Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1998-11-23
2003-05-27
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S330000, C310S332000, C310S364000
Reexamination Certificate
active
06570300
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a piezoelectric bending transducer with an electrically insulating substrate that carries a piezoelectret and to a method for producing such a piezoelectric element. The invention, in particular, is concerned with the problem of ensuring reliable electrical contacting for a piezoelectric element with an insulating substrate, in particular in the context of severe mechanical loading.
Bending transducers of the type with which the invention is concerned serve—hereafter referred to, more generally, as piezoelectric elements—principally utilize the indirect or reciprocal piezoelectric effect, i.e., the conversion of electrical energy into mechanical energy. However, such piezoelectric elements are still also suitable for converting mechanical into electrical energy. The latter case utilizes the direct piezoelectric effect.
There is a multiplicity of technical applications for a piezoelectric element as described above. Such applications are, for example, as a piezoelectric printhead for an ink-jet printer, as a sound receiver or sound generator for microphones or loudspeakers, as a sensor for measuring acceleration or pressure, as an actuator in Braille lines in reading units for the visually impaired, in textile machines, in pneumatic valves, in recording measuring instruments or in non-contact surface-measuring instruments and so on.
The piezoelectric elements in accordance with U.S. Pat. No. 5,126,615 (EP-0 455 342 B1), and U.S. Pat. Nos. 5,210,455 and 5,681,410 (EP-0 468 796 A1), issued to Takeuchi et al., are formed of a layered structure. There, a piezoelectrically active material which exhibits the piezoelectric and/or the electrostrictive effect and is to be designated below as a piezoelectret, is applied to a carrier or substrate in order to improve the mechanical stability of the element or for the purpose of more effectively converting electrical energy into mechanical energy or vice versa. The piezoelectret is electrically contacted on both sides with electrodes in the form of a planar coating made from a conductive material. Depending on the application, the substrate can be provided on one or two sides with the layer sequence described. According to German patent DE-34 34 726 C2, it is also possible for a plurality of plies of piezoelectrets, including the electrodes, to be stacked one above the other. Depending on the number of the piezoelectrically active layers, the art refers to a monomorph, a bimorph, a trimorph, and so on, or, in general, to a multimorph piezoelectric element.
Piezoelectric elements with a non-conducting or electrically insulating substrate are disclosed in German patent DE-43 37 265 C1 and in German published patent application DE-40 25 436 A1. In the case of a bimorph piezoelectric element, it is possible by using an insulating substrate to connect the sides of the piezoelectrets which face the substrate, or the inner electrodes located thereupon to a different potential. It is thereby possible to drive such a piezoelectric element exclusively via the internal sides of the piezoelectrets which are insulated with respect to one another by the substrate, or via the inner electrodes, while the exterior sides or the outer electrodes are connected to zero potential or frame. The result is a shockproof piezoelectric element. However, non-conducting materials can be just as suitable for the substrate, because of specific properties other than the properties of electrical insulation, for example because of a coefficient of thermal expansion matched to the piezoelectret for the purpose of avoiding thermal stresses.
The piezoelectric element with an electrically insulating substrate is also shown in the post-published German application DE 195 20 796 A1 (not prior art). The substrate of that disclosure is provided with a metallization layer which leads from the edge of the substrate to the opposite side. This causes the inner electrodes of the piezo-ceramic elements carried on the two sides of the substrate to be pulled to the same electrical potential.
A non-conducting substrate naturally leads, however, to difficulty in making contact on the side facing the substrate, i.e., the inner side of the piezoelectret. It is known from DE-40 25 436 A1 in this regard to provide the inner side of the piezoelectret with an inner electrode in the form of a layer of metallization, to lead the metallization out beyond the edge of the piezoelectret, and to make contact with it there with the aid of a solder contact.
However, in the case of a piezoelectric element having a substrate made from an electrically insulating material, making contact with the inner side of the piezoelectret is ensured exclusively via the material of an inner electrode. A rupture of the inner electrode therefore disconnects a part of the piezoelectric element from the voltage supply for the inner electrode. The piezoelectric element is no longer serviceable. Since the inner electrode is rupture-prone, the service life of a piezoelectric element having an insulating substrate cannot compete with the service life of a piezoelectric element having a conductive substrate.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a piezoelectric bending transducer and a production method, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which, with regard to the device, provides a substrate of electrically insulating material that remains completely serviceable (operationally functional) even in the case of material fatigue of an inner electrode. It is also the object of the invention to specify a method for producing such a piezoelectric element.
With the foregoing and other objects in view there is provided, in accordance with the invention, a piezoelectric bending transducer, comprising:
a substrate of electrically insulating material;
a piezoelectret carried on the substrate, the piezoelectret having an inner electrode towards the substrate; and
an electrically conductive coating between the inner electrode and the substrate forming an electrical contact at a plurality of locations with the inner electrode, wherein the conductive coating does not extend to a side of the substrate opposite from the piezoelectret.
In other words, the above-noted objects with regard to the piezoelectric element are satisfied by virtue of the fact that the substrate is provided with an electrically conductive coating which is in electrical contact at a number of sites with an inner electrode of a piezoelectret applied to the substrate and that the coating does not extend to the distal side of the substrate.
The invention proceeds in this case from the knowledge that, in the case of a piezoelectric element having a substrate, material fatigue, for example due to constant mechanical loading, leads during operation to rupturing or cracking of an inner electrode located between the substrate and the piezoelectret. Such material fatigue of the inner electrode occurs frequently, since the material of the inner electrode is additionally loaded by compressive or thermal stresses at the interface between the piezoelectret and the substrate. If the substrate consists of an electrically conductive material, such a rupture does not lead to functional failure of the piezoelectric element, since contacting continues to be ensured via the substrate. This requires only that the substrate makes electrical contact with the inner electrode at a sufficient number of sites. Such material fatigue of the inner electrode is therefore not observed at all in the case of a conductive substrate.
If, on the other hand, the substrate is electrically non-conducting, there is inserted between the substrate and the inner electrode of the piezoelectret an electrically conductive coating which is in electrical contact with the inner electrode at a number of sites. Contacting of the inner electrode is ensured in addition by the coating. The piezoelectric element remains serviceable
Riedel Michael
Sestak Hans-Jürgen
Budd Mark O.
Greenberg Laurence A.
Mayback Gregory L.
Siemens Aktiengesellschaft
Stemer Werner H.
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