Ultrahigh Performance Piezoelectric Energy Harvester Using Lead-Free Piezoceramics with Large Electromechanical Coupling Factor

Chae, Yeon-Gyeong; Chae, Seok-June; Go, Su-Hwan; Kim, Eun-Ji; Park, Seok-Jung; Song, Hyunseok; Nahm, Sahn

doi:https://doi.org/10.1155/2023/6177201

International Journal of Energy Research

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Abstract Introduction Results and Discussion Conclusions Data Availability Conflicts of Interest Authors’ Contributions Acknowledgments Supplementary Materials References Copyright Related Articles

Research Article | Open Access

Volume 2023 | Article ID 6177201 | https://doi.org/10.1155/2023/6177201

Ultrahigh Performance Piezoelectric Energy Harvester Using Lead-Free Piezoceramics with Large Electromechanical Coupling Factor

Yeon-Gyeong Chae,¹Seok-June Chae,¹Su-Hwan Go,¹Eun-Ji Kim,¹Seok-Jung Park,¹Hyunseok Song,²and Sahn Nahm¹

Academic Editor: Pawan Kumar Kulriya

Received10 Jan 2023

Revised27 Feb 2023

Accepted13 Mar 2023

Published30 Mar 2023

Abstract

The output power of piezoelectric energy harvester (PEH) at the resonance frequency is dependent on the electromechanical coupling factor () of the piezoceramic, which is proportional to , where and are the piezoelectric change and dielectric constants, respectively. Therefore, the piezoceramic for the PEH should have a large and a small . [001]-texturing can be used for developing piezoceramics for PEH because it generally increases without increasing . The 0.96(K_0.5Na_0.5)(N_1-zS_z)O₃-0.03(Bi_0.5Ag_0.5)ZrO₃-0.01SrZrO₃ [KN(N_1-zS_z)-BAZ-SZ] piezoceramics were textured along the [001] orientation, and the piezoceramic () showed a large of 0.77, which is the largest for KNN-related piezoceramics reported in the literature. The cantilever-type PEH manufactured using the KN(N_0.99S_0.01)-BAZ-SZ piezoceramic exhibited a large output power density of 7.86 mW/cm³ at resonance frequency because of its large . To date, this is the largest power density for PEHs manufactured utilizing lead-free piezoceramics. Hence, the [001]-textured KN(N_0.99S_0.01)-BAZ-SZ piezoceramic is an excellent candidate for PEH, and [001]-texturing is a very efficient method for developing piezoceramics for PEH.

1. Introduction

Piezoceramics, which facilitate the transformation of mechanical energy (or electrical energy) into electrical energy (or mechanical energy), have been used in various electronic devices such as sensors, actuators, and transformers [1, 2]. Recently, the Internet of things (IoT) has attracted significant attention, and the essential devices of IoT are wireless transducers and sensors [3, 4]. The power to these devices is generally supplied by batteries [3, 4]. However, as batteries should be connected to the electronic devices of the IoT through complicated wiring to supply electrical power, and they must be regularly replaced, there is a limitation to using batteries as the power source for IoT devices. This implies that permanent power sources should be utilized to operate IoT devices. Previous studies have reported that the power required to operate IoT devices is relatively low, ranging from 10–100 μW [5–7]. Therefore, the piezoelectric energy harvester (PEH) can be used as the perpetual power resource for IoT components; thus, the interest in PEH has considerably increased with the development of IoT.

The power of the PEH operating at resonance is significantly influenced by , where , , and are the mechanical quality factor, electromechanical coupling factor, and elastic compliance of the piezoelectric ceramic, respectively [8]. However, the value of the whole PEH () is mainly affected by its metal plate that contains the piezoceramic [9]. Hence, although piezoceramics with a large are used to produce the PEH, the whole PEH exhibits small and similar values because the metal substrate has a very small and occupies a large portion of the PEH system [9]. Moreover, most piezoceramics exhibit similar values [10]. Therefore, is the most important piezoelectric constant for producing high-power PEH under resonance conditions [9]. The value is the same as , where and are the piezoelectric charge and dielectric constant of the piezoelectric ceramic, respectively [11, 12]. Therefore, piezoceramics with large and small values should be used to fabricate PEH with high power at the resonance frequency. Moreover, the value significantly influences the output power of PEH at off resonance, where and are the piezoelectric voltage constant and dielectric loss of the piezoelectric ceramic, respectively [8, 10]. Because is equivalent to , piezoceramics with a large and small should also be used for the fabrication of PEH operating at off-resonance. Therefore, piezoceramics with large and and small must be utilized for the fabrication of high-power PEH.

PbO-related piezoceramics have generally been used in piezoelectric devices owing to their good piezoelectricity. However, they contain a considerable PbO (>60 wt%), resulting in environmental problems; thus, lead-free piezoceramics must be developed. (K_0.5Na_0.5)NbO₃ (KNN)-related lead-free piezoceramics have been intensively studied as substitutes for PbO-based piezoceramics since the beginning of this century owing to their promising piezoelectric characteristics with a high Curie temperature () [2, 13, 14]. However, because their piezoelectricity is still low, several studies have attempted to enhance it. The formation of polymorphic phase boundary (PPB) structures such as tetragonal-orthorhombic (T-O), orthorhombic-rhombohedral (O-R), T-R, and T-O-R structures has been used to enhance the piezoelectricity of KNN-related piezoceramics [15–22]. Particularly, the KNN-related piezoceramics with an ideal T-O-R structure, where each structure has an identical proportion of approximately 33%, have been reported to show a very large value [20–22]. However, as the value of KNN-related piezoceramics with the ideal T-O-R structure is generally large, their and values are relatively low [20–22]. Therefore, the formation of the PPB structure is not an effective approach to developing KNN-related piezoelectric ceramics for PEH.

The piezoelectric characteristics of KNN-related piezoceramics were also improved using the reactive template grain growth (RTGG) technique. The KNN-related piezoceramics aligned toward the [001] orientation using NaNbO₃ (NNO) templates exhibited a very large value; however, the enhancement of the value was not large because of the small polarization along the [001] orientation in the perovskite materials [22–25]. Therefore, texturing along the [001] direction may be an effective technique for obtaining KNN-based piezoceramics with large and values for application in high-power PEH. According to a previous study, the 0.96(K_0.5Na_0.5)(Nb_0.94Sb_0.06)O₃-0.03(Bi_0.5Ag_0.5)ZrO₃-0.01SrZrO₃ [KN(N_1-zS_z)-BAZ-SZ with ] piezoceramic, which has an ideal T-O-R structure, exhibited a large (650 pC/N) [21]. However, it also yielded a large value of approximately 3836, with a small of 0.52 [21]. Therefore, its value must be enhanced for high-power PEH, probably through the decrease in the value. Because the of this sample is relatively low at 182°C, the large value of this sample may be because of its low , suggesting that the value can be reduced by increasing . Furthermore, it is generally accepted that the of KNN-related piezoelectric ceramics can be increased by decreasing the Sb⁵⁺ content [2, 25, 26]. Therefore, the of the KN(N_1-zS_z)-BAZ-SZ piezoceramic () can be increased with a decrease in the (or Sb⁵⁺ content), resulting in a decrease in the value. Moreover, the [001]-texturing can enhance the and values of the KNN-related piezoelectric ceramics [22–25, 27]. Consequently, the [001]-textured KN(N_1-zS_z)-BAZ-SZ piezoelectric ceramics with a low Sb⁵⁺ content () are good candidates for PEH.

This study investigated various physical properties of the untextured KN(N_1-zS_z)-BAZ-SZ piezoelectric ceramics with . The samples with were not studied because the sample with already had small and values. Furthermore, they were textured along the [001] orientation to improve the and values without increasing the value for application in the PEH. The textured sample () yielded a large value of 0.77, which, to the best of our knowledge, is the largest of the KNN-based piezoceramics reported in the literature. Further, the PEH manufactured utilizing the textured sample () exhibited a large output power density of 7.86 mW/cm³ at resonance frequency because of its large value. This is the largest power density reported for PEHs manufactured using lead-free piezoelectric ceramics. Therefore, the [001]-textured KN(N_1-zS_z)-BAZ-SZ piezoelectric ceramic () is a good candidate for high-output-power PEH. Although it is good to texture the KN(N_1-zS_z)-BAZ-SZ piezoceramics along the various directions, it is difficult to develop proper templates to texture the piezoceramics along the different directions.

2. Experimental Procedure

The 0.96(K_0.5Na_0.5)(Nb_1-zSb_z)O₃-0.03(Bi_0.5Ag_0.5)ZrO₃-0.01SrZrO₃ piezoceramics (), which are abbreviated as KN(N_1-zS_z)-BAZ-SZ piezoceramics, were produced using a traditional solid-state process. Appropriate amounts of K₂CO₃, Na₂CO₃, Nb₂O₅, Sb₂O₃, Bi₂O₃, Ag₂O, SrCO₃, and ZrO₂ (>99%, High-Purity Chemicals, Saitama, Japan) precursor powders were mixed in a Nalgene bottle using the ball-milling method for 24 h using anhydrous ethanol and yttria-stabilized zirconia (YSZ) balls. The ball-milled powders were then dried at 80°C in an oven. Thereafter, the dried powder was calcined at 800°C for 5 h at a heating rate of 5°C/min. Subsequently, 0.5 mol% Fe₂O₃ (>95%, Kanto Chemical, Japan) additive was doped to aid the sintering of the specimens and reduce their sintering temperature. The calcined powders were ball milled again for 24 h and then dried. Following the 2^nd ball milling, the samples were dried and granulated using a 40-mesh sieve. Disk-shaped specimens were produced by applying a uniaxial pressure of 50 MPa and sintering at 1090°C for 6 h. The samples sintered at 1090°C showed the best results; thus, the sintering temperature was determined as 1090°C. Further, NNO seeds were synthesized via topochemical conversion and the molten-salt method [28, 29]. The NNO seeds have a homogeneous perovskite phase with dimensions of approximately and a high aspect ratio (>10), as shown in Figures S1(a) and (b), which are the XRD pattern and SEM image of the NNO templates, respectively. Further, the [001]-textured KN(N_1-zS_z)-BAZ-SZ piezoelectric ceramics () were produced using a tape-casting procedure, as explained in Section 1 of the Supplementary Materials.

The cantilever-type PEHs were manufactured using [001]-textured KN(N_1-zS_z)-BAZ-SZ piezoelectric ceramics () (). The [001]-textured piezoceramic plates were bonded to the steel substrates (SUS304) using DP-420 epoxy. The dimensions of the steel substrates were , and Figure S1(c) in Section 1 of the Supplementary Information shows a schematic of the PEH manufactured in this study. In addition, the PEH manufactured in this study was a type I PEH, wherein the piezoceramic plates were attached to the metal plate away from the part that was clamped to the exciter. Various physical properties of the untextured and textured piezoceramics and the output properties of the PEHs were investigated and are explained in Section 1 of the Supplementary Information.

3. Results and Discussion

The untextured KN(N_1-zS_z)-BAZ-SZ piezoceramics () densified at 1090°C for 6 h exhibited a pure and homogeneous perovskite structure (Figure S2 (a); XRD patterns of the untextured KN(N_1-zS_z)-BAZ-SZ piezoceramics). The Rietveld refinement was performed on the XRD patterns of the untextured KN(N_1-zS_z)-BAZ-SZ piezoelectric ceramics () to clearly identify their crystal structures, as shown in Figures 1(a)–1(f). The diverse parameters obtained from the Rietveld analysis are listed in Table 1. The piezoelectric ceramic with exhibited a T-O structure composed of the P4mm (99) T structure (9.5%) and the Amm2 (38) O structure (90.5%), as shown in (Figure 1(a)). However, owing to the proportion of the O structure being very large, this sample was expected to have a structure similar to the O structure. The R3m (160) R structure appeared in the sample with , suggesting that it had a T-O-R structure composed of the P4mm (99) T structure (12.3%), the Amm2 (38) O structure (79.7%), and the R3m (160) R structure (8%), as shown in Figure 1(b). The amount of the O structure decreased; however, those of the T and R structures increased with an increase in (Figures 1(b)–1(f)). Finally, the piezoelectric ceramic () exhibited a T-O-R structure with the large portions of the P4mm (99) T (45%) and the R3m (160) R structures (38.8%) and a small portion of the Amm2 (38) O structure (16.2%) (Figure 1(f)). Moreover, the piezoelectric ceramic () exhibited an ideal T-O-R structure, where each phase had a similar proportion, close to 33%, as shown in Figure 1(e). Therefore, the crystal structure of the untextured KN(N_1-zS_z)-BAZ-SZ piezoelectric ceramic () was altered from the T-O to the T-O-R structures with the increase in . The microstructures of these samples were also studied using SEM, and they exhibited a dense microstructure (Figures S2(b)–(g); SEM images of the untextured KN(N_1-zS_z)-BAZ-SZ piezoceramics). The sample () contained large grains with an average grain size of approximately 30 μm. In addition, it was slightly reduced with the increase in to 25 μm for the sample with . Thus, the effect of the microstructure on the physical properties may be the same for all samples.

The versus temperature curves detected at different frequencies for the untextured KN(N_1-zS_z)-BAZ-SZ piezoceramics () are shown in Figures 2(a)–2(f), and the inset shows the versus temperature curves obtained from a broad temperature range of –50–400°C. The piezoceramic () had a high of 338°C, which decreased when was increased to 119°C for the specimen (). The T-O transition temperature () and the O-R transition temperature () of the sample () were 108 and -48°C, respectively (Figure 2(a)). With an increase in the (or Sb⁵⁺ content), the peak slightly reduced; however, the peak increased and merged into the T-O-R peak at 48°C, which is the T-O-R transition temperature (), for the piezoceramic with (Figure 2(e)). The peak was slightly reduced to 40°C for this sample (). The above results clearly indicate that and (or ) decreased with an increase in (or Sb⁵⁺ content), resulting in the enhancement of the value at RT for the samples with , as shown in Figures 2(e) and 2(f). Hence, these samples are expected to yield small and values because they are inversely proportional to the value. In addition, the KN(N_1-zS_z)-BAZ-SZ piezoelectric ceramics () are expected to exhibit typical ferroelectric properties because they show normal polarization versus electric field () hysteresis curves, as displayed in Figures S3(a)–(g) that show the hysteresis curves of the untextured KN(N_1-zS_z)-BAZ-SZ piezoceramics.

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Figure 3(a) provides several physical characteristics of the untextured KN(N_1-zS_z)-BAZ-SZ piezoceramics () sintered at 1090°C for 6 h. All the specimens exhibited a comparatively large relative density (≥95% of the theoretical density), indicating that they may be well densified at 1090°C. The of the piezoceramic () was small (933), and it slightly increased with the increase in to 1676 for the specimen with . However, it increased considerably when exceeded 0.05, and the piezoceramic () exhibited a large value of 5616. The increase in the value of the samples (≥0.06) may be attributed to the decrease in and the presence of the peak close to RT, as shown in Figures 2(e) and 2(f). The dielectric loss () of the samples was less than 0.035, implying that its effect on the piezoelectric properties could be limited. Further, the value of the specimen () was small at 160 pC/N, and it was enhanced with an increase in . Subsequently, the maximum of 660 pC/N was obtained for the piezoelectric ceramic with , because it had an ideal T-O-R structure, where each structure exhibited a similar proportion of approximately 33% (Figure 1(e)). This value is similar to the largest value obtained for untextured KNN-based piezoceramics [21]. It decreased when exceeded 0.05, but the piezoelectric ceramic () maintained a relatively large value of 580 pC/N.

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The of the piezoceramic () was small (0.35), possibly owing to its small value. The value enhanced with the increase in , and the piezoelectric ceramics () exhibited the value in the range of 0.54–0.57 possibly owing to their small value. However, when exceeded 0.05, despite the increase in the value, the value decreased below 0.5, owing to their large value. The values of the piezoelectric ceramics are shown in Figure 3(a). The sample () exhibited a low value (3.1 pm²/N), which increased with increasing , although the enhancement was not that large. Hence, all the samples showed relatively low values, probably because the samples () had a small value, whereas the samples () had a large value. Figure 3(b) shows the electric field-induced strain () plots of the untextured KN(N_1-zS_z)-BAZ-SZ piezoceramics with , and the largest supplied electric field was 4.0 kV/mm. The piezoceramic () exhibited a small strain of 0.081%, which was enhanced with an increase in . However, the enhancement of the strain was not large, and the piezoceramics with also exhibited a comparatively small strain of approximately 0.133%. Hence, the strain of untextured specimens must be enhanced for application in piezoelectric actuators.

The untextured KN(N_1-zS_z)-BAZ-SZ piezoelectric ceramics () showed relatively small and values, although they had small value. Thus, their piezoelectric properties must be improved for PEH applications. Therefore, the KN(N_1-zS_z)-BAZ-SZ piezoelectric ceramics () were textured toward the [001] orientation to ameliorate their piezoelectricity without the increase in the value. Figure 4(a) shows the XRD patterns of the [001]-textured KN(N_1-zS_z)-BAZ-SZ piezoelectric ceramics () using 3.0 mol% NNO seeds. All samples exhibited large (001) and (002) peaks. Further, Lotgering factor () of samples was calculated using the XRD patterns to determine the degree of texturing, and all the specimens yielded large s of ≥98% (Figure 4(b)). Hence, all the specimens were well textured along the [001] orientation with 3.0 mol% NNO seeds. The electron backscatter diffraction (EBSD) image was obtained from the piezoceramic (), as shown in Figure 4(c). Most of the grains were oriented along the [001] direction (red parts), and the black parts may be defects developed on the surface of the [001]-textured sample. Furthermore, Figure 4(d) shows the (001) pole figure image of the sample (), which indicates the high intensity of the (001) reflection. Hence, the piezoceramic with was well textured along the [001] orientation, and identical results are expected from the other textured samples.

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The Rietveld refinement was conducted on the XRD patterns of the [001]-textured KN(N_1-zS_z)-BAZ-SZ piezoelectric ceramics () to identify their crystal structures, as shown in Figures 5(a)–5(f). Table 2 lists the physical parameters obtained from the Rietveld refinement. The piezoceramic with exhibits a T-O structure with a P4mm (99) T structure (9.3%) and an Amm2 (38) O structure (90.7%), as shown in Figure 5(a). In contrast, the piezoceramic with shows a T-O-R structure comprising the P4mm (99) T structure (12.6%), the Amm2 (38) O structure (82.6%), and the R3m (160) R structure (4.8%) (Figure 5(b)). When was enhanced, the portion of the O structure was reduced, but those of the T and R structures were enhanced (Figures 5(b)–5(f)). Hence, the textured piezoceramic with exhibited a T-O-R structure with a small portion of the O structure and a large portion of the T and R structures (Figure 5(f)). The XRD peaks at 2θ–66.5°, measured using the slow scanning technique and deconvoluted using the Voigt function, exhibited identical results (Figures S4(a)–(f); XRD reflections at 2θ = ~66.5° measured by the low-speed scanning method and deconvoluted using the Voigt function for the textured KN(N_1-zS_z)-BAZ-SZ piezoceramics). Therefore, the crystal structure of the textured KN(N_1-zS_z)-BAZ-SZ piezoelectric ceramics was altered from the T-O structure to the T-O-R structure with an increase in the Sb⁵⁺ content (or value). This indicated that the structures of the textured samples were the same as those of the untextured samples. The microstructures of the textured KN(N_1-zS_z)-BAZ-SZ piezoelectric ceramics () were investigated using SEM, as shown in Figures 6(a)–6(f). Most of the grains in the samples were aligned along the [001] direction, indicating that all samples were well textured toward the [001] direction. The average grain size of the samples was nearly 35 μm, and the variation in grain size with an increase in was negligible. In addition, all the samples show the holes indicated by the arrows in Figures 6(a)–6(f), and they were located where the NNO templates existed. Hence, it may be assumed that holes were produced, possibly because of the diffusion of the NNO seeds into the matrix of the sample. However, further studies are necessary to understand the formation mechanism of the holes.

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Figures 7(a)–7(f) show the plots of the textured KN(N_1-zS_z)-BAZ-SZ piezoceramics (), and all the samples provide the normal hysteresis plots. The changes in the , , and of the textured piezoceramics are shown in Figure 7(g). The sample () exhibited large , , and values of 24.9 μC/cm², 20.8 μC/cm², and 0.95 kV/mm, respectively. However, the , , and values were slightly reduced with the increase in (0.08) to 15.0 μC/cm², 9.4 μC/cm², and 0.55 kV/mm, respectively. Further, the and values of the [001]-textured piezoceramics are smaller than those of the untextured piezoceramics (Figures S3(a)–(g)) owing to the averaging of the polarization of each domain in three-dimensional space, and the identical results were obtained in the [001]-textured KNN-related and Na_1/2Bi_1/2TiO₃-BaTiO₃ ceramics [24, 30]. In addition, the decrease in the value was relatively small for the samples with but was large for the samples with (Figure 7(g)). When an electric field is applied to the [001] direction, the piezoceramics with the O and R structure have large values, whereas the piezoceramic with the T structure has a relatively small value [24, 30]. The proportion of the T structure increased as increased (Figures 5(a)–5(f)). Moreover, for the samples with , the increase in the proportion of the T structure was large compared with that of the samples with , as shown in Figure S4(g), which shows the proportions of the O, R, and T structures in the KN(N_1-zS_z)-BAZ-SZ piezoceramics. Therefore, the large decrease in the value for the samples with can be explained by the large increase in the proportion of the T structure. The plots of the samples are shown in Figures 7(a)–7(f). Two peaks were observed in the plots because of the domain switching with the supplication of an electric field. Therefore, the curve results also suggested that textured samples exhibit good ferroelectric characteristics. In addition, the versus temperature curves obtained from the textured samples were similar to those of the untextured samples (Figures S5(a)–(f); versus temperature curves for the textured KN(N_1-zS_z)-BAZ-SZ piezoceramics).

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Various physical characteristics of textured KN(N_1-zS_z)-BAZ-SZ piezoelectric ceramics () were investigated, as shown in Figure 8(a). All the samples had a slightly low relative density (92–95% of the theoretical density), probably owing to the existence of holes. The of the piezoceramic () was low at 780 and was enhanced with the increase in (0.08) to 2990 owing to the decrease in the and the presence of the peak close to RT. However, of the [001]-textured sample was smaller than that of the untextured sample (Figure 3(a)). Consequently, the decrease in the value of the textured samples may have contributed to the increase in their and values. All the samples () exhibited a relatively small value in the range of 0.02–0.04; thus, its effect on the piezoelectric properties could be minimal. The piezoceramic with provided an improved value of 415 pC/N, which was enhanced with an increase in , and the largest of 705 pC/N was detected in the sample with . However, it decreased as exceeded 0.05, and the textured piezoceramic () exhibited a decreased of 460 pC/N. The increase of following the [001]-texturing was large for the samples with (Figure S6 shows the increase of the value after [001]-texturing for KN(N_1-zS_z)-BAZ-SZ piezoceramics) because they exhibited a T-O-R structure with a small amount of T structure (or a large amount of the O-R structure). However, the increase in was insignificant after [001]-texturing for the samples with (Figure S6), because they had a T-O-R structure with a relatively large T structure. Similar results were also reported in previous study [22].

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The variation in the value is shown in Figure 8(a), and the piezoceramic () yielded a large of 0.77, which is the largest value obtained from the KNN-based piezoceramics reported in the literature [22, 25, 27]. Moreover, the samples with also exhibited large values in the range of 0.71–0.75. The large values of these samples can be explained by their small and increased values because the value was proportional to the value. was considerably reduced when exceeded 0.04, which may be owing to its large value. Figure 8(a) shows the values of the samples. The piezoceramic () exhibited a relatively large value of 24.9 pm²/N, and it was enhanced with 36.7 pm²/N when was increased to 0.05 because it has the largest (705 pC/N). However, it decreased when was larger than 0.05 because of the decreased and large values. In addition, the sample () yielded a large (31.2 pm²/N).

Figure 8(b) shows the electric field-induced strains of the textured KN(N_1-zS_z)-BAZ-SZ piezoelectric ceramics (), and all the samples exhibited a large strain (>0.15%) at 4.0 kV/mm. The piezoelectric ceramic (), which had the largest , also exhibited the largest strain of 0.183%. Moreover, the piezoceramic (), which provided the maximum , yielded a comparatively large strain of 0.16%. Therefore, textured KN(N_1-zS_z)-BAZ-SZ piezoceramics () can be used for piezoelectric actuators. Figure 8(c) shows the and of the piezoceramic () detected at different temperatures. was directly measured at the corresponding temperature; however, of the specimen was measured after annealing the specimen in an oven. A large of 0.77 was maintained up to 60°C. However, beyond this, it reduced to 0.54 at 90°C probably due to the presence of the O-T phase transition at approximately 91°C which induced an increase in the value. Further, the relatively large of 0.54 slightly decreased to 0.48 at 280°C, and it suddenly decreased to almost zero when the measuring temperature exceeded 280°C because the was close to 280°C. The value of this sample was approximately 500 pC/N at RT and continuously decreased to 374 pC/N at 280°C. In contrast, the effect of the O-T phase transition on the value was not significant. Further, the piezoceramic strain () was also detected at different temperatures (Figure 8(d)). The strain at RT was 0.16%, which increased with the measuring temperature to 0.2% at 125°C, and it slightly reduced to 0.17% at 225°C. The strain was expected to be reduced when the measuring temperature exceeded 280°C; however, the strain could not be measured at temperatures higher than 225°C because of the limitations of the facility. The above results show that the sample with maintained relatively large piezoelectric properties up to 270°C, although the value was slightly reduced after 60°C because of the enhancement of the owing to the presence of the O-T phase transition. The fatigue characteristics of the specimens were also investigated. Figure 8(e) shows the unipolar plots of the textured KN(N_1-zS_z)-BAZ-SZ piezoceramic () detected after a number of electric field cycles. The inset of Figure 8(e) shows the electric field used for the fatigue test (the largest electric field of 1.0 kV/mm at 100 Hz). Because the applied electric field is similar to the of this piezoceramic (1.0 kV/mm), it was sufficiently large for the fatigue test [31]. The strain of the piezoceramic (0.16% at 4.0 kV/mm) did not change after 10⁶ cycles, indicating that this specimen exhibited outstanding fatigue characteristics.

Textured KN(N_1-zS_z)-BAZ-SZ piezoelectric ceramics () were used to fabricate cantilever-type PEHs. Figure S1(c) shows the schematic of the PEH. The stress developed in the PEH was calculated using COMSOL, as illustrated in Figure 9(a). The PEH manufactured in this study corresponded to a type I PEH, wherein the piezoceramic was affixed to the metal plate away from the clamped part. Hence, mechanical stress was generated in the metal plate and conveyed to the piezoceramic in the PEH [9, 32]. The equivalent circuit of the PEH and voltage measurement system are shown in Figure 9(b). The voltage outputs of PEHs manufactured using textured KN(N_1-zS_z)-BAZ-SZ piezoceramics () detected at different frequencies are shown in Figure 10(a), and they were measured at load resistance () of 0.91 MΩ. The was calculated using the following equation: , where is the applied resistance. The maximum that can be applied with the present setup is 10 MΩ, and the of 0.91 MΩ can be obtained at MΩ. Hence, the of 0.91 MΩ was used to obtain the output voltage. Because 0.91 MΩ is slightly less than 1.0 MΩ, corresponding to the open circuit voltage condition, the of 0.91 MΩ is considered to be good enough to obtain the output voltages. All PEHs exhibited the maximum voltage output at approximately 120 Hz, corresponding to the resonance frequency of the PEHs. Figure 10(b) shows the root-mean-square values of the voltages () obtained at s and a resonance frequency of 120 Hz. The PEH manufactured utilizing the sample () exhibited the largest of 48.4 V at an of 0.91 MΩ. The current output of the PEHs calculated using is shown in Figure 10(c), and all the PEHs exhibited the largest output current at 1.0 kΩ. In particular, the PEH manufactured using the sample () showed a maximum current of 200 μA because of its large value. Moreover, the PEH manufactured using this sample () also showed a relatively large output current of 177 μA. Figure 10(d) shows the change in the output power as a function of for the PEHs manufactured using the [001]-textured KN(N_1-zS_z)-BAZ-SZ piezoceramics (). A maximum power output of roughly 3.21 mW was obtained at an of 0.32 MΩ from the PEH manufactured using the sample (). According to a previous study, the output power at the resonance frequency for type I PEH is mostly influenced by the value [9]. Figure 10(e) shows the power densities of the PEHs and values of the textured piezoceramics. The change in the former was exactly the same as that of the latter. Hence, it can be concluded that the PEH manufactured using the sample () exhibits the largest output power because it has the largest value of 0.77. The output power density of the PEH manufactured using this sample was approximately 7.86 mW/cm³. Table 3 lists the output power densities of PEHs manufactured using various PZT- and KNN-related piezoelectric ceramics. The output power density of the PEH manufactured using the sample () was the highest because it had a very large value. Therefore, the textured KN(N_1-zS_z)-BAZ-SZ piezoceramic () is a promising piezoceramic for PEH.

(a)

(b)

(a)

(b)

(c)

(d)

(e)

The values of the PEHs were measured at 200 Hz (off-resonance frequency), and Figure 11(a) shows the change in the value with respect to at 200 Hz. The of the PEH was low at a small value, increased with an increase in , and saturates when exceeded 0.5 MΩ. The PEH manufactured using the sample () showed the largest of 1.3 V. The output current of the PEH was calculated using the at 200 Hz (Figure 11(b)). A large output current was obtained at a low value, and the PEH manufactured using this sample () also showed a maximum output current of 6.6 μA at 1.0 kΩ. Figure 11(c) shows the power output of the various PEHs measured at different values and 200 Hz. The PEH manufactured utilizing the piezoceramic () yielded the maximum output power of approximately 4.57 μW at of 0.2 MΩ. At the off-resonance frequency, the power of type I PEH has been reported to be influenced by the value of the piezoceramic of the PEH [9, 22]. The power densities of the PEH at 200 Hz and of the textured piezoelectric ceramics are shown in Figure 11(d). The variation in the former was identical to that in the latter. Therefore, it can be concluded that the output powers at the off-resonance frequency are influenced by the value of the textured KN(N_1-zS_z)-BAZ-SZ piezoceramics (). Notably, the matching impedance at the off-resonance frequency is different from that at the resonance frequency. According to a previous study, the type 1 PEH can be regarded as a piezoelectric material with a low piezoelectric resonance frequency [9]. Moreover, the impedance at the resonance frequency is different from that at the off-resonance frequencies. Thus, the matching impedance at the off-resonance frequency is different from that at the resonance frequency.

(a)

(b)

(c)

(d)

4. Conclusions

The and values of the untextured KN(N_1-zS_z)-BAZ-SZ piezoceramics () are small, which results in small and values. Thus, this study textured these samples along the [001] orientation with 3.0 mol% NNO seeds to enhance their and values for application to PEH. The structure of the textured KN(N_1-zS_z)-BAZ-SZ piezoelectric ceramics () changed from the T-O to the T-O-R structures with the increase in . The [001]-textured piezoceramic () provided a large of 0.77, which was the largest value among the KNN-based piezoelectric ceramics reported in the literature. The large value can be explained by its small and the increased value due to [001]-texturing. Further, the cantilever-type PEHs were manufactured using the textured KN(N_1-zS_z)-BAZ-SZ piezoceramics (). A maximum output power of 3.21 mW was obtained at the resonance frequency from the PEH manufactured using the sample () owing to its large value. The power density of the PEH manufactured using the sample () was approximately 7.86 mW/cm³, which, to the best of our knowledge, is the largest power density of the PEHs manufactured by the KNN-based piezoceramics. Therefore, the sample () is a promising candidate for PEH, and [001]-texturing is a suitable approach for developing piezoceramics for PEH. In addition, the output power density of the PEH was influenced by the value at off-resonance frequency, and the piezoceramic () exhibited the maximum output power of 4.57 μW at 200 Hz because of its large .

Data Availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Authors’ Contributions

Yeon-Gyeong Chae was responsible for the experiment, data curation, and validation. Seok-June Chae was responsible for the conceptualization, investigation, data curation, experiment, validation, and writing—original draft and review and editing. Su-Hwan Go was responsible for the investigation and validation. Eun-Ji Kim was responsible for the data curation and validation. Seok-Jung Park was responsible for the experiment and validation. Hyunseok Song was responsible for the experiment and investigation. Sahn Nahm was responsible for the conceptualization, methodology, investigation, data curation, writing (original draft and review and editing), supervision, and funding acquisition. Yeon-Gyeong Chae and Seok-June Chae contributed equally to this work.

Acknowledgments

This work was supported by the National R&D Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (Project No. 2020M3H4A3105596).

Supplementary Materials

Figure S1: (a) XRD pattern and (b) SEM image of the NNO templates. (c) Schematic of the cantilever PEH. Figure S2: (a) XRD patterns of the untextured KN(N_1-zS_z)-BAZ-SZ piezoceramics (). SEM images of the untextured KN(N_1-zS_z)-BAZ-SZ piezoceramics with (b) , (c) , (d) , (e) , (f) , and (g) . Figure S3: hysteresis curves of the untextured KN(N_1-zS_z)-BAZ-SZ piezoceramics with (a) , (b) , (c) , (d) , (e) , and (f) . (g) , , and values of the untextured specimens. Figure S4: XRD reflections at ° measured by the low-speed scanning method and deconvoluted using the Voigt function for the textured KN(N_1-zS_z)-BAZ-SZ piezoceramics with (a) , (b) , (c) , (d) , (e) , and (f) . (g) Proportions of the O, R, and T structures in the KN(N_1-zS_z)-BAZ-SZ piezoceramics (). Figure S5: the versus temperature curves for the textured KN(N_1-zS_z)-BAZ-SZ piezoceramics with (a) , (b) , (c) , (d) , (e) , and (f) . The inset shows the versus temperature curves measured over a wide temperature range between -50 and 350°C. Figure S6: (a) values of the untextured and textured KNN(N_1-zS_z)-BAZ-SZ piezoceramics with and (b) the increasing rate of value () after the [001]-texturing and the amount of the T structure. (Supplementary Materials)

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