Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-11-02
2003-04-15
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S331000
Reexamination Certificate
active
06548939
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a piezoelectric bending transducer with a support consisting of a thermosetting material that is reinforced with fibers and with a piezoceramic coating that is thermobonded to at least one side of the support.
A bending transducer of this type is known for example from U.S. Pat. No. 5,404,067 (European patent EP 0 542 772 B1), German patent DE 196 20 826 B2, U.S. Pat. No. 4,363,993 (German patent application DE 30 46 535 A1). In U.S. Pat. No. 5,404,067 (EP 0 542 772 B1) and U.S. Pat. No. 4,363,993 (DE 30 46 535 A1), graphite is mentioned as the fiber material. In DE 196 20 826 C2, glass is specified as a further fiber material. The piezoceramic is, for example, a lead-zirconate-titanium ceramic.
A piezoelectric bending transducer of the type stated at the beginning serves primarily for utilizing the indirect or reciprocal piezoelectric effect, i.e. for the conversion of electrical energy into mechanical energy. There are many technical applications for a bending transducer. Such applications are, for example, as a piezoelectric printing head for an inkjet printer, as an acoustic pickup or generator for microphones or loudspeakers, as a sensor for acceleration or pressure measurement, as an adjusting element in lines of Braille in reading devices for the visually impaired, in textile machines, in pneumatic valves, in recording instruments or in contactless surface measuring instruments.
According to U.S. Pat. No. 5,126,615 (European patent EP 0 455 342 B1) and U.S. Pat. No. 5,210,455 (European patent application EP 0 468 796 A1), a bending transducer is composed of a layered structure. The piezoceramic is in this case applied to a support to improve the mechanical stability or for the purpose of better conversion of electrical energy into mechanical energy. For the electrical bonding, the piezoceramic is provided, if required, on both sides with electrodes in the form of a sheet-like covering consisting of a conductive material.
Depending on the application, the support may be provided on one or both sides with the sequence of layers described.
According to U.S. Pat. No. 4,649,313 (German patent DE 34 34 726 C2), a plurality of layers of piezoceramics, including the electrodes, may also be stacked one on top of the other. Depending on the number of piezoceramic layers, they are referred to as mono-, bi-, tri-, etc. or generally multimorph piezoelectric bending transducers.
Piezoelectric bending transducers each with an electrically insulating support are to be found in German published patent DE 43 27 265 C1 and U.S. Pat. No. 5,404,067 (German patent application DE 40 25 436 A1). The use of an electrically insulating support in the case of a bimorph piezoelectric bending transducer allows the sides of the piezoceramics facing the support, or the internal electrodes located on them, to be connected to different potentials. As a result, the activation of the bending transducer can take place exclusively via the inner sides of the internal electrodes, insulated with respect to one another by the support, while the external electrodes are connected to zero potential or ground. This measure makes a piezoelectric bending transducer of this type safe to touch.
The use of a thermosetting material reinforced with fibers as a substrate offers.the advantage of easy and unproblematical processing. For example, the thermosetting material can be processed in the form of what is known as a prepreg (a not yet cured, soft blank containing preimpregnated fibers). The prepreg is played together with the piezoceramics loosely into a corresponding mold. By slight pressing, the prepreg wets the surfaces of the piezoceramics or the electrodes applied to them and adhesively bonds with them as a result. A final heat treatment then causes the prepreg to cure irreversibly to form the thermosetting material and a durable and stable bond of the components of the bending transducer is obtained in a simple way. If a support consisting of metal is used, there are huge difficulties with the construction of the bending transducer. For instance, the cutting to length of such a bending transducer involves major problems in particular, on account of the piezoceramic-metal material pairing. The problems do not occur if a thermosetting material reinforced with fibers is used.
If a bending transducer which is safe to touch is required, graphite is ruled out as the fiber material on account of its electrical conductivity. It is therefore necessary here to resort to glass and consequently, for the support, to a thermosetting material reinforced with glass fibers. With glass fibers, however, a limited deflection of the bending transducer is obtained. A bending transducer with a support consisting of a thermosetting material reinforced with glass fibers exhibits a smaller deflection under the same operating voltage than a bending transducer with a support consisting of a plastic reinforced with graphite fibers.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a piezoelectric bending transducer, which overcomes the abovementioned disadvantages of the heretofore-known devices and methods of this general type and which exhibits improved deflection capabilities in comparison with comparable priorart bending transducers with supports consisting of a thermosetting material reinforced with fibers.
With the foregoing and other objects in view there is provided, in accordance with the invention, a piezoelectric bending transducer, comprising:
a support formed of a thermosetting material reinforced with fibers of a fiber material; and
a piezoceramic coating thermobonded to at least one side of the support;
the support extending in a longitudinal direction and the fibers extending substantially unidirectionally and parallel to the longitudinal direction, and the fiber material having a coefficient of thermal expansion of less than −0.5·10
−6
/K.
In other words, the object is achieved according to the invention for a bending transducer of the type stated at the beginning by the fact that the fiber material has a coefficient of thermal expansion of less than −0.5·10
−6
/K and the fibers all extend in the longitudinal direction of the transducer support.
The invention is based here on the idea that the better deflection capabilities of a bending transducer with a support consisting of a thermosetting material reinforced with graphite fibers in comparison with a bending transducer with a support consisting of a thermosetting material reinforced with glass fibers have to do with the coefficient of thermal expansion of the fiber material. For instance, glass exhibits a coefficient of thermal expansion of between 5 and 6·10
−6
/K. Graphite, on the other hand, exhibits a coefficient of thermal expansion of approximately −0.5·10
−6
/K. While glass consequently has a positive coefficient of thermal expansion, graphite exhibits a negative coefficient of thermal expansion.
Furthermore, the invention is based on the idea that a prepreg is generally used for the production of a piezoelectric bending transducer with a support consisting of a thermosetting material reinforced with fibers. As mentioned at the beginning, the support is permanently bonded with the piezoceramic by the prepreg being placed onto the piezoceramic and subsequently compressed by pressure and thermobonded during a heat treatment involving curing of the thermosetting material. Since the piezoceramic has a positive response to temperature changes perpendicularly to the direction of polarization, the piezoceramic is prestressed to a certain extent during the heat treatment if fibers with a negative response to temperature changes are used. The prestress of the piezoceramic is also retained after the heat treatment, since the thermosetting material cures as this happens. Initial investigations indicate that the distortion of the lattice structure of the piezoceramic caused by the prestress acts in a way supporting polarization. T
Dougherty Thomas M.
Mayback Gregory L.
Siemens Aktiengesellschaft
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