*3.3. Electro-Mechanical Coupling Property*

*3.3. Electro-Mechanical Coupling Property*  Figure 4 shows the electro-mechanical resonance spectroscopy of the oxide-modified SBT ceramics. As can be seen, there are no resonance-antiresonance peaks in the pure SBT ceramic at the measured frequency range from 20 Hz to 2 MHz. The resonance-antiresonance peaks of SGBT and SCBT appear, respectively, at 184 kHz and 186 kHz. A high angle indicates the fully poled state of the specimen [38]. The position generated the resonance-antiresonance peak and the maximum phase angle also converts with the introduction of other additives. SCBT obtaind the maximum phase angle value (*θ* = −24.8°), Figure 4 shows the electro-mechanical resonance spectroscopy of the oxide-modified SBT ceramics. As can be seen, there are no resonance-antiresonance peaks in the pure SBT ceramic at the measured frequency range from 20 Hz to 2 MHz. The resonanceantiresonance peaks of SGBT and SCBT appear, respectively, at 184 kHz and 186 kHz. A high angle indicates the fully poled state of the specimen [38]. The position generated the resonance-antiresonance peak and the maximum phase angle also converts with the introduction of other additives. SCBT obtaind the maximum phase angle value (*θ* = −24.8◦ ), which indicates its more fully polarized degree.

which indicates its more fully polarized degree. Table 3 presents electro-mechanical properties of the oxide-modified SBT ceramics. Clearly, oxide additives also affect the electro-mechanical coupling properties of the SBT ceramic, especially as the addition of Cr2O3 has a significant impact on it. SGBT-Cr and SCBT-Cr obtain relatively high *k*p, low *Q*m and *N*p. The oxygen vacancies in piezoceramics usually result in the increase in *Q*m and the decrease in *k*p for ferroelectric ceramics [39]. A higher *k*p achieved by SGBT-Cr and SCBT-Cr can be attributed to the reduced oxygen va-Table 3 presents electro-mechanical properties of the oxide-modified SBT ceramics. Clearly, oxide additives also affect the electro-mechanical coupling properties of the SBT ceramic, especially as the addition of Cr2O<sup>3</sup> has a significant impact on it. SGBT-Cr and SCBT-Cr obtain relatively high *k*p, low *Q*<sup>m</sup> and *N*p. The oxygen vacancies in piezoceramics usually result in the increase in *Q*<sup>m</sup> and the decrease in *k*<sup>p</sup> for ferroelectric ceramics [39]. A higher *k*<sup>p</sup> achieved by SGBT-Cr and SCBT-Cr can be attributed to the reduced oxygen vacancy concentration caused by the addition of Gd2O<sup>3</sup> and CeO2.

cancy concentration caused by the addition of Gd2O3 and CeO2.

**Table 3.** Electro-mechanical properties of the oxide-modified SBT ceramics. **Compositions SGBT SGBT-Ce SGBT-Mn SGBT-Cr SCBT SCBT-Gd SCBT-Mn SCBT-Cr**  *k*p (%) 5.72 5.2 4.03 6.35 3.7 4.5 5.8 6.51 *Q*m 423 465 1240 355 1885 787 443 374 *N*p (Hz·m) 2655 2745 2711 2481 2751 2740 2694 2613

**Figure 4.** Electro-mechanical resonance spectroscopy of the oxide-modified SBT ceramics at room temperature. **Figure 4.** Electro-mechanical resonance spectroscopy of the oxide-modified SBT ceramics at room temperature.

*3.4. Lower Limiting Frequency*  **Table 3.** Electro-mechanical properties of the oxide-modified SBT ceramics.


## between *RC* time constant and *fLL* is as follows: *3.4. Lower Limiting Frequency*

*f LL <sup>=</sup> <sup>1</sup> <sup>2</sup>πRC* (2) where *C* is the capacitance (1 kHz) and R is the insulation resistance. Low values of *fLL* allow the dynamic bandwidth to be extended to sonic frequencies [40]. The addition of different oxides decreases the *fLL* of SBT as shown in the inset of Figure 5 at room temperature. The result indicates that the addition of oxides could improve the resistivity of SBT Piezoelectric ceramic materials not only generate charges under the condition of applied stress or strain, but also ensure that the charges must be maintained for a period of time to be monitored by the system in actual engineering applications. The time of the maintained charge is proportional to the *RC* time constant. The minimum available frequency of sensor is considered to be the lower limiting frequency (*fLL*). The relationship between *RC* time constant and *f LL* is as follows:

$$f\_{LL} = \frac{1}{2\pi RC} \tag{2}$$

temperatures are also compared with each other in Figure 5. The *fLL* values of all compositions gradually increase with the rise in temperature, which may be attributed to the decrease in resistivity of the samples with increasing temperature. SCBT shows a lower *fLL* value in the measured temperature range as compared to others. High resistivity can where *C* is the capacitance (1 kHz) and R is the insulation resistance. Low values of *fLL* allow the dynamic bandwidth to be extended to sonic frequencies [40]. The addition of different oxides decreases the *fLL* of SBT as shown in the inset of Figure 5 at room temperature. The result indicates that the addition of oxides could improve the resistivity of SBT ceramics. Due to superfluous electrons generated by higher valence, Gd3+ and Ce4+ substituted lower

valence Sr2+ can neutralize the oxygen vacancies, which increases the resistivity of SBT. The lower limiting frequency of the oxide-modified SBT ceramics at different temperatures are also compared with each other in Figure 5. The *fLL* values of all compositions gradually increase with the rise in temperature, which may be attributed to the decrease in resistivity of the samples with increasing temperature. SCBT shows a lower *fLL* value in the measured temperature range as compared to others. High resistivity can prevent applied electrical signals from leaking away in the process of using, only the modified SBT ceramics with high resistivity can be used in high-temperature piezoelectric fields. *Materials* **2021**, *14*, x FOR PEER REVIEW 8 of 11 prevent applied electrical signals from leaking away in the process of using, only the modified SBT ceramics with high resistivity can be used in high-temperature piezoelectric fields.

**Figure 5.** Lower limiting frequency of the oxide-modified SBT ceramics at different temperatures **(**the insert shows the *fLL* values of various compositions at room temperature). **Figure 5.** Lower limiting frequency of the oxide-modified SBT ceramics at different temperatures (the insert shows the *fLL* values of various compositions at room temperature).

### *3.5. Piezoelectric Properties 3.5. Piezoelectric Properties*

The thermal stability of the piezoelectricity of the oxide-modified SBT ceramics is displayed in Figure 6. As can be seen from the insert, before annealing, the piezoelectric properties of the SBT ceramic (*d*33 = 10 pC/N) can be improved notably by adding only one of CeO2 and Gd2O3, a higher *d*33 ~ 22 pC/N was achieved in SCBT and SGBT. When considering that the addition of CeO2 and Gd2O3 could reduce the concentration of oxygen vacancies as mentioned above, thus the less pinning of domain walls and the elevated resistivity tend to promote the sufficient orientation of ferroelectric domains along the applied electric field during polarization. It is noteworthy that the addition of Cr2O3 can further enhance the piezoelectric properties of the SBT ceramic that *d*33 up to 28 pC/N was observed for SGBT-Cr and 26 pC/N for SCBT-Cr. As shown in Table 1, a larger orthorhombic distortion is obtained for SGBT-Cr and SCBT-Cr, in which a larger spontaneous polarization is believed to form [33]. Further, the thermal stability of piezoelectricity of the oxide-modified SBT ceramics was investigated by the annealing experiment. In general, the *d*33 values of all compositions slowly decrease with increasing the annealing temperature from room temperature to 400 °C, and then drastically drop after 400 °C, until they reach zero when the annealing temperature exceeded their *T*C. The thermal degradation of piezoelectricity can be attributed to the decoupling of space charges at moderate temperatures and the depolarization of intrinsic dipoles at high temperatures [41]. It should be noted that the *d*33 values of SGBT-Cr were decreased by only 3.6% after being annealed at 500 °C and by 18% after being annealed at 550 °C (which is approaching *T*C). This result indicates the composition with a good thermally stable piezoelectricity. The thermal stability of the piezoelectricity of the oxide-modified SBT ceramics is displayed in Figure 6. As can be seen from the insert, before annealing, the piezoelectric properties of the SBT ceramic (*d*<sup>33</sup> = 10 pC/N) can be improved notably by adding only one of CeO<sup>2</sup> and Gd2O3, a higher *d*<sup>33</sup> ~ 22 pC/N was achieved in SCBT and SGBT. When considering that the addition of CeO<sup>2</sup> and Gd2O<sup>3</sup> could reduce the concentration of oxygen vacancies as mentioned above, thus the less pinning of domain walls and the elevated resistivity tend to promote the sufficient orientation of ferroelectric domains along the applied electric field during polarization. It is noteworthy that the addition of Cr2O<sup>3</sup> can further enhance the piezoelectric properties of the SBT ceramic that *d*<sup>33</sup> up to 28 pC/N was observed for SGBT-Cr and 26 pC/N for SCBT-Cr. As shown in Table 1, a larger orthorhombic distortion is obtained for SGBT-Cr and SCBT-Cr, in which a larger spontaneous polarization is believed to form [33]. Further, the thermal stability of piezoelectricity of the oxide-modified SBT ceramics was investigated by the annealing experiment. In general, the *d*<sup>33</sup> values of all compositions slowly decrease with increasing the annealing temperature from room temperature to 400 ◦C, and then drastically drop after 400 ◦C, until they reach zero when the annealing temperature exceeded their *T*C. The thermal degradation of piezoelectricity can be attributed to the decoupling of space charges at moderate temperatures and the depolarization of intrinsic dipoles at high temperatures [41]. It should be noted that the *d*<sup>33</sup> values of SGBT-Cr were decreased by only 3.6% after being annealed at 500 ◦C and by 18% after being annealed at 550 ◦C (which is approaching *T*C). This result indicates the composition with a good thermally stable piezoelectricity.

In final, dielectric and piezoelectric properties of the oxide-modified SBT ceramics were summarized in Table 4. The high piezoelectric constant, low dielectric loss, and high

cation on the SBT ceramic applied by the oxides. As compared to the modified SBT ceramics reported by other works [20,23,25], the optimized composition SGBT-Cr also possesses

**Figure 6.** Thermal stability of piezoelectricity of the oxide-modified SBT ceramics (the insert shows the *d***33** values of various compositions at room temperature). **Figure 6.** Thermal stability of piezoelectricity of the oxide-modified SBT ceramics (the insert shows the *d***<sup>33</sup>** values of various compositions at room temperature).

the competitive electrical properties with a combination of high *T*C~555 °C and a high

**Table 4.** Comparison of electrical properties between the oxide-modified SBT ceramics and other compositions reported. **Compositions** *ε***r (1 kHz)** *tan δ* **(1 kHz)** *T***c (°C)** *d***33 (pC/N)**  SBT 152 0.9 537 10 SGBT 145 0.4 557 22 SGBT-Ce 168 0.4 552 14 SGBT-Mn 156 0.3 551 22 In final, dielectric and piezoelectric properties of the oxide-modified SBT ceramics were summarized in Table 4. The high piezoelectric constant, low dielectric loss, and high Curie temperature presented by some compositions demonstrated the successful modification on the SBT ceramic applied by the oxides. As compared to the modified SBT ceramics reported by other works [20,23,25], the optimized composition SGBT-Cr also possesses the competitive electrical properties with a combination of high *T*<sup>C</sup> ~ 555 ◦C and a high *d*<sup>33</sup> ~ 28 pC/N.

SGBT-Cr 156 0.3 555 28 SCBT 175 0.1 531 22 **Table 4.** Comparison of electrical properties between the oxide-modified SBT ceramics and other compositions reported.


### SBT, which can be related to the larger orthorhombic distortion caused by the substitution **4. Conclusions**

*d*33~28 pC/N.

of Gd3+ with a smaller ionic radius for Sr2+ at A-site. In addition, the addition of CeO2 reduced the *T*C of SBT, based on the fact that the stability of oxygen octahedron tends to be weakened by Ce4+ with higher coordination number substituting for Sr2+ at A-site. The synergetic doping of CeO2 and Cr2O3 could significantly improve the temperature stability The effects of oxide additives (Gd2O3, CeO2, MnO<sup>2</sup> and Cr2O3) on the phase structures and electrical properties of the SBT ceramics were investigated in this work, some main results were obtained as follows: XRD patterns demonstrated that all the oxide-modified SBT ceramics were a single SrBi4Ti4O<sup>15</sup> phase. The SBT ceramics with the addition of MnO<sup>2</sup> presented a high relative density up to 97%. The addition of Gd2O<sup>3</sup> increased the *T*<sup>C</sup> of SBT, which can be related to the larger orthorhombic distortion caused by the substitution of Gd3+ with a smaller ionic radius for Sr2+ at A-site. In addition, the addition of CeO<sup>2</sup>

presented a high relative density up to 97%. The addition of Gd2O3 increased the *T*C of

reduced the *T*<sup>C</sup> of SBT, based on the fact that the stability of oxygen octahedron tends to be weakened by Ce4+ with higher coordination number substituting for Sr2+ at A-site. The synergetic doping of CeO<sup>2</sup> and Cr2O<sup>3</sup> could significantly improve the temperature stability of the dielectric properties of SBT. Cr2O<sup>3</sup> can significantly enhance the *k*<sup>p</sup> of SBT, at the same time, the addition of these oxides also reduced the *fLL* of SBT at high temperatures. The addition of oxides could improve the piezoelectric property of SBT (*d*<sup>33</sup> = 10 pC/N); in particular, SCBT-Cr and SGBT-Cr obtained a higher *d*<sup>33</sup> of 26 pC/N and 28 pC/N, respectively. Among these compositions, SGBT-Cr (Sr0.92Gd0.053Bi4Ti4O<sup>15</sup> + 0.2 wt% Cr2O3) presented the best electrical properties, such as: *T*<sup>C</sup> = 555 ◦C, *tan δ =* 0.4%, *k*<sup>p</sup> = 6.35%, *d*<sup>33</sup> = 28 pC/N, as well as a good thermally stable piezoelectricity that the values of *d*<sup>33</sup> was decreased by only 3.6% after being annealed at 500 ◦C for 4 h and retained 82% after being annealed at the temperature approaching *T*C.

**Author Contributions:** S.W. conceived and designed the experiments; H.Z. performed the experiments; D.W. analyzed the data; S.W. wrote the paper; L.L. and Y.C. revised the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Opening foundation from the Key Laboratory of Deep Earth Science and Engineering (Sichuan University), Ministry of Education (Grant number: 202007).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this paper can be provided at the request of the corresponding authors.

**Conflicts of Interest:** The authors declare no conflict of interest.
