Enhanced Electrical Properties of Lead-Free Piezoelectric KNLN-BZ-BNT Ceramics With the Modification of Sm3+ Ions

Environment-friendly lead-free piezoelectric ceramics with great properties and high thermal stability are desired in the industry. In this work, the Sm3+-modified lead-free 0.915(K0.45Na0.5Li0.05)NbO3–0.075BaZrO3–0.01(Bi0.5Na0.5)TiO3 (KNLN-BZ-BNT) ceramics are prepared. The piezoelectric properties are improved with the introduction of Sm3+, and the optimal properties (d 33 = 325 pC/N and d 33* = 384 pm/V) are achieved in the ceramic modified with 0.3 mol% Sm3+ ions. Meanwhile, this sample shows good thermal stability such that the values of d 33* decreased less than 20% when the temperature raised from 30 to 180oC. These results show the Sm3+-modified KNLN-BZ-BNT ceramics are good for further applications even under high temperature.


INTRODUCTION
In recent years, environmental problems are a major concern in the whole world Zeng et al., 2020;Zheng et al., 2021). Lead is widely used in various industrial products such as glasses, gasoline, and batteries (Uchino, 1996;Shung, 2015;Wu et al., 2015). Ceramics based on lead zirconate titanate [Pb(Zr, Ti)O 3 (PZT)], the most widely used piezoelectric material, also contain PbO more than 60% (Saito et al., 2004;Jiang et al., 2016). The Restriction of Hazardous Substances Directive (RoHS) has been implemented to minimize the use of toxic materials in the end products (Saito et al., 2004;Zhang et al., 2007). Consequently, researches on lead-free piezoelectric materials are highly needed now (Saito et al., 2004;Zhao et al., 2017;Zhao et al., 2019). Among several lead-free piezoelectric materials, such as BaTiO 3 -, (Bi, Na)TiO 3 -, and BiFeO 3 -based piezoelectric materials, the (K, Na)NbO 3 (KNN)-based piezoelectric materials, with high Curie temperature and high piezoelectric properties, are considered a potential candidate for PZT-based materials in the industry Zhang et al., 2007).
The morphotropic phase boundary (MPB) between the rhombohedral phase and the tetragonal phase contributes to the excellent performance of PZT-based materials Zhang et al., 2007). Furthermore, as to the vertical MPB, the PZT and PZT-based ceramics show excellent thermal stability. For the lead-free piezoelectric KNN ceramics, a phase boundary between the orthorhombic phase and the tetragonal phase called the polymorphic phase transition appeared at about 200 o C (Egerton and Dillon, 1959;Karaki et al., 2013;Wang et al., 2013). By decreasing the temperature to near room temperature, the electrical properties, especially the piezoelectric property, were dramatically enhanced. Unfortunately, the high piezoelectric coefficients only remain stable in a narrow temperature range (Karaki et al., 2013).
To solve this problem, Karaki et al. constructed an MPB of rhombohedral and tetragonal phases by the introduction of BaZrO 3 (Karaki et al., 2013). The piezoelectric properties were expectedly improved. Meanwhile, by introducing Bi 0.5 Na 0.5 TiO 3 in an appropriate amount, a vertical MPB has appeared in 0.915(K 0.45 Na 0.5 Li 0.05 )NbO 3 -0.075BaZrO 3 -0.01(Bi 0.5 Na 0.5 )TiO 3 (KNLN-BZ-BNT) ceramics, and excellent thermal stability was manifested. In this work, to further improve the properties of KNLN-BZ-BNT ceramics, the doped ceramics with 0.1, 0.3, and 0.5 mol% Sm 3+ ions are prepared. The crystalline phase, micromorphologies, electrical properties, and thermal stability are investigated. For comparison, the KNLN-BZ-BNT ceramics in our previous work are discussed together (Quan et al., 2018;Quan et al., 2019;Quan et al., 2020).

EXPERIMENTAL DETAILS
0.915(K 0.45 Na 0.5 Li 0.05 )NbO 3 -0.075BaZrO 3 -0.01(Bi 0.5-x Sm x Na 0.5 ) TiO 3 , where x 0.1, 0.3, 0.5, ceramics were synthesized by a solid oxide reaction process. Reagent-grade oxide/carbonate powders, K 2 CO 3 (99%), Na 2 CO 3 (99.8%), Li 2 CO 3 (98%), Nb 2 O 5 (99.5%), BaCO 3 (99%), ZrO 2 (99%), Bi 2 O 3 (99%), TiO 2 (98%), and Sm 2 O 3 (99%), were selected as starting raw materials. The powders were weighed according to stoichiometry and mixed through ball milling, with partially stabilized ZrO 2 balls as media, in alcohol for 15 h at 300 rpm. After drying at 80 o C, the powder mixtures were calcined at 800 o C for 2 h. The calcined powders were re-milled for 15 h and then pressed into disks of 8 mm diameter and 1 mm thickness at 200 MPa. The green disks were heated at 600 o C for 2 h to remove the organics and then sintered at 1,200 o C for 4 h in a sealed alumina curable. To minimize the volatilization of volatile elements, the green compacts were embedded in the calcined powders during sintering. The final pellets were polished and coated with silver paste on both sides, to characterize the electrical properties.
The crystalline phase structure was evaluated using an X-ray diffractometer (D/MAX-2400, Rigaku, Cu Kα radiation, Japan). The temperature dependence of the dielectric constant and dielectric loss was measured using an LCR meter (4980A, Agilent Technologies, Inc.). A ferroelectric testing system (TF Analyzer 2000E, aixACCT) was used to characterize the piezoelectric strain and the P-E and S-E hysteresis loops. The piezoelectric coefficients were measured by a piezoelectric testing system (ZJ-1, CAS), after poling in a silicon oil bath at 30 kV/cm for 10 min.

RESULTS AND DISCUSSION
The X-ray diffraction (XRD) patterns of un-doped and doped KNLN-BZ-BNT ceramics with different amounts of Sm 3+ ions are shown in Figure 1A. It can be seen that all the samples show a pure perovskite structure. The introduction of Sm 3+ ions (less than 0.5 mol%) hardly changes the crystalline phase of ceramics. The details of (200) and (002) peaks for all samples are shown in Figure 1B, showing an invisible difference, attributed to fewer Sm 3+ ions. Besides, the splits of (002) and (200) become a little bit wider with the addition of Sm 3+ , suggesting a phase close to the tetragonal one after Sm 3+ addition. Figure 2A shows the room-temperature polarization-electrical field (P-E) hysteresis loops of the un-doped and doped samples with different amounts of Sm 3+ ions. All the samples show a well-saturated P-E loop. The un-doped KNLN-BZ-BNT ceramic shows the lowest remanent polarization, P r , of 9.70 μC/cm 2 . The 0.1 mol% Sm 3+ -doped ceramic shows the highest P r of 12.3 μC/cm 2 . The decreased P r values for 0.3 and 0.5 mol% Sm 3+ -doped KNLN-BZ-BNT ceramics could be attributed to the more tetragonal phase. The variation of maximum polarization (P max ) with the amount of Sm 3+ ions shows the same trend as the P r . The lowest and highest P max appeared in the un-doped and 0.1 mol % Sm 3+ ion-doped KNLN-BZ-BNT ceramics, respectively. The bipolar electric field-strain (S-E) loops are shown in Figure 2B. Those S-E loops show typical butterfly shapes with high strain, suggesting a typical ferroelectric property. It can be seen that the KNLN-BZ-BNT ceramics show improved piezoelectric strains after doping with Sm 3+ ions. The highest strain appeared in the 0.3 mol% and 0.5 mol% Sm 3+ -doped ceramics, which is around 0.12%.
The temperature dependence of dielectric constants and dielectric losses at the frequency of 1 kHz for all samples is shown in Figure 3. At room temperature, the dielectric constant of the un-doped KNLN-BZ-BNT ceramic is 1,441. Doping with the Sm 3+ ions, all the ceramics show a high dielectric constant of 1800 and a low dielectric loss of ∼3%, suitable for further applications. It can be noticed that introducing Sm 3+ ions did not affect the Curie temperatures (T C ) of KNLN-BZ-BNT ceramics, and all the samples show a T C of 240 o C. On further inspection, all the samples did not show dielectric anomaly before the temperature was up to T C , indicating a good ferroelectricstable characteristic. The temperature of 240 o C is enough for some high-temperature applications, such as actuators in car engines (Turner et al., 1994).
To measure the d 33 * of the samples, the unipolar strains are measured and shown in Figure 4A. The d 33 * was calculated by Li et al., 2018) where S is the strain under E (electric field). The un-doped KNLN-BZ-BNT ceramic shows the lowest strain of 0.087%. The Sm 3+ -doped ceramics show the enhanced unipolar strains. The strain of the 0.1% Sm 3+ -doped ceramic is 0.105%. A similar strain of 0.114% was obtained in the doped ceramics with 0.3 mol% and 0.5 mol% Sm 3+ . Figure 4B  The doping of Sm 3+ improved the piezoelectric and ferroelectric properties of KNLN-BZ-BNT ceramics. And the optimal performances were achieved in the sample with 0.3 mol% Sm 3+ addition. To investigate the thermal stability of the Sm 3+ -doped KNLN-BZ-BNT ceramics, the temperature dependence of unipolar strain and P-E loops is shown in Figures 5A,B, respectively. The strain under 30 kV/cm at room temperature is 0.114%; then, it decreased slightly with the increasing temperature. When the temperature went up to 180 o C, the unipolar strain remained 0.094%, which shows good thermal stability. The samples show good ferroelectric FIGURE 1 | XRD patterns of un-doped (Quan et al., 2018) and Sm 3+ -modified KNLN-BZ-BNT ceramics. (A) 2θ is between 10 and 60°, and (B) 2θ is between 44.5 and 46°.
Frontiers in Materials | www.frontiersin.org May 2021 | Volume 8 | Article 695330 properties even at the temperature of 180 o C. The P r decreased from 11.19 μC/cm 2 at room temperature (30 o C) to 7.91 μC/cm 2 at 180 o C, and the P max decreased from 19.29 μC/cm 2 to 16.15 μC/cm 2 . To manifest the temperature dependence of ferroelectric and piezoelectric properties of KNLN-BZ-BNT with 0.3 mol% Sm 3+ ions, the variation of P r and normalized d 33 * with temperature is plotted in Figure 5C. It can be found that the normalized d 33 * of the sample decreased less than 20% when the temperature raised from 30 to 180 o C, which is better than that in the PZT-5H ceramics (Fang et al., 2019).

CONCLUSION
The un-doped and doped KNLN-BZ-BNT ceramics with 0.1, 0.3, and 0.5 mol% Sm 3+ ions were prepared. The remanent polarization, P r , and piezoelectric coefficients, d 33 and d 33 *, were improved with the introduction of Sm 3+ ions. The best performances appeared in the sample with 0.3 mol% Sm 3+ ions, showing a d 33 of 325 pC/N, a d 33 * of 384 pm/V, a P r of 11.19 μC/ cm 2 , and a high strain of 0.114% at 30 kV/cm. Furthermore, the 0.3 mol% Sm 3+ -doped KNLN-BZ-BNT ceramic shows good thermal stability. The d 33 * values decreased less than 20% when the temperature raised from 30 to 180 o C. These excellent results show the Sm 3+ -modified KNLN-BZ-BNT ceramics are good for further applications even under high temperature.