**1. Introduction**

Piezoelectric ultrasonic and underwater sound transducers, by virtue of their excellent electromechanical properties, have been widely applied in medical imaging [1–3], non-destructive testing [4,5], underwater communications [6,7], and so on. In recent years, researchers have been trying to improve their design in hope of obtaining more excellent abilities to perform related tasks. Many attempts have been made and this research topic has received more and more attention.

One effective attempt is to add a flexible polymer phase when fabricating the piezoelectric/polymer composites. These composites can overcome the shortcomings of single-phase piezoelectric materials, such as brittleness, high acoustic impedance, and at the same time possess the advantages of both the piezoceramics and the polymers, including low acoustic impedance, high electromechanical coupling coefficient, low mechanical loss and large dielectric constant [8,9]. Through designing and optimizing various connectivity patterns, such as 1-3 type [10–18], 2-2 type [3,19–21], these transducers can be used in high-frequency, high-power, and high-temperature applications [9]. In addition, an addition of modified epoxy, i.e., aluminum load epoxy, can also enhance their dielectric, piezoelectric and acoustic

dampening [22–24], which is very useful for structural health monitoring, energy harvesting, and acoustic liners [22].

Another approach is to adopt new structural forms to realize multiple functionalities. The earlier transducers mainly adopted the approximated one-dimensional structures, which can excite one-dimensional longitudinal waves. The representative devices are the longitudinally sandwiched transducers [25–29] that are composed of axially polarized piezoceramic rings and end metal masses. Recently, two dimensional structures, i.e., cylindrical configurations, were proposed to increase the wave coverage area and output power. These structures include tubes, shells, disks and rings [30–47]. They can realize the radial wave radiations by utilizing their radial vibrations, and can be used as the omni-directional emitter and receiver in underwater sound and ultrasonic applications [48].

The above two methods can improve the performance of piezoelectric transducers to meet different requirements. However, both of these methods have certain limitations. In addition, in some special applications, such as endoscopic ultrasound (EUS), it is required that the transducer is able to realize 360◦ imaging [49]. To obtain a comprehensive performance, some efforts have been made to focus on combining the advantages of piezoelectric/polymer composites and those of cylindrical configurations. One attempt was adopting the 1-3 type piezocomposite to design the cylindrical EUS transducers, which can acquire high-resolution EUS imaging [50]. Another attempt was using the 2-2 type piezocomposite to develop the cylindrical underwater acoustic transducers, which can achieve high-frequency wideband ability [51]. In the latest work, a new type of radially layered cylindrical piezoceramic/epoxy composite transducer was developed by integrating two concentric axially polarized piezoceramic rings into cylindrical epoxy matrixes, which is expected to be utilized in developing novel ultrasonic and underwater sound techniques [52]. This preliminary study mainly focused on studying the effects of the external resistance on the electromechanical characteristics of the transducer, which lacked of clear understanding on the influences of other key factors, including material parameters of epoxy layers, piezoceramic material types, and locations of piezoceramic rings. To clearly understand the electromechanical characteristics of this type of transducer and to guide the device design, in this paper, parametric analysis was performed to reveal the effects of these multiple key factors on the electromechanical characteristics of the transducer.

The remainder of this paper is organized as follows. Section 2 exhibits a schematic representation of the radially layered cylindrical piezoceramic/epoxy composite transducer and gives its modified theoretical model based on the model developed in previous work. Section 3 validates the theoretical solution by comparing it to solutions from both ANSYS numerical simulation and experimental investigation. Section 4 discusses the effects of material parameters of epoxy layers, piezoceramic material types, and locations of piezoceramic rings on the electromechanical characteristics of the transducer through numerical analysis. Section 5 draws the conclusions of the paper.
