Recent Developments in (K, Na)NbO3-Based Lead-Free Piezoceramics

(K0.5Na0.5)NbO3 (KNN)-based ceramics have been extensively investigated as replacements for Pb(Zr, Ti)O3-based ceramics. KNN-based ceramics exhibit an orthorhombic structure at room temperature and a rhombohedral–orthorhombic (R–O) phase transition temperature (TR–O), orthorhombic–tetragonal (O–T) phase transition temperature (TO–T), and Curie temperature of −110, 190, and 420 °C, respectively. Forming KNN-based ceramics with a multistructure that can assist in domain rotation is one technique for enhancing their piezoelectric properties. This review investigates and introduces KNN-based ceramics with various multistructures. A reactive-templated grain growth method that aligns the grains of piezoceramics in a specific orientation is another approach for improving the piezoelectric properties of KNN-modified ceramics. The piezoelectric properties of the [001]-textured KNN-based ceramics are improved because their microstructures are similar to those of the [001]-oriented single crystals. The improvement in the piezoelectric properties after [001] texturing is largely influenced by the crystal structure of the textured ceramics. In this review, [001]-textured KNN-based ceramics with different crystal structures are investigated and systematically summarized.

Recently, KNN-based piezoceramics containing three structures, such as O-T-PC and R-O-T multistructures, have been reported, and their piezoelectric characteristics are significantly better than those of KNN-modified piezoceramics containing two phases [77,78,82,118,[120][121][122].However, their composition and piezoelectric characteristics are yet to be reviewed.Furthermore, [001]-textured KNN-modified piezoceramics that use NN templates exhibit an excellent d 33 value of 805 pC/N that is larger than or similar to that of PZT-based piezoceramics [118].However, [001]-textured KNN-modified ceramics are yet to be investigated.
Therefore, this review summarizes various KNN-modified piezoceramics wherein three structures coexist and discusses their piezoelectric properties.Furthermore, [001]textured KNN-modified piezoceramics are introduced, and the link between the crystal structure and enhancement of piezoelectric characteristics after [001]-texturing is discussed.CuO-added 0.96(Na 0.5 K 0.5 )(Nb 1−x Sb x )O 3 -0.04CaTiO 3 [NK(N 1−x S x )-CT] piezoceramics with 0.03 ≤ x ≤ 0.1 were well-sintered at 970 • C in the air.For a detailed analysis of the crystal structures of these ceramics, X-ray diffraction (XRD) peaks at ~67 • are obtained by slow-speed scanning and deconvoluted using the Voigt function, as shown in Figure 1a-f.Piezoceramics with x = 0.03 exhibit an O-T multistructure (Figure 1a), which changes to an O-T-C structure for the piezoceramic with x = 0.04.An identical structure is observed for the specimen with x = 0.05 (Figure 1b,c).The O structure is removed when x exceeds 0.05, and the piezoceramics with 0.07 ≤ x ≤ 0.1 have a T-C multistructure (Figure 1d-f).Therefore, the crystal structure of the NK(N 1−x S x )-CT piezoceramic transformed from an O-T multistructure to the O-T-C and T-C multistructures with an increase in the Sb 5+ content.The PC structure developed in the NK(N 1−x S x )-CT piezoceramics with x > 0.03 was understood as a T-C multistructure consisting of the P4mm T and Pm3m C structures based on the Rietveld analysis [78].The diverse electrical characteristics of the NK(N 1−x S x )-CT piezoceramics sintered at 970 • C were investigated, as shown in      The 0.96(Li x Na 0.5−x K 0.5 )(Nb 0.945 Sb 0.055 )-0.04SrZrO 3 [(L x N 0.5−x K)NS-SZ] piezoceramics (0.0 ≤ x ≤ 0.05) with 1.0 mol% CuO were densified at 1020 • C and showed a homogeneous perovskite phase with a dense microstructure.Rietveld analysis was carried out on the XRD patterns of the (L x N 0.5−x K)NS-SZ piezoceramics (0.0 ≤ x ≤ 0.05) to determine their crystal structure, as displayed in Figure 3a-c.The ceramic (x = 0.0) exhibited an O-R multistructure consisting of Amm2 O and R3m R structures (Figure 3a).The PC structure observed in the (L x N 0.5−x K)NS-SZ piezoceramics was identified as the R3m R structure [77].The P4mm T structure appeared in the ceramic with x = 0.03, which indicates that this ceramic has an O-T-R multistructure (Figure 3b).When x exceeded 0.03, the R (or PC) structure disappeared, and the ceramic (x = 0.05) exhibited an O-T multistructure containing P4mm T and Amm2 O structures (Figure 3c).The crystal structure and piezoelectric properties of the (K 0.5 Na 0.5-z Li z )(Nb 0.92 Sb 0.08 )-(Ca 0.5 Sr 0.5 )ZrO 3 [(KN 0.5-z L z )NS-CSZ] ceramics with 0.0 ≤ z ≤ 0.05 densified at 1060 • C have been investigated [82].Figure 5a-c shows the Rietveld refinement analysis results of the XRD profiles of the (KN 0.5-z L z )NS-CSZ piezoceramics with 0.0 ≤ z ≤ 0.05.The ceramic (z = 0.0) exhibits an O-R multistructure composed of an Amm2 O structure (30.8%) and an R3m R structure (69.2%), as provided in Figure 5a.The P4mm T structure appears in the ceramic (z = 0.03), and therefore, this piezoceramic has an O-T-R multistructure (Figure 5b).The R structure disappears in the ceramic with z = 0.05, which indicates that this piezoceramic has an O-T multistructure (Figure 5c).The PC structure formed in the (KN 0.5-z L z )NS-CSZ piezoceramics is understood as the R3m R structure [82].Figure 6a provides the diverse electrical properties of the (KN 0.5-z L z )NS-CSZ ceramics (0.0 ≤ z ≤ 0.06).All ceramics exhibited a large relative density (≥95% of the theoretical density), suggesting that they were well-densified.The piezoceramic (z = 0.03) showed large ε T 33 /ε 0 , d 33 , and k p values of 3800, 560 pC/N, and 0.49, respectively, indicating that the (KN 0.47 L 0.03 )NS-CSZ ceramic with an O-T-R multistructure had excellent piezoelectric properties.Moreover, it provided a high electric field-induced strain of 0.16% at 4 kV/mm (Figure 6b), and therefore, this ceramic is a suitable Pb-free piezoceramic for use in piezoelectric actuators.Multilayer ceramics composed of five layers of the (KN 0.47 L 0.03 )NS-CSZ thick film are fabricated (Figure 7a) and utilized to produce a cantilever-style piezoelectric actuator (Figure 7b) [82].The accelerations are obtained at different frequencies, and the largest acceleration of 42.96 G is detected at 57 Hz (resonance frequency) and ±250 V/mm (Figure 7c).The displacement of the actuator is measured at various electric fields at 57 Hz, as shown in Figure 7d.The displacement increased with an increase in the applied electric field, and a large displacement of 3500 µm was obtained at ±250 V/mm.This displacement is larger than that of the piezoelectric actuators fabricated using PZT-and KNN-modified piezoceramics [82].Hence, the (KN 0.47 L 0.03 )NS-CSZ piezoceramic can be used in various piezoelectric actuators.Adding the ABO 3 -type substance to the KNNS ceramic increased T R-O and decreased T O-T simultaneously; therefore, an R-O-T multistructure could exist [120].The O structure had 12 spontaneous polarization (P S ) directions along the {110} direction, whereas the R structures had eight possible P S directions along the <111> direction.The T structure had six possible Ps directions along the <100> directions [118], and therefore, the formation of the R-O-T multistructure considerably reduced the energy barrier for polarization rotation in KNN-modified piezoceramics, which resulted in high piezoelectric properties.The (Bi 0.5 Na 0.5 )HfO 3 (BNH) perovskite phase was added to the (K 0.48 Na 0.52 )(Nb 0.96 Sb 0.04 )O 3 (KNNS) ceramic to form an R-O-T multistructure [122].The (1−x)KNNS-xBNH piezoceramics were formed using a conventional solid-state reaction technique.A two-step sintering process was used to densify the (1−x)KNNS-xBNH piezoceramics, wherein the temperature was raised to 1160-1180 • C, maintained for 1 min, and then cooled down to 1040-1065 • C and kept for 15 h before cooling in the air [122].

KNN-Modified Ceramics with the O-T-PC Multistructure
The (200) reflections at approximately 45.5 • were obtained from the unpoled piezoceramic (x = 0.045) and theoretically fitted with five Lorentz curves, as shown in Figure 8a.Tetragonal (020) T and (200) T peaks, orthorhombic (200) O and (020) O peaks, and rhombohedral (200) R peaks were observed, indicating that the specimen with x = 0.045 has the R-O-T multistructure.Similar results were attained for the (200) reflections of the poled piezoceramics (Figure 8b).For the unpoled ceramic, the intensities of the T reflections were higher than those of the O and R reflections; however, for the poled ceramic, the intensities of the orthorhombic reflections were higher than those of the T and R reflections.Identical results were also obtained from the (1−x)KNNS-xBNH ceramics (0.035 ≤ x ≤ 0.047), which indicate that these piezoceramics have R-O-T multistructures.Table 1 lists the various physical properties of the (1−x)KNNS-xBNH piezoceramics with 0.035 ≤ x ≤ 0.047.The d 33 of the ceramic with x = 0.035 was low, at 350 pC/N, and it enhanced with the increase in x to the largest value of 540 pC/N for the piezoceramic with x = 0.045, and then, it decreased to 475 pC/N for the ceramic (x = 0.047).The ceramic with x = 0.045 had a relatively large k p value of 0.56, which indicates that the 0.955 KNNS-0.045BNH ceramic with the R-O-T multistructure showed promising piezoelectric properties (d 33 = 540 pC/N and k p = 0.56).

(1−x)(K, Na)(Nb, Sb)O3−x(Bi, Na)HfO3 Piezoceramics with the R-O-T Multistructure
Adding the ABO3-type substance to the KNNS ceramic increased TR-O and decreased TO-T simultaneously; therefore, an R-O-T multistructure could exist [120].The O structure had 12 spontaneous polarization (PS) directions along the {110} direction, whereas the R structures had eight possible PS directions along the <111> direction.The T structure had six possible Ps directions along the <100> directions [118], and therefore, the formation of the R-O-T multistructure considerably reduced the energy barrier for polarization rotation in KNN-modified piezoceramics, which resulted in high piezoelectric properties.The (Bi0.5Na0.5)HfO3(BNH) perovskite phase was added to the (K0.48Na0.52)(Nb0.96Sb0.04)O3 (KNNS) ceramic to form an R-O-T multistructure [122].The (1−x)KNNS-xBNH piezoceramics were formed using a conventional solid-state reaction technique.A two-step sintering process was used to densify the (1−x)KNNS-xBNH piezoceramics, wherein the temperature was raised to 1160-1180 °C, maintained for 1 min, and then cooled down to 1040-1065 °C and kept for 15 h before cooling in the air [122].
The (200) reflections at approximately 45.5° were obtained from the unpoled piezoceramic (x = 0.045) and theoretically fitted with five Lorentz curves, as shown in Figure 8a.Tetragonal (020)T and (200)T peaks, orthorhombic (200)O and (020)O peaks, and rhombohedral (200)R peaks were observed, indicating that the specimen with x = 0.045 has the R-O-T multistructure.Similar results were attained for the (200) reflections of the poled piezoceramics (Figure 8b).For the unpoled ceramic, the intensities of the T reflections were higher than those of the O and R reflections; however, for the poled ceramic, the intensities of the orthorhombic reflections were higher than those of the T and R reflections.Identical results were also obtained from the (1−x)KNNS-xBNH ceramics (0.035 ≤ x ≤ 0.047), which indicate that these piezoceramics have R-O-T multistructures.Table 1 lists the various physical properties of the (1−x)KNNS-xBNH piezoceramics with 0.035 ≤ x ≤ 0.047.The d33 of the ceramic with x = 0.035 was low, at 350 pC/N, and it enhanced with the increase in x to the largest value of 540 pC/N for the piezoceramic with x = 0.045, and then, it decreased to 475 pC/N for the ceramic (x = 0.047).The ceramic with x = 0.045 had a relatively large kp value of 0.56, which indicates that the 0.955 KNNS-0.045BNH ceramic with the R-O-T multistructure showed promising piezoelectric properties (d33 = 540 pC/N and kp = 0.56).
The existence of the R-O-T multistructure reduces the energy barrier for polarization rotation, thereby enhancing the piezoelectric properties.Furthermore, the presence of polar nanoregions (PNRs) in the KNN-modified ceramics improved their piezoelectric properties because the domain boundary energy decreased with a decrease in domain size, which led to easy domain rotation.Nanodomains are observed in relaxor ceramics [107].Figure 9a-c shows the schematics of PNRs developed in various relaxor ceramics [107].In traditional relaxor ceramics such as Pb(Mg 1/3 Nb 2/3 )O 3 (PMN) and (Pb, La)(Zr, Ti)O 3 , PNRs exist in the nonpolar matrix, as shown in Figure 9a.In relaxor-ferroelectric piezoceramics with a morphotropic phase boundary (MPB) structure that includes PMN-PbTiO 3 and Pb(Zn 1/3 Nb 2/3 )O 3 -PbTiO 3 ceramics, relaxor properties can be explained by PNRs in a long-range ordered matrix, as shown in Figure 9b.These ceramics exhibited excellent piezoelectric properties.Furthermore, PNRs can exist in KNN-modified ceramics with a nanoscale R-O-T multistructure, as provided in Figure 9c.Such a slush polar state is expected to have a very low energy barrier and induce easy polarization, thereby resulting in excellent piezoelectric properties [107].
further, this piezoceramic (x = 1.6%) has a relatively large TC of ~200 °C compared to the other soft piezoceramics [107].Figure 10c shows the STEM ABF image along the [110] direction obtained from the KNNS-BNKZ-Fe-xAs (x = 1.6%) ceramic, and it shows the polarization rotation from R to O to T phases, indicating the existence of PNRs in the nanoscale R-O-T multistructure.Further, the large d33 value of this piezoceramic (x = 1.6%) can be explained by the coexistence of the R-O-T nanophases and PNRs (Figure 10c).11a).The T structure appeared for the ceramic (x = 0.01), which indicates that this ceramic had an R-O-T multistructure (Figure 11b).An identical structure is also observed in ceramics with (0.02 ≤ x ≤ 0.03), as shown in Figure 11c,d.Finally, a ceramic (x = 0.04) has an R-T multistructure (Figure 11e), and therefore, ceramics (0.01 ≤ x ≤ 0.03) have an R-O-T multistructure.Figure 11f provides the Rietveld analysis results of the XRD pattern of the KNNS-0.01CZ-0.03BAZ(x = 0.03) ceramic.This ceramic had an R-O-T multistructure consisting of 28% R3m R, 34% Amm2 O, and 38% P4mm T structures, suggesting that the amounts of the R, O, and T structures were similar.The piezoceramics (x = 0.01 and 0.02) have an R-O-T multistructure; however, the amount of the Amm2 O structure is considerably larger than that of the other structures [121].Therefore, the ceramic (x = 0.03) has large piezoelectric properties because its structure is close to the ideal R-O-T multistructure, wherein the proportions of the R, O, and T structures are similar.11a).The T structure appeared for the ceramic (x = 0.01), which indicates that this ceramic had an R-O-T multistructure (Figure 11b).An identical structure is also observed in ceramics with (0.02 ≤ x ≤ 0.03), as shown in Figure 11c,d.Finally, a ceramic (x = 0.04) has an R-T multistructure (Figure 11e), and therefore, ceramics (0.01 ≤ x ≤ 0.03) have an R-O-T multistructure.Figure 11f provides the Rietveld analysis results of the XRD pattern of the KNNS-0.01CZ-0.03BAZ(x = 0.03) ceramic.This ceramic had an R-O-T multistructure consisting of 28% R3m R, 34% Amm2 O, and 38% P4mm T structures, suggesting that the amounts of the R, O, and T structures were similar.The piezoceramics (x = 0.01 and 0.02) have an R-O-T multistructure; however, the amount of the Amm2 O structure is considerably larger than that of the other structures [121].Therefore, the ceramic (x = 0.03) has large piezoelectric properties because its structure is close to the ideal R-O-T multistructure, wherein the proportions of the R, O, and T structures are similar.
Figure 12a shows the change of the ε T 33 /ε 0 values with respect to the temperature of the ceramic (x = 0.0) measured at various frequencies.The ε T 33 /ε 0 value decreases with increasing frequency; however, the variation of the T O-T can be negligible, which indicates that this ceramic is a normal ferroelectric material.However, T O-T increases with an increasing frequency for the ceramic with x = 0.03 (Figure 12b), and therefore, this ceramic exhibits relaxor properties, indicating that this ceramic is expected to contain nanodomains.Figure 12c provides a TEM bright-field image of the ceramic (x = 0.03); nanodomains with an average size of 3 × 20 nm were present in this ceramic.The ceramic with x = 0.03 had an ideal R-O-T multistructure and nanodomains with relaxor properties near the T O-T corresponding to the ferroelectric-to-ferroelectric transition temperature.Therefore, this ceramic (x = 0.03) was expected to exhibit excellent piezoelectric properties.Figure 13a displays the diverse physical characteristics of the KNNS-(0.04−x)CZ-xBAZpiezoceramics (0.0 ≤ x ≤ 0.04) densified at 1090 • C. The relative density of the ceramic (x = 0.0) is comparatively high (~92.5% of the theoretical density), which does not change significantly with increasing x.The tan δ values of all specimens are relatively low, ranging between ~3% and 4%.A comparatively large ε T 33 /ε 0 of 3752 was detected in the ceramic (x = 0.03).Further, the piezoceramic (x = 0.03) has a very large d 33 of 680 ± 10 pC/N because of the existence of an ideal R-O-T multistructure, wherein the three structures had similar proportions.This d 33 is higher than the maximum d 33 value reported in the literature for KNN-modified piezoelectric ceramics (Figure 13b).The k p s of the piezoceramics shows a trend similar to that observed for the d 33 s of the specimens, and the piezoceramic (x = 0.03) has a large k p of 0.5.Further, the piezoelectric and structural characteristics of the KNNS-(0.04−x)MZ-xBAZ(M = Sr and Ba) ceramics were investigated [118,120].The KNNS-0.01SZ-0.03BAZceramic exhibited a large d 33 of 650 pC/N and a k p of 0.51, as shown in Figure 14a.This ceramic had an ideal R-O-T multistructure, in which the three structures had similar proportions (Figure 14b).Moreover, this ceramic had nanodomains, as shown in Figure 14c.Therefore, the large d 33 value of the KNNS-0.01SZ-0.03BAZceramic can be explained by the existence of an ideal R-O-T multistructure and nanodomains.The KNNS-0.01BZ-0.03BAZceramic show a similar piezoelectricity: d 33 = 640 pC/N, k p = 0.49, and strain = 0.16%, as shown in Figure 14d.The existence of an ideal R-O-T multistructure with nanodomains is responsible for the high piezoelectricity [120].Figure 12a shows the change of the ε T 33/ε0 values with respect to the temperature of the ceramic (x = 0.0) measured at various frequencies.The ε T 33/ε0 value decreases with increasing frequency; however, the variation of the TO-T can be negligible, which indicates that this ceramic is a normal ferroelectric material.However, TO-T increases with an increasing frequency for the ceramic with x = 0.03 (Figure 12b), and therefore, this ceramic exhibits relaxor properties, indicating that this ceramic is expected to contain nanodomains.Figure 12c provides a TEM bright-field image of the ceramic (x = 0.03); nanodomains with an average size of 3 × 20 nm were present in this ceramic.The ceramic with x = 0.03 had an ideal R-O-T multistructure and nanodomains with relaxor properties near the TO-T corresponding to the ferroelectric-to-ferroelectric transition temperature.explained by the existence of an ideal R-O-T multistructure and nanodomains.The KNNS-0.01BZ-0.03BAZceramic show a similar piezoelectricity: d33 = 640 pC/N, kp = 0.49, and strain = 0.16%, as shown in Figure 14d.The existence of an ideal R-O-T multistructure with nanodomains is responsible for the high piezoelectricity [120].

[001]-Textured (K, Na)(Nb, Sb)O3-MZrO3 (M = Sr and Ca) Piezoceramics with an O-PC (or O-R) Multistructure
The [001]-textured 0.96(K0.5Na0.5)(Nb1−ySby)O3-0.04SrZrO3[KN(Nb1−ySy)-SZ] + xNN piezoceramics (0.0 ≤ x ≤ 0.05 and 0.045 ≤ y ≤ 0.08) were produced via a tape-casting method and sintered at 1060 °C for 6 h. Figure 16a shows a scanning electron microscopy image of the NN seeds utilized to texture the KN(Nb1−ySy)-SZ ceramics.The dimensions of the NN seeds are ~12 × 12 × 1.0 μm 3 with a high aspect ratio of 12.The XRD patterns of the KN(Nb0.945S0.055)-SZ+ xNN piezoceramics are provided in Figure 16b.A homogeneous perovskite phase was formed in all samples, and the LFs of the textured ceramics were calculated using XRD patterns, as shown in Figure 16c.The ceramic (x = 0.01) showed a low LF of 81%, which increased with an increase in x to 97% for the specimen with x = 0.03, indicating that 3.0% is the optimum amount of NN templates for texturing the specimens.16b.A homogeneous perovskite phase was formed in all samples, and the LFs of the textured ceramics were calculated using XRD patterns, as shown in Figure 16c.The ceramic (x = 0.01) showed a low LF of 81%, which increased with an increase in x to 97% for the specimen with x = 0.03, indicating that 3.0% is the optimum amount of NN templates for texturing the specimens.

(or O-R) Multistructure
The [001]-textured 0.96(K0.5Na0.5)(Nb1−ySby)O3-0.04SrZrO3[KN(Nb1−ySy)-SZ] + xNN piezoceramics (0.0 ≤ x ≤ 0.05 and 0.045 ≤ y ≤ 0.08) were produced via a tape-casting method and sintered at 1060 °C for 6 h. Figure 16a shows a scanning electron microscopy image of the NN seeds utilized to texture the KN(Nb1−ySy)-SZ ceramics.The dimensions of the NN seeds are ~12 × 12 × 1.0 μm 3 with a high aspect ratio of 12.The XRD patterns of the KN(Nb0.945S0.055)-SZ+ xNN piezoceramics are provided in Figure 16b.A homogeneous perovskite phase was formed in all samples, and the LFs of the textured ceramics were calculated using XRD patterns, as shown in Figure 16c.The ceramic (x = 0.01) showed a low LF of 81%, which increased with an increase in x to 97% for the specimen with x = 0.03, indicating that 3.0% is the optimum amount of NN templates for texturing the specimens.17b).The PC structure was determined to be an R3m R structure; therefore, the [001]-textured  after [001]-texturing.Further, the large d33 value (≥580 pC/N) was preserved up to 110 °C, as provided in Figure 17d.Moreover, the [001]-textured KN(Nb0.91S0.09)-CaZrO3ceramic also showed an ideal O-R (or O-PC) multistructure composed of the Amm2 O (59%) and the R3m R (41%) structures [114].This ceramic exhibited a large d33 value of 610 pC/N; therefore, the KN(Nb1−ySy)-MZ ceramics (M = Sr and Ca) with an ideal O-PC (or O-R) multistructure exhibited a large d33 value of 610-620 pC/N.The 0.96(K 0.5 Na 0.5 )(Nb 0.965 Sb 0.035 )O 3 -0.01CaZrO 3 -0.03(Bi0.5 K 0.5 )HfO 3 (96KNNS-1CZ-3BKH) piezoceramic was textured using x mol% NN templates (x = 0, 1, 3, 5) [115].Two-step sintering was used to fabricate the [001]-textured 96KNNS-1CZ-3BKH ceramics.The specimens were heated from RT to 1190 • C at a heating rate of 5 • C/min and rapidly cooled at a cooling rate of 10 • C/min to 1090 • C and held for 10 h [115].Figure 18a provides the XRD patterns of the [001]-textured 96KNNS-1CZ-3BKH ceramics with various amounts of NN templates.All ceramics show a single perovskite phase and the intensity of the [001] reflection is much stronger than those of the other peaks in the textured ceramics and LF was calculated using the XRD patterns.The values of d 33 , k p , and orientation degree F as functions of the amount of the NN seeds are displayed in Figure 18b.The orientation degree F was the same as the LF and the ceramic textured with 3.0 mol% NN template showed the largest orientation degree F of 98%, thereby suggesting an optimized NN template content of 3 mol% [115].The [001]-textured 96KNNS-1CZ-3BKH piezoceramic using 3.0 mol% NN templates showed a high d 33 (704 pC/N), k p (76%), and T C (242 • C), as shown in Figure 18b.Further, these piezoelectric properties are similar to those of soft PZT-5H piezoceramics and considerably larger than those of the lead-free piezoelectric materials reported in the literature (Figure 18c), thereby indicating that this ceramic can replace PZT-based piezoceramics [115].
plate content of 3 mol% [115].The [001]-textured 96KNNS-1CZ-3BKH piezoceramic using 3.0 mol% NN templates showed a high d33 (704 pC/N), kp (76%), and TC (242 °C), as shown in Figure 18b.Further, these piezoelectric properties are similar to those of soft PZT-5H piezoceramics and considerably larger than those of the lead-free piezoelectric materials reported in the literature (Figure 18c), thereby indicating that this ceramic can replace PZT-based piezoceramics [115].Figure 19a shows variations in the reflections at ~17.5° and 36° when an electric field is applied to the [001]-textured 96KNNS-1CZ-3BKH piezoceramic.Reflections at 36° can be fitted using the Lorentz function and analyzed as a mixture of O and R reflections,   [115].Therefore, the [001]-textured 96KNNS-1CZ-3BKH piezoceramic with the O-R multistructure is considered to possess a large d 33 because eight P S s contribute to the polarization along the [001] direction when an electric field is applied to the [001]-textured direction, four P S s along the <110> direction from the O phase, and four P S s along the <111> direction from the R phase.Furthermore, when an electric field was applied, a monoclinic (M) reflection appeared because of splitting the (002) peak, as shown in Figure 19a.This intermediate M phase disappeared and transformed into the O phase at a higher electric field of 20 kV/cm.The intermediate M phase serves as a bridge, thereby facilitating the polarization rotation between the rhombohedral [112] and orthorhombic [103] polar axes in the (10-1) plane, as shown in Figure 19b, which results in increased piezoelectricity in the [001]-textured 96KNNS-1CZ-3BKH piezoceramic.Figure 19c 21c).However, the textured piezoceramic (x = 0.03) had an ideal R-O-T structure, in which the proportion of R, O, and T structures was ~33% (Figure 21d).Finally, an R-T multistructure was developed in the textured piezoceramic with x = 0.04, as shown in Figure 21e.425 pC/N.The textured KNNS-0.02BZ-0.02BAZceramic, which had an R-O-T multistructure with a large fraction of R-O phase ((~80%), as shown in Figure 21c), exhibited a high d33 (805 pC/N) after texturing owing to the large d33 (506 pC/N) of the untextured piezoceramic (x = 0.02).The increase in d33 after the [001]-texturing is insignificant for the ceramic with x ≥ 0.03 (Figure 22b), and therefore, an increase in the d33 is high for the specimens (x ≤ 0.02); however, it is low for the specimens (x ≥ 0.03).

2. 2 .
(Li, Na, K)(Nb, Sb)-SrZrO 3 Ceramics with an O-T-PC (or O-T-R) Multistructure T R-O changed to T O-T-R and T O-T with an increase in the Li content.Therefore, the crystal structure of the (L x N 0.5−x K)NS-SZ ceramics changed from the O-R multistructure to the O-T-R and O-T multistructures.The diverse electrical characteristics of the (L x N 0.5−x K)NS-SZ piezoceramics (0.0 ≤ x ≤ 0.05) are displayed in Figure 4.The piezoceramic (x = 0.03), which shows an O-T-R multistructure, has a comparatively small d 33 (330 pC/N) and k p (0.4) [77].Therefore, the (L x N 0.5−x K)NS-SZ piezoceramics with an O-T-R multistructure showed low piezoelectric properties.ing P4mm T and Amm2 O structures (Figure 3c).TR-O changed to TO-T-R and TO-T with an increase in the Li content.Therefore, the crystal structure of the (LxN0.5−xK)NS-SZceramics changed from the O-R multistructure to the O-T-R and O-T multistructures.The diverse electrical characteristics of the (LxN0.5−xK)NS-SZpiezoceramics (0.0 ≤ x ≤ 0.05) are displayed in Figure 4.The piezoceramic (x = 0.03), which shows an O-T-R multistructure, has a comparatively small d33 (330 pC/N) and kp (0.4) [77].Therefore, the (LxN0.5−xK)NS-SZpiezoceramics with an O-T-R multistructure showed low piezoelectric properties.

Figure 7 .
Figure 7. (a) SEM image of a (KN0.47L0.03)NS-CSZmultilayer.(b) Schematic of the cantilever-style piezoelectric actuator.(c) Change of the acceleration as a function of the frequency measured at 25 V for the (KN0.47L0.03)NS-CSZMC actuator.The inset provides accelerations obtained and simulated at different applied voltages.(d) Change in displacement with respect to the applied voltage detected at 57 Hz for the (KN0.47L0.03)NS-CSZMC actuator.The inset provides the displacements obtained and simulated at diverse applied voltages [82].

Figure 7 .
Figure 7. (a) SEM image of a (KN0.47L0.03)NS-CSZmultilayer.(b) Schematic of the cantilever-style piezoelectric actuator.(c) Change of the acceleration as a function of the frequency measured at 25 V for the (KN0.47L0.03)NS-CSZMC actuator.The inset provides accelerations obtained and simulated at different applied voltages.(d) Change in displacement with respect to the applied voltage detected at 57 Hz for the (KN0.47L0.03)NS-CSZMC actuator.The inset provides the displacements obtained and simulated at diverse applied voltages [82].

Figure 7 .
Figure 7. (a) SEM image of a (KN 0.47 L 0.03 )NS-CSZ multilayer.(b) Schematic of the cantilever-style piezoelectric actuator.(c) Change of the acceleration as a function of the frequency measured at 25 V for the (KN 0.47 L 0.03 )NS-CSZ MC actuator.The inset provides accelerations obtained and simulated at different applied voltages.(d) Change in displacement with respect to the applied voltage detected at 57 Hz for the (KN 0.47 L 0.03 )NS-CSZ MC actuator.The inset provides the displacements obtained and simulated at diverse applied voltages [82].

Figure 9 .
Figure 9. Schematic of PNRs in various relaxor states.(a) Conventional relaxor state with PNRs in a nonpolar matrix.(b) Relaxor−ferroelectric solid solution with PNRs in a long-range ordered matrix.(c) New relaxor state displaying PNRs with multiphase coexistence [107].

Figure 9 .
Figure 9. Schematic of PNRs in various relaxor states.(a) Conventional relaxor state with PNRs in a nonpolar matrix.(b) Relaxor−ferroelectric solid solution with PNRs in a long-range ordered matrix.(c) New relaxor state displaying PNRs with multiphase coexistence [107].

Figure 10 .
Figure 10.Composition dependence of (a) d33 and d33 * and (b) d33 values of the various lead-free and typical PZT piezoceramics with high TC (>150 °C).(c) Atomically resolved contrast-reserved STEM ABF image along the [110] direction showing polarization rotation from R to O to T [107].

Figure 10 .
Figure 10.Composition dependence of (a) d 33 and d 33 * and (b) d 33 values of the various lead-free and typical PZT piezoceramics with high T C (>150 • C).(c) Atomically resolved contrast-reserved STEM ABF image along the [110] direction showing polarization rotation from R to O to T [107].
001]-Textured (K, Na, Li)(Nb, Sb, Ta)O 3 -CaZrO 3 Ceramics with an O-T Multistructure The enhancement in the piezoelectric properties of the [001]-textured KNN-modified ceramics was influenced by their crystal structure.Therefore, the piezoelectric properties of [001]-textured KNN-based ceramics with various structures were investigated [81,111-119].The 0.99(K 0.49 Na 0.49 Li 0.02 )(Nb 0.97−x Sb 0.03 Ta x )O 3 -0.01CaZrO 3 [KNL(N 0.97−x ST x )-CZ] ceramics (0.03 ≤ x ≤ 0.25) were textured along the [001] direction using 3.0% NaNbO 3 (NN) templates using a two-step sintering process [113].The ceramic tapes containing NN templates were produced using the tape-casting technique and heated from RT to 1180 • C with a heating rate of 3 • C/min, and then, they were rapidly cooled at the cooling rate of 10 • C/min to 1065-1080 • C and held for 10 h. Figure 15a shows the XRD patterns of the [001]-textured piezoceramics (0.03 ≤ x ≤ 0.25).The shapes of the (002) and (200) reflections at approximately 45.5 • changed when x increased, thereby implying a transformation of the crystal structure [113].The crystal structure of the specimens was determined using the ratio of the (002) and (200) peak intensities (I (002) /I (200) ).The ratio is 2 for the O structure; however, it decreases to 0.5 for the T structure.The specimen has an O-T multistructure when the I (002) /I (200) ratio is close to 1.0 [113].The I (002) /I (200) value of the [001]-textured piezoceramic (x = 0.25) was approximately 1.45, and therefore, it was considered to have an O-T multistructure [113].Although the Lotgering factor (LF) of the ceramic (x = 0.25) was relatively low, ~82.1% (Figure 15a), it was textured along the [001] direction.The ε T 33 /ε 0 versus temperature curves show that the T O-T is found at ~31 • for the ceramic with x = 0.25 (Figure 15b), which confirms that the [001]-textured KNL(N 0.97−x ST x )-CZ ceramic (x = 0.25) has the O-T multistructure.Figure 15c displays the d 33 and d 33 * values of the untextured and [001]-textured KNL(N 0.97−x ST x )-CZ ceramics (0.03 ≤ x ≤ 0.25); the d 33 * values were calculated at 30 kV/mm.The d 33 and d 33 * values of the [001]-textured piezoceramics were higher than those of the untextured piezoceramics.Further, the [001]-textured ceramic (x = 0.25), which had an O-T multistructure, provided the largest d 33 and d 33 * values of 391 pC/N and 578 pm/V, respectively.The d 33 value of the untextured ceramic (x = 0.25) was ~278 pC/N, indicating that the increase in the d 33 value after [001] texturing was not large because of the presence of a large amount of the T structure [113].Micromachines 2024, 15, x FOR PEER REVIEW 15 of 27 ~278 pC/N, indicating that the increase in the d33 value after [001] texturing was not large because of the presence of a large amount of the T structure [113].

Figure
Figure 17a shows the XRD reflections at 66.5° detected by slow-speed scanning and deconvoluted using the Voigt function for the ceramic (y = 0.065) textured along the [001] direction using 3.0 mol% NN templates.This ceramic has an O-PC multistructure because it provides a pseudocubic (220)P reflection and orthorhombic (004)O, (400)O, and (220)O reflections.Rietveld refinement analysis has been conducted on the XRD pattern of the [001]-textured ceramic (y = 0.065) to clearly identify its PC structure (Figure17b).The PC structure was determined to be an R3m R structure; therefore, the [001]-textured

Figure
Figure 17a shows the XRD reflections at 66.5 • detected by slow-speed scanning and deconvoluted using the Voigt function for the ceramic (y = 0.065) textured along the [001] direction using 3.0 mol% NN templates.This ceramic has an O-PC multistructure because it provides a pseudocubic (220) P reflection and orthorhombic (004) O , (400) O , and (220) O reflections.Rietveld refinement analysis has been conducted on the XRD pattern of the [001]textured ceramic (y = 0.065) to clearly identify its PC structure (Figure 17b).The PC structure was determined to be an R3m R structure; therefore, the [001]-textured KN(Nb 0.935 S 0.065 )-SZ (y = 0.065) ceramic has an O-R (or O-PC) multistructure consisting of the Amm2 O (51%) and the R3m R (49%) structures.Furthermore, the [001]-textured ceramic (y = 0.065) has an ideal O-R (or O-PC) multistructure, wherein the amount of the O structure is nearly the same as that of the R (or PC) structure.Figure 17c shows the various physical properties of the [001]textured KN(Nb 1−y S y )-SZ ceramics (0.045 ≤ y ≤ 0.08) using 3.0 mol% NN templates.The piezoceramic (x = 0.065), which possesses an O-R (or O-PC) multistructure, revealed a large d 33 value of 620 pC/N and a k p value of 0.53.The d 33 values of untextured KN(Nb 0.935 S 0.065 )-SZ ceramics (y = 0.065) are yet to be reported.However, the d 33 value of the untextured KN(Nb 0.94 S 0.06 )-SZ ceramic (y = 0.06) has been reported to be 265 pC/N; therefore, it can be assumed that the d 33 value of the untextured KN(Nb 0.935 S 0.065 )-SZ ceramic (y = 0.065) is ~260 pC/N [81].Hence, the d 33 value of the untextured KN(Nb 0.935 S 0.065 )-SZ (y = 0.065) improved by more than twice the original value after [001]-texturing.Further, the large d 33 value (≥580 pC/N) was preserved up to 110 • C, as provided in Figure 17d.Moreover, the [001]-textured KN(Nb 0.91 S 0.09 )-CaZrO 3 ceramic also showed an ideal O-R (or O-PC) multistructure composed of the Amm2 O (59%) and the R3m R (41%) structures [114].This ceramic exhibited a large d 33 value of 610 pC/N; therefore, the KN(Nb 1−y S y )-MZ ceramics (M = Sr and Ca) with an ideal O-PC (or O-R) multistructure exhibited a large d 33 value of 610-620 pC/N.

Figure
Figure19ashows variations in the reflections at ~17.5 • and 36 • when an electric field is applied to the [001]-textured 96KNNS-1CZ-3BKH piezoceramic.Reflections at 36 • can be fitted using the Lorentz function and analyzed as a mixture of O and R reflections, thereby indicating that the [001]-textured KNNS-CZ-BKH piezoceramic has an O-R multistructure.The Rietveld refinement analysis of the XRD patterns and convergent beam electron diffraction patterns of the [001]-textured 96KNNS-1CZ-3BKH piezoceramic indicate that this ceramic exhibits an O-R multistructure[115].Therefore, the [001]-textured 96KNNS-1CZ-3BKH piezoceramic with the O-R multistructure is considered to possess a large d 33 because eight P S s contribute to the polarization along the [001] direction when an electric field is applied to the [001]-textured direction, four P S s along the <110> direction from the O phase, and four P S s along the <111> direction from the R phase.Furthermore, when an electric field was applied, a monoclinic (M) reflection appeared because of splitting the (002) peak, as shown in Figure19a.This intermediate M phase disappeared and transformed into the O phase at a higher electric field of 20 kV/cm.The intermediate M phase serves as a bridge, thereby facilitating the polarization rotation between the rhombohedral[112] and orthorhombic[103] polar axes in the (10-1) plane, as shown in Figure19b, which results in increased piezoelectricity in the [001]-textured 96KNNS-1CZ-3BKH piezoceramic.Figure19cprovides a TEM bright-field image of the untextured 96KNNS-1CZ-3BKH piezoceramic and the large (micrometer or submicrometer scale) strip-like domains exist in this ceramic.A lamellar domain configuration composed of a high density of nanodomains is observed in the [001]-textured 96KNNS-1CZ-3BKH piezoceramic, as shown in Figure 19d.The presence of nanodomains with decreased domain wall energy and increased domain wall mobility contributed to the large piezoelectric properties of the [001]-textured 96KNNS-1CZ-3BKH piezoceramic.
Figure19ashows variations in the reflections at ~17.5 • and 36 • when an electric field is applied to the [001]-textured 96KNNS-1CZ-3BKH piezoceramic.Reflections at 36 • can be fitted using the Lorentz function and analyzed as a mixture of O and R reflections, thereby indicating that the [001]-textured KNNS-CZ-BKH piezoceramic has an O-R multistructure.The Rietveld refinement analysis of the XRD patterns and convergent beam electron diffraction patterns of the [001]-textured 96KNNS-1CZ-3BKH piezoceramic indicate that this ceramic exhibits an O-R multistructure[115].Therefore, the [001]-textured 96KNNS-1CZ-3BKH piezoceramic with the O-R multistructure is considered to possess a large d 33 because eight P S s contribute to the polarization along the [001] direction when an electric field is applied to the [001]-textured direction, four P S s along the <110> direction from the O phase, and four P S s along the <111> direction from the R phase.Furthermore, when an electric field was applied, a monoclinic (M) reflection appeared because of splitting the (002) peak, as shown in Figure19a.This intermediate M phase disappeared and transformed into the O phase at a higher electric field of 20 kV/cm.The intermediate M phase serves as a bridge, thereby facilitating the polarization rotation between the rhombohedral[112] and orthorhombic[103] polar axes in the (10-1) plane, as shown in Figure19b, which results in increased piezoelectricity in the [001]-textured 96KNNS-1CZ-3BKH piezoceramic.Figure19cprovides a TEM bright-field image of the untextured 96KNNS-1CZ-3BKH piezoceramic and the large (micrometer or submicrometer scale) strip-like domains exist in this ceramic.A lamellar domain configuration composed of a high density of nanodomains is observed in the [001]-textured 96KNNS-1CZ-3BKH piezoceramic, as shown in Figure 19d.The presence of nanodomains with decreased domain wall energy and increased domain wall mobility contributed to the large piezoelectric properties of the [001]-textured 96KNNS-1CZ-3BKH piezoceramic.

Figure
Figure 22a provides the diverse physical characteristics of the KNNS-(0.04−x)BZ-xBAZceramics (0.0 ≤ x ≤ 0.04) textured toward the [001] orientation.The relative densities of the [001]-textured piezoceramics range between 92% and 94% of the theoretical density.The ε T 33 /ε 0 values of the textured ceramics range between 2100 and 2500, with a comparatively low tan δ (0.03-0.045).The k p of the textured ceramic (x = 0.0) was ~0.65 (Figure 22a).An improved k p of 0.68 was obtained from the [001]-textured ceramic (x = 0.01) and it decreased to 0.55 for the piezoceramic with x = 0.04.The [001]-textured ceramic with x = 0.02 retained a high k p of 0.64.The [001]-textured ceramic (x = 0.0) possessed a relatively large d 33 value of 630 pC/N, which increased with the increase in x.The textured piezoceramic (x = 0.02) provided a very high d 33 of 805 pC/N, which is the largest d 33 value for the KNN-related lead-free piezoceramics reported to date [118].Figure 22b shows the d 33 value for the untextured and [001]-textured KNNS-(0.04−x)BZ-xBAZpiezoceramics (0.0 ≤ x ≤ 0.04).The d 33 value of the textured KNNS-0.04BZpiezoceramic with the R-O structure was relatively small (630 pC/N) because the untextured KNNS-0.04BZceramic has a small d 33 of 425 pC/N.The textured KNNS-0.02BZ-0.02BAZceramic, which had an R-O-T multistructure with a large fraction of R-O phase ((~80%), as shown in Figure21c), exhibited a high d 33 (805 pC/N) after texturing owing to the large d 33 (506 pC/N) of the untextured piezoceramic (x = 0.02).The increase in d 33 after the [001]-texturing is insignificant for the ceramic with x ≥ 0.03 (Figure22b), and therefore, an increase in the d 33 is high for the specimens (x ≤ 0.02); however, it is low for the specimens (x ≥ 0.03).

Figure
Figure22cshows changes in the proportion of the R-O phase and the increasing rate of the d33 value after [001]-texturing for the KNNS-(0.04−x)BZ-xBAZceramics.The increase in the d33 value can be provided by d33 T /d33 UT , where d33 T and d33 UT indicate the d33 values of the textured and untextured piezoceramics, respectively.The amount of the R-O phase reduced with the increase in x; however, a large amount of the R-O phase (>80%) existed in the piezoceramics (x ≤ 0.02).The amount of the R-O phase greatly reduced when x was larger than 0.02.The increasing rate of d33 increased slightly for piezoceramics (x ≤ 0.02), although the amount of the R-O phase reduced.The increasing rate of d33 decreased considerably when x was larger than 0.02 (Figure22c) because of the presence of a large amount of the T phase.Therefore, it can be concluded that the largest piezoelectricity is

Figure
Figure 22c shows changes in the proportion of the R-O phase and the increasing rate of the d 33 value after [001]-texturing for the KNNS-(0.04−x)BZ-xBAZceramics.The increase in the d 33 value can be provided by d 33 T /d 33 UT , where d 33 T and d 33 UT indicate the d 33 values of the textured and untextured piezoceramics, respectively.The amount of the R-O phase reduced with the increase in x; however, a large amount of the R-O phase (>80%) existed in the piezoceramics (x ≤ 0.02).The amount of the R-O phase greatly reduced when x was larger than 0.02.The increasing rate of d 33 increased slightly for piezoceramics (x ≤ 0.02), although the amount of the R-O phase reduced.The increasing rate of d 33 decreased considerably when x was larger than 0.02 (Figure 22c) because of the presence of a large amount of the T phase.Therefore, it can be concluded that the largest piezoelectricity is obtained after the [001]-texturing for the piezoelectric ceramics with an R-O-T multistructure containing a large amount of the R-O phase (~80%).The identical results were observed from the [001]-textured NKNS-(0.04−x)MZ-xBAZ(M = Ca and Sr) piezoceramics, which showed large d 33 values of ~760 ± 20 pC/N [116,117].Figure 22d displays the d 33 values of the KNN-modified piezoelectric ceramics reported in the literature.The untextured KNNS-0.01CZ-0.03BAZpiezoceramic with an ideal R-O-T multistructure exhibited the largest d 33 value of 680 pC/N among the untextured KNNmodified piezoceramics.The [001]-textured KNNS-0.02BZ-0.02BAZpiezoceramic, which had an R-O-T multistructure with a large fraction of the R-O phase, provided the largest d 33 value of 805 pC/N among the [001]-textured KNN-modified piezoceramics reported to date.