Influence of Al 2 O 3 Nanoparticles' Incorporation on the Structure and Electrical Properties of Pb 0.88 Sr 0.12 Zr 0.54 Ti 0.44 Sb 0.02 O 3 Ceramics

Abstract

In this research, the effects of an Al₂O₃ nanoparticle additive on the structure and electrical properties of Pb_0.88Sr_0.12Zr_0.54Ti_0.44Sb_0.02O₃ (PSZST) ceramics were investigated. The PSZST ceramics with the addition of 0 – 2.0 vol % Al₂O₃ were fabricated via a solid-state mixed oxide method and sintering at 1250°C for 2 h to obtain dense ceramics. X-ray diffraction indicated that all compositions exhibited a single perovskite structure. Phase identification showed coexisting mixed rhombohedral and tetragonal phases for the modified ceramics, while the unmodified ceramics showed a rhombohedral-rich phase. The addition of Al₂O₃ nanoparticles was also found to improve the densification and the electrical properties of the PSZST ceramics, such as dielectric constant and polarization. The 2.0 vol% sample showed the highest low-field piezoelectric coefficient (d₃₃) value of 646 pC/N, which was 90% higher than that of the unmodified sample, suggesting that this composition had the potential to be one of the promising piezoelectric ceramic candidates for further use in actual applications.

1. Introduction

Lead zirconate titanate (abbreviated to Pb(Zr _x TiO_1-x)O₃ or PZT) ceramics have been studied extensively since their discovery in the 1950s [1-4]. They are well-known materials for various sensors and actuators because of their excellent dielectric and piezoelectric properties (typically, d₃₃ ∼ 300 pC/N and k₃₃ ∼ 0.5) [5]. PZT has a perovskite structure with the general formula of ABO₃. PZT compositions are ferroelectric when the ZrTi ratio is < 95: 5. Above the Curie temperature (T_c), the unit cell of PZT is cubic, but below T_c, it is distorted to either tetragonal (when Zr: Ti > 53: 47) or rhombohedral (when Zr: Ti > 53: 47). The boundary between these two phases (rhombohedral and tetragonal) is known as the morphotropic phase boundary (MPB) [6–7]. It has been reported that high piezoelectric properties of PZT have been observed for compositions near the MPB where the Zr/Ti ratio is ∼ 52: 48 at room temperature [7].

PZT and PZT modified with acceptors and donors have important technological applications as transducers, detectors and electro-optic devices, and for induced charge release devices [8]. Most commercial ferroelectric ceramics have thus been designed in the vicinity of the MPB with various forms of doping in order to achieve improved properties [1–2]. For example, doping with ions of alkaline-earth metals, e.g., Sr²⁺, Ca²⁺ and Ba²⁺, have frequently been used to substitute for Pb²⁺ [8]. Hu et al. [9] studied the effect of Sb²⁺ and Ba²⁺ addition on the piezoelectric properties of PZT ceramics. They found that the addition of 0.6 wt% Sb and 0.2 wt% Ba showed the best electrical properties of d₃₃ = 438 pC/N, k_p = 66.3% and ε_r = 1740. Zheng et al. [10] reported that Sr substitution in Nb-doped PZT ceramics generally had higher dielectric and piezoelectric properties than pure PZT. Such Sr substitutions on the A-site in PZT ceramics tended to shift the MPB composition towards the tetragonal phase. Zheng et al. [6] also found that co-doping of Sr and Sb in PZT produced soft piezoelectric ceramics, and the composition of Pb_0.88Sr_0.12Zr_0.54Ti_0.44Sb_0.02O₃ exhibited an excellent d₃₃ value of 600 pC/N. In recent years, many researchers have attempted to improve the electrical properties of PZT-based ceramics by the addition of doping elements [11–12] such as Nd³⁺ [13] or by forming some composites such as PZT/BNT [14] and PZT/BNLT [15]. Moreover, it has been reported that the electrical properties of many ceramics such as BaTiO₃-based ceramics, i.e., BZT/NiO [16], and Pb-based ceramics such as PZT/Al₂O₃ [17–18], PbZrO₃/Al₂O₃ [19] and PZT/SiC [20] can be improved by the addition of the second-phase nanoparticles.

In the present work, we chose a modified PZT with a composition of Pb_0.88Sr_0.12Zr_0.54Ti_0.44Sb_0.02O₃ as the base material, as it shows interesting piezoelectric properties. In order to meet the requirements for specific applications, the studied PZT-based ceramics were added with Al₂O₃ nanoparticles. Therefore, the effects of the added Al₂O₃ nanoparticles on structure and electrical properties, i.e., the dielectric, ferroelectric and piezoelectric properties of Pb_0.88Sr_0.12Zr_0.54Ti_0.44Sb_0.02O₃ (PSZST) ceramics, were investigated in this research. The PSZST/Al₂O₃ ceramics were expected to exhibit better properties than the single phase of PSZST or Al₂O₃.

2. Experiment

The Pb_0.88Sr_0.12Zr_0.54Ti_0.44Sb_0.02O₃/Al₂O₃ or PSZST/Al₂O₃ ceramics were synthesized via a conventional mixed oxide method. The analytical grade reagents of PbO, SrCO₃, ZrO₂, Sb₂O₃ TiO₂ and Al₂O₃ nanoparticles were used as the starting raw materials. High-purity oxide and carbonate starting materials were weighed based on the stoichiometric formula of Pb_0.88Sr_0.12Zr_0.54Ti_0.44Sb_0.02O₃ and then mixed by ball milling for 24 h in an ethanol solution. The slurries were dried for 24 h in an oven. The obtained PSZST powder was calcined in an air atmosphere at 875 – 975°C for 2 h with a heating/cooling rate of 10°C/min. After that, Al₂O₃ nanoparticles were added to calcined PSZST powder with different ratios (0, 0.5, 1.0 and 2.0 vol%) and then milled for 24 h in ethanol. A few drops of 1 wt% polyvinyl alcohol (PVA) binders were then added to the mixed powders before they were uniaxially pressed into 10 mm diameter discs. The green pellets were then preheated in air at 500°C for 2 h to remove the organic binder and then sintered at a temperature of 1250°C for 2 h of dwell time, with a heating/cooling rate of 10°C/min, in sealed crucibles that had a PbO-rich atmosphere from PbZrO₃ powder.

The bulk density of all the ceramics was determined using the Archimedes' method. Phase identification of powders and sintered ceramics was investigated using an X-ray diffractometer with CuKα radiation over a 2θ scan range of 10 – 60°. A scanning electron microscope (SEM, JEOL JSM-6335F) was used to obtain the morphologies of thermally etched surfaces of the samples. For thermally etched surfaces' determination, the disc-shaped samples were polished to a mirror finish and then thermally etched at a temperature lower than that of the sintering temperature by ∼ 150 – 200°C, for 15 minutes with a heating/cooling rate of 5°C/min. The grain size was determined via the mean linear interception method from the SEM micrographs. The average grain size was estimated by counting the number of grains intercepted by one or more straight lines. For the electrical measurements, the samples were polished to obtain very smooth and parallel surfaces, and silver paste was used to coat both sides of the samples, which were fired at 650°C for 30 min. Measurement of dielectric properties as a function of temperature were carried out, using a 4284A LCR-meter connected to a high-temperature furnace with temperatures from room temperature to 300°C, and with various frequencies from 1 – 1000 kHz. A ferroelectric test system based on Radiant Technology was used to measure the polarization-electric field (P-E) hysteresis behaviour. An AC electric field of 20 kV/cm at a frequency of 1 Hz was utilized in the hysteresis measurement. To study the piezoelectric properties, all samples were poled at room temperature (25°C) in a stirred silicone oil bath by applying a DC electric field of 2 kV/mm for 30 min. The piezoelectric coefficient (d₃₃) of the samples was measured using a d₃₃-meter (KCF technologies S5865) at a frequency of 50 Hz.

3. Results and discussion

The Pb_0.88Sr_0.12Zr_0.54Ti_0.44Sb_0.02O₃ or PSZST powder was calcined for 2 h with a heating/cooling rate of 10°C/min in air, at various temperatures from 875 to 975°C, in order to find out the optimum calcination temperature. All calcined powders were characterized via X-ray diffraction patterns (XRD). The XRD patterns of PSZST-calcined powders are shown in Figure 1. At the calcination temperature of 875°C, the starting chemicals (PbO, SrCO₃, ZrO₂, Sb₂O₃ and TiO₂) did not react completely to form a single phase. The structure was not perfectly ordered and the XRD pattern still presented some peaks of the secondary phase. Upon reaching the calcination temperature of 925°C (for 2 h), XRD patterns indicated a formation of a single perovskite structure with sharp and high intensity XRD peaks. The symmetry of the samples was characterized as rhombohedral, which could be matched with the JCPDS file number 73–2022 of the PZT phase. However, on increasing the calcination temperature to 975°C, a small amount of the secondary phase was observed. Based on the results, the single phase of the PSZST powder was successfully obtained at the calcination temperature of 925°C (for 2 h).

Figure 1.

X-ray diffraction patterns of PSZST powder calcined at 875 – 975°C for 2 h

X-ray diffraction patterns of PSZST/A1₂O₃ ceramics (2θ = 10 – 60°) sintered at the temperature of 1250°C are shown in Figure 2. A single perovskite structure was observed for all compositions. No trace of Al₂O₃ or an impurity phase was detected. This may be due to the amount of Al₂O₃ in the ceramics being too small to be detected by the XRD instrument. An XRD pattern in a narrow 2θ range of 43 – 45.5° was performed as shown in Figure 3. In general, PZT with compositions near to MPB at room temperature consists of two phases, i.e., rhombohedral and tetragonal phases. The peak of (200)_R reflection in the XRD pattern suggests the rhombohedral-rich phase, whereas this peak splits into two peaks of (002)_T and (200)_T, which are characteristics of the tetragonal phase [8]. Based on the XRD peaks within a 2θ range of 43 – 45.5° in this work, a pure PSZST sample could be indexed according to a rhombohedral-rich phase that matches the JCPDS file number 73–2022. This is evidenced by unclear splitting of two tetragonal peaks of (002)_T and (200)_T, and the XRD peak showing only a broad (200)_R reflection. On increasing the Al₂O₃ content up to 2 vol%, however, the XRD patterns show a stronger peak splitting of (002)_T and (200)_T, and it seems to coexist with mixed rhombohedral and tetragonal phases [21–22].

Figure 2.

X-ray diffraction patterns of PSZST/Al₂O₃ ceramics sintered at 1250°C, where 2θ = 10–60°

Figure 3.

X-ray diffraction patterns of PSZST/Al₂O₃ ceramics sintered at 1250°C, where 2θ = 41 – 45.5°

Thermally etched surfaces of PSZST/Al₂O₃ ceramics observed by SEM are shown in Figure 4, and their grain sizes (which were calculated based on a mean linear interception method) are also summarized in Table 1. The SEM observation confirmed that all samples were of high quality and were densely sintered at 1250°C. The grains of all samples had regular shapes with clear grain boundaries and almost no pores were found, which corresponded to the high-density values of 7.40 – 7.51 g/cm³. The pure PSZST ceramic was well crystallized, with clear equiaxed grain shape with a diameter of around 9.73 μm. The addition of a small amount of Al₂O₃ into a PSZST ceramic of 0.5 – 1.0 vol%, however, slightly inhibited grain-growth behaviour. This can be seen from a slight drop in the grain size value, from 9.73 μm for pure PSZST to around 8.88 μm for the 1.0 vol% ceramic. On increasing the Al₂O₃ up to 2.0 vol%, the grain size rapidly decreased to the minimum value of 3.08 μm. We believe that the observed grain morphology and reduced grain size for these samples was due to a solute drag effect of the dissolved Al₂O₃. Since solute diffusion near the grain-boundary region was usually slower than the intrinsic diffusion of host atoms across the boundary plane, it became rate limiting for grain boundary movement. This seemed to be the main mechanism governing the observed microstructure [23].

Table 1.

Physical, microstructure and electrical properties of PSZST containing different Al₂O₃ concentrations

Al2O3 mol %	Density (gW3)	Grain size (μm)	εr	tan δ	(°C)	εmax	(μC/cm2)	Pmax (μC/cm2)	Ec (kV/cm)	Rsq	d33 (pC/N)
0	7.40	9.73	1479	0.030	205	12327	19.82	21.63	11.68	1.15	339
0.5	7.49	9.34	1860	0.029	204	19709	25.21	29.32	9.19	1.35	412
1.0	7.48	8.88	1900	0.031	203	19872	28.97	33.43	9.08	1.30	561
2.0	7.51	3.08	1970	0.038	203	18479	29.53	33.89	8.75	1.29	646

Figure 4.

SEM micrographs of thermally etched surfaces of PSZST/Al₂O₃ ceramics sintered at 1250°C, where (a) PSZST, (b) PSZST + 0.5 vol% Al₂O₃, (c) PSZST + 1.0 vol% Al₂O₃ and PSZST + 2.0 vol% Al₂O₃

Figure 5 displays the temperature dependence of ε_r and tanδ of PSZST/Al₂O₃ ceramics, measured at various frequencies from 1 – 1000 kHz. The dielectric properties are also listed in Table 1. It can be seen that the dielectric curves of all samples are rather similar. The ε_r increased with increasing temperature and reached the highest value at T_c and then gradually decreased, with a further increase in the temperature over T_c. The T_c value of unmodified PSZST ceramic in this study was 205°C, which was in agreement with the value of 187°C previously reported by Zheng et al. [6]. The Al₂O₃ additive (0.5 – 2 vol%) slightly shifted T_c to lower temperatures of ∼ 203 – 204°C. Furthermore, the additive also produced an increase in ε_max at the dielectric peak (at T_c). The ε_max value increased from 12300 for the unmodified ceramic to 19900 for the 1.0 vol% ceramic, and then slightly decreased to 18500 for the 2.0 vol% ceramic. Room-temperature dielectric constant (ε_r,RT) and dielectric loss (tanδ) values are also summarized in Table 1. The ε_r,RT of unmodified PSZST ceramic was found to be 1480. After adding the additive, the ε_r,RT increased to the maximum value of 1970 for the 2.0 vol.% ceramics. The tanδ value of the ceramics (at RT) slightly increased with the additive, but the tanδ values of all ceramics were lower than 0.038.

Figure 5.

Temperature dependence on dielectric constant (ε_r) and dielectric loss (tanδ) of PSZST/Al₂O₃ ceramics sintered at 1250°C and measured at various frequencies, where (a) PSZST, (b) PSZST + 0.5 vol% Al₂O₃, (c) PSZST + 10 vol% Al₂O₃ and PSZST + 2.0 vol% Al₂O₃

Figure 6 displays P-E hysteresis loops of PSZST/Al₂O₃ ceramics measured at a frequency of 1 Hz. Well-saturated square shaped P-E loops were observed for all compositions at room temperature under a maximum electric field of 20 kV/cm. For higher applied fields over 20 kV/cm, the ceramics were undergoing breakdown. Remanent polarization (P_r), maximum polarization (P_max), coercive field (E_c) and loop squareness (R_sq) values were determined from the hysteresis loops. The R_sq ratio of the hysteresis loop was calculated using the ratio of P_r at the zero electric field to maximum polarization (P_max), obtained at some finite field strength below dielectric breakdown, i.e., P_r/P_max. According to Haertling [4], the squareness can be used to measure not only the deviation in the polarization axis but also that in the electric field axis, using an empirical equation: R_sq = (P_r/P_s) + (P_1:1Ec/P _r ) [4]. The obtained ferroelectric properties are listed in Table 1. The unmodified ceramic had P_r ∼ 19.82 μC/cm², E_c ∼ 11.68 kV/cm and R_sq ∼ 1.15. However, the addition of Al₂O₃ produced a significant influence on loop shape, P_r and E_c values. Plots of P_r′ P_max and E_c values as a function of Al₂O₃ content are shown in Figure 7. It can be seen that P_r and P_max tended to increase with an increasing Al₂O₃ content, and reached the values of 29.53 μC/cm² and 33.89 μC/cm², respectively, for the 2.0 vol% ceramic. However, the E_c value decreased from 11.68 kV/cm for the unmodified ceramic to around 8.7 kV/cm for the 2.0 vol% ceramic. Based on the XRD result, the crystalline structure was considered to contain coexisting rhombohedral and tetragonal phases for the PSZST/Al₂O₃ system. The tetragonal phase had six different polarizations in the (001) direction for reorientation, while there are eight different (111) directions in the rhombohedral phase [24–25]. Therefore, both of these phases may produce 14 possible polarization directions. It seemed that the large number of polarization directions caused an enhancement of crystallographic orientations under the electric field and, in turn, resulted in high dielectric and polarization properties. It can be seen that ferroelectric properties exhibited a strong compositional and phase dependence within the MPB region. This phenomenon was similar to that observed in a conventional Pb-based piezoelectric ceramic [2]. Beside the MPB composition that had a strong effect on the ferroelectric properties, density is often considered to affect the electrical properties of PZT-based ceramics. In the case of density, higher density ceramics often result in better electrical properties, such as higher ferroelectric, and piezoelectric properties [26–27]. This fact is true for the present work. Therefore, the improvement of density contributed to the improvement of the ferroelectric properties. Grain size is also known to affect many properties of PZT-based ceramics. Many reports have suggested that coarser grained piezoceramics have higher ferroelectric and piezoelectric properties [7,28]. However, the grain size of the present ceramics decreased with the amount of the additive. It is therefore believed that grain size contributed less to the improvement of ferroelectric properties in this work, as compared to other factors (MPB composition and density).

Figure 6.

Polarization-electric field (P-E) hysteresis loops of PSZST/Al₂O₃ ceramics sintered at 1250°C and measured at a frequency of 1 Hz where (a) PSZST, (b) PSZST + 0.5 vol% Al₂O₃, (c) PSZST + 1.0 vol% Al₂O₃ and PSZST + 2.0 vol% Al₂O₃

Figure 7.

Plots of P_r, P_max and E_c values as a function of the Al₂O₃ content of PSZST/Al₂O₃ ceramics

The low-field piezoelectric coefficient (d₃₃) values obtained after poling are listed in Table 1. The unmodified ceramic had a low-field d₃₃ value of 339 pC/N. The d₃₃ value increased with increasing the Al₂O₃ content and reached 646 pC/N for the 2.0 vol% ceramic, which is a ∼ 2-fold increase as compared to the unmodified ceramic. This improvement may be because the Al₂O₃ additive moved the phase composition from the rhombohedral-rich phase to a more tetragonal phase, which may close to the MPB region. In this work, it should be noted that the trend for a low-field d₃₃ value is similar to the trend for the P_r value (Figure 8). This behaviour can be explained by the thermodynamic theory of ferroelectrics [29]. According to this theory, d₃₃ can be expressed as d₃₃ = 2ε₃₃ε₀Q₁₁P_r, where ε₃₃ represents the dielectric constant of the material and ε₀ is the vacuum permittivity. Q₁₁ represented the electrostrictive coefficient, which was a constant for perovskite materials [29]. From this theory, it can be seen that the low-field d₃₃ value is proportional to the P_r value. Therefore, significant enhancement in the low-field d₃₃ value can be related to the improvement of the P_r value after adding the additive. Furthermore, the improvement of the low-field d₃₃ value can be linked with the decrease in E_c, as the lower E_c can result in an easier poling [3]. This was due to the ease of ionic and domain-wall motions that facilitated the poling process and improved the polarizability of the system [30]. Density is also a factor that often relates to many electrical properties. In this work, density tended to increase with the Al₂O₃ content. Therefore, the improvement in density may be another factor that helps with the improvement of the low-field d₃₃ value. Thus, it seemed that all mentioned factors played a role in the piezoelectricity's improvement and effectively enhanced the d₃₃ value in our PSZST/Al₂O₃ samples.

Figure 8.

Plots of P_r and d₃₃ values as a function of the Al₂O₃ content of PSZST/Al₂O₃ ceramics

From this investigation, improvement of the electrical properties of PSZST/Al₂O₃ ceramics was achieved. It clearly showed that samples containing Al₂O₃ nanoparticles exhibited better electrical properties than the single phase of PSZST or Al₂O₃.

4. Conclusion

The ceramics with a composition of Pb_0.88Sr_0.12Zr_0.54Ti_0.44Sb_0.02O₃/Al₂O₃ or PSZST/Al₂O₃ were successfully synthesized via a simple conventional mixed-oxide and ordinary sintering method. Examinations of the effects of Al₂O₃ nanoparticles' addition on the structure and electrical properties of the ceramics were carried out. A single perovskite structure was observed for all compositions. The addition of Al₂O₃ nanoparticles moved the phase composition from a rhombohedral-rich phase for the unmodified ceramics to a more tetragonal phase for the modified ceramics. The highest values of dielectric (T_c = 203°C, ε_r = 1970, tanδ = 0.038), ferroelectric (P_r = 29.53 μC/cm², E_c = S.75 kV/cm, R_sq = 1.29) and piezoelectric properties (d₃₃ = 646 pC/N) was observed for the 2.0 vol% ceramics. Based on our results, it is suggested that the addition of Al₂O₃ significantly enhances the electrical properties of the ceramics, especially piezoelectric properties, meaning that it can be considered as one of the promising candidate materials for piezoelectric applications.