Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2002-10-10
2004-07-13
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S310000, C310S331000
Reexamination Certificate
active
06762536
ABSTRACT:
FIELD OF THE INVENTION
The invention generally relates to a piezocerainic bending transducer having a support body and having a stack of layers. Preferably, the layers include piezoceramic and planar electrodes arranged between the layers, wherein the stack is applied to the support body. The invention also generally relates to the use of a bending transducer of this type.
BACKGROUND OF THE INVENTION
A piezoceramic bending transducer is known from DE 293 918 A5 and WO 99/17383. According to WO 99/17383, to actuate the piezoceramnic bending transducer, the electrodes arranged between the layers of piezoceramic are alternately set to positive and negative potential, as seen in the stacking direction. Respectively adjacent layers of piezoceramic are polarized in opposite directions, so that the stack as a whole, when the operating voltage is applied, undergoes either a contraction or an expansion on account of the piezoelectric effect of the piezoceramic.
Further possible ways of actuating a stack comprising layers of piezoceramic of this type are known from DE 34 34 726 C2.
Furthermore, DE 34 34 726 has disclosed, as material for the piezoceramic in the layers, lead titanate, barium titanate, lead zirconium titanate or modifications of the ceramic substances. As material for the support body, DD 293 918 A5 has disclosed spring steel, and WO 97/17383 has disclosed a fiber composite material or glass. The support body made from a fiber composite material or from glass leads to a good efficiency for the conversion of electrical energy into mechanical energy.
A piezoelectric bending transducer with a support body is generally constructed as what is known as a trimorph. By this, the support body is coated on both sides with, in each case, at least one piezoelectrically active layer of piezoceranric. On account of the symmetrical structure, the temperature-induced internal bending of a piezoceramic bending transducer of this type is lower than if the support body were to be coated on only one side.
If a stack including a large number of piezoceramlc layers is used instead of a single piezoceramic layer, the same mechanical energy is provided at a lower operating voltage. The reason for this is that, on account of the low thickness of the individual piezoceramic layers in a stack, at the same operating voltage in accordance with E=U/d, where E is the electric field, U is the applied voltage and d is the thickness of the ceramic layer, a greater electrical field strength results than if a single layer of the same thickness of the stack is used. Building up the piezoelectrically active substance in the form of a stack with a large number of individual layers of piezocerarnic, i.e. in multilayer technology, is advantageous if short displacement paths and high displacement forces are required for the piezoceramic bending transducer.
For the latter reason, piezoceramic bending transducers of stacked or multilayer design are preferred in particular for applications in a valve. However, a drawback is that the production and materials costs for a piezoceramic bending transducer of multilayer design are relatively high. The piezoceramic layers have to be drawn as films; a large number of individual electrode layers are required, and this fact increases the materials costs (AgPd). Accordingly, when a piezoceramic bending transducer of multilayer design is used, a valve, despite having better actuation properties, would not be competitive compared to a similar valve of conventional design, on account of the high unit price.
SUMMARY OF THE INVENTION
Therefore, it is an object of an embodiment of the invention to provide a piezoceramic bending transducer of multilayer design which can be produced at low cost. A further object of an embodiment of the invention is to provide a use for a piezoceramic bending transducer of this type.
For a piezoceramic bending transducer, an object may be achieved, according to an embodiment of the invention, by the fact that a matching layer including a material which has substantially the same coefficient of thermal expansion as the piezoceramic bending, may be applied to that side of the support body which is remote from the stack.
An embodiment of the invention is based on the consideration that, when the piezoceramic bending transducer is used in a valve, only two defined positions of the bending transducer are required. In one defined position of the bending transducer, the valve must be closed, and in the other defined position of the bending transducer, the valve must be open. There is no need for a further, third defined position of the bending transducer. Depending on the way in which the bending transducer is actuated, one speaks of a normally open valve, in the valve is open when the bending transducer is not actuated, and of a normally closed valve, if the valve is closed when the bending transducer is not actuated.
Furthermore, an embodiment of the invention is based on the consideration that the two positions of the piezoceramic bending transducer which are required in order to control a valve are defined by its at-rest position when voltage is not applied and by a diverted position when voltage is applied. Accordingly, all that is required is for the bending transducer to deviate in one direction. Therefore, for a bending transducer which is used in a valve, it is sufficient for the stack including layers of piezoceramic, referred to below as the piezostack, to be applied to one side of the support body. This is because a second piezostack, which is actuated in the opposite direction to the polarization direction, makes only a small contribution to the deviation, since the field strength has to be limited on account of dipolarization effects. Accordingly, one piezostack can be dispensed with without restricting the performance of the bending transducer for use in valves. This represents a cost-cutting measure, since the production of a piezostack, comprising a large number of individual piezoceramic layers with electrodes between them, is expensive. Furthermore, an embodiment of the invention may be based on the consideration that a piezoceramic bending transducer having a support body and a piezostack applied to one side thereof, on account of its asymmetrical structure, has a higher thermally induced internal bending than a bending transducer having a support body and piezostacks applied to both sides of this body, and to this extent would be unsuitable for use in a valve. This problem is solved by the fact that a matching layer including a material with substantially the same coefficient of thermal expansion as the piezoceramic is applied to that side of the support body which is remote from the stack.
The matching layer advantageously includes a glass or an aluminum oxide. These two materials have a similar coefficient of thermal expansion to the lead zirconate titanium oxide ceramic which is customarily used as piezoceramic.
A piezoceramic generally acquires its piezoelectric properties through being polarized in a homogeneous electrical field. A change in the coefficient of thermal expansion of the piezoceramic is associated with the polarization. In a further advantageous configuration of an embodiment of the invention, therefore, the matching layer includes a polarized piezoceramic, in order to compensate for the thermally induced internal bending of the bending transducer. In this case, the coefficient of thermal expansion of the matching layer is identical to the coefficient of thermal expansion of the individual layers of piezoceramic in the stack which has been applied to the other side of the support body. In this case, the matching layer consists of a monolithic polarized piezoceramic, i.e. of a single layer of piezoceramic.
By way of example, the material used for the support body may be glass, metal or a fiber composite material. However, with a view to ease of processing and to achieving a permanent bond between piezoceramic and support body, it has proven advantageous for the support body to consist of a fiber composite materia
Hoffmann Markus
Lubitz Karl
Riedel Michael
Weinmann Michael
Dougherty Thomas M.
Harness & Dickey & Pierce P.L.C.
Siemens Aktiengesellschaft
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