US 7268471 B2
An electrical component includes a base having ceramic layers and internal electrodes between at least some of the ceramic layers. The electrical component also includes an external electrode on a face of the base, which contacts at least some of the internal electrodes. The external electrode includes a layer having at least one local minimum.
1. An electrical component comprising:
a base comprising:
ceramic layers, and
internal electrodes among at least some of the ceramic layers; and
an external electrode on a face of the base, the external electrode contacting at least some of the internal electrodes, the external electrode comprising a layer that has an indentation, wherein a thickness of the layer at the indentation is a local minimum thickness.
2. The electrical component of
3. The electrical component of
4. The electrical components of
5. The electrical component of
6. The electrical component of
7. The electrical components of
8. The electrical component of
9. The electrical component of
10. The electrical component of
11. The electrical component of
12. The electrical component of
13. A method for producing an electrical component, comprising:
producing a base, the base comprising:
ceramic layer, and
internal electrodes among at least some of the ceramic layers, a face of the base comprising an external electrode that contacts at least some internal electrodes, the external electrode comprising a layer that has an indentation, wherein a thickness of the layer at the indentation is a local minimum thickness:
establishing contact between the external electrode and a contact element; and
exerting a shearing force between the external electrode and the face of the base containing the external electrode.
14. The method of a
wherein soldering is used to establish contact between the external electrode and the contact element.
15. The method of
attaching wires to the external electrode by soldering at a temperature that is greater than 200° C.
16. The method of
17. An electrical component comprising:
electrodes among at least some of the ceramic layers, the ceramic layers and the electrode layers together forming a stack having a first surface and a second surface, the electrode layers comprising alternating first electrodes and second electrodes, the first electrodes extending to the first surface but not to the second surface, the second electrodes extending to the second surface but not to the first surface; and
an external electrode on the first surface, the external electrode contacting the first electrodes, and the external electrode comprising a layer having one or more local indentations, wherein a thickness of the layer at an indentation is a local minimum thickness.
18. The electrical component of
19. The electrical component of
20. The electrical component of
21. The electrical component of
22. The electrical component of
23. The electrical component of
This patent application describes an electrical multi-layer component, in particular a peizo actuator. This patent application also describes a method for producing such an electrical multi-layer component.
Piezo actuators are known that have a base body, with a stack of stratified ceramic layers, and internal electrodes lying between them. The internal electrodes are made from a mixture of silver and palladium. The ceramic layers contain a ceramic based on lead zirconium titanate, which has a piezoelectric effect because of its ferroelectric properties. Because of the piezoelectric effect, the ceramic expands when electrical potential is present, so that it is possible to make actuators from such a multi-layer ceramic.
Also present in the known piezo actuators are outer electrodes, which are applied continuously to one lateral face of the base body and contact the internal electrodes.
In order to reduce the costs of producing the piezo actuators, an effort is made to replace the material of the internal electrodes and the material of the outer electrode with copper. In known piezo actuators, the outer electrode has the form of a continuous layer. This form of the outer electrode is not suitable for outer electrodes made of copper. Under thermal demands, which occur, for example, when contact elements are soldered onto the outer electrode, with a continuous layer there is a shearing force between the outer electrode and the base body of the piezo actuator, which leads to damage to the boundary layer between the outer electrode and the base body.
This damage to the boundary layer is accompanied by a reduction of the effective bonding area between the outer electrode and the base body.
A piezo actuator produced in this manner is subjected, in the course of its use, to a multitude of mechanical strains, which in turn lead to shearing forces between the base body and the outer electrode. Because of the relatively high strength of the copper electrode, these additional loads result in the damage that arises when the thermal load spreads across the surface of the boundary layer and leads to detachment of the outer electrode. Thus the piezo actuator fails and can no longer be used.
Described herein an electrical multi-layer components and a method for producing it, where the danger of detachment of the outer electrode is reduced.
An electrical multi-layer component is described that includes a base body. The base body contains a stack of stratified ceramic layers, and internal electrodes lying between them. An outer electrode is placed on one lateral face of the base body, for contacting internal electrodes.
The outer electrode has the form of a layer in which at least one indentation is provided.
At the place of the indentation, the thickness of the layer can be reduced. That results in an intended tear point at this location, at which the tensile and compressive strength of the layer or outer electrode is reduced. As a result, even relatively small shearing forces of the outer electrode against the base body are sufficient to produce a tear through the layer at the intended tear point. As a result, the shearing load acting on the entire layer can be significantly reduced, since the tensile and compressive forces that produce the shearing load along the layer or along the surface of the base body can only add up over a correspondingly shorted distance.
In particular, the layer can have a local minimum thickness at the location of the indentations.
In just the same way, however, it is also to design the indentation in the outer electrode in such a way that the outer electrode is interrupted at the location of the indentation. As a result, in a sense a tear is provided already when the outer electrode is produced, so that it is inherently impossible for great shear forces to result from addition of tensile and compressive forces over long distances.
The outer electrodes can have areas with essentially constant layer thickness, which has the benefit that outer electrode can be applied via a screen-printing present.
In an embodiment of the electrical multi-layer component, the outer electrode contains copper, enabling the material costs for the component to be reduced.
Furthermore, it is beneficial if the ceramic layers are piezoelectrically active. This is beneficial because the electrical multi-layer component can be used as a piezo actuator.
A piezoelectric effect is obtained, for example, through the use of a ceramic based on lead zirconate titanate.
The indentations can run in the form of troughs, with the troughs extending along a longitudinal axis. The projection of these longitudinal axes on the lateral face of the stack of stratified ceramic layers or internal electrodes on the side of the outer electrode intersects the internal electrodes at an angle. That increases the chances that every internal electrode that runs along the corresponding outer surface of the stack can be contacted.
This means that the angle α takes on a value that 0 and 180°.
It is possible to advantageously arrange a plurality of indentations at equal distances. This has the benefit that the outer electrode is divided uniformly into a plurality of areas, which means a corresponding reduction in the maximum occurring shearing force.
In addition, a plurality of indentations can be distributed uniformly over the layer.
Furthermore, ti is also possible to arrange a plurality of indentations in such a way that they form a periodically recurring structure (for example a rhombus or square).
In order to promote the function of the indentation as an intended tear point, it is advantageous for the minimum layer thickness at the point of the indentation to be a maximum of 75% of the layer thickness in the area where the layer thickness is essentially constant.
The outer electrode can be applied in the form of a screen-printing paste that contains copper powder.
Applying the outer electrode in the form of a screen-printing paste has the benefit that by using appropriate screen-printing masks; a reliable and uniform arrangement of indentations can be achieved.
When applying the outer electrode in the form of a screen printing paste or by a screen printing process, the indentations may have a minimum width of 200 μm, since otherwise it may not be possible to guarantee a defined indentation on the basis of the consistency of the paste or the mesh number used. Smaller indentations can be realized only with increased effort and expense, due to the low viscosity of the screen-printing paste normally used. It would be conceivable, however, to produce indentations with smaller lateral extents by using a more structurally viscous or more thixotropic screen-printing paste.
In addition, a method for producing an electrical multi-layer component is specified that includes the following:
a) Production of a base body with a stack of stratified ceramic layers and internal electrodes lying between them, and with an outer electrode placed on a lateral face of the base body for contacting internal electrodes, which has the form of a layer and in which at least one indentation is provided.
b) Contacting of the outer electrode with a contact element while exerting a shearing load between the outer electrode and the lateral face of the base body.
Normally materials are used for the ceramic layers and the outer electrode whose thermal expansion coefficients differ from each other.
The method for producing the multi-layer electrical component has the advantage that in spite of exerting the shearing load, because of the intended tear points or already existing tears formed by the indentation, the danger of detachment of the outer electrode is reduced.
Despite the use of materials to their particular function with normally differing thermal expansion coefficients for the outer electrode and ceramic layers, the foregoing method allows the contacting of the outer electrode with a contact element to take place by soldering.
When copper is used for the outer electrode and when a PZT ceramic is used for the ceramic layers of the stack, the possible exists of performing the soldering at a temperature >200° C., which allows the use of correspondingly high-melting solders. Such high-melting solders have the advantage that they permit use of the electronic components, for example a piezo actuator, at temperatures of up to 150° C., such as occur, for example, on the combustion engine of a motor vehicle.
Embodiments will now be explained in greater detail with reference to corresponding figures.
An outer electrode 5 is placed on the upper lateral face 4 of the base body 1, for contacting internal electrodes 3. The outer electrode 5 has indentations 6. It can be seen from the detailed view of
The minimum layer thickness dmin in the area of indentation 6 should be not more than 75% of the layer thickness d of the outer electrode 5, since it may not otherwise be possible to guarantee the intended tear point.
The minimum layer thickness dmm in the area of indentation 6 should be not more than 75% of the layer thickness d of the outer electrode 5, since it may not otherwise be possible to guarantee the intended tear point.
An additional embodiment is shown in
Since the tensile or compressive stresses add up over a longer distance in the case of the outer electrode 5 a, the grater maximum shear load also arise for the outer electrode 5 a, identified in
A favorable minimum width b is around 200 μm. A typical layer thickness d producible by screen printing processes is around 15–25 μm.
When the outer electrode 5 is applied in the form of a screen printing paste, it is usual to use a screen printing paste that contains glass frit. This glass frit is used to improve the mechanical bonding of the outer electrode 5 to the base body 1. Depending on the glass frit used and depending on the ceramic used, or depending on the setting of additional process parameters, it can occur that the glass frit forms an intermediate layer 9 lying between the outer electrode 5 and the edge of the base body 1, which intermediate layer 9 is interrupted in the area of the internal electrodes 3. When considering the problem of the shearing off or separation of the outer electrode 5, the intermediate layer 9 of glass frit must be allocated to the base body 1, as can be seen from
For the glass frit it is possible, for example, to use a combination that contains lead oxide, silicon oxide, boric oxide and possibly additional components.
The apparatus and methods described herein are not limited to use in piezo actuators, but may be applied to all multi-layer components, for example also to capacitors.