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

doi:10.21203/rs.3.rs-839903/v1

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

https://doi.org/10.21203/rs.3.rs-839903/v1

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Fe₂O₃-added 0.96(K_0.5Na_0.5)(Nb_0.94Sb_0.06)-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.

Electronic Materials and Devices

Magnetics Materials and Devices

KNN-based ceramics

Lead-free piezoceramics

domain structure

crystal structure

Lead zirconium titanate (PZT)-based piezoceramics have been utilized in various electronic devices [1–4]. 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 [5–7]. (K_1 − xNa_x)NbO₃- (KNN)-related lead-free piezoceramics have attracted much attention as substitutions for PZT-based piezoceramics because they exhibit promising piezoelectricity [8–10]. 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, 11–14]. NaNbO₃ templates have been employed to texture the grains of the KNN-related piezoceramics along the [001] direction, and their piezoelectric charge constant (d₃₃) was found to be significantly improved [5, 13, 14]. Previously, a large d₃₃ (700 pC/N) was obtained from the [001]-oriented (K, Na)(Nb, Sb)O₃-(Bi, K)HfO₃-CaZrO₃ 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) [15–18]. Various structures, such as O-R, T-O, rhombohedral-tetragonal (R-T), and T-O-R multi-structures, have been synthesized by transferring the T_O−R and T_T−O to RT [9, 19–26]. Pseudocubic-orthorhombic (PC-O) and PC-T multi-structures have also been developed in KNN-based piezoceramics by decreasing the Curie temperature (T_C) and T_T−O to near RT [27–31]. KNN-based piezoceramics with the O-R multi-structure show relatively low d₃₃ values (190–230 pC/N) [19–21]. However, KNN-based piezoceramics with other multi-structures exhibit a comparatively large d₃₃ value (418–570 pC/N) [9, 22–26]. Furthermore, KNN-based piezoceramics with nanodomains have been reported to have a large d₃₃ value because the domain boundary energy of the nanodomains is low, resulting in easy domain rotation with high piezoelectricity [32, 33]. The nanodomains were generally observed in KNN-based piezoceramics with relaxor properties [34–36]. In particular, KNN-based piezoceramics that have a T-O-R multi-structure and nanodomains exhibited a large d₃₃ (650 ± 20 pC/N) [36]. Because KNN-related piezoceramics with a multi-structure and nanodomains can be easily synthesized using the traditional solid-state sintering method, the formation of a multi-structure with nanodomains is an easy and convenient technique to enhance the piezoelectric properties of KNN-related piezoceramics.

Various ABO₃-type additives (A = Ca/Sr/Ba, B = Ti/Zr) have been employed to develop such a structure [16, 27, 29, 37–42]. (Bi_0.5M_0.5)NO₃ (M = K/Na/Ag, N = Hf/Zr) additives have also been utilized to form various multi-structures with nanodomains [34, 36, 43–47]. Li⁺ and Sb⁵⁺ ions have also been used to develop such a multi-structure by transferring T_O−R and T_T−O to RT [18, 48–50]. Furthermore, the (Bi, Ag)ZrO₃ (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 d₃₃ values of 235–490 pC/N [47, 51–53]. 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_0.5Na_0.5)(Nb_0.93Sb_0.07)O₃-(0.04-x)SrZrO₃-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 d₃₃ (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 films to demonstrate their applicability to piezoelectric devices. This actuator shows excellent actuating properties, confirming that the KNNS-0.01SZ-0.03BAZ piezoceramic developed in this study is a promising lead-free piezoelectric material.

KNNS-(0.04-x)SZ-xBAZ piezoceramics (0.0 ≤ x ≤ 0.04) were produced using the traditional solid-state method. K₂CO₃, Na₂CO₃, Nb₂O₅, Sb₂O₃, SrCO₃, ZrO₂, Bi₂O₃, and Ag₂O (> 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% Fe₂O₃ (> 95%, Kanto Chemical, Japan) was added to assist the densification 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 films 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 densified 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 films were measured as described in Supporting Information 1.

A KNNS-0.01SZ-0.03BAZ thick film with dimensions of 20 mm × 20 mm × 0.3 mm was affixed 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.

3.1 Piezoelectric properties

The relative densities, ε^T₃₃/ε_o, tan δ, d₃₃, and k_p values of the KNNS-(0.04-x)SZ-xBAZ piezoceramics (0.0 ≤ x ≤ 0.04) are shown in Fig. 1(a), and these samples were sintered at 1090°C–1100°C for 3 h. All the samples have comparatively large relative densities (≥ 94.5% of the theoretical density). Hence, all the specimens were well densified. The ε^T₃₃/ε_o of the piezoceramic (x = 0.0) was approximately 1864, and its variation was not significant when x was less than 0.03. However, a very large ε^T₃₃/ε_o of approximately 3836 was observed for the sample (x = 0.03) because the T_T−O−R was developed at approximately RT for this sample, which will be discussed later. The sample (x = 0.04) exhibited similar results. The tan δ of the sample (x = 0.0) was approximately 4.5%, and it slightly decreased with an increase in x. The d₃₃ of the piezoceramic (x = 0.0) was 360 pC/N and increased with increasing x, and the largest d₃₃ of 650 pC/N was observed in the piezoceramic (x = 0.03). However, it decreased to 523 pC/N for the specimen (x = 0.04). Figure 1(b) shows the d₃₃ values for KNN-related lead-free ceramics that have been reported in the literature. The largest d₃₃ value of 650 ± 20 pC/N was observed in the (K, Na)(Nb, Sb)O₃-(Bi, Na, K)ZrO₃-Fe₂O₃-AgSbO₃ piezoceramics [36]. Therefore, the d₃₃ value observed from the sample (x = 0.03) is almost the same as the largest d₃₃ value reported in the literature. The k_p of the sample (x = 0.0) was approximately 0.44, which increased with increasing x, and the largest k_p of 0.52 was observed from the sample (x = 0.02). The sample (x = 0.03) exhibited a similar k_p of 0.51. The k_p value is proportional to d_ij²/ε^T₃₃. Therefore, the large k_p of the piezoceramic (x = 0.02) is ascribed to its small ε^T₃₃ and relatively large d₃₃, and the large k_p of the sample (x = 0.03) is attributed to its large d₃₃ value. The d₃₃ 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 d₃₃ slightly decreased with an increase in the measuring temperature, but a relatively high d₃₃ (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 d₃₃ value with respect to the poling temperature for the sample with x = 0.03. The sample poled at 20°C displayed the largest d₃₃ 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 [54–56]. The piezoceramic (x = 0.03) exhibited the largest d₃₃ 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 field (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/cm², a remnant polarization (P_r) of 16.3 µC/cm², and a coercive electric field (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)).

3.2 Structural properties

Figures 2(a)–(d) show the SEM images of the thermally etched surface of the KNNS-(0.04-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 [57–59]. Hence, these results suggest that the addition of BAZ can enhance the piezoelectric characteristics of the samples by increasing their grain size.

The piezoelectric properties of the KNN-based ceramics were considerably influenced 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 investigate the crystal structure of the samples (0.0 ≤ x ≤ 0.04), as displayed in Figs. 3(a)–(e). The sample with x = 0.0 shows rhombohedral (220)_R and (2–20)_R peaks, strong orthorhombic (004)_O, (400)_O, and (222)_O peaks, and weak tetragonal (202)_T and (220)_T peaks, indicating that this sample has a T-O-R multi-structure. However, the intensity of the orthorhombic peaks was much larger than those of the rhombohedral and tetragonal peaks for the specimen (x = 0.0) (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 significant. 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 multi-structure, with a large proportion of the tetragonal structure (Fig. 3(e)).

Rietveld refinement was also performed to determine the detailed crystal structure of the piezoceramic (x = 0.03). Various models were used for the Rietveld analysis (Figs. 4(a)–(c)): O-R multi-structure, T-O multi-structure, and T-O-R multi-structure. 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(c). The samples with x = 0.0, 0.01, and 0.02 also have a T-O-R multi-structure, but the amount of orthorhombic structure is larger than those of the other structures, as shown in Figs. S4(a)-(e). Therefore, Rietveld analysis confirmed that the piezoceramic (x = 0.03) has an ideal T-O-R multi-structure, in which the three structures have similar proportions. Moreover, it can be suggested that the presence of an ideal T-O-R multi-structure can contribute to the large d₃₃ value of this sample. In addition, the piezoceramic (x = 0.04) has an R-T multi-structure, as shown in Figs. S5(a) and (b), which is identical to the R-T morphotropic phase boundary structure of PZT-based piezoceramics. Hence, this piezoceramic also has a comparatively large d₃₃ of 523 pC/N.

Table 1

Atomic coordinates, site occupancies, lattice parameters, and R-values of the KNNS-0.01SZ-0.03BAZ piezoceramic for O-R, T-O, and T-O-R multi-structure models.
Phase	Structural model (SG)	Site label	x	y	z	Site occupancy	Lattice parameter (Å)	R factor
O-R multi-structure	Orthorhombic (Amm2) (69%) + Rhombohedral (R3m, H) (31%)	K/Na/Sr /Bi/Ag	0(-)	0(-)	0(-)	0.114(-)/0.123(-)/0.002(-) /0.004(-)/0.004(-)	a = 3.9894(2) b = 5.6153(3) c = 5.6223(3) α = β = γ = 90°	R_p/R_wp/R_exp R_b/R_f 4.70 / 6.09 / 3.00 5.49 / 2.80
		Nb/Sb/Zr	0.5(-)	0(-)	0.5250(37)	0.224(-)/0.016(-)/0.010(-)
		O1	0(-)	0(-)	0.5205(94)	0.250(-)
		O2	0.5(-)	0.2249(67)	-0.2105(84)	0.500(-)
		K/Na/Sr /Bi/Ag	0(-)	0(-)	0.4742(57)	0.076(-)/0.082(-)/0.002(-) /0.004(-)/0.002(-)	a = b = 5.6289(18) c = 6.8957(42) α = β = 90° γ = 120°	R_p/R_wp/R_exp R_b/R_f 4.70 / 6.09 / 3.00 5.48 / 3.36
		Nb/Sb/Zr	0(-)	0(-)	0(-)	0.150(-)/0.010(-)/0.007(-)
		O1	0.5057(-)	-0.5057(-)	0.4805(51)	0.500(-)
T-O multi-structure	Tetragonal (P4mm) (44%) + Orthorhombic (Amm2) (56%)	K/Na/Sr /Bi/Ag	0(-)	0(-)	-0.0483(31)	0.057(-)/0.062(-)/0.001(-) /0.002(-)/0.002(-)	a = 3.9768(1) c = 3.9801(1) α = β = γ = 90°	R_p/R_wp/R_exp R_b/R_f 4.26 / 5.76 / 3.01 4.06 / 2.86
		Nb/Sb/Zr	0.5(-)	0.5(-)	0.5(-)	0.112(-)/0.008(-)/0.005(-)
		O1	0.5(-)	0.5(-)	0.0155(55)	0.125(-)
		O2	0.5(-)	0(-)	0.4476(63)	0.250(-)
		K/Na/Sr /Bi/Ag	0(-)	0(-)	0(-)	0.114(-)/0.123(-)/0.002(-) /0.004(-)/0.004(-)	a = 3.9932(2) b = 5.6134(2) c = 5.6176(2) α = β = γ = 90°	R_p/R_wp/R_exp R_b/R_f 4.26 / 5.76 / 3.01 3.75 / 2.62
		Nb/Sb/Zr	0.5(-)	0(-)	0.4686(58)	0.224(-)/0.016(-)/0.010(-)
		O1	0(-)	0(-)	0.4829(199)	0.250(-)
		O2	0.5(-)	0.2499(88)	0.1981(89)	0.500(-)
T-O-R structure	Tetragonal (P4mm) (36%) + Orthorhombic (Amm2) (33%) + Rhombohedral (R3m, H) (31%)	K/Na/Sr /Bi/Ag	0(-)	0(-)	-0.0680(117)	0.057(-)/0.062(-)/0.001(-) /0.002(-)/0.002(-)	a = 3.9772(1) c = 3.9805(1) α = β = γ = 90°	R_p/R_wp/R_exp R_b/R_f 4.06 / 5.38 / 3.00 2.75 / 2.06
		Nb/Sb/Zr	0.5(-)	0.5(-)	0.5(-)	0.112(-)/0.008(-)/0.005(-)
		O1	0.5(-)	0.5(-)	0.0218(63)	0.125(-)
		O2	0.5(-)	0(-)	0.4741(69)	0.250(-)
		K/Na/Sr /Bi/Ag	0(-)	0(-)	0(-)	0.114(-)/0.123(-)/0.002(-) /0.004(-)/0.004(-)	a = 3.9948(2) b = 5.6147(2) c = 5.6189(2) α = β = γ = 90°	R_p/R_wp/R_exp R_b/R_f 4.06 / 5.38 / 3.00 3.13 / 2.85
		Nb/Sb/Zr	0.5(-)	0(-)	0.5094(29)	0.224(-)/0.016(-)/0.010(-)
		O1	0(-)	0(-)	0.5724(-)	0.250(-)
		O2	0.5(-)	0.2011(130)	0.2585(-)	0.500(-)
		K/Na/Sr /Bi/Ag	0(-)	0(-)	0.4467(27)	0.076(-)/0.082(-)/0.002(-) /0.002(-)/0.002(-)	a = b = 5.6339(11) c = 6.8980(25) α = β = 90° γ = 120°	R_p/R_wp/R_exp R_b/R_f 4.06 / 5.38 / 3.00 3.06 / 2.51
		Nb/Sb/Zr	0(-)	0(-)	0(-)	0.150(-)/0.010(-)/0.007(-)
		O	-0.5109(-)	-0.5109(-)	0.4841(-)	0.500(-)

3.3 Temperature dependence of dielectric properties and domain structure

Figures 5(a)–(e) show the ε^T₃₃/ε_o versus temperature plots for the KNNS-(0.04-x)SZ-xBAZ ceramics with 0.0 ≤ x ≤ 0.04, ranging between − 60°C and 90°C. Variations in the T_T−O and T_O−R peaks are shown in these figures. For the sample (x = 0.0), the T_T−O and T_O−R are approximately 70°C and 0.0°C, respectively, as shown in 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^T₃₃/ε_o versus temperature curves are similar to those of the XRD analysis. In addition, the inset of each figure also shows the ε^T₃₃/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 d₃₃ value of up to 120°C.

The ε^T₃₃/ε_o values of the KNNS-(0.04-x)SZ-xBAZ piezoceramics (0.0 ≤ x ≤ 0.04) were measured at different frequencies to investigate the relaxor properties of these samples. Relaxor ceramics generally have nanodomains with a small domain boundary energy [32–34]. Hence, relaxor piezoceramics with nanodomains have been reported to exhibit high piezoelectric characteristics [32–34]. Figure 6(a) shows the ε^T₃₃/ε_o values measured at various frequencies for the sample (x = 0.0). A change in T_T−O was not observed with an increase in frequency, implying that this sample is a normal ferroelectric ceramic. For the sample (x = 0.03), however, the T_T−O−R was enhanced with an increase in the measuring frequency, as shown in Fig. 6(b). The temperature difference (ΔT) between the T_T−O−R measured at the lowest frequency and the T_T−O−R measured at the highest frequency was approximately 11.3°C for the sample with x = 0.03. Therefore, the sample with x = 0.03 is considered to be a relaxor ceramic, and this ceramic is expected to have ferroelectric nanodomains. The ε^T₃₃/ε_o values were also measured at various frequencies for the samples (x = 0.01, 0.02, and 0.04), as shown in Figs. S6(a)-(c). The ΔT value of the specimen (x = 0.01) was 4.5°C, and it increased with the increase in x. Therefore, it is considered that the samples with x = 0.01, 0.02, and 0.04 also exhibit relaxor characteristics. TEM analysis was performed on the piezoceramics (x = 0.0, 0.03) to investigate the domain structure of these samples. Figure 6(c) shows the TEM bright-field image for the sample with x = 0.0. The domain size of this sample was approximately 50 nm × 400 nm, indicating that this sample had relatively large domains. A TEM bright-field image was also observed from the piezoceramic (x = 0.03), as shown in Fig. 6(d). Nanodomains with a size of 2 nm × 15 nm were formed in this sample, possibly because this sample showed relaxor properties. This result suggests that the presence of ferroelectric nanodomains also contributes to the large d₃₃ value of this sample.

3.4 planar-type actuator

A KNNS-0.01SZ-0.03BAZ thick film (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 film was determined using Rietveld analysis of the XRD pattern, as shown in Fig. 7(a), and was identified 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 film was the same as that of the KNNS-0.01SZ-0.03BAZ piezoceramic. The microstructures of the KNNS-0.01SZ-0.03BAZ thick films sintered at various temperatures were also studied. The thick film 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 film 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 films densified 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 films 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^T₃₃/ε_o, tan δ, d₃₃, and k_p values of the KNNS-0.01SZ-0.03BAZ thick films sintered at various temperatures. The thick film sintered at 1080°C showed a low relative density (90% of the theoretical density), but the thick films 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 film must be densified at temperatures higher than 1080°C. The ε^T₃₃/ε_o of the thick film 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 film 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 d₃₃ value of the thick film sintered at 1080°C was low (410 pC/N), but increased with an increase in the sintering temperature. The thick film sintered at 1090°C exhibited the largest d₃₃ value, 630 pC/N. This d₃₃ 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 films sintered at 1100°C and 1110°C exhibited slightly reduced d₃₃ 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 film densified at 1090°C. Therefore, the optimum sintering temperature of the KNNS-0.01SZ-0.03BAZ thick film was considered to be 1090°C. This thick film was also sintered at 1090°C for various amounts of time; the thick film sintered for 6.0 h showed the best piezoelectric properties (Fig. S7(d)), and this thick film was used to fabricate planar-type piezoelectric actuators.

The planar-type actuator was fabricated using a KNNS-0.01SZ-0.03BAZ thick film 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 field, and a large acceleration of 335 G was obtained under a comparatively small electric field 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 fields 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 field, 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 field 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 field of 750 V/mm [60], which is almost six times larger than the electric field 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 field, 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.

KNNS-(1-x)SZ-xBAZ piezoceramics (0.0 ≤ x ≤ 0.04) were well densified at 1090°C, and they exhibited large relative densities (≥ 94.5% of the theoretical density). The d₃₃ value improved with the addition of BAZ, and the largest d₃₃ of 650 pC/N was found for the piezoceramic (x = 0.03). This d₃₃ value is equivalent to the largest d₃₃ value of the KNN-related piezoceramics reported in the literature. The large d₃₃ value was maintained up to 120°C, suggesting that this piezoceramic has good thermally stable piezoelectricity. According to the Rietveld analysis, the piezoceramic with x = 0.03 has an ideal T-O-R multi-structure, in which each structure has similar proportions. The sample (x = 0.0) is a normal ferroelectric ceramic, but relaxor properties were observed when BAZ was added. Moreover, nanodomains with a size of 2 nm × 15 nm were found in the sample (x = 0.03). Therefore, the piezoceramic (x = 0.03) shows a large d₃₃ value of 650 pC/N because of the presence of the ideal T-O-R multi-structure and nanodomains. A KNNS-0.01SZ-0.03BAZ thick film was fabricated to produce a planar-type piezoelectric actuator. This thick film also has a T-O-R multi-structure, large d₃₃ (630 pC/N), and large k_p (0.51), which are similar to those of the KNNS-0.01SZ-0.03BAZ piezoceramics. The planar-type actuator was fabricated using a KNNS-0.01SZ-0.03BAZ thick film with dimensions of 20 mm × 20 mm × 0.2 mm. This actuator showed a large acceleration of 335 G and a displacement of 231 µm at a low electric field of 120 V/mm. Therefore, the KNNS-0.01SZ-0.03BAZ piezoceramic is an excellent material for piezoelectric actuators.

Acknowledgements

This work was supported by National R&D Programs through the National Research Foundation of Korea (NRF) funded by ICT (Project No. 2020M3H4A3105596). We also thank the KU-KIST Graduate School Program of Korea University.

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

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1. Introduction

2. Experimental Procedures

3. Results And Discussion

3.1 Piezoelectric properties

3.2 Structural properties

3.3 Temperature dependence of dielectric properties and domain structure

3.4 planar-type actuator

4. Conclusions

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References

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