Metal working – Piezoelectric device making
Reexamination Certificate
1998-10-19
2001-07-17
Young, Lee (Department: 3729)
Metal working
Piezoelectric device making
C029S851000, C310S323060, C310S331000
Reexamination Certificate
active
06260248
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a monolithic piezoelectric actuator of multilayer design which has a high aspect ratio of more than two and which is built up from a plurality of smaller stacks in a multilayer design.
Piezoelectric actuators normally include a plurality of piezoelectric elements arranged in a stack. Each of these elements, in turn, includes a piezoceramic layer which is provided on both sides with metallic electrodes. If a voltage is applied to these electrodes, the piezoceramic layer reacts with a lattice distortion which leads to a usable lengthwise expansion along a major axis. Since this amounts to less than two parts per thousand of the layer thickness along the major axis, a correspondingly higher layer thickness of active piezoceramic must be provided in order to achieve a desired absolute lengthwise expansion. With increasing layer thickness of the piezoceramic layer within one piezoelectric element, however, the voltage necessary for the response of the piezoelectric element also rises. In order to keep the voltage within manageable limits, the thicknesses of individual piezoelectric elements normally lie between 20 and 200 &mgr;m. A piezoelectric actuator must therefore have an appropriate number of individual elements or layers for a desired lengthwise expansion.
Known piezoelectric actuators of multilayer design therefore summarily include numerous individual layers. For their production, piezoceramic green films are alternatively arranged with electrode material to form a stack. The layers are then laminated and sintered together to form a monolithic composite. Such a process is known, for example, from an article by S. Takahashi et al. in Ferroelectrics, 1983, Vol. 90, pages 181 to 190. Larger actuators with larger absolute deflection are obtained, for example, by adhesively bonding a plurality of such stacks. Such a process is disclosed, for example, by U.S. Pat. No. 5,438,242. However, such bonded stacks have too low a stiffness for many applications, in particular if high forces have to be transmitted using the piezoelectric actuator. Sufficiently high stiffnesses are possessed only by piezoelectric actuators of fully monolithic multilayer design. Only the latter exhibit a sufficiently solid composite of the individual layers in the stack.
In the production of monolithic actuators of multilayer design, however, additional problems occur with increasing height. The plates which are laminated to form a stack and which contain numerous individual actuators must be divided up before sintering. Here, the stacks of the relatively large-area green films are divided up into smaller stacks with the desired actuator area. While low stacks can be stamped in automatic machines, such as is possible, for example in the case of multilayer capacitors, in the case of higher stacks this must be replaced by a multiple sawing process along the separating lines.
In the case of low stacks, the lamination can take place in automatic machines with short cycle times. Higher stacks must be laminated with increased care in order, in particular, to maintain the vertical structural accuracy during lamination. In the process there is always the risk that, as a result of the applied pressure, transverse flow processes in the green films will lead to a displacement of individual layers in relation to one another. In the zones with which contact is to be made later, the accuracy of contact is thereby destroyed.
The organic binders which are used in multilayer technology are primarily matched to the requirements of the green film production and also lamination. They also must be removed before sintering by means of a complicated binder removal process in furnaces with a monitored atmosphere. However, the diffusion paths for the binder or its decomposition products during the binder removal process are multiplied with increasing stack height. In order to prevent destruction of the stack as a result of too high an internal pressure of the decomposing binder, it is necessary to take very elaborate technical measures, much more sophisticated than those in the case of the production of multilayer capacitors.
Because of the many problems to be solved in the production of piezoelectric actuators of multilayer design, known monolithic piezoelectric actuators in the cost-effective technology which is normal in the manufacture of multilayer capacitors were until now restricted to a maximum height of about 2.5 to 5 mm. In addition, for the reasons mentioned, these known piezoelectric actuators only achieve an aspect ratio (height/width) of a maximum of about 2. Higher actuators and actuators with a higher aspect ratio were previously obtained by adhesively bonding together a plurality of smaller stacks; the stiffness of the stack and hence its mechanical loadability being reduced.
It is therefore an object of the present invention to provide a simple and reliable production process for a piezoelectric actuator of monolithic multilayer design which possesses a high aspect ratio of more than 2, which can be produced monolithically in stack heights of more than 5 mm and which possesses optimum piezoelectric properties and a high mechanical composite strength.
SUMMARY OF THE INVENTION
This object is achieved according to the present invention with a method, and apparatus according to the hereafter appended claims.
The fundamental idea of the present invention is to carry out the lamination in a blank with a limited stack height. Following the separation of the laminates, that is to say, the dividing up of the large-area laminated first stacks (=first composites) into smaller second stacks (=second composites) of the desired actuator base area, the binder removal takes place. A plurality of these second composites are then stacked up to a height which is a multiple of the first blank and then sintered under light pressure to make the piezoelectric actuator of the present invention. In this case, piezoelectric actuators according to the present invention having an overall height of more than 5 mm and an aspect ratio of more than 2 can be obtained. As a result of the lamination of the first stacks with a relatively low blank thickness and, hence, a relatively low stack height, it is possible to maintain a high vertical accuracy of the internal electrode structures. This is particularly important since the electrodes are printed in a structured way onto the green films so that cutouts remain in the electrode layers; the exact positioning of such cutouts in the stack being particularly important for the correct making of contact. This high vertical accuracy within a composite can be maintained in a simple way during the stacking on one another of a plurality of these composites and the subsequent joint sintering. Slight displacements which can occur during the stacking of the composites on one another are of no significance in this case. The dividing up of the first composites of limited stack height can be carried out in a known cost-effective way by cutting, stamping or shearing. The removal of the binder from the optionally separated composites takes place at a low stack height of, for example, up to 2 mm. This means that maximum diffusion paths of about 1 mm have to be overcome by the binder or its decomposition products. It is therefore possible to use a conventional process without additional complicated process monitoring for the binder removal.
The production of first composites with electrode structures which are unified in the composite makes it possible in a simple way to produce a piezoelectric actuator having different layer areas through the combination of a plurality of composites of different electrode structure. The layer areas may differ in this case in the exact electrode structure, the layer thicknesses of the individual layers or the making of electrical contact. It is thus possible to implement mechanically coupled, but electrically decoupled, actuator and sensor functions alongside one another in the piezo
Cramer Dieter
Hellebrand Hans
Lubitz Karl
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
Tugbang A. Dexter
Young Lee
LandOfFree
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