Method for producing a piezoelectric multilayer component and a piezoelectric multilayer component

Abstract

A piezoelectric multilayer component having a stack of sintered piezoelectric layers and inner electrodes arranged between the piezoelectric layers. A region which has poling cracks is present on the surface of at least one electrode, and the poling cracks are separated from a surface of at least one of the inner electrodes by the region having the poling cracks.

Claims

1. A piezoelectric multilayer component comprising: a stack of sintered piezoelectric layers and inner electrodes arranged between the piezoelectric layers, wherein a region that comprises cracks is present on a surface of at least one of the inner electrodes, and wherein the cracks are separated from a surface of at least one of the inner electrodes by the region comprising the cracks; and wherein the region that comprises the cracks has at least one of a PbO content that is greater than a PbO content of the piezoelectric layers, and traces of PbPdO.sub.2.

2. The multilayer component according to claim 1, wherein the cracks are substantially free of branching.

3. The multilayer component according to claim 1, wherein the inner electrodes comprise Pd.

4. The multilayer component according to claim 1, wherein the piezoelectric layers comprise lead zirconate titanate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The specified method and the specified multilayer component and their advantageous configurations will be explained below with the aid of schematic figures, which are not true to scale, and with the aid of exemplary embodiments.

(2) FIG. 1 shows the schematic side view of the multilayer component;

(3) FIG. 2a shows an enlarged detail of the schematic side view of the multilayer component;

(4) FIG. 2b shows another enlarged detail of the schematic side view of the multilayer component;

(5) FIG. 3a shows another enlarged detail of the schematic side view of the multilayer component; and

(6) FIG. 3b shows another enlarged detail of the schematic side view of the multilayer component.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(7) FIG. 1 shows a schematic side view of a piezoelectric multilayer component in the form of a piezo actuator. The component comprises a stack 1 of piezoelectric layers 10 arranged above one another and inner electrodes 20 lying between them. The inner electrodes 20 are formed as electrode layers. The piezoelectric layers 10 and the inner electrodes 20 are arranged above one another.

(8) The piezoelectric layers 10 contain a ceramic material, for example lead zirconate titanate (PZT) or a lead-free ceramic. The ceramic material may also contain dopants. The inner electrodes 20 contain, for example, a mixture or an alloy of Ag and Pd or Cu and Pd. The Pd has a proportion by weight of up to 40% in the electrode material.

(9) In order to produce the stack 1, for example, green films which contain a ceramic powder, an organic binder and a solvent are produced by film drawing or film casting. On some of the green films, an electrode paste is applied by means of screen printing in order to form the inner electrodes 20. The green films are stacked above one another along a length direction and compressed. The intermediate products of the components are separated in the desired shape from the film stack. Lastly, the stack of piezoelectric green films and electrode layers is sintered.

(10) Outer electrodes 30, which are also shown in FIG. 1, are furthermore applied after the sintering.

(11) In the embodiment shown here, the outer electrodes 30 are arranged on opposite side faces of the stack 1 and extend in the form of strips along the stack direction. The outer electrodes 30 contain, for example, Ag or Cu and may be applied onto the stack 1 as a metal paste and burnt in.

(12) Alternately along the stack direction, the inner electrodes 20 are led up to one of the outer electrodes 30 and separated from the second outer electrode 30. In this way, the outer electrodes 30 are electrically connected to the inner electrodes 20 alternately along the stack direction. In order to produce the electrical connection, a connection element (not shown here) may be applied onto the outer electrodes 30, for example by soldering.

(13) FIG. 2a shows an enlarged detail of the schematic side view of the multilayer component.

(14) The component expands in the length direction when a voltage is applied between the outer electrodes 30. In a so-called active zone, in which neighboring inner electrodes 20 overlap in the stack direction, an electric field is set up when a voltage is applied to the outer electrodes 30, so that the piezoelectric layers 10 expand in the length direction. In inactive zones, in which neighboring electrode layers 20 do not overlap, the piezo actuator expands only slightly.

(15) Owing to the different expansion of the component in the active and inactive zones, mechanical stresses occur in the stack 1. Such stresses can lead to poling and/or relaxation cracks 25 in the stack 1.

(16) FIG. 2a shows a detail of a stack 1 of piezoelectric layers 10 and inner electrodes 20, in which a crack 25 has been formed in the stack 1. The crack 25 extends parallel to the inner electrodes 20 inside the inactive zone, changes direction at the transition into the active zone and extends through neighboring inner electrodes 20 of different polarity in the active zone. This can lead to a short circuit of the inner electrodes 20.

(17) FIG. 2b shows a detail of a stack 1 of piezoelectric layers 10 and inner electrodes 20, in which a crack 25 has likewise been formed. Here, the crack 25 extends parallel to the inner electrodes 20. With such a profile of cracks 25, the risk of short circuits is reduced.

(18) In order to promote such a profile of cracks 25, at least one layer of electrode material is provided with a PbO coating during the production of the multilayer component. The PbO may be applied in the form of PbO powder, Pb.sub.3O.sub.4 or another material containing PbO. Liquid forms of chemical compounds containing Pb are likewise possible. The grain size of the PbO may have a median value d.sub.50 (the particle size distribution) of from 0.1 μm to 2 μm, preferably from 0.3 μm to 1.5 μm.

(19) In addition or as an alternative, PbO may also be mixed with the electrode material. In this case, the PbO content relative to the sum of PbO and the other metals of the electrode material is up to 100 wt %, preferably up to 50 wt %.

(20) A layer of electrode material may be provided with the coating or contain PbO; preferably, all layers may have a coating or contain PbO.

(21) An intermediate phase, which contains PbPdO.sub.2 and is spatially restricted to the region in which the PbO was present, is then formed during the sintering. Owing to the different volume and the different expansion behavior of this intermediate phase and of the contiguous piezoelectric layer, microcracks are formed in the region of the intermediate phase. After the conclusion of the sintering, when the PbPdO.sub.2 has been reconverted, the region weakened by microcracks remains between the inner electrode and the piezoelectric layer. Relaxation and/or poling cracks, which are likewise spatially restricted, can then be formed from these microcracks. Branching of these cracks is thereby reduced or prevented.

(22) FIG. 3a shows another detail of a schematic side view of the multilayer component. Here again, a crack 25 has been formed along an inner electrode 20. The region 21, in which the crack is spatially restricted, is additionally shown.

(23) This region 21 lies between the inner electrode 20 and the piezoelectric layer 10, specifically where the coating containing PbO was arranged on the electrode paste during the production method. In this region, the PbO reacted with the Pd from the electrode material to form PbPdO.sub.2, and thereby formed the intermediate phase. Microcracks were able to form in the intermediate phase, as described above, and opened to form cracks during the polarization. The crack can only propagate in this region 21, so that it does not branch.

(24) FIG. 3b shows a similar detail to FIG. 3a of a schematic side view of the multilayer component. Here, the cracks 25 extend around the inner electrodes 20. Such a distribution may occur above all when the PbO has been mixed with the electrode material, so that the region in which PbPdO.sub.2 is formed occurs uniformly around the electrode layer.

(25) A multilayer component produced in this way may contain pure lead zirconate titanate (PZT) or a PZT modified with dopants.

(26) An example of a multilayer component comprises the piezoelectric layer 10 (Pb.sub.1−x+a,Nd.sub.x)((Zr.sub.1−z,Ti.sub.z).sub.1−y,Ni.sub.y)O.sub.3 as PZT ceramic. In this case, the values may be x=0.0001 to 0.06, a=0 to 0.05, z=0.35 to 0.60 and y=0.0001 to 0.06. A PbO powder, which has a grain size of 1 μm, is used as a coating for the electrode layer. The inner electrodes contain Cu and Pd and a proportion by weight of PbO of from 10% to 50% in the electrode material. All electrode layers are printed with PbO for the production method.

(27) By the description with the aid of the exemplary embodiments, the invention is not restricted thereto but covers any new feature and any combination of features. This in particular comprises any combination of features in the patent claims, even if this feature or this combination is not itself indicated explicitly in the patent claims or exemplary embodiments.