Multilayer components encompass capacitors and piezo-actuators, containing in each case alternating dielectric layers and internal electrodes. In the case of the piezo-actuators, the dielectric layers are additionally piezoelectric. Therefore, piezo-actuators are included among the piezo-elements.
Piezo-elements are, inter alia, for positioning elements, ultrasonic transducers, sensors and in inkjet printers and also in automotive engineering for driving fuel elements is based on the deformation of piezo-ceramic materials, such as e.g., lead zirconate titanate, under the action of an electric field. If an electrical voltage is applied to the piezo-element, then the latter expands in a direction perpendicular to the electric field generated (inverse piezo-effect).
Advantages of piezo-elements include, inter alia, the relatively high speed thereof, the relatively high effectiveness thereof and, if used as piezo-actuator, the relatively small actuating travel thereof.
However, if a relatively large actuating travel is intended to be achieved with the piezo-actuator, then a piezo-stack comprising a plurality of alternately successive piezo-electric layers and internal electrode layers is used for the piezo-actuator, as is disclosed e.g., in JP 03174783 A.
The piezo-actuator disclosed in JP 03174783 A is embodied in such a way that the internal electrode layers are electrically connected alternately to external electrodes arranged at opposite outer surfaces of the piezo-stack. The internal electrode layers, which are electrically connected to one of the two external electrodes, are therefore led as far as the outer side at which said external electrode is arranged, for the electrical connection to the external electrode. In order that the internal electrode layers are electrically insulated from the other external electrode, however, the internal electrode layers do not extend as far as the outer side of the piezo-stack at which the further external electrode is arranged. In these regions, the internal electrode layers are set back from the outer side. This is achieved by the piezo-stack being provided with slots filled with silicone resin in these regions.
By virtue of the set-back internal electrode layers, so-called inactive zones arise in piezo-electric layers assigned to these regions. The inactive zones are usually produced during the layered production of the piezo-stack. Owing to tolerances of the processes, stacking, separation, binder removal and grinding during the production of the piezo-stack with inactive zones and on account of the stipulation of a reliable electrical insulation of the internal electrode layers with respect to the corresponding external electrode, relatively large inactive zones of typically up to 10 percent of the piezo-stack cross section arise. The inactive zones, which are permeated by a reduced electric field strength when an electrical voltage is applied to the external electrode layers or internal electrode layers and therefore expand to a lesser extent than the other, so-called active zones of the piezo-electric layers when an electrical voltage is applied. This leads to mechanical stresses in particular in the inactive zones and the edge regions with respect to the inactive zones and can lead to so-called poling cracks in the inactive and active zones of the piezo-electric layers, and also in the external electrodes. The risk of poling cracks is all the higher, the larger the inactive zones.
USPTO Backgrounds
USPTO Backgrounds