EP0056549B1 - Electromechanical transducer structure - Google Patents

Description

The invention relates to transducer devices using polymer films capable of exhibiting piezoelectricity phenomena by application of a polarization field.

It is known to produce an electromechanical transducer composed of a median sheet of material absorbing vibrational energy, on the two faces of which adhere active elements constituted by a film of piezoelectric polymer metallized on its two faces. The interior metallizations are connected to a first terminal and the exterior metallizations are connected to a second terminal. Such a structure is mentioned in document DE-A-2803168.

The invention is particularly applicable to structures composed of piezoelectric polymer films associated with other polymer films, in particular to a structure comprising at least one layer of polar polymer associated with another layer of polymer, such as with equal thickness. , the mechanical response to an electrical control voltage is increased compared to that which a homogeneous film of the same polar polymer would produce.

Certain films of polar polymers such as polyvinyl chloride (PVC), polyvinyl fluoride (PVF), polyvinylidene fluoride (PVF ₂ ) and certain polymers such as PVF ₂ - PTFE (polyvinylidene fluoride - polyethylene tetrafluoride) ), are known to have piezoelectric properties, and find their applications in electroacoustic transducers and sensors for example.

In general, the piezoelectric properties of these films are described by a tensorial relationship between the P components; of the polarization and the components X _jk of the mechanical stresses. We then define a tensor called the tensor of the piezoelectric coefficients D _ijk . As a first approximation, in the case of a uniaxial remanent polarization, a piezoelectric polymer has piezoelectric coefficients that are higher as the value of the remanent polarization is high, and as its mechanical flexibility, in a direction considered, is higher .

For example, in the case of polyvinylidene fluoride, for a remanent polarization of P = 6.10-2 Cm- ² perpendicular to the film with a thickness of 20 μm, a piezoelectric coefficient is obtained in a direction parallel to the film of 20 pCN -1 and a relative elongation in one direction of the film plane of 10- ⁶ per volt applied. The electromechanical coupling factor K also defines the electromechanical transducer effect. More precisely, K ² is equal to the ratio of the mechanical energy transformed by the piezoelectric effect, to the stored electrical energy.

In the case of polyvinylidene fluoride cited above as an example, the electromechanical coupling coefficient is: K2 = 1.2x 10-2.

In most applications of electroacoustic transducers, loudspeaker, earphone, transmitter for sonar or ultrasound, the excitation voltage available is limited, but not the intensity.

The advantage of the invention compared to the devices existing in the prior art lies in the maximum energy coupling. electric, low voltage, to get the maximum mechanical energy.

Indeed, a monolithic device has a certain stiffness which, during an electrical excitation, counteracts the mechanical deformations and therefore limits, by this fact, the piezoelectric effects perceived externally. The results can be improved by increasing the value of the electrical energy given to the device. For the same piezoelectric material, this effect can be achieved by reducing the thickness of the piezoelectric film, but in this case, the mechanical resistance of the device is no longer satisfactory.

A device according to the invention makes it possible to maintain good mechanical strength, since the total thickness of the materials is unchanged, to significantly increase the piezoelectric effects by reducing the influence of the stiffness inherent in the piezoelectric layers used and to deliver more electrical energy to the active layers of the device.

The present patent application relates to a structure comprising at least one piezoelectric film in which the relative elongation per applied volt, as well as the mechanical energy supplied by applied volt, are greater than those of the homogeneous polar polymer of the same thickness.

Consequently, the subject of the invention is an electromechanical transducer structure composed of a passive lift element surrounded by two active polymer films made of piezoelectric material having internal faces which adhere to the load-bearing faces of said lift element and external faces coated with 'electrodes; an equipotential electrical connection being established between said load-bearing faces, characterized in that said load-bearing faces are the faces of a film of polymer material ensuring, by own electrical conductivity, said equipotential electrical connection; said polymer material offering a mechanical rigidity lower than that of said piezoelectric material.

• la figure 1 représente les comportements respectifs d'un dispositif transducteur suivant l'art antérieur et d'un dispositif suivant l'invention;
• la figure 2 représente un mode de réalisation d'un dispositif conforme à l'invention;
• la figure 3 est une variante de la figure 2;
• la figure 4 représente l'évolution des caractéristiques d'un dispositif selon l'invention.
• La figure 1 représente les comportements, lors de sollicitations extérieures électriques, d'un dispositif transducteur suivant l'art antérieur, et d'un dispositif suivant l'invention.

The invention will be better understood using the following description and the appended figures, among which:

• FIG. 1 represents the respective behaviors of a transducer device according to the prior art and of a device according to the invention;
• Figure 2 shows an embodiment of a device according to the invention;
• Figure 3 is a variant of Figure 2;
• FIG. 4 represents the evolution of the characteristics of a device according to the invention.
• FIG. 1 represents the behaviors, during electrical external stresses, of a transducer device according to the prior art, and of a device according to the invention.

A device according to the prior art is shown in the left part of the figure, that according to the invention was shown in the right part.

A device, as it exists in the prior art, consists of a monolithic film of piezoelectric material, for example a polymer, as shown in FIG. 1a. To visualize the effects of mechanical deformation, imagine a separation line 14 which limits two layers (10 and 11) in the film.

A device according to the invention, which will be described more precisely in the following description, comprises a lifting film 1 of flexible non-piezoelectric material, sandwiched between two films of piezoelectric materials 3 and 3 ', as indicated in FIG. 1b. Films 3 and 3 'are mechanically integral with film 1.

When, via appropriate electrodes, the piezoelectric layers work in bending, as shown in FIG. 1c by a device of the prior art and in FIG. 1d by a device according to the invention, it is seen that the overall thickness differences of the films, as well as the stiffness differences at the level of the separation of the different layers, make it possible to obtain a greater driving torque in the device according to the invention assuming that the excitation voltage is exclusively applied to layers 3 and 3 '.

When working in compression compression of a device according to the prior art, as shown in FIG. The, the stiffness of the material limits the mechanical effects due to electrical excitations. One can then think of reducing the thickness of the film to increase the ceded electrical energy, under identical voltage values, but in this case the mechanical resistance becomes insufficient.

The same effect on a device according to the invention is shown in Figure 1f. The thickness of the piezoelectric films 3 and 3 ′ can be reduced considerably, because the overall mechanical resistance is reinforced by the presence of the lift film 1. The electrical energy transferred to this lift film must be zero, which can be achieved by metallizing the contact surfaces between layers 1 and 3 and layers 1 and 3 'then by electrically joining these metallizations. In addition to this condition, it is expected that the mechanical flexibility of the lift film is greater than that of the piezoelectric polymers used so that the elongations generated by them are not greatly reduced by the need to lengthen the lift element.

FIG. 2 represents an embodiment of a device according to the invention.

A layer of flexible material 6, which is a polymer, receives on each of its two main faces layers 3 and 3 ′ of piezoelectric polymer, for example polyvinylene fluoride PVF ₂ . Each of these layers is covered with an electrode 4 and 4 ', connected respectively by connections 5 and 5', to an electric generator delivering a voltage V. The electric energy delivered by this electric generator is only supplied to the layers active 3 and 3 '. At equal voltage, the transformed mechanical energy is greater than in a piezoelectric polymer device of the same thickness, since the capacity of the piezoelectric layers is higher, due to the reduction in thickness. According to a particularly advantageous embodiment, a symmetrical structure is adopted which provides the assembly with a stable shape during the effects of thermal expansion.

In Figure 2, layer 6 is made of a conductive polymer.

Such conductive polymers are generally prepared from an elastomer, in which particles of carbon, or of metals: copper, silver copper, etc. are included.

It should be noted that in the case of FIG. 2, the remanent polarizations of the piezoelectric layers 3 and 3 ′ are oriented in the same direction to obtain the most intense traction and compression effects; taking into account that the electric control fields have the same direction. However, by reversing one of the polarizations one can obtain an operation in bending.

FIG. 3 is a variant of FIG. 2.

The conductive polymer therefore acts as an electrode for the piezoelectric layers 3 and 3 '.

In the device of FIG. 3, the remanent polarizations of the layers 3 and 3 ′ must be in the opposite direction, for traction-compression effects.

FIG. 4 represents the evolution of the characteristics of a device according to the invention. For flexion operation, it suffices to change the direction of a polarization. The gain in relative elongation of a composite film compared to a homogeneous film is represented by the ordinate G of the curve of FIG. 4. The magnitude on the abscissa Φ represents the ratio of the sum of the thicknesses of the piezoelectric films 3 and 3 ', to the total thickness of the stratified structure.

The coefficient <7 represents the flexibility ratio of the active piezoelectric material composing the layers 3 to 3 ′, to the flexibility of elastic material, composing the layer 1 or 6.

Figure 4 shows the variation of the gain as a function of the thickness ratio Φ, according to certain values of the coefficient α. We note that if a = 1, the gain G is minimal whatever the value of the quantity tP,

The curves are difficult to use in practice for the too large differences between the flexibility and thickness of the materials. They are then indicated in dotted lines in these regions.

An example of a device according to the invention can be produced in the following manner, without limiting.

The piezoelectric layers are polyvinylidene fluoride PYF ₂ deposited on the intermediate polymer by dipping. PVF ₂ is dissolved in dymethyl formamide DMF in an amount of 100 to 200 g / 1. The intermediate polymer film is coated by passage through a tank of PVF ₂ + DMF solution. The solvent is evaporated at the temperature of 70 to 80 ° C by jets of hot air, or radiation of electrical resistances. The low temperature favors the appearance of the non-oriented y phase. The film can then be stretched, by a factor of 2 to 3 at 90 - 100 ° C: the phase there is therefore transformed into oriented β phase; if the composite film is not stretched, its mechanical and piezoelectric properties will be isotropic in its plane. The composite film is then metallized on its two faces, using for example the deposition of aluminum under vacuum. It is then polarized between these latter electrodes, by applying an electric field of 0.5 to 1 MV / cm in the PVF ₂ layers, at 80 ° C. for a few minutes: the polarization configuration essentially depends on the results that the 'we wish to obtain. The voltage generator of Figures 2 and 3 is then replaced by a bias voltage generator. The thicknesses of the achievable piezoelectric layers are greater than or equal to 1 μm. In the case described, using a conductive intermediate polymer, the same polymers can be used, loaded with carbon or metal particles.

The flexible material has been specially described as a polymer having a low stiffness. It is also possible, without departing from the scope of the invention, to use other extensible materials.

The invention relates to all the applications of piezoelectric films and transducers, in particular electroacoustic ones such as loudspeakers, headphones, various transmitters.

Claims (7)

1. Electromechanic transducer structure composed of a passive suspension member (6) framed by two active polymer films (3, 3') of piezoelectric material having internal faces adhering to the supporting faces of said suspension member and external faces covered with electrodes (4, 4'); an electric equipotential connection being established between said supporting faces, characterized in that said supporting faces are the faces of a film (6) of a polymer material assuring said electric equipotential connection by its inherent electric conductivity; said polymer material offering a mechanical rigidity lower than that of said piezoelectric material.

2. Structure according to claim 1, characterized in that said electrodes (4, 4') are formed of metallizations exclusively lying on said external faces.

3. Structure according to claim 2, characterized in that said metallizations are electrically interconnected; said film (6) forming a common counterelectrode of said active polymer films (3, 3').

4. Structure according to any of claims 1 to 3, characterized in that said polymer material is a polymer loaded with carbon or metal particles.

5. Structure according to claim 1, characterized in that the remanent polarizations of said active polymer films have the same direction and the same sense.

6. Structure according to claim 1, characterized in that the remanent polarizations of said active polymer films have the same direction and opposite senses.

7. Structure according to any of the preceding claims, characterized in that said piezoelectric polymer material is polyvinylidene fluoride.

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