Interpretation of the Thickness Resonances in Ferroelectret Films Based on a Layered Sandwich Mesostructure and a Cellular Microstructure

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 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 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 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.

Modification of Mechanical and Electromechanical Resonances of Cellular Ferroelectret Films Depending on the External Load

“…Transmission coefficient measurements of these films in air have been previously studied and reported (see Refs. [ 15 , 42 ]). The only difference in this case, compared with previously published results, is the presence of a sputtered Au layer.…”

“…As this is of interest for this work, and for the potential use of these films for medical transducers and hydrophones, a more detailed analysis of this point has been performed. In particular, the studied films in [ 42 ], the same that we have used to fabricate the samples for this work, were used to measure transmission coefficient in water. For these samples, the transmission coefficient in air is well described by a theoretical model based on a layered structure, in particular, a sandwich structure.…”

“…We have measured transmission coefficient measurements for these samples (HS03 and HS06), but in this case, in water. Then, we have used the same material parameters obtained in [ 42 ], from the air-coupled measurements, and used them to calculate the expected response in water. If the FE film remains unmodified, then the calculated transmission coefficient of the film in water using the material parameters obtained from air-coupled measurements should match the experimental measurements.…”

“…Ref. [ 42 ] supposed a step forward, as it showed that air-loaded thickness resonances of these films can be better explained if a sandwich meso-structure together with a cellular microstructure is assumed. This conclusion is consistent with SEM images of the FE film structure.…”

“…First, we measured the transmission coefficient in water immersion at normal incidence and for a frequency band that, at least, covers the first order of the thickness resonances—in most cases, the first two orders. Similar measurements were performed in air for the same materials [ 15 , 42 ]; in the latter case, material parameters were extracted by assuming a sandwich structure for the film and solving the inverse problem. Observed resonances in air and in water have been compared and differences have been analyzed.…”