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Article

High-Temperature Piezoelectric Response and Thermal Stability of BiGaO3 Modified BiFeO3–BaTiO3 Lead-Free Piezoelectric Ceramics

by
Shibo Guan
1,2,3,4,
Huabin Yang
1,2,3,*,
Shuai Cheng
1,2,3,*,
Hua Tan
5,6,
Guanjun Qiao
4,*,
Qiaohong Chen
1,2,3,
Jiwen Xu
1,2,3,
Linna Yuan
1,2,3,
Xueting Wang
1,2,3 and
Ling Yang
1,2,3
1
Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, China
2
School of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, China
3
Engineering Research Center of Electronic Information Materials and Devices, Ministry of Education, Guilin University of Electronic Technology, Guilin 541004, China
4
School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China
5
School of Materials Science and Engineering, State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China
6
Guangdong HUST Industrial Technology Research Institute, Dongguan 523808, China
*
Authors to whom correspondence should be addressed.
Crystals 2023, 13(7), 1026; https://doi.org/10.3390/cryst13071026
Submission received: 5 June 2023 / Revised: 22 June 2023 / Accepted: 27 June 2023 / Published: 28 June 2023
(This article belongs to the Special Issue Research Progress of Perovskite Ferroelectric Materials)
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Versions Notes

Abstract

:
BiGaO3 doped BiFeO3–BaTiO3 ceramics were prepared by the traditional solid-phase synthesis process. The phase analysis, microstructure, piezoelectric, ferroelectric, dielectric properties, and thermal stability of 0.7BiFeO3-(0.3 − x)BaTiO3-xBiGaO3 (Abbreviated as BF–BT-xBG) were investigated. The results show that the ceramics have rhombohedral (R) and tetragonal (T) structures. Particle dimensions gradually get bigger with the increase of BiGaO3 concentration, and dense ceramic grains were observed through SEM. Electrical properties of BF–BT-xBG are improved after adding a small amount of BiGaO3: piezoelectric constants d33 = 141 pC/N, electromechanical coupling coefficient kp = 0.314, mechanical Quality Factor Qm = 56.813, dielectric loss tanδ = 0.048, residual polarization intensity Pr = 18.3 µC/cm2, Curie temperature Tc = 485.2 °C, depolarization temperature Td = 465 °C for x = 0.003. The “temperature-piezoelectric performance” curve under in situ d33 indicates that piezoelectric properties d33 increase rapidly with increasing temperature. Remarkably, the piezoelectric response d33 reaches a maximum of 466 pC/N at a temperature T = 340 °C, and afterward, reduces gradually to zero with increasing temperature until 450 °C.

1. Introduction

High-temperature piezoelectric ceramics are used in aircraft, aviation, military, oil prospecting, and other fields [1,2,3]. PbNb2O6 and BiScO3–PbTiO3 are the most commonly used materials in the area of high-temperature piezoelectric ceramics. The performances of PbNb2O6 system piezoelectric ceramics are d33 = 70–190 pC/N and Tc = 370–610 °C [4,5]. BiScO3-bTiO3 piezoelectric ceramics have a high properties d33 = 400 pC/N and Tc = 450 °C [1,6]. Following increased awareness of environmental protection during the 20th century, the use of lead became more and more regulated by law. Therefore, the research on low-cost, non-toxic, and high-quality lead-free piezoelectric ceramics is of major significance. So far, potential materials like (K, Na) NbO3 (KNN), Bi1/2Na1/2TiO3 (BNT), and BaTiO3(BT), each with their own strengths and weaknesses [7,8,9]. KNN has a high d33, but the piezoelectric properties gradually decrease as the temperature rises [10,11]. BNT has a large strain but a low depolarization temperature [12,13]. BiFeO3–BaTiO3 ceramics are attracting more attention due to their excellent properties.
BiFeO3 is a material with high Tc = 870 °C and high residual polarization strength [14,15]. At room temperature, BaTiO3 is a perovskite material with a tetragonal structure, which has a high dielectric constant (εr) and low tanδ. BiFeO3 and BaTiO3 can form a perovskite solid solution with excellent piezoelectric properties and thermal stability [16,17]. BF–BT is a kind of piezoelectric ceramic with high Tc and high Td [18,19]. Kumar et al. Found that BF–BT has a rhombohedral structure and tetragonal structure, respectively, when the content of BiFeO3 is higher than 70% and lower than 4%. It changes to a cubic structure when the content of BiFeO3 is higher than 4% and lower than 70% [19]. In the process of synthesizing pure BF, some second phases will be produced, such as Bi2O3, Bi2Fe4O9, Bi25FeO39, Bi25FeO40, or Bi46Fe2O72 [20,21]. On the other hand, doping other elements will enhance the electrical performance of BF–BT. It has been reported that many ions doped to enhance the electric performances of BF–BT, such as Cr3+, Sc3+, Nd3+, Ga3+, and so on [22,23,24,25]. BiGaO3 has huge tetragonal distortion and a P4mm space group [26]. Liu et al. have shown that the resistivity of 0.7Bi(GaxFe1−x)O3-0.3BaTiO3 ceramics increases with the Ga content increasing [27]. Zhou et al. synthesized 0.71Bi(Fe1−xGax)O3-0.29BaTiO3 ceramics with good Curie temperature, and they obtained good electrical performances: d33 = 157 pC/N, kp = 0.326, Tc = 467 °C [28]. Myang Hwan Lee et al. studied 0.67Bi1.05-(Fe1–xGax)O3-0.33BaTiO3 ceramic and obtained d33 = 402 pC/N, Tc = 454 °C by quenching process [29]. Akram et al. prepared (1 − x)(0.65Bi1.05FeO3-0.35BaTiO3)-xBiGaO3 ceramics and obtained d33 = 165 pC/N, kp = 0.25 for this system of ceramics at x = 0.01 [30]. Guan et al. has been reported that 0.67BiFeO3–0.33BaTiO3–xBiGaO3 ceramics have good piezoelectric performances: d33 = 170 pC/N and Tc = 434 °C [25]. Recently, Myang Hwan Lee et al. increased the d33 of 1 mol% BiGaO3-doped BF33BT (BG) ceramics from 402 to 454 pC/N [31]. As an additive, MnO2 was added to BF–BT ceramics to enhance the DC resistance and electrical properties of the ceramics [32,33]. Recently, in situ d33 has been used to characterize the d33 in the actual operating state of piezoelectric ceramics [34,35].
In this work, 0.7BiFeO3-(0.3 − x)BaTiO3-xBiGaO3 + 0.01MnO2 (BF–BT-xBG) piezoelectric ceramics were produced by the traditional solid reactive method. BiGaO3 influences on the crystalline structure, piezoelectric, ferroelectric, dielectric properties, and thermal stability have been systematically investigated. By designing this experiment, it is expected to obtain high performances at high temperatures, and at room temperature, polarization still has good piezoelectric properties of ceramics. The results showed that BF–BT-0.003BG ceramics have good piezoelectric performances with d33 = 466 pC/N at 340 °C. When MnO2 is added to the pre-fired ceramics, it also has good piezoelectric properties after polarization at room temperature. It represents the piezoelectric performance of ceramics in the actual working state. These findings show that BF–BT ceramics have a great possibility of replacing PZT ceramics.

2. Experimental Methods

0.7BiFeO3-(0.3 − x)BaTiO3-xBiGaO3 + 0.01MnO2 ceramics are synthesized by traditional solid-state sintering. Bi2O3 (99.99%), Fe2O3 (99.99%), BaCO3 (99.99%), TiO2 (99.99%), MnO2 (99.99%) (Xilong Chemical Plant, Shantou, China) and Ga2O3 (99.99% Macklin) were used as raw materials for synthesis. Due to the volatilization of Bi during the sintering process, an excess of 2 mol% Bi was added. The powder was weighed following a certain stoichiometric ratio into a bottle, mixed with alcohol, and ball mill for 12 h with 1000 r/min. The mixed particles were calcined at 800 °C for 6 h with a heating speed of 5 °C/min. MnO2 was added to the calcined powder and poured into the bottle for the second grinding. Then, sintered at 1020 °C for 9 h with a heating rate of 5 °C/min. Both sides of the fired ceramic sheets were coated with silver electrodes and fired at 600 °C for a holding time of 30 min for electrical properties testing. The thickness of the measured sample is about 0.8 mm.
The crystal structure of the ceramic was tested by using an X-ray diffractometer with Cu Kα (Smart Lab 9 kw, Rigaku, Tokyo, Japan). The morphology of the ceramic surface was photographed using a scanning electron microscope (JSM-7610FPlus). Density was calculated by Archimedes’s drainage method. Piezoelectric properties were tested by the quasi-static d33 tester (Institute of Acoustics, Chinese Academy of Sciences). P-E curves were measured by a ferroelectric test system (aixACCT TF Analyzer 1000, Aachen, Germany) at room temperature. Thermal stability was measured by the LCR analyzer (keysight, 4980A) from 25 °C to 550 °C. The depolarization temperature was measured ex situ. The in situ d33 was obtained by using a high-temperature in situ d33 test instrument (Wide-temperature-range d33 m: TZFD-900, Harbin Julang Technology Co., Ltd., Harbin, China, Figure S1). In the in situ d33 test method: the ceramic plate is placed in the apparatus, and the d33 operating state of the ceramic piece is tested in the furnace as the temperature rises. In ex situ d33, the ceramic piece is placed in a furnace, heated to a certain temperature, removed and cooled to room temperature, and then tested for its d33.

3. Results and Discussion

Figure 1a shows the XRD of BF–BT-xBG ceramics and the standard diffraction peaks for BF (R3c, PDF#71-2494) with the R phase and BT (P4 mm, PDF#75-1169) with the T phase. The ceramics display a perovskite structure with a few Bi25FeO40 impurity phases in Figure 1a, which has been reported in other studies [36,37,38]. The structure of BF–BT-xBG ceramics is a coexistence of the R phase and T phase. Figure 1b shows the BF–BT-xBG XRD pattern of 39°. The peak of 39° has no obvious change as the BG concentration increases.
As shown in Figure 2, the Rietveld refinement method was used to analyze the phase structure of the BF–BT-xBG ceramics. The R phase is R3c (PDF#71-2494), and the T phase is P4mm (PDF#75-1169) through fitting analysis. The R phase weight fraction increases from 29.478% to 41.662% as BG concentration increases from 0 to 0.006. Then, the R phase content decreases as BG concentration increases from 0.006 to 0.012. Table 1 shows the Rietveld refinement structure parameters. The XRD refinement data are consistent with the results in Figure 1b.
Figure 3 shows the microstructure of the BF–BT-xBG ceramic after sintering at 1020 °C for 9 h. The results show that all ceramic surfaces are dense without obvious pores, that the grains are regular polygons, and that the grain boundaries are clear. Figure 3 shows the particle dimension distribution of the BF–BT-xBG ceramics after sintering at 1020 °C for 9 h. The particle dimensions of the BF–BT-xBG ceramics were measured using Nano Measurer software. It was shown that the particle dimensions of ceramics increase gradually as the BG concentration increases, and the average particle dimensions increase from 7.02 μm at x = 0 to 13.37 μm at x = 0.012. It can be concluded that the addition of BiGaO3 promotes an increase in particle dimensions. There are two reasons for this: one is that the incorporation of Ga3+ ions boosts the formation of the liquid phase and the sintering of the ceramic, which increases the size of the grain [28]. The other one is that more Bi2O3 is added with the addition of BiGaO3, resulting in the generated impurity phase Bi25FeO40 leading to the generation of a more liquid phase and promoting the growth of ceramic grains [36,37]. On the other hand, in pure dense ceramics, the parabolic law indicates that the grain boundary mobility controls grain growth [39], and the doping of BiGaO3 may promote grain boundary migration. The relative density of BF–BT-xBG ceramics is shown in Figure 3. It was shown that the relative density of ceramics first grew and then reduced as the BG concentration increased, reaching the highest value of 95.79% at x = 0.003.
Figure 4a shows the d33 of BF–BT-xBG ceramics polarization at room temperature (rp) and polarization at 100 °C (100p), of which the d33 has little difference. It can be seen that the d33 of ceramics grew at first and then reduced as the BG concentration grew, which reaches the highest d33 = 141pC/N (rp) when x = 0.003. There are two reasons for this: one is that the ion radius Bi3+ = 1.38 Å (CN = 12), Ga3+ = 0.62 Å (CN = 6), Ba2+ = 1.61 Å (CN = 12), Ti4+ = 0.605 Å (CN = 6). A small amount of BiGaO3 doped into BaTiO3 will lead to lattice distortion, which promotes the movement of ferroelectric domains and enhances the piezoelectric performances. Another reason is that the addition of a small amount of BiGaO3 may produce polar nano micro-regions (PNRs). It destroys the long-range ordered ferroelectric state, strengthens the electromechanical coupling effect, and improves the intrinsic piezoelectric activity [40]. Meanwhile, BiGaO3 doping promotes the formation of liquid phase and grain growth, and the relative density is maximum at x = 0.003, when the piezoelectric performance is the best. Ga3+ replaces Ti4+ to produce oxygen vacancy when the BiGaO3 content is high [41]. The movement of the oxygen vacancy pinning domain causes the decrease of d33. MnO2 decomposes to Mn2O3 above 900 °C. In our experiment, MnO2 is added after the pre-combustion, so that more MnO2 becomes Mn2O3 [33,42]. According to this reaction:
4MnO2→2Mn2O3 + O2↑ (≥900 °C)
Mn3+ + Fe2+→Mn2+ + Fe3+
Mn2O3 can better inhibit the conversion from Fe3+ to Fe2+, and improve the thermal stability of ceramics. Therefore, ceramics still have excellent piezoelectric properties under rp conditions.
Figure 4b shows the kp and Qm of BF–BT-xBG ceramics. The kp rises first and then drops as the BG concentration increases, and reaches the highest 0.314 when x = 0.003. Qm changes little as the BG concentration increases. The change of kp is consistent with d33, and the reason is the same as that of d33 described above. Figure 4c shows the change of dielectric constant εr with BG concentration. The εr increases first from 751.843 at x = 0 to 853.149 at x = 0.003, and then decreases to 670.483 at x = 0.012. The addition of a small amount of Ga3+ causes lattice distortion and contributes to the enhancement of εr. Figure 4d shows the change of dielectric loss tanδ with BG concentration. It can be concluded that tanδ decreases first to 0.048 at x = 0.003 and then increases as the BG concentration grows, indicating that a small amount of BiGaO3 incorporation is conducive to enhancing the dielectric properties of ceramics.
Figure 5a–f show the variation in impedance and phase angle θ with frequency at 25 °C. It has been shown that the polarization phase angle θ reaches the maximum value θ = 58.917 when x = 0.003, which corresponds to the component point when d33 is at the maximum value, indicating that the ceramic has sufficient polarization and the highest performance when x = 0.003.
Figure 6a–e shows the P-E hysteresis loop diagram of the BF–BT-xBG ceramic at room temperature. It can be seen that the morphology of the hysteresis loop tends to saturate with increasing electric field at the same component point, and gradually changes from flat to well-saturated. It shows that the ferroelectric performances of ceramic increase gradually with the enhancement of the electric field. Figure 6f shows the P-E hysteresis loops of different component points under the same electric field of 50 kv/cm. All hysteresis loops are saturated. The asymmetric shape of the P-E hysteresis line is due to the internal bias field of the ceramic during the test [2]. Figure S2 shows the internal bias field at different composition points of 50 kV/cm. The Ei at all component points is between 2 and 3 kV/cm. The main doped ferroelectric produces oxygen vacancies to maintain its own electrical neutrality, which in turn leads to the formation of defective dipoles, which are oriented in the polarization direction after sufficient polarization aging, so that the directionally arranged defective dipoles form the internal bias field Ei.
Figure 7a shows the changes in residual polarization intensity Pr and coercive field Ec at different composition points at room temperature. The results show that Pr rises first and then drops as the BG concentration grows, reaching the highest value, Pr = 18.3 µC/cm2, when x = 0.003. Ec does not change much as the BG concentration increases. The reason why Pr reaches the maximum at x = 0.003 is that a few doping of BiGaO3 improves the lattice aberration of the ceramic, which increases the ferroelectric activity. Another reason may be that a few doping of BiGaO3 improves the order of ceramic domains and then improves Pr. Figure 7b shows the P-E hysteresis loop of BF–BT-0.003BG ceramics at 40 kV/cm with different temperatures. Pr rises, and Ec drops gradually with the increase in temperature. The reason for this is that the high-temperature environment diminishes the pegging effect of the defective dipole and contributes to the flipping of the ferroelectric domains. Because of the internal bias field in this ceramic, there is an asymmetry in the P-E hysteresis loop [2].
Figure 8a–e shows the temperature-dependent εr and tanδ of the ceramic with different BG concentrations as a function of temperature. The test frequencies are 1, 10, and 100 kHz, respectively. There is a high-temperature dielectric anomaly peak and no obvious frequency dependence at low temperatures. As the temperature increases, the frequency dependence becomes more pronounced. This behavior is associated with the chemical heterogeneity of the material and the decomposition of the macro-nano domain into a nanodomain structure near the ferroelectric paraelectric phase transition. Ferroelectrics can be classified as normal ferroelectrics, dispersion ferroelectrics, and relaxor ferroelectrics [43]. Normal ferroelectrics are distinguished by a sharp phase change peak, dispersed ferroelectrics by a broad phase change peak, and relaxor ferroelectrics by a broad phase change peak, which gradually shifts to higher temperatures as the frequency increases. Figure 8 shows that the ceramics in this system exhibit all the characteristics of a relaxor ferroelectric. There are many defects in the ceramic sintering process. These defects have little influence on the dielectric performances of ceramics at low temperatures and have no obvious frequency dependence. The influence of the defects on the ceramic becomes stronger, and the ceramic shows an evident dependence on frequency as the temperature continues to increase. The εr initially remains unchanged and then increases rapidly with increasing temperature, and then drops gradually after arriving at the peak. Oxygen vacancies require less energy to be excited and can be excited at low temperatures. Cation excitation requires more energy and is not easy to excite at a lower temperature; therefore, the εr is low. The energy of the excited cation is satisfied at high temperatures, so it may be excited, leading to a high dielectric constant [44]. On the other hand, it may be due to the directional arrangement of ferroelectric domains in the polarization process of ceramics, and the lattice energy in the stable state is locked. At this time, the energy is difficult to make the long-range ordered macro domain move, and the domain wall cannot move as the electric field is applied. The relaxing time is long, resulting in a very low dielectric constant. Then, the energy required for the thermal motion is achieved as the temperature continues to increase, which makes the electric domain of the ceramics change from the long-range ordered state to the short-range disorder state, resulting in a large dielectric response and a rapid increase in εr [23]. The temperature of this peak is the Tc (phase change temperature from the ferroelectric phase to the paraelectric phase). Ceramics exhibit a ferroelectric phase below Tc, and the ferroelectric domains remain in an ordered state with piezoelectric properties. Ceramic is a paraelectric phase above Tc, and the electrical domain is disordered without piezoelectric properties. Tanδ rises slightly at low temperatures, and it rises quickly at high temperatures. The reason is that there are a lot of oxygen vacancies in the ceramic during the sintering process, and the energy needed to excite the cations of these oxygen vacancies is more, which has no effect on the tanδ at low temperatures. Cations gain more energy and are excited at higher temperatures, leading to a rapid increase in tanδ. Tanδ increases slowly with the BG concentration increasing at low temperatures, and the loss is mainly relaxation loss. Then, the tanδ increases rapidly when a definite temperature is achieved, which is mainly the leakage loss. Figure 8f shows the change in the ceramic dielectric constant at different BG concentrations under 1 kHz. As the BG concentration grows, the curve shifts first to high temperatures and then to low temperatures. Figure S3 shows the local enlargement of the dielectric temperature spectrum of BF–BT-xBG ceramics and the variation of Tf at different composition points. The temperature at which the normal ferroelectric transforms into a non-ergodic relaxor ferroelectric is the freezing temperature (Tf). Tf reduces and then rises with the increase of BG concentration. The temperature at which the transition from the paraelectric state to the ergodic relaxor state is called Burns Temperature (TB). In this system of ceramics, Tc corresponds to the TB.
Figure 9a–f shows the curve of ln(1/εr − 1/εm) as a function of ln(T − Tm) for the BF–BT-xBG ceramics under 1 kHz. These points are almost in a straight line. Perovskite ferroelectrics are generally divided into normal ferroelectrics, dispersed ferroelectrics, and relaxor ferroelectrics [43]. According to Curie Weiss’s law:
1/εr − 1/εm= (T − Tm)γ/C
where γ, Tm, εm, and C represent the diffusion coefficient, phase transition temperature, maximum εr, and Curie constant, respectively [45,46].
It can judge what kind of ferroelectric the ceramics is. All γ values are higher than 1, showing that the ceramics are relaxor ferroelectrics. The γ value decreases first and then increases with the BG concentration increasing, and reaches the minimum value γ = 1.538 when x = 0.006. A few doping of BiGaO3 reduces the relaxation characteristics of the ceramics.
Figure 10a displays the variation of d33 with temperature for BF–BT-xBG ceramic. The d33 at the same component point x remains stable with rising temperature, and drops abruptly when it reaches a definite temperature. This temperature is determined as the depolarization temperature Td [34]. The domains are arranged orderly when the temperature is low, and the ceramics have high d33. The domain progressively returns to a disorderly state when the temperature rises to a certain value, which d33 decreases progressively. Figure 10b shows the Tc and the Td at different BG concentrations of BF–BT-xBG ceramics. The Tc reaches the maximum of 485.9 °C when x = 0.006. The Td increases first when the BG concentration grows, achieving the highest of 465 °C at x = 0.003 and x = 0.006, and then drops when the BG concentration grows. The higher the amount of R phase, the higher the Tc and Td, which is in agreement with the XRD refinement results (Table 1). It has been shown that a few doping of BiGaO3 increases the Tc and Td of ceramics. Because a few doping of BiGaO3 enhances the lattice distortion and anisotropy of the ceramic, it increases the Tc and Td. At the same time, lattice distortion means a higher phase transition barrier, resulting in a higher Tc [47]. Part of Bi3+ and Ga3+ accumulate on the grain boundary surface when too much Bi3+ and Ga3+ are doped, which reduces the lattice distortion and reduces the Tc and Td. On the other hand, too much Bi3+ and Ga3+ doping may introduce defects and disrupt the long-range ordered structure of the ceramic, reducing Tc and Td [48].
Figure 11a shows the variation of εr and tanδ with temperature for x = 0, x = 0.003, and x = 0.012; the εr grows slowly at low temperatures and grows rapidly when it reaches a certain temperature. The tanδ increases gradually with increasing temperature, decreases after achieving a definite temperature, and then rises rapidly. The specific mechanism is explained in Figure 8. Figure 11b shows the ex situ depolarization plots for x = 0, x = 0.003, and x = 0.012. The d33 of different components remained stable with the rise of temperature, and when reaching a certain temperature (Td) [34], d33 decreases quickly. Figure 11c shows the in situ d33 of BF–BT-xBG ceramic with high temperature. This in situ d33 represents the variation of d33 with temperature in the actual working condition of the ceramic. It shows that the d33 of the ceramic gradually rises with rising temperature, and decreases quickly after achieving the maximum at high temperature. Interestingly, ceramics have a large piezoelectric response d33 = 466 pC/N at 340 °C when x = 0.003. According to the formula d33 = 2QεPs [49], where Q is the electrostrictive coefficient, ε is the dielectric constant, and Ps is the spontaneous polarization. The enhancement of the piezoelectric properties is related to the ferroelectric and dielectric properties. The increase from d33 at low temperature corresponds to the change in residual polarization intensity Pr in Figure 7b. The d33 rises when the temperature rises because the rise of temperature will increase ε and Ps. As the ceramic is cooled in the furnace, oxygen vacancies will be generated, resulting in lattice defects. Oxygen vacancies will gather at grain boundaries and domain walls, preventing the movement of ferroelectric domains. The movement of ferroelectric domains and domain walls becomes active when temperature increases, improving piezoelectric response and piezoelectric properties. The flattening of the Gibbs free energy curve caused by the temperature variation leads to an increase in the dielectric sensitivity and piezoelectric response of the material under test [50,51]. The in situ d33 increase is influenced by both Ps and ε factors. It is mainly affected by Ps at low temperatures and εr at high temperatures, so it increases nonlinearly [52]. The depolarization process starts with a further increase in temperature. The microdomain returns to its initial state with a further increase in temperature. At this time, the ferroelectric domain changes from normal ferroelectric to non-ergodic relaxor ferroelectric, and the local microdomain is decomposed into randomly oriented nanodomains. It promotes the transition of the ferroelectric relaxor phase, so d33 decreases sharply [35]. It shows that a small amount of BiGaO3 can improve the thermal stability of ceramics. By comparing Figure 11a–c, it can be found that the ceramics of this system have an extremely high piezoelectric response in practical work. GuO and Wang reported PZT ceramics with d33 = 910 pC/N, Tc = 184 °C and d33 = 680 pC/N, Tc = 330 °C, respectively [53,54]. Compared with PZT ceramics, the present work has a high Curie temperature and good d33 at high temperatures. It provides great research value for the use of lead-free piezoelectric ceramics. Table 2 shows the piezoelectric properties of the BF–BT system synthesized using the conventional solid-state reaction and quenching process. A high d33 of 466 pC/N was achieved in this work.

4. Conclusions

Lead-free high-temperature piezoelectric 0.7BiFeO3-(0.3 − x)BaTiO3-xBiGaO3 (BF–BT-xBG) after pre-sintering with MnO2 system ceramics were fabricated by solid-state sintering technique and their phase analysis, microstructure, piezoelectric, ferroelectric, dielectric properties and thermal stability, were studied. XRD results show that BF–BT-xBG ceramics have co-existed R and T phases structure. SEM shows that the particle dimensions of ceramics gradually grow with the increase of Bi3+ and Ga3+ incorporation. The electrical property reaches the maximum value when x = 0.003: d33 = 141 pC/N, kp = 0.314, Qm = 56.813, Pr = 18.3 µC/cm2 Tc = 485.2 °C, Td = 465 °C. Lattice distortion leads to improved room-temperature piezoelectric and dielectric properties. The excitation of cations at high temperatures leads to the improvement of dielectric properties at high temperatures. The ceramic d33 = 466 pC/N at 340 °C when x = 0.003 through in situ d33 test. The improvement of Ps and εr leads to the improvement of piezoelectric properties at high temperatures. This study shows that doping BiGaO3 into BF–BT has excellent electrical properties, making it a potential application for high-temperature piezoelectric devices.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst13071026/s1, Figure S1: Wide-temperature-range d33 meter (TZFD-900, Harbin Julang Technology Co., Ltd., Harbin, China). Figure S2: Internal bias field Ei for different component points at 50 kV/cm. Figure S3: Local enlargement of the dielectric temperature spectrum of BF-BT-xBG ceramic and the variation of Tf at different composition points. (a) x = 0, (b) x = 0.003, (c) x = 0.006, (d) x = 0.009, (e) x = 0.012, (f) the variation of Tf at different composition points.

Author Contributions

Conceptualization, S.G., H.Y., S.C. and H.T.; methodology, S.G., H.Y. and G.Q.; software, S.G.; validation, S.G. and H.Y.; formal analysis, S.G., S.C.; investigation, S.G., S.C., H.T., Q.C., J.X., L.Y. (Linna Yuan), X.W. and L.Y. (Ling Yang); resources, H.Y. and G.Q.; data curation, S.G.; writing—original draft preparation, S.G.; writing—review and editing, S.G., H.Y. and S.C.; visualization, S.G.; supervision, H.Y. and G.Q.; project administration, H.Y. and G.Q.; funding acquisition, H.Y. and G.Q.; All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (52162016, 52172069, 52062007), Natural Science Foundation of Guangxi, China (2021GXNSFAA220020, 2022CXNSFBA035612, AD19245084), Guangxi Key Laboratory of Information Materials, the Key Research and Development Plan (BE2019094), Thanks to Engineering Research Center of Electronic Information Materials and Devices, Ministry of Education, Guilin University of Electronic Technology for help with related tests.

Data Availability Statement

The data used to support the findings of this study are available fromthe corresponding author upon request.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Crystals 13 01026 g001 550
Figure 1. The XRD patterns of BF–BT-xBG ceramics from (a) 20–80°, (b) 38–40°, * is impurity phases Bi25FeO40.
Figure 1. The XRD patterns of BF–BT-xBG ceramics from (a) 20–80°, (b) 38–40°, * is impurity phases Bi25FeO40.
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Crystals 13 01026 g002 550
Figure 2. Rietveld refinement results for BF–BT-xBG ceramics by GSAS (a) x = 0, (b) x = 0.003, (c) x = 0.006, (d) x = 0.009, (e) x = 0.012.
Figure 2. Rietveld refinement results for BF–BT-xBG ceramics by GSAS (a) x = 0, (b) x = 0.003, (c) x = 0.006, (d) x = 0.009, (e) x = 0.012.
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Crystals 13 01026 g003 550
Figure 3. The microstructure of BF–BT-xBG ceramics after sintering at 1020 °C for 9 h (a) x = 0 mol%, (b) x = 0.3 mol%, (c) x = 0.6 mol%, (d) x = 0.9 mol%, (e) x = 1.2 mol%, (f) the relative density (five-pointed star) of BF–BT-xBG ceramics.
Figure 3. The microstructure of BF–BT-xBG ceramics after sintering at 1020 °C for 9 h (a) x = 0 mol%, (b) x = 0.3 mol%, (c) x = 0.6 mol%, (d) x = 0.9 mol%, (e) x = 1.2 mol%, (f) the relative density (five-pointed star) of BF–BT-xBG ceramics.
Crystals 13 01026 g003
Crystals 13 01026 g004 550
Figure 4. (a) The d33 of BF–BT-xBG ceramics polarization at room temperature (rp) and polarization at 100 °C (100p), (b) the changes of kp and Qm of ceramics with BG concentration, (c) the change of dielectric constant εr with BG concentration, (d) the change of dielectric loss tanδ with BG concentration.
Figure 4. (a) The d33 of BF–BT-xBG ceramics polarization at room temperature (rp) and polarization at 100 °C (100p), (b) the changes of kp and Qm of ceramics with BG concentration, (c) the change of dielectric constant εr with BG concentration, (d) the change of dielectric loss tanδ with BG concentration.
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Crystals 13 01026 g005 550
Figure 5. (a–f) The variation of impedance and phase angle with frequency at room temperature.
Figure 5. (a–f) The variation of impedance and phase angle with frequency at room temperature.
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Figure 6. (a–e) The P-E hysteresis loop diagram of BF–BT-xBG ceramic at room temperature, (f) the P-E hysteresis loops of different component points under the same electric field 50 kV/cm.
Figure 6. (a–e) The P-E hysteresis loop diagram of BF–BT-xBG ceramic at room temperature, (f) the P-E hysteresis loops of different component points under the same electric field 50 kV/cm.
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Crystals 13 01026 g007 550
Figure 7. (a) The changes of residual polarization intensity Pr and coercive field Ec at different composition points at room temperature, (b) P-E hysteresis loop of BF–BT-0.003BG ceramic variable with temperature.
Figure 7. (a) The changes of residual polarization intensity Pr and coercive field Ec at different composition points at room temperature, (b) P-E hysteresis loop of BF–BT-0.003BG ceramic variable with temperature.
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Crystals 13 01026 g008 550
Figure 8. (a–e) Temperature dependences of εr and tanδ for BF–BT-xBG ceramics at 1, 10, and 100 kHz; (f) temperature dependences of dielectric constant εr for BF–BT-xBG ceramics at 1 kHz.
Figure 8. (a–e) Temperature dependences of εr and tanδ for BF–BT-xBG ceramics at 1, 10, and 100 kHz; (f) temperature dependences of dielectric constant εr for BF–BT-xBG ceramics at 1 kHz.
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Crystals 13 01026 g009 550
Figure 9. (a–e) The curve of ln(1/εr − 1/εm) as a function of ln(T − Tm) for the BF–BT-xBG ceramics under 1 kHz. The blue squares are the corresponding points.
Figure 9. (a–e) The curve of ln(1/εr − 1/εm) as a function of ln(T − Tm) for the BF–BT-xBG ceramics under 1 kHz. The blue squares are the corresponding points.
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Crystals 13 01026 g010 550
Figure 10. (a) Variation of d33 with the temperature at different composition points x of BF–BT-xBG ceramic, (b) the Tc and the Td at different composition points x of BF–BT-xBG ceramics.
Figure 10. (a) Variation of d33 with the temperature at different composition points x of BF–BT-xBG ceramic, (b) the Tc and the Td at different composition points x of BF–BT-xBG ceramics.
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Figure 11. (a) The variation of εr and tanδ with temperature for x = 0, x = 0.003, and x = 0.012, (b) the ex situ depolarization plots for x = 0, x = 0.003 and x = 0.012, (c) the in situ d33 of BF–BT-xBG ceramics for x = 0, x = 0.003 and x = 0.012 at high temperature.
Figure 11. (a) The variation of εr and tanδ with temperature for x = 0, x = 0.003, and x = 0.012, (b) the ex situ depolarization plots for x = 0, x = 0.003 and x = 0.012, (c) the in situ d33 of BF–BT-xBG ceramics for x = 0, x = 0.003 and x = 0.012 at high temperature.
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Table
Table 1. Rietveld refinement structure parameters for unpoled BF–BT-xBG ceramics.
Table 1. Rietveld refinement structure parameters for unpoled BF–BT-xBG ceramics.
CompositionPhaseLatticeVolumeFitting
fractionparameters(VR/VT) (Å3)parameter
a c (Å) Rwp/Rp
x = 0R3c 29.4785.64780 (3)386.48 (6)Rwp = 0.0402
13.99083 (6) Rp = 0.0296
P4 mm 70.5223.99613 (1)63.98 (2)
4.00659 (1)
x = 0.003R3c 38.8085.65280 (2)388.76 (2)Rwp = 0.0425
14.04836 (9) Rp = 0.03
P4 mm 61.1923.99815 (5)64.11 (8)
4.01106 (1)
x = 0.006R3c 41.6625.68791 (5)388.58 (9)Rwp = 0.0422
13.86928 (7) Rp = 0.0299
P4 mm 58.3383.99785 (9)64.25 (1)
4.02001 (1)
x = 0.009R3c 37.1815.69049 (9)388.10 (4)Rwp = 0.0439
13.83939 (5) Rp = 0.0317
P4 mm 62.8193.99542 (0)63.89 (6)
4.00266 (2)
x = 0.012R3c 35.1025.65378 (9)389.47 (8)Rwp = 0.0358
14.06932 (2) Rp = 0.0253
P4 mm 64.8983.99647 (3)63.95 (2)
4.00407 (4)
Table
Table 2. Electrical properties of reported BiFeO3–BaTiO3 Lead-free piezoelectric ceramics. Room Temperature: RT.
Table 2. Electrical properties of reported BiFeO3–BaTiO3 Lead-free piezoelectric ceramics. Room Temperature: RT.
Systemsd33 (pC/N)kpEc (kV/cm)Pr (μc/cm2)Tc (°C)Td (°C)Ref.
0.725BiFe0.98Sc0.02O3 − 0.275BaTiO3 + 0.01MnO21270.3664819.1636450[55]
0.67BiFeO3 − 0.33BaTiO3 + 0.02BiGaO3 + 0.0035MnO21700.30622.05925434422[25]
0.75 BiFeO3 − 0.25BaTiO3 + MnO2116-39.322.9619469[32]
0.75 BiFeO3 − 0.25BaTiO3 + 0.01NdCoO3 + 0.01MnO2110--8.2605525[56]
0.7BF − 0.3BT(SBT)2100.343031.2514400[57]
0.67Bi1.05(Fe0.97Ga0.03) − 0.33BaTiO3 (water-quenching)402---454 [29]
0.67BiFeO3 − 0.33BaTiO3 + 0.01BiGaO3(water-quenching)454 451 [31]
0.7BiFeO3 − 0.297BaTiO3 − 0.003BiGaO3 + 0.01MnO2466 (340 °C) 141 (RT)0.31430.8418.3485.2465This work
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Guan, S.; Yang, H.; Cheng, S.; Tan, H.; Qiao, G.; Chen, Q.; Xu, J.; Yuan, L.; Wang, X.; Yang, L. High-Temperature Piezoelectric Response and Thermal Stability of BiGaO3 Modified BiFeO3–BaTiO3 Lead-Free Piezoelectric Ceramics. Crystals 2023, 13, 1026. https://doi.org/10.3390/cryst13071026

AMA Style

Guan S, Yang H, Cheng S, Tan H, Qiao G, Chen Q, Xu J, Yuan L, Wang X, Yang L. High-Temperature Piezoelectric Response and Thermal Stability of BiGaO3 Modified BiFeO3–BaTiO3 Lead-Free Piezoelectric Ceramics. Crystals. 2023; 13(7):1026. https://doi.org/10.3390/cryst13071026

Chicago/Turabian Style

Guan, Shibo, Huabin Yang, Shuai Cheng, Hua Tan, Guanjun Qiao, Qiaohong Chen, Jiwen Xu, Linna Yuan, Xueting Wang, and Ling Yang. 2023. "High-Temperature Piezoelectric Response and Thermal Stability of BiGaO3 Modified BiFeO3–BaTiO3 Lead-Free Piezoelectric Ceramics" Crystals 13, no. 7: 1026. https://doi.org/10.3390/cryst13071026

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