Fabrication and Electrical Properties of PSZT Piezoelectric Ceramic Ring for Ultrasonic Welding Application

Abstract

In recent years, ultrasonic welding technology has developed rapidly and is applied in many fields. The research and application of piezoelectric ceramic materials in ultrasonic welding have received significant attention from researchers and commercial manufacturers. In this work, we have studied, designed and fabricated piezoelectric ceramic materials described by the general formula Pb_(1−x)Sr_x(Zr_0.53Ti_0.47)O₃ or (PSZT) with x = 0.6 mol%. In addition, the experimental fabrication process of PSZT piezoelectric ceramic transducer in the form of a ring by ball milling and solid-state reaction process are also presented. Adding hard dopants Sr²⁺ to the A (of Pb²⁺) position in the perovskite structure is considered to improve the dielectric and piezoelectric properties of Pb(Zr_0.53Ti_0.47)O₃. The results show that the dielectric constant (ε_r) is 1707.4, and the dielectric loss (tan δ) is 0.0034 at room temperature and 1 kHz. The hysteresis loop of the non-polar PSZT piezoelectric ceramic sample shows the same behaviour as the hard PZT ceramic with a remnant polarization of 2.0 µC/cm² and a force field of 5.5 kV/cm. The Curie temperature (T_C) is determined to be about 342°C. The piezoelectric properties of the PSZT piezoelectric ceramic ring were also measured: the effective electromechanical coupling coefficient (k_eff) of 0.40, electromechanical coupling coefficient (k_p) of 0.48, mechanical quality factor (Q_m) of 270.92, piezoelectric coefficient (d₃₃) is 314 pC/N, respectively. This result demonstrates that the fabrication process of Pb_1−xSr_x(Zr_0.53Ti_0.47)O₃ or (PSZT) ceramic rings with x = 0.6 mol%, exhibiting superior characteristics, is entirely suitable for practical applications such as the production and substitution of piezoelectric sensors for high-power ultrasonic welding technology operating at a resonant frequency ranging from 30 to 40 kHz.

1. Introduction

Ultrasonic welding technology is one of the most popular welding techniques today.¹⁾ Ultrasonic welding has developed rapidly and is widely used in most of the major industries such as automotive, electronics and medical, airspace, home appliances, non-woven fabrics, packaging, etc.^1–3) The essential component that is considered to be the core of this ultrasonic welding technology is the piezoelectric transducer (the part that converts electrical energy into mechanical vibrations).^4,5) However, the various piezoelectric ring transducer types are also consumable components, susceptible to damage, and often require replacement due to the high-power ultrasonic welding process.⁵⁾

In many countries, ultrasonic welding equipment and piezoelectric ceramic components are almost entirely imported at high costs due to the inability for domestic production.^6,7) Additionally, the import and export of items, materials, or components related to hazardous Pb elements face considerable challenges. Therefore, the application and development of various dielectric ceramic materials and technology for manufacturing high-power piezoelectric ceramic rings to meet the production demands is indeed essential. This is truly a technology-driven autonomous solution that significantly enhances labor productivity.

Among the high-power ultrasonic transducer piezoelectric materials, the traditional ceramic material lead zirconate titanate [Pb(Zr_xTi_1−x)O₃ or PZT] is widely studied and considered the best choice. The reason is that they possess outstanding ferroelectric, piezoelectric properties such as high dielectric constant (ε_r) and electromechanical coupling coefficient (k_p), high piezoelectric constant (d₃₃), Curie temperature (T_C) is 328°C as well as many other properties.⁷⁾ Moreover, Pb(Zr_xTi_1−x)O₃ has many different ratios of Zr/Ti (x) components, most notably Pb(Zr_0.53Ti_0.47)O₃, which has optimized properties close to the morphological phase boundary (MPB) boundary between the tetragonal and rhombohedral phases, exhibiting outstanding piezoelectric properties.^7,8) On the other hand, to improve the piezoelectric properties to suit different ultrasonic applications, more complex ceramics have also been studied by dopants such as hard dopants (acceptor type): K⁺, Na⁺ for Pb²⁺ site or Fe³⁺, Cu²⁺, Al³⁺, Mg²⁺ for Zn⁴⁺ and Ti⁴⁺ site; soft dopants (donor type): Sb³⁺, La³⁺, Nd³⁺, Th⁴⁺ for Pb²⁺ or Nb⁵⁺, Sb⁵⁺ for Zn⁴⁺ and Ti⁴⁺ site.^7,9) In particular, the isovalent dopant: Sr²⁺ for Pb²⁺ (A-site of PZT), for example [Pb_0.94Sr_0.06(Zr_0.52Ti_0.48)O₃], making it ferroelectric harder and is widely used for high power applications such as ultrasonic transducers. This application requires low dielectric loss, high electromechanical coupling coefficient (k_p) and high-quality mechanical coefficient (Q_m).^10–15) However, most of these studies only focus on the fabrication of small pellet disk samples (EN 50324 standard, with diameters and thicknesses of 10 mm and 1 mm, respectively) to investigate the properties of different piezoelectric ceramic materials.^10–12) Some other studies perform computational simulations to optimize the design of PZT ceramic rings, investigate the coupling of piezoelectric variables, or utilize commercial PZT powder materials.^16,17) To the best of our knowledge, there is no published work on the development of a complete manufacturing process for Pb_0.94Sr_0.06(Zr_0.52Ti_0.48)O₃ (PSZT) piezoelectric ring, as specifically presented.

Various chemical or physical methods can prepare PZT powder. Compared with wet chemical processes such as hydrothermal, co-precipitation and sol-gel, physical methods such as ball milling are considered to have many advantages: (i) using input materials from low-cost oxide precursors (ii) simple operation process, no chemical treatment or discharge into the environment (iii) low temperature and (iv) can be produced on a large scale.^18,19)

The primary objective of this study is to develop a refined process for fabricating circular-ring-type lead zirconate titanate (PSZT) piezoelectric actuators (with an outer diameter of 5 mm, inner diameter of 17 mm, and thickness of 6.5 mm) from initial oxide precursors, thereby eliminating the need for importing raw materials or commercial ring components, which can sometimes pose challenges. In this research, we approach and focus on investigating lead strontium zirconate titanate (Pb_1−xSr_x(Zr_0.53Ti_0.47)O₃ or PSZT) piezoelectric ceramics with a composition ratio of x = 6.0 mol% using both ball milling and solid-state reaction methods. The characteristics, processing techniques, fabrication procedures, and effectiveness testing as well as applications of the annular piezoelectric ceramic components are also discussed. Despite the necessity of producing a significant amount of PSZT powder for ring compaction, the proposed annular PSZT ceramic product maintains high-quality standards and is compared against previously published or commercially available small disk configurations. Comparative results affirm that the PSZT ceramic ring product exhibits properties that are nearly equal to or even surpass those of the smaller disk counterparts, meeting the requirements of practical applications, particularly in high-power ultrasonic welding. This research holds substantial significance in achieving mastery over high-power ultrasonic welding technology.

2. Experiments

2.1 Materials

Chemicals used: strontium carbonate (SrCO₃, 99.9%); zirconium oxide (ZrO₂, 99.9%); lead oxide (PbO, 99.9%), titanium dioxide (TiO₂, 99.9%), and polyvinyl alcohol (PVA, 87%) were purchased from Sigma-Aldrich.

2.2 Manufacturing process: PSZT piezoelectric ceramic ring

Piezoelectric ceramic material (Pb_1−xSr_x)(Zr_0.53Ti_0.47)O₃ (PSZT) where x = 0.6 mol% was synthesized from the prepared chemicals SrCO₃, ZrO₂, PbO and TiO₂ by process ball mill and solid phase reaction. The process of manufacturing piezoelectric ceramic transducer PSZT - in the form of a ring can be detailed step by step: First, the chemical raw materials were weighed according to the stoichiometric ratio of Pb_0.94Sr_0.06(Zr_0.53Ti_0.47)O₃. An excess of 5% by weight of PbO has also been added to compensate for the loss of lead ions, which often occurs during high-temperature annealing.²⁰⁾ Next, the powder was mixed by grinding with zirconia balls in the medium in ethanol for 4 h. The mixture was then dried, and the pellets were hydraulically compressed and then calcined in an aluminium oxide crucible at 800°C for 3 h. After that, the sample was ground again with a ball mill for 16 hours, and the resulting powder was dried for the following ceramic sintering process. Before ceramic sintering, PSZT powder, after two grindings, was mixed with 3% by weight of PVA and pressed into rings. PVA is used as a binding agent and is often added during the pelletizing process, which not only enhances the compressive strength of the pressed pellets but also restricts potential microstructural defects that could occur during the sintering process forming ceramics.²¹⁾ Next, the sample was heat treated at 550°C for 1 h to remove PVA, formed a phase and then raised to a sintering temperature at 1200°C for 3 h on an aluminum oxide ceramic plate. The heating rate and temperature reduction to room temperature of the kiln are both set to 5°C/min.

2.3 Measurement methods

The PSZT ceramic sample, after partial sintering, was pre-ground into powder to investigate the properties. The crystal structure and phase of the powder samples were determined by X-ray diffraction (XRD) using a Bruker D5005 diffractometer with Cu Kα radiation (λ = 1.5406 Å). The size distribution, morphology on the halved surface and the chemical element composition of the PSZT ceramic sample after sintering were also determined by scanning electron microscope (SEM), which incorporates measurements (EDS). After the remaining rings were polished with a polishing machine (model UNIPOL-802, MIT), a silver paste was coated on both sides of the samples and then calcined at 700°C for 30 min to form the upper electrodes and below, as shown in Fig. 1. The ring was then placed in a silicone oil beaker heated at 150°C and electrically polarized by applying a DC electric field of 20 kV/cm for 30 min. The electrical properties of the PSZT ceramic ring, such as dielectric constant (ε_r) and dielectric loss (tan δ), were measured with an LCR Meter (model 3550, TEGAM Inc., US) over a frequency range from 1 kHz to 1 MHz at room temperature (RT). The temperature dependence with dielectric properties such as Curie temperature (T_C) was also investigated at 1 kHz. The effective electromechanical coupling coefficient (k_eff) and the electromechanical coupling coefficients (k_p) are calculated by eqs. (1) (2):¹⁸⁾

  

\begin{equation} k_{\text{eff}} = [1 - (f_{\text{r}})^{2}/(f_{\text{r}})^{2}]^{1/2} \end{equation} (1)

  

\begin{equation} k_{\text{p}} = [2.529 \cdot (f_{\text{a}} - f_{\text{r}})/f_{\text{r}}]^{1/2} \end{equation} (2)

Fig. 1

The PSZT ceramic ring has been fabricated after sintering and polishing (a) 3D design image of the PSZT ring with Ag electrode coated on two sides of the ring (b).

The resonant frequency (f_a) and antiresonance frequency (f_r) are obtained from the LCR Meter. The mechanical quality factor (Q_m) of the ceramic ring is also calculated based on eq. (3).^12,18)

  

\begin{equation} Q_{\text{m}} = (f_{\text{a}})^{2}/[2\pi Z_{\text{m}}C \cdot f_{\text{r}}((f_{\text{a}})^{2} - (f_{\text{r}})^{2})] \end{equation} (3)

Z_m and C are the impedance at the first resonant frequency and the capacitance value of the sample, respectively. The hysteresis loops (P-E) of the ceramic ring were measured with the Sawyer Tower Precision LC II ferroelectric test system (Radiant Technologies. Inc.). The piezoelectric coefficient (d₃₃) was measured with a semi-static d₃₃ m Meter (ZJ-3A, China).

3. Results and Discussion

3.1 The structure and morphology

Figure 2 shows the crystal structure of Pb_1−xSr_x(Zr_0.53Ti_0.47)O₃ ceramic (PSZT) when x = 6.0 mol%. The diffraction pattern of the sample in Fig. 2(a) is very consistent with the standard Pb(Zr_0.53Ti_0.47)O₃ sample with lattice parameters a = b = 4.03530 Å, c = 4.13120 Å, α = β = γ = 90° and volume unit cell V = 67.27 ų (according to PDF data number: 01-070-4264). It can be seen that the sample exhibits a unique perovskite structure, which is consistent with the literature.^22,23) There is no evidence of any secondary phase or unusual impurity peaks, often present in Pb-deficient samples.²²⁾ This result indicates that it is appropriate to use the initial mass of 5% of excess PbO added during ceramic fabrication to compensate for lead ions that may be lost during sintering. It is essential to ensure the complete morphological phase according to the selected material formula.²⁰⁾ Apparent peak splitting was also observed as (001) and (100); (002) and (200) correspond to the 2θ angles from 21°–22.5° and 30.5°–32°, respectively. The single peak at an angle of 2θ = 38.2° can be perfectly indexed to (111). With the above crystal structure, it is concluded that the tetragonal phase dominates in PSZT ceramics.^8,9) In addition, a slight shift in the intensity of the peaks is observed in the Pb_0.94Sr_0.06(Zr_0.53Ti_0.47)O₃ ceramic. It indicates the distortion of the lattice due to the incorporation of impurities Sr²⁺ into the lattice without disturbing the basic perovskite structure (Pb²⁺ and Sr²⁺ can be captured and replaced).²²⁾ In particular, the maximum relative intensity (200) is almost three times the maximum relative intensity (002) in the XRD configuration. For the standard sample Pb(Zr_0.53Ti_0.47)O₃, the intensity ratio between these two peaks is less than 2 times. This observation suggests that the direction in the (200) direction becomes the dominant crystal property with the contribution of Sr²⁺, which is consistent with the results of previous studies.¹⁴⁾

Fig. 2

XRD patterns of PSZT ceramic at 2θ = 20°–80° (a), and the 3D crystal structure diagram of PSZT (space group- P4mm; # 99 – 1) is prepared by using the VESTA package (b).

From the XRD results, the Scherrer equation was used to calculate the mean crystal size (L) of the sample:

  

\begin{equation} L = K\cdot\lambda/\beta\cdot\cos\theta \end{equation} (4)

Where λ (= 1.54056 Å) is the wavelength of the X-ray radiation source, and K is a constant related to the crystal shape, usually taken as 0.94. The half-peak width (FWHM) and θ are the Bragg maximum diffraction angle. The mean crystal size value of PSZT ceramic L ∼ 32.07 nm. Figure 2(b) shows the 3D crystal structure diagram of PSZT ceramic with a symmetric tetragonal structure that fits well with the P4mm space group.

Figure 3(a) shows the SEM image and grain size distribution graph of the broken PSZT ceramic ring surface after sintering at 1200°C for 3 h. The results show that the sintered PSZT ceramic has a relatively uniform grain size and clear and sharp boundaries with a high density of approximately 7.6 g/cm³. Moreover, the grain size distribution graph of PSZT ceramics after firing is also shown in the inset. This analysis confirms that about 72% of the grain size is between 1.8 and 3.0 µm and that the average grain size of the sintered PSZT ceramic is about 2.35 µm. This tight, uniform grain distribution is thought to play an essential role in enhancing piezoelectric efficiency by reducing porosity and local stress.²⁴⁾

Fig. 3

SEM image with inset showing the grain size distribution (a) and EDS analysis (b) of PSZT ceramics sintered at 1200°C for 3 h.

Figure 3(b) is an EDS analysis, showing all the expected chemical element compositions of the PSZT ceramic sample, pre-ground after sintering at 1200°C for 3 h, without the appearance of any element other than the original precursor. This EDS result also proves that the PSZT sample has been fabricated successfully and agrees with the previous XRD results.

To further verify the presence and distribution of the elements: Pb, Sr, Zr, Ti, and O within the PSZT ceramic ring, elemental mapping analysis was employed, as illustrated in Fig. 4. The results indicate that all five expected elements were observed across the mapping area with a uniform distribution.

Fig. 4

Elemental mapping analysis of PSZT ceramic ring sintered at 1200°C for 3 h.

3.2 Dielectric and ferroelectric properties

Figure 5 shows the dielectric properties of the PSZT piezoelectric ceramic ring studied as a function of frequency and temperature. Specifically, Fig. 5(a) shows the frequency dependence between 100 Hz and 1 MHz of the relative dielectric constant (ε_r) values of the PSZT ceramic sample measured at room temperature. The value of ε_r tends to decrease with increasing measurement frequency. The results show that, at the frequency of 1 kHz, the relative dielectric constant (ε_r) and dielectric loss (tan δ) of the 1200°C sintered PSZT piezoelectric ceramic ring reached the maximum value of 1707.4 and 0.0034, respectively.

Fig. 5

The dielectric constant (ε_r) value was measured over the frequency range from 1 kHz to 1 MHz at room temperature (a), and the Curie temperature (T_C) was investigated at frequency 1 kHz (b) of PSZT ceramic ring.

On the other hand, an anomaly of the dielectric constant curve when the temperature is increased from room temperature to 400°C is also observed, as shown in Fig. 5(b). At the temperature of 342°C, the relative dielectric constant value spiked and peaked at 16,444. It is considered the Curie temperature T_C, corresponding to the characteristic ferroelectric-paraelectric phase transition, which is very important in determining the heat treatment temperature.¹⁴⁾ It should also be noted that the PSZT ceramic with a high T_C meets the actual use requirements of high-power piezoelectric ceramics. The temperature-dependent constant measurements can be found to be consistent with the XRD analysis.

Figure 6 shows the P-E hysteresis curve of the PSZT ceramic ring with an electric field value of 9 kV/cm at 50 Hz at room temperature as measured by the Sawyer-Tower method. The results show that the hysteresis curve is typical for typical ferroelectric materials. From the hysteresis curve, remnant polarization P_r and force field E_C values were also determined to be 2.0 µC/cm² and 5.5 kV/cm, respectively. The results show that adding hard dopants Sr²⁺ to the A site of the perovskite structure of PZT improves the dielectric and ferroelectric properties of PZT.^10–12)

Fig. 6

Room temperature P–E hysteresis loops of the PSZT ceramic ring at an electric field of ±9 kV/cm and frequency of 1 kHz, the inset shows the circuit diagram of Sawyer Tower.

3.3 Piezoelectric properties

Figure 7 shows the resonance spectrum of the PSZT ceramic ring in the thickness direction analyzed by an LCR meter 3550. The impedance dependence of the material and phase of the resonant vibration spectrum on the excitation frequency range has been shown. The PSZT sample displays the resonant and antiresonant frequencies as 32.914 kHz and 35.875 kHz, respectively. Furthermore, near the resonant frequency the phase changes from −87.8° to +84.8° and it again varies from +84.5° to −89.6° in the near antiresonant frequency range. The characteristic of the resonance oscillation spectrum between these two ranges has a sharp squareness because the phase of the PSZT ceramic remains close to +90°. According to the above results, the PSZT ceramic sample showed an inductive behaviour consistent with the equivalent circuit and good piezoelectric properties.

Fig. 7

Resonant spectroscopy of PSZT ceramic ring in a radial direction at room temperature.

From the impedance spectrum of the PSZT ceramic ring, we can determine the electromechanical coupling coefficient values according to eqs. (1) (2). The effective electromechanical coupling coefficient (k_eff) and the electromechanical coupling coefficients (k_p) of the PSZT ceramic ring are 0.40 and 0.48, respectively. In addition, the mechanical quality factor Q_m of the PSZT ceramic ring is also determined to be around 270.92 with an impedance at the resonant frequency Z_m of 46.22 Ω. Furthermore, the piezoelectric constant (d₃₃) of the PSZT ceramic ring was also determined using a piezoelectric d₃₃ Meter with a vibration frequency of 110 Hz and an applied force of 0.25 N. Accordingly, the d₃₃ value obtained for the PSZT ceramic ring was 314 pC/N. The PSZT ceramic ring with high d₃₃ values (> 265 pC/N) have met the requirements for their inclusion in industrial applications.²⁵⁾ Although larger quantities of PSZT powder must be produced for ring pressing (with an outer diameter of 5 mm, inner diameter of 17 mm, and thickness of 6.5 mm), the proposed PSZT piezoelectric ring product has also been compared with previously published small pellet disk configurations or commercial materials (with diameters and thicknesses of 10 mm and 1 mm, respectively), as illustrated in Table 1.^{10–12,14,26,27)} The fabricated PSZT ceramic ring can meet the requirements of practical applications, especially for high-power ultrasonic welding.

Table 1 Comparison of the properties of PZST ceramic ring with other hard piezoelectric ceramics pellet disk (published or commercial).

4. Conclusions

In summary, a fabrication process of piezoelectric ceramic ring based on the lead zirconate titanate material Pb_1−xSr_x(Zr_0.53Ti_0.47)O₃ (PSZT), where x = 6.0 mol%, has been directly synthesized using a ball milling method and solid-state reaction from a mixture of precursor oxides. The PSZT ceramic ring, after sintering at 1200°C for 3 hours, possesses with a thickness of 6.5 mm and outer and inner diameters of 50 mm and 17 mm, respectively. At room temperature (RT), the PSZT ceramic ring exhibits the relative dielectric constant (ε_r) of 1707.4 and dielectric loss (tan δ) of 0.0034 at 1 KHz, the remaining polarization P_r of 2.0 µC/cm², and the coercive field E_C of 5.5 kV/cm. The T_C of the PSZT ceramic ring is 342°C corresponding to the maximum relative dielectric constant value of 16,444. The characteristic piezoelectric parameters of the PSZT ring are as follows: k_eff is 0.40, k_p is 0.48, Q_m is 270.92, and d₃₃ is 314 pC/N. The manufacturing of the PSZT piezoelectric ceramic ring using this method and process is considered to be relatively straightforward, cost-effective, and amenable to industrial-scale expansion. The fully fabricated PSZT ceramic ring holds potential for use in high-power ultrasonic transducer fabrication or other practical applications.

Acknowledgements

This research has been done under the research project 11/2021/HĐ-ĐTCB. Nguyen Dang Co, ID VNU.2021.NCS.06, thanks The Development Foundation of Vietnam National University, Hanoi for sponsoring this research. Nguyen Dang Co was funded by the Master, PhD Scholarship Programme of Vingroup Innovation Foundation (VINIF), code VINIF.2022.TS.016.

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