Based on the piezoelectric effect, piezoelectric ceramics having a suitable crystal structure can expand or contract internally when an electric field is generated. This longitudinal change takes place virtually without delay relative to the controlling electric signal and furthermore is precisely controllable. For that reason piezoelectric multilayer actuators are used as final control elements.
Since the controlling electric field strength in such multilayer actuators lies in the range of several kV/mm, but moderate drive voltages are desired when the multilayer actuators are deployed in practice, the piezoceramic multilayer actuators are produced with layer thicknesses in the range of 100 μm. Typical production methods are slip casting or dry pressing. The individual ceramic layers are metallized and stacked one on top of another so that internal electrodes of opposite polarity disposed between two ceramic layers can generate the piezoelectric effect. All the internal electrodes of the same electric potential are electrically driven in alternation via an outer metallization parallel to the stacking direction or, as the case may be, longitudinal axis of the piezoceramic multilayer actuator. In order to ensure that the internal electrodes of opposite electric potential in each case are not shorted, these are arranged offset in relation to the other electrodes in the area of the outer metallization of the internal electrodes of opposite potential in each case. Due to design factors, piezoelectrically inactive zones are created as a result of this layer structure within the piezoceramic multilayer actuator. Because the piezoceramic multilayer actuator only expands or contracts in length in piezoelectrically active zones, since the internal electrodes of different polarity are arranged directly on top of one another in these zones, mechanical stresses occur inside the multilayer actuator in the transition region to piezoelectrically inactive zones. Said mechanical stresses resulted in the formation of cracks in the layer structure of the multilayer actuator which adversely affect the electrical contact of the outer metallization to the individual electrodes or can even lead to the total failure of the multilayer actuator. The problem of crack formation is reduced by the use of fully active piezoelectric multilayer actuators. No piezoelectrically inactive or passive zones are present in said fully active piezoelectric multilayer actuators, because the internal electrodes of both electric potentials extend on all sides over the entire cross-section of the multilayer actuator as far as the edge of the latter.
EP 1 206 804 B1 discloses a production method for piezoelectric multilayer actuators. Electrically conductive materials are deposited electrochemically on the lateral surfaces of the electrodes of the same polarity on the individual multilayer structures. Said materials form bridges which are electrically connected with one another after the lateral surfaces of the electrodes arranged between said bridges have been insulated.
DE 42 24 284 A1 discloses the production of a multilayer bar and its subsequent sintering as a preliminary stage for the production of piezoceramic multilayer actuators. A bar of said kind has for example the depth vertically and the length parallel to its stacking direction in accordance with a subsequent multilayer actuator. Its width vertically to the stacking direction is a multiple of a subsequent width of a piezoceramic multilayer actuator. After the sintering, the electrode structure of the individual piezoceramic multilayer actuators is created jointly on the multilayer bar and subsequently the latter is split into individual multilayer actuators. In order to produce the electrode structure, electrochemically conducting material is deposited on the lateral surfaces of every second electrode. The intermediate regions, i.e. ceramic and electrode, are set back into the sintered multilayer bar by etching or mechanical removal. These operations are complex and time-consuming, since the sintered multilayer bar is resistant to mechanical and chemical attack. An insulation layer is subsequently applied in the set-back regions and external electrodes are created for each subsequent multilayer actuator. Finally the sintered multilayer bar is subdivided into individual piezoceramic multilayer actuators.
U.S. Pat. No. 5,568,679 and WO 2005/075113 A1 also describe the creation of structured external electrodes on an already sintered multilayer preform body. An electrically insulating material is electrochemically deposited on each internal electrode of the same polarity such that the lateral surfaces of said electrodes are insulated toward the exterior. An electrically conductive material which interconnects the still exposed lateral surfaces of the electrodes of the other polarity is subsequently applied to the exteriors. Finally the sintered multilayer bar with structured electrode is split into individual piezoceramic multilayer actuators.
USPTO Backgrounds
USPTO Backgrounds