Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-03-30
2001-11-13
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S359000, C310S366000
Reexamination Certificate
active
06316865
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a piezoelectric element having a first and a second stack containing a plurality of piezoelectric ceramic layers and having an intermediate layer disposed between the first and second stacks. Electrodes are disposed in each stack between the respective ceramic layers and on the respective outer side of the ceramic layers bounding the stack.
A piezoelectric element of this type is disclosed in Published, Non-Prosecuted German Patent Application DE 35 18 055 A1 and in German Patent DE 34 34 726 C2. In order to operate the piezoelectric element, the electrodes have a voltage applied to them in such a way that the piezoelectric ceramic layers of the first stack respectively expand, while the piezoelectric ceramic layers of the second stack respectively contract, or vice versa. This leads to the bending of the piezoelectric element, which can be utilized to operate a mechanical or electrical switch. The intermediate layer disposed between the two stacks is likewise a ceramic layer, but this itself does not execute any active movement. For this purpose, the ceramic of the intermediate layer is either unpolarized and is consequently not piezoelectrically active, or else it is not subjected to any electric field when the piezoelectric element is driven. The intermediate layer serves to balance out the opposite movements of the two stacks and therefore increases the service life of the piezoelectric element.
A piezoelectric element that contains a large number of ceramic layers stacked on one another is also referred to as a so-called multimorphous piezoelectric element. By contrast with this, a piezoelectric element in which the stacks are replaced by a single coherent ceramic layer is referred to as a bimorphous or as a trimorphous piezoelectric element, depending on whether an intermediate layer is disposed between the two ceramic layers or not. A multimorphous piezoelectric element offers the advantage, by comparison with a bimorphous or trimorphous piezoelectric element, that the same mechanical energy is provided at a lower applied voltage. The reason for this is that the individual ceramic layers of a multimorphous piezoelectric element have a significantly lower thickness than the ceramic layers of a comparable bimorphous or trimorphous piezoelectric element, so that at the same voltage a higher electrical field is established in accordance with E =U/d, E indicating the electrical field, U the applied voltage and d the thickness of the ceramic layer.
Moreover, Published, Japanese Patent Application JP 60-178677 A2 discloses a piezoelectric multimorphous element, the material of the intermediate layer differs from the material of the ceramic layers.
However, because of the many, comparatively thin individual ceramic layers stacked on one another, a multimorphous piezoelectric element disadvantageously exhibits a high susceptibility to fracture and a comparatively low mechanical stability. Therefore, under load, mutual drift of the individual layers often occurs. Furthermore, a multimorphous piezoelectric element, by comparison with a bimorphous or trimorphous piezoelectric element, exhibits a lower efficiency. In addition, its production entails higher costs.
For the aforementioned reasons, hitherto preference was given to a bimorphous or trimorphous piezoelectric element in technical applications, although a multimorphous piezoelectric element may be operated with a lower voltage. Technical applications for a piezoelectric element are, for example, as a piezoelectric printing head for an ink jet printer, as a sound pick-up or generator for a microphone or a loudspeaker, as a sensor for measuring acceleration or pressure, as a measured value pick-up for gas meters and in particular as a flexural transducer, for example as an actuating element in Braille lines, in readers for blind persons, in textile machines, in pneumatic valves, in recording measuring instruments or in non-contacting surface measuring instruments and so on. In this case, a low operating voltage would permit the use of the piezoelectric element even in locations at risk of explosion, such as in the chemical industry or in mining.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a piezoelectric element which overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which a multimorphous piezoelectric element which, by comparison with a bimorphous or trimorphous piezoelectric element, has the advantage of a lower operating voltage.
With the foregoing and other objects in view there is provided, in accordance with the invention, a piezoelectric element, including:
two stacks, including a first stack and a second stack, each of the stacks having a plurality of piezoelectric ceramic
layers and electrodes disposed between the ceramic layers and on an outer side of the ceramic layers bounding each of the stacks, the ceramic layers formed of a given material; and
an intermediate layer disposed between the first stack and the second stack, the intermediate layer formed as a supporting body made of a material selected from the group consisting of a fiber composite material and glass, the material of the intermediate layer differing from the given material of the ceramic layers.
The fact that the intermediate layer is formed as a supporting body made of a fiber composite material or glass provides a multimorphous piezoelectric element made of a material composite which, by comparison with a multimorphous piezoelectric element which is purely ceramic, apart from the electrodes, has a significantly higher mechanical stability. An intermediate layer of this type serves not only to equalize stresses but in addition performs a load-bearing and supporting function for the entire piezoelectric element.
The use of a supporting body made of the fiber composite material or glass for a multi-morphous piezoelectric element also results in that the efficiency of the conversion of electrical into mechanical energy is improved. Replacing part of the ceramic material of a multimorphous piezoelectric element by such a supporting body significantly reduces the susceptibility to fracture of the piezoelectric element, which offers an inestimable advantage, in particular during installation or during operation. In addition to this, production costs are saved in this way, since piezoceramic material, in particular the multi-layer material of a multimorphous piezoelectric element, is comparatively expensive.
The invention provides a multimorphous piezoelectric element which may be driven with low voltages, has a high efficiency, has a low susceptibility to fracture with, at the same time, a high mechanical stability and, because of the improved efficiency, manages with less of the comparatively expensive ceramic multi-layer material than a comparable purely ceramic multimorphous piezoelectric element. A ceramic multi-layer material in this case refers to that material which is composed of ceramic layers (polarized or not) stacked one above another and electrodes disposed in between, for example in the form of a metalization.
A saving in ceramic multi-layer material, with simultaneous enhancement of the mechanical stability and of the efficiency of the piezoelectric element, may advantageously be achieved by the thickness of the supporting body being selected to be greater than the thickness of an individual ceramic layer in a stack or in the ceramic multi-layer material. In this connection, it has been shown in particular that the piezoelectric element has excellent properties if the thickness of the supporting body is essentially equal to the respective thickness of a stack. In this way, in particular by comparison with a purely ceramic multimorphous piezoelectric element, it is possible to save up to one third of the material costs for the ceramic multi-layer material.
During production, the fiber composite material is used in the form of a so-called prepreg (a not yet cured, soft, pre-impregn
Budd Mark O.
Greenberg Laurence A.
Lerner Herbert L.
Siemens Aktiengesellschaft
Stemer Werner H.
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