Piezoactuator with a predetermined breaking layer

A piezoactuator of multilayer design includes piezoelectric layers and electrode layers to form a stack. A predetermined breaking layer for the targeted origination and guiding of cracks is introduced between two adjacent electrode layers. The predetermined breaking layer has a barrier region, in which the formation of continuous electrically conductive paths or the formation of cracks leading through the barrier region is impeded.

TECHNICAL FIELD

A piezoactuator is specified, which is constructed from a multiplicity of piezoelectric layers with electrode layers lying therebetween. When an electrical voltage is applied to the electrode layers, the piezoelectric layers expand, as a result of which a stroke is generated. Piezoactuators of this type are used, for example, for actuating an injection valve in a motor vehicle.

BACKGROUND

Mechanical stresses can occur during operation of the piezoactuator, as a result of which cracks can arise. In particular, such stresses occur in the boundary region between so-called active zones and inactive zones, in which the piezoelectric layers expand to different extents. In order that cracks do not arise in an uncontrolled fashion in the stack and thus cause, for example, a short circuit between electrode layers of different polarities, the stacks are provided with predetermined breaking layers. The predetermined breaking layers are embodied such that cracks occur particularly easily in the predetermined breaking layers and propagate within the predetermined breaking layers.

The PCT document WO 2004/077583 A1 describes a piezoactuator having predetermined breaking layers.

SUMMARY OF THE INVENTION

In one embodiment, the invention provides a piezoactuator with a predetermined breaking layer which has an increased reliability with respect to failure of the piezoactuator.

A piezoactuator of multilayer design is specified, wherein piezoelectric layers and electrode layers arranged therebetween are arranged to form a stack.

Preferably, the piezoactuator is a monolithic multilayer actuator produced from thin films of a piezoelectric material, for example, lead zirconate titanate (PZT). In order to form the electrode layers, a metal paste, for example, a silver-palladium paste or a copper-containing paste, can be applied to the films by means of a screen printing method. The films are subsequently stacked, pressed and jointly sintered. In this case, an electrode layer need not be applied to every piezoelectric layer. By way of example, a plurality of piezoelectric layers can be situated between two electrode layers.

Preferably, external electrodes are applied on two opposite outer areas of the piezoactuator. An external electrode comprises a base metallization, by way of example, which can be produced by means of a stoving paste analogously to the electrode layers. The electrode layers are connected to the external electrodes alternately, for example, along the stacking direction of the piezoactuator. For this purpose, the electrode layers are led, for example, alternately to one of the external electrodes and at a distance from the second external electrode. In this way, the electrode layers of one polarity are electrically connected to one another via a common external electrode.

In one embodiment, the piezoactuator has an enclosure. The enclosure can protect the piezoactuator against external influences, e.g., ingress of moisture, or prevent mechanical damage. By way of example, the enclosure covers the external electrodes and is applied on side areas of the piezoactuator. Preferably, the enclosure contains an elastic material.

The piezoactuator specified has a predetermined breaking layer the tear strength of which is at least in part lower than the tear strength of adjoining piezoelectric layers. By way of example, the tensile strength of the predetermined breaking layer is at least in part less than ⅔ of the tensile strength of the composite assembly comprising the piezoelectric layers and electrode layers.

In one embodiment, the predetermined breaking layer is configured inhomogeneously laterally, i.e., in a plane perpendicular to the stacking direction.

Preferably, the predetermined breaking layer has at least one partial region that differs from a further region of the predetermined breaking layer with regard to its chemical or physical properties. By way of example, the material in the partial region differs from the material in the further region of the predetermined breaking layer with regard to its chemical composition or its inner structure.

Such an inhomogeneous configuration makes it possible to impede, for example, the origination of conductive paths or the origination of a segmentation of the piezoactuator. In this case, the partial region can be regarded as a barrier region or part of a barrier region.

Such a barrier region impedes the formation of electrically conductive paths leading through the barrier region. As an alternative or in addition thereto, the formation of cracks leading through the barrier region can be impeded in the barrier region. *

Short circuits between electrode layers of different polarities which are adjacent in the stacking direction are intended to be prevented by the insertion of a predetermined breaking layer.

In order for the piezoactuator to expand when a voltage is applied to the electrode layers the piezoelectric layers should be polarized. For this purpose, by way of example, a DC voltage is applied via the external electrodes between adjacent electrode layers and the stack is heated. In inactive zones in which adjacent electrode layers of different polarities do not overlap in the stacking direction, the piezoelectric material does not expand or expands only partly in the same direction as in the active zones. As a result of the different expansion of the piezoelectric layers in active and inactive zones, mechanical stresses arise, which can lead to cracks during polarization or during operation of the piezoactuator.

The insertion of a predetermined breaking layer makes it possible to control the cracking in a targeted manner, such that cracks originate only in the predetermined breaking layer and then also propagate within the predetermined breaking layer. As a result, the cracks run in a plane perpendicular to the stacking direction and therefore cannot lead to a short circuit between adjacent electrode layers of different polarities.

In one preferred embodiment, the predetermined breaking layer has at least in part a greater average porosity than an adjoining piezoelectric layer.

By way of example, the predetermined breaking layer contains a piezoelectric material, the porosity of which is greater than the porosity of an adjoining piezoelectric layer. A greater porosity can be produced, for example, by additives being introduced into a basic material, the additives producing voids during the sintering process. By way of example, the voids arise as a result of the additive being evaporated. By way of example, the predetermined breaking layer is formed by a porous ceramic layer.

In one embodiment, the predetermined breaking layer contains a metal such as, for example, silver, palladium, copper or an alloy of the metals. In this case, the composition of the metal can be chosen such that diffusion occurs during the sintering of the piezoactuator, as a result of which pores likewise arise in the predetermined breaking layer. By way of example, the predetermined breaking layer runs within a metallic layer comprising a material similar or identical to that of an electrode layer.

The mechanical strength of the predetermined breaking layer is preferably determined by the degree of porosity. If the predetermined breaking layer has a lower mechanical strength than the adjoining piezoelectric layers, then cracks preferably originate in the predetermined breaking layer and propagate within the predetermined breaking layer.

However, electrically conductive paths can originate in such predetermined breaking layers, for example, as a result of the ingress of water during operation in the case of high moisture or as a result of substances emerging from a passivation layer. This is critical particularly when a conductive path leads to the short circuit of the external electrodes. As a result, the operation of the piezoactuator is greatly impaired or it is even possible for failure of the device to occur.

Moreover, such predetermined breaking layers can promote a segmentation of the piezoactuator during operation. A segmentation can arise, for example, by virtue of a crack extending over the entire cross-sectional area of a piezoactuator, such that the piezoactuator is divided into two partial stacks. Such a segmentation can have the effect, for example, that the external contact-connection of the piezoactuator is interrupted. Moreover, a segmentation can lead to the origination of conductive paths between the external electrodes and hence a short circuit. If the piezoactuator has an enclosure, a segmentation can result in an increased mechanical loading of the enclosure and damage, e.g., tearing of the enclosure.

A barrier region in the predetermined breaking layer can prevent the origination of electrically conductive paths which electrically connect the external electrodes. For this purpose, the barrier region is configured in such a way that it impedes the formation of electrically conductive paths leading through the barrier region.

In addition or as an alternative thereto, the occurrence of a segmentation of the piezoactuator can be prevented by a barrier region in the predetermined breaking layer. For this purpose, the barrier region is configured in such a way that it impedes the formation of cracks leading through the barrier region.

Preferably, the barrier region is arranged in such a way that, within the predetermined breaking layer, each connecting line between the external electrodes passes through the barrier region. Correspondingly, within the predetermined breaking layer, each connecting line between the inactive zones passes through the barrier region.

In one embodiment, at least one partial region of the barrier region is provided in which the formation of electrically conductive paths or the formation of cracks is impeded to a greater extent than in a further region of the predetermined breaking layer.

The further region of the predetermined breaking layer has a lower tear strength than an adjoining piezoelectric layer. Preferably, this region lies at least in the region of the inactive zones.

By way of example, the partial region of the barrier region has a lower average porosity than the further region of the predetermined breaking layer.

This reduces the probability of conductive substances diffusing into the partial region and electrically conductive paths being formed. Given a suitable arrangement of the partial region in the barrier region, it is thus possible to prevent an electrically conductive path from leading through the barrier region.

Moreover, the lower average porosity leads to an increased tear strength, as a result of which a segmentation of the piezoactuator is made more difficult.

By way of example, the partial region contains the same material as an adjoining piezoelectric layer. In this case, the partial region can also be regarded as an interruption of the predetermined breaking layer. In order to produce such a layer, it is possible to apply a predetermined breaking layer to a piezoelectric layer which does not extend over the entire cross section of the piezoelectric layer. Preferably, the interruptions of the predetermined breaking layer are filled by the material of the adjoining piezoelectric layers during the pressing and sintering.

In one preferred embodiment, the predetermined breaking layer has no interruptions of this type, but rather has a lower tear strength than an adjoining layer of the stack over the entire cross section of the stack. In this case, the formation of electrically conductive paths or the formation of cracks can be impeded, for example, by a variation in the porosity of the predetermined breaking layer. In one embodiment, the porosity of the partial region of the barrier layer is greater than that of an adjoining piezoelectric layer and less than the porosity in a further region of the predetermined breaking layer.

In one embodiment, the barrier region has a continuous path, in which the tear strength is lower than in the partial region of the barrier region. The continuous path is preferably longer than that section of each straight line connecting the external electrodes which lies in the barrier region.

Ideally, the barrier layer is formed in such a way that electrically conductive paths or cracks can form in the barrier layer only along the continuous path. The longer the continuous path, the lower the probability of the path becoming conductive over its entire length, for example, as a result of the indiffusion of conductive substances, or of the path leading to a segmentation of the piezoactuator. Consequently, the probability of an electrically conductive path or a crack leading completely through the barrier layer is also lower, the longer the continuous path. By way of example, the continuous path contains the same material as the further region of the predetermined breaking layer.

In one embodiment, the material within the path is more porous than in the partial region of the barrier layer surrounding the path. Preferably, the continuous path is sinuous and is significantly longer than that section of each straight line connecting the external electrodes which lies in the barrier region, that is to say that its length is significantly greater than the width of the barrier layer.

Furthermore, such an embodiment has the advantage that a crack that has originated in the predetermined breaking layer can propagate well within the predetermined breaking layer in a plane perpendicular to the stacking direction of the electrode layers. This is intended to prevent the origination of a crack edge at which a crack present propagates into the adjoining piezoelectric layers.

In one preferred embodiment, the partial region of the barrier region is formed in such a way that, within the predetermined breaking layer, each connecting line between the external electrodes passes through the partial region.

By way of example, the partial region of the barrier region surrounds islands having a lower tear strength than the partial region.

Within the islands, cracks can originate in a targeted manner and a mechanical strain of the device can thus be prevented.

By way of example, the islands have a greater porosity than their surroundings. Preferably, the islands contain the same material as the further region of the predetermined breaking layer. The islands are formed in a circular fashion, for example, but they can also be formed in a rectangular fashion or have any other shape.

Preferably, at least one inactive zone is formed between two electrode layers of different polarities that are adjacent in a stacking direction, in which at least one inactive zone the electrode layers do not overlap in the stacking direction. In one preferred embodiment, the barrier region is situated outside the inactive zone.

Since, in exemplary embodiments of the piezoactuator, the greatest mechanical stresses occur in the region of the inactive zone, it is advantageous if the predetermined breaking layer is configured optimally there with regard to its ability with respect to crack formation and crack guiding. However, this can be accompanied by an increased probability of electrically conductive substances penetrating into the predetermined breaking layer. By way of example, electrically conductive substances penetrate into the predetermined breaking layer more easily in the case of an increased porosity.

In one embodiment, two separate inactive zones are formed between two adjacent electrode layers of different polarity.

By way of example, external electrodes are arranged on two opposite outer areas of the piezoactuator. The electrode layers are led in the stack direction alternately to one external electrode and are at a distance from the second external electrode. In this way, two separate inactive zones adjoining the external electrodes arise between two adjacent electrode layers of different polarities.

Furthermore, a piezoactuator with a predetermined breaking layer is specified wherein additives that impede the formation of electrically conductive paths or the formation of cracks are situated in the barrier region.

Such additives can be introduced into the predetermined breaking layer externally, for example, by doping of the material of the predetermined breaking layer, by printing or by diffusion. The additives bind, for example, penetrating substances or bring about a catalyst effect, such that the penetrating substances are converted into substances which do not lead to conductive paths. As an alternative or in addition thereto, the additives can lead to an increased tear strength of the barrier region.

In the case of a porous predetermined breaking layer, suitable additives can at least partly fill the pores.

In this way it is possible to prevent moisture or other electrically conductive substances from penetrating into the predetermined breaking layer and leading to the formation of an electrically conductive path. In addition or as an alternative thereto, it is possible to increase the tear strength in this region in this way.

In one embodiment, the barrier region is arranged at least in the outer edge region of the predetermined breaking layer. In this way, the predetermined breaking layer is outwardly sealed against penetrating moisture. In addition or as an alternative thereto, in this way the occurrence of cracks in the edge region of the piezoactuator can be made more difficult and mechanical stresses can thus be prevented.

In a further embodiment, the entire predetermined breaking layer constitutes a barrier region that impedes the formation of electrically conductive paths leading through the predetermined breaking layer. In this case, therefore, the barrier region extends over the entire predetermined breaking layer.

By way of example, additives that impede the formation of electrically conductive paths are introduced into a predetermined breaking layer, which can extend over the entire cross section of the piezoactuator, with a homogeneous concentration distribution.

The following list of reference symbols may be used in conjunction with the drawings:1piezoactuator11stack12a,12bside area2,2a,2b,2cpiezoelectric layer3,3a3belectrode layer4predetermined breaking layer4afurther region4bconductive path5barrier region5apartial region of the barrier region5bislands5ccontinuous path5dbridge6a,6binactive zone7additive8a,8bexternal electrode9active zone10enclosureS stacking direction

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1Ashows a piezoactuator1of multilayer design, wherein a multiplicity of piezoelectric layers2, for example, ceramic layers, are arranged one above another along a stacking direction S. Electrode layers3are arranged between some piezoelectric layers2. A respective external electrode8a,8bis applied on opposite side areas12a,12bof the stack11. In order to make electrical contact with the external electrodes8a,8b, leads can be soldered onto the external electrodes8a,8b(not shown). The electrode layers3are connected to the two external electrodes8a,8balternately in the stacking direction S. For this purpose, an electrode layer3ais alternately led to one of the external electrodes8a, while it is spaced apart from the second external electrode8b. The adjacent electrode layer3bin the stacking direction S is electrically connected to the second external electrode8band spaced apart from the opposite external electrodes8a. When a voltage is applied between the external electrodes8a,8b, the polarity of the electrode layers3a,3bthus alternates in the stack direction S.

The piezoelectric layers2expand along the field lines when a voltage is applied. The piezoactuator has inactive zones6a,6b, in which adjacent electrode layers of different polarities3a,3bin the stacking direction S have no overlap. Therefore piezoelectric layers2expand to a lesser extent in the inactive zones6a,6bthan in an active zone9, in which adjacent electrode layers of different polarities3a,3boverlap. This leads to mechanical stresses, as a result of which cracks can originate in the piezoactuator1. For the targeted formation and guiding of the cracks, predetermined breaking layers4are arranged between adjacent electrode layers3a,3b.

FIG. 1Bshows a piezoactuator1surrounded by an enclosure10at its side areas. The enclosure10covers the external electrodes8a,8band protects them against external influences. By way of example, the enclosure10serves for protection against the ingress of moisture and against mechanical damage. The enclosure10contains an elastic material, for example silicone. The piezoactuator1has predetermined breaking layers4for the targeted formation and guiding of cracks.

FIG. 2Ashows an excerpt from a longitudinal section through a piezoactuator1, in which an electrically conductive path4bhas originated within a predetermined breaking layer4. The predetermined breaking layer4contains a porous material and has a lower tear strength than the adjoining piezoelectric layers2a,2c. By way of example, the predetermined breaking layer4contains a ceramic material, the porosity of which is higher than the porosity of the adjoining piezoelectric layers2a,2c. By way of example the piezoelectric layers2a,2clikewise contain a ceramic material. On account of the increased porosity of the predetermined breaking layer4, moisture can penetrate into the predetermined breaking layer4more easily than into adjoining piezoelectric layers2a,2c. As a result, an electrically conductive path4bshown here can originate within the predetermined breaking layer4, which path connects the external electrodes8a,8bto one another and thus leads to a short circuit.

FIG. 2Bshows an excerpt from a piezoactuator1in accordance withFIG. 1B, in which a crack has originated within a predetermined breaking layer4and has propagated over the entire cross-sectional area of the predetermined breaking layer4. In this way, the piezoactuator1is divided into two partial stacks1a,1b, which can be displaced relative to one another. This leads to an intensified mechanical stressing of the enclosure10of the piezoactuator1, which can lead to tearing of the enclosure10.

FIGS. 3A to 3F,4,5and6A to6G show exemplary embodiments of predetermined breaking layers4which are configured inhomogeneously laterally. The origination of conductive paths, as shown inFIG. 2A, or the origination of a segmentation of the piezoactuator1, as shown inFIG. 2B, can be prevented or made more difficult by such an inhomogeneous configuration.

The predetermined breaking layer4has at least one partial region5awhich differs from a further region4aof the predetermined breaking layer4with regard to its chemical or physical properties. In this case, the subdivision of the predetermined breaking layer4into at least one partial region5aand a further region4aruns in a plane perpendicular to the stacking direction S. By way of example, a barrier region5contains a partial region5ain order thus to impede the propagation of cracks and of electrically conductive paths over the cross-sectional area of the predetermined breaking layer4through the partial region5a.

InFIGS. 3A to 3F, the predetermined breaking layers4have a barrier region5with a partial region5a, in which the formation of electrically conductive paths or the formation of cracks is impeded to a greater extent than in a further region4aof the predetermined breaking layer4. The further region4acontains a porous material. The partial region5acontains the same material as the adjoining piezoelectric layers2a,2c. In particular, the porosity of the material in the partial region5aand in the adjoining piezoelectric layers2a,2cis identical and the porosity of the material in the further region4aof the predetermined breaking layer4is greater than in the partial region5a. Hereinafter, the term “porous material” denotes a material whose porosity is higher than the porosity of adjoining layers of the stack. By way of example, both the partial region5aand the further region4acontain a ceramic material.

The partial region5a, on account of its lower porosity, has a higher average strength than the further region4aof the predetermined breaking layer4. As a result, both the origination of conductive paths and the segmentation of the piezoactuator1can be prevented.

FIG. 3Ashows a predetermined breaking layer4in which the barrier region5is arranged in a region between the two inactive zones6a,6b. The barrier region5extends over the entire diagonal90of the predetermined breaking layer4, which runs transversely with respect to the inactive zones6a,6b, that is to say perpendicularly to a connecting line between the inactive zones6a,6b. In this way, within the predetermined breaking layer4, each connecting line between the external electrodes8a,8bpasses through the barrier region5. As a result, it is possible, for example, to prevent the formation of an electrically conductive path that electrically interconnects the external electrodes8a,8b, which are arranged at the edge of the piezoactuator in the region of the inactive zones6a,6b. Moreover, it is possible to prevent the origination of a crack that connects the external electrodes8a,8bto one another. In the further region4a, in which the inactive zones6a,6bare also situated, the predetermined breaking layer4contains a porous material that is configured in an optimized manner with regard to crack formation and crack guiding.

FIG. 3Bshows a further embodiment of the arrangement of a barrier layer5in a predetermined breaking layer4, wherein, in contrast toFIG. 3A, the barrier layer5does not extend along the diagonal90, but rather connects two side areas of the piezoactuator1. In this case, too, each connecting line between the external electrodes8a,8band hence also each connecting line between the inactive zones6a,6bpasses within the predetermined breaking layer4through the barrier region5.

FIG. 3Cshows a further embodiment of a predetermined breaking layer4, with a partial region5ain the barrier region5. In this case, too, the partial region5ais embodied in such a way that, within the predetermined breaking layer4, each connecting line between the external electrodes8a,8bpasses through the partial region5a. The partial region5aforms a continuous sinuous path extending over the diagonal90of the predetermined breaking layer4. The material of the further region4aof the predetermined breaking layer4extends in a finger-like manner into the barrier region5and is intermeshed with the partial region5a. Mechanical stresses of the piezoactuator1can be compensated for particularly well in this way.

FIG. 3Dshows an embodiment of the predetermined breaking layer4in which circular islands5bcomposed of porous material are situated in the barrier region5. The islands5bare surrounded by the partial region5aof the barrier region5. Within the islands5b, cracks can originate in the piezoactuator1in a targeted manner.

FIG. 3Eshows an embodiment of the predetermined breaking layer4in which rectangular islands5bcomposed of porous material are situated in the barrier region5.

Instead of the circular and rectangular islands shown inFIGS. 3D and 3E, such islands can also have any other shape.

FIG. 3Fshows an embodiment of the predetermined breaking layer4in which porous material forms a continuous path5cin the barrier region5. The continuous path5cis longer than that section of each connecting straight line between the external electrodes8a,8bwhich lies in the barrier region5. This decreases the probability of penetrating moisture giving rise to an electrically conductive path that leads completely through the barrier region5and can thus connect the external electrodes8a,8b. In addition thereto, a segmentation of the piezoactuator1can be prevented.

In the embodiments of the predetermined breaking layer4as shown inFIGS. 3A to 3F, the diagonal90running transversely through the predetermined breaking layer4with respect to the inactive zones6a,6bintersects both the partial region5aof the barrier region5and the further region4aof the predetermined breaking layer4. This can contribute to the reduction of mechanical stresses in the piezoactuator1.

FIG. 4shows a predetermined breaking layer4with a barrier region5. The barrier region5extends along a diagonal90of the predetermined breaking layer4and contains a material that is less porous than the material of a further region4aof the predetermined breaking layer4, but is more porous than adjoining piezoelectric layers. Penetration of moisture into the barrier region5can be impeded in this way. Moreover, the origination of cracks in the barrier region5can be made more difficult and a segmentation of the piezoactuator1can thus be prevented. In the barrier region5, too, the tear strength is reduced by comparison with adjoining piezoelectric layers. In the inactive zones6a,6b, the predetermined breaking layer4, on account of its higher porosity, has a reduced tear strength and is thus optimized in the inactive zones6a,6bwith regard to its crack formation and crack guiding capability.

FIG. 5shows a further embodiment of a predetermined breaking layer4, in which additives7are introduced into the barrier region5. The predetermined breaking layer4consists of a porous material. In the barrier region5, the pores are largely filled by the additives7, and the penetration of moisture into the barrier region5is thus impeded. In addition or as an alternative thereto, such additives7can lead to an increased tear strength of the barrier region5. In an alternative embodiment, the additives7can also be introduced only into the edge region of the predetermined breaking layer4, such that the barrier region5is arranged in the edge region.

FIGS. 6A to 6Gshow further examples of structured predetermined breaking layers4having at least one partial region5awhich differs from a further region in its chemical or physical properties. The partial region5ais, for example, part of a barrier region5that limits the formation of conductive paths or of cracks within the predetermined breaking layer4. In contrast to the embodiments shown inFIGS. 3A to 3F, here the barrier region5does not extend over the entire diagonal90.

FIG. 6Ashows a predetermined breaking layer4with a partial region5awhich has a rectangular form and extends principally along the diagonal90of the predetermined breaking layer4. In contrast to the exemplary embodiment of a barrier region5as shown inFIG. 3A, here the barrier region does not extend as far as the corners of the predetermined breaking layer4.

FIG. 6Bshows a predetermined breaking layer4with two partial regions5a, which principally run along the diagonal90and in this case, in particular, in the corners of the predetermined breaking layer4which are not associated with the inactive zones6a,6b. The two partial regions5aare separated from one another by a bridge5din the center. The bridge5dcontains the material of a further region4aof the predetermined breaking layer4and therefore has a reduced tear strength in comparison with the partial regions5a.

FIG. 6Cshows an exemplary embodiment of a predetermined breaking layer4in which partial regions5aextend in bar-type fashion along the diagonal90. The partial regions5aare separated from one another by bridges5dcomposed of porous material.

FIG. 6Dshows a predetermined breaking layer4containing a partial region5aembodied in a rectangular fashion, similar to that inFIG. 6A. Here, however, the partial region5ais interrupted by circular islands5bcomposed of porous material.

FIG. 6Eshows an embodiment of the predetermined breaking layer4in which the partial region5ahas a similar geometry to the partial region5ashown inFIG. 6B. Here, however, the partial region5ais interrupted by rectangular islands5bcomposed of porous material.

FIG. 6Fshows an embodiment of the predetermined breaking layer4in which the partial region5a, as inFIG. 3B, does not extend along the diagonal90, but rather is arranged parallel to the external electrodes8a,8b. Here, however, the partial region5adoes not extend as far as the side areas. The partial regions shown inFIGS. 6A to 6Eand6G can also have such a direction of extent.

FIG. 6Gshows an embodiment of the predetermined breaking layer4in which the partial region5ahas an elliptical contour. The longitudinal axis of the partial region5aextends along the diagonal90.

The forms of the partial regions5aand barrier regions5described here are not restricted to the geometries shown here. Thus, by way of example, the boundary between the barrier region5and the further region4aof the predetermined breaking layer4can also have a curved profile instead of a rectilinear profile. Moreover, the barrier region can run not just diagonally, as shown inFIG. 3A, or parallel to the side areas of the piezoactuator, as shown inFIG. 3B, but can also have some other orientation.

FIG. 7shows a further embodiment of a predetermined breaking layer4, in which a barrier region5which impedes the formation of electrically conductive paths leading through the barrier region extends over the entire predetermined breaking layer4. Additives7that make it more difficult for moisture to penetrate into the predetermined breaking layer4are introduced into the predetermined breaking layer4.

The invention is not restricted to the exemplary embodiments by the description on the basis of the exemplary embodiments, but rather encompasses any novel feature and also any combination of features. This includes, in particular, any combination of features in the patent claims, even if this feature or this combination itself is not explicitly specified in the patent claims or exemplary embodiments.