Method for producing a multilayer element

Abstract

A method for producing a ceramic multilayer element is disclosed. In an embodiment the method includes forming a plurality of multilayer segments in a green state, wherein each multilayer segment is formed by pressing together a plurality of ceramic layers in the green state and pressing together the multilayer segments in the green state to form a multilayer element that is in the green state. The method further includes sintering the multilayer element that is in the green state to form a ceramic multilayer element that includes the ceramic layers and electrode layers arranged one on top of another, wherein at least one or more of a temperature at which the multilayer segments are pressed together, a pressing force applied during the pressing of the multilayer segments, and/or a duration of the pressing of the multilayer segments are adjusted.

Claims

1. A method for producing a ceramic multilayer element, the method comprising: forming a plurality of multilayer segments in a green state, wherein each multilayer segment has a plurality of ceramic layers in the green state and a plurality of electrode layers, wherein, while forming the plurality of multilayer segments, the multilayer segments are not located one on top of the other but are separate from each other, wherein each multilayer segment is formed separately by pressing together the plurality of ceramic layers and electrode layers at one pressing force; stacking the plurality of multilayer segments in the green state one on top of the other after forming the plurality of multilayer segments; pressing together the stacked multilayer segments in the green state thereby forming a multilayer element that is in the green state, wherein the one pressing force is greater than a pressing force applied during the pressing together of the stacked multilayer segments; and sintering the multilayer element that is in the green state thereby forming the ceramic multilayer element, the ceramic multilayer element comprising the ceramic layers and electrode layers arranged one on top of another, wherein at least one or more of a temperature at which the stacked multilayer segments are pressed together, the pressing force applied during the pressing together of the stacked multilayer segments, and a duration of the pressing together of the stacked multilayer segments, are adjusted such that the sintered ceramic multilayer element includes a boundary region between the stacked multilayer segments, the boundary region having a tensile strength that causes the boundary region to function as a predetermined breakage region, wherein the tensile strength is determined by adjusting at least the pressing force applied during the pressing together of the stacked multilayer segments.

2. The method as claimed in claim 1, wherein forming the multilayer segments comprises pressing together the plurality of ceramic layers into a film stack, each ceramic layer containing an organic binder.

3. The method as claimed in claim 2, wherein pressing together the plurality of ceramic layers comprises pressing the ceramic layer together at a temperature that is lower than a temperature at which the multilayer segments are pressed together.

4. The method as claimed in claim 3, wherein the temperature at which the ceramic layers are pressed together deviates by a maximum of 25% from room temperature, and the temperature at which the multilayer segments are pressed together is between 75° C. and 95° C.

5. The method as claimed in claim 2, wherein a binding effect of the organic binder during the pressing together of the multilayer segments differs from a binding effect during the pressing of the ceramic layers.

6. The method as claimed in claim 2, wherein forming the multilayer segments further comprises separating the multilayer segments from the film stack using a cutting tool.

7. The method as claimed in claim 6, wherein the multilayer segments are separated from the film stack with a contour shape.

8. The method as claimed in claim 7, wherein the multilayer segments have one of the following contour shapes: rounded, circular with flattened sides, circular, or oval.

9. The method as claimed in claim 6, wherein the cutting tool transports the separated multilayer segments.

10. The method as claimed in claim 9, wherein the cutting tool transports the separated multilayer segments into a cavity for pressing.

11. The method as claimed in claim 10, wherein the cutting tool comprises a stamping tool and wherein the multilayer segments are pressed together by the stamping tool pressing on a face surface of the multilayer segment that was last inserted into the cavity.

12. The method as claimed in claim 11, wherein the multilayer segments are pressed together applying a press pin that presses on an undermost multilayer segment that is in the cavity, toward the stamping tool.

13. The method as claimed in claim 6, wherein the cutting tool comprises a stamping tool.

14. The method as claimed in claim 2, wherein the ceramic layers comprise ceramic layers with imprinted metallizations.

15. The method as claimed in claim 1, wherein the ceramic multilayer element includes the boundary region between the stacked multilayer segments in an end multilayer element that provides the ceramic multilayer element with a function.

16. The method as claimed in claim 15, further comprising separating the stacked multilayer segments from a film stack using a cutting tool.

17. The method as claimed in claim 16, wherein the ceramic layers comprise ceramic layers with imprinted metallizations and wherein the multilayer element forms at least a part of a piezoelectric multilayer element.

18. The method as claimed in claim 1, wherein the tensile strength being determined by adjusting the duration of the pressing together of the stacked multilayer segments.

19. The method as claimed in claim 1, wherein the tensile strength is determined by adjusting the temperature at which the stacked multilayer segments are pressed together and adjusting the pressing force applied during the pressing together of the stacked multilayer segments.

20. The method as claimed in claim 18, wherein the tensile strength is determined by adjusting the temperature at which the stacked multilayer segments are pressed together and adjusting the duration of the pressing together of the stacked multilayer segments.

21. The method as claimed in claim 1, wherein the tensile strength is determined by adjusting the pressing force applied during the pressing together of the stacked multilayer segments and adjusting the duration of the pressing together of the stacked multilayer segments.

22. The method as claimed in claim 1, wherein the tensile strength is determined by adjusting the temperature at which the stacked multilayer segments are pressed together, adjusting the pressing force applied during the pressing together of the stacked multilayer segments, and adjusting the duration of the pressing together of the stacked multilayer segments.

23. The method as claimed in claim 1, wherein the stacked multilayer segments are pressed together with a height of 0.8 mm to 1.2 mm.

24. The method as claimed in claim 1, wherein the stacked multilayer segments are pressed to produce the ceramic multilayer element with a height of 70 mm to 100 mm.

25. The method as claimed in claim 1, wherein the stacked multilayer segments are pressed together with a cross-sectional area of less than 110 mm.sup.2.

26. The method as claimed in claim 1, wherein the stacked multilayer element is debinded.

27. The method as claimed in claim 1, wherein the stacked multilayer element forms at least a part of a piezoelectric multilayer element.

28. The method as claimed in claim 1, wherein the predetermined breakage region runs parallel to the ceramic layers and has reduced tensile strength, the predetermined breakage region being localized between adjacent multilayer segments and in pails of the ceramic layers of the adjacent multilayer segments.

29. The method as claimed in claim 28, wherein the predetermined breakage region is partially contained in an electrode layer that is arranged between adjacent ceramic layers.

30. The method as claimed in claim 28, wherein the predetermined breakage region is one of a plurality of breakage regions that are distributed over a height of the stacked multilayer element at regular distances.

31. The method as claimed in claim 28, wherein the predetermined breakage region runs between adjacent multilayer segments and is partially contained in them.

32. The method as claimed in claim 28, wherein the predetermined breakage region has a porosity that is higher than an average porosity of the ceramic layers in the stacked multilayer element.

33. A method for producing a ceramic multilayer element, the method comprising: forming a plurality of multilayer segments in a green state, wherein each multilayer segment has a plurality of ceramic layers in the green state and a plurality of electrode layers, wherein, while forming the plurality of multilayer segments, the multilayer segments are not located one on top of the other but are separate from each other, wherein each multilayer segment is formed separately by pressing together the plurality of ceramic layers and electrode layers at one pressing force; stacking the plurality of multilayer segments in the green state one on top of the other after forming the plurality of multilayer segments; pressing together the stacked multilayer segments in the green state thereby forming a multilayer element that is in the green state, wherein the one pressing force is greater than a pressing force applied during the pressing together of the stacked multilayer segments; and sintering the multilayer element that is in the green state thereby forming the ceramic multilayer element, the ceramic multilayer element comprising the ceramic layers and electrode layers arranged one on top of another, wherein at least one or more of a temperature at which the stacked multilayer segments are pressed together, the pressing force applied during the pressing together of the stacked multilayer segments, and a duration of the pressing together of the stacked multilayer segments, are adjusted such that the sintered ceramic multilayer element includes a boundary region between the stacked multilayer segments, the boundary region having a tensile strength that causes the boundary region to function as a predetermined breakage region, and wherein the tensile strength is determined by adjusting at least the duration of the pressing together of the stacked multilayer segments.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The described embodiments are described in more detail by means of the following embodiment examples. Here:

(2) FIG. 1, which includes FIGS. 1a and 1b, shows a multilayer element with a predetermined breakage site in a first arrangement;

(3) FIG. 2, which includes FIGS. 2a and 2b, shows a multilayer element with a predetermined breakage site in a second arrangement;

(4) FIG. 3 shows a cross-sectional view of a multilayer element;

(5) FIG. 4 shows a perspective view of a part of an essentially massive block comprising a cavity, with a partially inserted punching tool;

(6) FIG. 5 shows a top view of an essentially massive block comprising a cavity; and

(7) FIG. 6 shows a side view of an essentially massive block comprising a cavity, with a partially inserted punching tool.

(8) The following list of reference symbols may be used in conjunction with the drawings:

(9) 1 Multilayer element

(10) 2 Ceramic layer

(11) 3 Electrode layer

(12) 4 Multilayer segment

(13) 5 Predetermined breakage site

(14) 6 External contact

(15) 7 Massive block

(16) 8 Cavity

(17) 9 Punch tool

(18) 10 Bottom-side opening of cavity

(19) 11 Vertical fastener drilling

(20) 12 Horizontal drilling

(21) 13 Upper side opening of cavity

(22) A-A Section line

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(23) FIG. 1a shows a multilayer element 1 with ceramic layers 2 and electrode layers 3 arranged alternately one above the other. FIG. 1b shows a magnified section from FIG. 1a. The multilayer element has a plurality of multilayer segments 4, each of which has a plurality of ceramic layers and electrode layers. Between each two adjacent multilayer segments 4 is a predetermined breakage region 5, which is designed as a region with reduced strength compared to the surface areas between other ceramic layers deeper into the interior of each multilayer segment 4. The predetermined breakage region 5 is contained in the boundary region between two adjacent multilayer segments and merges with its reduced mechanical strength into an undermost and uppermost ceramic layer of adjacent multilayer segments.

(24) The predetermined breakage region 5 is realized, by means of the method of producing the multilayer element, as a region of reduced porosity by comparison with the porosity of other ceramic layers within each multilayer segment 4. The elevated porosity in the boundary region between two multilayer segments 4 can be determined by adjusting the combination of the following parameters: Temperature and/or applied pressure and duration of pressing force at which the ceramic films are pressed together to form the precursor product film stack, Choice of a binder used for the ceramic films, its binding effect in dependence on the parameters mentioned above, Temperature and/or pressure and duration of pressure at which the multilayer segments are pressed together.

(25) FIG. 1 shows in particular how the boundary region between two multilayer segments 4 can be designed. The boundary region with reduced tensile strength runs, according to one embodiment, between two edge- or face-side ceramic layers 2 of adjacent multilayer segments 4, where one of the ceramic layers is provided with an imprinted internal electrode layer 3. This means that the predetermined breakage site 5 runs through the internal electrode layer 2, or cracking caused by certain tensile stresses runs through the internal electrode layer.

(26) FIG. 2a shows a multilayer element 1 with ceramic layers 2 and electrode layers 3 arranged alternately one above the other. FIG. 2b shows an enlarged section from FIG. 1a. In composition, this embodiment of the multilayer element corresponds to that of FIG. 1. The predetermined breakage region 5 on two adjacent multilayer segments, however, is designed differently here. It merges into two edge- or face-side and adjacent ceramic layers 2 of adjacent multilayer segments 4, and there is no electrode layer 3 between these ceramic layers 2. A construction of this kind can be achieved, for example, by stacking the multilayer segments stamped out of a film stack with different orientations for pressing. This means that, for example, two multilayer segments 4 that are adjacent and pressed together can have face surfaces turned toward each other, with these face surfaces free of internal electrode layers 3.

(27) FIG. 3 shows a multilayer element 1 with a preferred contour in a top view. The multilayer element or each multilayer segment 4 of the multilayer element in this case is circular in cross section, with flat sides. In a particularly favorable way, external electrodes 6 can be applied to or arranged on the flattened side, where the electrodes are each in contact with a set of internal electrode layers 3 arranged one above the other and having the same electric polarity.

(28) The multilayer element or its multilayer segments preferably have a diameter of 8-10 mm; the flattened sides each have a length of 2-4 mm.

(29) Preferably, the following operation is chosen to produce the multilayer element.

(30) According to one embodiment, in producing a multilayer element, after the addition of a suitable binder and disperser system in the form of a slurry, a ceramic powder with piezoelectric properties is processed into films.

(31) The films are imprinted according to the desired design with an electrode paste, in particular, screen-printed, so that an isolation zone on the flattened segments of the multilayer element is made available. The isolation zone comprises a nearly field-free region, where adjacent internal electrodes do not overlap. Each film is again imprinted, where the printing of adjacent films of a film stack takes place with an offset.

(32) This is followed by the lamination or stacking of, preferably, 25-50 films. They are pressed together to a thickness of 1-2 mm by means of a pressing operation at about room temperature under a weight of 100 metric tons, with respect to an area of 105×105 mm.sup.2. The dielectric thickness of each ceramic film is then about 100-120 μm. The low thickness of the film stack enables multilayer segments of any kind to be cut from the film stack, preferably ones with a round, oval or octagonal cross section.

(33) Multilayer segments with the desired cross-sectional shape are stamped out of the pressed film stack with a stamp. The stamp or stamping tool comprises a sharp projecting edge for stamping out multilayer sequences; further inward, it has a flat area that presses on the multilayer segment surface and separates it from the film stack.

(34) Advantageously, the stamped-out multilayer segments, in contrast to individual ceramic layers (which each come from a single ceramic film) are much easier to handle in the process of manufacturing the multilayer element. For example, they can be grabbed and transported better. In this case, the risk of damage to these multilayer segments is also reduced. The effectiveness of these advantages is especially apparent when the cross-sectional area of the stamped out multilayer segment is 20 mm.sup.2 or smaller.

(35) The multilayer segments with low height that are stamped from the film stack or stacks are preferably stacked in a cavity by means of the stamping tool. A multilayer segment is pressed onto a multilayer segment or partial multilayer element that is already in the cavity, at a force of 1500 N at about 85° C. The operation is repeated until a piezoelectric multilayer element with any desired height, preferably between 70 and 100 mm, is made.

(36) The advantage of pressing small volumes becomes clear in this case, since the scatter of the applied pressure is relatively small with respect to a small area. Frictional forces on the inner walls of the cavity that arise are smaller than in the case of larger multilayer elements, for example, ones that have a height of 10-1000 mm. Thus, multilayer elements with extremely high symmetry can be made; pressing warpages can no longer be detected in a completed multilayer element. Moreover, the production process provides that internal electrodes that may be present in a multilayer element are not affected or are only minimally affected by warpage or bending. In the absence of the pressing warpage noted above, internal electrode parts that are creased or bent over at the edge of the multilayer element are no longer seen.

(37) Preferably used criteria for establishing the absolute breaking force at the seam between the multilayer segments are the applied pressure, temperature or hold time in a pressing operation.

(38) A still-green multilayer element produced by pressing multilayer segments can then be debinded and sintered. External contacts can then be applied to the side surfaces of the multilayer element.

(39) Measurement of multilayer elements made by pressing multilayer segments show a seam region between two adjacent multilayer segments with lower strength than the bonds between individual ceramic layers lying in the multilayer segments. An advantage of this effect was seen to be that the multilayer element, because of the weaker seam regions, contains one or more predetermined breakage sites that favor a stable failure or controlled failure of the multilayer element. Additional process or manufacturing steps to introduce predetermined breakage sites into the element can thus be omitted. From the standpoint of manufacturing technology, the number of predetermined breakage sites alone is already determined by the height of the multilayer element, since a certain height of the multilayer element implies a number of multilayer segment boundaries and thus predetermined breakage regions.

(40) The predetermined breakage region responds at certain tensile stresses, where it forms a crack running parallel to the ceramic layers or electrode layers. Since the dielectric cannot be entirely broken through in the direction between two internal electrodes by the crack, a short circuit between two electric poles of the multilayer element supported by internal electrodes situated one above the other caused by certain tensile stresses can be avoided.

(41) FIGS. 4-6 show different perspective views of a preferably massive metallic block 7, in which multilayer segments are pressed together to form a multilayer element 1 in a cavity 8 by means of a stamping tool 9.

(42) For illustration, FIG. 4 shows a section of a preferably massive and metallic block 7 in a perspective view.

(43) The block has the following dimensions: width=130-170 mm, height=115-155 mm, depth=30-70 mm. A cavity 8, which is realized as a drilling, runs centrally and vertically through block 7. The cavity 8 has an opening 10 at the bottom that allows the insertion of a press pin, not shown, which pushes against a stamping tool 9, which is shown inserted into the cavity. The cavity 8 has an internal clear diameter that is dimensioned so that a multilayer segment can be inserted into the cavity 8 together with the transport means or stamping tool 9 that surrounds and transports the multilayer segment.

(44) Multilayer segments arranged between the press pin, which pushes from below, and the stamping tool 9, inserted from above, are pressed together.

(45) According to one embodiment, the press block 7 has a number of parts that are positioned on locating pin(s) and secured with screws. However, the press block can also be made in one piece, i.e., from a casting.

(46) Preferably, the press block is made of steel, and other materials such as ceramic, sintered materials or other hard metals can be used.

(47) The press block 7 has a plurality of vertically running drillings 11, which according to one embodiment serve to secure the block 7 by means of screws or other fastening means to another object, for example, another housing. Horizontal fastener drillings 12 are also shown. By means of suitable fasteners such as screws, the horizontal fastener drillings 12 serve for assembly of the possibly several parts of the block 7 (of which only one part is shown by this figure) into a block unit.

(48) According to an advantageous embodiment, heating elements are inserted into the vertically running drillings 11 to heat the press block. The heating elements can be realized as heating resistance wires. The heating elements can be contained in drillings 11 together with a fastening element if the drillings 11 are also used to fasten the press block.

(49) FIG. 5 shows a top view of block 7 that is shown only partially in perspective view in FIG. 4. An opening of cavity 8 is shown on the top side and centrally arranged in the block. The section line A-A is also shown; a section of the entire block through this section line was shown in the previous figure. The lateral course of the horizontal fastener drillings 12 and the openings of the vertical drillings 11 are shown.

(50) FIG. 6 is a side view of block 7. The partially inserted stamping tool 9 is shown at the top. The lateral course of a fastener drilling 12 is shown at the bottom.