PIEZOELECTRIC ELEMENT FOR ATOMIZERS WITH IMPROVED LIFESPAN

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to the technical field of spray devices capable of producing a mist of microdroplets from a liquid, and more particularly to devices in which microdroplets are generated by a piezoelectric element. More particularly, the invention relates to such a miniaturized device, which can be integrated into a small volume, and / or which allows very fine dosing of the quantity of mist generated. STATE OF THE ART

[0002] Spray devices capable of producing a mist of micro-droplets from a liquid by piezoelectric excitation are known as such. In these systems the piezoelectric element can be combined with a micro-perforated membrane or an acoustic concentrator to promote mist production.

[0003] More particularly, nebulization devices are known in which a piezoelectric element in contact with a volume of liquid generates ultrasonic waves which cause the formation of a mist of very small droplets. This size typically depends on the acoustic frequency of the excitation. It is typically between 2 µm and 10 µm for an acoustic frequency typically between 0.5 MHz and 5 MHz, and decreases as the acoustic frequency increases.

[0004] Such devices are described for example in WO 2017 / 093655 (ARECO). They are typically in the form of a cylindrical wafer with a circular diameter of about 5 mm to about 50 mm and a thickness of about 1 mm to 2 mm.

[0005] These piezoelectric wafers are commonly found in specialist shops, in various diameters and thicknesses. They are manufactured by sintering powders of a piezoelectric material. Two electrodes are then deposited on this wafer (namely: one on each side) to allow their subsequent connection within an electrical circuit. Before these wafers can be used as piezoelectric elements, they must still undergo strong polarization under the influence of a stationary voltage of the order of one kilovolt per square millimeter.

[0006] Typically, in normal operating conditions, the electronics of such a piezoelectric element are powered by a direct voltage (for example at a voltage of 24 V DC). The excitation signal transmitted to the piezoelectric element is typically composed of a pulse train each composed of individual pulses at a frequency close to the resonant frequency of the piezoelectric element, said pulse trains being spaced apart in time, this spacing being defined by a second frequency and by a duty cycle. This modulation is generated by a frequency generator, which drives the gate of a transistor which produces a periodic square (on / off) signal of determined frequency; this signal is then amplified by a signal amplifier. Said frequency can be variable and adjustable (typically of the order of 1 MHz to 3 MHz). The ratio between the "on" and "off" durations of the pulse trains defines the nebulization power.The on / off ratio of the high frequency (e.g. 1.7 MHz) allows to compose the asymmetry of the electronic components and the vibration response of the piezoelectric wafer taking into account its geometry (including the dimension of the electrodes). The signal at the terminals of the piezoelectric wafer is therefore a sinusoid generated by the power stage as described above. As used in nebulization devices, its amplitude is typically of the order of 70 V.

[0007] The peak-to-peak power of the pulses can be high. For example, an individual pulse train can represent a power of between 30 W and 70 W (this value refers to a continuous regime, i.e. over 100% of the cycles) for an active surface with a diameter of between about 15 mm and about 25 mm; the total power absorbed by the device then depends on the time spacing between two pulse trains.

[0008] When the emitting surface of the piezoelectric element is covered with water, the current consumed by the device increases with the applied voltage, over a useful operating range which is quite wide; at a frequency close to the resonance frequency the piezoelectric element has an essentially resistive operation.

[0009] By way of example, in a typical operating mode for such a piezoelectric wafer with an active surface diameter of between approximately 10 mm and approximately 20 mm, a thickness of approximately 1 mm and a resonance frequency of approximately 1.7 MHz, the control electronics of the piezoelectric element are powered with a voltage of approximately 24 V. The power consumption in continuous mode of the power supply is of the order of 2.3 A. As described in the applicant's document WO 2017 / 093655 cited above, the operating zone of the device is quite wide and extends in this example from approximately 5 V to approximately 40 V; a particularly useful zone is between approximately 15 V and approximately 30 V. In other words, in normal operation, such a wafer absorbs a power of the order of a few tens of Watts, for a surface area of ​​the order of a few square centimeters.As an example, a wafer with a diameter of 20 mm and a surface area of ​​3.14 cm 2 is used, which absorbs a power of approximately 50 W, which corresponds to a power density of approximately 16 W / cm 2 .

[0010] Also of interest is DE 198 20 208 A1, which relates to a piezoelectric transducer of the type used for the propagation of ultrasound, for example in acoustic flow measuring devices or level detection devices. This piezoelectric transducer consists of a piezoelectric ceramic which has a first electrode on a first surface and a second electrode on an opposite surface. An electrodeless edge region, on which a temperature-sensitive component is mounted, is arranged on the first surface of the piezoelectric ceramic. This component is conductively connected to at least one of the electrodes.

[0011] The applicant noted that in industrial nebulizing devices, the service life of the piezoelectric elements is sometimes quite limited. They can be replaced; this is a routine operation, but requires the intervention of a specialized maintenance technician at the site where the nebulizing device is installed (i.e., for example, in a supermarket, if the device is used for nebulizing fresh food products).

[0012] It has been found that the wear of piezoelectric wafers can be reduced by reducing their dissipated power per unit of emitting surface. However, we do not want to reduce fog production: reducing the power consumption of the device is not a viable solution. On the other hand, we can use a wafer with a larger active surface for the same operating power, but this means making the fogging device more bulky and more expensive, which is also not desirable.

[0013] The problem that the present invention seeks to solve is to reduce the failure rate of piezoelectric wafers in service. OBJECTS OF THE INVENTION

[0014] The applicant has realized that a common failure mode is a failure at the level of the contacts between the piezoelectric element and the nebulizing device in which it is integrated.

[0015] Typically, the piezoelectric element comprises a plate of piezoelectric material whose rear face, opposite the acoustic emission face, has at least two electrical contact zones, which have typically been deposited by screen printing or by bonding a metal sheet; one of these zones represents a positive pole, the other a negative pole. These zones may typically comprise a metal layer on which a connection member is fixed. Most often, said connection member is an electric wire which has been soldered onto said metal layer. Here, a "soldering zone" is a fixing point by which an electrical connection member, such as an electric wire, is fixed onto a metal layer.This attachment point is most often achieved by a suitable soldering technique; the term "soldering area" as used here also encompasses bonding points, in the case where the connection member has been fixed to the metal layer using a conductive adhesive. These techniques for connecting an electrical wire to a conductive surface are known to those skilled in the art, and will not be explained here in greater detail. (The term "welding" is sometimes used, improperly, to designate what is in reality soldering).

[0016] The applicant has realized that in many cases, it is at this soldering area that the device shows early failures, and more specifically, this failure is observed more often at the soldering area on the phase electrode than on the ground electrode. Without wishing to be bound by this theory, the inventors believe that the prolonged passage of a high intensity of high-frequency pulsed current through a soldering area on a vibrating wafer weakens said soldering area, which leads to a defect in the electrical contact between the soldering area and the metal layer deposited by screen printing. This defect may or may not be visible; it may be small cracks or a clear break. The end result is that the nebulizing device no longer functions.This failure mode accounts for approximately half of the early failures of piezoelectric wafers that the applicant has observed over the years in thousands of nebulization installations.

[0017] Given that said piezoelectric wafers are devices that are commonly found in specialist shops, but whose price increases with their diameter, it is neither desirable to enlarge them nor to modify their shape or structure. The applicant has also not succeeded in solving this problem by seeking to optimize the soldering process of the contact elements; nor by improving the system for protecting the soldering area against humidity or mechanical stress.

[0018] According to the invention, the problem is solved in a very simple way by modifying the electrical contacts.

[0019] Thus, a first subject of the invention is a piezoelectric element for a device for emitting acoustic waves, said piezoelectric element comprising a plate made of a piezoelectric material, of substantially cylindrical shape, with a circular diameter preferably between approximately 5 mm and approximately 100 mm and with a thickness preferably between 0.4 mm and 2.0 mm, said plate comprising on a first face, called the "front face", which is the face for emitting acoustic waves, a first layer of metallic conductivity capable of acting as a first electrode, forming a disc and extending over at least 50%, preferably at least 70%, and even more preferably at least 80% of the surface area of ​​said front face, and said wafer comprising on a second face, called "rear face", a second layer of metallic conductivity capable of acting as a second electrode and extending over at least 10%, preferably at least 20% and even more preferably at least 50% of the surface area of ​​said rear face, said piezoelectric element being characterized in that: said second electrode forms a disc comprising, on said second face, at least two second extension zones each extending in a radial direction. The piezoelectric element according to the invention comprises at least three electrical connection elements, namely a wire fixed on each of the two second extension zones (or on at least two of the second extension zones), and a wire fixed on said first electrode, preferably on a soldering zone.More particularly, it may comprise at least three electrical connection elements, namely a wire fixed to each of the two second extension zones, and a wire fixed to said first extension zone.

[0020] According to preferred embodiments: Said cylindrical wafer has a circular diameter of between about 10 mm and about 50 mm; Said cylindrical wafer has a thickness of between about 0.5 mm and about 2.0 mm, and preferably between about 1 mm and about 2 mm; Said second metallic conductivity layer capable of acting as a second electrode extends over at least 60% of the surface area of ​​said rear face.

[0021] Typically, said wafer further comprises a first extension zone of said first electrode which extends over the edge of said wafer as well as over a peripheral portion of said second face.

[0022] According to other aspects of the invention, the piezoelectric element according to the invention comprises, on each of the extension zones of said second electrode, a brazing zone by which an electrical connection element such as a wire is fixed.

[0023] According to another aspect of the invention, the piezoelectric element further comprises a first protective coating applied to the front face, and a second protective coating applied to the rear face, each of these protective coatings covering said metallic conductivity layers and, where appropriate, said soldering zones.

[0024] The piezoelectric element according to the invention may comprise electrical connection elements, which are all fixed on the rear face of said wafer.

[0025] Advantageously, the piezoelectric element according to the invention may also comprise a flexible peripheral seal capable of protecting the edge of said wafer; this facilitates its integration into a nebulization device.

[0026] Another object of the invention is the use of a piezoelectric element according to any one of the embodiments or variants of the invention in a nebulization device. FIGURES

[0027] There figure 1 relates to the state of the art, whereas the figures 2 to 5 illustrate embodiments of the invention. [ Fig. 1 ] schematically shows a sequence of steps for manufacturing a piezoelectric wafer according to the state of the art. [ Fig. 2 ] schematically shows the rear face of a piezoelectric element according to a first embodiment of the invention. Fig. 3] schematically shows two views of a piezoelectric element according to a first embodiment of the invention, namely a front view of the rear face (bottom) and a cross-section (top). [ Fig. 4 ] schematically shows the rear face of a piezoelectric element according to a second embodiment of the invention. Fig. 5 ] schematically shows the rear face of a piezoelectric element according to another embodiment of the invention.

[0028] The following reference numerals are used in the figures and in the description that follows: 1;21;41;61Piezoelectric wafer 2;42First electrode (ground) 3;23,43Extension area of ​​the first electrode 4;24;44Second electrode (phase) 5First protective coating 6Stationary high voltage source (DC) 7,8First and second connecting wire 9,10First and second soldering area 11Second protective coating 12Printing of the reference 28,58Electrical connecting wire (ground) 33,34;52,53,54Extension areas of the second electrode 35,36;55,56,57Electrical connecting wire (phase electrode) 62Gasket 63Tube DETAILED DESCRIPTION

[0029] There figure 1 illustrates the state of the art and schematically shows the manufacture of a piezoelectric element ready to be integrated into a nebulization device from a wafer manufactured by sintering a powder of a piezoelectric material.

[0030] A wafer 1 is supplied with a piezoelectric material, a first electrode 2 (in this case the ground electrode) is deposited on a first face (here called the "front face"), an extension zone 3 of said first electrode 2 is deposited on the edge of the wafer 1 (this extension zone being commonly called "wrap-up electrode"), a second electrode 4 (in this case the phase electrode) is deposited on the second face (here called the "back face") of the wafer, and a first protective coating 5 is deposited on said first electrode.

[0031] In the advantageous embodiment of the state of the art shown in this figure 1 , said extension zone 3 of said first electrode extends over a peripheral portion of said second face. This makes it possible to make contact between the first electrode and the second face of the wafer.

[0032] Then the piezoelectric plate is polarized by applying a high stationary voltage 6 (of the order of kilovolts per millimeter). Then we fix (typically by brazing) on ​​the second electrode 4 as well as on the extension zone 3 from the first electrode a first and second connecting wire 7 , 8 , knowing that it is advantageous that the soldering points 9,10 of these two connecting wires 7 , 8 are located on the same side of the plate 1 This makes wiring easier when the piezoelectric wafer is mounted in a nebulizing device, and it avoids having to run a connecting wire through the water while ensuring a good seal of said nebulizing device.

[0033] Then we deposit on the second electrode 4 as well as on the extension zone 3 a second protective coating11 ; this coating also covers the soldering areas 9 , 10 Manufacturing typically ends with the writing of the product reference. 12 on one of the protective coatings; this can be done by inkjet.

[0034] As for the said protective coatings, it is advantageous to use a high-temperature annealed varnish or glass on the front side. However, this cannot be done on the back side because the high temperature would depolarize the ceramic that was polarized just after the annealing of the front side varnish; therefore, a simple varnish is applied to the back side that dries or crosslinks at room temperature or at a very moderate temperature.

[0035] This piezoelectric element is then ready for use in electrical devices as an ultrasonic transmitter. The ultrasound is emitted from the front side, the rear side being used for the electrical connection. As can be seen in the figure 1 , said first and second electrodes each have the shape of a substantially circular disc.

[0036] As indicated above, it is at the level of the electrical contacts that the invention brings a modification to the structure of the device and to its manufacturing method. figure 2 schematically illustrates an embodiment of the invention. It shows the rear face of a piezoelectric wafer 21 with its second electrode 24 (in this case the phase) and the extension zone 23of the first electrode (deposited on the front face of the wafer, not visible in the figure) which has been deposited on the edge of the wafer. Said extension zone of the first electrode serves as a contact zone for a first connection wire.

[0037] According to the invention, said second electrode 24 forms a disc which has at least two extension zones 33,34 on the second side. Said extension zones 33,34 each extend in a substantially radial direction. On each of these extension zones 33,34 an electrical connection wire is attached 35 , 36.Said second electrode is the phase. Thus, the current of the phase electrode is distributed over two connections. The applicant has found that this reduces the failure rate of the connections as well as the number of failed connections. This is surprising since in the piezoelectric element according to the invention, the number of electrical connections is increased, which would be likely to increase the number of failures. It seems that the failure rate of piezoelectric elements is directly linked to the amount of current (expressed in Amperes) which passes through a soldering point. It is found that for a piezoelectric element according to the invention, the average lifetime is increased by approximately 30% due to the reduction in the current of the phase electrode through the connections; this figure was established as an average value for a significant number of wafers.

[0038] Furthermore, it is observed that in the case where in the piezoelectric element according to the invention a connection becomes faulty, the presence of a second connection point on the phase electrode prolongs the lifetime of this piezoelectric element.

[0039] The applicant observed that this effect of the number of connection points on the lifetime of the piezoelectric element is only shown for the phase electrode. Failures of the connection to the ground electrode are in any case rare. Compared to the constraint of having to duplicate the positive connections (which generates additional costs during the manufacture of the wafer and during its wiring), the duplication of the ground connections is not of significant interest.

[0040] The back side of the plate shown in the figure 2 can then receive a coating, as explained in relation to the figure 1 .

[0041] In a particular embodiment, as shown in the figure 2 , the extension zones 33,34 are located in opposite positions on the phase electrode 24 (for example at radial positions which differ by about 140° to 220°, and more especially by about 180°, as in the figure). The latter advantageously (and in a manner known as such) has the shape of a circular disc in the case where the piezoelectric plate 21 has a circular disc shape. In order to be able to give as large a surface area as possible to the phase electrode 44 , the connection point 33 closest on this phase electrode relative to the extension zone 23 of the ground electrode is offset from the latter (for example at approximately 45°).

[0042] In another embodiment, shown in the figure 3, the extension areas are located at radial positions of approximately 90° (± 30°) and 180° (± 30°) relative to the extension area of ​​the ground electrode. The embodiment shown in this figure has two other advantageous features which are compatible with all embodiments of the present invention. The wafer 61 , after receiving the protective coatings (not shown in this figure) on the first and second electrode, is surrounded by a flexible seal 62 , for example made of silicone. This seal protects the wafer edge, and it can also serve as a sealing element in the device (for example a nebulizing device) in which said piezoelectric element is incorporated.

[0043] Furthermore, a tube 63flexible or semi-rigid unites the three electrical wires to prevent their uncontrolled movement which can lead to their tearing. In the embodiment according to the figure 3 , this tube 63 ensures the passage of wires through the joint.

[0044] In another embodiment of the invention, shown in the figure 4 , three connection points are used on the phase electrode 44 . This second electrode 44 therefore presents three extension zones 52 , 53 , 54. Compared to the embodiment with two connections of the figure 2 , no additional reduction in the failure rate that would be significant was observed; the additional cost of this solution is only justified in specific cases.

[0045] In yet another embodiment of the invention, shown in the Figure 5 , the soldering areas 35a , 36aelectrical connection wires on the extension areas 33 , 34 of the second electrode are located very close to the edge of the piezoelectric plate 21 , and preferably the center of said brazing zones 35a,36a is located at a distance of less than 10% of the plate diameter 21 , and preferably at a distance of less than 5% of the wafer diameter 21 , or at a distance of less than 3 mm, and preferably at a distance of less than 2 mm. The extension zones 33,34 on which these brazing areas are brought 35a , 36a can be very close to the edge of the wafer 21 , or may even touch the edge of the wafer 21 , as can be seen on the Figure 5 . It is understood that these brazing areas 35a , 36a fix the connecting wires 35 , 36of the phase electrode. Optionally, the same arrangement can be chosen for the center of the brazing area 28a of the connecting wire 28 of the ground electrode, as shown in the Figure 5 , and in this case the extension zone 23 of the first electrode can be very close to the edge of the wafer 21 , or may even touch the edge of the wafer 21 .

[0046] The inventors found that positioning the solder areas as close to the edge of the wafer as possible decreased the failure rate of solder connections; this is probably related to the fact that the further away from the center of the wafer, the lower the vibration amplitude.

[0047] This embodiment can also be implemented in the case where three phase wires are used, as in the figure 4 .

[0048] The invention applies in particular to nebulization systems intended to generate a so-called dry mist with a droplet size typically between approximately 1 µm and approximately 10 µm. It is advantageously produced with piezoelectric elements having the shape of a cylindrical plate, the circular diameter of which is between approximately 10 mm and approximately 30 mm, preferably between approximately 12 mm and approximately 25 mm, and even more preferably between 12 mm and 25 mm. Their thickness determines their resonance frequency; it is advantageously between approximately 0.5 mm and approximately 2 mm, preferably between approximately 0.8 mm and approximately 1.3 mm. In an advantageous embodiment, the plate has a circular diameter of between 15 mm and 25 mm and a thickness of between 0.5 mm and 2.0 mm, and preferably between 0.6 mm and 1.7 mm.

[0049] For example, for a thickness of approximately 1 mm the resonant frequency is of the order of 1.7 MHz. For a greater thickness the resonant frequency decreases, while for a smaller thickness it increases. Such a piezoelectric element with a diameter of 20 mm can absorb a current which is typically between approximately 2 A and approximately 2.5 A, the absorbed power being of the order of 40 W to 50 W. Generally speaking, for use in a nebulization device a piezoelectric element with an acoustic emission frequency of between 0.5 MHz and 10 MHz is advantageously chosen.

Claims

1. A piezoelectric element for an acoustic wave emitting device, said piezoelectric element comprising a plate (1, 21, 41, 61) made of a piezoelectric material, of substantially cylindrical shape, preferably with a circular diameter of between 5 mm and 100 mm, and preferably between 10 mm and 50 mm, and preferably with a thickness of between 0.4 mm and 2.0 mm, and even more preferably between 0.5 mm and 2.0 mm, said plate comprising on a first face, referred to as the 'front face', which is the acoustic wave emission face, a first layer of metallic conductivity capable of acting as a first electrode (2, 42), forming a disc and extending over at least 50%, and preferably at least 70%, of the surface area of the said front face, said plate (1, 21, 41, 61) comprising on a second face, referred to as the 'rear face', a second layer of metallic conductivity capable of acting as a second electrode (4, 24, 44) and extending over at least 10%, preferably at least 20%, and even more preferably at least 50% of the surface area of said rear face, and said second electrode (4, 24, 44) forming a disc comprising, on said second face, at least two second extension zones (33, 34; 52, 53, 54) each extending in a radial direction, said piezoelectric element being characterized in that it comprises at least three electrical connection elements, namely at least two wires (35, 36; 55, 56, 57) fixed to at least two of the second extension zones (33, 34; 52, 53, 54), and a wire (28, 58) fixed to said first electrode (2; 42), said electrical connection elements preferably being fixed by a welding zone.

2. A piezoelectric element according to claim 1, characterized in that said plate (1, 21, 41, 61) has a circular diameter of between 12 mm and 25 mm, and a thickness of between 0.5 mm and 2.0 mm, and preferably a circular diameter of between 15 mm and 25 mm and a thickness of between 0.6 mm and 1.7 mm.

3. A piezoelectric element according to claim 1 or 2, characterized in that said plate (1, 21, 41, 61) further comprises a first extension zone (3, 23, 43) of said first electrode (2, 42) which extends over the edge of said plate (1, 21, 41, 61) as well as over a peripheral portion of said second face.

4. A piezoelectric element according to any one of claims 1 to 3, characterized in that it comprises, on each of the extension zones (33, 34; 52, 53, 54) of said second electrode (4, 24, 44), a welding zone by means of which an electrical connection element such as a wire is fixed.

5. Piezoelectric element according to any one of claims 1 to 4, characterized in that the centre of said welding zone (35a, 35b) on said second extension zone (33, 34) is at a distance of less than 10% of the diameter of the plate (21), and preferably at a distance of less than 5% of the diameter of the plate (21), or at a distance of less than 3 mm, and preferably at a distance of less than 2 mm.

6. Piezoelectric element according to any one of claims 3 to 5, characterized in that it comprises at least three electrical connection elements, namely a wire (35, 36; 55, 56, 57) fixed to each of the two second extension zones (33, 34; 52, 53, 54), and a wire (28, 58) fixed to the said first extension zone (3; 23, 43).

7. Piezoelectric element according to any one of claims 3 to 6, characterized in that it also comprises a first protective coating (5) applied to the front face, and a second protective coating (11) applied to the rear face, each of these protective coatings covering said layers of metallic conductivity and, where appropriate, the said welding zones.

8. Piezoelectric element according to any one of claims 1 to 7, characterized in that it comprises electrical connection elements, all of which are fixed to the rear face of said plate.

9. Piezoelectric element according to any one of claims 1 to 8, characterized in that it further comprises a flexible peripheral seal (62) capable of protecting the edge of said plate.

10. Piezoelectric element according to any one of claims 1 to 9, characterized in that it has an acoustic emission frequency of between 0.5 MHz and 10 MHz.

11. Piezoelectric element according to any one of claims 1 to 10, characterized in that said first electrode (2) is ground, and said second electrode (4, 24, 44) is the phase.

12. Use of a piezoelectric element according to any one of claims 1 to 11 in a nebulization device.