Thick films with nominal composition (K₀.₅Na₀.₅)₀.₉₉Sr₀.₀₀₅NbO₃ (KNNSr) on porous ceramics with identical nominal composition were investigated as potential candidates for environmentally benign ultrasonic transducers composed entirely of inorganic materials. In this paper, the processing of the multilayer structure, namely, the thick film by screen printing and the porous ceramic by sacrificial template method, is related to their phase composition, microstructure, electromechanical, and acoustic properties to understand the performance of the devices. The ceramic with a homogeneous distribution of 8 μm pores had a sufficiently high attenuation coefficient of 0.5 dB/mm/MHz and served as an effective backing. The KNNSr thick films sintered at 1100 °C exhibited a homogeneous microstructure and a relative density of 97%, contributing to a large dielectric permittivity and elastic constant and yielding a thickness coupling factor k_t of ~30%. The electroacoustic response of the multilayer structure in water provides a centre frequency of 15 MHz and a very large fractional bandwidth (BW) of 127% at −6 dB. The multilayer structure is a candidate for imaging applications operating above 15 MHz, especially by realising focused-beam structure through lenses to further increase the sensitivity in the focal zone.

The acoustical modelling of multilayered systems is crucial for researchers and engineers aiming to evaluate and control the behaviour of complex media and to determine their internal properties. In this work, we first develop a forward model describing the propagation of acoustic waves through various types of materials, including fluids, solids, and poroelastic media. The model relies on the classical theoretical frameworks of Thomson and Haskell for non-porous layers, while Biot’s theory is employed to describe wave propagation in poroelastic materials. The propagation is mathematically treated using the transfer matrix method, which links the acoustic displacement and stress at the extremities of each layer. Appropriate boundary conditions are applied at each interface to assemble all local matrices into a single global matrix representing the entire multilayer system. This forward model allows the calculation of theoretical transmission coefficients, which are then compared to experimental measurements to validate the approach proposed. Secondly, this modelling framework is used as the basis for solving inverse problems, where the goal is to retrieve unknown internal parameters, such as mechanical or acoustic properties, by minimizing the discrepancy between simulated and experimental transmission spectra. This inverse problem approach is essential in non-destructive evaluation applications, where direct measurements are often unfeasible.

The purpose of this study is to characterise piezoceramic materials using their hysteretic behaviours under high electrical loading. In previous works, a phenomenological model has been constructed by bridging characteristics of microscopic domain wall distribution into macroscopic behaviours. In this present work, an inverse problem solving is proposed to retrieve electromechanical parameters of piezoceramic materials (relative permittivity and piezoelectric coefficient) and microscopic parameters (spontaneous polarisation and spontaneous strain). Optimisation is performed on dielectric and butterfly curves using a genetic algorithm procedure in order to ensure robust convergence even if several local solutions exist. Method is validated on comparing results obtained for PLZT8/65/35 and P188, whose parameters are well known in literature. Good agreements observed between retrieved parameters and values reported in literature show that domain wall modelling is a promising way to characterise piezoceramic materials.