric performance of typical relaxor ferroelectric (type-I) is highly unstable with changing temperatures (Malik et al., 2016; Habib et al., 2022). It is because thermal migration of defect charges induces high variation in the functional properties (Habib et al., 2020; Habib et al., 2022; Habib et al., 2023). In this work, the type-II composition is designed near the MPB and then doped by a donor dopant in order to suppress defect charges to achieve thermally stable, excellent piezoelectric performance. The room temperature -S_neg = 0.17% value significantly increased to 0.45% and E_C of the bipolar S-E loop dropped with rising temperature up to 140°C, see Fig. 7(c). This decrease in E_c or increase in S_neg is mainly related to thermal improvement of domain-switching behavior. The S_max = 0.23% with d₃₃^* of 450 pC/N is varied 32% over a wide temperature over a range of 25-140°C, as shown in Fig. 7(d). These results indicate promising findings in eco-friendly materials for elevated-temperature piezoelectric device applications.

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Fig. 7.

(a, b) Temperature-dependent bipolar and unipolar S-E curves under the 50 kV/cm and (c) temperature dependence variation of piezoelectric Ec and -Sneg and (d) d33* in the temperature range 25°C to 140°C.

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4. Conclusions

The primary aim of the work is to make echo-friendly piezoceramics for high-temperature applications. A new lead-free donor-doped BF30BT polycrystalline ceramic is successfully designed, and its electrical properties are optimized. In this work, a high d₃₃ of 344 pC/N is realized by applying a new AC-bias poling method. Furthermore, a large d₃₃^* of 450 pm/V with thermal variation of Δd₃₃^* = 32% over a wide temperature range of 25-140°C, together with a high T_C of 530°C were achieved simultaneously in the same composition. The phase structure and microstructural properties strongly support the high piezoelectric characteristics. The composition design strategy and sintering condition optimization for the BF-BT ceramics show significant progress towards real device applications.