Multilayer device

Abstract

The present description relates to a multilayer device comprising an electroactive stack (100) comprising at least three electroactive layers (103), at least two of the electroactive layers (103) differing from one another by at least one of the parameters selected from thickness, Young's modulus, piezoelectric coefficient, and electrostrictive coefficient.

Description

[0001] DESCRIPTION

[0002] Multilayer device

[0003] This application is based on, and claims priority from, French patent application number FR2412419 filed on November 14, 2024, entitled "Multilayer Device", which is considered to form an integral part of this description within the limits provided by law.

[0004] technical field

[0005]

[0001] The present description relates in general to multilayer devices, that is to say systems in which several layers (at least two) of materials are stacked one on top of the other.

[0006] Previous technique

[0007]

[0002] Multilayer devices are stacks of several layers of materials. For example, there are multilayer stacks based on metals, polymers, cellulose, ceramics, gels (ionic, rheological, etc.) or composite materials.

[0008]

[0003] Such materials find applications in many fields: manufacture of bulletproof vests, waterproof devices, thermal insulation, etc.

[0009]

[0004] Actuators, for example used in the field of microsurgery, can also be formed of a multilayered device. In the article by Toinet et al. ("Design optimization of printed multilayered electroactive actuators used for steerable guidewire in micro-invasive surgery", Materials (Basel) 2024; 17 ( 9): 2135), actuators comprising several layers of electroactive polymer materials are proposed.

[0010]

[0005] Regardless of the field, multilayer devices comprise several layers of the same material, the material having been chosen so as to have the properties best suited to the intended application. In other words, all the layers of the multilayer have the same physical and mechanical properties (for example: the same electrical resistance, conductivity, dielectric permittivity, Young's modulus, piezoelectric coefficient, etc.).

[0011] Summary of the invention

[0012]

[0006] There is a need for devices with improved properties, particularly in the piezoelectric field.

[0013]

[0007] This goal is achieved by a piezoelectric device comprising a stack comprising at least three electroactive layers, at least two of the electroactive layers differing from each other by at least one of the parameters chosen from the thickness, the Young's modulus, the piezoelectric coefficient and the electrostrictive coefficient.

[0014]

[0008] According to a particular embodiment, the electroactive layers are in P (VDF-TrFE) and the parameter is chosen from the thickness, the Young's modulus and the piezoelectric coefficient.

[0015]

[0009] According to a particular embodiment, the electroactive layers are made of PVDF terpolymer, for example PVDF-TrFE-CFE or PVDF-TrFE-CTFE, or of a PVDF tetrapolymer, and the parameter is chosen from the thickness, the Young's modulus and the electrostrictive coefficient.

[0016]

[0010] According to a particular embodiment, the parameter varies according to an increasing gradient.

[0017]

[0011] According to a particular embodiment, the parameter varies according to a decreasing gradient.

[0018]

[0012] According to a particular embodiment, the parameter varies according to a first increasing gradient from a first electroactive layer to a second electroactive layer and then according to a decreasing gradient from the second intermediate electroactive layer to a third electroactive layer.

[0019]

[0013] According to a particular embodiment, the parameter varies according to a first decreasing gradient from a first electroactive layer to a second electroactive layer and then according to an increasing gradient from the second electroactive layer to a third electroactive layer.

[0020]

[0014] According to a particular embodiment, the device comprises between 3 and 15 electroactive layers.

[0021]

[0015] According to a particular embodiment, the electroactive stack is arranged on a first face of a substrate, for example a polyimide substrate.

[0022]

[0016] According to a particular embodiment, the device includes an additional electroactive stack disposed on a second face of the substrate.

[0023]

[0017] This goal is also achieved by a method for manufacturing a piezoelectric device as defined above, comprising the following steps:

[0024] a) form a first electrode,

[0025] b) deposit an electroactive layer on the first electrode, for example by wet annealing, from a solution containing one or more solvents and the electroactive material, c) form a second electrode on the electroactive layer, d) perform a thermal annealing step,

[0026] repeat the steps so as to form at least three electroactive layers, each electroactive layer being disposed between a first electrode and a second electrode, at least two of the electroactive layers being deposited with different deposition parameters and / or with different thermal annealing parameters, for example by varying the nature of the solvent(s) during step b) and / or by varying the duration and / or temperature of the thermal annealing during step d).

[0027] Brief description of the drawings

[0028]

[0018] These features and advantages, as well as others, will be described in detail in the following description of particular embodiments, given by way of non-limiting example, in relation to the accompanying figures, among which:

[0029]

[0019] Figure IA schematically represents, in cross-section, a multilayer device, according to a particular embodiment;

[0030]

[0020] Figure IB schematically represents, in cross-section, a multilayer device, according to another particular embodiment;

[0031]

[0021] Figure 2A, Figure 2B, Figure 2C, Figure 2D, Figure 2E, Figure 2F, Figure 2G and Figure 2H are graphs schematically representing the variation of a parameter P in different multilayers comprising N EAP layers according to several specific embodiments;

[0032]

[0022] Figure 3 is a graph representing the proportion of beta phase in a piezoelectric layer as a function of the number of annealings;

[0033]

[0023] Figure 4 is a graph representing the bending angle as a function of the substrate thickness for multilayer devices exhibiting different configurations of variation of the parameter d31 in the multilayer;

[0034]

[0024] Figure 5 is a graph representing the bending angle as a function of the Young's modulus of the substrate for multilayer devices exhibiting different configurations of variation of the parameter d31 in the multilayer;

[0035]

[0025] Figure 6 is a graph representing the bending angle as a function of the substrate thickness for multilayer devices exhibiting different configurations of variation of the parameter £r (permittivity) in the multilayer;

[0036]

[0026] Figure 7 is a graph representing the bending angle as a function of the Young's modulus of the substrate for multilayer devices exhibiting different configurations of variation of the parameter £r (permittivity) in the multilayer;

[0037]

[0027] Figure 8 is a graph representing the bending angle as a function of the substrate thickness for multilayer devices exhibiting different configurations of variation of the YpvDF-TrFE parameter (Young's modulus) in the multilayer;

[0038]

[0028] Figure 9 is a graph representing the bending angle as a function of the Young's modulus of the substrate for multilayer devices exhibiting different configurations of variation of the YpvDF-TrFE parameter (Young's modulus) in the multilayer;

[0039]

[0029] Figure 10 is a graph representing the bending angle as a function of the thickness of P (VDF-TrFe) for multilayer devices exhibiting different configurations of variation of the parameter YpvDF-TrFE (Young's modulus) in the multilayer;

[0040]

[0030] Figure 11 is a graph representing the bending angle as a function of the substrate thickness for multilayer devices exhibiting different configurations of parameter variation t PV DF-TrFE (thickness) in the multilayer;

[0031] Figure 12 is a graph representing the bending angle as a function of the Young's modulus of the substrate for multilayer devices exhibiting different configurations of variation of the parameter t PV DF-TrFE (thickness) in the multilayer.

[0041]

[0032] The different elements are not necessarily on a uniform scale in order to make the figures more legible.

[0042] Description of the implementation methods

[0043]

[0033] The same elements have been designated by the same reference numerals in the different figures. In particular, the structural and / or functional elements common to the different embodiments may have the same reference numerals and may have identical structural, dimensional and material properties.

[0044]

[0034] For the sake of clarity, only the steps and elements useful for understanding the described embodiments have been represented and are detailed.

[0045]

[0035] Unless otherwise specified, when two elements are referred to as connected together, this means directly connected without any intermediate elements other than conductors, and when two elements are referred to as coupled together, this means that these two elements can be connected or linked through one or more other elements.

[0046]

[0036] In the following description, when reference is made to absolute positional qualifiers, such as "front", "back", "top", "bottom", "left", "right", etc., or relative positional qualifiers, such as "above", "below", "superior", "lower", etc., or to orientational qualifiers, such as "horizontal", "vertical", etc., unless otherwise specified, reference is made to the orientation of the figures.

[0037] Unless otherwise specified, the expressions "approximately", "roughly", "about", and "in the order of" mean to the nearest 10%, preferably to the nearest 5%.

[0047]

[0038] By included between X and Y, it is meant that the bounds X and Y are included.

[0048]

[0039] When referring to an electroactive element, it means an element that can be electrically activated. In particular, under the action of an electric field (application of a voltage, for example), a mechanical deformation of the element is obtained.

[0049]

[0040] Although the invention is particularly described for piezoelectric multilayer devices, it could be used in all fields concerning multilayer systems: actuators / sensors, heat sinks or wave propagators, thermal insulation, sealing, etc. It can also be used in the field of acoustics or vibration detection.

[0050]

[0041] For example, in the case of thermal insulation, these variations can make it possible to choose in which layer the condensation of water will take place in order to recover it.

[0051]

[0042] In the case of cellular materials, the size of the cells can vary in order to modify the convection coefficient. In this same field of thermal insulation, the concentration of gases (Argon, Krypton, Xenon) in the air gaps can also be subject to gradient / variations.

[0052]

[0043] In the field of electrical conduction, these variations can allow the transport of several different currents or voltages within the same cable.

[0053]

[0044] In sensor applications, a polarization gradient enables the use of a three-dimensional sensor. Also within this same sensor domain, a geometric gradient between layers (surface or thickness gradient) allows the cutoff frequency to be modulated.

[0054]

[0045] On the other hand, since the charge generated by the sensor depends directly on the displacement, a gradient of Young's modulus in the sensor makes it possible to enlarge the range of use of the sensor.

[0055]

[0046] We will now describe the multilayer device in more detail with reference to Figure IA and Figure IB.

[0056]

[0047] The multilayer device comprises an electroactive stack 100. The electroactive stack 100 comprises at least 3 electroactive layers 103. In other words, it can comprise at least N electroactive layers 103 with N an integer greater than or equal to 3.

[0057]

[0048] At least one of the parameters of the electroactive layers 103 varies. Several parameters may vary. The variations may concern the intrinsic properties of the material and / or its geometry. The parameter(s) that vary are chosen from the thickness, the Young's modulus, the piezoelectric coefficient, and the electrostrictive coefficient.

[0049] Within the multilayer stack, it is possible to vary a parameter P according to several configurations.

[0058]

[0050] By way of illustration and not limitation, different configurations of variation of a parameter P between a minimum value a and a maximum value b in a 100 multilayer stack comprising N layers (with N a positive integer strictly greater than 3) are shown in Figures 2A to 2H:

[0059] - variation according to an "increasing gradient": increase of P from the value a (layer 1 ) to the value b (layer N) ( figure 2 A);

[0060] - variation according to a "decreasing gradient": decrease of P from the value b (layer 1) to the value a (layer N) (figure 2B);

[0061] - V-shaped variation: a decreasing gradient and an increasing gradient with, for example, a decrease in P from the value b (layer 1) to a minimum of P (value a) at the level of one of the intermediate layers then an increase in P up to the value b on layer N (figure 2C);

[0062] - Inverted V variation: a decreasing gradient and an increasing gradient with, for example, an increase in P from the value a (layer 1) up to a maximum of P (value b) at the level of one of the intermediate layers then a decrease back to the value a up to layer N (figure 2D); - W variation: with two increasing gradients and two decreasing gradients with, for example, a first gradient decreasing to a first intermediate value at the level of one of the intermediate layers, a second gradient increasing to a second intermediate value at the level of another intermediate layer, a third gradient decreasing to a third intermediate value at the level of another intermediate layer and a fourth increasing gradient (figure 2E);

[0063] - variation in inverted W: with two decreasing gradients and two increasing gradients with, for example, a first increasing gradient up to a first intermediate value at the level of an intermediate layer, a second decreasing gradient up to a second intermediate value at the level of another intermediate layer, a third increasing gradient up to a third intermediate value at the level of another intermediate layer and a fourth decreasing gradient (figure 2F).

[0064]

[0051] The intermediate layer 103 can be one of the layers at the center of the stack. In particular, it can be the central layer of the stack.

[0052] The variations can locally include plateaus. By plateau, we mean that the value of the parameter is constant over at least two consecutive layers 103. The value can be constant at an intermediate value (Figure 2D), a minimum value (Figure 2G), or a maximum value (Figure 2H).

[0065]

[0053] In these different figures, the abscissa represents the number of electroactive layers 103 of the stack 100 ( 'N EA p' ) and the ordinate represents the value of the parameter P in arbitrary units.

[0066]

[0054] Preferably, the stack 100 comprises between 3 and 15 electroactive layers 103, and even more preferably between 3 and 15 layers 103 and even more preferably between 8 and 12 electroactive layers 103.

[0067]

[0055] For example, an increasing gradient or a decreasing gradient may be chosen for a stack comprising between 3 and 15 layers, preferably between 8 and 12 layers.

[0068]

[0056] The stack comprises an alternation of electroactive layers 103 and electrodes 101, 102. The stack 100 begins with a first electrode 101 and ends with a second electrode 102.

[0069]

[0057] The electroactive layers 103 have a thickness 'tEAP' that can vary from one layer to another. The thickness 'tEAP' is, for example, between 3 and 50 pm, preferably between 3 and 15 pm, preferably between 3 and 10 pm. With such thicknesses, the applied voltages can be relatively low. The thickness corresponds to the dimension along the z-axis.

[0070]

[0058] The electroactive material can be chosen, for example, from polymers (conductive, piezoelectric, ferroelectric, electrostrictive, ionic, etc.), ceramics (in particular lead zirconate titanosilicate (PZT) or Sr-Ba-Nb oxide (SBN)), shape memory materials, and rheological fluids. It can also be a composite material, for example a polymer material comprising ceramic particles (in particular PZT particles).

[0071]

[0059] Preferably, each electroactive layer 103 is an organic electroactive layer. Each electroactive layer 103 comprises, preferably, a polymeric matrix of PVDF, a PVDF copolymer, a PVDF terpolymer or a PVDF tetrapolymer.

[0072]

[0060] It may be a copolymer of vinylidene fluoride and at least one other monomer copolymerizable with VDF. Advantageously, the copolymer comprises at least 50% by mol, preferably at least 70% by weight, and even more preferably at least 80% by mol of VDF.

[0073]

[0061] By way of illustration, the copolymerizable monomer(s) are, for example, chosen from chlorotrif luoroethylene (CTFE), chlorof luoroethylene (CFE), hexaf luoropropylene (HEP), trif luoroethylene (VF3), methyl methacrylate (MMA), tetraf luoroethylene (TFE), and perfluoro (alkyl vinyl) ethers such as perfluoro (methyl vinyl) ether (PMVE).

[0074]

[0062] For example, the copolymer is a copolymer of poly(vinylidene fluoride-trifluoroethylene) PVDF / TrFe, also noted as P(VDF-TrFe) or PVDF-CTFE.

[0075]

[0063] It can also be a terpolymer. For example, a PVDF / TRFE / CTFE or PVDF / TRFE / CFE terpolymer might be chosen. Such materials are called electrostrictive materials.

[0076]

[0064] According to another embodiment, the polymer is not a ferroelectric polymer: it may be PVDF-HFP.

[0065] In particular, as an electroactive material, a piezoelectric material (such as P (VDF-TrFE)) or an electrostrictive material (such as a PVDF terpolymer, for example PVDF-TrFE-CTFE or PVDF-TrFE-CFE, or a PVDF tetrapolymer) may be chosen.

[0077]

[0066] For an electrostrictive material, the parameter P can be chosen from the Young's modulus, the thickness and the electrostrictive coefficient.

[0078]

[0067] For a piezoelectric material, the parameter P can be chosen from the Young's modulus, the thickness and the piezoelectric coefficient.

[0079]

[0068] PVDF-TrFE comprises an amorphous portion and a crystalline portion. The crystalline portion can be crystallized in one or more phases (a, p, y). The p phase is the steepest phase (i.e., the one with the highest Young's modulus). It is possible to create variations in material properties by varying the proportion of the p phase. Indeed, the piezoelectric coefficient, the relative permittivity, and the Young's modulus of PVDF-TrFE are directly related to the proportion of the p phase.

[0080]

[0069] To vary the proportion of phase p, different annealing cycles can be used. Indeed, the temperature, time, number of annealing cycles, and pressure used during annealing all affect the amount of phase p (Figure 3). The relative proportion of phase p can be determined by Raman scattering spectroscopy.

[0081]

[0070] It is also possible to vary the proportion of the p-phase by using different solvents for each layer. Indeed, certain solvents promote the crystallization of PVDF-TrFE in the p-phase.

[0071] As mentioned previously, the electroactive organic layer 103 can be a composite material. For example, layer 103 can comprise, in addition to the polymer matrix, ferroelectric particles and, optionally, PEDOT:PSS particles in order to increase the relative permittivity of the material and thus improve its electrical behavior.

[0082]

[0072] For example, ferroelectric particles are made of BaTiO₃ (BTO), PZT (lead zirconate titanate), AIN, ZnO, or even SBN (Sr-Ba-Nb oxide) or SBT (Sr-Ba-Ti oxide). Such particles are used, for example, with a crosslinkable epoxy matrix. The layer thus exhibits a certain stiffness, which makes it possible to efficiently convert the electrical impulse into mechanical displacement.

[0083]

[0073] The use of composite materials can also allow for a variation of properties. In the case of P (VDF-TrFE), ceramic inclusions (such as PZT, BaTiO₃, ZnO, etc.) allow for variation of the piezoelectric coefficients, the Young's modulus, or the permittivity.

[0084]

[0074] Each electroactive layer 103 is arranged between a first electrode 101 and a second electrode 102.

[0085]

[0075] The electrodes 101, 102 have, for example, a 'telec' thickness of between 0.1 and 10 pm, preferably between 0.1 and 3 pm, even more preferably between 1 and 2.5 pm.

[0086]

[0076] Preferably, the electrodes 101, 102 are made of an electrically conductive polymer, preferably PEDOT-PSS (poly(3,4-ethylenedioxythiophene).

[0087]

[0077] Even more preferably, the stack 100 comprises electrodes 101, 102 in PEDOT-PSS and electroactive layers in P (VDF-TrFe) whose properties vary from one layer to another.

[0088]

[0078] According to one embodiment, the device may include a substrate 10 on which the electroactive stack 100 is positioned. The substrate 10 comprises a first main face 10a and a second main face 10b. The stack 100 is positioned on the first face 10a of the substrate 10.

[0089]

[0079] The substrate 10 is a flexible substrate, that is to say, it can deform when the piezoelectric stack is subjected to a voltage. The material is chosen so as to allow reversible deformation.

[0090]

[0080] The substrate 10 is, for example, a polymer such as polyimide (PI) or poly(ethylene naphthalate) (PEN). It may also be a polyarylate (PAR). It could also be a natural, bio-based material.

[0091]

[0081] The substrate 10 has a low thickness, typically between 15 pm and 1 mm, preferably between 20 pm and 200 pm, even more preferably between 20 and 150 pm, for example 125 pm.

[0092]

[0082] The substrate 10 has a width, for example, between 0.1 mm and 20 mm, preferably between 0.2 and 5 mm.

[0093]

[0083] The length / width ratio is preferably between 2 and 150, even more preferably between 5 and 100, for example between 50 and 100. The higher the ratio, the lower the tension required to deform the substrate 10 of the distal part 1.

[0094]

[0084] According to a particular embodiment, the device may comprise two electroactive stacks, each stack being arranged on one of the principal faces 10a, 10b of the substrate 10.

[0085] The stack(s) 100 have, for example, a length between 0.5 and 3 cm, preferably between 1.5 and 2.5 cm. The length is the dimension defined along a Y-axis (perpendicular to the X and Z axes in Figure IB).

[0095]

[0086] The stack(s) 100 have, for example, a width between 0.1 mm and 3 cm, preferably between 0.2 mm and 5 mm.

[0096]

[0087] When tension is applied to the stack(s) 100, they are mechanically deformed, creating tension and compression zones on the faces 10a, 10b of the substrate 10, which bends the substrate or modifies its curvature if it was initially subjected to prestressing. The curvature of the substrate 10 can be characterized, for example, by measuring a bending angle.

[0097]

[0088] The piezoelectric stack 100 can be implemented as follows:

[0098] a) form a first electrode 101, preferably on one face of a substrate 100,

[0099] b) to form an electroactive layer 103 on the first electrode 101, for example by wet process, from a solution containing one or more solvents and the electroactive material,

[0100] c) form a second electrode 102 on the electroactive layer 103,

[0101] d) perform thermal annealing.

[0102]

[0089] In step b), the electroactive material layer 103 can be deposited by spin coating. Other types of localized deposition can be used, such as screen printing, spraying, dispensing, or even ink deposition. Preferably, the electroactive layer is deposited by screen printing. In a single pass, the deposited thickness is between 1 and 20 µm. It is possible to superimpose several layers by screen printing until the desired final thickness of the electroactive layer 103 is achieved.

[0103]

[0090] Advantageously, between 3 and 15 electroactive layers 103 will be deposited intercalated between the electrodes 101, 102, each electroactive layer 103 being arranged between a first electrode 101 and a second electrode 102.

[0104]

[0091] Steps a) to d) are repeated to form the stack 100. For example, the following sequence can be carried out to form 3 electroactive layers: a), b), c), d), b), a), d), b), c), d).

[0105]

[0092] The process also includes a crystallization step of the electroactive layers 103 to improve its performance. This irradiation is, for example, carried out with UV flash light, with a flash duration, or pulse duration, of between approximately 500 ps and 2 ms, and a fluence (energy delivered per unit area) of between approximately 15 J / cm² 2 and 25 J / cm² 2and with light of wavelength between approximately 200 nm and 380 nm. The number of UV light flashes, or pulses, during this irradiation varies depending on the thickness to which the material is to be crystallized. For example, for a P(VDF-TrFe) thickness of approximately 2 pm, the irradiation can be carried out with a fluence of approximately 17 J / cm². 2 , a pulse duration of approximately 2 ms and a number of pulses of 5.

[0106]

[0093] A crystallization step can be carried out after the deposition of each electroactive layer 103, after the deposition of several electroactive layers 103 (for example, two electroactive layers 103), or after the deposition of all the electroactive layers 103.

[0094] The material, possibly having undergone previous crystallization, is then subjected to annealing, for example, at approximately 130°C for approximately 60 minutes, to complete the total crystallization of the material. Annealing can be carried out at ambient pressure or at low pressure.

[0107]

[0095] The crystallization of the material can therefore be carried out in two stages: firstly, irradiation by UV light pulse to properly crystallize the second face of the layer in material in order to increase its thermal conductivity, then a thermal annealing completing the crystallization for the rest of the material not crystallized by the previous irradiation.

[0108]

[0096] When the material is a P-based copolymer (VDF-TrFe), a material polarization step is performed before use. This step can be carried out, for example, by applying an alternating or direct current voltage across its terminals, via the electrodes, in order to improve the piezoelectric coefficient of the material. This polarization is performed only once for the entire lifespan of the material. This alternating and / or direct current polarization can be performed at room temperature or at high temperature (up to approximately 100°C). When the polarization is performed at room temperature, it is possible to apply a voltage of up to approximately 150 V / m or even 200 V / m of the electroactive layer thickness for a duration, for example, ranging from a few seconds to a few minutes. For example, a voltage of 120 V / m could be applied for 20 seconds.When polarization is performed hot, for example at a temperature of approximately 90°C, a DC voltage, for example between approximately 50 V / pm and 80 V / pm, can be applied to the dielectric layer for a duration, for example, between approximately 1 and 5 minutes. The temperature is then lowered to ambient temperature, and the electric field applied to the material via the applied DC voltage is then stopped. Such polarizations allow PVDF to achieve remanent polarization values ​​of 8 pC / cm. 2 .

[0109]

[0097] The molecules inside the layer remain oriented in this way, even when the material is no longer subjected to this electric field. The material can thus be polarized by applying an initial polarization voltage across the electrodes. Preferably, a piezoelectric material thickness of between 3 and 4 pm is chosen to promote the polarization of the piezoelectric material by this capacitance, and the level of the electric voltage applied between the electrodes to achieve the initial polarization of the piezoelectric material (when the piezoelectric material must be initially polarized) is also chosen.

[0110]

[0098] Annealing is advantageously carried out at the end of the process, or between the different steps. Annealing is, for example, at a temperature between 100°C and 150°C, preferably around 100°C to remove residual traces of solvent and / or finalize the crystallization of the piezoelectric material.

[0111]

[0099] Annealing can be carried out under vacuum. For example, a vacuum on the order of mbar. The annealing time, for Imbar, can be at least 1 minute, preferably 3 minutes.

[0112]

[0100] At least two of the electroactive layers 103 are deposited with different deposition parameters and / or with different annealing parameters, for example by varying the nature of the solvent(s) in step b) and / or by varying the duration and / or temperature of the thermal annealing. This results in at least two layers 103 having different physical parameters.

[0101] Illustrative and non-limiting example

[0113]

[0102] In this example, the device is a piezoelectric device comprising a stack of 20 elements: a layer of polyimide (Kapton) acting as substrate 10, ten layers of PEDOT:PSS to form the electrodes 101, 102 and nine layers of P (VDF TrFE) to form the electroactive layers 103.

[0114]

[0103] On the substrate 10, the electrodes 101, 102 in PEDOT-PSS and the piezoelectric layers 103 in P (VDF-TrFE) are stacked alternately.

[0115]

[0104] This stack 100 forms a bending actuator, the performance of which is evaluated by means of the bending angle of the substrate 10.

[0116]

[0105] The substrate 10 and the electrodes 101, 102 are identical for the different tests. Only the physical and mechanical properties of the active layers 103 within the stack 100 have been modified.

[0117]

[0106] Several sample simulations were carried out:

[0118] sample 'situation l' ("increasing gradient"): increase in the value X from a (layer 1) to b (layer 9);

[0119] sample 'situation 2' ("decreasing gradient"): decrease of X from b (layer 1) to a (layer 9);

[0120] - 'situation 3' sample (V-shaped variation): Maximum of X (value b) at the center then decrease to the value a on layers 1 and 9;

[0121] sample 'situation 4' (inverted V variation): minimum of X (value a) at the center a then increase up to the value b on layers 1 and 9;

[0122] - 'situation 5' sample - comparative example (constant at a minimum or "Extreme Case 1"): minimum of X (value a) on all layers; - 'situation 6' sample - comparative example (constant at a maximum or "Extreme Case 2"): a maximum of X (value b) on all layers.

[0123]

[0107] The influence of several parameters was studied. The maximum and minimum values ​​of the parameters studied are listed in Table 1 below.

[0124]

[0108] [Table 1]

[0125] Maximum Property (b) Minimum Property (a) Piezoelectric coefficient (dsi) 22 pC / N 6 pC / N Relative permittivity (er ) 14 8

[0126] Young's modulus (YPVDF-TrFE) 3 GPa 10 MPa

[0127]

[0128] Thickness 13 µm 1 µm

[0129]

[0109] Simulations of the properties of the different samples were carried out. The results obtained are presented in the form of graphs (figures 4 to 12).

[0130]

[0110] The increasing gradient (situation 1) shows significantly better performance than the others in certain windows, particularly when the substrate is thin and soft or when the P(VDF-TrFE) is thick (in other words, when the P(VDF-TrFE) becomes mechanically or geometrically dominant over the substrate). There are configurations in which a gradient from 6 pC / N to 22 pC / N for the piezoelectric coefficient allows for better curvature than a constant coefficient at 22 pC / N.

[0131]

[0111] On the other hand, the decreasing gradient (situation 2) exhibits negative bending angles in certain windows, particularly when the substrate is thin and soft or when the P(VDF-TrFE) is thick (in other words, when the P(VDF-TrFE) becomes predominant from a mechanical or geometric point of view compared to the substrate). These negative bending angles correspond to the other direction of curvature.

[0112] In the classical configuration where all the layers are identical (situation 5 and situation 6), this type of actuator bends downwards, i.e., the substrate is in compression while the upper layers are in tension. This configuration, in our sign convention, leads to a positive angle.

[0132]

[0113] To date, the only known way to reverse this direction of curvature is to first bias the P (VDF-TrFE) with an electric field. However, this method for reversing the direction of curvature is only usable at low fields (typically less than 45 V / pm). Above this value, the excitation voltage realigns the dipoles and thus erases the initial bias.

[0133]

[0114] The configuration of a variation of the parameter according to a decreasing gradient (situation 2) therefore makes it possible to reverse the direction of curvature and this without limit of applied field.

[0134]

[0115] Figures 6 and 7 show that the use of 103 layers having different permittivities has little influence on the angle of curvature.

[0135]

[0116] Various embodiments and variations have been described. A person skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will become apparent to a person skilled in the art.

[0136]

[0117] Finally, the practical implementation of the embodiments and variants described is within the reach of a person skilled in the art, based on the functional indications given above.

Claims

DEMANDS 1. Piezoelectric device comprising a stack (100) comprising at least three electroactive layers (103), at least two of the electroactive layers (103) differing from each other by at least one of the parameters chosen from the thickness, the Young's modulus, the piezoelectric coefficient and the electrostrictive coefficient.

2. Device according to claim 1, wherein the electroactive layers (103) are in P (VDF-TrFE) and wherein the parameter is chosen from the thickness, the Young's modulus and the piezoelectric coefficient.

3. Device according to claim 1, wherein the electroactive layers (103) are made of PVDF terpolymer, for example PVDF-TrFE-CFE or PVDF-TrFE-CTFE, or of a PVDF tetrapolymer, and wherein the parameter is chosen from the thickness, the Young's modulus and the electrostrictive coefficient.

4. Device according to any one of claims 1 to 3, wherein the parameter varies according to an increasing gradient.

5. Device according to any one of claims 1 to 3, wherein the parameter varies according to a decreasing gradient.

6. Device according to any one of claims 1 to 3, wherein the parameter varies according to a first increasing gradient from a first electroactive layer to a second electroactive layer and then according to a decreasing gradient from the intermediate second electroactive layer to a third electroactive layer.

7. A device according to any one of claims 1 to 3, wherein the parameter varies according to a first decreasing gradient from a first electroactive layer to a second electroactive layer and then according to a increasing gradient from the second electroactive layer to a third electroactive layer.

8. Device according to any one of the preceding claims, wherein the device comprises between 3 and 15 electroactive layers (103).

9. Device according to any one of the preceding claims, wherein the electroactive stack (100) is arranged on a first face (10a) of a substrate (10), for example a polyimide substrate.

10. Device according to the preceding claim, comprising an additional electroactive stack disposed on a second face (10b) of the substrate (10).

11. A method for manufacturing a piezoelectric device as defined in any one of claims 1 to 10, comprising the following steps: a) form a first electrode (101), b) depositing an electroactive layer (103) on the first electrode (101), for example by wet method, from a solution containing one or more solvents and the electroactive material, c) form a second electrode (102) on the electroactive layer (103), d) perform a thermal annealing step, repeat the steps so as to form at least three electroactive layers (103), each electroactive layer (103) being arranged between a first electrode (101) and a second electrode (102), at least two of the electroactive layers (103) being deposited with different deposition parameters and / or with different thermal annealing parameters, for example by varying the nature of the solvent(s) during step b) and / or by varying the duration and / or temperature of the thermal annealing during step d).

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