Our last paper in UFFC-IEEE reveals the hidden sandwich mesostructure of ferroelectret films

Interpretation of the thickness resonances in ferroelectret films based on a layered sandwich mesostructure and a cellular microstructure. Ultrasound, Ferroelectrics and Freq. Control Transactions

DOI: 10.1109/TUFFC.2020.3025358

Abstract:

Transmission coefficient spectra of two ferroelectret
films (showing several thickness resonances) measured with aircoupled

ultrasound (0.2–3.5MHz) 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 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
presented 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 tends to be more
flattened (supporting the proposed sandwich model). The fact
that more flattened cells contributes 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 which is
governed, mainly, by cell wall bending. Consistency of extracted
parameters with trends shown by 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.