Device for directly measuring curvature by flexoelectricity, and associated manufacturing method

Abstract

The invention relates to a device (1) which comprises an active electromechanical layer made of a semiconductor material (40), with two polymers, at least one of which is conjugated, arranged between two metal electrodes (2a, 2b) so as to be capable of generating a voltage between the two electrodes (2a, 2b) when the device is folded. The electromechanical layer (4) has a thickness of between 10 and 160 micrometres, at least one of the two electrodes (2a, 2b) being directly bonded to this layer. The electrodes are configured to be flexible and less thick than the active electromechanical layer within a multilayer structure of the device. The conductive polymer PEDOT may be one of the conjugated polymers. The flexoelectric device (1) obtained, protected if necessary in a protective assembly (7) by a flexible encapsulation material of submillimetric thickness, can be used in the form of a band in a sensor (C) for detecting a curvature or a position of an object.

Description

Device for direct curvature measurement by flexoelectric effect and associated manufacturing process technical field

[0001] The present invention relates to the field of flexoelectricity and more particularly proposes a flexible device for direct measurement of curvature, as well as a method for manufacturing such a device. Previous technique

[0002] The phenomenon of electromechanical coupling is widespread in various artificial and natural materials. In a piezoelectric material, electrical polarization occurs when the material is subjected to mechanical stress, and conversely, mechanical deformation is triggered in response to an electric field.

[0003] The piezoelectric effect results from the relative displacement between atoms within the crystal, and only asymmetric crystals can produce the piezoelectric effect. The flexoelectric effect refers to the phenomenon of electrical polarization generated by a non-uniform strain field or strain gradient. Unlike the piezoelectric effect, the flexoelectric effect can also generate electrical polarization in materials with centrosymmetric crystals. In a crystalline solid, the strain gradient causes a non-uniform relative displacement of atoms, which breaks the inversion symmetry of the crystal and generates polarization within the structure. While the piezoelectric effect is produced by uniform deformation, the flexoelectric effect is produced by non-uniform deformation (gradient).

[0004] The flexoelectric effect can be simply defined as the coupling between the strain gradient and polarization in solid dielectrics. It is thus an alternative transduction mechanism to the piezoelectric effect, allowing, for example, the direct detection / measurement of structural curvature through a measurement performed in bending.

[0005] A number of devices using flexoelectricity have been proposed, particularly in curvature or position measurement applications. Highly flexible structures are desirable when measuring large curvatures. US patent 2011094306 describes a complex textile incorporating multiple strips that can be placed in contact with an object's surface, so that each strip bends along the object's surface contour. The bending of the flexoelectric strip generates a polarization voltage between its opposite edges, which can be measured by electrodes to determine the local contour of the object's surface. Each strip can be made of boron nitride (BN) or... Boron and carbon nitride (BC2N). This type of structure is complex and, at the level of each lamella forming an elementary sensor, the bending is reduced.

[0006] In the case of large curvatures, soft and flexible sensors are required, and soft polar elastomers can be suitable for curvature measurement. A method for developing a thin sensor that utilizes flexoelectricity in polyurethane is described in the article "Flexoelectric response in soft polyurethane films and their use for large curvature sensing" by V. I. Merupo, B. Guiffard, R. Seveno, M. Tabellout, and A. Kassiba; Journal of Applied Physics 122 (2017). Sensor systems using other polymers (e.g., PVDF) have also been studied. Overall, the flexoelectric response coefficients obtained with such highly flexible materials remain modest, for example, not exceeding the range of 10⁻¹⁰ 10 Cm' 1 at 10' 8 C.rrr 1 .

[0007] Several sensor structures have been proposed, for example, in document WO 2022081672, which utilize a central layer (made from an ionic electroactive polymer or a liquid crystal elastomer) and an organic semiconductor distributed across two outer layers (acting as flexible electrodes). The organic semiconductor can be doped (and even heavily doped), as in the case of PEDOT:PSS. This allows for the creation of highly sensitive bending sensors using a mechanically robust central layer that supports the semiconductor electrodes (which are significantly thinner). The following two documents describe methods for manufacturing structures with good flexoelectric response coefficients, using a stainless steel element that forms an electrode and serves as a support for the deposition of a PEDOT:PSS layer. - “Enhancing flexoelectricity in PEDOT:PSS polymer with soft treatments” - Saadeh et al. (Journal of Applied Physics, vol. 130), - “Revealing the flexoelectric-like response of poly(3, 4 -ethylenedioxythiophene): poly(styrenesulfonate) thin films” - Saadeh et al. (Polymers for Advanced Technologies, vol. 31). Technical problem

[0008] There is still a need to design, in an accessible and simple way, sensors compatible with a high flexoelectric coefficient (generally greater than 10 pC.nr). 1 (typically) with the aim of obtaining direct sensors with high curvature, possibly greater than 50 or 60 m' 1 by making the best use of the flexoelectric effect (proportional to the curvature) which is present in all dielectrics and does not require the application of an electric field to give the material its macroscopic electromechanical activity (polarization step). Solution technique

[0009] To improve the situation, a flexoelectric device is proposed comprising an active electromechanical layer made of a semiconductor material inserted between two metallic electrodes capable of (intrinsically) generating a voltage between the two electrodes when bent, the active electromechanical layer being able to have a thickness between 10 and 160 micrometers, the semiconductor material being based on a mixture of polymers containing at least one conjugated conducting polymer and a polymer charged oppositely to the conjugated conducting polymer, at least one of the two electrodes being directly linked to the active electromechanical layer, the two electrodes being flexible and thinner than the active electromechanical layer within a multilayer structure of the flexoelectric device.The semiconductor material, typically designed before the electrodes, is adapted to carry the two electrodes without relying on a support structure (mineral substrate for example). The PEDOT compound or similar polymer with repeating thiophene units may be the conjugated polymer(s), preferably with a positive charge (protonated / oxidized conjugated polymer). Another polymer with an opposite charge, negative in the case where PEDOT forms the first polymer in the mixture, is used in the semiconductor material composition. The active layer may be elongated / have a specific elongation direction (to define a length and width, for example) and extend planarly in a resting configuration, in which the charges are distributed substantially uniformly throughout the semiconductor material. The semiconductor material of the active layer, which may extend from one electrode to the other, is configured to exhibit a transverse flexoelectric coefficient greater than 20 or 30 pC / m, as measured in response to an induction frequency of 2 Hz or less.The stress can correspond to a bending action against the surface of a cylinder / roller, by winding against the external surface of the roller.

[0010] With these arrangements, by using conjugated polymers in a layer that is structural and / or has a thickness of at least 10 micrometers, we ensure the realization of a self-supporting multilayer structure, typically without metallic substrate / layer with an ability to generate a current (therefore a voltage between the electrodes), for example which exceeds the nanoampere for a winding on half of a cylindrical surface. More broadly, it is understood that the active electromechanical layer can allow easy flow of charges for a flexoelectric effect enabling the use of A device such as a curvature sensor or similar characteristic parameter representative of an object's spatial configuration. The semiconductor material exhibits significant flexibility, made possible by a plastic composition including a conjugated polymer. This semiconductor material comprises a mixture of two electrically conductive polymers that can constitute at least 95% by mass of the material (preferably at least 98% or 99%), with at least one of these polymers being a conjugate type. At least one of these two polymers is a p-type conductor, i.e., an electron acceptor.

[0011] In one particular design, the semiconductor material forms a central layer of the device structure, which is devoid of a metallic or mineral substrate. Only the electrodes within this structure contain a metallic composition, with a thickness (electrode thickness) of, for example, approximately 0.5 micrometers and / or electrode thicknesses at least 10 times smaller than the thickness of the active layer. This allows for a thin, highly flexible device in which the active layer lacks a thick layer (thus, the active layer does not exceed 160 micrometers), which forms the supporting substrate from below and potentially the lower electrode.

[0012] In some embodiments, one or more of the following features may be used: - the active electromechanical layer has a thickness greater than 9 or 10 micrometers and less than or equal to 50 micrometers. - the electrodes have the same thickness or a thickness of the same order of magnitude (not varying by more than a factor of 10 between them). - the semiconductor material, typically designed before the electrodes, is adapted to carry the electrodes, thus being self-supporting at least up to the (inclusive) integration / deposition stage of the two electrodes. - the mixture combining the two polymers also includes a polyol added as an additive at less than 1% by weight of the mixture. - the polymer blend is the PEDOT:PSS blend, preferably also including a polyol added as an additive at less than 1% by weight of the blend. - the polyol is xylitol, added in a proportion of between 0.2 and 0.6% by weight of the mixture of two polymers. - the active electromechanical layer has a flat outer surface partially covered by one of the two electrodes. - the multilayer structure of the flexoelectric device is waterproofly enveloped by an outer protective assembly. - the two electrodes form respectively: a lower electrode covered with a lower film portion of the outer protective assembly, and an upper electrode covered with an upper film portion of the outer protective assembly.

[0013] In embodiments that protect the active electromechanical layer, it is understood that two non-functional film portions in the flexoelectric effect can protect the conjugated polymer and, more generally, the semiconductor material including such a conjugated polymer, particularly if it is PEDOT. The two film portions can be directly joined to each other, possibly by heat welding, at least in a peripheral margin area surrounding a peripheral edge of the semiconductor material.

[0014] Depending on the design options for the two electrodes, one or more of the following features are anticipated: - each of the two electrodes is directly formed on the active electromechanical layer. - one of the two electrodes is deposited / formed on a first surface of the active electromechanical layer, while the other electrode is deposited / formed on a second opposite surface of the active electromechanical layer. - the thickness of the active electromechanical layer is constant between the two electrodes in a planar configuration of the flexoelectric device. - at least one of the two electrodes (typically both electrodes) is deposited under vacuum by a spray technique. - the two electrodes are deposited using the same metallic source material, by a spraying technique. - spraying can be carried out in two stages and using at least one inert transparent support, for example glass. - an upper electrode can be completed before starting the spraying step to obtain the lower electrode.

[0015] In another approach, a curvature sensor in the form of a headband, typically a waterproof headband, is proposed for determining a parameter representative of the curvature of an object's surface, the sensor comprising: - the flexoelectric device as described above; and - two opposite ends selectively formed by a material which covers, between the two ends, the active electromechanical layer and, preferably also the two electrodes, of the flexoelectric device; with the particularity that the band, which extends between the two ends, is foldable in an intermediate region of the band between the two, the band having a thickness less than or equal to 150 pm.

[0016] A curvature determination can be made accurately, even for a large curvature of a surface having locally a radius of curvature less than 20 mm, thanks to the high flexibility of the band, while having interesting properties in terms of charge mobility in the semiconductor material forming the active electromechanical layer of the flexoelectric device integrated into the sensor.

[0017] In some embodiments, the band is foldable in the intermediate region by at least 80°, which is thus an angle of deviation from the straight angle. Considering, for example, that one end of the band is held fixed, possibly horizontal, while the intermediate region of the band follows the surface of the object whose curvature is to be determined, it is understood that the other end of the band (locally unfolded because it is outside the intermediate region) can extend in a general direction that makes a potentially significant angle with the first end. Preferably, the angle that can be obtained by folding the strip can be greater than or equal to 10° (at least 120° of deviation from the flat angle).

[0018] The band has a homogeneous composition comprising two oppositely charged polymers, including at least one conjugated polymer, to form an active electromechanical layer consisting solely of a semiconductor material. Optionally, the opposite faces of the band can be made of an electrically insulating (non-metallic) plastic material to form the external surfaces of the sensor (thus creating two non-functional layers, possibly fused or welded around the perimeter of the active electromechanical layer). The electrically insulating plastic material can include or consist of PET or another similar polymer that is robust even in a thin layer, for example, not exceeding one hundred micrometers.

[0019] The active (flexoelectric) layer can be self-supporting between the two flexible outer layers, meaning it does not rely on a support structure (such as a thicker structure or an ionic / metallic support structure) before encapsulation. A PEDOT:PSS can be used to form this layer, optionally with at least one additive, such as a polyol, homogeneously distributed within the active layer.

[0020] The two electrodes can be connected to a conditioning circuit, for example, one equipped with an oscilloscope and a current amplifier. This allows the recovery of a signal representing the current or voltage applied through the thickness of the active electromechanical layer. Optionally, the connections / junctions of these wires are located at or near one end of the electrode. in order to leave a significant length of the electrode available in an intermediate region (between its two ends) of the flexoelectric device.

[0021] In some embodiments, the active electromechanical layer, which includes the PEDOT:PSS mixture, for example with a molar ratio greater than or equal to 1:7 between these two polymers (optionally with 6 times more PSS), is capable of exhibiting a flexoelectric response coefficient greater than 1000 or 10000 pC.nr 1 .

[0022] The deformable headband is equipped with a protective assembly that may include or consist of two external protective layers, each with a thickness greater than approximately 10 µm (e.g., 35 µm in the case of PET) and optionally less than 80 µm, to protect the active layer, which includes at least one conjugated polymer. This protective assembly allows the sensor to be configured in a curved shape, attached to a solid surface of the object, corresponding to the surface whose curvature is to be determined, with a good flexoelectric response coefficient and water resistance.

[0023] In one particular aspect, a manufacturing process for a direct curvature measurement device using the flexoelectric effect is also proposed, the process comprising the steps essentially consisting of: - to mix two polymers in fluid form, with an additive, such as a polyol, representing less than 1% by weight of the mixture obtained, so as to form a homogeneous solution capable of being plasticized, one of the polymers being conjugated by being electrically conductive; - deposit the homogeneous solution in liquid form onto a rigid substrate by distributing the deposit over the substrate, such as a PTFE plate as a non-limiting example, in order to form an elongated layer of a semiconducting material which is based on said mixture with said conjugated polymer; - after drying the layer of semiconductor material which is in a solid and flexible state, with a thickness between 10 and 160 micrometers, deposit in two stages a first electrode in the form of a flexible layer and then a second electrode in the form of a flexible layer, in order to obtain a structure of a flexoelectric device whose thickest layer is formed by the layer of semiconductor material (in other words for this device, the previously designed semiconductor material serves to carry the two electrodes, directly / without intermediate element); - after or before a connection of conductive wires linking respectively the first electrode and the second electrode in order to make detectable a voltage or a current which is generated by the semiconductor material between the two electrodes in response to a bending of the latter, protect said structure by a protective assembly which isolates, in a watertight manner, the semiconductor material from the outside of the flexoelectric device.

[0024] This methodology allows for the design of a thin film made of a high-performance semiconductor material with respect to the flexoelectric effect, using a temporary support / substrate during the deposition of the layer that forms the active electromechanical layer of the device. The substrate used for the deposition / application of the composite material as a thin film is a support / substrate chosen with surface properties that facilitate dry peeling of the layer. Such a substrate, for example, PTFE, is separated from the semiconductor material no later than just before the deposition of the second electrode. Optionally, the deposit is made by forming a layer with a width of at least 6 mm and a length greater than or equal to 40 or 60 mm, with a thickness between 10 and 100 micrometers.

[0025] According to one embodiment, the two polymers are poly(3,4-ethylenedioxythiophene) and sodium poly(styrene sulfonate), so that the semiconductor material is a PEDOT:PSS-based composite material, optionally with a molar concentration at least double for PSS in the precursor solution used for layer deposition. In embodiments, the deposition of the elongated layer of semiconductor material is carried out in such a way as to obtain a homogeneous composition of the semiconductor material, preferably with a substantially constant thickness (except for edge areas, depending on the options) which is at least 100 times or at least 200 times greater than the thickness of any of the electrodes which are metallic.

[0026] In some options, the two-step deposition of the first and second electrodes is performed directly onto the semiconductor material, on two opposite faces of the elongated layer. The electrode length is, for example, greater than 30 mm. When the device is protected by a protective assembly (which may be transparent), a band at least twice as long as the electrodes and / or at least twice as long as the active electromechanical layer can be formed. To facilitate handling of the strip out of the area where the layer of semiconductor material is formed, an overhang of at least 15 or 20 mm can be provided on either side of the opposite short edges of the active electromechanical layer to form strip ends without composite material, with for example the sheets of the protective material directly attached to each other. Brief description of the drawings Other features, details, and advantages will become apparent upon reading the detailed description below and analyzing the attached drawings, on which: Figure 1 is a view of a curvature sensor having an ultrathin layered structure simple intercalated between two electrodes, according to one embodiment of the invention. Figure 2 is a diagram illustrating steps in a method for preparing a flexoelectric plastic film. Figure 3 is a perspective view of a sensor capable of determining curvature, in a state of bending with the flexoelectric functional structure encapsulated in a flexible protective assembly. Figure 4 illustrates by means of a curve the evolution of the flexoelectric coefficient of a device according to the invention, as a function of the excitation frequency. Description of the implementation methods

[0027] Several examples of non-limiting embodiments are described in detail below. In the various figures, identical reference numerals indicate identical or similar elements. Some dimensions may be exaggerated for illustrative purposes.

[0028] In the drawings, the direction of an arrow Ah indicates an upward direction in a vertical direction passing through the thickness of the device. The terms "upper" and "lower" in this description correspond to a top part and a bottom part, considering a direction passing through the thickness of the device 1, passing through each of its constituent layers.

[0029] The flexoelectric sensor of this first embodiment, as illustrated in Fig. 1, has a three-layer structure 6 with a layer 4 that may be much thicker than the two outer layers used to form electrodes 2a, 2b. Device 1 may consist of a superposition of layers in which the electrodes cover only a portion of the corresponding surface of layer 4, which is the active electromechanical layer, including a polymer-based semiconductor material of which: - one is a conjugated polymer; and - another may have ionic bonds (for example with a negatively charged group linked to the sodium ion which thus enters into its composition). More generally, a semiconductor material 40 is prepared that is capable of distributing charges within its structure along its thickness (parallel to the direction Ah). This material can be obtained by casting a mixture called a precursor solution. Typically, the two aforementioned polymers are organic polymers with carbon chains, whose structure facilitates charge flow, so that the solid layer obtained by deposition on a suitable substrate / support S forms a semiconductor that is highly sensitive to bending, thus providing an easily measurable flexoelectric response.

[0030] The active electromechanical layer 4, which includes, for example, PEDOT or another doped conjugated polymer with similar properties as an electronically conductive material, can have a generally rectangular film format, with flexibility resulting from a thickness e40 of less than 200 micrometers, more particularly less than or equal to 100 or 160 micrometers. The electrodes 2a and 2b, each metallic and made of a material comparatively more rigid than the semiconductor material, are deposited in an ultrathin layer on the lower and upper sides of such a layer 4, respectively.

[0031] In the case of a PEDOT conjugated polymer, an association with the PPS polymer can be expected, such that PEDOT and PSS form bonds between themselves, between the (SOa)' groups of PSS and S +of PEDOT. More broadly, two polymers suitable for charge circulation are chosen, for example to form a semiconductor material 40 in which interactions are formed between the conjugated polymer and a negatively charged part in the complementary polymer.

[0032] Semiconductor material 40 may also include in its composition a polyol, in a proportion of less than 1% by weight, which can act as a stabilizer and plasticizer of the polymer mixture including the conjugated polymer and a complementary polymer capable of being in an ionic form interacting with a part of the conjugated polymer.

[0033] Referring to Figure 3, device 1 can be part of a sensor C having a protective assembly, external in this case, completely covering device 1, which is sandwiched between two sheets of a material, possibly transparent, constituting the protective assembly 7. The transparency allows for visual verification of the integrity of device 1. In the illustrated case, the two opposite ends E1, E2 of sensor C are selectively formed from the same material, which covers, between the two ends E1, E2, the active electromechanical layer made of the semiconductor material 40, and also the two electrodes (2a, 2b). All or part of the sheets or films 7a, 7b of the protective assembly 7a, 7b can be in direct contact with the mixture comprising the conjugated polymer and a polymer with complementary ionic bonds.

[0034] The headband, which extends between the two ends E1, E2, can be folded at least 80° away from the flat angle, preferably at least 120° away as illustrated in Figure 3, at least in an intermediate region 8 of the headband between the two ends E1, E2, which is a region where the flexoelectric device 1 extends. The headband can have a thickness less than or equal to 150 µm, with a device 1 not exceeding 50 or 80 µm in thickness in options allowing a high level of flexibility compatible with measurements of the curves exceeding 60m' 1 More broadly, device 1 is capable of bending with a level of flexibility allowing it to follow a pronounced curvature.

[0035] In embodiments, doping can be carried out to optimize the flexoelectric response of the semiconductor material 40 which can spontaneously generate a polarization during a transverse bending deformation process, for example during a bend in which the device 1 wraps around an axis transverse / perpendicular to the direction Ah (as in the case of Figure 3). Non-exhaustive examples of manufacturing

[0036] One possible manufacturing technique involves supplying polymers, at least one of which is conjugated, that together can have a flexible structure, using, for example, PEDOT as the conductive component (the most conductive in the mixture). For example, the two polymers could be PEDOT and PSS, without the need for a structure or layer made of a metallic or mineral material complementing these two polymers. Optionally, only an ionic form of an alkaline earth metal, for example, present in PSS, could be included in the mixture. Only the constituent material of electrodes 2a, 2b can be chosen from metals commonly considered current-conducting, such as gold, as a non-limiting example. Most of the manufacturing process steps can be carried out under a controlled atmosphere and / or in a standard cleanroom.

[0037] In one example, the fabrication of device 1 uses a PEDOT:PSS mixture and aims to create a core layer using a PEDOT:PSS-based semiconductor material while exhibiting high flexibility / plasticity (typically with an elastic self-returning effect). Without a protective assembly, the core layer can thus be self-supporting, with electrodes 2a and 2b being deposited directly onto this layer.

[0038] With reference to Figure 2, a manufacturing process for a film of semiconductor material such as PEDOT:PSS is described. A PEDOT / PSS solution in a specific ratio is first supplied, with a greater proportion of PSS in relation to the molar ratio between these two components being mixed. Experimentally, a first commercial solution with a PEDOT:PSS ratio of 1:6 (Al 4083 - OSSILA) or a second commercial solution with a PEDOT:PSS ratio of 1:2.5 (PH1000 - OSSILA) was used to deposit an elongated layer onto an inert substrate, thus obtaining a film. After step 50, which involves supplying such a solution, step 51, which involves mixing the commercial PEDOT:PSS solution in a stirred container, xylitol 99% [C5H12O5] (ACROS ORGANICS®) is introduced into the commercial PEDOT:PSS solution. The fraction thus added represents, for example, approximately 0.3% by weight to form a precursor solution. More generally, this type of additive, here a polyol (any polymer having several OH functions in its elementary motif), is used to promote the softening of the PEDOT:PSS film (plasticizing effect) while also decreasing the electrostatic interaction between the PEDOT+ chains and the PSS chains via the hydrogen bonds they impose, thus increasing the mobility of charge carriers in the polymer.

[0039] The duration of mixing step 51 can exceed 60 minutes, for example, approximately 2 hours, using an internal stirring method, such as magnetic stirring with a solid stirrer like a stir bar or a paddle element. Step 51 may be carried out at room temperature, without heating, using a stirring method that maintains the precursor solution in a liquid (sufficiently fluid) state.

[0040] In variants, at least one other polyol can be used as an additive (e.g. sorbitol or other similar molecule) in a liquid stage, in order to promote the circulation of charges in the PEDOT:PSS.

[0041] As seen in Figure 2, a deposition step 52 of the precursor solution is deposited onto a plate or similar support S, for example, by drip dispensing using a nozzle or needle and piston instrument 3, or a similar actuator. In the experiment described above, the PEDOT:PSS precursor solution can be deposited onto a polytetrafluoroethylene (PTFE) polymer plate or other equivalent material that facilitates film peeling while being chemically inert. Deposition by 3D printing is possible in certain scenarios.

[0042] At the end of deposition step 52, a composite PEDOT:PSS film with a large surface area is obtained, for example greater than 2 cm 2The substrate S, such as a polytetrafluoroethylene polymer plate (chemically inert even when heated), does not interact with the PEDOT:PSS polymer deposited on it. The substrate S also exhibits an extremely low coefficient of friction, which facilitates the removal of the deposited film.

[0043] The support S is then placed in an oven for a drying step 53, which involves heating to a temperature below 40 or 50°C, for example, at approximately 30°C in an oven for a sufficient duration to remove the water present in the precursor solution. This duration can exceed 10 hours, for example, be on the order of 24 hours. In alternative configurations, the drying step can be carried out without a heating element, for example, by circulating a gas stream along the composite film.

[0044] After step 54 of detaching the support S to separate the film constituting the layer of semiconductor material 40, the electrode pair 2a, 2b can be designed in a phase 55 of deposition of an electrode material G, typically metallic. In the non-limiting case of Figure 2, the following is carried out during this phase 55: - on one side of the semiconductor material 40, a first electrode 2a is deposited by sputtering metallic grains, atoms and / or particles by arranging the layer of semiconductor material 40 on a glass support V, opposite a metallic element such as gold (as the source of electrode material G); and - on one side of the semiconductor material 40, a deposit of a second electrode 2b by spraying metallic grains, atoms and / or particles by arranging the layer of semiconductor material 40 in reverse, possibly on the same type of support (glass support V), optionally using the same metallic element used to make the first electrode.

[0045] Each deposition is carried out in a vacuum chamber at a scale of a few tens of nanometers, for example, to create a gold cathode approximately 70 nm thick. For each electrode 2a or 2b deposition, the semiconductor material 40 can optionally extend between a lower anode and an upper cathode, and sputtering is activated. This can be achieved by using charges (ions) to eject / sputter the grains, atoms, or particles from the impact surface of the metallic element, so that the sputtering occurs at least partially towards the available surface (here, the upper surface opposite the glass support V) of the semiconductor material 40. A contour of the first electrode 2a can be defined using one or more SM masks. The same applies to a contour of the second electrode 2b. In some variations, simple vacuum heating may be sufficient for the deposition of the electrode material (for example in the case of aluminum electrodes).

[0046] In one option, electrodes 70 nm thick and 34 mm x 4 mm in area are symmetrically deposited on both surfaces of the PEDOT:PSS composite film using a spray technique, with the aid of a perforated or suitably cut SM mask. More generally, elongated cathodes can be produced that only partially cover the corresponding face of the material 40. The layer 4 can thus extend beyond the cathodes in its longitudinal dimension, and also in its width in some embodiments. Once the electrodes 2a, 2b have been produced, the film is, for example, left to rest, possibly for at least 3 hours, before the respective conductive wires 5a, 5b are deposited using an adhesive (CW2400 - Chemtronics®, by way of non-limiting example) as the bonding material 5c.For the two sides of the film with a thin layer forming the electrode, a respective conductive wire 5a, 5b is deposited on the corresponding electrode 2a, 2b, and the adhesive is allowed to dry. The adhesive drying time. is suitable, which can reach or exceed 24 hours (between each side and before proceeding with any characterization / measurement).

[0047] In some experiments, the films obtained have a maximum thickness of approximately 25 µm (not taking into account the connection of the conductive wires 5a, 5b) to obtain flexoelectric devices 1 that are extremely flexible, as clearly visible, for example, in Figure 3. Since the PEDOT:PSS composite film is sensitive to humidity (due to the hygroscopic nature of PSS), it may be possible to cover the external faces of the device 1 with a protective material such as PET (polyethylene terephthalate), sandwiching the three-layer structure as shown in Figure 1. Figure 3 illustrates an example of encapsulation, covering the two opposite faces of the device 1 with a first film 7a or thin polymer sheet and a second film 7b or thin polymer sheet, which is a water-resistant polymer.

[0048] In some embodiments, these films are made of PET or an equivalent robust polymer to protect the material 40, here including PEDOT and PSS, from mechanical stress and moisture. Before or after step 55, the composite film / semiconductor material 40 and the electrodes 2a, 2b are completely encapsulated between two 35 µm thick sheets of PET. A laminator can be used to ensure good adhesion between the outer faces of the device 1 and the PET sheets. More generally, the flexoelectric structure of device 1 can be encapsulated in an external protection assembly, defining in particular two main external faces of a sensor C, extending beyond the longitudinal edges L4 of layer 4 supporting electrodes 2a, 2b.

[0049] Once encapsulated, the devices 1 can withstand significant deformation, for example, bringing the two opposite ends of the device 1 into contact by accentuating the bend with a radius of curvature that can locally fall below 10 or 15 mm. Furthermore, they can be fully immersed in an aqueous medium without any loss of aptitude in their flexoelectric function / response coefficient, thus expanding their range of applications when integrating PSS as a conjugated polymer in the mixture used to obtain the semiconductor material 40.

[0050] Regarding the Young's modulus of material 40, it was measured at 424 or 430 MPa, respectively for a 1:6 and 1:2.5 ratio for PEDOT:PSS. These are therefore cases with high flexibility, which are quite equivalent. The in-plane electrical conductivity, as measured on average (by taking at least four measurements), is good in both cases, being greater than 0.10 S / m. The lower PSS content in the composite obtained with the PEDOT:PSS mixture at a 1:2.5 molar ratio results in an in-plane electrical conductivity greater than 0.50 S / m, which is comparatively higher. than in the case with a molar ratio of 1:6. Furthermore, it has been verified that encapsulation in a sufficiently flexible protective film, such as PET or a similar plastic, with a thickness of the same order of magnitude as the thickness of device 1 (without multiplying the total thickness of material 40 by 10, for example), with the protective film applied to both sides of device 1, does not significantly / negatively influence the aforementioned properties: there is neither a decrease in the flexoelectric response coefficient nor a loss of flexibility limiting the application of a bending sensor. This type of protection can limit dielectric losses by reducing the influence of the external environment on device 1.

[0051] Tests using rollers (with a fixed radius) defining a predetermined curvature were performed on devices 1 having a trilayer structure with electrodes 2a, 2b directly attached to the semiconductor material 40 based on a PEDOT:PSS (1:6) mixture and not supplying significant current before deformation (thus with a uniform ion distribution in the composite film). Device 1 follows the curvature of the roller to reach a predefined bending state.

[0052] It was observed, during the measurement between the two terminals formed by wires 5a and 5b, that the succession of the following four phases: Bending by the roller / Release / Further bending / Further release, with current variations reaching the same extremes at each rapid cycle, by linking these four phases over a total duration of 1 second. The peak of current decrease and then increase reaches, for example, 3.5 microamperes (-3.5 and +3.5) for a length of device 1 between 25 and 80 mm, for example, on the order of 50 mm with electrodes made of gold and extended to 34 mm, with a width of device 1 not exceeding 10 mm.

[0053] Effective flexoelectricity coefficients (transverse coefficient Coeff) could be determined experimentally with the following relation (R1): [Math. 1] where i is the induced flexoelectric current, f is the excitation frequency of the roller, b is the width of each electrode, I is the length of each electrode and Cech is the curvature imposed on the sample constituted by a device 1, respectively. By repeating the experiments for different frequencies f and for different roller radii, a linear relationship between the current and the curvature was established (tests carried out with frequencies f of 0.5 Hz, 1 Hz, 1.5 Hz, and 2 Hz, respectively). The linearity is very good for the sample containing the composite material based on the PEDOT / PSS mixture (1:6).

[0054] Using the aforementioned equation (R1), the flexoelectric coefficient Coeff is calculated for different frequencies, as shown in Figure 4. It can be observed that the Coeff, reflecting the flexoelectric response, varies with frequency; it decreases as frequency increases. However, it is independent of the degree of curvature achieved (here, a deviation of + / - 1.6 pC.nr). 1 It is interesting to note that the flexoelectric coefficient is greater than 50 pC.m' 1 provided that the frequency used for the test remains greater than or equal to 1 Hz. Even at a frequency of 2 Hz for the test, the coefficient is significantly greater than 30 pC. 1 This is significantly higher compared to coefficients determined with known materials (on the order of 0.01 pC.m'). 1), while allowing for significant bending, for example with radii of curvature ranging from 10 to 18 mm (corresponding to the case with a 55 m curvature -1 or 62 rrr 1 (see figure 4).

[0055] It is noteworthy that the flexoelectric device 1 thus constructed can dispense with a support layer of the semiconductor material 40 between the electrodes, which helps to optimize the thinness of the active electromechanical layer. The distribution of layers 2a, 4, and 2b can be symmetrical in the structure 6 of the device, which facilitates setup for measurements and avoids errors. It also makes it possible to obtain a highly sensitive and reliable semiconductor material for measurements.

[0056] The embodiments described above are examples used to describe one or more ways of obtaining the device, without limitation. Furthermore, each part of this disclosure is not limited to the corresponding embodiment, and various variations may be made within the same technical framework.

Claims

Claims

1. A flexoelectric device (1) comprising an active electromechanical layer (4) made of a semiconductor material inserted between two metal electrodes (2a, 2b) capable of generating a voltage between the two electrodes (2a, 2b) when bent, characterized in that the semiconductor material carries the two electrodes (2a, 2b) without resting on a support structure, the active electromechanical layer (4) being a layer with a thickness of between 10 and 160 micrometers, the semiconductor material (40) being based on a polymer mixture containing a conjugated conductive polymer and a polymer oppositely charged compared to said conjugated conductive polymer, at least one of the two electrodes (2a, 2b) being directly bonded to the active electromechanical layer (4), the two electrodes (2a, 2b) being flexible and less thick than the active electromechanical layer (4) within of a multilayer structure of the flexoelectric device (1).

2. Device according to claim 1, in which the semiconductor material (40) constitutes a central layer of the structure of the device which is a structure devoid of metallic or mineral substrate, only the electrodes (2a, 2b) having a metallic composition in said structure, the active electromechanical layer (4) having a thickness greater than 10 micrometers and less than or equal to 50 micrometers.

3. Device according to claim 2, wherein the polymer mixture is the PEDOT:PSS mixture, preferably also including a polyol added as an additive at less than 1% by weight of the mixture, and wherein the electrode thicknesses are about 10 times less, at least, than the thickness of the active electromechanical layer (4).

4. Device according to claim 3, in which the semiconductor material which extends from one to the other of the electrodes is configured to have a transverse flexoelectric coefficient which is greater than 20 pC / m, preferably greater than 30 pC / m, as measured in response to a stress frequency less than or equal to 2 Hz, and in which the polyol is preferably xylitol, added in a proportion of between 0.2 and 0.6% by weight of said mixture.

5. Device according to any one of the preceding claims, in which the active electromagnetic layer (4) has a flat outer surface partially covered by one of the two electrodes (2a, 2b), and in which the multilayer structure of the flexoelectric device (1) is impermeably enveloped by an outer protective assembly (7), so that the two electrodes (2a, 2b) respectively form: - a lower electrode covered with a lower film portion (7a) of the outer protective assembly (7); and - an upper electrode covered with an upper film portion (7b) of the outer protective assembly (7).

6. Device according to any one of the preceding claims, in which each of the two electrodes (2a, 2b) is: - directly formed on the active electromechanical layer (4), one on a first surface and the other on a second opposite surface of the active electromechanical layer (4), - preferably deposited by a spray technique.

7. Curvature sensor (C) in the form of a waterproof strip, for determining a parameter representative of a curvature of an object surface, the sensor (C) comprising: - the flexoelectric device (1) according to any one of claims 1 to 6; and - two opposite ends (E1, E2) selectively formed by a material which covers, between the two ends, the active electromechanical layer (4) and, preferably also the two electrodes (2a, 2b), of the device (1); in which the strip, which extends between the two ends (E1, E2), is foldable at least 80° apart from the flat angle, preferably at least 120° apart, in an intermediate region (8) of the strip between the two ends (E1, E2), the strip having a thickness less than or equal to 150 μm.

8. A bend sensor according to claim 7, wherein the two electrodes (2a, 2b) are connectable to a conditioning circuit, for example provided with an oscilloscope and a current amplifier, while the active electromechanical layer (4), which includes the PEDOT:PSS mixture with a ratio greater than or equal to 1:7 for the molar distribution between these two polymers, is capable of exhibiting a transverse flexoelectric response coefficient greater than 20 pC / m, as measured in response to a stress frequency less than or equal to 2 Hz.

9. Method of manufacturing a device (1) for direct measurement of curvature by flexoelectric effect, the method comprising the steps essentially consisting of: - mixing (51) in fluid form two polymers with an additive, such as a polyol, representing less than 1% by weight of the mixture obtained, so as to form a homogeneous solution capable of plasticizing, one of the polymers being a conjugated polymer and being electrically conductive; - deposit (52) in liquid form the homogeneous solution on a substrate (S) preferably rigid, distribute the deposit on the substrate (S), such as a PTFE plate, in order to form an elongated layer of a semiconductor material (40) which is based on said mixture with said conjugated polymer; - after drying the layer of semiconductor material (40) which is in a solid and flexible state, with a thickness of between 10 and 160 micrometers, depositing in two steps a first electrode (2a) in the form of a flexible layer then a second electrode (2b) in the form of a flexible layer, in order to obtain a structure of a flexoelectric device (1) whose thickest layer is formed by the layer of semiconductor material (40), said substrate (S) having been separated from the semiconductor material (40) at least before the deposition of the second electrode (2b), so that the semiconductor material, designed before the electrodes, carries the two electrodes (2a, 2b), said elongated layer forming the active electromechanical layer (4) of the device (1); - after or before a connection of conductive wires (5a, 5b) respectively connecting the first electrode (2a) and the second electrode (2b) in order to make detectable a voltage which is a voltage generated by the semiconductor material (40) between the two electrodes (2a, 2b) in response to a folding thereof, protecting said structure by a protection assembly (7) which insulates, in a waterproof manner, the semiconductor material (40) from the outside of the flexoelectric device (1).

10. Manufacturing method according to claim 9, wherein the two polymers are poly(3,4-ethylenedioxythiophene) and sodium poly(styrene sulfonate), so that the semiconductor material (40) is a composite material based on PEDOT:PSS, wherein the deposition (52) in liquid form to obtain the elongated layer of the semiconductor material (40) is carried out so as to obtain a homogeneous composition of the semiconductor material (40) with a substantially constant thickness (e40), which is at least 200 times greater than the thickness (e2) of any one of the electrodes (2a, 2b) which are metallic, and wherein the two-step deposition of the first electrode (2a) and the second electrode (2b) is carried out directly on the semiconductor material (40), on two opposite faces of the active electromechanical layer (4).

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