Antimony Oxide-Doped 0.99Pb(Zr_0.53Ti_0.47)O₃–0.01Bi(Y_1−xSb_x)O₃ Piezoelectric Ceramics for Energy-Harvesting Applications

Abstract

The effects of doping antimony oxides (Sb₂O₃/Sb₂O₅) on the ferroelectric/piezoelectric and energy-harvesting properties of 0.99Pb(Zr_0.53Ti_0.47)O₃–0.01BiYO₃ (PZT–BY) have been studied. The feasibility of doping Sb₂O₃ and Sb₂O₅ into the PZT–BY ceramics has also been compared by considering factors such as sintering condition, grain size, density, and electrical properties etc. This work discusses a detailed experimental observation using Sb₂O₃, because Sb₂O₅ is relatively expensive and does not follow the stoichiometric reaction mechanism when doped in PZT–BY. The Sb₂O₃-doped specimens were well sintered by oxygen-rich sintering and reached a maximum density of 99.1% of the theoretical value. X-ray diffraction (XRD) analysis showed a complete solid solution for all the specimens. Scanning electron microscope (SEM) observation revealed that the addition of Sb₂O₃ inhibits grain growth, and exhibits a denser and finer microstructure. The 0.1 moles of Sb₂O₃-doped ceramic shows a sharp decrease in the dielectric constant (ε₃₃^T = 690), while the piezoelectric charge constant (d₃₃) and electromechanical coupling factor (k_p) maintained high values of 350 pC/N and 66.0% respectively. The relatively higher value of d₃₃ and lower ε₃₃^T of the 0.99Pb(Zr_0.53Ti_0.47)O₃–0.01Bi(Y_0.9Sb_0.1)O₃ ceramic resulted in an optimum value of piezoelectric voltage constant (g₃₃ = 57.4 × 10⁻³ Vm/N) and a high figure of merit (d₃₃ × g₃₃ = 20075 × 10⁻¹⁵ m²/N). These values are high compared to recently reported works. Therefore, Sb₂O₃-doped PZT–BY ceramic could be a promising candidate material for the future study of power-harvesting devices.

1. Introduction

The use of renewable energy from different technologies to make a contribution to the world’s total energy supply has grown fast. Among different energy conversion technologies, piezoelectric energy-harvesting has received considerable attention for powering small electronic components. The piezoelectric effect is a unique property that allows materials to convert mechanical energy to electrical energy or vice versa. Piezoelectric material selection is crucial in the design of piezoelectric energy harvesters. Numerous researchers are working in this area, but most of the solutions being employed have low power (nW) and high frequency [1,2]. Among the various piezoelectric materials, lead zirconate titanate (PZT) based ceramic is one of the most widely used because of its fast electro-mechanical response, relatively low power requirements, high generative force, and inherent durability [3,4,5,6,7]. Extensive research and development of PZT-based ceramics have been conducted to find such piezoelectric materials [1,7,8,9,10]. A variety of parameters is used to compare piezoelectric materials for energy-harvesting, such as electromechanical coupling factor, piezoelectric charge constant, output voltage, power density, energy-conversion efficiency, and cost and frequency of operation. Among them, energy-conversion efficiency is the most important when designing piezoelectric energy harvesters for real applications [11]. Xu et al. proposed a new figure of merit (FOM), which combines the transduction efficiency and the energy-conversion capacity, for comparing piezoelectric materials for energy-harvesting applications [11]. Priya et al. proposed a FOM for a polycrystalline piezoelectric energy-harvesting device, which can be expressed by the following Equations (1) and (2) [12]:

u = 1/2 (d.g) (F/A)²

(1)

g = d/ε₃₃^T

(2)

where d is the piezoelectric charge constant, g is the piezoelectric voltage constant, ε₃₃^T is the dielectric permittivity, F is the applied force, and A is the area.

According to Equations (1) and (2), a material with a high d and a lower ε₃₃^T will generate high FOM. In general, the piezoelectric charge constant and the dielectric constant increase or decrease concurrently upon modification with dopants or processing techniques [7,8,9,10]. However, by modifying the fabrication technique and choosing suitable dopants, it is possible to produce high d₃₃ with low ε₃₃^T. Previous work has reported on such a piezoelectric ceramic composition of (1 − x)Pb(Zr_0.53Ti_0.47)O₃–xBiYO₃ [PZT–BY] and its fabrication process [13].

In the present study, PZT–BY composition is further tuned using antimony oxide (both Sb₂O₃ and Sb₂O₅) to improve the FOM. The Sb₂O₃- and Sb₂O₅-doped PZT has had a long history whereby Sb₂O₃/Sb₂O₅ was doped with other oxides for tailoring piezoelectric/dielectric properties [14,15,16,17,18,19,20,21,22]. Lonker et al. reported a high electrical output in the lanthanum-doped Pb(Ni_1/3Sb_2/3)–PbZrTiO₃ system which was comparable to the PZT type 5A [16]. Zhou et al. reported a high electrical output in the Sb₂O₃-doped Pb(Zr_0.54Ti_0.46)O₃ system [14]. They explained that the ionic radius of Sb³⁺ is smaller than that of other trivalence “soft” ions; this may be the most important factor for improving the piezoelectric activity. S. Zahi et al. reported a high electromechanical coupling factor and dielectric constant with relatively higher T_c in the xPbZrO₃–(90 − x)PbTiO₃–10Pb(Ni_1/3, Sb_2/3)O₃ ternary system [17]. Dutta et al. studied the structural and electrical properties of Sb³⁺-modified [Pb_0.92(La_1−zSb_z)_0.08(Zr_0.65Ti_0.35)_0.98]O₃ (PLZT) ceramics and provided some interesting ferroelectric and sensing properties [20,21]. Furthermore, Zhou et al. studied the effect of doping Sb₂O₃ in Pb(Zr_0.54Ti_0.46)O₃ and obtained an optimum value of piezoelectric activity with d₃₃ = 570 × 10⁻¹² C/N and piezoelectric voltage constant, g₃₃ = 32 × 10⁻³ Vm/N [14].

In this work, the effects of doping Sb₂O₃ and Sb₂O₅ on 0.99Pb(Zr_0.53Ti_0.47)O₃–0.01Bi[Y_(1−x)Sb_x]O₃ (abbreviated as PZT–BYS(x) from now on, where 0 ≤ x ≤ 0.6) ceramics have been compared in terms of sintering condition and piezoelectric/dielectric properties. Based on comparative studies, Sb₂O₃ is selected as a dopant into the PZT–BYS(x) system. Therefore, detailed experimental observations of Sb₂O₃ doping into PZT–BYS(x) regarding piezoelectric/dielectric, ferroelectric and energy-harvesting properties are evaluated and discussed.

2. Materials and Methods

At the first stage, a precursor of PbZr_0.53Ti_0.47O₃ (PZT) and Sb₂O₃/Sb₂O₅-doped BiY_1−xSb_xO₃ [BYS(x), where x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6] was prepared by a conventional solid state method. In the final stage, the precursors of PZT and BYS(x) was mixed stoichiometrically to fabricate 0.99Pb(Zr_0.53Ti_0.47)O₃–0.01Bi[Y_(1−x)Sb_x]O₃ (abbreviated as PZT–BYS(x)) containing various amounts of Sb₂O₃/Sb₂O₅ (0 ≤ x ≤ 0.6). The pre-synthesis method was considered for the fabrication of PZT–BYS(x) to eliminate the effects associated with the direct co-doping. The stoichiometric amount of the analytical reagent (AR) grade Bi₂O₃ (99.9%, High Purity Chemicals, Japan), Sb₂O₃ (99.9%, High Purity Chemicals, Japan) and Y₂O₃ (99.9%, High Purity Chemicals, Japan) were weighed to synthesize Bi(Y_1−xSb_x)O₃ [BYS(x)] (where, x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6). The BYS(x) was then ball milled in distilled water using a ZrO₂ media for 24 h. The same specimens of BYS(x) were also prepared by using Sb₂O₅ (99%, Samchun Pure Chemical, Mokock-dong Pyeongtaek, Korea). According to the thermogravimetric and differential scanning calorimetry (TG/DSC-Mettler Toledo) analysis, both Sb₂O₃ and Sb₂O₅-doped BYS(x) powders were calcined at 800 °C for 2 h in alumina crucibles. A detailed synthesis process of Pb(Zr_0.53Ti_0.47)O₃ (PZT) and BiYO₃ (BY) were described in previous work [13]. The pre-synthesized PZT and BYS(x) powders were weighed according to reaction stoichiometry to fabricate PZT–BYS(x), and ball-milled for 72 h. The dried powders were pressed as a disk of 15 mm diameter and then cold isostatically pressed (CIP) under a pressure of 147.2 MPa. The pressed pellets were sintered at various temperatures between 1050 °C and 1250 °C in air and an oxygen (O₂) rich atmosphere. An equimolar mixture of PbO and ZrO₂ inside a covered alumina crucible was used to limit PbO loss from pellets during sintering. The sintered specimens were then polished to obtain parallel surfaces and suitable dimensions (diameter/thickness ≥15) to estimate the piezoelectric properties.

The specimens were characterized using an X-ray diffractometer (XRD, D/Max-2500H, Rigaku, Tokyo, Japan and D8 advance, Bruker, Karlsruhe, Germany) with Cu Kα radiation after sintering. The morphologies and the microstructures for all of the powders and sintered samples were investigated using a scanning electron microscope (Hitachi S-2400, Tokyo, Japan). In order to measure the electrical properties, we used silver paste to form electrodes on both sides of the sample, which was then fired at 560 °C for 30 min. For investigating the proper poling conditions, we poled each specimen in stirred silicon oil at 120 °C by applying a DC electric field of 3–6 kV/mm for 45 min; subsequently, the sample was aged at 120 °C for 3 h. The dielectric and the piezoelectric properties of the aged samples were then measured and evaluated. A preliminary study indicated that the spontaneous polarization was fully saturated under a DC electric field of 4 kV/mm. Therefore, the optimum poling voltage was chosen as 4.0 kV/mm. A standard deviation of the electric properties could exist in the measured samples, so five specimens were prepared for each batch and tested. The piezoelectric coefficient was determined using a d₃₃ meter (IACAS, Model ZJ-6B, Bolingbrook, IL, USA), and the electromechanical and the dielectric properties were calculated by using a resonance/anti-resonance measurement method [15] and an impedance/gain phase analyzer (HP-4194A, Palo Alto, CA, USA). The temperature dependences of the dielectric constant and the dissipation factor over the temperature range from −25 to 500 °C were measured using an automated system at 1 kHz, in which an HP-4194A and temperature-control box (−40 °C to 150 °C: Delta 9023 chamber, 150–500 °C: Lindberg tube furnace) were controlled by using a computer system. The temperature was measured using a Keithley 740 thermometer via a K-type thermocouple mounted on the samples. The polarization-electric field (P-E) behavior was determined using a Precision LC system (Radiant Technology, Model: 610E, Walden, NY, USA).

3. Results and Discussion

3.1. X-ray Diffraction (XRD) Analysis of Pre-Synthesized PZT and Bi[Y_(1−x)Sb_x]O₃

Figure 1a,b shows the X-ray diffraction (XRD) patterns of pre-synthesized Sb₂O₃-doped Bi[Y_(1−x)Sb_x]O₃ [BYS(x)] (where x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) and Pb(Zr_0.53Ti_0.47)O₃ [PZT], respectively. For clarity, only Sb₂O₃-doped BYS(x) is presented here. The reaction mixture of BYS(x) was characterized by thermogravimetry and differential scanning calorimetry (TG–DSC) to locate the formation temperature. Based on the TG–DSC analysis, undoped BiYO₃ (BY) and PZT was calcined at 850 °C, whereas both Sb₂O₃ and Sb₂O₅-doped BY was calcined at 800 °C for 2 h. In Figure 1a it can be seen that the major phase formed for the BYS(x) could be related only to the diffraction peak given for BiYO₃ (JCPDS No. 271047). However, a tiny peak of Y₃Sb₅O₁₂ [JCPDS No. 0430160], Bi₂O₃ [JCPDS No. 0270052], Y₂O₃ [JCPDS No. 0431036], and BiSbO₄ [JCPDS No. 0480469] was observed in the XRD patterns. Moreover, it can be seen that the intensity and the number of second phases increase with increasing Sb₂O₃ content. Therefore, the XRD studies indicate that the solubility of Sb₂O₃ in BY is limited to a certain range. According to Shannon’s effective radius [23], Y³⁺ and Sb³⁺ with a coordination number of six have radii of 0.90 and 0.76 Å, respectively. The difference in radii between Y³⁺ and Sb³⁺ ions is greater than 15%. Therefore, it is not easy for Sb³⁺ to enter into Y³⁺, and the solubility of Sb³⁺ in the system would be limited. On the other hand, the calcined PZT powder was fully stabilized to the perovskite structure (JCPDS No. 01-070-4264) without any second phases.

3.2. Comparative Study of Sb₂O₃ and Sb₂O₅ Doping on the PZT–BY System

0.99Pb(Zr_0.53Ti_0.47)O₃–0.01Bi[Y_(1−x)Sb_x]O₃ (abbreviated as PZT–BYS(x) from now on, where x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 moles) ceramics were sintered in air and an O₂-rich atmosphere to investigate the Sb₂O₃ and Sb₂O₅-doping effects on the microstructure and electrical properties. Figure 2 shows the microstructure for the fracture surfaces of Sb₂O₃ and Sb₂O₅-doped PZT–BYS(0.1) ceramic sintered at 1170 °C in different sintering atmospheres. In addition, the piezoelectric, dielectric, ferroelectric and energy-harvesting parameters of same specimens have been listed in

Table 1. The piezoelectric, dielectric and energy-harvesting parameters of Sb₂O₃ and Sb₂O₅-doped PZT-BYS(0.1) ceramics sintered in air and in an O₂-rich atmosphere. PZT-BYS: 0.99Pb(Zr_0.53Ti_0.47)O₃–0.01Bi[Y_(1−x)Sb_x]O₃.

Doping	Sintering Temp. (°C)	Sintering Condition	Grain Size (µm)	Density (g/cc)	d33 (pC/N)	ε33T	kp (%)	g33 × 10−3 (Vm/N)	d33 × g33 (10−15 m2/N)
Sb2O5	1170	air	2.4	7.94	346	753	65.1	51.9	17,971
Sb2O3		air	1.4	7.88	335	793	63.6	47.7	15,987
Sb2O3		oxygen	2.3	7.96	350	738	64.8	53.3	18,519

. From Table 1, it is clear that the piezoelectric properties of Sb₂O₅-doped specimens sintered in an air atmosphere and Sb₂O₃-doped specimens sintered in an O₂-rich atmosphere show higher values than the Sb₂O₃-doped specimen sintered in air. It may be considered that the presence of oxygen during sintering reduced the porosity and resulted in dense microstructures. Therefore, among other reasons like pre-synthesis, doping, etc., sintering in an oxygen-rich atmosphere is an essential factor for relatively higher piezoelectric properties as well as higher FOM.

In Figure 2, uniform, fine-grained, and almost pore-free microstructures can be seen for Sb₂O₅-doped specimens when sintered in the air atmosphere and for Sb₂O₃-doped samples when sintered in the O₂-rich atmosphere. On the other hand, Sb₂O₃-doped specimens sintered in the air have small (approx. 1.4 µm) and loosely bonded grains. Since the dielectric properties are directly associated with grain size, the air-sintering of the Sb₂O₃ sample has a smaller average grain size and shows a higher value of ε₃₃^T. A similar observation was also explained by Uchino et al. [24]. Subsequently, higher ε₃₃^T produces a lower FOM according to Equation (1), therefore, doping Sb₂O₃ and sintering in the presence of air is not favorable or advantageous in the present case. On the contrary, Sb₂O₅ is relatively expensive and does not follow the stoichiometric reaction mechanism when doped in PZT–BY. Therefore, according to the experimental results and discussions above, Sb₂O₃ is considered as a substituent for Y³⁺ in the PZT–BYS(x) and also an O₂-assisted sintering atmosphere was selected. Hence, a detailed analysis of the crystal structure, microstructure, piezoelectric/dielectric properties, and ferroelectric properties of the Sb₂O₃-doped PZT–BYS(x) specimens sintered in the O₂ atmosphere will be discussed in the next sections.

3.3. Effects of Pre-Synthesized BYS(x) Content on the Densities of PZT–BYS(x) Ceramics

Figure 3 shows the density of PZT–BYS(x) ceramics sintered at various temperatures as a function of Sb₂O₃ content. It is well known that electrical properties are directly related to density. Therefore, a detailed experiment was conducted to find out the optimum sintering temperature at which these ceramics reached their maximum density. From Figure 3, it is clear that the ceramics are well sintered to high densities between 1150 and 1170 °C. At x = 0.1–0.3, PZT–BYS(x) ceramics showed the highest density at about 1150 °C. At x = 0.4–0.6, the density reached its maximum at 1170 °C. As the piezoelectric and dielectric properties are closely related to the microstructure, and sintered density, a detailed investigation of crystal structure, microstructure, piezoelectric/dielectric and ferroelectric properties of PZT–BYS(x) ceramics sintered at their optimum temperature will be considered and discussed in the next sections.

3.4. Effects of Pre-Synthesized BYS(x) on the Crystal Structure and Microstructure of PZT–BYS(x) Ceramics

The XRD patterns of PZT–BYS(x) ceramics sintered at their optimum temperatures are shown in Figure 4a. In these patterns, no detectable traces of the pyrochlore or other impurities were observed. Additionally, asymmetry and/or peak splitting in 111 and 200 reflections indicate all of the compositions are exhibiting rhombohedral–tetragonal phase coexistence [14,15,17]. Quantitative analysis of possible change in relative phase fraction as a function of BYS is not within the scope of this work. Nevertheless, the enlarged XRD patterns in Figure 4b indicated that the decrease in 002 reflection with an increasing amount of Sb₂O₃ is possibly related to the decrease in volume fraction of the long c-axis of the tetragonal unit cell.

Figure 5 shows the microstructures of the fracture surface containing various amounts of Sb₂O₃ content sintered at an optimum temperature at which maximum density was obtained. A significant change in grain size was observed with the addition of Sb₂O₃. The microstructures of the sintered samples are uniform, dense and consistent with the measured density. In the previous work, it was reported that the addition of a small amount of BY abruptly decreases the grain size of PZT due to the pinning effect of the second phase [13]. In the present case, the addition of Sb₂O₃ doping on PZT–BYS(x) shows a further decrease in grain size. The average grain size of specimens is decreased with the increasing amount of Sb₂O₃. From Figure 5b, it can be seen that the microstructure of the specimen doped with 0.1 moles of Sb₂O₃ shows higher uniformity with fine grain size, and an average grain size of 2.2 µm was observed. On further addition of Sb₂O₃ content, as shown in Figure 5c–g, the average grain size abruptly decreased to approximately ~1.7 µm at x = 0.2–0.3 moles and approximately 1.4 µm for x = 0.4–0.6 moles. This reduction of grain size can be interpreted as a pinning effect of the secondary phases (Y₃Sb₅O₁₂, Bi₂O₃, Y₂O_3, and BiSbO₄) as indicated in XRD patterns in Figure 1a.

The optimized density of sintered PZT–BYS(x) ceramics is listed in

Table 2. The ferroelectric, dielectric and piezoelectric properties of the PZT–BYS(x) ceramics at the optimum sintering temperature.

Composition	Sintering Temp. (°C)	Grain Size (µm)	ρ (g/cc)	d33 (pC/N)	ε33T	kp (%)	Pr (µC/cm2)	EC (kV/cm)	TC (°C)	tanδ (1 kHz) 25 °C [10−3]
PZT–BYS(0.0)	1150	2.5	7.91	324	742	63.5	45.6	11.5	373	0.0197
PZT–BYS(0.1)	1150	2.2	7.96	350	689	66	45.8	12.0	369	0.0213
PZT–BYS(0.2)	1150	1.7	7.95	352	774	65.5	46.3	12.9	365	0.0226
PZT–BYS(0.3)	1150	1.6	7.95	355	804	65.2	46.9	13.6	361	0.0201
PZT–BYS(0.4)	1170	1.6	7.91	360	835	64.9	47	13.8	361	0.0286
PZT–BYS(0.5)	1170	1.4	7.9	366	867	64.7	47.2	14.0	357	0.0215
PZT–BYS(0.6)	1170	1.4	7.86	370	920	64.5	47.8	14.1	354	0.0208

. The relative densities of the Sb₂O₃-doped specimens were higher than 98% of the theoretical density, indicating the development of dense microstructure in all of these compositions. However, it can be observed that the compositions with x = 0.1 show relatively higher density than other compositions. This may be attributed to the relatively uniform grain size with scarcer porosity at x = 0.1, which promotes densification. Further addition of Sb₂O₃ into the compositions decreases the density again. The SEM investigations, as in Figure 5, revealed supporting evidence that the ceramics with higher Sb₂O₃ (x > 0.1 moles) compositions contain small and loosely bonded grains. Evidently, this could be a reason for the much lower density of these compositions. Among all the compositions, a peak density of 7.96 g/cm³ (99.1% of the theoretical density) was obtained at x = 0.1.

3.5. The Piezoelectric and Dielectric Properties of PZT–BYS(x) Ceramics

Figure 6a,b shows the effects of Sb₂O₃ contents on d₃₃, k_p, ε₃₃^T and grain size. In addition, the electrical properties of PZT–BYS(x) samples sintered at their optimum temperature have been listed in Table 2. From Table 2, it is clear that doping Sb₂O₃ into the PZT–BY increased the d₃₃ and k_p sharply compared to the undoped specimen. This can be explained by factors like the density, grain size and soft effect of Sb₂O₃. Previously, it was observed that pre-synthesized BiYO₃ acts as a donor into the PZT ceramics and improves the piezoelectric and dielectric properties. In the present work, incorporating Sb₂O₃ into the PZT ceramic further improved the piezoelectric and dielectric properties because it is possible that the Sb³⁺ entered into the A-site and facilitated the domain wall movement. A similar observation was also reported by Dutta et al. where they doped Sb³⁺ into the Pb²⁺ site [20]. Additionally, The Sb₂O₃-doped specimen shows higher sintered density (7.96 g/cm³ at x = 0.1) than the undoped specimen (7.91 g/cm³ at x = 0.0). As a result, adding Sb₂O₃ into the PZT improved the d₃₃ and k_p sharply (at x = 0.1) compared to the undoped specimen (at x = 0.0). A detailed explanation of increasing piezoelectric, dielectric and ferroelectric behavior of the Sb₂O₃-doped sample will be discussed in the next paragraph.

In Figure 6a, it is clear that the d₃₃ and the k_p increase with increasing amount of Sb₂O₃. The increased density and homogeneous grain size might be a reason for high d₃₃ and k_p. The increasing behavior of d₃₃ with the addition of Sb₂O₃ into PZT was also explained by Zhou et al. [14]. In general, donor doping in PZT is expected to increase the d₃₃ and the k_p values, which is a soft effect [25]. In the previous work, it was observed that BiYO₃ acts as a grain growth inhibitor as well as a donor in the vicinity of grains which improve d₃₃ and k_p in the PZT–BY system [13]. In the present case, similar trends of d₃₃ and k_p were also observed. Furthermore, the increasing tendency of d₃₃ can be explained by the presence of second phases, as shown in Figure 1a.

It is possible that the second phases of Y₃Sb₅O₁₂, Bi₂O₃, Y₂O_3, and BiSbO₄, as shown in Figure 1a, dissociate at the higher sintering temperature. It can be speculated that Bi³⁺, Y³⁺ and Sb³⁺ enter into the Pb²⁺ site as donor ions in the PZT system to create A-site vacancies in the lattice. Similarly, a donor doping effect of Sb³⁺ was reported by Dutta et al. [20]. They studied the structural, dielectric and electrical properties of PZT ceramic extensively to find out the effect of double-doping (La³⁺ and Sb³⁺) at the Pb²⁺-site of PZT. Therefore, in the present work, A-site vacancies reduce the concentration of intrinsic oxygen vacancies during sintering and introduce lead vacancies to maintain the charge neutrality. These cation vacancies would, therefore, increase the mobility of the domain wall [26]. The change in the mobility of the domain wall by cation vacancies improves the d₃₃, because displacement is sufficient. Therefore, d₃₃ increases with increasing Sb₂O₃ content due to the increased concentration of the second phase in the PZT–BYS(x) system.

This increase in d₃₃ by doping was also observed in a typical soft type of PZT system [26,27]. On the contrary, the value of k_p reached the maximum value of 66.0% at x = 0.1, whereby it decreased slightly on further addition of Sb₂O₃. The grain size effect could be the predominant factor in this case. Ichinose and Kimura [28] reported that Nb and partially substituted Sb-doped PZT shows the maximum electrical properties at a grain size of 2–3 µm. The deterioration of k_p with decreasing grain size is probably related to the fewer domains and less mobile domain walls. As a result, a slight decrease in k_p was observed at higher Sb₂O₃ (x > 0.1) contents.

Figure 6b shows the variation of ε₃₃^T and grain size as a function of Sb₂O₃ contents. It is clear that ε₃₃^T varies from those of the piezoelectric properties. The ε₃₃^T sharply decreases as the grain size decreases from ~2.5 µm (at x = 0) to 2.2 µm (at x = 0.1), while it increases slightly with a decreasing grain size of ~1.7 µm at x = 0.2. A further increase in the Sb₂O₃ content decreases the grain size to ~1.4 µm, and dielectric constants increased sharply. This type of dielectric anomaly was previously explained by the effect of grain sizes [13,29,30,31,32,33,34]. Martirenat et al. [29] reported that the dielectric constant of BaTiO₃ and PZT could be varied at different ranges of grain size. Okazaki and Nagata [34,35] reported that ε₃₃^T at room temperature increases within a critical range of grain size. Similar behavior was also explained by many earlier investigations of grain size effects [35,36]. They reported that there was a microstructure transition region between 1.3 μm and 2.7 μm. Within this range, ε₃₃^T decreases with decreasing grain size and increases with increasing grain size [36]. Therefore, in the present work, ε₃₃^T decreases with the PZT–BYS(0.1) composition because grain size decreases from ~2.5 µm to 2.2 µm. As the grain size decreases below critical values, the structure is changed, and the internal stress would be expected to increase. This in turn suggests that the dielectric constant increases with decreasing grain size for higher Sb₂O₃ content.

Further evidence for the Sb₂O₃ doping and grain size effects are obtained by examining the polarization-electric field (P-E) curve at room temperature and shown in Figure 7a. The values of P_r and E_c are determined from the measured P-E loops. The variation of P_r and E_c for undoped and Sb₂O₃-doped PZT–BYS(x) samples sintered at optimum temperatures are shown in Figure 7b. At 0.1 moles of Sb₂O₃ content, a relatively higher P_r, and lower E_c were observed. The relatively higher P_r and lower E_c at x = 0.1 could be explained by high mobility of domain walls achieved by suppression of the oxygen vacancies. Upon further increase of Sb₂O₃ content, E_c and P_r increases sharply. Above 0.1 moles of Sb₂O₃, the grain size decreases from 2.2 μm to around 1.7~1.4 μm. As a result, the clamping effect caused by internal stress below a critical grain size increases the E_c. By contrast, the increasing behavior of P_r with increasing Sb₂O₃ content can be explained by the presence of second phases, which is similar to the behavior of d₃₃. It is believed that, at the higher sintering temperature, the second phases are dissociated. According to the similarities of ionic radius, the dissociated phase produces cation vacancies in PZT–BYS(x) as explained for the behavior of d₃₃ as a function of Sb₂O₃ content (see Figure 6a). As a result of cation vacancies, the mobility of the domain wall increases [26] due to the sufficient displacement, which finally increases the P_r. Therefore, P_r increases at higher Sb₂O₃ content due to the higher concentration of the second phase in the PZT–BYS(x) system. These results are in agreement with a previous analysis of piezoelectric/dielectric properties.

The dielectric constant (ε₃₃^T) of PZT–BYS(x) ceramics as a function of temperature is shown in Figure 8. The ε₃₃^T was almost constant up to 300 °C, and then increased gradually to a maximum value ε₃₃^T_(max) at Curie temperature (T_C). The T_C of pure PZT is about ~360 °C. It is well known that cubic phase addition to PZT reduces T_C, but in our recent work where BY was added into the PZT system, T_C increased [13]. This anomaly was explained in terms of internal stress, which retarded the ferroelectric and paraelectric transition. In the present case, Sb₂O₃-doping has been shown to produce a linear reduction in T_C. In Figure 8, it is clear that the ε₃₃^T_(max) sharply decreases as the content of Sb₂O₃ increases and the dielectric peaks broaden around T_C, which is similar to previous observations [13,37]. Similarly, Dutta et al. also explained that the broadening of the dielectric peak and variation of ε₃₃^T_(max) could be attributed to the variation of grain size and some kinds of structural disorder [20]. It has already been established that, with decreasing grain size, ε₃₃^T_(max) decreases, which results in the broadening of the dielectric peak, and shifts in ferroelectrics transition to higher or lower temperature sides [20,29,38].

3.6. Feasibility of Sb₂O₃-Doped PZT–BYS(x) Ceramics for Use in an Energy Harvester

The calculated values of g₃₃ and d₃₃×g₃₃ as a function of Sb₂O₃ contents are illustrated in Figure 9. The data of g₃₃ and d₃₃ × g₃₃ for PZT–BYS(x) has also been shown in

Table 3. The evaluated values of g₃₃ and d₃₃ × g₃₃ of PZT–BYS(x) ceramics as a function of Sb₂O₃ content at optimum sintered temperature.

Composition	Sintering Temp. (°C)	g33 × 10−3(Vm/N)	d33 × g33 (10−15 m2/N)
PZT–BYS(0.0)	1150	49.1	15,824
PZT–BYS(0.1)	1150	57.4	20,075
PZT–BYS(0.2)	1150	51.3	18,044
PZT–BYS(0.3)	1150	49.9	17,686
PZT–BYS(0.4)	1170	48.7	17,513
PZT–BYS(0.5)	1170	47.7	17,431
PZT–BYS(0.6)	1170	45.6	16,908

. From Figure 9, it is clear that the addition of Sb₂O₃ to the PZT–BYS(x) system significantly improved d₃₃ × g₃₃. In general, d₃₃ and ε₃₃^T increase or decrease simultaneously depending on doping or processing conditions [24]. In the present case, it is observed that increasing behaviors of d₃₃ and ε₃₃^T/ε₀ were not similar at x = 0.1. For a small amount of Sb₂O₃ (x = 0.1), a considerable increase in d₃₃ was observed, while ε₃₃^T/ε₀ decreased to its lowest value of 690. Therefore, at x = 0.1 moles of Sb₂O₃, the g₃₃ and d₃₃ × g₃₃ values reached a maximum value of 57.4 × 10⁻³ Vm/N and 20076 × 10⁻¹⁵ m²/N, respectively. On the other hand, the g₃₃ and d₃₃ × g₃₃ values decreased rapidly at x > 0.1 due to the considerably slower rate of increase in d₃₃ and rapid increase of ε₃₃^T/ε₀.

A list of piezoelectric energy-harvesting parameters reported recently is provided in

Table 4. A comparison between the present work and recently reported piezoelectric ceramic materials for energy-harvesting applications.

Composition	g33 × 10−3 (Vm/N)	d33 × g33 (10−15 m2/N)	TC (°C)	Ref.
0.9Pb(Zr0.56Ti0.44)O3–0.1Pb[(Zn0.8Ni0.2)1/3Nb2/3]O3 + 2mol% MnO2	83	18,456	~315	41
(0.65 + y)Pb(Zr0.47Ti0.53)–(0.35 − y)Pb[(Ni1−xZnx)1/3Nb2/3]O3	36	20,056	…	42
0.72Pb(Zr0.47Ti0.53)O3–0.28Pb[(Zn0.45Ni0.55)1/3Nb2/3]O3	35	13,051	…	43
Pb0.98La0.02(NiSb)0.05[(Zr0.52Ti0.48)0.995]0.95O3	36.8	16,570	…	16
0.99Pb(Zr0.53Ti0.47)O3–0.01BiYO3	53	18,549	373	13
0.99Pb(Zr0.53Ti0.47)O3–0.01Bi(Y0.9Sb0.1)O3	57.4	20,075	369	present

for comparison with the present results. In Table 4, clearly, PZT–BYS(x) possesses a considerably higher figure of merit with a high g₃₃ value in comparison to the commercially available and recently reported materials. Furthermore, most of the recent works reported a high figure of merit after doping relaxor-type materials into the PZT system. Doping relaxor-type materials into the PZT system evidently causes an abrupt decrease in the T_C, which limits the application temperature range of these materials [39,40,41,42,43]. On the other hand, Sb₂O₃-doped PZT–BYS(0.1) shows a high transduction coefficient without much lowering the T_C. Therefore, it might be concluded that PZT–BYS(x) with x = 0.1 moles of Sb₂O₃ content could be considered as a good candidate material for piezoelectric energy-harvesting devices.

4. Conclusions

The present study has shown that the addition of Sb₂O₃ to 0.99Pb(Zr_0.53Ti_0.47)O₃–0.01Bi(Y_1−xSb_x)O₃ [PZT–BYS(x)] ceramics (0 ≤ x ≤ 0.6) has significantly improved their energy-harvesting properties. The comparative study between doping Sb₂O₃ and Sb₂O₅ into the PZT–BYS(x) revealed the feasibility of both ceramics for energy harvesting when an appropriate sintering condition is maintained. It was observed that doping Sb₂O₃ into PZT–BYS(x) while maintaining the oxygen-rich sintering atmosphere had a significant influence on the dielectric and piezoelectric characteristics. All the Sb₂O₃-doped PZT–BYS(x) ceramics could be well sintered to high densities at temperatures between 1150 and 1170 °C. The XRD analysis of the sintered sample indicated a pure solid solution of PZT–BYS(x) without any second phases. The microstructures of the sintered samples were uniform and dense and consistent with the measured density. The Curie temperature of the ceramics was shown to produce a linear reduction with increasing Sb₂O₃ contents, but it is still high enough to use for high-temperature applications. The observed piezoelectric properties were in agreement with the measured P-E hysteresis loops. A maximum d₃₃ of 370 pC/N at x = 0.6 moles and a maximum k_p of 66% at x = 0.1 moles were observed. It was also observed that the ε₃₃^T (690) sharply decreased at x = 0.1 while maintaining relatively higher d₃₃ (350 pC/N), which is a highly demanding piezoelectric characteristic for energy-harvesting. The relatively higher d₃₃ and lower ε₃₃^T at x = 0.1 resulted in an optimum value of g₃₃ = 57.4 × 10⁻³ Vm/N and d₃₃ × g₃₃ = 20075 × 10⁻¹⁵ m²/N, which is high among recently reported work. Therefore, the newly reported ceramic composition of 0.99Pb(Zr_0.53Ti_0.47)O₃–0.01Bi[Y_(1−x)Sb_x]O₃ could be a promising material for future study to be used in an energy harvester.