Device for direct curvature measurement by flexoelectric effect and associated manufacturing method

Abstract

The device (1) 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) to be capable of generating a voltage between the two electrodes (2a, 2b) when the device is bent. The electromechanical layer (4) has a thickness of between 10 and 160 micrometers, at least one of the two electrodes (2a, 2b) being directly bonded to this layer. The electrodes are designed 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 where appropriate in a protective assembly (7) by a flexible encapsulating material of submillimeter thickness, can be used in the form of a strip, in a sensor (C) making it possible to detect a curvature or positioning of an object. Fig.3.

Description

Title of the invention: Device for direct measurement of curvature by flexoelectric effect and associated manufacturing method Technical field

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

[0002] The phenomenon of electromechanical coupling is widespread in various artificial and natural materials. In a piezoelectric material, electrical polarization occurs when this 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 in 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 having centrosymmetric crystals. In a solid with a crystalline structure, the strain gradient results in a non-uniform relative displacement of the atoms, which breaks the inversion symmetry of the crystal and generates polarization in the structure. While the piezoelectric effect is produced by uniform strain, the flexoelectric effect is produced by non-uniform (gradient) strain.

[0004] The flexoelectric effect can be simply defined as the coupling between the deformation gradient and the polarization in solid dielectrics. It is thus an alternative transduction mechanism to the piezoelectric effect, allowing for example to directly detect / measure the curvature of structures, by a measurement carried out in bending.

[0005] A number of devices using flexoelectricity have been proposed, particularly in curvature or position measurement applications. Particularly flexible structures are desirable when it comes to being able to measure large curvatures. Document US 2011094306 thus provides for producing a complex textile, incorporating a plurality of lamellae, which can be placed in contact with the surface of an object, so that each lamella bends along the contour of the surface of the object. The bending of the flexoelectric lamella generates a polarization voltage between its opposite edges, which can be measured by the electrodes to determine the local contour of the surface of the object. Each lamella can be designed with boron nitride. (BN) or carbon boron 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 necessary and soft polar elastomers can be suitable for measuring the curvature. Thus, from the article Flexoelectric response in soft polyurethane films and their use for large curvature sensing; VI Merupo, B. Guiffard, R. Seveno, M. Tabellout, and A. Kassiba; Journal of Applied Physics 122 (2017), a method for developing a thin sensor that uses flexoelectricity in polyurethane is known. Sensor systems using other polymers (PVDF for example) have also been studied. Overall, the flexoelectric response coefficients obtained with such highly flexible materials remain modest, for example not exceeding the range from 10 10 Cm 1 to 108 Cm1.

[0007] Certain sensor structures using a central layer (formed from an ionic electroactive polymer or a liquid crystal elastomer) and an organic semiconductor distributed in two outer layers (acting as flexible electrodes) have been proposed, for example by document WO 2022081672. The organic semiconductor can be doped (and even heavily doped), as in the case of PEDOT:PSS. It is then possible to obtain bending sensors with good sensitivity and using a mechanically robust central layer carrying the electrodes made of the semiconductor (these electrodes being much thinner). Technical problem

[0008] There is still a need to design, in an accessible and simple manner, sensors compatible with a high flexoelectric coefficient (generally greater than 10 pC.m ', typically) with the aim of obtaining direct sensors with high curvature, possibly greater than 50 or 60 m1, 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). Technical solution

[0009] In order to improve the situation, a flexoelectric device is proposed comprising an active electromechanical layer consisting of a semiconductor material inserted between two metal electrodes capable of generating (intrinsically) a voltage between the two electrodes when it is bent, the active electromechanical layer being able to have a thickness of between 10 and 160 micrometers, the semiconductor material being based on a mixture of polymers containing at least one conjugated conductive polymer and a polymer charged oppositely compared to the conjugated conductive polymer, at least one of the two electrodes being directly linked to the active electromechanical layer, the two electrodes being flexible and less thick than the active electromechanical layer within a multilayer structure of the flexoelectric device. The PEDOT compound or similar polymer with a repeating thiophene unit may be the or one of the conjugated polymers, preferably with a positive charge (conjugated polymer in protonated / oxidized form). Another charged polymer with an opposite charge, negative in the case where PEDOT forms the first polymer of the mixture, is used in the composition of the semiconductor material. The active layer may be elongated / have an elongation direction (so as to define a length and a width for example) and extend planarly in a rest configuration, in which the charges are distributed substantially uniformly in the semiconductor material. The semiconductor material of the active layer, which may extend from one of the electrodes to the other, is configured to have a transverse flexoelectric coefficient that is greater than 20 or 30 pC / m, as measured in response to a loading frequency of less than or equal to 2 Hz.The stress may correspond to a bending action against the surface of a cylinder / roller, by winding against the external surface of the roll.

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

[0011] According to a particular feature, the semiconductor material constitutes a central layer of the structure of the device which is a structure devoid of metallic or mineral substrate, only the electrodes having a metallic composition in said structure without exceeding for example a thickness (electrode thickness) of the order of 0.5 micrometers and / or having electrode thicknesses approximately 10 times lower, at less than the thickness of the active layer. This allows a thin, particularly flexible device to be obtained, the active layer of which is devoid of a thick layer (thus, the active layer does not reach more than 160 micrometers) forming a support substrate from below and possibly a lower electrode.

[0012] In embodiments, one or more of the following features may be used: - the active electromechanical layer has a thickness greater than 9 micrometers and less than or equal to 50 micrometers. - the electrodes have an identical 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 step (inclusive) of integration / deposition of the two electrodes. - the mixture bringing together 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 multi-layer structure of the flexoelectric device is impermeably wrapped by an external protective assembly. - the two electrodes respectively form: 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 making it possible to protect the active electromechanical layer, it is understood that two film parts which are not functional in the flexoelectric effect can make it possible to protect the conjugated polymer and more generally the semiconductor material including such a conjugated polymer, in particular if it is PEDOT. The two film parts can be directly joined to each other, possibly by being heat-welded, at least in a peripheral margin zone surrounding a peripheral edge of the semiconductor material.

[0014] According to the options for producing the two electrodes, one or more of the following features are provided: - 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 using a sputtering technique. - the two electrodes are deposited using the same metallic source material, using a sputtering technique. - the spraying can be divided into two stages and carried out 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] According to another aspect, there is provided a curvature sensor in the form of a strip, typically a waterproof strip, for determining a parameter representative of a curvature of an object surface, the sensor comprising: - the flexoelectric device as mentioned 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 strip, which extends between the two ends, is foldable in an intermediate region of the strip between the two, the strip having a thickness less than or equal to 150 μm.

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

[0017] In embodiments, the strip is foldable in the intermediate region at least at 80°, which is thus a spacing angle relative to the flat angle. Considering for example that one end of the strip is kept fixed, possibly horizontal, while the intermediate region of the strip follows the surface of the object whose curvature is to be determined, it is understood that the other end of the strip (locally not folded because outside the intermediate region) can extend in a general direction which makes an angle with the first end which can be substantial. Preferably, the angle obtainable by bending the strip may be greater than or equal to 10° (at least 120° away from the flat angle).

[0018] The strip has a homogeneous composition including two oppositely charged polymers, including at least one conjugated polymer, in order to form an active electromechanical layer consisting solely of a semiconductor material. Optionally, the opposite faces of the strip may be made of electrically insulating (non-metallic) plastic material, to form external surfaces of the sensor (thus forming two non-functional layers, possibly fused or welded on a periphery of the active electromechanical layer). The electrically insulating plastic material may include or consist of PET or other similar robust polymer even in a layer of reduced thickness, for example not exceeding a hundred micrometers.

[0019] The (flexoelectric) layer which is active can be self-supporting between the two flexible outer layers, i.e. without resting on a support structure (without a thicker structure and / or without an ionic / metallic support structure for example) before its encapsulation. A PEDOT:PSS can be used to form this layer, with optionally at least one additive such as a polyol distributed homogeneously in the active layer.

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

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

[0022] The deformable strip is provided with a protective assembly which may include or consist of two external protective layers each with a thickness greater than approximately 10 μm (for example 35 μm in the case of PET) and optionally less than 80 μm, in order to protect the active layer including at least one conjugated polymer. With this protective assembly, it is possible for the sensor to have a curved configuration and attached to a solid surface of the object, corresponding to the surface whose curvature is to be determined, with a good flexoelectric response coefficient while benefiting from water resistance.

[0023] According to a particular aspect, there is also provided a method of manufacturing a device for direct measurement of curvature by flexoelectric effect, the method comprising the steps essentially consisting of: - mixing 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 plasticizing, one of the polymers being conjugated while being electrically conductive; - depositing the homogeneous solution in liquid form on a rigid substrate by distributing the deposit on the substrate, such as a PTFE plate by way of non-limiting example, in order to form an elongated layer of a semiconductor 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 of between 10 and 160 micrometers, depositing in two stages a first electrode in the form of a flexible layer then a second electrode in the form of a flexible layer, in order to obtain a structure of a flexoelectric device of which the thickest layer is formed by the layer of semiconductor material; - after or before a connection of conductive wires respectively connecting 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 folding thereof, protect said structure by a protection assembly which insulates, in a waterproof manner, the semiconductor material from the outside of the flexoelectric device.

[0024] With this methodology it is possible to design a thin layer made of a high-performance semiconductor material with regard to the flexoelectric effect, using a temporary support / substrate at the time of deposition of the layer which forms the active electromechanical layer of the device. The substrate used for the deposition / application of the composite material in the form of a thin layer is a support / substrate chosen with surface properties facilitating the dry detachment of the layer. Such a substrate, for example made of PTFE, is separated from the semiconductor material at the latest just before the deposition of the second electrode. Optionally, the deposit is carried out by forming a layer with a width at least equal to 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 composite material based on PEDOT:PSS, optionally with at least double the molar concentration for PSS in the precursor solution used for the deposition of the layer. In embodiments, the deposition of the elongated layer of the semiconductor material is carried out so as to obtain a homogeneous composition of the semiconductor material, preferably with a substantially constant thickness (with the exception of edge zones, depending on the options) which is at least 200 times greater than the thickness of any of the electrodes which are metallic.

[0026] In options, the two-step deposition of the first electrode and the second electrode is carried out directly on the semiconductor material, on two opposite faces of the elongated layer. The length of the electrodes is for example greater than 30 mm. When the device is protected by a protective assembly (which may be transparent), a strip at least twice as long as the electrodes and / or at least twice as long as the active electromechanical layer may be formed. In order to facilitate handling of the strip outside the area where the layer of semiconductor material is formed, an overhang of at least 15 or 20 mm may be provided on either side of the opposite short edges of the active electromechanical layer to form strip ends without composite material, for example with the sheets of protective material directly attached to each other. Brief description of the drawings

[0027] Other characteristics, details and advantages will appear on reading the detailed description below, and on analyzing the attached drawings, in which: [Fig.l] is a view of a curvature sensor having an ultrathin single-layer structure sandwiched between two electrodes, in accordance with one embodiment of the invention. [Fig.2] is a diagram illustrating steps in a method for preparing a flexoelectric plastic film. [Fig.3] is a perspective view of a sensor capable of determining curvature, in a flexural state with the flexoelectric functional structure encapsulated in a flexible protective assembly. [Fig.4] illustrates by a curve the evolution of the flexoelectric coefficient of a device according to the invention, as a function of the frequency of stress. Description of the embodiments

[0028] Several examples of non-limiting embodiments are set out below in detail. In the various figures, identical references indicate identical or similar elements. Certain dimensions may be exaggerated for the purposes of illustration.

[0029] In the drawings, the direction of an arrow Ah indicates a top in a vertical direction passing through the thickness of the device. The terms "upper" and "lower" in the present description correspond to a top portion and a bottom portion, in considering a through direction following the thickness of the device 1, passing through each of the constituent layers thereof.

[0030] The flexoelectric effect sensor of this first embodiment has, as illustrated in [Fig.l], a three-layer structure 6 with a layer 4 which may be much thicker than two outer layers used to form electrodes 2a, 2b. The device 1 may consist of a superposition of layers in which the electrodes only cover a part of the corresponding surface of the layer 4 which is the active electromechanical layer, including a polymer-based semiconducting material including: - 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 which is capable of distributing charges in its structure along the direction of its thickness (parallel to the Ah direction), this material being able to be obtained by casting a mixture called precursor solution. Typically, the two aforementioned polymers are organic polymers, with carbon chains, the structure of which facilitates the circulation of charge, so that the solid layer obtained by deposition on a suitable substrate / support S forms a semiconductor which is very sensitive to bending, thus providing an easily measurable flexoelectric response.

[0031] The active electromechanical layer 4, which includes for example PEDOT or another doped conjugated polymer with similar properties as an electronically conductive material, may 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, 2b, each metallic and made of a material comparatively more rigid than the semiconductor material, are added as an ultrathin layer on the lower and upper side of such a layer 4, respectively.

[0032] In the case of a PEDOT conjugated polymer, an association with the PPS polymer can be provided, so that the PEDOT and the PSS establish bonds between them, between the (SO3) groups of PSS and S+ of PEDOT. More broadly, two polymers are chosen which are suitable for the circulation of charges, 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.

[0033] The semiconductor material 40 may also include in its composition a polyol, in a proportion of less than 1% by weight, which may act as a stabilizer and a plasticizer of the mixture of polymers including the conjugated polymer and a complementary polymer capable of being in an ionic form interacting with a part of the conjugated polymer.

[0034] With reference to [Fig. 3], the device 1 may be part of a sensor C having a protective assembly, external here, by completely covering the device 1 which is sandwiched between two sheets of a material, possibly transparent, constituting the protective assembly 7. The transparency may allow the integrity of the device 1 to be visually verified. In the illustrated case, the two opposite ends E1, E2 of the sensor C are selectively formed by the same material which covers, between the two ends E1, E2, the active electromechanical layer consisting of the semiconducting 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 may be directly in contact with the mixture including the conjugated polymer and a complementary ionically bonded polymer.

[0035] The strip, which extends between the two ends E1, E2, can be bendable at least 80° apart from the flat angle, preferably at least 120° apart as in the case illustrated in [Fig. 3], at least in an intermediate region 8 of the strip between the two ends E1, E2, which is a region where the flexoelectric device 1 extends. The strip can have a thickness less than or equal to 150 μm, with a device 1 which does not exceed 50 or 80 μm in thickness in options making it possible to achieve a high level of flexibility compatible with measurements of curvatures exceeding 60 μm *. More broadly, the device 1 is able to bend with a level of flexibility allowing it to follow a pronounced curvature.

[0036] In embodiments, doping may be performed to optimize the flexoelectric response of the semiconductor material 40 which may spontaneously generate polarization during a transverse bending deformation process, for example during bending in which the device 1 wraps around an axis transverse / perpendicular to the Ah direction (as in the case of [Fig.3]). Non-limiting examples of manufacturing

[0037] A possible manufacturing technique may involve the provision of polymers, at least one of which is conjugated, which together may have a flexible structure, for example using PEDOT as the conductive component (the most conductive in the mixture). For example, the two polymers may be PEDOT and PSS, without recourse to a structure or layer consisting of a metallic or mineral material completing these two polymers. Optionally, only an ionic form of an alkaline-earth metal, for example present in PSS, may be involved in the mixture. Only one / the constituent material of the electrodes 2a, 2b may be chosen from among the metals usually considered as current conductors, such as gold by way of non-limiting example. Most of the steps of the manufacturing process may be carried out in a controlled atmosphere and / or in a standard clean room.

[0038] In one example, the manufacture of the device 1 uses a PEDOT:PSS mixture and aims to constitute a central layer using a PEDOT:PSS-based semiconductor material while exhibiting great flexibility / plasticity (typically with an elastic return effect which can be exerted on its own). In the absence of a protective assembly, the central layer can thus be self-supporting, the deposition of electrodes 2a, 2b being carried out directly on this layer.

[0039] With reference to [Fig. 2], a process for manufacturing a film of semiconductor material such as PEDOT:PSS is described. A PEDOT / PSS solution is provided beforehand in a determined ratio with more PSS as regards the molar ratio between these two components which are mixed. Experimentally, a first commercial solution with a PEDOT:PSS ratio of 1:6 (AI 4083 - OSSILA company) or a second commercial solution with a PEDOT:PSS ratio of 1:2.5 (PH1000 - OSSILA company) was thus used, in order to deposit an elongated layer on an inert substrate, and thus obtain a film. After a step 50 of providing such a solution, a step 51 of mixing is carried out in a stirred container, by introducing xylitol 99+% [C5Hi2O5] (ACROS ORGANICS®) 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 unit), is used to promote the softening of the PEDOT:PSS film (plasticizing effect) while also reducing the electrostatic interaction between the PEDOT+ chains and the PSS chains via the hydrogen bonds they impose, thus increasing the mobility of the charge carriers in the polymer.

[0040] The duration of the mixing step 51 may be greater than 60 min, for example of the order of 2 hours, using a stirring mode from inside the container, for example magnetic stirring with a solid stirrer such as a bar or a paddle stirring element. Step 51 is optionally carried out at room temperature, without heating with a stirring mode maintaining the precursor solution in the liquid state (sufficiently fluid).

[0041] In variants, at least one other polyol may be used as an additive (for example sorbitol or other similar molecule) at a liquid stage, in order to promote the circulation of charges in the PEDOT:PSS.

[0042] As seen in the case of [Fig.2], a deposition step 52 of the precursor solution is deposited on a plate or similar support S, for example by drop-by-drop casting using an instrument 3 with a nozzle or needle and a piston or similar actuator. In the case experimented above, the precursor solution of PEDOT:PSS can be deposited on a plate of polytetrafluoroethylene (PTFE) polymer or other equivalent material which facilitates the detachment of the film while being chi- inert. A deposition by 3D printing can be carried out in certain options.

[0043] At the end of the deposition step 52, a composite film of PEDOT:PSS is obtained having a large surface area, for example greater than 2 cm2. The support S such as a polytetrafluoroethylene polymer plate (chemically inert even when heated) does not interact with the PEDOT:PSS polymer deposited thereon. The support S also has an extremely low coefficient of friction, which facilitates the detachment of the film deposited thereon.

[0044] The support S is then placed in an oven for a drying step 53 with heating to a temperature below 40 or 50°C, for example carried out at approximately 30°C in an oven for a sufficient time to eliminate the water present in the precursor solution. This duration may exceed 10 hours, for example of the order of 24 hours. In variants, the drying step may be carried out without a heating element, for example by consisting of circulating a gas flow along the composite film.

[0045] After a step 54 of detaching the support S to separate the film constituting the layer of semiconductor material 40, the pair of electrodes 2a, 2b can be designed in a phase 55 of deposition of an electrode material G, typically metallic. In the non-limiting case of [Fig.2], the following are carried out during this phase 55: - on a front of the semiconductor material 40, a deposition of a first electrode 2a by spraying of grains, atoms and / or metallic particles by arranging the layer of semiconductor material 40 on a glass support V, opposite a metallic element such as gold (as a source of electrode material G); and - on a back of the semiconductor material 40, a deposition of a second electrode 2b by spraying grains, atoms and / or metallic particles by arranging the layer of semiconductor material 40 in an inverted manner, possibly on the same type of support (glass support V), optionally using the same metallic element used to produce the first electrode.

[0046] Each deposit is made, in a vacuum chamber, on a scale of a few tens of nanometers, for example to make a gold cathode about 70 nm thick. For each electrode deposition 2a or 2b, the semiconductor material 40 can optionally extend between a low anode and a high cathode, and sputtering is activated. This can occur by using charges (ions) to eject / sputter the grains, atoms or particles from the impact surface of the metal element, so that the sputtering is carried out at least partly in the direction of the available surface (here upper surface opposite the glass support V) of the semiconductor material 40. A contour of the first electrode 2a can be delimited using one or more SM masks. The same applies to a contour of the second electrode 2b. In variants, simple vacuum heating may be sufficient for the deposition of the electrode material (e.g. in the case of aluminum electrodes).

[0047] In one option, electrodes with a thickness of 70 nm and an area of ​​34 mm x 4 mm are deposited symmetrically on both surfaces of the PEDOT:PSS composite film by the sputtering technique, using a perforated SM mask or suitably cut mask. More generally, cathodes of elongated shape can be produced, covering only partially the corresponding face of the material 40. The layer 4 can thus exceed the cathodes in its longitudinal extension, and also in the direction / extension of the width in embodiment options. Once the electrodes 2a, 2b have been produced, the film is for example left to rest, possibly allowing at least 3 hours to pass before depositing the respective conductive wires 5a, 5b using an adhesive (CW2400 - Chemtronics®, as a non-limiting example) as connection material 5c.For the two faces of the film provided with a thin layer constituting the electrode, a respective conductive wire 5a, 5b is deposited on the corresponding electrode 2a, 2b, allowing the glue to dry. The glue drying time is adapted, being able to reach or exceed 24 hours (between each face and before carrying out any characterization / measurement).

[0048] In some experiments, the films obtained have a maximum thickness of about 25 μm (without taking into account the connection of the conductive wires 5a, 5b) to obtain flexoelectric devices 1 which are extremely flexible, as clearly visible for example in [Fig. 3]. The PEDOT:PSS composite film being sensitive to humidity (because of the hygroscopic PSS), it may be provided to cover the external faces of the device 1 with a protective material such as PET (polyethylene terephthalate), sandwiching the tri-layer structure as shown in [Fig. 1]. [Fig. 3] illustrates an example of encapsulation, by covering the two opposite faces of the device 1 with a first film 7a or thin sheet of polymer and a second film 7b or thin sheet of polymer, which is a water-resistant polymer.

[0049] In exemplary embodiments, these films are made of PET or equivalent robust polymer in order to protect the material 40, here including PEDOT and PSS, from mechanical stresses and humidity. 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 laminating device can ensure good adhesion between the external faces of the device 1 and the PET sheets. More generally, the flexoelectric structure of the device 1 can be encapsulated in an external protective assembly, defining in particular two main external faces of a sensor C, extending on either side of the longitudinal edges L4 of the layer 4 supporting the electrodes 2a, 2b.

[0050] Once encapsulated, the devices 1 can withstand large deformations, going for example as far as joining the two opposite ends of the device 1 by accentuating the bending with a radius of curvature which can locally fall below 10 or 15 mm. They can also be fully immersed in an aqueous medium without loss of aptitude in their flexoelectric function / response coefficient, which makes it possible to extend their field of use when it comes to integrating PSS as a conjugated polymer in the mixture used to obtain the semiconductor material 40.

[0051] Concerning the Young's modulus of the material 40, it was measured at 424 or 430 MPa, respectively for the case of a ratio of 1:6 and 1:2.5 for PEDOT:PSS. These are thus cases with great flexibility, entirely equivalent. The electrical conductivity in the plane as measured on average (by carrying out 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 in a molar ratio of 1:2.5 makes it possible to have an electrical conductivity in the plane greater than 0.50 S / m and 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 sheet such as PET or similar plastic at a thickness of the same order of magnitude as the thickness of the device 1 (without multiplying by 10 the total thickness of the material 40 for example), with protection deposited on both sides of the device 1, does not significantly / negatively influence the aforementioned properties: there is neither a lowering of the flexoelectric response coefficient nor a loss of flexibility limiting the application of the bend sensor. This type of protection can limit dielectric losses, by reducing the influences of the external environment on the device 1.

[0052] Tests using rollers (with fixed radius) defining a curvature known in advance were carried out for devices 1 having a tri-layer structure with the electrodes 2a, 2b directly fixed on the semiconductor material 40 based on a PEDOT:PSS mixture (1:6) and not providing significant current before deformation (therefore with a uniform distribution of ions in the composite film). The device 1 follows the curvature of the roller to reach a predefined bending state.

[0053] It was observed, during the measurement between the two terminals formed by the wires 5a, 5b, that the succession of the following four phases: Flexion drive by the roller / Release / New bending / New release, current variations reaching the same extremes at each rapid cycle, by chaining these four phases together over a total duration of 1 second. The peak of decrease then increase of current reaches for example 3.5 microamperes (-3.5 and +3.5) for a length of the device 1 between 25 and 80 mm, for example of the order of 50 mm with electrodes made of gold and elongated over 34 mm, having a width of the device 1 not exceeding not 10 mm.

[0054] Effective coefficients of flexoelectricity (transverse coefficient Coeff) could be determined experimentally with the following relation (RI): [Math. 1] Coeff = i / (2 <bl ) (R 1 ) where i is the induced flexoelectric current, f is the excitation frequency of the roller, b is the width of each electrode, 1 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 radii of the roller, it was possible to establish a linear relationship between the current and the curvature (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 including the composite material based on the PEDOT / PSS mixture (1:6).

[0055] By the aforementioned relation (RI), the flexoelectric coefficient Coeff is calculated for the different frequencies, as reported in [Fig.4] with the observation that the coefficient Coeff reflecting the flexoelectric response evolves as a function of the frequency; it decreases with the increase in frequency. On the other hand, it is independent of the level of curvature reached (here deviation of + / - 1.6 pC.m *). It is interesting to see that the flexoelectric coefficient is greater than 50 pC.m *, as long as 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.m *. This is significantly higher compared to coefficients determined with known materials (of the order of 0.01 pC.m *), while allowing significant bending, for example with radii of curvature ranging from 10 to 18 mm (corresponding to the case with curvature at 55 m 1 or 62 m 1 in [Fig.4]). .

[0056] It is notable that the flexoelectric device 1 thus formed can dispense with a support layer of the semiconductor material 40, between the electrodes, which contributes to optimizing the thinness of the active electromechanical layer. The distribution of the layers 2a, 4, 2b can be symmetrical in the structure 6 of the device, which facilitates the installation for measurements and avoids errors. It is also possible to obtain a very sensitive and reliable semiconductor material for measurements.

[0057] The description of the embodiments presented above corresponds to examples to describe one or more ways of obtaining the device, in a non-limiting manner. In addition, each part constituting the present disclosure is not limited to the corresponding embodiment, and various variants can 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, typically designed before the electrodes, is adapted to carry 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 mixture of polymers 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 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 wherein the active electromagnetic layer (4) has a flat outer surface partially covered by one of the two electrodes (2a, 2b), and wherein 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 by a lower film portion (7a) of the outer protective assembly (7); and - an upper electrode covered by an upper film portion (7b) of the outer protective assembly (7).

6. Device according to any one of the preceding claims, wherein 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 sputtering 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 (El, 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); wherein the strip, which extends between the two ends (El, 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 (El, 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 demand 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; - depositing (52) in liquid form the homogeneous solution on a preferably rigid substrate (S) by distributing 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. A manufacturing method according to claim 9, wherein the two polymers are poly(3,4-ethylenedioxythiophene) and sodium poly(styrene sulfonate), such that the semiconductor material (40) is a PEDOT:PSS-based composite material, wherein the deposition (52) in liquid form to obtain the layer elongated portion of the semiconductor material (40) is made 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 in which 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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