For transducer design it is essential to know the acoustic properties of the materials in their operating conditions. At frequencies over 15 MHz, standard methods are not well adapted because layers are very thin and backings have very high attenuation. In this paper we report on an original method for measuring the acoustic properties in the 15 to 25 MHz frequency range, corresponding to typical skin imaging applications, using a backing/piezoelectric multilayer structure. Onto a porous Pb(Zr0.53Ti0.47)O3 (PZT) substrate, a piezoelectric PZT-based layer with a thickness of ~ 20 μm was deposited and directly used to excite an acoustic signal into water. Here, the measured signal corresponds to the wave that is first reflected on a target in water, then propagates back to the multilayer structure, is transmitted through the thick film and further to the rear face of the porous backing, where it is again reflected and returns to the piezoelectric thick film, thus avoiding overlap with the electrical excitation signal. Two types of PZT backings with similar porosity of ~ 20 % and spherical pores with size of 1.5 μm and 10 μm were processed. The ultrasound group velocities were measured at ~ 3500 m/s for both samples. The acoustic attenuation of the backings with pore size of 1.5 μm and 10 μm were 12 dB/mm and 33 dB/mm, respectively, measured at 19 MHz. This advanced measuring technique demonstrated potential for simple measurements of acoustic properties of backing at high frequencies in operating conditions. Importantly, this method also enables rapid determination of the minimum required thickness of the backing in order to act as a semi-infinite medium, for high-frequency transducer applications.

The transducer is an essential part of all ultrasonic systems used for applications such as medical diagnostics, therapy, non-destructive evaluation, cleaning…because its health condition is vital to their proper operation. Defects within the active element, backing or other constitutive elements, loss of adhesion between layers can significantly weaken the performance of a transducer. The objective of this work is to determine procedures to monitor the behavior of a single-element probe during its lifetime and detect degradations before they significantly affect the performance of the system. To achieve this, electromechanical admittance (EMA) based method is envisaged numerically and experimentally. A simplified single-element transducer consisting of a piezoceramic disc, a bonding layer, and a backing is studied and the influence of bonding delamination on EMA is investigated. This study considers three different types of delaminations, which are named respectively “center” (circular delamination from the center of the disc towards the peripheric zone), “peripheric” (annular delamination from the peripheric zone towards the center), and “wedge” (wedge-shaped delamination with a given angle). For each case, a numerical model based on the finite element method is developed: a two-dimensional (2D) finite element analysis is implemented for the first two types of delaminations, taking advantage of their axisymmetric structure, and “wedge” delamination is modelled in three dimensions (3D). Then, transducers with different shapes of 3D printed backings are mounted and experiments are conducted using an impedance analyzer. Finally, experimental results are found to be in good agreement with numerical solutions and it shows that changes in EMA can particularly reveal the occurrence and extent of delamination in an ultrasound probe.

Symmetric reflector ultrasonic transducers (SRUTs) are a new type of ultrasonic transducer that take advantage of the ultrasonic wave emitted on the rear face of the active element. In this work, the electroacoustic modeling and characterization of such a structure is reported. Using the well-known Krimholtz, Leedom, et Matthaei (KLM) model, the electroacoustic response of a SRUT based on a piezoelectric ceramic with one matching layer is calculated. Simulations show that the optimal acoustic impedance of the matching layer should be lower than for conventional transducers, leading to a relative bandwidth of approximately 50%. The characterization of such a transducer based on a piezoceramic plate has been carried out. Bandwidth and sensitivity are reported. They are found to be close to the simulation results and demonstrate that these new types of transducers need to be designed according to new rules compared to conventional ones.

Ultrasonic transducers performance can be seriously deteriorated by loss of adhesion between some constitutive elements such as the active element, the backing, or the matching layer. In the present work, the influence of bonding delaminations on the performance of a single-element ultrasonic transducer, which is composed of a piezoelectric disk, a backing, and a matching layer, is studied numerically and experimentally. Based on the positions between layers, two cases, i.e., delaminations between ceramic and backing or between ceramic and matching layer, are considered. Each case involves three different types of delaminations, which are marked as delamination type (DT)-I, II, and III. DT-I, a circular shape delamination, starts from the center and expands towards the peripheric zone; DT-II, an annular shape delamination, starts from the peripheric zone and expands towards the center; DT-III is a sector shape delamination with a given angle. The numerical simulations are performed by the finite element method and the influence of delaminations on the electromechanical admittance (EMA) of the transducer is investigated. 3D printed backings and matching layers are mounted on a PZT sample to assemble delaminated single-element transducers. An impedance analyzer is used for experimental measurements. Comparison between numerical and experimental results shows a reasonable agreement making changes in EMA an interesting indicator to inform about the occurrence and severity of delaminations in a single-element ultrasonic transducer.