A novel PZT-based high-power piezoelectric vibrator for ultrasonic motors: properties enhanced by Yb institution

1. Introduction

Spaceflight is a highly demanding and prevalent technology in the world today (Wang et al., 2022; Wang and Cao, 2022). Spacecrafts serve as the carriers of spaceflight applications and are equipped with various high-precision mechanisms to meet diverse needs such as precise control, accurate observation and stable operation. Electric motors act as the driving source for these mechanisms and play a crucial role in the overall performance of spacecraft mechanisms (Bi et al., 2021). In recent years, ultrasonic motor (USM) has attracted much attention to the advantages of light weight, large torque and fast response (Tian et al., 2020). The USM differs from conventional electromagnetic motors in that they generate controlled microscopic vibrations through the inverse piezoelectric effect of the piezoelectric ceramics inside the piezoelectric vibrator (see Figure 1). These vibrations are then transformed into macroscopic rotations through frictional transmission (Zhang et al., 2021). Therefore, the performance of piezoelectric materials plays a crucial role in the mechanical output characteristics and environmental applicability of USM (Dong and Shu, 2012). At present, the most commonly used piezoelectric materials in USM are mainly Pb(Zr, Ti)O₃ (PZT) based ceramics, especially, “hard” type PZT-8 commercial piezoelectric ceramics (Li et al., 2014a, b). However, harsher applications such as outer space environments require more advanced USM with larger torque output and longer utilization. Hence, it is urgently necessary to develop piezoelectric ceramics with a large piezoelectric coefficient and electromechanical coupling coefficient, high mechanical quality factor and mechanical strength, as well as low dielectric loss (Guo et al., 2011). Uchino et al. carried out systematic rare earth doping investigations on PZT-Pb(Mn,Sb)O3 (PMS) (Zhu et al., 2005; Gao and Uchino, 2016; Shekhani et al., 1998) and found that certain transition metal substituents show a combination of softening and hardening effects in piezoelectric ceramics. The vibration velocity of the piezoelectric vibrators was significantly increased by these rare earth substituents (from Yb to Cerium (Ce)) under high driving conditions, and Yb has the most significant effect on improving the high-power performance of ternary piezoelectric ceramics such as PZT-PMS. The Pb(In_1/3Nb_2/3)O₃ (PIN) was introduced into PMS-PZT to form a quaternary system, and its mechanical quality factor was higher than that of Pb(Zn,Nb)O3 (PZN)-PMS-PZT (Cheng et al., 2010; Wu et al., 2018), PZN-Pb(Ni,Nb)O3 (PNN)-PZT (Wang et al., 2017; Zhang et al., 2016) and other quaternary high-power piezoelectric ceramics (Li et al., 2019; Feng, 2019). However, to match the requirements of outer space applications, the performance of the piezoelectric ceramics needs to be further improved.

Element substitutions are usually utilized to achieve “soft” and “hard” combination effects in PZT-based piezoelectric ceramics (Xin et al., 2020; Jin et al., 2022). The radius of doping element ions must be between the perovskite structure's A-site ions and B-site ions (Gao and Uchino, 2016). In this paper, 0.92Pb(Zr_0.48Ti_0.52)O₃-0.05Pb(Mn_1/3Sb_2/3)O₃-0.03Pb(In_1/2Nb_1/2)O₃ high-power piezoelectric ceramic was chosen as the base composition, and a novel piezoelectric vibrator was developed, which demonstrated exceptional high-power performance. The incorporation of the element Yb played a crucial role in enhancing the vibrator's functionality `and the effect of Yb³⁺ doping on the crystal structure, micromorphology and the piezoelectric performance of the ceramic system is studied. And then the vibration velocity and the corresponding high-power characteristics of the vibrators were discussed.

2. Experimental procedures

The ceramics 0.92PZT-0.05PMS-0.03PIN-x mol% Yb (x = 0, 0.1, 0.2, 0.4, 0.6, 0.7) modified by Yb³⁺ were synthesized by conventional solid-phase reaction method, and the experimental steps are shown in Figure 2. Starting oxides of TiO₂ (99.80%), MnO₂ (99.95%), ZrO₂ (99.90%), PbO (99.90%), In₂O₃ (99.99%), Nb₂O₅ (99.99%) and Sb₂O₃ (99.99%) were weighed stoichiometrically according to the compositional formula 0.92Pb(Zr_0.48Ti_0.52)O₃-0.05Pb(Mn_1/3Sb_2/3)O₃-0.03Pb(In_1/2Nb_1/2)O₃, and 3 wt% excess PbO was added to compensate the possible lead loss during sintering.

InNbO₄ precursor was synthesized firstly by calcining In₂O₃ and Nb₂O₅ mixture at 850 °C for 4 h. Then the obtained precursor and the other powders were wet-milled for 12 h in an ethanol medium. After drying the mixture was calcined at 850 °C for 2 h, causing the water in the original ingredient oxide to evaporate. Following this, the resulting calcined powder was crushed through ball milling for 16 h. To this milled powder, a 5 wt% polyvinyl alcohol (PVA) binder was added, and the mixture was then pressed into discs under 250 MPa pressure. The ceramics underwent sintering in a sealed alumina crucible at a temperature of 1,150 °C for a duration of 2 h. To minimize the loss of lead, the samples were covered with their original powder. As a result of this process, the volume of the samples decreased while the density increased. To achieve the desired thickness of approximately 500 mm, sandpaper polished the ceramic sheet on both sides. Following this, silver electrodes were brushed onto both sides and the sheet was calcined in a furnace at 700 °C for 30 min to dry and set the electrodes. The sample was poled in silicone oil at a temperature of 120 °C for 20 min while applying an electric field of 4 kV/mm. The electrical properties were measured 24 h after the polling process.

The crystal structures of the sintered samples were analyzed by using an X-ray diffractometer (Panalytical Empyrean, λ = 1.506 nm). For microstructure observation, samples were thermally etched at 1,120 °C for 15 min and observed by scanning electron microscope (SEM, Quanta 200, FEI Company). The quasi-static piezoelectric coefficient meter (ZJ-3AN, Institute of Acoustics, Chinese Academy of Sciences) was used for piezoelectric measurement. Dielectric properties were measured by utilizing an impedance analyzer (Agilent 4294A) with a laboratory heating unit. Ferroelectric hysteresis loops were obtained using a ferroelectric analyzer (Radiant, Multiferroic 100). A Doppler laser vibrometer (Polytec PSV-300F, Germany) and a power amplifier (DPO2014, Tek, America) were used for piezoelectric vibration measurements, during which the temperature rise was determined by an infrared radiation thermometer (Victor 303, China).

3. Result and discussion

1. Crystal structure and microstructure

Figure 3(a) shows the XRD patterns of PZT-PMS-PIN-x mol% Yb (0 ≤ x ≤ 0.7) ceramics sintered at 1,150 °C. All samples present a pure perovskite structure. The local peaks of (002)_T/(200)_T and (200)_R for tetragonal and rhombohedral symmetry around 43°–46° are presented in Figure 3(b). We analyzed the phase content variations of samples sintered at different Yb³⁺ concentrations by comparing the ratio of the peak intensities of the tetragonal (t) and rhombohedral (r) phases. The rhombohedral phase content (Χ_r) was calculated using the formula Χ_r = I_{200, t} + I_{200, r} + I_{002, t}. (where Χ_r denotes the content of the rhombohedral phase, I_{200, r} is the peak intensity of the (200)_R peak, I_{200, t} is the peak intensity of the (200)_T peak and I_{002, t} is the peak intensity of the (002)_T peak). The values of I_{200, t} I_{200, r} and I_{002, t} peaks were 21.56%, 23.05%, and 23.42%, 23.74%, 27.16% 62.93%, respectively, for Yb concentrations ranging from 0 to 0.7 mol%. At x = 0, the ceramic of 0.92PZT-0.05PMS-0.03PIN presents mainly tetragonal symmetry. The composition of PZT-PMS-PIN-0.7 mol% Yb shows a rhombohedral symmetry mainly. With the increase of Yb content, the content of rhombohedral overall upward trend, and at Yb additions of x = 0.1 and 0.4, the amount of Χ_r of this composition is almost unchanged and exhibit coexistence of rhombohedral and tetragonal symmetry.

Figure 4(a) shows the rhombohedral lattice parameters of PZT-PMS-PIN-x mol% Yb ceramics calculated by unit-cell software. The calculation illustrates that the a-axis of the lattice increases and the c-axis decreases with increasing Yb³⁺ content, hence leading to a decrease in the c/a ratio (Yan et al., 2012). The value of c/a in piezoelectric ceramic lattice determines its tetragonal degree. A c/a value of 1 indicates a tripartite phase, while a value greater than 1 indicates the existence of a tetragonal phase. The decreased c/a ratio indicates the tetragonality of ceramic decreases. According to the element ion radii shown in Table 1 (Liu and Li, 2020), the Yb³⁺ ion has a radius smaller than the Pb²⁺ ion, when it locates in A-site to replace the Pb²⁺ ion, the unit cell parameters will reduce. When Yb³⁺ replaces B-site ions, the unit cell parameters will enlarge since the radius of Yb³⁺ is larger than that of B-site ions. According to the change in lattice parameters, it is inferred that Yb³⁺ trends to dope on B sites, as shown in Figure 4(b).

Figure 5 presents the SEM micrographs of PZT-PMS-PIN-x mol% Yb ceramics, and it can be found that all samples show a dense microstructure. As can be seen from the figure, there are no impurities formed in the ceramic, and as the Yb content increases, the grain size of the ceramics decreases and the size distribution becomes more uniform. At higher doping contents of 0.6 and 0.7 mol%, a few small pores on the grain boundary are observed, which may be due to the component aggregation and volatilization at high doping contents.

1. Ferroelectric characteristics

Figure 6(a) shows P-E loops of the ceramics under different external electric fields. It can be found that saturated P-E loops can be achieved for all the ceramics under a 4.5 kV/mm electric field. For these ceramics with Yb doping content x ≤ 0.2 mol%, the P-E loops show a pinched shape, indicating the possible existence of oxygen vacancies, defect dipoles alignment pins domain switch during P-E loop measurement (Li et al., 2014b; Eichel, 2010; Shrout and Zhang, 2007; Slabki et al., 2021). With the increase of Yb addition, more rectangular P-E loops are observed. From the asymmetrical P-E loops, the values of the internal electric field (E_i) of the ceramics are obtained. The remnant polarization P_r, coercive field E_c, and the internal electric field E_i of the PZT-PMS-PIN-x mol% Yb ceramics are summarized in Figure 6(b). The P_r increases significantly with the increase of Yb amount, showing that Yb substitution helps to reduce the pinning effects and leads to more complete switch of the domains. The E_c and E_i increase first and then decrease with more amount of Yb addition. At Yb content of 0.4 mol%, the ceramic reaches maxima of E_c and E_i, displaying “hard” characters of Yb doping.

Yb³⁺ ions tend to preferentially occupy positions with smaller ionic radius differences at a low content. It can be inferred that the Yb³⁺ ions in this doping range occupy the B-position and combine with oxygen vacancy to form defect dipoles so that the internal electric field of the sample shows an increasing trend with the increase of doping. As the doping amount continues to increase, some of the Yb³⁺ ions start to occupy the A-site, which results in a reduction of oxygen vacancies inside the sample and a reduction of E_i, and when the doping amount increases to 0.6 mol%, the replacement of the A-site Yb³⁺ ions increase significantly, the internal electric field decreases and the bundle waist disappears. As reported in PZT-PMS-Yb ceramic, the enhancement of the internal electric field can be explained by an additionally generated oxygen vacancy by Yb doping (Yang et al., 2021), which could increase or stabilize the localized dipole moment. And the enhanced E_i is essential to the piezoelectric ceramic for high-power functions (Yang et al., 2006).

1. Dielectric and piezoelectric properties

The dependences of room temperature dielectric constant and dielectric loss on the Yb³⁺ doping contents in PZT-PMS-PIN-x mol% Yb ceramics are illustrated in Figure 7. It shows that the dielectric constant reaches the maximum of 1,496 at 0.1 mol% of Yb content and then decreases monotonously with the increase of Yb addition. Usually, the compositions in the morphological phase boundary (MPB) present a large dielectric constant since the coexistence of rhombohedral and tetragonal phases benefits the transitions between both phases under an external electric field or stress (Zhang et al., 2015). Hence, the ions' motion and polarization are easier, and it results in a larger dielectric response. However, other factors such as grain size, domain structure and electric field bias also exert complicated effects on the dielectric properties of the piezoelectric ceramics (Qi et al., 1999; Li et al., 2005). The enhanced internal electric field in 0.4 mol% Yb-doped ceramic may be responsible for the decreased dielectric constant at room temperature.

The temperature-dependent dielectric constant of PZT-PMS-PIN-x mol% Yb ceramics at 1 kHz frequency is shown in Figure 8. It can be seen that no position shift of the dielectric peak is induced with the addition of the Yb dopant (Liao et al., 2019). The Curie temperature T_c of the ceramics is around 326 °C. The inset shows the maximum ε_r of the ceramics in terms of Yb contents. With the increase of Yb doping amount, the maximum dielectric constant increases first and then decreases. At 0.2–0.4 mol% of Yb doping contents, the ceramics in the MPB region present a relatively large dielectric maximum.

The piezoelectric properties of PZT-PMS-PIN-x mol% Yb ceramics are presented in Figure 9. For the compositions in the MPB area with Yb from 0.1 to 0.4 mol% enhance piezoelectric coefficient of d₃₃ values are obtained compared to the ceramic without Yb doping. Maximum d₃₃ of 467 pC/N and planar coupling factor k_p of 0.67 is achieved in the ceramic of 0.2 mol% Yb addition. However, for x = 0.4 mol% Yb ceramic, the largest mechanical factor Q_m of 1,692 is achieved because of the enhanced E_i induced by Yb doping in the ceramic. As can be seen from Figure 6(b) and Figure 9, these curves of E_i and Q_m show high consistency concerning Yb doping contents. Despite its slightly decreased d₃₃ and k_p values, to a certain degree, large Q_m is more beneficial to high-power applications.

1. High-power properties

Figure 10 shows the measured vibration velocities of PZT-PMS-PIN-x mol% Yb ceramics under different external electric fields. As can be seen, the vibration velocity linearly increases with the driven field. However, the ceramics present different slopes with different Yb contents.

For a round shape sample vibrating in k_p mode, the vibration velocity υ_m is calculated as follows (Li et al., 2005):

The vibration velocity of piezoelectric vibrators using PZT-PMS-PIN-x mol% Yb ceramics at a driving voltage of 20V is illustrated in the inset of Figure 10, the black line lists the measured data from Figure 10 and the red line lists the calculated velocities according to Eq. (1). It can be seen that these two curves are highly consistent. With the increase of Yb doping amount, the vibration velocity under the same driving field increases first and then decreases. The sample with 0.4 mol% Yb presents the highest velocity than other ceramics since the sample displays an improved mechanical quality factor.

When piezoelectric vibrators work with high vibration velocity, considerable heat will be generated. To prevent possible performance deterioration from overheating the devices, the vibration velocity of piezoelectric vibrators must be under control during work. Therefore, another parameter to characterize the high-power performance of the piezoelectric ceramic component inside the vibrator is the maximum vibration velocity at the constraint condition of 20 °C temperature rise (Doshida et al., 2019). During the measurement, the piezoelectric vibrator vibrates at different velocities when driven by different electric fields. The corresponding temperature rising is recorded by an IR thermometer. Figure 11 shows temperature rises of PZT-PMS-PIN-x mol% Yb during vibrations. As shown, the temperature of the piezoelectric vibrators rises sharply with the increase in vibration velocity. The inset of Figure 11 displays the reached maximum vibration velocity of the ceramics under 20 °C temperature rise conditions. Among them, the ceramic inside the vibrator with 0.4 mol% Yb content reaches the maximum vibration of 0.81 m/s, which is much higher than that of the conventional PZT ceramics, as we can see the purple lines in Figure 11, where v_m is about to 0.4 m/s (Yoon et al., 2010). According to Eq. (1), high Q_m benefits high v_m. The ceramic PMT-PMS-PZT-0.4 mol% Yb presents the highest mechanical quality factor and thus higher v_m under the same driving field or less heat at the same vibration velocity is generated in the vibrator.

4. Conclusion

For the major needs of aerospace development, this paper carried out a study on the modification of piezoelectric vibrators, one of the core components inside the USMs of special equipment for spaceflight. The main conclusions are drawn as follows.

The perovskite structure contains A-site and B-site ions, and the ionic radius of Yb³⁺ falls between these two ions. When Yb³⁺ is used as a dopant in ceramics, it replaces the B-site ion due to its smaller size compared to Pb²⁺. This replacement results in the introduction of oxygen vacancy pinning electric domains. The ceramics exhibit enhanced piezoelectric performance at 0.1–0.4 mol% of Ytterbium (Yb) doping contents since the compositions locate in the MPB region with the coexistence of rhombohedral and tetragonal phase and the addition replaces B-site ions to obtain an enhanced internal bias field. The ceramic of 0.4 mol% Yb reaches the maximal internal bias field and presents a larger mechanical quality factor of 1,692. The vibrator with this large mechanical quality factor ceramic displays a vibration velocity of up to 0.81 m/s under the constraint of 20 °C temperature rising, which is a 103% improvement compared to commercially commercial ceramic PZT-8. This improved high-power characteristic is attributed to the enhanced inner electric-field bias by Yb substitution, and this novel vibrator may provide a potential application for high-performance USMs.