Method for producing a multi-layered structural element, and a multi-layered structural element produced according to said method

Abstract

A multi-layered structural element and a method for producing a multi-layered structural element are disclosed. In an embodiment dielectric green sheets, at least one ply containing an auxiliary material which contains at least one copper oxide and layers containing electrode material are provided and arranged alternately one above another. These materials are debindered and sintered. The copper oxide is reduced to form the copper metal and the at least one ply is degraded during debindering and sintering.

Claims

1. A method for producing a multi-layered structural element, the method comprising: providing an electrode material comprising copper and green sheets containing a dielectric material; providing an auxiliary material containing at least one copper oxide; forming a stack comprising the dielectric green sheets, at least one ply containing the auxiliary material and layers containing the electrode material arranged alternately one above another; and debindering and sintering the stack comprising diffusing and reducing the at least one copper oxide of the at least one ply so that copper ions diffuse from the auxiliary material to adjacent electrode layers thereby forming the multi-layered structural element comprising at least one weakening layer and dielectric ceramic layers with internal electrode layers arranged between them, the internal electrode layers consisting essentially of a metal comprising copper, wherein the weakening layer has a reduced fracture strength compared to the dielectric ceramic layers, and wherein a concentration of the copper in the electrode material is present in a smaller proportion than a concentration of copper in the auxiliary material, the copper in the auxiliary material being part of the at least one copper oxide.

2. The method according to claim 1, wherein debindering and sintering comprises applying oxygen partial pressure thereby reducing the at least one copper oxide to a copper metal.

3. The method according to claim 2, wherein reducing the at least one copper oxide to the copper metal is concluded after debindering.

4. The method according to claim 1, wherein the copper oxide is CuO.

5. The method according to claim 1, wherein the copper oxide is Cu.sub.2O.

6. The method according to claim 1, wherein the auxiliary material consists of one component.

7. The method according to claim 1, wherein the dielectric material is a piezoelectric material.

8. A method for producing a multi-layered structural element, the method comprising: providing green sheets comprising a dielectric material; providing an electrode material comprising copper on some of the green sheets; providing plies, each ply comprising an auxiliary material containing at least one copper oxide; forming a stack comprising the dielectric green sheets and a ply between adjacent green sheets with the electrode material; and debindering and sintering the stack comprising reducing and diffusing the at least one copper oxide of the plies so that copper ions diffuse from the auxiliary material to the adjacent electrode material thereby forming the multi-layered structural element comprising electrode layers formed from the electrode material, weakening layers formed from the plies and dielectric ceramic layers formed from the green sheets, the electrode layers comprising a copper metal, wherein the weakening layers have a reduced fracture strength compared to the dielectric ceramic layers, and wherein a concentration of the copper in the electrode material is present in a smaller proportion than a concentration of copper in the auxiliary material, the copper in the auxiliary material being part of the at least one copper oxide.

9. The method according to claim 8, wherein the at least one copper oxide is CuO.

10. The method according to claim 8, wherein the at least one copper oxide is Cu.sub.2O.

11. The method according to claim 8, wherein the at least one copper oxide consists of CuO.

12. The method according to claim 8, wherein the at least one copper oxide consists of Cu.sub.2O.

13. The method according to claim 8, wherein the dielectric ceramic layers comprise lead zirconate titanate (PZT), and wherein the lead zirconate titanate (PZT) is Pb(Zr.sub.xTi.sub.1-x)O.sub.3, where o≤x≤1.

14. The method according to claim 8, wherein the dielectric ceramic layers comprise a lead free ceramic.

15. The method according to claim 8, wherein the electrode material comprises a CuPd paste.

16. The method according to claim 8, wherein the weakening layers are porous layers.

17. The method according to claim 8, wherein debindering comprises debindering at a temperature between 500° C. and 600° C.

18. The method according to claim 17, wherein sintering comprises sintering at a temperature of 1010° C.

19. The method according to claim 18, further comprising forming external electrodes on opposite faces of the stack after sintering, wherein forming the external electrodes comprises applying a metal paste comprising copper or silver on opposite faces of the stack and firing the metal paste.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The stated method and the advantageous configurations thereof will be explained herein below on the basis of schematic figures, which are not true to scale, and also on the basis of an exemplary embodiment.

(2) FIG. 1 shows the schematic side view of a multi-layered structural element produced by the method;

(3) FIGS. 2a and 2b show enlarged schematic side views of a multi-layered structural element;

(4) FIG. 3a shows an optical micrograph (magnification 1:170) of a multi-layered structural element in profile after debindering, the copper oxide being 100% Cu.sub.2O and the copper present in the layers have been blackened;

(5) FIG. 3b shows an optical micrgraph (magnification 1:150) of a multi-layered structural element in profile after debindering, the coppper oxide being 100% CuO and the copper present in the layers have been blackened;

(6) FIG. 3c shows an optical micrograph (magnification 1:170) of a multi-layered structural element in profile after debindering, the coppper oxide being 50%Cu/50% CuO and the copper present in the layers have been blackened;

(7) FIG. 3d shows an optical micrograph (magnificaton 1:150) of a multi-layered structural element in profile after debindering for reference, no weakening layer being present; and

(8) FIG. 4 shows equilibrium curves of the systems Cu.sub.2O/Cu, Pb/PbO, Zr/ZrO.sub.2, Pb+TiO.sub.2+O.sub.2/PbTiO.sub.3 and Ni/NiO for different oxygen partial pressures and temperatures.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(9) FIG. 1 shows a schematic side view of a piezoelectric multi-layered structural element in the form of a piezo actuator. The structural element comprises a stack 1 of piezoelectric layers 10 arranged one above another and internal electrodes 20 lying between them. The internal electrodes 20 are formed as electrode layers. The piezoelectric layers 10 and the internal electrodes 20 are arranged one above another. Weakening layers 21 are shown in FIG. 1 between piezoelectric layers 10 and parallel to the internal electrodes 20. The weakening layer 21 is to be understood to be a region in which predetermined breaking points are present in the structural element.

(10) 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 internal electrodes 20 contain, for example, a mixture or an alloy of Cu and Pd.

(11) In order to produce the stack 1, for example, green sheets which contain a ceramic powder, an organic binder and a solvent are produced by sheet drawing or sheet casting. An electrode paste is applied by screen printing to some of the green sheets, in order to form the internal electrodes 20. Furthermore, plies containing an auxiliary material having a first and a second component are applied to one or more green sheets in order to form weakening layers 21. The green sheets are stacked one above another along a length direction and compressed. The intermediate products of the structural elements are separated in the desired shape from the sheet stack. Lastly, the stack of piezoelectric green sheets, plies of auxiliary material and electrode layers is debindered and sintered. External electrodes 30, which are also shown in FIG. 1, are furthermore applied after the debindering and sintering.

(12) In the embodiment shown here, the external electrodes 30 are arranged on opposing side faces of the stack 1 and extend in the form of strips along the stacking direction. The external electrodes 30 contain, for example, Ag or Cu and may be applied to the stack 1 as a metal paste and fired in.

(13) The internal electrodes 20 are led along the stacking direction alternately as far as one of the external electrodes 30 and are at a distance from the second external electrode 30. In this way, the external electrodes 30 are electrically connected alternately to the internal electrodes 20 along the stacking direction. In order to produce the electrical connection, a connection element (not shown here) may be applied to the external electrodes 30, for example, by soldering.

(14) The production of the multi-layered structural element shown in FIG. 1, which contains weakening layers 21, is to be explained in more detail with the aid of the following exemplary embodiment:

(15) To form the stack 1, a ply containing an auxiliary material is applied to at least one green sheet. A CuPd paste is printed onto green sheets as the electrode material. The auxiliary material contains CuO.

(16) If a temperature of 500 to 600° C., at which compaction of the stack 1 still does not occur, is then reached during debindering under a reducing atmosphere, i.e. using an equilibrium partial pressure of the oxygen lying between the curves Pb/PbO and Cu.sub.2O/Cu (such that PbO and Cu are present at the same time) (see, e.g., FIG. 4), copper ions diffuse from the auxiliary material in the direction of the adjacent electrode layers 20. Moreover, the copper ions and further copper oxide from the auxiliary material are reduced to form copper metal. As a result, the ply containing the auxiliary material is degraded and therefore the weakening layer 21 is formed. The porosity of the weakening layer can be set or influenced, for example, by the grain size of the CuO used.

(17) The diffusion per se is promoted by the difference in concentration of Cu which is present in the ply containing the auxiliary material or containing the components formed from the auxiliary material and in the electrode material.

(18) FIG. 2a shows an enlarged excerpt of the schematic side view of the multi-layered structural element. The formation of cracks in multi-layered structural elements is to be explained with the aid of this figure.

(19) The structural element expands in the longitudinal direction when a voltage is applied between the external electrodes 30. In a so-called active zone, in which adjacent internal electrodes 20 in the stacking direction overlap, an electric field arises when a voltage is applied to the external electrodes 30, such that the piezoelectric layers 10 expand in the longitudinal direction. In inactive zones, in which adjacent electrode layers 20 do not overlap, the piezo actuator expands only slightly.

(20) On account of the different expansion of the structural element in the active and inactive zones, mechanical stresses occur in the stack 1. Such stresses can lead to poling cracks and/or relaxation cracks 25 in the stack 1.

(21) FIG. 2a shows an excerpt from a stack 1 composed of piezoelectric layers 10 and internal electrodes 20, wherein a crack 25 has arisen in the stack 1. The crack 25 runs within the inactive zone parallel to the internal electrodes 20, bends away at the transition into the active zone and runs in the active zone through adjacent internal electrodes 20 of differing polarity. This can lead to a short circuit of the internal electrodes 20.

(22) FIG. 2b shows an excerpt from a stack 1 composed of piezoelectric layers 10 and internal electrodes 20, in which a crack 25 has likewise arisen. Here, the crack 25 runs parallel to the internal electrodes 20. In the case of such a course of cracks 25, the risk of short circuits is reduced.

(23) In order to promote such a course of cracks 25, the multi-layered structural element is produced according to the aforementioned method, so that the cracks 25 form in a targeted manner in the region of the weakening layer 21, in which there is a predetermined breaking point.

(24) FIGS. 3a to 3d show optical micrographs (magnification 1:170 (FIGS. 3a to 3c) and 1:150 (FIG. 3d))) of a multi-layered structural element in profile after the debindering method has been carried out under a reducing atmosphere. Here, 100% Cu.sub.2O was used in FIG. 3a, 100% CuO was used in FIG. 3b and 50% Cu/50% CuO was used in FIG. 3c. FIG. 3d shows a multi-layered structural element without weakening layers for comparison. The degree of blackening in FIGS. 3a to 3d is in direct correlation with the copper content in the ceramic layers. It was possible to make the verification using laser ablation in combination with ICP/MS. The reaction which takes place here is as follows: CuO->Cu+0.5 O.sub.2 or Cu.sub.2O->2 Cu+0.5 O.sub.2. Copper cations diffuse faster than metallic copper. The greater the oxygen proportion of the added Cu component, the greater the extent of diffusion in the debindering process. In case d), there is no copper oxide component, and therefore the diffusion is not stronger or weaker than in the case of all the other Cu electrodes.

(25) In FIG. 3a, the blackening proceeding from the auxiliary material reaches as far as the two adjacent electrode layers. In FIG. 3b, the blackening even encompasses two adjacent electrode layers on each side of the auxiliary layer. It can therefore be seen from a comparison of FIGS. 3a and 3b that the region in which blackening occurs is approximately twice as large in FIG. 3b than in FIG. 3a. It follows from this that CuO (oxidation number of the copper: 2) is preferred to Cu.sub.2O as auxiliary material, since divalent copper has a higher degree of diffusion and a greater length of diffusion.

(26) In FIG. 3c, in which 50% Cu/50% CuO is used as the auxiliary material, the blackening is locally limited and does not extend as far as the adjacent electrode layers. The degree of diffusion and length of diffusion of the copper in the ceramic material adjacent to the auxiliary layer therefore decrease when the auxiliary material additionally contains copper metal (oxidation number 0). It is therefore preferable to use an auxiliary material which does not contain any copper metal.

(27) The invention is not limited by the description with reference to the exemplary embodiments. Instead, the invention encompasses any new feature and also any combination of features which, in particular, contains any combination of features in the patent claims, even if this feature or this combination itself is not explicitly indicated in the patent claims or exemplary embodiments.