Functional piezoelectric / pyroelectric / ferroelectric film devices deposited by AD at room temperature on rigid or flexible substrates

Abstract

The present invention relates to the fabrication of devices comprising films with piezoelectric, pyroelectric, and / or ferroelectric properties, obtained at room temperature by Aerosol Deposition (AD). Advantageously, the devices according to the invention are functional without requiring further heat treatment and can be produced on various types of substrates, rigid or flexible, with varying thicknesses, whether as a thin film (< 10 µm) or a thick film (> 10 µm). More particularly, the invention relates to ceramic-polymer composite films with piezoelectric, pyroelectric, or ferroelectric properties, obtained by AD, in which the polymer content is controlled to obtain films that are directly functional after deposition. The interconnectivity of the phases and the diameter of the ceramic grains are also controlled, ensuring the maintenance of the desired functional properties.The invention further relates to a method AD for the production of such devices.

Description

Title of the invention: Functional piezoelectric / pyroelectric / ferroelectric film devices deposited by AD at room temperature on rigid or flexible substrates. TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to the field of ceramic-polymer films with piezoelectric, pyroelectric, and / or ferroelectric properties, deposited by aerosol onto any type of substrate. More particularly, the invention relates to rigid or flexible devices incorporating these functional films, as well as an innovative Aerosol Deposition (AD) deposition process. This invention offers the significant advantage of enabling the formation of directly functional films on a wide variety of substrates, at room temperature, without requiring post-deposition heat treatment. STATE OF THE ART

[0002] Piezoelectric, pyroelectric, and ferroelectric materials possess unique functional properties that make them essential in numerous technological applications. Piezoelectric materials have the ability to convert mechanical stress into electrical charge, and vice versa, enabling their use in devices such as pressure sensors and actuators. Pyroelectric materials, on the other hand, react to temperature changes with a change in electrical charge, making them suitable for applications such as infrared sensing and temperature measurement. Finally, ferroelectric materials are characterized by spontaneous polarization that can be reoriented under the influence of an electric field, which is crucial for non-volatile memories and certain types of sensors.

[0003] The fabrication of these materials in the form of thin or thick films is essential for their integration into advanced electronic devices. Traditional methods, such as chemical and physical vapor deposition (CVD, PVD) or screen printing, generally require high temperatures to ensure the crystallization and consolidation of the layers. These requirements limit their compatibility with heat-sensitive substrates, such as polymers or certain metals, and also increase production costs.

[0004] The Aerosol Deposition (AD) method represents an innovative alternative, enabling the deposition of consolidated and dense films at room temperature. This process is based on the projection of dry particles at high speed (100-600 m / s) onto a substrate using an aerosol jet. During impact, complex phenomena collectively known as Room Temperature Impact Consolidation (RTIC) occur. These phenomena include particle fragmentation, intense cracking generated by kinetic energy, the formation of new, highly reactive surfaces, and their rapid consolidation under pressure and inertial forces. These mechanisms enable the formation of high-density (>95%) and highly adherent films on a variety of substrates without the need for heating.

[0005] However, the use of the AD method for piezoelectric, pyroelectric and ferroelectric materials has significant limitations. Excessive particle fragmentation during deposition often results in grain sizes smaller than 50 nm, which, due to a phenomenon known as the "size effect," leads to a loss of functional properties. Furthermore, films deposited using this method typically exhibit high residual stresses, which can affect their mechanical and electrical stability. These drawbacks frequently necessitate post-deposition heat treatment to restore the films' functional properties, thus negating one of the main advantages of the AD method: room-temperature deposition. These limitations reduce the attractiveness of this technology, particularly for heat-sensitive substrates, and restrict its potential applications in emerging fields such as flexible electronics and implantable medical devices.

[0006] In the prior art, certain approaches have been explored to overcome the limitations inherent in the Aerosol Deposition (AD) process, notably by introducing polymers into the ceramic powder mixture. This solution aims to use the polymeric particles as a protective cushion upon impact of the particles on the substrate, thereby reducing the mechanical stresses experienced by the ceramic grains and limiting their fragmentation. By plastically deforming upon impact, the polymers dampen the kinetic energy of the ceramic particles, preserving a sufficient average grain size to maintain the piezoelectric, pyroelectric, and ferroelectric properties.However, this approach, while effective in mitigating certain problems related to grain fragmentation, presents challenges related to the precise control of the proportion of polymers in the powder mixture, in order to avoid excessive reduction in film density or loss of functional properties due to excessive damping.

[0007] A notable example of this approach is described in the article "Growth of BaTiO3-PTFE Composite Thick Films by Using Aerosol Deposition" published by Kim et al. in 2011. In this study, the authors explored the use of aerosol deposition to fabricate BaTiO3-PTFE composite films at room temperature, in incorporating 0.1 wt% PTFE into the ceramic powder mixture as a protective cushioning polymer. The initial mixture consists of 0.45 µm BaTiO3(BT) particles and 0.15 µm PTFE particles dispersed in ethanol, then ground for 24 hours and dried at 80 °C for 48 hours. The powders are then conveyed into a deposition chamber by a helium flow at rates between 4 and 7 L / min and sprayed through a 10 mm nozzle onto copper or glass substrates. The working pressure in the chamber is maintained between 5.5 and 21 Torr, and the deposition time varies from 1 minute 30 seconds to 5 minutes depending on the conditions. The particles are accelerated to speeds of At 200-300 m / s, the particles impact the substrate, where the elasticity of the PTFE particles acts as a protective cushion, reducing the fragmentation of the ceramic grains and limiting the distortion of the crystalline structure. This methodology made it possible to maintain an average BaTiO3 grain size of 24.6 nm in the composites, compared to 11.2 nm for pure BaTiO3 films.

[0008] However, despite these advances, BaTiO3-PTFE composite films exhibit a significantly lower relative permittivity than films composed purely of BaTiO3. This reduction is attributed to insufficient connectivity between the ceramic particles (active phase conferring functional properties) within the polymer matrix, considerably limiting the electrical performance of the films for applications requiring high dielectric properties.

[0009] Another relevant example is described in the article entitled “Growth of BaTiO3-PVDF Composite Thick Films by Using Aerosol Deposition” published in 2016 by Sung Hwan Cho et al. This article proposes a method for manufacturing BaTiO3-PVDF composite films at room temperature by Aerosol Deposition (ADM), using BaTiO3 particles coated with a PVDF layer obtained by a wet process. This polymeric layer, with a thickness of 10 to 20 nm, improves the compatibility between the ceramic and polymeric powders and facilitates their transport in the deposition chamber. The films thus produced have a composite 0-3 structure, where the BaTiO3 particles are well dispersed (active phase) in a PVDF matrix (passive phase), with ceramic grain sizes reaching 100 to 500 nm, i.e. 5 to 50 times greater than those of pure BaTiO3 films obtained by the same method.This increase in grain size is attributed to the elasticity of PVDF, which reduces impact energy and minimizes the fragmentation of ceramic particles. However, despite these structural improvements, the composite films exhibit a significantly lower relative permittivity than pure BaTiO3 films (10.6 vs. 68.4 at 1 MHz), which is attributed to the weak connectivity between the ceramic particles dispersed in the polymer matrix. Although this approach improves certain... While it offers properties such as leakage current and provides an efficient method for manufacturing composite films, it remains limited in terms of electrical performance, reducing its applicability to devices requiring high ferroelectric or dielectric properties.

[0010] Thus, despite the advances described, current room-temperature aerosol deposition methods fail to fully preserve the piezoelectric, pyroelectric, and ferroelectric functional properties of materials. Therefore, there is a need to develop a process capable of maintaining these properties without requiring heat treatment, while also allowing integration onto heat-sensitive substrates for advanced applications such as flexible electronics, sensors, and energy harvesting devices. Description of the invention

[0011] The present invention relates to the manufacture of new ceramic film devices with piezoelectric, pyroelectric or ferroelectric properties deposited at room temperature by Aerosol Deposition (AD), and being functional without requiring a post-deposition heat treatment step.

[0012] In particular, the invention proposes a device comprising a composite film with piezoelectric, pyroelectric or ferroelectric properties, formed on a substrate by Aerosol Deposition (AD), and having at least one ceramic phase (active phase and matrix) and at least one polymer phase (passive phase) comprising a polymer with plastic or thermoplastic properties, said device being characterized in that the volume fraction of said polymer phase in the composite film is >9%vol and <18%vol, preferably is >10%vol and <17%vol.

[0013] In a novel approach, the inventors have demonstrated that by controlling the polymer content within the aforementioned concentrations, it is possible to obtain consolidated and densified films by AD deposition at room temperature. The polymer acts as a protective cushion in the film compositions to limit the fragmentation of ceramic grains during AD deposition, and enables high film densification and the formation of interconnectivity within the ceramic phase, resulting in piezoelectric, pyroelectric, or ferroelectric properties directly after deposition, without a post-deposition annealing step.

[0014] In a preferred embodiment, the volume fraction of said polymer phase in the composite film is >11%vol and <16%vol.

[0015] Similarly, the device of the invention may also incorporate any of the following features or combinations of features:

[0016] - said composite films have an average diameter of ceramic grains said film determined by SEM surface images is >90 nm, preferably >100 nm and <1000 nm;

[0017] - the film predominantly exhibits 3-0 phase connectivity according to the Newnham nomenclature, 3 for ceramic and 0 for polymer, and according to which the polymer is dispersed in a continuous matrix of ceramic phase, and with or without minor interconnectivity between the polymer phase.

[0018] - observation of the film surface by scanning electron microscopy (SEM) reveals a surface essentially devoid of significant porosity;

[0019] - at least one ceramic phase is chosen from: • ceramics of the lead titano-zirconate family, such as PZT, PLZT, PZN-PT; • ceramics of the barium titanate family, such as BaTiO3; • ceramics of the alkali-bismuth titanate family, such as NaO>5 Bi0j5TiO3 called NBT, K0j5Bi0j5TiO3 called KBT, or NBT-6BT; • potassium and sodium niobates, such as Ko.5NaO.5NbO3, also known as KNN; and • inorganic piezoelectric ceramic materials such as ZnO, LiNbO3, LiTaO3 and / or mixtures thereof; - the polymer is chosen from among fluoropolymers such as polytetrafluoroethylene PTFE, polyvinylidene fluoride PVDF and its copolymers PVDF TrFE and PVDF TFE, as well as organic polymers such as polyethylene terephthalate PET, polyimide PI, polycarbonate PC, polymethyl methacrylate PMMA, polylactic acid PLA, polyetheretherketone PEEK, polyvinyl acetate PVA and polybutylene terephthalate PBT, or mixtures thereof; - the polymer is chosen from among the fluorinated polymers, and is preferably polyvinylidene fluoride PVDF, and the ceramic phase is from the barium titanate family and / or from the alkali-bismuth titanate family; - the substrate is of rigid or flexible type, and / or of polymeric, ceramic or metallic type; - the creation of an energy recovery demonstrator incorporating a device according to the invention.

[0020] The invention also relates to a method for manufacturing a device comprising a ceramic-polymer film by means of an Aerosol Deposition (AD) device as described above, said film having piezoelectric, pyroelectric or ferroelectric functional properties, and comprising the following steps

[0021] - the preparation of powders comprising the mixing of at least one powder ferro-, pyro- or piezoelectric ceramic and at least one polymer powder with plastic properties, a mass content of polymer in the powder mixture being pre-defined to obtain, after deposition, a volume fraction of polymer in the film >9%vol and <18%vol,

[0022] - the generation of an aerosol from said powder mixture by means of said AD device

[0023] - the Aerosol Deposition of a film onto a substrate by projection of said aerosol onto said substrate.

[0024] - obtaining a film exhibiting ferro-, pyro- or piezoelectric properties directly after Aerosol Application.

[0025] According to one embodiment, the preparation of powders includes the planetary grinding of a ceramic powder with a polymer powder in a humid environment, and the drying and sieving of the powder mixture to remove agglomerates.

[0026] For example, the particle size diameter determined by SEM images of the surface mixture of ceramic polymer powders has an average diameter of about 1 pm, in particular less than 1 pm.

[0027] Finally, to improve the dispersion of the polymer in said aerosol, the invention proposes adjusting the pressure applied to a vibrating table of said device AD, and the flow rate of a carrier gas during the generation of the aerosol.

[0028] Other features and advantages of the invention will become apparent from the following supplementary description, which refers to the accompanying figures. It is understood that this supplementary description is given only by way of non-limiting illustration of the subject matter of the invention. LIST OF FIGURES

[0029] [Fig.1] Cross-sectional SEM images of BT-PVDF films deposited on: I) Kovar® with powders of (a) 0.1 wt%, (b) 0.5 wt%, (c) 1 wt% and (d) 2 wt% in PVDF, II) Polyimide with powders of a) 0.25 wt%, (b) 0.5 wt%, (c) 1 wt% in PVDF, and III) Silicon with powders of a) 0.25 wt%, (b) 0.5 wt%, (c) 1 wt% in PVDF.

[0030] [Fig.2] Grain size distribution calculated from SEM images of BT-PVDF films deposited on: I) Kovar® with powders at (a) 0.1 wt%, (b) 0.5 wt%, (c) 1 wt% and (d) 2 wt% in PVDF, II) Polyimide with powders at a) 0.25 wt%, (b) 0.5 wt%, (c) 1 wt% in PVDF, and III) Silicon with powders at a) 0.25 wt%, (b) 0.5 wt%, (c) 1 wt% in PVDF.

[0031] [Fig.3] Cross-sectional SEM images of BT-PVDF films deposited on: I) Kovar® with powders of (a) 0.1 wt%, (b) 0.5 wt%, (c) 1 wt% and (d) 2 wt% in PVDF, II) Polyimide with powders of a) 0.25 wt%, (b) 0.5 wt%, (c) 1 wt% in PVDF, and III) Silicon with powders of a) 0.25 wt%, (b) 0.5 wt%, (c) 1 wt% in PVDF.

[0032] [Fig.4] Binary (B / W) images obtained by ImageJ processing from the images SEM of [Fig.3] for BT-PVDF films deposited on Kovar® with powders of (a) 0.1 wt%, (b) 0.5 wt%, (c) 1 wt% and (d) 2 wt% in PVDF.

[0033] [Fig. 5] Comparison of PVDF volume percentages in films BT-PVDF deposited on the three substrates tested.

[0034] [Fig. 6] PE hysteresis cycles measured for BT-PVDF films on Kovar® with (a) 0-0.1 %m, (b) 0.25-1 %m and (c) 2-3 %m in PVDF.

[0035] [Fig.7] SE deformation-electric field cycles measured for films BT-PVDF on Kovar® with powders at 0.25-1 wt% PVDF.

[0036] [Fig.8] PE and SE cycles measured for BT-PVDF films deposited on polyimide with powders at (a) 0.25 wt%, (b) 0.5 wt%, (c) 0.75 wt% and (d) 1 wt% in PVDF.

[0037] [Fig.9] PE and SE cycles measured for BT-PVDF films deposited on silicon with powders at (a) 0.25 wt%, (b) 0.5 wt%, (c) 0.75 wt% and (d) 1 wt% in PVDF.

[0038] [Fig. 10] PE and SE cycle of films a) BT-PVDF and b) NBT-6BT-PVDF on Kovar®.

[0039] [Fig. 11] SEM images of the surface of the BT-PVDF and NBT6BT-PVDF films shown in [Fig. 10].

[0040] [Fig. 12] Cross-sectional SEM images of films a) BT-PVDF and b) NBT6BT-PVDF shown in [Fig. 10].

[0041] [Fig. 13] Power recovered curves as a function of frequency and resistive load for demonstrator D (BT-PVDF at 0.25%m of PVDF).

[0042] [Fig. 14] Power recovered curves as a function of frequency and resistive load for demonstrator E (BT-PVDF at 0.75%m of PVDF). DETAILED DESCRIPTION OF THE INVENTION

[0043] The present invention relates to the manufacture of devices for films with piezoelectric, pyroelectric, or ferroelectric properties deposited at room temperature by Aerosol Deposition (AD). Advantageously, the devices of the invention are functional without the need for post-deposition heat treatment and can be deposited on various types of substrates, rigid or flexible, in thin (<10 µm) or thick (>10 µm) layers.

[0044] More particularly, the invention provides ceramic-polymer composite films with piezoelectric, pyroelectric or ferroelectric properties obtained by AD. The volume-controlled polymer in the film helps to dampen the impact energy during AD projection, thus limiting the fragmentation of the ceramic powders.

[0045] Controlling the percentage of polymer in the powder mixture before projection proves to be a determining factor for the functional properties of the resulting films. At excessively high polymer levels, two main problems arise The following problems can occur: on the one hand, ceramic particles undergo little or no fragmentation upon impact, preventing their efficient consolidation within the film. This leads to the formation of a compacted powder with a low densification rate, significantly altering the functional properties of the films. On the other hand, high polymer contents reduce the connectivity of the matrix active phase, resulting in a loss of the ferroelectric, pyroelectric, and piezoelectric properties essential for the intended applications. Conversely, at excessively low polymer contents, excessive fragmentation of the ceramic particles causes a significant reduction in grain size and thus a loss of functional properties due to the "grain size effect."

[0046] With an optimal polymer content, moderate fragmentation of ceramic particles can be achieved during aerosol deposition. This controlled fragmentation promotes a high densification rate, associated with the preservation of the functional properties of the resulting films. However, to the inventors' knowledge, a polymer content in the powder mixture that maintains piezoelectric, pyroelectric, or ferroelectric properties for films obtained by aerosol deposition has not yet been defined, and the results reported in the literature vary from case to case.

[0047] Furthermore, the inventors have observed that the polymer incorporation rate in the film varies according to the parameters of the AD process, and the substrate used to deposit the film, so that it does not seem possible to generalize an optimal polymer rate in the powder mixture to obtain functional films directly after AD deposition at room temperature.

[0048] Distinctively, the present invention provides polymer ceramic composite films obtained by additive manufacturing (AD) with a controlled polymer volume fraction in the film obtained after deposition. Indeed, after numerous studies, the inventors determined the relationship between the polymer content in the powders and the percentage incorporated into different types of films. They also characterized the microstructure and functional properties of the films obtained as a function of the polymer content in the film. These studies made it possible to define, for the first time, the characteristics of functional polymer ceramic composite films obtained at room temperature and without heat treatment, and exhibiting piezoelectric / pyroelectric / ferroelectric properties.

[0049] The invention also relates to a deposition method by AD for obtaining said films.

[0050] Detailed examples of these studies will be described in the following section; these examples are given for illustrative purposes only and do not limit the scope of the invention. EXPERIMENTAL SECTION

[0051] METHODOLOGY BT-PVDF Powder Preparation

[0052] BaTiO3 (BT) powders with different PVDF mass concentrations were first tested, including 5 wt%, 3 wt%, 2 wt%, and 1 wt%. Four powders containing 0.75 wt%, 0.5 wt%, 0.25 wt%, and 0.1 wt% were also studied. Unless otherwise stated, the results cited refer to the mass percentage of polymer present in the BT-PVDF powder mixture.

[0053] BaTiO3 (BT) powder with an average particle size of 200 nm to 3 µm is mixed in a moist medium with PVDF powder (initial average diameter approx. < 1 µm) using a planetary mill, so as to obtain a particle diameter of the powder mixture of approximately < 3 µm. The suspension is then dried for 12 to 48 hours at a temperature of 70-80°C before being sieved to remove large agglomerates, ensuring that they do not exceed a size of 10 µm. BT-PVDF powder deposition by ADM

[0054] Aerosol deposition of BT-PVDF powders was tested on three substrates, Kovar®, polyimide (PI), and silicon.

[0055] The table below shows the ADM parameters used. In general, a significant adjustment was necessary regarding the pressure of the AD device's vibrating table, which aids in aerosol formation, due to the different behaviors of the composite powders during deposition depending on the PVDF content. The highest pressures were used at lower PVDF content levels. Substrate Kovar® sheets (125 µm) Polyimide sheets (150 µm) Silicon sheets (0.6-5 mm) with gold layer Projected powders BT-PVDF (0.1-5 wt%) BT-PVDF (0.25-1 wt%) BT-PVDF (0.25-1 wt%) Carrier gas Helium (He) Helium (He) Helium (He) Gas flow rate 12.5 L.min 1 10 L.min 1 12.5 L.min 1 Projection distance 5 mm 5 mm 5 mm Deposition area 5 x 10 mm2 5x10 mm2 5 x 10 mm2 Number of scans 10 10 20 Scanning speed 1.5 mm.s 1 1.5 mm.s 1 1.5 mm. s 1 Vibrating table pressure 3 to 6 bar 4-6 bar 4-6 bar Nozzle 0.7 x 5 mm2 0.7 x 5 mm2 0.7 x 5 mm2 RESULTS I. Surface observations by SEM BT-PVDF film on Kovar®

[0056] Figure 1 shows SEM images of the surface of BT-PVDF films obtained with powder mixtures containing different amounts of PVDF (0.1, 0.5, 1, and 2 wt%). The images reveal significant changes in the surface morphology of the deposits. The film formed from 0.1 wt% PVDF has a surface similar to that of ceramic films produced by ADM, characterized by a smooth surface and a cratered texture visible at the submicron scale. However, unlike pure ceramic films where the grains are excessively fragmented and indistinguishable, distinct grains are visible in this film. As the PVDF content increases, the surface morphology changes: the cratered structures disappear above 1 wt% PVDF in the BT-PVDF powder mixture, while larger craters appear, resulting in increased roughness in the surface profiles.The 5%m PVDF content did not allow for the formation of a consolidated and densified film; the results are not shown.

[0057] An increase in grain size is also noted with higher PVDF contents. Figure 2 shows the grain size distribution calculated from SEM images for these films. The film formed from 0.1 wt% PVDF in the starting mixture exhibits an average grain size of 99.5 ± 39.3 nm (Figure VL15a), consistent with the crystallite size of 86 nm determined by the WH method. With a PVDF content of 0.5 wt% (Figure VL15b), the average size increases to 133.0 ± 55.4 nm.

[0058] Figures 1 and 2 also show SEM images of the surface and grain size of BT-PVDF films deposited on polyimide (PI) sheets with 0.25 wt%, 0.5 wt%, and 1 wt% of PVDF in the starting powder. These images reveal that, unlike the results obtained on Kovar®, the grain size remains more modest. For example, with 0.25 wt% of PVDF, the average grain size is 65.9 ± 32.2 nm, increasing to 81.9 ± 46.1 nm for 0.5 wt% of PVDF, and then reaching 120.8 ± 77.0 nm for 1 wt% of PVDF. At the lower PVDF content (0.25 wt%), fragmentation remains marked, while increasing the PVDF content significantly increases the average grain size, thus improving surface homogeneity. Nevertheless, these grain sizes remain generally smaller than those achieved on Kovar® substrate with comparable PVDF content in the powders. BT-PVDF film on silicon

[0059] Figures 1 and 2 also show SEM images of the surface and grain size distribution of BT-PVDF films deposited on a silicon substrate. In this case, deposition on a rigid substrate promotes higher PVDF incorporation into the films. For example, at 0.25 wt% PVDF in the powder, the average grain size is approximately 106.8 ± 48.5 nm. Increasing the PVDF content to 0.5 wt% increases the average grain size to 138.3 ± 59.2 nm, and to 152.0 ± 62.6 nm at 1 wt% PVDF, but this is accompanied by increased porosity. Thus, on silicon, increasing the PVDF content initially increases the grain size, but an excess of PVDF can degrade densification and alter the surface morphology.

[0060] IL Cross-sectional observations by FIB-SEM

[0061] 2.1 Connectivity in composite materials according to the nomenclature of Newnham

[0062] Phase connectivity in composite materials can be described according to the classical Newnham nomenclature, which uses two numerical indices to characterize the interconnectivity of the active and passive phases. The first index represents the connectivity of the active phase, while the second describes that of the passive phase. The values ​​typically used are 0-3: Active phase with no interconnection and dispersed within the continuous passive phase. 1-3: Active phase interconnected in one direction, passive phase continuous in three directions. 2-3: Active phase interconnected in two directions, passive phase continuous in three directions. 3-3: Active phase interconnected in three directions, passive phase continuous in three directions.These values ​​typically describe composite materials, whether in bulk or in the form of films or coatings, in which the passive phase acts as a matrix, while the active phase is exclusively functional. However, in certain specific cases, such as porous materials (where porosity constitutes the passive phase) or within the framework of the invention described here (active phase: ceramic(s); passive phase: polymer(s)), the active phase simultaneously fulfills the roles of functional phase and matrix. So-called 3-O connectivity therefore means that the matrix active phase is three-dimensionally connected in all directions, forming a continuous network, while the passive phase is dispersed in an isolated manner without significant connection between its inclusions. BT-PVDF film on Kovar®

[0063] Figure 3 shows cross-sectional SEM images of BT-PVDF films. All the films exhibit a dense composite microstructure with a homogeneous distribution of the polymer phase (dark phase: passive phase). Depending on their composition, the BT-PVDF films exhibit a particulate composite-type microstructure, where the connectivity of the passive phase varies progressively. depending on the PVDF content. This configuration is primarily 3-0 for films with low PVDF content (well dispersed), and relative to the ceramic phase (matrix active phase in all three directions, discontinuous polymer inclusions in all three directions), i.e., 3 for the ceramic, 0 for the polymer. A possible partial transition to a configuration close to 3-1 or 3-2 is also observed for the samples. Above 1 wt% PVDF in the BT-PVDF powder blend, the beginnings of more interconnected polymer networks are observed in the films, although they are not yet fully continuous. The differences observed in the shape of the polymer inclusions reflect increased plastic deformation for PVDF particles when their percentage is low. Conversely, their deformation is less pronounced in films with 1 and 2 wt% PVDF. BT-PVDF film on Polyimide

[0064] As shown in the cross-sectional SEM images ([Fig. 3], middle), the distribution of PVDF in the film on PI remains homogeneous, but the volume proportion of PVDF in the films is lower than that obtained on Kovar® for similar starting powders. This difference suggests a less efficient deposition of the polymer particles onto the polymer substrate, probably due to its flexibility reducing the plastic deformation of the PVDF particles. Despite this, increasing the PVDF content in the powder compensates for this less efficient deposition, ensuring a sufficient quantity of polymer to reduce the fragmentation of the ceramic grains. At 1 wt% of PVDF in the ceramic-polymer powder mixture, the connectivity remains essentially of the 3-0 type, which promotes the maintenance of the functional properties of the matrix active phase without going through a transition to less favorable 3-1 or 3-2 connectivities. BT-PVDF film on silicon

[0065] Cross-sectional SEM images ([Fig. 3], bottom) show that the distribution of PVDF in the silicon films is homogeneous, with an overall higher volume percentage than with Kovar® or polyimide, for the same mass content in the starting powder. This abundance of PVDF promotes mechanical damping and grain growth, but also results in greater porosity and a less dense microstructure when the PVDF content is high in the powders. The more numerous and deformed PVDF particles alter the overall connectivity and thus limit the functional performance beyond a certain PVDF threshold. 2.2 PVDF content of BT-PVDF films

[0066] To accurately quantify the PVDF content of the analyzed BT-PVDF films, the inventors used ImageJ software (function: binary image segmentation particle analysis) to estimate the PVDF volume percentage from various cross-sectional SEM images ([Fig. 4]). [Fig. 5] summarizes the comparison of the PVDF volume percentage in the powders with the PVDF volume percentage in the films on the three substrates. The various results are detailed below. BT-PVDF film on Kovar®

[0067] Figure 4 illustrates an example of binary (B&W) images obtained after processing the SEM images from [Fig. 3] were used to calculate the relative proportions of BT and PVDF in the film microstructure. The calculated PVDF volume percentages are consistently higher than those of the starting powders. For example, while the starting powder for the 0.1 wt% BT-PVDF film contains 0.33 wt% PVDF, the corresponding film has an average content of 9.9 wt%. Advantageously, the results show a linear relationship between the volume percentage of PVDF added to the starting powders and that estimated in the films, illustrated by a linear regression line with the equation y = 2.14x + 9.20, where y represents the wt% PVDF in the films and x represents that in the starting powders. The coefficient of determination R² of 0.9969 indicates a strong correlation between these values.

[0068] Starting from the postulate that 0.1 %m of PVDF in the powder corresponds to approximately 0.33 %vol, we obtain for 0.25 %m, 0.5 %m, 1 %m, 2 %m and 5 %m, respectively 0.825 %vol, 1.65 %vol, 3.3 %vol, 6.6 %vol and 16.5 %vol of PVDF in the powder. Applying the equation, we can calculate the following volume fractions in the film: for 0.25%m (0.825%vol) approximately 11%vol, for 0.5%m (1.65%vol) approximately 12.7%vol, for 1%m (3.3%vol) approximately 16.3%vol, for 2%m (6.6%vol) approximately 23.3%vol, and for 5%m (16.5%vol) approximately 44.5%vol. BT-PVDF film on Polyimide

[0069] Overall, a linear increase in the vol% of PVDF in the films compared to the vol% in the starting powders is observed, corroborating the observations made on the films deposited on Kovar®. The relationship between the two gives a linear regression line whose equation is y = 1.97x + 7.38, where y represents the vol% of PVDF in the films and x that in the starting powders (R² = 0.9982).

[0070] For the powders studied with 0.25 wt%, 0.5 wt% and 1 wt% of PVDF in the starting powder, the respective volume correspondences are 8-9 wt%, 9-10 wt% and 11-12 wt% in the film deposited on polyimide. BT-PVDF film on silicon

[0071] A linear increase in the %vol of PVDF in the films compared to the %vol in the starting powders is also observed, but also an increase compared to the substrates of Kovar® and polyimide (y = 2.39x + 10.95; R2=0.9246).

[0072] For the powders studied with 0.25% wt, 0.5% wt and 1% w of PVDF in the starting powder, the respective volume correspondences are «12-13% vol, «14-15% vol and «18-19% vol in the film deposited on silicon.

[0073] III. Measurements of ferro- and piezoelectric cycles

[0074] 3.1 Ferroelectric hysteresis (PE) cycles and deformation-field cycles Electrical (SE) BT-PVDF film on Kovar®

[0075] Figure 6 summarizes the results of the ferroelectric hysteresis (PE) cycle measurements for the different BT-PVDF films studied. The 0 and 0.1 wt% PVDF films were polarized under a 10 kV.mm⁻¹ field for 30 min at room temperature, while the remaining samples were polarized under 20 kV.mm⁻¹ for the same duration and at room temperature.

[0076] As shown in Figure 6a, the 0.1 wt% PVDF film exhibits linear leakage behavior, similar to the ceramic films as deposited, but with a significantly increased maximum polarization (Pmax) of 3.0 pC·cm², compared to 0.5 pC·cm² for the BT film without PVDF. In the range between 0.25 and 1 wt% PVDF (Figure 6b), the film behavior changes significantly: the various films analyzed then display PE cycles characteristic of nonlinear ferroelectric behavior. The 0.25 wt% film exhibits the best ferroelectric cycle among these samples, with a clear polarization reversal and a remanent polarization (Pr) of 9.2 pC·cm². This value gradually decreases for films with 0.5, 0.75 and 1%m of PVDF, reaching 6.6 pC.cm2 for the film with 0.5%m of PVDF and 5.4 pC.cm2 for the films with 0.75 and 1%m of PVDF.Above 1%m of PVDF (6c), the behavior of BT-PVDF films becomes linear again with increased leakage, and polarization values ​​decrease as the PVDF content increases. For example, the film with 2%m of PVDF shows a Pmax of 3.8 pC.cm2, while the film with 3%m of PVDF shows a Pmax of only 1.9 pC.cm2.

[0077] Strain-electric field (SE) cycles were measured for BT-PVDF films in the 0.25–1 wt% PVDF range. As illustrated in [Fig. 7], all these films exhibit SE cycles indicative of piezoelectric behavior. Indeed, the measured SE cycles have a shape close to the classic “butterfly” shape of piezoelectric ceramics, although they are asymmetrical due to prior polarization. These results indicate that the ferro- Piezoelectric properties are preserved in BT-PVDF films with PVDF contents of 0.25 to 1 wt% in the powders, resulting in a PVDF volume content in the film of approximately 10 wt% to 16 wt%. This is a novel result, never before achieved with ADM films as deposited (unannealed). The preservation of functional properties within a specific PVDF content range demonstrates the importance of controlling the polymer content in the powders, but primarily in the resulting composite films. Similarly, to maintain the functional properties of BT, it appears crucial to keep grain sizes sufficiently large (>90 nm, preferably >100 nm and <1000 nm) to prevent loss of properties due to size effects.

[0078] Furthermore, the connectivity between phases in the microstructure plays a crucial role in the functional performance of the films. The active matrix phase, composed of functional ceramic (e.g., BaTiO3), must predominantly exhibit three-dimensional (3-x) connectivity, i.e., continuous interconnection in all three dimensions, to maintain optimal ferroelectric, pyroelectric, and piezoelectric properties. Conversely, a decrease in this interconnectivity, leading to lower configurations such as 2-x, 1-x, or 0-x, results in a progressive degradation of performance due to the partial or total isolation of the ceramic grains.

[0079] The passive phase, consisting of polymer(s) such as PVDF, plays a secondary role by acting as a mechanical damper to limit the fragmentation of the ceramic grains during aerosol deposition. However, in order not to disrupt the continuity of the active matrix phase, the polymer connectivity must remain low, ideally, in theory, of the x-0 type (homogeneous dispersion without connection between the polymer inclusions). However, in practice, low connectivity of the passive phase (polymer), whether of the x-1, x-2, or even x-3 type, also contributes to better functional performance of the films, provided that the active matrix phase remains well interconnected and continuous.It should also be noted that, although these characteristics are preferential, different connectivities can coexist within the volume of functional films, provided that the fundamental principles are respected, including an appropriate grain size (reduced fragmentation upon ADM impact), high connectivity of the matrix active phase (3-x, which is equivalent to a majority connectivity 3-0 according to the Newnham nomenclature) or reduced connectivity of the passive phase (i.e., with little or no interconnectivity of the polymeric phase), as well as the principle of essential volumetric proportionality between the two phases and already established previously. BT-PVDF film on Polyimide

[0080] Figure 8 shows the PE and SE cycles of BT-PVDF films deposited on PI. Unlike the results obtained with Kovar®, a higher PVDF content is required to restore the ferro- and piezoelectric properties without annealing. Thus, at 0.25 wt% PVDF, the film exhibits linear behavior with leakage, without a net polarization reversal. At 0.5 wt% PVDF (approximately 9-10 vol% in the film), ferroelectric behavior emerges, accompanied by a detectable piezoelectric response. With 1 wt% PVDF, the PE and SE cycles are significantly improved, showing high remanent polarization (Pr ~5 pC.cm²) and a characteristic "butterfly" type deformation cycle, with a maximum deformation of ~0.4%. Thus, a PVDF mass content of around 1%m (approximately 11-12%vol in the film) proves optimal to compensate for the lower polymer deposition rate on PI and maintain the ferro- and piezoelectric properties without annealing.The principles established in terms of connectivity between the active and passive phases to promote functional properties in films deposited on Kovar® also apply to films deposited on polyimide. BT-PVDF film on silicon

[0081] The PE and SE measurements ([Fig.9]) indicate that the ferro- and Piezoelectric properties are preserved at moderate PVDF concentrations in silicon-based powders. For example, at 0.25 wt. PVDF (12.9 vol. in the film) and 0.5 wt. PVDF (14.9 vol. in the film), the films exhibit polarization reversal and a detectable piezoelectric response, with maximum polarization around 8 pC·cm² and measurable deformation. In contrast, at 1 wt. PVDF (18.8 vol. in the film), the excess polymer and associated porosity significantly reduce polarization and piezoelectricity. IV. Conclusions

[0082] These tests made it possible to understand the relationship between the polymer content in the starting powders and the volumetric polymer content in the resulting films. A significant increase in the polymer content incorporated into the films was observed compared to the mass content of the polymer in the prepared powders. Similarly, the incorporation rate into the film also varies depending on the substrate used and is strongly influenced by the control of the operating parameters of the AD process. Consequently, the powder optimization approach only allows for the optimization of results on a case-by-case basis and does not allow for the generalization of a technical solution applicable to different types of films and different operating parameters.

[0083] However, the correlation of film characterization results with the volumetric polymer content in the film made it possible to define the essential characteristics Functional films obtained by AD at room temperature. In particular, to obtain piezoelectric / pyroelectric / ferroelectric properties, the polymer must be present in the film at a volume concentration <18% vol, preferably <17% vol. Indeed, below these levels, it is possible to obtain a consolidated, densified film with predominantly 3-0 interconnectivity according to the Newnham nomenclature (also called 3-x of the active matrix phase), with very little connectivity between polymer inclusions (passive phase) and low porosity.

[0084] Functional films with piezoelectric / pyroelectric / ferroelectric properties obtained by AD also exhibit average ceramic grain sizes (matrix active phase) greater than >90 nm, preferably greater than >100 nm. A minimal polymer content in the film, in particular >9% vol, prevents fragmentation of the ceramic grains and allows this size to be achieved.

[0085] The invention thus proposes a polymer content by volume in the film >9% vol. and <18% vol., and preferably between >11% and <16% for optimal properties. In the upper optimum range, the ceramic grain size increases, and good connectivity of the 3-x type matrix active phase is also observed, allowing the functional properties to be maintained when the maximum polymer threshold is respected. Conversely, when the volume threshold of the polymer passive phase is exceeded, a preferential increase in x-3 and x-2 type connectivities is observed, leading to a progressive loss of functional properties, until their total disappearance, or even a lack of consolidation or densification of the ceramic-polymer films.

[0086] In the initial ceramic-polymer powder mixture, a suitable size of ceramic particles (200 nm - 3 pm) and agglomerates (maximum 10 pm) is desirable for film formation by the AD process at room temperature. The particles in the ceramic-polymer mixture will have an approximate average size of between 100 nm and 3 pm. As for the films obtained, the average size of the ceramic grains generally does not exceed one micrometer (1 pm), due to the typical fragmentation of the ceramic particles or agglomerates upon impact with the substrate. The films obtained therefore generally have an average ceramic grain size < 1000 nm, < 800 nm, or < 600 nm.

[0087] Of course, the optimal polymer content for a specific composition will also vary from case to case and depending on the nature of the polymer / ceramic, its size, the operating parameters of the AD process, and the presence of additives. A person skilled in the art, with their general knowledge, will adapt the AD process to ensure a consolidated / homogeneous dispersion and deposition of the film, with a controlled polymer content. In particular, the invention proposes adapting the pressure of the vibrating table. depending on the polymer content in the powders, higher levels require less pressure to ensure uniform projection.

[0088] Figure 10 shows another example of the composite films a) BT-PVDF and b) NBT-6BT-PVDF on Kovar® manufactured at room temperature by AD according to the invention, and exhibiting piezoelectric / pyroelectric / ferroelectric properties without the need for post-deposition annealing. In particular, the BT-PVDF film incorporates 14.9 vol% PVDF and has an average grain size of 133 nm, and the NBT6BT-PVDF film incorporates 12.8 vol% PVDF and has an average grain size of 119 nm. Figures 11 and 12 show the SEM images of the surface and cross-section of said films a) BT-PVDF and b) NBT-6BT-PVDF on Kovar®.

[0089] For the first time, the inventors also demonstrated the possibility of producing several energy recovery demonstrators obtained by AD without a post-deposition annealing step, and measured their ability to recover vibrational energy. The results obtained with two examples of these demonstrators (D and E) are shown in Figures 13 and 14. The demonstrators were fabricated with two compositions of BT-PVDF powders, one containing 0.25 wt% PVDF (demonstrator D, [Fig. 13]) and the other 0.75 wt% PVDF (demonstrator E, [Fig. 14]), i.e., to achieve a polymer volume fraction in the film of approximately 11 wt% and 14.5 wt%, respectively. For demonstrator D ([Fig. 14]), the results show a maximum recovered power of approximately 0.176 nW, obtained at a frequency of 46.2 Hz for a load of 128 kQ. The measured bandwidth (BW) is 0.42 Hz, centered around 46 Hz. Demonstrator E (Fig. 15) showed slightly higher power recovery, reaching 0.253 nW at a frequency of 45.2 Hz for a 256 kQ load. The measured bandwidth (BW) for this demonstrator is 0.44 Hz.These results confirm the viability of the anneal-free method for obtaining functional piezoelectric films, confirming the possibility of energy harvesting by demonstrators based on BT-PVDF composite layers as deposited.

Claims

Demands

1. A device comprising a composite film having piezoelectric, pyroelectric, or ferroelectric properties, formed on a substrate by Aerosol Deposition (AD), and having at least one ceramic phase and at least one polymer phase comprising a polymer having plastic or thermoplastic properties, said device being characterized in that the volume fraction of said polymer phase in the composite film is >9% vol and <18% vol, preferably >10% vol and <17% vol.

2. A device according to claim 1, wherein the volume fraction of said polymer phase in the composite film is >11% vol and <16% vol.

3. A device according to any one of the preceding claims, wherein an average diameter of the ceramic grains of said film determined by SEM surface imaging is >90 nm, preferably >100 nm and <1000 nm.

4. Device according to any one of the preceding claims, wherein the film predominantly exhibits 3-0 phase connectivity according to Newnham nomenclature, 3 for the ceramic and 0 for the polymer, and wherein the polymer is dispersed in a continuous matrix of ceramic phase, and with or without minor interconnectivity between the polymer phase.

5. Device according to any one of the preceding claims, wherein observation of the film surface by scanning electron microscopy (SEM) reveals a surface essentially devoid of notable porosity.

6. A device according to any one of the preceding claims, wherein the ceramic phase is selected from: - ceramics of the lead titano-zirconate family, such as PZT, PLZT, PZN-PT; - ceramics of the barium titanate family, such as BaTiO3; - ceramics of the alkali-bismuth titanate family, such as NaosBio sTiOs (NBT), Ko sBio sTiOs (KBT), or NBT-6BT; - potassium and sodium niobates, such as Ko^Nao^NbCb (KNN); and - inorganic piezoelectric ceramic materials such as ZnO, LiNbO3, LiTaO3

7. Device according to any one of the preceding claims, wherein the polymer is selected from fluoropolymers such as polytetrafluoroethylene PTFE, polyvinylidene fluoride PVDF and its copolymers PVDF TrFE and PVDF TFE, as well as organic polymers such as polyethylene terephthalate PET, polyimide PI, polycarbonate PC, polymethyl methacrylate PMMA, polylactic acid PLA, polyetheretherketone PEEK, polyvinyl acetate PVA and polybutylene terephthalate PBT.

8. Device according to any one of the preceding claims, wherein the polymer is selected from fluorinated polymers, and is preferably polyvinylidene fluoride PVDF, and the ceramic phase is from the barium titanate family and / or the alkali-bismuth titanate family.

9. Device according to any one of the preceding claims, wherein the substrate is of a rigid or flexible type, and / or of a polymeric, ceramic or metallic type.

10. A method for manufacturing a device comprising a polymeric ceramic film using an Aerosol Deposition Device (AD), said film having piezoelectric, pyroelectric, or ferroelectric functional properties, and comprising the following steps: - the preparation of powders comprising mixing at least one ferro-, pyro-, or piezoelectric ceramic powder and at least one polymer powder with plastic properties, a predefined mass content of polymer in the powder mixture to obtain, after deposition, a polymer volume fraction in the film >9% vol and <18% vol; - the generation of an aerosol from said powder mixture using said AD; - the aerosol deposition at room temperature of a film onto a substrate by spraying said aerosol onto said substrate; - obtaining a film having ferro-, pyro-, or piezoelectric properties directly after aerosol deposition.

11. A method according to claim 10, wherein the powder preparation comprises the planetary grinding of ceramic powder(s) with polymer powder(s) in a humid medium, and drying and sieving the powder mixture to remove clumps.

12. A method according to claim 11, wherein the particle size determined by SEM images of the mixture of ceramic polymer powders has an average diameter of about 1 pm, in particular less than 1 pm.

13. A method according to any one of claims 10 to 12, wherein the generation of the aerosol comprises adjusting the pressure applied to a vibrating table of said device AD, and the flow rate of a carrier gas to improve the dispersion of the polymer in said aerosol.

14. A method according to any one of claims 10 to 13, wherein the device obtained is a device according to any one of claims 1 to e

15. O. Energy recovery demonstrator comprising a device according to any one of claims 1 to 9.

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