Battery separator me3mbranes are thin polymeric microporous films placed between
battery electrodes. They are a key component that strongly affects both battery performance and
security and controlling its main properties is critical for the industry. In particular, the
manufacturing process demands tests that allow checking both the mechanical integrity
(porosity, thickness, pore size, etc.) of the materials and the consistency of these properties
during the construction of the batteries. The use of an air-coupled ultrasonic system is proposed
in this work. It is expected that this will make possible the non-destructive and quick testing of
these materials, in order to evaluate thickness, stiffness, pore distribution and pore size of the
separator. By means of three pairs of transducers, the modulus and phase of the transmission
coefficient was measured from 0.35 to 1.4 MHz in different materials. The measurements show
a clear modification of the film response produced by the presence of porosity. This suggests
that the transmission of ultrasonic waves in battery separators using air-coupled ultrasound may
have two contributions: one corresponds to the propagation though the solid part, the other
corresponds to propagation in the pores. The former allows to determine properties like film
thickness and elastic constants, while the latter allows to determine porosity and pore size and
pore tortuosity. This work set the basis for the development of a quality test for the in-line
inspection of these materials during their fabrication. The observed ultrasonic response in
microporous separators suggest the possibility to use low frequency air-coupled transducers with
limited sensitivity, where the use of MEM transducers at industrial scale can be advantageous, m project Listen2Future. Under grant 101096884, Listen2Future is
co-funded by the European Union and by the Project Ref. PCI2022-135060-2 funded by
CIN/AEI/10.13039/501100011033 and by EU “NextGenerationEU”/PRTR., Peer reviewed

. This paper introduces the concept of ultrasonic venting. Similar to acoustic vents,
ultrasonic vents refer to the aperture in air-coupled MEMS ultrasonic transducers (either PMUT
or CMUT) intended to allow the equalization of internal and external pressures, the transfer of
heat and the pass of ultrasonic waves, while impeding the penetration of fluids or particles that
can affect the transducer membrane. To that end, vents are covered with a porous membrane
whose properties are tuned to meet the afore mentioned requirements. The main difficulty in
ultrasonic venting, compared with acoustic venting, is that the required “transparency” to
ultrasonic waves is much more difficult to achieve. This involves two main problems as both
transmission loss and frequency distortion are much larger at ultrasonic frequencies than in the
audio range. The objectives of this paper are: to measure the response of acoustic venting
materials in the ultrasonic frequency range, to determine the usability of these materials in
ultrasonic vents, and to extract useful information for the design of efficient ultrasonic venting
materials. Transmission coefficient spectra of different acoustic venting materials is measured
in the frequency range 0.2 – 2.7 MHz. The origin of the ultrasonic losses and frequency distortion
are analysed as well as the role of mode conversion, internal interferences, modes interference,
etc. Results reveal that none of the acoustic venting materials analysed can be used in ultrasonic
venting applications, but the obtained knowledge about the response of these materials in the
ultrasonic frequency range permit to advance in the selection of successful candidate materials
for this application., Project Listen2Future funding is acknowledged. Under grant 101096884, Listen2Future is co-funded by
the EU and by the Project Ref. PCI2022-135060-2 funded by CIN/AEI/10.13039/501100011033 and by
EU “Next Generation EU”/PRTR, Peer reviewed

13 páginas, 9 figuras, 3 tablas., Non-contact methods are useful to improve the quality control of particle filtration media. The purpose of this paper is to investigate the correlation between the filtration efficiency of a porous sheet and its ultrasonic properties obtained using a non-contact technique. An air-coupled ultrasonic technique is used to obtain rapid measurements without affecting the integrity of the material. High frequencies (from 0.1 to 2.5 MHz) are used to improve technique sensitivity, and transmitted waves are measured to probe the internal properties of the material. Measurements of transmission coefficient spectra (amplitude and phase) and the corresponding ultrasound velocity and attenuation coefficient at different frequencies are obtained for a set of filtration media with well-characterized properties. Results show that the ultrasonic properties of filtration media vary as a function of basis weight, and therefore filtration efficiency, for a given charge state. However, the effect of electrostatic charge on ultrasonic propagation is almost negligible, as expected. We conclude that ultrasonic transmission may provide a valuable tool for the continuous online monitoring of material quality during fabrication and as a method to tease apart mechanical and electrostatic contributions to particle filtration., The Spanish National Research Council (CSIC) acknowledges funding from project Listen2Future. Under grant
101096884, Listen2Future is co-funded by the European Union and by the Project Ref. PCI2022-135060-2
funded by CIN/AEI/https://doi.org/10.13039/501100011033 and by EU “NextGenerationEU”/PRTR. The project is supported by the Key Digital Technologies Joint Undertaking
and its members (including top-up funding by Austria, Belgium, Czechia, Germany, Netherlands, Norway, and
Spain).’ The National Research Council Canada (NRC) acknowledges funding from the Public Health Agency of
Canada and the NRC Pandemic Response Challenge Program., Peer reviewed

Ultrasonic transmission coeffcient (magnitude), in dB, (measured in air at normal incidence) versus frequency of SABEU membranes used for acoustic venting applications. These data permit to determine the response of these vents in the frequency range 180 kHz - 2.8 MHz.
Data for three types of membranes are collected: 040-290; 040-300; 040-470
Data of interest for the packaging of MEMs air-coupled ultrasonic transducers. In particular for the venting protection.
Data obtained within the Listen2Future project., Project Listen2Future funding is acknowledged. Under grant 101096884, Listen2Future is co-funded by the EU and by the Project Ref. PCI2022-135060-2 funded by MCIN/AEI/10.13039/501100011033 and by EU “Next Generation EU”/PRTR., Peer reviewed

8 páginas, 4 figuras, 2 tablas, Air-coupled ultrasonic methods allow for the rapid and noninvasive characterization of porous materials
extracting valuable information regarding their performance and structure directly from their ultrasonic
response. Nevertheless, in some cases, both the accuracy and the amount of information can be increased if
measurements are taken over a wide frequency range. This can be a significant limitation depending on the
transduction technology to be employed. In particular, for the use of PMUTs and CMUTs that are better suited for
scaling up industrial deployment of this technology, as these are, typically, narrowband and low frequency
transducers when operated in air. To enable the use of low-frequency and narrowband transducers for the
characterization of filtration membranes using air-coupled ultrasound, we propose to make use of large ampli-
tude narrowband and low frequency signals and non-linear harmonic generation in the air. To validate the
potential of this approach, we characterized several filter materials by applying the proposed technique using
different excitation and setup configurations including different ultrasonic sensors: high-sensitivity and broad-
band piezoelectric self-made transducers (235 kHz, 650 kHz and 1000 kHz); narrowband, low-frequency and
low-cost transducers (flexural 40 kHz and radial 120 kHz); and an ultra-wideband receiver self-fabricated based
on PVDF film. The results obtained were compared with conventional linear wideband air-coupled ultrasonic
measurements, which allowed us to validate this approach. This innovation is expected to have a positive impact
on the applicability of ultrasonic techniques, as it will ease the measurement process being a less demanding
alternative in terms of equipment needs and time required., Authors acknowledges funding from project Listen2Future. Under
grant 101096884, Listen2Future is co-funded by the European Union
and by the Project Ref. PCI2022-135060-2 funded by CIN/AEI/
10.13039/501100011033 and by EU “NextGenerationEU”/PRTR.
Views and opinions expressed are however those of the author(s) only
and do not necessarily reflect those of the European Union or Key Digital
Technologies Joint Undertaking. Neither the European Union nor the
granting authority can be held responsible for them. The project is
supported by the Key Digital Technologies Joint Undertaking and its
members (including top-up funding by Austria, Belgium, Czechia, Ger-
many, Netherlands, Norway, and Spain). L. Fari˜nas acknowledges
financial support from MICINN (Spain) through the programme Juan de
la Cierva-Incorporaci´on [JC2020-043487-I] and by European Union
[NextGenerationEU]., Peer reviewed

15 páginas, 7 figuras, 9 tablas., Separator membranes are one of the components in batteries and, although being a passive component, they may
strongly affect both battery performance and security. Hence, effective control of both its main properties and
their variability are critical issues for the battery industry. This work proposes testing these membranes by the
measurement of the spectra of the ultrasonic transmission coefficient (magnitude and phase) using high-
frequency and wideband air-coupled ultrasonic pulses. This non-conventional ultrasonic technique for mate-
rials testing offers several advantages: it is completely contactless and non-invasive, it is very fast, and it has the
potential to be deployed in the fabrication line to test the 100 % of the production in real time. This work shows
that it is possible: a) to propagate high-frequency, wideband air-coupled ultrasonic pulses in the frequency range
0.15–2.4 MHz in battery separators, b) to obtain the transmission coefficient spectra, both magnitude and phase;
c) to theoretically model this propagation and the transmission coefficient spectra in thin films and then d) to
solve the inverse problem to extract some separator properties of interest. The obtained properties depend on the
theoretical model use to describe the propagation. For separators with porosity <60 % and pore size big enough
(approximately <1 μm for the separators studied here), effective medium approach can be successfully used,
while for the other cases, the porous nature of the separator has to be taken into account and the Biot’s theory is
used., Authors acknowledges funding from project Listen2Future. Under
grant 101096884, Listen2Future is co-funded by the European Union
and by the Project Ref. PCI2022-135060-2 funded by CIN/AEI/
10.13039/501100011033 and by EU “NextGenerationEU”/PRTR.
Views and opinions expressed are however those of the author(s) only
and do not necessarily reflect those of the European Union or Key Digital
Technologies Joint Undertaking. Neither the European Union nor the
granting authority can be held responsible for them. The project is
supported by the Key Digital Technologies Joint Undertaking and its
members (including top-up funding by Austria, Belgium, Czechia, Ger-
many, Netherlands, Norway, and Spain)., Peer reviewed

10 páginas, 11 figuras, 1 tabla, Mechanical properties of nanocellular polymers remain under debate, as reported enhancements are not
consistently observed across different systems and the underlying mechanisms are not universally established. In
this work, air-coupled ultrasonic measurements are employed as a non-destructive technique to extract longi-
tudinal and shear wave velocities of polyurethane aerogels, conventional polymers, micro and nanocellular
polymethyl methacrylate (PMMA). The work focuses on these last ones, with cell sizes from 4 μm to 14 nm and
relative densities between ∼ 0.1 and 0.5, determining not only the longitudinal and shear wave velocities but
also the effective elastic moduli and Poisson's ratio of these materials.
The results reveal that the mechanical response at the nanoscale deviates from classical geometrical scaling.
While microcellular polymers follow the expected density-modulus scaling, nanocellular materials with cell sizes
below ∼ 100 nm progressively deviate from this trend. While the ratio between cell wall thickness and cell size
increases, as cell size reduces, promoting stiffening, a smaller amount of mass in the vertices and the open cell
structure counteracts this effect. As a consequence, elastic modulus increases from values below 10 MPa in
aerogels to around 2000 MPa in the stiffest nanocellular structures, although this value is lower than expected
from density scaling alone.
It is hypothesized that these competing structural mechanisms govern the effective elastic response and
explain why nanoscale stiffening cannot be interpreted solely as a pure size effect but rather as a structural
reorganization phenomenon. Overall, this work highlights air-coupled ultrasound as a powerful tool to elucidate
structure-property relationships in cellular polymers., Financial support Grant PID2024-157392OB-I00 funded by MICIU/
AEI/10.13039/501100011033 and, by “ERDF/EU”. Grant PDC2025-
165502-I00 funded by MICIU/AEI/10.13039/501100011033 and, by
the “European Union NextGenerationEU/PRTR”.
Financial assistance from Ministerio de Ciencia, Innovaci´on y Uni-
versidades (MCIU) (Spain) PTQ2023-013073 (I. Sanchez-Calder´on) is
gratefully acknowledged.
Authors acknowledges funding from project Listen2Future. Under
grant 101096884, Listen2Future is co-funded by the European Union
and by the Project Ref. PCI2022-135060-2 funded by CIN/AEI/
10.13039/501100011033 and by EU “NextGenerationEU”/PRTR.
Views and opinions expressed are however those of the author(s) only
and do not necessarily reflect those of the European Union or Key Digital
Technologies Joint Undertaking. Neither the European Union nor the
granting authority can be held responsible for them. The project is
supported by the Key Digital Technologies Joint Undertaking and its
members (including top-up funding by Austria, Belgium, Czechia, Ger-
many, Netherlands, Norway, and Spain).
During the preparation of this work, the authors used ChatGPT and
Copilot in order to help generate Figs. 8 and 10. After using this tool/
service, the author(s) reviewed and edited the content as needed and
take(s) full responsibility for the content of the published article, Peer reviewed