Enhancing the Performance of PZT-5H Piezoelectric Ceramics by Vacuum Sintering

Abstract

This study comparatively investigates the effects of vacuum sintering and traditional sintering on the structure and electrical properties of lead zirconate titanate (PZT) 5H (PZT-5H) piezoelectric ceramics. The density of the vacuum-sintered ceramics increases from 7.67 g/cm³ (for traditionally sintered ceramics) to 7.98 g/cm³. Importantly, the dielectric constant (ε_r), remnant polarization (P_r), planar electromechanical coupling coefficient (k_p), and piezoelectric coefficient (d₃₃) for the PZT-5H ceramics increase by 35%, 20%, 9%, and 12%, respectively, when vacuum sintering is employed instead of traditional sintering. Over a temperature range from room temperature to 180 °C, the d₃₃ variation measured by the resonant method is only about 4% for the vacuum-sintered PZT-5H ceramics. High-temperature impedance spectroscopy analysis reveals that vacuum sintering reduces the hole concentration in PZT-5H ceramics, leading to significant improvements in their dielectric and piezoelectric performance. This research demonstrates that vacuum sintering is a simple and effective method to enhance the density, dielectric, and piezoelectric properties of PZT-5H ceramics.

1. Introduction

Piezoelectric materials are smart materials which can achieve mutual conversion between mechanical energy and electrical energy [1,2]. PbZrO₃-PbTiO₃-based (PZT-based) ceramics were discovered in the 1950s and have since gained significant attention and widespread application in various piezoelectric devices such as sensors, transducers, and actuators due to their exceptional dielectric, piezoelectric, and electromechanical properties [3,4,5,6]. The global market for piezoelectric ceramics is estimated to be USD 2 billion, with PZT-based materials dominating this industry [7]. To meet the demands of industrial applications, PZT ceramics are often modified with donor or/and acceptor materials, resulting in a series of PZT-based ceramics, including PZT-4, PZT-5, PZT-5A, PZT-5H, and PZT-8 [8,9,10,11,12]. Among these, PZT-5H is a class of typical “soft” piezoelectric ceramics, which are widely used in piezoelectric transducers, energy harvesters, and sensors [13,14,15,16,17,18].

The sintering atmosphere significantly affects the microstructure and electrical properties of piezoelectric ceramics [19,20,21,22,23]. For example, Feng et al. [22] sintered PZT-based ceramics in oxygen, air, and nitrogen to study the effect of sintering atmosphere on their microstructure and electrical properties and obtained a maximum piezoelectric constant d₃₃ of 500 pC/N and an inverse piezoelectric coefficient d₃₃* of 856 pm/V. The use of low-oxygen partial pressure in vacuum sintering reduces hole concentration in p-type piezoelectric ceramics, leading to improved electric properties, such as high resistivity, and excellent dielectric and piezoelectric properties [24,25,26,27,28]. However, there is a lack of systematic investigations on the effects of vacuum sintering on the structure, dielectric, ferroelectric, and piezoelectric properties of PZT ceramics, as well as their defect chemistry.

In this study, our focus is to investigate the effects of vacuum sintering on commercial PZT-5H ceramics. We successfully achieved high-density PZT-5H ceramics through vacuum sintering. To compare their performance, we systematically analyzed the dielectric, ferroelectric, and piezoelectric properties of both vacuum-sintered and traditionally sintered PZT-5H ceramics. Additionally, we examined the high-temperature complex impedance spectrum of PZT-5H ceramics to gain insights into their conduction mechanism. Based on our experimental findings, we propose a possible explanation for the improved dielectric, ferroelectric, and piezoelectric properties observed in vacuum-sintered PZT-5H ceramics.

2. Materials and Methods

PZT-5H powders obtained from Xi’an Kanghong New Material Technology Co., Ltd. (Xi’an, China), were manually formed into pellets, followed by cold isostatic pressing at 200 MPa. A sintering temperature of 1150 °C and a time of 2 h are enough for sintering of PZT-5H ceramics [9,29]. The vacuum furnace was pumped to 0.2 Pa for vacuum sintering. And then, the samples were sintered at 1150 °C for 2 h in the vacuum furnace. For comparison, another set of samples were heated to 1150 °C and maintained for 2 h in a cased resistance furnace in air. The heating rate was 3 °C/min and the furnace was naturally cooled from sintering temperature to room temperature for all samples.

Scanning electron microscopy (SEM) (TM3030Plus) was performed to study the morphology of PZT-5H ceramics. The pores and grain size of the samples were analyzed by ImageJ 1.52p (a public domain software for processing and analyzing scientific images) [30]. The density of the samples was measured by the Archimedes method. The phase structure of the PZT-5H ceramics were determined by an X-ray diffractometer (SmartLab, Tokyo, Japan) with Cu Kα radiation. The Kα2 was stripped by the Jade 6 program [31].

Prior to electrical measurements, the samples were coated with a silver paste, followed by burning at 600 °C for 10 min in air to form electrodes. Samples for electrical measurement were poled in a silicone oil bath with a dc electric field of 15 kV/cm at around 100 °C for 30 min. The piezoelectric coefficients were measured by the resonant method, electric field-induced strain, and a quasi-static d₃₃ m (ZJ-6A, Institute of Acoustics, Chinese Academy of Sciences, Beijing, China). Dielectric temperature spectra and impedance measurements were carried out using an LCR meter (E4980AL, Keysight, Santa Rosa, CA, USA) attached to a computer-controlled temperature chamber. The ferroelectric hysteresis loops and strain curves were measured using a ferroelectric evaluation system (TF analyzer 2000, aixACCT Systems, Aachen, Germany). Traditionally sintered and vacuum-sintered samples with diameters of 7.50 and 7.46 mm and thicknesses of 0.763 and 0.737 mm, respectively, were used for the planar electromechanical coupling characterization. Bars with a size of 1.5 × 1.5 × 5.0 mm³ were used for longitudinal electromechanical coupling characterization.

3. Results and Discussion

Figure 1a,b show SEM images of the surfaces of traditionally sintered and vacuum-sintered PZT-5H ceramics, respectively. Some pores were observed on the surface of the traditionally sintered PZT-5H ceramics, while a dense microstructure was obtained for the vacuum-sintered PZT-5H ceramics. The ceramic density was 7.98 and 7.69 g/cm³ for vacuum sintering and traditional sintering, respectively, at 1150 °C for 2 h. The theoretical density of PZT-5H ceramics was about 8.1 g/cm³. The porosity (p) was about 5.1% and 1.5% for traditionally sintered and vacuum-sintered PZT-5H ceramics, respectively, calculated by the ratio of the difference between theoretical density and measured density to theoretical density. For vacuum sintering, the gas within the pores of the ceramic body is forced to diffuse out through interconnected pores [32]. The porosity (p*) was calculated by the ratio of the area of pores to the total area according to the SEM image of the surface. The values of p* were about 1% and almost zero for traditionally sintered and vacuum-sintered PZT-5H ceramics, respectively. The values of p* were less than those of p due to the easier elimination of pores near the surface compared to those in the bulk. The traditionally sintered PZT-5H ceramic was yellow, while the vacuum-sintered PZT-5H ceramic was black, as shown in the inset in Figure 1a,b. Figure 1c,d show the grain size histograms of the traditionally sintered and vacuum-sintered PZT-5H ceramics, respectively. The average grain size is 6.1 and 6.8 μm for the traditionally sintered and vacuum-sintered PZT-5H ceramics, respectively. Grain growth was promoted by decreasing pores. On the other hand, grain growth was promoted by the PbO liquid phase [33]. The PbO liquid phase decreased more quickly due to faster volatilization of PbO at an elevated temperature in the vacuum environment, which hindered grain growth. The composite effect of pores and the PbO liquid phase led to increased average grain size, but it was relatively modest in the vacuum-sintered PZT-5H ceramic.

Further, X-ray diffraction was employed to identify the phase structure of PZT-5H ceramics. All the samples were dominated by a perovskite structure, and no detectable secondary phase were observed, as shown in Figure 2a. To further investigate the crystal structure, the {002} peaks were magnified and are presented in Figure 2b. The splitting of the diffraction peaks of (200) and (002) suggested a dominant tetragonal phase for both vacuum-sintered and traditionally sintered PZT-5H ceramics. The diffraction profiles in Figure 2 are similar, which suggests that there is no notable difference in phase structure between vacuum-sintered and traditionally sintered PZT-5H ceramics.

The dielectric constant temperature spectra of traditionally sintered and vacuum-sintered PZT-5H ceramics were measured and are shown in Figure 3a. Only one dielectric constant peak was observed for each sample, corresponding to the tetragonal–cubic phase transition. Moreover, the dielectric constant peaks were located at around 227 and 232 °C for the traditionally sintered and vacuum-sintered PZT-5H ceramics, respectively, indicating that vacuum sintering hardly affects the Curie temperature of the PZT-5H ceramics. Notably, the maximum dielectric constant ε_m of the vacuum-sintered sample (ε_m ~ 40,940) was about 40% larger than that of the traditionally sintered sample (ε_m ~ 28,484), and the dielectric constant peak of the vacuum-sintered sample was sharper. At room temperature, ε_r of the vacuum-sintered sample (ε_r ~ 4228) was also about 35% larger than that of the traditionally sintered sample (ε_r ~ 3117). Larger εm and εr at room temperature for the vacuum-sintered sample suggested easier domain wall motion in the vacuum-sintered PZT-5H ceramic compared to the traditionally sintered PZT-5H ceramic.

To further investigate the influence of the two sintering methods on dielectric relaxation behavior, during phase transition, the dielectric constant above the Curie temperature was fitted by the modified Curie–Weiss law, which can be described by the following equation [34,35,36]:

$${\displaystyle \frac{1}{{\mathrm{\epsilon }}_{\mathrm{r}}}}-{\displaystyle \frac{1}{{\mathrm{\epsilon }}_{\mathrm{m}}}}={\displaystyle \frac{{(\mathrm{T}-{\mathrm{T}}_{\mathrm{m}})}^{\mathrm{\gamma }}}{\mathrm{C}}}$$

(1)

where ε_r, ε_m, C, and γ are the relative dielectric constant, the maximum dielectric constant at temperature Tm, the Curie–Weiss constant, and the dispersion factor of the diffuseness degree (1 ≤ γ ≤ 2). Generally, γ = 1 means typical normal ferroelectrics, while γ = 2 suggests complete relaxation ferroelectrics. By fitting Equation (1), the values of γ are 1.68 and 1.61 for the traditionally sintered sample and vacuum-sintered sample, respectively. The difference in the value of γ could be caused by the different response time of domain wall motion to the mutative electric field [37]. When the domain wall is easy to move, the response time to the electric field is short, resulting in a decrease in the dispersion factor, and the dielectric peak of the dielectric temperature spectrum is sharper [37]. The γ of the vacuum-sintered sample is smaller than that of the traditionally sintered sample, suggesting easier domain wall motion in vacuum-sintered PZT-5H ceramics compared to traditionally sintered PZT-5H ceramics.

Figure 4a shows the ferroelectric hysteresis loops for traditionally sintered and vacuum-sintered PZT-5H ceramics at room temperature. Well-saturated hysteresis loops were observed at 15 kV/cm for both samples. At room temperature, the coercive fields (E_C) were about 7 kV/cm for both samples, while the remnant polarization (P_r) of the vacuum-sintered sample (38.1 μC/cm²) is about 20% higher than that of the traditionally sintered sample (31.3 μC/cm²). Figure 4b shows P_r as a function of temperature for traditionally sintered and vacuum-sintered PZT-5H ceramics. P_r gradually decreases with increasing temperature, and the value of P_r is 20.1 and 22.6 μC/cm² at 160 °C for traditionally sintered and vacuum-sintered samples, respectively. The intrinsic piezoelectric ${\mathrm{d}}_{33}^{\mathrm{I}}$ and dielectric response ${\mathrm{\epsilon }}_{\mathrm{r}}$ are related through the electro-strictive coefficient ${\mathrm{Q}}_{11}$ and the remanent polarization P_r [38,39,40]:

$${\mathrm{d}}_{33}^{\mathrm{I}}=2{\mathrm{\epsilon }}_{\mathrm{r}}{\mathrm{\epsilon }}_0{\mathrm{Q}}_{11}{\mathrm{P}}_{\mathrm{r}}$$

(2)

where ${\mathrm{\epsilon }}_0$ is vacuum permittivity. Compared to traditionally sintered PZT-5H ceramics, an enhanced piezoelectric constant d₃₃ can be expected in vacuum-sintered PZT-5H ceramics due to the improved ${\mathrm{\epsilon }}_{\mathrm{r}}$ and ${\mathrm{P}}_{\mathrm{r}}$ obtained in the vacuum sintering process.

Further, the electromechanical coupling performance was investigated by impedance spectra. To calculate the planar electromechanical coupling factor (k_p), the impedance spectra of the kp mode were measured using the disk samples, as shown in Figure 5a. The resonance frequencies (${\mathrm{f}}_{\mathrm{r}}$) and anti-resonance frequencies (${\mathrm{f}}_{\mathrm{a}}$) were about 252 and 256 kHz and 308 and 331 kHz for the two PZT-5H ceramics, respectively. k_p was calculated according to the formula [41]

$${\mathrm{k}}_{\mathrm{p}}^2={\displaystyle \frac{1}{0.806}}{\displaystyle \frac{{\mathrm{f}}_{\mathrm{a}}^2-{\mathrm{f}}_{\mathrm{r}}^2}{{\mathrm{f}}_{\mathrm{a}}^2}}$$

(3)

The ${\mathrm{k}}_{\mathrm{p}}$ of the vacuum-sintered sample (${\mathrm{k}}_{\mathrm{p}}$ ~ 0.70) is about 9% higher than that of the traditionally sintered sample (${\mathrm{k}}_{\mathrm{p}}$ ~ 0.64). Figure 5b shows the temperature dependence of k_p. The value of k_p gradually decreases with increasing temperature and drops rapidly above 200 °C.

Figure 5. (a) Impedance and phase spectrum of k_p mode at room temperature and (b) temperature dependence of k_p for traditionally sintered and vacuum-sintered PZT-5H ceramics.

Figure 5. (a) Impedance and phase spectrum of k_p mode at room temperature and (b) temperature dependence of k_p for traditionally sintered and vacuum-sintered PZT-5H ceramics.

Figure 6a shows the impedance spectra of k₃₃ mode for both traditionally sintered and vacuum-sintered PZT-5H ceramics. The resonance frequency (${\mathrm{f}}_{\mathrm{r}}$) is about 277 kHz for both types of ceramics, while the anti-resonance frequency (${\mathrm{f}}_{\mathrm{a}}$) is about 375 and 394 kHz for traditionally sintered and vacuum-sintered PZT-5H ceramics, respectively. The longitudinal electromechanical coupling factor (${\mathrm{k}}_{33}$) and d₃₃ were obtained by the formulae [42,43,44]

$${\mathrm{\epsilon }}_{33}^{\mathrm{T}}={\displaystyle \frac{\mathrm{C}\mathrm{t}}{\mathrm{l}\mathrm{w}}}$$

(4)

$${\mathrm{k}}_{33}={\displaystyle \frac{\mathrm{\pi }}{2}}{\displaystyle \frac{{\mathrm{f}}_{\mathrm{r}}}{{\mathrm{f}}_{\mathrm{a}}}}\tan \left({\displaystyle \frac{\mathrm{\pi }}{2}}{\displaystyle \frac{{{\mathrm{f}}_{\mathrm{a}}-\mathrm{f}}_{\mathrm{r}}}{{\mathrm{f}}_{\mathrm{a}}}}\right)$$

(5)

$${\mathrm{S}}_{33}^{\mathrm{D}}={\displaystyle \frac{1}{4\mathsf{\rho }{{\mathrm{f}}_{\mathrm{a}}}^2{\mathrm{t}}^2}}$$

(6)

$${\mathrm{S}}_{33}^{\mathrm{E}}={\displaystyle \frac{{\mathrm{S}}_{33}^{\mathrm{D}}}{1-{{\mathrm{k}}_{33}}^2}}$$

(7)

$${\mathrm{d}}_{33}={\mathrm{k}}_{33}\sqrt{{\mathrm{S}}_{33}^{\mathrm{E}}{\mathrm{\epsilon }}_{33}^{\mathrm{T}}}$$

(8)

where ${\mathrm{\epsilon }}_{33}^{\mathrm{T}}$ and $\mathrm{C}$ are the free permittivity and capacitance at 1 kHz; $\mathrm{l}$, $\mathrm{w}$, and t are the size of the sample; ${\mathrm{S}}_{33}^{\mathrm{D}}$ and ${\mathrm{S}}_{33}^{\mathrm{E}}$ are the elastic compliance constants; and $\mathsf{\rho }$ is the density of the sample.

${\mathrm{k}}_{33}$ is 0.71 and 0.74 for traditionally sintered and vacuum-sintered PZT-5H ceramics, respectively. Only a slight variation of less than 10% in the ${\mathrm{k}}_{33}$ constant is observed over the temperature range from room temperature to 160 °C, as depicted in Figure 6b.

Figure 7a shows the d₃₃ obtained by the resonant method as a function of temperature for traditionally sintered and vacuum-sintered PZT-5H ceramics. At room temperature, the d₃₃ of the vacuum-sintered sample obtained by the resonant method (613 pC/N) is about 20% higher than that of the traditionally sintered sample (511 pC/N). Over the temperature range of room temperature to 180 °C, the d₃₃ variation is about 10 and 4% for traditionally sintered and vacuum-sintered PZT-5H ceramics, respectively, as shown in Figure 7a. The piezoelectric coefficient was also obtained by electric field-induced strain and a quasi-static measurement.

Figure 7b shows the unipolar electric field-induced strain at room temperature. The effective piezoelectric coefficient d₃₃* was calculated by the formula [45]

$${\mathrm{d}}_{33}^{\ast }={\displaystyle \frac{{\mathrm{S}}_{\mathrm{m}\mathrm{a}\mathrm{x}}}{{\mathrm{E}}_{\mathrm{m}\mathrm{a}\mathrm{x}}}}$$

(9)

where ${\mathrm{S}}_{\mathrm{m}\mathrm{a}\mathrm{x}}$ and ${\mathrm{E}}_{\mathrm{m}\mathrm{a}\mathrm{x}}$ are the maximum electric field-induced strain and the applied electric field. The d₃₃* of the vacuum-sintered sample (1383 pm/V) is about 18% higher than that of the traditionally sintered sample (1171 pm/V) at 5 kV/cm. A quasi-static d₃₃ m (measured at 110 Hz) was also employed, with d₃₃ values of 708 and 791 pC/N for traditionally sintered and vacuum-sintered PZT-5H ceramics, respectively. The d₃₃ contributed by domain wall motion increases with decreasing measurement frequency and/or increasing applied electric field; this could be responsible for the difference in d₃₃ values measured by the resonant method, electric field-induced strain, and quasi-static measurement [46,47].

Figure 8a,b show the Cole–Cole plots [48,49,50] of traditionally sintered and vacuum-sintered PZT-5H ceramics at 460–540 °C. A semicircle Cole–Cole plot was observed at 460–540 °C for both samples. The bulk resistance (R_b) of traditionally sintered and vacuum-sintered PZT-5H ceramics was estimated from the Cole–Cole plots as the intercept on the Z′ axis. The R_b of vacuum-sintered ceramics is much larger than that of traditionally sintered ceramics: the R_b of the vacuum-sintered ceramic sample is about 3.8 times that of the traditionally sintered ceramic sample over the measurement temperature range, which indicates p-type conduction [24] for the PZT-5H ceramics and lower carrier concentration in vacuum-sintered samples. DC conductivity σ as a function of temperature for the samples was fitted by the Arrhenius law, which can be described by the following equation [51,52,53]:

$$\mathsf{\sigma }={\mathsf{\sigma }}_0\mathrm{e}\mathrm{x}\mathrm{p}\left({\displaystyle \frac{-{\mathrm{E}}_{\mathrm{c}\mathrm{o}\mathrm{n}}}{{\mathrm{k}}_{\mathrm{B}}\mathrm{T}}}\right)$$

(10)

where ${\mathsf{\sigma }}_0$, ${\mathrm{E}}_{\mathrm{c}\mathrm{o}\mathrm{n}}$, ${\mathrm{k}}_{\mathrm{B}}$, and T are the high-temperature limit of conductivity, activation energy for conduction, Boltzmann constant, and absolute temperature on the Kelvin scale, respectively. The activation energy obtained ${\mathrm{E}}_{\mathrm{c}\mathrm{o}\mathrm{n}}$ is 1.20 and 1.24 eV for traditionally sintered and vacuum-sintered PZT-5H ceramics, respectively, as shown in Figure 8c.

Figure 9a,b show the variation in the imaginary part of impedance (Z″) with frequency at 460–540 °C. The occurrence of peaks in Z″ indicates the presence of a relaxation process. The frequency of the peak maximum (${\mathrm{f}}_{\mathrm{m}}$) of the vacuum-sintered ceramics is lower than that of traditionally sintered ceramics at the same temperature, which suggests that relaxation has been enhanced by vacuum sintering. The relaxation time (${\mathrm{\tau }}_{\mathrm{c}}$) can be determined by $2\mathrm{\pi }{\mathrm{f}}_{\mathrm{m}}{\mathrm{\tau }}_{\mathrm{c}}=1$. ${\mathrm{\tau }}_{\mathrm{c}}$ as a function of temperature for the samples was fitted by the Arrhenius law, which can be described by the following equation [54,55,56]:

$${\mathrm{\tau }}_{\mathrm{c}}={\mathrm{\tau }}_0\mathrm{e}\mathrm{x}\mathrm{p}\left({\displaystyle \frac{-{\mathrm{E}}_{\mathrm{r}\mathrm{e}\mathrm{l}}}{{\mathrm{k}}_{\mathrm{B}}\mathrm{T}}}\right)$$

(11)

where ${\mathrm{\tau }}_0$ and ${\mathrm{E}}_{\mathrm{r}\mathrm{e}\mathrm{l}}$ are the characteristic relaxation time constant and the activation energy for conduction relaxation. The values of ${\mathrm{\tau }}_0$ and ${\mathrm{E}}_{\mathrm{r}\mathrm{e}\mathrm{l}}$ are 1.57 $\times {10}^{-15}$ and 3.69 $\times {10}^{-15}$ s and 1.48 and 1.52 eV for traditionally sintered and vacuum-sintered PZT-5H ceramics, respectively, as shown in Figure 9c. For vacuum-sintered PZT-5H ceramics, there is a slight increase in ${\mathrm{E}}_{\mathrm{r}\mathrm{e}\mathrm{l}}$ but an increment in ${\mathrm{\tau }}_0$ by a factor of 2.3 compared to traditionally sintered samples. The relaxation time is inversely proportional to carrier mobility [57]. And the activation energy (E_con, E_rel) of the vacuum-sintered sample (1.24 eV, 1.52 eV) is larger than that of the traditionally sintered sample (1.20 eV, 1.48 eV), which means vacuum-sintered ceramics have lower hole mobility. Thus, an increased characteristic relaxation time constant was observed in the vacuum-sintered sample.

For PZT-based ceramics, volatilization of PbO is an important aspect at elevated temperature, which can be expressed as following equation [58,59,60]:

$${\mathrm{P}\mathrm{b}}_{\mathrm{P}\mathrm{b}}^{\times }+{\mathrm{O}}_{\mathrm{O}}^{\times }\rightleftharpoons {\mathrm{V}}_{\mathrm{P}\mathrm{b}}^{″}+{\mathrm{V}}_{\mathrm{O}}^{··}+\mathrm{P}\mathrm{b}\mathrm{O}\left(\mathrm{g}\right)$$

(12)

The formation of holes possibly occurs in the grain boundary regions,

$${\mathrm{V}}_{\mathrm{O}}^{··}+{\mathrm{O}}_2\rightleftharpoons {\mathrm{O}}_{\mathrm{O}}^{\times }+2{\mathrm{h}}^{·}$$

(13)

resulting in p-type conduction.

High temperature and low-oxygen partial pressure promote oxygen loss from the perovskite lattice [61]. With decreasing-oxygen partial pressure, Equation (13) shifts towards the left, resulting in decreasing hole concentration and conductivity, as shown in Figure 8c. Decreasing hole concentration and mobility in p-type piezoelectric ceramics has a positive effect on dielectric and piezoelectric effects [26]. Accordingly, enhanced dielectric and piezoelectric properties were obtained in the vacuum-sintered PZT-5H ceramics, as shown in

Table 1. Comparison of electric performance of PZT-5H ceramics obtained by different sintering methods.

Sintering Method	εr	d33 (pC/N) by Resonance	d33 (pC/N) by d33 Meter	d33* (pm/V)	kp	k33
Traditional sintering	3117	511	708	1171	0.64	0.71
Vacuum sintering	4228	613	791	1383	0.70	0.74

. In addition, the density of vacuum-sintered PZT-5H ceramics is higher than that of traditionally sintered ceramics, which also contributes to an improvement in their electrical properties.

4. Conclusions

In summary, vacuum sintering was used to produce dense PZT-5H ceramics, and a comprehensive investigation was conducted to understand the effects of this process on their structure and electrical properties. The density of the vacuum-sintered PZT-5H increased to 7.98 g/cm³. Compared to traditionally sintered ceramics, the vacuum-sintered PZT-5H ceramics exhibited excellent electrical properties, including a high dielectric constant (ε_r ~ 4228), high electromechanical coupling coefficients (k_p ~ 0.70, k₃₃ ~ 0.74), and large piezoelectric coefficients (d₃₃ ~ 613 pC/N by the resonant method, 791 pC/N by d₃₃ m; d₃₃* ~ 1383 pm/V at 5 kV/cm). The d₃₃ variation measured by the resonant method was only about 4% for vacuum-sintered PZT-5H ceramics at a temperature range from room temperature to 180 °C. Impedance analysis confirmed that the PZT-5H ceramics exhibited p-type conduction. The higher density and lower hole concentration of the vacuum-sintered ceramics are believed to contribute to their improved dielectric and piezoelectric properties. This study demonstrates that vacuum sintering is an effective method to produce highly dense PZT-5H ceramics with enhanced electrical properties, making it promising for use in piezoelectric devices.