Outstanding Piezoelectricity of (K, Na)(Nb, Sb)O3-SrZrO3-(Bi, Ag)ZrO3 Piezoceramics and Their Application to a Planar-Type Actuator


 Fe2O3-added 0.96(K0.5Na0.5)(Nb0.94Sb0.06)-0.01SrZrO3-0.03(BiAg)ZrO3 [KNNS-0.01SZ-0.03BAZ] piezoceramic was well densified at 1090 °C. It exhibited a large piezoelectric charge constant (d33) of 650 pC/N, which is similar to the largest d33 value reported in the literature. This sample has a tetragonal-orthorhombic-rhombohedral (T-O-R) structure, in which each structure has a similar proportion. Moreover, nanodomains (2 nm × 15 nm) with low domain boundary energy were found in this sample, which exhibited relaxor properties. Therefore, the presence of a T-O-R multi-structure and nanodomains is responsible for the large d33 value of this sample. A KNNS-0.01SZ-0.03BAZ thick film was synthesized, and it also had a T-O-R multi-structure with a large d33 (630 pC/N). A planar-type actuator was produced using this thick film, and this actuator exhibited a large acceleration (335 G at 120 V/mm) and displacement (231 μm at 120 V/mm). Hence, the KNNS-0.01SZ-0.03BAZ thick film is a good candidate for lead-free piezoelectric actuators.


Introduction
Lead zirconium titanate (PZT)-based piezoceramics have been utilized in various electronic devices [1][2][3][4]. However, they are harmful to human health and the environment because they contain more than 60 wt% PbO. Thus, many efforts have been made to develop lead-free piezoceramics as a substitute for PZT-based piezoceramics [5][6][7]. (K 1 − x Na x )NbO 3 -(KNN)-related lead-free piezoceramics have attracted much attention as substitutions for PZT-based piezoceramics because they exhibit promising piezoelectricity [8][9][10]. However, the piezoelectricity of KNN-based piezoceramics is still lower than that of PZT-based piezoceramics. Therefore, various techniques have been applied to further improve the piezoelectricity of KNN-based piezoceramics. The reactive template grain growth (RTGG) technique is an effective method for increasing the piezoelectricity of KNN-based piezoceramics [5,[11][12][13][14]. NaNbO 3 templates have been employed to texture the grains of the KNN-related piezoceramics along the [001] direction, and their piezoelectric charge constant (d 33 ) was found to be signi cantly improved [5,13,14]. Previously, a large d 33 (700 pC/N) was obtained from the [001]-oriented (K, Na)(Nb, Sb)O 3 -(Bi, K)HfO 3 -CaZrO 3 piezoceramic [14]. In the RTGG method, however, single-crystal templates have to be used, and the tape casting process should be employed. Therefore, the RTGG process for KNN-based ceramics is complicated compared with the traditional solid-state method.
Synthesis of KNN-based piezoceramics with a multi-structure, in which its constituent structures coexist, is another method to enhance the piezoelectricity of KNN-related piezoceramics. A multi-structure has been developed by transferring the orthorhombic-rhombohedral (O-R) phase transition temperature (T O−R ) and the tetragonal-orthorhombic (T-O) phase transition temperature (T T−O ) to room temperature (RT) [15][16][17][18]. Various
Various ABO 3 -type additives (A = Ca/Sr/Ba, B = Ti/Zr) have been employed to develop such a structure [16, 27,29,[37][38][39][40][41][42]. (Bi 0.5 M 0.5 )NO 3 (M = K/Na/Ag, N = Hf/Zr) additives have also been utilized to form various multi-structures with nanodomains [34,36,[43][44][45][46][47]. Li + and Sb 5+ ions have also been used to develop such a multi-structure by transferring T O−R and T T−O to RT [18,[48][49][50]. Furthermore, the (Bi, Ag)ZrO 3 (BAZ) compound was also utilized to improve the piezoelectricity of KNN-related piezoceramics through the development of a multi-structure with nanodomains, and KNN-based piezoceramics with T-O-R, O-R, R-T, and T-O multi-structures were produced using the BAZ compound; they showed comparatively large d 33 values of 235-490 pC/N [47,[51][52][53]. Moreover, relaxor properties with nanodomains were also observed in the BAZ-added KNN-based piezoceramics. Hence, BAZ is considered to be a promising compound for enhancing the piezoelectric properties of KNN-related piezoceramics through the formation of the multi-structure and nanodomains. In addition, although many KNN-based piezoceramics have been exploited, piezoelectric devices produced by lead-free piezoceramics have rarely been reported. It is thus important to evaluate the applicability of lead-free piezoceramics through the fabrication of piezoelectric devices with good electrical properties.
In this study, therefore, 0.96(K 0.5 Na 0.5 )(Nb 0.93 Sb 0.07 )O 3 -(0.04-x)SrZrO 3 -xBAZ [KNNS-(0.04-x)SZ-xBAZ] piezoceramics were synthesized and their piezoelectric characteristics were studied. The structural properties of these samples were investigated in detail to identify the relationship between the structural properties and piezoelectricity of these samples. In particular, the piezoceramic (x = 0.03) displayed a large d 33 (650 pC/N), which was attributed to the existence of the ideal T-O-R multi-structure with nanodomains. Moreover, planar-type actuators were fabricated using KNNS-0.01SZ-0.03BAZ thick lms to demonstrate their applicability to piezoelectric devices. This actuator shows excellent actuating properties, con rming that the KNNS-0.01SZ-0.03BAZ piezoceramic developed in this study is a promising lead-free piezoelectric material.

Experimental Procedures
KNNS-(0.04-x)SZ-xBAZ piezoceramics (0.0 ≤ x ≤ 0.04) were produced using the traditional solid-state method. K 2 CO 3 , Na 2 CO 3 , Nb 2 O 5 , Sb 2 O 3 , SrCO 3 , ZrO 2 , Bi 2 O 3 , and Ag 2 O (> 99%, High Purity Chemicals, Saitama, Japan) powders with the appropriate compositions were ball-milled in Nalgene bottles using yttria-stabilized zirconia balls and anhydrous ethanol for 24 h. Then, 0.5 mol% Fe 2 O 3 (> 95%, Kanto Chemical, Japan) was added to assist the densi cation of the samples. After drying, the milled powders were calcined at 850°C for 6 h. The calcined powders were ball-milled again for 24 h and then dried. After granulating the milled powders, disc-shaped pellets were produced by applying a uniaxial pressure of 50 MPa and sintering at 1090°C − 1100°C for 3 h in air.
The calcined KNNS-0.01SZ-0.03BAZ powder (x = 0.03) was used to fabricate thick lms through the tape-casting process. The calcined KNNS-0.01SZ-0.03BAZ powders, a dispersant, anhydrous ethanol, and toluene were ball-milled for 12 h, and plasticizer and binder were supplied during the second ball-milling process, which was conducted for 36 h. These slurries were utilized to fabricate KNNS-0.01SZ-0.03BAZ green sheets by the tape casting method, after which the green sheets were dried at 65°C. The dried green sheets were cut, stacked, screen-printed, and warm isostatically pressed under 30 MPa at 70°C for 20 min. These sheets were heated to 550°C at 0.2°C /min and maintained for 5 h to eliminate the organics. They were then re-pressed under 200 MPa using a cold isostatic press and were densi ed at 1090°C-1110°C for various sintering times. The piezoelectric, structural, dielectric, and ferroelectric properties of the KNNS-(0.04-x)SZ-xBAZ piezoceramics (0.0 ≤ x ≤ 0.04) and the KNNS-0.01SZ-0.03BAZ thick lms were measured as described in Supporting Information 1.
A KNNS-0.01SZ-0.03BAZ thick lm with dimensions of 20 mm × 20 mm × 0.3 mm was a xed to an SUS304 panel using DP-420 epoxy to fabricate a planar-type actuator. A laser Doppler vibrometer (LDV, Polytec, PSV-400, OFV-5000) was used to measure the actuating characteristics of this planar-type actuator, including the vibration acceleration and displacement under a sine wave input voltage. These values were obtained at different frequencies and voltages. The resonance frequencies of the actuators were measured at different voltages. COMSOL software was used to simulate the actuating characteristics, and the experimental values were evaluated using the simulated values.

Piezoelectric properties
The relative densities, ε T 33 /ε o , tan δ, d 33  Therefore, the large k p of the piezoceramic (x = 0.02) is ascribed to its small ε T 33 and relatively large d 33 , and the large k p of the sample (x = 0.03) is attributed to its large d 33 value. The d 33 of the sample (x = 0.03) was measured at various temperatures to investigate the temperature dependence of the piezoelectricity of this sample ( Fig. 1(c)). The d 33 slightly decreased with an increase in the measuring temperature, but a relatively high d 33 (500 pC/N) was observed at 120°C, implying that this sample preserved comparatively high piezoelectricity up to 120°C. Figure 1(d) shows the variation in the d 33 value with respect to the poling temperature for the sample with x = 0.03. The sample poled at 20°C displayed the largest d 33 of 650 pC/N, and a similar result was observed for the sample poled at 30°C. However, it decreased when the poling temperature exceeded 30°C. It has been reported that KNN-based specimens show better piezoelectric properties when they are poled near the phase transition temperature because many structures coexist at the phase transition temperature [54][55][56].
The piezoceramic (x = 0.03) exhibited the largest d 33 value of 650 pC/N when it was poled at 20°C-30°C because this sample has a T T−O−R near RT, which will be shown later. In addition, the polarization-electric eld (P-E) hysteresis curves of these piezoceramics were also measured (Figs. S1(a)-(f)]. The piezoceramic (x = 0.03) shows a normal P-E curve and has a comparatively high saturated polarization (P S ) of 21.6 µC/cm 2 , a remnant polarization (P r ) of 16.3 µC/cm 2 , and a coercive electric eld (E C ) of 0.65 kV/mm (Fig. S1(d)), indicating that it has good ferroelectric properties. Moreover, other piezoceramics also exhibited similar ferroelectric properties (Fig. S1(f)). 04-x)SZ-xBAZ piezoceramics (0.0 ≤ x ≤ 0.04). All the samples had a dense microstructure, resulting in a large relative density ( Fig. 1(a)). The sample (x = 0.0) has a microstructure consisting of two types of grains: large grains with an average grain size of approximately 20 µm and small grains with an average grain size of 1 µm, as shown in the inset of Fig. 2 (a). As BAZ was added, the samples began to exhibit large grains without small grains (Figs. 2(b)-(d)), suggesting that the BAZ assisted the grain growth of the samples. Grain size did not change with an increase in BAZ content, and the average grain size of the samples with 0.01 ≤ x ≤ 0.04 was 23-25 µm. SEM images were also obtained from the fractured surfaces of the samples, and they showed similar results (Figs. S2(a)-(e)). It has been generally accepted that samples with large grains exhibit better piezoelectric properties [57][58][59]. Hence, these results suggest that the addition of BAZ can enhance the piezoelectric characteristics of the samples by increasing their grain size.

Structural properties
The piezoelectric properties of the KNN-based ceramics were considerably in uenced by the crystal structure of the samples. Piezoceramics generally exhibit large piezoelectric properties when they have a T-O-R (or R-T) multi-structure. Hence, it is important to clarify the crystal structure of the KNNS-(0.04-x)SZ-xBAZ ceramics with 0.0 ≤ x ≤ 0.04. According to the XRD patterns, all the samples have a homogeneous perovskite phase, without a secondary phase (Figs. S3(a)-(e)). However, it is not possible to determine the crystal structure of these samples using the normal XRD pattern. Hence, the XRD peaks at 66.5°, which were obtained by the slow-speed scanning method, were deconvoluted using the Voigt function to  (Fig. 3(a)). As the amount of BAZ increased, the intensity of the orthorhombic peaks decreased, and that of the tetragonal structure was enhanced, but the variation in the rhombohedral peak intensity was not signi cant. For the samples (0.0 ≤ x ≤ 0.02), however, the proportion of the orthorhombic structure was still larger than those of other structures (Figs. 3(a)-(c)). However, each structure had a similar proportion in the sample (x = 0.03), as illustrated in Fig. 3(d). Moreover, it can be suggested that this sample has an ideal T-O-R multi-structure because the rhombohedral, orthorhombic, and tetragonal structures have similar proportions. Finally, the sample (x = 0.04) exhibited an R-T multistructure, with a large proportion of the tetragonal structure ( Fig. 3(e)).
Rietveld re nement was also performed to determine the detailed crystal structure of the piezoceramic (x = 0.03). Various models were used for the  Table 1 shows the atomic coordinates, site occupancies, R-values, and lattice parameters of these models. The T-O-R multi-structure that consists of 31% R3m rhombohedral, 33% Amm2 orthorhombic, and 36% P4mm tetragonal structures shows the lowest R-value, as shown in Fig. 4 Fig. 5(a). The T T−O decreases and the T O−R increases with an increase in x, and they meet at approximately 25°C for the sample with x = 0.03, resulting in the formation of T T−O−R at RT. When x exceeded 0.03, the orthorhombic phase disappeared and a broad T R−T was formed at approximately 10°C for the sample with x = 0.04. Therefore, the results of the ε T 33 /ε o versus temperature curves are similar to those of the XRD analysis. In addition, the inset of each gure also shows the ε T 33 /ε o versus temperature curve of the corresponding sample that includes T C . The T C of the sample (x = 0.0) was approximately 154°C, and it increased with an increase in x to 185°C for the sample (x = 0.04). In particular, the T C of the sample (x = 0.03) was comparatively high at 182°C. Therefore, this sample can maintain a high d 33 value of up to 120°C.

planar-type actuator
A KNNS-0.01SZ-0.03BAZ thick lm (x = 0.03) with dimensions of 20 mm × 20 mm × 0.3 mm was fabricated to produce a planar-type piezoelectric actuator. The crystal structure of this thick lm was determined using Rietveld analysis of the XRD pattern, as shown in Fig. 7(a), and was identi ed as a T-O-R multi-structure consisting of R3m rhombohedral (29.7%), Amm2 orthorhombic (32.9%), and P4mm tetragonal (37.4 %) structures. Hence, the crystal structure of this thick lm was the same as that of the KNNS-0.01SZ-0.03BAZ piezoceramic. The microstructures of the KNNS-0.01SZ-0.03BAZ thick lms sintered at various temperatures were also studied. The thick lm sintered at 1080°C shows two types of grains: large and small grains with average grain sizes of 25 µm and 0.5 µm, respectively ( Fig. S7(a)). An SEM image of the thick lm sintered at 1090°C is displayed in Fig. 7(b), showing a dense microstructure with large grains with an average grain size of 25 µm, which is similar to the microstructure of the KNNS-0.01SZ-0.03BAZ piezoceramic. The thick lms densi ed at 1100°C and 1110°C also displayed an equivalent microstructure (Figs. S7(b) and (c)].
Therefore, it can be concluded that the structural properties of the KNNS-0.01SZ-0.03BAZ thick lms sintered at temperatures higher than 1080°C are similar to those of the KNNS-0.01SZ-0.03BAZ piezoceramic.
Figure 7(c) shows the relative densities, ε T 33 /ε o , tan δ, d 33 , and k p values of the KNNS-0.01SZ-0.03BAZ thick lms sintered at various temperatures. The thick lm sintered at 1080°C showed a low relative density (90% of the theoretical density), but the thick lms sintered at temperatures higher than 1080°C exhibited a large relative density (≥ 93% of the theoretical density). Hence, the KNNS-0.01SZ-0.03BAZ thick lm must be densi ed at temperatures higher than 1080°C. The ε T 33 /ε o of the thick lm sintered at 1080°C is relatively small at 1666, probably because of its low density, and it increased with increasing sintering temperature to 3390 for the thick lm sintered at 1110°C. The tan δ of the sample sintered at 1080°C was approximately 3.3%, and it increased slightly with an increase in sintering temperature. The d 33 value of the thick lm sintered at 1080°C was low (410 pC/N), but increased with an increase in the sintering temperature. The thick lm sintered at 1090°C exhibited the largest d 33 value, 630 pC/N. This d 33 value is slightly smaller than that of the KNNS-0.01SZ-0.03BAZ piezoceramic (650 pC/N), but the difference is not large. The thick lms sintered at 1100°C and 1110°C exhibited slightly reduced d 33 values of 610 pC/N. The k p value showed an equivalent trend, and a maximum k p value of 0.51 was obtained from the thick lm densi ed at 1090°C. Therefore, the optimum sintering temperature of the KNNS-0.01SZ-0.03BAZ thick lm was considered to be 1090°C. This thick lm was also sintered at 1090°C for various amounts of time; the thick lm sintered for 6.0 h showed the best piezoelectric properties (Fig. S7(d)), and this thick lm was used to fabricate planar-type piezoelectric actuators.
The planar-type actuator was fabricated using a KNNS-0.01SZ-0.03BAZ thick lm with dimensions of 20 mm × 20 mm × 0.3 mm, as shown in the schematic diagram in Fig. 8(a). The COMSOL program was used to simulate the actuating characteristics. Figure 8(b) shows a simulated image of the KNNS-0.01SZ-0.03BAZ planar-type actuator, along with its strain. The accelerations of this actuator were measured at various frequencies and applied voltages, as shown in Fig. 8(c). The maximum accelerations were obtained at 610 Hz, indicating that 610 Hz is the resonance frequency of the actuator. The maximum acceleration increased with an increase in the applied electric eld, and a large acceleration of 335 G was obtained under a comparatively small electric eld of 120 V/mm (36 V) at 610 Hz. The maximum accelerations, which were measured at various applied voltages and 610 Hz, are displayed in the inset of Fig. 8(c) as black circles. A similar resonance frequency of 620 Hz was obtained from the simulation (Fig. S8(a)). Accelerations of this actuator were calculated at various applied voltages, and the maximum accelerations, which were calculated at the resonance frequency and various applied voltages, are displayed as red circles in the inset of Fig. 8(c). The calculated accelerations are similar to the measured values. Figure 8(d) shows the change in the displacement as a function of the frequency obtained at various applied electric elds for the KNNS-0.01SZ-0.03BAZ planar-type actuator. The maximum displacements were also obtained at 610 Hz and are shown in the inset of Fig. 8(d) as black circles. The displacement increased with an increase in the electric eld, as illustrated by the black circles in the inset of Fig. 8(d). The largest displacement of 231 µm was obtained with the application of a low electric eld of 120 V/mm. Displacements were also simulated ( Fig. S8(b)), and the maximum displacements at different applied voltages are indicated by red circles in the inset of Fig. 8(d).
The simulated displacements were also similar to the measured displacements, implying that the experimental results were in good agreement with the calculated results. The PZT-based planar-type haptic actuator with a circular shape showed a large displacement of 427 µm with an applied electric eld of 750 V/mm [60], which is almost six times larger than the electric eld used in this study. The displacement of the planar-type KNNS-0.01SZ-0.03BAZ actuator can be further enhanced by increasing the applied electric eld, indicating that this actuator is capable of generating a larger displacement than the PZT-based planar-type actuator. Therefore, the KNNS-0.01SZ-0.03BAZ lead-free piezoceramic is a promising material for piezoelectric actuators.