Multifunctional Experimental Studies of Sm-Ion-Influenced Pseudo-Cubic Morphotropic Phase Boundary Regional BiFeO₃-xSrTiO₃ Ceramics for High-Temperature Applications

Abstract

In this manuscript, for the first time, the exploration of the microstructural, ferroelectric, piezoelectric, and dielectric performances are measured for Sm-ion-influenced pseudo-cubic, morphotropic phase boundary (MPB) regional 0.62BiFeO₃−0.38SrTiO₃:xwt%Sm₂O₃ (BFST:xSm) ceramics with x = 0–0.25. All the compositions maintained their pseudo-cubic MPB structural stability. The composition of BFST:0.15Sm ceramics exhibited an excellent remnant polarization (P_r) of ~52.11 μC/cm², an enhanced d₃₃ of 101 pC/N, and the highest relative dielectric constant (ɛ_r) of ~1152, which are much improved as compared to that of pure BFST ceramics. BFST:0.15Sm ceramics demonstrated a Curie temperature (T_C) of 378 °C. Moreover, the composition exhibited high thermal stability for d₃₃ 72 pC/N (only a 28% decrease), even at a high temperature of 300 °C. Such outstanding outcomes make BFST:0.15Sm ceramics an ideal applicant for high-temperature piezoelectric applications.

1. Introduction

Recently, perovskite-based ceramics have gained significant importance for high-temperature piezoelectric applications due to their tremendous thermal stability and high piezoelectric coefficient (d₃₃). They are considered ideal for offering enhanced durability in harsh and fatigued conditions for aerospace, energy sector, and automotive applications. Perovskite-based materials are renowned due to their unique structures and versatile properties; the novel crystalline structural arrangement of such materials makes them ideal to dope/add or substitute suitable elements, resulting in a wide range of amplified tunable properties [1,2,3,4]. Researchers have paid attention to analyzing the thermal stability of their piezoelectric performances in the high-temperature range and utilizing them for applications in harsh or fatigued conditions, like high-temperature piezoelectric devices, industrial processes, automotive applications, aerospace applications, and energy harvesting systems [5,6,7]. Scientists are focused on exploring and stimulating the properties of lead-free dielectric ceramics due to the rising health concerns associated with lead-based ceramics [8,9].

Among perovskite-based ceramics, BiFeO₃ (BF) has special importance due to its ability to demonstrate electro-magnetic effects simultaneously. It possesses a rhombohedral structure with space group R3c. BF shows the ferroelectric to paraelectric transition near its Curie temperature (T_C) of ~820 °C [10]. The reported d₃₃ of pure ceramic BF is ~40 pC/N [11]. However, the voltaic nature of Bi-ions at high temperatures always makes it a challenging task for researchers to fabricate single-phase BF ceramics without the appearance of a secondary phase [12]. To overcome the challenge of low d₃₃, BF is mostly doped with its A- or B-site elements, with the suitable elements keeping the valance and ionic radii in consideration for d₃₃ improvement. Another significant effort made to utilize the best merit of BF (its high T_C) is to combine one of its ends with another different phased structure perovskite to explore the MPB regions and amplify the ferroelectric, dielectric, and piezoelectric performances, like BaTiO₃ [13], Bi_0.5K_0.5TiO₃ [14], Bi_0.5Na_0.5TiO₃ [15], SrTiO₃ [16], etc. Among such counterparts of BF, SrTiO₃ (ST) has rarely been explored by scientists; it possesses a cubic structure with the space group Pm3m [17]. ST is regarded among the linear dielectric perovskites, which exhibit unique properties like a moderate relative dielectric constant (ɛ_r) and low dielectric loss (tanδ) [18]. ST is reported to be an incipient ferroelectric material that acts paraelectric at 0K, but there is the possibility of ferroelectricity by modifying the A- or B-sites with some chemical substitution or strain [19]. From a literature survey, it is observed that very limited research has been performed on BiFeO₃-SrTiO₃ ceramics at their MPB vicinity. Makarovic et al. have reported a d₃₃ of 69 pC/N for BF–0.375ST ceramics and a P_r of ~30–50 μC/cm² [20]. Recently, Wang et al. fabricated BF–0.38ST ceramics to exploit the MPB region and reported a P_r of 51.2 μC/cm² and a d₃₃ of 72 pC/N [21], which is the maximum reported to date according to the best of our knowledge.

Rare-earth (RE)-ion doping/addition is found to improve the dielectric permittivity and limit the dielectric loss. RE-ions are considered to be good substitutes at the A-site cations of perovskite materials due to their suitable ionic radii. Such comparable ionic radii of RE-ions as dopants/additives can accommodate lattice sites well, as it involves little formation energy to simply inhabit the A-site [22,23,24]. Samarium (Sm) is found to be a soft metal that possesses a silvery white color, and it gradually oxidizes to Sm₂O₃ in open air [25]. Hence, the introduction of Sm₂O₃ as an additive in the pseudo-cubic MPB regional BFST host matrix will possibly reduce the defected concentrations (occurring due to the voltaic nature of Bi) and increase the density of ceramics, which will result in diffusion control at the Bi-lattice sites during high-temperature sintering and will avoid undesirable phase transitions. The addition of Sm-ions will also lead to well-organized grain boundaries, resulting in the variation of dipole moments and strong ferroelectric and piezoelectric properties.

Herein, 0.62BiFeO₃–0.38SrTiO₃:xwt%Sm₂O₃ (BFST:xSm) with x = 0–0.25 ceramic series was synthesized to investigate its microstructural, ferroelectric, piezoelectric, and dielectric performances. BFST:0.15Sm’s ceramic composition demonstrated outstanding multifunctional properties with a P_r of ~52.11 μC/cm², a d₃₃ of 101 pC/N, and an ɛ_r of ~1152, which are better than those of pure BFST ceramics. Moreover, its composition demonstrated a thermally stable piezoelectric response of 72 pC/N at 300 °C, which makes it a potential candidate for high-temperature sensors or actuators.

2. Fabrication and Characterization

The conventional solid-state reaction technique was followed for the fabrication of 0.62BiFeO₃–0.38SrTiO₃:xwt%Sm₂O₃ (BFST:xSm) with x = 0–0.25 ceramic series. High-purity (~99.9%) Sigma-Aldrich chemical Bi₂O₃, Fe₂O₃, SrO, and TiO₂ powders were weighted with BFST stoichiometric ratios, mixed in ethanol, and milled for 15 h. The mixed powder was dried at 150 °C overnight and then calcinated at 850 °C for 2 h with 20 °C/min increase/decreases in temperature. Calcinated powder was ground, and Sm₂O₃ (99.9%, Sigma-Aldrich, St. Louis, MO, USA) was added to the ground BFST powder with the stoichiometric ratios of xwt%Sm₂O₃ (x = 0–0.25), milled in ethanol for 12 h, and then dried at 150 °C overnight. The dried powder was pressed into disc-shaped ceramics of diameter 11 mm under 150 MPa hydraulic pressure and sintered at 1050 °C for 3 h with a rapid increase/decrease in sintering temperature (20 °C/min), and the extra content of compositional powders was spread around the ceramic discs to avoid the undesired diffusion of Bi-ions.

Structural stability of ceramic series, phase purity, and detection of any secondary phases were characterized by X-ray Diffraction (XRD, PANalytical, Almelo, The Netherlands, 40 kV) Cu Ka 1:l = 1.54056 Å and step size: 0.02°. The Rietveld method was used in Material Studio for the simulation of the achieved XRD patterns. Morphological analysis and grain size variations were analyzed using a scanning electron microscope (FE-SEM, FEI Quanta 200, Hillsboro, OR, USA). For the ferroelectric, piezoelectric, and dielectric measurements, ceramics were polished to a thickness of 0.7 mm, and Ag electrodes of specific diameters were pasted on both faces of the ceramic samples. Polarization versus electric field (P-E) analysis was performed for the ferroelectric measurements by a (aix-ACCT TF Analyzer 2000; Germany) ferroelectric analyzer at room temperature and 1 Hz frequency. BFST:xSm ceramics were poled in a silicon oil bath, applying a 100 kV/cm DC electric field, and at 60 Hz, the piezoelectric d₃₃ coefficient was measured by (IAAS ZJ-30, Institute of Acoustics of CAS, Beijing, China) piezo-d₃₃ m. The thermal piezoelectric stability of the ceramic series was measured by annealing the poled ceramics in a temperature range of 50–400 °C with a step size of 50 °C for 30 min, and then d₃₃ was measured at room temperature. The dielectric properties of the fully poled ceramic series, like dielectric permittivity versus frequency (ɛ_r-f) and dielectric loss versus frequency (tanδ-f), were measured for the frequency range 10²–10⁷ Hz by an HP4284A precision impedance analyzer (Hewlett-Packard, Palo Alto, CA, USA). For the measurement of the Curie temperature of the ceramic samples, dielectric permittivity versus temperature (ɛ_r-T) and dielectric loss versus temperature (tanδ-T) were measured by an LCR analyzer (HP4980A, Agilent, Santa Clara, CA, USA) attached to a programmable furnace and computer.

3. Results and Discussion

XRD patterns of BFST:xSm with x = 0–0.25 ceramics were measured at room temperature and are presented in Figure 1a. All the samples maintained their MPB-existent pseudo-cubic perovskite structures; no phase impurities or secondary phases were detected in the patterns. For confirmation of the dual structural rhombohedral R from BiFeO₃ and cubic C from the SrTiO₃ phases, an amplified XRD image from 37 to 48° was taken and is presented in Figure 1b, where the duality of structures is confirmed by the dual (111) and (200) peaks. Herein, it can be clearly described that the additive ions of xwt%Sm₂O₃ have incorporated at the lattice sites of the BFST host successfully without damaging the structure of the material. All the visible peaks matched well with the PDF# 71–2494 for BiFeO₃ and PDF# 89–4934 for SrTiO₃. For further confirmation of the structural stability, XRD patterns were further simulated by the Rietveld method by using Material Studio, keeping the rhombohedral R3c and cubic Pm3m as reference structures (Figure 2a–d). The R_WP, R_P, and χ were the simulation parameters, which remained less than 10%, confirming the consistency of the refined data. The refinement outcomes disclose the co-existence of dual phases (R and C) for all the fabricated compositions, demonstrating the strong existence of an MPB region, which is not disturbed by the addition of Sm-ions even at varying contents.

SEM micrographs of BFST:xSm with x = 0–0.2 ceramics are presented in Figure 3a–d. No holes or cracks were observed in all the images, confirming the compactness and high density of the ceramic compositions. The grain size dispersion and average grain size of the ceramics were calculated using ImageJ software (version 1.53v). The average grain size of the compositions varied from 4.35 μm to 5.2 μm (Insets Figure 3) for pure BFST ceramics to BFST:0.15Sm ceramics by increasing the Sm content, and then a slight decrease occurred for the BFST:0.20Sm ceramics to 4.86 μm for BFST:0.20Sm ceramics. The Sm-ions can have a strong influence on the grain size of the host BFST; it only depends on how evenly the Sm-ions are dispersed in the host matrix. A low content of Sm-ions promotes grain growth due to the pinning effect and helps to establish the uniform structure. But, as the Sm-ion content increases to a specific ratio (percolation threshold), the ions can form a cluster or form other phases, reducing the grain size at a higher sintering temperature.

Ferroelectric measurements of the ceramics were performed to investigate the ability of the materials in several applications, like sensors, actuators, capacitors, energy harvesting, and memory devices. The P-E loops of pseudo-cubic structural MPB regional BFST:xSm with x = 0–0.20 ceramics were measured at room temperature, 1 Hz frequency, and 120 kV/cm electric field. Well-saturated and open P-E loops of the ceramics were achieved and are presented in Figure 4a. All the compositions demonstrated typical ferroelectric behavior without signs of any leakage current. Maximum remnant polarization (P_r~52.11 μC/cm²) and saturated polarization (P_S~58.4 μC/cm²) were observed for the BFST:0.15Sm ceramic composition (Figure 4b), which are much improved over those of pure BFST ceramic (P_r~42 μC/cm² and P_S~44.55 μC/cm²). From Figure 4b, it can be observed that for the BFST:0.20Sm ceramic composition, the polarization values (P_r~47.1 μC/cm² and P_S~55.6 μC/cm²) have reduced; this reduction in the polarization values can be associated with the percolation threshold value [26,27]. Keeping the Sm-ion content below percolation threshold values, the Sm-ions incorporate the BFST host ceramics and adjust at the vacant lattice sites successfully, causing an improvement in the ferroelectric behavior till BFST:0.15Sm ceramics. Afterwards, by performing complete diffusion at the lattice sites, Sm-ions start to accumulate at the grain boundaries, which increases the conduction of the ceramics and results in the production of a leakage current to reduce the ferroelectric behavior. From Figure 4c, the corrosive electric field (E_C) as a function of x values of Sm-ions in BFST ceramics is presented, which shows the reduction in the values of E_C with the variation in the Sm-ion content, confirming that fabricated ceramics are well oriented and Sm-ions helped the electric domains saturate easily at lower electric fields to make the ceramic compositions completely saturated. From the ferroelectric analysis, it can be suggested that BFST:0.15Sm ceramics have the best composition and are capable of being utilized in capacitors, energy harvesting, and memory devices.

The piezoelectric coefficient (d₃₃) as a function of the x values of Sm-ions in the BFST host matrix is presented in Figure 5a. Here, it can be analyzed that the BFST:0.15Sm ceramic composition demonstrates a maximum d₃₃ of 101 pC/N, which is 22% higher than the pure BFST ceramic, consistent with the ferroelectric analysis. Exceptional ferroelectric and piezoelectric performances can be associated with the dual-structure MPB regional composition of the R and C phases, which is consistent for all the compositions and facilitates domain switching and polarization rotation because of an applied electric field [28]. The reduction in the d₃₃ value (95 pC/N) for the BFST:0.20Sm ceramics is attributed to the percolation threshold limit, where the leakage current plays an important role in reducing the piezoelectric properties [29]. For the practical utilization of BFST:0.15Sm ceramics, the thermal stability of the piezoelectric performances for all the BFST:xSm (x = 0–0.2) ceramics is measured by annealing the ceramics for 50–400 °C with a step size of 50 °C for 30 min each, followed by the measurement of d₃₃ at room temperature. A similar kind of trend is observed for all the compositions; the d₃₃ value of BFST:0.15Sm ceramics is reduced to 72 pC/N at 300 °C, which is just a 28% reduction from the initial value but still strong enough to operate, which confirms the piezoelectric thermal stability of the ceramics at high temperatures. The thermal stability of piezoelectric materials is no doubt an important parameter in their performance and is strongly associated with the diffusion temperature (T_d). T_d marks the specific value of temperature where ferroelectric properties start the transition towards paraelectric properties and the reduction of piezoelectric properties. High temperatures cause variations in the thermal vibration within the crystal lattice, resulting in the disturbance of dipoles’ alignments and causing the diminution of piezoelectric performances. The specific explanation in terms of a schematic diagram is presented in Supplementary Figure S1. This analysis confirms the ability of the ceramic composition for high-temperature piezoelectric applications. Moreover, for the utilization of such ceramics in frequency-dependent sensors, the dielectric permittivity as a function of the frequency range of 10² Hz to 10⁷ Hz of poled BFST:xSm (x = 0–0.2) ceramics was measured (Figure 5c). The maximum dielectric constant (ɛ_r of ~1152) was observed for the BFST:0.15Sm ceramic composition, which is much better than all other compositions, consistent with the ferroelectric and piezoelectric properties of the BFST:0.15Sm ceramics. Dielectric anomalies were observed for all the ceramic compositions at the frequency ~2.1 × 10⁵ Hz. Poled ceramics demonstrate deformation at an applied 0.5 V AC voltage because of the piezoelectric effect present in them. Such deformation appears in the form of propagation and resonance, causing the corresponding dielectric anomalies at a specific frequency [30]. The frequency range where such dielectric anomalies appear due to the planar or thickness piezoelectric effect is dependent on the diameter and thickness of the ceramic discs and acoustic velocity. The dielectric loss values of the ceramics define the compactness and density of the fabricated ceramics. Herein, the dielectric losses as a function of frequency plots are presented in Figure 5d, where BFST:0.15Sm ceramics exhibit the lowest dielectric loss, confirming the limited leakage current, high density, excellent ferroelectric properties, outstanding piezoelectric properties, and better dielectric outcomes.

For the measurement of the Curie temperature, the dielectric permittivity as a function of temperature plots of the BFST:xSm (x = 0–0.20) ceramics measured at 1 kHz is presented in Figure 6a, demonstrating the relaxor ferroelectric behavior. As the content of xwt%Sm₂O₃ increases, the peak shape of the dielectric peak gradually becomes sharp (Figure 6a), meaning that the change in Sm₂O₃ content regulates the dispersion characteristic of the BFST:xSm system. In general, as the additive content increases, the long-range ordered state of the ceramics is disrupted to a greater extent, which causes the system to become more relaxed (expressed by green arrow in Figure 6a) [31,32,33]. The T_C values of BFST, BFST:0.1Sm, BFST:0.15Sm, and BFST:0.20Sm ceramics are 395 °C, 388 °C, 378 °C, and 367 °C, respectively. However, the dielectric permittivity (ɛ_r) values improve until the BFST:0.15Sm ceramic composition (Figure 6b), consistent with the dielectric permittivity versus frequency plots. The values of dielectric permittivity versus the temperature of the frequency plots for all the compositions showed the same trend at the frequency of 1 kHz and room temperature (Supplementary Figure S2). Moreover, thermally stable behavior of dielectric loss as a function of temperature is observed (Figure 6c); even at the high temperature of 350 °C, the lowest dielectric loss of BFST:0.15Sm ceramic is observed (Figure 6d). The thermally lowest dielectric loss and the high ferroelectric, good piezoelectric, and significant dielectric permittivity performances of the BFST:0.15Sm ceramics confirm the potential of the material for energy harvesting and memory-based and high-temperature piezoelectric devices.

A detailed comparison of our article’s outcomes with the literature survey is presented in

Table 1. Piezoelectric co-efficient comparison of the present study with the literature.

Sr. No	Material	Fabrication Technique	Piezoelectric Coefficient d33 (pC/N)	Ref.
1	(1 − x)Bi1−ySmyFeO3–xBiScO3	Rapid thermal quenching	51	[16]
2	0.625BiFeO3–0.375SrTiO3	Conventional solid-state synthesis	69	[20]
3	0.62BiFeO3–0.38SrTiO3	Conventional solid-state synthesis	72	[21]
4	0.66BiFeO3–0.34BaTiO3	Conventional solid-state synthesis	217	[32]
5	K+ and Nb5+ co-doped (0.75 − x)BiFeO3–0.25BaTiO3	Conventional solid-state synthesis	163	[34]
6	0.62BiFeO3–0.38SrTiO3:0.15wt%Sm2O3	Conventional solid-state synthesis	101	This work

. It is evident from the analysis that the BFST:0.15Sm ceramics in this study achieved the highest d₃₃ value as compared to other ST-based ceramics. But it is still lower than that of the BiFeO₃–xBaTiO₃-based ceramics. Efforts are continuing towards the improvement, but in this manuscript the BFST:0.15Sm ceramics demonstrated a strong ability to be utilized in high-temperature piezoelectric applications.

4. Conclusions

A successful fabrication of the pseudo-cubic structural MPB regional BFST:xSm (x = 0–0.25) ceramic series was fabricated by the conventional solid-sate reaction method. Among different compositions, BFST:0.15Sm ceramics demonstrated the best merits, like a P_S of ~58.4 μC/cm², a P_r of ~52.11 μC/cm², a d₃₃ of 101 pC/N, and an ɛ_r of ~1152, making the composition suitable for many applications, like sensors, actuators, and memory-based devices. The BFST:0.15Sm ceramics exhibited a Curie temperature of 378 °C. Moreover, the thermally stable piezoelectric performance of the BFST:0.15Sm ceramics was observed. The composition showed a high d₃₃ of 72 pC/N even at the high temperature of 300 °C with just a 28% decrement from the initial value, making it an ideal candidate for high-temperature piezoelectric devices.