Method for manufacturing a piezoelectric stack actuator and piezoelectric stack actuator

Abstract

The present invention relates to a method for manufacturing a piezoelectric stack actuator, and preferably to a piezoelectric stack actuator manufactured according to said method. In order to extend the service life of a piezoelectric stack actuator consisting of individual actuators, especially when used in a wet environment, the present invention proposes a method for manufacturing a piezoelectric stack actuator (1) as set forth in claim 1, comprising: step A: providing at least two actuators (2), each designed to produce a deflection along an axis (A) when electrically activated; and step B: coupling at least two actuators (2) to form a stack actuator (1) such that, when the actuators (2) are electrically activated, their deflections overlap along a stack axis (S) and force coupling of the actuators (2) occurs over at least one coupling area (K) that is smaller than the protrusion area (P) of the actuator (1) in a plane (E) perpendicular to the stack axis.

Description

[Technical field]

[0001] The present invention relates to a piezoelectric stack actuator and a method for manufacturing the same. [Background technology]

[0002] Piezoelectric stack actuators consisting of alternating piezoelectric layers and electrodes are known, for example, from US Pat. No. 4,384,230 and US Pat. No. 4,721,447.

[0003] US Patent Application No. 2010 / 0140379 A1, US Patent Application No. 2006 / 0066178 A1, JP 2006-179525 A, JP 2006-216850 A, JP 2006-229068 A, US Patent Application No. 2010 / 0139621 A1 and US Pat. No. 7,309,945 disclose inter alia a monolithic block consisting of a piezoelectric layer and an electrode sintered together.

[0004] From WO 2003 / 105246 A2 the principle of selectively introduced and controlled break zones to prevent the build-up of tension forces in monolithic actuators is known.

[0005] According to an alternative design, which represents the starting point of the invention, piezoelectric stack actuators are assembled from actuators, which alone already contain one or several piezoelectric ceramic layers and are preferably designed as functional multilayer actuators, so that the deflections of the individual actuators are summed up on top of each other in the stacking direction. Unlike monolithic blocks of alternating piezoelectric layers and electrical contacts, these so-called chip stacks are glued together, since sintering is excluded, since at temperatures above 300 ° C the actuators lose their polarization. For gluing, there is glue between the individual actuators along the stacking axis, so that the actuator parts are spaced apart from each other in the stacked assembly. Moreover, large actuators cannot be reliably produced as defect-free monoliths due to organic burnout problems. Furthermore, gluing facilitates the construction of stacks of variable length with minimal production times.

[0006] WO 2006 / 100247 A1 discloses a stacked arrangement of multi-layer actuators, which are connected by a global adhesive bond across a thermally conductive metal layer.

[0007] In stack actuators where adjacent piezoelectric layers are entirely connected, mechanical tension is generated during electrical actuation as the bonded piezoelectric layers resist each other upon deformation of the piezoelectric layers, a problem that increases as the stack height increases.

[0008] During the manufacture of such stack actuators, also called bonded chip stacks of single or multi-layer actuators, uncontrolled crack formation can occur. This occurs even if the individual parts have been previously 100% polarized and checked for functionality and cracks. After bonding these crack-free actuators (chips) and operating again, cracks appear in the outer passive insulating layers of the actuators (chips).

[0009] These cracks affect the resistivity or service life of the actuator, especially in wet environments. Therefore, during quality control, affected parts are usually rejected, thereby causing loss. Moreover, the cracks that cause loss only occur at the end of a process that lasts several weeks in total, thereby disrupting production plans.

[0010] Even in actuators that are crack-free at the time of shipment, cracks regularly occur in the passive boundary layers under application conditions, especially during dynamic operation. These can likewise affect reliability or service life and in extreme cases lead to the shutdown of high-volume production plants. Mixed operation of static and dynamic loads has a particularly significant impact on the service life of actuators. Alternating loads promote cracking of the passive layers, which in turn allow electromigration as a damage mechanism due to moisture ingress during static operation, without the actuator self-heating.

[0011] A further important mode of operation is operation at low frequencies (a few Hz), where self-heating of the actuator via the ambient temperature does not occur, so moisture may be present in direct proximity to the actuator and alternating loads further promote crack formation.

[0012] Experiments have shown that the frequency of such cracks increases with increasing actuator cross section.

[0013] Even in high voltage actuators, where single layer actuators and contact plates are arranged in an alternating stack and glued together, short circuits occur due to cracks in the edge regions of the actuator parts or in the transitions between active and passive areas.Unlike multi-layer actuators, single layer actuators require high voltages to electrically actuate the piezoelectric ceramic material to produce deformation. Summary of the Invention [Problem to be solved by the invention]

[0014] SUMMARY OF THE PRESENT EMBODIMENT The underlying objective of the present invention is to increase the useful life of piezoelectric stack actuators made of individual actuators by preventing the above-mentioned crack formation, especially when used in wet environments. [Means for solving the problem]

[0015] This object is achieved by a method for manufacturing a piezoelectric stack actuator according to claim 1, which method comprises the following steps: - Step A: providing at least two actuators designed to cause a deflection along an axis when electrically actuated; - step B: coupling at least two actuators to form a stack actuator such that deflections of the actuators occurring when the actuators are electrically actuated overlap along a stack axis and force coupling of the actuators occurs over at least one coupling area that is smaller than the protruding area of ​​the actuators on a plane perpendicular to the stack axis.

[0016] According to the invention, a piezoelectric stack actuator is produced by bonding individual actuators together. From these already functional actuators, a piezoelectric stack actuator can be constructed without much effort, which achieves a deflection along the stack axis that is significantly larger than each individual actuator. Furthermore, stack actuators can be produced substantially more easily from functional individual actuators than stack actuators designed as a monolithic block and having a significant axial extension.

[0017] In each actuator, tensile stresses arise in the so-called passive areas, which are devoid of electrodes, during expansion, since they do not expand in the same way as the so-called active areas. Due to the low height of the actuator and the deformation of the passive areas, the tensile stresses are below the critical load in each individual actuator. However, if adjacent actuators are bonded together as a whole, deformation of the passive areas is not possible, since these deformations occur in different directions relative to the bonding plane and therefore resist each other. This effect increases continuously as the number of actuators bonded together increases. The tensions generated by the deformations then sum up to exceed the strength of the passive areas, which leads to crack formation at weak points.

[0018] The solution to the object mentioned at the beginning is analogously to selectively decoupling adjacent actuators, preferably in the edge region. According to the invention, the individual actuators are bonded to one another on a bonding area that is smaller than the projection area of ​​the actuator in a plane perpendicular to the stacking axis. The flat expansion area of ​​the bonding area preferably corresponds to the flat expansion area of ​​the active area of ​​the actuator, i.e. the flat expansion area of ​​the stack actuator in a plane perpendicular to the stacking axis minus the area of ​​the passive area. The passive area is usually located at the edge of the actuator. The actuator used here itself consists of a single- or multi-layer pack with alternating piezoelectric ceramic layers and electrodes / contacts that run parallel to one another in a plane perpendicular to the stacking axis and thus produce a deflection along the stacking axis. The area of ​​the piezoelectric layer that is covered by the electrodes / contacts and is electrically activated defines the so-called active area of ​​the actuator, which is actively deformed when electrically activated. The non-actuated areas (without electrodes) of the piezoelectric layer, which do not actively deform but optionally passively deform when the actuator is electrically actuated, are referred to as insulators, insulating layers or passive areas.

[0019] By selectively connecting or disconnecting the actuators on the connecting area, the peak loads in the passive areas of the connected actuators are reduced, so that the crack formation mentioned at the beginning can be effectively prevented and the service life of the stack actuator produced according to the method of the present invention is significantly increased.

[0020] Advantageous developments are the subject matter of the dependent claims. It may be advantageous if step A comprises at least one of the following sub-steps:

[0021] - Sub-step A1: Providing the piezoelectric ceramic layers and electrical contacts for constructing the actuator, preferably as a single-layer actuator or as a multi-layer actuator. Such components for stack actuators are usually available at low cost and can be used to manufacture stack actuators of various sizes. The electrical contacts for electrically activating the piezoelectric ceramic layers are also called electrode layers or internal electrodes. The flat extensions of the electrical contacts or internal electrodes are preferably smaller than the flat extensions of the piezoelectric ceramic layers. In the laminated assembly, the edge regions of the piezoelectric layers are then not covered with electrical contacts or internal electrodes and are not electrically activated. These edge regions of the piezoelectric layers are thereby not actively deformed, but optionally passively deformed, forming passive regions of the actuator, also called "insulators" or "insulating layers". This ceramic "insulator" or "insulating layer" ensures an effective and permanent protection of the internal electrodes, especially also in wet environments. The bonding of the actuator on the bonding area according to the invention makes it possible to protect this effective ceramic protective layer, which is brittle and hard, from defects.

[0022] - substep A2: The piezoelectric layers are preferably stacked along the axis such that two adjacent piezoelectric layers face each other with one electrical contact inserted each, preferably with an electrode layer or contact plate having poles of the same polarity. This design has been found to be particularly compact and advantageous, taking into account the connection of the piezoelectric layers for electrical actuation.

[0023] - sub-step A3: connecting the electrical contacts to electrodes of corresponding polarity, the electrodes of different polarity preferably spaced apart around the circumference of the actuator and preferably located on diametrically opposite sides of the actuator, the electrodes mentioned here also being referred to as side electrodes or external electrodes, among other terms.

[0024] - Sub-step A4: The piezoelectric ceramic layer and the electrical contacts are coated with a preferably ceramic insulating material, which is preferably air-tight and / or moisture-proof, this design particularly facilitating the application of the actuator in wet environments.

[0025] Sub-step A5: forming at least one coupling area for force-coupling an actuator to an adjacent actuator, preferably according to at least one of the following sub-steps:

[0026] Substep A5-1: forming at least one coupling area in the active area of ​​the actuator, which is embedded or covered with electrodes in places and is actively deformed by the electrical activation of the electrodes. The edges of the actuator around the active area then form so-called passive areas, which are not actively deformed by the electrical activation of the piezoceramic layers, but passively follow the deformations of the active area. In the passive areas, the load reduction caused by the reduction of the coupling area compared to the extension of the actuator in a plane perpendicular to the axis is particularly effective, since the load reduction allows a passive deformation of the insulators of the actuators coupled to each other, so that the peak loads in the area of ​​the insulator are reduced. According to this feature, the active area includes the coupling area, i.e. the coupling area is located within the active area. In other words, the coupling area projects on a plane perpendicular to the stack axis and is completely located within the projecting area of ​​the electrical contacts or internal electrodes.

[0027] Substep A5-2: Form at least one coupling region in the additional element to be arranged between the two actuators to be coupled, which additional element is preferably designed as an electrode. With this design, e.g. cylindrical or rectangular actuators, whose extension in the plane perpendicular to the axis is not reduced on the front side, can be coupled to each other via an additional element, which allows a passive deformation of the insulators of the actuators coupled to each other and thus reduces the peak loads in the region of the insulators. If designed as an electrode, the piezoelectric ceramic layer can be electrically actuated via the additional element. This design is particularly suitable for building so-called high-voltage stacks.

[0028] Substep A5-3: Forming a coupling area at one or both axial ends of the actuator or the additional element, preferably such that the coupling area forms one or both axial ends of the actuator or the additional element, respectively. With this design, coupling of the actuator is particularly easy.

[0029] Sub-step A5-4: A chamfer or a step is respectively formed in the adjacent region and / or the surrounding region, preferably the edge region, of the actuator or the additional element, while forming a bonding range. In these embodiments, the bonding range can be created particularly easily, for example, by slight material removal or material application on the front side of the actuator.

[0030] Sub-step A5-5: The bonding range is formed such that it protrudes on a plane perpendicular to the stacking axis and is completely located within the protruding range of the actuator, preferably surrounded by the protruding range over its entire circumference. In this design, since the passive edge regions of adjacent actuators are separated from each other, when the actuator is electrically actuated, passive deformation of the passive edge regions becomes possible, and the peak load within the region of the passive edge regions is reduced.

[0031] Sub-step A5-6: The bonding range is formed in a plane extending perpendicular to the axis. In this design, particularly simple bonding of the actuator is possible.

[0032] Sub-step A5-7: With respect to the protruding range (P) of the actuator, the bonding range (K) is formed such that the following area ratio applies, i.e., 0.7*P ≦ K < P, preferably 0.8*P ≦ K ≦ 0.99*P, preferably 0.9*P ≦ K ≦ 0.95*P. The bonding range ideally has the same flat extension as or is slightly smaller than the electrode or the active region of the actuator. Thus, the passive region of the actuator that protrudes on a plane perpendicular to the axis and is located radially outside the electrode is located outside the bonding range. In other words, the separation region in the bonding plane outside the bonding range is substantially or completely the same width as or slightly larger than the passive region of the actuator. Thereby, on the one hand, an ideal force coupling between the actuators in the active region of the actuator can be achieved through the bonding range, and on the other hand, the passive edge regions of the actuator are ideally separated to enable passive deformation of the piezoelectric ceramic layer.

[0033] Sub-step A5-8: Forming the coupling area as an area through which the shaft passes and / or which is arranged coaxially with respect to the shaft, the coupling area being preferably formed circular or annular, with this design making it possible to particularly effectively reduce peak loads, since the load acting on the insulator is distributed particularly uniformly over the entire circumference of the coupling area.

[0034] Sub-step A5-9: Forming the bonding area from adhesive: In this design, the adhesive surface between two actuators is smaller than the flat extension of the actuators perpendicular to the stacking axis.

[0035] - Sub-step A6: Providing at least two identical actuators. With this design, the assembly of stack actuators is particularly easy.

[0036] However, it may also be useful if step B comprises at least one of the following sub-steps:

[0037] - Sub-step B1: Preferably, at least two actuators are arranged in a regular and / or identical orientation with respect to the stacking axis, such that the electrodes of the actuators of the same polarity are oriented in a row parallel to the stacking axis and / or such that the opposing sides of adjacent actuators are parallel. With this design, the assembly of the stack actuators is also particularly easy, in particular since the electrodes of the individual actuators can be easily connected to one another.

[0038] Substep B2: Arranging an additional element between two adjacent actuators. In this embodiment, in particular cylindrical or cuboidal individual actuators can be particularly easily coupled to one another without separately forming a reduced coupling area compared to the flat extension of the actuator in a plane perpendicular to the axis.

[0039] - sub-step B3: connecting at least two actuators such that each of two adjacent actuators is directly actively connected or indirectly actively connected via an additional element, this connection preferably being made while forming a solid adhesive connection between adjacent actuators or between each of adjacent actuators and an additional element arranged between them, preferably by adhesive, particularly preferably the adhesive completely bonds at least one connection area. In this design, unlike the prior art mentioned at the beginning, the piezoceramic layers are not connected, for example by sintering, into a monolithic block, but rather the stack actuator is assembled by stacking and bonding functional individual actuators, which are preferably bonded to each other. The piezoceramic layers of the stack actuator are thus arranged in a pack and axially spaced apart by adhesive between the actuators bonded to each other.

[0040] - substep B4: Separating the two actuators from each other outside at least one bonding area, preferably in the edge area, preferably in the bonding plane, particularly preferably by removing the piezoceramic material and / or adhesive and / or by applying a release agent. According to this embodiment, a reduced bonding area in a plane perpendicular to the axis compared to the flat extension of the actuator can also be created after bonding of the actuator, for example by selective material removal in the piezoceramic layer or by selective material removal in the material connecting two adjacent piezoceramic layers. Removal of the adhesive without residues can be facilitated by applying a release agent to the area to be separated outside the bonding area. It is also conceivable to subsequently fill the separation area with a soft material that allows passive deformation of the passive area of ​​the actuator, while the force bonding of the actuator still takes place via the bonding area.

[0041] - Substep B5: Connecting the electrodes of the actuators of the same polarity to one another by means of connections, preferably by applying the connections onto the electrodes. The side electrodes of the individual actuators can be connected particularly easily by means of these connections. In principle, one contact point per actuator is sufficient. However, multiple connection points or even linear connections over the entire height of the part are also conceivable.

[0042] The object stated at the outset is also achieved by a piezoelectric stack actuator, preferably manufactured according to a method as claimed in one of the method claims, which comprises at least two actuators designed to produce a deflection along an axis when electrically activated, the at least two actuators being stacked along the stacking axis such that the deflections of the actuators which occur when the actuators are electrically activated overlap along the stacking axis, and the at least two actuators being coupled such that a coupling area, in which a force coupling between an actuator and an adjacent actuator takes place, is smaller than the protruding area of ​​the actuators in a plane perpendicular to the axis.

[0043] It may be further useful if each of the actuators includes at least one of the following features:

[0044] The actuator preferably consists of a piezoelectric ceramic layer and electrical contacts, either as a single layer actuator or as a multi-layer actuator.

[0045] - the piezoelectric ceramic layers are preferably stacked along the axis such that two adjacent piezoelectric ceramic layers face each other with one electrical contact inserted each, preferably an electrode layer or contact plate with poles of the same polarity.

[0046] The electrical contacts are connected to electrodes of corresponding polarity, the electrodes of different polarity being preferably spaced apart around the periphery of the actuator, preferably on diametrically opposite sides of the actuator.

[0047] - The piezoelectric ceramic layer and the electrical contacts are preferably coated with a ceramic insulating material, and the coated portion using this insulating material preferably has airtightness and / or moisture resistance.

[0048] However, it may be found useful when the bonding range includes at least one of the following features.

[0049] - At least one bonding range is formed in the active region of the actuator. - At least one bonding range is formed in an additional element to be disposed between two actuators to be bonded, and this additional element is preferably formed as an electrode.

[0050] - At least one bonding range is preferably formed at one or both axial ends of the actuator or the additional element such that the bonding range forms one or both axial ends of the actuator or the additional element, respectively.

[0051] - At least one bonding range is formed while forming a chamfer or a step in the adjacent region and / or the surrounding region, preferably the edge region, of the actuator or the additional element.

[0052] - At least one bonding range is formed such that the bonding range protrudes on a plane perpendicular to the stacking axis and is completely located within the protruding range of the actuator, preferably surrounded by the protruding range over its entire circumference.

[0053] - At least one bonding range is formed in a plane extending perpendicular to the axis. - At least one bonding range (K) is formed such that the following area ratio, i.e., 0.7*P ≦ K < P, preferably 0.8*P ≦ K ≦ 0.99*P, preferably 0.9*P ≦ K ≦ 0.95*P, applies with respect to the protruding range (P) of the actuator.

[0054] At least one of the connection regions is formed as a region through which the axis passes and / or which is arranged coaxially relative to the axis, the connection region being preferably formed circularly or annularly.

[0055] - at least one bonding area is formed by adhesive. It may also be useful if the stack actuator has at least one of the following features:

[0056] - Preferably, at least two actuators are arranged in a regular and / or identical orientation with respect to the stack axis, such that the electrodes of the actuators of the same polarity are oriented in a row parallel to the stack axis and / or such that the facing sides of adjacent actuators are parallel.

[0057] at least one additional element is arranged between two adjacent actuators; - at least two adjacent actuators are directly actively connected or indirectly actively connected via an additional element, this connection preferably being made by a firm adhesive connection, preferably by adhesive, between the adjacent actuators or between each of the adjacent actuators and the additional element arranged between them, particularly preferably the adhesive completely bonds at least one bonding area.

[0058] - At least two adjacent actuators are separated from one another, preferably in the edge region, preferably in the bonding plane, except for at least one bonding area, particularly preferably by removing the piezoelectric ceramic material and / or the adhesive and / or by applying a release agent before bonding.

[0059] The electrodes of the actuators of the same polarity are connected to one another by connections, which are preferably applied onto the electrodes.

[0060] Advantageous developments arise from the feature combinations disclosed in the claims, the description and the drawings.

[0061] Terms and Definitions The terms "insulating layer" and "insulator" refer to the passive regions of the actuator. This is the edge region of the piezoelectric ceramic layer or material that is not electrically excited by the electrodes and therefore does not deform. In addition to this "insulating layer" and "insulator" of piezoelectric material, the actuator may include a covering of insulating material. [Brief description of the drawings]

[0062] [Figure 1] 1 is a schematic diagram of steps of a method according to the present invention for manufacturing a piezoelectric stack actuator, in which (a) shows a cross-sectional view of two actuators in a non-connected state and an additional element to be inserted between them, and (b) shows a cross-sectional view of the two actuators in a directly connected state with the additional element inserted. [Diagram 2] FIG. 2 is a schematic diagram of the steps of a method according to the present invention for manufacturing a piezoelectric stack actuator, which, unlike FIG. 1, has chamfered upper and lower sides and the actuators are directly bonded to each other without inserting additional elements. [Diagram 3] In figures (a) to (d) are shown schematic cross-sectional views of further embodiments of actuators having a coupling range that is smaller than the protruding range of the actuator on a plane perpendicular to the axis. [Figure 4] FIG. 1 is a top plan view of a piezoelectric stack actuator of identical actuators stacked along a stacking axis where the coupling area between adjacent actuators is less than the protrusion area of ​​the actuators in a plane perpendicular to the axis or stacking axis, respectively. [Diagram 5] 1 is a perspective view, partially in section, of an actuator for use in a method according to the present invention for manufacturing a piezoelectric stack actuator; [Figure 6]Schematic cross-section of a so-called high-voltage stack with a single-layer actuator of piezoelectric ceramic and electrical contacts in the form of contact plates (internal electrodes), in which the piezoelectric ceramic layers are stacked on top of each other with one contact plate inserted each with poles of the same polarity facing each other, which contact plates are respectively connected to external or side electrodes via which the piezoelectric ceramic layers can be actuated simultaneously. [Figure 7] FIG. 6 is a perspective view, partially in section, of a stack actuator made from a plurality of the actuators according to FIG. 5; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0063] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0064] Each actuator 2 of the stack actuator 1 is constructed as a single-layer or multi-layer actuator of a piezoelectric ceramic 2a and electrical contacts or electrode layers (internal electrodes) 2c, 2e. The piezoelectric ceramic layers 2a are stacked along an axis A such that two adjacent piezoelectric ceramic layers 2a face each other with one electrical contact (internal electrode) inserted therein in the form of an electrode layer or disk 2c, 2e, respectively, having poles 2b, 2d of the same polarity.

[0065] The electrical contacts (internal electrodes) 2c, 2e are in turn connected to (external) electrodes (or respective side electrodes) 25, 26 of corresponding polarity, the electrodes 25, 26 being spaced apart around the periphery of the actuator 2 on diametrically opposite sides of the actuator 2. However, it may also be advantageous to arrange the (external) electrodes (or respective side electrodes) 25, 26 directly adjacent to each other. In this example, the electrode layer 2c is connected as an internal electrode to the positive pole 2b of the piezoelectric ceramic layer 2a and is connected to the positive external electrode 25 (right side of FIG. 6), whereas the electrode layer 2e is connected as an internal electrode to the negative pole 2d of the piezoelectric ceramic layer 2a and is connected to the negative external electrode 26.

[0066] The piezoelectric ceramic layer 2a and the electrical contacts or electrode layers (internal electrodes) 2c, 2e are of different sizes and are stacked on top of each other such that the piezoelectric ceramic layer 2a extends radially with respect to the axis A beyond the electrical contacts or electrode layers (internal electrodes) 2c, 2e. The electrical contacts or electrode layers (internal electrodes) 2c, 2e are thereby surrounded by piezoelectric ceramic material at their edges, creating an airtight and moisture-proof "insulator" that allows the actuator 2 to be used in wet environments. The parts of the piezoelectric ceramic layer 2a that are covered by and can be actuated by the electrical contacts or electrode layers (internal electrodes) 2c, 2e, or the parts that protrude on a plane oriented perpendicular to the axis A and are located radially inward of the electrical contacts or electrode layers (internal electrodes) 2c, 2e, form the active area of ​​the actuator (shown as a shaded area in Figures 1-3) that deforms when electrically actuated. The parts of the piezoelectric ceramic layer 2a that are not covered with electrical contact or electrode layers (internal electrodes) 2c, 2e and therefore inoperable, or the parts that protrude onto a plane E oriented perpendicular to the axis A and are situated radially outside the electrical contact or electrode layers (internal electrodes) 2c, 2e, form passive edge regions of the actuator 2 that passively follow the deformations of the active region. On the circumferential, diametrically opposite sides of the covering, slightly radially protruding (external) electrodes (or lateral electrodes, respectively) 25, 26 are arranged to facilitate electrical bonding.

[0067] The actuator 2, represented in FIG. 5 as a partial cross-sectional perspective view, includes a total of 20 piezoelectric ceramic layers 2a, with 10 electrical contacts (internal electrodes) 2e connected to an (external) negative electrode (or respective side electrode) 26 (on the left side in FIG. 5) and 10 electrical contacts (internal electrodes) 2c connected to an (external) positive electrode 25 (or respective side electrode, on the right side in FIG. 5, not shown).

[0068] In a preferred embodiment, a monolithically sintered ceramic insulating layer 20 surrounds the periphery of the actuator 2 except for the negative and positive electrodes 25 and 26 .

[0069] In this example, a circular bonding area K is formed on the upper side 21 of each actuator 2 of piezoelectric ceramic material. The bonding area K extends in a plane E perpendicular to and coaxial with the axis A of the actuator 2 and is arranged axially offset at its apex by a step relative to the adjacent surrounding edge region of the actuator 2, thereby forming the axial upper end of the actuator 2.

[0070] Instead of or in addition to the upper connecting region K, a corresponding connecting region K can also be formed on the underside of the actuator 2 (see FIG. 3a). Instead of a step, the connecting region K can also transition via a chamfer into the adjacent or surrounding region, in particular into the edge region (see FIGS. 3b-3d).

[0071] The coupling range K is, according to the invention, smaller than the projection range P of the actuator 2 on a plane E perpendicular to the axis A. The meaning of this feature will be explained with reference to Figs.

[0072] When electrically activated, each actuator 2 deflects along its axis A. The direction of this deflection corresponds to the axis A along which the piezoelectric ceramic layers 2a are stacked one on top of the other. When electrically activated, an active area of ​​the actuator 2 (represented by hatching in Figures 1 to 3) deforms. A passive area of ​​the actuator 2 (represented outside the hatching in Figures 1 to 3) does not actively deform, but passively follows the deformation of the active area.

[0073] In this example, the coupling area K protrudes onto a plane E perpendicular to the lamination axis S, and is located completely within the protruding area P of the actuator 2, and is surrounded by the protruding area P over its entire periphery (see FIG. 4).

[0074] In order to explain the principle of the present invention with reference to the drawings, a stack actuator 1 including such an actuator 2 will be described below in particular. The number of actuators 2 combined in the stack actuator 1 is not limited.

[0075] For example, the piezoelectric stack actuator 1 shown in FIG. 7 is made up of a total of six individual actuators 2, each of which functions independently to cause a deflection along an axis A when each is electrically actuated.

[0076] In the stack actuator 1 depicted in FIG. 7 , the individual actuators 2 are coupled such that when the actuators 2 are electrically actuated, their deflections overlap along the stacking axis S, and the force coupling of each of the actuators 2 occurs on an upper coupling area K that is smaller than the protruding area P of the actuator 2 on a plane E perpendicular to the stacking axis S.

[0077] Identical designed actuators 2 (see FIG. 5) are arranged with identical orientation relative to the stacking axis S, such that their coupling areas K face upwards and the (external / side) electrodes 25, 26 of the actuators 2 of the same polarity are oriented in line parallel to the stacking axis S. The facing front sides of adjacent actuators 2 are parallel.

[0078] In the example according to FIG. 7, two adjacent actuators 2 are each directly actively connected by an adhesive forming a solid adhesive connection, each being completely bonded in one bonding area K.

[0079] The (external / side) electrodes 25 , 26 of the actuators 2 of the same polarity are connected to each other by connections 11 , which are fixed to the corresponding electrodes of each actuator 2 at respective fixing points 12 .

[0080] In the bonding plane including the bonding area K, the edge regions of the actuators 2 are separated from one another and freely movable with respect to one another outside the bonding area K. As a result, peak loads due to passive deformations of the piezoelectric ceramic layers 2a can be reduced without preventing the bonding of the actuators 2 to one another in the edge regions. Thereby, crack formation is prevented and the service life of the stack actuator 1 is significantly increased.

[0081] In the present example, each coupling range K is designed as a plane extending perpendicular to the axis A or S, respectively. The range dimension of the coupling range K is approximately 0.9 times or 90% of the projection range P of the actuator 2 on the plane perpendicular to the axis A.

[0082] In other words, the advantage of the invention of the reduced bonding area K compared to the flat extension of the actuator 2 in a plane perpendicular to the axis A is analogous to the selective separation of adjacent actuators 2 from each other in the edge regions formed by passive insulator edges. In each actuator 2, tensile stresses arise in the so-called passive regions when it expands, since this region does not contain the electrodes 2b and therefore does not actively deform. Due to the low height of the actuator 2 and the passive deformation of the edge regions, the tensile stresses are below the critical load in each individual actuator 2. If the actuator 2 were bonded entirely over its entire cross-sectional area (extension in a plane perpendicular to the axis A), the bonding of the actuators would prevent and hinder this deformation. This effect increases with the number of actuators 2 bonded to each other. The stresses generated by the deformations would add up, if no measures were taken, and would exceed the ultimate strength of the passive boundary layer. Without the features according to the invention, crack formation would occur at the weakest points.

[0083] To achieve high actuator stiffness, stiff adhesives and very thin adhesive gaps are used, so stress relief by the adhesive layer is usually insufficient. Nevertheless, soft damping adhesives may also be used within the scope of the present invention.

[0084] Therefore, the bonding is preferably only performed in the bonding area K and not in the edge areas of the actuator 2 surrounding the bonding area K in the bonding plane. The bonding area K is smaller than the flat extension of the actuator 2 to be bonded in a plane E perpendicular to the stacking axis S or smaller than the protruding area P of the actuator 2 to be bonded on the plane E perpendicular to the axis A or S, respectively (see FIG. 4). Due to the deformation of the passive areas of the actuator 2, the deformation of the actuator 2 takes place strictly along this axis A or S, respectively, which may result in a corresponding reduction in the peak load.

[0085] Various modifications of the embodiment are possible in order to realize the subject matter of the present invention. On the one hand, it is possible to leave the adhesive in the edge area. From a technical point of view, this is complicated, since the adhesive flows under pressure when curing and often at elevated temperature. For this reason, a defined and reproducible bond over an unlimited range in a plane is almost impossible. Therefore, it is preferable that the bonding range K is arranged offset or limited with respect to the adjacent ranges.

[0086] Therefore, it is preferred to apply a release agent to the area of ​​the insulator edge that delimits the bonding area K by preventing contact with the adhesive or at least its adhesion to at least one connection destination.

[0087] Alternatively, a thin additional element 3 with a smaller cross section can be placed between each of the two actuators 2, which additional element 3 forms a coupling region K to the adjacent actuator 2 (FIG. 1 or FIG. 6). For example, a thin metal film with a perimeter smaller than the actuators 2 (at least by an amount equal to the width of the insulator edge) can be glued between the actuators 2. The insulator edge thus remains free and can deform in the same way as in the case of a single actuator 2.

[0088] It is also possible to create a small step on the axial front side of each actuator 2 (Fig. 3a). This step is approximately as wide as the insulator edge and a few tens of μm high. This achieves the same result as inserting an additional thin additional element 3 (Fig. 1 or Fig. 6), but simplifies assembly and positioning and reduces the number of joints. The step can be on only one side (not shown) or on both sides (Fig. 3a). A one-sided design reduces the manufacturing effort and a two-sided design reduces the required thickness of the passive end layers since less material is removed per side.

[0089] Instead of steps, chamfers or other recesses can also be used (FIGS. 3a-3d).Furthermore, actuators 2 with arched surfaces are possible.

[0090] In high-voltage actuators, thin plates with reduced cross-section are usually glued between the actuator parts (FIG. 6). They act as electrodes 2b, 2d for later contacting the external electrodes 25, 26 and the electrodes on the surface of the stack, and at the same time as additional elements 3 by reducing the coupling area between the actuators 2 compared to their surface extension in a plane oriented perpendicular to the axes A, S. However, the actuators 2 are coated with adhesive to protect them from external influences, so that the remaining gaps in the edge regions of the actuators 2 are filled with adhesive.

[0091] One solution for this type of actuator is to apply a protective layer that does not completely fill the gap. This is possible on the one hand by applying a very thin layer (e.g. by evaporation, spraying, etc.). Furthermore, it is possible to apply a film instead of a liquid.

[0092] A further alternative is to fill the gaps with an elastic material, after which the stack can be coated normally.

[0093] For both types of actuator 2, the above solution is applicable to any cross-section, in terms of shape (circular, elliptical, rectangular, etc.) and size. Here, not only solid cross-sections are conceivable, but also hollow cross-sections, for example in the form of a hollow cylinder.

[0094] Testing with a variety of actuators 2 has shown that the service life under static tension of individual chip actuators is typically significantly higher than the service life of bonded chip stacks, in some cases at least two times higher.

[0095] Isolation of the critical areas of the chips according to the present invention maintains the lifetime advantages of bonded stacks.

[0096] For an actuator 2 with a circular cross section in a plane perpendicular to the stacking axis S, a ratio can be specified between the diameter D and the height H. In an advantageous development of the above-mentioned embodiment, this ratio is D / H≧1, preferably D / H>5 and particularly preferably D / H=6.4. It can be even more advantageous if D / H<50.

[0097] For an actuator 2 with a rectangular cross section in a plane perpendicular to the stacking axis S, it is possible to specify a ratio of edge length L to height H. In an advantageous development relating to an actuator 2 with a rectangular cross section, this ratio is L / H≧1, preferably L / H>5, particularly preferably L / H=6.4. It may be even more advantageous if L / H<50.

[0098] Such D / H and L / H ratios provide an actuator that has high rigidity and at the same time high deflection, and further has a minimal tendency to crack, resulting in a highly reliable actuator.

[0099] An actuator 2 with a circular cross section is generally advantageous compared to an actuator 2 with a rectangular cross section in that a more uniform deformation occurs in the transition region between the active and passive regions, since the passive region has a constant radial distance from the stacking axis S (this is not the case for square or rectangular actuator cross sections). [Explanation of symbols]

[0100] List of Reference Numbers 1 Stack Actuator 2 Actuators 2a Piezoelectric ceramic layer 2b positive electrode 2c Positive contact (plate) or (inner) electrode 2d negative electrode 2e Negative contact (plate) or (internal) electrode 3 Additional elements 20 Insulators 21 Upper 22 Lower side 23 Upper step / chamfer 24 Lower step / chamfer 25 (External / Side) Electrode Positive 26 (external / side) electrode negative electrode A-axis D is the diameter of the actuator with a circular cross section E Plane perpendicular to the lamination axis H Actuator height K bond range L is the edge length of the actuator with a rectangular cross section Projection range on a plane perpendicular to the P axis S laminated shaft

Claims

1. A method for manufacturing a piezoelectric stack actuator (1), comprising the steps of: a. Step A: Providing at least two actuators (2) designed to cause a deflection along an axis (A) when electrically actuated, Step A comprising the following sub-steps: Sub-step A1: providing a piezoelectric ceramic layer (2a) and electrical contacts (2c, 2e) for constructing the actuator (2) as a multi-layer actuator; Sub-step A4: covering the piezoelectric ceramic layer (2a) and the electrical contacts (2c, 2e) with a ceramic insulating material (20); The method further comprises: b. Step B: The method includes coupling at least two actuators (2) to form the stack actuator (1) such that the deflections of the actuators (2) caused by the electrical actuation of the actuators (2) overlap along a stacking axis (S) and force coupling of the actuators (2) occurs on at least one coupling area (K) that is smaller than a protruding area (P) of the actuators (2) on a plane (E) perpendicular to the stacking axis (S).

2. Step A comprises the following sub-steps: a. Sub-step A2: Laminating the piezoelectric ceramic layers (2a) along the axis (A); b. Sub-step A3: connecting the electrical contacts (2c, 2e) to electrodes (25, 26) of corresponding polarity; c. Sub-step A4-1: Coating the piezoelectric ceramic layer (2a) and the electrical contacts (2c, 2e) with a monolithically sintered ceramic insulating material (20). d. Sub-step A5: forming at least one coupling area (K) for force-coupling the actuator (2) to an adjacent actuator (2); The method of claim 1 , comprising at least one of:

3. Step B comprises the following sub-steps: a. Sub-step B2: Placing an additional element (3) between two adjacent actuators (2); b. Sub-step B3: connecting at least two actuators (2) such that each of two adjacent actuators (2) is directly actively connected or indirectly actively connected via an additional element (3); c. Sub-step B4: Separating the two actuators (2) from each other outside the at least one coupling area (K); d. Sub-step B5: Connecting together the electrodes (25, 26) of the actuator (2) of the same polarity by means of a connection (11).

3. The method according to claim 1, further comprising at least one of the following steps:

4. 1. A piezoelectric stack actuator comprising at least two actuators (2), each of which is a multi-layer actuator and is made of a piezoelectric ceramic layer (2a) and electrical contacts (2c, 2e), and is designed to deflect along an axis (A) when electrically activated, the piezoelectric ceramic layer (2a) and the electrical contacts (2c, 2e) being coated with a ceramic insulating material (20), the at least two actuators (2) being stacked along a stacking axis (S) such that the deflections of the actuators (2) that occur when the actuators (2) are electrically activated overlap along the stacking axis (S), and the at least two actuators (2) are coupled such that a coupling area (K) where a force coupling between an actuator (2) and an adjacent actuator (2) occurs is smaller than a protrusion area (P) of the actuator (2) on a plane perpendicular to the axis (A).

5. Each of said actuators (2) has the following characteristics: a. The piezoelectric ceramic layers (2a) are stacked along an axis (A); b. the electrical contacts (2c, 2e) are connected to electrodes (25, 26) of corresponding polarity; c. the piezoelectric ceramic layer (2a) and the electrical contacts (2c, 2e) are coated with a monolithically sintered ceramic insulating material (20); d. the actuator has a substantially circular or nearly circular cross-sectional shape in a plane perpendicular to the stacking axis S; e. the actuator has a solid or hollow cross-section in a plane perpendicular to the stacking axis S; f. For actuators (2) with a circular cross section, the ratio of diameter to height is D / H≧1; g. For actuators (2) having a rectangular cross section, the edge length to height ratio L / H is ≥ 1; 5. The piezoelectric stack actuator (1) according to claim 4, characterized in that it comprises at least one of the following:

6. The bond range (K) has the following characteristics: a. said at least one coupling area (K) is in said covering part of said actuator (2); b. said at least one coupling region (K) is formed on an additional element (3) to be arranged between two actuators (2) to be coupled; c. said at least one coupling region (K) is formed at an axial end of one or both of said actuators (2) or said additional elements (3), respectively; d. The at least one coupling area (K) is designed with respect to the protruding area (P) of the actuator (2) such that the following area ratio applies: 0.1*P<K<P. A piezoelectric stack actuator (1) according to claim 4 or 5, characterized in that it comprises at least one of the following:

7. The stack actuator (1) has the following features: a. at least one additional element (3) is disposed between two adjacent actuators (2); b. at least two adjacent actuators (2) are directly actively connected or indirectly actively connected via an additional element (3); c. at least two adjacent actuators (2) are separated from each other except for said at least one coupling area (K); d. the electrodes (25, 26) of the actuator (2) of the same polarity are connected to each other by a connection (11); A piezoelectric stack actuator (1) according to any one of claims 4 to 6, characterized in that it comprises at least one of the following:

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