Compressive force measuring device with adaptable sensitivity

Abstract

Compressive force measuring device with adaptable sensitivity The present description relates to a force measuring device (10), intended to be subjected to compressive forces, comprising a substrate (100) on which is arranged at least one sensor comprising an organic piezoelectric film (120) arranged between a first electrode (110) and a second electrode (130), the device (10) further comprising an adaptability layer (150) made of a material having a Young's modulus lower or higher than the Young's modulus of the organic piezoelectric film (120). Figure for abstract: Fig. 1

Description

Title of the invention: Compressive force measuring device with adaptable sensitivity Technical field

[0001] The present description relates generally to force measuring devices, and in particular to compression force measuring devices. More particularly, these are devices having an organic piezoelectric film. Such devices can be used, for example, for measurements on complex and / or deformable surfaces. The devices have a sensitivity that can be easily adapted, either increased or decreased, depending on the intended applications. Prior art

[0002] Piezoelectric sensors can be used to measure forces, including compressive forces. Generally, the sensor is positioned on a surface so that the mechanical stress is not only uniform across the surface of the sensor but also unidirectional in the direction transverse to the sensor. Thus, it is possible to have a proportionality between the electrical charge Q (in pC) generated by the sensor and the force F (in N) applied: Q = -d33 F with d 55 is the piezoelectric coefficient (in pC / N) of the piezoelectric material, characterizing the compression performance of the piezoelectric material.

[0003] To make lightweight devices, making them compatible with many applications, particularly medical ones, piezoelectric materials are used in the form of thin films (typically between 1 and 100 pm). In addition, in order to have flexible and / or biocompatible devices, piezoelectric polymers are generally used rather than piezoelectric ceramics, which are rigid and toxic, particularly lead-based ceramics such as lead zirconate titanoates (known by the abbreviation PZT).

[0004] However, polymers perform less well in compression than ceramics (d33= 30 pC / N for PVDF polymer, d33= 590 pC / N for PZT ceramic).

[0005] In order to have a sensor with good mechanical strength and which can be easily handled, the piezoelectric thin films are generally positioned on a rigid substrate which serves as a mechanical support. However, these substrates can influence the piezoelectric properties of the piezoelectric thin films.

[0006] For example, a very rigid substrate can decrease the piezoelectric performance in compression. This has been demonstrated, for example, for a 200 μm thick PZT film on an alumina substrate having a thickness of 0.64 mm, in the article by Torah et al. ('Experimental investigation into the effect of substrate clamping on the piezoelectric behavior of thick-film PZT elements', J. Phys. Appl. Phys. 37, 1074 (2004)). The model used only concerns the extreme case where the rigidity of the piezoelectric film is negligible compared to that of the substrate, and the latter therefore imposes its mechanical properties on the overall system.

[0007] Conversely, it has been demonstrated by Chakhchaoui et al. ('An enhanced power harvesting from woven textile using piezoelectric materials', IOP Conf. Ser. Mater. Sci. Eng. 827, 012046 (2020)) that a very flexible textile substrate can improve the piezoelectric performance of a PVDF piezoelectric film. Here, too, the model concerns the extreme case where the mechanical properties of the piezoelectric film are negligible compared to that of the substrate in the overall system. In addition, the proposed model shows limitations and is not perfectly validated by experimental data. The thickness of the textile substrate is 500pm, that of the PVDF film is not mentioned. Summary of the invention

[0008] There is therefore a need to provide a measuring device whose sensitivity can be adapted according to the compressive forces to be measured.

[0009] This aim is achieved by a force measuring device, intended to be subjected to compressive forces, comprising a substrate on which is arranged at least one sensor comprising an organic piezoelectric film arranged between a first electrode and a second electrode, the device further comprising an adaptability layer having a Young's modulus lower or higher than the Young's modulus of the organic piezoelectric film, the Young's modulus of the adaptability layer being at least 10 times lower or at least 10 times higher than the Young's modulus of the organic piezoelectric film.

[0010] According to a particular embodiment, the adaptability layer is arranged between the substrate and the first electrode.

[0011] According to a particular embodiment, the adaptability layer covers the second electrode.

[0012] According to a particular embodiment, the substrate is made of polyimide.

[0013] According to a particular embodiment, the adaptability layer is made of PDMS or TPU.

[0014] According to another particular embodiment, the adaptability layer is made of epoxy or acrylate.

[0015] According to a particular embodiment, the adaptability layer has a thickness of between 10 μm and 100 μm.

[0016] According to a particular embodiment, the device comprises two substrates and two adaptability layers.

[0017] According to a particular embodiment, the device comprises two piezoelectric films, separated by an intermediate electrode, the adaptability layers being arranged on either side of the assembly formed by the first electrode, the first piezoelectric film, the intermediate electrode, the second piezoelectric film and the second electrode.

[0018] According to a particular embodiment, an encapsulation layer covers the sides of the adaptability layers and of the piezoelectric film(s).

[0019] According to a particular embodiment, the device comprises several sensors, which can be arranged in lines or in the form of a matrix on the substrate.

[0020] This aim is also achieved by a method for determining the compression performance of a force measuring device, intended to be subjected to compressive forces, the device comprising a substrate on which is arranged at least one sensor comprising an organic piezoelectric film arranged between a first electrode and a second electrode, the device further comprising an adaptability layer, arranged between the substrate and the first electrode or covering the second electrode, the method comprising a step during which the generated electric charge Q is calculated from the following equation: — { AOA \ j-, If - - - YP} with Q electric charge, in pC, generated by the sensor, F the force (in N) applied to the sensor, d jjet d 3i the piezoelectric coefficients of the piezoelectric film, in pC / N, Yp and Ym the elastic moduli, in Pa, respectively, of the piezoelectric film and the adaptability layer, vp and vm are the Poisson's ratios, respectively, of the piezoelectric film and the adaptability layer. Brief description of the drawings

[0021] These characteristics and advantages, as well as others, will be explained in detail in the following description of particular embodiments given without limitation in relation to the attached figures among which:

[0022] [Fig.l], [Fig.2], [Fig.3], [Fig.4] and [Fig.5] represent, schematically and in section, a device according to different particular embodiments of the invention;

[0023] [Fig. 6] represents, schematically and in three dimensions, a piezoelectric film arranged on an adaptability layer on which compressive forces (represented by arrows) are applied, according to another particular embodiment of the invention;

[0024] [Fig.7] represents, schematically and in top view, a device according to another particular embodiment of the invention;

[0025] [Fig.8] is a graph representing the sensitivity of a sensor on a substrate in PI, given for comparison, and of a sensor on a PI substrate, an adaptability layer being positioned between the substrate and the sensor, according to another particular embodiment of the invention; and

[0026] [Fig.9] is a photographic image of different piezoelectric sensors, with from left to right: a sensor printed on PI, a sensor printed on TPU and a sensor, according to the invention, printed on TPU then positioned on PI. Description of the embodiments

[0027] The same elements have been designated by the same references in the different figures. In particular, the structural and / or functional elements common to the different embodiments may have the same references and may have identical structural, dimensional and material properties.

[0028] For the sake of clarity, only the steps and elements useful for understanding the described embodiments have been shown and are detailed.

[0029] Unless otherwise specified, when referring to two elements connected to each other, this means directly connected without intermediate elements other than conductors, and when referring to two elements connected (in English "coupled") to each other, this means that these two elements can be connected or be connected by means of one or more other elements.

[0030] In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as the terms "above", "below", "upper", "lower", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made unless otherwise specified to the orientation of the figures.

[0031] Unless otherwise specified, the expressions "about", "approximately", "substantially", and "of the order of" mean to within 10%, preferably to within 5%.

[0032] By between X and Y, we mean that the limits are included.

[0033] The invention is particularly interesting for applications requiring lightweight, flexible and / or deformable force sensors seeking to measure low forces. The sensors are portable on a person. This is of interest, for example, in the medical sector, for sensors for measuring coaptation forces in a heart valve, or for tactile sensors on the skin.

[0034] We will now describe the device in more detail with reference to Figures 1, 2, 3, 4 and 5.

[0035] The piezoelectric device 10 comprises at least one substrate 100 on which at least one sensor is arranged. The substrate comprises a first face 100a, on which the sensor(s) is / are arranged, and a second face 100b. Each sensor comprises a piezoelectric film 120 arranged between a first electrode 110 (also called the lower electrode) and a second electrode 130 (also called the upper electrode).

[0036] Each piezoelectric film 120 has a thickness, for example, between 1 and 11 μm, preferably between 2 and 5 μm, for example 3 μm. Such thicknesses make it possible to obtain layers 120 having good flexibility.

[0037] Preferably, each piezoelectric film 120 is an organic piezoelectric film. Each piezoelectric film 120 preferably comprises a polymer matrix made of PVDF, a PVDF copolymer or a PVDF terpolymer. It may be a copolymer of vinylidene fluoride and at least one other monomer copolymerizable with VDF. Advantageously, the copolymer comprises at least 50 mol%, preferably at least 70 weight%, even more preferably at least 90 mol% of VDF.

[0038] By way of illustration, the copolymerizable monomer(s) are, for example, chosen from chlorotrifluoroethylene (CTFE), chlorofluoroethylene (CFE), hexafluoropropylene (HFP), trifluoroethylene (VF3), methyl methacrylate (MMA), tetrafluoroethylene (TFE), and perfluoro(alkyl vinyl) ethers such as perfluoro(methyl vinyl) ether (PMVE).

[0039] For example, the copolymer is a copolymer of poly(vinylidene fluoride-trifluoroethylene) PVDF / TrFe, also noted P(VDF-TrFe) or PVDF-CTFE.

[0040] It can also be a terpolymer. For example, a PVDF / CTFE / CFE terpolymer will be chosen.

[0041] According to another embodiment variant, the polymer is not a ferroelectric polymer: it may be PVDF-HFP.

[0042] According to another embodiment variant, the polymer is polylactic acid (also noted PLA or PLLA).

[0043] Preferably, the piezoelectric films 120 are piezoelectric polymers chosen from PVDF, P(VDF-TrFE) and PLLA.

[0044] Each organic piezoelectric film 120 may be a composite material. For example, the film 120 may comprise, in addition to a piezoelectric or non-piezoelectric polymer matrix, piezoelectric particles and / or PEDOT:PSS particles. The particles will be small enough not to alter the roughness.

[0045] For example, the ferroelectric particles are made of BaTiO3 (BTO), PZT (lead zirconate titanoate), AIN, ZnO, or even SBN (Sr-Ba-Nb oxide) or SBT (Sr-Ba-Ti oxide).

[0046] For example, the piezoelectric films 120 are formed from composite materials comprising, for example, a polymer (piezoelectric type PVDF, or non-piezoelectric type TPU, PDMS, PVDF HFP) and piezoelectric nanoparticles or nanofibers (PVDF, PZT, BaTiO3, AIN).

[0047] The piezoelectric films 120 are arranged between electrodes 110, 130. The electrodes 110, 130 are flexible electrodes.

[0048] Preferably, the electrodes 110, 130 are made of an electrically conductive polymer or a metal such as gold or silver. Electrically conductive polymers have a better interface with the P(VDF-TrFE). Preferably, it is PEDOT-PSS (poly(3,4-ethylenedioxythiophene).

[0049] They can be deposited by screen printing methods to ensure good mechanical cohesion, without risk of detachment, at the interface between the sensor and the adaptability layer.

[0050] The electrodes 110, 130 preferably have a thickness of between 0.1 and 3 μm, even more preferably between 0.5 and 1.5 μm, for example approximately 1 μm).

[0051] The metal electrodes 110, 130 have, for example, a thickness of between 30 and 200 nm.

[0052] Preferably, the mechanical properties of the electrodes 110, 130 are close to those of the piezoelectric film 120. Preferably, their Young's moduli vary by less than 30%. There is then no particular constraint on the thickness of the electrodes 110, 130. Preferably, electrodes 110, 130 will nevertheless be chosen having a thickness less than that of the piezoelectric film 120.

[0053] In the case where the mechanical properties of the electrodes 110, 130 are very different from those of the piezoelectric film 120, in particular in the case where their Young's moduli vary by more than 30%, the thickness of the electrodes 110, 130 will be chosen so that it is negligible compared to that of the piezoelectric film. The thickness of the electrodes is at least 10 times less than that of the piezoelectric film 120.

[0054] The substrate 100 preferably has a modulus of elasticity greater than 1GPa and / or a thickness greater than 25 μm, for example between 10 and 500 μm, preferably between 20 and 250 μm, in order to have a device having sufficient mechanical strength.

[0055] The thickness of the sensor is relatively small (typically less than 10 μm). The substrate 100 ensures the mechanical strength of the device for its manufacture and during its use.

[0056] The nature of the substrate 100 will depend on the applications. The substrate 100 may be made of a polymer material, preferably chosen from polyimide (PI), poly(ethylene naphthalate) (PEN), polyethylene terephthalate (PET), thermoplastic polyurethane (TPU) or polydimethylsiloxane (PDMS). Preferably, it is made of PI.

[0057] The substrate 100 may also be made of glass or steel.

[0058] The device further comprises an adaptability layer 150. The adaptability layer 150 is made of a material different from that of the substrate 100.

[0059] The adaptability layer 150 may be made of polymer, sol-gel material or oxide. For example, it is made of epoxy, (meth)acrylate, thermoplastic polyurethane (TPU), polydimethylsiloxane (PDMS), alumina, silica, titanium oxide, or zirconium oxide.

[0060] According to a first variant embodiment, shown in [Fig. 1], the adaptability layer 150 modifying the piezoelectric performance of the sensor is arranged between the substrate 100 and the first electrode 110. In other words, the device 10 successively comprises: the substrate 100, the adaptability layer 150, the first electrode 110, the piezoelectric film 120 and the second electrode 130.

[0061] This configuration allows for a wide choice of substrate 100.

[0062] According to a second variant embodiment, shown in [Fig.2], the adaptability layer 150 is arranged on the second electrode 130. In other words, the device 10 successively comprises: the substrate 100, the first electrode 110, the piezoelectric film 120, the second electrode 130 and the adaptability layer 150.

[0063] The adaptability layer 150 modifying the piezoelectric performance is thus arranged above the sensor. It can also act as an encapsulation layer by protecting it from the external environment.

[0064] In the devices previously described, the adaptability layer 150 is positioned on or under the piezoelectric film 120. The material of the adaptability layer 150 is made of a different material than that of the piezoelectric film 120. Its mechanical properties are different from those of the piezoelectric film 120, which makes it possible to control the performance of the piezoelectric film 120 in compression. It is the mechanical interactions between the adaptability layer 150 and the piezoelectric film 120 which modify the piezoelectric performance.

[0065] It is thus possible to increase the piezoelectric performance in compression (d33 eff) by choosing an adaptability layer 150 made of a material having a Young's modulus lower than the Young's modulus of the material of the piezoelectric film 120 (in other words by choosing an adaptability layer 150 more flexible than the piezoelectric film 120). This makes it possible to improve the sensitivity of the piezoelectric film 120, in particular for compressive forces. For example, its performance is multiplied by a factor between 2 and 10.

[0066] Preferably, the Young's modulus of the adaptability layer 150 is between 10 and 1000 times lower than the Young's modulus of the piezoelectric film 120.

[0067] For example, for a substrate 100 made of PI, the adaptability layer 150 may be made of TPU or PDMS.

[0068] It is also possible to reduce the piezoelectric performance in compression (d33 eff) by choosing an adaptability layer 150 made of a material having a Young's modulus greater than the Young's modulus of the material of the piezoelectric film 120 (in other words by choosing an adaptability layer 150 that is more rigid than the piezoelectric film 120). This makes it possible, for example, to adapt the performance of the piezoelectric film 120 to the measurement range of measurement electronics connected to the piezoelectric device 10.

[0069] Preferably, the Young's modulus of the adaptability layer 150 is between 10 and 1000 times greater than the Young's modulus of the piezoelectric film 120.

[0070] For example, for a PI substrate 100, the adaptability layer 150 may be made of epoxy or acrylate.

[0071] In addition, to increase the piezoelectric performance, it is possible to increase the thickness of the adaptability layer 150 and / or the difference in elastic moduli between the adaptability layer 150 and the piezoelectric film 120.

[0072] The adaptability layer 150 has a thickness greater than the thickness of the piezoelectric film 120. The thickness of the adaptability layer 150 is between 10 μm and 1 mm, preferably between 10 μm and 100 μm. Such a thickness makes it possible to modify the properties of the piezoelectric film 120 while remaining flexible (even if its modulus of elasticity is quite high).

[0073] The adaptability layer 150 has a surface area greater than or equal to the surface area of ​​the piezoelectric film 120 (surface area in the plane - directions 1 and 2 of [Fig.6]). Preferably, the surfaces are identical.

[0074] As shown in Figures 3, 4 and 5, the device 10 may comprise two substrates 100, 170 and two adaptability layers 150, 160. The device 10 successively comprises: the first substrate 100, the first adaptability layer 150, the first electrode 110, the piezoelectric film 120, the second electrode 130, the second adaptability layer 160 and the second substrate 170.

[0075] The two substrates 100, 170 may be made of the same material or different materials. The two substrates 100, 170 may have the same dimensions or different dimensions. Preferably, the two substrates 100, 170 are made of the same material. They preferably have the same dimension.

[0076] The two adaptability layers 150, 160 may be made of the same material or different materials. The adaptability layers 150, 160 preferably each have a Young's modulus greater (or conversely smaller) than that of the piezoelectric film 120. The two adaptability layers 150, 160 may have the same dimensions or different dimensions. Preferably, the two adaptability layers 150, 160 are made of the same material and have the same dimensions.

[0077] The presence of an adaptability layer 150, 160 on either side of the piezoelectric film 120 makes it possible to further increase or decrease its piezoelectric performance.

[0078] The presence of a substrate 150, 160 on each face of the sensor makes it possible to protect the latter well.

[0079] Furthermore, in the case where the device is perfectly symmetrical with the axis of symmetry passing through the piezoelectric film 120, it is possible to divide the piezoelectric film into two parts 120, 125 by adding an intermediate electrode 140 as shown in [Fig. 5]. In other words, the device 10 comprises two piezoelectric films 120, 125. More particularly, the device 10 successively comprises: the first substrate 100, the first adaptability layer 150, the first electrode 110, the first piezoelectric film 120, the intermediate electrode 140, the second piezoelectric film 125, the second electrode 130, the second adaptability layer 160 and the second substrate 170.

[0080] A piezoelectric bimorph is thus formed and it is possible to take advantage of the benefits of this type of device (in particular by polarizing the piezoelectric films in an identical or opposite manner).

[0081] The device 10 may, in addition, comprise an encapsulation layer (FIGS. 4 and 5). It covers the sides of the elements arranged between the substrates 100, 170 (i.e. the sides of the adaptability layers 150, 160, of the electrodes 110, 130 and of the piezoelectric film(s) 120, 125).

[0082] The encapsulation layer 180 completely isolates the sensor from the outside. Such a layer may be biocompatible and / or waterproof. It may be a dielectric material such as non-polarized PVDF or the product marketed under the reference ED AG 455B by Loctite.

[0083] The encapsulation layer 180 is preferably made of PDMS.

[0084] The device 10 may comprise a layer (not shown), forming a ground plane between two dielectric layers to reduce measurement noise.

[0085] The ground plane makes it possible to greatly reduce measurement noise, improve measurement accuracy and therefore open up the field of applications more widely.

[0086] The ground plane is flexible. It has a thickness between 3 pm and 10 pm, for example 5 pm.

[0087] The ground plane is preferably printed. It is integrated into the device during its manufacture by printing.

[0088] To make the ground plane, any flexible conductive ink can be used.

[0089] The ground plane is preferably made of carbon. Carbon is highly conductive and has good biocompatibility, which is advantageous when it is in contact with the human body because it is one of the layers closest to the external environment.

[0090] Alternatively, the ground plane may be gold.

[0091] The dielectric layers serve to electrically insulate the sensor from the ground plane. The dielectric layers are also flexible. They have a thickness of, for example, between 3 μm and 30 μm, preferably between 3 and 10 μm, for example 5 μm.

[0092] They are thick enough not to snap during polarization, nor too thick so as not to stiffen the sensor.

[0093] The dielectric material presents few constraints. A printable and crosslinkable material will preferably be chosen. For example, it may be non-piezoelectric crosslinked PVDF or EDAG. Preferably, EDAG is preferred because, at the time of printing, it does not contain a solvent that can damage the lower layers (unlike the TEP solvent of PVDF which damages the silver).

[0094] Such layers have mechanical properties close to those of the piezoelectric film 120: their presence in the device does not alter the predictive capacity of the model.

[0095] The device 10 may comprise several sensors arranged on the same face 100a of the substrate 100. They may be arranged in line or in the form of a matrix.

[0096] For example, a line may include between X and Y sensors.

[0097] The matrices are matrices of nxm sensors with n and m positive integers, greater than or equal to 2, and preferably between 2 and 10.

[0098] The adaptability layer 150 may be common to all the sensors of the matrix or the line, or specific to each sensor.

[0099] As shown in [Fig.7], the device 10 comprises electrical tracks 190 arranged to carry the electrical signal from the sensors, preferably to amplifiers, and then to measuring devices, preferably electronic measuring devices. Voltage measuring devices can also be used to measure the signals from the sensors.

[0100] The electrical tracks 190 are electrically conductive tracks. They can be printed on the substrate. Any highly conductive ink that can be printed with a fairly fine resolution (typically the track width is less than 100 μm for high-density matrices) can be used.

[0101] The tracks 190 may be made of a metal (gold for example), or of an electrically conductive polymer material. For example, the electrically conductive polymer material is PEDOT:PSS. It may also be a polymer in which electrically conductive particles are dispersed, for example particles carbon. Silver can also be used in ink form, preferably formulated with a solvent-resistant polymer matrix (e.g., silicone, TPU, or acrylate base).

[0102] Preferably, a silver-type ink (in particular a silicone matrix with silver particles) is used for the electrical connections between the sensor and the measuring electronics.

[0103] Gold electrical tracks 190 are not only highly conductive but also biocompatible. The thickness of such tracks is preferably less than 200 nm to obtain flexible tracks. The thickness is preferably between 10 nm and 200 nm, for example 100 nm.

[0104] Gold can be deposited by photolithography, which makes it possible to achieve very high resolution and thus obtain matrices with a high density of sensors.

[0105] Preferably, gold is used with a PI substrate, which is resistant to the solvent used during photolithography (typically acetone).

[0106] The method of manufacturing such a device may comprise the following steps: - providing a substrate 100, - depositing an adaptability layer 150 on the substrate 100, - forming the different layers of the sensor (electrodes, piezoelectric film).

[0107] Alternatively, the method may comprise the following steps: - form the different layers of the sensor (electrodes, piezoelectric film) on an adaptability layer 150, - deposit the assembly obtained on a substrate 100.

[0108] The method may further comprise a step during which an encapsulation layer is formed.

[0109] When the device 10 comprises 2 substrates, it is possible to form the layers successively from the first substrate 100 to the second substrate 170. Alternatively, it is possible to position on a first assembly (formed by the first substrate 100, the first adaptability layer 150, the first electrode 110, the piezoelectric film 120, the second electrode 130) a second assembly (formed by the second substrate 170, the second adaptability layer 160). The second assembly may optionally further comprise the intermediate electrode 140 and the second piezoelectric film 125.

[0110] Preferably, two identical assemblies (substrate 100 or 170, adaptability layer 150 or 160, electrode 110 or 130, piezoelectric film 120 or 125) are manufactured then assembled to the electrode 140, for example by means of a conductive adhesive.

[0111] The method also includes a step of crystallization (or alignment of the dipoles) of the layer of piezoelectric material, to improve its performance. piezoelectrics. This irradiation is for example implemented with UV flash light, with a flash duration, or pulse, of between approximately 500 ps and 2 ms, a fluence (energy delivered per unit area) of between approximately 15 J / cm2 and 25 J / cm2, and with light of wavelength between approximately 200 nm and 380 nm. The number of flashes, or pulses, of UV light produced during this irradiation varies according to the thickness over which the piezoelectric material is to be crystallized. For example, for a thickness of P(VDF-TrFe) equal to approximately 2 pm, the irradiation can be implemented with a fluence equal to approximately 17 J / cm2, a pulse duration equal to approximately 2 ms and a number of pulses equal to 5.

[0112] The piezoelectric material, having possibly undergone a previous crystallization, is then subjected to an annealing step, for example, carried out at approximately 130°C for approximately 60 min, to finalize the total crystallization of the piezoelectric material.

[0113] The crystallization of the piezoelectric material can therefore be carried out in two stages: firstly, irradiation by UV light pulse to properly crystallize the second face of the layer of piezoelectric material in order to increase its thermal conductivity, then thermal annealing completing the crystallization for the remainder of the piezoelectric material not crystallized by the previous irradiation.

[0114] When the piezoelectric material is a P(VDF-TrFe)-based copolymer, a step of polarizing the piezoelectric material is carried out before its use. This step can be carried out, for example, by applying a direct electric voltage to its terminals, via the electrodes, in order to improve the piezoelectric coefficient of this material. This polarization is carried out only once for the entire lifetime of the piezoelectric material. This polarization by electric field can be carried out at room temperature or hot (up to approximately 100°C). When the polarization is carried out at room temperature, it is possible to apply a direct voltage up to approximately 150V / pm of thickness of the piezoelectric layer for a duration, for example, of between a few seconds and a few minutes. For example, a voltage of 120V / pm will be applied for 20s.When the polarization is carried out hot, for example at a temperature of about 90°C, a direct voltage for example between about 50 V and 120 V per micron of thickness of the piezoelectric layer can be applied for a duration for example between about 1 min and 5 min. The temperature is then lowered until it reaches room temperature, then the electric field applied to the piezoelectric material, via the applied direct voltage, is stopped. Such polarizations allow the PVDF to achieve piezoelectric coefficients between about 10 and 40 pC / N.

[0115] The molecules inside the piezoelectric layer remain oriented in this way, even when the material is no longer subjected to this electric field. The material can be thus polarized by applying an initial polarization voltage across the electrodes. A thickness of piezoelectric material less than or equal to approximately 3 μm will preferably be chosen in order to promote the polarization of the piezoelectric material of this capacity, and the level of the electrical voltage applied between the electrodes to achieve the initial polarization of the piezoelectric material (when the piezoelectric material must be initially polarized).

[0116] For example, we will choose an ideal remanent polarization at 8 pC / cm2.

[0117] Annealing is advantageously carried out at the end of the process, or between the different stages. The annealing is, for example, at a temperature between 100°C and 150°C, preferably around 100°C to remove residual traces of solvent and / or finalize the crystallization of the piezoelectric material.

[0118] Each sensor of the device is preferably printed by screen printing on the substrate 100. The printing manufacturing method makes it possible to simplify and reduce the cost of the manufacturing process.

[0119] All sensors on the same side of the substrate can be printed at the same time.

[0120] The performance, more particularly the compression performance, of a device 10 as described above can be determined. For this, the conditions of [Fig. 6] are used, and it is considered that the piezoelectric film 120 and the adaptability layer 150 in contact are both elastic, linear and isotropic materials. This is almost always the case for low compression forces, typically less than 100 kPa.

[0121] We define the following variables: - mechanical quantities: T the stress in the piezoelectric film in N.m2 (T 3 therefore corresponds to the uniform stress applied in compression), S the deformation of the piezoelectric film; - Electrical quantity: D the electrical displacement in the piezoelectric film in pC.m2; - geometric parameters \hp and hm thicknesses in m, respectively of the piezoelectric film 120 and of the material of the adaptability layer 150 allowing the performance of the device to be modified; - mechanical properties: Y p and Ym the elastic moduli respectively of the piezoelectric film 120 and of the material of the adaptability layer 150 in Pa, vp and vm their Poisson's ratio. - piezoelectric properties: d 33 and d 31 the piezoelectric coefficients of the piezoelectric film 120.

[0122] In the configuration of [Fig.6], the compression performance of the piezoelectric film 120 is characterized by an effective d33 (d 33:

[0123] Q=-d33effF

[0124] [Math.l] - ^33- W1MÇ \ h / lv^Y^l-VpW ~ T; )

[0125] Indeed, according to the theory of piezoelectricity for a compressive stress:

[0126] [Math 3] D3^d3V{T +d33I3 (1)

[0127] By isotropy of the piezoelectric film, we set T\ = T2 = T and = S2 = S. According to the law of Hooke in the piezoelectric film:

[0128] [Math 4] S==^T_±T (2)

[0129] According to Hooke's law in the equivalent material consisting of the piezoelectric film and the contacting material:

[0130] [Math 5] 5^ _^r(3) ° ïe <r* 3

[0131] where Y eq and veq are respectively the modulus of elasticity in direction 3 and the Poisson's ratio of the equivalent material.

[0132] Combining equation (2) and equation (3):

[0133] [Math 6] T _ _ Il ( 1 T (4) 1- Pp \ Yeq ip / 3

[0134] Combining equation (4) and equation (1):

[0135] [Math7] D (d 2d II Æ .ÿ. 1 \T^ (5)

[0136] Y eq and v eq are calculated using the improved version of the Reuss equations for a composite material under isostressed conditions (equations from the article by Y. Luo ('proved Voigt and Reuss Formulas with the Poisson Effect'. Materials. (2022)):

[0137] [Math.8] y — ........................................................................................................................................................ (6)

[0138] [Math.9] , pF7^p(i~vBi)Fp^^x q Y pi nJ^XFv»i)+^WEvp)]+ / tpÀnl[( lr,„« 1-2 p+4vpVmypI m+( l+Vp)(l-2vp)I (7)

[0139] By injecting equations (6) and (7) into (5) and then normalizing by the surface area of ​​the piezoelectric film, we obtain the charge Q:

[0140] [Math. 10] ilnl YP ( Vp \ \ y - - ^33-^3] h^lv^Y^k,^ -Yp /

[0141] This model makes it possible to determine the compression performance of the piezoelectric film 120 by varying the modulus of elasticity of the material of the adaptability layer 150.

[0142] In the case of a piezoelectric polymer for which d33 and dM are of opposite signs, the model shows that: - the piezoelectric performance is improved if Ym < Yp (adaptability layer 150 more flexible than the piezoelectric film 120), - the piezoelectric performance decreases if Yp< Ym (adaptability layer 150 more rigid than the piezoelectric film 120), - the piezoelectric performance does not vary or varies little if Yp ~ Ym (adaptability layer 150 mechanically close to the piezoelectric film 120), - the greater the thickness of the material of the adaptability layer 150 compared to that of the piezoelectric film 120, and the more the elastic moduli are different, the more the piezoelectric performance is impaired.

[0143] The dimensions and materials of the electrodes 110, 130 will be chosen so as not to affect the model: - the thickness of the electrodes 110, 130 is negligible compared to that of the piezoelectric film 120 (typically at least 10 times lower), - the mechanical properties of the electrodes 110, 130 are close to those of the piezoelectric film 120 (less than 30% difference between the elastic moduli) so that it can be considered that hp is the thickness corresponding to the sum of the thickness of the piezoelectric film 120 and the thickness of the electrodes 110, 130.

[0144] From this model, it is possible to predict whether an adaptability layer 150 will decrease or increase the performance of the piezoelectric device 10.

[0145] Illustrative and non-limiting example

[0146] Several devices have been produced.

[0147] First, a device was manufactured for comparison purposes. It comprises a sensor on a PI substrate (Ym = 6 GPa, hm = 50 pm, Upilex from UBE). The geometry is that shown in [Fig.7] (except for the presence of the adaptability layer). The sensors have a diameter between 2 mm and 3 cm, preferably less than 1 cm so that they can be easily subjected to a uniform compressive force.

[0148] The sensor is printed by screen printing on the substrate. The lower electrode is first printed. This is a layer of PEDOT:PSS (product marketed by the company Heraeus). An annealing at 135°C for 30 minutes is used to evaporate the solvent (final thickness approximately 1 μm). The material is then printed piezoelectric, P(VDF-TrFE) 80 / 20 (marketed by Arkema Piezotech), then a 3-minute annealing at 135°C under vacuum (3pm thickness). The upper electrode of PEDOT:PSS is then deposited in the same way as the lower electrode. Tracks are printed with silver ink (marketed by Taiyo Ink) and then a 10-minute annealing at 135°C is carried out. These tracks will provide the link between the sensor and the measurement electronics.

[0149] The piezoelectric film is then polarized. It is connected to a voltage source via its electrodes, and a first low DC voltage is applied (electric field in the piezoelectric material ~ 30 V / pm) for 3 minutes to evacuate as much electrical charge as possible from the sensor. A first voltage increase is then carried out in AC at 10 Hz (from approximately 30 V / pm to 120 V / pm) to eliminate certain defects in the sensor. The same voltage increase is then carried out again but at 1 Hz to finalize the alignment of the dipoles.

[0150] P(VDF-TrFE) and PEDOT:PSS have very similar mechanical properties and can therefore be considered as a single layer in the model (Yp = 2.5 GPa). To experimentally test the device in compression, it is placed on a rigid glass plate with a double-sided adhesive (marketed by Teraoka under the reference 9030W) and then pressed with different forces at 1 Hz with a flat indenter perfectly parallel to the surface of the sensor and with an area greater than that of the sensor. The electrical charges generated by the piezoelectric film under compressive stress are measured by a Kistler 5015 charge amplifier with a 10 Hz low-pass filter to eliminate noise.

[0151] The same device was made by adding a TPU adaptability layer 150 (Ym = 50 MPa, hm = 100 pm, Intexar TE-1 IC from DuPont) between the substrate 100 and the sensor ([Fig.7]).

[0152] The TPU used has a thin layer of glue on its lower face which is activated at high temperature. After screen printing, the assembly formed by the sensor and the TPU layer 150 is placed on the PI substrate (larger than the sensor) and then pressed with a heating plate at 150°C for 10s to bond the PI and the TPU. The sensor printed on TPU then bonded to PI has a better performance (approximately 3 times better) than the sensor printed directly on PI ([Fig.8]). The different devices are in [Fig.9].

[0153] It is likely that the high temperature process slightly degrades the performance of P(VDF-TrFE). The expected performance was an improvement of a factor of 10, but the achieved performance is an improvement of a factor of 3. Optimization of the process will improve the sensor performance. Nevertheless, the feasibility is demonstrated.

[0154] Various embodiments and variations have been described. Those skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will occur to those skilled in the art.

[0155] Finally, the practical implementation of the embodiments and variants described is within the reach of those skilled in the art from the functional indications given above.

Claims

Claims

1. A force measuring device (10), intended to be subjected to compressive forces, comprising a substrate (100) on which is arranged at least one sensor comprising an organic piezoelectric film (120) arranged between a first electrode (110) and a second electrode (130), the device (10) further comprising a conformability layer (150) having a Young's modulus lower or higher than the Young's modulus of the organic piezoelectric film (120), the Young's modulus of the conformability layer (150) being at least 10 times lower or at least 10 times higher than the Young's modulus of the organic piezoelectric film (120).

2. Device according to claim 1, characterized in that the adaptability layer (150) is arranged between the substrate (100) and the first electrode (110).

3. Device according to claim 1, characterized in that the adaptability layer (150) covers the second electrode (130).

4. Device according to one of claims 1 to 3, characterized in that the substrate (100) is made of polyimide.

5. Device according to any one of claims 1 to 4, characterized in that the adaptability layer (150) is made of PDMS or TPU.

6. Device according to any one of claims 1 to 4, characterized in that the adaptability layer (150) is made of epoxy or acrylate.

7. Device according to any one of the preceding claims, characterized in that the adaptability layer (150) has a thickness of between 10 pm and 100 pm.

8. Device according to any one of the preceding claims, characterized in that it successively comprises: the substrate (100), the adaptability layer (150), the first electrode (110), the piezoelectric film (120), the second electrode (130), a second adaptability layer (160) and a second substrate (170).

9. Device according to claim 8, characterized in that it comprises two piezoelectric films (120, 125), separated by an intermediate electrode (140), the adaptability layers (150, 160) being arranged on either side of the assembly formed by the first electrode (110), the first piezoelectric film (120), the electrode intermediate (140), the second piezoelectric film (125) and the second electrode (130).

10. Device according to one of claims 8 and 9, characterized in that an encapsulation layer (180) covers the sides of the adaptability layers (150, 160) and of the piezoelectric film(s) (120, 125).

11. Device according to any one of the preceding claims, characterized in that it comprises several sensors, which can be arranged in lines or in the form of a matrix on the substrate (100).

12. A method for determining the compression performance of a force measuring device (10), intended to be subjected to compressive forces, the device comprising a substrate (100) on which is arranged at least one sensor comprising an organic piezoelectric film (120) arranged between a first electrode (110) and a second electrode (130), the device (10) further comprising an adaptability layer (150), arranged between the substrate (100) and the first electrode (110) or covering the second electrode (130), the method comprising a step during which the generated electric charge Q is calculated from the following equation: [Math.10] Q - - [di3-2d3^ ( h / ^Yp+h^\-vp)Ym ~Yp) with Q electric charge, in pC, generated by the sensor (10), F the force (in N) applied to the sensor (10), d jjet d 3i the piezoelectric coefficients of the piezoelectric film (120), in pC / N, Yp and Ym the elastic moduli, in Pa, respectively, of the piezoelectric film (10) and the adaptability layer (150), vp and vm the Poisson's ratios, respectively, of the piezoelectric film (120) and the adaptability layer (150).

Images