Transmission coefficient spectra of two ferroelectret films (showing several thickness resonances) measured with air-coupled ultrasound (0.2-3.5 MHz) are presented, and an explanation for the observed behavior is provided by proposing a film layered sandwich mesostructure (skin/core/skin) and by solving the inverse problem, using a simulated annealing algorithm. This permits us to extract the value of the ultrasonic parameters of the different layers in the film, as well as overall film parameters. It is shown that skin layers are thinner, denser, and softer than core layers and also present lower acoustic impedance. Similarly, it is also obtained that the denser film also presents lower overall acoustic impedance. Scanning electron microscopy was employed to analyze the films' cross section, revealing that both denser films and film layers present more flattened cells and that close to the surface cells tend to be more flattened (supporting the proposed sandwich model). The fact that more flattened cells contribute to a lower elastic modulus and acoustic impedance can be explained, as it has been made previously by several authors, by the fact that the macroscopic film elastic response is furnished by cell micromechanics, is governed, mainly, by cell wall bending. The consistency of extracted parameters with trends shown by a simple model based on a honeycomb microstructure is discussed, as well as the possibilities that this sandwich mesostructure and the associated impedance gradient could offer to improve the performance of FE films in ultrasonic transducers.
Ministerio de Economíay Competitividad (Grant Number: DPI2016-78876-R (AEI/FEDER, UE))