**3.2 Piezoelectric materials in USM**

Piezoelectric materials used in USM, plays an important role in determining its performance. In the following table we tabulated some prominently used material & their properties (**Table 3**).

Wu et al. explored how elliptical shapes and force factors of the polymerbased vibrators vary as several key structural parameters were changed [229]. Subsequently, attempt to improve the maximum torques of the polymer-based USMs by adjusting several key dimensions, and the reason for their relatively low output torques and power compared to the metal-based USMs were done. Further he employed a high-order bending mode in the polymer-based cylindrical ultrasonic motor, because this mode yields a relatively high electromechanical coupling factor, which may lead to high output power of the motor. Additionally, in contrast with the low-order modes with only vertical nodal lines, the high-order mode has both horizontal and vertical nodal lines on the circumferential outer surface of the polymer-based vibrator [230]. Similarly Wu et al. also researched on, polyphenylene sulfide (PPS)-based bimodal piezoelectric motor. Considering the viscoelasticity of PPS, the electromechanical coupling analytical model was established to describe the dynamics of the PPS-based motor by using the Kelvin-Voigt viscoelastic model. Based on the proposed model, the Taguchi method was adopted to match the resonance frequencies of the longitudinal and bending vibration. The performance test demonstrates that the PPS-based motor could yield the maximal torque of 2 mNm with the stator weight of 5.4 g [232]. Wang et al., fabricated a lead zirconate titanate (PZT) thick-film piezoelectric micro stator based on a



*The Roles of Piezoelectric Ultrasonic Motors in Industry 4.0 Era: Opportunities & Challenges DOI: http://dx.doi.org/10.5772/intechopen.100560*

#### **Table 3.** *The list of piezoelectric materials for USM.*

high-performance PZT thick film by electrohydrodynamic jet (E-jet) printing in order to simplify the manufacturing process and enhanced the performance of the piezoelectric stator. The thick-film micro stator produced a traveling wave with an amplitude of 345 nm, and the mechanical quality factor was found to be 736 [236]. Zhao et al., proposed an oblate-type ultrasonic micro-motor with multilayer piezoelectric ceramic with chamfered driving tips. The micro-motor works based on the standing-wave principle and has a higher rotary speed than the traditional standing-wave. The experimental results showed that the rotary speed was around 2000 r/min at the voltage of 20 Vp − p [242].
